Sclerostin inhibits osteoblast differentiation without affecting BMP2/SMAD1/5 or Wnt3a/β-catenin signaling but through activation of platelet-derived growth factor receptor signaling in vitro.

Sclerostin inhibits bone formation mostly by antagonizing LRP5/6, thus inhibiting Wnt signaling. However, experiments with genetically modified mouse models suggest that a significant part of sclerostin-mediated inhibition of bone formation is due to interactions with other binding partners. The objective of the present work was to identify signaling pathways affected by sclerostin in relation with its inhibitory action on osteogenic differentiation of C3H10T1/2 cells, MC3T3-E1 cells and primary osteoblasts. Sclerostin inhibited BMP2-induced osteoblast differentiation without altering SMAD1/5 phosphorylation and transcriptional activity. Moreover, sclerostin prevented Wnt3a-mediated osteoblastogenesis without affecting LRP5/6 phosphorylation or β-catenin transcriptional activity. In addition, sclerostin inhibited mineralization promoted by GSK3 inhibition, which mimics canonical Wnt signaling without activation of LRP5/6, suggesting that sclerostin can prevent osteoblast differentiation without antagonizing LRP5/6. Finally, we found that sclerostin could activate platelet-derived growth factor receptor (PDGFR) and its downstream signaling pathways PLCγ, PKC, Akt and ERK1/2. PDGFR inhibition could reverse sclerostin-mediated inhibitory activity on BMP2-induced osteoblast differentiation. Therefore, our data suggest that sclerostin can activate PDGFR signaling by itself, and this functional interaction may be involved in the negative effect of sclerostin on osteoblast differentiation.