The estrogen receptor alpha:insulin receptor substrate 1 complex in breast cancer: structure-function relationships

Background: Insulin receptor substrate 1 (IRS-1) is a signaling molecule that exerts a key role in mediating cross talk between estrogen receptor a (ERa) and insulin-like growth factor 1 (IGF-1) in breast cancer cells. Previously, we demonstrated that a fraction of IRS-1 binds ERa, translocates to the nucleus, and modulates ERa-dependent transcription at estrogen response elements (ERE). Here, we studied structure–function relationships of the ERa:IRS-1 complex under IGF-1 and/or estradiol (E2) stimulation. Materials and methods: ERa and IRS-1 deletion mutants were used to analyze structural and functional ERa/IRS-1 interactions. IRS-1 binding to ERE and IRS-1 role in ERa-dependent ERE transcription was examined by chromatin immunoprecipitation and gene reporter analysis, respectively. The requirement for IRS-1 in ERa function was tested with RNAi technology. Results: Nuclear translocation of IRS-1 was induced by E2, IGF-1, and a combination of both stimuli. ERa/IRS-1 binding was direct and involved the activation function-1 (AF-1)/DNA binding domain (DBD) region of ERa and two discrete regions of IRS-1 (the N-terminal pleckstrin homology domain and a region within the C-terminus). IRS-1 knock down abrogated IGF-1-dependent transcriptional activity of unliganded ERa, but induced the activity of liganded ERa. Conclusions: ERa/IRS-1 interactions are direct and involve the ERa AF-1/DBD domain and IRS-1 domains mapping within N- and C-terminus. IRS-1 may act as a repressor of liganded ERa and coactivator of unliganded ERa.