Deregulation of Cdk2 causes Bim-mediated apoptosis in p53-deficient tumors following actin damage

We previously reported that actin damage by treatment with an actin-depolymerizing agent including pectenotoxin-2 induces Bim-mediated apoptosis in p53-deficient human tumors. In this study, we investigated a molecular mechanism underlying Bim-mediated apoptosis of p53-deficient tumor cells following actin damage. We found that actin inhibitors increased the protein levels of p53 and p21 and thereby inactivated both Cdk2 and Cdc2 kinases. However, p53- or p21-knockout cells fail to induce p21 and hence kept both Cdk2 and Cdc2 kinases active even after treatment with actin inhibitor. The p53- or p21-knockout cells became multinucleate and polyploidy in association with induction of apoptosis. Expression of Bcl-x(L) resulted in accumulation of polyploid cells in association with inhibition of apoptosis. However, expression of a dominant negative mutant ( Cdk2dn) and treatment with chemical inhibitors for Cdk2 suppressed not only accumulation of multinucleated cells, but also induction of Bim expression and apoptosis. Therefore, these results suggest that Bim-mediated apoptosis following actin damage due to deregulation of Cdk2 and the cell cycle by the absence of functional p53.