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Mitogenic Signal Transduction in Pancreatic Beta-Cells

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It has now been realized that type-2 diabetes is a disease of insulin insufficiency. Type-2 diabetes is associated with a decrease in functional pancreatic ss-cell mass that no longer compensates for the peripheral insulin resistance. As such, maintaining an optimal ss-cell population for the insulin secretory demand, especially by promoting ss-cell survival, is key for delaying the onset of type-2, as well as type-1, diabetes. In this regard, IRS-2 has been shown to play a pivotal role in ss-cell growth and survival. Increased IRS-2 expression promotes ss-cell growth and survival, whereas insufficient IRS-2 expression leads to spontaneous ss-cell apoptosis. Although IRS-2 protein and mRNA half-life is short in islet ss-cells, this is countered by efficient and highly regulated control of IRS-2 expression, predominately mediated at the transcriptional level. Under basal conditions, ss-cell IRS-2 gene transcription is controlled by a FoxO transcription factor via an insulin response element (IRE) in the IRS-2 promoter. When IRS-2/PI3K/PKB signaling is activated in ss-cells, FoxO transcription factors are consequently inactivated and IRS-2 expression is reduced, in what appears to be a temporal negative feedback mechanism to prevent IRS-2 signaling from being sustained. However, IRS-2 expression can be independently controlled in ss-cells by alternative means. Glucose, in the physiologically relevant range, is a major regulator of ss-cell IRS-2 gene transcription. This requires glucose metabolism and is Ca2+-dependent. It likely provides a mechanism to preserve ss-cell well-being during acute changes in metabolic demand, and is important since other factors, like incretins, only increae IRS-2 expression in ss-cells in a glucose-dependent fashion. However, these early findings need substantiating. This proposal means to gain a better insight into the control of IRS-2 expression in pancreatic ss-cells at the molecular level. It is intended to better characterize control of IRS-2 gene transcription under basal conditions with an emphasis on identifying which particular FoxO transcription factor downstream of PI3K/PKB signaling increases IRS-2 expression. In addition, we will pinpoint which particular secondary signals emanating from increased glucose metabolism in ss-cells link to increased IRS-2 expression (especially via Ca2+/CaMK). It is intended to define a glucose-regulatory cis-element(s) (GREs) in the IRS-2 gene promoter and then identify a trans-acting factor(s) that specifically associates with the GRE glucose-regulatory manner. Thus, a much deeper insight into the molecular mechanism that controls IRS-2 expression in normal, obese and type-2 diabetic primary ss-cells will emerge from these proposed studies. Obesity-linked type-2 diabetes is a major health problem in the US and caused by loss of pancreatic ss-cells that produce insulin. Novel therapeutic approaches are needed which are aimed at protecting the endogenous ss-cell population to produce enough insulin to delay, perhaps indefinitely, the onset of diabetes. IRS-2 is a gene key to ss-cell survival, and it is anticipated that new insight into the control of IRS-2 expression will lead to a novel means of maintaining adequate ss-cell numbers and sufficient insulin production in vivo, that in turn will alleviate, or perhaps even prevent, symptoms of type-2 diabetes.
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