Adhesion Properties and Integrin Expression of Cultured Human Osteoclast-like Cells

Osteoclast interaction with extracellular matrix drives the sequential events that end with bone resorption. However, the role of matrix proteins is not yet fully understood. We studied this problem on human osteoclast-like cells derived from giant cell tumors of bone (GCT cells). On GCT cells we considered cytoskeletal organization, adhesion properties, and integrin expression upon plating in serum-free medium onto fibronectin (FN), collagen (COL), thrombospondin (TSP), bone sialoprotein (BSPII), and osteopontin (OPN). GCT cells promptly adhered and spread on FN, BSPII, and OPN, while only 50% adhered on COL and none on TSP. The integrin β₁ chain was always associated to focal adhesions, while the α_vβ₃ heterodimer was detected in focal contacts only upon plating on BSPII, OPN, and FN. The focal clustering of β₁ was impaired by monensin treatment, indicating that endogenous FN secretion was required to drive β₁ into focal contacts. Conversely, α_vβ₃ clustering was also not affected by monensin when cells were plated onto plasma FN. Immunoprecipitation of metabolically labeled GCT cell lysates showed that three different heterodimers (α_vβ₃, α₃β₁, and α₅β₁) were assembled. Adhesion to FN was completely inhibited by β₁ antibodies at dilutions up to 1:400, while β₃ antibodies, at similar dilutions, impaired spreading but not adhesion. We conclude that α_vβ₃3 is the main integrin used by GCT cells in bone recognition. We also suggest that selected substrata may induce the release and the organization of endogenous FN that eventually drives the recruitment of a β₁ integrin receptor into focal contacts.