The Role of Metabolic Flexibility in the Regulation of the DNA Damage Response by Nitric Oxide

Abstract

In this report, we show that nitric oxide suppresses DNA damage response (DDR) signaling in the pancreatic β-cell line INS 832/13 and rat islets by inhibiting intermediary metabolism. Nitric oxide is known to inhibit complex IV of the electron transport chain and aconitase of the Krebs cycle. Non-β cells compensate by increasing glycolytic metabolism to maintain ATP levels; however, β cells lack this metabolic flexibility, resulting in a nitric oxide-dependent decrease in ATP and NAD+. Like nitric oxide, mitochondrial toxins inhibit DDR signaling in β cells by a mechanism that is associated with a decrease in ATP. Non-β cells compensate for the effects of mitochondrial toxins with an adaptive shift to glycolytic ATP generation that allows for DDR signaling. Forcing non-β cells to derive ATP via mitochondrial respiration (replacing glucose with galactose in the medium) and glucose deprivation sensitizes these cells to nitric oxide-mediated inhibition of DDR signaling. These findings indicate that metabolic flexibility is necessary to maintain DDR signaling under conditions in which mitochondrial oxidative metabolism is inhibited and support the inhibition of oxidative metabolism (decreased ATP) as one protective mechanism by which nitric oxide attenuates DDR-dependent β-cell apoptosis.

Inhibition of oxidative metabolism by nitric oxide restricts EMCV replication selectively in pancreatic beta-cells

“…β-cells lack this metabolic flexibility, as the coupling of glycolytic and mitochondrial oxidation of glucose is an essential regulatory feature of glucose-induced insulin secretion (26). A consequence of this lack of flexibility is the decrease in ATP levels in β-cells experiencing impaired mitochondrial oxidative capacity in response to nitric oxide (27,29) or inhibitors of mitochondrial respiration such as rotenone ( Figure 1E). In cells that maintain metabolic flexibility such as MEFs, the inhibition of mitochondrial respiration does not modify ATP levels ( Figure 1E), consistent with our previous observations (27).…”

“…A consequence of this lack of flexibility is the decrease in ATP levels in β-cells experiencing impaired mitochondrial oxidative capacity in response to nitric oxide (27,29) or inhibitors of mitochondrial respiration such as rotenone ( Figure 1E). In cells that maintain metabolic flexibility such as MEFs, the inhibition of mitochondrial respiration does not modify ATP levels ( Figure 1E), consistent with our previous observations (27). These data correlate the β-cell selective inhibition of EMCV replication by mitochondrial respiratory chain inhibitors with decreases in ATP levels that are due to an inability of b-cells to compensate for the inhibition of mitochondrial oxidative metabolism with increases in glycolytic flux (27).…”

Regulation of ATR-dependent DNA damage response by nitric oxide

“…Nitric oxide mediates the inhibitory actions of cytokines on insulin secretion ( 23 , 24 , 25 ) by attenuating the Krebs cycle enzyme aconitase and complex IV of the electron transport chain ( 26 , 27 ). Because of the coupling of aerobic and anaerobic metabolisms, β-cell ATP levels are decreased when mitochondrial respiration is inhibited because of a lack of glycolytic compensation ( 17 , 28 , 29 , 30 , 31 ). In non–β-cells, nitric oxide does not decrease ATP because they have the metabolic flexibility to increase glycolysis ( 28 ).…”

“…Because of the coupling of aerobic and anaerobic metabolisms, β-cell ATP levels are decreased when mitochondrial respiration is inhibited because of a lack of glycolytic compensation ( 17 , 28 , 29 , 30 , 31 ). In non–β-cells, nitric oxide does not decrease ATP because they have the metabolic flexibility to increase glycolysis ( 28 ). Recently, we have shown that the same metabolic pathways responsible for the control of glucose-stimulated insulin secretion are inhibited by nitric oxide, and this results in an attenuation in ATM signaling and the inhibition of DNA damage–induced β-cell apoptosis ( 14 , 28 , 32 ).…”

“…In non–β-cells, nitric oxide does not decrease ATP because they have the metabolic flexibility to increase glycolysis ( 28 ). Recently, we have shown that the same metabolic pathways responsible for the control of glucose-stimulated insulin secretion are inhibited by nitric oxide, and this results in an attenuation in ATM signaling and the inhibition of DNA damage–induced β-cell apoptosis ( 14 , 28 , 32 ).…”