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