Letter to the Editor: Natural product intervention in metabolic dysfunction-associated steatotic liver disease: From allyl isothiocyanate to signaling networks

Abstract

The pursuit of effective therapeutic agents against metabolic dysfunction-associated steatotic liver disease continues, with natural products offering promise due to their multi-targeting capabilities. A study published in the World Journal of Hepatology by Gao et al demonstrates that allyl isothiocyanate (AITC) alleviates hepatic steatosis and insulin resistance by upregulating the hepatic vitamin D receptor (VDR), which activates the hepatocyte nuclear factor 4 alpha/microsomal triglyceride transfer protein/apolipoprotein B axis to promote lipid export and β-oxidation while suppressing lipogenesis. While compelling, this VDR-centric model exists in isolation, lacking integration with other known AITC targets like the sirtuin 1/AMP-activated protein kinase axis. Furthermore, although the experimental model provides insightful data, it relies exclusively on in vitro studies, awaiting validation in vivo and direct genetic evidence from VDR knockout models to solidify the causal relationship. This commentary aims to elucidate the multifaceted mechanisms of AITC against metabolic dysfunction-associated steatotic liver disease, underscoring its systemic effects on lipid metabolism, insulin sensitivity, and hepatocyte transcriptional reprogramming, which collectively provide a foundation for developing precise, safe, and sustainable therapeutic strategies.

Key Words: Allyl isothiocyanate; Metabolically-dysfunction-associated steatotic liver disease; Vitamin D receptor; Multitarget therapy; Network pharmacology

Core Tip: This comment highlights that allyl isothiocyanate (AITC) exemplifies the principle of multi-target natural product therapy for metabolic dysfunction-associated steatotic liver disease. In addition to the novel vitamin D receptor/hepatocyte nuclear factor 4 alpha pathway identified by Gao et al, we integrate this axis with established AITC targets, such as sirtuin 1/AMP-activated protein kinase and nuclear factor erythroid 2-related factor 2, to construct a multi-signaling network. This network collectively reprograms lipid metabolism, insulin sensitivity, and oxidative stress. Future research must validate this integrative mechanism in vivo and leverage systems biology to advance AITC as a precise, network-based therapeutic strategy.

TO THE EDITOR

Metabolic dysfunction-associated steatotic liver disease (MASLD) has emerged as a prevalent chronic liver condition worldwide, affecting approximately one-quarter of adults and up to 75% of individuals with obesity[1,2]. The pathogenesis of MASLD involves multiple interacting factors, including excessive intrahepatic lipid accumulation, systemic insulin resistance, and low-grade chronic inflammation. These factors form a vicious cycle, collectively driving disease progression from simple steatosis to metabolic dysfunction-associated steatohepatitis, liver fibrosis, and potentially to cirrhosis or hepatocellular carcinoma[3]. Currently, no approved drugs can directly reverse hepatic steatosis or inflammatory fibrosis[4]. Although lifestyle interventions are the first-line therapy, their poor long-term adherence highlights the need for additional pharmacological strategies. Against this background, natural products represent a promising option for treating multifactorial complex diseases like MASLD, given their broad accessibility, favorable safety profile, and characteristic multi-component, multi-target synergistic actions[5]. In contrast to single-target chemical drugs that correct isolated aspects, natural active molecules can simultaneously regulate multiple key nodes, including lipid metabolism, insulin signaling, and oxidative stress/inflammatory responses, thus achieving a more comprehensive restoration of metabolic homeostasis.

Allyl isothiocyanate (AITC) is a natural organic sulfur compound characterized by its pungent, spicy odor. It primarily originates from cruciferous plants, where it is produced as a defensive substance through the enzymatic breakdown of glucosinolates upon tissue damage[6,7], which has garnered significant attention for its notable anti-inflammatory, antioxidant, and metabolism-modulating biological effects[8]. A recent study published in World Journal of Hepatology by Gao et al[9] investigated the role of AITC in MASLD and identified a mechanism involving vitamin D receptor (VDR) regulation. This study demonstrates that AITC increases VDR expression in hepatocytes, subsequently activating the hepatocyte nuclear factor 4 alpha (HNF-4α)/microsomal triglyceride transfer protein (MTTP)/apolipoprotein B (ApoB) signaling axis. This activation promotes hepatic lipid export, enhances fatty acid β-oxidation, and suppresses de novo lipogenesis, suggesting that the VDR pathway mediates the hepatic effects of AITC.

