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

Department of Molecular Genetics and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 12/2011; 108(51):20503-8. DOI: 10.1073/pnas.1112831108
Source: PubMed


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 membrane-bound 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 sterol-induced 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.

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    • "c o m / l o c a t e / b b a d i s TRC8 is an ER-resident E3 ligase and its gene disruption has been associated with hereditary renal cancer [10]. In addition to its tumor suppressive function [11] [12], TRC8 has been shown to regulate cholesterol and fatty acid biosynthesis by stimulating the destabilization of the ER-bound sterol regulatory element-binding proteins (SREBPs) [13], hindering ER to Golgi transport of SREBP-2 [14], and mediating the degradation of HMG-CoA reductase [15]. Moreover, studies in Drosophila and mammalian cells revealed that TRC8 suppresses protein translation through targeting eIF3f and eIF3h for degradation [11]. "
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    • "TRC8 function likely extends beyond the degradation of TA proteins. Its sterol sensing domain suggests an involvement in membrane lipid homeostasis, though the role of TRC8 in the processing of SREBP transcription factors and sterol-induced degradation of HMG-CoA reductase remains controversial (Irisawa et al., 2009; Jo et al., 2011; Lee et al., 2010; Tsai et al., 2012). The study of US2 has therefore uncovered a novel branch of mammalian ERAD with an E3 ligase involved in channel-independent degradation and lipid homeostasis. "
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    • "AMFR/gp78 is an E3 ubiquitin ligase and INSIG is an insulin signaling factor (Flury et al., 2005; Chen et al., 2012). Both are localized to the ER membrane and when activated function together to degrade HMG COA reductase, a cholesterol synthesis enzyme (Jo et al., 2011; Tsai et al., 2012). "
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