The estrogen receptor α: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 alpha (ER alpha) and insulin-like growth factor 1 (IGF-1) in breast cancer cells. Previously, we demonstrated that a fraction of IRS-1 binds ER alpha, translocates to the nucleus, and modulates ER alpha-dependent transcription at estrogen response elements (ERE). Here, we studied structure-function relationships of the ER alpha: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 ER alpha/IRS-1 interactions. IRS-1 binding to ERE and IRS-1 role in ER alpha-dependent ERE transcription was examined by chromatin immunoprecipitation and gene reporter analysis, respectively. The requirement for IRS-1 in ER alpha function was tested with RNA technology. Results: Nuclear translocation of IRS-1 was induced by E2, IGF-1, and a combination of both stimuli. ER alpha/IRS-1 binding was direct and involved the activation function-1 (AF-1)/DNA binding domain (DBD) region of ER alpha 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 ER alpha, but induced the activity of liganded ER alpha. Conclusions: ER alpha/IRS-1 interactions are direct and involve the ER alpha AF-1/DBD domain and IRS-1 domains mapping within N- and C-terminus. IRS-1 may act as a repressor of liganded ER alpha and coactivator of unliganded ER alpha. Key words: estrogen receptor alpha (ERa), Insulin receptor substrate 1 (IRS-1), breast cancer