Abstract

We examined the effect of overexpression of p21^waf1 on cytotoxicity of paclitaxel, a microtubule stabilizer, using a tetracycline-inducible expression system in human sarcoma cells (SaOs-2) that lack both functional retinoblastoma protein and p53. Under normal growth conditions, p21^waf1 is not detectable in SaOs-2 cells. Upon p21^waf1 induction by tetracycline withdrawal, we observed a reduced apoptotic response to paclitaxel with a 3- to 6-fold increase in IC₅₀ values compared with that of cells not induced by p21^waf1. We also observed a 5-fold increase in the IC₅₀ value when cytotoxicity to vincristine, another microtubule-disrupting agent, was assessed, whereas we observed a marked decrease in the IC₅₀ value after p21^waf1 induction in response to etoposide, a topoisomerase II inhibitor. After treatment with paclitaxel, less accumulation of G₂-M was observed in p21^waf1-induced cells compared with non-p21^waf1-induced cells (57% versus 74%). p21^waf1 induction also inhibited the increased cyclin B1-associated kinase activity induced by paclitaxel. Overexpression of p21^waf1 in SaOs-2 cells lacking both p53 and functional retinoblastoma protein may decrease the G₂-M arrest induced by paclitaxel due to suppression of the S-G₂ checkpoint, resulting in a decreased apoptotic response of cells to paclitaxel.