Cholesterol 7α‐hydroxylase (CYP7A1) plays a critical role in preventing hepatic inflammation and fibrosis

Abstract

IntroductionCholesterol 7α‐hydroxylase (CYP7A1) is the rate‐limiting enzyme in the bile acid biosynthetic pathway in the liver and controls cholesterol and bile acid homeostasis. Accumulation of toxic bile acids in liver causes cholestasis and inflammatory liver injury. The mechanism of bile acid homeostasis in hepatic inflammation and fibrosis remains unclear.MethodsWe investigated the roles of CYP7A1 in hepatic inflammation and fibrosis using lipopolysaccharides (LPS)‐induced acute liver injury and methionine/choline deficient (MCD) diet‐induced liver injury models. Cyp7a1−/− mice in pure C57BL6 background, bile acid‐activated nuclear receptor farnesoid X receptor deficient (Fxr−/−) mice, and G protein‐coupled bile acid receptor deficient (Tgr5−/−) mice, and adenovirus‐mediated transduction of Cyp7a1 were used to study the roles of CYP7A1 and bile acid receptors in hepatic inflammation and fibrosis. Real‐time PCR, western blot, immunohistochemistry, luciferase report and chromatin immunoprecipitation (ChIP) assays were used in the study.ResultsAdenovirus‐mediated overexpression of CYP7A1 ameliorated LPS‐induced inflammatory cell infiltration and pro‐inflammatory cytokines production in WT and Tgr5−/−, but not Fxr−/− mice suggesting that FXR played a key role in protection against LPS‐induced hepatic inflammation. Luciferase report assay and real‐time PCR showed that FXR agonists, GW4064 or INT‐747 significantly inhibited NF‐κB activation and pro‐inflammatory cytokine mRNA expression in HepG2 cells and human primary hepatocytes. Mammalian two‐hybrid assay, co‐immunoprecipitation and ChIP assays showed that activation of FXR promotes FXR interaction with p65, which in turn blocked p65 binding to NF‐κB response elements on the TNF‐α and IL‐1β gene promoters. Serum AST and ALT levels were significantly increased in the liver of Cyp7a1−/− mice fed with MCD diet compared with WT mice. Pro‐inflammatory cytokine mRNA expression and immunohistochemistry of α‐SMA showed that Cyp7a1−/− mice fed with MCD diet developed more severe hepatic inflammation and fibrosis than WT mice. Cholesterol is aggravated in Cyp7a1−/− mice fed with MCD diet, and may increase reactive oxidizing species and cell death in the liver.ConclusionThese data suggest that increased bile acids ameliorated hepatic inflammation through activation of FXR but not TGR5 signaling. On the other hand, reduced bile acid pool increased susceptibility to MCD diet‐induced hepatic inflammation and fibrosis. Maintaining bile acid homeostasis is important in protection against liver injury and non‐alcoholic fatty liver disease.