UCN-01 abrogates G₂ arrest through a Cdc2-dependent pathway that is associated with inactivation of the Wee1Hu kinase and activation of the Cdc25C phosphatase

We have previously demonstrated that UCN-01, a potent protein kinase inhibitor currently in phase I clinical trials for cancer treatment, abrogates G₂ arrest following DNA damage. Here we used murine FT210 cells, which contain temperature-sensitive Cdc2 mutations, to determine if UCN-01 abrogates G₂ arrest through a Cdc2-dependent pathway. We report that UCN-01 cannot induce mitosis in DNA-damaged FT210 cells at the nonpermissive temperature for Cdc2 function. Failure to abrogate G₂ arrest was not due to UCN-01-inactivation at the elevated temperature because parental FM3A cells, which have wild-type Cdc2, were sensitive to UCN-01-induced G₂ checkpoint abrogation. Having established that UCN-01 acted through Cdc2, we next assessed UCN-01's effect on the Cdc2-inhibitory kinase, Wee1Hu, and the Cdc2- activating phosphatase, Cdc25C. We found that Wee1Hu was indeed inactivated in UCN-01-treated cells, possibly just prior to Cdc2 activation and entry of DNA-damaged cells into mitosis. This inhibition appeared, however, to be a consequence of a further upstream action since in vitro studies revealed purified Wee1Hu was relatively resistant to UCN-01-inhibition. Consistent with such an upstream action, UCN-01 also promoted the hyperphosphorylation (activation) of Cdc25C in DNA-damaged cells. Our results suggest that UCN-01 abrogates G₂ checkpoint function through inhibition of a kinase residing upstream of Cdc2, Wee1Hu, and Cdc25C, and that changes observed in these mitotic regulators are downstream consequences of UCN-01's actions.