TO THE EDITOR
In this article, we discuss the research findings recently published by Niu et al[1]. Metabolic dysfunction-associated steatotic liver disease (MASLD) previously known as non-alcoholic fatty liver disease is the most prevalent chronic liver disease, affecting up to 38% of the global population[2]. It is a progressive liver disease ranging from benign fat accumulation in the liver (hepatosteatosis) to an inflammatory and fibrotic state called metabolic dysfunction-associated steatohepatitis (MASH). MASLD not only increases the risk for the development of severe liver diseases such as cirrhosis, liver failure, and hepatocellular carcinoma, but also contributes to the development of various cardiovascular diseases, chronic kidney disease, and certain types of non-hepatic cancers[3-7]. As its new name suggests, MASLD is generally identified in individuals with metabolic dysfunction suffering from obesity, type 2 diabetes, and dyslipidemia[8]. Moreover, single nucleotide polymorphisms in several genes have been associated with the development and progression of MASLD[9]. However, until recently, there was no Federal Drug Administration-approved pharmacological treatment for MASLD or MASH[10]. In March 2024, the Federal Drug Administration approved resmetirom, a liver-targeted agonist for thyroid hormone receptor (THR)-β, for the treatment of MASH in noncirrhotic adults with fibrotic liver[11,12]. THR-β is highly expressed in hepatocytes and regulates metabolic pathways, which are impaired in MASLD/MASH patients. It plays a crucial role in hepatic lipid metabolism by stimulating hepatocyte fatty acid uptake, enhancing hepatic adipogenesis, promoting hydrolysis of hepatic triglycerides, and inducing fatty acid β-oxidation. The net effect of THR-β agonism is an increase in fatty acid metabolism that exceeds fatty acid synthesis, leading to a reduction in total hepatic triglycerides[13,14]. However, resmetirom treatment is associated with several side effects, including diarrhea, nausea, constipation, gastrointestinal discomfort, headache, dizziness, and pruritus. It may also induce liver toxicity and have gallbladder-related side effects. Given the high prevalence and incidence of MASLD/MASH and associated hepatic and extrahepatic complications, the development of additional novel therapeutic drugs with minimal side effects is essential.
PATHOGENESIS OF MASLD
The pathogenesis of MASLD is complex and involves several key factors contributing to hepatic lipid deposition and inflammation. Excess delivery of free fatty acids to the liver by lipolysis of triglycerides in adipose tissue, increased de novo lipogenesis driven by excess carbohydrates (glucose and fructose), and reduced mitochondrial β-oxidation of fatty acids in hepatocytes, cause dysfunctional hepatic metabolism[15]. These processes collectively promote lipid accumulation and the formation of lipotoxic species in the liver[15]. Within the liver, fatty acids undergo either mitochondrial β-oxidation or esterification to form triglycerides, and the formation of triglycerides is considered a compensatory response to an excess supply of fatty acids[16]. Impaired mitochondrial function and the failure to export triglycerides as very low-density lipoprotein into the bloodstream further promote hepatic lipid accumulation and hepatocellular injury[17,18]. This metabolic stress combined with the accumulation of lipotoxic species activates inflammasomes and results in the production of pro-inflammatory cytokines and caspase-1-mediated hepatocyte death and subsequent fibrosis[19-21]. Additionally, insulin resistance and hyperinsulinemia intensify lipid accumulation by redirecting glucose from glycogen synthesis to hepatic lipogenesis and altering lipolytic pathways[15]. Several proteins, including adenosine-5’-monophosphate-activated protein kinase, acetyl-CoA carboxylase (ACC), steroyl-CoA response element binding protein-1c, farnesoid X receptor, and carbohydrate response element-binding protein play important roles in de novo lipogenesis[22,23]. Under normal conditions, insulin inhibits lipolysis and facilitates triglyceride storage in adipose tissue. However, in the setting of insulin resistance, lipolysis in adipose tissue increases, leading to an augmented influx of free fatty acids into the liver for triglyceride synthesis. Simultaneously, insulin resistance promotes de novo lipogenesis via steroyl-CoA response element binding protein-1c activation. Elevated glucose levels further induce dephosphorylation of carbohydrate response element-binding protein and stimulate its nuclear translocation to regulate the expression of lipogenic (ACC and fatty acid synthase, etc.) proteins, thereby promoting lipid synthesis in hepatocytes[24]. Pharmacologically targeting these proteins may inhibit hepatic de novo lipogenesis and reduce hepatosteatosis. Moreover, proteins involved in redirecting fatty acids and carbohydrates from the liver to extrahepatic tissues such as adipose tissue and muscles, for storage and mitochondrial respiration also have significant roles in the pathogenesis of MASLD. Genetic polymorphisms in genes such as PNPLA3 and TM6SF2, significantly influence susceptibility to MASLD. These genetic variants affect lipid metabolism, inflammation, and fibrosis in the liver. Overall, the pathogenesis of MASLD is regulated by a combination of genetic, metabolic, and environmental factors, making it a challenging disease to manage and treat.
