Estrogen receptors and growth response in cultured human periodontal ligament cells

The periodontal ligament (PDL) is a connective tissue involved in the remodeling process associated with tooth development and positioning. PDL cells grown in culture were analyzed for the capacity to specifically bind steroid hormones and for growth response to estradiol-17 beta. Using [3H]estradiol-17 beta as the ligand, PDL cells in first passage cultures exhibited a specific estrogen binding capacity of 881 fmol/mg cell protein. With [3H]dexamethasone as a ligand, the binding capacity of the glucocorticoid receptor was 143 fmol/mg protein. With [3H]R5020 as a ligand, the progestin receptor exhibited a binding capacity of 5 pmol/mg protein. Scatchard analysis of estradiol binding at 37 degrees revealed a dissociation constant of 2.7 X 10(-9) M, representative of the estrogen receptor. The addition of estradiol-17 beta at concentrations of 10(-9) and 10(-8) M to culture media induced a dose-dependent decrease in growth (DNA content) to 62% and 38% control values, respectively. The addition of the antiestrogens tamoxifen and 4-hydroxytamoxifen at concentrations of 10(-7) and 10(-6) M similarly depressed cell growth. These results show that PDL cells contain high affinity receptors for several steroid hormones and further that these cells are targets for the action of estrogens.     

Specific progesterone receptors in dimethylbenzanthracene (DMBA)-induced mammary tumors.

Properties of a progesterone receptor present in the cytosol (105,000 xg supernatant) of dimethylbenzanthracene (DMBA)-induced mammary tumors were studied using the highly potent progestin [3H]R 5020 (17, 21-dimethyl-19-nor-pregna-4, 9-diene-3,20-dione). As shown by sucrose gradient analysis, specific binding of [3H] R 5020 is associated with components migrating at 7-8S and 4S. Low affinity binding of the synthetic progestin is eliminated by treatment with dextran-coated charcoal. [3H] R 5020 binding is highly progestin-specific since it is easily displaced by unlabelled norgestrel, R 5020 and progesterone while estradiol-17beta, dihydrotestosterone, testosterone, testosterone and diethylstilbestrol have much lower activity. Dexamethasone and cortisol have little, if any, effect on [3H] R 5020 binding.     

Estrogens augment the stimulation of ovarian aromatase activity by follicle-stimulating hormone in cultured rat granulosa cells

The effects of estrogens on ovarian aromatase activity were investigated in vitro using granulosa cells from immature hypophysectomized estrogen-primed rats. The cells were cultured for 3 days in an androgen-free medium in the presence of follicle-stimulating hormone (FSH), with or without the specified estrogen. After washing, the cells were reincubated for 5 h with 10(-7) M androstenedione, and the formation of estrogens was measured. Estrogen production by control and diethylstilbestrol-treated cells was negligible, while FSH stimulated aromatase activity. Furthermore, concomitant treatment with diethylstilbestrol led to dose-dependent increases in the FSH-induced aromatase activity with an ED50 value of 4 X 10(-9) M and an apparent Vmax value 12- to 16-fold higher than those induced by FSH alone. The direct stimulatory effect of estrogens was time-dependent and was not accounted for by increases in cell protein. Various native and synthetic estrogens also augmented the FSH induction of aromatases (native estrogens: estradiol-17 beta = estrone greater than estradiol-17 alpha greater than estriol; synthetic estrogens: hexestrol greater than moxestrol greater than ethinyl estradiol much greater than chlorotrianisene and mestranol). The effect of estradiol-17 beta was dose-dependent with an ED50 value of 9 X 10(-9) M, which is within the physiological levels of follicular estradiol-17 beta. Although treatment with androgens also enhanced the FSH-induced aromatases, treatment with a progestin (R5020) or a mineralocorticoid (aldosterone) was without effect. Thus, estrogens directly augment the stimulation of granulosa cell aromatase activity by FSH. Follicular estrogens may activate intraovarian autoregulatory positive feedback mechanisms to enhance their own production, resulting in selective follicle maturation and the preovulatory estrogen surge.     

