Serotonin (5‐Hydroxytryptamine): Metabolism, Signaling, Biological Functions, Diseases, and Emerging Therapeutic Opportunities

ABSTRACT

Serotonin (5‐hydroxytryptamine; 5‐HT) is an evolutionarily conserved monoamine neurotransmitter that plays critical roles in various physiological systems, functioning as a neurotransmitter, hormone, and paracrine signaling molecule. This review synthesizes current research on 5‐HT metabolism (biosynthesis, transport, and degradation), 5‐HT receptor‐mediated signaling pathways (seven receptor families and 14 subtypes), and broad biological functions of 5‐HT. We emphasize the roles of 5‐HT in both health and disease, with a particular focus on its emerging significance in the tumor immune microenvironment. Studies have shown that dysregulated 5‐HT signaling is associated with various pathological conditions, including functional gastrointestinal disorders, psychiatric diseases, metabolic disorders, and cancer progression. Notably, this review describes novel mechanisms by which 5‐HT modulates tumor immunity, including its effects on macrophage polarization, dendritic cell function, T cell activity, and PD‐L1 expression, and it explores the therapeutic potential of targeting 5‐HT‐associated pathways. Promising therapeutic strategies that target 5‐HT include combining selective serotonin reuptake inhibitors with immune checkpoint inhibitors, inhibiting key metabolic enzymes (e.g., Tph1 and MAO‐A), and developing receptor subtype‐specific agents (e.g., 5‐HT₇R antagonists). These findings position the 5‐HT system as a pivotal target for next‐generation precision therapeutics across multiple disease domains.

This article reviews the roles of 5‐HT metabolism, 5‐HT receptors, and their related signaling pathways in normal physiology and various diseases, and explores their potential value in disease treatment, providing a reference basis for research and clinical treatment in related fields.

1. Introduction

Serotonin (5‐hydroxytryptamine; 5‐HT) research spans the fields of neurobiology, pharmacology, immunology, and systems physiology. 5‐HT functions as a key neurotransmitter, hormonal regulator, and paracrine signaling molecule, and it is widely involved in the regulation of multiple physiological systems [1, 2]. Moreover, 5‐HT is currently recognized as a central modulator of central nervous system (CNS) functions, such as mood, cognition, and pain perception, and peripheral processes, including gastrointestinal (GI) motility, cardiovascular homeostasis, platelet aggregation, and immune responses [3, 4]. In addition to its fundamental biological roles, 5‐HT has attracted enormous attention because of its clinical relevance in central and peripheral disorders, including depression, irritable bowel syndrome (IBS), neurodegenerative diseases, metabolism, and other syndromes [5, 6, 7, 8]. Additionally, recent studies have highlighted the increasingly prominent role of 5‐HT in tumor immunomodulation [9]. The important role of 5‐HT in many diseases emphasizes its emerging potential for use as a therapeutic target and places this field at the forefront of translational research.

There is broad consensus in the scientific community that 5‐HT functions through a complex network of metabolic pathways and receptor subtypes; at least 14 subtypes of 5‐HT receptors (5‐HTRs) have been identified to date, and each subtype mediates distinct physiological effects [10, 11]. Although the pathways that are involved in 5‐HT metabolism and 5‐HTR signaling are well characterized, their functional roles and mechanisms in the context of disease remain controversial [12, 13]. For example, the 5‐HT transporter (SERT) has been particularly well studied, and it is the cornerstone of current antidepressant pharmacotherapy [14]. However, the precise mechanisms underlying the role of 5‐HT in mood disorders, as well as the contribution of peripheral 5‐HT to systemic diseases such as metabolic syndrome or cardiovascular dysfunction, are still under debate [15]. Emerging evidence that 5‐HT plays dual roles in tumorigenesis has further complicated the traditional CNS‐centric understanding of 5‐HT, highlighting the need for integrative and systems research [16].

In light of these developments, this review aims to provide a comprehensive and up‐to‐date summary of the current knowledge about 5‐HT, including its metabolism, receptor‐mediated signaling, physiological functions, and pathophysiological roles [17, 18, 19]. Here, we systematically organize the advances in understanding how 5‐HT signaling modulates disease progression and how these pathways may be leveraged in the development of multitarget therapeutic strategies [20, 21, 22, 23, 24]. By incorporating insights from the fields of biology, immunoregulation, and molecular signaling, this review highlights the integrative role of the 5‐HT system in the pathophysiology of multiple organ systems, offering a comprehensive perspective for future mechanistic research.

This review is organized as follows. First, we provide an overview of 5‐HT metabolism and summarize its physiological roles. Next, we describe 5‐HTR‐mediated signaling cascades and receptor–receptor interactions. Then, we assess currently available therapeutic strategies and their limitations. Subsequent sections explore recent advances in 5‐HT‐related research in the fields of cancer, functional GI disorders, neuropsychiatric diseases, and metabolic syndromes, with a focus on its immunomodulatory functions. We further evaluate the potential of 5‐HT to be used as a biomarker of disease. Finally, we discuss emerging directions for targeting the 5‐HT system, including strategies for multitarget modulation.

2. 5‐HT Metabolism and Physiological Functions

5‐HT is widely distributed in both the CNS and peripheral tissues, where it performs diverse regulatory functions in the nervous, endocrine, and immune systems [1]. In recent years, advances in molecular biology, neuroscience, and immunology have further elucidated the mechanisms underlying 5‐HT synthesis and metabolism as well as its roles in the physiological regulation of multiple systems.

2.1. Biosynthesis, Transport, and Degradation

5‐HT is a substance that exerts a strong vasoconstrictor effect, and it was first isolated from the intestine [1]. Tryptophan is converted into 5‐hydroxytryptophan (5‐HTP) by tryptophan hydroxylase (Tph), and then, 5‐HT is further generated via the action of 5‐HTP decarboxylase [10]. In this biosynthetic process, Tph functions as the rate‐limiting enzyme in 5‐HT synthesis. There are two isoforms of Tph in the body: Tph1 and Tph2 [25]. Tph1 is located mainly in the chromaffin cells of the intestine, where it produces approximately 95% of the total 5‐HT molecules throughout the body, whereas Tph2 is expressed primarily in the brain within the CNS, where it synthesizes approximately 5% of the total 5‐HT molecules throughout the body [26]. Since 5‐HT cannot cross the blood‒brain barrier, the 5‐HT that is synthesized by Tphs is confined to either the peripheral or CNS [27] (Figure 1). Notably, in addition to the classical synthesis pathway, the cytochrome P450 enzyme system (especially CYP2D6) may participate in the synthesis of 5‐HT via the O‐demethylation of 5‐methoxytryptamine [28]

FIGURE 1.

Pathways of serotonin (5‐HT) metabolism. Within the cytoplasm of cells, tryptophan hydroxylase (Tph) catalyzes the conversion of l‐tryptophan (Trp) to 5‐hydroxytryptophan (5‐HTP), which is then converted to 5‐HT via aromatic amino acid decarboxylase (AADC). After crossing the cell membrane via exocytosis, 5‐HT binds to a 5‐HT receptor (5‐HTR). Extracellular 5‐HT is transported into the cytoplasm via the 5‐HT transporter (SERT). 5‐HT can be catabolized into 5‐hydroxyindoleacetic acid (5‐HAA), NH₃, and reactive oxygen species (ROS) by mitochondrial monoamine oxidase A (MAO‐A). Abbreviations: SLC1A5: solute carrier family 1 member 5; SCC7A5: solute carrier family 1 member 7.

In the circulatory system, the 5‐HT that is synthesized by the body is stored in platelets through SERT, where it performs important physiological functions [26, 29]. 5‐HT plays a physiological role by binding to specific receptors and then quickly dissociating from these receptors [11]. The dissociated 5‐HT is then reabsorbed into platelets or other related cells by SERT, resulting in a loss of its physiological activity, thereby terminating its effects in the body [30, 31].

In tissues, platelets, and other related cells, the metabolism of 5‐HT mainly depends on monoamine oxidase A (MAO‐A) in mitochondria, and the function of MAO‐A requires flavin adenine dinucleotide as a cofactor [32, 33]. 5‐HT is metabolized mainly into 5‐hydroxyindoleacetic acid (5‐HIAA) [34]. This metabolic pathway involves the initial oxidation of 5‐HT by MAO‐A to generate the corresponding aldehyde. This aldehyde is then further oxidized by aldehyde dehydrogenase to form 5‐HIAA, which is an indoleacetic acid derivative [34]. 5‐HIAA is subsequently excreted by the kidneys [34]. In addition, there is a minor metabolic pathway by which 5‐HIAL is converted to 5‐hydroxytryptol by acetaldehyde reductase, but this pathway is generally considered nondominant [35]. During the degradation of 5‐HT by MAO‐A, reactive oxygen species (ROS) are also generated, and these ROS may induce oxidative stress in cells, further regulating cellular functions or contributing to pathological responses [16, 36].

2.2. Physiological Functions

5‐HT, which is an important neurotransmitter, is widely distributed in various tissues, such as the CNS, GI tract, and platelets [37]. 5‐HT plays key roles in many physiological processes, especially in the CNS and peripheral systems [38]. 5‐HT is involved in mood regulation, cognitive function, sleep, appetite control, and so on, and it plays a crucial role in regulating peripheral systems, such as the cardiovascular and GI systems [38, 39].

The role of 5‐HT in the nervous system is particularly important, since it is involved in the regulation of various functions, such as mood, memory, learning, and pain transmission [40]. Research has shown that changes in 5‐HT levels are closely associated with several mental disorders [40]. 5‐HT deficiency or 5‐HTR dysfunction may lead to symptoms, such as depression and anxiety. Selective serotonin reuptake inhibitors (SSRIs) function by increasing the concentration of 5‐HT in the brain, thereby helping to treat depression and anxiety [41, 42]. In addition, 5‐HT plays dual roles in the perception, regulation and conduction of pain depending on the site of its action and its interaction with different receptors [43]. Peripherally, 5‐HT enhances pain perception by binding to the 5‐HTRs on peripheral nerves [44]. Especially during an inflammatory response, 5‐HT may exacerbate pain by promoting local vasodilation and increasing the excitability of local nerve conduction. In the CNS, the role of 5‐HT is more complex, since in this system, it can both alleviate and intensify pain [45]. 5‐Hydroxytryptaminergic (5‐HTergic) neurons communicate with the spinal thalamic tract through multisynaptic or monosynaptic connections, thereby modulating the transmission of pain signals [46]. Furthermore, the 5‐HT system interacts with other neurotransmitters, such as those in the norepinephrine (NE) and dopamine (DA) systems, to jointly regulate the transmission and modulation of pain [47, 48, 49].

Moreover, 5‐HT not only is involved in the regulation of emotions but also plays important roles in cognitive function, learning and memory [39]. 5‐HTergic neurons primarily project from the raphe nucleus to areas of the brain, such as the hippocampus and prefrontal cortex (PFC), as well as extending descending projections to regions such as the periaqueductal gray, striatum, and amygdala [50]. The function of 5‐HT in areas of the brain such as the hippocampus is considered to be closely related to synaptic plasticity and efficiency of information transmission during learning [49]. 5‐HT regulates hippocampal synaptic connectivity by altering the excitability of intracellular homeostasis and synaptic connections [51], and reduced release of 5‐HT leads to declarative memory dysfunction [52].

Furthermore, 5‐HT plays a crucial role in the peripheral nervous system, particularly in the regulation of GI function, the cardiovascular system, and platelet activity [53]. In the GI tract, 5‐HT regulates gut motility, secretion, and immune responses [54, 55, 56, 57]. An imbalance in the 5‐HT levels in the GI tract may be associated with a variety of GI disorders, such as IBS, constipation, or diarrhea [58, 59, 60]. Moreover, 5‐HT may have complex interactions with the gut microbiota. Indeed, 5‐HT affects the growth and metabolism of gut microbes, thereby indirectly influencing overall gut function and immune responses [61, 62]. Additionally, 5‐HT plays a significant role in the vascular system [63]. 5‐HT regulates vascular smooth muscle to induce vasoconstriction, thereby affecting the regulation of blood pressure [64]. Under normal physiological conditions, 5‐HT helps maintain the balance between blood vessel constriction and dilation, thus participating in the regulation of local blood flow [16]. However, under pathological conditions, the abnormal release or metabolism of 5‐HT may contribute to the development of diseases such as hypertension and arteriosclerosis [16, 65]. Additionally, 5‐HT plays a crucial role in platelets. During the coagulation process, platelets release 5‐HT, which aids in platelet aggregation and thrombus formation, thus contributing to hemostasis [66]. Changes in the concentration of 5‐HT in the blood are closely related to thrombus formation and dissolution, highlighting its significant role in the development of cardiovascular diseases [67, 68].

In addition to its traditional physiological functions, 5‐HT has recently been shown to play important roles in metabolic regulation and immune modulation [69]. Similar to the expression of the classic fasting hormone glucagon, the upregulation of Tph1 expression in the gut leads to a significant increase in 5‐HT levels during fasting [70]. This change effectively promotes the breakdown of fats in adipocytes, providing the necessary substrates for hepatic gluconeogenesis [70]. Furthermore, 5‐HT plays dual roles in inflammatory responses [71]. Research has shown that 5‐HT can modulate the intensity of inflammation by promoting or inhibiting the release of certain cytokines. This function makes 5‐HT a key focus in the study of autoimmune diseases and allergic reactions.

Overall, 5‐HT, which is an important neurotransmitter, not only regulates psychological and physiological functions such as mood, cognition, and sleep in the CNS but also participates in various physiological processes of the peripheral nervous system by regulating the GI tract, blood vessels, and platelet functions. As the understanding of the physiological functions of 5‐HT increases, more drugs that target the 5‐HT system are likely to emerge in the future, potentially leading to significant advances in the treatment of both neurological and peripheral system disorders.

3. Signaling Mechanisms

5‐HT plays a wide range of physiological roles in both the central and peripheral nervous systems [38]. 5‐HT must exert its effects through its corresponding receptors. Owing to the diversity of 5‐HTRs, the signaling mechanisms that are mediated by 5‐HT are complex, and different receptors activate distinct signaling pathways. The following is an overview of the signaling mechanisms that are mediated by 5‐HT.

3.1. 5‐HTRs

The classification of 5‐HTRs is complex. To date, 14 subtypes of 5‐HTRs have been identified in mammals, and these receptors are grouped into seven families: 5‐HT₁R to 5‐HT₇R [11, 72]. By activating different 5‐HTR subtypes, 5‐HT exerts diverse pharmacological effects (Figure 2).

FIGURE 2.

Pathways associated with serotonin (5‐HT) receptors. 5‐HT signals through seven receptors on membrane surfaces. These receptors activate several major interconnected signaling networks, such as those involving PKB/cAMP, PI3K/Akt, Ras/MEK1/2/MAPK, and PLC/PKCE, that trigger cell proliferation, migration, and invasion.

In the CNS, research on 5‐HT has focused primarily on 5‐HT₁Rs, 5‐HT₂Rs, 5‐HT₃Rs, and 5‐HT₆Rs [15, 73]. 5‐HT_1BR is predominantly distributed in the striatum and, to a lesser extent, in the basal ganglia, hypothalamus, substantia nigra, pituitary gland, and neocortex [74]. As a presynaptic autoreceptor, 5‐HT_1BR plays a key role in regulating the synthesis and release of 5‐HT. 5‐HT_1DR is abundantly expressed in the coronary arteries, cerebral arteries, and ganglia, where it mainly mediates vasodilation [74]. 5‐HT_1ER, which has a high affinity exclusively for 5‐HT, is diffusely distributed in the cerebral cortex and may be involved in various mental activities [75]. 5‐HT_1FR exhibits strong binding affinity for tritiated 5‐HT, and it is located primarily in layer V pyramidal neurons of the PFC; in the periphery, it is distributed mainly in the uterus and mesentery [76]. 5‐HT_2AR is widely expressed in the CA1, CA2, and CA3 regions of the hippocampus, and it is regulated by exogenous 5‐HT neurotransmission [77]. 5‐HT_2BR is expressed in multiple regions of the spinal cord and brain [77]. 5‐HT₆R, which is coupled to excitatory guanine nucleotide‐binding proteins (G proteins), activates adenylate cyclase (AC) to initiate intracellular signaling cascades [78, 79].

The role of 5‐HT in peripheral tissues has also received considerable attention. Studies have shown that 5‐HT can promote liver regeneration, 5‐HT_2AR plays a key role in signal transduction during this process [80]. 5‐HT_2BR is highly expressed in a wide range of peripheral tissues, including the heart, skeletal muscle, ovary, liver, kidney, lungs, pancreas, trachea, spleen, prostate, and salivary glands [81, 82]. 5‐HT₃R is expressed at high levels in enteric neurons, particularly in intestinal afferent neurons containing substance P [83]. In the human colon, 5‐HT₃Rs are most densely distributed in the myenteric plexus and less densely distributed in the muscular and mucosal layers [83]. In the GI tract of rats, 5‐HT₃Rs are located primarily in the neurons of the myenteric and submucosal plexuses, especially near the serosal membrane of neurons and circular and longitudinal muscle fibers; additionally, in the nerve fibers of the mucosa and submucosa, only low levels of 5‐HT₃RS expression are found in the interstitial cells of Cajal and enterochromaffin (EC) cells [84]. 5‐HT₄R is expressed in various GI cell types, including EC cells, smooth muscle cells, secretory cells, and neurons [85]. However, studies on the genetic and functional roles of 5‐HT₅R in the nervous and digestive systems remain limited. Finally, 5‐HT₇R is present in enteric neurons and smooth muscle cells [76, 86].

Notably, studies have revealed differential 5‐HT_2BR expression among patients with subtypes of IBS, suggesting that distinct IBS subtypes may have different underlying pathophysiological mechanisms [87]. Furthermore, 5‐HT and its receptors have been identified in hepatocellular carcinoma (HCC) cells. In these cells, 5‐HT and its receptors are predominantly localized in the cytoplasm, with strong positive expression near the cell membrane, indicating a possible autocrine mechanism of 5‐HT function in HCC [88, 89]. In particular, HCC cells expressing 5‐HT and its receptors demonstrate high proliferative potential [88, 89].

In summary, the distribution of 5‐HTRs varies significantly across different tissues and spatial dimensions. A thorough analysis of their spatial distribution patterns in various tissues and cell types will contribute to a more comprehensive understanding of the physiological functions of 5‐HTR and the signaling mechanisms they mediate.

3.2. 5‐HTR‐Associated Signal Transduction Pathways

Although 5‐HTRs are differentially distributed across various tissues and organs, the signaling pathways they mediate exhibit a certain degree of consistency. With the exception of 5‐HT₃Rs, all 5‐HTRs are coupled to G proteins, and thus, they activate second messenger systems and regulate cellular functions through the phosphorylation or dephosphorylation of intracellular proteins [90, 91]. These receptors primarily regulate two classical intracellular second messenger pathways. Moreover, an increasing number of studies have suggested that these receptor subtypes can also activate various other intracellular signaling cascades, indicating that they play more complex roles in the modulation of cellular functions.

3.2.1. 5‐HT₂R

The 5‐HT₂R family consists of three members: 5‐HT_2AR, 5‐HT_2BR, and 5‐HT_2CR [92]. These receptors are consistently coupled to the phospholipase C (PLC)‐β second messenger pathway in both native tissues and heterologous cells [93]. However, 5‐HT₂R can also be coupled to other second messenger pathways in a cell‐specific manner [92]. Although 5‐HT₂Rs are similar in terms of structure, pharmacology, and signaling pathways, there are some differences in their signaling properties [93].

