Lipid Metabolism in the Etiology of Type 2 Diabetes

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DESCRIPTION: (Scanned from the applicant's description) Type 2 diabetes mellitus has major clinical and social impact, but its underlying pathophysiology is poorly understood. Since the disease is diagnosed as a disorder of carbohydrate metabolism, i.e., hyperglycemia, the possible contribution of abnormal lipid metabolism to its etiology has been largely overlooked. The predominant, obesity-related form of diabetes is characterized by hyperinsulinemia, resistance to insulin-mediated glucose disposal in skeletal muscle, and elevated plasma free fatty acid and triglyceride levels. It has been suggested that a derangement of lipid metabolism is an early event contributing to the development of both hyperinsulinemia and insulin resistance. Our laboratory has demonstrated the essential role that plasma fatty acids play in sustaining normal glucose-stimulated insulin secretion in fasted subjects, and has also utilized novel 1-H NMR spectroscopic techniques to illustrate the strong correlation between intramyocellular lipid (IMCL) content and skeletal muscle insulin resistance. In the current proposal, we seek to expand on this theme by investigating the effects of high-fat feeding, pharmacologic inhibition of lipid oxidation, and leptin administration on insulin secretion and insulin sensitivity in rats and determining how these changes might be linked to alterations in muscle and islet triglyceride content. Because deficiencies of leptin and/or leptin signaling can precipitate the development of obesity/diabetes mellitus, it is conceivable that the primary function of leptin is to control lipid oxidation and lipolysis in a manner that prevents tissue lipid accumulation, thus maintaining normal glucose metabolism. We will administer leptin intracerebroventricularly to rats consuming a high fat diet and determine if this reverses the development of hyperinsulinemia and insulin resistance. Parallel measurements of IMCL, muscle P13-kinase activation and islet triglyceride levels will seek to establish a direct link between fat dissipation and improved function in these tissues. Future studies will explore the biochemical pathways through which leptin regulates lipid metabolism.