Specifically, the study’s core finding shows that AITC treatment upregulates VDR expression, and the activated VDR subsequently interacts with HNF-4α to enhance its transcriptional activity, leading to upregulation of its downstream target genes MTTP and ApoB. This two proteins are critical for the assembly and secretion of very-low-density lipoprotein in the liver[10]. Their upregulation indicates that the liver reduces intrahepatic lipid accumulation, associated with enhanced lipid export. Furthermore, this pathway is implicated in improved fatty acid β-oxidation and insulin resistance. Given the multi-target characteristics of AITC, this VDR-involved pathway not only offers molecular evidence for its hepatoprotective effects but also positions VDR within the mechanistic framework underlying the action of natural products in MASLD.

AITC’s known multi-target characteristics suggest that its biological effects are likely mediated through a synergistic network. The VDR pathway, as identified by Gao et al[9], is a crucial component of this network. However, this single mechanism alone cannot fully explain the anti-MASLD efficacy of AITC. Indeed, numerous prior studies have demonstrated that AITC is a molecule with complex regulatory functions[11-13]. Through specific binding and activation of the transient receptor potential ankyrin 1 (TRPA1) ion channel, the natural agonist AITC triggers a crucial influx of calcium ions (Ca²⁺)[14]. This initial Ca²⁺ signal acts as a central second messenger, initiating a multi-layered signalling cascade network that combats the complex pathology of MASLD from multiple angles. Based on this TRPA1/Ca²⁺-initiated regulatory foundation, AITC targets multiple specific molecules and pathways to alleviate hepatic steatosis, mitigate oxidative stress and inflammation, improve insulin resistance and exert anti-fibrotic effects[15]. Within this network, the TRPA1-mediated calcium influx serves as an essential prerequisite for activating the entire downstream cellular defense system. This highlights the unique mechanism by which natural products treat metabolic diseases through multi-target systematic intervention (Figure 1). The following sections will detail the specific molecular mechanisms by which it regulates lipid metabolism, enhances antioxidant defences, exerts anti-inflammatory and anti-fibrotic effects, and improves insulin sensitivity.

Figure 1 Schematic diagram of the molecular mechanism by which allyl isothiocyanate counteracts metabolic dysfunction-associated steatotic liver disease. MASLD: Metabolic dysfunction-associated steatotic liver disease; NCS: Isothiocyanate group; H₂C: Methylene group; AITC: Allyl isothiocyanate; TRPA1: Transient receptor potential ankyrin 1; VDR: Vitamin D receptor; SIRT1: Sirtuin 1; AMPK: AMP-activated protein kinase; NF-κB: Nuclear factor kappa B; TGF-β: Transforming growth factor-β; Samd: Smad family proteins; Nrf2: Nuclear factor erythroid 2-related factor 2; HNF-4α: Hepatocyte nuclear factor 4 alpha; MTTP: Microsomal triglyceride transfer protein; APOB: Apolipoprotein B; p-PARa: Phosphorylated peroxisome proliferator-activated receptor α; ACC: Acetyl-CoA carboxylase; SREBP-IC: Sterol regulatory element-binding protein-1c; TNF-α: Tumor necrosis factor-α; IL-6: Interleukin-6; a-sMA: Α-smooth muscle actin; ARE: Antioxidant response element.

Regarding the regulation of lipid metabolism, the core action of AITC is the activation of the sirtuin 1/AMP-activated protein kinase (SIRT1/AMPK) pathway. This activation inhibits sterol regulatory element-binding protein 1, the key transcriptional factor for de novo lipogenesis, along with its downstream target genes (such as stearoyl-CoA desaturase 1 and fatty acid synthase), while simultaneously upregulating proteins involved in fatty acid β-oxidation (e.g., peroxisome proliferator-activated receptor gamma coactivator 1 alpha, peroxisome proliferator-activated receptor alpha, carnitine palmitoyltransferase 1 alpha)[10]. This effectively reduces hepatic lipid synthesis and promotes its breakdown, alleviating hepatic steatosis. Loss-of-function experiments confirmed that knocking down SIRT1 or AMPK completely blocked the lipid-improving effects of AITC, establishing the central role of this pathway[10]. In studies on the anti-diabetic mechanisms of AITC, accumulating evidence further supports that AITC activates the TRPA1 receptor to induce Ca²⁺ influx, which in turn triggers the AMPK signaling pathway via calcium-sensitive upstream kinases such as Ca²⁺/calmodulin-dependent protein kinase kinase 2, thereby playing a key role in the regulation of glucose and lipid metabolism[16].

