MEF2B is a component of a smooth muscle-specific complex that binds an A/T-rich element important for smooth muscle myosin heavy chain gene expression

To understand smooth muscle-specific gene expression, we have focused our studies on the smooth muscle myosin heavy chain (SMHC) gene, a smooth muscle-specific marker. In this study, we demonstrate that the SMHC promoter region (-1594 to -1462 base pairs) containing the A/T-rich element can activate the heterologous thymidine kinase promotor in smooth muscle cells, but not in fibroblasts. Mutations of this A/T-rich element decreased SMHC promoter activity significantly. Both gel mobility shift assays and DNase I footprinting revealed that this region binds to specific protein complexes from smooth muscle nuclear extracts, whereas nuclear extracts form skeletal muscle and fibroblasts produced a different binding pattern. We also demonstrate that the protein complex obtained from smooth muscle nuclear extract reacts with MEF2B-specific antibody, but not with antibodies specific to MEF2A, MEF2C, or MEF2D, suggesting that only MEF2B protein binds to the A/T-rich element. Furthermore, MEF2B overexpression in smooth muscle cells up-regulated the SMHC promoter, suggesting that MEF2B is important for SMHC gene regulation. This is the first report demonstrating a role for MEF2 factors in smooth muscle-specific gene expression.