In the past two decades there has been a doubling of the prevalence of obesity in adults and more than a 50% increase in diabetes in the United States. The morbidity associated with obesity and diabetes may soon exceed that associated with cigarette smoking, and if not checked, United States citizens'life expectancy will decline. This application is the continuation of a project aimed at discovering the mechanisms by which persons at high risk for diabetes fail to achieve the full benefit of exercise training. The first phase of these studies evaluated the impact of strength training on insulin responsiveness and muscle adaptation in subjects with the metabolic syndrome. The second phase will apply endurance training of similar intensity and duration. The strength training study results suggest that stimulation of mitochondrial biogenesis in muscle was more important than increased expression of the insulin-responsive glucose transporter, GLUT4. Unlike controls, trained metabolic syndrome subjects did not improve their insulin response. In slow-twitch (type 1) muscle fibers, metabolic syndrome subjects predominantly activated the mammalian target of rapamycin (mTOR) pathway, in contrast to control subjects who predominantly activated the AMP-activated protein kinase (AMPK) pathway. Mitochondrial markers increased twice as much in the control subjects'muscle. The hypothesis to explain the lack of improvement in insulin action is that persons with the metabolic syndrome have a deficient response to training in the type 1 muscle fiber AMPK pathway. It is anticipated that endurance training will induce a larger difference from controls in the key parameters measured, which will reveal the inherently diminished response in mitochondrial biogenesis and give important clues to the mechanism. This dysfunction may be related to a higher proportion of fast-twitch (type 2) fibers that make up their skeletal muscle and as yet unidentified inhibitory cross-talk from the mTOR pathway to the AMPK pathway. The two specific aims of this proposal are: (1) The endurance training-driven adaptations in leg muscle that are associated with enhanced insulin response will be quantified in subjects with the Metabolic Syndrome and matched controls, and (2) In subjects at high risk for diabetes, we will determine changes in gene expression that occur in fast-twitch and slow-twitch muscle fibers in response to endurance or strength training of equivalent energy expenditure. Training-related changes in insulin responsiveness will be measured with euglycemic clamps. Muscle fiber-specific expression and activation of key intracellular pathways will be evaluated by immunoblotting and immunohistochemical studies of pre- and post-training muscle biopsies. Muscle fiber type samples will be obtained by laser capture microdissection to identify major changes in gene expression with Affymetrix gene chip sets. The long term goal of this project is to find genetically-determined risk factors for type 2 diabetes that can be ameliorated by interventions, either behavioral or pharmacological, such that clinical diabetes can be prevented in a portion of those who would otherwise develop this disease.
This proposal is the second phase in a quest to understand the reasons why exercise training is less effective in preventing diabetes among persons with metabolic syndrome. The hope is that this new information will help design better exercise interventions and may lead to medications that may make exercise more effective.
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