EFFECTS OF FANLIAN HUAZHUO FORMULA TREATMENT IN MASLD, FUTURE QUESTIONS AND CONCLUSIONS
In a recent study, Niu et al[1] examined the therapeutic effects of Fanlian Huazhuo Formula (FLHZF) in regulating hepatocyte lipid accumulation, body weight changes, serum lipid profile, and the levels of various antioxidants in mice following high-fat diet (HFD) feeding (10 weeks). FLHZF is a Chinese herbal medicine comprising Psidium guajava L. (Fanshiliuye), Ficus simplicissima Lour. (Wuzhimaotao), Morus alba L. (Sangye), Codonopsis pilosula N. (Dangshen), Coptis chinensis Franch. (Huanglian), Atractylodes lancea (Thunb.) DC. (Cangzhu), Crataegus pinnatifda Bunge. (Shanzha), Euonymus alatus (Thunb.) Siebold (Guijianyu), Monascus purpureus Went. (Hongqu) and Zingiber officinale R. (Ganjiang)[25]. Its beneficial effects have been previously demonstrated in reducing fasting blood glucose and improving lipid metabolism in diabetic rodent models[25]. Moreover, various individual components of FLHZF possess therapeutic effects in regulating hepatic lipid metabolism, body weight, adipose tissue inflammation, hepatic injury, and gut microbiota[1]. However, most of these studies have primarily focused on analyzing its effects in diabetic rodents, and there is limited information on the benefits of this combination medicinal preparation in diet-induced MASLD. Firstly, Niu et al[1] utilizing HepG2, a human hepatocyte cell line, derived from a hepatocellular carcinoma patient showed that FLHZF treatment improved hepatocyte viability and reduced lipid accumulation following exposure to fatty acids as demonstrated by triglyceride quantitation and Oil Red O staining. Additionally, male C57 black/6 mice treated with low and high doses of FLHZF had reduced HFD-induced body weight gain, and liver and adipose tissue content compared with control mice, and these protective effects were comparable to fenofibrate, a drug used to treat hypertriglyceridemia, primary hypercholesterolemia, or mixed dyslipidemia. Moreover, FLHZF (high dose) improved serum lipid profile, reduced alanine transaminase and aspartate transferase levels, attenuated hepatosteatosis, and prevented oxidative stress. Mechanistically, FLHZF treatment inhibited de novo lipogenesis possibly by increasing adenosine-5’-monophosphate-activated protein kinase expression/activation and reducing the levels of lipogenic genes fatty acid synthase and ACC. Besides, FLHZF reduced HFD-induced hepatocyte death. Therefore, the study provides compelling in vitro and in vivo evidence of the therapeutic effects of FLHZF in MASLD. However, certain aspects of the study warrant future investigation to better understand the molecular mechanisms regulated by FLHZF during MASLD progression.
FLHZF is a combination of various herbal medicines known for their therapeutic effects in various metabolic diseases and contains numerous active components. It is important to identify which specific component(s) or herb(s) are primarily responsible for the protective effects of FLHZF against hepatosteatosis, and whether FLHZF offers better outcomes than any individual herb. The combination therapy may provide additive or synergistic effects, which requires further exploration. Previous studies have reported sex differences in the incidence and progression of MASLD in humans and murine models[26,27]. Notably, post-menopausal females have a higher prevalence of MASLD in comparison to pre-menopausal females, suggesting the potential role of sex hormones in the onset of the disease[28,29]. Therefore, future studies are required to investigate the effects of FLHZF treatment in female and aged mice to better understand its efficacy across different sexes and age groups. MASLD is often asymptomatic in its early stages and can occur in lean individuals with normal liver function, highlighting the unmet need for treatment strategies that control or reverse this disease. It would be valuable to determine FLHZF’s effects on the reversal of the disease in mice with established MASLD and in other murine models of MASLD such as methionine and choline-deficient diet, atherogenic Western diet, and various genetic models[30]. Given the close links among hepatosteatosis, insulin resistance, and liver fibrosis, investigating insulin and glucose tolerance, as well as hepatic fibrosis following FLHZF treatment, will provide deeper insights into its therapeutic benefits for MASH. Additionally, safety studies examining the effects of FLHZF treatment on clinical (urea, creatinine, Na, K, and others) and hematological (complete blood count) parameters, and the histology of vital organs are essential for its translational use. Establishing an appropriate in vitro cell culture model that closely mimics in vivo cell type/conditions for mechanistic and other lipid accumulation studies is very crucial. Previous studies have shown that primary hepatocytes and cell line HepG2 respond differently when exposed to fatty acids, glucose, fructose, and other chemicals[31,32]. It remains to be determined whether FLHZF induces similar beneficial effects in primary (human and mouse) hepatocytes when exposed to fatty acids. Additionally, this study paves the way for new research focusing on the effects of FLHZF on whole-body energy expenditure, hepatocyte mitochondrial function and biogenesis, hepatic inflammation, adipose tissue phenotype, gut microbiome, THR-β activation, and the pharmacokinetics of FLHZF.
