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 α-smooth muscle actin (α-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. α-SMA expression was induced by culturing clonal endothelial cells in medium containing either transforming growth factor-β or low levels of serum and no basic fibroblast growth factor. Cells induced to express α-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 α-SMA, as seen by double-label immunofluorescence. Similarly, this co-expression of endothelial markers and α-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.