Serotonin as a Mitogen in the Gastrointestinal Tract: Revisiting a Familiar Molecule in a New Role

Abstract

Serotonin signaling is ubiquitous in the gastrointestinal (GI) system, where it acts as a neurotransmitter in the enteric nervous system (ENS) and influences intestinal motility and inflammation. Since its discovery, serotonin has been linked to cellular proliferation in several types of tissues, including vascular smooth muscle, neurons, and hepatocytes. Activation of serotonin receptors on distinct cell types has been shown to induce well-known intracellular proliferation pathways. In the GI tract, potentiation of serotonin signaling results in enhanced intestinal epithelial proliferation, and decreased injury from intestinal inflammation. Furthermore, activation of the type 4 serotonin receptor on enteric neurons leads to neurogenesis and neuroprotection in the setting of intestinal injury. It is not surprising that the mitogenic properties of serotonin are pronounced within the GI tract, where enterochromaffin cells in the intestinal epithelium produce 90% of the body’s serotonin; however, these proliferative effects are attributed to increased serotonin signaling within the ENS compartment as opposed to the intestinal mucosa, which are functionally and chemically separate by virtue of the distinct tryptophan hydroxylase enzyme isoforms involved in serotonin synthesis. The exact mechanism by which serotonergic neurons in the ENS lead to intestinal proliferation are not known, but the activation of muscarinic receptors on intestinal crypt cells indicate that cholinergic signaling is essential to this signaling pathway. Further understanding of serotonin’s role in mucosal and enteric nervous system mitogenesis may aid in harnessing serotonin signaling for therapeutic benefit in many GI diseases, including inflammatory bowel disease, malabsorptive conditions, and cancer.

Summary.

Binding of serotonin receptors on intestinal epithelial cells and enteric neurons activates intracellular proliferative pathways. Potentiation of serotonin signaling has been linked to mucosal proliferation, enteric neurogenesis in the setting of intestinal injury, and the pathogenesis of colon cancer.

Serotonin has long been viewed as a critical signaling molecule for gastrointestinal (GI) health, with a well-established role in GI motility and inflammation.^1,2 More recently, serotonin has come into focus for its mitogenic properties in the intestine, both physiologic and pathologic. Cellular proliferation has been linked to serotonin signaling in myriad cell types and organ systems.3, 4, 5 This effect is illustrated uniquely in the intestine as a result of the juxtaposition between serotonin signaling in the gut epithelium, which produces a majority of the body’s serotonin supply, and within the enteric nervous system.

In this review, we synthesize contemporary understanding of serotonin signaling as it relates to cellular proliferation in the intestine. Specifically, we describe the serotonin receptor–initiated pathways involved in intestinal epithelial proliferation, enteric neurogenesis, and intestinal neoplasia. Although further research is necessary to harness the translational potential of serotonin-mediated proliferation in the intestinal mucosa and enteric nervous system, the therapeutic promise of this effect is substantial. Chronic conditions of intestinal injury and insufficiency, for example, in patients with inflammatory bowel disease or short-bowel syndrome, are costly and difficult to treat because of the limitations of intestinal replacement therapies.⁶ Induction of intestinal mucosal proliferation through the modulation of serotonin signaling is already within the means of medicine, given the established arsenal of serotonin transporter (SERT) inhibitors and serotonin-receptor antagonists that exist, and offers a new therapeutic prospect for this clinical problem.

