MODULATION OF ENDOGENOUS ANTIOXIDANT ENZYMES BY NITRIC-OXIDE IN RAT C-6 GLIAL-CELLS

To understand the possible mechanism of nitric oxide (NO)-mediated cyt otoxicity, we investigated the effect of NO on the endogenous antioxid ant enzymes (AOEs) catalase, glutathione peroxidase (GPX), and CuZn- a nd Mn-superoxide dismutases (SODs) in rat C-6 glial cells under condit ions in which these cells expressed oligodendrocyte-like properties as evidenced by the expression of 2',3'-cyclic-nucleotide 3'-phosphohydr olase. The 24-h treatment with S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, decreased the activities and the protein levels of catala se, GPX, and Mn-SOD in a dose-dependent manner. Alternatively, the act ivity and the protein level of CuZn-SOD were increased. 2-Phenyl-4,4, 5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), a NO scavenger, bloc ked the effect of SNAP. Moreover, the treatment of C-6 cells with sodi um nitroprusside, another NO donor, or with a combination of lipopolys accharide (LPS) and interferon-gamma (IFN-gamma), which induce excessi ve production of NO, also significantly modulated the AOE activities i n a manner similar to that seen with SNAP treatment. The compounds/enz ymes that inhibit the production of NO (e.g., N-nitro-L-arginine methy l ester hydrochloride, arginase, and PTIO) blocked the effects of LPS and IFN-gamma on the activities of AOEs. Treatment with SNAP and a com bination of LPS and IFN-gamma also modulated the mRNA levels of AOEs, parallel to the changes in their protein levels and activities, except for Mn-SOD where the combination of LPS and IFN-gamma markedly stimul ated the mRNA expression. In spite of the stimulation of mRNA level, L PS and IFN-gamma significantly inhibited the activity of Mn-SOD within the first 24 h of incubation; however, Mn-SOD activity gradually incr eased with the increase in time of incubation. These results suggest t hat alterations in the status of AOEs by NO may be the basis of NO-ind uced cytotoxicity in disease states associated with excessive NO produ ction.