Breast tumorigenesis and breast cancer progression involves the deregulation or hyperactivation of intracellular signaling proteins that leads to uncontrolled cellular proliferation, invasion and metastasis. For example, the expression and cellular responses to estogen receptor (ER) and transforming growth factor beta (TGFbeta) signaling pathways change during breast tumorigenesis and breast cancer progression. While the expression and activity of ER and TGFbeta maybe significant in the development of breast cancer, alterations in the cross-talk between these pathways may be equally important. Autocrine and paracrine effects of TGFbeta on breast cancer cell growth have been known for some time, but only recently have direct interactions between ER and TGFbeta been described. The purpose of this article was to further characterize the cross-talk between ER and TGFbeta, by examining ER interaction with Smad3, a downstream mediator of TGFbeta signaling. Transient transfection of Cos1 cells with p3TP-lux, demonstrate that ERalpha and ERbeta(1) repress Smad3 transcriptional activity in an estradiol-dependent manner and that this effect is inhibited by antiestrogen treatment. The ERbeta variants, ERbeta(2) and ERbeta(5), did not have any effect on Smad3 transcriptional activity. Further experiments attempted to characterize the molecular mechanism by which activated ER inhibits Smad3 transcriptional activity. Results indicate that ligand-bound ER does not affect Smad3 protein expression levels and that ER does not form direct protein interactions with Smad3. Transient transfection of Cos1 cells with the Ap-1 transcription factor c-Jun but not c-Fos was able to rescue the inhibitory effect of estrogen on Smad3 transcriptional activity. Based on these results, a model is proposed whereby c-Jun is limiting in its ability to act as a Smad3 co-activator in the presence of E(2)-bound ER, possibly due to ER sequestering c-Jun away from the Smad3 responsive promoter.