Dissecting ER-Associated Degradation of a Membrane Protein in Drosophila S2 Cells

描述（由申请人提供）：在哺乳动物细胞中，酶HMG辅酶a还原酶催化HMG辅酶a还原为甲羟戊酸盐，这是合成胆固醇和非固醇类异戊二烯的速率决定步骤。还原酶通过包含八个跨膜螺旋的n端结构域整合到内质网膜中。还原酶的c端投射到细胞质中并发挥催化活性。甲羟戊酸代谢的最终产物加速er相关的还原酶降解（ERAD），作为确保胆固醇稳态的复杂反馈系统的一部分。过量的甾醇导致还原酶的膜结构域与内质网膜蛋白insg -1和insg -2结合，导致还原酶的多泛素化。这种泛素化对于还原酶的识别和传递到胞质26S蛋白酶体进行降解是必需的。通过过表达哺乳动物还原酶的膜结构域和insg -1或insg -2，在果蝇S2细胞中重建了该反应。作为研究生物学基础问题的模型系统，S2细胞具有许多优势。例如，转基因很容易在S2细胞中过表达以研究其功能，而RNAi在S2细胞中比在哺乳动物细胞中更简单、更有效。为了进一步了解insg介导的还原酶降解机制，我们提出了三个具体目标：1)确定S2细胞中胆固醇加速哺乳动物HMG CoA还原酶降解的机制；2)明确Hrd1泛素连接酶复合物组分在哺乳动物细胞中固醇加速还原酶降解中的作用；3)通过全基因组RNAi筛选S2细胞中还原酶降解所需的新基因。总的来说，这些研究将提供关于内质网中还原酶和其他多聚体蛋白降解机制的重要信息。此外，这些研究具有重要的临床意义。还原酶是他汀类药物的作用靶点，他汀类药物被广泛用于降低血液中的低密度脂蛋白胆固醇，减少冠状动脉疾病的发病率。他汀类药物引发反应，导致活性还原酶蛋白的积累，从而减弱其作用。这种增加部分是由于还原酶的降解减慢。因此，阐明还原酶ERAD的机制可能会导致新的治疗方法，提高他汀类药物的有效性，并最终降低心脏病发作的发生率。

DESCRIPTION (provided by applicant): In mammalian cells, the enzyme HMG CoA reductase catalyzes reduction of HMG CoA to mevalonate, a rate-determining step in the synthesis of cholesterol and non-sterol isoprenoids. Reductase is integrated into the ER membrane through an N-terminal domain that contains eight membrane-spanning helices. The C-terminus of reductase projects into the cytosol and exerts catalytic activity. End-products of mevalonate metabolism accelerate ER-associated degradation (ERAD) of reductase as part of a complex feedback system that ensures cholesterol homeostasis. Excess sterols cause binding of the membrane domain of reductase to ER membrane proteins called Insig-1 and Insig-2, resulting in the poly-ubiquitination of reductase. This ubiquitination is obligatory for recognition and delivery of reductase to cytosolic 26S proteasomes for degradation. The reaction has been reconstituted in Drosophila S2 cells by overexpressing the membrane domain of mammalian reductase and Insig-1 or Insig-2. As a model system to study fundamental questions in biology, S2 cells offer a number of advantages. For example, transgenes can be easily overexpressed in S2 cells for study of their function and RNAi is simpler and much more effective in S2 cells than in mammalian cells. To gain further insight into mechanisms for Insig-mediated degradation of reductase, we propose three specific aims: 1) Determine mechanism for sterol-accelerated degradation of mammalian HMG CoA reductase in S2 cells; 2) Define role of Hrd1 ubiquitin ligase complex components in sterol-accelerated degradation of reductase in mammalian cells; and 3) Identify novel genes required for degradation of reductase through a genome-wide RNAi screen in S2 cells. Collectively, these studies will provide crucial information regarding mechanisms for degradation of reductase and other polytopic proteins from the ER. In addition, these studies have significant clinical implications. Reductase is the target of statins, a family of widely prescribed drugs that lower blood LDL-cholesterol and reduce the incidence of coronary artery disease. Statins trigger responses that lead to accumulation of active reductase protein, thereby blunting their effects. Part of this increase is due to slowed degradation of reductase. Thus, elucidating mechanisms for the ERAD of reductase may lead to new therapies that increase the effectiveness of statins and ultimately reduce the incidence of heart attacks. PUBLIC HEALTH RELEVANCE: The key enzyme in cholesterol synthesis is HMG CoA reductase, which is tightly controlled through multiple mechanisms that includes regulation of protein stability. Competitive inhibitors of HMG CoA reductase called statins are routinely used to lower blood cholesterol, but they trigger regulatory responses that lead to the accumulation of reductase protein. This grant will investigate the mechanism for the degradation of reductase, the elucidation of which will provide insight into development of therapies to counteract statin-induced accumulation of reductase and improve the effectiveness of the drugs.