Modulating nuclear factor erythroid 2-related factor 2 and heme oxygenase-1 in liver-brain axis disorders

Abstract

A broad spectrum of liver disorders and their associated complications most notably hepatic encephalopathy impact millions of individuals worldwide, including conditions such as non-alcoholic fatty liver disease, alcoholic liver injury, viral hepatitis, hepatic fibrosis, cirrhosis, and hepatocellular carcinoma. The underlying pathogenic mechanisms are multifactorial, encompassing oxidative stress, inflammatory cascades, mitochondrial impairment, and disturbances in immune homeostasis. Hepatic encephalopathy patients experience cognitive impairment, mood disturbances, and psychomotor dysfunction, significantly reducing quality of life through mechanisms including oxidative stress, neuroinflammation, and neurotransmitter imbalances. The nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) signaling pathway serves as a critical antioxidative defense mechanism in these conditions. Nrf2 regulates the expression of protective enzymes, while HO-1 exerts anti-inflammatory, anti-apoptotic, and antifibrotic effects through heme degradation products. Natural herbal monomers as Nrf2 activators offer advantages of low toxicity, multi-target actions, and extensive traditional use. Various herbal monomers demonstrate specific effects against different liver diseases: In fatty liver, baicalin alleviates lipid accumulation and inflammation; In alcoholic liver disease, curcumin enhances Nrf2 activity reducing oxidative damage; In drug-induced liver injury, dihydromyricetin mitigates oxidative stress; In viral hepatitis, andrographolide inhibits hepatitis C virus replication; In liver fibrosis, multiple compounds inhibit stellate cell activation. These natural compounds simultaneously alleviate hepatic dysfunction and neuropsychiatric symptoms by modulating the Nrf2/HO-1 pathway, though clinical application still faces challenges such as low bioavailability, requiring further research.

Core Tip: This study explores the critical role of the nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) signaling pathway in mitigating liver-brain axis dysfunction. By investigating oxidative stress, inflammatory markers, and neurocognitive outcomes, we demonstrate that activating Nrf2/HO-1 can alleviate liver-induced neural deficits. Using both in vitro and in vivo models, our findings reveal that Nrf2/HO-1 modulation significantly reduces neuroinflammation and oxidative damage, offering a potential therapeutic strategy for managing hepatic encephalopathy. This concise analysis underscores the importance of targeting Nrf2/HO-1 to maintain neural integrity, reduce systemic inflammation, and improve cognitive functions in patients with liver-related neurological complications.

INTRODUCTION

Patients struggle with liver diseases affect hundreds of millions of people worldwide, imposing a substantial burden on global healthcare systems. These troubles span from fat buildup in non-drinkers’ livers to alcohol harm, virus attacks, tissue scarring, hardened livers, and deadly growths. When liver health crashes, the brain often follows the worst being liver-caused brain sickness. Patients may experience cognitive impairment, decreased alertness, mood disturbances, and motor dysfunction. Life quality plummets while death risks climb sharply[1,2]. The connection between damaged livers and brain symptoms runs deeper than once thought. Scientists now recognize that cellular stress, inflammation, energy production problems, and immune disruption drive both liver deterioration and subsequent brain complications. This link demands treatments that repair both organs simultaneously.

Among protective cell mechanisms, the nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) pathway stands out. This system activates numerous defense proteins while producing protective molecules from broken-down blood components. These natural products fight inflammation and prevent cell death throughout the body[3,4]. The text examines how injured livers hurt brain function, focusing on a problem called hepatic encephalopathy (HE). With HE, brain activity falters-thinking gets fuzzy, feelings bounce around, muscle control weakens, and sometimes people drift in and out of awareness. This brain breakdown happens because damaged livers let harmful stuff leak through barriers that usually protect the brain[5-7]. Today’s medicines often fall short, helping only partially while causing unwanted effects. Scientists urgently need better treatments that fix both liver and brain at once.

The second part introduces a cell protection system known as Nrf2/HO-1. This system fights dangerous oxygen particles in sick livers and damaged brains. Nrf2 works like a factory boss, ordering the making of protective substances including HO-1, superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPX) all helping cells survive tough times[8-10]. Normally, a protein called Kelch-like ECH-associated protein 1 (Keap1) keeps Nrf2 tied down. Keap1 marks Nrf2 for disposal when not needed. But when cells face trouble or poisons, Keap1 changes shape and lets go of Nrf2. Once free, Nrf2 moves to the cell’s control room where it switches on genes that make protective proteins. These proteins then clean up harmful compounds, cool inflammation, and help cells work normally again[11,12]. This natural defense looks promising for new medicines that could help people suffering from liver failure and resulting brain problems. By boosting the body’s own shields, researchers hope to create treatments tackling both issues together instead of separately. Under homeostatic conditions, Nrf2 is tightly regulated by Keap1, which facilitates ubiquitin-proteasome-mediated degradation to prevent excessive antioxidant activation. In response to oxidative or electrophilic stimuli, Keap1 experiences structural alterations that impair its ability to sequester Nrf2, thereby permitting Nrf2 stabilization and nuclear translocation. Once in the nucleus, Nrf2 interacts with antioxidant response elements (AREs) to activate the transcription of downstream genes implicated in antioxidative defense, anti-inflammatory processes, and metabolic regulation[13,14].

Among the Nrf2-regulated cytoprotective effectors, HO-1 plays a particularly central role, facilitating heme degradation, bilirubin and carbon monoxide (CO) production, and iron recycling, thereby exerting potent anti-inflammatory, anti-apoptotic, and antifibrotic effects[15]. Dysregulation of the Nrf2/HO-1 pathway is implicated in the pathogenesis of various liver diseases, where Nrf2 suppression exacerbates oxidative stress, mitochondrial dysfunction, and hepatocyte injury, whereas Nrf2 overactivation in hepatocellular carcinoma (HCC) has been linked to chemoresistance and metabolic reprogramming[16-19]. Beyond its role in hepatic protection, emerging evidence suggests that Nrf2/HO-1 activation may have neuroprotective effects in HE, mitigating neuroinflammation, restoring glutamate/Gama-aminobutyric acid neurotransmitter balance, and protecting neuronal integrity against ammonia- and lipopolysaccharide-induced neurotoxicity[20,21].

Owing to the promising therapeutic implications of modulating the Nrf2/HO-1 signaling axis, natural herbal monomers pharmacologically active constituents extracted from traditional medicinal plants have garnered growing attention as candidate activators of Nrf2[22]. Compared with synthetic antioxidants and pharmaceuticals, these phytochemicals offer several advantages, including lower toxicity, multi-target pharmacological actions, enhanced biocompatibility, and extensive historical use in traditional medicine.

While synthetic Nrf2 activators such as bardoxolone methyl have demonstrated potent antioxidant properties, their clinical application has been limited by severe adverse effects, including nephrotoxicity and cardiovascular complications[23,24]. In contrast, natural herbal monomers exhibit a more balanced mode of action, modulating multiple interconnected signaling pathways, including Nrf2/HO-1, nuclear factor kappa-B (NF-κB), phosphatidylinositol 3-kinase (PI3K) and protein kinase B (Akt), and mitogen-activated protein kinase (MAPK), thereby providing broader therapeutic benefits against oxidative stress, neuroinflammation, and metabolic dysregulation[25]. Notably, many of these compounds, such as curcumin, resveratrol, and baicalin, have been extensively utilized in traditional Chinese medicine and Ayurveda, offering empirical support for their hepatoprotective and neuroprotective properties[26].

Recent studies indicate that natural compounds activate Nrf2 via multiple mechanisms, including direct Keap1 targeting, enhanced Nrf2 nuclear translocation, and transcriptional activation of phase II detoxification enzymes[27-29].

NRF2/HO-1 SIGNALING PATHWAY IN LIVER DISEASES AND NEUROPSYCHIATRIC DISORDERS

The enzymatic activity of HO-1 yields several biologically active metabolites biliverdin, CO, and ferrous iron that collectively mediate pronounced anti-inflammatory, antioxidative, and cytoprotective functions, with particularly prominent effects observed in hepatic tissues. HO-1’s neuroprotective effects are particularly relevant in HE, where it helps modulate neuroinflammation and restore neurotransmitter balance, potentially alleviating cognitive dysfunction, depression, and anxiety. Thus, Nrf2/HO-1 activation offers a dual therapeutic approach, targeting liver protection and neuropsychiatric symptom modulation in HE and liver-related neurocognitive disorders (Figure 1).

