Age Related Insulin Resistance, Akt/PKB, and Skeletal Muscle Proteolysis

描述(由申请人提供)：肌肉大小和力量的减少与年龄的增加和2型糖尿病有关。由于胰岛素抵抗和2型糖尿病通常在老年人中观察到，与年龄相关的肌肉质量损失的很大一部分可能是由于胰岛素作用的减少。胰岛素的主要作用是抑制骨骼肌的蛋白分解，部分是通过激活蛋白激酶B(PKB)。PKB是一种关键的胰岛素信号分子，它控制着大量的细胞功能，包括葡萄糖摄取、糖原合成、蛋白质合成、蛋白质降解和基因转录。PKB的激活通过磷酸化叉头盒O(FOXO)转录因子，从而抑制阿托金-1和MuRF1的表达，从而减少骨骼肌的蛋白分解，这两种泛素连接酶被认为介导了骨骼肌的蛋白分解。我们的初步数据显示，与年轻小鼠相比，老年小鼠骨骼肌中总蛋白质降解率和泛素-蛋白质结合物增加。因此，我们提出这一领域建议的第一个目标是证明泛素-蛋白酶体蛋白分解途径的增加是老年骨骼肌总蛋白降解率增加的原因。我们的初步数据还表明，与年轻肌肉相比，老年肌肉中PKB活性和FOXO转录因子磷酸化水平降低。在这方面，我们的第二个目标是证明在衰老的骨骼肌中，胰岛素激活PKB和促进FOXO转录因子磷酸化的能力降低。由于PKB控制着几个与生长和代谢相关的关键细胞功能，不同的激酶亚型可能提供信号特异性。由于PKB-1缺失的转基因小鼠比野生型或PKB-2缺失的小鼠小，我们认为PKB-1比PKB-2在更大程度上控制骨骼肌的蛋白分解。因此，我们这一领域应用的第三个目标是通过使用同型特异性PKB基因敲除小鼠来证明PKB-1比PKB-2对泛素-蛋白酶体依赖的蛋白分解起到更大的控制作用。实现这些目标将建立一种新的机制，将胰岛素抵抗与年龄相关的肌肉质量损失联系起来，并确定PKB的异构体特异性功能。根据美国国立卫生研究院地区倡议，这项R15提案的目标将通过使本科生成为所有研究活动的组成部分来实现。

DESCRIPTION (provided by applicant): Decreases in muscle size and strength are associated with advancing age and type 2 diabetes. Since insulin resistance and type 2 diabetes are commonly observed in older individuals, a good portion of the age-related loss of muscle mass may be due to a decrease in insulin action. A major action of insulin is to suppress skeletal muscle proteolysis, in part, by activating protein kinase B (PKB). PKB is a critical insulin signaling molecule that controls a plethora of cellular functions including glucose uptake, glycogen synthesis, protein synthesis, protein degradation, and gene transcription. Activation of PKB reduces skeletal muscle proteolysis by phosphorylating forkhead box O (FOXO) transcription factors and thereby repressing the expression of atrogin-1 and MURF1, two ubiquitin ligases thought to mediate skeletal muscle proteolysis. Our preliminary data demonstrates increases in total protein degradation rates and ubiquitin-protein conjugates in skeletal muscles of aged mice compared to younger mice. Therefore, our first goal of this AREA proposal is to demonstrate that an increase in the ubiquitin-proteasome proteolytic pathway is responsible for the increase in total protein degradation observed in aged skeletal muscle. Our preliminary data also demonstrates decreases in PKB activity and FOXO transcription factor phosphorylation in aged muscle compared to young muscle. In this regard, our second goal is to demonstrate that the ability of insulin to activate PKB and promote FOXO transcription factor phosphorylation is reduced in aged skeletal muscle. Because PKB controls several critical cellular functions relating to both growth and metabolism, different isoforms of the kinase may provide signaling specificity. Since PKB-1 null transgenic mice are smaller than wildtype or PKB-2 null mice, we believe that PKB-1 controls skeletal muscle proteolysis to a greater extent than PKB-2. Therefore, our third goal of this AREA application is to demonstrate that PKB-1 exerts a greater control of ubiquitin-proteasome dependent proteolysis than PKB-2 by using isoform specific PKB knockout mice. Accomplishing these goals will establish a novel mechanism connecting insulin resistance to age-related loss of muscle mass as well as identify an isoform specific function of PKB. In accordance with the NIH-AREA initiative the goals of this R15 proposal will be accomplished by making undergraduate students an integral part of all research activities.