In vivo and in vitro studies on the regulatory link between 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase in rat liver

The activities of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCoA reductas e; EC 1.1.1.34), rate-limiting enzyme of cholesterol biosynthesis, and chol esterol 7 alpha-hydroxylase (EC 1.14.13.17), key enzyme of the neutral bile acid synthesis pathway, were measured in the microsomal fraction of rat li ver and in rat liver cells to investigate the coordinate regulation of the two pathways. Both enzyme activities exhibited the same diurnal rhythm and responded in a coordinate fashion to fasting or bile acid-feeding (decrease) and to chole styramine-feeding (increase). Cholesterol-feeding decreased the activity of HMGCoA reductase, increased that of cholesterol 7 alpha-hydroxylase, and c oncomitantly increased free cholesterol in microsomes. In an ex vivo setting using primary hepatocytes from animals fed a high cho lesterol diet the activity of HMGCoA reductase was initially low and that o f cholesterol 7 alpha-hydroxylase was elevated. Release of cholesterol into the medium with ongoing incubation caused HMGCoA reductase activity to inc rease, and that of cholesterol 7a-hydroxylase to decline. Incubation of hep atocytes with a cholesterol-containing lipoprotein fraction stimulated the activity of cholesterol 7a-hydroxylase, but left HMGCoA reductase activity unaffected. The results confirm the idea of a joint regulation of the two key enzymes o f cholesterol metabolism in response to the levels of substrate and metabol ites, and support the notion that with respect to bile acid and cholesterol levels, respectively, regulation of HMGCoA reductase activity may be secon dary to that of cholesterol 7a-hydroxylase. The in vitro studies supply evi dence that the effects of cholesterol and bile acid excess or deficiency ar e direct and do not involve accessory changes of hormone levels or mediator s.