Identification of antiâosteoclastogenic agents is important for the treatment of bone loss diseases that feature excessive osteoclast (OC) activity and bone resorption. Tranylcypromine (TCP), an irreversible inhibitor of monoamine oxidase (MAO), has been used as an antidepressant and anxiolytic agent in the clinical treatment of mood and anxiety disorders. TCP has been discovered to exert anabolic effect on osteoblasts, and MAOâA has also been verified as an important mediator in prostate cancer cells to accelerate osteoclastogenesis. In current study, we were focused on TCP and MAOâA effects on osteoclastogenesis. As illustrated by tartrateâresistant acid phosphatase staining, TCP was capable of inhibiting osteoclastogenesis induced by receptor activators of the NFâÎșB ligand (RANKL) in bone marrowâderived macrophage cells without any cytotoxicity. It was also shown to effectively suppress bone resorption of OCs. The subsequent study revealed that TCP inhibited osteoclastogenesisârelated genes in a timeâdependent manner through protein kinase B (AKT)âmediated mechanism followed by the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1)âcâfos pathway. And TCP could overcome the osteoclastogenic effects of AKT activator SC79. In addition, our results indicated that the expression and catalytic activity of MAOâA were upâregulated by RANKL stimulation and downâregulated by TCP in vitro and in vivo. Furthermore, the effects of MAOâA knockdown on OC differentiation indicated that MAOâA played an important role in osteoclastogenesis in vitro and might contribute to the inhibitory effects of TCP. And AKT, NFATc1, and câfos were involved in the MAOâA pathway. Notably, our in vivo study reflected that TCPs were capable of restoring the bone loss in LPSâinduced calvaria osteolysis and estrogen deficiencyâinduced osteoporosis models. Thus, our current work provided a potential option for the treatment of bone loss diseases and highlighted the important role of MAOâA in osteoclastogenesis as well.âLiu, Z., Yang, K., Yan, X., Wang, T., Jiang, T., Zhou, Q., Qi, J., Qian, N., Zhou, H., Chen, B., Huang, P., Guo, L., Zhang, X., Xu, X., Jiang, M., Deng, L. The effects of tranylcypromine on osteoclastogenesis in vitro and in vivo. FASEB J. 33, 9828â9841 (2019). http://www.fasebj.org