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Background: Global interferon-␤ deficiency causes osteoporosis. Lack of interferon-␤ production by osteoclast precursors is considered to induce excess osteoclastogenesis. Results: Isolated osteocytes express higher amount of interferon-␤ mRNA than osteoclast precursors and inhibit osteoclastogenesis partially in interferon-␤-dependent manner. Conclusion: Osteocytes produce interferon-␤ as an inhibitor of osteoclastogenesis. Significance: Osteocytic interferon-␤ might be involved in the regulation of bone homeostasis.

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Osteoporosis is associated with vessel diseases attributed to hyperlipidemia, and bone resorption by multinucleated osteoclasts is related to lipid metabolism. In this study, we generated low-density lipoprotein receptor (LDLR)/lectin-like oxidized LDL receptor-1 (LOX-1, also known as Olr1) double knockout (dKO) mice. We found that, like LDLR single KO (sKO), LDLR/LOX-1 dKO impaired cell-cell fusion of osteoclast-like cells (OCLs). LDLR/LOX-1 dKO and LDLR sKO preosteoclasts exhibited decreased uptake of LDL. The cell surface cholesterol levels of both LDLR/LOX-1 dKO and LDLR sKO osteoclasts were lower than the levels of wild-type OCLs. Additionally, the amount of phosphatidylethanolamine (PE) on the cell surface was attenuated in LDLR/LOX-1 dKO and LDLR sKO preosteoclasts, whereas the PE distribution in wild-type OCLs was concentrated on the filopodia in contact with neighboring cells. Abrogation of the ATP binding cassette G1 (ABCG1) transporter, which transfers PE to the cell surface, caused decreased PE translocation to the cell surface and subsequent cell-cell fusion. The findings of this study indicate the involvement of a novel cascade (LDLR∼ABCG1∼PE translocation to cell surface∼cellcell fusion) in multinucleation of OCLs.