Dose-dependent induction of HUVEC apoptosis by Sph or DMS. HUVECs were treated with various concentrations of Sph (○, •) or DMS (□, ▪) for 4 hours in the presence (○, □) or absence (•, ▪) of growth factors. Apoptotic cells were detected using Hoechst 33258 staining and visualized with fluorescence microscopy. Results were expressed as the percentage of apoptotic cells (apoptotic cells/total cells × 100).