Abstract
p27^kip1^ is a cyclin‐dependent kinase (CDK) inhibitor, which controls several cellular processes in strict collaboration with pRb. We evaluated the role of p27^kip1^ in paclitaxel‐induced apoptosis in the pRb‐defective SaOs‐2 cells. Following 48 h of exposure of SaOs‐2 cells to 100 nM paclitaxel, we observed an increase in p27^kip1^ expression caused by the decrease of the ubiquitin‐proteasome activity. Such increase was not observed in SaOs‐2 cells treated with the caspase inhibitors Z‐VAD‐FMK, suggesting that p27^kip1^ enhancement at 48 h is strictly related to apoptosis. Finally, we demonstrated that SaOs‐2 cells transiently overexpressing the p27^kip1^ protein are more susceptible to paclitaxel‐induced apoptosis than SaOs‐2 cells transiently transfected with the empty vector. Indeed, after 48 h of paclitaxel treatment, 41.8% of SaOs‐2 cells transiently transfected with a pcDNA3‐p27^kip1^ construct were Annexin V‐positive compared to 30.6% of SaOs‐2 cells transfected with the empty vector (P < 0.05). In conclusion, we demonstrated that transfection of the pRb‐defective SaOs‐2 cells with the p27^kip1^ gene via plasmid increases their susceptibility to paclitaxel‐induced apoptosis. The promoting effect of p27^kip1^ overexpression on apoptosis makes p27^kip1^ and proteasomal inhibitors interesting tools for therapy in patients with pRb‐defective cancers. J. Cell. Biochem. © 2006 Wiley‐Liss, Inc.