Prohibitin expression and function in ethanol treated pancreatic beta-cells
Lee, Jong Han
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Type 2 diabetes is now recognized as a worldwide epidemic. Pancreatic beta-cell decompensation in the presence of insulin resistance is a major mechanism for the development of type 2 diabetes and may be triggered by mitochondrial dysfunction. Alcoholism is a known risk factor for type 2 diabetes. Excessive or chronic alcohol consumption leads to increased oxidative stress and mitochondrial dysfunction in beta-cells. Prohibitin is a multifunctional protein that also regulates mitochondrial biogenesis and function. Although it has anti-oxidant effects in some cell types, its role in pancreatic beta-cells is not known. This study has investigated the effects of prohibitin in ethanol treated pancreatic beta-cells using RINm5F and INS-1E cell lines. Prohibitin was found to be expressed in pancreatic beta-cells with localization to the nucleus and the perinuclear area. Ethanol increased the expression of prohibitin and induced its translocation from the nucleus to the mitochondria. Ethanol, through its metabolism by alcohol dehydrogenase (ADH), increased oxidative stress and altered mitochondrial membrane potential, decreased the activity of mitochondrial respiratory complexes I and IV, and uncoupled energy production with resulting reduction in ATP production. This was associated with activation of the proinflammatory enzyme c-Jun N-terminal kinase and proapoptotic proteins Bax and caspase-3, leading to beta-cell apoptosis. Ethanol also reduced glucose induced insulin secretion without alteration of the beta-cell transcription factors PDX-1 and MafA. Treatment with exogenous prohibitin or cellular overexpression of endogenous prohibitin attenuated ADH activity, prevented the deleterious effects of ethanol on mitochondrial function and reduced apoptosis, whereas prohibitin knockdown enhanced ethanol-induced apoptosis. In addition, prohibitin per se increased PDX-1 and MafA levels. Through the above mechanisms, prohibitin restored glucose induced insulin secretion in ethanol exposed beta-cells. In brief, ethanol causes mitochondrial dysfunction and induces apoptosis in beta-cells, which result in a reduction of insulin secretion; whereas prohibitin prevents mitochondrial dysfunction, apoptosis, and -cell failure by stabilizing mitochondrial complexes I and IV and inhibiting ADH activity during ethanol metabolism. In addition, prohibitin in itself increases the levels of beta-cell transcription factors. As a consequence, prohibitin maintains normal pancreatic beta-cell function and could be useful in diabetes prevention and treatment.