Abrogation of an S-phase Checkpoint and Potentiation of Camptothecin Cytotoxicity by 7-hydroxystaurosporine (UCN-01) in Human Cancer Cell Lines, Possibly Influenced by P53 Function

7-Hydroxystaurosporine (UCN-01) is a selective protein kinase C inhibitor in clinical trial for cancer treatment. In this study, we found that nanomolar concentrations of camptothecin (CPT), a topoisomerase I inhibitor, arrest or delay cell cycle progression during the S and G2 phases in p53 mutant human colon carcinoma HT29 cells and that UCN-01 abrogates the S-phase arrest or delay induced by CPT. Under these conditions, CPT increased cyclin A levels and cyclin A/cyclin-dependent kinase 2 activity. UCN-01 prevented the increase of cyclin A/cyclin-dependent kinase 2 activity induced by CPT and enhanced Cdc2 kinase activity. Replication protein A (RPA2) was hyperphosphorylated after CPT treatment, and this effect was also abrogated by UCN-01. UCN-01 potentiated the cytotoxicity of CPT and reduced by 6-fold the concentration of CPT required to kill 50% of the HT-29 cells, as determined by clonogenic assays. This effect was observed at concentrations of UCN-01 that alone were not cytotoxic and had no detectable effect on cell cycle progression. UCN-01 markedly potentiated the cytotoxicity of CPT also in HCT116/E6 and MCF-7/ADR cells defective for p53 function, whereas significantly less potentiation was observed in p53-wild-type HCT116 and MCF-7 cells. These results suggest the existence of an S-phase checkpoint that delays replication and that may extend the time available for DNA repair. Thus, pharmacological abrogation of CPT-induced S- and G2-phase checkpoints by UCN-01 may provide an effective strategy for enhancing the chemotherapeutic activity of CPT, particularly against p53-defective tumors.