Regulation of Exercise-induced Increase in GLUT4 Expression

众所周知，在高脂饮食和db/db小鼠中，通过将肌肉组织中的GLUT4含量增加一倍，可以完全预防糖尿病的发生。事实上，有规律的运动训练增加了骨骼肌中GLUT4的含量，防止了糖尿病的发生。因此，我们正在研究运动介导的预防糖尿病的与Glut4调节相关的分子机制。先前的研究表明，小鼠Glut4基因的脂肪组织特异性元件(S)(ASE)位于5‘-侧翼序列的-551至-506之间，而用于下调Glut4基因的高脂反应元件(S)位于碱基-701至-552之间。核因子1(NF1)的结合位点位于碱基-700和-688之间，它介导胰岛素和cAMP诱导的3T3-L1脂肪细胞中GLUT4的抑制。为了研究NF1在体内白色脂肪组织(WAT)中对GLUT4基因表达的调控作用，我们建立了两种类型的转基因小鼠，它们在GLUT4微基因结构中含有NF1结合位点的5‘或3’半突变。在这两种情况下，GLUT4微型基因在Wat中不表达，而在棕色脂肪组织、骨骼肌和心脏中保持表达。这是一个意想不到的发现，因为在Wat中表达了一个没有NF1结合位点的-551GLUT4微型基因。我们提出了一个模型，解释了在WAT中表达GLUT4对ASE和NF1结合位点的要求。此外，我们还创建了在GLUT4微基因构建中含有MEF2结合位点的转基因小鼠。在这些转基因小鼠中，GLUT4基因在心脏中不表达，而在棕色脂肪组织、骨骼肌和Wat中表达较低。MEF-2结合位点是GLUT4在心脏表达所必需的。

It is known that the occurrence of diabetes mellitus can be completely prevented in high-fat diet and db/db mice by doubling the amount of GLUT4 in muscle tissues. Indeed, regular exercise training increased GLUT4 amount in skeletal muscles and prevented development of diabetes mellitus. We are therefore studying molecular mechanisms for exercise-mediated prevention of diabetes mellitus in relation to GLUT4 regulation.Previous studies demonstrated that an adipose tissue specific element(s) (ASE) of the murine GLUT4 gene is located between -551 and -506 in the 5'-flanking sequence and that a high-fat responsive element(s) (HFRE) for down-regulation of the GLUT4 gene is located between bases -701 and -552. A binding site for nuclear factor 1 (NF1), that mediates insulin and cAMP-induced repression of GLUT4 in 3T3-L1 adipocytes is located between bases -700 and -688. To examine the role of NF1 in the regulation of GLUT4 gene expression in white adipose tissues (WAT) in vivo, we created two types of transgenic mice harboring mutated either 5' or 3' half-site of NF1 binding sites in GLUT4 minigene constructs. In both cases, the GLUT4 minigene was not expressed in WAT, while expression was maintained in brown adipose tissue, skeletal muscle, and heart. This was an unexpected finding, since a -551 GLUT4 minigene that did not have the NF1 binding site, was expressed in WAT. We propose a model that explains the requirement for both the ASE and the NF1 binding site for expression of GLUT4 in WAT.In addition, we created of transgenic mice harboring MEF2 binding sites in GLUT4 minigene constructs. In these transgenic mice, the GLUT4 minigene was not expressed in heart, while expression was lower but maintained in brown adipose tissue, skeletal muscle, and WAT. MEF 2 binding site is necessary for GLUT4 expression in heart.