DNA damage mediated s and g 2 checkpoints in human embryonal carcinoma cells

Abstract

For mouse embryonic stem (ES) cells, the importance of the Sand 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 undiffer- entiated 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. ©AlphaMed Press.