Regulation of HMG-CoA reductase in mammals and yeast

Abstract

HMG-CoA reductase (HMGR), a highly conserved, membrane-bound enzyme, catalyzes a rate-limiting step in sterol and isoprenoid biosynthesis and is the primary target of hypocholesterolemic drug therapy. HMGR activity is tightly regulated to ensure maintenance of lipid homeostasis, disruption of which is a major cause of human morbidity and mortality. HMGR regulation takes place at the levels of transcription, translation, post-translational modification and degradation. In this review, we discuss regulation of mammalian, Saccharomyces cerevisiae and Schizosaccharomyces pombe HMGR and highlight recent advances in the field. We find that the general features of HMGR regulation, including a requirement for the HMGR-binding protein Insig, are remarkably conserved between mammals and ascomycetous fungi, including S. cerevisiae and S. pombe. However the specific details by which this regulation occurs differ in surprising ways, revealing the broad evolutionary themes underlying both HMGR regulation and Insig function.

Figure 1. The sterol biosynthetic pathway in mammals, Saccharomyces cerevisiae and Schizosaccharomyces pombe

Mammalian enzymes (blue) are listed by their full names; S. pombe enzymes (green) are named based on homology to S. cerevisiae enzymes (orange). † indicates enzymes not conserved between mammals and yeast.

Figure 2. Reaction catalyzed by HMG-CoA reductase

HMGR catalyzes the reduction of 3-hydroxy-3-methylglutaryl-CoA to mevalonate, thereby oxidizing two molecules of NADPH.

Figure 3. Mammalian HMGR catalytic activity is regulated by AMP kinase

A high AMP:ATP ratio stimulates AMP-activated protein kinase (AMPK), which phosphorylates HMGR at a conserved serine in the active site, thus inhibiting HMGR activity. Protein phosphatase 2A (PP2A) dephosphorylates HMGR, restoring enzyme activity.

Figure 4. Mammalian HMGR enzyme levels are regulated post-translationally by Insig-dependent and sterol-accelerated degradation

In sterol-replete cells, Insig binds HMGR and recruits gp78, a ubiquitin E3 ligase. gp78, in concert with the E2 conjugating enzyme Ubc7, ubiquitinates HMGR on K89 and K248, a process which also requires SPFH2 and TMUB1. After membrane extraction, which is enhanced by geranylgeraniol (GGOH), the hexameric ATPase p97/VCP allows proteasomal degradation of HMGR. Insig dissociates from HMGR when sterols are depleted, thus stabilizing HMGR by preventing enzyme ubiquitination.

Figure 5. Hmg2p levels are controlled post-translationally by geranylgeranyl pyrophosphate and the HRD complex in S. cerevisiae

Geranylgeranyl pyrophosphate alters the conformation of the Hmg2p N-terminal membrane domain, promoting recognition and ubiquitination by the membane-bound, multi-subunit HMG-CoA reductase degradation (HRD) complex. Hrd1p, a membrane-spanning ubiquitin E3 ligase with homology to the mammalian E3 ligase gp78, utilizes two E2 ubiquitin conjugating enzymes, Ubc7p and Ubc1p. Ubc7p, a soluble protein that interacts with the membrane through the integral membrane protein Cue1p, is the primary E2 for Hrd1. Hrd3 is involved in substrate recognition and delivery to the HRD complex. Usa1p plays a role in Hrd1p function and Hrd1p self-regulation. Ubx2p is an integral membrane protein that recruits the Cdc48p/Npl4p/Ufd1p complex. Cdc48p, a hexameric ATPase and homolog of mammalian p97/VCP, acts in retrotranslocation of both luminal and membrane-bound HRD substrates. The HRD complex contains other components, including Der1p, Kar2p and Yos9p, that are not required for Hmg2p degradation and are not shown.

Figure 6. Hmg1 activity is regulated post-translationally by phosphorylation under the control of the Insig homolog, Ins1, in S. pombe

Phosphorylation of Hmg1 decreases enzyme activity. Extracellular glucose suppresses activity of the phosphatase Ppe1 through Sds23 thereby preventing Hmg1 dephosphorylation. Osmotic stress stimulates Hmg1 phosphorylation through the MAP kinase Sty1. Ins1 is strictly required for Hmg1 phosphorylation. The enzymes that directly phosphorylate and dephosphorylate Hmg1 have not been identified.