Human osteoclasts express different CXC chemokines depending on cell culture substrate: molecular and immunocytochemical evidence of high levels of CXCL10 and CXCL12

Chemokines are important mediators of chemotaxis, cell adherence, and proliferation and exert specific functions in bone remodeling. Despite the potential intriguing role of chemokines in the regulation of osteoclast (OC) functions, little is known about the expression of chemokines and their receptors in human OCs at different stages of differentiation. Therefore, we analyzed the expression of CXC chemokine receptors (CXCR1, CXCR2, CXCR3, CXCR4 and CXCR5) and ligands (CXCL8, CXCL10, CXCL12 and CXCL13) both at molecular and protein levels, in human OCs grown on plastic or calcium phosphate-coated slides at different stages of differentiation. Real-time PCR showed that CXCR1, CXCR2, CXCR3, CXCR4, CXCR5 and CXCL8 were expressed in undifferentiated cells and significantly decreased during OC differentiation. By contrast, CXCL10 and CXCL12 were strongly upregulated from day 0 to day 8 in cells grown on calcium phosphate-coated slides. Immunocytochemistry showed that OCs grown on plastic expressed CXCR3, CXCR4, CXCR5, CXCL8 and CXCL12, while they were negative for CXCR1, CXCR2 and CXCL10. Interestingly, both at molecular and protein levels CXCL10 and CXCL12 significantly increased only when cells were differentiated on calcium phosphate-coated slides. These data suggest that the selection of a substrate that better mimics the tridimensional structure of bone tissue, thus favoring OC maturation and differentiation, may be necessary when studying osteoclastogenesis in vitro.