Receptor activator of NF-kappa B ligand induces the fusion of mononuclear preosteoclasts into multinucleated osteoclasts

The osteoclasts are bone-resorbing multinucleated cells formed by the fusio n of mononuclear preosteoclasts (pOCs) of hematopoietic origin. Although re ceptor activator of NF-kappa B ligand (RANKL) has been shown to regulate os teoclast differentiation and function, its effect on the fusion of pOCs int o multinucleated osteoclast-like cells (OCLs) has not been known. Using our fusion assay system, that is not contaminated with multinucleated cells (M NCs) and osteoblastic cells, we determined the effect of RANKL on the fusio n of pOCs into MNCs. When pOCs were cultured on the plates, most of pOCs di ed and disappeared from the plates within 24 h in the absence of additives, but pOCs were fused to MNCs within 6 h in the presence of RANKL. RANKL-ind uced MNCs showed typical properties of OCL such as tartrate-resistant acid phosphatase (TRAP) activity, actin ring formation, and bone-resorbing activ ity. The fusion of pOCs into OCLs induced by osteoblastic cells or RANKL wa s inhibited by OPG/OCIF, but that induced by IL-1 beta was not. Both RANKL- and IL-1 beta-induced OCL formation from pOCs was inhibited by ZLLL-H, a p eptide inhibitor of proteasome. These findings indicate that RANKL supports the survival of pOCs and induces the fusion of pOCs into OCLs and suggest that NF-kappa B activation is involved in these processes induced by RANKL and IL-1 beta.