Transcriptional Regulation of Metabolic and Hepatic Homeostasis by FoxO Signaling

描述（由申请人提供）：该提案“FoxO信号对代谢和肝脏稳态的转录调节”通过研究燃料激素在肝脏中的作用来解决糖尿病的基本机制。2型糖尿病患者血胰高血糖素水平升高和胰岛素作用缺陷是导致高血糖的原因，但其分子机制尚不明确。胰高血糖素和胰岛素相互控制代谢和细胞稳态，其中肝脏是执行其细胞功能的主要器官。在空腹肝脏中，胰高血糖素刺激糖异生，降解糖原、脂质和蛋白质等大分子，并促进调节细胞器更新的自噬途径。在喂养的肝脏中，胰岛素逆转胰高血糖素的分解代谢。从禁食到摄食的代谢和细胞适应需要通过两种激素的相反作用严格控制基因转录，而适应的失败导致糖尿病中的高血糖。调控多种生物过程的叉头转录因子Foxo1受到胰岛素信号的抑制。胰岛素通过PKB激活使小鼠Foxo1的Ser253位点或人类Foxo1的S256位点磷酸化，并触发Foxo1核输出和细胞质隔离以实现泛素化。相反，当胰岛素水平降低或胰岛素抵抗时，胰高血糖素促进空腹肝脏或糖尿病肝脏中Foxo1蛋白的稳定性。Foxo1可以介导胰高血糖素第二信使环AMP对糖异生酶和自噬基因表达的影响，但Foxo1及其受胰高血糖素调控，特别是与胰岛素抵抗协同作用在代谢中的作用尚不清楚

DESCRIPTION (provided by applicant): This proposal "Transcriptional Regulation of Metabolic and Hepatic Homeostasis by FoxO Signaling" addresses the fundamental mechanism of diabetes by studying fuel hormone's action in the liver. Elevated blood glucagon levels and defective insulin action in patients with type 2 diabetes are responsible for hyperglycemia, but the molecular mechanisms remain elusive. Glucagon and insulin reciprocally control metabolic and cellular homeostasis, in which the liver is a major organ that executes their cellular functions. In the fasting liver, glucagon stimulates gluconeogenesis, degrades macromolecules including glycogen, lipid, and protein, and promotes the autophagic pathway that regulates cellular organelle turnover. In the feeding liver, insulin reverses the catabolic metabolism of glucagon. The metabolic and cellular adaptation from fasting to feeding requires a tight control of gene transcription by opposing effects of the two hormones, and the failure of the adaptation causes hyperglycemia in diabetes. The forkhead transcription factor Foxo1 that regulates multiple biological processes is inhibited by insulin signaling. Insulin phosphorylates Foxo1 at Ser253 in mice or S256 in humans via PKB activation, and triggers Foxo1 nuclear export and cytoplasmic sequestration for ubiquitination. Conversely, glucagon promotes Foxo1 protein stability in the fasting liver or the liver of diabetes when insulin level is decreased or insulin resistance occurs. Foxo1 can mediate the effect of cyclic AMP, the second messenger of glucagon, on expression of gluconeogenic enzymes and autophagic genes, but the role of Foxo1 and its regulation by glucagon, particularly in concert with insulin resistance, in metabolic regulation and cellular function is completely unclear. In Aim 1, we will use Foxo1 liver-specific knockout mice and examine whether Foxo1 is a key mediator in glucagon signaling to regulate hepatic glucose production, glycogenolysis, lipid and protein homeostasis, mitochondrial turnover and function, autophagy and survival, whereas disruption of Foxo1 prevents the glucagon-induced biological processes that promote the development of diabetes. In Aim2, we will use mass-spectrometry and phospho-specific antibodies to determine whether Foxo1 phosphorylation at S153 by glucagon and protein kinase PKA promotes nuclear targeting and whether phosphorylation at S276 enhances transcriptional activity in cells. In Aim 3, we have generated Foxo1- S253A mutant mice mimicking insulin resistance at the Foxo1 level in vivo. Using this unique mouse model, we will determine whether glucagon stimulates the effect of dephosphorylated Foxo1, disrupting metabolic and cellular homeostasis and liver function. In overall, we use Foxo1 gene loss- and gain-of-function approaches to investigate the physiological role of Foxo1 in glucagon action and identify novel molecular mechanisms of Foxo1 activation, which will advance our understanding of the mechanism of diabetes and help develop strategies detecting and inhibiting the glucagon->Foxo1 pathway to control the disease.