Processing of the NF-κB2 precursor p100 to p52 is critical for RANKL-induced osteoclast differentiation

Gene targeting of the p50 and p52 subunits of NF-κB has shown that NF-κB plays a critical role in osteoclast differentiation. However, the molecular mechanism by which NF-κB regulates osteoclast differentiation is still unclear. To address this issue, we analyzed alymphoplasia (aly/aly) mice in which the processing of p100 to p52 does not occur owing to an inactive form of NF-κB-inducing kinase (NIK). Aly/aly mice showed a mild osteopetrosis with significantly reduced osteoclast numbers. RANKL-induced osteoclastogenesis from bone marrow cells of aly/aly mice also was suppressed. RANKL still induced the degradation of IκBα and activated classical NF-κB, whereas processing of p100 to p52 was abolished by the aly/aly mutation. Moreover, RANKL-induced expression of NFATc1 was impaired in aly/aly bone marrow. Overexpression of constitutively active IKKα or p52 restored osteoclastogenesis in aly/aly cells. Finally, transfection of either wild-type p100, p100ΔGRR that cannot be processed to p52, or p52 into NF-κB2-deficient cells followed by RANKL treatment revealed a strong correlation between the number of osteoclasts induced by RANKL and the ratio of p52 to p100 expression. Our data provide a new finding for a previously unappreciated role for NF-κB in osteoclast differentiation.