5‐HT₂R activates PLC‐β in various tissues and cells, leading to the accumulation of inositol phosphates and an increase in intracellular Ca²⁺, which subsequently activates l‐type Ca²⁺ channels and stimulates protein kinases C (PKC) [94]. Additionally, 5‐HT_2A/2CR can also activate other phospholipases, such as PLA2 and PLD [95, 96]. Moreover, in CHO and 1C11 cells, 5‐HT_2A/2CR promotes the release of arachidonic acid (AA) via the activation of PLA2 [97], whereas in rat glomerular mesangial cells, 5‐HT_2A/2CR is coupled to PLD [98], although there is little evidence suggesting that 5‐HT_2BR can perform the same function. For 5‐HT_2CR, compared with 5‐HT, some agonists preferentially activate PLA2 or PLC in terms of relative efficacy. 5‐HT_2A/2CR generally does not regulate cAMP formation in most cells or tissues, but it can either stimulate or reduce cAMP accumulation in specific cell types [99]. In A1 cells, cAMP production is amplified via PKC‐α/δ and Ca²⁺/calmodulin (CaM) [100]. In rat mesangial cells, 5‐HT_2A/2CR can inhibit intracellular cAMP accumulation and adenylate cyclase (AC) activity in washed membranes, and this effect occurs independently of PLC, Ca²⁺, and PKC [101]. 5‐HT₂R activates extracellular regulated protein kinases (ERK) in contractile cells, such as vascular smooth muscle cells and mesangial cells [102, 103]. In vascular smooth muscle cells, ERK activation involves a complex mechanism that requires input from PLC, L‐type Ca²⁺ channels, and mitogen‐activated protein kinase (MEK) 1 [104]. In mesangial cells, this pathway also involves PKC stimulation, NADPH oxidase‐like enzyme activation, and ROS (H₂O₂ and/or superoxide) production [105, 106, 107]. In mesangial cells, ERK activation via 5‐HT_2A/2CR depends on the production of ROS by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which serves as a critical upstream step. Furthermore, 5‐HT_2A/2CR is coupled to the Jak/STAT signaling pathway, promoting the phosphorylation of janus kinase (Jak) 2 and signal transducers and activators of transcription (STAT) 3, inducing the nuclear translocation of STAT3, and regulating the expression of genes that are related to myogenic differentiation [108, 109]. 5‐HT₂R can increase intracellular Ca²⁺ levels either by releasing Ca²⁺ from internal stores or by activating both voltage‐dependent and voltage‐independent Ca²⁺ channels [110]. This effect is associated with Ca²⁺‐activated K⁺ channels and Ca²⁺‐activated Cl⁻ channels in the cell [111]. 5‐HT_2A/2CR can both promote NO release and inhibit cytokine‐induced inducible nitric oxide synthase (iNOS) expression [112]. Additionally, 5‐HT_2A/2CR can regulate various transport processes, such as the activation of the Na⁺/H⁺ exchanger in mesangial cells and the Na⁺/K⁺‐ATPase in airway smooth muscle cells. In mouse LMTK⁻ fibroblasts stably expressing 5‐HT_2BR, rapid activation of the Ras and ERK1/2 MAPK pathways, which involves both the Gαq and Gβγ subunits, has been observed [113, 114]. This signaling axis not only promotes cell proliferation but also leads to tumor formation in nude mice, indicating that tumorigenic potential of 5‐HT_2BR is primarily mediated by the Ras‐ERK pathway [114]. 5‐HT_2BR can also drive cell cycle progression via an ERK‐dependent mechanism, upregulating and activating the cyclin D1/cyclin dependent kinase (cdk) 4 and cyclin E/cdk2 complexes, resulting in hyperphosphorylation of the Rb protein [115]. The activation of cyclin D1 depends on the involvement of platelet‐derived growth factor (PDGF) receptor kinase. Moreover, activation of the PDGF receptor, ERK, and cyclin D1/E as well as cell proliferation rely on the function of the nonreceptor tyrosine kinase Src [116].

3.2.2. 5‐HT₃R

To date, five different 5‐HT₃R subunits have been identified, and among these subunits, the 5‐HT_3AR and 5‐HT_3BR subunits are the mature components of the 5‐HTR [117]. Notably, only 5‐HT_3AR subunits can form functional isoforms, and at least one 5‐HT_3AR subunit is present in all isomeric receptors. Many researchers refer collectively to various subtypes of 5‐HT₃R, and studies have also indicated that the selective blockade of 5‐HT_3AR has the same effect as the complete blockade of 5‐HT₃R activity [118, 119]. 5‐HT₃R antagonists reportedly increase cytoplasmic Ca²⁺ levels and ERK1/2 phosphorylation and inhibit microtubule formation [120, 121, 122]. 5‐HT_3AR promotes the proliferation of lung adenocarcinoma cells by increasing the phosphorylation of ERK1/2 [123].

3.2.3. 5‐HT₁R

The 5‐HT₁R family consists of five members: 5‐HT_1AR, 5‐HT_1BR, 5‐HT_1DR, 5‐HT_1ER, and 5‐HT_1FR [124]. 5‐HT₁R primarily interact with Gi/o proteins, thereby inhibiting AC activity and regulating various other signaling pathways and effector molecules [125].

5‐HT_1AR can inhibit AC in the human dorsal raphe nucleus (DRN), but it is ineffective in rats [126]. Under specific conditions, 5‐HT_1AR and 5‐HT_1BR can cooperate with the βγ subunit to activate AC in order to promote cAMP expression, whereas 5‐HT_1DR can stimulate mild levels of cAMP accumulation [126, 127]. This effect is specific to particular cell types: 5‐HT₁A is effective in HeLa cells [128], whereas 5‐HT_1BR and 5‐HT_1DR can increase Ca²⁺ levels in specific cell types, thus contributing to K⁺ channel regulation [129]. 5‐HT_1AR may also activate AA metabolism via PC‐PLC and PLA2, indirectly regulating nuclear factor kappa‐B (NF‐κB) and ERK signaling [127]. 5‐HT₁R can activate the ERK signaling pathway, which depends on Gβγ subunits, PC‐PLC, phosphatidylinositol 3‐kinase (PI3K), endocytosis, and ROS [129, 130]. 5‐HT_1BR activates ERK, protein kinase (Akt), and p70 S6 kinase more efficiently than 5‐HT_1AR [131]. In 5‐HT_1DR‐expressing systems, DNA synthesis can also be stimulated, and the proliferative effect is associated with the ERK‐ and PI3K‐related pathways [132]. In CHO cells, 5‐HT_1AR can induce the production of H₂O₂ and superoxide anions, which are critical for ERK activation [133]. Additionally, 5‐HT_1AR and 5‐HT_1BR can regulate NO production [134]. The effect of 5‐HT_1AR on NO varies across systems; it may either promote or inhibit NO generation [134], whereas 5‐HT_1BR promotes endothelial nitric oxide synthase (eNOS) activity in vascular endothelial cells via a Ca²⁺‐dependent mechanism [135, 136]. Moreover, 5‐HT₁Rs modulate various ion channels. 5‐HT_1AR is a classic regulator of G‐protein inwardly rectifying K⁺ (GIRK) channels, and it promotes their opening via Gβγ subunits [137]. 5‐HT_1AR can also inhibit N‐type and P/Q‐type Ca²⁺ channels in a Goα protein‐dependent manner [138]. 5‐HT_1BR and 5‐HT_1DR can activate Ca²⁺‐dependent K⁺ channels, which participate in postsynaptic modulation and negative feedback mechanisms [139, 140]. 5‐HT_1AR can regulate various transport mechanisms, including Na⁺/K⁺‐ATPase, Na⁺/H⁺ exchange, and Na⁺‐dependent phosphate uptake, via the Ca²⁺ and PKC pathways, suggesting that 5‐HT_1AR plays important roles in cell volume regulation and energy metabolism [141]. 5‐HT_1AR promotes the proliferation of T cells, small‐cell lung cancer cells, and carcinoid cells in an ERK signaling‐dependent manner [127]. 5‐HT_1BR enhances the proliferation of aortic endothelial cells and neuroblastoma cells, whereas 5‐HT_1DR stimulates DNA synthesis in lung cancer cells [142]. Presynaptic 5‐HT_1AR and 5‐HT_1BR receptors have been shown to negatively regulate 5‐HT release [127]. Moreover, 5‐HT_1FR participates in regulating 5‐HT release and may play a physiological role in antimigraine effects [143]. The pharmacological function of 5‐HT_1ER remains unclear because of the lack of selective ligands.

3.2.4. 5‐HT₄R

5‐HT₄R primarily exerts its effects through the activation of AC [144]. The major functional roles of this receptor include prokinetic effects in the GI tract, as well as positive inotropic, chronotropic, and lusitropic effects in the atria but not in the ventricles [145].

5‐HT₄R typically activates AC in both endogenous tissues and heterologously expressed cells. In the presence of 5‐HT, 5‐HT₄R has similar abilities to stimulate AC activity [144]. Since the primary function of 5‐HT₄R is to increase cAMP levels, most of the downstream effects of 5‐HT₄R are mediated by PKA activation [146, 147]. Additionally, 5‐HT₄Rs can modulate various ion channels. For example, they regulate Ca²⁺ channels by increasing the intracellular cAMP level and activating PKA [148]. In addition, 5‐HT₄R enhances the If pacemaker current in atrial myocytes, stimulates Cl⁻ currents in the human jejunal mucosa and rat distal colon, and activates Na⁺ currents in type II dorsal root ganglion cells via a diffusible second messenger pathway that is independent of cAMP, indicating that the receptor can also modulate channel function through the noncAMP/PKA pathway [149, 150]. However, 5‐HT₄R does not universally activate ion channels. For example, 5‐HT₄R inhibits Ca²⁺‐activated K⁺ currents, and this effect is also mediated through increased cAMP and PKA activation [151]. Furthermore, the receptor inhibits delayed rectifier K⁺ currents and suppresses voltage‐activated K⁺ channels in colliculi neurons via the cAMP/PKA pathway [152]. In addition, 5‐HT₄Rs facilitate striatal dopamine (DA) release, acetylcholine (ACh) release from the frontal cortex, and 5‐HT release in the hippocampus [153, 154].

Although the four 5‐HT₄R variants have similar abilities to stimulate AC, some functional differences between the isoforms have been observed. For example, compared with longer variants such as 5‐HT_4AR and 5‐HT_4BR, the 5‐HT_4ER and 5‐HT_4FR isoforms can significantly increase AC activity even in the absence of agonists [155, 156]. One proposed explanation is that the C‐terminal region may be involved in rapid or constitutive desensitization, and removal of this region could increase basal activity by reducing desensitization. Recent findings about the B2 bradykinin receptor support this mechanism [157]. Additionally, 5‐HT_4BR splice variants differ in their inclusion of PDZ interaction domains, which may further influence their signaling properties, thus increasing the complexity of this regulatory mechanism [144].

3.2.5. 5‐HT₅R and 5‐HT₆R

Previous research on 5‐HT₅R has been relatively limited. Studies have identified two subtypes within the 5‐HT₅R family: 5‐HT_5AR and 5‐HT_5BR [158]. Both subtypes have been cloned from rats and mice; however, only the 5‐HT_5AR has been successfully cloned from humans [158]. 5‐HT_5AR is expressed primarily in several brain regions, including the cerebral cortex, hippocampus, habenula, olfactory bulb, and granule layer of the cerebellum, whereas no 5‐HT_5AR expression has been detected in peripheral tissues [159, 160]. To date, there is no conclusive evidence that 5‐HT_5ARs are directly linked to specific physiological effects or signal transduction pathways in mammalian cells [161]. Recent studies have indicated that human 5‐HT_5AR may be coupled to GIRK channels [162]. In addition, 5‐HT_5AR may inhibit cAMP accumulation [3]. In addition to its effects on cAMP levels, the receptor has been implicated in other signaling pathways and the regulation of secondary messengers [163]. Treatment of cells with pertussis toxin abolishes the receptor's ability to inhibit adenosine diphosphate (ADP)‐ribosyl cyclase activity, suggesting that the receptor may signal through Gi/Go proteins. Furthermore, 5‐HT_5AR has been shown to regulate intracellular Ca²⁺ mobilization via inositol triphosphate (IP3)‐sensitive calcium stores [164]. Nevertheless, the precise G protein subtypes, downstream signaling pathways, and physiological roles of 5‐HT₅R have yet to be fully elucidated and warrant further investigation.

5‐HT₆Rs are widely expressed in various brain regions, and they are predominantly distributed in the caudate nucleus, olfactory tubercle, striatum, hippocampus, and nucleus accumbens [165, 166]. Studies have shown that this receptor primarily regulates cholinergic neurotransmission in the CNS, suggesting its potential for use as a therapeutic target for learning and memory disorders [167]. Research has demonstrated that 5‐HT₆Rs can stimulate AC activity in cultured striatal neurons and pig caudate nucleus membranes [168].

3.2.6. 5‐HT₇R

5‐HT₇R is highly expressed in the CNS, particularly in the hippocampus, hypothalamus, and neocortex [169, 170]. Owing to its expression in the suprachiasmatic nucleus, the receptor may be involved in the regulation of circadian rhythms [171]. 5‐HT₇R is also expressed in glial cells, the spleen, vascular smooth muscle, and the intestine [172].

5‐HT₇R can activate AC, and most of its functions are likely mediated via its coupling with the G protein [173]. 5‐HT₇R can stimulate several AC isoforms, including the Gs‐sensitive AC5 isoform and the calcium‐sensitive AC1 and AC8 isoforms [173]. The activation of 5‐HT₇R leads to an increase in the intracellular Ca²⁺ concentration, which is consistent with the calcium sensitivity of AC1 and AC8. However, this increase in Ca²⁺ occurs independently of PKC, phosphoinositide signaling, or Gi proteins, suggesting the involvement of a nonclassical signaling pathway [174]. Furthermore, endogenously expressed 5‐HT₇R can activate ERK1 and ERK2 in the MAPK pathway, and this activation is insensitive to pertussis toxin, indicating that it likely does not involve Gi/o proteins [175]. 5‐HT₇R also contributes to vasorelaxation, potentially through mechanisms involving the release of NO or the cAMP‐mediated inhibition of myosin light chain kinase, leading to smooth muscle relaxation [170]. In the nervous system, 5‐HT₇R is thought to inhibit calcium spike‐induced slow afterhyperpolarization in CA3 hippocampal neurons [171]. However, other studies have shown that the receptor may actually promote afterdepolarization [176]. Research in neurons of the anterodorsal thalamic nucleus has shown that 5‐HT₇R‐mediated modulation of afterhyperpolarization is driven by cAMP and a hyperpolarization‐activated nonselective cation current [177]. This process occurs independently of PKA or changes in intracellular Ca²⁺ levels, suggesting that a novel signal transduction mechanism may be involved [178].

3.3. 5‐HTR Crosstalk and Integration

Recent studies have revealed that different 5‐HTR subtypes do not function in isolation but instead engage in functional “crosstalk” through various mechanisms [179]. This crosstalk can manifest as mutual regulation of signaling pathways, changes in ligand‐binding affinities, or even the physical formation of heterodimers or higher‐order receptor complexes [180]. These receptor–receptor interactions contribute to the high complexity and plasticity of the 5‐HT signaling network, playing a critical role in the fine‐tuning of neuronal functions.

For example, 5‐HT_1AR and 5‐HT₇R can undergo functional crosstalk or form heterodimers in specific brain regions, such as the hippocampus. The binding affinity of these compounds follows the order 5‐HT₇R–5‐HT₇R > 5‐HT₇R–5‐HT_1AR > 5‐HT_1AR–5‐HT_1AR [181]. Functionally, the activation of 5‐HT₇R inhibits 5‐HT_1AR‐mediated Gi signaling, significantly reduces GIRK channel activity, and promotes 5‐HT_1AR internalization via the β‐arrestin/MAPK pathway [182]. 5‐HT_2AR participates in crosstalk with several glutamate and DA receptors, such as metabotropic glutamate receptor (mGluR) 2 and D2 receptor (D2R) [183]. In these interactions, the activation of one receptor may alter the ligand‐binding affinity and downstream signaling of the other [183]. For example, D2 receptor activation enhances the affinity of 5‐HT_2AR for 5‐HT but simultaneously inhibits IP3 production, suggesting that Gq signaling is modulated by Gi pathways [184]. The activation of mGluR2 increases the affinity of 5‐HT_2AR for 5‐HT, whereas the activation of 5‐HT_2AR decreases the affinity of mGluR2 [179, 183]. Furthermore, 5‐HT_2AR and presynaptic mGluR2/3 mutually regulate each other through downstream signaling [185]. Although they are colocalized at glutamatergic terminals in the PFC, 5‐HT_2AR and mGluR2/3 do not form physical complexes but instead functionally interact in an antagonist‐like manner to modulate glutamate exocytosis [186, 187]. Crosstalk also occurs between ATP receptor (P2X4) and 5‐HT₃R via their intracellular and transmembrane domains [188]. In addition, mGlu4 and 5‐HT_1AR exhibit functional interactions, wherein mGlu4 activation suppresses 5‐HT_1AR activity and subsequently reduces cAMP production [189]. A functional heteromer composed of 5‐HT_2CR and oxytocin receptor (OTR), which functions as a 5‐HT_2CR antagonist and enhances OT‐mediated hypoactivity in mice, has been identified [190]. Moreover, phosphatase and tensin homolog deleted on chromosome ten (PTEN) physically interacts with the third intracellular loop (3L4F) of 5‐HT_2CR and thus activates the PI3K/Akt signaling pathway; this highlights additional layers of complexity in 5‐HTR‐mediated signaling [191]. In brainstem raphe nuclei, autoreceptors such as 5‐HT₁Rs negatively regulate 5‐HT release, whereas heteroreceptors that are located in the cortex influence emotion, cognition, and even sexual behavior. Chronic use of SSRIs can lead to desensitization of autoreceptors, indirectly enhancing heteroreceptor activity, which may explain the delayed onset of antidepressant efficacy. The activation of 5‐HTRs can initiate multiple intracellular signaling cascades, including the cAMP/PKA, PLC/PKC, and MAPK/ERK pathways. These pathways are often shared among different 5‐HTR subtypes, supporting a model of multipoint cooperative regulation [192].

In addition, inhibitory crosstalk between 5‐HT_2BR and 5‐HT₇R depends on NOX and PKA activity [193]. Similar inhibitory interactions have been observed among 5‐HT_2AR, 5‐HT₇R, A2A receptor, and CD73 in neuroblastoma cells [194]. These findings suggest that the mechanisms of action for atypical antipsychotics and antidepressants may not be limited to individual receptor signaling but rather involve the modulation of receptor heterodimers, such as 5‐HT_2AR–mGluR2 or 5‐HT_2AR–D2R.

Overall, a relatively clear understanding of the mechanisms of 5‐HT‐mediated signal transduction has been established, and studies have confirmed that 5‐HTRs play important roles in various tissues and organs. On the basis of known 5‐HT signaling pathways, the development of targeted drugs is providing new approaches and strategies for the treatment of multiple diseases.

3.4. Termination of Signaling

While the mechanisms by which 5‐HTRs are activated have been extensively studied, increasing attention has only recently been given to how 5‐HTR signaling is effectively terminated to prevent prolonged activation, which may lead to toxicity or functional dysregulation [195]. Previous research indicated that 5‐HTR signaling termination involves a series of finely tuned regulatory mechanisms, including receptor desensitization, phosphorylation, β‐arrestin recruitment, endocytosis/recycling and degradation, negative feedback loops, and modulation through receptor heteromerization [196, 197].

Receptor desensitization and phosphorylation represent the primary steps in signal termination [198]. When 5‐HT persistently stimulates its receptors, especially G protein‐coupled 5‐HTRs (e.g., 5‐HT_1AR, 5‐HT_2AR, and 5‐HT₇R), the intracellular regions of these receptors are phosphorylated by G protein‐coupled receptor kinases, thereby promoting the recruitment of β‐arrestin, which in turn disrupts G protein coupling and leads to signal cessation [199, 200]. Moreover, β‐arrestin itself functions as a signaling scaffold, mediating noncanonical pathways such as MAPK/ERK signaling, and thus resulting in biased signaling [201].

