Tumor necrosis factor stimulates osteoclastogenesis from human bone marrow cells under hypoxic conditions

Bone homeostasis is maintained by the balance between osteoblastic bone formation and osteoclastic bone resorption. In this study, we used human bone marrow cells (BMCs) to investigate the role of hypoxic exposure on human osteoclast (OC) formation in the presence of tumor necrosis factor (TNF). Exposing the BMCs to 3%, 5%, or 10% O₂ in the presence of receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) generated tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells, consistent with OCs. The addition of TNF under hypoxic conditions generated significantly greater numbers of mature OCs with more nuclei than OCs generated under normoxic conditions. Longer initial hypoxic exposure increased the number of OC precursor cells and facilitated the differentiation of OC precursor cells into multinucleated OCs. Quantitative RT-PCR analysis revealed that RANKL and TNFR1 were expressed at higher levels in non-OC cells from BMCs under hypoxic conditions than under normoxic conditions. Furthermore, to confirm the involvement of TNF-induced signaling, we examined the effects of blocking antibodies against TNFR1 and TNFR2 on OC formation under hypoxic conditions. The TNFR1 antibody was observed to significantly suppress OC formation. These results suggest that hypoxic exposure plays an important role in TNF-induced osteoclastogenesis from human BMCs.