Objective. Cartilage homeostasis dysregulation during osteoarthritis (OA) has been linked to an increased rate of apoptosis of chondrocytes, the only cell type resident in the cartilage. In addition, the CD95-CD95 ligand (the Fas system) has emerged as one of the major pathways of cell death in the cartilage. We undertook the present study to investigate the role of interferon-γ (IFNγ) in the regulation of the Fas system by analyzing the modulation of intracellular signaling molecules (FLICE inhibitory protein [FLIP] and caspases 3 and 8) in primary cultures of human OA chondrocytes. Methods. CD95-induced apoptotic death of human OA chondrocytes was analyzed in the presence or absence of IFNγ using cell death immunoassay for apoptosis, real-time polymerase chain reaction for FLIP and caspase 8 expression, Western blotting for FLIP, and proteolytic activity for caspases 3 and 8. Results. CD95-induced apoptotic death of human OA chondrocytes was strongly counteracted by IFNγ treatment, although the surface expression of CD95 was slightly up-regulated by this cytokine. The messenger RNA (mRNA) expression of FLIP and caspase 8, mediators involved in CD95 signaling, revealed that FLIP expression in human OA chondrocytes was significantly up-regulated (2-fold increase) by IFNγ treatment. Moreover, the FLIP:caspase 8 mRNA ratio increase significantly. FLIP up-regulation by IFNγ was con firmed at the protein level. Caspase 8 and caspase 3 proteolytic activities, both induced in these cells by stimulation with anti-CD95, were also significantly down-modulated by IFNγ. Conclusion. These findings suggest that IFNγ impairs CD95-mediated signaling and apoptotic death in human chondrocytes. Its mechanism of action in volves down-regulation of caspase 8 and caspase 3 activities and increased expression of the antiapoptotic protein FLIP, suggesting an interesting mechanism for the inhibition of chondrocyte apoptosis.
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