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 G(2) checkpoint and enhance cytotoxicity. Howeve r, 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 G(2) arrest caused by the DNA-damaging a gent cisplatin, Here, using Chinese hamster ovary cells, we show that cisplatin causes predominantly an S-phase arrest; UCN-01 abrogates thi s S-phase arrest, causing progression of cells to G(2) and, subsequent ly, apoptotic cell death, In searching for an explanation for this acc elerated DNA synthesis, we discovered that UCN-01 caused translocation of proliferating cell nuclear antigen (PCNA) to the detergent-insolub le, DNA-bound fraction, PCNA acts as a sliding clamp for DNA polymeras e delta. Sequestering of PCNA by p21(waf1/cip1) is required for p53-de pendent G(1) arrest in damaged cells, However, the S-phase arrest occu rs independently of p53 and p21(waf1/cip1). Our results suggest that P CNA is also a component of this S-phase checkpoint, despite the fact t hat CHO cells are defective for p53, and no increase in p21(waf1/cip1) was observed. The mechanism by which PCNA is sequestered in the absen ce of p21(waf1/cip1) and the mechanism by which UCN-01 disrupts this s equestration remain to be elucidated.