Endogenously expressed estrogen receptor and coactivator AIB1 interact in MCF-7 human breast cancer cells

Coactivators are believed to mediate estrogen-induced gene responses via in teraction with estrogen receptors (ER), Currently, a major challenge is to determine the importance of each coactivator in a specific cell type and pr omoter context in response to a particular ligand, The potential of ER to i nteract with a growing list of coactivators has been shown in a variety of in vitro and gene transfer assays, yet very few data have demonstrated the interaction of endogenous coactivators with ER in intact cells. We report h ere a ligand-specific interaction of endogenous human ER (hER) and the AIB1 coactivator in MCF-7 human breast cancer cells by using immunoprecipitatio n analyses. Complexes between endogenously expressed hER and AIB1 were dete cted in estradiol-treated cells and to a much lesser extent in cells treate d with the partial agonist, monohydroxytamoxifen. We were unable to detect an hER-SRC-1 complex in our immunoprecipitations from MCF-7 cells. The in v itro-binding affinity for mouse ER interaction with AIB1 was estimated to b e 40-120 nM. We conclude that AIB1 is a major coactivator for hER in MCF-7 human breast cancer cells.