S-PHASE CELL-CYCLE ARREST FOLLOWING DNA-DAMAGE IS INDEPENDENT OF THE P53 P21(WAF1) SIGNALING PATHWAY/

It is now likely that the cyclin-kinase inhibitor, p21(WAF1/SDI1), is, key effector of p53-mediated cell-cycle arrest at the G(1)/S checkpoin t following DNA damage. More recently, however, in vitro data has sugg ested that this pathway may also mediate the acute inhibition of DNA s ynthesis seen in cells already in S phase. Here we address this questi on in an intact cell system using normal human diploid fibroblasts in which p53 function is manipulated by expression mutant (ala(143)) intr oduced vector. Induction of DNA strand breaks in normal control fibrob lasts by exposure to bleomycin led as expected to G(1)/S cell cycle ar rest, induction of p21(WAF1) and a rapid reduction in the rate of DNA synthesis in cells already in S phase. Stable expression of mutant p53 abrogated the G(1)/S (but not the G(1)/M) cell cycle checkpoint and a bolished the induction of p21(WAF1), but had no significant effect on the inhibition of DNA replication in S phase nuclei. We conclude that, despite the in vitro evidence for inhibitory activity on PCNA/polymer ase delta, p21(WAF1) induction does not appear to be essential for the acute inhibition of DNA synthesis in the intact cell following strand -break damage in S phase.