Effect of Nitric Oxide on Naphthoquinone Toxicity in Endothelial Cells: Role of Bioenergetic Dysfunction and Poly(ADP-ribose) Polymerase Activation

When produced at physiological levels, reactive oxygen species (ROS) can act as signaling molecules to regulate normal vascular function. Produced under pathological conditions, ROS can contribute to the oxidative damage of cellular components (e.g., DNA and proteins) and trigger cell death. Moreover, the reaction of superoxide with nitric oxide (NO) produces the strong oxidant peroxynitrite and decreases NO bioavailability, both of which may contribute to activation of cell death pathways. The effects of ROS generated from the 1,4-naphthoquinones alone and in combination with NO on the activation status of poly(ADP-ribose) polymerase (PARP) and cell viability were examined. Treatment with redox cycling quinones activates PARP, and this stimulatory effect is attenuated in the presence of NO. Mitochondria play a central role in cell death signaling pathways and are a target of oxidants. We show that simultaneous exposure of endothelial cells to NO and ROS results in mitochondrial dysfunction, ATP and NAD(+) depletion, and cell death. Alone, NO and ROS have only minor effects on cellular bioenergetics. Further, PARP inhibition does not attenuate reduced cell viability or mitochondrial dysfunction. These results show that concomitant exposure to NO and ROS impairs energy metabolism and triggers PARP-independent cell death. While superoxide-mediated PARP activation is attenuated in the presence of NO, PARP inhibition does not modify the loss of mitochondrial function or adenine and pyridine nucleotide pools and subsequent bioenergetic dysfunction. These findings suggest that the mechanisms by which ROS and NO induce endothelial cell death are closely linked to the maintenance of mitochondrial function and not overactivation of PARP.