In terms of enhancing antioxidant defense, AITC is a classic activator of nuclear factor erythroid 2-related factor 2 (Nrf2), effectively inducing the expression of a suite of antioxidant and detoxifying genes, thereby strengthening the hepatocyte’s ability to resist oxidative damage[17]. It is noteworthy that research suggests activation of the VDR may also enhance the Nrf2 signaling pathway[18]. This implies that under AITC treatment, the VDR pathway may synergize with the Nrf2 pathway, jointly bolstering antioxidant defense and providing protection at more advanced disease stages (metabolic dysfunction-associated steatohepatitis).

Its anti-inflammatory and anti-fibrotic effects are primarily mediated through the modulation of relevant signaling pathways. AITC upregulates the inhibitor of nuclear factor kappa-B (NF-κB) alpha protein and inhibits the phosphorylation and activation of the IκB kinase/NF-κB pathway, effectively reducing the production of key pro-inflammatory cytokines such as tumor necrosis factor-α and interleukin-1β, thereby alleviating chronic liver inflammation[11]. This anti-inflammatory action synergizes with AITC’s mechanism for improving lipid metabolism, jointly mitigating the pathological progression of MASLD. Furthermore, by activating TRPA1 and subsequently modulating the transforming growth factor-β1/small mother against decapentaplegic signaling, AITC can inhibit the differentiation of fibroblasts into myofibroblasts (manifested as reduced α-smooth muscle actin expression), demonstrating potential anti-fibrotic effects[19].

Regarding the improvement of insulin sensitivity, AITC acts through multi-target synergy. On one hand, it regulates energy metabolism and lipid flux by activating the AMPK/SIRT1 axis, directly alleviating lipotoxicity. On the other hand, it mitigates the damage caused by oxidative stress and inflammation to insulin signaling pathways by activating Nrf2 and inhibiting NF-κB. Simultaneously, AITC upregulates key insulin signaling molecules, including glucose transporter 2, peroxisome proliferator-activated receptor-γ, and phosphorylated insulin receptor substrate 1, and comprehensively improves blood glucose and lipid levels, thereby systematically restoring insulin sensitivity[11,20]. These pathways are interconnected and form a positive feedback network, systematically reshaping the metabolic homeostasis of the liver and the entire body, and ultimately restoring insulin sensitivity. This demonstrates its unique advantage as a natural product in the multi-targeted collaborative treatment of metabolic disorders.

The study by Gao et al[9] revealed a new mechanism by which AITC promotes liver lipid output by upregulating the VDR and activating the HNF-4α/MTTP/ApoB axis. This provides an important perspective for understanding its hepatoprotective effect and officially incorporates VDR into the mechanism network of AITC’s action[10]. However, it must be pointed out that this mechanism centered on VDR may not be able to fully explain all the effects exhibited by AITC, especially its extensive anti-inflammatory, antioxidant, and improvement of insulin resistance properties. The existing evidence suggests that the efficacy of AITC is more likely attributed to the aforementioned synergistic network triggered by TRPA1/Ca²⁺, covering multiple pathways such as SIRT1/AMPK, Nrf2, and NF-κB. The VDR pathway may be an important component of this network and may interact with other pathways such as Nrf2[18].

CONCLUSION

In summary, AITC may act as an initial signal by activating the TRPA1 receptor and triggering Ca²⁺ influx. It then forms a multi-targeted network to exert its effects. This network simultaneously regulates several core pathways such as SIRT1/AMPK, Nrf2, VDR, and NF-κB, thereby systematically interfering with the complex pathological process of MASLD from multiple dimensions including reducing liver lipid accumulation, combating oxidative stress and inflammation, improving insulin resistance, and inhibiting fibrosis. This not only fully demonstrates the unique advantages and potential of natural products in treating complex metabolic diseases through multi-target systemic intervention, but also broadens our understanding of the mechanism of action of AITC, and provides a theoretical basis for formulating a synergistic treatment strategy targeting the multi-factor pathological characteristics of MASLD. Future research needs to rely on more in-depth in vivo experiments and genetic tools such as VDR knockout models to further verify the specific contribution of the VDR pathway to the overall effect of AITC and its interaction with other pathways, thereby more completely and systematically elucidating the multi-target synergistic mechanism of AITC in the treatment of MASLD.