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
