Role of gut microbiota and Toll-like receptors in nonalcoholic fatty liver disease

Emerging data have shown a close association between compositional changes in gut microbiota and the development of nonalcoholic fatty liver disease (NAFLD). The change in gut microbiota may alter nutritional absorption and storage. In addition, gut microbiota are a source of Toll-like receptor (TLR) ligands, and their compositional change can also increase the amount of TLR ligands delivered to the liver. TLR ligands can stimulate liver cells to produce proinflammatory cytokines. Therefore, the gut-liver axis has attracted much interest, particularly regarding the pathogenesis of NAFLD. The abundance of the major gut microbiota, including Firmicutes and Bacteroidetes, has been considered a potential underlying mechanism of obesity and NAFLD, but the role of these microbiota in NAFLD remains unknown. Several reports have demonstrated that certain gut microbiota are associated with the development of obesity and NAFLD. For instance, a decrease in Akkermansia muciniphila causes a thinner intestinal mucus layer and promotes gut permeability, which allows the leakage of bacterial components. Interventions to increase Akkermansia muciniphila improve the metabolic parameters in obesity and NAFLD. In children, the levels of Escherichia were significantly increased in nonalcoholic steatohepatitis (NASH) compared with those in obese control. Escherichia can produce ethanol, which promotes gut permeability. Thus, normalization of gut microbiota using probiotics or prebiotics is a promising treatment option for NAFLD. In addition, TLR signaling in the liver is activated, and its downstream molecules, such as proinflammatory cytokines, are increased in NAFLD. To data, TLR2, TLR4, TLR5, and TLR9 have been shown to be associated with the pathogenesis of NAFLD. Therefore, gut microbiota and TLRs are targets for NAFLD treatment.