Osteoclasts are cells of monocyte/macrophage origin that erode bone matrix: regulation of their differentiation is central to the understanding of the pathogenesis and treatment of bone diseases such as osteoporosis^1,2. Signalling by RANKL (receptor activator of NF-κB ligand), also known as Tnfsf11, is essential for the induction of osteoclast differentiation^3,4,5, and it must be strictly regulated to maintain bone homeostasis. But it is not known whether RANKL signalling to the cell interior is linked to any regulatory mechanisms. Here we show that RANKL induces the interferon-β (IFN-β) gene in osteoclast precursor cells, and that IFN-β inhibits the differentiation by interfering with the RANKL-induced expression of c-Fos, an essential transcription factor for the formation of osteoclasts. This IFN-β gene induction mechanism is distinct from that induced by virus, and is dependent on c-Fos itself. Thus an autoregulatory mechanism operates—the RANKL-induced c-Fos induces its own inhibitor. The importance of this regulatory mechanism for bone homeostasis is emphasized by the observation that mice deficient in IFN-β signalling exhibit severe osteopenia (loss of bone mass) accompanied by enhanced osteoclastogenesis. Our study places the IFN-β system in a new context, and may offer a molecular basis for the treatment of bone diseases.

Osteoclasts are specialized macrophages that resorb bone. Mice lacking the AP-1 component c-Fos are osteopetrotic because of a lack of osteoclast differentiation and show an increased number of macrophages. The nature of the critical function of c-Fos in osteoclast differentiation is not known. Microarray analysis revealed that Nfatc1, another key regulator of osteoclastogenesis, was down-regulated in Fos^-/- osteoclast precursors. Chromatin immunoprecipitation assay showed that c-Fos bound to the Nfatc1 and Acp5 promoters in osteoclasts. In vitro promoter analyses identified nuclear factor of activated T-cells (NFAT)/AP-1 sites in the osteoclast-specific Acp5 and Calcr promoters. Moreover, in Fos^-/- precursors gene transfer of an active form of NFAT restored transcription of osteoclast-specific genes in the presence of receptor activator of the NF-κB ligand (RANKL), rescuing bone resorption. In the absence of RANKL, however, Fos^-/- precursors were insensitive to NFAT-induced osteoclastogenesis unlike wild-type precursors. These data indicate that lack of Nfatc1 expression is the cause of the differentiation block in Fos^-/- osteoclast precursors and that transcriptional induction of Nfatc1 is a major function of c-Fos in osteoclast differentiation.

Postmenopausal osteoporosis and rheumatoid joint destruction result from increased osteoclast formation and bone resorption induced by receptor activator of NF-κB ligand (RANKL) and tumor necrosis factor (TNF). Osteoclast formation induced by these cytokines requires NF-κB p50 and p52, c-Fos, and NFATc1 expression in osteoclast precursors. c-Fos induces NFATc1, but the relationship between NF-κB and these other transcription factors in osteoclastogenesis remains poorly understood. We report that RANKL and TNF can induce osteoclast formation directly from NF-κB p50/p52 double knockout (dKO) osteoclast precursors when either c-Fos or NFATc1 is expressed. RANKL- or TNF-induced c-Fos up-regulation and activation are abolished in dKO cells and in wild-type cells treated with an NF-κB inhibitor. c-Fos expression requires concomitant RANKL or TNF treatment to induce NFATc1 activation in the dKO cells. Furthermore, c-Fos expression increases the number and resorptive capacity of wild-type osteoclasts induced by TNF in vitro. We conclude that NF-κB controls early osteoclast differentiation from precursors induced directly by RANKL and TNF, leading to activation of c-Fos followed by NFATc1. Inhibition of NF-κB should prevent RANKL- and TNF-induced bone resorption.