Recognition of osteopontin and related peptides by an α_vβ₃ integrin stimulates immediate cell signals in osteoclasts

We have investigated the nature of immediate cell signals produced by occupancy of the chicken osteoclast α_vβ₃ integrin. Synthetic osteopontin and peptides from the osteopontin and bone sialoprotein sequences containing Arg-Gly-Asp stimulated immediate reductions in osteoclast cytosolic Ca²⁺. The changes in cytosolic Ca²⁺ required the Arg-Gly-Asp sequence and were blocked by a monoclonal antibody to the α_vβ₃ integrin, LM609. Osteoclast stimulation by the proteins through the integrin did not require immobilization since soluble peptides produced changes in cytosolic Ca²⁺ and inhibited osteoclast binding to bone particles and bone resorption. The decrease in cytosolic Ca²⁺ stimulated by osteopontin and related peptides appeared to be due to activation of a plasma membrane Ca²⁺-ATPase by calmodulin. Thus, the data suggest that ligand binding to the osteoclast α_vβ₃ integrin results in calmodulin-dependent reduction in cytosolic Ca²⁺ which participates in regulation of osteoclast function.