Regulatory mechanism of osteoclastogenesis by Wnt signaling

Abstract

Bone-resorbing osteoclasts develop from monocyte-macrophage lineage cells under the regulation of bone-forming osteoblasts. Osteoblasts express two cytokines essential for osteoclastogenesis, macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Osteoblasts also produce osteoprotegerin (OPG), a decoy receptor for RANKL, which inhibits the interaction between RANKL and RANK, a receptor of RANKL. Wnt proteins (Wnts) play a central role in the development of organs and tissues. There are two pathways of Wnt signaling. β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. The discovery that loss-of-function mutations in low density lipoprotein receptor-related protein 5 (LRP5), a Wnt co-receptor, led to low bone mass in humans revealed the possible role of Wnt signaling in bone formation: Wnts act on osteoblast precursor cells and promote their differentiation into osteoblasts through the β-catenin-dependent canonical pathway. In addition, Wnts suppress bone resorption by the up-regulation of OPG expression and the down-regulation of RANKL expression in osteoblasts through the same pathway. In contrast, the activation of the β-catenin-independent noncanonical pathway enhances the RANKL-induced osteoclastogenesis. Recent studies have shown that the β-catenin independent noncanonical pathway is also involved in bone resorption induced by arthritis. This review summarizes the regulatory mechanism of bone resorption by Wnt signals.