Paclitaxel induces apoptosis in Saos-2 cells with CD95L upregulation and Bcl-2 phosphorylation

We examined the effect of paclitaxel on human osteoblastic cells Saos-2 to determine if paclitaxel can affect proliferation and apoptosis. We used a p 53-negative cell line in order to mimic the loss of function frequently obs erved at the clinical level. Paclitaxel induced cell death in a dose- and t ime-dependent manner. Marked nuclear condensation and fragmentation of chro matin were observed by Hoechst 33258 stain, DNA ladder formation, electron microscopy, and flow cytometry at concentrations as low as 100 nM, a concen tration which can be achieved by infusion in human plasma. At 100 nM, pacli taxel induced a G2 arrest at 8 h of treatment, The cells then continued to accumulate in G2 until 72 h when the percentage of apoptotic events reached 54%. At the molecular level, Bcl-2 protein was phosphorylated at 16 h and PARP protein was cleaved, indicating the activation of caspase-3-like prote ases. Caspase inhibitors Z-VAD-FMK and Z-DEVD-FMK rescued Saos-2 cells from paclitaxel-induced apoptosis. CD95 expression was constantly high, while C D95L showed a threefold increase in expression. This suggests that, followi ng the G2 arrest, apoptosis is induced through the CD95/CD95L system (C) 19 99 Academic Press.