Differential activation of wild-type estrogen receptor α and C-terminal deletion mutants by estrogens, antiestrogens and xenoestrogens in breast cancer cells

17β-Estradiol (E2), diethylstilbestrol (DES) and several synthetic (or xenoestrogenic) compounds induced transactivation in MCF-7 or MDA-MB-231 cells transfected with wild-type estrogen receptor α (ERα) and a construct (pERE₃) containing three tandem estrogen responsive elements (EREs) linked to a luciferase gene. In contrast, the antiestrogens ICI 182,780 and 4-hydroxytamoxifen (4-OHT) were inactive in this assay. We have investigated the effects of these compounds and several structurally-diverse estrogenic compounds on transactivation in cells transfected with pERE₃ and wild-type ERα, mutant ERα (1-553), and ERα (1-537) containing deletions of amino acids 595-554 and 595-538, respectively. These constructs were used to develop an in vitro assay to distinguish between different structural classes of estrogenic compounds. The results obtained using these constructs were highly cell context- and structure-dependent. Neither E2- nor diethylstilbestrol-induced transactivation in MCF-7 (or MDA-MB-231) cells transfected with pERE₃/ERα (1-537) due to partial deletion of helix 12; however, octylphenol and nonlylphenol, resveratrol (a phytoestrogen), kepone and 2′,3′,4′,5′-tetrachloro-4-biphenylol were "estrogenic" in MCF-7 cells transfected with pERE₃/ERα (1-537). Moreover, the structure-dependent estrogenic activities of several synthetic estrogens (xenoestrogens) in MDA-MB-231 cells were different than those observed in MCF-7 cells. These results demonstrate that the estrogenic activity of many synthetic compounds do not require activation function 2 (AF-2) of ERα and are mechanistically different from E2. These data suggest that xenoestrogens are selective ER modulators (SERMs).