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1 Department of Exercise and Sport Science, East Carolina University, Greenville, NC, USA; Department of Physiology, East Carolina University, Greenville, NC, USA
2 Department of Biochemistry, The University of Oklahoma, Oklahoma City, OK, USA
3 Department of Physiology and Experimental Biology, Brigham Young University, Provo, UT, USA
4 Department of Physiology, East Carolina University, Greenville, NC, USA
* To whom correspondence should be addressed. E-mail: bfh0530{at}email.arizona.edu.
As the primary glucose transporter in skeletal muscle, GLUT4 is an important factor in the regulation of blood glucose. We previously reported that stimulation of AMP-activated protein kinase (AMPK) with 5-aminoimidazole-4-carboxamide-1-
-D-ribofuranoside (AICAR) increased GLUT4 expression in muscle. GLUT4 Enhancer Factor (GEF) and Myocyte Enhancer Factor 2 (MEF2) have been shown to be important for normal GLUT4 expression because deletion or truncation of the consensus sequences on the promoter causes depressed GLUT4 mRNA expression. This led to the current study to investigate possible roles for GEF and MEF2 in mediating the activation of GLUT4 gene transcription in response to AMPK. Here we show that while AMPK does not appear to phosphorylate MEF2A, AMPK directly phosphorylates the GEF protein in vitro. MEF2 and GEF are activated in response to AMPK as we observed translocation of both to the nucleus after AICAR treatment. Nuclear MEF2 protein content was increased after 2 h and GEF protein was increased in the nucleus 1 and 2 h post AICAR treatment. Lastly, GEF and MEF2 increase in binding to the GLUT4 promoter within 2 h after AICAR treatment. Thus, we conclude that GEF and MEF2 mediate the AMPK induced increase in transcription of skeletal muscle GLUT4. AMPK can phosphorylate GEF and in response to AICAR, GEF and MEF2 translocate to the nucleus and have increased binding to the GLUT4 promoter.
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