TNF-α and RANKL promote osteoclastogenesis by upregulating RANK via the NF-κB pathway

Although tumor necrosis factor alpha (TNF-alpha) is known to serve a critical role in the pathogenesis of inflammatory osteolysis, the exact mechanisms underlying the effects of TNF-alpha on osteoclast recruitment and differentiation remain unclear. To investigate the mechanisms by which TNF-alpha influences osteoclast differentiation, mouse bone marrow-derived macrophages (BMMs) were used as osteoclast precursors, and osteoclastogenesis was induced by macrophage colony-stimulating factor and receptor activator of nuclear factor (NF)-kappa B ligand (RANKL) with or without TNF-alpha for 4 days. Then, NF-kappa B was inhibited using the inhibitor, BAY 11-7082. The results indicated that treatment with TNF-alpha alone did not induce osteoclastogenesis of BMMs. However, TNF-alpha in combination with RANKL dramatically stimulated the differentiation of osteoclasts and positively regulated the expression of mRNA markers of osteoclasts. Finally, treatment of BMMs with BAY 11-7082 prevented the formation of mature osteoclasts by BMMs treated with TNF-alpha only or with RANKL, as well as the upregulation of osteoclast marker genes. Therefore, although TNF-alpha does not induce osteoclastogenesis alone, it does work with RANKL to induce osteoclastic differentiation, and the NF-kappa B pathway may serve an important role in this process.