Bile acid synthesis. VI. Regulation of cholesterol 7 alpha-hydroxylase by taurocholate and mevalonate.

Taurocholate, a relatively hydrophobic bile salt, is a potent down-regulator of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase (C7 alpha H), the rate-determining enzymes of the cholesterol and bile acid biosynthetic pathways, respectively. Inhibition of cholesterol synthesis with a bolus dose of mevinolin (lovastatin) a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, profoundly decreases the specific activity of cholesterol 7 alpha-hydroxylase and rate of bile acid synthesis in rats with complete biliary diversion. It is therefore conceivable that taurocholate may suppress cholesterol 7 alpha-hydroxylase primarily by down-regulating the activity of HMG-CoA reductase. To test this hypothesis, taurocholate was coinfused simultaneously to rats with chronic bile fistula with mevalonate (administered as mevalonolactone), an intermediate in the cholesterol biosynthetic pathway. Mevalonolactone was administered to provide a constant supply of newly synthesized cholesterol to cholesterol 7 alpha-hydroxylase, in order to overcome any inhibitory effect of taurocholate on HMG-Coa reductase. Infusions were started 72 h after biliary diversion, and carried out for an additional 48 h. Complete biliary diversion resulted in an increase in C7 alpha H specific activity (510%), protein mass (550%), steady-state mRNA levels (1430%), and transcriptional activities (330%) as compared to control rats with intact enterohepatic circulations. When rats with biliary diversion were infused intraduodenally with taurocholate, the specific activities of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities decreased by 75% (P less than 0.001) and 73% (P less than 0.001), respectively. Cholesterol 7 alpha-hydroxylase mass, mRNA, and transcriptional activity decreased after intraduodenal infusion of taurocholate to levels similar to those of rats with an intact enterohepatic circulation. The combination of constant infusion of mevalonate and taurocholate failed to reverse the inhibitory effects of taurocholate on cholesterol 7 alpha-hydroxylase activity, mRNA levels, and in vitro transcriptional rates. These data provide evidence that taurocholate represses cholesterol 7 alpha-hydroxylase at the level of gene transcription, and not via down-regulation of HMG-CoA reductase. Infusion of mevalonate alone to biliary diverted rats did not alter cholesterol 7 alpha-hydroxylase activity or mRNA levels, while leading to a 57% decrease in C7 alpha H gene transcription. This latter finding suggests that mevalonate or its metabolites may be capable of stabilizing C7 alpha H mRNA levels while down-regulating transcriptional activity.