Structure-activity relationships of estrogens. Effects of 14-dehydrogenation and axial methyl groups at C-7, C-9 and C-11

Thirty compounds were evaluated in the rat for uterotropic effects, inhibition of gonadotropin release, and competitive displacement of (3H) estradiol-17 beta from uterine cytosolic preparations. 7 alpha-Methylestradiol-17 beta was 150% as active as estradiol-17 beta as an uterotropic agent. Estradiol-17 beta was the most active inhibitor of gonadotropin release. 11 beta-Methylestradiol-17 beta had 124% of the activity of estradiol-17 beta in displacing (3H) estradiol-17 beta from the "estrogen receptor." The 9 alpha-methyl group considerably decreased the potency of estrogens in any of the three assays. The 14-dehydro modification was advantageous only in the estradiol-17 beta 3-methyl ether series. Uterotropic activities and inhibition of gonadotropin release did not parallel. The best compound for inhibiting gonadotropin release, as compared to uterotropic activity, was estrone. The "estrogen receptor" assay data correlated fairly well with uterotropic assay data, but only for compounds having free 3-hydroxyl groups; even so, some exceptions were noted.     

Nongenomic inhibition of oxytocin binding by progesterone in the ovine uterus

Progesterone (P4) has been reported to inhibit oxytocin (OT) binding to its receptor in isolated murine endometrial membranes. The purpose of the present research was to 1). examine the in vivo and in vitro effect of P4 on the binding of OT to its receptor in the ovine endometrium and 2). determine whether the endometrial plasma membranes have high-affinity binding sites for P4. Ovariectomized ewes were pretreated with a sequence of estradiol-17beta (2 days) and P4 (5 days) before being treated with estradiol-17beta plus either vehicle (corn oil), P4, or P4 + mifepristone (RU 486) for 3 consecutive days. Treatment of ewes with 10 mg P4/day for 3 days suppressed binding of OT (P < 0.01) compared with that of controls, whereas concomitant treatment with the progestin antagonist RU 486 (10 mg/day) blocked the effect of P4. Similarly, incubation of endometrial plasma membranes with P4 (5 ng/ml) inhibited binding of OT (P < 0.05), whereas this effect of P4 was blocked by the presence of RU 486 (10 ng/ml). By radioreceptor assay, the endometrial plasma membranes were found to contain a high-affinity binding site for P4 and the progestin agonist promegestone (Kd 1.2 x 10-9 and 1.74 x 10-10M, respectively). Incubation of endometrial plasma membranes with P4 (5 ng/ml) significantly increased the concentration of progestin binding sites. Binding of labeled promegestone (R 5020) was competitively inhibited by excess unlabeled R 5020, P4, RU 486, and OT but not by estradiol-17beta, cortisol, testosterone, and arginine vasopressin. These data suggest a direct suppressive action of P4 on the binding of OT to OT receptors in the ovine endometrial plasma membrane.     

Binding of 2-hydroxyestradiol and 4-hydroxyestradiol to estrogen receptors from human breast cancers

The binding of catechol estrogens, epoxyenones and methoxyestrogens was evaluated using estrogen receptors in cytosol prepared from human breast cancers. The relative affinity of 2-hydroxyestradiol, a metabolite formed in vitro from estradiol-17 beta by breast cancer cells, was indistinguishable from that of estradiol-17 beta. 4-Hydroxyestradiol, which is also a metabolite of estradiol-17 beta, associated with the estrogen receptor with a relative affinity approximately 1.5-fold greater than that of estradiol-17 beta. Epoxyenones and methoxyestrogens were weak competitors compared to the binding of estradiol-17 beta, exhibiting relative affinities 3% or less than the affinity of estradiol-17 beta. Sucrose density gradient centrifugation revealed that both 2- and 4-hydroxyestradiol inhibited the binding of estradiol-17 beta to both the 4S and 8S isoforms of the estrogen receptor in a competitive manner, with a Ki = 0.94 nM for 2-hydroxyestradiol and a Ki = 0.48 nM for 4-hydroxyestradiol. It can be concluded that these data demonstrate a specific receptor-mediated estrogenic action for both of these catechol estrogens.     

Characterization of estrogen binding sites in the liver of the newt Pleurodeles waltl (Urodela:Amphibia): demonstration of a sex-linked heterogeneity

Saturation analysis over a wide range of [3H]estradiol-17 beta concentrations (1-40 nM) in cytosols prepared from liver of the newt Pleurodeles waltl of both sexes revealed a sex-linked heterogeneity of the estradiol-17 beta binding sites. In females, one type of binding site has been identified as a classical receptor. It exhibited a high affinity for estradiol-17 beta (Kd = 9 X 10(-9) M), had a high specificity for estrogenic compounds and was stabilized by monothioglycerol. In males, in addition to the receptor found in females, a second estrogen binding component was detected, not observed in female cytosols. It exhibited a Kd of 4.8 X 10(-8) M for estradiol 17 beta, higher capacity and displayed the same highly specific estrogen binding as does the estrogen receptor. It was affected by monothioglycerol and its binding was found to be significantly increased on cytosol dilution, as well as by estrogen-treatment.     

