Aspects of Novel Sites of Regulation of the Insulin Stimulus-Secretion Coupling in Normal and Diabetic Pancreatic Islets

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Abstract
Noninsulin-depenent diabetes mellitus (NIDDM), a major health care problem in the Western world, is a disease typified by a relative deficiency of insulin, leading to vast derangements in glucose and lipid homeostasis with disastrous vascular complications. Despite immense research efforts aimed at a clear understanding of the etiology of this complex disease, the molecular mechanisms causing the disorder still remain elusive. This article reviews extant data from recent publications implicating novel signal transduction pathways as important regulators of the insulin stimulus-secretion coupling in the pancreatic β-cell. The significance of nitric oxide and serine/threonine protein phosphatases, and their inactivation by insulin secretagogues, glucose metabolites, ATP, GTP, glutamate, and inositol hexaphosphate in this arena is scrutinized. Additionally, also presented is the growing concept that an important signal for insulin secretion may reside in the inextricable interplay between glucose and lipid metabolism, specifically the generation of malonyl-CoA, which inhibits carnitine palmitoyl-transferase 1 with the attendant accumulation of long-chain acyl CoA esters. Moreover, attention is directed towards novel intracellular actions of hypoglycemic sulfonylureas in the β-cell. Finally, the importance of “lipotoxicity” and aberrations in glucose uptake and metabolism in β-cell dysfunction is given consideration. Future research efforts should aim at further characterization of effects of second messengers on protein phosphorylation elements in β-cells. Additionally, long-term regulation by glucose and the diabetic state (e.g., fatty acids and ketones) on β-cell protein phosphatases, pyruvate dehydrogenase, and carnitine palmitoyltransferase 1 needs to be explored in greater depth. Clearly, the detrimental impact of diabetic hyperlipidemia on β-cell function has been a relatively neglected area, but future pharmacological approaches directed at preventing lipotoxicity may prove beneficial in the treatment of diabetes.

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