SUPPRESSION OF STEROL 27-HYDROXYLASE MESSENGER-RNA AND TRANSCRIPTIONAL ACTIVITY BY BILE-ACIDS IN CULTURED RAT HEPATOCYTES

In previous work we have demonstrated suppression of cholesterol 7 alp ha-hydroxylase by bile acids at the level of mRNA and transcription, r esulting in a similar decline in bile acid synthesis in cultured rat h epatocytes [Twisk, Lehmann and Princen (1993) Biochem. J. 290, 685-691 ]. In view of the substantial contribution of the 'alternative' or '27 -hydroxylase' route to total bile acid synthesis, as demonstrated in c ultured rat hepatocytes and in vivo in humans, we here evaluate the ef fects of various bile acids commonly found in bile of rats on the regu lation of sterol 27-hydroxylase in cultured rat hepatocytes. Addition of taurocholic acid, the predominant bile acid in rat bile, to the cul ture medium of rat hepatocytes resulted in a 72% inhibition of sterol 27-hydroxylase activity. The effect was exerted at the level of sterol 27-hydroxylase mRNA, showing a time- and dose-dependent decline with a maximal suppression (-75%) at 50 mu M taurocholic acid after 24 h of culture. The decline in mRNA followed first-order kinetics with an ap parent half-life of 13 h. Under these conditions cholesterol 7 alpha-h ydroxylase mRNA (-91%) and bile acid synthesis (i.e. chenodeoxycholic and beta-muricholic acid, -81%) were also maximally suppressed. In con trast, no change was found in the level of lithocholic acid 6 beta-hyd roxylase mRNA. Assessment of the transcriptional activity of a number of genes involved in routing of cholesterol towards bile acids showed similar suppressive effects of taurocholate on expression of the stero l 27-hydroxylase and cholesterol 7 alpha-hydroxylase genes (-43% and - 42% respectively), whereas expression of the lithocholic 6 beta-hydrox ylase gene was not affected. Taurocholic acid and unconjugated cholic acid were equally as effective in suppressing sterol 27-hydroxylase mR NA. The more hydrophobic bile acids, chenodeoxycholic acid and deoxych olic acid, also produced a strong inhibition of 57%, and 76% respectiv ely, whereas the hydrophilic beta-muricholic acid was not active. We c onclude that (1) a number of bile acids, at physiological concentratio ns, suppress sterol 27-hydroxylase by down-regulation of sterol 27-hyd roxylase mRNA and transcriptional activity and (2) co-ordinated suppre ssion of both sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylas e results in inhibition of bile acid synthesis in cultured rat hepatoc ytes.