Historical Perspective

Serotonin, named for the location in which it was first discovered (Latin serum) and one of its earliest known functions (Greek tonic), has a storied scientific history. Initial reports of the substance later identified as serotonin date back to 1868, when Ludwig and Schmidt⁷ observed that perfusion of dog muscle with defibrinogenated blood resulted in vasoconstriction. In the following decades, other investigators proposed that epinephrine was the responsible vasoactive chemical.⁸ It was not until O’Connor⁹ observed that a substance stored in platelets exerted a vasoconstrictive effect on the intestine that a molecule distinct from epinephrine, which relaxes intestinal smooth muscle, was hypothesized to exist.¹⁰ The third and fourth decades of the 20th century saw substantial advancements in the understanding of serotonin physiology. Rapport et al¹¹ are credited with isolating serotonin from blood and describing its structure as 5-hydroxytryptamine (5-HT) in 1948. However, it subsequently became clear that 5-HT was identical to the substance enteramine, which the Italian scientists Erspamer, Aspero, and Vialli^12,13 had identified a decade earlier. Erspamer had selected this term to reflect the molecule’s site of production, in enterochromaffin (EC) cells in the gut mucosa, and contractile effect on intestinal smooth muscle.¹² Discovery of serotonin’s excitatory neurotransmitter function originated in the hard-shell clam in 1953, which led soon thereafter to the identification of serotonergic cells in the vertebrate brain.^14,15 Although the name serotonin has endured, the historical significance of enteramine has been realized as subsequent research showed the varied role of 5-HT in GI function, which we describe later.

Overview of Serotonin Physiology

Serotonin, or 5-HT, is a ubiquitous signaling molecule that has been ascribed diverse functions, including neurotransmission, vascular tone, hemostasis, bone resorption, GI function, and cellular proliferation.16, 17, 18, 19 5-HT is derived from the amino acid tryptophan in a 2-step enzymatic process involving tryptophan hydroxylase (TPH), followed by L-amino acid decarboxylase.²⁰ TPH exists in 2 distinct isotypes in vertebrates: TPH1 is expressed in several non-neuronal cell types, most abundantly in the intestine, pineal gland, and pituitary gland, whereas TPH2 is expressed in serotonergic neurons in the central and enteric nervous systems (Figure 1).21, 22, 23, 24 L-amino acid decarboxylase is an abundant and fast-acting enzyme that also is involved in catecholamine synthesis. As such, TPH is the rate-limiting step for 5-HT synthesis in peripheral tissues where tryptophan is readily available; in the central nervous system (CNS), 5-HT synthesis is rate-limited by transport of tryptophan across the blood-brain barrier.²⁰ 5-HT also is unable to cross the blood-brain barrier, thus lending spatial and functional separation based on where biosynthesis occurs.

Figure 1.

Life cycle of serotonin in the intestinal epithelium and ENS. 5-HT is synthesized from tryptophan in enterochromaffin cells and in enteric neurons by distinct isoforms of TPH, respectively. TPH1-derived 5-HT in the mucosa enters the portal circulation, where it is stored in platelets that enter the systemic circulation. Serotonergic neurons in the ENS produce TPH2 to synthesize 5-HT, which is stored in synaptic vesicles. Upon release from intracellular vesicles or platelets, 5-HT binds to 5-HTRs on cell membranes. The majority of 5-HTRs are G-protein–coupled receptors, with the exception of the 5-HT₃–receptor family, which are ligand-gated ion channels. The SERT is important in inactivating 5-HT by relocating the molecule intracellularly, where it subsequently is degraded by monoamine oxidase into 5-hydroxyindoleacetic acid (5-HIAA). IPAN, intrinsic primary afferent neuron; MAO, monoamine oxidase; Trp, tryptophan.

Irrespective of where it is synthesized, serotonin exerts biologic action by binding to cell membrane–bound serotonin receptors (5-HTR). Numerous 5-HTR genes have been identified, each with the potential to produce distinct splice variants and isoforms of the final receptor protein. To date, 18 unique serotonin receptors have been identified and grouped into 7 families (5-HTR₁ through 5-HTR₇) on the basis of genetic homology and associated second messenger systems (Table 1).²⁵ All 5-HTR subtypes are G-protein–coupled receptors with the exception of the 5-HTR₃ family, which are ligand-gated ion channels.²⁶

Table 1.