HPLC separation of anti-estrogen and estrogen receptor binding components of mouse plasma

Proestrous mouse plasma and urine were subjected to diethyl ether extraction, enzyme hydrolysis and HPLC separation of estrogen components. Radioimmunoassay of the treated proestrous samples with a broad spectrum anti-estrogen serum failed to detect estradiol-17 beta, estrone or estriol. HPLC chromatograms contained two peaks of immunoreactive and estrogen receptor binding material with polarities between those of estriol and estradiol-17 beta. Similar peaks were detected in HPLC chromatograms of urinary extracts from ovariectomized and ovariectomized-adrenalectomized mice. The least polar of the two peaks produced a mass spectrum identical to that of authentic equol [7-hydroxy-3-(4'-hydroxyphenyl)chroman], a phytoestrogen metabolite. The presence of significant quantities of circulating equol in all strains studied, combined with apparently low plasma levels of endogenous classical estrogens during proestrus, confound attempts to study estrogen secretion in the mouse.     

Cytoplasmic progesterone receptors in the hypothalamus-preoptic area of the mouse: effect of estrogen priming

A synthetic progestin, R5020, was used to identify cytoplasmic progestin receptors in the hypothalamuspreoptic area (HPOA) of ovariectomized mice. These high-affinity receptors exhibited an apparent dissociation constant of approx. 1 nM. The receptors were specific for progestins. [3H]R5020 binding was inhibited by more than 50% with a 50-fold excess of either radioinert R5020 or progesterone. 5 alpha-Dihydroprogesterone inhibited binding to a lesser extent. 3 alpha-Hydroxy-5 alpha-pregnane-20-one and cortisol did not compete for [3H]R5020 binding. Administration of estradiol benzoate (10 micrograms), 48 h prior to death, resulted in a 54% increase in the HPOA progestin receptor concentration when compared to oil-injected controls. These data demonstrate that there are specific and saturable cytoplasmic progestin receptors in the mouse HPOA and that the concentration of these receptors is increased after estrogen treatment.     

Boron estrogens: Synthesis,biochemical and biological testing of estrone and estradiol-17β 3-carboranylmethyl ethers

Synthesis, biochemical and biological testing of the first carborane derivatives of estrogens are described. Estrone 3-carboranylmethyl ether was synthesized in two steps from estrone. Reduction of estrone 3-carboranylmethyl ether with sodium borohydride provided estradiol-17β 3-carboranylmethyl ether. Enzyme kinetic measurements showed that estrone 3-carboranylmethyl ether is a substrate for human placental 17β-hydroxy-steroid dehydrogenase with Km = 5×10^?6M, and Vmax = 0.016 μmol min^?1 μg^?1. The relative affinity constant of estradiol-17β 3-carboranylmethyl ether for rat uterine estrogen receptor was 0.5 (compared with a value of 100 for estradiol-17β). Consistent with its low affinity for estrogen receptor, the dose-dependent uterotropic response to estradiol-17β 3-carboranylmethyl ether in castrated female rats was one sixtieth that of estradiol-17β. None of the tested rats had a toxic reaction to estradiol-17β 3-carboranylmethyl ether. These results demonstrate that exceptionally stable carborane derivatives of estrogens can be synthesized with preservation of their biochemical and biological properties. Boron-containing estrogens may be useful for thermal neutron capture therapy of cancers with estrogen receptors to concentrate boron in the cell nucleus.     

Specific progesterone receptors in human breast cancer.

We have identified a specific progesterone receptor in 11 of 33 human breast cancer cytosols. Since progesterone itself binds to glucocorticoid receptor, to corticosteroid binding globulin (CBG), and to nonspecific components as well as to its own receptor, we have used a synthetic progestin, R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione), whose binding specificity is restricted to progesterone receptor. Bound R5020 sediments at 8 S in sucrose gradients; binding is competed by excess unlabeled R5020 or progesterone. The receptor is distinct from glucocorticoid receptor and CBG as determined by competition studies using dexamethasone and hydrocortisone. The dissociation constant for R5020 obtained by Scatchard analysis of dextran-coated charcoal assays is approximately 2 times 10- minus 9 M.     

