Transport of Cholesterol into Mitochondria Is Rate-limiting for Bile Acid Synthesis via the Alternative Pathway in Primary Rat Hepatocytes

Bile acid synthesis occurs mainly via two pathways: the “classic” pathway, initiated by microsomal cholesterol 7α-hydroxylase (CYP7A1), and an “alternative” (acidic) pathway, initiated by sterol 27-hydroxylase (CYP27). CYP27 is located in the inner mitochondrial membrane, where cholesterol content is very low. We hypothesized that cholesterol transport into mitochondria may be rate-limiting for bile acid synthesis via the “alternative” pathway. Overexpression of the gene encoding st eroidogenic a cute r egulatory (StAR) protein, a known mitochondrial cholesterol transport protein, led to a 5-fold increase in bile acid synthesis. An increase in StAR protein coincided with an increase in bile acid synthesis. CYP27 overexpression increased bile acid synthesis by