Nitric oxide attenuates cellular hexose monophosphate shunt response to oxidants in articular chondrocytes and acts to promote oxidant injury.

Nitric oxide (NO) has been implicated in both cartilage degradation and cell survival. Importantly, NO has been shown, in a cell-type-dependent manner, to directly cause cell death or indirectly promote cell death by compromising the ability of cells to detoxify intra- or extracellular oxidants. In this study we examined the role of NO in the survival of bovine chondrocytes exposed to catabolic cytokines (interleukin-1 (IL-1); tumor necrosis factor [TNF]) with or without the addition of an exogenous oxidant stress (e.g., H2O2, HOOCl, etc.). The exposure of chondrocytes to a mixture of IL-1 and TNF (IL-1/TNF) results in the release of NO but did not alter cell viability. However, there was evidence of NO-dependent oxidative responses in the IL-1/TNF group, as we observed an increased level of intracellular oxidants as well as the appearance of a 55 kD nitrated protein which reflects the formation of peroxynitrite. We next analyzed viability with H2O2. The LD50 for IL-1/TNF-treated cells was 0.1 mM (vs. 1 mM for control). The enhanced sensitivity was completely reversed when cells were incubated with the NO synthase inhibitor 1-n5-1-iminoethylornithine (NIO). To test whether cell death was caused by compromising the ability of cells to detoxify extracellular oxidants, we examined the hexose monophosphate shunt (HMPS) response in cells given H2O2. Treatment of control cells with H2O2 resulted in a fourfold increase in HMPS activity. In contrast, IL-1/TNF cells exhibited no increase in HMPS activity. The attenuation of stimulated HMPS activity was reversed by the coaddition of NIO. Thus, these data indicate that 1) endogenous NO mediates cytokine-dependent susceptibility to oxidant injury and 2) this effect is in part due to impaired activation of the HMPS. In inflamed joints replete with cytokines and oxidants, NO may contribute to chondrocyte death and progressive joint destruction.