AMPK在胰岛素抑制SREBP-1c活化而改善骨骼肌脂毒性胰岛素抵抗中的作用机制研究

Lipotoxicity plays an important role in the association between obesity and type 2 diabetes mellitus. Our recent study indicated for the first time that sterol regulatory element binding protein 1c (SREBP-1c), a transcription factor that controls cellular lipogenesis, participates in fatty acid-induced insulin resistance through a direct effect of suppressing the transcription of insulin receptor substrate-1 in skeletal muscle cells, addressing its potential importance in metabolic disease. Earlier studies suggested that insulin promotes SREBP-1c activation, whereas we found that insulin reduced SREBP-1c nuclear translocation and improved insulin resistance in vivo and in vitro, and little is known about the mechanism. AMP-activated protein kinase (AMPK) is known as an intracellular energy sensor. We found that the inhibition of SREBP-1c activity by insulin was significantly attenuated with AMPK dominant negative lentivirus. Meanwhile, we found that insulin increased AMPK phosphorylation and increased the AMP/ATP ratio in palmitate-pretreated L6 myotubes. These studies suggested that the activation of AMPK is necessary for insulin suppression of SREBP-1c activity in lipid nutrient excess. Based on these results, further studies will be performed to provide the molecular insight into the mechanism by which insulin activates AMPK to inhibit SREBP-1c activity and regulates mitochondrial function to increase AMP/ATP ratio. This study will extend the understanding of lipotoxicity and provide clues into the specific therapeutic target for improving insulin resistance and timing for insulin treatment in type 2 diabetes mellitus.

脂毒性是肥胖导致2型糖尿病的主要病理生理机制。我们近期发现促进脂质合成的转录因子固醇调节元件结合蛋白1 c（SREBP-1c）可直接抑制胰岛素受体底物-1的基因启动子转录而导致骨骼肌胰岛素抵抗。既往研究表明胰岛素增强SREBP-1c转录活性，而我们体内体外实验均发现胰岛素可抑制SREBP-1c活化所导致的骨骼肌胰岛素抵抗，但机制不明确。我们进一步研究发现通过显性失活慢病毒抑制腺苷酸活化蛋白激酶（AMPK）活性则可拮抗胰岛素上述作用，同时胰岛素激活AMPK，升高AMP/ATP比值，表明胰岛素可能通过激活AMPK而拮抗SREBP-1c活化，但机制尚不明确。本项目通过建立基因敲除的小鼠模型和细胞模型等方法，进一步阐明AMPK在胰岛素抑制骨骼肌SREBP-1c活化而改善脂毒性胰岛素抵抗中的作用及其分子机制。研究结果将有助于推进对脂毒性机制的认识，并为早期胰岛素治疗2型糖尿病的靶点研究奠定理论基础。

胰岛素是常见的糖尿病治疗药物之一，2型糖尿病早期给予胰岛素强化治疗，在改善糖毒性同时还可减轻脂毒性。固醇调节元件结合蛋白-1c（sterol regulatory element binding protein-1c, SREBP-1c）是调控脂质合成的关键转录因子，可直接抑制胰岛素受体底物-1（insulin receptor substrate-1，IRS-1）基因启动子转录，从糖脂代谢多条途径促进胰岛素抵抗发生。有意义的是，胰岛素治疗可抑制高脂诱导的SREBP-1c活化，改善骨骼肌胰岛素信号通路转导，其机制有待明确。我们发现高脂条件下，胰岛素通过腺苷酸活化蛋白激酶 （AMP-activated protein kinase, AMPK）抑制高脂诱导的SREBP-1c活化，减少脂质合成、促进脂肪酸氧化，从而减少骨骼肌内脂质沉积，减轻胰岛素抵抗。进一步研究提示，胰岛素恢复AMPK磷酸化水平的机制包括：①上调AMPK经典上游激酶——肝激酶B1（liver kinase B1, LKB1）表达；②抑制线粒体解偶联蛋白3（uncoupling protein 3, UCP3）泛素化降解，减少腺苷三磷酸（adenosine triphosphate, ATP）合成，降低细胞能荷。基于此，临床研究也提示，早期胰岛素治疗可改善初治2型糖尿病患者骨骼肌线粒体氧化磷酸化效率，其改变量与基础胰岛功能及平均血糖水平改善呈正相关；并且，基线空腹血糖、糖化水平高，以及有2型糖尿病家族史患者获益更明显。本研究补充了早期胰岛素治疗获益机制，并为2型糖尿病个体化治疗提供理论依据。