Avian osteoclast cells are stimulated to resorb calcified matrices by and possess receptors for leukotriene B4.

Leukotriene B4 (LTB4) is elevated in inflammatory conditions and appears to be a potential mediator of inflammation. We have recently shown that this 5-lipoxygenase metabolite of arachidonic acid stimulates bone resorption in vitro and in vivo. In order to determine the mechanism whereby LTB4 causes bone resorption, avian osteoclasts were examined for the effects of LTB4 and for the presence of LTB4 receptors. Isolated avian osteoclast mononuclear precursor cells, which fuse in culture to form multinucleated cells, were chosen for receptor binding studies because this population is a morphologically similar source of osteoclasts, and large numbers of these cells can be obtained from egg-laying hens. Binding of LTB4 and activation would support the hypothesis of a direct effect of this compound on osteoclasts. LTB4 stimulated isolated avian osteoclasts to form resorption lacunae on calcified matrices and to increase their content of tartrate-resistant acid phosphatase (TRAP), a marker of activated osteoclasts. Receptor binding studies were performed at day 1, when the cells were mononuclear, at day 4, when mononuclear precursors were actively fusing, and at day 7, when fusion has slowed. Scatchard analysis of saturation binding data showed two classes of binding sites, a high- and low-affinity binding site with dissociation constants (KD) of 0.2-0.4 nM and 5. 6-24 nM. Association studies showed rapid binding of LTB4 to the cells within 10 minutes. These data show that LTB4 accelerates fusion and activates highly enriched populations of avian osteoclasts and that LTB4 receptors are present in this cell population.