D-flow induces p53 SUMOylation and apoptosis via PIASy activation. (A) HUVECs were transfected with PIASy siRNA (si-PIASy) or control siRNA for 48 h and then stimulated with d-flow for the indicated times. p53 SUMOylation, expression of PIASy, p53, and SUMO2/3 were detected as described in Material and methods. Densitometric analyses of p53 SUMOylation were performed as described in Fig. 1. (B and C) HUVECs were transfected with PIASy or control siRNA for 48 h. After treatment with d-flow for 36 h, apoptotic nuclei were detected by TUNEL staining (B, bottom), and Western blotting with anti–cleaved caspase 3 (C, top) was performed. Immunoblots of PIASy conformed depletion of PIASy by the specific siRNA (B, top). Densitometry analysis of cleaved caspase 3 expression was performed as described in Fig. 2 C (bottom). The experiments were performed in triplicate using three different batches of d-flow–stimulated HUVECs. (D) HUVECs were transduced with an adenovirus vector containing p53, p53-K386R (KR; sumoylation defect mutant), or p53-ΔNES (L348,350A; NES mutant) for 24 h and then stimulated with d-flow for 36 h followed by TUNEL staining as described in Materials and methods. (E, top) Quantification of apoptosis shown as the percentage of TUNEL-positive cells. Bars, 30 µm. (bottom) Equal expressions of p53, p53-K386R, and p53-ΔNES were analyzed by Western blotting in ECs. Data are from three separate experiments using two or more different EC preparations. Error bars show means ± SD; *, P < 0.05; **, P < 0.01. Molecular masses are given in kilodaltons. IB, immunoblot. IP, immunoprecipitation. WT, wild type.