A molecular basis for endotoxin mediated insulin resistance

描述(由申请人提供)：骨骼肌中的胰岛素抵抗是2型糖尿病(T2D)的一个主要标志，也是这种疾病发展过程中早期可检测到的异常。然而，T2D肌肉胰岛素抵抗的分子基础还不完全清楚。我们的实验室(初步数据)和其他人发现，胰岛素抵抗(肥胖和T2D)受试者血浆中脂多糖(脂多糖或内毒素)浓度增加。脂多糖是革兰氏阴性细菌细胞壁外膜的一种成分，通过激活Toll样受体4(TLR4)诱导炎症反应。有研究认为，肠道微生物区系是胰岛素抵抗受试者(代谢性内毒素血症)中观察到的过量内毒素的来源。TLR4和受该受体调控的炎症信号转导通路和丝裂原活化蛋白激酶(MAPKs)在胰岛素抵抗的发病机制中起重要作用。尽管越来越多的证据表明胰岛素抵抗受试者血浆内毒素浓度增加，但目前尚不清楚代谢性内毒素血症是否会导致肌肉中的炎症和胰岛素抵抗(胰岛素信号转导和胰岛素刺激的葡萄糖摄取受损)，以及这些影响是否由TLR4介导。拟议的研究将通过利用WT和TLR4突变小鼠的代谢性内毒素血症模型来解决这一差距。总的假设是，代谢性内毒素血症通过激活TLR4和该受体下游的信号通路而导致胰岛素抵抗。为了检验这一假设，我们将探讨以下具体目标。具体目标1将确定高脂饮食诱导的胰岛素抵抗是否涉及肠道通透性增加，血浆内毒素水平升高，以及体内是否需要TLR4来实现内毒素的有害影响。具体目标2将确定在生理范围内循环内毒素的实验性升高是否在体内导致肌肉胰岛素抵抗，以及这种影响是否由TLR4介导。综上所述，这些研究将利用WT和TLR4突变小鼠在长期高脂饮食和急性内毒素输注下的高胰岛素、正血糖钳夹技术，为胰岛素抵抗的发病机制提供新的见解。如果阳性，我们的结果将表明，旨在降低血浆内毒素浓度和/或阻断TLR4的策略将有助于减轻炎症和改善T2D患者的胰岛素作用。因此，我们的结果可能导致开发治疗这种疾病的新方法。

DESCRIPTION (provided by applicant): Insulin resistance in skeletal muscle is a major hallmark of type 2 diabetes (T2D) and an early detectable abnormality in the development of this disease. However, the molecular basis for the muscle insulin resistance of T2D is not fully understood. Our laboratory (preliminary data) and others have found that insulin resistant (obese and T2D) subjects have increased concentrations of lipopolysaccharide (LPS or endotoxin) in plasma. LPS is a component of the outer membrane of gram negative bacteria cell walls which induces an inflammatory response by activating toll-like receptor-4 (TLR4). It has been proposed that intestinal microbiota is the source of the excessive LPS observed in insulin resistant subjects (metabolic endotoxemia). TLR4 and inflammatory signaling pathways regulated by this receptor [inhibitor ?B kinase IKK-nuclear factor-?B (?F??) and mitogen activated protein kinases (MAPKs)] have been implicated in the pathogenesis of insulin resistance. Despite the accumulating evidence that insulin resistant subjects have increased concentrations of LPS in plasma, it is unclear whether metabolic endotoxemia causes inflammation and insulin resistance (impaired insulin signaling and insulin-stimulated glucose uptake) in muscle and whether these effects are mediated by TLR4. The proposed studies will address this gap by utilizing a model of metabolic endotoxemia in WT and TLR4-mutant mice. The overall hypothesis is that metabolic endotoxemia causes insulin resistance by activating TLR4 and signaling pathways downstream this receptor. To test this hypotheses the following specific aims will be addressed. Specific Aim 1 will determine whether high fat diet-induced insulin resistance involves increased intestinal permeability, elevated plasma levels of LPS, and whether TLR4 is required for the deleterious effects of LPS in vivo. Specific Aim 2 will determine whether an experimental elevation in circulating LPS, within a physiologic range, causes muscle insulin resistance in vivo and whether this effect is mediated by TLR4. In summary, the proposed studies will utilize the hyperinsulinemic, euglycemic clamp technique in WT and TLR4 mutant mice subjected to chronic high fat diet and acute endotoxin infusion, to provide new insights into the molecular mechanisms responsible for the pathogenesis of insulin resistance. If positive, our results would indicate that strategies aimed at lowering plasma LPS concentrations and/or blocking TLR4 will help to reduce inflammation and improve insulin action in patients with T2D. As such, our results could lead to the development of novel approaches for the treatment of this disease.