The stability of the uterine estrogen receptor when complexed with estrogens or antiestrogens

As probes for possible conformational differences in estrogen receptor complexes when liganded with estrogen agonists or estrogen antagonists (antiestrogens), we have examined ligand-mediated thermal stability and resistance to trypsin-mediated proteolysis of uterine receptors occupied by a variety of steroidal and nonsteroidal estrogens and antiestrogens displaying a wide range of binding affinities for receptor. The estrogens examined (with chemical names and relative binding affinities in parentheses) were diethylstilbestrol (α,α'-diethyl-4,4'-stilbenediol (126%)), estradiol-17β (estra-1,3,5(10)-triene-3,17β-diol (100%)), 11β-methoxy-17-α-ethynylestradiol, R2858 (11β-methoxy-17α-ethynyl-estra-1,3,5(10)triene-3,17β-diol (65%)), zearalanol, P-1496 (6-(6,10-dihydroxyundecyl)-β-resorcylic acid μ-lactone (14%)), estriol (estra-1,3,5(10)-triene-3,16α,17β-triol (19%)), estrone (3-hydroxy-estra-1,3,5(10)-triene-17-one (15%)), and estradiol-17α (estra-1,3,5(10)-triene-3,17α-diol (5%)); the antiestrogens examined were CI-628 demethylated (α-[4-pyrrolidinoethoxy]phenyl4-hydroxy-α'-nitrostilbene (70%)), CI-628 (α-[4-pyrrolidinoethoxy]-phenyl-4-methoxy-α'-nitrostilbene (5%)), 11α-methoxyethynylestradiol, RU16117 (11α-methoxy-17α-ethynylestra-1,3,5(10)-triene-3,17β-diol (4%)), tamoxifen (trans-1 (p-dimethylaminoethoxyphenyl)-1,2-diphenyl-1-butene (5%)), and U23,469 ((±)-cis-3-[p-(1,2,3,4-tetrahydro-6-methoxy-2-phenyl-1-naphthyl)phenoxy]-1,2-propanediol (0.3%)). Ligand-mediated thermal stability and resistance to tryptic proteolysis were observed to be closely related to binding affinity for receptor and not to whether the ligand was an agonist or antagonist. Consequently, while the receptor may distinguish an agonist from an antagonist by some resulting conformational perturbation that is ultimately translated into different biological responses, this change is not manifested in any characteristic fashion by ligand-mediated thermal stability or protection against tryptic proteolysis.