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1 UCT/MRC Research Unit for Exercise Science & Sports Medicine, Human Biology, University of Cape Town, Capetown, Western Province, South Africa
2 Center for Cardiovascular Research, Washington University, St. Louis, Missouri, United States
* To whom correspondence should be addressed. E-mail: eojuka{at}sports.uct.ac.za.
In vitro binding assays have indicated that the exercise-induced increase in muscle GLUT4 is preceded by increased binding of MEF2A to its cis-element on the Glut4 promoter. Since 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 non-exercised controls or at 24h post exercise, but was significantly elevated ~ 6h post exercise. Interestingly, the increase in bound MEF2A was preceded by increases in phosphorylation and 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 ~100% more MEF2A was bound to the Glut4 promoter in CA compared to DN CaMK IV-expressing cells. GLUT4 protein increased ~ 70% 24h post exercise but was unchanged by over-expression 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.
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