Nonalcoholic Steatohepatitis: Role of Gut Microbiota is Dependent on Lipopolysaccharide-Toll-like Receptor-4 Axis

Hepatic steatosis is the hepatic manifestation of metabolic syndrome which is increasingly becoming a health problem worldwide, especially in the western hemisphere. Hepatic steatosis is benign, but long standing hepatic steatosis can lead to non-alcoholic steatohepatitis (NASH). NASH is a form of nonalcoholic fatty liver disease where excessive fat accumulation in the liver leads to chronic inflammation of a patient without any history of alcohol abuse. However, the mechanism underlying the progression of hepatic steatosis to NASH is unclear. Role of gut microbiota in metabolic syndrome has long been reported. Here, we hypothesized that gut microbiota plays an important role in the modulation of NASH through the involvement of lipopolysaccharide (LPS)-toll-like receptor-4 (TLR-4) pathway in dietary fat mediated hepatic steatosis. To test this hypothesis, germ free or broad-spectrum antibiotics-treated mice were fed high fat diet, which resulted in decreased level of inflammation in their fatty liver compared to the specific pathogen free control mice. This result demonstrated the involvement of gut microbiota in mediating NASH. To address the role of LPS-TLR-4 pathway, broad spectrum antibiotics-treated mice fed high fat diet were injected i.p. with low dose LPS, resulting in an increased level of inflammation in the liver which was decreased upon the co-treatment with TAK-242, an inhibitor of TLR-4. To investigate the role of TLR-4- expressing kupffer cells in mediating NASH, wild-type kupffer cells in the liver of wild-type mice were replaced with TLR-4 KO kupffer cells by bone marrow transplantation, which resulted in a decreased level of inflammation in liver upon NASH induction. It was also investigated whether the peroxisomal anti-oxidative function is altered in the livers of mice fed high fat diet in a TLR-4 dependent manner. Indeed, the level and function of catalase, the principal antioxidative enzyme in peroxisomes, was decreased in mice fed high fat diet, which was reversed in TLR-4 KO mice fed high fat diet. This individual piece of data demonstrated the role of TLR-4 pathway in modulating NASH through the alteration of peroxisomal anti-oxidative function. In conclusion, this project established the role of gut microbiota in modulating NASH which is dependent on LPS-TLR-4 pathway