Mitogenic Signal Transduction in Pancreatic Beta-Cells

描述(申请人提供)：现在已经认识到2型糖尿病是一种胰岛素缺乏的疾病。2型糖尿病与功能性胰腺细胞团的减少有关，后者不再补偿外周胰岛素抵抗。因此，保持胰岛素分泌需求的最佳细胞群，特别是通过促进细胞存活，是延缓2型糖尿病和1型糖尿病发病的关键。在这方面，IRS-2已被证明在细胞生长和存活中发挥关键作用。IRS-2表达增加会促进细胞的生长和存活，而IRS-2表达不足则会导致自发性细胞凋亡。虽然IRS-2蛋白和信使核糖核酸在胰岛细胞中的半衰期很短，但IRS-2的表达受到有效和高度调控的控制，主要是在转录水平上调节的。在基础条件下，细胞IRS-2基因转录受转录因子FoxO通过IRS-2启动子中的胰岛素反应元件(IRE)控制。当IRS-2/PI3K/PKB信号在细胞中被激活时，FoxO转录因子失活，IRS-2表达减少，这似乎是一种暂时的负反馈机制，阻止了IRS-2信号的持续。然而，IRS-2的表达可以通过替代手段在-细胞中独立调控。在生理相关范围内，葡萄糖是细胞IRS-2基因转录的主要调节因子。这需要葡萄糖代谢，而且依赖于钙离子。它可能提供了一种在代谢需求急剧变化时保持细胞健康的机制，而且很重要，因为其他因素，如胰岛素，只会以葡萄糖依赖的方式增加细胞中IRS-2的表达。然而，这些早期的发现需要证实。这一建议意味着在分子水平上更好地了解胰腺细胞IRS-2表达的调控。目的是为了更好地描述基础条件下IRS-2基因转录的调控，重点是确定PI3K/PKB信号下游的哪个特定FoxO转录因子促进IRS-2的表达。此外，我们还将确定细胞中葡萄糖代谢增加所产生的特定次级信号与IRS-2表达增加(特别是通过钙/钙调素)有关。本研究的目的是在IRS-2基因启动子中定义一个葡萄糖调节顺式元件(S)(GREs)，然后确定一个与GREs糖调节方式特异相关的反式作用因子(S)。因此，对控制正常、肥胖和2型糖尿病患者IRS-2表达的分子机制有了更深入的了解 ？-细胞将从这些拟议的研究中涌现。肥胖相关的2型糖尿病在美国是一个主要的健康问题，由分泌胰岛素的胰腺细胞丧失引起。需要新的治疗方法，旨在保护内源性细胞群体，以产生足够的胰岛素来无限期地延缓糖尿病的发生。IRS-2是细胞存活的关键基因，预计对IRS-2表达控制的新见解将导致一种新的方法来维持体内足够的细胞数量和足够的胰岛素产生，这反过来将缓解甚至预防2型糖尿病的症状。公共卫生相关性：2型糖尿病是由于功能性胰腺细胞群减少，不再能够补偿外周胰岛素抵抗，因此通过促进细胞存活和保护来维持有效的细胞群是延缓2型糖尿病发病的关键。IRS-2在细胞生长和存活中起关键作用，其表达受到严格控制(主要是在转录水平)，但对这种调控知之甚少。这项应用的总体目标是更好地了解IRS-2在细胞中转录控制背后的分子机制，这可能最终导致一种新的治疗方法，通过保持IRS-2的最佳表达来促进细胞存活，从而随后可能无限期地推迟糖尿病的发生。

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.