OSTEOCLAST RECRUITING ACTIVITY IN BONE-MATRIX

An activity that recruits osteoclasts has been identified and partiall y characterized from bone matrix. Bone-derived osteoclast recruiting a ctivity (BORA) was co-purified with osteogenin, a bone inductive prote in. Osteogenin was extracted from bovine bone with 6 M urea and purifi ed by chromatography on hydroxyapatite, heparin-Sepharose and Sephacry l S-200 gel filtration. The biologically active osteoclast formation-s timulating material was further purified by C-18 reverse phase HPLC. B ORA is obviously distinct from osteogenin and transforming growth fact or beta (TGF-beta), since further purified osteogenin and pure TGF-bet a did not stimulate the formation of osteoclast-like cells. BORA (0.1- 10 mu g/ml) stimulated the formation of tartrate-resistant acid phosph atase (TRAP)-positive multinucleated cells (MNC) in a dose-dependent m anner. These multinucleated cells resorbed bone when cultured on bovin e bone slices. The effect of BORA is primarily directed to differentia te osteoclast precursors, since it did not stimulate osteoclast functi on in in vitro resorption assay where disaggregated rat osteoclasts we re cultured on bovine bone slices. However, after 24 h preincubation w ith 50 nM PTH in the mouse calvaria assay, BORA at 10 mu g/ml signific antly stimulated bone resorption.