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, α 4, α5, α6, and αv were highly expressed on primary osteoblasts. 2) Engagement of β 1 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 β 1 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.