Roles of macrophage-colony stimulating factor and osteoclast differentiation factor in osteoclastogenesis

We have isolated osteoclast precursors (OCPs) from cocultures of mouse calvarial cells and bone marrow cells without adding any osteotropic factors. OCPs expressed Mac-1, Mac-2, and Gr-1 antigens but not osteoclast markers such as tartrate-resistant acid phosphatase (TRAP) and calcitonin receptors, and they differentiated into TRAP-positive cells within 48 h on a fixed calvarial cell layer pretreated with osteotropic factors such as 1 alpha, 25-dihydroxyvitamin D3. In the present study, we investigated the regulatory mechanisms of OCP formation from hemopoietic cells and TRAP-positive cell formation from OCPs. Calvarial osteoblasts obtained from macrophage-colony stimulating factor (M-CSF)-deficient op/op mice failed to support OCP formation or the differentiation of OCPs into TRAP-positive cells. Both OCP formation and TRAP-positive cell formation supported by osteoblasts were completely inhibited by osteoclastogenesis inhibitory factor (OCIF, also called OPG), which is a decoy receptor of osteoclast differentiation factor (ODF; also called TRANCE, RANKL, and OPGL). When bone marrow cells were cultured for 4 days with soluble ODF (sODF/sRANKL) together with M-CSF, OCPs were formed even in the absence of osteoblasts. When OCPs were treated with sODF/sRANKL and M-CSF in the absence of osteoblasts, they differentiated into TRAP-positive cells within 48 h even in the presence of hydroxyurea. Northern blotting analysis revealed that osteoblasts constitutively expressed a certain level of ODF/RANKL mRNA. These results indicated that M-CSF and sODF/sRANKL produced by osteoblasts are two essential factors for both OCP formation and TRAP-positive osteoclast formation.