Receptor endocytosis followed by either degradation or recycling is another crucial mechanism that modulates signal strength [202, 203]. After being phosphorylated and bound to β‐arrestin, a receptor is internalized into clathrin‐coated vesicles and trafficked to early endosomes [204]. Once internalized, receptors may follow one of two fates: degradation via lysosomal pathways, which reduces receptor availability at the membrane and thus increases cellular sensitivity to 5‐HT [196], or recycling back to the membrane to facilitate renewed signaling [203]. For example, 5‐HT_2AR and 5‐HT_2CR tend to be routed toward degradation after prolonged stimulation, whereas 5‐HT_1AR receptors often exhibit high recycling capacity in certain cell types [203, 205, 206].

Negative feedback regulation also plays a critical role in signal termination. In the raphe nuclei, autoreceptors such as 5‐HT_1BR and 5‐HT_1DR sense extracellular 5‐HT levels and inhibit further 5‐HT release, thereby preventing overexcitation and potential neurotoxicity or network instability [207]. In recent years, studies on interreceptor crosstalk and heteromerization have expanded our understanding of the mechanisms that regulate signal termination. For example, 5‐HT₇R and 5‐HT_1AR can form heterodimers that influence each other's signaling efficiency and internalization rates [181]; additionally, functional coupling between 5‐HT_2AR and receptors such as mGluR2 or D2R not only modulates downstream pathways but also may affect desensitization, internalization, or overall signaling duration [179, 183]. These receptor complexes do not always require direct physical interactions; functional synergy or antagonism alone can significantly alter receptor activity status, signaling persistence, and termination dynamics.

Importantly, signal termination is not merely a “shut‐off” process but rather lays the foundation for reshaping future responsiveness, including receptor resensitization, downregulation, or signal switching to alternative pathways. In summary, 5‐HTR signaling termination is a multilayered, dynamic, and highly regulated process that involves molecular‐level events (such as phosphorylation and endocytosis) as well as system‐level mechanisms (such as feedback control and receptor‒receptor interactions).

In conclusion, significant progress has been made in the study of 5‐HT‐mediated signaling mechanisms. However, further exploration is needed in areas such as the functional characterization of receptor subtypes, the integration of signaling networks, and the development of personalized therapeutic strategies to better understand their physiological roles and advance innovative treatments for related diseases.

4. Currently Available Therapeutics and Limitations in the Targeting of 5‐HT and its Receptors

On the basis of the signaling networks that are mediated by 5‐HT and 5‐HTR, as well as their extensive regulatory roles in both central and peripheral physiological processes, researchers have gradually developed a variety of modulators that target the 5‐HT system [208]. In recent years, research on the effects of 5‐HT modulators on multiple systems, including the nervous system, cardiovascular system, and endocrine system, has made significant progress. Currently, the therapeutic effects of various 5‐HT‐related drugs in disease treatment have been evaluated. Therefore, further exploration of the potential of these drugs for use in clinical application is essential. The therapeutic effects of 5‐HT modulators in cancer treatment are among the currently popular research topics in the field of oncology (Table 1).

TABLE 1.

Effects of agonists and antagonists of 5‐HT metabolism‐related enzymes and 5‐HTRs.

Molecule	Target	Cancer type	Clinical development for cancer treatment	Mechanism	Effects	Clinical diseases	References
LP‐533401	‐Tph1	Breast cancer	In preclinical development	Inhibits the synthesis of 5‐HT by breast tumor cells, thereby reducing the effect of 5‐HT through autocrine or paracrine pathways	Reduces breast cancer stem cell frequency	Functional gastrointestinal disorders	[209]
Telotristat ethyl (TE)	‐Tph1	Pancreatic cancer, colorectal cancer, NET, cholangiocarcinoma	In phase II and III clinical trials	(a) Enhances the accumulation and effect of CD8+ T cells; (b) decreases 5‐HT‐mediated PD‐L1 expression	Inhibits tumor growth	Carcinoid syndrome	[210, 211, 212, 213]
LX‐1031	‐Tph1	Glioma	In phase I clinical trial	Inhibits 5‐HT‐induced activation of the L1CAM/NF‐κB signaling pathway	Inhibits tumor cell proliferation, migration and chemoresistance	Functional gastrointestinal disorders	[214]
p‐CPA	‐Tph1	Cholangiocarcinoma	In preclinical development	Inhibits 5‐HT‐induced activation of 5HT1AR, 5HT2AR, 5HT2BR, 5HT4R, and 5HT6R	Inhibits tumor growth		[215]
Fluoxetine	‐SERT	Various cancer types	In preclinical development	(a) Decreases tumor cyclin D3, cyclin E and cyclin B expression; (b) increases the number of circulating CD8+ T lymphocytes and promotes IFN‐γ and TNF‐α secretion	(a) Inhibits tumor growth; (b) promotes tumor apoptosis; (c) promotes T cell‐mediated immunity	Depression, anxiety disorder, obsessive–compulsive disorder, bulimia	[216, 217, 218]
Citalopram	‐SERT	Colon cancer	In preclinical development	Inhibits TGF‐β signaling in tumors	Inhibits tumor cell proliferation and migration	Depression, anxiety disorders	[219]
Sertraline	‐SERT	Breast cancer, lung cancer, lymphoma	In preclinical development	(a) Inhibits autocrine or paracrine pathways; (b) decreases tumor cyclin D3, cyclin E, and cyclin B expression; (c) inhibits AMPK/mTOR pathway signaling in NSCLC cells	(a) Inhibits tumor growth; (b) promotes antitumor immunity	Depression, social phobia, obsessive–compulsive disorder, posttraumatic stress disorder	[209, 220, 221, 222]
Paroxetine	‐SERT	Breast cancer	In phase II and III clinical trials	Suppresses autocrine or paracrine pathways	Reduces breast cancer stem cell frequency	Depression, anxiety disorders, obsessive–compulsive disorder, menopausal mood disorders	[209]
Clorgyline	‐MAO‐A	Various cancer types	In preclinical development	(a) Reduces the expression of oncogenes FOS, JUN, NF‐κB, and Myc, as well as cell cycle regulators CCND1, CCNE1, and CDK4/6; (b) inhibits tumor T cell and TAM auto secretion of 5‐HT signaling to promote antitumor immunity	(a) Inhibits tumor cell proliferation, metastasis, and invasion; (b) decreases tumor microvessel density; (c) increases immune cell infiltration		[223, 224, 225, 226, 227, 228, 229]
Phenelzine	‐MAO‐A	Prostate cancer, melanoma, colorectal cancer	In phase II and III clinical trial	(a) Suppresses Enz/ARv7/hair‐A signaling in tumor cells; (b) inhibits the secretion of 5‐HT by tumor‐infiltrating T cells and TAMs to promote antitumor immunity	(a) Inhibits the growth and increase in tumor in vitro and in vivo; (b) promotes immune cell infiltration		[227, 229, 230]
NAN‐190	−5‐HT1AR	Various cancer types	In preclinical development	(a) Blocks MAPK and PI3K/Akt signaling pathways; (b) blocks signaling pathways that are involved in protein translation and survival, such as the Akt/mTOR pathway	(a) Inhibits tumor cell proliferation and metastasis; (b) inhibits tumor microvessel density; (c) increases immune cell infiltration		[231, 232]
WAY‐100635	−5‐HT1AR	Lymphoma	In preclinical development	(a) Inhibits cell proliferation and metabolism through transcription and translation (e.g., Akt, GSK‐3β, cMyc, and p53); (b) regulates mitochondrial activity by reducing mitochondrial membrane potential and decreasing dehydrogenase activity	(a) Inhibits tumor cell proliferation; (b) promotes apoptosis		[233]
SB216641	−5‐HT1BR	Uterine leiomyoma, liver cancer	In preclinical development	(a) Decreases cyclin D1 and α‐SMA expression and decreases MAPK, ERK, and EF2K pathway activation; (b) induces caspase‐8, caspase‐9, and caspase‐3 activation	(a) Inhibits tumor cell proliferation; (b) promotes apoptosis		[131, 234]
GR127935	−5‐HT1DR	Colorectal cancer	In preclinical development	Blocks the Axin1/β‐catenin/MMP‐7 signaling pathway	Inhibits tumor metastasis and invasion		[235]
BRL54443	−5‐HT1ER	Ovarian cancer	In preclinical development	Activates SRC‐mediated downstream signaling pathways and significantly promotes cell proliferation and EMT	Promotes tumor growth and peritoneal spread		[236]
Ketanserin	−5‐HT2AR	Choriocarcinoma, breast cancer, lung cancer	In preclinical development	(a) Inhibits MEK‐ERK1/2 and Jak2–STAT3 signaling; (b) inhibits Jak1/STAT3/ERK1/2 and AC/PKA signaling	(a) Inhibits tumor cell proliferation and viability; (b) inhibits tumor cell glycolysis and mitochondrial biogenesis	Hypertension, Raynaud's phenomenon	[108, 237]
SB204741	−5‐HT2BR	Various cancer types	In preclinical development	(a) Downregulates FOXO3a expression in tumor cells; (b) blocks the 5‐HT–5‐HT2BR–pERK–Yap axis; (c) blocks 5‐HT induced phosphorylation of ERK1/2 and eNOS; (d) inhibits 5‐HT‐mediated upregulation of STAB1 and SERPINB2 gene expression	(a) Inhibits tumor cell proliferation; (b) promotes macrophage polarization toward the M1 phenotype; (c) reduces tumor microvessel density		[238, 239, 240, 241, 242]
PRX08066	−5‐HT2BR	Neuroendocrine tumors	In preclinical development	(a) Inhibits many signaling pathways (e.g., WNT, focal adhesion kinase, and Jak/STAT); (b) inhibits ERK1/2 phosphorylation and profibrotic growth factor synthesis and TGFβ1, CTGF, and FGF2 secretion	(a) Inhibits tumor cell proliferation, metastasis and invasion; (b) inhibits MET fibroblast proliferation		[243, 244]
SB215505	−5‐HT2BR	Prostate cancer	In preclinical development	Regulates the production of IL‐6, IL‐1β, and TNF‐α by fibroblasts	(a) Inhibits tumor cell proliferation, metastasis and invasion; (b) inhibits MET fibroblast proliferation		[245, 246]
Palonosetron	−5‐HT3R	Lung cancer	CINV/RINV	(a) Blocks ERK pathway‐induced autophagy in tumor cells; (b) inhibits LC3 protein expression	(a) Inhibits cell proliferation and migration; (b) suppresses the formation of tumor colonies		[247]
Ramosetron	−5‐HT3R	Lung cancer	CINV/RINV	(a) Blocks ERK pathway‐induced autophagy in tumor cells; (b) inhibits LC3 protein expression	(a) Inhibits cell proliferation and migration; (b) suppresses the formation of tumor colonies		[247, 248]
Tropisetron	−5‐HT3R	Colorectal cancer, melanoma, lung cancer	CINV/RINV	(a) Inhibits NLRP3 inflammasome activation; (b) promotes apoptosis, microtubule depolymerization, ERK activation, and NF‐κB downregulation	(a) Inhibits tumor cell proliferation; (b) alleviates inflammation; (c) promotes apoptosis; (d) promotes microtubule depolymerization	Nausea and vomiting induced by radiotherapy/chemotherapy	[120, 123, 249]
Ondansetron	−5‐HT3R	Pancreatic cancer	CINV/RINV	Inhibits Ca2+ mobilization in tumor cells	Inhibits tumor cell proliferation	Nausea and vomiting induced by radiotherapy/chemotherapy	[249, 250, 251]
RS23597‐190	−5‐HT4R	Prostate cancer	In preclinical development	Blocks cell cycle progression in the M phase	Inhibits tumor cell proliferation		[245]
Prucalopride	+5‐HT4R	Glioma	In preclinical development	Induces autophagy via the Akt–mTOR pathway	(a) Inhibits tumor cell proliferation, migration and invasion; (b) promotes apoptosis; (c) promotes cell autophagy	Chronic constipation	[252]
Mosapride	+5‐HT4R	Various cancer types	In preclinical development	Blocks cell cycle progression	Inhibits angiogenesis	Indigestion, gastroparesis	[253]
SB699551	−5‐HT5AR	Breast cancer	In preclinical development	Inhibits Gαi/o signaling and PI3K/AKT/mTOR signaling	(a) Inhibits tumor cell proliferation; (b) reduces tumorsphere frequency		[162]
Valerenic Acid	+5‐HT5AR	Glioblastoma	In preclinical development	(a) Inhibits 5‐HT‐induced angiogenesis and tumor growth through the PI3K/NOX pathway; (b) increases ROS levels in GBM cells; (c) activates the AMPK pathway	(a) Inhibits tumor cell proliferation, migration, and invasion; (b) promotes tumor cell death	Auxiliary treatments for insomnia, anxiety	[254]
SB258719	−5‐HT7R	Hepatocellular carcinoma	In preclinical development	Inhibits Wnt/β‐catenin signaling	(a) Inhibits tumor cell proliferation; (b) promotes tumor cell autophagy		[255]
SB269970	−5‐HT7R	Various cancer types	In preclinical development	(a) Inhibits PI3K/Akt phosphorylation; (b) inhibits cAMP/PKA and CREB activity; (c) inhibits PKCɛ‐ and P38‐induced IL‐6 production	(a) Inhibits tumor cell proliferation; (b) promotes tumor apoptosis; (c) promotes the polarization of macrophages toward the M1 phenotype		[242, 256, 257, 258, 259]
LP211	+5‐HT7R	Non‐small cell lung cancer	In preclinical development	Activates the Akt and P38 signaling pathways	(a) Promotes tumor cell colony formation; (b) promotes tumor cell migration and invasion		[260]

⁺agonist, ⁻antagonist.

4.1. Drugs Targeting 5‐HT Metabolic Enzymes

The biosynthesis and metabolism of 5‐HT are crucial for the regulation of its functions in the body [208]. Changes in the activity of metabolic enzymes not only affect the concentration of 5‐HT but also are directly related to the occurrence and progression of various diseases.

4.1.1. Tph Inhibitors Reduce 5‐HT Synthesis

Tph1, which is the rate‐limiting enzyme in 5‐HT synthesis, has gained attention because of its role in biology [208]. Although 5‐HT production can be increased by Tph1 agonists, no Tph1 agonists are currently available for use in the clinic. Antagonists of Tph1 aim to inhibit 5‐HT synthesis. Telotristat ethyl (TE), which is an oral inhibitor of Tph1, has been approved by the United States Food and Drug Administration (US FDA) to relieve diarrhea symptoms in patients with carcinoid syndrome [210]. Additionally, a Teleace study revealed that cancer patients who were treated with TE experienced a reduction in tumor size [211]. LX‐1031 is a selective peripheral Tph1 inhibitor that was developed for the treatment of diarrhea‐predominant IBS (IBS‐D) [261]. The drug entered phase II clinical trials and demonstrated a certain degree of efficacy and safety because it inhibited intestinal 5‐HT synthesis to alleviate hypermotility symptoms [262]. However, owing to its limited therapeutic efficacy, the development of LX‐1031 has not continued beyond this stage [263]. Moreover, pCPA has been widely used in animal studies to create 5‐HT depletion models for investigations of the roles of 5‐HT in anxiety, depression, aggression, and cognitive function [264]. However, its poor selectivity, irreversible inhibition, and severe side effects have limited its potential for clinical application [265] (Figure 3).

FIGURE 3.

Effects of serotonin (5‐HT) metabolic enzyme antagonists on tumors. Tph1 antagonists block 5‐HT synthesis, resulting in increased tumor cell apoptosis and decreased tumor cell proliferation and migration. MAO‐A antagonists block 5‐HT degradation, resulting in a decrease in intracellular reactive oxygen species (ROS) production and the suppression of tumor cell autophagy and DNA repair.

In conclusion, the Tph1 inhibitor TE is currently used in clinical practice to treat carcinoid syndrome. However, the clinical use of other inhibitors has been discontinued because of severe side effects. Therefore, the development of drugs with fewer side effects and greater targeting specificity has become a possible future research direction.

4.1.2. SSRIs Inhibit 5‐HT Reuptake

The use of these medications varies slightly among different populations. For example, fluoxetine, sertraline, fluvoxamine, and escitalopram are approved for use in patients under 25 years of age, whereas citalopram is generally not used in children [266]. Vilazodone, which has partial 5‐HT_1AR agonistic activity, is approved for the treatment of major depressive disorder (MDD) in adults [267]. Paroxetine is also used to treat menopausal hot flashes and premature ejaculation [268]. Furthermore, SSRIs have applications in migraine, fibromyalgia, IBS, and other nontypical indications [269].

Although SSRIs are effective, their side effects require attention. Common side effects include GI discomfort (such as nausea and diarrhea), headache, insomnia or somnolence, and sexual dysfunction [270]. Citalopram may lead to QT interval prolongation [271, 272]. Owing to its short half‐life, pahroxetine may cause significant discontinuation syndrome, whereas fluoxetine, which has a longer half‐life, has a milder withdrawal effect [273]. Overall, citalopram, fluvoxamine, and paroxetine are associated with slightly higher rates of adverse effects, with citalopram exhibiting the best GI tolerability [267]. In clinical practice, differences among individual patients, drug metabolism characteristics, and potential drug interactions should be considered in the selection of SSRIs. For example, paroxetine may antagonize the metabolism of tamoxifen via CYP2D6, potentially influencing its antitumor effect [274, 275]. Additionally, SSRIs should be used with caution in children, pregnant women, and patients with cardiovascular diseases to ensure drug safety [276].

In summary, SSRIs have been widely used in clinical practice. Recent studies have revealed their potential value in other diseases, such as cancer; thus, further expanding their range of applications will become an important direction for future research.

4.1.3. MAO‐A Inhibitors Block 5‐HT Degradation

MAO‐A is primarily responsible for the breakdown of neurotransmitters, such as 5‐HT, norepinephrine (NE), and DA; therefore, MAO‐A regulates the concentrations of these neurotransmitters in the nervous system and affects mood, stress responses, and various physiological functions [277]. MAO‐A inhibitors (MAOIs) function by inhibiting the activity of MAO‐A, thereby increasing the concentrations of these neurotransmitters and ameliorating mood disorders such as depression and anxiety [278]. Currently, the major MAOIs that have been approved by the US FDA include hydrazine derivatives (isoniazid, iproniazid, and phenelzine), nonhydrazine derivatives, and selective MAOIs (moclobemide, toloxatone, and brofaromine) [279].

MAOIs have a long history of use in treating depression, particularly in patients who do not respond to conventional antidepressants such as SSRIs [278]. Medications such as phenelzine and moclobemide increase the concentrations of 5‐HT, NE, and other neurotransmitters by inhibiting MAO‐A, thereby ameliorating depressive symptoms [280]. MAOIs are also used to treat generalized anxiety disorder and panic disorder [281]. By increasing 5‐HT levels in the brain, MAOIs can effectively alleviate anxiety symptoms. Although MAOIs have potential for use in the treatment of anxiety, they are typically not used as first‐line therapies because of their notable side effects. Some MAOIs have been shown to have therapeutic effects on migraines, particularly in patients with migraines accompanied by depressive symptoms [282]. In patients with depression, MAOIs can ameliorate symptoms such as appetite loss and weight loss [283]. These medications alleviate appetite issues related to depression by increasing 5‐HT levels [283].

Although MAOIs have significant therapeutic effects in the treatment of many conditions, they also have several limitations and side effects [284]. MAOIs can cause severe adverse reactions when combined with foods containing tyramine (such as cheese, cured meats, and soy sauce) and various other medications. These adverse reactions include dizziness, insomnia, muscle weakness, blurred vision, hyperreflexia, respiratory difficulties, and changes in blood counts [285, 286, 287]. Specifically, owing to their shorter half‐life, MAOIs such as paroxetine may result in significant withdrawal symptoms, making discontinuation more challenging [288].

In conclusion, MAOIs have certain clinical application value in the treatment of depression, anxiety, social phobia, and other psychiatric disorders. Although MAOIs can effectively increase neurotransmitter levels and ameliorate mood and behavioral symptoms in patients, owing to the risks of side effects and drug interactions, MAOIs are typically used as second‐line treatment options. Close monitoring and consideration of differences among individual patients are necessary when these medications are used.

