Microbiota Derived Metabolites as Modulators of Inflammation in Non-alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is the most prevalent chronic liver disease in western countries and characterized by a spectrum of liver injuries ranging from simple steatosis to steatohepatitis (NASH) with or without fibrosis. Among the patients with NAFLD, about 10-20% of them have NASH and may eventually progress to fibrosis, cirrhosis and liver carcinoma. Accumulating evidences indicate gut microbiota plays an important role in NAFLD and different bacterial taxa have been correlated to disease. However, the specific metabolites through which the microbiota exert their effects and the target pathways in host cells are not fully understood. Our previous work demonstrated that intestinal microbiota are capable of producing bioactive molecules that engage host cellular pathway. The goal of this study is to investigate the hypothesis that gut microbiota derived metabolites could directly act on the liver macrophages to modulate inflammation and this affects the progression of NAFLD. Utilizing liquid chromatography-mass spectrometry metabolomics we identify two tryptophan-derived metabolites – tryptamine (TA) and indole-3-acetate (I3A) – that require the microbiota and are depleted in high-fat diet mice compared to low fat diet mice. Both I3A and TA reduced palmitate and LPS induced production of pro-inflammatory cytokines (TNFα, IL-1β and MCP-1) in macrophages, as well as inhibited macrophage migration toward the chemokine MCP-1. We identified that both palmitate and LPS reduced levels of p-AMPK and the p-AMPK activator AICAR reduced palmitate and LPS induced inflammatory cytokine production. The addition of I3A and TA reversed p-AMPK reduction induced by palmitate and LPS. siRNA knockdown of AMPK in macrophages partially abrogated I3A and TA’s anti-inflammatory effects suggests that I3A and TA’s effect is dependent on AMPK signaling. In summary, these studies demonstrated that gut microbiota derived metabolites can modulate inflammatory responses in macrophages by engaging the AMPK signaling pathway