Licorice isoliquiritigenin suppresses RANKL-induced osteoclastogenesis in vitro and prevents inflammatory bone loss in vivo

Osteoclasts, bone-specialized multinucleated cells, are responsible for bone destructive diseases such as osteoporosis, periodontitis, and rheumatoid arthritis. Natural plant-derived products have received substantial attention given their potential therapeutic and preventive activities against human diseases. In the present study, we investigated the effects of isoliquiritigenin (ISL), a natural flavonoid isolated from licorice, on receptor activator of nuclear factor-κB ligand (RANKL)-induced in vitro osteoclastogenesis and inflammation-mediated bone destruction in vivo. We observed that ISL dose-dependently inhibited RANKL-induced osteoclast formation from RAW 264.7 and primary mouse bone marrow-derived macrophages (BMMs), as well as decreased the extent of lacunar resorption. Specifically, ISL targeted RANKL-induced osteoclastogenesis and F-actin rings formation at an early stage. The RANKL-stimulated mRNA expression of osteoclast-related genes and transcription factors were also diminished by ISL. Mechanistically, ISL blocked the RANKL-triggered RANK-TRAF6 association, phosphorylation of mitogen-activated protein kinases (MAPKs), inhibitor of κBα (IκBα) phosphorylation and degradation, nuclear factor-κB (NF-κB) p65 nuclear translocation, as well as activator protein (AP)-1 activation. ISL almost abrogated the nuclear factor of activated T cells (NFATc1) expression and inhibited its nuclear translocation specifically in pre-osteoclasts. Furthermore, the ectopic introduction of NFATc1 into osteoclast precursors almost reversed the ISL-elicited anti-osteoclastogenic effects. Consistent with the in vitro results, administration of ISL prevented inflammatory bone loss in mice by attenuating osteoclast activity. Taken together, our results demonstrated that ISL suppresses RANKL-induced osteoclastogenesis and inflammatory bone loss via RANK-TRAF6, MAPK, IκBα/NF-κB, and AP-1 signaling pathways. Therefore, ISL may be considered as a novel therapeutic and/or preventive strategy against lytic bone diseases.