4.2. Drugs Targeting 5‐HTRs

In recent years, the development of drugs that target 5‐HTRs has made significant progress, especially in the fields of neuro‐psychiatric disease, cardiovascular disease, and cancer treatment, demonstrating great application prospects. This section summarizes the role of drugs that target 5‐HTRs in disease treatment.

4.2.1. 5‐HT₂R Agonists and Antagonists

The 5‐HT₂R family includes three subtypes, namely, 5‐HT_2AR, 5‐HT_2BR, and 5‐HT_2CR, which play key roles in various physiological and pathological processes. These receptors are regulated by agonists or antagonists and have broad clinical application potential [289].

5‐HT_2AR agonists directly stimulate 5‐HT_2AR, thus affecting cognitive, emotional, and perceptual functions. In recent years, research on 5‐HT_2AR agonists has focused primarily on treating mental disorders such as depression, anxiety, and schizophrenia [290]. Lysergic acid diethylamide and psilocybin are typical 5‐HT_2AR agonists and have shown potential in the treatment of treatment‐resistant depression, posttraumatic stress disorder, and anxiety, especially in the fields of palliative care and psychological counseling [291, 292]. However, 5‐HT_2AR agonists are often accompanied by intense hallucinogenic effects; thus, their clinical application requires strict regulation and supervision. 5‐HT_2CR agonists, particularly lorcaserin, are used as antiobesity drugs on the market [293]. However, owing to its association with risk of cancer development, lorcaserin has been withdrawn from the market [293, 294].

5‐HT_2AR antagonists have become first‐line drugs for the treatment of schizophrenia, depression, and bipolar disorder. For example, clozapine, olanzapine, and risperidone, which are typical atypical antipsychotics, antagonize 5‐HT_2AR to ameliorate symptoms such as hallucinations and delusions in psychiatric patients, with fewer side effects than traditional drugs such as chlorpromazine [295, 296, 297]. 5‐HT_2BR antagonists (e.g., sarpogrelate) are used to treat cardiovascular diseases, especially ischemic symptoms such as ulcers, pain, and cold sensations caused by chronic arterial occlusion [298, 299]. 5‐HT_2CR antagonists have been used to ameliorate depression and anxiety and have shown potential for appetite control. Some antidepressants, such as azelastine, are both antihistamines and 5‐HT_2CR antagonists and can be used to alleviate appetite loss and weight loss caused by depression [300].

The 5‐HT₂R agonist DOI has been shown to stimulate dendritic cell activation [301]. The 5‐HT_2AR antagonist ketanserin can inhibit the proliferation of JEG‐3 and BeWo choriocarcinoma cells [108]. The 5‐HT_2BR antagonist SB204741 significantly inhibits the proliferation of Huh7 cells [238]. Additionally, SB204741 reduces microvessel density in the context of lung cancer and melanoma [239]. The 5‐HT_2BR agonist BW‐723C86 enhances the anti‐inflammatory properties of macrophages [302].

Significant research progress has been made in the use of 5‐HT₂R agonists and 5‐HT₂R antagonists in the clinical treatment of mental illnesses, cardiovascular diseases, and obesity. Although these drugs have shown considerable efficacy in treating certain diseases, agonists and antagonists are also associated with side effects and risks; in particular, 5‐HT_2AR agonists are associated with hallucinogenic effects, and 5‐HT_2BR agonists are associated with cardiac side effects. With further clinical research and drug safety evaluations, drugs that regulate 5‐HT₂R still hold vast application potential.

4.2.2. 5‐HT₃R Agonists and Antagonists

5‐HT₃R is an important neurotransmitter receptor that plays a key role in both the central and peripheral nervous systems. It is particularly involved in regulating nausea, vomiting, GI motility, mood, and other physiological functions [303]. Unlike other G protein‐coupled receptors, 5‐HT₃R is unique in that it is a ligand‐gated ion channel that directly influences the cellular membrane potential by modulating ion flow [119]. In recent years, research on 5‐HT₃R antagonists has made significant progress, showing broad potential for the use of these agents in the clinic.

These antagonists function by preventing the activation of 5‐HT₃Rs, thereby reducing the physiological responses associated with receptor activation, especially under conditions such as nausea, vomiting, and pain [304, 305]. 5‐HT₃R antagonists, such as ondansetron, granisetron, and dolasetron, are widely used to prevent and treat the nausea and vomiting that are caused by chemotherapy, radiotherapy, and surgery [306]. These agents effectively reduce the incidence of nausea and vomiting by blocking the 5‐HT₃R‐mediated emetic reflex [307]. Alosetron, which is another 5‐HT₃R antagonist, alleviates diarrhea and abdominal pain in patients with IBS by inhibiting 5‐HT₃R in the gut [308]. Although its efficacy is well established, the drug is associated with rare but serious side effects, such as severe constipation and ischemic colitis, and therefore requires strict monitoring during use [308]. Studies have also indicated that 5‐HT₃R antagonists may offer auxiliary analgesic benefits in patients with conditions such as cancer pain and postoperative pain [309]. However, this indication is still in the preclinical or early clinical research stages. Emerging evidence also suggests that 5‐HT₃R antagonists may have modulatory effects on anxiety and depressive‐like behaviors. Animal studies and small‐scale clinical trials have shown promising results, although larger clinical studies are needed to confirm these findings [310].

The 5‐HT₃R agonist 2‐methylserotonin enhances 5‐HT₃R‐dependent Ca²⁺ influx in immature and mature dendritic cells (DCs) [301]. In addition, the 5‐HT₃R agonist 2‐methylserotonin promotes T cell activation and proliferation from the S phase to the G2/M phase of the cell cycle [311]. The 5‐HT₃R antagonists palonosetron and ramosetron effectively inhibit tumor growth and colony formation [247].

In conclusion, 5‐HT₃R antagonists have become essential therapeutic agents in the management of nausea, vomiting, and IBS, whereas the use of agonists remains largely in the exploratory phase. Both classes of drugs show promising potential for treating various diseases, but further clinical trials and long‐term safety evaluations are necessary to support their broader application.

4.2.3. 5‐HT₁R Agonists and Antagonists

5‐HT₁R plays a crucial role in regulating neurotransmitter release, and it is involved in processes such as emotional regulation, vasoconstriction, and pain transmission [124]. In recent years, research on 5‐HT₁R agonists and antagonists has continued to progress, revealing the broad therapeutic potential of these agents in the treatment of psychiatric disorders, neuropathic pain, and cardiovascular diseases [312, 313].

5‐HT_1AR agonists are the most extensively studied subtype of 5‐HT₁R‐targeting drugs, and they are widely used to treat psychiatric disorders such as anxiety, depression, and schizophrenia [314]. A typical agent, buspirone, is a partial 5‐HT_1AR agonist with antidepressant and anxiolytic effects, and it is associated with a low risk of dependence [315]. Newer antidepressants, such as vilazodone and vortioxetine, combine SSRIs with partial 5‐HT_1AR agonism, and these antidepressants are thought to more effectively alleviate depressive symptoms and cognitive impairment in patients with mood disorders [316, 317, 318]. 5‐HT_1AR antagonists such as WAY‐100635 are mainly used in experimental research to investigate receptor function, and they have limited clinical application [319]. 5‐HT_1B/1DR agonists, also known as “triptans,” including sumatriptan, rizatriptan, and zolmitriptan, are the primary medications that are used for migraine treatment [320, 321, 322]. These drugs alleviate headache symptoms by constricting dilated intracranial blood vessels and inhibiting proinflammatory neuropeptide release [323]. Despite their efficacy, triptans can cause vasoconstriction‐related side effects, such as coronary artery spasms, and they should therefore be used cautiously in patients with a history of cardiovascular disease [324]. To mitigate cardiovascular risk, 5‐HT_1FR agonists such as lasmiditan have been developed as a new generation of antimigraine medications. Unlike triptans, lasmiditan does not induce vasoconstriction, and it has been approved by the US FDA for the treatment of acute migraine attacks, for which it has demonstrated both efficacy and safety [312, 325].

Research on 5‐HT₁R antagonists is relatively limited, and these antagonists are primarily used to investigate the physiological and pathological roles of 5‐HT1Rs [326]. The 5‐HT₁R antagonist NAN‐190 induces G2/M phase arrest in HT‐29 cells, whereas the 5‐HT_1BR antagonist SB224289 causes apoptosis [326]. The 5‐HT_1BR agonist AnHcl has been shown to stimulate DCs activation [301]. The 5‐HT_1DR antagonist GR127935 effectively inhibits tumor metastasis [235]. Currently, there are no widely used clinical drugs that function as selective 5‐HT₁R antagonists.

In summary, 5‐HT₁R agonists, particularly agonists of the 5‐HT_1AR and 5‐HT_1B/1DR subtypes, have significant therapeutic value in the treatment of anxiety, depression, and migraine. The emergence of 5‐HT_1FR receptor agonists offers a novel option for migraine treatment without cardiovascular risk. Research on other subtypes and a greater understanding of receptor function may further expand the applications of 5‐HT₁R‐targeted drugs in the treatment of neurological, psychiatric, and vascular disorders.

4.2.4. 5‐HT₄R Agonists and Antagonists

5‐HT₄Rs are widely distributed in the CNS, GI tract, and cardiovascular system, where they regulate various physiological functions, including GI motility, cognitive function, neurotransmitter release, and cardiac function [327]. In recent years, research on 5‐HT₄R agonists and antagonists has continued to advance.

5‐HT₄R agonists are primarily used to enhance GI motility [328]. Drugs such as mosapride, prucalopride, and tegaserod have demonstrated significant efficacy in treating chronic constipation, gastroparesis, and constipation‐predominant IBS (IBS‐C) [329, 330]. These agents activate 5‐HT₄Rs in the GI tract, promoting ACh release and thereby enhancing intestinal peristalsis. Prucalopride, which is a selective 5‐HT₄R agonist, has better cardiac safety, whereas tegaserod has been associated with cardiovascular side effects [331]. In addition to GI indications, 5‐HT₄R agonists have shown promise in ameliorating cognitive impairments and depressive symptoms [332]. Studies have demonstrated that 5‐HT₄R agonists can enhance learning and memory functions and may have potential antidepressant and antidementia effects [333]. Animal studies also suggest that these drugs may augment the effects of SSRIs and accelerate the onset of antidepressant action [334]. 5‐HT₄R antagonists are mainly used in experimental models.

In summary, the use of 5‐HT₄R agonists for the treatment of GI motility disorders has been well established, but their applications in the neuropsychiatric and cardiovascular fields remain at the preclinical or early clinical trial stages. With further understanding of the mechanisms underlying receptor‐mediated functions and the development of new selective drugs, 5‐HT₄R‐related therapeutics are expected to play broader roles in the treatment of various systemic diseases.

4.2.5. 5‐HT₅R and 5‐HT₆R Agonists and Antagonists

5‐HT₅R mainly refers to the 5‐HT_5AR subtype. Research suggests that 5‐HT_5AR may play a role in regulating circadian rhythms, cognition, mood, depression, and schizophrenia [335]. Research on agonists of this receptor is still in its early stages, and no drugs have been marketed yet. Research on antagonists of this receptor, however, is relatively more advanced. For example, SB‐699551 has been shown to have antidepressant, cognition‐enhancing, and anxiolytic effects in animal studies [159].

5‐HT₆Rs are closely related to learning, memory, and cognitive disorders. 5‐HT₆R antagonists have been used to treat alzheimer’s disease (AD) and cognitive impairments [336]. Idalopirdine has been investigated in multiple clinical trials to assess its ability to ameliorate cognitive function in AD patients, but large‐scale phase III trials have failed to meet primary endpoints [337]. SUVN‐502 and intepirdine were once highly anticipated as adjunctive treatments for AD, but their efficacy was disappointing in later‐stage studies, and their development was hindered [337, 338, 339].

Currently, both 5‐HT_5AR and 5‐HT₆R remain relatively underexplored but promising targets, with related drugs still in preclinical research stages.

4.2.6. 5‐HT₇R Agonists and Antagonists

5‐HT₇Rs are widely involved in regulating various physiological functions, including mood, cognition, sleep, and temperature regulation [171]. In recent years, research on 5‐HT₇R agonists and antagonists has increased, revealing their potential therapeutic value in various neuropsychiatric disorders [169].

Research on 5‐HT₇R agonists is relatively limited and has focused mainly on exploring their effects on neurofunction, mood regulation, and cognition [175]. In animal models, 5‐HT₇R agonists, such as AS19, have shown promising effects in ameliorating cognitive deficits and mood disorders [340]. For example, AS19 promotes M2 phenotypic polarization and cytokine production in macrophages [302]. Additionally, AS19 promotes T cell activation and proliferation [341]. However, despite the significant research potential of agonists, their clinical application remains at an early stage and lacks sufficient clinical validation.

In contrast, research on 5‐HT₇R antagonists is more developed, and they have been used in the treatment of various diseases. Some 5‐HT₇R antagonists, such as SB‐269970 and LY‐3154207, have been shown to ameliorate depression, anxiety, and cognitive dysfunction in animal models [342, 343]. In addition, 5‐HT₇R antagonists have been shown to improve sleep quality, helping alleviate insomnia and other sleep‐related symptoms [344]. Some studies also suggest that 5‐HT₇R antagonists may play a role in temperature regulation. The 5‐HT₇R antagonist SB258719 can effectively inhibit the proliferation of liver cancer cells [255]; SB‐258719 also inhibits the growth of xenograft tumors in patients with primary liver cancer [256, 345], modulates cytokine release from DCs, and inhibits ERK signaling in T cells [341].

Overall, both 5‐HT metabolism and 5‐HTR agonists and antagonists have broad potential in the treatment of various diseases. Despite the promising efficacy of these drugs, further clinical trials and drug safety evaluations are needed to advance their clinical application.

5. Emerging Therapeutic Opportunities

In recent years, with the successive development of various related drugs, the role of 5‐HT in the treatment of various systemic diseases has attracted increasing attention [290]. Studies have shown that targeting 5‐HT has extensive therapeutic potential, particularly in cancer, immune disorders, neuropsychiatric conditions, metabolic syndromes, and GI diseases [346]. As research continues to elucidate the mechanisms underlying the 5‐HT signaling network under pathological conditions, its potential for use as a target for multitarget intervention platforms is gradually emerging, resulting in the development of a series of novel therapeutic strategies (Figure 4).

FIGURE 4.

Effects of serotonin (5‐HT) receptor (5‐HTR) regulators on diseases. Blocking 5‐HTRs on the cell surface regulates the progression of a variety of diseases. These include psychiatric diseases, gastrointestinal diseases, liver diseases, diabetes, and tumors. In tumors, blockade of 5‐HTRs inhibits the proliferation and migration of tumor cells and induces their apoptosis.

5.1. The 5‐HT System and Cancer

Recent studies have shown that 5‐HT and 5‐HTRs play significant roles in the occurrence, development, and treatment of cancer [347, 348]. The interactions between the 5‐HT system and the TME, as well as processes such as tumor cell proliferation, migration, and invasion and tumor immune escape, are closely related to this system and have become emerging topics in research in the field of cancer treatment.

5.1.1. Role of 5‐HT Metabolism in Cancer

5‐HT synthesis, transport, and degradation are collectively referred to as 5‐HT metabolism [346]. Tph1 increases the intracellular 5‐HT level, SERT promotes the transport of 5‐HT from the extracellular space to the intracellular space, and MAO‐A reduces the intracellular 5‐HT level by degrading 5‐HT [346]. This review includes a summary of the roles of 5‐HT metabolism‐related enzymes in various types of cancer. Various enzymes that are related to 5‐HT metabolism are abnormally expressed in many tumors and cause 5‐HT metabolic disorders [349]. Therefore, the study of 5‐HT metabolism is very important because 5‐HT metabolism is a potential target for cancer treatment [348].

Recent studies have shown that cancers, such as pancreatic ductal adenocarcinoma, cholangiocarcinoma, breast cancer, glioblastoma, and colorectal cancer, exhibit 5‐HT accumulation, which is accompanied by increased tumor cell proliferation and growth [214, 215, 258, 350, 351, 352]. Clinical and experimental data indicate that increased 5‐HT levels lead to glioma progression and shortened overall survival times. Mechanistically, 5‐HT promotes cell proliferation, invasive migration, and resistance [214]. In 5‐HT‐related pancreatic neuroendocrine tumors, exposure to 5‐HT activates the transforming growth factor (TGF)‐β signaling pathway in pancreatic stellate cells, which is associated with extracellular matrix remodeling; thus, exposure to 5‐HT promotes tumor progression [353]. A reduction in 5‐HT expression can significantly reduce the proliferation of cholangiocarcinoma cells, leading to increased tumor necrosis, enhanced tumor fibrosis, and slowed tumor growth [215]. Additionally, a reduction in 5‐HT can suppress the growth of tumor‐initiating cells and exert a synergistic effect with chemotherapy on the growth of xenografted breast cancer tissue [209]. The use of SSRIs to increase 5‐HT levels before and after the diagnosis of breast cancer has been shown to increase the mortality rate in patients with breast cancer [354]. Clinical data indicate that men with depression who are receiving long‐term SSRI treatment‐induced 5‐HT accumulation and have a significantly increased risk of developing malignant breast cancer [355]. 5‐HT also increases the Ki67 expression and tumor weight of breast cancer and carcinoid tumors [356, 357]. In Tph1‐knockout mice, peripheral 5‐HT levels are significantly decreased, and in Tph1‐knockout mice with colorectal or pancreatic cancer, tumor growth is markedly slower. Furthermore, the expression of the programmed death 1 (PD‐1) ligand 1 (PD‐L1) in tumor cells is reduced, and the accumulation of functional CD8⁺ T cells is increased [212]. In Tph^−/− mice, colorectal cancer xenografts exhibit reduced tumor growth and angiogenesis, leading to slower subcutaneous tumor growth, spontaneous tumor necrosis, and tumor hypoxia [350]. Moreover, Tph1 knockout effectively reduces carbon tetrachloride (CCl₄)‐induced liver tumor formation [358]. 5‐HT enters cells via SERT and activates the Ras homolog family member A (RhoA)/Rho‐associated kinases (ROCKs)/Yes‐associated protein (Yap) signaling pathway, thereby promoting the development of colon cancer [359]. Furthermore, colorectal cancer stem cells synthesize large amounts of 5‐HT, promoting the self‐renewal of cancer stem cells and playing a significant role in the development and metastasis of colorectal cancer [360]. Therefore, inhibiting peripheral 5‐HT synthesis may have therapeutic applications, such as in the prevention of cancer progression [361]. The elevated levels of circulating free 5‐HT induced by SSRI treatment promote the progression of cholangiocarcinoma [362]. 5‐HT promotes the apoptosis of Burkitt lymphoma cells [363]. 5‐HT in neutrophils is activated through histone H3 modification, promoting liver metastasis of neuroendocrine prostate cancer [364]. SSRIs block 5‐HT transport into neutrophils and inhibit 5‐HT‐mediated H3 modification, reversing prostate cancer liver metastasis [365]. 5‐HT also inhibits the neural invasion of prostate cancer cells through semaphorin 3C (SEMA3C)/PlexinA2/neuropilin‐1 (NRP1)/cellular‐mesenchumal epithelial transition factor (cMET) signaling, which reduces the density of tumor‐infiltrating nerve fibers and promotes the growth and migration of mouse xenograft tumors [366]. 5‐HT suppresses the induction of MAO‐A expression; promotes cancer cell proliferation and migration by activating downstream molecules, such as p21, matrix metalloproteinases 2 (MMP2), and vascular endothelial growth factor (VEGF); and promotes epithelial‐to‐mesenchymal transition (EMT) and hypoxia inducible factor‐1 (HIF1) α protein accumulation [367, 368]. Additionally, 5‐HT is highly expressed in classical Hodgkin lymphoma cells. 5‐HT suppresses the induction of MAO‐A expression, inhibits tumor cell proliferation and is efficacious in the majority of patients when combined with adriamycin, bleomycin, vinblastine, or dacarbazine (ABVD) [228]. This regimen presumably sensitizes tumor cells to ABVD treatment. However, approximately 20% of patients experience refractory disease or relapse after initial treatment [228]. Additionally, 5‐HT reduces the ability of induced MAO‐A to affect the TME and hinder tumor immunotherapy. Wang et al. [226] reported that suppressing the 5‐HT‐mediated induction of MAO‐A promotes the polarization of TAMs toward the M2 phenotype via oxidative stress. Furthermore, 5‐HT suppresses the induction of MAO‐A expression, inhibits tumor‐associated macrophage (TAM) reprogramming, induces tumor growth in xenograft tumor models, and reduces synergistic antitumor effects when combined with anti‐PD‐1 therapy [226]. Wang et al. [227] reported that 5‐HT suppresses the induced MAO‐A‐mediated suppression of antitumor T cell immunity by regulating autocrine 5‐HT signaling in tumor‐infiltrating CD8⁺ T cells [227].

