Permanent cell cycle exit in G2 phase after DNA damage in normal human fibroblasts

Abstract

Although the Cdk inhibitor p21Waf1/Cip1, one of the transcriptional targets of p53, has been implicated in the maintenance of G 2 arrest after DNA damage, its function at this stage of the cell cycle is not really understood. Here, we show that the exposure of normal human fibroblasts (NHFs) to genotoxic agents provokes permanent cell cycle exit in G2 phase, whereas mouse embryo fibroblasts and transformed human cells progress through mitosis and arrest in G1 without intervening cytokinesis. p21Waf1/Cip1 exerts a key role in driving this G 2 exit both by inhibiting cyclin B1-Cdk1 and cyclin A-Cdk1/2 complexes, which control G2/M progression, and by blocking the phosphorylation of pRb family proteins. NHFs with compromised pRb proteins could still efficiently arrest in G2 but were unable to exit the cell cycle, resulting in cell death. Our experiments show that, when under continuous genotoxic stress, normal cells can reverse their commitment to mitotic progression due to passage through the restriction point and that mechanisms involving p21Waf1/Cip1 and pocket proteins can induce exit in G2 and G1.