p53 Modulates base excision repair activity in a cell cycle-specific manner after genotoxic stress

To elucidate the nature of the cross-talk between the p53 protein and the D NA repair machinery, we have investigated the relationship between the two throughout the cell cycle. Base excision repair (BER) was analyzed in cell cycle phase-enriched populations of lymphoid cells expressing wild-type p53 . Our study yielded the following novel findings: (a) BER exhibited two dis tinct peaks of activity, one associated with the G(0)-G(1) checkpoint and t he second with the G(2)-M checkpoint; (b) although the overall BER activity was reduced after exposure of cells to 400R, there was an augmentation of the G(0)-G(1)-associated BER activity and a reduction in the G(2)-M-associa ted BER activity; and (c) modulations in these patterns of BER after genoto xic stress were found to be p53 regulated. p53 protein levels induced after gamma -irradiation were distributed evenly in the various cell cycle popul ations (analyzed by the PAb-248 anti-p53 monoclonal antibody). However, bot h the dephosphorylation of serine 376 of p53 (contained in the PAb-421 epit ope) and the specific DNA binding activity, as well as apoptosis, were enha nced toward the G(2)-M populations. Furthermore, inactivation of wild-type p53, mediated by mutant p53 expression, abolished the alterations in the BE R pattern and showed no induction of a G(2)-M-associated apoptosis after ga mma -irradiation. These results suggest that after genotoxic stress, stabil ized p53 enhances the G(0)-G(1)-associated BER activity, whereas it predomi nantly reduces BER activity at the G(2)-M-enriched populations and instead induces apoptosis. After genotoxic stress, p53 functions as a modulator tha t determines the pattern of BER activity and apoptosis in a cell cycle-spec ific manner.