Role of Stearoyl-CoA Desaturase in Metabolism

描述(由申请人提供)：本提案的目的是揭示硬脂酰辅酶A去饱和酶(SCD)表达在储存和氧化之间分割脂质的分子机制。了解脂平衡的基础是开发抗击肥胖、糖尿病和其他脂代谢异常疾病的新策略的基础。SCD是催化合成单不饱和脂肪酸的中心酶，主要是油酸(C18：1)，甘油三酯、胆固醇酯、蜡酯和磷脂的主要单不饱和脂肪酸。在之前的拨款申请中的研究表明，具有SCD1亚型靶向干扰(SCD1-/-)的小鼠减少了脂肪生成基因的表达，增加了脂肪酸氧化，并增强了胰岛素敏感性。SCD1-/-小鼠比对照组小鼠更瘦，对饮食和瘦素缺乏导致的肥胖都有抵抗力，并减少了肝脏脂肪变性。SCD1缺乏引起上述表型的机制仍然知之甚少。我们最近观察到，尽管喂食高碳水化合物脱脂饲料的SCD-1-/-小鼠血浆胰岛素水平增加，但在喂食状态下，包括脂肪酸B氧化和糖异生基因在内的过氧化体增殖物激活受体y共激活物(PGC-1a)的基因表达增加。此外，成熟的固醇调节元件结合蛋白-1(SREBP-1)的表达不会增加，SREBP-1c靶基因的表达也不会被诱导。有趣的是，insig-2a的基因表达和蛋白水平增加，insig-2a是一种蛋白质，通过将SREBPs保留在内质网来阻止它们的成熟，通常被胰岛素抑制。转基因的人SCD在小鼠肝脏中的高表达增加了成熟的SREBP-1蛋白和成脂基因的表达。我们假设细胞的18：1/18：0比例通过调节PGC-1a和insig-2a蛋白的表达来调节餐后状态下的肝脏脂质代谢。我们将设计实验来解决这一假设，使用体内和体外的方法。该方案的两个具体目的是：1.检测肝脏SCD1缺乏是否通过调节PGC-1a的表达来增加餐后状态下线粒体脂肪酸B的氧化。2.检测肝脏SCD_1缺乏是否通过增加insig-2a的表达来阻断SREBP-1的加工，下调餐后脂肪生成。餐后脂肪储存是饮食诱导肥胖和2型糖尿病的主要原因之一。我们已经发现了一种机制，可以将脂肪从储存中分离出来，在美联储状态下进行燃烧。我们的研究将有助于治疗人类肥胖、糖尿病和其他代谢性疾病。

DESCRIPTION (provided by applicant): The objective of this proposal is to reveal molecular mechanisms by which stearoyl-CoA desaturase (SCD) expression partitions lipids between storage and oxidation. Understanding the basis of lipid homeostasis is fundamental to developing new strategies to combat obesity, diabetes and other diseases of abnormal lipid metabolism. SCD is a central enzyme catalyzing the synthesis of monounsaturated fatty acids, mainly oleate (C18:1), the major monounsaturated fatty acid of triglycerides, cholesterol esters, wax esters and phospholipids. Studies in the previous grant application demonstrated that mice with a targeted disruption of SCD1 isoform (SCD1-/-) have reduced lipogenic gene expression, increased fatty acid oxidation and enhanced insulin sensitivity. The SCD1-/- mice are leaner than control mice, resistant to both diet- and leptin deficiency-induced obesity and have reduced liver steatosis. The mechanisms by which SCD1 deficiency elicits the aforementioned phenotypes are still poorly understood. We have recently observed that despite an increase in plasma insulin upon feeding SCD1-/- mice with a high carbohydrate fat-free diet, the gene expression of peroxisome proliferator-activated receptor y co-activator (PGC-1a) including the fatty acid B-oxidation and gluconeogenic genes is increased in the fed state. Furthermore, the expression of the mature sterol regulatory element binding protein-1 (SREBP-1) is not increased and that of the SREBP-1c target genes is not induced. Interestingly, the gene expression and protein levels of insig-2a, a protein that blocks maturation of SREBPs by retaining them in the ER and is normally repressed by insulin is increased. Transgenic over expression of human SCD in mouse liver increases the expression of the mature SREBP-1 protein and that of lipogenic genes. We hypothesize that the cellular 18:1/18:0 ratio regulates hepatic lipid metabolism in the postprandial state by modulating the expression of PGC-1a and insig-2a proteins. We will design experiments to address this hypothesis using in vivo and in vitro approaches. The two specific aims of this proposal are: 1. To test whether hepatic SCD1 deficiency increases mitochondrial fatty acid B-oxidation in the postprandial state by regulating the expression of PGC-1a. 2. To test whether hepatic SCD1 deficiency blocks SREBP-1 processing and down regulates lipogenesis in the postprandial state by increasing insig-2a expression. Fat storage after a meal is one of the major causes of diet induced-obesity and type 2 diabetes. We have discovered a mechanism that partitions fat away from storage towards its burning in the fed state. Our research will contribute to the treatment of human obesity, diabetes and other metabolic diseases.