The role of phosphoinositide 3-kinase in spreading osteoclasts induced by colony-stimulating factor-1

Department Clinical Research, Bone Biology, University of Bern, Bern, Switzerland.

BACKGROUND: Colony-stimulating factor-1 (CSF-1), a growth and survival factor for osteoclasts, stimulates these cells to spread and migrate towards a gradient of CSF-1. This may support the translocation of osteoclasts to new sites on the bone surface to be resorbed. Phosphoinositide 3-kinase (PI 3-K) is a lipid kinase participating in various signal transduction pathways. OBJECTIVE: To investigate the role of PI 3-K in the CSF-1-induced spreading of osteoclasts. METHODS: In isolated rat osteoclasts treated with or without CSF-1, the distribution of PI 3-K and proteins phosphorylated on tyrosine were investigated using immunofluorescence. In murine osteoclast-like cells grown from bone marrow cells co-cultured with osteoblasts, the activation of the PI 3-K by CSF-1 was determined both in vivo and in vitro. In vivo, the enzyme product in the cell was determined after extraction and separation with thin layer chromatography; in vitro, PI 3-K activity was measured in the pellet immunoprecipitated from the cell lysate. RESULTS: Inhibition of PI 3-K blocked the CSF-1-induced spreading of osteoclasts. In spreading osteoclasts, a portion of PI 3-K was translocated to the periphery where proteins phosphorylated on tyrosine appeared simultaneously. In osteoclast-like cells, CSF-1 stimulated PI 3-K activity. This activity could be immunoprecipitated with antibody against phophotyrosine residues. CONCLUSION: PI 3-K participates in the CSF-1-induced spreading of osteoclasts. The activated PI 3-K may induce the reorganization of the cytoskeleton resulting in spreading and migration.