MYOCYTE-SPECIFIC ENHANCER-BINDING FACTOR (MEF-2) REGULATES ALPHA-CARDIAC MYOSIN HEAVY-CHAIN GENE-EXPRESSION IN-VITRO AND IN-VIVO

A myocyte-specific enhancer-binding factor (MEF-2) DNA binding site wa s identified in the rat alpha-myosin heavy chain (MHC) gene adjacent t o the E-box binding site for alpha-MHC binding factor-2 (BF-2). Mutati on of the MEF-2 site, within the context of the full-length promoter, reduced activity by 85 and 80% in neonatal cardiomyocytes and the adul t heart, respectively. Mutation of the BF-2 site reduced activity appr oximately 70% in both models. A MEF-2/BF-2 double mutant gave signific antly less activity than the BF-2 mutant but not the MEF-2 mutant, sug gesting the possibility that BF-2 and MEF-2 interact. Mutations in MEF -2, which decreased functional activity, also abolished MEF-2 DNA bind ing activity. MEF-2 DNA binding activity was present in the developing heart, reached a peak in the late fetal and early neonatal stages, an d then declined to low levels in the adult heart. The adult levels wer e sufficient to support alpha-MHC gene expression. MEF-2 activity was increased 2-3-fold in the adult heart subjected to a pressure or volum e overload. Two working models are proposed as possible explanations o f the antithetic relationship between MEF-2 levels and alpha-MHC gene expression.