Osteoprotegerin reduces the serum level of receptor activator of NF-kappa B ligand derived from osteoblasts

Osteoprotegerin (OPG) is a decoy receptor for receptor activator of NF-kappa B ligand (RANKL). We previously reported that OPG deficiency elevated the circulating level of RANKL in mice. Using OPG(-/-) mice, we investigated whether OPG is involved in the shedding of RANKL by cells expressing RANKL. Osteoblasts and activated T cells in culture released a large amount of RANKL in the absence of OPG. OPG or a soluble form of receptor activator of NF-kappa B (the receptor of RANKL) suppressed the release of RANKL from those cells. OPG- and T cell-double-deficient mice showed an elevated serum RANKL level equivalent to that of OPG(-/-) mice, indicating that circulating RANKL is mainly derived from bone. The serum level of RANKL in OPG(-/-) mice was increased by ovariectomy or administration of 1 alpha,25-dihydroxyvitamin D-3. Expression of RANKL mRNA in bone, but not thymus or spleen, was increased in wild-type and OPG(-/-) mice by la,25-dihydroxyvitamin D,. These results suggest that OPG suppresses the shedding of RANKL from osteoblasts and that the serum RANKL in OPG(-/-) mice exactly reflects the state of bone resorption.