Insulin dysregulation drives mitochondrial cholesterol metabolite accumulation: initiating hepatic toxicity in nonalcoholic fatty liver disease

CYP7B1 catalyzes mitochondria-derived cholesterol metabolites such as (25R)26-hydroxycholesterol (26HC) and 36-hydroxy-5-cholesten-(25R)26-oic acid (36HCA) and facilitates their conversion to bile acids. Disruption of 26HC/ 36HCA metabolism in the absence of CYP7B1 leads to neonatal liver failure. Disrupted 26HC/36HCA meta-bolism with reduced hepatic CYP7B1 expression is also found in nonalcoholic steatohepatitis (NASH). The current study aimed to understand the regulatory mechanism of mitochondrial cholesterol metabolites and their contribution to onset of NASH. We used Cyp7b1-/-mice fed a normal diet (ND), Western diet (WD), or high-cholesterol diet (HCD). Serum and liver cholesterol metabolites as well as hepatic gene ex-pressions were comprehensively analyzed. Interest-ingly, 26HC/36HCA levels were maintained at basal levels in ND-fed Cyp7b1-/-mice livers by the reduced cholesterol transport to mitochondria, and the upre-gulated glucuronidation and sulfation. However, WD -fed Cyp7b1-/-mice developed insulin resistance (IR) with subsequent 26HC/36HCA accumulation due to overwhelmed glucuronidation/sulfation with facili-tated mitochondrial cholesterol transport. Meanwhile, Cyp7b1-/-mice fed an HCD did not develop IR or subsequent evidence of liver toxicity. HCD-fed mice livers revealed marked cholesterol accumulation but no 26HC/36HCA accumulation. The results suggest 26HC/36HCA-induced cytotoxicity occurs when increased cholesterol transport into mitochondria is coupled to decreased 26HC/36HCA metabolism driven with IR. Supportive evidence for cholesterol metabolite-driven hepatotoxicity is provided in a diet -induced nonalcoholic fatty liver mouse model and by human specimen analyses. This study uncovers an insulin-mediated regulatory pathway that drives the formation and accumulation of toxic cholesterol metabolites within the hepatocyte mitochondria, mechanistically connecting IR to cholesterol metabolite-induced hepatocyte toxicity which drives nonalcoholic fatty liver disease.

Automated summary

CYP7B1 catalyzes the metabolism of cholesterol metabolites and its disrupted expression can lead to liver failure in newborns and nonalcoholic steatohepatitis. This study investigates the regulatory mechanism of mitochondrial cholesterol metabolites and their contribution to the onset of NASH.