Mechanism for down-regulation of GLUT4 by high-fat diet

GLUT4是胰岛素反应性葡萄糖转运蛋白，介导骨骼肌和脂肪组织中葡萄糖代谢的限速步骤。高脂肪喂养下调GLUT4 mRNA并降低白色脂肪组织（WAT）中胰岛素刺激的葡萄糖摄取。通过使用5缺失的小鼠GLUT4小基因转基因小鼠，我们发现小鼠GLUT4的脂肪细胞特异性元件(s)相对于转录起始位点位于-551和-506之间的45 bps范围内，而高脂肪响应元件(s)位于-701和-551之间的150 bps范围内。DNase I足迹分析显示，来自高碳水化合物和高脂肪饮食的小鼠WAT核提取物对DNase I消化的-705至682（对应NF-1位点）和-673至651区域具有保护作用。结果表明，无论饮食如何，核蛋白都会与这些区域结合。转基因动物研究表明，701附近NF-1位点的突变使GLUT4的表达减少，特别是在白色脂肪组织中。这些数据表明，一种假定的脂肪细胞特异性增强子在551和506之间结合NF-1和45 bps，而高脂肪饮食反应性增强子在701和551之间结合150 bps，并与脂肪细胞特异性增强子有一些相互作用。

GLUT4, the insulin responsive-glucose transporter, mediates the rate limiting step of glucose metabolism in skeletal muscles and adipose tissues. High-fat feeding downregulates GLUT4 mRNA and decreases in insulin-stimulated glucose uptake in white adipose tissues (WAT). By using 5 -deleted mouse GLUT4 minigene transgenic mice, we have demonstrated that adipocyte specific element (s) of mouse GLUT4 are located within 45 bps between -551 and -506 relative to transcription initiation site, while high fat responsive element (s) is located within 150 bps between -701 and -551.DNase I footprinting analysis revealed that nuclear extract in WAT from both high-carbohydrate and high-fat diet fed mice protected the region from -705 to 682 (correspond to NF-1 site) and from -673 to 651 from DNase I digestion. This result showed that nuclear proteins bind to these regions regardless diet. Transgenic animal study indicated that mutation of NF-1 site near 701 abolished GLUT4 expression specially in white adipose tissues. These data suggest that a putative adipocyte specific enhancer binds both NF-1 and 45 bps between 551 and 506, while high-fat diet responsive enhancer binds 150 bps between 701 and 551 with some interaction to adipocyte specific enhancer.