CHOLESTEROL 7-ALPHA-HYDROXYLASE - EVIDENCE FOR TRANSCRIPTIONAL REGULATION BY CHOLESTEROL OR METABOLIC PRODUCTS OF CHOLESTEROL IN THE RAT

Cholesterol 7alpha-hydroxylase, the rate-determining enzyme in the bil e acid biosynthesis pathway, is regulated in a negative feedback manne r by hydrophobic bile salts returning to the liver via the portal circ ulation. The role of cholesterol in the regulation of cholesterol 7alp ha-hydroxylase and the interrelationship between the cholesterol and b ile acid biosynthesis pathways remain controversial. The objective of the present study was to define the role of cholesterol in the regulat ion of cholesterol 7alpha-hydroxylase and determine the molecular leve l of its control. In order to avoid intestinal or intravenous administ ration of cholesterol, we manipulated the flow of cholesterol within t he hepatocytes by decreasing cholesterol synthesis with lovastatin in bile fistula rats (bile acid synthesis is up-regulated), or by increas ing cholesterol supply by administering mevalonate, a precursor of cho lesterol, to rats with intact enterohepatic circulation (bile acid syn thesis is normal). In the first series of studies, lovastatin was admi nistered as a single intravenous bolus (10 mg/kg) to rats with chronic bile fistula and to rats with intact enterohepatic circulation (chole sterol and bile acid synthesis is normal). Three hours after lovastati n administration, cholesterol 7alpha-hydroxylase specific activity, en zyme mass, mRNA, and gene transcriptional activity were decreased by 3 5%, 32%, 56%, and 34%, respectively, in rats with chronic bile fistula . In rats with intact enterohepatic circulation, lovastatin administra tion resulted in a similar decrease (34%) of cholesterol 7alpha-hydrox ylase specific activity. In the second group of experiments, rats with intact enterohepatic circulation were administered a 180 mum bolus of mevalonate followed by a continuous infusion of 180 mumol/h for 1.5, 3, 4.5, and 24 h prior to being killed. Continuous infusion of mevalon ate increased cholesterol 7-alpha-hydroxylase specific activity, mRNA levels, and transcriptional activity by an average of 2- to 3-fold at all time intervals. We conclude that under circumstances in which chol esterol is present in excess, cholesterol 7alpha-hydroxylase transcrip tional activity is up-regulated and removal of cholesterol from the he patocytes is facilitated by an increase of bile acid synthesis. When c holesterol availability is decreased, cholesterol 7alpha-hydroxylase t ranscriptional activity is down-regulated leading to a decreased elimi nation of cholesterol via bile acid synthesis. In both instances, hepa tic cholesterol homeostasis is effectively maintained.