Distribution of Serotonin Receptors on Cells Showing Serotonin-Induced Proliferation

Receptor	Cell type – signaling pathway (if known)	Reference
5-HT1A	Neurons Prostate cancer: MAPK/ERK, PI3K/Akt	Segi-Nishida, 201792 Dizeyi et al, 2011116
5-HT1B	Pulmonary artery smooth muscle cells: ERK1/2, PDK	Liu et al, 201385
5-HT1D	Intestinal epithelium: WNT/β-catenin	Sui et al, 2015108
5-HT2A	Pulmonary artery fibroblasts: p38 MAPK Hepatocytes: protein kinase C Mouse fibroblast tumor: ERK1/2	Welsh et al, 200481 Balasubramanian and Paulose,88 1998; Lesurtel et al, 20064 Launay et al, 199680
5-HT2B	Neural crest cells Myocardial precursor cells Fibroblast: MAPK/cyclin D1, MAPK/cyclin E	Choi et al, 199798 Choi et al, 199798 Nebigil et al, 200079
5-HTR2C	Hepatocytes: protein kinase C	Balasubramanian and Paulose,88 1998; Lesurtel et al, 20064
5-HT3	Intestinal epithelium Enteric neurons	Bertrand et al, 200053 Cunningham et al, 198755
5-HT4	Neurons in hippocampus Intestinal epithelium Enteric neurons Mesangial kidney cells: protein kinase A, ERK1/2 Ovarian tumor cells	Segi-Nishida 201792 Pauwelyn and Lefebvre, 201770 Liu et al, 200967 Norum et al, 200378 Henriksen et al, 201277
5-HT7	Osteosarcoma: p38 MAPK, ERK2	Ballou et al, 2018114

NOTE. Italics indicate neoplastic cell types.

ERK, extracellular-signal regulated kinase; PDK, pyruvate dehydrogenase kinase; PI3K, phosphoinositide-3 kinase.

Tight regulation of serotonin signaling is achieved by rapid intracellular reuptake of 5-HT by SERT on cell membranes. SERT is a sodium-dependent monoamine transporter protein responsible for removing free serotonin from the extracellular space, thus terminating the downstream effects of 5-HTR activation. Once within the cell, 5-HT can be degraded by a number of mechanisms. The most common pathway for serotonin catabolism involves the activity of monoamine oxidases A and B that produce the renally excreted metabolite 5-hydroxyindoleacetic acid.²⁷

Serotonin Production and Receptor Activation in the Intestine

More than 90% of the body’s serotonin is produced by TPH1 in EC cells in the mucosa of the small intestine. From here, it either acts in a paracrine fashion on the intestinal epithelium or enters portal circulation where it subsequently is stored in circulating platelets.28, 29, 30 Enteroendocrine cells are encountered throughout the epithelium of the GI tract from the stomach to the rectum, and produce hormones involved in several GI functions including digestion and motility.³¹ 5-HT–producing EC cells comprise the largest subset of enteroendocrine cells, and are present along the length of the crypt–villus axis.³² Recent profiling of enteroendocrine cells in the gut have shown significant overlap between hormones secreted by cells that once were thought to be distinct subtypes.³³ EC cells, for example, produce substance P and tachykinin in addition to 5-HT.³² The nearby enteric nervous system (ENS), termed the local nervous system for its ability to function without CNS input, contains serotonin-producing neurons in both a submucosal plexus that controls mucosal secretions and blood flow and a myenteric plexus that regulates motility. Similar to serotonergic neurons in the CNS, ENS neurons produce serotonin through the TPH2 enzymatic pathway.^28,34

The specific paracrine effects of 5-HT within the gastrointestinal tract are determined by the distribution and location of various 5-HTRs. Receptors from the 5-HT₁, 5-HT₂, 5-HT₃, 5-HT₄, and 5-HT₇ receptor families have been identified in the intestine, with variable expression in the epithelium, ENS, and intestinal smooth muscle (reviewed by Mawe and Hoffman³⁵). The 5-HTR₄ and 5-HTR_2A have been localized to the intestinal epithelium.^36,37 Although the exact position of the 5-HT₄ receptor along the crypt–villus axis has not been described definitively, there is convincing evidence that proliferative cells within the intestinal crypt are the targets of 5-HTR₄ signaling effects.³⁸ 5-HTR_2A expression is most prominent in Paneth cells within the intestinal crypt.

