Soluble glucocorticoid-induced tumor necrosis factor receptor stimulates osteoclastogenesis by down-regulation of osteoprotegerin in bone marrow stromal cells

Soluble glucocorticoid-induced tumor necrosis factor receptor (sGITR) is a potent stimulator of osteoclastogenesis. The mechanism by which it induces osteoclastogenesis was studied by culturing bone-marrow-derived macrophages (BMM) with conditioned medium from mouse bone marrow stromal cells. GITR and GITR ligand (GITRL) were expressed on the surface of bone marrow stromal cells, and sGITR-induced osteoclastogenesis was inhibited by anti-GITRL Ab, indicating that stimulatory effect of osteoclastogenesis by sGITR involved signaling via GITRL. Bone marrow stromal cells up-regulated cyclooxygenase-2 (COX-2) and produced prostaglandin E2 (PGE2) early in their response to sGITR, and the stimulation of osteoclastogenesis was markedly inhibited by NS398, a COX-2 inhibitor. Later, sGITR markedly reduced the steady-state level of osteoprotegerin (OPG) mRNA and increased receptor activator of nuclear factor-κB ligand (RANKL) mRNA. NS398 blocked the sGITR-induced reduction of OPG mRNA but did not significantly affect the sGITR-induced rise in RANKL mRNA. This suggests that down-regulation of OPG by PGE2 is involved in sGITR-induced osteoclast (OC) formation in the presence of conditioned medium from mouse bone marrow stromal cells.