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

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Abstract
Objective To examine the potential consequences of overproduction of nitric oxide (NO) and nitrite (NO) 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 (NO) and nitrate (NO) 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 NO but not NO substantially decreased HOCl-dependent cellular toxicity even when NO was added at low (μM) concentrations. In sharp contrast, NO (1 mM) did not inhibit superoxide-, hydroxyl radical–, H2O2-, or peroxynitrite-mediated cytotoxicity. Furthermore, culture media from cells treated with interleukin-1β (to generate NO and NO) offered significantly more protection against HOCl-mediated cytotoxicity than culture media from untreated cells. Conclusion These data suggest that NO accumulation at chronically inflamed sites where both HOCl and NO are overproduced may be cytoprotective against damage induced by HOCl. Accumulation of NO could represent a novel cytoprotective role of NO in inflamed joints. A mechanism for this is suggested.
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