Abstract： Objective To explore the effect of ataxia telanglectasia mutated and RAD3 related protein (ATR) expression and ATR kinase activity on the sensitivity to cisplatin in ovarian cancer SKOV3 cells.Methods SiRNA targeting ATR was transfected into SKOV3 cells for 48 h to reduce the ATR protein level,and ATR kinase inhibitor VE-821 was used for 12 h to inhibit the ATR pathway activity.The alteration of cell viability was examined by CCk-8 assay.Expression levels of ATR,p-ATR and γ-H2AX proteins were detected by Western blot.The DNA double strand breaks (DSB) marker γ-H2AX and homologous recombination repair key protein RAD51 and their co-localization in the cells were examined under the confocal microscope.The status of DNA double strand breaks (DSB) in single cells was visualized by alkaline comet assay.Finally,the cell cycle distribution was assessed using flow cytometry.Results DDP caused evident DNA double strands breaks and activated ATR kinase pathway.ATR-siRNA notably reduced ATR protein level,the 48 h IC50 value of DDP was 72.12 μmol/L and 41.25 μmol/L,respectively,in the NC-siRNA and ATR-siRNA groups (P ＜ 0.05).Confocal microscopic assay presented decreased recruitment of RAD51 at the DSB loci and comet assay showed enhanced DSB in the cells after ATR knocking down.After the inhibition of ATR kinase by VE-821,the 48 h IC50 value of DDP was 75.32 μmol/L and 45.64 μmol/L,respectively,in the DMSO and VE-821 groups (P ＜0.05 for both),confocal microscopic assay demonstrated reduced RAD51 recruitment,and comet assay showed increased DSB in cells after ATR kinase inhibition.Flow cytometry showed that percentage of cells distributed in G0/G1,S and G2/M phases was 71.2％,13.4％ and 15.4％,repectively,after 40 μmol/L DDP treatment for 24 h.Compared with that of control group (G0/G1:54.2％,S:21.3％ and G2/M:24.4％),DDP induced G0/G1 phase arrest.DDP intervention resulted in the cell cycle status (G0/G1:43.2％,S:20.4％,G2/M:36.4％) in the ATR-siRNA group and (G0/G1:40.2％,S:22.5％,G2/M:37.3％) in the VE 821 group,indicating that the inhibition of ATR or ATR kinase could abrogate the effect of G0/G1 phase arrest induced by DDP.Conclusions Suppression of ATR can affect the homologous recombination repair in ovarian cancer cells,leading to accumulation of DNA double strand breaks in the cell nuclei as well as reduction of DDP-caused G0/G1 phase arrest,finally enhances the sensitivity to cisplatin in the ovarian cancer SKOV3 cells.