JNK-mediated phosphorylation of Cdc25C regulates cell cycle entry and G₂/M DNA damage checkpoint

c-Jun NH₂-terminal Kinases (JNKs) play a central role in the cellular response to a wide variety of stress signals. After their activation, JNKs induce phosphorylation of substrates, which control proliferation, migration, survival, and differentiation. Recent studies suggest that JNKs may also play a role in cell cycle control, although the underlying mechanisms are largely unexplored. Here we show that JNK directly phosphorylates Cdc25C at serine 168 during G₂ phase of the cell cycle. Cdc25C phosphorylation by JNK negatively regulates its phosphatase activity and thereby Cdk1 activation, enabling a timely control of mitosis onset. Unrestrained phosphorylation by JNK, as obtained by a cell cycle-stabilized form of JNK or as seen in some human tumors, results in aberrant cell cycle progression. Additionally, UV irradiation-induced G₂/M checkpoint requires inactivation of Cdc25C by JNK phosphorylation. JNK phosphorylation of Cdc25C as well as Cdc25A establishes a novel link between stress signaling and unperturbed cell cycle and checkpoint pathways.