EFFECTS OF DIFFERENT BILE-SALTS ON STEADY-STATE MESSENGER-RNA LEVELS AND TRANSCRIPTIONAL ACTIVITY OF CHOLESTEROL 7-ALPHA-HYDROXYLASE

Cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in the bile a cid synthesis pathway, is down-regulated by taurocholate by way of neg ative feedback control at the level of gene transcription. The molecul ar basis of regulation of cholesterol 7alpha-hydroxylase by other hydr ophobic bile salts and under more physiological conditions is not know n. The aim of this study was to investigate the molecular basis of reg ulation of cholesterol 7alpha-hydroxylase by several naturally occurri ng bile salts in rats with intact enterohepatic circulation. Male Spra gue-Dawley rats were pair-fed for 14 days normal chow (control), chole styramine (5% of diet), cholic acid (1%), chenodeoxycholic acid (1%) o r deoxycholic acid (0.25%). When rats were killed, livers were harvest ed and HMG-CoA reductase specific activity and cholesterol 7alpha-hydr oxylase specific activities, steady-state mRNA levels and transcriptio nal activity were determined and compared with those of control rats f ed normal chow. Compared with results in paired controls, cholestyrami ne feeding led to an approximate threefold increase in HMG-CoA reducta se specific activity. Feeding of hydrophobic bile salts profoundly dec reased the specific activity of HMG-CoA reductase. Cholestyramine led to a three-fold increase in cholesterol 7alpha-hydroxylase specific ac tivity, steady-state mRNA levels and gene transcriptional activity. Th e feeding of cholic (1%), chenodeoxycholic (1%) and deoxycholic acid ( 0.25%) led to significant decreases in cholesterol 7alpha-hydroxylase specific activities (62%, 84% and 97%, respectively), steady-state mRN A levels (72%, 29% and 61%, respectively) and transcriptional activiti es (44%, 43% and 54%, respectively). Down-regulation of cholesterol 7a lpha-hydroxylase specific activity was in order of increasing hydropho bicity of bile salts (cholic < chenodeoxycholic < deoxycholic acid). N o such clear correlation was observed between bile salt hydrophobicity and steady-state mRNA levels or gene transcriptional activity. We con clude that down-regulation of cholesterol 7alpha-hydroxylase activity by cholic, chenodeoxycholic and deoxycholic acids occurred as the resu lt of decreased transcriptional activity of the cholesterol 7alpha-hyd roxylase gene. Because chenodeoxycholic and deoxycholic acids led to g reater fractional suppression of cholesterol 7alpha-hydroxylase specif ic activity than of gene transcriptional activity, we postulate the ex istence of posttranscriptional regulation of cholesterol 7alpha-hydrox ylase by these two hydrophobic bile salts.