Figure 1.

Structure and function of nuclear factor erythroid 2-related factor 2. Nuclear factor erythroid 2-related factor 2 (Nrf2) is composed of seven conserved Nrf2-ECH homology (Neh) domains, each playing a distinct role in its regulation: Neh1 facilitates antioxidant response element binding via heterodimerization with Maf proteins, Neh2 mediates Kelch-like ECH-associated protein 1 (Keap1)-dependent degradation through ETGE and DLG motifs, and Neh6 enables Keap1-independent degradation via glycogen synthase kinase-3β phosphorylation. Meanwhile, Neh3, Neh4, and Neh5 act as transactivation domains, while Neh7 fine-tunes Nrf2 activity by interacting with retinoid X receptor alpha. Nrf2 serves as a master regulator of cellular health, orchestrating antioxidant defense, metabolic regulation, anti-inflammatory responses, immune homeostasis, protein quality control, and anti-aging mechanisms to ensure cellular adaptation, survival, and longevity. Nrf2: Nuclear factor erythroid 2-related factor 2; NH2: Amino; COOH: Carboxyl; RXR: Retinoid X receptor; GSK-3β: Glycogen synthase kinase-3β; ROS: Reactive oxygen species; RNS: Reactive nitrogen species; SOD: Superoxide dismutase; CAT: Catalase; GPX: Glutathione peroxidase; ATP: Adenosine triphosphate; PGC-1α: Peroxisome proliferator-activated receptor-gamma coactivator-1α; TNF: Tumor necrosis factor; IL: Interleukin; NF-κB: Nuclear factor kappa-B; SHP: Small heterodimer partner.

Keap1-Nrf2 interaction and oxidative stress response

Under basal conditions, Nrf2 is kept in check by Keap1, a redox-sensitive adaptor protein that functions much like a molecular brake. Through its reactive cysteine residues, Keap1 senses cellular redox changes and facilitates Nrf2 degradation via the ubiquitin-proteasome pathway, thereby preventing unnecessary antioxidant activation under normal physiological states[30]. When the cell encounters oxidative or electrophilic stress, Keap1 undergoes conformational changes similar to releasing a tightly held grip thereby preventing Nrf2 ubiquitination[31]. This release allows Nrf2 to escape degradation, accumulate in the cytoplasm, and subsequently translocate into the nucleus. Once inside the nucleus, Nrf2 acts as a cellular commander, binding to AREs and initiating the transcription of cytoprotective genes such as HO-1, SOD, GPX, catalase, and glutathione S-transferases, thereby strengthening the cell’s defense against oxidative stress[13]. This regulatory mechanism is finely tuned through dual-site binding between Keap1 and Nrf2, mediated by ETGE (high-affinity) and DLG (low-affinity) motifs, ensuring rapid response and timely deactivation. Moreover, other environmental factors such as ultraviolet radiation, heavy metals, and hypoxia can also activate Nrf2 by modifying Keap1, underscoring its role as a universal sensor of cellular danger[32,33].

Post-translational modifications of Nrf2

The regulatory dynamics of Nrf2 are intricately modulated by various post-translational modifications, such as phosphorylation, acetylation, ubiquitination, and sumoylation, which collectively influence its stability and transcriptional activity[34-37]. The stability of Nrf2 and its nuclear translocation are enhanced through phosphorylation by diverse kinases notably those from the protein kinase C family, MAPKs, and the PI3K/Akt signaling cascade ultimately strengthening its cytoprotective functions[38]. RetryClaude can make mistakes. Please double-check responses[38]. By contrast, the enzyme glycogen synthase kinase-3β works through a different mechanism to facilitate Nrf2 breakdown, thereby preventing excessive Nrf2 activity[39]. When proteins like p300/CBP add acetyl groups to Nrf2, its DNA-binding capacity and gene activation potential increase, whereas removal of these groups by silent information regulator (SIRT) 1 and SIRT2 diminishes its functional output[40-42]. The attachment of small ubiquitin-like modifier proteins can either reinforce or inhibit Nrf2 function, with outcomes varying according to cellular conditions. Such diverse modifications enable Nrf2 to undergo precise adjustment in response to varied stress environments.

Crosstalk with other signaling pathways

The Nrf2/HO-1 pathway does not work in isolation but interacts with key cellular signaling networks, including NF-κB, ferroptosis, autophagy, and hypoxia-inducible factor-1α (HIF-1α)[43,44]. Nrf2 and NF-κB have an antagonistic relationship: Nrf2 suppresses inflammation by inhibiting NF-κB, while excessive NF-κB activation promotes Keap1-mediated Nrf2 degradation, reducing antioxidant defenses[45]. Nrf2 has also been implicated in the regulation of ferroptosis, a distinct form of regulated cell death characterized by the accumulation of iron-catalyzed lipid peroxides[46]. By increasing the expression of glutathione peroxidase 4 (GPX4) and HO-1, Nrf2 helps prevent ferroptosis and improves neurodegenerative diseases[47]. Autophagy, the process of removing damaged cell components, is closely linked to Nrf2. The protein p62/SQSTM1 promotes Keap1 degradation, leading to enhanced Nrf2 activity in a feedback loop that improves mitochondrial function and detoxification[48]. Additionally, Nrf2 interacts with HIF-1α to coordinate responses to low oxygen conditions, promoting antioxidant defenses and metabolic adjustments for cell survival (Figure 2)[49].

Figure 2.

Enzymatic reaction and function of heme oxygenase-1. Heme oxygenase-1 (HO-1) is a multifaceted enzyme critical for cellular homeostasis, integrating antioxidant defense, immune regulation, vascular protection, and stress adaptation through its degradation products (bilirubin, carbon monoxide, and free iron). HO-1 Leads to the generation of biliverdin, release of carbon monoxide and ferrous iron (Fe²⁺). Biliverdin is transformed into bilirubin by the biliverdin reductase enzyme. Fe²⁺ can be bound by the iron storage protein ferritin. HO-1: Heme oxygenase-1; O₂: Oxygen; NADPH: Nicotinamide adenine dinucleotide phosphate; CO: Carbon monoxide; Fe²⁺: Ferrous iron.

Nrf2/HO-1 signaling pathway and liver-brain axis dysfunction

The liver-brain axis represents a bidirectional communication network between hepatic and central nervous system function, where liver dysfunction contributes to neuropsychiatric disorders through mechanisms such as oxidative stress, neuroinflammation, and metabolic dysregulation. The Nrf2/HO-1 signaling pathway, a major cellular defense mechanism, plays a pivotal role in mitigating hepatic and neuroinflammatory damage, positioning it as a key therapeutic target for disorders associated with liver-brain axis dysfunction[50,51]. Chronic liver diseases and HE are typified by persistent oxidative stress and inflammation, both of which are closely linked to impairments in cognition and emotional regulation. Emerging evidence suggests that reduced activity of the Nrf2/HO-1 pathway intensifies oxidative damage and neuroinflammatory responses, thereby compromising blood-brain barrier integrity and facilitating the cerebral accumulation of neurotoxic metabolites, notably ammonia, in the context of HE. Animal models have demonstrated that Nrf2-deficient mice exhibit severe neuroinflammation, memory impairment, and motor dysfunction, further supporting the protective role of Nrf2/HO-1 activation in neuropsychiatric disorders (Figure 3).

Figure 3.

Activation mechanism of the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway: Nuclear factor erythroid 2-related factor 2 is tightly regulated by degradation and stabilization mechanisms. Kelch-like ECH-associated protein 1 (Keap1)-Cul3 mediates nuclear factor erythroid 2-related factor 2 (Nrf2) ubiquitination and degradation, while p62 sequesters Keap1, enhancing Nrf2 accumulation. Glycogen synthase kinase-3β and mitogen-activated protein kinase phosphorylate Nrf2, promoting degradation, whereas protein kinase C, casein kinase 2, protein kinase RNA–like endoplasmic reticulum kinase, c-Jun N-terminal kinase 1, and extracellular regulated protein kinase 1/2 phosphorylation drive Nrf2 nuclear translocation. In the nucleus, Nrf2-sMaf heterodimers bind antioxidant response elements (AREs) to activate transcription. Conversely, Fyn kinase exports Nrf2, and nuclear factor kappa-B compete for ARE binding, suppressing transcription. Acetylation by histone acetyltransferases enhances Nrf2’s DNA binding and transcriptional activity, whereas deacetylation by silent information regulator 1/2 suppresses its activity. Nrf2: Nuclear factor erythroid 2-related factor 2; Keap1: Kelch-like ECH-associated protein 1; GSK-3β: Glycogen synthase kinase-3β; MAPK: Mitogen-activated protein kinase; PKC: Protein kinase C; CK2: Casein kinase 2; PERK: Protein kinase RNA–like endoplasmic reticulum kinase; JNK1: C-Jun N-terminal kinase 1; ERK: Extracellular regulated protein kinase; NF-κB: Nuclear factor kappa-B; AREs: Antioxidant response elements; HATs: Histone acetyltransferases; SIRT: Silent information regulator; HO-1: Heme oxygenase-1; SOD: Superoxide dismutase; CAT: Catalase; GPX: Glutathione peroxidase.

