Bone morphogenetic protein 2 stimulates osteoclast differentiation and survival supported by receptor activator of nuclear factor-kappa B ligand

Bone is a major storage site for TGF beta superfamily members, including TG F beta and bone morphogenetic proteins. It is believed that these cytokines are released from bone during bone resorption. Recent studies have shown t hat both RANKL and macrophage colony-stimulating factor are two essential f actors produced by osteoblasts for inducing osteoclast differentiation. In the present study we examined the effects of bone morphogenetic protein-2 o n osteoclast differentiation and survival supported by RANKL and/or macroph age colony-stimulating factor. Mouse bone marrow-derived macrophages differ entiated into osteoclasts in the presence of RANKL and macrophage colony-st imulating factor. TGF beta superfamily members such as bone morphogenetic p rotein-2, TGF beta, and activin A markedly enhanced osteoclast differentiat ion induced by RANKL and macrophage colony-stimulating factor, although eac h cytokine alone failed to induce osteoclast differentiation in the absence of RANKL. Addition of a soluble form of bone morphogenetic protein recepto r type 1A to the culture markedly inhibited not only osteoclast formation i nduced by RANKL and bone morphogenetic protein-2, but also the basal osteoc last formation supported by RANKL alone. Either RANKL or macrophage colony- stimulating factor stimulated the survival of purified osteoclasts. Bone mo rphogenetic protein-2 enhanced the survival of purified osteoclasts support ed by RANKL, but not by macrophage colony-stimulating factor. Both bone mar row macrophages and mature osteoclasts expressed bone morphogenetic protein -2 and bone morphogenetic protein receptor type IA mRNAs. An EMSA revealed that RANKL activated nuclear factor-kappaB in purified osteoclasts. Bone mo rphogenetic protein-2 alone did not activate nuclear factor-kappaB, but rat her inhibited the activation of nuclear factor-kappaB induced by RANKL in p urified osteoclasts. These findings suggest that bone morphogenetic protein -mediated signals cross-communicate with RANKL-mediated ones in inducing os teoclast differentiation and survival. The enhancement of RANKL-induced sur vival of osteoclasts by bone morphogenetic protein-2 appears unrelated to n uclear factor-kappaB activation.