Aortic valve endothelial cells undergo transforming growth factor-β-mediated and non-transforming growth factor-β-mediated transdifferentiation in vitro

Cardiac valves arise from endocardial. cushions, specialized regions of the developing heart that are formed by an endothelial-to-mesenchymal cell transdifferentiation. Whether and to what extent this transdifferentiation is retained in mature heart valves is unknown. Herein we show that endothelial cells from mature valves can transdifferentiate to a mesenchymal phenotype. Using induction of alpha -smooth muscle actin (alpha -SMA), an established marker for this process, two distinct pathways of transdifferentiation were identified in clonally derived endothelial cell populations isolated from ovine aortic valve leaflets. alpha -SMA expression was induced by culturing clonal endothelial cells in medium containing either transforming growth factor-beta or low levels of serum and no basic fibroblast growth factor. Cells induced to express alpha -SMA exhibited markedly increased migration in response to platelet-derived growth factor-BB, consistent with a mesenchymal phenotype. A population of the differentiated cells co-expressed CD31, an endothelial marker, along with alpha -SMA, as seen by double-label immunofluorescence. Similarly, this coexpression of endothelial markers and alpha -SMA was detected in a subpopulation of cells in frozen sections of aortic valves, suggesting the transdifferentiation may occur in vivo. Hence, the clonal populations of valvular endothelial. cells described here provide a powerful in vitro model for dissecting molecular events that regulate valvular endothelium.