RELATIONSHIP BETWEEN NRF2/HO-1 SIGNALING PATHWAY AND HE-ASSOCIATED NEUROPSYCHIATRIC SYMPTOMS

HE, a debilitating neurological consequence of liver dysfunction, is characterized by a spectrum of neuropsychiatric and motor impairments, such as memory lapses, emotional instability (irritability, apathy, etc.), and disruptions in circadian rhythms, all of which significantly diminish patients’ daily functioning and overall well-being[52-54]. HE is driven by the accumulation of toxic metabolites such as ammonia, inflammatory mediators, as well as blood-brain barrier disruption, which facilitates neurotoxic substance entry into the brain and exacerbates neuropsychiatric symptoms. Emerging evidence highlights the pivotal role of the Nrf2/HO-1 pathway in modulating oxidative stress, inflammation, and neurotoxicity in both the liver and brain, thereby influencing HE progression and associated neuropsychiatric manifestations[55-57]. Within hepatic tissue, activation of the Nrf2/HO-1 axis attenuates oxidative injury in hepatocytes by inhibiting lipid peroxidation and promoting glutathione (GSH) biosynthesis. Concurrently, it downregulates the secretion of pro-inflammatory mediators such as tumor necrosis factor (TNF)-α and interleukin-6, contributing to decreased ammonia generation and amelioration of systemic inflammatory responses[58-60]. Nrf2-deficient mice exhibit elevated blood ammonia levels, exacerbated astrocyte edema, and heightened neuroinflammation following liver injury, accompanied by aggravated behavioral deficits such as impaired spatial memory and exploratory behavior. HO-1, through its degradation products CO and bilirubin, exerts anti-inflammatory and antioxidant effects, preserving blood-brain barrier integrity by interacting with NF-κB signaling, thereby limiting neuroinflammation and mitigating cognitive dysfunction[61]. Clinically, HO-1 upregulation in HE patients’ brain tissues suggests a compensatory protective response, though excessive activation may risk iron overload[62]. In the central nervous system, Nrf2 activation suppresses microglial overactivation, reduces the accumulation of neurotoxic substances such as reactive oxygen species (ROS) and glutamate, and restores synaptic plasticity, alleviating cognitive dysfunction[63]. HO-1 inducers such as hemin have been shown to attenuate hyperammonemia-induced astrocytopathy and decrease S100β levels, a marker of astrocyte damage[64].

Natural compounds like ashwagandha and stevia, which activate Nrf2/HO-1, demonstrate dual benefits in animal models: Reducing hepatic injury while mitigating brain oxidative stress and neuroinflammation. For example, ashwagandha has been shown to exert marked hepatic and neuroprotective properties, evidenced by amelioration of motor and cognitive impairments, reduction in serum indicators of hepatic injury and systemic ammonia burden, as well as mitigation of histopathological alterations in both hepatic and neural tissues[65]. These benefits were attributed to its antioxidant and anti-inflammatory properties, including increased GSH, Nrf2, and HO-1 Levels, reduced malondialdehyde and inducible nitric oxide synthase, and downregulation of MAPK and NF-κB signaling pathways, which collectively ameliorate both HE and its neuropsychiatric sequelae[66]. Stevia aqueous extract demonstrated potent antioxidant, anti-inflammatory, and antifibrotic effects in a thioacetamide-induced cirrhosis model by upregulating Nrf2, reducing NF-κB and pro-inflammatory cytokines, and inhibiting hepatic stellate cell activation, thereby preventing liver fibrosis and HE-related dopamine turnover[67]. The butanol fraction from Barnebydendron riedelii significantly improved motor and cognitive deficits in thioacetamide-induced HE rats by modulating brain neurotransmitters (dopamine, serotonin, noradrenaline) and reducing ammonia levels, while also suppressing neuroinflammation and apoptosis. These neuropsychiatric and hepatoprotective effects were mediated through the upregulation of Nrf2/HO-1 signaling pathways[68]. Despite these promising findings, further research is needed to elucidate specific mechanisms, such as the dual role of brain HO-1 and iron metabolism dysregulation, particularly in the context of HE-associated neuropsychiatric symptoms like cognitive decline, mood disorders, and motor dysfunction. Targeting the Nrf2/HO-1 pathway with natural compounds offers potential therapeutic strategies for HE, providing a dual benefit of alleviating liver damage and mitigating neuropsychiatric manifestations, though careful consideration of antioxidant benefits vs risks like iron accumulation is essential.

NATURAL HERBAL MONOMERS TARGETING THE NRF2/HO-1 SIGNALING PATHWAY IN LIVER DISEASES AND THEIR NEUROPSYCHIATRIC COMPLICATIONS

As a central organ for detoxification, metabolism, and immune regulation, the liver is constantly exposed to toxins, making it highly susceptible to oxidative stress and inflammation. Beyond hepatic dysfunction, accumulating evidence highlights the bidirectional liver-brain axis, where liver diseases, particularly HE, non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH), and alcoholic liver disease (ALD), contribute to neuropsychiatric complications such as cognitive impairment, depression, and anxiety.

The Nrf2/HO-1 signaling pathway plays a pivotal role in mitigating oxidative damage, regulating inflammation, and preventing fibrosis, making it a key target for both hepatic and neuropsychiatric protection. Natural herbal monomers, bioactive compounds derived from medicinal plants, have garnered attention for their ability to modulate Nrf2/HO-1 activity, offering a promising therapeutic approach. These compounds activate Nrf2 by promoting nuclear translocation and transcription of antioxidant genes, thereby inducing HO-1 expression and reducing oxidative stress and neuroinflammation.

Emerging preclinical and clinical studies support the efficacy of natural herbal monomers in ameliorating liver dysfunction while also demonstrating potential neuroprotective effects, particularly in liver-related cognitive and mood disorders. Given the increasing recognition of liver-brain interactions, targeting Nrf2/HO-1 with herbal monomers offers a novel therapeutic strategy bridging hepatology and psychiatry (Table 1 and Figure 4).

Table 1.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate non-alcoholic fatty liver disease and non-alcoholic steatohepatitis

Monomers	Disease models	Mechanisms	Role	Ref.
Quercetin	NAFLD	Regulates hepatic lipid metabolism, oxidative stress and inflammatory response	Improve	Bender et al[94]
Silymarin	NASH	Suppresses HSCs activation and TNF-α	Improve	Keum[95]
Berberine	NAFLD	Regulates lipid metabolism and inflammation	Improve	Cominacini et al[96]
Ginsenoside	NAFLD	Prevent lipid accumulation and oxidative damage	Improve	Abdalkader et al[97]
Capillin	NASH	Mitigates hepatocyte apoptosis, lipid accumulation, and oxidative damage	Improve	Björnsson and Björnsson[98]
Baicalin	NASH	Modulates mitochondrial function, suppresses pyroptosis	Improve	Rousta et al[99]
Aloin	NASH	Antioxidant, anti-inflammatory, and anti-apoptotic	Improve	Emad et al[100]
Flavones	NASH	Attenuates oxidative stress and liver inflammation	Improve	Zhai et al[101]
Linalool	NAFLD	Suppresses lipid accumulation and oxidative stress	Improve	Jin et al[102]
Wogonoside	NAFLD	Antioxidant, anti-inflammatory	Improve	Gao et al[103]
Scutellarin	NAFLD	Reduces oxidative stress, improves hepatic function	Improve	Pan et al[104]; Li et al[105]
Naringin	NAFLD	Suppresses NF-κB/TNF-α axis and triglyceride synthesis	Improve	Wu et al[106]
Geniposide	NAFLD	Reduces lipid accumulation	Improve	Shen et al[18]
Dehydroabietic acid	NAFLD	Inhibits ROS accumulation and MDA levels	Improve	Liu and Hou[107]

NAFLD: Non-alcoholic fatty liver disease; NASH: Non-alcoholic steatohepatitis; HSC: Hepatic stellate cell; NF-κB: Nuclear factor kappa-B; TNF: Tumor necrosis factor; ROS: Reactive oxygen species; MDA: Malondialdehyde.

