Advancing age is typically accompanied by insulin resistance. Relatively little, however, is known concerning the cellular mechanism(s) responsible for age-related insulin resistance, particularly in the primary target tissue for insulin action, skeletal muscle. In the current proposal, the hypothesis to be tested is that the insulin signal leading to glucose transport is impaired with the aging process in human skeletal muscle, which subsequently contributes to insulin resistance. Our working hypothesis is that insulin signal transduction is reduced in aged skeletal muscle due to inhibition from increased muscle lipid content and/or reduced oxidative capacity. As decrements during the aging process can be a consequence of physical inactivity, our secondary hypothesis is that age-related insulin resistance is compensated for with exercise training by enhanced insulin signaling. The following aims will test these hypotheses.
Specific Aim 1 : Determine if insulin signal transduction is impaired with aging in human skeletal muscle. We will examine if insulin signal transduction in skeletal muscle is impaired with the aging process in humans and if muscle lipid accumulation and/or impaired oxidative capacity contribute to the decrement.
Specific Aim 2 : Determine if lipid accumulation and or reduced muscle oxidative capacity impairs insulin signal transduction in human skeletal muscle. Using a primary human skeletal muscle cell culture system we will determine if lipid exposure impairs insulin signaling and if decrements in lipid oxidation (akin to aging) induce insulin resistance. With PPARa agonist treatment we will conversely increase muscle oxidative capacity in muscle cells and in aged individuals and determine if this increase reverses lipid-induced insulin resistance.
Specific Aim 3 : Determine the cellular mechanism(s) by which insulin action is enhanced with physical activity in aged skeletal muscle. We will determine if enhanced insulin signal transduction contributes to the improvement in insulin action seen with endurance- and resistance-oriented exercise training in aged individuals and if changes in muscle lipid content and/or oxidative capacity are mechanisms that can explain such changes. ? ? ?
