Prostaglandin G/H synthase-2 is required for maximal formation of osteoclast-like cells in culture

We examined the effect on osteoclast formation of disrupting the prostaglan din G/H synthase genes PGHS-1 and-2. Prostaglandin E-2 (PGE(2)) production was significantly reduced in marrow cultures from mice lacking PGHS-2 (PGHS -2(-/-)) compared with wild-type (PGHS-2(+/+)) cultures. Osteoclast formati on, whether stimulated by 1,25-dihydroxyvitamin D-3 (1,25-D) or by parathyr oid hormone (PTH), was reduced by 60-70% in PGHS-2(-/-) cultures relative t o wild-type cultures, an effect that could be reversed by providing exogeno us PGE(2). Cultures from heterozygous mice showed an intermediate response. PGHS inhibitors caused a similar drop in osteoclast formation in wild-type cultures. Coculture experiments showed that supporting osteoblasts, rather than osteoclast precursors, accounted for the blunted response to 1,25-D a nd PTH. This lack of response appeared to result from reduced expression of RANK ligand (RANKL) in osteoblasts. We cultured spleen cells with exogenou s RANKL and found that osteoclast formation was 50% lower in PGHS-2(-/-) th an in wild-type cultures, apparently because the former cells expressed hig h levels of GM-CSF. Injection of PTH above the calvaria caused hypercalcemi a in wild-type but not PGHS-2(-/-) mice. Histological examination of bone f rom 5-week-old PGHS-2(-/-) mice revealed no abnormalities. Mice lacking PGH S-1 were similar to wild-type mice in all of these parameters. These data s uggest that PGHS-2 is not necessary for wild-type bone development but play s a critical role in bone resorption stimulated by 1,25-D and PTH.