TNF-α-induced osteoclastogenesis is central to post-menopausal and inflammatory bone loss, however, the effect of phytoestrogens on TNF-α-induced bone resorption has not been studied. The phytoestrogens genistein, daidzein, and coumestrol directly suppressed TNF-α-induced osteoclastogenesis and bone resorption. TRAP positive osteoclast formation and resorption area were significantly reduced by genistein (10(-7) M), daidzein (10(-5) M), and coumestrol (10(-7) M), which was prevented by the estrogen antagonist ICI 182,780. TRAP expression in mature TNF-α-induced osteoclasts was also significantly reduced by these phytoestrogen concentrations. In addition, in the presence of ICI 182,780 genistein and coumestrol (10(-5) -10(-6) M) augmented TNF-α-induced osteoclast formation and resorption. However, this effect was not observed in the absence of estrogen antagonist indicating that genistein's and coumestrol's ER-dependent anti-osteoclastic action normally negates this pro-osteoclastic effect. To determine the mechanism mediating the anti-osteoclastic action we examined the effect of genistein, coumestrol, and daidzein on caspase 3/7 activity, cell viability and expression of key genes regulating osteoclast differentiation and fusion. While anti-osteoclastic phytoestrogen concentrations had no effect on caspase 3/7 activity or cell viability they did significantly reduce TNF-α-induced c-fos and NFATc1 expression in an ER dependent manner and also inhibited NFATc1 nuclear translocation. Significant decreases in NFκB and DC-STAMP levels were also noted. Interestingly, constitutive c-fos expression prevented the anti-osteoclastic action of phytoestrogens on differentiation, resorption and NFATc1. This suggests that phytoestrogens suppress TNF-α-induced osteoclastogenesis via inhibition of c-fos-dependent NFATc1 expression. Our data provides further evidence that phytoestrogens have a potential role in the treatment of post-menopausal and inflammatory bone loss directly inhibiting TNF-α-induced resorption.