Estrogen Inhibits ATR Signaling to Cell Cycle Checkpoints and DNA Repair

DNA damage activates the ataxia telangiectasia-mutated and Rad3-related (ATR) kinase signal cascade. How this system is restrained is not understood. We find that in estrogen receptor (ER)-positive breast cancer cells, UV or ionizing radiation and hydroxyurea rapidly activate ATR-dependent phosphorylation of endogenous p53 and Chk1. 17-beta-estradiol (E-2) substantially blocks ATR activity via plasma membrane-localized ER alpha. E-2/ER reduces the enhanced association of ATR andTopBP1 proteins that follows DNA damage and strongly correlates to ATR activity. E-2 inhibits ATR activation through rapid PI3K/AKT signaling: AKT phosphorylates TopBP1 at Serine 1159, thereby preventing the enhanced association of ATR with TopBP1 after DNA damage. E-2 also inhibits Claspin: Chk1 protein association via AKT phosphorylation of Chk1, preventing Chk1 signaling to the G2/M checkpoint. ATR-phosphorylation of p53 induces p21 transcription, prevented by E-2/ER. E-2 delays the assembly and prolongs the resolution of gamma H2AX and Rad51 nuclear foci and delays DNA repair. E-2/ER also increases the chromosomal damage seen from cell exposure to IR. Therefore, the restraint of ATR cascade activation may be a novel estrogen action relevant to breast cancer.