Exercise and CaMK activation both increase the binding of MEF2A to the Glut4 promoter in skeletal muscle in vivo

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 approximately 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 C(2)C(12) myotubes expressing constitutively active (CA) or dominant negative (DN) CaMK IV proteins. We found that approximately 75% more MEF2A was bound to the Glut4 promoter in CA compared with DN CaMK IV-expressing cells. GLUT4 protein increased approximately 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.