Hepatic Rictor Knockout Mouse Model and the Metabolic Syndrome

描述（由申请人提供）：伴随着肥胖和糖尿病的并行流行，高血糖症（肾病、神经病变、视网膜病变和血管疾病）和高脂血症（动脉粥样硬化和心血管疾病）的并发症给我们的医疗保健系统带来了越来越大的负担。胰岛素抵抗是肥胖、糖尿病和代谢综合征的病理生理学以及许多相关并发症的共同贡献者。肝脏是葡萄糖和脂质代谢的中心，胰岛素起着关键的调节作用。肝葡萄糖生成升高导致高血糖症;而极低密度脂蛋白（VLDL）-甘油三酯（TG）生成升高导致致动脉粥样硬化性血脂异常（TG升高、小而密的低密度脂蛋白（LDL）和高密度脂蛋白（HDL）降低）。由此产生的高血糖症和血脂异常归因于肝脏中胰岛素作用的改变。 肝脏中的“选择性”胰岛素抵抗的特征在于胰岛素不能抑制脂肪生成（受损的FOXO 1调节），同时继续刺激从头脂肪生成和VLDL-TG分泌（SREBP 1c的完整调节）。因此，选择性胰岛素抵抗是高血糖症和高脂血症可能发生的潜在机制;然而，所涉及的分子缺陷尚不清楚。我推测“选择性”胰岛素抵抗是由于AKT上丝氨酸473（S473）和苏氨酸308（T308）磷酸化位点的解偶联导致的部分AKT活化; S473磷酸化的丧失导致AKT不能磷酸化FOXO 1并抑制关键的促凋亡酶的表达，而部分活化的AKT（仅T308）足以磷酸化和抑制GSK 3，导致SREBP 1c活化和高甘油三酯血症。我将通过研究小鼠模型来验证这一假设，在该模型中，mTORC 2调节蛋白rictor在肝细胞中被基因删除，导致S473磷酸化受损，T308磷酸化完整。我将验证rictor在肝脏中的遗传丢失、rictor基因表达的基因剂量特异性效应和rictor/mTORC 2功能（AKT磷酸化）。将采用正葡萄糖钳夹和示踪剂稀释技术定量胰岛素抑制肝葡萄糖生成的能力。将通过泰洛沙泊和放射性标记脂质研究评估高脂血症，以定量VLDL-TG产生和清除速率以及对脂蛋白谱（VLDL、LDL、HDL和apoB）的影响。将通过肝脏脂质定量评估肝脂肪变性的可能性。 公共卫生相关性：“选择性”肝胰岛素抵抗被假设为导致高血糖和高脂血症;与肥胖、糖尿病和代谢综合征相关的发病率和死亡率的代谢危险因素。我推测肝脏中“选择性”胰岛素抵抗的分子机制涉及胰岛素信号分子AKT水平的失调。本文的研究旨在提高我们对高血糖症和高脂血症发病机制的理解，并可能最终产生改善的治疗和/或预防方法。

DESCRIPTION (provided by applicant): Concomitant with the parallel obesity and diabetes epidemics, an increasing burden on our healthcare system is due to the complications of hyperglycemia (nephropathy, neuropathy, retinopathy and vascular disease) and hyperlipidemia (atherosclerosis and cardiovascular disease). Insulin resistance is a common contributor to the pathophysiology of obesity, diabetes and metabolic syndrome, and many of their associative complications. The liver is central to glucose and lipid metabolism and insulin plays a key regulatory role. Elevated hepatic glucose production contributes to hyperglycemia; whereas, increased very low-density lipoproteins (VLDL)- Triglyceride (TG) production contributes to atherogenic dyslipidemia (elevated TGs, small and dense low- density lipoproteins (LDL) and reduced high-density lipoproteins (HDL)). The resultant hyperglycemia and dyslipidemia are attributed to altered insulin action in the liver. "Selective" insulin resistance in liver is characterized by inability of insulin to suppress gluconeogenesis (impaired FOXO1 regulation) while continuing to stimulate de novo lipogenesis and VLDL-TG secretion (intact regulation of SREBP1c). Thus, selective insulin resistance is a potential mechanism by which hyperglycemia and hyperlipidemia can ensue; however, the molecular defect involved is not known. I hypothesize that "selective" insulin resistance is due to partial AKT activation that results from the uncoupling of Serine473 (S473) and Threonine308 (T308) phosphorylation sites on AKT; loss of S473 phosphorylation results in the failure of AKT to phosphorylate FOXO1 and suppress expression of key gluconeogenic enzymes, whereas partially activated AKT (only T308) is sufficient to phosphorylate and inhibit GSK3, leading to SREBP1c activation and hypertriglyceridemia. I will test this hypothesis by studying a mouse model in which the mTORC2 regulatory protein, rictor, has been genetically deleted in hepatocytes leading to impaired S473 phosphorylation with intact T308 phosphorylation. I will validate genetic loss of rictor in the liver, gene dosage specific effects of rictor gene expression and rictor/mTORC2 function (AKT phosphorylation). Euglycemic clamp and tracer dilution techniques will be employed to quantify the ability of insulin to suppress hepatic glucose production. Hypertriglyceridemia will be assessed by utilizing tyloxapol and radiolabeled lipid studies to quantify the rate of VLDL-TG production and clearance, and effects on the lipoprotein profile (VLDL, LDL, HDL and apoB). The possibility of hepatic steatosis will be assessed by liver lipid quantification. PUBLIC HEALTH RELEVANCE: "Selective" hepatic insulin resistance is hypothesized to contribute towards hyperglycemia and hyperlipidemia; metabolic risk factors for morbidity and mortality related to obesity, diabetes and metabolic syndrome. I hypothesize that a molecular mechanism for "selective" insulin resistance in the liver involves dysregulation at the level of the insulin signaling molecule, AKT. The studies herein are designed to improve our understanding of the pathogenesis of hyperglycemia and hyperlipidemia and may ultimately yield improved therapeutic and/or preventive approaches.