Hydrogen peroxide mediates interleukin-1beta-induced AP-1 activation in articular chondrocytes: implications for the regulation of iNOS expression

The pro-inflammatory cytokine interleukin-1beta (IL-1) induces articular chondrocytes to produce reactive oxygen species (ROS), including hydrogen peroxide (H2O2), which mediate some IL-1-induced responses. This study aimed at elucidating the role of ROS, particularly H2O2, in mediating IL-1-induced activation of the transcription factor activator protein-1 (AP-1) in primary cultures of articular chondrocytes. AP-1 may function either as an inducer or as a repressor of the inducible nitric oxide synthase (iNOS) gene promoter. Since we observed that AP-1 is not required for iNOS expression in chondrocytes, we also investigated whether it is a repressor of this gene. The results of electrophoretic mobility shift assays showed that both IL-1 and H2O2 activated AP-1 and that inhibition of IL-1-induced ROS production abrogated AP-1 activation. The AP-1 complexes, induced by either IL-1 or H2O2, contained c-Fos/c-Jun and c-Fos/JunD heterodimers, but IL-1 activated AP-1 with a kinetics slower than that observed with H2O2. Pre-activation of AP-1, before stimulation of the cells with IL-1, did not inhibit iNOS mRNA and protein synthesis, relative to cells treated with IL-1 alone. These results indicate that H2O2 is a major mediator of IL-1-induced AP-1 activation in articular chondrocytes and that inhibition of ROS production is an effective strategy to block this IL-1-induced response. This study also identifies c-Fos/c-Jun and c-Fos/JunD heterodimers as the AP-1 transcription factors induced by IL-1, which, although not involved in the transcriptional regulation of the iNOS gene, may be important for the regulation of other genes also relevant in arthritic diseases, namely the collagenase-1 and IL-8 genes.