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1University of Cape Town/Medical Research Center Research Unit for Exercise Science and Sports Medicine, Department of Human Biology, University of Cape Town, Cape Town South Africa; 2Center for Cardiovascular Research, Washington University School of Medicine, St. Louis, Missouri
Submitted 1 January 2006 ; accepted in final form 13 September 2006
In vitro binding assays have indicated that the exercise-induced increase in muscle GLUT4 is preceded by increased binding of myocyte enhancer factor 2A (MEF2A) to its cis-element on the Glut4 promoter. Because in vivo binding conditions are often not adequately recreated in vitro, we measured the amount of MEF2A that was bound to the Glut4 promoter in rat triceps after an acute swimming exercise in vivo, using chromatin immunoprecipitation (ChIP) assays. Bound MEF2A was undetectable in nonexercised controls or at 24 h postexercise but was significantly elevated 
6 h postexercise. Interestingly, the increase in bound MEF2A was preceded by an increase in autonomous activity of calcium/calmodulin-dependent protein kinase (CaMK) II in the same muscle. To determine if CaMK signaling mediates MEF2A/DNA associations in vivo, we performed ChIP assays on C2C12 myotubes expressing constitutively active (CA) or dominant negative (DN) CaMK IV proteins. We found that 75% more MEF2A was bound to the Glut4 promoter in CA compared with DN CaMK IV-expressing cells. GLUT4 protein increased 70% 24 h after exercise but was unchanged by overexpression of CA CaMK IV in myotubes. These results confirm that exercise increases the binding of MEF2A to the Glut4 promoter in vivo and provides evidence that CaMK signaling is involved in this interaction.
rats; C2C12 myotubes; chromatin immunoprecipitation assay; autonomous calcium/calmodulin-dependent protein kinase activity; myocyte enhancer factor 2A; glucose transporter-4
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