Aortic valve endothelial cells undergo transforming growth factor-beta-mediated and non-transforming growth factor-beta-mediated transdifferentiationin vitro

Cardiac valves arise from endocardial. cushions, specialized regions of the developing heart that are formed by an endothelial-to-mesenchymal cell tra nsdifferentiation. 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 establishe d marker for this process, two distinct pathways of transdifferentiation we re identified in clonally derived endothelial cell populations isolated fro m ovine aortic valve leaflets. alpha -SMA expression was induced by culturi ng 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 mesench ymal phenotype. A population of the differentiated cells co-expressed CD31, an endothelial marker, along with alpha -SMA, as seen by double-label immu nofluorescence. Similarly, this coexpression of endothelial markers and alp ha -SMA was detected in a subpopulation of cells in frozen sections of aort ic valves, suggesting the transdifferentiation may occur in vivo. Hence, th e clonal populations of valvular endothelial. cells described here provide a powerful in vitro model for dissecting molecular events that regulate val vular endothelium.