Multiple roles of M-CSF in human osteoclastogenesis

Although the critical role of M‐CSF in osteoclastogenesis is well documented, there has been no detailed analysis of how it regulates human osteoclast formation and function in vitro. We used a human osteoclastogenesis model employing CFU‐GM osteoclast precursors cultured for 14 days on dentine with RANKL, with varying exposure to exogenous human M‐CSF. Short‐term treatment of precursors with M‐CSF (10–100 ng/mL) resulted in increased proliferation with or without RANKL. Treatment with M‐CSF (1–100 ng/mL) for 14 days caused a biphasic concentration‐dependent stimulation of formation, fusion, and resorption peaking at 10–50 ng/mL and almost complete abolition of resorption at 100 ng/mL. Time‐course studies using M‐CSF (25 ng/mL) showed that osteoclast size, nuclei/cell, and resorption increased with longer duration of M‐CSF treatment. When treatment was restricted to the first 4 days, M‐CSF (25–100 ng/mL) stimulated formation of normal numbers of osteoclasts that resorbed less. Blockade of endogenous M‐CSF signaling with neutralizing M‐CSF antibody during the first week of culture extensively inhibited osteoclastogenesis, whereas blockade during the second week produced only a small reduction in resorption. Treatment with M‐CSF during the second week of culture caused a small increase in osteoclast number and a concentration‐dependent increase in cytoplasmic spreading with inhibition of resorption. We have shown that M‐CSF modulates multiple steps of human osteoclastogenesis, including proliferation, differentiation and fusion of precursors. In the later stages of osteoclastogenesis, M‐CSF modulates osteoclast‐resorbing activity, but is not required for survival. Modulation of M‐CSF signaling is a potential therapeutic target for conditions associated with excess bone resorption. J. Cell. Biochem. 102: 759–768, 2007. © 2007 Wiley‐Liss, Inc.