Processing of the NF-kappa B2 precursor p100 to p52 is critical for RANKL-induced osteoclast differentiation.

Gene targeting of the p50 and p52 subunits of NF-kappaB has shown that NF-kappaB plays a critical role in osteoclast differentiation. However, the molecular mechanism by which NF-kappaB 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-kappaB-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 I kappaB alpha and activated classical NF-kappaB, 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 IKK alpha or p52 restored osteoclastogenesis in aly/aly cells. Finally, transfection of either wild-type p100, p100 Delta GRR that cannot be processed to p52, or p52 into NF-kappaB 2-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-kappaB in osteoclast differentiation.