Hypoxia Promotes Osteoclast Differentiation through Weakening the USP18-Mediated Suppression on NF-κB Signaling Pathway

Background: Osteoporosis, a prevalent metabolic bone disorder, is characterized by reduced bone density and increased fracture risk. The pathogenesis of osteoporosis is closely associated with an imbalance in bone remodeling, where the resorption function of osteoclasts exceeds the formation function of osteoblasts. Hypoxia has been implicated in the promotion of osteoclast differentiation and the subsequent development of osteoporosis. The ubiquitin proteasome system (UPS) and its regulatory enzymes, deubiquitinating enzymes (DUBs), play a significant role in bone homeostasis. In this study, we investigated the contribution and mechanism of USP18, a DUB, in osteoclast differentiation under hypoxic conditions. Methods: BMDMs and RAW264.7 cells were treated with RANKL to induce osteoclastogenesis and were subjected to overexpression or knockdown of USP18 under normoxic or hypoxia conditions. Osteoclast formation was assessed using TRAP staining, and the expression of osteoclast marker genes was determined by qRT-PCR. The activation of the NF-κB signaling pathway was evaluated by immunoblotting. Results: We found that hypoxia significantly enhanced the differentiation of BMDMs and RAW264.7 cells into osteoclasts, accompanied by a notable downregulation of USP18 expression. Overexpression of USP18 inhibited RANKL-induced osteoclast differentiation, while knockdown of USP18 promoted that process, unveiling the inhibitory effect of USP18 in osteoclastogenesis. Furthermore, overexpression USP18 rescued the hypoxia-induced increase in osteoclast differentiation. Mechanistic insights revealed that USP18 inhibits osteoclastogenesis by suppressing the NF-κB signaling pathway, with a potential target on TAK1 or its upstream molecules. Conclusion: Our study indicates that hypoxia promotes osteoclast differentiation through the downregulation of USP18, which in turn relieves the suppression on the activation of NF-κB signaling pathway. The USP18 emerges as a potential therapeutic target for osteoporosis treatment, highlighting the importance of the hypoxia-DUBs axis in the pathogenesis of the disease.