Inhibition of RIPK1/RIPK3 ameliorates osteoclastogenesis through regulating NLRP3-dependent NF-κB and MAPKs signaling pathways

Osteoblast-induced bone formation and osteoclast-regulated bone resorption are the essential events contributing to bone homeostasis. It is critical to investigate the underlying molecular mechanisms. In this study, we explored the effects of receptor-interacting serine-threonine kinases (RIPKs) on osteoclastogenesis and bone loss in vitro and in vivo. We found that both RIPK1 and RIPK3 expression levels were highly up-regulated during osteoclastogenesis. Inhibiting RIPK1 and RIPK3 by their inhibitors Necrostatin-1 (Nec-1) and GSK-872, respectively, showed effective activities against osteoclast differentiation and bone resorption induced by receptor activator of nuclear factor-κB ligand (Rankl). Osteoclast-specific gene expression levels were also impeded by RIPK1/RIPK3 blockage in a time-dependent manner. Subsequently, we found that the pyrin domain-containing protein 3 (NLRP3) inflammasome stimulated by Rankl during osteoclastogenesis was greatly inhibited by Nec-1 and GSK-872. Additionally, reducing RIPK1/RIPK3 overtly reduced the activation of NF-κB (p65) and mitogen-activated protein kinases (MAPKs) signaling during Rankl-induced osteoclast formation. Notably, adenovirus-regulated NLRP3 over-expression significantly abrogated the inhibitory effects of Nec-1 and GSK-872 on NF-κB and MAPKs signaling pathways, as well as the osteoclastogenesis. Finally, the in vivo studies indicated that suppressing RIPK1/RIPK3 could effectively ameliorate ovariectomy (OVX)-induced bone loss in mice through repressing osteoclastogenesis, as proved by the clearly down-regulated number of osteoclasts via histological staining. In conclusion, our study elucidated that restraining RIPK1/RIPK3 could hinder osteoclastogenesis and attenuate bone loss through suppressing NLRP3-dependent NF-κB and MAPKs signaling pathways. Therefore, targeting RIPK1/RIPK3 signaling might be a potential therapeutic strategy to develop effective treatments against osteoclast-related bone lytic diseases.