Wnt signalling in osteoblasts regulates expression of the receptor activator of NFκB ligand and inhibits osteoclastogenesis in vitro

Abstract

Reports implicating Wnt signalling in the regulation of bone mass have prompted widespread interest in the use of Wnt mimetics for the treatment of skeletal disorders. To date much of this work has focused on their anabolic effects acting on cells of the osteoblast lineage. In this study we provide evidence that Wnts also regulate osteoclast formation and bone resorption, through a mechanism involving transcriptional repression of the gene encoding the osteoclastogenic cytokine receptor activator of NFκB ligand (RANKL or TNFSF11) expressed by osteoblasts. In co-cultures of mouse mononuclear spleen cells and osteoblasts, inhibition of GSK3β with LiCl or exposure to Wnt3a inhibited the formation of tartrate-resistant acid phosphatase-positive multinucleated cells compared with controls. However, these treatments had no consistent effect on the differentiation, survival or activity of osteoclasts generated in the absence of supporting stromal cells. Activation of Wnt signalling downregulated RANKL mRNA and protein expression, and overexpression of fulllength β-catenin, but not transcriptionally inactive β-catenin ΔC(695-781), inhibited RANKL promoter activity. Since previous studies have demonstrated an absence of resorptive phenotype in mice lacking LRP5, we determined expression of a second Wnt co-receptor LRP6 in human osteoblasts, CD14+ osteoclast progenitors and mature osteoclasts. LRP5 expression was undetectable in CD14-enriched cells and mature human osteoclasts, although LRP6 was expressed at high levels by these cells. Our evidence of Wnt-dependent regulation of osteoclastogenesis adds to the growing complexity of Wnt signalling mechanisms that are now known to influence skeletal function and highlights the requirement to develop novel therapeutics that differentially target anabolic and catabolic Wnt effects in bone.

Wnt signaling and skeletal development

“…However, in this in vitro model, osteoblasts lacking β-catenin additionally showed impaired maturation and mineralization with elevated expression of the osteoclast differentiation factor receptor activated by nuclear factor-κB ligand (RANKL) [94]. In co-cultures of mouse spleen cells and osteoblasts, activation of Wnt signaling downregulated RANKL expression, inhibiting formation of tartrate-resistant acid phophatase-positive multinucleated cells [107].…”

β-Catenin induces immortalization of melanocytes by suppressing p16INK4a expression and cooperates with N-Ras in melanoma development

“…Immortalization of melanocytes almost always involves inactivation of the p16 INK4a gene, and, consistent with this, our results suggest that activated ␤-catenin directly represses the expression of p16 INK4a through an evolutionarily conserved LEF/Tcf site in its promoter. While ␤-catenin is more usually an activator of transcription, as it is on the Mitf-M promoter, increasing evidence has emerged to support its role as a transcriptional repressor (Kahler and Westendorf 2003;Kim et al 2005;Spencer et al 2006). However, it is yet unclear what signals or cofactors mediate the switch from activator to repressor and whether the effects seen are promoter specific.…”

A Phytoestrogen Diarylheptanoid Mediates Estrogen Receptor/Akt/Glycogen Synthase Kinase 3β Protein-dependent Activation of the Wnt/β-Catenin Signaling Pathway

“…E2-induced prevention of bone loss is mediated through the suppression of osteoclastogenesis by stimulating the production of insulin-like growth factor 1 (IGF-1), modulating the expressions of osteoprotegerin and receptor activator of NF-B ligand (RANKL) in osteoblastic cells (38). Indeed, Wnt signaling regulates the expression of receptor activator of NF-B ligand and inhibits osteoclastogenesis (23). Therefore, it is likely that ASPP 049 exerts its action on osteoblastic cell proliferation and differentiation via similar mechanisms.…”