Non-alcoholic fatty liver disease (NAFLD), commonly known as fatty liver, is a disease characterized by an abnormal accumulation of fats [triglycerides (TGs)] inside liver cells (1). Thus far, the prevalence of NAFLD has consistently increased with lifestyle changes (2). Steatohepatitis is histologically characterized by a significant accumulation of hepatic lipids and lobular necro-inflammation in NAFLD, which may be progressive and eventually induce liver fibrosis and cirrhosis (3,4).

Although much progress has been made in recent years, the pathogenesis of steatohepatitis has not been fully elucidated. It is known that defects in fat metabolism are responsible for the pathogenesis of NAFLD, which may be due to an imbalance in lipid storage and consumption in hepatocytes. Peroxisome proliferator-activated receptor-γ (PPARγ) signaling is important in hepatic fat accumulation, transport and utilization (5). PPARγ, also known as the thiazolidines ligand, plays a critical role in regulating fatty acid storage and metabolism. A previous study identified that PPARγ exhibited an anti-steatohepatitic effect by reducing the production of hepatic pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) (6). Furthermore, the activation of PPARγ may inhibit inflammatory responses by preventing the activation of nuclear transcription factors, such as nuclear factor-κB (NF-κB) (7). For these reasons, PPARγ may be a potential therapeutic target in the treatment of steatohepatitis. Additional clinical studies have demonstrated that PPARγ agonists, such as thiazolidinediones (TZDs), not only increase insulin sensitivity in adipose, liver and skeletal muscle tissue, but also affect hepatic fat accumulation by enhancing fatty acid oxidation (8,9). Despite their validated efficacy and widespread use, TZDs possess a number of side-effects, including significant weight gain and peripheral edema (10,11). In serious cases, TZDs may result in severe hepatotoxicity (12). In this regard, it is necessary to develop novel agents that target PPARγ, but with reduced adverse effects.

Medicinal plants, also called herbal medicines, have been traditionally used for treating liver disease worldwide (13). There are numerous herbal products that are believed to have therapeutic benefit on NAFLD, such as silymarin (milk thistle), glycyrrhizin (licorice root extract) and curcumin (turmeric extract) (14). Among them, curcumin (Fig. 1B) is the best known and has been used for centuries in Asia as a dietary spice, a food coloring and a treatment for inflammation, wounds, and gastrointestinal and liver disorders (15). A previous study demonstrated that curcumin may alleviate steatohepatitis and inhibit liver fibrogenesis (16). The possible mechanisms of curcumin may involve the stimulation of PPARγ activity and the inhibition of hepatic stellate cell (HSC) activation (17).

Previous studies have investigated the anti-oxidative and anti-inflammatory activities effects of puerarin (Fig. 1A) (18). Puerarin has been shown to exhibit therapeutic effects on NAFLD by acting as an antioxidant, lowering cholesterol levels and improving leptin signal transduction (19,20). In the present study, the intervening actions of puerarin and curcumin were compared on mice models of steatohepatitis induced by a methionine- and choline-deficient (MCD) diet. With regard to previous studies concerning the roles of curcumin and puerarin in PPARγ signaling, it may be hypothesized that curcumin and puerarin have potential as therapeutic agents for the treatment of steatohepatitis.

Nonalcoholic fatty liver disease (NAFLD) is a multi-factorial disorder resulting from a variety of genetic and environmental factors. At present, the pathogenesis of NAFLD is not fully understood and therapeutic clinical trials are ongoing (22). Although lipid accumulation in the liver is the major hallmark of NAFLD, the mechanisms resulting in steatohepatitis remain elusive (23). Researchers have suggested the ‘2-hit’ hypothesis to explain NAFLD pathogenesis (24,4). Briefly, the ‘first hit’ involves hepatic TG accumulation or steatosis. The ‘second hit’ relates to the induction of inflammatory cytokines and oxidative stress. Hepatic TG accumulation may occur as a result of increased fat synthesis, increased fat delivery, decreased fat export and/or decreased fat oxidation (25). To a certain degree, PPARγ is important for the ‘first hit’ stage (25). Inflammatory cytokines, such as TNF-α and IL-6, are involved in the ‘second hit’ stage, which mediates steatohepatitis in patients with NAFLD (26). TNF-α, a pro-inflammatory cytokine, not only promotes insulin resistance, but also mediates cholesterol and TG metabolism (27). Similarly, the involvement of IL-6 has also been identified in animal models of and patients with NAFLD, with elevated serum levels of IL-6 being correlated with increasing steatohepatitis (28). It is worth stressing that the elevated expression levels of TNF-α and IL-6 were mediated by activation of the NF-κB signaling pathway (29). NF-κB, a critical transcription factor, may promote hepatic steatosis, hepatic injury and fibrosis by upregulating serum and hepatic levels of TNF-α and IL-6 (30). An increasing number of studies have suggested that PPARγ links the NF-κB signaling pathway through being mediated by regulating adipokines via controlled by suppressing TNF-alpha and IL-6, which closely connect to NF-κB and is critical for the treatment of NAFLD (6,7,31).

In the present study, mice models of steatohepatitis induced by a MCD diet were employed to compare the efficacies of puerarin and curcumin against steatohepatitis. The MCD diet-induced animal model is an internationally-recognized model for the study of the inflammation and fibrosis associated with NAFLD (32). Methionine and choline are precursors of phosphatidylcholine (PC). PC is an essential substrate for very low density lipoproteins (VLDL). Deficiency of methionine and choline may reduce VLDL production or secretion, which limits lipid packaging and export (33,34).

As has been observed in previous studies (32,35), the MCD was noted to increased plasma TNF-α and IL-6 levels, increase hepatic tissue NF-κB expression, but decrease hepatic tissue PPARγ expression. The present study also demonstrated that there were no noticeable changes in the plasma TG levels of mice fed an MCD (32). However, the present study identified that puerarin and curcumin had promising hepatoprotective and anti-steatohepatitis activities. In addition, curcumin significantly reduced serum levels of TNF-α and IL-6, and hepatic tissue levels of NF-κB and PPARγ. The effects of puerarin differed from those of curcumin. Puerarin downregulated the serum levels of TNF-α and hepatic tissue levels of PPARγ, but demonstrated no significant effects on the levels of IL-6 and NF-κB. Moreover, compared with curcumin, puerarin indicated notable anti-hyperlipidemic effects (Fig. 2A and B). These results suggest that curcumin and puerarin affect NAFLD by different mechanisms. It is hypothesized that puerarin may be involved in the early pathological stage (first hit) through regulating the lipid metabolism mediated by PPARγ. By contrast, curcumin may impact multiple nodes, particularly the inflammatory response stage (second hit). These results suggest a novel strategy for preventing NAFLD and for the development of novel agents with anti-steatohepatitis effects.

In conclusion, curcumin and puerarin induce favorable effects on steatohepatitis through different mechanisms. Puerarin may regulate lipid metabolism in the ‘first hit’ stage through the PPARγ pathway, whereas curcumin may inhibit the inflammatory response in the ‘second hit’ stage through the PPARγ/NF-κB pathway. Further experiments focusing on the molecular mechanisms of curcumin and puerarin using different blocking agents and advanced experimental techniques are required.