SC-19220, Antagonist of Prostaglandin E2 Receptor EP1, Inhibits Osteoclastogenesis by RANKL

Abstract

We examined the direct effect of SC-19220, an EP1 prostaglandin (PG) E2 receptor antagonist, on osteoclastogenesis induced by RANK/RANKL signaling in mouse cell cultures. We found that SC-19220 inhibited RANKL-induced osteoclastogenesis by suppression of the RANK/RANKL signaling pathway in osteoclast precursors. Bone growth is accomplished by a dynamic equilibrium between formation by osteoblasts and resorption by osteoclasts, which are regulated by many systemic and local osteotropic factors that induce osteoclast formation from hematopoietic precursors through RANK/RANKL signaling. There are four subtypes of prostaglandin E (PGE) receptors, EP1, EP2, EP3, and EP4, and PGE2 facilitates bone resorption by a mechanism mediated by EP2/EP4. It is well known that SC-19220 is an EP1-specific antagonist. We previously found that SC-19220 inhibited osteoclastogenesis induced by osteotropic factors, including PGE2; however, the inhibitory mechanism is not clear. In this study, we investigated the inhibitory effects of SC-19220 on osteoclastogenesis induced by RANK/RANKL signaling in mouse cell cultures and analyzed the mechanism involved. A bone marrow culture system and bone marrow macrophages were used to examine the effects of SC-19220 on PGE2-, 11-deoxy-PGE1-, and RANKL-induced osteoclastogenesis. We analyzed RANKL expression in osteoblasts induced by PGE2 using RT-PCR. We also examined the effects of SC-19220 on the macrophage-colony-stimulating factor (M-CSF) receptor (c-Fms) and RANK expression in osteoclast precursors as well as RANK/RANKL signaling using RT-PCR and Western blotting analyses. SC-19220 dose-dependently inhibited osteoclast formation induced by PGE2, 11-deoxy-PGE1, and RANKL in the mouse culture system; however, it had no influence on RANKL expression in osteoblasts induced by PGE2. Furthermore, the expression of RANK and c-Fms in osteoclast precursors was decreased by SC-19220 at the mRNA and protein levels. In RANK signaling networks, SC-19220 inhibited c-Src and NFAT2 expression. Our findings indicated that SC-19220 inhibits RANKL-induced osteoclastogenesis through the suppression of RANK, c-Fms, c-Src, and NFAT2, suggesting that this EP1-specific antagonist inhibits osteoclast formation induced by RANKL from the early stage of osteoclastogenesis.