Although our focus is on the mitogenic properties of serotonin, 5-HTR binding is essential for multiple intestinal functions. First, 5-HT mediates the inflammatory response in the intestinal mucosa. Animal models have shown that potentiation of serotonin signaling, either through pharmacologic inhibition or genetic modification, results in more severe colitis and increased levels of proinflammatory cytokines in the intestinal mucosa.³⁹ Proliferation of EC cells from intestinal epithelial progenitors is under immunologic control; specifically, an intact mucosal immune response, including the recruitment of CD3+ and CD4+ T lymphocytes and production of interleukin 13, is necessary for EC cell hyperplasia and consequent enhanced mucosal 5-HT production seen in states of enteric infection and inflammation.40, 41, 42, 43 Inflammatory states also result in down-regulation of SERT, further potentiating 5-HT signals.⁴⁴ Conversely, depletion of serotonin, modeled with TPH1 knockout mice, is protective against mucosal inflammation.⁴⁵ Interestingly, TPH2 knockout mice showed more severe colitis than wild-type littermates, suggesting an anti-inflammatory role of serotonin signaling in the ENS.^39,46 5-HTR₇ on enteric neurons and intestinal myeloid cells and 5-HTR_1B on lymphocytes are the specific serotonin receptors activated during intestinal inflammation; although contrary to what one would expect, 5-HTR₇ binding appears to confer an anti-inflammatory effect.^2,47, 48, 49

Second, the peristaltic reflex, or the intrinsic propulsive motility of the intestine, is regulated by mucosal serotonin.^50,51 Animal studies have shown prolonged gastric transit in Tph1^-/- mice and mice treated with 5-HTR antagonists.^19,50 This prokinetic effect is apparent in carcinoid syndrome, in which EC cell proliferation and the consequent increase in serotonin production manifests with symptoms of hypermotility and diarrhea.⁵² Neuronal 5-HT produced in the ENS has an apparent inhibitory effect on gastric emptying, but its role is more subtle than that of mucosal 5-HT in overall gut motility.⁵⁰

The 5-HTR₃ subtype warrants particular attention for its role in gut motility. In the myenteric plexus, 5-HTR₃ activation is involved in initiating the peristaltic reflex⁵³ and antagonism of this receptor opposes carcinoid-associated diarrhea.⁵⁴ Inhibition of 5-HTR₃ also has a potent anti-emetic effect, as shown by the 5-HTR₃ antagonist ondansetron, which is used to treat chemotherapy-induced nausea and vomiting. This pharmacologic effect is mediated by competitive inhibition of 5-HTR₃ on visceral afferent neurons and neurons in the vomiting center of the brainstem, as well as by inhibition of 5-HT production by EC cells.^55,56

Serotonin-Mediated Growth of the Intestinal Epithelium

Evidence linking intestinal epithelial proliferation to serotonergic regulation was presented by Tutton and Barkla⁵⁷ in the 1970s, who showed that serotonin blockade was associated with inhibition of colonic adenocarcinoma cell division in rats. Later, Gross et al⁵ showed that serotonin signaling enhances proliferation of non-neoplastic epithelial cells. Maintenance of the intestinal mucosal epithelium is achieved via a balance between cell production and cell death. Multipotent intestinal stem cells at the base of epithelial crypts proliferate and give rise to all epithelial cell types, including absorptive enterocytes, Paneth cells, goblet cells, and EC cells. Epithelial cell apoptosis and extrusion into the intestinal lumen, a process known as shedding, occurs at the tips of intestinal villi.⁵⁸ Alterations in the fine regulation of these processes underlie phenotypic changes seen in the mucosal response to injury, adaptation after bowel resection, and the development of intestinal neoplasia.⁵⁹ The findings by Gross,⁵ and others subsequently, showed that serotonin potentiation in mice modulated this intestinal homeostasis, leading to increased crypt cell proliferation, elongation of intestinal villi and crypts, and overall expansion of mucosal surface area in the small intestine.⁶⁰ Interestingly, epithelial proliferation is linked specifically to the activity of neuronal serotonin, or 5-HT synthesized by TPH2; in contrast, the depletion of EC cell–produced 5-HT in experiments with TPH1 knockout mice did not alter mucosal proliferation.⁵

