Absence of platelet-activating factor receptor protects mice from osteoporosis following ovariectomy

While platelet-activating factor (PAF) is produced in various diseases associated with bone resorption, its functions in bone metabolism remain unknown.Using PAF receptor-deficient mice, we evaluated the role of PAF in the development of bone resorption following ovariectomy, a model of postmenopausal osteoporosis.Through observations of bone mineral density and histomorphometric parameters, it was found that bone resorption was markedly attenuated in PAF receptor-deficient mice, indicating that PAF links estrogen depletion and osteoporosis in vivo.Osteoclasts expressed higher amounts of the enzymes required for PAF biosynthesis than osteoblasts.TNF-α and IL-1β increased the acetyl-coenzyme A:lyso-PAF acetyltransferase activity in osteoclasts.Osteoclasts, but not osteoblasts, expressed the functional PAF receptor.PAF receptor stimulation prolonged the survival of osteoclasts in vitro.Furthermore, osteoclasts treated with a PAF receptor antagonist, and also those from PAF receptor-deficient mice, showed reductions in survival rate and Ca resorption activity.Consistently, in organ cultures, bone resorption was significantly suppressed by a PAF receptor antagonist treatment or genetic PAF receptor deficiency.Thus, these results suggest that, through the inflammatory cytokines, estrogen depletion enhances PAF production as a unique autocrine factor for osteoclast functions.Inhibition of PAF function might pave the way for a new strategy to prevent postmenopausal bone loss without disturbing osteoblast functions. ResultsBone density in ovariectomized PAFR-KO mice.In soft x-ray radiograph analysis, radiolucency of the excised femurs and tibiae was increased in ovariectomized PAFR-WT mice compared with sham-operated PAFR-WT mice (Figure 1A).However, ovariectomized PAFR-KO mice showed little increase in radiolucency.To quantitate these observations, the bone mineral density (BMD) of the entire femurs in the ovariectomized mice was compared with that of the sham-operated mice.Differences in BMD observed by dual x-ray absorptiometry (DXA) measurements were significant between the ovariectomized group and sham-operated group in PAFR-WT mice (Figure 1B).DXA analysis also demonstrated that the BMD of the femurs in ovariectomized PAFR-KO mice was comparable to that in sham-operated PAFR-KO mice.There were no differences in the levels of Ca, phosphate, or parathyroid hormone in the sera of both genotypes, either with or without ovariectomy (data not shown).Bone histomorphometry in ovariectomized PAFR-KO mice.It was apparent from the histologic observations that the bone volume in the metaphyseal region of the tibiae was reduced in ovariectomized PAFR-WT mice, but ameliorated in ovariectomized PAFR-Nonstandard abbreviations used: acetyl-CoA:lyso-PAF acetyltransferase (lyso-PAF acetyltransferase); acetyl-coenzyme A (acetyl-CoA); 4-amidinophenylmethanesulfonyl fluoride (APMSF); bone mineral density (BMD); cytosolic phospholipase A2α (cPLA2α); 1,25-dihydroxyvitamin D3 (1,25-[OH]2D3); dual x-ray absorptiometry (DXA); 1-O-hexadecyl-sn-glycero-3-phosphocholine (lyso-PAF); intracellular Ca 2+ concentration ([Ca 2+ ]i); methylcarbamoyl PAF (mc-PAF); osteoclast surface per bone surface (Oc.S/BS); osteoid thickness (O.Th); platelet-activating factor (PAF); PAF receptor (PAFR); PAFR knockout (PAFR-KO); PAFR wild type (PAFR-WT); receptor activator of NF-κB ligand (RANKL); tartrate-resistant acid phosphatase (TRAP); trabecular bone volume (BV/TV); trabecular number (Tb.N); trabecular separation (Tb.Sp).