Prostaglandin E2 Strongly Inhibits Human Osteoclast Formation

Prostaglandin E2 (PGE2) enhances osteoclast formation in mouse macrophage cultures treated with receptor activator of nuclear factor-κB ligand (RANKL). The effects of PGE2 on human osteoclast formation were examined in cultures of CD14+ cells prepared from human peripheral blood mononuclear cells. CD14+ cells differentiated into osteoclasts in the presence of RANKL and macrophage colony-stimulating factor. CD14+ cells expressed EP2 and EP4, but not EP1 or EP3, whereas CD14+ cell-derived osteoclasts expressed none of the PGE2 receptors. PGE2 and PGE1 alcohol (an EP2/4 agonist) stimulated cAMP production in CD14+ cells. In contrast to mouse macrophage cultures, PGE2 and PGE1 alcohol inhibited RANKL-induced human osteoclast formation in CD14+ cell cultures. H-89 blocked the inhibitory effect of PGE2 on human osteoclast formation. These results suggest that the inhibitory effect of PGE2 on human osteoclast formation is mediated by EP2/EP4 signals. SaOS4/3 cells have been shown to support human osteoclast formation in cocultures with human peripheral blood mononuclear cells in response to PTH. PGE2 inhibited PTH-induced osteoclast formation in cocultures of SaOS4/3 cells and CD14+ cells. Conversely, NS398 (a cyclooxygenase 2 inhibitor) enhanced osteoclast formation induced by PTH in the cocultures. The conditioned medium of CD14+ cells pretreated with PGE2 inhibited RANKL-induced osteoclast formation not only in human CD14+ cell cultures, but also in mouse macrophage cultures. These results suggest that PGE2 inhibits human osteoclast formation through the production of an inhibitory factor(s) for osteoclastogenesis of osteoclast precursors.