Regulation of oxysterol 7α‐hydroxylase (CYP7B1) in primary cultures of rat hepatocytes

Conversion of cholesterol into 7α‐hydroxylated bile acids is a principal pathway of cholesterol disposal. Cholesterol 7α‐hydroxylase (CYP7A1) is the initial and rate‐determining enzyme in the “classic” pathway of bile acid synthesis. An “alternative” pathway of bile acid synthesis is initiated by sterol 27‐hydroxylase (CYP27) with subsequent 7α‐hydroxylation of 27‐hydroxycholesterol by oxysterol 7α‐hydroxylase (CYP7B1). The regulation of CYP7B1, possibly a rate‐determining enzyme in the alternative pathway, has not been thoroughly studied. The aims of this study were to (1) study the regulation of liver CYP7B1 by bile acids, cholesterol, adenosine 3', 5'‐cyclic monophosphate (cAMP), and phorbol myristate acetate (PMA) in primary rat hepatocytes and (2) determine the effect of CYP7B1 overexpression on rates of bile acid synthesis. The effects of different bile acids (3‐150 μmol/L), cAMP (50 μmol/L), PMA (100 nmol/L; protein kinase C stimulator), cholesterol (200 μmol/L), and squalestatin (1 μmol/L; cholesterol synthesis inhibitor) on CYP7B1 expression in primary rat hepatocytes were studied. Taurocholic acid and taurodeoxycholic acid decreased CYP7B1 activity by 45% ± 10% and 36% ± 7%, respectively. Tauroursodeoxycholic acid and taurochenodeoxycholic acid did not alter CYP7B1 activity. Inhibition of cholesterol synthesis with squalestatin decreased CYP7B1 activity by 35%, whereas addition of cholesterol increased activity by 39%. Both PMA and cAMP decreased CYP7B1 activity by 60% and 34%, respectively, in a time‐dependent fashion. Changes in CYP7B1 messenger RNA (mRNA) levels correlated with changes in specific activities. Overexpression of CYP7B1 led to a marked increase in CYP7B1 mRNA levels and specific activity but no change in rates of bile acid synthesis. In conclusion, in the rat, CYP7B1 specific activity is highly regulated but does not seem to be rate limiting for bile acid synthesis.