Underlying mechanisms for sterol-induced ubiquitination and ER-associated degradation of HMG CoA reductase

Abstract

Accelerated ubiquitination and subsequent endoplasmic reticulum (ER)-associated degradation (ERAD) constitute one of several mechanisms for feedback control of HMG CoA reductase, the rate-limiting enzyme in synthesis of cholesterol and nonsterol isoprenoids. This ERAD is initiated by the accumulation of certain sterols in ER membranes, which trigger binding of reductase to ER membrane proteins called Insigs. Insig-associated ubiquitin ligases facilitate ubiquitination of reductase, marking the enzyme for extraction across the ER membrane through a reaction that is augmented by nonsterol isoprenoids. Once extracted, ubiquitinated reductase becomes dislocated into the cytosol for degradation by 26S proteasomes. In this review, we will highlight several advances in the understanding of reductase ERAD, which includes the discovery for a role of the vitamin K₂ synthetic enzyme UBIAD1 in the reaction and demonstration that sterol-accelerated ERAD significantly contributes to feedback regulation of reductase and cholesterol metabolism in livers of whole animals.

Keywords: Cholesterol; ER-associated degradation; Endoplasmic reticulum (ER);; Golgi; Isoprenoid; Prenyltransferase; Proteasome; Ubiquitin; Vitamin K.

Figure 1.

Biosynthesis of cholesterol and nonsterol isoprenoids in mammalian cells.

Figure 2.

Mevalonate-mediated multivalent feedback regulation of mammalian HMG CoA reductase. Sterol and nonsterol isoprenoids differentially inhibit HMG CoA reductase through three mechanisms – transcription, translation, and protein degradation. Transcriptional regulation is mediated by the sterol-regulated transcription factors called SREBPs that bind to SRE-1 sequences in the promoter of the HMG CoA reductase gene. Translational regulation is mediated by an unidentified nonsterol isoprenoid through an unknown mechanism that may involve the complex 5-untranslated region (UTR) in the reductase mRNA. Sterol and nonsterol isoprenoids combine to accelerate degradation of reductase protein. This figure was adapted from Goldstein and Brown, 1980 (1).

Figure 3.

Domain structure of mammalian HMG CoA reductase. (A) As discussed in the text, HMG CoA reductase consists of two domains: an N-terminal domain with eight transmembrane helices that plays an essential role in sterol-accelerated ERAD and a C-terminal domain that exerts enzymatic activity. (B) Amino acid sequence and topology of HMG CoA reductase. Lysine residues that are required for Insig-mediated, sterol-induced ubiquitination are highlighted in red and denoted by arrows. The YIYF sequence required for sterol-regulated binding to Insigs is highlighted in yellow.

Figure 4.

Insig-mediated, sterol-accelerated ERAD of HMG CoA reductase.

Figure 5.

Role of UBIAD1 in sterol-accelerated ERAD of HMG CoA reductase. The intracellular accumulation of sterols causes reductase to bind to Insigs, resulting in its ubiquitination by Insig-associated ubiquitin ligases and association with UBIAD1. Exogenously added GGOH becomes phosphorylated to produce GGpp, which enhances reductase ERAD by binding to UBIAD1 and stimulating release from reductase-Insig. This release allows for transport of UBIAD1 from ER to Golgi and membrane extraction, cytosolic dislocation, and proteasomal degradation of reductase. SCD-associated variants of UBIAD1 resist GGpp-induced release from reductase and remain in the ER to block reductase ERAD.