Osteoclastogenesis is dependent on distinct stimuli that prime and activate osteoclast differentiation. One cytokine needed to prime monocytes for osteoclastogenesis is TGF-beta, which enables and augments RANKL and TNF-alpha-induced osteoclast differentiation. However, the precise time-period during which this occurs and the molecular mechanism mediating this action are unknown. We report here TGF-beta prime monocytes for osteoclast formation within 24 h by regulating expression of NFATc1, a key osteoclastic transcription factor. TGF-beta directly induces cytoplasmic NFATc1 expression within 24 h, but is unable to stimulate NFATc1 nuclear translocation. Furthermore, RANKL-induced NFATc1 expression is dependent on the presence of TGF-beta during the early stages of osteoclastogenesis. Similarly, TNF-alpha activates osteoclastogenesis by stimulating translocation of TGF-beta-induced NFATc1. In light of these findings, it is apparent that osteoclast formation is dependent on coordinated interactions between TGF-beta and RANKL/TNF-alpha that regulate the expression and intracellular distribution of NFATc1 during early stages of osteoclast differentiation. (C) 2007 Elsevier Inc. All rights reserved.