SMAD3 and breast cancer: To study the cross-talk between the cAMP and the TGFβ signaling pathways in breast cancer cells we chose MDA-MB-231 cells, because they express functional TGFβ receptors I and II and respond to TGFβ by nuclear translocation of Smad3 and upregulation of TGFβ-responsive genes, such as PTHrP and PAI-1 [16], [20], [22]–[24].