M-CSF Augments Resorption by Human Osteoclasts Figure 5. MEK inhibitor PD98059 blocks M-CSF induced stimulation of bone resorption by harvested OCs. Mature OCs where cultured for 72 h in the presence of RANKL (125 ng/mL) 6 M-CSF (25 ng/mL) 6 MEK inhibitor PD98059 (5 mM); representative data from 4 independent experiments. Results expressed as mean 6 SEM (n = 6 dentine slices/group). Groups with different annotations are significantly different; p = 0.001; Oneway ANOVA; Fishers multiple comparison test. doi:10.1371/journal.pone.0021462.g005 In healthy individuals, serum M-CSF increases from approxi- mately 12 ng/mL in the early twenties to 20 ng/mL in the eighties, corresponding to the concentration range where we observed in vitro a three- to four-fold increase in resorption [39]. In women, the serum bone resorption marker, C-telopeptide, increases in a near linear manner from age 21 to 71 years and bone mineral density declines substantially with ageing [40]. Thus, the possibility exists that increasing circulating M-CSF is involved in ‘‘normal’’ age- related bone loss. Furthermore, as we have previously reviewed, systemic and/or local M-CSF concentrations are increased in a variety of inflammatory and neoplastic conditions known to be associated with bone loss [11]. Karsdal et al. [41] pointed out that osteopetrotic mutations resulting in reduced or absent OCs are associated with decreased bone formation, whereas osteopetrotic mutations that result in increased numbers of non-resorbing OCs are associated with increased bone formation and suggested that nonresorbing osteoclasts References provide anabolic signals for osteoblasts [41]. It has subsequently been shown that secreted products of OCs, spingosine-1-phosphate and bone morphogenic protein 6 stimulate migration and osteoblast differentiation of human mesenchymal stem cells [42]. We hypothesize that incomplete blockade of c-fms signalling, or downstream signalling pathways, may provide a potential means to attenuate bone loss. In this situation, OC differentiation will proceed but the proportion activated to resorb will be lower, resulting in reduced bone resorption yet maintenance of OC- derived anabolic signals to osteoblasts. Author Contributions Conceived and designed the experiments: JMH FMC MAK GCN. Performed the experiments: JMH FMC NJP. Analyzed the data: JMH FMC. Contributed reagents/materials/analysis tools: JMH FMC NJP. Wrote the paper: JMH FMC NJP GCN. 1. Hattersley G, Owens J, Flanagan AM, Chambers TJ (1991) Macrophage colony stimulating factor (M-CSF) is essential for osteoclast formation in vitro. Biochem Biophys Res Commun 177: 526–531. 2. Marks SC, Jr., Lane PW (1976) Osteopetrosis, a new recessive skeletal mutation on chromosome 12 of the mouse. J Hered 67: 11–18. 3. Tanaka S, Takahashi N, Udagawa N, Tamura T, Akatsu T, et al. (1993) Macrophage colony-stimulating factor is indispensable for both proliferation and differentiation of osteoclast progenitors. J Clin Invest 91: 257–263. 4. Wiktor-Jedrzejczak W, Bartocci A, Ferrante AW, Jr., Ahmed-Ansari A, Sell KW, et al. (1990) Total absence of colony-stimulating factor 1 in the PLoS ONE | www.plosone.org 7 June 2011 | Volume 6 | Issue 6 | e21462