Comparison of the mechanisms of bone resorption induced by 1 alpha,25-dihydroxyvitamin D3 and lipopolysaccharides.

The mechanisms of increase in bone resorption induced by 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] and bacterial lipopolysaccharides (LPS) were compared in an in vitro dead bone assay and a living bone assay. 1 alpha,25(OH)2D3 at concentrations of 0.05-5 ng/ml dose-dependently enhanced the ability of alveolar macrophages to release 45Ca from prelabeled dead bone particles (dead bone assay). In addition, the vitamin promoted fusion of the macrophages to form multinucleated cells and also enhanced glucose consumption, a marker of activation of macrophages. LPS at 0.05-5 micrograms/ml similarly enhanced the release of 45Ca from the dead bone particles and glucose consumption by alveolar macrophages, but it did not induce fusion of the cells at any concentration. Both 1 alpha,25(OH)2D3 and LPS dose-dependently stimulated the release of 45Ca from fetal mouse calvaria prelabeled with 45Ca (living bone assay). Compared to control bone, there were several times as many osteoclasts per given length of trabecular bone surface in calvaria treated for 5 days with either 5 ng/ml of 1 alpha,25(OH)2D3 or 5 micrograms/ml of LPS. Indomethacin (10(-5) M) completely inhibited the LPD-induced increase of osteoclasts, but not the 1 alpha,25(OH)2D3-induced increase. These results suggest that 1 alpha,25(OH)2D3 and LPS similarly stimulate bone resorption by activating macrophages as well as by promoting fusion of precursor cells to form multinucleated cells. 1 alpha,25(OH)2D3 induced formation of multinucleated cells with bone-resorbing activity directly, whereas LPS appeared to induce multinucleated cells through prostaglandin synthesis by some other types of cells present in living bone tissues.