Rangos Research Center, Children's Hospital of Pittsburgh of University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania 15224, USA.
Endocrinology. 2010 Aug;151(8):3521-35. doi: 10.1210/en.2009-1306. Epub 2010 May 25.
Forkhead box O1 (FoxO1) is a transcription factor that mediates the inhibitory effect of insulin on target genes in hepatic metabolism. Hepatic FoxO1 activity is up-regulated to promote glucose production during fasting and is suppressed to limit postprandial glucose excursion after meals. Increased FoxO1 activity augments the expression of insulin receptor (IR) and IR substrate (IRS)2, which in turn inhibits FoxO1 activity in response to reduced insulin action. To address the underlying physiology of such a feedback loop for regulating FoxO1 activity, we delivered FoxO1-ADA by adenovirus-mediated gene transfer into livers of adult mice. FoxO1-ADA is a constitutively active allele that is refractory to insulin inhibition, allowing us to determine the metabolic effect of a dislodged FoxO1 feedback loop in mice. We show that hepatic FoxO1-ADA production resulted in significant induction of IR and IRS2 expression. Mice with increased FoxO1-ADA production exhibited near glycogen depletion. Unexpectedly, hepatic FoxO1-ADA production elicited a profound unfolded protein response, culminating in the induction of hepatic glucose-regulated protein 78 (GRP78) expression. These findings were recapitulated in primary human and mouse hepatocytes. FoxO1 targeted GRP78 gene for trans-activation via selective binding to an insulin responsive element in the GRP78 promoter. This effect was counteracted by insulin. Our studies underscore the importance of an IR and IRS2-dependent feedback loop to keep FoxO1 activity in check for maintaining hepatic glycogen homeostasis and promoting adaptive unfolded protein response in response to altered metabolism and insulin action. Excessive FoxO1 activity, resulting from a dislodged FoxO1 feedback loop in insulin resistant liver, is attributable to hepatic endoplasmic reticulum stress and metabolic abnormalities in diabetes.
叉头框蛋白 O1（FoxO1）是一种转录因子，可介导胰岛素对肝代谢靶基因的抑制作用。在禁食期间，肝 FoxO1 活性上调以促进葡萄糖生成，而在餐后则被抑制以限制餐后葡萄糖波动。FoxO1 活性增加会增强胰岛素受体（IR）和胰岛素受体底物（IRS）2 的表达，而这反过来又会抑制 FoxO1 活性，以响应胰岛素作用的降低。为了解释调节 FoxO1 活性的这种反馈环的潜在生理学机制，我们通过腺病毒介导的基因转移将 FoxO1-ADA 递送到成年小鼠的肝脏中。FoxO1-ADA 是一种组成性激活的等位基因，对胰岛素抑制具有抗性，使我们能够确定在小鼠中脱离 FoxO1 反馈环的代谢效应。我们发现肝 FoxO1-ADA 的产生导致 IR 和 IRS2 表达的显著诱导。FoxO1-ADA 产生增加的小鼠几乎耗尽了肝糖原。出乎意料的是，肝 FoxO1-ADA 的产生引起了严重的未折叠蛋白反应，最终导致肝葡萄糖调节蛋白 78（GRP78）表达的诱导。这些发现可在原代人和鼠肝细胞中重现。FoxO1 通过选择性结合 GRP78 启动子中的胰岛素反应元件，靶向 GRP78 基因进行反式激活。这种效应被胰岛素抵消。我们的研究强调了 IR 和 IRS2 依赖性反馈环对于维持 FoxO1 活性以维持肝糖原稳态和促进代谢改变和胰岛素作用下适应性未折叠蛋白反应的重要性。由于胰岛素抵抗性肝中 FoxO1 反馈环的脱离而导致的 FoxO1 活性过度增加，归因于肝内质网应激和糖尿病中的代谢异常。
