ChREBP Isoforms in Pancreatic Beta Cells

功能性胰腺β细胞群的扩增是1型和2型糖尿病的主要治疗目标。葡萄糖是适应性β细胞群扩增的关键生理驱动因素，在体外和体内均促进β细胞增殖。近二十年来，我们一直在研究葡萄糖调节基因表达的机制。在2012年，我们发现碳水化合物反应元件结合蛋白（ChREBP; Mlxipl）是啮齿动物和人类β细胞中葡萄糖刺激的β细胞增殖所必需的。同年，我们的合作研究者（Mark赫尔曼博士）发现ChREBP以2种主要亚型存在。ChREBP α是主要在细胞质中并在低葡萄糖下被抑制的全长形式。ChREBP β是一种选择性剪接产物，其核输出信号和低葡萄糖抑制结构域被去除。因此，ChREBP β主要是核，并且具有组成性和潜在活性。葡萄糖响应性ChREBP α激活后，强有力的碳水化合物响应元件可诱导ChREBP β转录，形成强有力的前馈环（图1）。值得注意的是，在每个详细研究的系统中，ChREBP β的诱导是驱动ChREBP主要生理作用的分子引擎。在脂肪组织或肝脏中，ChREBP β的诱导分别增加新生脂肪生成和增加或降低胰岛素敏感性。在β细胞中，我们最近发现，葡萄糖刺激的增殖需要ChREBP β的指数诱导-从几乎检测不到的水平到与ChREBP α相当的量。然而，引人注目的是，β细胞中ChREBP β的过度表达，如可能发生在长期高血糖或异位病毒表达中，导致β细胞凋亡。与此形成鲜明对比的是，ChREBP α的过表达放大了葡萄糖刺激的β细胞增殖，而没有细胞死亡。我们的总体假设是，ChREBP α与ChREBP β丰度的比率对于β细胞功能至关重要，无论是对于β细胞质量扩增还是当失调时，作为葡萄糖毒性的介体。我们将测试我们的假设， （1）确定增加或减少以下物质的丰度的影响： β细胞中ChREBP β特异性对β细胞质量和葡萄糖稳态的影响;（2）确定人胰岛中ChREBP α或ChREBP β的外源性表达是否改善或抑制胰岛移植结果;（3）确定ChREBP α与ChREBP β的比例如何控制β细胞的命运。该应用使用新的工具，将提供对转录葡萄糖传感的基本机制理解，这将为扩大和保护β细胞群的疗法提供信息。

Expansion of functional pancreatic beta cell mass is a major therapeutic goal for both Type 1 and Type 2 diabetes. Glucose is a key physiological driver of adaptive beta cell mass expansion, promoting beta cell proliferation both in vitro and in vivo. We have been studying mechanisms of glucose-regulated gene expression for nearly two decades. In 2012 we found that carbohydrate response element binding protein (ChREBP; Mlxipl) is required for glucose-stimulated beta cell proliferation in both rodent and human beta cells. That same year, our co-investigator (Dr. Mark Herman) discovered that ChREBP exists as 2 major isoforms. ChREBPα is the full-length form that is mostly cytoplasmic and repressed in low glucose. ChREBPβ is a product of alternative splicing, with the nuclear export signals and the low glucose inhibitory domain removed. Consequently, ChREBPβ is mostly nuclear, and is constitutively and potently active. ChREBPβ transcription is induced by a powerful carbohydrate response element after activation of the glucose-responsive ChREBPα, creating a vigorous feed-forward loop (Fig. 1). Remarkably, in every system studied in detail, the induction of ChREBPβ is the molecular engine that drives the major physiological effects of ChREBP. In adipose tissue or liver, induction of ChREBPβ increases de novo lipogenesis and either increases or decreases insulin sensitivity, respectively.. In beta cells, we recently showed that the exponential induction of ChREBPβ - from a nearly undetectable level to amounts comparable to ChREBPα - is required for glucose-stimulated proliferation. Strikingly, however, overexpression of ChREBPβ in beta cells, as may happen with prolonged hyperglycemia or by ectopic viral expression, results in beta cell apoptosis. In stark contrast, overexpression of ChREBPα amplifies glucose-stimulated beta cell proliferation without cell death. Our overarching hypothesis is that the ratio of ChREBPα to ChREBPβ abundance is critically important for beta cell function, both for beta cell mass expansion and, when dysregulated, as a mediator of glucose toxicity. We will test our hypothesis by performing the following Specific Aims: (1) Determine the effects of increasing or decreasing the abundance of ChREBPβ specifically in beta cells on beta cell mass and glucose homeostasis; (2) Determine if exogenous expression of ChREBPα or ChREBPβ in human islets improves or worsens islet transplantation outcomes; and (3) Determine how the ratio of ChREBPα to ChREBPβ controls the fate of beta cells. This application uses novel tools that will provide an essential mechanistic understanding of transcriptional glucose sensing that will inform therapies to expand and protect beta cell mass.