(2S)-2'-Methoxykurarinone inhibits osteoclastogenesis and bone resorption through down-regulation of RANKL signaling.

(2S)-2'-Methoxykurarinone (MK), a compound isolated from the roots of Sophora flavescens, has various physiological properties, such as anti-inflammatory, antipyretic, antidiabetic, and antineoplastic effects. However, the effect of S. flavescens-derived MK on osteoclastogenesis remains unknown. Therefore, we examined the effect and mechanism of action of MK on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation and bone resorption. MK inhibited osteoclast differentiation in bone marrow cell-osteoblast cocultures but did not affect the RANKL-to-osteoprotegerin ratio induced by osteoclastogenic factors in osteoblasts. MK also inhibited RANKL-induced osteoclast differentiation from bone marrow macrophages in a dose-dependent manner, without cytotoxicity. Pretreatment with MK significantly suppressed the Akt, p38, c-Jun N terminal kinase (JNK), c-Fos, and nuclear factor of activated T cells c1 (NFATc1) pathways and inhibited the bone-resorbing activity of mature osteoclasts. These results collectively suggest that MK inhibits osteoclast differentiation and bone resorption through RANKL-induced mitogen-activated protein kinases (MAPKs) and c-Fos-NFATc1 signaling pathways.