The role of non-esterified cholesterol concentration in endoplasmic-reticular membranes in the regulation of hydroxymethylglutaryl-CoA reductase.

initially high but declined as HMG-CoA reductase became activated. Glucagon prevented the loss of activity, whereas insulin promoted inactivation of the enzyme. Similar affects on the activity of HMG-CoA reductase kinase. and the state of phosphorylation of HMG-CoA reductase in liver have been reported after injection of rats with glucagon (Beg el al., 1979). Thus insulin and glucagon have concerted effects on the activities of both HMG-CoA reductase and HMG-CoA reductase kinase. In liver. the binding of glucagon to specific cell-surface receptors results in the activation of adenylate cyclase and an elevation in cytoplasmic cyclic A M P concentrations. The resulting effects of the increase in cyclic A M P are thought t o be a consequence of the phosphorylation of a variety of cellular proteins by an enzvme termed cyclic AMP-dependent protein kinase (EC 2.7.1.37) (Greengard. 1978). Although the activation of this protein kinase I S likely to underlie the effects of glucagon on the HMG-CoA reductase phosphorylation system, both HMG-CoA reductase kinase and HMG-CoA reductase kinase appear to be distinct from cyclic AMP-dependent protein kinase (Ingebritsen et al.. 1981). Another mechanism by which glucagon could regulate the phosphorylation of HMG-CoA reductase and HMG-CoA reductase kinase is through the phosphorylation of inhibitor1 by cyclic AMP-dependent protein kinase (Fig. 2). In principle, this would prevent the dephosphorylation of both HMG-CoA reductase and HMGCoA reductase kinase by protein phosphatase 1 and could account for the observed changes in the state of phosphorylation of the two enzymes. A complication with this hypothesis. however, arises from the fact that protein phosphatases 2A and 2C are also active on HMG-CoA reductase and HMG-CoA reductase kinase (lngebritsen & Cohen, 1983). Thus further experiments are necessary to establish the possible roles of protein phosphatases 2A and 2C in regulating the reductase system. In summary. it is now clear that the phosphorylation of HMG-CoA reductase and HMG-CoA reductase kinase underlies the acute regulation of hepatic cholesterol synthesis by glucagon and insulin. With glucagon. it is likely that cyclic AMP-dependent protein kinase, is involved, but the precise mechanism remains to be established. Insulin is known to promote the dephosphorylation of a number of important regulatory enzymes in addition to HMG-CoA reductase and HMG-CoA reductase kinase. However, the mechanism of insulin action in liver and other tissues remains a major unsolved problem. It is noteworthy, however, that insulin appears to stimulate liver protein phosphatase activity (Curnow & Larner. 1979). and it is possible that this stimulation may be involved in the regulation of HMG-CoA reductase and HMG-CoA reductase kinase by this hormone.