Hyperinsulinemia, mTOR activity and plasma lipoproteins

描述（由申请人提供）：在2型糖尿病和代谢综合征中，肝脏似乎对胰岛素对脂肪生成、VLDL脂质和载脂蛋白ob分泌的影响保持敏感。在与Accili博士的合作研究中，我们发现肝脏中胰岛素受体（IRs）基因水平降低的Ldlr-/-小鼠的VLDL分泌和动脉粥样硬化减少。然而，在LDLR功能正常的小鼠中，肝脏胰岛素信号传导受限也导致LDLR水平降低，这表明胰岛素信号传导增加VLDL分泌的能力被LDLR水平的增加所抵消。最近的研究表明，胰岛素信号对VLDL分泌和LDLR水平的调节可能是通过影响肝脏mTOR活性来介导的。有趣的是，mTOR信号被发现抑制Sort（一种最近在CAD和LDL水平的GWAS中发现的基因）的表达，导致VLDL甘油三酯和载脂蛋白ob分泌增加。初步结果表明，这些作用可能是由mtor诱导的内质网应激介导的，导致Sort的转录抑制因子ATF3的表达增加。相反，LDLR的水平似乎受到mTOR下游独特途径的调节，该途径导致Pcsk9的表达降低和转录后LDLR的增加。拟议的研究将验证肝脏mTOR信号作为整合胰岛素和营养因子信号的中心枢纽来调节VLDL分泌和LDLR水平的假设。这一假设将使用最近可用的小鼠进行测试，这些小鼠具有肝脏特异性敲除调节肝脏mTOR活性的关键分子，即Li-Tsc1KO （mTOR1活性增加）和Li-RapKO小鼠（mTOR1活性降低）。与Accili和Tabas博士一起，我们将寻求证明影响VLDL分泌和LDLR的胰岛素信号的遗传操纵作用于mTOR的上游，而ER应激、ATF3和Sort作用于mTOR的下游，调节VLDL载脂蛋白ob和脂质分泌。与Tabas博士一起，我们将分析肥胖和瘦弱受试者的肝脏样本，以确定人类是否也存在类似的Sort调节。这些研究将为肥胖和高胰岛素血症患者VLDL分泌和LDLR水平的调节提供新的见解。

DESCRIPTION (provided by applicant): In Type 2 diabetes and metabolic syndrome, the liver appears to remain sensitive to the effects of insulin on lipogenesis and VLDL lipid and apoB secretion. In collaborative studies with Dr Accili, we showed that Ldlr-/- mice with genetically reduced levels of insulin receptors (IRs) in liver had decreased VLDL secretion and atherosclerosis. However, in mice with functioning LDLRs, restricted hepatic insulin signaling also led to diminished LDLR levels, indicating that the ability of insulin signaling to increase VLDL secretion is offset by an increase in LDLR levels. Recent studies have shown that the regulation of VLDL secretion and LDLR levels by insulin signaling may be mediated via effects on hepatic mTOR activity. Interestingly, mTOR signaling has been found to repress expression of Sort (a gene recently identified in GWAS of CAD and LDL levels), leading to increased VLDL triglyceride and apoB secretion. Preliminary results indicate that these effects may be mediated by mTOR-induced ER stress, leading to increased expression of ATF3, a transcriptional repressor of Sort. In contrast, the levels of LDLR appear to be regulated by a distinctive pathway downstream of mTOR that leads to decreased expression of Pcsk9 and a post-transcriptional increase in LDLR. The proposed studies will test the hypothesis that hepatic mTOR signaling acts as a central hub integrating signals from insulin and nutritional factors to regulate VLDL secretion and LDLR levels. This hypothesis will be tested using recently available mice with liver-specific knock-outs of key molecules regulating hepatic mTOR activity i.e. Li-Tsc1KO (increased mTOR1 activity) and Li-RapKO mice (reduced mTOR1 activity). With Drs Accili and Tabas, we will seek to show that genetic manipulations of insulin signaling that affect VLDL secretion and LDLR act upstream of mTOR, while ER stress, ATF3 and Sort act downstream of mTOR to regulate VLDL apoB and lipid secretion. With Dr Tabas, we will analyze liver samples from obese and lean subjects to determine if similar regulation of Sort occurs in humans. These studies should provide new insights into the regulation of VLDL secretion and LDLR levels in subjects with obesity and hyperinsulinemia. PUBLIC HEALTH RELEVANCE: The dyslipidemia of Type 2 diabetes and metabolic syndrome is characterized by excessive production of VLDL but relatively normal levels of LDL cholesterol. However, the underlying mechanisms have remained poorly understood. The proposed studies should provide new insights into the regulation of VLDL secretion and LDLR levels in these conditions.