Targeting the Enterohepatic Bile Acid Signaling to Modulate Hepatic Autophagic Activity in Non‐alcoholic steatohepatitis

Abstract

Non‐alcoholic steatohepatitis is associated with hepatic free cholesterol accumulation and defective autophagy activity, which sensitize hepatocytes to injury and inflammation. Bile acid synthesis is the only quantitatively significant mechanism for hepatic cholesterol catabolism. Stimulating bile acid synthesis prevented hypercholesterolemia and hepatic steatosis in mice. Autophagy delivers intracellularly stored cholesterol to the lysosome where cholesterol ester is hydrolyzed for subsequent cellular distribution, efflux and bile acid synthesis. The objective of this study is to investigate the molecular link between cholesterol and bile acid metabolism and autophagic activity in hepatocytes and in mouse models of fatty liver disease. Confocal microscopy, immunoblot and cathespin B activity assay showed that excessive intracellular free cholesterol accumulation caused by free cholesterol loading and inhibition of cholesterol esterification in the endoplasmic reticulum (ER) with an ACAT inhibitor blocked autophagosome/lysosome fusion and impaired lysosome function in cholesterol‐laden HepG2 cells. In contrast, cholesterol ester accumulation induced by purified human low‐density lipoprotein treatment increased lysosome function without inhibiting autophagy activity. In addition, bile acid treatment blocked autophagosome/lysosome fusion without affecting key autophagy gene expression in HepG2 cells. Interestingly, over‐expression of CYP7A1 by adenoviral gene delivery, which induced ER cholesterol catabolism and caused relative ER cholesterol depletion, strongly induced autophagy flux in both human hepatocytes and HepG2 cells. Based on this novel regulatory relationship between ER cholesterol sensing and autophagy activity, confocal microscopy, electron microscopy and immunoblot further revealed that cholestyramine administration strongly induced hepatic autophagy in lean mice, and restored hepatic autophagy activity and improved hepatic steatosis in obese mice. The cholestyramine‐mediated hepatic autophagy induction can be attributed to CYP7A1‐mediated ER cholesterol catabolism and decreased hepatic bile acid signaling. In conclusion, these results suggest that hepatic autophagy is regulated by an intracellular cholesterol sensing mechanism, and is an integral part of the complex regulatory network that maintains cholesterol homeostasis in hepatocytes. This study supports a new concept of targeting the enterohepatic bile acid‐CYP7A1‐cholesterol‐autophagy axis to selectively modulate hepatic autophagy activity in fatty liver disease.Support or Funding InformationNIDDK and NIAAA