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Prostaglandin D2 Receptors Control Osteoclastogenesis and the Activity of Human Osteoclasts

We recently showed that human osteoblasts synthesize prostaglandin D2 (PGD2) and express both DP and CRTH2 receptors. Activation of the DP receptor decreased osteoprotegerin production, whereas activation of the CRTH2 receptor induced osteoblast chemotaxis and decreased RANKL expression. Our objectives in this study were to determine the presence, distribution, and action of these receptors in the functions of human osteoclasts and in osteoclastogenesis. Immunohistochemistry was used to detect the presence of DP and CRTH2 in in vitro–differentiated human osteoclasts in culture and in osteoclasts in situ. The effects of the activation of PGD2 receptors on the cytoskeleton were determined by fluorescence microscopy. Specific agonists and antagonists allowed the study of the roles of these receptors on bone resorption and osteoclast differentiation. Our results show that in vitro–differentiated human osteoclasts and authentic fetal osteoclasts express both DP and CRTH2 receptors, as shown by immunocytochemistry. Similar results were obtained in osteoclasts from normal, osteoporotic, pagetic, and osteoarthritic adult bone tissues. Stimulation of osteoclasts with PGD2 induced a robust reorganization of the cytoskeleton with a decrease in the number of cells presenting actin rings and an increase of lamellipodia, effects mediated by the DP and CRTH2 receptors, respectively. PGD2 showed an inhibitory effect on bone resorption activity acting through the DP receptor. In vitro osteoclastogenesis from peripheral blood mononuclear cells cultured in the presence of RANKL and macrophage‐colony stimulating factor was decreased by activation of either DP or CRTH2 receptors. These results suggest that PGD2 receptors could be useful targets in certain bone diseases because their specific activation/inhibition leads to a decrease in osteoclastogenesis and to inhibition of bone resorption by osteoclasts.