Splice donor site mutations in the 3-hydroxy-3-methylglutaryl coenzyme A reductase gene cause a deficiency of the endoplasmic reticulum 3-hydroxy-3-methylglutaryl coenzyme A reductase protein in UT2 cells

In mammalian cells, the enzyme 3-hydroxy-3-methylglutarylcoenzyme A reductase (HMGR), which catalyzes the rate-limiting step in the mevalonate pathway, is ubiquitylated and degraded by the 26 S proteasome when mevalonate-derived metabolites accumulate, representing a case of metabolically regulated endoplasmic reticulum-associated degradation (ERAD). Here, we studied which mevalonate-derived metabolites signal for HMGR degradation and the ERAD step(s) in which these metabolites are required. In HMGR-deficient UT-2 cells that stably express HMGal, a chimeric protein between ␤-galactosidase and the membrane region of HMGR, which is necessary and sufficient for the regulated ERAD, we tested inhibitors specific to different steps in the mevalonate pathway. We found that metabolites downstream of farnesyl pyrophosphate but upstream to lanosterol were highly effective in initiating ubiquitylation, dislocation, and degradation of HMGal. Similar results were observed for endogenous HMGR in cells that express this protein. Ubiquitylation, dislocation, and proteasomal degradation of HMGal were severely hampered when production of geranylgeranyl pyrophosphate was inhibited. Importantly, inhibition of protein geranylgeranylation markedly attenuated ubiquitylation and dislocation, implicating for the first time a geranylgeranylated protein(s) in the metabolically regulated ERAD of HMGR.Mammalian cells satisfy most of their cholesterol requirements through receptor-mediated endocytosis of cholesterolrich plasma lipoproteins. However, when exogenous cholesterol supply is short, or when there is an increased demand for particular nonsterol isoprenoid(s), cells up-regulate the activity of the mevalonate (MVA) 3 pathway ( Fig. 1), in which sterols and essential MVA-derived nonsterol isoprenoids are produced (1, 2). Intracellular MVA levels are finely tuned and tightly regulated by a feedback mechanism that governs the rate of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reduction into MVA, the major rate-limiting step for the entire pathway (3, 4). This reaction is catalyzed by HMG-CoA reductase (HMGR), an integral glycoprotein resident to the endoplasmic reticulum (ER). Each of the 97-kDa subunits of HMGR is embedded in the ER membrane via a ϳ350 residue N-terminal noncatalytic region that spans the membrane 8 times (5-7), whereas the rest of the polypeptide faces the cytosol where it tetramerizes to form the active sites of the enzyme (5, 8, 9). To allow sufficient production of isoprenoids yet avoid overaccumulation of potentially toxic products, the activity of HMGR is stringently controlled and varies according to the cellular needs for sterol and the nonsterol products. HMGR activity increases when cells are starved for cholesterol or MVA. Conversely, abundant cholesterol and/or MVA acutely suppress the enzyme (4, 10). At the post-translational level, regulation of HMGR mainly involves modulation of its stability. Thus, in cells deprived of sterols and/or MVA, HMGR is a rather stable protein that turns over with a half-l...

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Early Embryonic Lethality Caused by Targeted Disruption of the 3-Hydroxy-3-methylglutaryl-CoA Reductase Gene

Ohashi

Osuga

Tozawa

et al. 2003

Journal of Biological Chemistry

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The endoplasmic reticulum (ER) enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which converts HMG-CoA to mevalonate, catalyzes the ratelimiting step in cholesterol biosynthesis. Because this mevalonate pathway also produces several non-sterol isoprenoid compounds, the level of HMG-CoA reductase activity may coordinate many cellular processes and functions. We used gene targeting to knock out the mouse HMG-CoA reductase gene. The heterozygous mutant mice (Hmgcr؉/؊) appeared normal in their development and gross anatomy and were fertile. Although HMG-CoA reductase activities were reduced in Hmgcr؉/؊ embryonic fibroblasts, the enzyme activities and cholesterol biosynthesis remained unaffected in the liver from Hmgcr؉/؊ mice, suggesting that the haploid amount of Hmgcr gene is not rate-limiting in the hepatic cholesterol homeostasis. Consistently, plasma lipoprotein profiles were similar between Hmgcr؉/؊ and Hmgcr؉/؉ mice. In contrast, the embryos homozygous for the Hmgcr mutant allele were recovered at the blastocyst stage, but not at E8.5, indicating that HMG-CoA reductase is crucial for early development of the mouse embryos. The lethal phenotype was not completely rescued by supplementing the dams with mevalonate. Although it has been postulated that a second, peroxisome-specific HMG-CoA reductase could substitute for the ER reductase in vitro, we speculate that the putative peroxisomal reductase gene, if existed, does not fully compensate for the lack of the ER enzyme at least in embryogenesis.The mevalonate pathway produces isoprenoids that are essential for diverse cellular functions, ranging from cholesterol synthesis to growth control. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) 1 reductase (EC 1.1.1.34), which catalyzes the conversion of HMG-CoA to mevalonate, is the rate-limiting enzyme in the mevalonate pathway (1). Because of its major role in cholesterol biosynthesis, the regulation of HMG-CoA reductase has been intensely studied. To ensure a steady mevalonate supply, the non-sterol and sterol end-products of mevalonate metabolism exert feedback regulation on the activity of this enzyme through multivalent mechanisms, including inhibition of transcription of the HMG-CoA reductase mRNA, blocking of translation, and acceleration of protein degradation, thus regulating the amount of reductase protein over a several hundred-fold range (reviewed in Refs.

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Splice donor site mutations in the 3-hydroxy-3-methylglutaryl coenzyme A reductase gene cause a deficiency of the endoplasmic reticulum 3-hydroxy-3-methylglutaryl coenzyme A reductase protein in UT2 cells

Cited by 7 publications

References 28 publications

Explication of interactions between HMGCR isoform 2 and various statins through In silico modeling and docking

Explication of interactions between HMGCR isoform 2 and various statins through In silico modeling and docking

Metabolically Regulated Endoplasmic Reticulum-associated Degradation of 3-Hydroxy-3-methylglutaryl-CoA Reductase

Early Embryonic Lethality Caused by Targeted Disruption of the 3-Hydroxy-3-methylglutaryl-CoA Reductase Gene

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