Treatment of CHO-K1 cells with 25-hydroxycholesterol produces a more rapid loss of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity than can be accounted for by enzyme turnover

A key enzyme in the regulation of mammalian cellular cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). It is well established that treatment with the compound 25-hydroxycholesterol lowers HMG-CoA reductase activity in cultured Chinese hamster ovary (CHO-K1) cells. After brief incubation (0-4 h) with 25-hydroxycholesterol (0.5 microgram/ml), cellular HMG-CoA reductase activity is decreased to 40% of its original level. This also occurs in the presence of exogenous mevinolin, a competitive inhibitor of HMG-CoA reductase which has previously been shown to inhibit its degradation. The inhibition of HMG-CoA reductase activity by 25-hydroxycholesterol is complete after 2 h. Radio-immune precipitation analysis of the native enzyme under these conditions shows a degradation half-life which is considerably longer than that of the observed inhibition. Studies with sodium fluoride, phosphatase 2A, bacterial alkaline phosphatase and calf alkaline phosphatase indicate that the observed loss of activity is not due to phosphorylation. These data are not consistent with described mechanisms of HMG-CoA reductase activity regulation by phosphorylation or degradation but are consistent with a novel mechanism that regulates the catalytic efficiency of this enzyme.