7-Hydroxystaurosporine (UCN-01) causes redistribution of proliferating cell nuclear antigen and abrogates cisplatin-induced S-phase arrest in Chinese hamster ovary cells.

A variety of agents, such as caffeine, have been shown to abrogate the DNA damage-dependent G2 checkpoint and enhance cytotoxicity. However, these agents are too toxic for clinical use. We have reported that the potent protein kinase inhibitor 7-hydroxystaurosporine (UCN-01) at nontoxic doses abrogates the G2 arrest caused by the DNA-damaging agent cisplatin. Here, using Chinese hamster ovary cells, we show that cisplatin causes predominantly an S-phase arrest; UCN-01 abrogates this S-phase arrest, causing progression of cells to G2 and, subsequently, apoptotic cell death. In searching for an explanation for this accelerate DNA synthesis, we discovered that UCN-01 caused translocation of proliferating cell nuclear antigen (PCNA) to the detergent-insoluble, DNA-bound fraction. PCNA acts as a sliding clamp for DNA polymerase delta. Sequestering of PCNA by p21waf1/cip1 is required for p53-dependent G1 arrest in damaged cells. However, the S-phase arrest occurs independently of p53 and p21waf1/cip1. Our results suggest that PCNA is also a component of this S-phase checkpoint, despite the fact that CHO cells are defective for p53, and no increase in p21waf1/cip1 was observed. The mechanism by which PCNA is sequestered in the absence of p21waf1/cip1 and the mechanism by which UCN-01 disrupts this sequestration remain to be elucidated.