Modification of DNA damage mechanisms by nitric oxide during ionizing radiation

Nitric oxide ((NO)-N-center dot) is a very effective radiosensitizer of hypoxic mammalian cells. In vivo (NO)-N-center dot may have effects on tumor vasculature and hence on tumor oxygenation and it may also interact with radiation-produced radicals to modify DNA lesions. Few studies have addressed this last aspect, and we report here specific base modifications that result from reaction of (NO)-N-center dot with radicals in DNA bases and in plasmid DNA after irradiation. 2'-Deoxyxanthosine monophosphate and 2'-deoxy-8-azaguanosine monophosphate (8azadGMP) are formed upon gamma-irradiation of 2'-deoxyguanosine monophosphate (dGMP) in the presence of micromolar levels of (NO)-N-center dot in anoxia. In addition, the presence of (NO)-N-center dot at physiological pH inhibits the formation of the well-described (OH)-O-center dot-induced oxidation product of dGMP, 8-oxo-2'-deoxyguanosine monophosphate. Single-strand breaks are induced in plasmid DNA when gamma-irradiated in anoxia, whereas in the presence of (NO)-N-center dot the number of breaks is reduced by approximately threefold, and evidence is shown for the formation of 8azadGMP in these plasmids. The consequence of the base modifications by (NO)-N-center dot are as yet unknown although additional breaks are revealed in irradiated plasmid DNA after treatment with glycosylases involved in base excision repair. V79-4 cells irradiated in anoxia show an enhancement in the number of gamma H2AX foci when (NO)-N-center dot is present, particularly evident a few hours postirradiation, indicative of the formation of replication-induced DNA damage. We propose that the consequence of (NO)-N-center dot-induced base modifications in anoxia contributes to its radiosensitization of cells. (C) 2013 Elsevier Inc. All rights reserved.