ACTIVATION OF TRANSCRIPTIONALLY INACTIVE HUMAN ESTROGEN-RECEPTORS BY CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE AND LIGANDS INCLUDING ANTIESTROGENS

We show that some transcriptionally inactive human estrogen receptor ( ER) mutants can be activated by 17 beta-estradiol (E(2)), and sometime s by antiestrogens, in the presence of elevated levels of intracellula r cAMP. ER-deficient Chinese hamster ovary or 3T3 mouse fibroblast cel ls were transfected with mutant ERs (the point mutant L540Q, the frame shift mutant S554fs, or the carboxy-terminal truncated receptor ER1-53 0) and various estrogen response element-containing reporter genes. In dividual treatments with E(2), the antiestrogens trans-hydroxytamoxife n and ICI 164,384, or with 3-isobutyl-1-methyl-xanthine plus cholera t oxin (IBMX plus CT) which raise intracellular cAMP, generally do not a ctivate the mutant receptors. However, cotreatment with IBMX/CT and on e of the three ligands (E(2), trans-hydroxytamoxifen, or ICI 164,384) results in the unexpected recovery of strong activation of the L540Q o r S554fs receptors, the magnitude of which is dependent upon promoter- and cell-contexts. Unlike L540Q and S554fs, the transcriptionally ina ctive ER1-530 is not activated by any combination of ligands and IBMX/ CT. These data demonstrate that some ER mutants that form transcriptio nally nonproductive ER-E(2) complexes can be successfully activated by the combination of an agonist or antagonist ligand and an agent thoug ht to act via phosphorylation pathways. Also highlighted is the promot er- and cell-specific nature of the transcriptional response to differ ent ligand-ER complexes. Lastly, the enhanced transcriptional activity of wild type ER and some ER mutants in the presence of antiestrogens and elevated intracellular cAMP may provide a partial explanation of t he ability of some estrogen-dependent human breast tumors to resist an tiestrogen therapies currently employed.