Oxide-mediated Radiosensitization of Hypoxic EMT-6 Tumor Activation of Inducible Nitric Oxide Synthase Results in Nitric

EMT-6 cells treated for 16 h with 1-10 units/ml IFN-y showed a gradual activation ofinducible nitric oxide synthase (¡NOS)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 radioseasitization were counteracted by the NO scavenger oxyhemoglobin, by the specific ¡NOSinhibitor aminoguanidine, and by the L-arginine analogue A^'-monomethyl-i.-arginine. Collectively, these data demonstrate that IFN-y can radiosensitize EMT-6 cells through ¡NOSinduction and that NO is the effector molecule responsible for radiosensitization. Compared with the spon taneous NO releaser (2)-l-[A'-(3-ammoniopropyl)-A'-(H-propyl)amino)diazenl-ium-l,2-diolate|. the ¡NOS-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 i.-arginine/iNOS pathway repre sents a novel approach to exploit the radiosensitizing properties of NO.