Herbacetin inhibits RANKL-mediated osteoclastogenesis in vitro and prevents inflammatory bone loss in vivo.

Herbacetin is an active flavonol (a type of flavonoid) that has various biologic effects such as antioxidant, antitumor, and anti-inflammatory activities. However, one of its novel effects remains to be investigated, that is, the induction of osteoclastogenesis by the receptor activator of nuclear factor-κB ligand (RANKL). In this study, we examined the effects and mechanisms of action of herbacetin on osteoclastogenesis in RANKL-treated bone marrow-derived macrophages (BMMs) and murine macrophage RAW264.7 cells in vitro and on lipopolysaccharide (LPS)-induced bone destruction in vivo. Herbacetin significantly inhibited RANKL-induced osteoclast formation and differentiation in BMMs and RAW264.7 cells in a dose-dependent manner. Moreover, the suppressive effect of herbacetin resulted in a decrease in osteoclast-related genes, including RANK, tartrate-resistant acid phosphatase, cathepsin K, and matrix metalloproteinase-2 and -9 (MMP-9). Consistent with mRNA results, we confirmed that herbacetin treatment downregulated protein expression of MMP-9 and cathepsin K. Herbacetin also decreased induction of the osteoclastogenic transcription factor c-Fos and nuclear factor of activated T cells c1 (NFATc1) and blocked RANKL-mediated activation of Jun N-terminal kinase (JNK) and nuclear factor-κB. Herbacetin clearly inhibited the bone resorption activity of osteoclasts on plates coated with fluorescein-labeled calcium phosphate. More importantly, the application of herbacetin significantly reduced LPS-induced inflammatory bone loss in mice in vivo. Taken together, our results indicate that herbacetin has potential for use as a therapeutic agent in disorders associated with bone loss.