Inhibitory effects of Stewartia koreana on osteoclast differentiation and bone resorption

Osteoclasts are responsible for bone lysis in several bone diseases such as osteoporosis and rheumatoid arthritis. Natural products from plants have been invaluable source in discovery of compounds for new therapies. In this study, we screened plant products for potential application to therapy for bone loss using a primary osteoclastogenesis culture system and found that extract of Stewartia koreana (SKE) had a strong inhibitory effect on osteoclast formation. To gain molecular insights, we examined the effect of SKE on signaling pathways and transcription factors stimulated by the osteoclast differentiation factor RANKL. SKE suppressed the induction of c-Fos and NFATc1 by RANKL. However, SKE did not inhibit NF-kappaB activation by RANKL. Among the MAPKs stimulated by RANKL, SKE significantly reduced the activation of ERK and p38. Therefore, the anti-osteoclastogenic effect of SKE is likely to be elicited by interference with RANKL signaling to ERK and p38, which mediate the induction of c-Fos and subsequently that of NFATc1. Consistent with the in vitro effect on osteoclast differentiation, SKE showed a great inhibitory effect on in vivo bone loss in LPS-challenged mice. Taken together, we demonstrated that SKE has inhibitory effects on osteoclast differentiation in vitro and confirmed its in vivo efficacy in prevention of inflammatory bone loss.