Dual inhibition of Cdc2 protein kinase activation during apoptosis in Xenopus egg extracts

When intracellular damage accumulates in proliferating somatic cells, the cell cycle usually arrests in G1 or G2 in a checkpoint-dependent manner, either to repair the damage or to die by apoptosis. In contrast, early embryonic cells lack checkpoint-mediated cell-cycle arrest, and it is not clear whether apoptosis in early embryonic cells occurs at a specific cell cycle stage or at random points. Here, we examined the functional molecular link between the embryonic cell cycle and apoptosis using Xenopus egg extracts. When apoptosis was induced in egg extracts by addition of exogenous cytochrome c during cell-cycle progression, cyclin B accumulation was inhibited, Cdc2 was not activated, and the cell cycle arrested at interphase. However, addition of recombinant cyclin B failed to activate Cdc2 due to the strong inhibitory phosphorylation of Cdc2 Tyr15 in apoptotic egg extracts. We found that endogenous Cdc25C, which activates the Cdc2-cyclin B complex by dephosphorylating Cdc2 Tyr15, was inactivated by caspase-mediated cleavage at two sites in the N-terminal regulatory domain. When the hyperactive Cdc25A catalytic fragment was added together with recombinant cyclin B to artificially dephosphorylate Cdc2 Tyr15, M-phase induction was restored in apoptotic egg extracts, indicating that the blockage of cyclin B accumulation and the caspase-mediated inactivation of Cdc25C dually inhibited Cdc2 activation. Apoptosis induction in cytostatic factor-arrested metaphase egg extracts resulted in inactivation of Cdc2 without cyclin B degradation. These results suggest that apoptotic inactivation of Cdc25C plays an important role in arresting the embryonic cell cycle at interphase during apoptosis.