高脂诱导骨骼肌胰岛素抵抗的分子调控机制研究

Although lipotoxicity has been associated with type 2 diabetes mellitus, the precise occurrence mechanism of lipotoxicity is not yet known. We found that sterol regulatory element binding protein 1c (SREBP1c), as a master transcription factor that control lipogenesis, has directly inhibitory effect on transcription of insulin receptor substrate 1(IRS-1) in L6 myotube cells and suppress skeletal muscle insulin signaling. These studies suggest that SREBP-1c could be central to the pathogenesis of fatty acids-induced insulin resistance in skeletal muscle, however, much less is known about transcriptional control mechanism of SREBP-1c expression in that setting. Liver X receptor (LXR), which form heterodimers with retinoid X receptor (RXR), is known to be inducer of the SREBP-1c gene. We demonstrated by chromatin immunoprecipition (CHIP-qPCR) assay that the acetylation of histone H3 and H4 at the promoter of SREBP-1c especially in LXR response element (LXRE) was increased in palmitic acid (PA)-treated L6 cells, which was decreased by histone deacetylase sirtuin 1(SirT1) agonist resveratrol treatment.Our results suggest that fatty acids promote SREBP-1c transcription involved in insulin resistance potentially through the mechanism of histone acetylation to allow increased binding of key transcription factors to the promoter of SREBP-1c. Thus, further studies will be performed to investigate the chromatin events and molecular mechanism in lipotoxicity conditions in vivo and in vitro, and to examine the dysregulation of histone acetylation to the development of SREBP-1c transcription and insulin resistance, which will provide clues into the specific therapeutic target for improving insulin resistance in type 2 diabetes mellitus.

胆固醇调节元件结合蛋白-1 c（sterol regulatory element-binding protein 1c，SREBP-1c）是脂肪酸合成的关键转录因子。我们的最新研究发现:SREBP-1c高表达也可抑制骨骼肌胰岛素受体底物-1转录，促进细胞胰岛素抵抗，表明SREBP-1c可能是脂毒性形成的重要环节。肝脏X受体（liver X receptor, LXR）可诱导SREBP-1c转录，我们通过CHIP-qPCR分析发现：高脂使SREBP-1c转录调控区特别是LXR结合原件位点的组蛋白乙酰化水平增加，而去乙酰化酶（Sirtuin1）激动剂使其水平降低，表明高脂可能通过乙酰化组蛋白调控LXR诱导的SREBP-1c转录。本课题拟在前期工作基础上研究高脂对组蛋白乙酰化修饰的作用及其对SREBP-1c转录、骨骼肌胰岛素抵抗形成的病理作用机制，为胰岛素抵抗特异性治疗提供新的思路。

已有研究表明骨骼肌胰岛素抵抗与肌细胞内脂质沉积密切相关。临床与基础研究表明早期胰岛素治疗可以改善2型糖尿病患者及动物模型中血糖，并缓解脂毒性，但机制不明。调控脂质合成的关键转录因子固醇调节元件结合蛋白-1c（SREBP-1c）可促进肌细胞内脂质沉积而导致骨骼肌胰岛素抵抗，在脂毒性及胰岛素抵抗的形成中起了重要作用，同时受腺苷酸活化蛋白激酶（AMPK）负调控。为此，我们建立了棕榈酸（PA）诱导的骨骼肌脂毒性细胞模型，并通过转染及ChIP等技术在细胞水平上确定 SREBP-1c 调控肌细胞胰岛素信号通路的目标分子及作用，同时在体内体外脂毒性模型中，通过siRNA及负性失活转染等技术明确了胰岛素经AMPK/SREBP-1c通路改善骨骼肌脂毒性胰岛素抵抗的的分子机制，进一步研究了AMPK/SREBP-1c通路在抗糖尿病药物二甲双胍对骨骼肌胰岛素抵抗的作用分子机制和靶点。本研究在国际上首次发现 SREBP-1c 具有直接调控骨骼肌细胞胰岛素信号的作用，同时明确了胰岛素及二甲双胍等抗糖尿病药物改善骨骼肌胰岛素抵抗的分子机制和靶点，为研发改善胰岛素抵抗的特异性治疗靶点奠定了理论基础。