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DNA Protein Kinase–Dependent G₂ Checkpoint Revealed following Knockdown of Ataxia-Telangiectasia Mutated in Human Mammary Epithelial Cells

Environmental Stress and Cancer Group, National Institute of Environmental Health Sciences, NIH, Research Triangle Park, North Carolina

Requests for reprints: Richard S. Paules, Environmental Stress and Cancer Group, National Institute of Environmental Health Sciences, P.O. Box 12233, Mail Drop D2-03, Research Triangle Park, NC 27709. Phone: 919-541-3710; Fax: 919-316-4771; E-mail: paules{at}niehs.nih.gov.

Members of the phosphatidylinositol 3-kinase–related kinase family, in particular the ataxia-telangiectasia mutated (ATM) kinase and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), regulate cellular responses to DNA double-strand breaks. Increased sensitivity to ionizing radiation (IR) in DNA-PKcs– or ATM-deficient cells emphasizes their important roles in maintaining genome stability. Furthermore, combined knockout of both kinases is synthetically lethal, suggesting functional complementarity. In the current study, using human mammary epithelial cells with ATM levels stably knocked down by >90%, we observed an IR-induced G₂ checkpoint that was only slightly attenuated. In marked contrast, this G₂ checkpoint was significantly attenuated with either DNA-PK inhibitor treatment or RNA interference knockdown of DNA-PKcs, the catalytic subunit of DNA-PK, indicating that DNA-PK contributes to the G₂ checkpoint in these cells. Furthermore, in agreement with the checkpoint attenuation, DNA-PK inhibition in ATM-knockdown cells resulted in reduced signaling of the checkpoint kinase CHK1 as evidenced by reduced CHK1 phosphorylation. Taken together, these results show a DNA-PK–dependent component to the IR-induced G₂ checkpoint, in addition to the well-defined ATM-dependent component. This may have important implications for chemotherapeutic strategies for breast cancers. [Cancer Res 2008;68(1):89–97]