Effects of endothelial growth media on proepicardial cell gene expression and morphogenesis in 3D collagen matrices

Proepicardial cells (PE) contribute to embryonic coronary vessel and epicardial development. Cells from the PE region can differentiate into coronary vascular smooth muscle cells and fibroblasts in vitro, but the endothelial specification capability of these cells is controversial. We sought to examine the effects of endothelial cell growth media on gene expression and the morphogenic properties of proepicardial cells in three-dimensional (3D) matrices. A primary culture of avian PE cells was subjected to molecular characterization with selected endothelial specific markers. Morphogenic properties of PE cells were assessed by in vitro assays of coronary vasculogenesis and invasion, which utilized highly defined, serum free, three-dimensional matrix conditions. PE cells maintained mixed cell population properties in the culture based on morphogenic features, immunohistochemistry, and the gene expression data. When suspended in a 3D vasculogenesis in vitro assay, PE cells formed intracellular vacuoles and assembled into multicellular tubes. Further, ultrastructural analysis revealed the presence of pinocytic vacuoles, intercellular junctions, and endothelial specific Weibel Palade bodies. In the invasion assay, PE cells spontaneously invaded control matrices. This invasion was markedly enhanced by lysophosphatidic acid (94 +/- 9.6 vs. 285.6 +/- 54.9, p < 0.05) and was completely blocked with synthetic broad-spectrum metalloproteinase inhibitor GM6001. Isolated PE cells grown in endothelial cell media represent mixed-cell population, characterized by both smooth muscle and endothelial gene expression. When placed in 3D in vitro assays, PE cells manifest morphogenic properties, including multicellular tube assembly and invasion.