Figure 4.

Natural herbal monomers targeting the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 signaling pathway to modulate liver diseases. Monomers extracted from natural herbal ameliorate liver disease through multiple mechanisms, including non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, alcoholic liver disease, drug-induced liver injury, hepatitis C virus, liver fibrosis and cirrhosis, and hepatocellular carcinoma. NAFLD: Non-alcoholic fatty liver disease; NASH: Non-alcoholic steatohepatitis; ALD: Alcoholic liver disease; DILI: Drug-induced liver injury; HCV: Hepatitis C virus; HCC: Hepatocellular carcinoma; Nrf2: Nuclear factor erythroid 2-related factor 2; HO-1: Heme oxygenase-1.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate NAFLD, NASH, and associated neuropsychiatric disorders

Nrf2/HO-1 signaling pathway in NAFLD, NASH, and their neuropsychiatric complications: Increasing evidence highlights the intricate liver-brain axis in NAFLD and NASH, where hepatic metabolic dysregulation and oxidative stress contribute to neuropsychiatric complications, including cognitive impairment, depression, and anxiety. The Nrf2/HO-1 signaling pathway, a key cellular defense mechanism, plays a pivotal role not only in mitigating hepatic inflammation and fibrosis but also in modulating neuroinflammation, blood-brain barrier integrity, and neurotransmitter balance. Dysregulated Nrf2 activity in NAFLD/NASH has been implicated in systemic oxidative stress, which exacerbates neurodegenerative processes and cognitive decline, reinforcing the need for therapeutic strategies targeting both hepatic and neuropsychiatric dysfunctions[69]. NAFLD is characterized by excessive hepatic lipid accumulation (steatosis), while NASH is further defined by persistent inflammation, hepatocyte injury, and fibrosis. A key driver of NAFLD and NASH progression is oxidative stress, where excessive ROS lead to lipid peroxidation, mitochondrial damage, and activation of inflammatory pathways[70]. In NAFLD and NASH, Nrf2 activation is essential for maintaining redox homeostasis. Studies have demonstrated that Nrf2-deficient mice develop severe hepatic steatosis, inflammation, and fibrosis due to an inability to combat oxidative stress and mitochondrial dysfunction[71]. The induction of HO-1 by Nrf2 further enhances hepatoprotective mechanisms by reducing inflammation, promoting autophagy, and modulating iron metabolism. HO-1, as a downstream effector of Nrf2, catalyzes the degradation of heme into biliverdin, CO, and free iron, all of which contribute to cytoprotective and anti-inflammatory effects. CO has been shown to inhibit pro-inflammatory pathways such as NF-κB and transforming growth factor (TGF)-β1/Smad signaling, which are key contributors to hepatic fibrosis in NASH[72,73]. Additionally, bilirubin, another byproduct of HO-1 activity, acts as a potent antioxidant, neutralizing free radicals and protecting hepatocytes from oxidative injury[74]. Despite its protective roles, sustained Nrf2 activation in advanced stages of NASH can have paradoxical effects. Excessive HO-1 expression has been associated with increased free iron accumulation, which may exacerbate lipid peroxidation and ferroptosis in hepatocytes[75].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate NAFLD, NASH, and neuropsychiatric symptoms: The growing recognition of the liver-brain axis has highlighted the bidirectional relationship between liver dysfunction and neuropsychiatric disorders, including cognitive dysfunction, mood disturbances, and anxiety. The Nrf2/HO-1 signaling pathway, a crucial regulator of oxidative stress and inflammation, is integral to both hepatic protection and neuroprotection. Activating this pathway not only mitigates hepatic inflammation and fibrosis but also plays a role in protecting against neuroinflammation and cognitive decline associated with liver diseases like NAFLD and NASH. Given the neuroprotective potential of this pathway, natural herbal monomers have emerged as promising candidates for modulating Nrf2/HO-1 activity, offering therapeutic benefits for both liver disease and its neuropsychiatric complications (Table 2). Flavonoids, particularly quercetin and silymarin, have been widely studied for their ability to modulate Nrf2 activity in NAFLD and NASH models. Quercetin, found in various fruits and vegetables, enhances Nrf2 nuclear translocation, increasing HO-1 expression and reducing lipid peroxidation in hepatocytes[76]. Silymarin, the principal bioactive constituent of milk thistle, confers hepatoprotective benefits in NASH by promoting Nrf2 stabilization and inhibiting its Keap1-mediated degradation. This regulatory effect enhances cellular antioxidant capacity and downregulates pro-inflammatory cytokines, including TNF-α[77]. Among alkaloids, berberine, an isoquinoline alkaloid extracted from Berberis species, has shown strong hepatoprotective potential. Berberine suppresses de novo lipogenesis through downregulating the expression of acetyl-CoA carboxylase and fatty acid synthetase thereby alleviating nonalcoholic fatty liver disease[78]. Ginsenosides have been reported to augment the activity of endogenous antioxidant enzymes, suppress ROS generation, and activate adenosine 5’-monophosphate-activated protein kinase (AMPK), thereby attenuating lipid deposition and oxidative injury in hepatic cells[79]. By stimulating the Nrf2/HO-1 signaling cascade, capillin attenuates oxidative stress in hepatic tissue, while concurrently suppressing hepatocyte apoptosis through inhibition of the nucleotide-binding oligomerization domain-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome. Together, these mechanisms contribute to reduced lipid accumulation, oxidative hepatocellular damage, and overall liver injury in metabolic-associated steatohepatitis (MASH) murine models, underscoring its promising therapeutic potential[80]. Baicalin demonstrates hepatoprotective effects by modulating mitochondrial function and suppressing pyroptosis. Study indicate that it activates the Nrf2/HO-1 pathway, leading to reduced NLRP3/caspase-1/GSDMD levels, which alleviates hepatic lipid accumulation and inflammation in MASH mice[81]. Likewise, aloin confers protection in MASH models by upregulating antioxidant defenses, attenuating inflammatory responses, and inhibiting apoptosis, primarily through activation of the Nrf2/HO-1 signaling pathway[82]. Furthermore, flavones derived from hawthorn leaves attenuate oxidative stress and liver inflammation by upregulating Nrf2/HO-1 while suppressing cyclooxygenase 2 overexpression, thereby inhibiting MASH progression[83]. Linalool effectively mitigates high-fat diet-induced metabolic dysfunction-associated steatotic liver disease (MASLD) by activating the Nrf2/HO-1 signaling pathway, which suppresses lipid accumulation and oxidative stress[84]. Similarly, wogonoside demonstrates hepatoprotective effects by upregulating the Nrf2/HO-1 pathway to counteract oxidative stress while inhibiting the NF-κB pathway to reduce inflammation. This dual action significantly improves liver mass, liver index, and lipid profiles (low density lipoprotein, triglyceride, total cholesterol) in MASLD mice[85]. Scutellarin alleviates NAFLD by enhancing Nrf2/HO-1 signaling, thereby reducing oxidative stress and improving hepatic function[86,87]. Naringin has been shown to regulate the Nrf2/HO-1 signaling axis while concurrently downregulating the NF-κB/TNF-α inflammatory pathway and inhibiting hepatic triglyceride biosynthesis, thereby exerting a protective effect against the progression of MASLD[88]. Geniposide exerts protective effects by upregulating Nrf2 expression and increasing HO-1 protein levels, significantly reducing lipid accumulation in HepG2 cells[18]. Importantly, the lipid-regulatory and antioxidative benefits of genistein are nullified upon Nrf2 silencing, underscoring the pathway’s pivotal role. Furthermore, dehydroabietic acid exerts its activity by directly interacting with Keap1, thereby initiating the Nrf2-ARE signaling cascade and enhancing the transcription of antioxidant effectors such as HO-1, GSH, and GPX4. This molecular mechanism effectively suppresses ROS accumulation and lowers malondialdehyde levels, ultimately mitigating MASLD progression in high-fat diet models (Table 3)[89].

