IGF‐I secreted from osteoblasts as a chemotactic factor for osteoblasts

Abstract

Osteoblast recruitment to the site of future bone formation is essential for skeletal development, bone remodeling and fracture healing, but the mechanism of which remains to be clarified. Here, we hypothesized that osteoblasts secrete a chemoattractant(s) for osteoblast recruitment and examined the serum-free conditioned medium of mouse osteoblast-like cell line MC3T3-E1 for its ability to induce osteoblast migration. Using a modified Boyden chamber assay, we found that the medium induced MC3T3-E1 chemotaxis in a dose-dependent manner. Employing several chromatographic procedures and liquid chromatography equipped with tandem mass spectrometry analysis, we identified IGF-I as a potent chemotactic factor from the conditioned medium. By neutralization of IGF-I activity with specific anti-mouse IGF-I antibody, both osteoblast monolayer wound healing and cellular polarization were impaired, whereas human IGF-I replenishment reversed these inhibitory effects. IGF-I also promoted cell spreading on fibronectin in an integrin β1-dependent manner. IGF-I induced Akt and ERK phosphorylation in MC3T3-E1 cells and a PI3K inhibitor, LY294002, but not a MEK inhibitor, PD98059, inhibited IGF-I-induced cell migration and wound healing. Together, these findings suggest that IGF-I secreted from osteoblasts regulates osteoblast migration through the activation of PI3K signaling.