Association of steroid receptor coactivator AIB1 with estrogen receptor-alpha in breast cancer cells

The steroid receptor coactivator AIB1 (amplified in breast cancer-1) is a t ranscriptional coactivator which has been found to be amplified in breast c ancer. We have now investigated the role of the AIB1 protein in breast canc er cell lines. Although detectable levels of AIB1 were present in most cell lines, high levels of AIB1 expression were observed only in the ER-positiv e cell lines MCF-7 and BT-474 by western blot analysis. Newly developed mon oclonal antibodies (mAbs) were used in several assays to show an associatio n between AIB1 and estrogen receptor-alpha (ER). AIB1 and ER co-localized t o the nucleus of ER positive cell lines as shown by immunofluorescence micr oscopy, and a functional association of native AIB1 and ER in MCF-7 nuclear extracts was shown by EMSA. Recombinant ER also recruited AIB1 protein fro m nuclear extracts, shown by EMSA and by precipitation of ER-complex protei ns bound to a biotinylated-ERE DNA target. Additionally, anti-AIB1 mAbs wer e able to immunoprecipitate ER from nuclear extracts of chemically cross-li nked cells but not from uncross-linked cells. Both immunoprecipitation and oligonucleotide precipitation studies demonstrated the presence of p300 and CBP as part of the ER transcriptional complex. These results suggest that AIB1 and ER do associate physically in ER-positive breast cancer cell lines . We propose that in AIB1 amplified breast cancers, a heightened AIB1/ER as sociation may play a crucial role in the progression of these tumors.