Mitogenic Signal Transduction in Pancreatic Beta-Cells

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DESCRIPTION (provided by applicant): 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 ¿-cell mass that no longer compensates for the peripheral insulin resistance. As such, maintaining an optimal ¿-cell population for the insulin secretory demand, especially by promoting ¿-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 ¿-cell growth and survival. Increased IRS-2 expression promotes ¿-cell growth and survival, whereas insufficient IRS-2 expression leads to spontaneous ¿-cell apoptosis. Although IRS-2 protein and mRNA half-life is short in islet ¿-cells, this is countered by efficient and highly regulated control of IRS-2 expression, predominately mediated at the transcriptional level. Under basal conditions, ¿-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 ¿-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 ¿-cells by alternative means. Glucose, in the physiologically relevant range, is a major regulator of ¿-cell IRS-2 gene transcription. This requires glucose metabolism and is Ca2+-dependent. It likely provides a mechanism to preserve ¿-cell well-being during acute changes in metabolic demand, and is important since other factors, like incretins, only increase IRS-2 expression in ¿-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 ¿-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 ¿-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 ¿-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 ¿-cells that produce insulin. Novel therapeutic approaches are needed which are aimed at protecting the endogenous ¿-cell population to produce enough insulin to delay, perhaps indefinitely, the onset of diabetes. IRS-2 is a gene key to ¿-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 ¿-cell numbers and sufficient insulin production in vivo, that in turn will alleviate, or perhaps even prevent, symptoms of type-2 diabetes. PUBLIC HEALTH RELEVANCE: Type-2 diabetes is caused by a decrease in functional pancreatic ¿-cell mass that is no longer able to compensate for the peripheral insulin resistance, and thus maintaining an effective ¿-cell population by promoting ¿-cell survival and protection is key for delaying the onset of type-2 diabetes. IRS-2 plays a pivotal role in ¿-cell growth and survival, and its expression is tightly controlled (predominately at the transcriptional level), but little is known about this regulation. The overall goal of this application is to get better insight into the molecular mechanism behind transcriptional control of IRS-2 in ¿-cells, that may eventually lead to a novel therapeutic means of promoting ¿-cell survival via maintaining optimal IRS-2 expression to subsequently delay, perhaps indefinitely, the onset of diabetes.