SUBSTRATES AND INSULIN RECEPTOR ENDOCYTOSIS

胰岛素作用受损或胰岛素抵抗是非胰岛素依赖型糖尿病（NIDDM）的标志。 由于胰岛素抵抗是NIDDM发病机制中的主要因素，因此了解胰岛素抵抗的机制对于确定改善NIDDM易感个体的胰岛素敏感性的新方法具有潜在意义。 胰岛素与其受体结合会激活受体的酪氨酸激酶，导致受体和其他底物（例如ppl 20（肝细胞中的质膜糖蛋白））的磷酸化。在不存在胰岛素的情况下，pp120在胞内结构域中的Ser503上被cAMP依赖性激酶磷酸化，并且这种磷酸化是响应胰岛素的胰岛素受体激酶在Tyr488上磷酸化所需的。 ppl20在胰岛素作用中的作用还不清楚。 培养细胞中的PP 120表达与通过受体介导的内吞作用机制从培养基中清除胰岛素的速率增加相关，表明PP 120在从门静脉循环中清除胰岛素的过程中是重要的。相比之下，磷酸化缺陷型pp120亚型（截短型和Y488F和S503A定点突变体）的表达并没有增加受体介导的胰岛素内化，这表明p120对胰岛素内吞作用的影响取决于其磷酸化状态。免疫荧光和生物素标记的研究表明，pp120发挥其作用，通过受体介导的内在化反应胰岛素。 因此，看来pp120参与了一个蛋白质复合物，靶向胰岛素受体的内吞囊泡。 PP120（Tyr488）和胰岛素受体（Tyr960）之间的复合物形成似乎是由细胞内蛋白介导的。 我们在此提出鉴定这些蛋白质。此外，我们建议解决ppl 20在体内胰岛素作用机制中的作用。 为此，我们已经产生了在肝脏中过表达磷酸化缺陷型S503A同种型ppl20的转基因小鼠。 转基因系将解决磷酸化缺陷型pp 120的表达是否与从门静脉循环中清除过量胰岛素的能力减弱有关，从而引起外周高胰岛素血症。 由于高胰岛素血症导致靶组织受体下调，因此通常与胰岛素抵抗相关。 这些拟议中的研究将为高胰岛素血症、胰岛素抵抗和糖尿病的潜在机制提供新的见解。

Impaired insulin action, or insulin resistance, is a hallmark of Non-Insulin-Dependent Diabetes Mellitus (NIDDM). Because insulin resistance is a major factor in the pathogenesis of NIDDM, understanding the mechanisms of insulin resistance has potential implications in identifying novel means to improve insulin sensitivity in individuals predisposed to NIDDM. Insulin binding to its receptor activates the tyrosine kinase of the receptor to cause phosphorylation of the receptor and of other substrates, such as ppl20, a plasma membrane glycoprotein in the hepatocyte. pp120 is phosphorylated on Ser503 in the intracellular domain by cAMP-dependent kinase in the absence of insulin, and this phosphorylation is required for its phosphorylation on Tyr488 by the insulin receptor kinase in response to insulin. The role of ppl20 in insulin action is not well understood. pp120 expression in cultured cells was correlated with increased rate of insulin clearance from the medium through a mechanism of receptor-mediated endocytosis, suggesting that pp120 is important in the process of insulin clearance from the portal circulation. In contrast, expression of phosphorylation-defective pp120 isoforms (truncated and the Y488F and S503A site-directed mutants) did not increase receptor-mediated insulin internalization, suggesting that the effect of p120 on insulin endocytosis depends on its phosphorylation state. Immunofluorescence and biotin-labeling studies suggested that pp120 exerts its effect by undergoing receptor-mediated internalization in response to insulin. Thus, it appears that pp120 takes part in a complex of proteins that target the insulin receptor to endocytosis vesicles. The complex formation between pp120, at Tyr488, and the insulin receptor, at Tyr960 of its juxtamembrane domain, appears to be mediated by intracellular proteins. We herein propose to identify these proteins. Additionally, we propose to address the role of ppl20 in the mechanism of insulin action in vivo. To this end, we have generated a transgenic mouse overexpressing a phosphorylation-defective S503A isoform of ppl20 in liver. The transgenic line will address whether expression of a phosphorylation-defective pp 120 is associated with a blunted ability to remove excess insulin from the portal circulation, causing peripheral hyperinsulinemia. Since hyperinsulinemia leads to receptor down-regulation on target tissues, it is usually associated with insulin resistance. These proposed studies should provide novel insights into a potential mechanism of hyperinsulinemia, insulin resistance and diabetes.