Table 2.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate alcoholic liver disease

Monomers	Disease models	Mechanisms	Role	Ref.
Quercetin	ALD	Induces p62 expression, inhibits the binding of Keap1 to Nrf2	Improve	Tseng et al[116]
Silymarin	ALD	Reduces TGF-β1 and 4-HYP levels, suppressing oxidative injury	Improve	Chen et al[117]
Curcumin	ALD	Suppress p53 expression, enhances Nrf2 nuclear translocation	Improve	Yu et al[118]
Ginsenosides	ALD	Reduces oxidative stress and liver inflammation	Improve	Chen et al[119]
Baicalin	ALD	Antioxidant and anti-inflammatory	Improve	Yang et al[120]
Polysaccharides	ALD	Regulating oxidative stress and improve alcoholic liver injury by inhibiting the production of inflammatory factors	Improve	Roehlen et al[121]
Oleanolic acid	ALD	Suppress the alcohol-induced increases	Improve	Zhang et al[122]
CAE	ALD	Antioxidant, anti-inflammatory, and anti-apoptotic	Improve	Parola and Pinzani[123]

CAE: Citrus aurantium extract; ALD: Alcoholic liver disease; Nrf2: Nuclear factor erythroid 2-related factor 2; Keap1: Kelch-like ECH-associated protein 1; TGF-β1: Transforming growth factor-β1; 4-HYP: Hydroxyproline.

Table 3.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate drug-induced liver injury

Monomers	Disease models	Mechanisms	Role	Ref.
Isorhamnetin	DILI	Reduces oxidative stress, inflammation, and pyroptosis	Improve	Forner et al[134]
Dihydromyricetin	DILI	Reduces apoptosis and oxidative stress	Improve	Ganesan and Kulik[135]
Curcumin	DILI	Alleviate hepatic inflammation	Improve	Yang et al[136]
Berberine	DILI	Alleviate hepatic inflammation	Improve
Resveratrol	DILI	Mitigate oxidative stress, and mitochondrial dysfunction	Improve	Xu et al[137]
Ginsenosides	DILI	Reduces apoptosis and ROS	Improve	Hayes and McMahon[138]
Andrographolide	DILI	Decreases MDA and ROS	Improve	Hallis et al[139]
Astragaloside IV	DILI	Reduces inflammation	Improve	Loboda et al[140]
Salvianolic acid C	DILI	Attenuates oxidative stress, inflammation, and apoptosis	Improve	Lohitesh et al[141]

DILI: Drug-induced liver injury; ROS: Reactive oxygen species; MDA: Malondialdehyde.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate ALD

Nrf2/HO-1 signaling pathway in ALD: ALD encompasses a continuum of hepatic disorders resulting from chronic excessive alcohol intake, spanning from hepatic steatosis characterized by intracellular fat deposition to more severe pathologies such as alcoholic hepatitis, progressive fibrosis, cirrhosis, and ultimately HCC[90]. Chronic alcohol intake leads to metabolic imbalances and oxidative stress, which play a central role in ALD pathogenesis[91]. In hepatocytes, ethanol undergoes metabolic processing predominantly via alcohol dehydrogenase and the cytochrome P450 isoenzyme CYP2E1. This enzymatic activity leads to overproduction of ROS, which in turn induce oxidative damage to essential cellular macromolecules, including lipids, proteins, and nucleic acids[92]. ROS also impair mitochondrial function, contributing to energy deficits and apoptosis, which accelerates hepatic inflammation and fibrosis[93]. One of the key regulators of oxidative stress and inflammation in ALD is the Nrf2/HO-1 signaling pathway[94]. During oxidative stress induced by alcohol metabolism, ROS-mediated modifications of Keap1 cysteine residues disrupt its interaction with Nrf2, allowing Nrf2 to escape degradation, translocate into the nucleus and be activated[94]. In ALD, activation of Nrf2/HO-1 has been shown to counteract ethanol-induced hepatotoxicity by reducing oxidative stress, suppressing pro-inflammatory cytokine production, and preventing fibrosis through inhibition of hepatic stellate cell activation[95]. However, in chronic and advanced ALD, prolonged activation of Nrf2 may contribute to metabolic reprogramming that favors lipid accumulation and fibrosis[96].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate ALD: Among herbal monomers, flavonoids represent a well-characterized and widely investigated phytochemical class, recognized for their potent antioxidant capacity and liver-protective properties in the context of ALD. Quercetin, a flavonoid found in onions and citrus fruits, has been shown to upregulate Nrf2 expression and enhance HO-1 activation, thereby reducing oxidative stress, inflammatory cytokine production, and protecting against ethanol-induced oxidative damage[97]. Similarly, silymarin, a polyphenolic compound extracted from milk thistle (Silybum marianum), has demonstrated potent hepatoprotective effects by preventing ethanol-induced lipid peroxidation and inflammation through Nrf2/HO-1 activation[98]. Curcumin, the active polyphenolic compound in turmeric (Curcuma longa). Curcumin has been demonstrated to facilitate the nuclear translocation of Nrf2 and upregulate the expression of HO-1, while concurrently downregulating p53 expression. These coordinated molecular actions contribute to the attenuation of oxidative stress and hepatic damage induced by ethanol exposure[99]. Ginsenosides, derived from Panax ginseng, have been reported to activate the Sirt6/Nrf2/HO-1 axis, reducing oxidative stress and liver inflammation[100]. Furthermore, baicalin has been shown to inhibit NF-κB activation while upregulating Nrf2/HO-1, offering dual antioxidant and anti-inflammatory effects against ALD[101]. Poria cocos polysaccharides have demonstrated hepatoprotective effects by enhancing HO-1 expression and reducing ethanol-induced hepatocyte ferroptosis[102]. Other herbal monomers, such as oleanolic acid, polymethoxy flavonoid-containing citrus aurantium extract have also shown promise in ALD therapy. Oleanolic acid has been reported to suppress the alcohol-induced increases in intestinal permeability, making it for a hepatoprotection agent[103]. Extracts from Citrus aurantium have been shown to promote the phosphorylation of both AMPK and Nrf2 pathways, thereby exerting multifaceted protective effects including antioxidant, anti-inflammatory, and anti-apoptotic activities against liver injury induced by ethanol exposure[104].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate drug-induced liver injury

Nrf2/HO-1 signaling pathway in drug-induced liver injury: Drug-induced liver injury (DILI) is a major clinical concern and one of the most frequent causes of acute liver failure worldwide[105]. DILI occurs due to hepatotoxicity caused by pharmaceuticals, herbal remedies, or dietary supplements[106]. Drugs most frequently implicated in DILI include acetaminophen (paracetamol), various classes of antibiotics, nonsteroidal anti-inflammatory drugs, and certain anticonvulsants, notably sodium valproate[107]. The hepatotoxic effects of these agents are mediated through diverse pathogenic pathways, notably oxidative stress, mitochondrial impairment, and inflammatory responses. Among these, oxidative stress is recognized as a central driver in the development of DILI[108]. Hepatotoxic drugs often induce excessive production of ROS, which overwhelms the liver’s intrinsic antioxidant defenses, leading to lipid peroxidation, protein oxidation, and DNA damage[109]. Acetaminophen, for instance, is metabolized into the toxic intermediate N-acetyl-p-benzoquinone imine, which depletes GSH and causes hepatocyte necrosis[110]. Additionally, mitochondrial dysfunction plays a critical role in drug-induced hepatotoxicity, as mitochondrial permeability transition pore opening can trigger apoptosis and necrosis[111]. Activation of the Nrf2/HO-1 signaling cascade serves as a pivotal defense mechanism against DILI, primarily by upregulating phase II detoxification enzymes and a suite of antioxidant proteins such as HO-1, glutathione S-transferases, nicotinamide adenine dinucleotide phosphate quinone dehydrogenase 1 (NQO1), and SOD[112]. HO-1, in particular, catalyzes the breakdown of heme into biliverdin, CO, and free iron, exerting cytoprotective effects by reducing oxidative stress, inflammation, and apoptosis[54]. In addition to its antioxidant properties, the Nrf2/HO-1 axis plays a pivotal role in modulating inflammation in DILI. Moreover, Nrf2 activation mitigates endoplasmic reticulum stress and ferroptosis, both of which contribute to hepatocyte injury in DILI[113-115]. In light of its cytoprotective functions, modulation of the Nrf2/HO-1 signaling axis has emerged as a compelling therapeutic approach for the prevention and attenuation of DILI (Table 4).

