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Unlabelled: (-)-DHMEQ, a newly designed NF-kappaB inhibitor, inhibited RANKL-induced osteoclast differentiation in mouse BMMs through downregulation of the induction of NFATc1, an essential transcription factor of osteoclastogenesis., Introduction: Bone destruction is often observed in advanced case of rheumatoid arthritis and neoplastic diseases, including multiple myeloma. Effective and nontoxic chemotherapeutic agents are expected for the suppression of these bone destructions. RANKL induces activation of NF-kappaB and osteoclastogenesis in bone marrow-derived monocyte/macrophage precursor cells (BMMs). Targeted disruption or pharmacological suppression of NF-kappaB result in impaired osteoclastogenesis, but how NF-kappaB is involved in the regulation of osteoclastogenesis is not known., Materials and Methods: The effect of (-)-dehydroxymethylepoxyquinomicin [(-)-DHMEQ] on osteoclast differentiation was studied using a culture system of mouse BMMs stimulated with RANKL and macrophage colony-stimulating factor. The mechanism of the inhibition was studied by biochemical analysis such as immunoblotting and retroviral transfer experiments., Results: (-)-DHMEQ strongly inhibited RANKL-induced NF-kappaB activation in BMMs and inhibited RANKL-induced formation of TRACP(+) multinucleated cells. Interestingly, (-)-DHMEQ specifically inhibited the RANKL-induced expression of NFATc1 but not the expressions of TRAF6 or c-fos. Inhibition of osteoclast differentiation by (-)-DHMEQ was rescued by overexpression of NFATc1, suggesting that the inhibition is not caused by a toxic effect. Moreover, pit formation assays showed that (-)-DHMEQ also inhibited the bone-resorbing activity of mature osteoclasts., Conclusion: The inhibition of NF-kappaB suppresses osteoclastogenesis by downregulation of NFATc1, suggesting that NFATc1 expression is regulated by NF-kappaB in RANKL-induced osteoclastogenesis. Our results also indicate the possibility of (-)-DHMEQ becoming a new therapeutic strategy against bone erosion.