Myocyte-specific enhancer-binding factor (MEF-2) regulates alpha-cardiac myosin heavy chain gene expression in vitro...

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