Endothelial Cell Apoptosis Induces TGF-β Signaling-Dependent Host Endothelial-Mesenchymal Transition to Promote Transplant Arteriosclerosis

Endothelial cells (ECs) apoptosis is an initial event in transplant arteriosclerosis (TA), resulting in allograft function loss. To elucidate the precise mechanisms of ECs apoptosis leading to neointimal smooth muscle cells (SMCs) accumulation during TA. We induced apoptosis in cultured ECs by overexpressing p53 through lentivirus-mediated transfection. ECs apoptosis induced the production of transforming growth factor (TGF)-beta 1 in both apoptotic and neighboring viable cells, leading to increased TGF-beta 1 in the culture media. Conditioned media from Ltv-p53-transfected ECs further promoted transition of cultured ECs to SM-like cells by activating TGF-beta/Smad3, PI3K/Akt/mTOR, and MAPK/ERK signaling in a TGF-beta-dependent manner. In transgenic rat aorta transplantation models, inhibition of ECs apoptosis in Bcl-xL(+/+) knock-in rat aortic allografts significantly reduced TGF-beta 1 production both in allograft endothelia and in blood plasma, which in turn decreased accumulation of SM22 alpha+ cells from transgenic recipient ECs originally marked with EGFP knock-in in neointima and alleviated TA. Systemic treatment with SIS3, AP23573, or PD98059 also prevented recipient ECs-originated SM-like cells accumulation and intima hyperplasia in aortic allografts. These data suggest that allograft EC apoptosis induced recipient endothelial-mesenchymal (smooth muscle) transition via TGF-beta signaling, resulting in recipient EC-derived SMC accumulation as a major mechanism of vascular remodeling during TA.