Furthermore, some studies have suggested that 5‐HT overproduction can inhibit tumor development [369, 370]. 5‐HT can inhibit the proliferation of colon cancer cells in mice [371]. Additionally, in colon cancer cells, 5‐HT production is induced by dimethylhydrazine, which inhibits microvascular formation in the TME and affects mitochondrial function and energy production in cancer cells [372]. In melanoma, 5‐HT can slow tumor growth by inhibiting the production of interleukin (IL)‐10 and interferon (IFN)‐γ and increasing the production of IL‐1β [216, 373]. In non‐small cell lung cancer, SSRIs limit tumor growth and increase tumor sensitivity to erlotinib by inhibiting the adenosine 5‐monophosphate adenosine 5‘‐monophosphate (AMP)‐activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling pathway [220]. Additionally, 5‐HT can modulate immunity, reversing the suppression of antitumor immune responses due to chronic stress [374]. 5‐HT reduces the expression of IL‐4I1 and PD‐L1 and decreases the numbers of inhibitory immune cells, MDSCs, Tregs, and CD39⁺/PD‐1⁺ CD8⁺ T cells. Additionally, the SSRI amitriptyline (AMI) enhances the cytotoxic potential of CD8⁺ T cells and exerts a synergistic effect with PD‐1 [375]. Furthermore, research has shown that low 5‐HT expression is associated with poor clinical outcomes in prostate cancer patients [376]. Reduced 5‐HT levels promote prostate tumorigenesis and development [376, 377]. 5‐HT reduces glioma cell proliferation and invasion as well as microvessel density while reducing macrophage infiltration [224, 225].

Thus, these reports suggest that the dysregulation of 5‐HT, which is regulated by 5‐HT metabolic enzymes, is involved in tumor growth and immune regulation in the TME. Therefore, reducing intratumoral 5‐HT levels by blocking Tph1 and SERT or promoting MAO‐A expression may be a target for cancer therapy as well as for the design and development of antineoplastic drugs. However, other studies have shown that the aberrant expression of MAO‐A in many cancers promotes tumor progression by reducing tumor 5‐HT levels, which may be related to the heterogeneity of the tumors studied. Therefore, targeting 5‐HT metabolism to regulate tumor 5‐HT levels has become a potential target for the design and development of cancer therapeutic drugs.

5.1.2. Roles of 5‐HTRs in Cancer

5‐HT‐induced 5‐HTR activation can affect the development, recurrence and metastasis of tumors [347]. This section focuses on the relationships between 5‐HTRs and the occurrence, development, and prognosis of tumors. The expression of 5‐HTRs in various cancer tissues is shown in Table 2. The role of 5‐HTRs in cancer is discussed further below.

TABLE 2.

Roles of 5‐HTRs in normal and tumor microenvironments.

Receptor	Normal vs. tumor	Tumor type	Mechanism	Response	References
5‐HT1B/1D/1FR		Colorectal cancer, pancreatic ductal carcinoma	5‐HT activates 5‐HTRs to initiate Wnt/b‐catenin signaling in order to promote the self‐renewal of colorectal cancer stem cells, integrin/Src/Fak‐mediated signaling, uPAR/MMP‐2 signaling, and zinc finger ZEB1 and Snail protein expression.	Tumor cell proliferation and migration	[360, 378]
5‐HT1AR	Upregulated in tumors	Lung cancer, lymphoma	5‐HT activates 5‐HT1AR, and then the 5‐HT1AR/autophagy/p‐STAT3 axis increases the abundance of Tregs and reduces the Th1/Th2 ratio. )5‐HT activates 5‐HT1AR, and then inhibits DNA damage and ROS‐independent caspase activation.	Tumor cell proliferation, tumor recurrence, and tumor metastasis	[233, 379]
5‐HT1DR	Upregulated in tumors	Hepatocellular carcinoma	5‐HT1DR activated by 5‐HT inhibits PIK3R1 ubiquitin degradation, thereby increasing FoxO6 expression via the PI3K/Akt pathway.	Increased probability of tumor recurrence	[380]
5‐HT1ER	Upregulated in tumors	Ovarian cancer	5‐HT1DR activated by 5‐HT inhibits the activation of factors related to SRC	Inhibits tumor cell proliferation and EMT	[236]
5‐HT1BR	Upregulated in tumors	Uterine leiomyoma, colorectal cancer, lymphoma	5‐HT1BR activated by 5‐HT activates the MAPK/ERK pathway, EF‐2 kinase, the cyclin D1 pathway and α‐smooth muscle antigen; inhibits the caspase pathway; and inhibits apoptosis in tumor cells.	Promotes tumor cell proliferation and tumor metastasis	[131, 233, 235, 326]
5‐HT2BR	Upregulated in tumors	Colorectal cancer, hepatocellular carcinoma, prostate cancer, uveal melanoma, pancreatic ductal adenocarcinoma, gastric cancer	5‐HT2BR activated by 5‐HT activates TGF‐β signaling and promotes ERK phosphorylation, thereby promoting the upregulation of Yap expression. 5‐HT2BR activated by 5‐HT activates Jak/STAT and adhesion kinase; 5‐HT2BR activated by 5‐HT increases the Warburg effect and PI3K/Akt/mTOR signaling by increasing the protein levels of HIF1 α and Myc. 5‐HT2BR activated by 5‐HT activates the FoxO3a pathway in liver cancer cells	Inhibits early CAC initiation but promotes late CAC progression Promotes the proliferation, invasion and metastasis of liver cancer, prostate cancer, melanoma, and pancreatic ductal cancer cells Inhibits tumor apoptosis	[238, 240, 241, 243, 245, 381, 382]
5‐HT2A/2CR	Upregulated in tumors	Colorectal cancer, choriocarcinoma, breast cancer	5‐HT2A/2CR activated by 5‐HT activates the ERK‐mediated pathways (c‐Jun and Ki67 transcription). 5‐HT2A/2CR activated by 5‐HT activates the STAT3 and ERK1/2 signaling pathways through Jak2.	Promotes tumor cell proliferation Promotes choriocarcinoma cell growth and survival	[383, 384]
5‐HT3R	Upregulated in tumors	Pancreatic cancer, neuroblastoma, lung cancer, esophageal squamous cell carcinoma, colorectal cancer, melanoma, gastric cancer	5‐HT3R activated by 5‐HT inhibits the expression of BAD and Bax and upregulates the expression of Bcl‐2, inhibits cAMP expression, inhibits ERK phosphorylation, and increases intracellular ([Ca2+]i) and causes Ca2+ influx, which in turn causes CaMKIIa phosphorylation and activation, leading to NLRP3 phosphorylation and inflammasome assembly.	Promotes tumor cell proliferation Promotes metastasis and spread of esophageal squamous cell carcinoma tumor cells	[123, 352, 385, 386, 382, 387, 388, 389]
5‐HT4R	Upregulated in tumors	Prostate cancer, adrenal cancer, glioblastoma	5‐HT4R activated by 5‐HT activates the Akt/mTOR pathway, thereby inhibiting cleaved caspase 3 and Bax and promoting Bcl‐2, reduces LC3‐II and Beclin 1 expression.	Promotes tumor cell proliferation, migration, and invasion	[245, 252]
5‐HT5R	Upregulated in tumors	Breast cancer, glioblastoma, prostate cancer	5‐HT5R activated by 5‐HT enhances ROS levels and activates the AMPK pathway. Activates the PI3K/Akt/mTOR axis. 5‐HT5R activated by 5‐HT promotes the PKA pathway to increase androgen receptor activation.	Inhibits glioblastoma cell proliferation and migration Promotes the proliferation of breast and prostate cancer cells	[162, 254, 390]
5‐HT7R	Upregulated in tumors	Non‐small cell lung cancer, prostate cancer, adrenocortical carcinoma, glioblastoma, liver cancer, breast cancer	5‐HT7R activated by 5‐HT activates the P38 MAPK and NF‐κB signaling pathway and activates the cAMP⁄PKA signaling axis, leading to increased phosphorylation of CREB and ERK⁄Akt. Simultaneously, it activates the kinase signaling pathway and the PI3K/Akt signaling axis.	Promotes the proliferation, migration, and spread of non‐small cell lung cancer, prostate cancer, and breast cancer cells	[256, 258, 260, 391]

5.1.2.1. 5‐HT₂R

5‐HT₂R affects tumors [20]. The 5‐HT₂R family consists of three subtypes, namely, 5‐HT_2AR, 5‐HT_2BR, and 5‐HT_2CR. 5‐HT_2AR expression is significantly upregulated in tumor cells, and 5‐HT_2AR enhances proliferation by altering cell cycle progression [108, 392]. 5‐HT_2AR activation also promotes skin cancer progression. Furthermore, 5‐HT_2AR‐overexpressing mice exhibit a significantly increased incidence of skin cancer, and platelet‐activating factor halts skin cancer progression [393]. Peripheral 5‐HT phosphorylates ERK through 5‐HT_2AR, thereby increasing the Yap/vestigial like family member 4 (VGLL4) ratio [394]. The 5‐HT_2AR–p‐ERK–Yap axis promotes liver cancer progression [395, 396]. Furthermore, the expression of 5‐HT_2BR has been detected in breast cancer and is closely associated with the activation of estrogen receptor‐α [397]. Studies have shown that 5‐HT, through 5‐HT_2BR, inhibits autophagy and downregulates the expression and phosphorylation of FoxO3 in HCC cells, thereby promoting the proliferation and growth of these cells [238, 398]. In prostate cancer, 5‐HT_2BR expression is upregulated in high‐grade pathological subtypes [245]. Compared with normal pancreatic tissue, 5‐HT_2BR is overexpressed in pancreatic cancer tissues and is closely associated with poor clinical prognosis [241, 399]. Previous studies have confirmed the oncogenic role of 5‐HT_2BR in pancreatic cancer [241]. In colorectal cancer, the expression of 5‐HT_2BR is reduced in early colorectal cancer and promotes tumor growth [381]. Knocking out 5‐HT_2BR in late‐stage colorectal cancer inhibits tumor development via tumor cell proliferation and migration [381]. In uveal melanoma, the transcription levels of 5‐HT_2BR are significantly elevated, leading to increased intracellular expression of the 5‐HT_2BR protein [400]. 5‐HT_2BR promotes tumor cell proliferation and migration through the activation of multiple signaling pathways, such as focal adhesion kinase and Jak [243]. Moreover, 5‐HT_2BR can promote the nuclear translocation of cAMP response element‐binding protein 1 (CREB1), which further enhances the transcription of zinc finger e‐box binding homoeobox 1 (ZEB1), thereby increasing the migration and promoting the EMT process of colorectal cancer cells [401].

Studies have shown that 5‐HT can be involved in tumor progression via signaling through various subtypes of 5‐HT2Rs [402]. Both 5‐HT_2AR and 5‐HT_2CR are also expressed in breast cancer cells and tissue samples [403, 404]. Research has shown that 5‐HT can increase oxidative phosphorylation and glycolytic activity in MCF‐7 cells via the 5‐HT_2A/2CR signaling pathway, thereby promoting cell proliferation and inhibiting apoptosis [405]. In addition, this signaling pathway activates the transcriptional coactivator PPAR gamma coactivator 1alpha (PGC‐1α), thus promoting mitochondrial biogenesis in MCF‐7 cells [406, 407]. In contrast to other studies, a study by Muller et al. [408] revealed that human cutaneous melanoma cells exhibit high 5‐HT_2AR, 5‐HT_2BR, and 5‐HT_2CR expression. 5‐HT can enhance the inhibition of melanoma cell proliferation induced by radiation, possibly through the release of 5‐HT by mast cells in response to ionizing radiation [408]. These findings suggest that 5‐HT₂R plays an important role in tumor progression. Systematic analysis of the regulation of 5‐HT₂R expression in various cancers thus has substantial implications for the development of effective targeted drug treatments.

5.1.2.2. 5‐HT₃R

Many researchers refer collectively to various subtypes of 5‐HT₃R, and studies have also indicated that the selective blockade of 5‐HT_3AR has the same effect as the complete blockade of 5‐HT₃R activity [118]. Early investigations often combined 5‐HT₃R antagonists with other chemotherapeutic drugs to ameliorate the vomiting that is caused by anticancer chemotherapy [409]. In recent years, studies have shown that in addition to reducing vomiting, 5‐HT₃R directly affects tumor progression [410]. Clinical data have shown that the expression of the 5HT_3AR gene is upregulated in tumor tissue compared with that in tumor margin tissue in patients with breast cancer [411]. The breast cancer cell line MCF‐7 expresses 5‐HTR_3AR isoforms [403], and 5‐HT enhances the proliferation of MCF‐7 cells via 5‐HT_3AR signaling. The inhibition of 5‐HT_3AR with chemical antagonists can reduce the proliferation and induce the apoptosis of MCF‐7 cells [403]. 5‐HT₃R is also expressed in human colon cancer cells, and activation of this receptor by 5‐HT promotes mitosis while inhibiting apoptosis [412]. Similarly, 5‐HT₃R antagonists inhibit the proliferation of melanoma cells, promote apoptosis and exert a synergistic effect when combined with paclitaxel. In addition, 5‐HT₃R antagonists reportedly increase cytoplasmic Ca²⁺ levels and ERK1/2 phosphorylation and inhibit microtubule formation [120]. Lung adenocarcinoma cells also express 5‐HT_3AR, which promotes their proliferation [123]. In colon cancer, 5‐HT_3AR is expressed at different levels across several cell lines, including SW1116, HCT116, DLD‐1, LoVo, SW62, and RKO cells, and promotes the proliferation of these cells [413]. Knockdown of 5‐HT_3AR causes cell cycle arrest and increases the expression levels of proapoptotic proteins, including Bcl2 associated X protein (Bax), that promote apoptosis [385]. Similarly, both colon cancer tissue and colon cancer cells exhibit high levels of 5‐HT₃R expression, and inhibiting 5‐HT₃R slows tumor growth [352, 414]. Huang et al. [386] reported that 5‐HT_3AR promotes the proliferation and migration of esophageal squamous cell carcinoma cells, mainly via the long noncoding RNA linc01305, the stability of which is maintained via its interaction with 5‐HT_3AR mRNA.

These findings suggest that 5‐HT₃R may have significant effects on tumors. Considering that it is the only ion channel receptor among 5‐HTRs, it may play a different role than other receptor subtypes do. The exact role of 5‐HT₃R in tumors still needs to be elucidated through large‐scale studies and clinical trials.

5.1.2.3. 5‐HT₁R

Various 5‐HT₁R isoforms have been studied in tumors, and their expression has been shown to be correlated with cancer progression and prognosis. However, the distribution of 5‐HT₁R isoforms in tumors is heterogeneous [415, 416]. 5‐HT_1AR and 5‐HT_1BR are upregulated in benign and noninvasive tumors but downregulated in invasive tumors [417]. Multiple ovarian cancer cell lines (ES2, OV90, SW626, CaOV3, 2774, TOV112D, UWB1.298, SKOV3, and HEYA8 cells) also overexpress 5‐HT_1AR, 5‐HT_1BR, and 5‐HT_1DR; moreover, two ovarian cancer cell lines (2774 and CaOV3 cell) also express the 5‐HT_1ER isoform [417]. In contrast, the expression of 5‐HT_1ER is markedly reduced in peritoneally disseminated ovarian cancer cells, and this reduction significantly increases tumor cell proliferation via the activation of Src‐mediated downstream signaling pathways [236]. Siddiqui et al. [418] also measured the expression of 5‐HT_1AR and 5‐HT_1BR in human bladder cancer tissues and in HT1376 cells. Clinical data also indicate that the 5‐HT_1AR subtype is highly expressed in lung cancer tissues from patients with depression [379]. Additionally, 5‐HT_1DR has been implicated in lung cancer cell proliferation [419]. Importantly, 5‐HT_1AR expression in lung cancer cells is negatively correlated with the activity of cytotoxic T lymphocytes (CTLs) among tumor infiltrating lymphocytes (TILs). Patients with increased intertumoral 5‐HT_1AR expression exhibit increased Treg infiltration and a decreased Th1/Th2 cell ratio, as well as upregulated PD‐L1 expression in the tumor microenvironment (TME) [379].

Additionally, 5‐HT₁R has been reported to be associated with liver cancer. 5‐HT levels are elevated in a chemically induced HCC mouse model, and 5‐HT expression levels are correlated with HCC prognosis and progression [420]. In vitro studies have demonstrated that 5‐HT promotes FoxO6 expression via 5‐HT_1BR and 5‐HT_1DR, thereby increasing liver cancer cell proliferation [380]. Moreover, 5‐HT_1BR signaling promotes autophagy and activates Notch signaling by upregulating the expression of light chain 3 (LC3) β and autophagy‐related effector proteins, such as elF4E‐binding protein 1 (4EBP1), autophagy related 3 (ATG3), Beclin1, and s65, thereby inducing liver cancer cell death [421].

In mouse models of colorectal cancer, 5‐HT_1DR overexpression has been shown to promote tumor cell invasion [235]. Human HT29 colon cancer cells and uterine leiomyoma cells express high levels of 5‐HT_1BR [131, 326]. Furthermore, the 5‐HT_1B/1D/1FR isoforms are highly expressed in colorectal cancer stem cells, and the binding of 5‐HT to these receptors promotes self‐renewal, tumor initiation, and migration in these cells [360].

5‐HT_1BR and 5‐HT_1DR are more highly expressed in pancreatic cancer cells than in normal pancreatic cells [378]. Overexpression of 5‐HT_1DR modulates β1 integrin activity, promoting the recruitment of the Src–FAK complex, which enhances cancer cell proliferation and migration. Furthermore, 5‐HT_1CR is highly expressed in chordomas in transgenic mice and promotes tumor growth [422].

In summary, these findings indicate that 5‐HT₁R promotes the occurrence and development of tumors. Abnormally high expression levels of 5‐HT₁R lead to the occurrence and development of various cancers. However, previous research on 5‐HT₁Rs has focused mainly on tumor cells themselves. The effects of 5‐HT₁R on immune cells, especially the regulatory role of immune cells in the TME, have not been studied thoroughly and require further exploration.

5.1.2.4. 5‐HT₄R

Compared with the large number of studies on 5‐HT_{1, 2, 3}R in cancer, relatively few studies have explored the role of 5‐HT₄R in tumors [423]. Current evidence indicates that 5‐HT₄R is upregulated in various tumor types, including cholangiocarcinoma, colon cancer, and breast cancer, where 5‐HT promotes tumor cell growth and proliferation through 5‐HT₄R‐mediated signaling [412]. In addition, 5‐HT₄R is upregulated in high‐grade prostate cancer cell lines, such as DU145 and LNCaP [245, 424]. These 5‐HT₄R isoforms have been shown to promote tumor cell proliferation even under androgen‐deficient conditions [424]. In the ovary, 5‐HT₄R is expressed in both normal tissues and ovarian malignancies. Notably, 5‐HT₄R is overexpressed in benign and noninvasive ovarian tumors, whereas its expression is downregulated in more invasive ovarian cancers [417].