Synthesis and study of the estrogenic activity of 2,4-bis(bromomethyl)estradiol-17 beta 3-methyl ether.

Synthesis of 2,4-bis(bromomethyl)estradiol-17 beta 3-methyl ether (BBE2M) was accomplished by reducing a methanolic solution of 2,4-bis(bromomethyl)estrone methyl ether with sodium borohydride. In 0.5 M phosphate buffer, pH 7.0, 25 degrees, BBE2M readily reacts with Ellman's anion and alkylates cysteine to form a steroid-amino acid conjugate. Stoichiometry of the reaction indicates that the bromosteroid is divalent with cysteine. Tryptophan and histidine react more slowly with the bromosteroid. Estrogenic activity of BBE2M was evaluated in ovariectomized rats by uterine intraluminal administration and quantitation of glucose-6-phosphate dehydrogenase (D-glucose-6-P:NADP+ oxidoreductase, EC 1.1.1.49) activity in the uterus. BBE2M induced glucose-6-phosphate dehydrogenase activity as did estradiol-17 beta or estradiol-17 beta 3-methyl ether (E2M). BBE2M was more persistent in activity than E2M. Histological examination of uterus following BBE2M treatment shows classic estrogenic morphology. BBE2M covalently binds to the cytoplasmic estrogen receptor of calf uterus. Such binding is prevented by pretreatment of the receptor protein with estradiol-17 beta. The covalently bound steroid-receptor complex appears to stimulate RNA synthesis in isolated nuclei from calf endometrium.     

Radioautographic study of the effect of estradiol-17 , estrone, estriol, progesterone, testosterone and corticosterone on the in vitro uptake of 2,4,6,7- 3 H estradiol-17 by uterine eosinophils of the rat

The in vitro uptake of 2,4,6,7-tritiated estradiol-17beta in uterine eosinophils of the rat was inhibited by the presence of nonradioactive estradiol-17beta, estrone, and estriol, but not by progesterone, testosterone, or corticosterone. This action is attributed to competition between tritiated estradiol and the various estrogenic compounds for the same binding site. Compounds without any estrogenic activity do not compete. The proposal is made that the eosinophil binding system and the 8S-5S binding system are involved in different mechanisms of estrogen action. The parallelism between the doses of estradiol and estriol needed to promote certain estrogenic early effects in the uterus, and the affinity of these steroids for the eosinophil uptake sites, suggests that uterine eosinophils might be responsible for some of these early effects, such as water imbibition, histamine releasing activity, and estrogen priming effect.     

Regulation of uterine progesterone receptors by the nonsteroidal anti-androgen hydroxyflutamide

We have recently reported that the anti-androgen hydroxyflutamide causes delayed implantation and exhibits antideciduogenic activity in the rat. The present experiments were conducted to examine whether hydroxyflutamide binds to the uterine progesterone receptors and/or alters the progesterone binding sites in the uterus. Cytosol and nuclear fractions from decidualized rat uterus were incubated with [3H]-R5020 without or with increasing concentrations of radioinert R5020, RU486, dihydrotestosterone, or hydroxyflutamide. From the log-dose inhibition curves, the relative binding affinity of both hydroxyflutamide and dihydrotestosterone was less than 0.1% and 2%, compared with R5020 (100%) for displacing [3H]-R5020 bound to uterine cytosol and nuclear fractions, respectively. Injection of estradiol-17 beta (1 microgram/rat) to ovariectomized prepubertal rats induced a 1.85-fold increase in uterine weight by 24 h. Hydroxyflutamide at 2.5 or 5.0 mg did not significantly alter the estrogen-induced increase in uterine weight. Compared to vehicle alone, estrogen induced an approximately 5-fold increase in uterine cytosolic progesterone binding sites. Hydroxyflutamide at both 2.5- and 5.0-mg doses significantly attenuated the estrogen-induced elevation in uterine progesterone binding sites. These studies demonstrate that hydroxyflutamide does not bind with high affinity to progesterone receptors, but suppresses the estrogen-induced elevation in progesterone receptor levels in the uterus.     

