Nitric oxide (NO) donors have recently been evaluated as radiosensitizers of hypoxic cells in a model of metabolisminduced hypoxia at high cell densities. Mitchell et al (1993Mitchell et al ( , 1996 demonstrated that 2-(N,N,-diethylamino)-diazenolate-2-oxide-Na + (DEA/NO), SNAP and S-nitroso-L-glutathione (GSNO) at a concentration of 1 mM, radiosensitized Chinese hamster V79 lung fibroblasts to a similar extent as oxygen. Griffin et al (1996) reported comparable activity for DEA/NO and (Z)-1-{N- [3-aminopropyl]-N-[4-(3-aminopropylammonio)butyl-amino}-diazen-1-ium-1,2-diolate (SPER/NO) with enhancement ratios of 2.8Ð3.0 in SCK mammary carcinoma cells exposed to 1Ð2 mM radiosensitizer. Our laboratory investigated the radiosensitizing activity of SNP in a panel of eight human pancreatic tumour cell lines and found an overall enhancement ratio of 1.9 at 0.1 mM (Verovski et al, 1996). At 0.3Ð1 mM, SNP caused almost complete radiosensitization in hypoxic PSN1/ADR cells that was accompanied by restoration of radiation-induced DNA breakage up to the level in aerated cells.These data collectively confirm the high radiosensitizing efficiency of NO donors at 1Ð2 mM and suggest a role of NO in the fixation of DNA damage caused by radiation. Whether DNA is the main target for NO, as was postulated by Howard-Flanders (1957), or whether other mechanisms contribute to the enhanced DNA damage remains unclear. As NO can interact with ironÐsulphur containing enzymes, this may result in the inhibition of cellular respiration and sparing of the natural sensitizer oxygen (Mitchell et al, 1996). This sensitizing effect mediated by residual oxygen can be minimized under hypoxic conditions induced by nitrogen gassing, a general approach to study radiosensitizers. Moreover, the latter model of hypoxia allows the use of a broad range of cell densities, which is a necessary step to establish whether the rate of NO release is enhanced by the presence of cells. The NO donors DEA/NO and SPER/NO are known to liberate NO by a purely spontaneous mechanism (Maragos et al, 1991). Some NO donors, such as SNAP and GSNO, have generally been assumed to release NO in a spontaneous manner although the slow decomposition rate of S-nitrosothiols appeared to underestimate their biological effects (Kowaluk and Fung, 1990). In fact, the chemistry of Snitrosothiols containing NO as a nitrosonium cation (Stamler et al, 1992), supports the idea of reductive catalysis of NO release, which may occur on the cellular membrane (Bates et al, 1991;Kowaluk et al, 1990;Rochelle et al, 1994). In the report of Mitchell et al (1996), the possibility of bioreductive generation of NO from SNAP has not been explored, but NO output in a cellfree system was apparently too low to account for radiosensitization. The same conclusion was drawn for another nitrosonium-like NO donor, SNP, whose bioreductive activation was found to be responsible for radiosensitization (Verovski et al, 1996).The objective of the present study was to examine whether bioreductive release of NO from SNAP...

Summary Chronic hypoxia up-regulated the mRNA and protein expression of inducible nitric oxide synthase (iNOS) in EMT-6 tumour cells exposed to interferon (IFN)-gamma and interleukin (IL)-I beta. Low concentrations of cytokines (1 unit ml -1 ) in 1% but not in 21% oxygen induced a remarkable increase in NO production and a 1.8-fold hypoxic cell radiosensitization. Therefore, chronic hypoxia may potentially be exploited to increase tumour cell radioresponse through the cytokine-inducible iNOS pathway.