In contrast to its tumor‐promoting effects in many cancer types, 5‐HT₄R activation has antitumor effects on gliomas. Specifically, 5‐HT₄R agonists inhibit the proliferation, migration, and invasion of glioma cells; promote the apoptosis and autophagy of glioma cells; and significantly increase the expression of Beclin1 and LC3‐II [252].

These findings illustrate the role of 5‐HT₄R in cancer progression. While its tumor‐promoting effects are evident in several cancers, such as breast, prostate, and colon cancers, 5‐HT₄R activation may have inhibitory effects on gliomas. Despite these findings, the mechanisms by which 5‐HT₄R functions in tumor cells have not been fully studied and need to be explored.

5.1.2.5. 5‐HT₅R and 5‐HT₆R

Although the role of 5‐HT₄Rs in cancer has been demonstrated in several studies, the roles of 5‐HT₅Rs and 5‐HT₆Rs in tumors have rarely been studied. Recent studies have shown that a 5‐HT_5AR antagonist reduces the frequency of tumor microsphere‐initiating cells among breast cancer cells and synergizes with chemotherapy to inhibit the growth of xenograft tumors by targeting the Gα‐coupling pathway [162]. Additionally, high 5‐HT_5AR expression is also observed in prostate cancer. 5‐HT_5AR increases androgen receptor activity by activating PKA signaling, which promotes tumor growth [390]. In contrast, although 5‐HT_5AR isoforms are expressed in glioma tissues, their levels are lower in glioma cells than in normal brain tissues [254]. Interestingly, the activation of 5‐HT_5AR with the agonist valine has been shown to inhibit the proliferation and invasion of glioma cells [254].

Furthermore, 5‐HT₆R is expressed in cholangiocarcinoma cells, and 5‐HT promotes their proliferation through 5‐HT₆R signaling [215].

These findings suggest that 5‐HT_5AR and 5‐HT₆R may have significant effects on tumors. However, the role of 5‐HT₅R and 5‐HT₆R in tumors has not yet been explored in depth. Thus, the mechanisms underlying these processes in tumors are unclear and need further exploration. Furthermore, the potential value of this receptor as a therapeutic target remains unclear.

5.1.2.6. 5‐HT₇R

Although the roles of 5‐HT₅R and 5‐HT₆R in tumor cells have rarely been studied, the role of 5‐HT₇R in cancer has been demonstrated in several studies [425]. Multiple glioblastoma cell lines (including U‐373MG, H4, CCF‐STTG1, U‐138MG, Hs683, DBTRG‐05MG, T98G, and U‐87MG) express 5‐HT₇R subtypes [426]. In U‐373MG cells, 5‐HT promotes IL‐6 secretion via 5‐HT₇R signaling, contributing to tumor progression by fostering a proinflammatory TME [259]. Clinical data have shown that 5‐HT₇R mRNA expression in prostate tissue is approximately 200 times greater than that in healthy tissue [257]. Cinar et al. [257] reported that 5‐HT₇R mRNA expression is significantly upregulated in PC‐3 prostate cancer cells. 5‐HT promotes the proliferation of tumor cells and inhibits apoptosis by downregulating the expression of caspase3, caspase9, Bax, and tumor protein 53 (TP53) through 5‐HT₇R signaling [257]. Similarly, compared with its expression in paratumoral lung tissue, 5‐HT₇R is overexpressed in non‐small cell lung cancer tumor tissue [260]. This effect of 5‐HT₇R is mediated through activation of the MAPK and Src signaling pathways [260]. Additionally, 5‐HT₇R expression is elevated triple‐negative breast cancer (TNBC) cells. 5‐HT activates Src/ERK through 5‐HT₇R signaling, which promotes tumor cell invasion but does not participate in tumor spread [258]. However, 5‐HT₇R signaling activates the adenyl cyclase, MAPK, and PI3K/Akt pathways, which inhibit the proliferation of TNBC cells [427].

These findings suggest that blocking 5‐HT₇R may have significant potential for treating tumors. However, owing to the lack of highly selective drugs that target 5‐HT₇R, implementing this treatment approach is difficult; thus, developing highly selective antagonists has become a potential therapeutic strategy.

Among the various 5‐HTR subtypes, certain receptors have been assigned greater priority for research and therapeutic use because of their critical roles in tumor progression and potential as drug targets. Among them, 5‐HT₂Rs have the greatest therapeutic relevance. These receptors are highly expressed in multiple tumor types, and they are involved in regulating cell proliferation, angiogenesis, immune evasion, and even resistance to treatment [20]. The direct roles of 5‐HT₃Rs in cancer have not yet been fully elucidated; however, emerging studies suggest that they may influence tumor progression [410]. Clinically, 5‐HT₃R antagonists (e.g., ondansetron) are widely used to alleviate chemotherapy‐induced nausea and vomiting [409], and their indications could be expanded into antitumor applications in the future. 5‐HT_1AR also represents a high research priority. 5‐HT_1AR is involved in regulating the cell cycle, apoptosis, and proliferation‐related signaling in cancers such as HCC [420]. The promigratory function of 5‐HT₇Rs in various cancer cells has made these receptors promising targets for future therapeutic intervention [425]. Currently, the development of drugs that target 5‐HT₇R is still in its early stages, and further studies are warranted. In summary, on the basis of the current evidence and drug development potential, 5‐HT₂R, 5‐HT_1AR, and 5‐HT₇R represent high‐priority targets in cancer therapy and merit further mechanistic investigation and therapeutic exploration. Although the mechanisms by which 5‐HT₃R functions in cancer remain to be clarified, its established clinical use has translational potential. Other 5‐HTR subtypes may also contribute to specific cancer types, but further validation is needed, as most studies remain in the exploratory phase.

5.2. The 5‐HT System and Functional GI Disorders

In addition to its role in tumors, 5‐HT, which functions as an enteric neurotransmitter and a paracrine signaling molecule, plays a vital role in regulating GI motility and secretion. In the gut, 5‐HT is released primarily by EC cells and exerts paracrine effects on nearby target cells, inducing various physiological responses, such as nausea, vomiting, increased intestinal secretion, and enhanced motility. Dysregulation of the 5‐HT system can lead to abnormalities in GI motility and function, potentially resulting in GI disorders such as chronic constipation, IBS, and carcinoid syndrome.

5.2.1. IBD

IBD comprises two chronic inflammatory conditions of the GI tract with unknown etiology and recurrent episodes: ulcerative colitis (UC) and Crohn's disease (CD). Previous research on the pathogenesis of IBD has focused mainly on immunological and genetic factors; however, studies have also suggested that functional factors are closely involved in its development. Elevated levels of gut‐derived 5‐HT are strongly associated with the onset and progression of IBD [428]. In patients with active IBD, both EC cell numbers and 5‐HT levels are significantly increased [429]. Animal studies have confirmed that abnormalities in the 5‐HT signaling system can lead to the dysregulation of intestinal motility in IBD models [430]. Specifically, the expression of SERT is markedly downregulated in inflamed regions, leading to the local accumulation of 5‐HT, which exacerbates the inflammatory response and is accompanied by an increased number of EC cells [430]. Research has demonstrated that in UC mouse models induced by dextran sulfate sodium (DSS) and CD models induced by trinitrobenzene sulfonic acid, 5‐HT levels are significantly elevated in inflamed colonic tissues [431]. These findings suggest that increased 5‐HT levels may promote inflammatory responses and affect local tissue inflammation, thereby exacerbating colitis [431]. Further studies have shown that reducing 5‐HT expression in the gut can significantly alleviate the severity of colitis in mice, indicating that 5‐HT plays an important regulatory role in the inflammatory process [432]. Moreover, clinical studies have revealed significantly elevated 5‐HT levels in the colonic tissues of UC patients. Experimental interventions involving Lactobacillus have been shown to effectively reduce abnormally elevated 5‐HT levels and relieve inflammation in colonic tissues [433, 434]. Clinical studies have shown that blocking 5‐HT₃Rs can effectively alleviate IBS‐D‐like symptoms that are common in patients with quiescent IBD [435]. This effect is achieved by inhibiting the expression of tight junction proteins in intestinal epithelial cells, thereby increasing intestinal mucosal permeability. As a result, microbial translocation and immune cell activation are triggered, further exacerbating the inflammatory response and mucosal damage associated with IBD. In addition, 5‐HT₇R antagonists significantly reduce the severity of colonic inflammation and decrease the expression of proinflammatory cytokines in both DSS‐induced colitis and T cell transfer‐induced colitis models [436].

5.2.2. Gastroesophageal Reflux Disease

5‐HT is a key neurotransmitter that plays a crucial role in regulating GI motility, and it is involved in the pathogenesis of gastroesophageal reflux disease (GERD). The functional response of esophageal smooth muscle is mediated by two main signaling mechanisms: receptor‐dependent and receptor‐independent mechanisms. Among these pathways, the receptor‐dependent pathway includes signal transduction mediated by ACh and 5‐HTRs. In children with aromatic Tph deficiency, the conversion of tryptophan to 5‐HT is impaired, resulting in significantly reduced 5‐HT levels and subsequent esophageal motility disorders; thus, Tph deficiency contributes to the development of GERD [437]. Histological studies have shown that in patients with reflux esophagitis, 5‐HT levels are significantly elevated in diseased tissues, whereas SERT mRNA and 5‐HT₄R expression is markedly decreased in both reflux esophagitis and nonerosive reflux disease [438]. Further research has demonstrated that inhibition of 5‐HT₄R activity can weaken the contraction of the lower esophageal sphincter, thereby increasing the risk of gastric content reflux [439].

Thus, 5‐HT₄R agonists may be novel therapeutic options for GERD patients who are unresponsive to proton pump inhibitors. Moreover, selective blockade of specific 5‐HTR subtypes may help reduce intestinal inflammation, indicating potential clinical value in the comprehensive treatment of GERD.

5.2.3. IBS

IBS is among the most common functional GI disorders, it and is characterized primarily by abdominal pain, altered bowel habits, and abnormal stool consistency, with symptoms that are either persistent or intermittently recurrent [440]. In recent years, the understanding of IBS has shifted from a purely biological model to a more integrated biopsychosocial‐genetic model [441]. Among the various mechanisms proposed, abnormalities in the 5‐HT signaling system have emerged as a prominent focus of research worldwide [442].

Previous studies have shown that the mRNA expression of SERT and Tph1 is reduced in IBS patients, leading to impaired 5‐HT reuptake and elevated plasma 5‐HT levels. This increase may contribute to the development of visceral hypersensitivity, which is considered a key mechanism underlying the pathology of IBS [5]. Furthermore, chronic psychological stress can disrupt the gut–brain axis, increasing the number of EC cells in the intestinal mucosa and increasing both the synthesis and release of 5‐HT [443]. Acting through various 5‐HTR subtypes that are located in the myenteric and submucosal plexuses, 5‐HT increases gut sensitivity and worsens IBS symptoms [443]. A study revealed that fasting plasma 5‐HT levels are significantly greater in IBS patients than in healthy controls and that this elevation is positively correlated with the numbers of mast cells and the severity of abdominal pain [444]. These findings suggest that increased 5‐HT release may trigger immune responses in the mucosa, contributing to the sensation of pain. Overall, visceral hypersensitivity is recognized as a core pathophysiological feature of IBS, and elevated 5‐HT levels play a crucial role in its development [444].

Moreover, gut microbiota‐derived metabolites also significantly influence the pathogenesis of IBS [445, 446]. The microbial metabolite acetone can promote 5‐HT release, increase plasma 5‐HT levels, inhibit intestinal water reabsorption, and subsequently cause diarrhea and visceral hypersensitivity [447, 448]. Additionally, short‐chain fatty acids, which are key microbial fermentation products, have been shown to stimulate 5‐HT secretion, increase colonic smooth muscle contraction, and accelerate colonic transit [449]. Studies have also reported that in IBS‐C, probiotic therapy increases intestinal 5‐HT secretion and leads to significant symptom amelioration. Conversely, in IBS‐D, the inhibition of 5‐HT synthesis and release has proven effective in controlling symptoms [450]. These findings suggest that the modulation of 5‐HT signaling could be a potential strategy for the individualized treatment of patients with IBS. Specifically, suppressing 5‐HT synthesis and release is beneficial for IBS‐D patients, whereas increasing 5‐HT levels through probiotics or SSRIs may help alleviate symptoms in IBS‐C patients [450].

At the genetic level, the number of molecular studies on IBS continues to increase. Zhang et al. [451] reported that miRNA‐510 and 5‐HT_3ER gene expression is significantly increased in the colonic mucosa of Chinese female IBS‐D patients, suggesting that targeting the expression of these molecules may provide therapeutic benefit. Research has shown that polymorphisms in the SERT gene are closely associated with the development of IBS‐C [452]. In a meta‐analysis, Areeshi et al. [453] confirmed that polymorphisms in the SERT gene SLC6A4 are associated with increased susceptibility to IBS among both American and Asian populations.

5.2.4. Functional Dyspepsia

Functional dyspepsia (FD) is a common functional GI disorder. The pathogenesis of FD remains incompletely understood and may involve multiple factors, including GI motility disturbances, visceral hypersensitivity, psychological and social factors, and genetic polymorphisms [454]. Notably, the severity of FD symptoms is not primarily determined by GI pathophysiology, but it is closely associated with psychological factors [455, 456]. Some studies have shown that the S allele of the SERT gene‐linked polymorphic region is significantly associated with the postprandial distress syndrome subtype of FD and is correlated with an increased risk of comorbid psychological symptoms, such as anxiety and depression, in FD patients [457]. However, previous research on FD‐related genetic polymorphisms remains limited, and the genetic mechanisms underlying FD development require further investigation. In recent years, with an increased understanding of the brain–gut axis, SSRIs have gradually been introduced into the clinical management of FD [458, 459]. One study using the SSRI itopride to treat FD patients demonstrated that the drug not only significantly ameliorates dyspeptic symptoms but also effectively alleviates anxiety and depressive states, thereby improving overall quality of life [458]. Nonetheless, SSRIs are also associated with potential risks, including a more than twofold increase in the incidence of GI bleeding, highlighting the need for further large‐scale clinical trials to evaluate their safety profile [460]. FD is also closely associated with chronic psychological stress, which can disrupt brain‒gut axis function and contribute to visceral hypersensitivity [461]. Research suggests that 5‐HT plays a central role in this process by activating specific receptors that regulate neurotransmitter release, increasing GI mucosal and epithelial permeability, and activating pain signaling pathways, ultimately leading to persistent discomfort or pain perception [462]. Abnormal 5‐HT signaling has also been implicated in the pathophysiology of FD. Studies have shown that patients with FD exhibit a significantly attenuated short‐circuit current response to exogenous 5‐HT stimulation. This is accompanied by upregulated expression of 5‐HT_3ER and SERT and downregulated expression of 5‐HT₇R and Tph1, suggesting the localized dysfunction of mucosal 5‐HT signaling in patients with FD [463]. A recent meta‐analysis indicated that 5‐HTR agonists can significantly alleviate symptoms in FD patients, with an odds ratio (OR) of approximately 2.99; in particular, these agents can improve early satiety and epigastric fullness [464, 465]. Among these agents, 5‐HT₄R agonists improve gastric motility, thereby relieving symptoms related to delayed gastric emptying. Furthermore, studies have shown that FD patients exhibit reduced 5‐HT₃R activity and increased 5‐HT₄R expression in their stomachs, further suggesting a pivotal role of altered 5‐HT signaling in FD pathogenesis and treatment [466, 467, 468].

Therefore, increasing plasma 5‐HT levels and enhancing 5‐HT activity may be effective therapeutic strategies for treating FD. The combined use of 5‐HTR agonists or SSRIs on the basis of conventional treatment represents a novel direction and therapeutic approach for FD. However, the efficacy and safety of this strategy require further validation in high‐quality clinical trials to support its standardized application in routine clinical practice.

5‐HT plays a significant role in the occurrence and development of functional GI disorders. Drugs that target 5‐HT metabolism and 5‐HTRs represent new agents for treating these diseases. With an increased understanding of the mechanism of action of 5‐HT, more precise and personalized treatment plans are expected to be developed in the future.

5.3. The 5‐HT System and Psychiatric Diseases

5‐HTergic neurons regulate a wide range of physiological and behavioral functions and play crucial roles in the onset and progression of various neuropsychiatric disorders [15]. In recent years, in‐depth studies of the serotonergic system, especially its precise mechanisms of action in different brain regions, have increasingly revealed the pivotal role of 5‐HT in mental illnesses [469]. Abnormal 5‐HT signaling is closely associated with numerous neuropsychiatric conditions, including depression, anxiety disorders, bipolar disorder, schizophrenia, autism spectrum disorders, and obsessive‒compulsive disorder [15]. In these disorders, dysfunction of the 5‐HT system typically involves abnormalities in 5‐HT synthesis and metabolism, imbalances in receptor expression, impaired transporter function, and disruptions in downstream signaling pathways [470, 471].

5.3.1. Depression

The 5‐HT h ypothesis of depression posits that depression is associated with impaired serotonergic neuronal function [472]. Studies have demonstrated that alterations in 5‐HTR and SERT, as well as increased activity of presynaptic autoreceptors, are observed in patients with major depressive disorder (MDD) [473, 474]. Moreover, transient depletion of tryptophan in remitted MDD patients can induce a relapse of acute depressive symptoms, further suggesting a critical role of the serotonergic system in depression [475]. SSRIs and other antidepressants exert therapeutic effects by prolonging the half‐life of 5‐HT, thereby alleviating depressive symptoms [14].

Both presynaptic and postsynaptic 5‐HT_1ARs are believed to play key roles in the regulation of depression‐like behaviors [476]. Researchers have proposed that selectively targeting 5‐HT_1AR and 5‐HT_1BR populations through heteroreceptor activation and autoreceptor blockade may contribute to the antidepressant and antipsychotic effects of certain drugs [477]. Some 5‐HT_1AR agonists, such as buspirone, have been applied in the clinic to treat depression [478, 479]. Other 5‐HTR subtypes also play important roles in the neurobiology of depression. For example, selective blockade of 5‐HT_2AR [480], 5‐HT_2CR [481], and 5‐HT₃R [482], as well as activation of 5‐HT_2BR [483] and 5‐HT₄R [484], is considered to have antidepressant potential. Notably, SSRI treatment activates 5‐HT_2AR on GABAergic neurons, leading to reduced NE firing [485]. This interaction between 5‐HT_2AR and the noradrenergic system may explain the limited efficacy of SSRIs in treatment‐resistant depression [480, 486]. In addition, 5‐HT_2CR antagonism has been shown to increase NE and DA levels in the prefrontal cortex (PFC). Therefore, the suboptimal therapeutic response of some patients to SSRIs may be partially due to the suppression of ventral tegmental area (VTA) dopaminergic activity via the 5‐HT_2CR [481]. Furthermore, the antidepressant‐like effects of 5‐HT₃R antagonists are thought to be mediated via the hypothalamic–pituitary–adrenal (HPA) axis [487]. Studies also suggest that both agonists and antagonists of 5‐HT₆R exert antidepressant‐ and anxiolytic‐like effects in rodent models. However, whether these convergent outcomes occur due to distinct neurochemical effects or to region‐specific brain mechanisms remains unclear. Further research is needed to elucidate the role of 5‐HT₆Rs in antidepressant responses. Moreover, blockade of 5‐HT₇R has been proposed to be a promising novel and faster‐acting strategy for treating depression. Interestingly, the inhibition of 5‐HT₇R has been shown to induce and accelerate antidepressant effects [488]. The microbiota–gut–brain axis is also increasingly being recognized as playing a pivotal role in the pathogenesis of depression [489]. The gut microbiota may help alleviate depressive symptoms by regulating 5‐HT levels [490].