Properties of estrogen and hydroxysteroid sulphotransferases in human mammary cancer

Partial purification (approximately x 140-fold) of estrogen sulphotransferase (EC 2.8.2.4) in human mammary estrogen receptor positive cancer tissue was achieved by affinity chromatography on adenosine-3',5'-diphosphate-agarose. It had a Mr of approximately 70,000 by gel filtration and upon electrophoresis on concave gradient polyacrylamide gels, showed a major (Mr 70,000) and a minor (Mr 200,000) peak of activity. Kinetics of this preparation (estradiol-17 beta and estrone as substrates), and also that of hydroxysteroid sulphotransferase (EC 2.8.2.2) contained in the cytosol of human mammary cancer MCF-7 cells (5-androstene-3 beta,17 beta-diol and dehydroepiandrosterone as substrates), were compared. The enzymes showed very similar behaviour, characterized by high affinity for their steroid substrates (low nM range) and co-operativity in their binding. For hydroxysteroid sulphotransferase, the adrenal-derived estrogen 5-androstene-3 beta,17 beta-diol was the preferred substrate compared to dehydro-epiandrosterone in the 0-40 nM concentration range. Such properties of the enzymes might be designed to limit the exposure of nuclear receptor to free ligand. Alternatively, a defined subcellular location would perhaps involve the enzymes in the elimination of estrogen after processing of the ligand-bound receptor.     

MCF-7; a human breast cancer cell line with estrogen, androgen, progesterone, and glucocorticoid receptors.

We have identified receptors for glucocorticoids, progestins, and androgens in a human breast tumor cell line (MCF-7) known to have estrogen receptor. Sucrose density gradients show that MCF-7 cytosol contains approximately 100 fm/mg protein estradiol (E2-3H) receptor, more than 300 fm/mg protein progesterone receptor (measured with R5020-3H), about 40 fm/mg protein 5alpha-dihydrotestosterone (5alpha-DHT-3H) receptor, and 800 fm/mg glucocorticoid receptor (measured with dexamethasone-3H). Dissociation constants obtained by Scatchard analyses were approximately 0.6 x 10(-10)M (E2), 1 x 10(-9)M (R5020), 2.8 x 10(-10)M (5alpha-DHT) and 8 x 10(-9)M (dexamethasone). No cross competition was found for estrogen receptor, but progestins competed for androgen and glucocorticoid binding. The androgen, but not the glucocorticoid, partially competed for R5020 binding to progesterone receptor. This first demonstration of 4 classes of steroid receptors in human breast cancer means that MCF-7 may be an excellent in vitro model for studying the mechanism of tumor response to endocrine therapy as well as the complex relationships between binding and biological actions of these hormones.     

Interaction of progestins with steroid receptors in human uterus

Norethindrone (17β-hydroxy-19-nor-17α-pregn-4-en-20-yn-3-one) and norethindrone acetate (17β-acetoxy-19-nor-17α-pregn-4-en-20-yn-3-one) interfered to a varying degree, by competitive inhibition, with the binding of progesterone and oestradiol to respective cytoplasmic receptors in the human uterus. Progesterone binding to 4S macromolecule was saturable and co-specific for progestins. Competitors like norgestrel (17β-hydroxy-18-methyl-19-nor-17α-pregn-4-en-20-yn-3-one), 19-norprogesterone, medroxyprogesterone acetate (17α-acetoxy-6α-methylpregn-4-ene-3,20-dione) and compound R₅₀₂₀ (17,21-dimethyl-19-norpregna-4,9-diene-3,20-dione) possessed higher binding affinities for the progestin receptor. The dissociation constant (K_d) for the progesterone–receptor interaction was 0.6–1.6nm and the receptor concentration ranged between 6600 and 8200 sites/cell. Norethindrone and norethindrone acetate competed for the progesterone receptor with inhibition constants (K_i) of 6.8 and 72nm respectively. Gradient displacement and competitive-receptor assays indicated that norethindrone acetate-binding affinity for progestin receptor was approximately one-tenth that of norethindrone and progesterone. The progestins also inhibited oestradiol binding to 4.6S oestrogenic receptor by 8–12%, involving interaction at the oestradiol-binding site with a calculated K_i value of 0.5–0.8μm. The competitive interaction of progestins with steroid receptors may be of putative importance in explaining the progestin action at the target site.     

