Neither p21WAF1 nor 14-3-3σ prevents G2 progression to mitotic catastrophe in human colon carcinoma cells after DNA damage, but p21WAF1 induces stable G1 arrest in resulting tetraploid cells

p21(WAF1) and 14-3-3 sigma, which are both transcriptional products of p53, have been reported to play a role in the G(2) DNA damage checkpoint in mammalian cells. Human colon carcinoma cells, isogenic except for the presence or absence of either p21(WAF1) or 14-3-3 sigma (T. A. Chan et al., Genes Dev., 14: 1584-1588, 2000), are useful models for analysis of the role of these proteins in checkpoint control. Here, we have examined mitotic behavior within a single cell cycle after DNA damage in these cell lines. Our results show that p21(WAF1) but not 14-3-3 sigma, imposes a significant G(2). delay after DNA damage. After G(2) delay, we found that all isogenic cells, including those competent for both p21(WAF1) and 14-3-3 sigma, adapt to the DNA damage checkpoint and progress into mitosis, where they undergo incomplete chromosome segregation and reenter G(1) with a tetraploid DNA content. Strikingly, our results show that p21(WAF1), but not 14-3-3 sigma, activates a checkpoint in response to DNA damage that prevents continued cycling of the tetraploid cells that result from a mitotic catastrophe characterized by failure to complete cell division. These results demonstrate that a tetraploid DNA content is not a reliable criterion to establish that arrest occurs in G(2). Also, the DNA damage checkpoint mediated by p53-dependent induction of p21(WAF1) assures neither G(2) arrest nor DNA repair sufficient to enable accurate chromosome segregation in human colon carcinoma cells. We conclude that p21(WAF1), but not 14-3-3 sigma, has a unique role in the induction of G(1) arrest in tetraploid cells that results from mitotic catastrophe after DNA damage.