Syndecan-2 is involved in the mitogenic activity and signaling of granulocyte-macrophage colony-stimulating factor in osteoblasts

Abstract

We previously showed that granulocyte-macrophage colony-stimulating factor (GM-CSF) binds to heparan sulfate proteoglycans expressed at the surface of osteoblastic cells and that the mitogenic activity of this cytokine is dependent on the presence of fully sulfated proteoglycans. In this study, we determined if GM-CSF interacts with syndecans, a family of cell surface heparan sulfate proteoglycans. Human primary osteoblasts were found to express syndecan-2 and -4 but few syndecan-1 transcripts and proteins. Recombinant human GM-CSF coupled to biotin was found to bind to syndecan-2. Immunocytochemical transmission electron microscope analysis showed co-localization of syndecan-2 and GM-CSF at the cell membrane surface. Syndecan-2 also co-localized at the cell surface and co-immunoprecipitated with the GM-CSF receptor alpha chain, suggesting a strong interaction between the cytokine, its receptor, and syndecan-2. Phosphorylation of tyrosine residues in syndecan-2 associated with the alpha chain of the GM-CSF receptor was increased after cell stimulation by GM-CSF. Antisense oligonucleotides that reduced specifically the expression of syndecan-2 inhibited the mitogenic activity of GM-CSF and the activation of extracellular signal-regulated kinase-1 induced by the cytokine. Our results indicate functional interactions between syndecan-2 and GM-CSF in osteoblasts, and we propose that syndecan-2 plays a role as a co-receptor for this cytokine.