Modification of DNA damage mechanisms by nitric oxide during ionizing radiation.

Nitric oxide ((•)NO) is a very effective radiosensitizer of hypoxic mammalian cells. In vivo (•)NO 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 with radicals in DNA bases and in plasmid DNA after irradiation. 2'-Deoxyxanthosine monophosphate and 2'-deoxy-8-azaguanosine monophosphate (8azadGMP) are formed upon γ-irradiation of 2'-deoxyguanosine monophosphate (dGMP) in the presence of micromolar levels of (•)NO in anoxia. In addition, the presence of (•)NO at physiological pH inhibits the formation of the well-described (•)OH-induced oxidation product of dGMP, 8-oxo-2'-deoxyguanosine monophosphate. Single-strand breaks are induced in plasmid DNA when γ-irradiated in anoxia, whereas in the presence of (•)NO 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 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 γH2AX foci when (•)NO is present, particularly evident a few hours postirradiation, indicative of the formation of replication-induced DNA damage. We propose that the consequence of (•)NO-induced base modifications in anoxia contributes to its radiosensitization of cells.