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 an d cell survival. Importantly, NO has been shown, in a cell-type-depend ent manner, to directly cause cell death or indirectly promote cell de ath by compromising the ability of cells to detoxify intra- or extrace llular oxidants. In this study we examined the role of NO in the survi val of bovine chondrocytes exposed to catabolic cytokines (interleukin -l (IL-l); tumor necrosis factor [TNF]) with or without the addition o f an exogenous oxidant stress (e.g., H2O2, HOOCl, etc.). The exposure of chondrocytes to a mixture of IL-l and TNF (IL-1/TNF) results in the release of NO but did not alter cell viability. However, there was ev idence of NO-dependent oxidative responses in the IL-1/TNF group, as w e observed an increased level of intracellular oxidants as well as the appearance of a 55 kD nitrated protein which reflects the formation o f 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 s ensitivity was completely reversed when cells were incubated with the NO synthase inhibitor 1-n5-1-iminoethylornithine (NIO). To test whethe r cell death was caused by compromising the ability of cells to detoxi fy extracellular oxidants, we examined the hexose monophosphate shunt (HMPS) response in cells given H2O2. Treatment of control cells with H 2O2 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 cyto kines and oxidants, NO may contribute to chondrocyte death and progres sive joint destruction. (C) 1997 Wiley-Liss, Inc.