Ohashi Ken, Osuga Jun-ichi, Tozawa Ryuichi, Kitamine Tetsuya, Yagyu Hiroaki, Sekiya Motohiro, Tomita Sachiko, Okazaki Hiroaki, Tamura Yoshiaki, Yahagi Naoya, Iizuka Yoko, Harada Kenji, Gotoda Takanari, Shimano Hitoshi, Yamada Nobuhiro, Ishibashi Shun
Department of Metabolic Diseases, Faculty of Medicine, University of Tokyo, Tokyo 113-8655, Japan.
J Biol Chem. 2003 Oct 31;278(44):42936-41. doi: 10.1074/jbc.M307228200. Epub 2003 Aug 14.
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.
内质网（ER）酶3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶可将HMG-CoA转化为甲羟戊酸，催化胆固醇生物合成中的限速步骤。由于这条甲羟戊酸途径还会产生几种非甾醇类异戊二烯化合物，HMG-CoA还原酶的活性水平可能会协调许多细胞过程和功能。我们利用基因打靶技术敲除了小鼠的HMG-CoA还原酶基因。杂合突变小鼠（Hmgcr+/-）在发育和大体解剖结构上看起来正常，并且具有生育能力。尽管Hmgcr+/-胚胎成纤维细胞中的HMG-CoA还原酶活性降低，但Hmgcr+/-小鼠肝脏中的酶活性和胆固醇生物合成并未受到影响，这表明单倍体的Hmgcr基因在肝脏胆固醇稳态中并非限速因素。同样，Hmgcr+/-和Hmgcr+/+小鼠的血浆脂蛋白谱相似。相比之下，Hmgcr突变等位基因纯合的胚胎在囊胚期可以回收，但在胚胎发育第8.5天则无法回收，这表明HMG-CoA还原酶对小鼠胚胎的早期发育至关重要。给母鼠补充甲羟戊酸并不能完全挽救致死表型。尽管有人推测，第二种过氧化物酶体特异性HMG-CoA还原酶在体外可以替代内质网还原酶，但我们推测，假定的过氧化物酶体还原酶基因（如果存在）至少在胚胎发生过程中不能完全补偿内质网酶的缺失。
