Differential feedback regulation of cholesterol 7α-hydroxylase mRNA and transcriptional activity by rat bile acids in primary monolayer cultures of rat hepatocytes

We have used primary monolayer cultures of rat hepatocytes to study the effects of physiological concentrations of various bile acids, commonly found in bile of normal rats, on the mechanism of regulation of cholesterol 7α-hydroxylase and bile acid synthesis. Addition of taurocholic acid, the most predominant bile acid in rat bile, to the culture medium suppressed cholesterol 7α-hydroxylase activity and mRNA time- and dose-dependently. The decrease in enzyme activity paralleled the changes in mRNA. Maximal suppression of cholesterol 7α-hydroxylase mRNA (-91%) and enzyme activity (-89%) was observed after a 16 h incubation period with 50 uM taurocholic acid. The declines in mRNA and enzyme caused by taurocholic acid were tightly coupled and followed first-order kinetics with a half-life of 4 h. Transcriptional activity, as assessed with nuclear run-on assays, was decreased by 44% at 50 μM taurocholic acid. Mass production of bile acids (chenodeoxycholic acid and β-muricholic acid) was inhibited to a similar extent as the cholesterol 7α-hydroxylase when different concentrations of taurocholic acid were used, giving maximal inhibition (-81%) at 50 μM taurocholic acid. Glycocholic acid and unconjugated cholic acid were equally effective as taurocholic acid in suppressing cholesterol 7α-hydroxylase mRNA. The more hydrophobic bile acids (chenodeoxycholic acid and deoxycholic acid) showed profound suppression of the cholesterol 7α-hydroxylase mRNA by 85% and 75% respectively, whereas the other trihydroxy bile acids in rat bile, α- and β-muricholic acid, were not or only marginally active. We conclude that rat bile acids, in particular the more hydrophobic ones, in concentrations commonly observed in portal blood, exert negative feedback control at the level of cholesterol 7α-hydroxylase mRNA in cultured rat hepatocytes through a direct effect on the hepatocytes, and that down-regulation of transcription is only one of the mechanisms involved in this regulation. Chemicals/CAS: chenodeoxycholic acid, 474-25-9; cholesterol 7alpha monooxygenase, 9037-53-0; cholic acid, 32500-01-9, 361-09-1, 81-25-4; deoxycholic acid, 83-44-3; glycocholic acid, 475-31-0; taurocholic acid, 145-42-6, 59005-70-8, 81-24-3; Bile Acids and Salts; Chenodeoxycholic Acid, 474-25-9; Cholesterol 7-alpha-Hydroxylase, EC 1.14.13.17; Cholic Acid, 81-25-4; Cholic Acids; Glycocholic Acid, 475-31-0; muricholic acid, 39016-49-4; RNA, Messenger; Taurocholic Acid, 81-24-