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

In the developing heart, the epicardium is essential for coronary vasculogenesis as it provides precursor cells that become coronary vascular smooth muscle 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 collagen gel assay, we show that serum, FGF-1, -2, and -7, VEGF, and EGF stimulate epicardial EMT. TGF beta -1 stimulates EMT only weakly, while TGF beta -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-conditioned medium. TGF beta -3 does not block expression of vimentin, a mesenchymal marker, but appears to inhibit EMT by blocking epithelial cell dissociation and subsequent extracellular matrix invasion. Blocking antisera directed against FGF-1, -2, or -7 substantially inhibit conditioned medium-stimulated 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 organ 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 growth factors to the myocardium and epicardium of stage-appropriate embryos by immunofluorescence. Our results strongly support a model in which myocardially 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.