Currently, commonly used antidepressant drugs include tricyclic antidepressants (TCAs), SSRIs, and 5‐HT‐NE reuptake inhibitors (SNRIs). These medications act by increasing the synaptic concentrations of monoamine neurotransmitters, primarily 5‐HT and NE, and activating corresponding postsynaptic receptors [491]. The long‐term use of antidepressants has also been shown to enhance hippocampal neurogenesis. Among TCAs, drugs such as AMI and imipramine, as well as MAOIs, are widely prescribed owing to their safety and tolerability profiles. Recent evidence suggests that SNRIs may have lower tolerability than SSRIs in some patients with depression. However, clinical observations indicate that tolerability varies considerably between individuals. SSRIs are suitable for long‐term treatment and are considered relatively effective at managing moderate to severe depression in adults [492].

5.3.2. Schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by chronic or recurrent episodes of psychosis [493]. The clinical manifestations of schizophrenia include positive symptoms (such as hallucinations and delusions), negative symptoms (such as affective flattening or social withdrawal), and cognitive impairments involving attention, memory, and executive function [494]. There is evidence that neurodevelopmental abnormalities that occur during early brain development may represent key factors in the pathogenesis of schizophrenia [495]. Moreover, dysregulation of various neurotransmitter systems is closely associated with the progression of this disorder. In addition to the classical DA hypothesis, chronic stress mediated by the dorsal raphe (DRN) can lead to hyperactivation of the 5‐HT system, thereby disrupting neuronal activity in the cortex, anterior cingulate cortex, and dorsolateral PFC [496]. In addition to changes in 5‐HT activity, postmortem and in vivo studies of patients with schizophrenia have revealed changes in the expression of 5‐HTR and SERT, with most studies focused on the roles of 5‐HT_1AR and 5‐HT_2AR. However, findings about the role of 5‐HT_1AR in schizophrenia remain inconsistent. A meta‐analysis revealed significantly increased 5‐HT_1AR density in the PFC of patients with schizophrenia [497], whereas another study reported no significant difference in immunoreactivity between patients and healthy controls [498]; these findings indicate that the role of 5‐HT_1AR in the pathophysiology of schizophrenia remains under debate. In contrast, the role of 5‐HT_2AR in the pathology of schizophrenia has been extensively documented [295]. Studies have demonstrated significantly reduced 5‐HT_2AR binding potential in the PFC of patients with schizophrenia compared with that in healthy individuals, suggesting that downregulation or decreased activity of 5‐HT_2AR may contribute to this disorder [499]. These receptors regulate DA release in the nigrostriatal pathway; the administration of 5‐HT_2AR, such as olanzapine and risperidone, can increase striatal DA release by blocking the inhibitory effect of 5‐HT [500, 501]. Moreover, excessive activation of 5‐HT_2AR may increase the release of glutamate in the ventral tegmental area (VTA), which activates the mesolimbic pathway and leads to excessive DA levels in the ventral striatum [502]. Selective 5‐HT_2AR antagonists have largely failed as monotherapies for schizophrenia, whereas multitarget antagonists with greater affinity for 5‐HT_2AR than DA for D2R have proven more effective [503].

5‐HT₃R and 5‐HT₆R have emerged as promising therapeutic targets for the development of antipsychotic agents, especially for ameliorating the cognitive deficits that are associated with schizophrenia [504, 505, 506]. Studies have shown that the 5‐HT₃R antagonist ondansetron, when used as an adjunctive treatment, can alleviate symptoms in patients with treatment‐resistant disease and enhance visual memory performance [506, 507]. In addition, preclinical studies have demonstrated that blockade of 5‐HT₅Rs may have beneficial effects on negative symptoms and cognitive deficits in patients with schizophrenia [508]. 5‐HT₇R, which has high affinity for several antipsychotic and antidepressant drugs, has also been identified as a potential target [324]. Postmortem studies have revealed reduced expression of 5‐HT₇R in the PFC of patients with schizophrenia, and polymorphisms in the 5‐HT₇R gene are significantly associated with this disorder [509]. Blockade of 5‐HT₇R may help alleviate the negative symptoms of schizophrenia and has been shown to ameliorate ketamine‐induced social withdrawal behavior in mice [510].

Research has indicated that SERT density is not significantly decreased in patients with schizophrenia [511, 512]. These findings suggest that SERT may play a limited role in the pathophysiology of schizophrenia.

In summary, accumulating evidence suggests the presence of serotonergic dysfunction in individuals with depression and schizophrenia. However, further research is needed to develop 5‐HT‐based pharmacotherapies that specifically target the mechanisms underlying this disorder.

5.4. The 5‐HT System and Hepatobiliary Diseases

5‐HT not only regulates mental disorders in the CNS but also plays an important role in the periphery. In recent years, the role of 5‐HT in various pathological and physiological processes of the liver, as well as its underlying mechanisms, has attracted significant attention. 5‐HT plays a role in promoting liver regeneration and is closely associated with liver diseases such as viral hepatitis, liver fibrosis, cirrhosis, nonalcoholic steatohepatitis (NASH), primary liver tumors, liver ischemia‒reperfusion injury, and chronic cholestasis [380, 513].

5.4.1. Liver Regeneration

5‐HT not only functions as a neurotransmitter in the nervous system but also promotes cell mitosis and participates in tissue remodeling. Multiple studies have shown that 5‐HT plays an important regulatory role in hepatocyte proliferation during liver regeneration [514, 515, 516]. Platelets are the primary carriers of 5‐HT in the bloodstream, and they transport it to various organs. In a mouse model of liver regeneration following partial hepatectomy (PH), a reduction in platelet count or inhibition of platelet activity significantly suppresses hepatocyte proliferation [516, 517]. In Tph1^−/− mice, which lack peripheral 5‐HT, hepatocyte proliferation is markedly reduced. After PH, Tph1^−/− mice exhibit impaired liver regeneration and more severe liver injury, which are accompanied by significant upregulation of the transcriptional regulator Yap [518]. The inhibition of Yap expression significantly diminishes the proliferative effect mediated by 5‐HT, suggesting that 5‐HT promotes liver regeneration by regulating Yap expression [518, 519]. In addition, 5‐HTRs plays crucial roles in this process. In the livers of mice subjected to PH, the mRNA expression levels of 5‐HT_2AR and 5‐HT_2BR increase, which contributes to liver regeneration [520]. The underlying mechanism involves the IL‐33/growth stimulation expressed gene 2 (ST2) pathway‐mediated release of gut‐derived 5‐HT into the bloodstream, which subsequently activates the 5‐HT_2AR/p70S6K signaling pathway in hepatocytes [521]. Further research indicated that 5‐HT₂R may act as a cofactor in DNA synthesis, enhancing hepatocyte proliferation by promoting the G1/S phase cell cycle transition. Moreover, the 5‐HT₇R receptor is involved in hepatocyte mitosis and plays a role in liver regeneration [256]. In animals subjected to PH, elevated 5‐HT levels in the brainstem and cerebral cortex, along with the upregulation of 5‐HT_2CR receptors, suggest that this pathway may indirectly promote hepatocyte proliferation by stimulating sympathetic nerve activity [522].

5.4.2. Liver Fibrosis and Cirrhosis

Activation of hepatic stellate cells (HSCs) is a key step in the initiation and progression of liver fibrosis. 5‐HT regulates the activation, proliferation, and apoptosis of HSCs by binding to various receptors that are expressed on their surface, thereby influencing the fibrotic process. Multiple 5‐HTR subtypes are upregulated in activated HSCs and are predominantly distributed within fibrotic tissues [523]. Among these receptors, 5‐HT_2AR plays a critical role in HSC‐mediated fibrosis [524]. In vitro experiments have indicated that activation of 5‐HT_2AR can stimulate the Smad signaling pathway, leading to changes in the expression of fibrosis‐related markers, such as α‐smooth muscle actin (α‐SMA) and collagen [102, 524]. Additionally, 5‐HT can activate the ERK and JunD signaling pathways via 5‐HT_2BR, upregulate TGF‐β1 expression, and synergize with PDGF signaling to promote HSC activation [525, 526]. 5‐HT₂R antagonists significantly inhibit HSC proliferation and induce apoptosis while downregulating the expression of key fibrogenic proteins, including TGF‐β1, PKC, p‐Smad3, and p‐ERK1/2, in liver tissue, thus effectively alleviating liver fibrosis [527]. Moreover, emerging evidence suggests that 5‐HT₇R may have opposing effects on HSC function. Ruddell et al. [528] reported a marked reduction in 5‐HT₇R mRNA expression in activated rat HSCs. Similarly, Polat et al. [529] reported decreased hepatic expression of 5‐HT₇R in a CCl₄‐induced mouse model of cirrhosis. Activation of 5‐HTRs attenuates liver fibrosis and cirrhosis and slows HCC progression [529]. Notably, HSCs also express SERT, which allows them to actively take up and release 5‐HT, forming an autocrine/paracrine regulatory loop that helps maintain the local stability and persistence of 5‐HT signaling [528].

In summary, 5‐HT may play dual roles in the development of liver fibrosis and cirrhosis through different receptor subtypes. On the one hand, 5‐HT promotes HSC activation and proliferation via 5‐HT₂R, thereby facilitating fibrogenesis; on the other hand, 5‐HT may exert antifibrotic effects through 5‐HT₇R. The differential expression patterns and regulatory mechanisms of various 5‐HTRs in distinct hepatic cell populations during the progression of fibrosis warrant further investigation.

5.4.3. NASH

5‐HT plays a critical role in the pathogenesis of NASH [530]. 5‐HT levels are significantly elevated in both the peripheral blood and portal veins of NASH patients, suggesting a close association with disease onset [531]. Research has indicated that MAO‐A expression is upregulated in patients with NASH [531]. In methionine–choline‐deficient diet‐induced animal models of NASH, 5‐HT is metabolized by MAO‐A in mitochondria, leading to the production of large amounts of ROS and lipid peroxides [532]. This oxidative stress results in mitochondrial dysfunction and triggers inflammatory responses, thereby exacerbating hepatocellular injury and the pathological progression of NASH [532]. Moreover, inhibition of MAO‐A activity in wild‐type mice alleviates liver damage. Although Tph1^−/− mice exhibit levels of hepatic steatosis that are similar to those in wild‐type controls, impaired peripheral 5‐HT synthesis in these mice leads to reduced ROS production and milder hepatocellular injury, consequently attenuating liver inflammation [533]. In addition, 5‐HT contributes to NASH progression by activating 5‐HT_2AR on hepatocyte membranes, thereby disrupting lipid metabolism and promoting hepatic fat accumulation and inflammatory responses [534]. The underlying mechanisms involve the upregulation of peroxisome proliferators‐activated receptors (PPARs) γ2 and lipogenesis‐related genes, such as factor‐related apoptosis (Fas), Cd36, and Perilpin2, which increase lipid synthesis and deposition [535]. Studies have demonstrated that hepatosteatosis is significantly reduced in intestinal‐specific Tph1^−/− mice and liver‐specific 5‐HT_2AR^−/− mice that are fed a high‐fat diet (HFD); these results suggest that inhibiting gut‐derived 5‐HT synthesis or blocking hepatic 5‐HT_2AR signaling can ameliorate lipid metabolism disorders [536]. Furthermore, 5‐HT_2AR antagonists can prevent HFD‐induced hepatic steatosis [537], and the combined use of 5‐HT synthesis inhibitors and 5‐HT₂R antagonists markedly reduces hepatic steatosis and inflammation in mouse models of diabetes [535, 538]. Moreover, studies have revealed that 5‐HT₄R expression in both liver and adipose tissue significantly decreases after HFD intervention in mice. Blocking 5‐HT₄R not only mitigates hepatic steatosis but also reduces adipose tissue mass and suppresses proinflammatory cytokine expression and inflammasome complex formation, highlighting a potential anti‐inflammatory and metabolic regulatory effect [539].

In addition to its direct hepatic effects, 5‐HT plays an essential role in modulating intestinal barrier function. 5‐HT₃R regulates the expression of tight junction proteins such as claudin‐1 (CLDN1) and occludin in intestinal epithelial cells. Disruption of 5‐HT₃R activity increases gut permeability, allowing LPS to enter the liver, where it activates immune responses and accelerates the transition from NAFLD to NASH [540]. Additionally, the gut microbiota participates in the pathogenesis of NASH by regulating the levels of 5‐HT and its metabolites. Gut bacteria can influence 5‐HT synthesis by metabolizing tryptophan and modulating the inflammatory status of the gut–liver axis, thereby indirectly affecting hepatic lipid accumulation and inflammation, which further exacerbates the progression of NASH [541].

In conclusion, 5‐HT is involved in various aspects of NASH pathology, including lipid metabolism dysregulation, oxidative stress, inflammatory activation, and disruption of the gut–liver barrier, via multiple targets and signaling pathways. These multifaceted mechanisms reveal novel targets and strategies for NASH treatment. Future interventions that focus on the comprehensive regulation of 5‐HT synthesis, metabolism, and receptor signaling may offer promising directions for the effective treatment of NASH.

5.4.4. Viral Hepatitis

Clinical studies have shown that serum 5‐HT levels in patients with hepatitis C virus (HCV) infection who are receiving interferon antiviral therapy can serve as a predictor of treatment efficacy [542]. Patients with higher 5‐HT levels tend to respond better to interferon treatment, suggesting that 5‐HT may be closely associated with the onset and progression of viral hepatitis [543]. In mouse models of viral hepatitis, the number of activated platelets in the liver is significantly increased [544]. These platelets release 5‐HT, which can reduce hepatic blood flow, impair the recruitment of CD8⁺ T cells that are needed for viral clearance, and thus exacerbate liver injury. In contrast, Tph1^−/− mice, which lack the ability to synthesize 5‐HT in the periphery, exhibit significantly reduced liver damage; these results further suggest a pathogenic role of 5‐HT in viral hepatitis [545]. Additionally, studies have shown that SSRIs, such as fluoxetine, can effectively inhibit platelet uptake of 5‐HT, thereby reducing circulating 5‐HT levels. This phenomenon improves hepatic microcirculation, enhances CD8⁺ T cell recruitment, and accelerates hepatitis viral clearance [545]. Clinical observations have also shown that SSRIs can reduce the risk of HCC in patients with hepatitis B virus (HBV) infection in a dose‐dependent manner [546]. Moreover, in vitro studies involving HCV‐infected hepatocyte cultures have suggested that 5‐HTR activation participates in the viral entry process. Specifically, the activation of 5‐HT_2AR enhances the membrane localization of CLDN1, which is a key tight junction protein, thereby promoting endocytosis and facilitating HCV entry into cells [542].

In conclusion, 5‐HT plays a crucial role in the occurrence and progression of various liver diseases. Modulating the level of 5‐HT or blocking signaling pathways mediated by its receptors may provide new therapeutic approaches and potential therapeutic targets for liver diseases.

5.5. The 5‐HT System and Diabetes

5‐HT, which is a key neurotransmitter, also plays a widespread role in regulating energy metabolism, particularly in the maintenance of glucose homeostasis, in which it performs dual functions. On the one hand, some studies have reported that 5‐HT can increase blood glucose levels and promote insulin resistance; on the other hand, evidence suggests that 5‐HT can stimulate insulin secretion, increase insulin sensitivity, and ameliorate metabolic disorders.

The hyperglycemic effect of 5‐HT is likely mediated by 5‐HT₂R. Gut‐derived 5‐HT can act on 5‐HT_2BR in hepatocytes, thereby promoting gluconeogenesis [547, 548]. Blocking 5‐HT_2BR improves glucose metabolism and facilitates the restoration of normoglycemia [549, 550]. When 5‐HT binds to 5‐HT₂R, it induces serine phosphorylation of insulin receptor substrate‐1, reduces its activity, and promotes its ubiquitin‐mediated degradation, thereby disrupting insulin signaling pathways [551]. On the basis of these mechanisms, 5‐HT₂R antagonists have been explored in the clinic as potential treatments for type 2 diabetes and metabolic syndrome.

Recent studies have also revealed the hypoglycemic effects of 5‐HT. Animal experiments have shown that 5‐HT increases serum insulin levels, reduces plasma glucose concentrations, improves glucose tolerance and lipid metabolism in mice with type 2 diabetes, and reduces body weight in a dose‐dependent manner [552]. Additionally, 5‐HT activates 5‐HT₇R in the adrenal gland, promoting the release of β‐endorphin, which in turn activates opioid receptors, enhances peripheral glucose uptake and utilization, and significantly decreases blood glucose levels in type 1 diabetes models [553]. Clinical studies have shown that 5‐HT₄R agonists also exert glucose‐lowering effects, primarily by increasing insulin sensitivity [554]. Furthermore, 5‐HT activates 5‐HT_2A/2CR, increasing the membrane expression of glucose transporters (e.g., GLUT4) and thereby promoting glucose uptake by cardiomyocytes [555]. In addition, Tph1^−/− mice, which lack peripheral 5‐HT, exhibit reduced insulin secretion and elevated blood glucose levels, highlighting the crucial role of 5‐HT in pancreatic islet function [556].

In addition to insulin signaling, 5‐HT plays important roles in regulating insulin secretion and β‐cell growth. 5‐HT is stored along with insulin in pancreatic β‐cell granules, and it is released simultaneously in response to glucose stimulation. Studies suggest that intracellular 5‐HT can regulate the fusion of insulin‐containing vesicles with the cell membrane through serotonylation, thus facilitating insulin secretion [551]. Extracellular 5‐HT can regulate insulin release via the activation of 5‐HT_1AR. Additionally, 5‐HT may act in an autocrine or paracrine manner to activate 5‐HT_2BR, thus promoting β‐cell proliferation [557]. 5‐HT also suppresses the expression of α‐cell markers (such as Arx and Gcg) while increasing β‐cell marker expression in murine α‐cell lines, suggesting its potential to induce α‐to‐β‐cell phenotype conversion [558].

In addition to its direct effects on glucose metabolism, 5‐HT is involved in the regulation of diabetic complications. Peripheral 5‐HT_5AR has been shown to play a significant role in inhibiting cardiac sympathetic nerve activity in rats with type 1 diabetes [559]. The 5‐HT_1AR agonist NLX‐112 has been shown to ameliorate urinary dysfunction in rats with diabetes, and this mechanism is likely associated with the upregulation of 5‐HTR expression in the L6–S1 spinal dorsal lateral nucleus [560]. Treatment with a 5‐HT_2BR agonist has been shown to improve colonic migrating motor complex activity in males with diabetes and enhance colonic transit in ovariectomized female patients with diabetes [561]. In type 1 diabetes models, the downregulation of presynaptic 5‐HT_1AR enhances spinal 5‐HT release, reduces quinolinic acid levels, inhibits tryptophan 2,3‐dioxygenase (Tdo), indoleamine 2,3‐dioxygenase (Ido) 1, and Ido2 expression, and improves neuronal degeneration and pain‐related behaviors [562]. Furthermore, the contractile response of the pancreatic and mesenteric arteries to 5‐HT is significantly reduced in mice with diabetes, possibly due to decreased 5‐HTR activity and increased eNOS activation, leading to elevated nitric oxide release and reduced vascular tone [563].

In summary, the 5‐HT signaling pathway, which is a complex and highly tunable biological network, includes factors that are emerging as therapeutic targets for a wide range of systemic diseases. Future research should continue to integrate multiomics data, drug screening platforms, and clinical validation to advance the development of 5‐HT‐related therapies toward precision medicine, combination treatments, and targeted delivery, thereby promoting translational advancements in disease management.

6. Role of 5‐HT in Tumor Immunity

In addition to its role in a variety of diseases that have been widely reported or applied in clinical treatment, an increasing number of studies in recent years have focused on the function of 5‐HT in immune regulation. Owing to its multifaceted effects on immune cells, the 5‐HT signaling axis represents a promising direction for research on tumor immunity (Figure 5 and Table 3).

FIGURE 5.

