Sterol-induced degradation of HMG CoA reductase depends on interplay of two Insigs and two ubiquitin ligases, gp78 and Trc8

Youngah Jo, Peter C W Lee, Peter V. Sguigna, Russell A. DeBose-Boyd

Research output: Contribution to journalArticle

80 Scopus citations

Abstract

Accumulation of sterols in membranes of the endoplasmic reticulum (ER) leads to the accelerated ubiquitination and proteasomal degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-limiting enzyme in synthesis of cholesterol and nonsterol isoprenoids. This degradation results from sterol-induced binding of reductase to the Insig-1 or Insig-2 proteins of ER membranes. We previously reported that in immortalized human fibroblasts (SV-589 cells) Insig-1, but not Insig-2, recruits gp78, a membranebound RING-finger ubiquitin ligase. We now report that both Insig-1 and Insig-2 bind another membrane-bound RING-finger ubiquitin ligase called Trc8. Knockdown of either gp78 or Trc8 in SV-589 cells through RNA interference (RNAi) inhibited sterolinduced ubiquitination of reductase and inhibited sterol-induced degradation by 50-60%. The combined knockdown of gp78 and Trc8 produced a more complete inhibition of degradation (>90%). Knockdown of gp78 led to a three to fourfold increase in levels of Trc8 and Insig-1 proteins, which opposed the inhibitory action of gp78. In contrast, knockdown of Trc8 had no effect on gp78 or Insig-1. The current results suggest that sterol-induced ubiquitination and proteasomal degradation of reductase is dictated by the complex interplay of at least four proteins: Insig-1, Insig-2, gp78, and Trc8. Variations in the concentrations of any one of these proteins may account for differences in cell- and/or tissue-specific regulation of reductase degradation.

Original languageEnglish (US)
Pages (from-to)20503-20508
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume108
Issue number51
DOIs
StatePublished - Dec 20 2011

Keywords

  • Cholesterol metabolism
  • ER-associated degradation

ASJC Scopus subject areas

  • General

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