Vascular Endothelial Growth Factor Induces MEF2C and MEF2-Dependent Activity in Endothelial Cells

Vascular endothelial growth factor (VEGF, also referred to as VEGF-A) is a key regulator in physiological angiogenesis and in pathologic angiogenic processes, such as those associated with tumor growth and retinal neovascularization.1 The proangiogenic activity of VEGF derives from its ability to regulate many steps in the angiogenic process, including proliferation, migration, survival, and permeability. Much is known about the upstream events mediating the effect of VEGF on endothelial cells, including the major signaling pathways that are activated.2–4 Relatively less is known about the key transcription factors involved in the angiogenic effects of VEGF. The myocyte enhancer factor (MEF)-2 family of transcription factors consists of a group of transcriptional activators (MEF2A, -B, -C, and -D) that show homology in a MADS (MCM1, Agamous, Deficiens, serum response factor) box and an adjacent motif known as the MEF2 domain.5 MEF2 factors play a pivotal role in the morphogenesis and myogenesis of skeletal, cardiac, and smooth muscle cells.6 Of the MEF2 family members, MEF2C appears to have a particularly important role in vasculogenesis and angiogenesis during vascular development. MEF2C is expressed in endothelial cells in vivo7 and in vitro.8 Targeted deletion of the MEF2C gene in mice results in embryonic lethality between embryonic day (E) 9.5 and E10,7,9 with significant cardiovascular defects.7,10 These mice exhibit severe vascular abnormalities, including the absence of yolk sac vascularization.7 In these mice, endothelial cells are able to differentiate but are unable to become organized into a vascular network.7 The overall phenotype of MEF2C mutant mice is similar to that of mice lacking VEGF or Flt-1.7 MEF2C appears to be a direct transcriptional target of Ets transcription factors in the developing and the adult vasculature. A transcriptional enhancer from the MEF2C gene containing four consensus Ets transcription factor binding sites is sufficient to direct gene expression to the vascular endothelium in developing and adult vasculature in transgenic LacZ reporter mice.11 The regulation of MEF2C gene expression by Ets transcription factors is of particular interest because VEGF is known to upregulate Ets-1 in endothelial cells.12 In light of the important role of MEF2C in vasculogenesis and angiogenesis, we were interested in characterizing the regulation of MEF2C by VEGF and the potential role of MEF2 in endothelial cells. We found that VEGF induces MEF2C gene expression in a protein kinase C–dependent manner. In addition, VEGF activates MEF2-dependent transcription, and this activation is regulated by several signaling pathways. Finally, MEF2C may play an important role in endothelial cell migration.