Degradation of 3-Hydroxy-3-methylglutaryl-CoA Reductase in Endoplasmic Reticulum Membranes Is Accelerated as a Result of Increased Susceptibility to Proteolysis

Abstract

The endoplasmic reticulum (ER) membrane protein 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is subject to regulated degradation when cells are presented with an excess of sterols or mevalonate. In this report, we demonstrate the degradation of HMGCoA reductase in ER membranes prepared from cells which have been pretreated with mevalonate or sterols prior to membrane purification. Degradation of HMGCoA reductase in membranes prepared from pretreated cells is more rapid than in membranes prepared from cells which have received no regulatory molecules. In vitro degradation is blocked by protease inhibitors previously shown to inhibit reductase degradation in vivo and is specific for intact HMG-CoA reductase. The lumenal contents of the ER membranes are dispensible for the regulated proteolysis and the proteases responsible for reductase degradation are stably associated with the ER membrane. Regulated proteolysis of HMG-CoA reductase is inhibited by lactacystin, a newly defined inhibitor of the multicatalytic protease, the proteasome, and in vitro degradation of reductase correlates with the presence of proteasome subunits in purified ER membranes. The ubiquitin system for protein degradation, which has recently been shown to be required for the degradation of several ER membrane proteins, is not required for the degradation of HMG-CoA reductase. Finally, we conclude that the regulated proteolysis of HMG-CoA reductase in response to regulatory molecules such as mevalonate or sterols is mediated by increased susceptibility of the reductase to ER proteases, rather than the induction of a new proteolytic activity.

Squalene synthase inhibitors suppress triglyceride biosynthesis through the farnesol pathway in rat hepatocytes

“…In yeast, the Hmg2P isozyme of HMG-CoA reductase is regulated through ubiquitination-dependent degradation by proteasome, and an FPP-derived signal increased the ubiquitination and consequently the degradation of HmgP2 (54,55). In mammalian cells, both 25-hydroxycholesterol and mevalonic acid accelerated the degradation of HMG-CoA reductase, and the process was inhibited by lactacystin, a proteasome inhibitor, even though ubiquitination is not involved in sterol-induced HMG-CoA reductase degradation (56). It is possible that farnesol and its derivatives also induce the degradation of enzymes that are related to fatty acid biosynthesis.…”

Degradation of Hepatic Stearyl CoA Δ9-Desaturase

“…Multiple types of evidence suggest that the proteosome plays a key role in the processing of antigens for the major histocompatibility complex class I presentation (Chiechanover, 1994) and is involved in generating the active forms of molecules such as the production of the 50-kDa subunit of the transcription factor NF-B from the 105-kDa precursor (Palombella et al, 1994). The proteosome is also thought to be responsible for the degradation of the HMG-CoA reductase (McGee et al, 1996) and of the cystic fibrosis gene product (CFTR) in the endoplasmic reticulum (Rock et al, 1994;Ward et al, 1995). The proteosome is a 26S (2000-kDa) complex, containing the 20S proteosome as a key proteolytic component (Rechsteiner et al, 1993;Jentsch and Schlenker, 1995;Lowe et al, 1995).…”

Involvement of Heat Shock Protein 90 in the Degradation of Mutant Insulin Receptors by the Proteasome

“…Therefore, Asp 1179 and Leu 1193 mutant insulin receptor proteins were degraded by the proteasome without direct ubiquitination in the transfected COS-7 cells and the patient's cells. In fact, the proteasome degradation of certain proteins does not require ubiquitination (19,33,34).…”