title: Differential regulation of HMG-CoA reductase and Insig-1 by enzymes of the ubiquitin-proteasome system.
abstract: The endoplasmic reticulum (ER)-resident enzyme 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase catalyzes the rate-limiting step in sterol production and is the therapeutic target of statins. Understanding HMG-CoA reductase regulation has tremendous implications for atherosclerosis. HMG-CoA reductase levels are regulated in response to sterols both transcriptionally, through a complex regulatory loop involving the ER Insig proteins, and posttranslationally, by Insig-dependent protein degradation by the ubiquitin-proteasome system. The ubiquitin ligase (E3) gp78 has been implicated in the sterol-regulated degradation of HMG-CoA reductase and Insig-1 through ER-associated degradation (ERAD). More recently, a second ERAD E3, TRC8, has also been reported to play a role in the sterol-accelerated degradation of HMG-CoA reductase. We interrogated this network in gp78(-/-) mouse embryonic fibroblasts and also assessed two fibroblast cell lines using RNA interference. Although we consistently observe involvement of gp78 in Insig-1 degradation, we find no substantive evidence to support roles for either gp78 or TRC8 in the robust sterol-accelerated degradation of HMG-CoA reductase. We discuss factors that might lead to such discrepant findings. Our results suggest a need for additional studies before definitive mechanistic conclusions are drawn that might set the stage for development of drugs to manipulate gp78 function in metabolic disorders.
authors.affiliation: Laboratory of Protein Dynamics and Signaling, National Cancer Institute, Frederick, MD 20712, USA.
authors.forename: Yien Che
authors.initials: YC
authors.lastname: Tsai
authors.forename: Gil S
authors.initials: GS
authors.lastname: Leichner
authors.forename: Margaret M P
authors.initials: MM
authors.lastname: Pearce
authors.forename: Gaye Lynn
authors.initials: GL
authors.lastname: Wilson
authors.forename: Richard J H
authors.initials: RJ
authors.lastname: Wojcikiewicz
authors.forename: Joseph
authors.initials: J
authors.lastname: Roitelman
authors.forename: Allan M
authors.initials: AM
authors.lastname: Weissman
mesh_terms: D000214:Acyl Coenzyme A / Q000235:genetics / Q000378:metabolism; D000818:Animals; D004721:Endoplasmic Reticulum / Q000201:enzymology* / Q000378:metabolism; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism; D059748:Proteolysis; D060285:Receptors, Autocrine Motility Factor / Q000235:genetics / Q000378:metabolism; D013261:Sterols / Q000378:metabolism; D025801:Ubiquitin / Q000235:genetics / Q000378:metabolism
chemical_list: D000214:Acyl Coenzyme A; C506119:Insig1 protein, mouse; D008565:Membrane Proteins; D013261:Sterols; D025801:Ubiquitin; C008047:3-hydroxy-3-methylglutaryl-coenzyme A; D006903:Hydroxymethylglutaryl CoA Reductases; C555002:Amfr protein, mouse; D060285:Receptors, Autocrine Motility Factor; D046988:Proteasome Endopeptidase Complex