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It has been reported that reactive oxygen species (ROS), such as hydrogen peroxide and superoxide, take part in osteoclast differentiation as intra-cellular signaling molecules. The current assumed signaling cascade from RANK to ROS production is RANK, TRAF6, Rac1, and then Nox. The target molecules of ROS in RANKL signaling remain unclear; however, several reports support the theory that NF-κB signaling could be the crucial downstream signaling molecule of RANKL-mediated ROS signaling. Furthermore, ROS exert cytotoxic effects such as peroxidation of lipids and phospholipids and oxidative damage to proteins and DNA. Therefore, cells have several protective mechanisms against oxidative stressors that mainly induce cytoprotective enzymes and ROS scavenging. Three well-known mechanisms regulate cytoprotective enzymes including Nrf2-, FOXO-, and sirtuin-dependent mechanisms. Several reports have indicated a crosslink between FOXO- and sirtuin-dependent regulatory mechanisms. The agonists against the regulatory mechanisms are reported to induce these cytoprotective enzymes successfully. Some of them inhibit osteoclast differentiation and bone destruction via attenuation of intracellular ROS signaling. In this review article, we discuss the above topics and summarize the current information available on the relationship between cytoprotective enzymes and osteoclastogenesis.

Abstract: Bone destructive diseases are common worldwide and are caused by dysregulation of osteoclast formation and activation. During osteoclastogenesis, reactive oxygen species (ROS) play a role in the intracellular signalling triggered by receptor activator of nuclear factor‐κB ligand (RANKL) stimulation. Previously, we demonstrated that induction of antioxidant enzymes by Nrf2 activation using Nrf2‐gene transfer, an ETGE‐peptide or polyphenols, successfully ameliorated RANKL‐dependent osteoclastogenesis. Dimethyl fumarate (DMF) has been shown to activate Nrf2 signalling and has been lately used in clinical trials for neurodegenerative diseases. In this study, we hypothesized that Nrf2 activation by DMF would inhibit osteoclastogenesis and bone destruction via attenuation of intracellular ROS signalling through antioxidant mechanisms. RAW 264.7 cells were used as osteoclast progenitor cells. We found that DMF induced Nrf2 translocation to the nucleus, augmented Nrf2 promoter‐luciferase reporter activity and increased antioxidant enzyme expression. Using flow cytometry, we found that DMF attenuated RANKL‐mediated intracellular ROS generation, which resulted in the inhibition of RANKL‐mediated osteoclastogenesis. Local DMF injection into the calvaria of male BALB/c mice resulted in attenuated bone destruction in lipopolysaccharide‐treated mice. In conclusion, we demonstrated in a preclinical setting that DMF inhibited RANKL‐mediated osteoclastogenesis and bone destruction via induction of Nrf2‐mediated transcription of antioxidant genes and consequent decrease in intracellular ROS levels. Our results suggest that DMF may be a promising inhibitor of bone destruction in diseases like periodontitis, rheumatoid arthritis and osteoporosis. [ABSTRACT FROM AUTHOR]

Reactive oxygen species (ROS) play a role in intracellular signaling during osteoclastogenesis. We previously reported that transcriptional factor nuclear factor E2-related factor 2 (Nrf2) was exported from the nucleus to the cytoplasm by receptor activator of nuclear factor-κB ligand (RANKL), and that Nrf2 negatively regulated osteoclastogenesis via antioxidant enzyme up-regulation. Knockout mice of BTB and CNC homology 1 (Bach1)-the competitor for Nrf2 in transcriptional regulation-was known to attenuate RANKL-mediated osteoclastogenesis, although the mechanism remains unclear. Therefore, we hypothesized that RANKL could be involved in the nuclear translocation of Bach1, which would attenuate Nrf2-mediated antioxidant enzymes, thereby augmenting intracellular ROS signaling in osteoclasts. RANKL induced Bach1 nuclear import and Nrf2 nuclear export. Induction of Bach1 nuclear export increased Nrf2 nuclear import, augmented antioxidant enzyme expression, and, thus, diminished RANKL-mediated osteoclastogenesis via attenuated intracellular ROS signaling. Finally, an in vivo mouse bone destruction model clearly demonstrated that induction of Bach1 nuclear export inhibited bone destruction. In this study, we report that RANKL favors osteoclastogenesis via attenuation of Nrf2-mediated antioxidant enzyme expression by competing with Bach1 nuclear accumulation. Of importance, induction of Bach1 nuclear export activates Nrf2-dependent antioxidant enzyme expression, thereby attenuating osteoclastogenesis. Bach1 nuclear export might be a therapeutic target for such bone destructive diseases as rheumatoid arthritis, osteoporosis, and periodontitis.-Kanzaki, H., Shinohara, F., Itohiya, K., Yamaguchi, Y., Katsumata, Y., Matsuzawa, M., Fukaya, S., Miyamoto, Y., Wada, S., Nakamura, Y. RANKL induces Bach1 nuclear import and attenuates Nrf2-mediated antioxidant enzymes, thereby augmenting intracellular reactive oxygen species signaling and osteoclastogenesis in mice., (© FASEB.)