Relaxed cell-cycle arrests and propagation of unrepaired chromosomal damage in cancer cell lines with wild-type p53

The role of the p53 protein in mediating G₁ and G₂ cell-cycle arrests after genotoxic insult has been clearly and reproducibly established in primary diploid fibroblasts, but data obtained from p53 wild-type (wt) cancer cell lines are inconsistent. Furthermore, a large proportion of human tumors have p53 wt genotypes but present genetic aberrations that may result from defective cell-cycle checkpoints. We therefore investigated the integrity of G₁/S and G₂/M cell-cycle arrests in p53 wt cancer cell lines. In the study presented here, we showed that in most cancer cells tested, G₁ arrest was relaxed or absent in comparison with arrest in normal diploid fibroblasts, despite seemingly normal p53 and p21 responses. Two cell lines (MCF7 and HCT116) were synchronized in G₀/G₁ by leucine starvation and subjected to genotoxic stress to determine more precisely the relative proportion of cells arresting in G₁ and G₂. Whereas the MCF7 cells showed consistent G₁ arrest, the HCT116 cells showed none at all. Furthermore, cell-cycle arrests in G₁ and G₂ in response to γ irradiation and bleomycin treatment were transient, as the cells resumed cycling after 48–72 h. The cells resuming proliferation suffered massive apoptosis, but a proportion of the cells were rescued and showed normal doubling times. These cells retained a p53 wt genotype but presented gross chromosomal aberrations in 15–20% of the analyzed metaphases. The aberrations were not clonal. These data show that p53 wt cancer cells have relaxed cell-cycle controls after genotoxic insult and tolerate unrepaired chromosomal damage, despite normal p53 function. Mol. Carcinog. 23:1–12, 1998. © 1998 Wiley-Liss, Inc.