FoxO6 in Glucose Metabolism

描述(申请人提供)：糖异生是一个维持生命的过程，在饥饿或长期禁食期间为大脑、睾丸和红细胞提供唯一的燃料来源。糖异生主要发生在肝脏中，代谢途径受到胰岛素的严格调控。当肝脏胰岛素信号失控时，糖异生作用不减，导致过量的葡萄糖产生，并导致糖尿病患者的空腹高血糖。我们的长期目标是确定将胰岛素作用受损与不受抑制的糖异生联系起来的因素。我们的研究发现FoxO6在糖异生中起着重要作用。FoxO6是前额蛋白O家族的新成员，具有未知的代谢功能。我们发现，肝脏FOXO6活性在摄食状态下维持在较低的基础水平，但对禁食的反应明显不受调节。在胰岛素抵抗的肝脏中可以检测到FoxO6活性增强，这与肥胖和糖尿病患者不受限制的糖异生有关。FoxO6刺激培养的肝细胞糖异生，这一作用可被胰岛素所抵消。胰岛素通过位点特异性磷酸化抑制FoxO6的活性，而不改变其亚细胞分布，这是FoxO6区别于FoxO家族其他成员的独特机制。我们的数据强调了FoxO6在葡萄糖代谢中的重要性；这促使我们假设FoxO6失调可能与胰岛素抵抗受试者空腹高血糖的发病机制有关。为了解决这一假说，我们提出了三个具体的目标：1)表征FoxO6在糖异生中的作用，并确定其对血糖代谢的贡献；2)研究FoxO6整合胰岛素信号与肝脏中糖异生的不同机制；以及3)确定FoxO6在肥胖和糖尿病空腹高血糖发病机制中的功能贡献。为了实现这些目标，我们将使用基因转移、转基因过表达、基因敲除和siRNA介导的基因沉默方法来实现正常小鼠和糖代谢改变小鼠的FoxO6功能获得和功能丧失。我们已经提供了原则性证明，并论证了该建议的可行性。该项目的完成将加深我们对胰岛素依赖的肝糖异生调控的理解，揭示了一条微调禁食和消退状态下肝脏葡萄糖生成速率的新的调控途径。虽然糖异生途径一直是抗高血糖治疗的主要靶点，但FoxO6依赖的糖异生途径的发现将为改善糖尿病的血糖控制提供潜在的治疗途径。 与公共卫生相关：肝脏中葡萄糖的过量产生可归因于空腹高血糖，这是一种常见于病态肥胖或糖尿病控制不佳的受试者的病理状态。人们对其背后的机制知之甚少。我们的目标是表征胰岛素作用受损和无限制葡萄糖产生的因素，以确定改善糖尿病血糖控制的新治疗靶点。

DESCRIPTION (provided by applicant): Gluconeogenesis is a life-sustaining process for providing the sole fuel source for brain, testes and erythrocytes during starvation or prolonged fasting. Gluconeogenesis takes place mainly in liver in a metabolic pathway that is tightly regulated by insulin. When hepatic insulin signaling goes awry, gluconeogenesis becomes unabated, resulting in excessive glucose production and contributing to fasting hyperglycemia in diabetes. Our long-term goal is to characterize factors that link impaired insulin action to unrestrained gluconeogenesis. Our research identified FoxO6 as an important player in gluconeogenesis. FoxO6 is a new member of the forehead box O family, with unassigned function in metabolism. We show that hepatic FoxO6 activity is maintained at low basal levels in fed states, but is markedly unregulated in response to fasting. Augmented FoxO6 activity is detectable in insulin resistant livers, correlating with unrestrained gluconeogenesis in obesity and diabetes. FoxO6 stimulates gluconeogenesis in cultured hepatocytes and this effect is counteracted by insulin. Insulin inhibits FoxO6 activity via site-specific phosphorylation without altering its subcellular distribution, a distinct mechanism that distinguishes FoxO6 from other members of FoxO family. Our data underscore the importance of FoxO6 in glucose metabolism; spurring the hypothesis that FoxO6 dysregulation may contribute to the pathogenesis of fasting hyperglycemia in insulin resistant subjects. To address this hypothesis, we propose three specific aims: 1) To characterize the role of FoxO6 in gluconeogenesis and determine its contribution to blood glucose metabolism; 2) To investigate the distinct mechanism by which FoxO6 integrates insulin signaling to gluconeogenesis in liver; and 3) To determine the functional contribution of FoxO6 to the pathogenesis of fasting hyperglycemia in obesity and diabetes. To achieve these goals, we will employ gene transfer, transgenic overexpression, gene knockout and siRNA- mediated gene-silencing approaches to achieve FoxO6 gain- vs. loss-of-function in normal mice and mice with altered glucose metabolism. We have provided proof-of-principle and demonstrated the feasibility for the proposal. Accomplishing this project will deepen our understanding of insulin-dependent regulation of hepatic gluconeogenesis, by revealing a new regulatory pathway for fine-tuning the rate of hepatic glucose production between fasting and receding states. While the gluconeogenic pathway has been a major target for anti- hyperglycemia therapies, revelation of FoxO6-dependent gluconeogenic pathway will provide a potential therapeutic avenue for improving glycemic control in diabetes. PUBLIC HEALTH RELEVANCE: Excessive glucose production in liver is attributable to fasting hyperglycemia, a pathological condition that is commonly seen in subjects with morbid obesity or poorly controlled diabetes. The underlying mechanism is poorly understood. Our goal is to characterize factors that couple impaired insulin action to unrestrained glucose production for identifying novel therapeutic targets for improving glycemic control in diabetes.