DNA Damage Mediated S and G 2 Checkpoints in Human Embryonal Carcinoma Cells

For mouse embryonic stem (ES) cells, the importance of the S and G 2 cell cycle checkpoints for genomic integrity is increased by the absence of the G 1 checkpoint. We have investigated ionizing radiation (IR)‐mediated cell cycle checkpoints in undifferentiated and retinoic acid‐differentiated human embryonal carcinoma (EC) cells. Like mouse ES cells, human EC cells did not undergo G 1 arrest after IR but displayed a prominent S‐phase delay followed by a G 2 ‐phase delay. In contrast, although differentiated EC cells also failed to arrest at G 1 ‐phase after IR, they quickly exited S‐phase and arrested in G 2 ‐phase. In differentiated EC cells, the G 2 ‐M‐phase cyclin B1/CDC2 complex was upregulated after IR, but the G 1 ‐S‐phase cyclin E and the cyclin E/CDK2 complex were expressed at constitutively low levels, which could be an important factor distinguishing DNA damage responses between undifferentiated and differentiated EC cells. S‐phase arrest and expression of p21 could be inhibited by 7‐hydroxystaurosporine, suggesting that the ataxia‐telangiectasia and Rad‐3‐related‐checkpoint kinase 1 (ATR‐CHK1), and p21 pathways might play a role in the IR‐mediated S‐phase checkpoint in EC cells. IR‐mediated phosphorylation of ataxia‐telangiectasia mutated, (CHK1), and checkpoint kinase 2 were distinctly higher in undifferentiated EC cells compared with differentiated EC cells. Combined with the prominent S and G 2 checkpoints and a more efficient DNA damage repair system, these mechanisms operate together in the maintenance of genome stability for EC cells. S TEM C ELLS 2009;27:568–576