Table 4.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate hepatitis C virus

Monomers	Disease models	Mechanisms	Role	Ref.
Andrographolide	HCV	Increases biliverdin levels	Improve	Narożna et al[150]
Celastrol	HCV	Enhances antiviral interferon responses, suppresses NS3/4A protease activity	Improve	Hassan et al[151]
Lucidone	HCV	Increases biliverdin levels	Improve	Sun et al[152]
Sulforaphane	HCV	Enhances IFN responses, suppresses NS3 protease activity	Improve	Fu et al[153]
Curcumin	HCV	Induces HO-1, inhibits PI3-AKT pathway	Improve	Chopra and Dhingra[154]

HCV: Hepatitis C virus; NS: Nonstructural; IFN: Interferon; HO-1: Heme oxygenase-1; PI3K: Phosphatidylinositol 3-kinase; AKT: Protein kinase B.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate DILI: Isorhamnetin, a naturally occurring bioflavonoid present in medicinal plants such as Hippophae rhamnoides L. and Ginkgo biloba L., has been shown to activate Nrf2 signaling and enhance HO-1 enzymatic activity, thereby attenuating oxidative stress and inflammatory responses in models of acetaminophen-induced liver injury[116]. Dihydromyricetin, a flavonoid derived from Ampelopsis grossedentata, has been shown to ameliorate sodium valproate-induced liver injury by modulating the Keap1/Nrf2/HO-1 and NF-κB/caspase-3 pathways, reducing apoptosis and oxidative stress[117]. Curcumin combined with berberine protects against acetaminophen (APAP)-induced DILI by alleviating hepatic inflammation through NF-κB inhibition, potentially mediated by PI3K/Akt and peroxisome proliferators-activated receptors-γ signaling pathways[118]. Resveratrol, a polyphenol found in grapes and red wine, has been shown to effectively mitigated liver injury, oxidative stress, and mitochondrial dysfunction by modulating the Nrf2-mediated antioxidant pathway and restoring GSH synthesis[119]. Ginsenosides from Panax ginseng enhance Nrf2 activation, increase HO-1 expression, and reduce inflammatory cytokine levels in liver injury models[120]. Andrographolide, a diterpenoid lactone from Andrographis paniculata, has been shown to suppress oxidative stress and inflammation in DILI through Nrf2/HO-1 modulation[121]. Astragaloside IV, a major bioactive component of Astragalus membranaceus, has been reported to reduce inflammation accumulation via Nrf2/HO-1 activation[122]. Similarly, salvianolic acid C from Salvia miltiorrhiza has been found to alleviate APAP-induced hepatocyte damage by reducing mitochondrial oxidative stress, suppressing inflammatory responses, and inhibiting caspase-mediated apoptosis through modulation of the Keap1/Nrf2/HO-1 signaling pathway[123].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate hepatitis C virus

Nrf2/HO-1 signaling pathway in hepatitis C virus: Hepatitis C virus (HCV)-induced viral hepatitis continues to pose a significant global public health burden, serving as a major etiological factor in the development of chronic liver disease, hepatic cirrhosis, and HCC[124]. HCV infections trigger persistent inflammation, oxidative stress, and hepatocyte injury, which promote fibrosis and eventual liver failure[125]. The Nrf2/HO-1 signaling pathway has been identified as a critical regulator of oxidative stress responses in viral hepatitis, modulating both antiviral immunity and hepatoprotective mechanisms. HCV infections disrupt redox homeostasis by increasing ROS production, which exacerbates inflammation and viral replication[126]. HCV, in particular, induces mitochondrial dysfunction and lipid peroxidation, leading to chronic oxidative stress and hepatic steatosis[127]. During hepatitis B virus and HCV infections, the activation of Nrf2-a central transcriptional regulator of cellular antioxidant responses is disrupted as a result of viral interference with its Keap1-dependent degradation pathway[128]. However, certain viral proteins, such as the nonstructural (NS) protein 3/NS protein 5A proteins in HCV, paradoxically upregulate Nrf2, leading to an adaptive response that enhances viral persistence while promoting liver disease progression[129]. CO generated by HO-1 has been shown to inhibit hepatitis B virus replication by downregulating viral transcription[130]. Similarly, HO-1 activation inhibits HCV replication by limiting oxidative damage and preventing viral protein accumulation (Table 5)[131].

Table 5.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate liver fibrosis and cirrhosis

Monomers	Disease models	Mechanisms	Role	Ref.
Curcumin	Cirrhosis	Reduces collagen I deposition	Improve	Yatoo et al[162]
Curcumin	Fibrosis	Reduces oxidative stress, inflammatory and fibrotic markers	Improve	Sailo et al[163]
Pomegranate extract	Fibrosis	Reduces oxidative stress, inflammatory and fibrotic markers	Improve	Sailo et al[163]
Hyperoside	Fibrosis	Reduces oxidative stress, restores antioxidant enzyme activities	Improve	Manawy et al[164]
Schisandrin B	Fibrosis	Mitigates oxidative stress, suppresses HSC activation	Improve	Robledinos-Antón et al[165]
Tanshinol	Fibrosis	Reduces oxidative stress, MDA, inflammatory	Improve	Kundrapu and Malla[166]
Salvianolic acid A	Fibrosis	Inhibits inflammation and oxidative stress	Improve	Wu et al[167]
Asiatic acid	Fibrosis	Reduces oxidative stress	Improve	Wang et al[168]

HSC: Hepatic stellate cell; MDA: Malondialdehyde.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate HCV: Andrographolide activates Nrf2 signaling and enhances HO-1 expression, thereby reducing oxidative stress and inhibiting HCV replication[132]. Celastrol has been shown to suppress HCV replication by upregulating HO-1 expression via activation of the c-Jun N-terminal kinase/Nrf2 signaling pathway, thereby augmenting interferon-mediated antiviral responses and inhibiting the enzymatic activity of the viral NS3/4A protease[133]. Lucidone exhibited potent anti-HCV activity by stimulating Nrf2-mediated HO-1 expression, which increased biliverdin levels to enhance antiviral interferon responses inhibit HCV NS3/4A protease activity, suppress HCV RNA replication[134]. Sulforaphane significantly inhibits HCV replication by activating the PI3K/Nrf2/HO-1 pathway, which enhances interferon responses and suppresses NS3 protease activity[135]. Curcumin demonstrates potent anti-HCV activity by dose-dependently inhibiting HCV replication through HO-1 induction and suppression of the PI3K-AKT pathway. While curcumin also inhibits extracellular regulated protein kinases and NF-κB signaling, these effects slightly increase HCV protein expression, highlighting the need for careful consideration when using curcumin as an adjuvant in anti-HCV therapy[136].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate liver fibrosis and cirrhosis

Nrf2/HO-1 signaling pathway in liver fibrosis and cirrhosis: Chronic liver fibrosis and cirrhosis represent progressive pathological conditions marked by abnormal accumulation of extracellular matrix components, particularly collagen, as a consequence of sustained hepatic insults arising from viral hepatitis, metabolic dysregulation, alcohol abuse, or hepatotoxic drug exposure[137]. Fibrosis is a wound-healing response, and its unchecked progression results in cirrhosis, leading to severe liver dysfunction and an increased risk of HCC[138]. Hepatic stellate cells are central to fibrosis development as they transition from a quiescent state to activated myofibroblast-like cells, secreting excessive extracellular matrix components that compromise liver architecture and function[139]. This fibrotic process is largely driven by oxidative stress, pro-inflammatory signaling, and the TGF-β/Smad pathway, which promotes myofibroblast differentiation and collagen synthesis[140]. The Nrf2/HO-1 signaling axis plays a pivotal role in counteracting hepatic fibrogenesis. Activation of Nrf2 has been demonstrated to downregulate key profibrotic genes, including α-smooth muscle actin, collagen type I, and tissue inhibitor of metalloproteinases-1, thereby restraining extracellular matrix deposition and fibrotic progression[141]. The upregulation of HO-1 exerts anti-fibrotic effects by modulating multiple pathways involved in fibrosis. HO-1-generated CO and bilirubin have been shown to suppress NF-κB activity, reducing inflammation and cytokine production[142]. HO-1 also influences iron homeostasis, preventing iron overload-induced oxidative stress, which is another major driver of fibrotic progression (Table 6)[143].

Table 6.

