Regulation of Insulin Sensitivity by the Ubiquitin Ligase Siah2

描述(申请人提供)：肥胖是一个主要的公共卫生问题，因为肥胖相关的慢性疾病，如2型糖尿病的增加。肥胖相关的胰岛素抵抗和2型糖尿病的发展可能与脂肪组织以甘油三酯形式充分储存能量的能力失调有关，导致非脂肪组织中的体外脂质堆积。脂肪细胞中的核激素受体过氧化物酶体增殖激活受体γ(PPARγ)是调节脂代谢、维持血糖稳态和胰岛素敏感性的关键因子。噻唑烷二酮类抗糖尿病药物对PPARγ的激活与脂肪组织的扩张和脂肪组织储存膳食脂肪的能力增强有关。这些药物改变了PPARγ活性和PPARγ蛋白水平，这表明了解PPARγ活性和PPARγ蛋白稳定性之间的联系可能会为肥胖如何导致非胰岛素依赖型糖尿病提供新的见解。PPARγ在脂肪细胞中的稳定性由泛素蛋白酶体途径的酶调节，泛素蛋白酶体途径是一种高度选择性的信号通路，通过用多个泛素多肽标记蛋白质来将蛋白质靶向蛋白酶体。通过基于siRNA的筛选，我们鉴定了一种泛素连接酶Siah2，它调节脂肪细胞中PPARγ蛋白水平和PPARγ活性。我们使用Siah2KO小鼠模型进行的初步研究表明，Siah2调节脂肪组织中PPARγ蛋白水平和炎症，消除Siah2可防止肥胖相关的胰岛素抵抗。我们假设，依赖于SIHA2对PPARγ活性的调节将肥胖与脂肪组织炎症和胰岛素抵抗联系在一起。我们的目标是从机制上深入了解SIHA2在控制PPARγ蛋白水平、胰岛素敏感性和脂肪组织炎症之间的关系中所起的作用。在具体目标1中，我们将测试 假设依赖Siah2的PPARγ修饰通过增加PPARγ的配体依赖的蛋白酶体降解来限制PPARγ的活性。具体目标2将评估Siah2对脂肪组织炎症的影响。在具体目标3中，我们将检验脂肪细胞中的Siah2通过调节PPARγ活性和脂肪与非脂肪组织之间的脂肪分配来调节全身胰岛素敏感性的假设。总之，这些研究将为PPARγ活性与脂肪组织在调节胰岛素敏感性中的作用之间的关系提供新的见解，并将推进我们的目标，即确定泛素-蛋白酶体系统是否代表着治疗肥胖相关的胰岛素抵抗和2型糖尿病的新的治疗靶点。

DESCRIPTION (provided by applicant): Obesity is a major public health problem due to the increase in obesity-related chronic diseases such as type 2 diabetes. The development of obesity-related insulin resistance and type 2 diabetes can be linked to dysregulation of the ability of adipose tissue to adequately store energy in the form of triglycerides, leading to ectopc lipid accumulation in non-adipose tissues. The nuclear hormone receptor peroxisome proliferation-activated receptor gamma (PPARγ) in adipocytes is a key factor in integrating lipid metabolism with glucose homeostasis and insulin sensitivity. Activation of PPARγ by the thiazolidinedione (TZD) class of anti-diabetic drugs is associated with expansion of the adipose tissue and enhanced ability of the adipose tissue to store dietary lipids. These drugs alter PPARγ activity and PPARγ protein levels, an indication that understanding the link between PPARγ activity and PPARγ protein stability may offer new insights into how obesity contributes to NIDDM. PPARγ stability in adipocytes is regulated by enzymes of the ubiquitin proteasome pathway, a highly selective signaling pathway that targets proteins to the proteasome by tagging the protein with multiple ubiquitin polypeptides. Using siRNA-based screening, we identified a ubiquitin ligase, Siah2 that regulates PPARγ protein levels and PPARγ activity in adipocytes. Our preliminary studies using a Siah2KO mouse model show Siah2 regulates PPARγ protein levels and inflammation in adipose tissue and that eliminating Siah2 prevents obesity-related insulin resistance. We hypothesize that Siah2-dependent regulation of PPARγ activity links obesity with adipose tissue inflammation and insulin resistance. Our goal is to provide mechanistic insight into the role of Siah2 in controlling the relationship between PPARγ protein levels, insulin sensitivity and inflammation in adipose tissue. In Specific Aim 1, we will test the hypothesis that Siah2-dependent modification of PPARγ limits PPARγ activity by increasing ligand-dependent proteasomal degradation of PPARγ. Specific aim 2 will assess the effect of Siah2 on adipose tissue inflammation. In specific aim 3, we will test the hypothesis that Siah2 in adipocytes regulates systemic insulin sensitivity via regulation of PPARγ activity and lipid partitioning between adipose and non-adipose tissue. Together, these studies will provide new insight into the relationship between PPARγ activity and the role of adipose tissue in regulating insulin sensitivity and will advance our goal of determining if the ubiquitin-proteasome system represents a novel therapeutic target in the treatment of obesity-related insulin resistance and type 2 diabetes.