Smad3/AP-1 interactions control transcriptional responses to TGF-beta in a promoter-specific manner

Smad proteins transduce signals from TGF-beta receptors and regulate transcription of target genes either directly or in combination with other sequence-specific transcription factors. AP-1 sites and their cognate transcription factors also play important roles in the gene regulatory activities of TGF-beta. In this report, we have investigated the functional interactions of the Smad and AP-1 transcription factors. We demonstrate that Smad and AP-1 complexes specifically bind to their cognate cis-elements and do not interact with each other on-DNA, whereas off-DNA interactions occur between Smad3 and both c-Jun and JunB. Using both artificial constructs specific for either the Smad or AP-1 signaling pathways or natural promoters known to be TGF-beta-responsive, we have determined that Jun family members downregulate Smad3-mediated gene transactivation whereas AP-1-dependent promoters are synergistically activated by Smad3 and Jun proteins. We propose a model where the presence of Smad- and/or AP-1-specific cis-elements within TGF-beta-responsive genes allows dynamic modulation of gene expression, in contrast to the existing model where interactions between Smad and AP-1 proteins are merely an on/off mechanism to regulate TGF-beta/Smad targets.