Role of αvβ3 integrin in osteoclast migration and formation of the sealing zone

ABSTRACT The αvβ3 integrin is abundantly expressed in osteoclasts and has been implicated in the regulation of osteoclast function, especially in cell attachment. However, in vivo studies have shown that echistatin, an RGD-containing disintegrin which binds to αvβ3, inhibits bone resorption without changing the number of osteoclasts on the bone surface, suggesting inhibition of osteoclast activity. The objective of this study was to examine how occupancy of αvβ3 integrins inhibits osteoclast function, using primary rat osteoclasts and murine pre-fusion osteoclast-like cells formed in a co-culture system. We show that: (1) echistatin inhibits bone resorption in vitro at lower concentrations (IC50 = 0.1 nM) than those required to detach osteoclasts from bone (IC50 ∼1 µM); (2) echistatin (IC50 = 0.1 nM) inhibits M-CSF-induced migration and cell spreading of osteoclasts; (3) αvβ3 integrins are localized in podosomes at the leading edge of migrating osteoclasts, whereas, with echistatin treatment (0.1 nM), αvβ3 disperses randomly throughout the adhesion surface; and (4) when bone resorption is fully inhibited with echistatin, there is visible disruption of the sealing zone (IC50 = 13 nM), and αvβ3 visualized with confocal microscopy re-distributes from the basolateral membranes to intracellular vesicular structures. Taken together, these findings suggest that αvβ3 integrin plays a role in the regulation of two processes required for effective osteoclastic bone resorption: cell migration (IC50 = 0.1 nM) and maintenance of the sealing zone (IC50 ∼10 nM).