DNA-PK-Dependent G2 Checkpoint Revealed Following Knockdown of ATM in Human Mammary Epithelial Cells

• Published in: Cancer research (Chicago, Ill.) Ill.), 2008-01, Vol.68 (1), p.89-97
• Main Authors: Arlander, Sonnet J. H., Greene, Bryan T., Innes, Cynthia L., Paules, Richard S.
• Format: Article
• Language: English
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-07-0675

Members of the phosphatidylinositol 3-kinase related kinase (PIKK) 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 (DSBs). 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 G 2 checkpoint that was only slightly attenuated. In marked contrast, this G 2 checkpoint was significantly attenuated with either DNA-PK inhibitor treatment or RNAi knockdown of DNA-PKcs, the catalytic subunit of DNA-PK, indicating that DNA-PK contributes to the G 2 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 phophorylation. Taken together these results demonstrate a DNA-PK-dependent component to the IR-induced G 2 checkpoint in addition to the well-defined ATM-dependent component. This may have important implications for chemotherapeutic strategies for breast cancers.