Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in the rat ileum: Effects of bile acids and lovastatin

We investigated the effects of intestinal bile acid flux, orientation of the 7-hydroxy group, and administration of lovastatin on the regulation of intestinal 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity in the rat ileum. HMG-CoA reductase activities in villous and crypt cells from the ileal mucosa were similar, and the study was performed on whole mucosa that contained both cell types. Taurocholate feeding decreased ileal reductase activity 48%, whereas tauroursocholate, the 7β-hydroxy epimer of taurocholate, had no effect. Feeding lovastatin (inhibitor of HMG-CoA reductase) stimulated total ileal HMG-CoA reductase activity threefold in washed microsomes, which were dissociated from the inhibitor. However, the proportion of active enzyme in the ileum of lovastatin-fed rats was 50% lower than in controls, whereas there was no change in the percentage of expressed enzyme with bile acid treatments. Interruption of the enterohepatic circulation (bile fistula) increased HMG-CoA reductase activity in the ileum 73%. Duodenal infusion of taurocholate to bile-fistula rats significantly decreased microsomal HMG-CoA reductase activity in the ileal mucosa. In contrast, infusion of the 7β-hydroxy epimer tauroursocholate failed to inhibit the derepressed HMG-CoA reductase activity in the ileum of bile-fistula rats. The inhibition of intestinal HMG-CoA reductase activity by taurocholate occurred without accumulation of mucosal cholesterol. Furthermore, the stimulation of total ileal HMG-CoA reductase activity by lovastatin treatment was observed without a decrease in mucosal cholesterol. In summary, the regulation of ileal HMG-CoA reductase activity by the intestinal luminal flux of bile acids is dependent on the orientation of the hydroxyl groups. Taurocholate is a potent inhibitor, whereas the 7β-hydroxylated bile acid tauroursocholate, which is more hydrophilic, fails to repress intestinal HMG-CoA reductase activity. Lovastatin, a competitive inhibitor of HMG-CoA reductase, stimulates the formation of inactive reductase in the ileum without a change in enterocyte cholesterol concentration.