Examining Mechanism and Physiological Significance of HMG CoA Reductase ER-Associated Degradation

项目摘要 多种反馈机制汇聚在多位的内质网定位酶HMG上 辅酶A还原酶（HMGCR），催化胆固醇和必需非固醇合成的关键步骤 类异戊二烯包括Fpp（焦磷酸法呢酯）和GGpp（焦磷酸香叶基香叶基酯）。Fpp和GGpp 可以附着于许多细胞蛋白质，并用于合成多萜醇、泛醌、血红素和 维生素K2 HMGCR反馈控制的一种机制涉及甾醇诱导的泛素化，其标志着HMGCR的表达。 用于提取穿过ER膜并随后释放到细胞溶质中的蛋白酶体的酶- 介导的ERAD（ER相关降解）。GGpp通过增强HMGCR的膜增强ERAD 萃取我们最近发现GGpp抑制固醇诱导的HMGCR与UBIAD 1（UbiA 异戊烯基转移酶结构域蛋白-1），其利用GGpp合成维生素K2。这种抑制 允许HMGCR的膜提取和UBIAD 1的ER至高尔基体转运。UBIAD 1突变导致 SCD（Schnyder corneal dystrophy）是一种常染色体显性遗传眼病，以角膜积聚为特征 胆固醇。SCD相关的UBIAD 1抵抗GGpp诱导的HMGCR释放，并被困在HMGCR中。 ER，它阻止HMGCR ERAD。 基于以上总结的发现，我们现在准备1）阐明 甾醇促进HMGCR的泛素化; 2）确定HMGCR的分子结构; 3）建立 泛素化的HMGCR如何从ER膜上去除以用于ERAD;以及4）确定ERAD的生理学机制。 HMGCR ERAD的意义。总的来说，这些研究将提供有关机制的关键信息， 通过该方法将多位HMGCR从ER膜上去除并递送用于ERAD的胞质蛋白酶体。在 此外，这些研究具有重要的临床意义。HMGCR是他汀类药物的靶点， 降低血浆LDL-胆固醇和减少心血管疾病的药物。然而，他汀类药物引发反应， 导致HMGCR的积累，从而减弱其临床效果。这一增长的部分原因是， HMGCR的ERAD。因此，阐明HMGCR ERAD的机制为开发新的药物提供了希望。 增加他汀类药物疗效并进一步减少心脏病发作的治疗。

Project Summary Multiple feedback mechanisms converge on the polytopic, ER (endoplasmic reticulum)-localized enzyme HMG CoA reductase (HMGCR), which catalyzes a key step in synthesis of cholesterol and essential nonsterol isoprenoids including Fpp (farnesyl pyrophosphate) and GGpp (geranylgeranyl pyrophosphate). Fpp and GGpp can become attached to many cellular proteins and are utilized in synthesis of dolichol, ubiquinone, heme, and vitamin K2. One mechanism for feedback control of HMGCR involves sterol-induced ubiquitination, which marks the enzyme for extraction across ER membranes and subsequent release into the cytosol for proteasome- mediated ERAD (ER-associated degradation). GGpp augments ERAD of HMGCR by enhancing its membrane extraction. We recently discovered that GGpp inhibits sterol-induced binding of HMGCR to UBIAD1 (UbiA prenyltransferase domain-containing protein-1), which utilizes GGpp to synthesize vitamin K2. This inhibition allows for membrane extraction of HMGCR and ER-to-Golgi transport of UBIAD1. Mutations in UBIAD1 cause SCD (Schnyder corneal dystrophy), an autosomal dominant eye disease characterized by corneal accumulation of cholesterol. SCD-associated UBIAD1 resists GGpp-induced release from HMGCR and becomes trapped in the ER where it blocks HMGCR ERAD. Building on discoveries summarized above, we are now poised to 1) elucidate mechanisms through which sterols promote ubiquitination of HMGCR; 2) determine the molecular structure of HMGCR; 3) establish how ubiquitinated HMGCR is removed from ER membranes for ERAD; and 4) ascertain the physiological significance of HMGCR ERAD. Collectively, these studies will provide key information regarding mechanisms by which polytopic HMGCR is removed from ER membranes and delivered cytosolic proteasomes for ERAD. In addition, these studies have significant clinical implications. HMGCR is the target of statins, widely prescribed drugs that lower plasma LDL-cholesterol and reduce cardiovascular disease. However, statins trigger responses that cause accumulation of HMGCR, which blunts their clinical effects. This increase results in part, from slowed ERAD of HMGCR. Thus, elucidating mechanisms for HMGCR ERAD holds promise for development of new therapies that increase the efficacy of statins and further reduce heart attacks.