Furthermore, the absorptive function of the intestine is enhanced in animals with potentiated serotonin signaling, correlating with the observed increase in mucosal surface area.61, 62, 63 This change in absorptive capacity does not reflect an increased proportion of absorptive enterocytes in villi. The fact that the cellular composition of the epithelium is not altered in these animals suggests that cellular division is induced in stem cells at the base of the intestinal crypt, or in the adjacent undifferentiated transit-amplifying cells.⁶¹

5-HT–mediated control over crypt cell division also has been observed in studies of intestinal injury. In animal models of intestinal ischemia and reperfusion, mice with enhanced serotonin signaling showed less severe mucosal injury and increased enterocyte proliferation after injury compared with control animals.^45,64 In addition, patients with inflammatory bowel disease show deficiencies in mucosal 5-HT signaling, including decreased EC cell populations in the intestinal epithelium.⁶⁵ Together, these findings have therapeutic implications for malabsorptive conditions such as short-bowel syndrome, and bowel injury more broadly, which warrants further investigation.

Serotonin Receptors in the Intestinal Epithelium

Compelling evidence that serotonin stimulates mitogenic pathways in the intestinal epithelium has led to efforts to characterize the signaling pathways responsible for these effects, starting with identification of the specific 5-HT receptors present on intestinal villi and on proliferative cells within the intestinal crypt. The 5-HTR₄ subtype has been localized to the intestinal epithelium throughout the small intestine and colon, and also is expressed in the ENS and on intestinal smooth muscle cells. The prolific expression of 5-HTR₄ in different cellular compartments contributes to its role in enteric neuroprotection, intestinal inflammation, and motility.^36,66, 67, 68, 69

There is substantial evidence of the intersection between serotonergic and cholinergic signaling pathways in the intestine, for example, in the regulation of intestinal motility through binding of 5-HTR₄ on myenteric cholinergic neurons (Figure 2).⁷⁰ Cholinergic regulation of intestinal epithelial cell proliferation is an active area of investigation, with data linking muscarinic acetylcholine receptor activation to colon cancer proliferation and intestinal stem cell division.71, 72, 73 Recent work by Greig et al^74,75 showed that stimulation of the muscarinic acetylcholine receptor M1 subtype leads to increased mucosal surface area and enterocyte proliferation in mice, similar to the effects seen with serotonin potentiation. Furthermore, the M1 receptor is localized to the stem cell niche in the intestinal crypt base. These findings, together with evidence that treatment with prucalopride, a pharmacologic 5-HTR₄ agonist, increased small intestine morphometric and proliferative markers in mice, suggest that serotonin signaling causes intestinal epithelial cell proliferation both directly and via a cholinergic pathway.³⁸

Figure 2.

Diagram of physiologic and pathologic processes in which serotonin exerts mitogenic effects. Potentiation of serotonin signaling leads to activation of cellular proliferation signaling pathways in diverse cell types. 5-HT receptors, namely 5-HTR_1B and 5-HTR_2A, and SERT play a role in smooth muscle proliferation seen in idiopathic pulmonary hypertension. 5-HTR_2A and 5-HTR_2C on hepatocytes are involved in liver regeneration after partial hepatectomy. 5-HT–receptor binding leads to uncontrolled cellular division and metastatic potential in multiple cancers. Intact 5-HT signaling is necessary for normal embryonic development, and 5-HT–receptor binding is central to neurogenesis.

