Regulation of RANKL (receptor activator of nuclear factor B ligand)-induced osteoclast differentiation is of current interest in the development of antiresorptive agents. Osteoclasts are multinucleated cells that play a crucial role in bone resorption. In this study, we investigated the effects of N-methylpyrrolidone (NMP) on the regulation of RANKL-induced osteoclastogenesis. NMP inhibited RANKL-induced tartrate-resistant acid phosphatase activity and the formation of tartrate-resistant acid phosphatase-positive multinucleated cells. The RANKL-induced expression of NFATc1 (nuclear factor of activated T cells, cytoplasmic 1) and c-Fos, which are key transcription factors for osteoclastogenesis, was also reduced by treatment with NMP. Furthermore, NMP induced disruption of the actin rings and decreased the mRNAs of cathepsin K and MMP-9 (matrix metalloproteinase-9), both involved in bone resorption. Taken together, these results suggest that NMP inhibits osteoclast differentiation and attenuates bone resorption. Therefore, NMP could prove useful for the treatment of osteoporosis or other bone diseases associated with excessive bone resorption.Bone remodeling is a physiological process that involves the resorption and synthesis of bone by osteoclasts and osteoblasts, respectively (1, 2). Osteoclasts are known to be formed by the fusion of hematopoietic cells of the monocyte-macrophage lineage during the early stage of the differentiation process (3). This process consists of multiple steps, including differentiation of osteoclast precursors into mononuclear osteoclasts, fusion of mononuclear preosteoclasts into mature multinucleated osteoclasts, and activation of osteoclasts to resorb bone (4 -7). The terminal differentiation in this lineage is characterized by acquisition of mature phenotypic markers such as expression of tartrate-resistant acid phosphatase (TRAP), 2 the calcitonin receptor, MMP-9, and cathepsin K, as well as morphological conversion into large multinucleated cells and the ability to form resorption lacunae on bone (8 -10). The essential signaling molecules for osteoclast differentiation include RANKL and M-CSF (macrophage colony-stimulating factor) in bone marrow-derived macrophage precursor cells (11,12).RANKL is a member of the TNF superfamily that is expressed in osteoblasts. It interacts with the osteoclast cell surface receptor RANK, which in turn recruits TNF receptor-associated factors and plays a crucial role in the osteoclast differentiation axis (13,14). The downstream intracellular signaling mediated by RANK in osteoclast progenitor cells includes TRAF6 (TNF receptor-associated factor 6)-dependent activation of NF-B via the IB kinase complex and MAPKs such as ERK, p38 MAPK, and JNK (7,11,15). In addition, RANKL induces the key transcription factors for osteoclastogenesis, NFATc1 and c-Fos (9, 16, 17). Therefore, chemical or natural compounds that specifically inhibit these steps could be developed as antiresorptive drugs for the treatment of metabolic bone disorders characterized by e...