Osteoprotegerin ligand regulates osteoclast adherence to the bone surface in mouse calvaria

The stimulators of bone resorption, prostaglandin E(2) (PGE(2)) and 1,25-dihydroxyvitamin D(3) (1,25D(3)), act through osteoblast-like cells to activate osteoclasts. One candidate for the intermediary produced by osteoblasts that subsequently stimulates the osteoclast is osteoprotegerin ligand (OPGL). OPGL has been shown to stimulate osteoclast differentiation and activation. The aim of the work reported here was to determine if soluble recombinant extracellular domain of human OPGL would bring about the change in osteoclast adhesion from the periosteum of mouse calvaria to the adjacent bone surface that occurs with the above-mentioned stimulators of resorption. This change in adherence or translocation of osteoclasts onto the bone surface required the expression and functioning of the integrin subunit, beta 3. We show that this soluble OPGL, like PGE(2) and 1,25D(3), stimulated the release of osteoclasts from the periosteum and their adherence to the bone surface accompanied by an increase in staining for immunolocalized integrin subunit beta 3. Recombinant human osteoprotegerin (OPG), which binds strongly to OPGL, inhibited this translocation of osteoclasts that occurred with PGE(2) and 1,25D(3), leaving integrin beta-3-negative osteoclasts on the periosteum. PGE(2) and 1,25D(3) increased the expression of messenger RNA for OPGL compared with indomethacin-treated controls after 6 h exposure. Evidence is presented that the change in the adhesion of osteoclasts from the periosteum to the bone surface, resulting in osteoclast activation, is mediated by OPGL.