The activity of 5-HTR₄ on other cell types supports its role in intestinal epithelial proliferation. For example, 5-HTR₄ on enteric neurons is implicated in neurogenesis through a pathway by which intracellular cyclic adenosine monophosphate production activates proteins involved in cellular proliferation—namely protein kinase A and the extracellular-signal regulated kinase pathway.^67,76 These same pathways are activated by 5-HTR₄ activation in non-neuronal cell types including human mesangial kidney cells and ovarian tissue.^77,78

There is evidence that the 5-HTR₂ family, which has been linked to mitogen-activated protein kinase (MAPK)-mediated cellular proliferation in mouse fibroblasts, also participates in mitogenic signaling in the intestinal epithelium.^79,80 5-HTR_2A is expressed in the intestinal mucosa and submucosal plexus,³⁷ and the proliferative effects of serotonin potentiation are absent when SERT knock-out mice are treated concomitantly with ketanserin, a selective 5-HT_2A antagonist.⁵

Serotonin-Mediated Mitogenic Pathways in Extraintestinal Tissues

To understand the potentially important intracellular pathways in serotonin-induced mitogenesis, it is useful to examine the role of 5-HT in pathologic cellular proliferation and induction of cellular division in nonproliferating cells (Figure 3). Nemecek et al³ described evidence of serotonin acting as a mitogen in their study of vascular remodeling. They determined that cellular division of bovine smooth muscle cells was enhanced by exposure to serotonin and platelet-derived growth factor stored within platelets. Further research linked idiopathic pulmonary hypertension to abnormal serotonin release from platelets, whereby 5-HT was shown to enhance pulmonary vascular remodeling. 5-HT–induced mitogenesis occurred both by stimulating the proliferation of and decreasing the apoptosis of pulmonary artery fibroblasts and vascular endothelial cells.81, 82, 83, 84, 85, 86 In addition, 5-HTR and SERT activity have been implicated in the up-regulation of mitogenic pathways and DNA replication in pulmonary artery smooth muscle cells.^81,85,87

Figure 3.

Schematic of serotonergic and cholinergic receptors in the intestinal epithelium. The intestinal crypt is composed of intestinal stem cells and undifferentiated transit-amplifying cells, whose division underlies epithelial proliferation. Neuronal serotonin, synthesized by TPH2 in the enteric nervous system, leads to epithelial proliferation both through direct binding of 5-HTR on epithelial cells, and indirectly via a cholinergic pathway that leads to muscarinic-receptor activation of intestinal crypt cells.

Later, Balasubramanian and Palouse⁸⁸ shed light on how serotonin induces differentiated hepatocytes to re-enter the cell cycle. Unlike fibroblasts in vascular endothelium and smooth muscle, hepatocytes typically are nonproliferating cells that can be induced to undergo cell division under specific conditions—namely, after a partial liver resection. Thus, the investigation of 5-HT–mediated hepatocyte division paved the way for understanding how serotonin signaling facilitates bypass of cell-cycle checkpoints. Balasubramanian and Palouse⁸⁸ observed increased binding of 5-HTRs during hepatocyte regeneration, and found that 5-HTR₂ activation was associated specifically with downstream activation of protein kinase C, an established intracellular messenger for cell growth and division. The 5-HTR_2A subtype subsequently was identified as the specific receptor activated during liver regeneration, drawing a parallel between what is known about 5-HT signaling during proliferation of hepatocytes and intestinal epithelial cells.⁴

TPH1 knockout mice show impaired liver regeneration after hepatectomy, further supporting the mitogenic role of 5-HT, and implicating non-neuronal sources of serotonin in this process.^4,89 However, CNS regulation also appears to play a role in this process, with data showing that autonomic nervous system activation of EC cells after partial hepatectomy increases peripheral 5-HT production and corresponding 5-HTR activation in regenerating hepatocytes.⁹⁰

Serotonin-Mediated Neurogenesis and Neuroprotection in the ENS

Serotonin reuptake inhibitor (SSRI)-mediated neurogenesis underlies the effectiveness of this drug class as an antidepressant, and has contributed to understanding of 5-HT as a regulator of cellular proliferation in both the CNS and ENS.91, 92, 93

