Insulin Control of Gene Transcription Through Sensitin

描述（由申请人提供）：导致2型糖尿病的胰岛素抵抗通常涉及受体后水平胰岛素信号传导的缺陷。PI 3 '-激酶活化为Akt通路在胰岛素信号传导中起重要作用，但Akt下游的事件在很大程度上是未知的。最近，我们发现了一种新的Akt底物，它与胰岛素样生长因子结合蛋白3（IGFBP-3）和其他胰岛素应答基因的胰岛素应答元件（IRES）结合。IRE结合蛋白“敏化素”cDNA编码120 kDa的胞质蛋白。加入胰岛素后，蛋白质被切割成50 kDa片段，其主要定位于细胞核并反式激活IRE。PI 3 '-激酶抑制降低了敏感素的Ser/Thr磷酸化和蛋白水解。基于我们的初步数据，我们研究的长期目标是采用IGFBP-3的IRE作为模型系统来描绘胰岛素通过与IRE结合的特定反式作用因子敏化素的作用诱导基因组效应的信号传导途径。本申请的目的如下：1）验证胰岛素调节敏化素磷酸化的假说，利用二维磷酸作图和质谱技术对HepG 2细胞中胰岛素调节敏化素磷酸化的残基进行定位，并通过突变分析评估磷酸化对胰岛素诱导的转录的影响。2)为了检验这一假设，即过敏素的蛋白水解裂解是必需的反式激活的IRE，和推论的假设，即核转位的过敏素是继发于蛋白质的裂解。我们将通过微测序确定切割位点，突变蛋白水解结构域以测试其在核转位和IRE转录中的作用;并突变Akt磷酸化位点以确定去磷酸化是否阻止敏化素的核转位或蛋白水解。3)为了确定在大鼠糖尿病模型中敏化素的磷酸化和蛋白水解的生理意义，我们将通过来自糖尿病、肥胖和瘦对照大鼠的肝细胞溶质提取物比较体外敏化素的磷酸化和蛋白水解，并确定赋予蛋白酶和磷酸化抗性的敏化素突变是否增加肥胖大鼠的糖尿病易感性，或恶化糖尿病大鼠的葡萄糖控制。本研究有助于更好地了解胰岛素调控的基因转录，并可能为2型糖尿病的发病机制提供新的见解。

DESCRIPTION (provided by applicant): Insulin resistance leading to type 2 diabetes generally involve defects in insulin signaling at the postreceptor level. Activation of the PI 3'-kinase to Akt pathway plays a significant role in insulin signaling, but the events downtream of Akt are largely unknown. Recently, we identified a novel Akt substrate which binds to the insulin-responsive elements (IREs) of insulin-like growth factor binding protein-3 (IGFBP-3) and other insulin-responsive genes. The IRE-binding protein "sensitin" cDNA encodes a 120 kDa cytoplasmic protein. After adding insulin, the protein is cleaved to a 50 kDa fragment, which localizes predominantly to the nucleus and transactivates IREs. PI 3'-kinase inhibition decreases both Ser/Thr phosphorylation and proteolysis of sensitin. Based on our preliminary data, the long-term goal of our investigation is to employ the IRE of IGFBP-3 as a model system to delineate the signaling pathway by which insulin induces genomic effects through the action of a specific trans-acting factor, sensitin, that binds to the IRE. This application has the following aims: 1) To test the hypothesis that sensitin is subject to regulated phosphorylation by insulin, using 2-D phosphomapping and mass spectrometry to localize residues of sensitin phosphorylated by insulin in HepG2 cells, and the effect of phosphorylation on insulin-induced transcription assessed by mutational analysis. 2) To test the hypothesis that proteolytic cleavage of sensitin is required for transactivation of the IRE, and the corollary hypothesis that nuclear translocation of sensitin is secondary to cleavage of the protein. We will identify the cleavage sites by microsequencing, mutate the proteolytic domain to test its role in nuclear translocation and IRE transcription; and mutate the Akt phosphorylation site to determine if dephosphorylation prevents nuclear translocation or proteolysis of sensitin. 3) To determine the physiological significance of phosphorylation and proteolysis of sensitin in a rat diabetes model, we will compare the phosphorylation and proteolysis of sensitin in vitro by hepatic cytosolic extracts from diabetic, obese, and lean control rats, and determine whether sensitin mutation conferring protease- and phosphorylation-resistance increase the diabetes susceptibility of obese rats, or worsen glucose control in diabetic rats. This study may lead to better understanding of insulin-regulated gene transcription, and may provide insights into linked to the pathogenesis of type 2 diabetes mellitus.