Specific interactions between Smad proteins and AP-1 components determine TGFβ-induced breast cancer cell invasion

Deregulation of the transforming growth factor beta (TGF beta) signal transduction cascade is functionally linked to cancer. In early phases, TGFb acts as a tumor suppressor by inhibiting tumor cell proliferation, whereas in late phases, it can act as a tumor promoter by stimulating tumor cell invasion and metastasis. Smad transcriptional effectors mediate TGF beta responses, but relatively little is known about the Smad-containing complexes that are important for epithelial-mesenchymal transition and invasion. In this study, we have tested the hypothesis that specific members of the AP-1 transcription factor family determine TGF beta signaling specificity in breast cancer cell invasion. Using a 3D model of collagen-embedded spheroids of MCF10A-MII premalignant human breast cancer cells, we identified the AP-1 transcription factor components c-Jun, JunB, c-Fos and Fra1 as essential factors for TGF beta-induced invasion and found that various mesenchymal and invasion-associated TGF beta-induced genes are co-regulated by these proteins. In situ proximity ligation assays showed that TGF beta signaling not only induces complexes between Smad3 and Smad4 in the nucleus but also complexes between Smad2/3 and Fra1, whereas complexes between Smad3, c-Jun and JunB could already be detected before TGF beta stimulation. Finally, chromatin immunoprecipitations showed that c-Jun, JunB and Fra1, but not c-Fos, are required for TGF beta-induced binding of Smad2/3 to the mmp-10 and pai-1 promoters. Together these results suggest that in particular formation of Smad2/3-Fra1 complexes may reflect activation of the Smad/AP-1-dependent TGF beta-induced invasion program.