MITOTIC PHOSPHORYLATION OF BCL-2 DURING NORMAL-CELL CYCLE PROGRESSIONAND TAXOL-INDUCED GROWTH ARREST

There is increasing evidence that prolonged mitotic arrest initiates a poptosis; however, little is known about the signaling pathways involv ed. Several studies have associated deregulated Cdc2 activity with apo ptosis, Herein, we report that the anti-apoptotic protein, Bcl-2, unde rgoes cell cycle-dependent phosphorylation during mitosis when there i s elevated Cdc2 activity. We found that paclitaxel (Taxol(R)) treatmen t of epithelial tumor cells induced a prolonged mitotic arrest, elevat ed levels of mitotic kinase activity, hyperphosphorylation of Bcl-2, a nd subsequent cell death. The Taxol-induced BcI-2 phosphorylation was dose-dependent. Furthermore, phosphorylated Bcl-2 remained complexed w ith Bar in Taxol-treated cells undergoing apoptosis, Immunoprecipitati on experiments revealed a Bcl-2-associated kinase capable of phosphory lating histone H1 in vitro. However, the kinase was likely not cyclin B1/Cdc2, since cyclin B1/Cdc2 was not detectable in Bcl-2 immunoprecip itates, nor was recombinant Bcl-2 phosphorylated in vitro by cyclin B1 /Cdc2, The results of this study further define a link between mitotic kinase activation and the apoptotic machinery in the cell. However, t he role, if any, of prolonged Bcl-2 phosphorylation in Taxol-mediated apoptosis awaits further definition of Bcl-2 mechanism of action. Taxo l may increase cellular susceptibility to apoptosis by amplifying the normal downstream events associated with mitotic kinase activation.