Inhibition of ATM and ATR kinase activities by the radiosensitizing agent,caffeine

Caffeine exposure sensitizes tumor cells to ionizing radiation and other ge notoxic agents. The radiosensitizing effects of caffeine are associated wit h the disruption of multiple DNA damage-responsive cen cycle checkpoints. T he similarity of these checkpoint defects to those seen in ataxia-telangiec tasia (A-T) suggested that caffeine might inhibit one or more components in an A-T mutated (ATM)-dependent checkpoint pathway in DNA-damaged cells, We now show that caffeine inhibits the catalytic activity of both ATM and the related kinase, ATM and Rad3-related (ATR), at drug concentrations similar to those that induce radiosensitization, Moreover, like ATM-deficient cell s, caffeine-treated A549 lung carcinoma cells irradiated in G(2) fail to ar rest progression into mitosis, and S-phase-irradiated cells exhibit radiore sistant DNA synthesis. Similar concentrations of caffeine also inhibit gamm a- and UV radiation-induced phosphorylation of p53 on Ser(15), a modificati on that may be directly mediated by the ATM and ATR kinases, DNA-dependent protein kinase, another ATM-related protein involved in DNA damage repair, was resistant to the inhibitory effects of caffeine, Likewise, the catalyti c activity of the G(2) checkpoint kinase, hChk1, was only marginally suppre ssed by caffeine but was inhibited potently by the structurally distinct ra diosensitizer, UCN-01, These data suggest that the radiosensitizing effects of caffeine are related to inhibition of the protein kinase activities of ATM and ATR and that both proteins are relevant targets for the development of novel anticancer agents.