Osteocytes Produce Interferon-β as a Negative Regulator of Osteoclastogenesis

Osteoclastogenesis is controlled by osteocytes; osteocytic osteoclastogenesis regulatory molecules are largely unknown. We searched for such factors using newly developed culture methods. Our culture system mimics the three-dimensional cellular structure of bone, consisting of collagen gel-embedded osteocytic MLO-Y4 cells, stromal ST2 cells on the gel as bone lining cells, and bone marrow cells. The gel-embedded MLO-Y4 cells inhibited the osteoclastogenesis induced by 1,25(OH) 2 D 3 without modulating receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG) production by ST2 cells, despite MLO-Y4 cells supported osteoclastogenesis in the absence of ST2 cells. In the bone marrow cell culture, the conditioned medium from MLO-Y4 cells decreased the capability of osteoclastic differentiation from the cells induced by macrophage colony-stimulating factor. This decreased capability was concomitant with an increase in protein kinase R mRNA expression and an inhibition of c-Fos translation. These changes were partially normalized by the simultaneous addition of an anti-interferon (IFN)-β neutralizing antibody to MLO-Y4 cell conditioned medium. To study primary osteocytes, we prepared non-osteocytic cell-free osteocyte-enriched bone fragments (OEBFs). When osteoclast precursors were induced by macrophage colony-stimulating factor in the presence of OEBFs, the generated cells exhibited a diminished capacity for osteoclastogenesis. OEBFs prepared from OPG-knock-out mice exhibited a similar effect, indicating OPG-independent inhibition. The addition of anti-IFN-β neutralizing antibody during the co-culture with OEBFs partially recovered the osteoclastogenic potential of the generated cells. The MLO-Y4 cells and OEBFs expressed IFN -β mRNA. Although osteocytic RANKL is known to be important for osteoclastogenesis, our data suggest that osteocytes also produce IFN-β as an inhibitor of osteoclastogenesis. Background: Global interferon-β deficiency causes osteoporosis. Lack of interferon-β production by osteoclast precursors is considered to induce excess osteoclastogenesis. Results: Isolated osteocytes express higher amount of interferon-β mRNA than osteoclast precursors and inhibit osteoclastogenesis partially in interferon-β-dependent manner. Conclusion: Osteocytes produce interferon-β as an inhibitor of osteoclastogenesis. Significance: Osteocytic interferon-β might be involved in the regulation of bone homeostasis.