A critical mechanism that has been proposed for transcription regulation by estrogen receptor α (ER) is the tethering of ER to DNA via other transcription factors, such as AP-1. However, genome-wide assessment of the overlap in chromatin binding repertoires of these two transcription factors has not been reported. Here, we show that the AP-1 transcription factor c-Jun interacts with ER and that c-Jun chromatin binding shows extensive overlap with ER binding at the global level. Further, we show that c-Jun overexpression reprograms ER chromatin binding and modulates ER-mediated gene regulation. Our data are consistent with a mechanism where estrogen/ER-dependent crosstalk with AP-1 at the transcriptional level is mediated through the tethering of ER to DNA bound AP-1. Additionally, in our system c-Jun overexpression causes reduced sensitivity to tamoxifen in ER+ breast cancer cells. Integrated cistrome, transcriptome, and clinical data reveal TGFBI as a candidate gene which may confer tamoxifen resistance by ER and AP-1 crosstalk. Further, we show that TGFBI expression is elevated in breast cancer compared to normal breast. Together, our data provide a novel genome-wide footprint of ER and AP-1 crosstalk and suggest AP-1 and TGFBI signaling as potential therapeutic targets in AP-1-overexpressing ER-positive breast tumors.
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