The ENS is derived from vagal- and sacral-level neural crest cells that migrate to and within the gut of the developing embryo, as well as from Schwann cells, which adopt a neuronal fate in the postnatal period.^94,95 5-HT signaling contributes to the development of the ENS, as evidenced by decreased myenteric neuronal density in TPH1 knockout mice compared with wild type.⁵⁰ Furthermore, the specific development of neurons responsive to the neurotransmitters dopamine and γ-aminobutyric acid in the ENS, is sensitive to serotonin signaling.⁵⁰

Serotonin signaling is critical during embryonic development outside of the nervous system as well. Various 5-HTR subtypes are expressed differentially throughout mouse embryogenesis, and appear to regulate the rate of cellular division and cleavage of the early embryo. Specifically, 5-HTR_2B binding and downstream signaling have been studied in detail and are necessary for normal myocardial development and neural crest differentiation.96, 97, 98 The role of serotonin signaling in intestinal epithelial development has not been well studied, although EC cells and 5-HT production are detected within the mouse intestinal epithelium as early as embryonic day 16.⁹⁹

Although the ganglia that comprise the ENS are present and functional at birth, the intricate enteric neural circuitry continues to develop and mature throughout adult life.100, 101, 102, 103 Serotonergic signaling plays an important role in this maturation. Much of what we know about 5-HT–mediated cellular proliferation in the ENS comes from work by Liu et al,⁶⁷ who studied neurogenesis in 5-HTR₄ knockout mice. As discussed earlier, 5-HTR₄ has an established role in gut motility and neurogenesis. Liu et al⁶⁷ found that wild-type mice treated with 5-HTR₄ agonists formed new neurons in the muscular layers of the small intestine that proceeded to migrate into the myenteric plexus; conversely, these newly generated neurons were absent when 5-HTR₄ knockout mice were treated with 5-HTR₄ agonists. Subsequent studies have recapitulated these findings and showed that 5-HT plays a neuroprotective role, with the capacity for 5-HTR₄ activation to induce enteric neurogenesis after insults to the intestine.104, 105, 106, 107

Serotonin and Proliferation of Cancer Cells

Serotonin-mediated induction of the cell cycle regulates physiologic cell division in the setting of the intestinal crypt cell, and a beneficial adaptive response in the setting of hepatocyte regeneration after liver resection. At the molecular level, these intracellular pathways are not far removed from those activated in cancer cells to facilitate evasion of cell-cycle checkpoints. Since Tutton and Barkla’s⁵⁷ discovery of serotonergic regulation of colorectal adenocarcinoma, our understanding of the role of 5-HT in intestinal stem cell proliferation and the pathogenesis of colonic neoplasia has grown substantially. Colorectal adenocarcinoma tumor specimens show 5-HTR overexpression when compared with normal colon tissue, specifically of the 5-HT_1D, 5-HTR_3C, and 5-HTR₄–receptor subtypes.¹⁰⁸ Serotonin-receptor binding has been linked to colorectal tumor angiogenesis, invasion, and migration in experimental models, correlating with data that circulating plasma serotonin levels are higher in colon cancer patients, and are associated with worse cancer prognosis.108, 109, 110 Recent research by Sakita et al¹¹¹ further elucidated the role of 5-HT signaling in colorectal carcinogenesis by showing that colonocytes in TPH1 knockout animals show more DNA damage and worse intestinal inflammation than wild-type animals—both of which are etiologic precursors to colon cancer.

Despite this evidence linking 5-HTR activation to cancer progression, SSRI use has been associated with a decreased risk of colorectal cancer, both in a large database analysis of human beings and in rodent models.^112,113 In the face of these seemingly contradictory data, study of serotonin’s role in the pathogenesis of other tumors supports the link between serotonin signaling and neoplasia. In a review of genomic data from multiple cancer cell lines, Ballou et al¹¹⁴ found that cancer cells showed clear gene overexpression of specific 5-HTR subtypes, with similarities seen among diverse cell lines that shared a tumor origin. Furthermore, they found that inhibition of serotonin signaling may have therapeutic benefit. In experimental models of breast cancer and sarcoma, pharmacologic inhibition with a variety of 5-HTR antagonists, with the exception of antagonists to the 5-HTR₃ family, resulted in reduced phosphorylation of cancer signaling molecules including Protein Kinase B, MAP kinases, and cyclin-dependent kinases, along with a reduction in tumor cell viability.¹¹⁴

