Induction of HUVEC apoptosis by various sphingolipids and staurosporine. HUVECs were treated without (C) or with 20 μmol/L Sph, DMS, or C₂-Cer or 1 μmol/L staurosporine (Ssp) for 4 hours in the presence (left panel) or absence (right panel) 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). Columns and error bars represent the mean ± SD (n = 3). *Statistically significant (t-test, P < .05) compared with the control cells (without treatment).