Epithelial-mesenchymal transition (EMT) is essential for organogenesis and is triggered during carcinoma progression to an invasive state. Transforming growth factor-beta (TGF-beta) cooperates with signalling pathways, such as Ras and Wnt, to induce EMT, but the molecular mechanisms are not clear. Here, we report that SMAD3 and SMAD4 interact and form a complex with SNAIL1, a transcriptional repressor and promoter of EMT. The SNAIL1-SMAD3/4 complex was targeted to the gene promoters of CAR, a tight-junction protein, and E-cadherin during TGF-beta-driven EMT in breast epithelial cells. SNAIL1 and SMAD3/4 acted as co-repressors of CAR, occludin, claudin-3 and E-cadherin promoters in transfected cells. Conversely, co-silencing of SNAIL1 and SMAD4 by siRNA inhibited repression of CAR and occludin during EMT. Moreover, loss of CAR and E-cadherin correlated with nuclear co-expression of SNAIL1 and SMAD3/4 in a mouse model of breast carcinoma and at the invasive fronts of human breast cancer. We propose that activation of a SNAIL1-SMAD3/4 transcriptional complex represents a mechanism of gene repression during EMT.

Jonas Fuxe was supported by grants from the Swedish Research Council, the Swedish Wenner-Gren Foundation, the Swedish Childhood Cancer Foundation and an International Union Against Cancer (UICC), American Cancer Society International Fellowship for Beginning Investigators. Theresa Vincent was supported by the Swedish Research Council. Philip Leopold and Ronald Crystal were supported by the National Institutes of Health (NIH) by PO1 HL59312 and Antonio García de Herreros, Joan Albanell and Federico Rojo by RD06/0020/109, RD06/0020/040, FIS PI061513, SAF2006-00339 and Fundació Privada Cellex