Importantly, serotonin potentiation has been linked to downstream activation of proliferative cellular pathways in non–colorectal cancer types. In human osteosarcoma cells, 5-HT exposure was associated with increased phosphorylation of p38 and p42 MAP kinases, which regulates expression of a number of transcription factors involved in cellular proliferation.^114,115 In prostate cancer cells, the MAP kinase/extracellular-signal regulated kinase and phosphoinositide-3 kinase/Protein Kinase B cellular pathways are up-regulated by activation of the 5-HT_1A receptor, facilitating cancer cell proliferation and migration.¹¹⁶

Therapeutic Considerations

Intestinal injury from inflammatory and ischemic disease and malabsorptive states in the setting of short-bowel syndrome pose a challenge to clinicians given the chronic nature of these conditions and the limitations of intestinal replacement therapies.⁶ Exploiting the mitogenic properties of serotonin offers a promising therapeutic approach to these difficult clinical problems. Fortunately, the agents that have enabled detailed study of the downstream effects of 5-HT signaling, namely SSRIs and 5-HTR agonists and antagonists, also are viable pharmacologic therapies. Citalopram, an SSRI, and prucalopride, a 5-HTR₄ agonist, have been shown to increase mucosal surface area and the absorptive capacity of the intestinal epithelium in mouse models.^38,60,62 In addition, citalopram treatment confers a protective effect to the intestinal epithelium in a mouse model of intestinal ischemia, with enhanced enterocyte renewal.⁶⁴

As mentioned previously, SSRI use has not been associated with increased rates of intestinal neoplasia, despite the evidence linking serotonin potentiation to enterocyte proliferation and colorectal cancer pathogenesis, and has in fact been shown to have therapeutic potential in reducing tumor viability in other malignancies.^109,110,114 This is reassuring for the therapeutic application of SSRIs to conditions of intestinal insufficiency, particularly when glucagon-like peptide 2 agonists, an alternate investigational treatment for this indication, carry risks of carcinogenesis.¹¹⁷

Indeed, as a class of drugs, SSRIs show a relatively benign side-effect and risk profile, despite the ubiquity of 5-HT and SERT in the body. When used for CNS indications, common off-target effects include GI symptoms and sexual dysfunction.^118,119 Although further research is needed to determine optimal dosing and administration of SSRIs for therapeutic use in intestinal insufficiency, and to characterize adverse off-target effects, current evidence suggests that the harm of long-term SSRI use is minimal.

Conclusions

A critical element of 5-HT signaling is its ability to exert long-term changes on cell fate and gene expression, as evidenced by its mitogenic effect on the intestinal mucosa and enteric nervous system. 5-HTR binding on diverse cell types, including intestinal stem cells, activates intracellular pathways that lead to cellular proliferation and mitogenesis. Division of undifferentiated stem cells and transit-amplifying cells in the intestinal crypt is mediated by serotonergic signaling in the ENS. This effect is observed in normal, injured, and neoplastic cells, and evidently is mediated through cholinergic pathways, as well as through direct binding of 5-HTRs on mucosal cells. Furthermore, serotonin is essential for neurogenesis and continued regeneration of the enteric neurons throughout life, underscoring the importance of serotonergic neurotransmission in GI function. SSRIs and 5-HTR binding agents hold promise as therapies to harness the translational potential of serotonin-mediated proliferation for conditions of intestinal injury or insufficiency. Modification of 5-HT signaling is already within the means of medicine, given the established arsenal of SERT inhibitors and 5-HTR antagonists that exist, but additional research is warranted to characterize the risks and benefits to patients with intestinal insufficiency in the clinical setting.