Activation of inducible nitric oxide synthase results in nitric oxide-mediated radiosensitization of hypoxic EMT-6 tumor cells.

EMT-6 cells treated for 16 h with 1-10 units/ml IFN-gamma showed a gradual activation of inducible nitric oxide synthase (iNOS) in Western and Northern blots, a simultaneous raise in NO output, and an increase in hypoxic cell radiosensitivity almost to the level of aerobic cells. Both the NO signal and radiosensitization were counteracted by the NO scavenger oxyhemoglobin, by the specific iNOS inhibitor aminoguanidine, and by the L-arginine analogue N(G)-monomethyl-L-arginine. Collectively, these data demonstrate that IFN-gamma can radiosensitize EMT-6 cells through iNOS induction and that NO is the effector molecule responsible for radiosensitization. Compared with the spontaneous NO releaser (2)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino)diazen-1-ium -1,2-diolate], the iNOS-generated NO signal appeared to be 10 times lower yet resulting in the same enhancement ratio of 2.4. Direct stimulation of NO synthesis in tumor cells through the L-arginine/iNOS pathway represents a novel approach to exploit the radiosensitizing properties of NO.