Nitric oxide, generated in β-cells in response to cytokines, induces DNA damage that activates ataxia-telangiectasia mutated protein (ATM)-dependent DNA damage response (DDR). While the DDR does not contribute to DNA repair in β-cells, it does regulate β-cell apoptosis when the nitric oxide production falls below a threshold level. Indeed, nitric oxide at high micromolar levels, while inducing DNA damage, also inhibits DDR and protects β-cells from apoptosis. This ability of nitric oxide, a potent inhibitor of mitochondrial oxidation, to attenuate DDR activation correlates with a 10-fold reduction in ATP and the actions are selective to β-cells. Unlike most other cell types, β-cells cannot compensate for mitochondrial inhibition by increasing glycolysis, leading to a loss in ATP levels which correlates with DDR inhibition. We now show that nitric oxide also inhibits the activation of a second DDR transducer kinase, ATR. ATR is activated in response to single-strand DNA breaks and replication stress. We showed that low levels of nitric oxide activate ATR-dependent DDR signaling by inducing replication stress. However, much like with ATM, at higher concentrations nitric oxide and other inhibitors of mitochondrial respiration that reduce ATP levels by more than 10-fold, also inhibit ATR selectively in β-cells. Nitric oxide and these mitochondrial poisons do not inhibit ATR signaling in non-β-cells as they maintain ATP levels by increasing glycolysis under mitochondrial inhibition. Importantly, when non-β-cells are forced to utilize oxidative phosphorylation for ATP generation, nitric oxide and other mitochondrial inhibitors attenuate ATR signaling in a manner similar to β-cells. Together, these studies suggest that inhibition of the DDR by nitric oxide in β-cells is dependent on its ability to inhibit mitochondrial oxidative metabolism and deplete ATP, and suggest that the inhibition of DDR signaling may be a protective response to prevent β-cell apoptosis.
C. Yeo: None. B. Oleson: None. J.A. Corbett: None. J.K. Schnuck: None.
National Institutes of Health (DK052194, AI-44458)