Inhibition of the ATR/Chk1 Pathway Induces a p38-Dependent S-phase Delay in Mouse ES Cells

Ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR)kinases, family members of the PI-3 kinase related proteins, play a key role in checkpointactivation and maintenance of genomic stability following DNA damage. We have usedwild type (WT) and p38?-deficient mouse embryonic stem (ES) cells to investigate therole of ATR and ATM kinases during embryonic cell cycle. We have found thatinhibition of ATR and ATM kinases with caffeine or Chk1 with UCN-01, results inactivation of a p38-dependent intra-S-phase checkpoint and activation of apoptosis in EScells. However, wortmannin at a concentration, that inhibits ATM kinase but not ATRkinase, did not affect cell cycle progression. Furthermore, the presence of caffeine resultsin activation of p38 kinase, accumulation of p21/Waf1 in a complex with Cdk2 anddecrease of Cdk2 kinase activity. In contrast, caffeine-treated p38?-/- ES cells show lessapoptosis, and fail to trigger an effective S-phase checkpoint and accumulation ofp21/Waf1. We conclude that ATR kinase activity is essential for normal cell cycleprogression of exponentially proliferating mouse ES cells even in the absence ofexogenous DNA damage, and ATR deregulation triggers p38?-dependent cell-cyclecheckpoint and apoptotic responses.