Interaction between ATM Kinase and p53 in determining glioma radiosensitivity

Glioblastoma multiforme (GBM) is the most common primary brain tumor. Studies have shown that targeting the DNA damage response can sensitize cancer cells to DNA damaging agents. Ataxia telangiectasia mutated (ATM) is involved in signaling DNA double strand breaks. Our group has previously shown that ATM inhibitors (ATMi) sensitize GBM cells and tumors to ionizing radiation. This effect is greater when the tumor suppressor p53 is mutated. The goals of this work include validation of a new ATM inhibitor, AZ32, and elucidation of how ATMi and p53 status interact to promote cell death after radiation. We propose that ATMi and radiation induce mitotic catastrophe in p53 mutants by overriding cell cycle arrest. We tested this hypothesis in human colon carcinoma and glioma cells that differ only in p53 status. We found that AZ32 effectively inhibits phosphorylation of ATM targets. In addition, AZ32 significantly sensitizes glioma cells to ionizing radiation. While HCT116 colon carcinoma cells fail to arrest the cell cycle after radiation, their response to ATMi differs from that in gliomas. Indeed, wild type HCT116 cells were more sensitive than p53 mutants to ionizing radiation in the presence of ATMi. In contrast, ATMi significantly radiosensitized glioma cells in which p53 is knocked down. Live cell imaging confirmed that radiation and ATMi preferentially induce mitotic catastrophe in p53-deficient cells. We conclude that p53-deficient cells rely on ATM signaling for G2/M cell cycle arrest. We propose a model of G2/M arrest whereby ATM and p53-dependent signaling pathways converge to ultimately inhibit Cdc25 phosphatases