The Roles of Matrix Stiffness and β-Catenin Signaling in Endothelial-to-Mesenchymal Transition of Aortic Valve Endothelial Cells

Valve stiffening is a hallmark of aortic valve stenosis caused by excess extracellular matrix accumulation by myofibroblasts. We aimed to elucidate whether matrix stiffness regulates endothelial-to-mesenchymal transition (EndMT) of adult valvular endothelial cells (VECs) to myofibroblasts as a mechanism to further promote valve fibrosis. In addition, we specifically examined the role of the Wnt/beta-catenin signaling pathway in the development of myofibroblasts during EndMT, as Wnt/beta-catenin signaling has been implicated in EndMT during heart development, is reactivated in valve disease, and is required for mechanically-regulated myofibrogenesis of valve interstitial cells. Clonally derived porcine VECs were cultured on soft (5 kPa) or stiff (50 kPa) silicone Sylgard 527 substrates and treated with transforming growth factor (TGF)-beta 1 to induce EndMT. Immunofluorescent staining revealed that TGF-beta 1 preferentially promoted EndMT in VECs on stiffer substrates, evidenced by a decrease in the endothelial marker VE-cadherin and an increase in the myofibroblast marker alpha-smooth muscle actin (alpha-SMA). These changes were accompanied by beta-catenin nuclear localization both in vitro and in vivo, assessed by immunostaining. Degradation of beta-catenin with endostatin reduced VEC myofibroblast transition, as indicated by decreased alpha-SMA fiber expression. We conclude that TGF-beta 1-induced EndMT in aortic VECs is dependent on matrix stiffness and Wnt/beta-catenin signaling promotes myofibrogenesis during EndMT.