Inhibition of CPT-1b in muscle: effects on glucose homeostasis

描述(申请人提供)：相当多的证据支持这样一种观点，即饮食中脂肪的过量供应超过了脂肪组织的储存能力，并导致异位脂肪堆积，导致骨骼肌、肝脏、胰腺和可能的 其他组织，导致胰岛素抵抗。一种流行的理论是，受损的骨骼肌脂肪酸氧化(FAO)导致细胞内脂质中间体的积累，而这些中间体与胰岛素信号的缺陷直接相关。其他人报告说，通过高脂肪IET提供的脂肪过剩实际上会增加FAO，以至于肉碱和TCA循环中间体受到限制，导致线粒体异常和骨骼肌胰岛素抵抗。因此，有证据表明，脂肪毒性和线粒体功能障碍都与骨骼肌胰岛素抵抗有关。确定这些是否以及如何相互交织在一起是2型糖尿病研究中最热门的话题之一，其中最重要的问题是：骨骼肌中FAO的抑制是否会导致胰岛素抵抗？为了解决这个问题，我们创造了肌肉中缺乏肉碱棕榈酰转移酶-1b(CPT-1b-/-)的小鼠。正如预测的那样，CPT-1bm-/-小鼠降低了线粒体FAO，增加了IMCL，增加了循环自由脂肪酸(FFA)和甘油三酯(TG)，并降低了体力活动和运动耐力。然而，CPT-1bm-/-小鼠没有胰岛素抵抗，并且降低了循环中的胰岛素和血糖，改善了胰岛素和葡萄糖耐量，增加了丙酮酸氧化，并增加了全身碳水化合物的氧化。乍一看，尽管有这种疾病的标志性预测指标，但缺乏胰岛素抵抗与流行的脂毒性理论不符。事实上，这表明CPT-1bm-/-小鼠经历了独特的适应，以在骨骼肌FAO减少的情况下保持胰岛素敏感性。初步研究揭示了潜在的显著变化，促进了脂质的吸收和储存，线粒体的生物发生，增强的过氧体粮农组织，以及刺激与mTOR信号级联相关的因子。具体目标1：在CPT-1bm-/-小鼠中使用饮食和遗传操作，以更好地了解线粒体FAO抑制的急性和慢性后果。具体目标2：评估CPT-1b降低对葡萄糖和脂肪酸摄取和储存、线粒体数量和功能以及过氧化体粮农组织的影响。具体目标3：研究能量缺乏信号如何通过营养敏感途径转导，从而影响胰岛素敏感性。这些创新性的研究将以更明确的方式检验脂毒假说和线粒体超载假说，为CPT-1b和FAO在线粒体功能和胰岛素抵抗中的作用提供关键的机制信息。

DESCRIPTION (provided by applicant): Considerable evidence supports the idea that oversupply of dietary fat exceeds the storage capacity of adipose tissue and leads to ectopic lipid accumulation resulting in "metabolic stress" in skeletal muscle, liver, pancreas and possibly other tissues, leading to insulin resistance. One prevailing theory is that impaired skeletal muscle fatty acid oxidation (FAO) leads to the cytosolic accumulation of lipid intermediates that are directly linked to defects in insulin signaling. Others report lipid oversupply via a high fat iet can actually increase FAO to the extent that carnitine and TCA cycle intermediates are limiting, leading to mitochondrial abnormalities and skeletal muscle insulin resistance. Thus, evidence exists that both lipotoxicity and mitochondrial dysfunction contribute to skeletal muscle insulin resistance. Determining if and how these are intertwined is one of the hottest topics in type 2 diabetes research, with the fundamentally important question being: Does inhibition of FAO in skeletal muscle contribute to insulin resistance? To address this question we created mice lacking Carnitine Palmitoyltransferase-1b (CPT-1b) in muscle (CPT-1bm-/-). As predicted, CPT-1bm-/- mice have decreased mitochondrial FAO, increased IMCL, increased circulating free fatty acids (FFA) and triglycerides (TG), and decreased physical activity and exercise endurance. However, CPT-1bm-/- mice are not insulin resistant and have decreased circulating insulin and glucose, improved insulin and glucose tolerance, increased pyruvate oxidation, and increased whole body carbohydrate oxidation. At first glance, the lack of insulin resistance in spite of having hallmark predictors of the disease is at odds with prevailing lipotoxic theories. Indeed, it indicates that CPT-1bm-/- mice undergo unique adaptations to maintain insulin sensitivity in the face of decreased skeletal muscle FAO. Preliminary studies reveal potentially significant alterations promoting lipid uptake and storage, mitochondrial biogenesis, enhanced peroxisomal FAO, and stimulation of factors linked to the mTor signaling cascade. Specific Aim 1: Employ dietary and genetic manipulations in CPT-1bm-/- mice to gain a better understanding of acute and chronic consequences of mitochondrial FAO inhibition. Specific Aim 2: To evaluate the effects of decreased CPT-1b on glucose and fatty acid uptake and storage, mitochondrial number and function, and peroxisomal FAO. Specific Aim 3: To investigate how energy deficit signals are transduced through nutrient sensitive pathways to influence insulin sensitivity. These innovative studies will test the lipotoxic hypothesis and the mitochondrial overload hypothesis in a more definitive manner, providing critical mechanistic information on the role of CPT-1b and FAO in mitochondrial function and insulin resistance.