MEVALONIC ACID-DEPENDENT DEGRADATION OF 3-HYDROXY-3-METHYLGLUTARYL-COENZYME-A REDUCTASE IN-VIVO AND IN-VITRO

The microsomal enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is subject to rapid degradation when cells are incubated wit h sterols or mevalonic acid (MVA). It has been shown that this rapid d egradation is dependent upon both a sterol and another MVA-derived met abolite (Nakanishi, M., Goldstein, J. L., and Brown, M. S. (1988) J. B iol. Chem. 258, 8929-8937). In the current study, inhibitors of the is oprene biosynthetic pathway were used to define further this mevalonic acid derivative involved in the accelerated degradation of HMG-CoA re ductase. The accelerated degradation of HMG-CoA reductase in met-18b-2 cells, which is induced by the addition of MVA, was inhibited by the presence of the squalene synthase inhibitor, zaragozic acid/squalestat in, or the squalene epoxidase inhibitor, NB-598. Accelerated degradati on of HMG-CoA reductase was observed when NB-598-treated cells were in cubated with both MVA and sterols. In contrast, the addition of MVA an d sterols to zaragozic acid/squalestatin-treated cells did not result in rapid enzyme degradation. This MVA- and sterol-dependent degradatio n of HMG-CoA reductase persisted in cells permeabilized with reduced s treptolysin O. Finally, the selective degradation of HMG-CoA reductase was also observed in rat hepatic microsomes incubated in vitro in the absence of ATP and cytosol. We conclude that the MVA-derived componen t that is required for the accelerated degradation of HMG-CoA reductas e is derived from farnesyl diphosphate and/or squalene in the isopreno id biosynthetic pathway. We propose that this component has a permissi ve effect and does not, by itself, induce the degradation of HMG-CoA r eductase. We also conclude that the degradation of HMG-CoA occurs in t he endoplasmic reticulum, and, once the degradation of HMGCoA reductas e has been initiated by MVA and sterols, all necessary components for the continued degradation of HMG-CoA reductase reside in the endoplasm ic reticulum.