Osteoclast Differentiation Is Impaired in the Absence of Inhibitor of κB Kinase α*

Signaling through the receptor activator of nuclear factor κB (RANK) is required for both osteoclast differentiation and mammary gland development, yet the extent to which RANK utilizes similar signaling pathways in these tissues remains unclear. Mice expressing a kinase-inactive form of the inhibitor of κB kinase α (IKKα) have mammary gland defects similar to those of RANK-null mice yet have apparently normal osteoclast function. Because mice that completely lack IKKα have severe skin and skeletal defects that are not associated with IKKα-kinase activity, we wished to directly examine osteoclastogenesis in IKKα^-/- mice. We found that unlike RANK-null mice, which completely lack osteoclasts, IKKα^-/- mice did possess normal numbers of TRAP⁺ osteoclasts. However, only 32% of these cells were multinucleated compared with 57% in wild-type littermates. A more profound defect in osteoclastogenesis was observed in vitro using IKKα^-/- hematopoietic cells treated with colony-stimulating factor 1 and RANK ligand (RANKL), as the cells failed to form large, multinucleated osteoclasts. Additionally, overall RANKL-induced global gene expression was significantly blunted in IKKα^-/- cells, including osteoclast-specific genes such as TRAP, MMP-9, and c-Src. IKKα was not required for RANKL-mediated IκBα degradation or phosphorylation of mitogen-activated protein kinases but was required for RANKL-induced p100 processing. Treatment of IKKα^-/- cells with tumor necrosis factor α (TNFα) in combination with RANKL led to partial rescue of osteoclastogenesis despite a lack of p100 processing. However, the ability of TNFα alone or in combination with transforming growth factor β to induce osteoclast differentiation was dependent on IKKα, suggesting that synergy between RANKL and TNFα can overcome p100 processing defects in IKKα^-/- cells.