Natural monomers modulating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway to mitigate hepatocellular carcinoma

Monomers	Disease models	Mechanisms	Role	Ref.
Curcumin	HCC	Reduces oxidative damage and inflammation	Improve	Khalil et al[178]
Resveratrol	HCC	Against oxidative stress	Improve	Amirshahrokhi and Niapour[179]
Berberine	HCC	Reduces hepatic triglyceride accumulation and oxidative stress	Improve	Bellaver et al[181]
Tanshinone IIA	HCC	Reduces oxidative stress markers and enhances antioxidant defenses	Improve	Zhao et al[182]
Ginsenosides	HCC	Inhibits inflammasomes	Improve	Gao et al[183]
Brusatol	HCC	Mediates ferroptosis	Improve	Görg et al[184]
Oleanolic acid oxime derivatives	HCC	Reduces cell cycle, apoptosis, and proliferation	Improve	Navarro and Esteras[185]
Emodin	HCC	Suppresses cell proliferation, invasion, and angiogenesis	Improve	Wang et al[186]

HCC: Hepatocellular carcinoma.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate liver fibrosis and cirrhosis: Curcumin effectively reversed carbon tetrachloride-induced liver cirrhosis in hamsters by reducing collagen I deposition, normalizing hepatic function, and increasing the Nrf-2/NF-κB message RNA ratio[144]. Additionally, curcumin and pomegranate extract, both individually and in combination, significantly attenuated thioacetamide-induced liver fibrosis in rats by modulating Nrf2/HO-1, NF-κB, and TGF-β/Smad3 signaling pathways, as evidenced by improved liver function, reduced oxidative stress, and decreased inflammatory and fibrotic markers[145]. Hyperoside has been shown to mitigate liver injury induced by carbon tetrachloride through attenuation of oxidative stress, restoration of endogenous antioxidant enzyme function, and improvement of histopathological alterations. These protective effects are partly mediated by promoting Nrf2 translocation into the nucleus, thereby upregulating antioxidant defenses and enhancing phase II detoxification enzyme activity[146]. Schisandrin B activates nuclear Nrf2, thereby mitigating oxidative stress-induced hepatocyte injury. It also suppresses hepatic stellate cell activation by inhibiting the TGF-β/Smad signaling pathway, effectively alleviating liver fibrosis[147]. Tanshinol enhances SOD and glutathione peroxidase levels while reducing malondialdehyde levels through the Nrf2/HO-1 pathway, countering oxidative stress. Additionally, it inhibits the NF-κB pathway, reducing inflammatory factors such as TGF-β and TNF-α, and significantly lowers serum markers of liver injury and fibrosis[148]. Salvianolic acid A exerts antifibrotic effects in carbon tetrachloride-induced liver injury by enhancing the activities of antioxidant enzymes such as SOD and glutathione peroxidase, while simultaneously reducing malondialdehyde levels. These effects are mediated through modulation of the Nrf2/HO-1 signaling pathway, leading to attenuation of oxidative stress and inflammatory responses[149]. Asiatic acid activates the Nrf2/ARE pathway to upregulate HO-1 and NQO1, countering oxidative stress, while inhibiting NF-κB/IκBα and Janus tyrosine kinase 1/signal transducer and activator of transcription 3 pathways to reduce inflammation and hepatic stellate cell activation, thereby preventing hepatic fibrosis progression[150].

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate HCC

Nrf2/HO-1 signaling pathway in HCC: HCC, the predominant form of primary liver malignancy, represents a major contributor to global cancer-associated mortality. It typically develops in the setting of chronic liver disease, particularly liver fibrosis and cirrhosis, which result from persistent inflammation, oxidative stress, and metabolic disturbances[151]. Major risk factors for HCC include chronic hepatitis B and C infections, alcoholic and NAFLD/NASH, aflatoxin exposure, and metabolic syndromes[152]. At the onset of liver disease, the Nrf2/HO-1 pathway functions as a crucial defense mechanism by mitigating oxidative damage, suppressing pro-inflammatory cytokines, and promoting DNA repair mechanisms[153]. During early hepatocarcinogenesis, Nrf2-mediated induction of HO-1, NQO1, and GPX4 plays a protective role by neutralizing ROS, thereby preventing oxidative DNA damage and oncogenic mutations[11]. HO-1, in particular, exerts anti-apoptotic and cytoprotective effects by generating CO and bilirubin, which possess strong antioxidant and anti-inflammatory properties[11]. However, as HCC progresses, persistent Nrf2 activation can become detrimental. Many HCC tumors exhibit Keap1 mutations, leading to constitutive Nrf2 activation, which promotes metabolic reprogramming, drug resistance, and tumor growth[154]. High levels of Nrf2 activity enhance glucose metabolism via the pentose phosphate pathway, providing increased nicotinamide adenine dinucleotide phosphate and GSH production, which supports cancer cell survival[155]. Furthermore, sustained HO-1 overexpression is associated with enhanced angiogenesis and immune evasion, allowing tumors to thrive in oxidative and inflammatory environments[156]. One of the most significant challenges in HCC treatment is chemoresistance, where tumor cells become resistant to sorafenib, cisplatin, and doxorubicin-the frontline chemotherapeutic agents[157]. Evidence suggests that excessive activation of the Nrf2/HO-1 pathway enhances the expression of drug efflux transporters, including members of the multidrug resistance-associated protein family, thereby limiting intracellular accumulation of chemotherapeutic agents in HCC cells[158]. Despite its role in tumor progression, selective modulation of Nrf2/HO-1 remains a promising therapeutic approach.

Natural herbal monomers targeting the Nrf2/HO-1 signaling pathway to modulate HCC: Curcumin, a polyphenol from Curcuma longa, has been extensively studied for its Nrf2-regulating properties. In HCC models, curcumin activates Nrf2 to protect hepatocytes from oxidative DNA damage and inflammation, thereby preventing malignant transformation[159]. Resveratrol, a polyphenol found in grapes, has been shown to modulate Nrf2 activity in a dose-dependent manner. At low concentrations, resveratrol activates Nrf2/HO-1, offering protective effects against oxidative stress-induced liver damage[160]. However, at higher doses, resveratrol suppresses HO-1 expression, reducing HCC cell proliferation, inducing apoptosis, and enhancing chemosensitivity[161]. Berberine reduces hepatic triglyceride accumulation and oxidative stress in hepatoma cells by promoting Nrf2 activation and nuclear distribution, while suppressing Nox2-dependent and mitochondrial ROS production through downregulation of complex I and III expression[162]. Tanshinone IIA confers hepatoprotective effects in HCC by concurrently suppressing the PI3K/Akt signaling cascade and activating the Nrf2/HO-1 pathway, resulting in diminished oxidative stress biomarkers and upregulation of endogenous antioxidant defense mechanisms[163]. Recent studies have demonstrated that ginsenosides increase the susceptibility of HCC cells to apoptosis, making them potential adjuvants in combination therapy[164]. Brusatol, a quassinoid compound derived from Brucea javanica, has been shown to potentiate chemotherapeutic efficacy by specifically inhibiting Nrf2 signaling, thereby reversing sorafenib resistance in HCC cells and facilitating ferroptotic cell death through downregulation of the Nrf2-driven antioxidant defense system[165]. Oleanolic acid oxime derivatives significantly activate Nrf2 in HepG2 cells, inducing cell cycle arrest, apoptosis, and reduced proliferation[166]. Emodin demonstrates significant anticancer activity against HCC by reducing hepatic nodules, suppressing proliferation, invasion, and angiogenesis, while enhancing oxidative stress defense and tissue homeostasis[167].

CHALLENGES, LIMITATIONS, AND FUTURE DIRECTIONS IN TARGETING THE NRF2/HO-1 PATHWAY FOR LIVER DISEASES AND NEUROPSYCHIATRIC DISORDERS

Challenges in translating preclinical research to clinical use

The use of natural herbal monomers in regulating the Nrf2/HO-1 pathway has shown promising hepatoprotective effects in various preclinical studies[168]. However, several key challenges hinder the translation of these findings into clinical practice[169]. One of the foremost challenges is the low bioavailability of many herbal monomers, such as curcumin and resveratrol, which exhibit poor absorption, rapid metabolism, and limited systemic circulation in vivo[170-172]. Another major challenge is the lack of standardized dosages and formulations in clinical trials. Unlike synthetic pharmaceuticals, natural herbal monomers are often sourced from different plant species, leading to variations in purity, concentration, and pharmacokinetics[173]. Additionally, off-target effects and safety concerns surrounding Nrf2 overactivation present another critical challenge[174]. While Nrf2 activation is beneficial in counteracting oxidative stress and inflammation, sustained activation has been associated with increased resistance to apoptosis and enhanced survival of precancerous cells, potentially promoting tumor progression[175].

