The role of sphingolipids in insulin signal transduction

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The peptide hormone insulin stimulates glucose uptake and storage in skeletal muscle while simultaneously inhibiting glucose efflux from the liver. In certain pathological conditions, most notably type II diabetes mellitus, these tissues become resistant to insulin's effects, leading to abnormalities in glucose homeostasis. Determining the signal transduction events that link insulin's arrival at the cell surface to its numerous physiological responses is critical for a complete understanding of the development of insulin resistance. Membrane lipids have emerged as important regulators of hormone action. For example, phosphoinositide products of PI3-kinase are critical for the anabolic effects of insulin. Conversely, the sphingomyelin derivative ceramide, which is elevated in diabetic tissues, antagonizes insulin-stimulation of glucose uptake by preventing PI3-kinase from activating downstream signaling molecules. Moreover, circulating factors implicated in diabetic onset, such as free fatty acids or tumor necrosis factor-alpha, promote ceramide biosynthesis, suggesting that aberrant ceramide accumulation might contribute to the development of insulin resistance. Interestingly, preliminary data obtained in our laboratory indicate that another sphingolipid, sphingosine 1- phosphate, may prevent the ceramide effect on insulin signaling. The project proposed herein will evaluate the molecular mechanisms underlying ceramide and sphingosine 1-phosphate regulation of insulin action. First, we will test several hypothetical mechanisms by which ceramide regulates PI3-kinase dependent signaling. Second, we will evaluate the effectiveness of sphingosine 1-phosphate as both an antagonist of ceramide signaling and a positive regulator of normal insulin action. Results obtained, in addition to providing insight into the contribution of sphingolipids to basic hormonal signal transduction, could have significant implications on our understanding and treatment of type II diabetes mellitus.