Regulation of human osteoclast function by redox and sRANKL-induced intracellular ROS production

In animal models, reactive oxygen species (ROS) have been shown to activate osteoclasts and have been implicated as second messengers in RANKL-mediated osteoclastogenesis. The aim of this study was to investigate the role of ROS in the function of mature human osteoclasts. Osteoclasts were generated by culture of CFU-GM with human M-CSF and RANKL for 14 days. Modulation of resorbing capacity and survival were assessed in mature osteoclasts dissociated from the plastic substrate and re-cultured on dentine slices. Treatment with RANKL enhanced the survival of mature osteoclasts on dentine and was absolutely required for resorption. Resorption was further enhanced by the presence of both M-CSF and RANKL. Co-treatment with hydrogen peroxide (50μM) for 24 hours resulted in a 130% increase in osteoclast size and a 34% increase in resorption. In contrast, co-treatment with the thiol antioxidant N-acetylcysteine (NAC; 10 mM) reduced osteoclast size by 22% and number by 50%, and resorption was completely inhibited. When mature osteoclasts were RANKL- and serum-starved for 3 hours and then stimulated with RANKL (150 ng/mL) for 30 minutes, increased intracellular ROS-production was detected using fluorescence microscopy and the ROS-sensitive dye CM-H2DCFDA. Pre-treatment with NAC blocked RANKL-induced intracellular ROS. Treatment with RANKL also stimulated a timedependent increase in intracellular hydrogen peroxide, as measured using Amplex Red reagent, with peak accumulation at 10 minutes. Electrophoretic mobility shift assay demonstrated that pre-treatment of osteoclast precursors with hydrogen peroxide (10μM) for 16 hours resulted in increased RANKL-induced NF-κB activity, whereas pre-treatment with NAC (10mM) blocked this activity. Human osteoclast function is stimulated by ROS and inhibited by antioxidant quenching of ROS. Stimulation of osteoclast survival and resorbing activity by RANKL may be mediated by ligand-mediated intracellular ROS production