Mechanisms involved in bone resorption

Osteoclasts, which are present only in bone, are multinucleated giant cells with the capacity to resorb mineralized tissues. These osteoclasts are derived from hemopoietic progenitors of the monocyte-macrophage lineage. Osteoblasts or bone marrow-derived stromal cells are involved in osteoclastogenesis through a mechanism involving cell-to-cell contact with osteoclast progenitors. Experiments on the osteopetrotic op/op mouse model have established that a product of osteoblasts, macrophage colony-stimulating factor (M-CSF), regulates differentiation of osteoclast progenitors into osteoclasts. Recent discovery of osteoclast differentiation factor (ODF)/receptor activator of NF-kappa B ligand (RANKL) allowed elucidation of the precise mechanism by which osteoblasts regulate osteoclastic bone resorption. Treatment of osteoblasts with bone-resorbing factors up-regulated expression of RANKL mRNA. In contrast, TNF alpha stimulates osteoclast differentiation in the presence of M-CSF through a mechanism independent of the RANKL system. IL-1 also directly acts on mature osteoclasts as a potentiator of osteoclast activation. In addition, TGF-beta super family members, such as bone morphogenetic proteins (BMPs) strikingly enhanced osteoclast differentiation from their progenitors and survival of mature osteoclasts induced by RANKL. These results suggest that BMP-mediated signals cross-communicate with RANKL-mediated ones in inducing osteoclast differentiation and function.