β1 Integrin/Focal Adhesion Kinase-mediated Signaling Induces Intercellular Adhesion Molecule 1 and Receptor Activator of Nuclear Factor κB Ligand on Osteoblasts and Osteoclast Maturation

Abstract

We have assessed characteristics of primary human osteoblasts, shedding light on signaling mediated by β₁ integrin. β₁ integrins are major receptors for these matrix glycoproteins. 1) Integrins β₁, α₂, α₃, α₄, α₅, α₆, and α_v were highly expressed on primary osteoblasts. 2) Engagement of β₁ integrins on osteoblasts by cross-linking with specific antibody or ligand matrices, such as fibronectin or collagen, augmented expression of intercellular adhesion molecule 1 (ICAM-1) and receptor activator of nuclear factor κB ligand (RANKL) on the surface. 3) Up-regulation of ICAM-1 and RANKL on osteoblasts by β₁ stimulation was completely abrogated by pretreatment with herbimycin A and genistein, tyrosine kinase inhibitors, or transfection of dominant negative truncations of focal adhesion kinase (FAK). 4) Engagement of β₁ integrins on osteoblasts induced tartrate-resistant acid phosphatase-positive multinuclear cell formation in the coculture system of osteoblasts and peripheral monocytes. 5) Up-regulation of tartrate-resistant acid phosphatase-positive multinuclear cell formation by β₁ stimulation was completely abrogated by transfection of dominant negative truncations of FAK. Our results indicate that β₁ integrin-dependent adhesion of osteoblasts to bone matrices induces ICAM-1 and RANKL expression and osteoclast formation via tyrosine kinase, especially FAK. We here propose that β₁ integrin/FAK-mediated signaling on osteoblasts could be involved in ICAM-1- and RANKL-dependent osteoclast maturation.