Role of wild type p53 in the G₂ phase: regulation of the γirradiation-induced delay and DNA repair

Abstract

Up-regulation of the p53 protein was found to induce cell cyle arrest at the G₁/S border and in some cases at the G₂/M border. Futhermore, it was suggested that p53 is associated with the induction of the various DNA repair pathways. Previously, we demonstrated that cells coexpressing endogenous wild type p53 protein, together with dominant negative mutant p53, exhibit deregulation of apoptosis, G₁ arrest and delay in G₂ following γ-irradiation. IN the present study, we investigated the role of p53 protein in the DNA damage response at the G₂ phase. Using p53-null, wild type p53 and mutant p53-producer cell lines, we found that the two C-terminally spliced p53 forms could prevent γ-irradiation induced muatgenesis prior to mitosis, at the G₂/M checkpoint. We found that at the G₂ phase, p53 may facilitate repair of DNA breaks giving rise to micronuclei, and regulate the exit from the G₂ checkpoint. At the G₁ phase, only the regularly spliced form of p53 caused growth arrrest. In contrast, both the regularly and the alternatively spliced p53 forms directed postmitotic micronucleated cells towards apoptosis. These results provide a functional explanation for the cell cycle-independent expression of p53 in mnormal cycling cells, as well as in cells where p53 is up-regulated, following DNA damage.