The MEF2A isoform is required for striated muscle-specific expression of the insulin-responsive GLUT4, glucose transporter

Previously, we have demonstrated that an MEF2 consensus sequence located be tween -473/-464 in the human GLUT4 gene was essential for both tissue-speci fic and hormonal/metabolic regulation of GLUT4 expression (Thai, M. V., Gur uswamy, S., Cao, K. T., Pessin, J. E., and Olson, A. L. (1998) J. Biol. Che m. 273, 14285-14292). To identify the specific MEF2 isoform(s) responsible for GLUT4 expression, we studied the pattern of expression of the MEF2 isof orms in insulin-sensitive tissues. Both heart and skeletal muscle were foun d to express the MEF2A, MEF2C, and MEF2D isoforms but not MEF2B. However, o nly the MEF2A protein was selectively downregulated in insulin-deficient di abetes. Co-immunoprecipitation with isoform-specific antibodies revealed th at, in the basal state, essentially all of the MEF2A protein was presented as a MEF2A-MEF2D heterodimer without any detectable MEF2A-MEF2A homodimers or MEF2A-MEF2C and MEF2C-MEF2D heterodimers. Electrophoretic mobility shift assays revealed that nuclear extracts from diabetic animals had reduced bi nding to the MEF2 binding site compared with extracts from control or insul in-treated animals. Furthermore, immunodepletion of the MEF2A-MEF2D complex from control extracts abolished binding to the MEF2 element. However, addi tion of MEF2A to diabetic nuclear extracts fully restored binding activity to the MEF2 element. These data strongly suggest that the MEF2A-MEF2D heter odimer is selectively decreased in insulin-deficient diabetes and is respon sible for hormonally regulated expression of the GLUT4 gene.