The
serine/threonine protein kinase ATM signals to cell cycle and DNA repair components by phosphorylating downstream targets such as p53, CHK2, NBS1, and BRCA1. Mutation of ATM occurs in the human autosomal recessive disorder
ataxia-telangiectasia, which is characterized by
hypersensitivity to ionizing radiation and a failure of cells to arrest the cell cycle after the induction of
DNA double-strand breaks. It has thus been proposed that ATM inhibition would cause cellular radio- and chemosensitization. Through screening a small molecule compound library developed for the
phosphatidylinositol 3'-kinase-like
kinase family, we identified an
ATP-competitive inhibitor,
2-morpholin-4-yl-6-thianthren-1-yl-pyran-4-one (KU-55933), that inhibits ATM with an IC(50) of 13 nmol/L and a Ki of 2.2 nmol/L.
KU-55933 shows specificity with respect to inhibition of other
phosphatidylinositol 3'-kinase-like
kinases. Cellular inhibition of ATM by
KU-55933 was demonstrated by the ablation of ionizing radiation-dependent phosphorylation of a range of ATM targets, including p53, gammaH2AX, NBS1, and SMC1.
KU-55933 did not show inhibition of UV light DNA damage induced cellular phosphorylation events. Exposure of cells to
KU-55933 resulted in a significant sensitization to the cytotoxic effects of ionizing radiation and to the
DNA double-strand break-inducing chemotherapeutic agents,
etoposide,
doxorubicin, and
camptothecin. Inhibition of ATM by
KU-55933 also caused a loss of ionizing radiation-induced cell cycle arrest. By contrast,
KU-55933 did not potentiate the cytotoxic effects of ionizing radiation on
ataxia-telangiectasia cells, nor did it affect their cell cycle profile after DNA damage. We conclude that
KU-55933 is a novel, specific, and potent inhibitor of the ATM
kinase.