αvβ Integrins Play an Essential Role in BMP‐2 Induction of Osteoblast Differentiation

Both integrins and BMP‐2 exert similar effects on osteoblasts. We examined the relationship between the αv‐containing integrins (αvβ) and BMP‐2 in osteoblast function. BMP‐2 stimulates αvβ expression. BMP‐2 receptors co‐localize/overlap with αvβ integrins, and the intact function of αvβ is essential in BMP‐2 activity. Introduction: Bone morphogenetic protein (BMP)‐2 not only induces osteoblast differentiation and bone matrix mineralization, but also stimulates osteoblast migration on and adhesion to bone matrix proteins. The αvβ‐ and β1‐ (αβ1) containing integrins mediate osteoblast interaction with many bone matrix proteins and play important roles in osteoblast adhesion, migration, and differentiation. Because αvβ integrins and BMP‐2 share common effects on osteoblasts, we analyzed their relationship in osteoblast function. Materials and Methods: The effects of BMP‐2 on integrin expression were determined by surface labeling/immunoprecipitation and cell adhesion to matrix proteins. Confocal analysis of the immunostained cells and co‐immunoprecipitation of cell extracts were used to study the spatial relationship between integrins and BMP‐2 receptors. A function‐blocking anti‐αvβ integrin antibody (L230) was employed to investigate the roles of αvβ integrins in BMP‐2 function. Results: Human osteoblasts (HOBs) express αβ1, αvβ3, αvβ5, αvβ6, and αvβ8 integrins at focal adhesion sites. BMP‐2 increases the levels of these integrins on osteoblast surface and enhances HOB adhesion to osteopontin and vitronectin. Immunoprecipitation and immunostaining analyses show that BMP‐2 receptors co‐localize or overlap with αvβ and αβ1 integrins. Incubation of HOBs with L230 abolishes the antiproliferative effect of BMP‐2 and reduces the capacity of BMP‐2 to stimulate alkaline phosphatase activity and the expression of osteocalcin, osteopontin, and bone sialoprotein. Furthermore, L230 prevents BMP‐2 induction of matrix mineralization. Although BMP‐2 retains its receptor‐binding capability in the presence of L230, BMP‐2 stimulation of Smad signaling is abolished by L230. Conclusion: BMP‐2 upregulates the expression of αvβ integrins, and these integrins, in turn, play a critical role in BMP‐2 function in osteoblasts.