Obstruction of Estrogen-Receptor Complex Formation. Further Analysis of the Nature and Steroidal Specificity of the Effect¹²

Abstract

Qualitative and quantitative aspects of inhibition of 17β-estradiol-receptor complex formation by a number of natural and synthetic hormonal agents have been investigated in rat anterior pituitary cytosol. The initial velocity of the interaction between 17β-estradiol and its receptors is impeded by preincubation with androgenic compounds in a dose-related manner, the order of inhibitory effectiveness being: 3β-androstanediol > 3α-androstanediol = 5α-dihydrotestosterone > testosterone. Equivalence of 3α-androstanediol and 5α-dihydrotestosterone effectiveness can be accounted for by high cytoplasmic activity of 3α-hydroxysteroid dehydrogenase. Both initial velocity and inhibitory response to a given level of androgen are lower in the male than in female cytosol; the variation appears to be the consequence of different endogenous androgen levels. Weak androgens (dehydroepiandrosterone and etiocholanolone) and anti-androgens (flutamide, cyproterone and Ro7-8117) are effective inhibitors, but the degree of inhibition is not dose-dependent as with other androgens. Anti-estrogens effectively impede 17β-estradiol-receptor association in relation to their anti-estrogenic potency; thus, dimethylstilbestrol > enclomiphene > MER-25. CI-628 displays an inhibitory pattern suggestive of an affinity-labeling mechanism. Progesterone, cortisol and aldosterone are completely without effect on the association reaction. Co-incubation (i.e., addition of inhibitor and 17β-estradiol at the same time) is generally less effective than preincubation and, with 5α-dihydrotestosterone, an initial phase of competition for binding sites can be distinguished from subsequent displacement of androgen. Kinetic analysis firmly establishes the inhibition as being competitive for the androgens and for the weak estrogen, estriol. The latter was far more effective than the androgens tested (KD values of 1 nM, 17 nM and 72 nM were calculated for estriol, 5α-dihydrotestosterone and testosterone, respectively). The results demonstrate that impedance of normal estrogen-receptor complex formation can be effected by various types of compounds, all of which appear capable of interacting with the estrogen receptor to varying degrees.