Activation of α(v)β₃ integrin on human osteoclast-like cells stimulates adhesion and migration in response to osteopontin

Integrins mediate cell adhesion and can induce different cellular responses, including changes in intracellular pH, changes and oscillation in intracellular free calcium, and protein phosphorylation on tyrosine. During bone resorption, the integrin α(v)β₃ regulates adhesion of osteoclasts to bone extracellular matrix proteins, such us osteopontin (Opn). Adhesion via α(v)β₃ is followed by osteoclast polarization onto the bone surface and by the onset of bone resorption. To characterize these events at the molecular level, we investigated the state of activation of α(v)β₃ on the human osteoclast-like cell line GCT23 using the monoclonal antibody AP5 which binds to and can induce, under low calcium conditions, activated α(v)β₃. By flow cytometry, approximately 50% of α(v)β₃ on the surface of the osteoclast-like cell line GCT23 was reactive with AP5 and was therefore in the activated state. Incubation with AP5 in the presence of low calcium concentrations increased activated α(v)β₃ to 90-100%. Activation of α(v)β₃ increased the efficiency of GCT23 adhesion to Opn by 2-fold. Furthermore, haptotactic migration on Opn was also enhanced about 40% compared to control. We propose that changes in the activation state of α(v)β₃ may be a regulation point for osteoclasts during bone resorption.