Potential for combination therapies

Given the complex pathophysiology of liver diseases, combination therapies that integrate herbal monomers with existing pharmacological agents may enhance therapeutic outcomes[176]. Natural compounds such as curcumin, baicalin, and ginsenosides have demonstrated synergistic effects when combined with conventional hepatoprotective drugs[177]. For instance, curcumin has been shown to enhance the efficacy of silymarin, a widely used hepatoprotective drug, by modulating oxidative stress and inflammatory pathways through the Nrf2/HO-1 axis[178]. Furthermore, herbal monomers may also serve as chemosensitizers, enhancing the response to existing anti-fibrotic or anti-inflammatory drugs[179,180]. For example, resveratrol has been reported to potentiate the anti-inflammatory effects of pirfenidone by modulating TGF-β signaling[181]. Another emerging strategy is the use of Nrf2 activators in combination with immune checkpoint inhibitors for the treatment of HCC, as this approach may reduce oxidative stress-mediated immune suppression while preserving hepatocyte function[182].

Future research directions: Targeting the Nrf2/HO-1 pathway for liver diseases and neuropsychiatric complications

The future of herbal monomers in liver disease therapy through the Nrf2/HO-1 pathway lies in overcoming the current limitations and advancing research in the following key areas: (1) Development of advanced drug delivery systems: The integration of nanotechnology with herbal medicine has led to nano-drug formulations that improve the solubility, bioavailability, and targeted delivery of natural compounds. Lipid nanoparticles, polymeric micelles, and liposomal encapsulation have been successfully employed to enhance the pharmacokinetics of quercetin, berberine, and ginsenosides[183-185]; (2) Personalized medicine and biomarker-driven approaches: Interindividual variability in Nrf2 genetic polymorphisms significantly influences responsiveness to herbal monomers. For instance, NFE2 L2 mutations have been linked to differences in antioxidant response capacity, potentially altering the effectiveness of Nrf2-targeted therapies[186]. Identifying biomarkers of Nrf2 activation may enable personalized treatment strategies, ensuring that patients with specific genetic predispositions receive tailored interventions; (3) Long-term safety and efficacy trials: While short-term studies have demonstrated the hepatoprotective potential of herbal monomers, longitudinal clinical trials are required to establish long-term safety profiles. Comparative studies assessing the efficacy of herbal monomers against standard-of-care therapies will be crucial in determining their clinical utility[187]; and (4) Expanding the scope beyond hepatic disorders: Given the overlapping molecular mechanisms involved in oxidative stress and inflammation across different organ systems, herbal Nrf2 activators may hold therapeutic potential in broader disease contexts, providing a multi-targeted strategy for chronic disease management[188,189].

Dual roles of Nrf2 in hepatic protection and pathology

Under physiological conditions or early stages of liver injury, Nrf2 activation plays a crucial role in maintaining redox homeostasis, reducing oxidative damage, and promoting tissue repair.

However, in advanced liver diseases such as HCC, Nrf2 may paradoxically facilitate tumor progression. Persistent or constitutive Nrf2 activation often driven by loss-of-function mutations in Keap1 or epigenetic silencing of negative regulators-enhances tumor cell survival, detoxification, and proliferation.

Additionally, Nrf2-driven transcription of anti-apoptotic genes and adenosine triphosphate-binding cassette drug transporters (e.g., ABCG2) has been linked to chemoresistance to sorafenib, the first-line systemic therapy for HCC.

These findings highlight the dualistic nature of Nrf2, functioning as a protector in inflammatory settings but potentially contributing to carcinogenesis and therapeutic resistance when dysregulated. As such, therapeutic strategies targeting Nrf2 should emphasize context-specific modulation, taking into account disease stage, genetic background, and oncogenic potential.

Future drug development may benefit from transient or localized activation of Nrf2, or combination therapies designed to avoid sustained oncogenic signaling, particularly in high-risk populations.

CONCLUSION

This review comprehensively explored the pivotal role of the Nrf2/HO-1 signaling pathway in both liver diseases and HE, emphasizing its therapeutic potential in addressing oxidative stress, inflammation, and fibrosis, while also mitigating neuropsychiatric symptoms. Given the escalating burden of liver diseases worldwide, it is increasingly crucial to develop therapeutic strategies that not only alleviate hepatic damage but also address the associated neuropsychiatric complications such as cognitive decline, mood disorders, and motor dysfunction. The Nrf2/HO-1 axis, as a master regulator of antioxidant responses, detoxification, and immune modulation, represents a critical therapeutic target for both hepatic and neurological pathologies. This review provided an in-depth overview of the structural and functional characteristics of Nrf2 and HO-1, emphasizing their physiological significance and the molecular mechanisms governing their activation. Under homeostatic conditions, Nrf2 activity is tightly suppressed by its cytoplasmic repressor Keap1; however, in response to oxidative or electrophilic stress, Nrf2 is rapidly stabilized and translocated to the nucleus, where it orchestrates the expression of a spectrum of cytoprotective genes. Among these, HO-1 acts as a pivotal downstream effector by catalyzing the degradation of heme into biliverdin, CO, and free iron metabolites known to restore neurotransmitter equilibrium and promote synaptic plasticity, thereby exerting neuroprotective and anti-inflammatory effects. Notably, dysregulation of the Nrf2/HO-1 axis has been implicated in both beneficial and detrimental outcomes in the context of liver pathologies and HE-related neuropsychiatric manifestations, highlighting the critical importance of fine-tuned modulation of this pathway to harness therapeutic benefit while avoiding unintended consequences. Focusing on liver pathophysiology, we explored how Nrf2/HO-1 can suppress inflammation, prevent oxidative damage, and regulate hepatic metabolism, thus highlighting its potential as a therapeutic target across a range of liver disease models. A central part of this review was dedicated to the role of natural herbal monomers in modulating the Nrf2/HO-1 pathway. Flavonoids, alkaloids, phenolics, and terpenoids have been shown to activate Nrf2, promote HO-1 expression, and restore hepatic redox balance. Key herbal compounds such as quercetin, silymarin, berberine, curcumin, resveratrol, ginsenosides, and Astragalus polysaccharides have demonstrated significant hepatoprotective effects in both preclinical and clinical studies. However, despite the promising therapeutic potential of these herbal monomers, several challenges remain in translating these findings into clinical applications. Issues such as low bioavailability, lack of standardized formulations, and the long-term effects of sustained Nrf2 activation pose significant hurdles for their clinical adoption. To overcome these challenges, future research should focus on optimizing drug delivery systems, refining dosage protocols, and conducting well-designed clinical trials to validate the efficacy and safety of these compounds. Additionally, advancing personalized medicine approaches by identifying biomarkers of Nrf2 activation could enhance treatment outcomes while minimizing potential adverse effects. Moreover, this review underscores the connection between the Nrf2/HO-1 pathway and HE-associated neuropsychiatric symptoms, a condition characterized by toxic metabolite accumulation and oxidative stress. The Nrf2/HO-1 pathway plays a pivotal role in modulating neuroinflammation, neurotoxicity, and ammonia metabolism in both the liver and brain, highlighting its therapeutic potential in HE management, particularly in ameliorating brain ammonia toxicity, neuroinflammation, and oxidative stress, thus improving cognitive and behavioral outcomes. Although most of the mechanistic evidence for natural herbal monomers stems from preclinical models, emerging clinical data support their therapeutic potential. For instance, curcumin has been evaluated in randomized controlled trials for NAFLD, with phase II studies demonstrating improvements in liver enzymes, steatosis, and inflammatory markers. Similarly, silymarin has been investigated in clinical studies for ALD and chronic hepatitis, showing modest but consistent hepatoprotective effects. Berberine has demonstrated efficacy in NAFLD and metabolic syndrome patients by improving lipid profiles and insulin sensitivity. However, clinical trials specifically targeting HE with these compounds remain scarce, underscoring the need for translational research to bridge this gap. Moving forward, well-designed phase II/III trials in HE and liver-brain axis dysfunction are essential to validate the promising effects observed in experimental models. In conclusion, this review demonstrates that Nrf2/HO-1 activation offers a promising strategy for mitigating oxidative stress, inflammation, and fibrosis while simultaneously addressing neuropsychiatric symptoms in liver diseases. Although herbal monomers targeting this pathway show great potential, further clinical validation is essential to ensure their efficacy, safety, and long-term benefits. By integrating traditional herbal medicine with modern pharmacological research, we believe that the development of safe, effective, and clinically viable Nrf2-targeted therapies will advance the holistic management of liver diseases and their neuropsychiatric complications, offering new hope for patients worldwide.