Positive and negative regulation of epicardial-mesenchymal transformation during avian heart development

In the developing heart, the epicardium is essential for coronary vasculoge nesis as it provides precursor cells that become coronary vascular smooth m uscle and perivascular fibroblasts. These precursor cells are derived from the epicardium via epithelial-mesenchymal transformation (EMT). The factors that regulate epicardial EMT are unknown. Using a quantitative in vitro co llagen gel assay, we show that serum, FGF-1, -2, and -7, VEGF, and EGF stim ulate epicardial EMT. TGF beta -1 stimulates EMT only weakly, while TGF bet a -2 and -3 do not stimulate EMT. TGF beta -1, -2, or -3 strongly inhibits transformation of epicardial cells stimulated with FGF-2 or heart-condition ed medium. TGF beta -3 does not block expression of vimentin, a mesenchymal marker, but appears to inhibit EMT by blocking epithelial cell dissociatio n and subsequent extracellular matrix invasion. Blocking antisera directed against FGF-1, -2, or -7 substantially inhibit conditioned medium-stimulate d EMT in vitro, while antibodies to TGF beta -1, -2, or -3 increase it. We confirmed FGF stimulation and TGF beta inhibition of epicardial EMT in orga n culture. Immunoblot analysis confirmed the presence of FGF-1, -2, and -7 and TGF beta -1, -2, and -3 in conditioned medium, and we localized these g rowth factors to the myocardium and epicardium of stage-appropriate embryos by immunofluorescence. Our results strongly support a model in which myoca rdially derived FGF-1, -2, or -7 promotes epicardial EMT, while TGF beta -1 , -2, or -3 restrains it. Epicardial EMT appears to be regulated through a different signaling pathway than endocardial EMT. (C) 2001 Academic Press.