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.