Differential interactions of estrogens and antiestrogens at the 17β-hydroxy or counterpart function with the estrogen receptor

The action of diethylpyrocarbonate on lamb uterine estrogen receptor produced an homogeneous population of the receptor (∼ 55%) which still bound triarylethylene antiestrogens such as 4-hydroxytamoxifen with a high affinity but bound classical potent estrogens such as estradiol or diethylstilbestrol with a very low affinity. To specify the structural features of the ligands involved in the decrease of ligand affinity upon modification of the estrogen receptor, we determined the relative affinity constants of 17 steroidal estrogens or antiestrogens (deriving from estradiol by a 7α- or 11β-substitution) and 14 nonsteroidal estrogens or antiestrogens (all including the 1,2-trans-diphenylethylene structure of diethylstilbestrol) for native and diethylpyrocarbonate-modified estrogen receptors. Then the ratio of the relative affinity constant for the native receptor to that for the modified receptor (δ) was calculated for each ligand, to compare the variation in the affinity of the ligand upon modification of the receptor to that of 4-hydroxytamoxifen (δ= 1). The results showed that the strong decrease of ligand affinity upon modification of the receptor displayed by classical estrogens (δ≥ 200) is strictly dependent on the presence of the 17β-hydroxyl group in steroidal compounds or its α-4- and β-4-counterparts in diethylstilbestrol-related compounds. However, for the 7α- or 11β-derivatives of estradiol displaying potent antiestrogenic properties, the relative decrease in affinity was much more limited (δ≤ 19). For 11β-derivatives displaying a relative estrogenic activity weaker than that of estradiol itself, an average decrease in affinity was observed (23 ≤δ≤ 62). With the diethylstilbestrol-related compounds, bearing or not the α-4-hydroxyl and/or the β-4-hydroxy functions and showing either weak relative estrogenic or antiestrogenic properties, the relative variation in affinity was weak (0.6 ≤δ≤ 24). These results indicate that the interaction of 7α- or 11β-substituted steroidal antiestrogens and of 1,2-trans-diphenylethylene or triphenylethylene derivatives, displaying either weak relative estrogenic or antiestrogenic properties with the receptor, differs at the 17β-hydroxy or at the α-4-/β-4-hydroxy functions from that of potent estrogens. They suggest that the strong decrease in the relative affinity of ligands upon receptor modification may reflect the high efficiency of the ligands to activate the receptor properly. Consequently, the screening of 17β-hydroxylated steroids or diethylstilbestrol-related compounds with native and diethylpyrocarbonate-modified receptors could be a useful method for distinguishing between potent estrogens (δ > 200) and weak estrogens or antiestrogens (δ < 20).