POSTTRANSCRIPTIONAL REGULATION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE BY MEVALONATE

We have examined the mechanisms of sterol-independent regulation of th e expression of 3-hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reduct ase by mevalonate in Chinese hamster ovary (CHO) cells. Serum lipoprot eins, 25-hydroxycholesterol, or mevalonate each repress HMG-CoA reduct ase activity by fivefold or more, and mevalonate lowers the rate of re ductase synthesis by twofold. However, while the expression of the HMG -CoA reductase promoter construct, T42 Delta CAT, in stable transfecta nts is also repressed by serum lipoproteins and 25-hydroxycholesterol, mevalonate is without effect. In addition, while 25-hydroxycholestero l reduces the steady-state level of endogenous HMG-CoA reductase mRNA by more than threefold, mevalonate again has no effect. Mevalonate doe s partially regulate the expression of both the artificial promoter co nstruct pTK-Kx3-CAT, containing three copies of the sterol regulatory element, SRE-1, and the full-length LDL receptor promoter construct, p LDLRCAT-6500 as well as the expression of functional LDL receptors. Th is transcriptional regulation appears to be mediated by sterol end pro ducts generated from added mevalonate. In CHO cells starved for mevalo nate due tee a mutation in the biosynthetic pathway, addition of 20 mM mevalonate accelerates the rate of degradation of HMG-CoA reductase b y threefold whether new sterol biosynthesis is blocked or not. In such cells, addition of 25-hydroxycholesterol, by itself, also decreases t he half-life of reductase from 11.6 to 2.3 h. In contrast, in cells ac utely treated with a reductase inhibitor, sterol-accelerated degradati on of reductase is only observed in the presence of submillimolar leve l of mevalonate. We conclude that large concentrations of exogenous me valonate fail to generate a transcriptional regulator of HMG-CoA reduc tase in CHO cells but do lead to the formation of translational regula tor(s) of reductase synthesis, In contrast, sterol regulators derived from exogenous mevalonate appear to be capable of downregulating the L DL receptor promoter. We further conclude that in the absence of pretr eatment with a reductase inhibitor, the regulatory signals generated b y sterols and nonsterols for accelerated degradation of HMG-CoA reduct ase are mutually independent. However, the enzyme synthesized in the p resence of reductase inhibitors appears to exhibit an obligatory coreq uirement for low-dose mevalonate for sterol-accelerated degradation. ( C) 1995 Academic Press, Inc.