Role of serotonin (5‐HT) signaling in tumor immunity and mechanistic cross‐talk between 5‐HT signaling and immune checkpoints. 5‐HT acts on 5‐HT receptors (5‐HTRs) on the surfaces of immune cells, promotes dendritic cell (DC) maturation, stimulates T‐cell activation, increases NK cell cytotoxicity, promotes B‐cell maturation, and stimulates macrophage polarization toward the M2 phenotype.

TABLE 3.

Roles of 5‐HT in tumor immunity.

Receptor	Cell type	Mechanism	Effect	References
5‐HT1AR	T cells, B cells, NK cells	5‐HT binds to 5‐HTR and increases the intracellular cAMP level in response to ATP in T cells. 5‐HT binds to 5‐HTR and increases IL‐10 production in B cells through the activation of STAT3 activation. 5‐HT protects NK cells from peroxidase inhibition.	T cell proliferation and stimulation B cell proliferation Promotes NK cell activation and IFN‐γ production by NK cells	[564, 565, 566]
5‐HT1BR	DCs	5‐HT induces 5‐HTR‐dependent Ca2+ influx in immature and mature DCs.	DC migration	[301]
5‐HT1E/1FR	DCs	5‐HT induces a 5‐HTR‐dependent intracellular Ca2+ spike in immature DCs.	DC migration	[301]
5‐HT2R	DCs, T cells, TAMs	5‐HT stimulates 5‐HTR to induce intracellular Ca2+ mobilization via Gi/o proteins in immature DCs and promote TLR3 activation. 5‐HT mediates the phosphorylation of the ERK1/2 and Iκβα in T cells. 5‐HT stimulates 5‐HTR to promote the ERK pathway and AhR axis activation.	Immature CD1a+ monocyte‐derived DC maturation T cell proliferation and stimulation M2 macrophage polarization	[242, 301, 302]
5‐HT3R	DCs, T cells	5‐HT induces a 5‐HTR‐dependent intracellular Ca2+ spike in immature DCs. 5‐HT stimulates 5‐HTR and increases intracellular Na+ concentrations.	DC maturation T cell activation	[301]
5‐HT4R	DCs	5‐HT activates 5‐HTR to induce an increase in cAMP levels in mature DCs, increase the release of the cytokines IL‐1β and IL‐8, and reduce the secretion of IL‐12 and TNF‐α.	DC maturation	[301]
5‐HT1/4/7R	DCs	5‐HT binds to 5‐HTR in DCs cells to promote the production of the pro‐inflammatory cytokine IL‐6, inhibit the production of the chemokine IP‐10/CXCL10, increase the secretion of CCL22/MDC, and promote the transformation of mature DCs into high IL‐10 and low IL‐12/p70 secretion phenotypes.	Cytokine release from DCs	[567]
5‐HT7R	DCs, T cells, TAMs	5‐HT binds to 5‐HTR on monocytes, induces changes in DC phenotypes, decreases the expression of costimulatory molecules and CD1a, increases the expression of CD14, significantly decreases the stimulating activity of allogeneic T cells, and affects small GTPase and Cdc45 activation. 5‐HT binds to 5‐HTR to induce the phosphorylation of ERK1/2 and IκBα in naive T cells. 5‐HT binds to 5‐HTR to inhibit the release of TNF‐α and IL‐12/p40 from monocyte‐derived macrophages stimulated by LPS.	Maturation and migration of bone marrow‐derived DCs T cell activation TAM polarization toward an anti‐inflammatory phenotype and infiltration	[242, 341, 568]

6.1. 5‐HT‐Mediated Regulation of Tumor‐Infiltrating Immune Cells

A variety of 5‐HTRs are expressed on the surfaces of immune cells [569]. 5‐HT interacts directly with distinct 5‐HTRs on immune cells that can profoundly influence the ability of the immune system to target and eliminate tumors.

6.1.1. Macrophages

Macrophages, particularly TAMs, are critical targets of 5‐HT signaling within the TME. Macrophages play pivotal roles in modulating inflammation, tissue remodeling, and immune surveillance [570, 571, 572]. Emerging evidence indicates that 5‐HT can influence macrophage polarization and cytokine production, thereby shaping the immune landscape in ways that either support or hinder tumor development.

5‐HT binds to 5‐HTR on the surfaces of monocytes and macrophages, promoting the polarization of TAMs toward the M2 phenotype and regulating their cytokine secretion [573]. Nocito et al. [162] reported that 5‐HT also reduces the expression of MMP12 in TAMs and promotes tumor progression in mice with colon cancer. MMP12 cleaves plasminogen to generate the angiogenesis inhibitor angiostatin, exerting an antiangiogenic effect. However, it has been reported that macrophages respond to stimulation with 5‐HT at different concentrations. Low levels of 5‐HT promote the production of IL‐6 and TNF‐α by macrophages and T cells through 5‐HT₂R, whereas high concentrations of 5‐HT inhibit the secretion of IL‐6 and TNF‐α, thereby inducing T cells to differentiate into immunosuppressive or nonfunctional types [242, 302].

6.1.2. DCs

In addition to its effects on macrophages, 5‐HT also exerts a significant regulatory effect on DCs, which serve as key antigen‐presenting cells that bridge innate and adaptive immunity. By modulating the phenotype and cytokine profile of DCs, 5‐HT can indirectly shape T‐cell responses and thereby influence tumor progression.

5‐HT signaling promotes the maturation and chemotaxis of bone marrow‐derived DCs [574]. 5‐HT promotes the development of anti‐inflammatory DCs, which in turn may lead to the polarization of Tregs [575]. Tregs often suppress cytotoxic T‐cell activity, thereby promoting tumor development [576]. 5‐HT binds to 5‐HTR on DCs, induces changes in the DC phenotype, decreases the expression of costimulatory molecules and CD1a, increases the expression of CD14, and significantly decreases the stimulating activity of allogeneic T cells [568]. Idzko et al. [301] reported differences in the expression of 5‐HTR on DCs at different stages; immature DCs preferentially express 5‐HT_1BR, 5‐HT_1ER and 5‐HT_2BR, whereas mature DCs mainly express 5‐HT₄R and 5‐HT7R. The activation of different types of receptors induces different functions in DCs. 5‐HT₁R and 5‐HT₂R stimulation induces intracellular Ca²⁺ mobilization via the Gi/o protein in immature DCs. 5‐HT₄R and 5‐HT₇R induce elevated cAMP levels in mature DCs. In addition, 5‐HT₄R and 5‐HT₇R increase the release of IL‐1β and IL‐8 while decreasing the secretion of IL‐12 and TNF‐α by mature DCs [301].

6.1.3. T Cells

T cells play a direct role in inhibiting and killing tumors [577, 578, 579, 580]. Recent research has highlighted the complex and sometimes contradictory roles of 5‐HT in modulating antitumor immunity, particularly through its influence on the behavior of T cells in the TME. On the one hand, 5‐HT has been shown to inhibit T‐cell function; on the other hand, under certain conditions, it may enhance T cell‐mediated immune responses.

5‐HT can inhibit the proliferation and activity of cytotoxic T cells through 5‐HT_5AR and 5‐HT7R [581]. The activation of 5‐HT₃Rs on the surface of T cells by 5‐HT can significantly suppress the CXCL12‐mediated migration of CD4⁺ T cells, indicating that increased 5‐HT levels may prevent the recruitment of T lymphocytes to tumor tissues [341]. The expression of 5HT_2A/2BR in breast cancer patients is positively correlated with the invasion of CD8⁺ T cells. In zebrafish and mouse models, 5HT_2AR activation promotes CD8⁺ T‐cell proliferation and inhibits breast cancer invasion and metastasis [582]. CD8⁺ T cells accumulate 5‐HT intracellularly to promote the formation of 5‐HT from GAPDH, thereby promoting the glycolytic metabolism and antitumor immune activity of CD8⁺ T cells [583]. In addition, treatment of chronically stressed mice with 5‐HT reduces the infiltration of CD8⁺ T cells into the TME, and the expression of IFN‐γ and granzyme B in CD8⁺ T cells is also reduced, whereas the expression of PD‐1on CD8⁺ T cells is increased [584]. Other groups have reported that 5‐HT increases PD‐L1 expression by tumor cells in vitro through serotonylation. Moreover, serum 5‐HT concentrations in patients with abdominal tumor metastases are negatively correlated with the number of tumor‐infiltrating CD8⁺ T cells. 5‐HT depletion suppresses the growth of syngeneic pancreatic and colorectal tumors in wild‐type mice, increases CD8⁺ T cell influx, and reduces PD‐L1 expression [212].

6.1.4. Other Cells

5‐HT can increase the cytotoxic potential of NK cells [564, 585]. 5‐HT also regulates the mitogen‐stimulated proliferation of mature B cells [565]. However, the detailed mechanisms by which 5‐HT affects various immune cells via downstream signaling pathways have not been elucidated. In the future, the use of various drugs with the potential to regulate 5‐HT signaling may play a key role in modulating tumor immunity.

6.2. 5‐HT‐Mediated Tumor Immunotherapy

Studies have shown that 5‐HT can upregulate PD‐L1 expression on the surface of tumor cells, thereby significantly reducing the infiltration of CD8⁺ T cells into the TME and weakening the antitumor immune response. Experimental results demonstrate that eliminating 5‐HT or blocking 5‐HT signaling leads to a reduction in PD‐L1 levels on tumor cells, increased T‐cell infiltration, and slower tumor progression; these results suggest that antidepressants (e.g., SSRIs) in combination with PD‐1/PD‐L1 inhibitors may exert synergistic antitumor effects [212]. In addition, MAOIs can act as immune modulators. Studies have shown that MAO‐A is overexpressed in tumor‐associated immune cells and that its genetic deletion or pharmacological inhibition (e.g., with phenelzine) can significantly increase the antitumor activity of CD8⁺ T cells. In murine tumor models, MAO‐A knockout or inhibition not only markedly suppresses tumor growth but also exhibits synergistic efficacy when combined with anti‐PD‐1 therapy [226]. In the context of CAR T‐cell immunotherapy, which is a novel T cell‐based therapy that involves Tph1‐engineered CAR T cells that can endogenously synthesize 5‐HT, has been developed. These modified cells significantly enhance CD8⁺ T‐cell activation and endoplasmic function. While traditional CAR T cells reduce tumor volume by approximately 32% on day 23, Tph1‐CAR T cells achieve a tumor reduction of approximately 78%, indicating markedly stronger antitumor activity [583]. Furthermore, recent studies in animal models revealed that SSRIs significantly enhance CD8⁺ T‐cell function in various tumor models, including melanoma, breast cancer, prostate cancer, and colorectal cancer models, leading to greater than 50% tumor growth inhibition. More importantly, when combined with PD‐1/PD‐L1 immune checkpoint inhibitors, this treatment resulted in complete tumor regression in all experimental animals, further highlighting the promising clinical translation potential of targeting the 5‐HT signaling pathway [586].

In summary, 5‐HT plays a crucial role in regulating tumor‐infiltrating immune cells, and its signaling axis has emerged as a novel target for tumor immunotherapy. Future studies should focus on elucidating the mechanisms by which different receptor subtypes regulate immune cells and validating their therapeutic potential across various tumor models and clinical settings.

7. The Potential of 5‐HT for Use as a Biomarker of Disease

5‐HT, also known as serotonin, is a crucial neurotransmitter, hormone, and paracrine signaling molecule [587, 588]. While it is best known for its role in mood regulation, cognitive function, and GI motility, an increasing number of studies have demonstrated that changes in 5‐HT levels, metabolic pathways, and receptor expression are closely associated with various pathological conditions [39]. As such, interest in the potential of 5‐HT for use as a biomarker of a wide range of diseases, particularly psychiatric disorders, inflammatory conditions, cancer, and metabolic syndromes, is increasing [589].

Decreased 5‐HT signaling in the CNS is considered a key mechanism underlying depressive symptoms [590]. 5‐HT levels, such as those in plasma or platelets, have been used as dynamic indicators for evaluating depressive states [591]. Some studies have reported significantly lower levels of 5‐HT in the cerebrospinal fluid of patients with depression than in that of healthy individuals, suggesting the potential of 5‐HT levels to be used as a diagnostic or prognostic biomarker [592]. However, findings on the effects of antidepressant treatment on peripheral 5‐HT levels remain inconsistent. A meta‐analysis of 15 clinical trials and 11 animal studies revealed that peripheral 5‐HT levels may increase, decrease, or remain unchanged after pharmacological intervention [593, 594]. This high degree of variability currently limits the reliability of peripheral 5‐HT levels as a stable biomarker for diagnosing depression or evaluating therapeutic efficacy.

5‐HT plays a critical role in GI motility, secretion, and local immune regulation [595]. Studies have shown that elevated 5‐HT levels in the GI mucosa or peripheral blood are strongly associated with conditions such as IBS, IBD, and carcinoid syndrome [596]. In patients with CD, serum 5‐HT levels are significantly elevated during phases of active disease [597, 598]. Moreover, serum 5‐HT levels outperform traditional inflammatory markers, such as C‐reactive protein and circulating cytokine levels, in distinguishing active versus refractory or remissive disease states [599]. Thus, the serum 5‐HT level is considered an effective biomarker for disease stratification and treatment guidance in patients with CD [600]. Additionally, 5‐HT is involved in platelet aggregation and vasoconstriction, indicating its potential clinical relevance in cardiovascular diseases such as hypertension, atherosclerosis, and myocardial infarction [601, 602, 603].

A growing body of evidence highlights the role of 5‐HT in tumor biology [604]. In addition to supporting tumor cell proliferation and angiogenesis, 5‐HT may promote immune evasion and metastasis. In solid tumors such as colorectal, breast, and prostate cancers, the overexpression of 5‐HTRs (e.g., 5‐HT_2AR and 5‐HT_2BR) and increased SERT levels have been correlated with tumor progression, stage, and prognosis [604, 605, 606]. Dynamic changes in the 5‐HT levels in blood and tumor tissues may serve as indicators for monitoring therapeutic response and recurrence risk [606]. In patients with carcinoid syndrome, urinary 5‐HIAA, which is an end metabolite of 5‐HT, has already become a clinically established biomarker for assessing tumor burden and monitoring treatment efficacy [607].

5‐HT also plays a pivotal role in modulating the immune system. Studies have shown that 5‐HT influences the functions of DCs, macrophages, and T and B lymphocytes [608]. In autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, and multiple sclerosis, abnormal expression of 5‐HTRs on immune cells may affect disease progression, suggesting the potential of these receptors to be used as supplementary biomarkers for monitoring disease activity [609, 610]. In the context of metabolic disorders, particularly type 2 diabetes and obesity, 5‐HT has been shown to regulate glucose metabolism, insulin sensitivity, and adipose tissue inflammation [611]. Studies have shown that changes in 5‐HT levels or disruptions in its metabolic pathway are associated with insulin resistance and chronic inflammation in adipose tissue, indicating that 5‐HT can be used as a potential biomarker for the early diagnosis of diabetic complications and related disorders [611].

Overall, the widespread involvement of 5‐HT and its signaling pathways across numerous physiological and pathological processes highlights its tremendous potential in biomarker development. Through direct measurement (e.g., serum or CSF levels of 5‐HT and 5‐HIAA), functional imaging (e.g., PET scans of SERT distribution), or functional assessment (e.g., receptor sensitivity and transporter expression), 5‐HT‐related indicators hold promise for early disease screening, disease progression monitoring, therapeutic response prediction, and the development of personalized treatment strategies [612]. However, the expression of 5‐HT is influenced by various factors, such as age, sex, diet, circadian rhythms, and methodological differences in sample collection and analysis. Therefore, further large‐scale and standardized studies are needed to optimize its clinical application.

8. Conclusion and Prospects

This review provides a comprehensive overview of the 5‐HT system, emphasizing 5‐HT metabolic pathways, 5‐HT receptor‐mediated signaling mechanisms, and the complex and widespread roles of 5‐HT across various physiological and pathological contexts. 5‐HT has multiple biological effects on emotional regulation, GI homeostasis, and immune modulation, and it is also involved in the development and progression of numerous diseases, including psychiatric disorders, metabolic syndrome, inflammatory conditions, and cancer [17, 18, 19]. In recent years, aberrant 5‐HT signaling has been increasingly recognized as a key feature of various pathologies, particularly neuroinflammation, tumor immune evasion, and vascular dysfunction [589]. While several therapeutic agents that target the 5‐HT system, such as SSRIs, MAOIs, and receptor‐specific antagonists, have been successfully applied in clinical practice [221, 279, 290], their broader implementation is hindered by limited target selectivity, systemic complexity, and frequent adverse effects.

Several fundamental scientific questions remain unresolved. The mechanisms by which 5‐HT regulates immune cell functions, such as macrophage polarization and T‐cell activation, within the tumor immune microenvironment remain poorly defined, and the causal relationships between 5‐HT signaling and immune activation or suppression require further clarification. Additionally, the interplay between 5‐HTRs and key signaling pathways (e.g., Wnt/β‐catenin and PD‐1/PD‐L1) is a potential basis for the dual tumor‐promoting and tumor‐inhibiting effects of 5‐HT, and this interplay warrants systematic investigation. Moreover, the influence of the gut microbiota on peripheral 5‐HT synthesis and metabolism has emerged as a novel research topic [613]. Increasing evidence suggests that microbial metabolites can modulate tryptophan metabolism in the gut, thereby regulating EC cell‐derived 5‐HT production, which in turn affects host neuroendocrine and immune homeostasis through the gut–brain–immune axis [614]. Thus, microbially mediated regulation of 5‐HT metabolism may act as a mechanistic link in the pathogenesis of autoimmune diseases and mood disorders.

Although preclinical studies have demonstrated the therapeutic potential of targeting 5‐HT pathways, significant barriers remain in translating these findings into clinical applications. On the one hand, the bidirectional role of 5‐HT in different disease contexts, especially in cancer, poses considerable challenges for drug design. On the other hand, currently available pharmacological agents lack receptor subtype selectivity and tissue specificity, which results in systemic side effects. The development of highly selective receptor modulators represents a promising direction for enhancing therapeutic specificity and minimizing off‐target effects. Immunometabolic combination strategies should also be prioritized, including the use of SSRIs to increase CD8⁺ T‐cell function in synergy with PD‐1/PD‐L1 immune checkpoint blockade. Future research to engineer CAR T cells with the ability to locally modulate 5‐HT signaling may offer new approaches to reshape the immunosuppressive TME. Alternatively, targeted nanocarrier systems could enable the precise delivery of 5‐HTR antagonists to specific tissues, thereby reducing systemic toxicity. Spatial and temporal modulation of 5‐HT levels, for example, by enhancing 5‐HT signaling in the gut while suppressing it in the CNS, could represent a strategy to maximize therapeutic efficacy while minimizing side effects.

In summary, 5‐HT signaling represents a central interface that links metabolic control, receptor activation, and immune responses across a spectrum of pathophysiological states. Therapeutically targeting the 5‐HT system not only advances mechanistic insights into disease progression but also offers a framework for the development of integrative, multitarget treatment strategies. Continued research into the biology of 5‐HT is expected to accelerate the discovery of novel therapeutics, ultimately improving the management of cancer, neuropsychiatric disorders, GI dysfunction, and metabolic disease.

Author Contributions

Yuxin Zhang: conceptualization, methodology, investigation, data curation, writing – original draft, and visualization. Nan Wang: methodology. Louqian Zhang: methodology. Yan Zhuang: methodology. Qilei Xin: investigation. Xiaosong Gu: supervision, visualization, project administration, and funding acquisition. Chunping Jiang: supervision, visualization, project administration, and funding acquisition. Junhua Wu: supervision, visualization, writing – review and editing, project administration, and funding acquisition. All the authors have approved the final version of this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Ethics Statement

The authors have nothing to report.

Zhang Y., Wang N., Zhang L., et al. “Serotonin (5‐Hydroxytryptamine): Metabolism, Signaling, Biological Functions, Diseases, and Emerging Therapeutic Opportunities.” MedComm 6, no. 9 (2025): 6, e70383. 10.1002/mco2.70383

Data Availability Statement

No data were used for the research described in the article.