NITRIC-OXIDE GENERATION DURING CELLULAR METABOLIZATION OF THE DIABETOGENIC N-METHYL-N-NITROSO-UREA STREPTOZOTOZIN CONTRIBUTES TO ISLET-CELLDNA-DAMAGE

The N-methyl-N-nitroso-urea streptozotocin is an antibiotic with diabe togenic, carcinogenic and antitumor activity thought to act via alkyla tion of DNA and proteins. Evidence points to a release of bioactive ni tric oxide (NO) from streptozotocin as an additional cytotoxic activit y of this drug. Here we show by EPR spectroscopy, that NO is not gener ated during spontaneous decay of streptozotocin but that its metaboliz ation in rat hepatocytes and pancreatic islet cells yields NO. This NO formation is not due to a NO synthase (NOS) activity since NO formati on in hepatocytes in the presence of streptozotocin is not blocked by the NOS inhibitor N-G-methyl-L-arginine. By iNOS-specific RT-PCR no po sitive signal for specific m RNA presence was obtained in streptozotoc in-treated cells, proving that iNOS activity was not induced during ce ll isolation procedures and did not account for the NO release, Furthe rmore, early DNA-strand breaks induced either by SZ or by the NO donor nitroprusside were both significantly reduced in the presence of an i ntracellular NO scavenger. In contrast, DNA damage found after incubat ion with the purely alkylating agent methyl-methanesulfonate was not i nhibited by the NO trap, These results prove that intracellular format ion of NO occurs during degradation of SZ within cells. This NO appear s to contribute significantly to streptozotocin-induced cytotoxicity.