Bone acidic glycoprotein 75 inhibits resorption activity of isolated rat and chicken osteoclasts

Matrix protein effects on the differentiated activity of osteoclasts were examined in order to understand the functional significance of bone protein interactions with osteoclasts. Bone acidic glycoprotein 75 (BAG 75) from rat calvariae inhibited the resorption of bone by isolated rat osteoclasts with IC50 = 1 nM compared to IC50 = 10 nM for chicken osteoclasts. By contrast, other phosphoproteins similarly isolated from bone were less effective in inhibiting resorption with IC50 = 100 nM osteopontin and IC50 greater than 100 nM bone sialoprotein. Likewise, RGD-containing matrix proteins vitronectin, thrombospondin, and fibronectin all displayed IC50 greater than or equal to 100 nM. Mechanistically, 10 nM BAG 75 marginally slowed, but did not block, the association of bone particles with chicken osteoclasts compared with osteopontin or control media. Pretreatment of osteoclasts with 50 nM BAG 75 had no effect on subsequent bone resorption; however, pretreatment of bone with BAG 75 before incubation with osteoclasts reduced the extent of resorption by 55%. These data suggest that a BAG 75/bone surface complex, rather than BAG 75 alone, represents the inhibitory form. Consistent with this hypothesis, direct binding studies provided no evidence of specific, high-affinity receptors on osteoclasts for BAG 75, nor was an excess of BAG 75 (100 nM) able to compete with 0.3 nM sechistatin for osteoclastic avB3-like receptors. However, BAG 75 displayed cooperative binding to tissue fragments and bone particles at concentrations greater than 10 nM, suggesting that BAG 75 self-associates into higher-order species on bone surfaces. Electron microscopy confirmed the time-dependent polymerization of BAG 75 into interconnecting filaments. These data suggest a novel, inhibitory activity for surface-bound BAG 75 on bone resorption that does not appear to involve the osteoclastic avB3-like integrin.