Gut Microbiota-Associated Activation of TLR5 Induces Apolipoprotein A1 Production in the Liver.

Rationale: Dysbiosis of gut microbiota plays an important role in cardiovascular diseases but the molecular mechanisms are complex. An association between gut microbiome and the variance in HDL-cholesterol (HDL-C) level was suggested in a human study. Besides, dietary fat was shown to increase both HDL-C and LDL-C levels. We speculate that certain types of gut bacteria responding to dietary fat may help to regulate HDL-C level and potentially affect atherosclerotic development. Objective: We aimed to investigate whether and how high-fat diet (HFD)-associated gut microbiota regulated HDL level. Methods and Results: We found that HFD increased gut flagellated bacteria population in mice. The increase in HDL-C levels was adopted by mice receiving fecal microbiome transplantion from HFD-fed mouse donors. HFD led to increased hepatic but not circulating flagellin, and deletion of TLR5, a receptor sensing flagellin, suppressed HFD-stimulated HDL-C and apolipoprotein A1 (ApoA1) levels. Overexpression of TLR5 in the liver of TLR5-knockout mice was able to partially restore the production of ApoA1 and HDL-C levels. Mechanistically, TLR5 activation by flagellin in primary hepatocytes stimulated ApoA1 production through the transcriptional activation responding to the binding of NFkappaB on Apoa1 promoter region. Furthermore, oral supplementation of flagellin was able to stimulate hepatic ApoA1 production and HDL-C level, and decrease atherosclerotic lesion size in apolipoprotein E-deficient (Apoe-/-) mice without triggering hepatic and systemic inflammation. The stimulation of ApoA1 production was also seen in human ApoA1 transgenic mice treated with oral flagellin. Conclusions: Our finding suggests that commensal flagellated bacteria in gut can facilitate ApoA1 and HDL productions in liver through activation of TLR5 in hepatocytes. Hepatic TLR5 may be a potential drug target to increase ApoA1.