One of the mediators of osteoclast differentiation is receptor activator of nuclear factor κB ligand (RANKL), which is produced by osteoblasts. Binding of RANKL to its receptor, RANK, activates several signaling pathways, including those involving mitogen-activated protein kinases (MAPKs), nuclear factor κB (NF-κB), nuclear factor of activated T cells c1 (NFATc1) and Ca 2 -calcineurin. In the present study, we found that tetrandrine, a bisbenzylisoquinoline alkaloid extracted from the root of Stephania tetrandra S. MOORE, significantly ameliorated the decrease of bone mass in sciatic-neurectomized osteoporosis model mice. It appears that tetrandrine acts directly on osteoclast precursors, since tetrandrine inhibited osteoclast differentiation not only in mouse bone marrow cells, but also in monocultures of murine macrophage RAW 264.7 cells without osteoblasts. Tetrandrine suppressed RANKL-induced amplification of NFATc1, a master regulator of osteoclast differentiation. However, it did not affect other signaling molecules such as MAPKs and NF-κB. These results suggest that tetrandrine is a candidate for the treatment of bone-destructive diseases, or at least a suitable lead compound for further development.Key words tetrandrine; osteoporosis; osteoclast; receptor activator of nuclear factor κB ligand; nuclear factor of activated T cell c1Bone remodeling is a physiological process that involves resorption of bone by osteoclasts and synthesis of bone matrix by osteoblasts.1) A relative increase of bone resorption over bone formation can result in osteoporosis, one of the most prevalent diseases in the aged population.2,3) 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.4) Terminal differentiation in this lineage is characterized by acquisition of mature phenotypic markers, such as expression of tartrate-resistant acid phosphatase (TRAP), calcitonin receptor, matrix metalloproteinase 9 (MMP9), and cathepsin K, as well as morphological conversion into large multinucleated cells and formation of resorption lacunae on bone. 5-7)The essential signaling molecules for osteoclast differentiation include receptor activator of nuclear factor κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), and both of them are produced by osteoblasts. 8)RANKL induces the signaling essential for precursor cells to differentiate into osteoclasts, 9) whereas M-CSF provides the survival signal for these cells.10) RANKL binding to RANK, a receptor of RANKL that is expressed in osteoclast precursors, mediates the biological effects of RANKL and leads to the recruitment of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), which in turn results in the activation of the downstream signaling pathway, including nuclear factor κB (NF-κB), c-Jun N-terminal protein kinase (JNK), p38, extracellular signal-regulated kinase (ERK), and phosphatidylinositol 3-kinase/AKT. [11][12][13][14] RANKL activation of JNK ph...