title: Involvement of calcium in the mevalonate-accelerated degradation of 3-hydroxy-3-methylglutaryl-CoA reductase.
abstract: 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase), the rate-limiting enzyme in the biosynthesis of cholesterol and isoprenoids, is subject to rapid degradation which is regulated by mevalonate (MVA)-derived metabolic products. HMG-CoA reductase is an integral membrane protein of the endoplasmic reticulum, the largest nonmitochondrial pool of cellular Ca2+. To assess the possible role of Ca2+ in the regulated degradation of HMG-CoA reductase, we perturbed cellular Ca2+ concentration and followed the fate of HMG-CoA reductase and of HMGal, a fusion protein consisting of the membrane domain of HMG-CoA reductase and the soluble bacterial enzyme beta-galactosidase. The degradation of HMGal mirrors that of HMG-CoA reductase, demonstrating that the membrane domain of HMG-CoA reductase is sufficient to confer regulated degradation (Skalnik, D.G., Narita, H., Kent, C., and Simoni, R.D. (1988) J. Biol. Chem. 263, 6836-6841; Chun, K.T., Bar-Nun, S., and Simoni, R.D. (1990) J. Biol. Chem. 265, 22004-22010). In this study we show that the MVA-dependent accelerated rates of degradation of HMG-CoA reductase and HMGal in cells maintained in Ca(2+)-free medium are 2-3-fold slower than the rate of degradation in cells grown in high (1.8-2 mM) Ca2+ concentration. This effect is reversed upon addition of Ca2+ to the medium. Furthermore, when cells maintained in high Ca2+ are treated with 1 microM ionomycin, the MVA-dependent accelerated degradation of HMG-CoA reductase and HMGal is also reduced about 2-3-fold. This inhibition is not due to a Ca(2+)-dependent uptake or incorporation of MVA into sterols, since these processes are not affected in the absence of external Ca2+. In addition, cobalt, a known antagonist of Ca(2+)-dependent cellular functions, totally abolishes (IC50 = 520 microM in the presence of 1.8 mM extracellular Ca2+) the MVA-accelerated degradation of HMGal. These results suggest that Ca2+ plays a major role in the regulated degradation of HMG-CoA reductase.
authors.affiliation: Department of Biological Sciences, Stanford University, California 94305-5020.
authors.forename: J
authors.initials: J
authors.lastname: Roitelman
authors.forename: S
authors.initials: S
authors.lastname: Bar-Nun
authors.forename: S
authors.initials: S
authors.lastname: Inoue
authors.forename: R D
authors.initials: RD
authors.lastname: Simoni
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D003035:Cobalt / Q000494:pharmacology; D006224:Cricetinae; D003412:Cricetulus; D006207:Half-Life; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism*; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D008798:Mevalonic Acid / Q000494:pharmacology*; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D003035:Cobalt; D006903:Hydroxymethylglutaryl CoA Reductases; D001616:beta-Galactosidase; D008798:Mevalonic Acid; D002118:Calcium