The G(2) checkpoint is maintained by redundant pathways

While p53 activity is critical for a DNA damage-induced G(1) checkpoint, it s role in the G(2) checkpoint has not been compelling because cells lacking p53 retain the ability to arrest in G(2) following DNA damage. Comparison between normal human foreskin fibroblasts (HFFs) and HFFs in which p53 was eliminated by transduction with human papillomavirus type 16 E6 showed that treatment with adriamycin initiated arrest in G(2) with active cyclin B/CD C2, kinase, regardless of p53 status. Both E6-transduced HFFs and control ( LXSN)-transduced cells maintained a prolonged arrest in G(2); however cells with functional p53 extinguished cyclin B-associated kinase activity. Down regulation was mediated by p53-dependent transcriptional repression of the CDC2 and cyclin B promoters. In contrast, cells lacking p53 showed a prolo nged G(2) arrest despite high levels of cyclin B/CDC2 kinase activity, at l east some of which translocated into the nucleus. Furthermore, the G(2) che ckpoint became attenuated as p53-deficient cells aged in culture. Thus, at late passage, E6-transduced HFFs entered mitosis following DNA damage, wher eas the age-matched parental HFFs sustained a G(2) arrest. These results in dicate that normal cells have p53-independent pathways to maintain DNA dama ge-induced G(2) arrest, which may be augmented by p53 dependent functions, and that cells lacking p53 are at greater risk of losing the pathway that p rotects against aneuploidy.