Ojuka Edward O, Jones Terry E, Nolte Lorraine A, Chen May, Wamhoff Brian R, Sturek Michael, Holloszy John O
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Am J Physiol Endocrinol Metab. 2002 May;282(5):E1008-13. doi: 10.1152/ajpendo.00512.2001.
There is evidence suggesting that adaptive increases in GLUT4 and mitochondria in skeletal muscle occur in parallel. It has been reported that raising cytosolic Ca(2+) in myocytes induces increases in mitochondrial enzymes. In this study, we tested the hypothesis that an increase in cytosolic Ca(2+) induces an increase in GLUT4. We found that raising cytosolic Ca(2+) by exposing L6 myotubes to 5 mM caffeine for 3 h/day for 5 days induced increases in GLUT4 protein and in myocyte enhancer factor (MEF)2A and MEF2D, which are transcription factors involved in regulating GLUT4 expression. The caffeine-induced increases in GLUT4 and MEF2A and MEF2D were partially blocked by dantrolene, an inhibitor of sarcoplasmic reticulum Ca(2+) release, and completely blocked by KN93, an inhibitor of Ca(2+)-calmodulin-dependent protein kinase (CAMK). Caffeine also induced increases in MEF2A, MEF2D, and GLUT4 in rat epitrochlearis muscles incubated with caffeine in culture medium. 5-Aminoimidazole-4-carboxamide ribonucleoside (AICAR), which activates AMP-activated protein kinase (AMPK), also induced approximately twofold increases in GLUT4, MEF2A, and MEF2D in L6 myocytes. Our results provide evidence that increases in cytosolic Ca(2+) and activation of AMPK, both of which occur in exercising muscle, increase GLUT4 protein in myocytes and skeletal muscle. The data suggest that this effect of Ca(2+) is mediated by activation of CAMK and indicate that MEF2A and MEF2D are involved in this adaptive response.
有证据表明，骨骼肌中葡萄糖转运蛋白4（GLUT4）和线粒体的适应性增加是同时发生的。据报道，提高心肌细胞胞质中的钙离子（Ca(2+)）会导致线粒体酶增加。在本研究中，我们验证了胞质Ca(2+)增加会诱导GLUT4增加这一假设。我们发现，通过将L6肌管暴露于5 mM咖啡因中，每天3小时，持续5天来提高胞质Ca(2+)，会导致GLUT4蛋白以及肌细胞增强因子（MEF）2A和MEF2D增加，MEF2A和MEF2D是参与调节GLUT4表达的转录因子。咖啡因诱导的GLUT4、MEF2A和MEF2D增加被丹曲林（一种肌浆网Ca(2+)释放抑制剂）部分阻断，并被KN93（一种Ca(2+)-钙调蛋白依赖性蛋白激酶（CAMK）抑制剂）完全阻断。咖啡因还会使在培养基中用咖啡因培养的大鼠肱三头肌中的MEF2A、MEF2D和GLUT4增加。5-氨基咪唑-4-甲酰胺核苷（AICAR）可激活AMP激活的蛋白激酶（AMPK），它也会使L6心肌细胞中的GLUT4、MEF2A和MEF2D增加约两倍。我们的结果提供了证据，证明运动肌肉中发生的胞质Ca(2+)增加和AMPK激活都会增加心肌细胞和骨骼肌中的GLUT4蛋白。数据表明，Ca(2+)的这种作用是由CAMK激活介导的，并且表明MEF2A和MEF2D参与了这种适应性反应。
