Nitrite-mediated protection against hypochlorous acid-induced chondrocyte toxicity - A novel cytoprotective role of nitric oxide in the inflamed joint?

Objective. To examine the potential consequences of overproduction of nitric oxide (NO) and nitrite (NO2-) in the inflamed rheumatoid joint. Methods. Human articular chondrocytes in culture were exposed to HOCl (hypochlorous acid, a physiologic oxidant formed in increased amounts at sites of chronic inflammation), and assays of cell viability, intracellular ATP and glutathione (GSH), and lactate dehydrogenase (LDH) were performed. HOCl-induced lipid peroxidation and activation of the MAP kinases ERK-1/2, JNK-1/2, and p38 were also measured. The modulatory effects of NO-derived nitrite (NO2-) and nitrate (NO3-) on HOCl-mediated chondrocyte toxicity were investigated. Results. Exposure of human articular chondrocytes to HOCl resulted in a concentration- and time-dependent loss of viability, decrease in ATP and GSH levels, LDH leakage, and cell death. HOCl induced significant lipid peroxidation as well as activation of the MAP kinases ERK-1/2 and p38 but not JNK-1/2. However, the presence of NO2- but not NO3- substantially decreased HOCl-dependent cellular toxicity even when NO2- was added at low (muM) concentrations. In sharp contrast, NO2- (1 mM) did not inhibit superoxide-, hydroxyl radical-, H2O2-, or peroxynitrite-mediated cytotoxicity. Furthermore, culture media from cells treated with interleukin-1beta (to generate NO and NO2-) offered significantly more protection against HOCl-mediated cytotoxicity than culture media from untreated cells. Conclusion. These data suggest that NO2- accumulation at chronically inflamed sites where both HOCl and NO are overproduced may be cytoprotective against damage induced by HOCl. Accumulation of NO2- could represent a novel cytoprotective role of NO in inflamed joints. A mechanism for this is suggested.