Down-regulation of osteoprotegerin production in bone marrow macrophages by macrophage colony-stimulating factor

Macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) induce the differentiation of bone marrow macrophages (BMMs) into osteoclasts. To delineate mechanisms involved, the effect of M-CSF on the production of osteoprotegerin (OPG), decoy receptor of RANKL, in BMMs was investigated. Mouse bone marrow cells were cultured with M-CSF for 4 days and adherent cells formed were used as BMMs. BMMs were cultured with or without M-CSF, and analyzed for expression of OPG and receptor activator of NF-κB (RANK; receptor for RANKL) mRNAs by real-time polymerase chain reaction and secretion of OPG by enzyme-linked immunosorbent assay. BMMs expressed macrophage markers, CD115 (c-fms), Mac-1 and F4/80, and showed phagocytotic activity. In addition, BMMs expressed OPG mRNA and secreted OPG into medium. M-CSF inhibited both the OPG mRNA expression and the OPG secretion dose-dependently and reversibly. The expression of RANK mRNA was not significantly affected by M-CSF. The results showed that M-CSF suppresses the OPG production in BMMs, which may increase the sensitivity of BMMs to RANKL.