Regulation of Skeletal Muscle Metabolism by Insulin Signaling

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Project Summary The number of individuals with type 2 diabetes mellitus (T2DM) remains at an all-time high and is predicted to increase over the next decade. Therefore, it is of significant medical interest to define the underlying mechanisms driving T2DM to improve therapeutic efficacy. Insulin resistance, a condition known as reduced effectiveness to the hormone insulin, is associated with altered glucose homeostasis and muscle dysfunction. Despite decades of investigation, critical knowledge gaps remain in the molecular mechanisms that are responsible for the initiation and propagation of insulin resistance. The skeletal muscle plays a significant role in glucose homeostasis and accounts for a majority of glucose disposal following a meal. Defects in the insulin signaling pathway in the skeletal muscle have been hypothesized to be the primary cause of insulin resistance leading to hyperglycemia, altered protein metabolism and cardiovascular disease. Accumulating evidence has implicated the serine/threonine kinase Akt (protein kinase B) as a critical regulator of insulin action. To directly test the hypothesis that reduced insulin signaling via AKT causes insulin resistance and alters muscle function, we generated mice that lack AKT signaling specifically in skeletal muscle and surprisingly found that insulin can stimulate skeletal muscle glucose uptake and utilization in the absence of AKT. These data are inconsistent with the canonical molecular model of insulin resistance and suggest AKT is not an obligate intermediate in the control of skeletal muscle glucose metabolism by insulin in all conditions. The identification of this AKT-independent pathway and its role carbohydrate homeostasis will be the focus of Aim 1 of this proposal. Although mice lacking AKT in skeletal muscle have normal glucose uptake and insulin sensitivity, we found that they nevertheless exhibit significant muscle atrophy and mitochondrial dysfunction with a corresponding defect in muscle performance, confirming that AKT is required for muscle growth and function in vivo. The downstream mechanisms responsible for AKT’s control of muscle growth and function will be defined in Aim 2. Collectively, this proposal will build upon these important observations and elucidate the Akt-dependent and independent pathways that control the metabolic actions of insulin in vivo. These experiments have the potential to profoundly affect our mechanistic understanding of the pathways underlying insulin resistance and will lead to the identification of new therapeutic targets for T2DM, cardiovascular and skeletomuscular diseases.