Progestin binding in mammary tissue of prepartum,nonlactating and postpartum,lactating cows

Binding of [³H]R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3, 20-dione) to bovine mammary cytosol indicated the presence of progestin binding sites of high-affinity and low-capacity in tissue from prepartum, nonlactating and from postpartum, lactating cows. To prevent binding of [³H]R5020 to glucocorticoid binding sites, a 200-fold molar excess of nonradioactive cortisol was included during all incubations, thus specific binding was limited to progestin binding sites. Nonradioactive R5020 and progesterone effectively inhibited [³H]R5020 binding to progestin binding sites, while estradiol-17β, dihydrotestosterone (17β-hydroxy-5α-androstan-3-one), dexamethasone (9-fluoro-11β, 17, 21-trihydroxy-16α-methyl-1,4-pregnadiene-3,20-dione) or additional cortisol were ineffective. Dissociation constants for specifically bound [³H]R5020 in cytosol from mammary tissue of nonlactating and lactating cows were nearly identical, averaging 1.9 ( ± 0.3) and 0.8( ± 0.2) × 10?⁹M, respectively. However, binding capacities (fmol/mg cytosolic protein) were greater in cytosol from prepartum, nonlactating (179 ± 53) than postpartum, lactating (41 ± 15) cows. Specific binding components in cytosol from lactating cows sedimented in the 6-7S region on linear sucrose density gradients. When subjected to isoelectric focusing, specific binders with isoelectric points (pI) of approximately 6.1, 7.9 and 8.3 were resolved. The decrease in number of binding sites during lactation was due to the virtual absence of the anionic binding species, suggesting that their presence is necessary for progesterone to inhibit milk secretion.     

Steroid levels after intramuscular injection of radioactive estradiol-17beta, estrone, progesterone and testosterone in the bovine

Eight 2 year old Hereford cows from days 8 to 12 of the estrous cycle were injected intramuscularly with 5 ml of corn oil containing 5 mg of estradiol-17beta (two cows), estrone (two cows), progesterone (two cows) or testosterone (two cows). Each cow treated with estradiol received 494 microc of estradiol-17beta-6, 7 H3 and each cow treated with estrone received 492 microc of estrone-6, 7 H3. Each cow treated with progesterone or testosterone received 400 muc of H3 compound labeled in the 7 position. Total urine was collected by urethral catheterization of the cows treated with estrogens. Blood samples for plasma and serum were collected via jugular cannulae. Blood and urine samples from estrogen-treated cows were collected hourly for the first 24 hr, at 2 hr intervals for the next 26 hr, at 4 hr intervals for the next 12 hr and at 12 hr intervals until background was reached. Blood samples were collected hourly from 1 to 8 hr after injection from progesterone or testosterone-treated cows. Plasma and serum levels of radioactive estradiol-17beta, estrone, progesterone and testosterone were similar. Blood levels of radioactivity peaked at 2 hr post-injection in cows receiving estradiol-17beta and at 3 hr in cows receiving estrone. Blood levels of labeled estradiol-17beta and estrone were nondetectable by 54 hr and 83 hr, respectively. Peak urinary excretion of radioactivity was reached at 7 hr for estradiol-17beta and at 14 hr for estrone and nondetectable levels were reached by 95 hr for estradiol-17beta and 14 hr for estrone. At these times, 15.5% of the total dose of radioactive estradiol-17beta and 17.5% of the injected estrone had been excreted in the urine. Peak blood and urinary excretion levels were reached earlier for radioactive estradiol-17beta than for estrone, and excretion of estradiol-17beta was completed more rapidly. No difference was found in plasma and serum levels for any steroid studies; thus, endogenous steroid titers in blood plasma and serum are not different in the cow.     

Binding of [3H] methyltrienolone (R 1881) in rat prostate and human benign prostatic hypertrophy (BPH).

Methyltrienolone (R 1881 - 17beta-hydroxy-17alpha-methyl-estra-4, 9, 11-trien-3-one) binding to rat ventral prostate cytosol has a specificity typical of an androgen receptor. In human benign prostatic hypertrophy (BPH) tissue, the specificity of [3H] R 1881 binding is different from that measured in rat prostate: progesterone and R 5020 (17, 21-dimethyl-19-nor-4, 9-pregnadiene-3, 20-dione) being more potent while 19-nortestosterone is less potent competitor. Moreover, the synthetic progestin [3H] R 5020 binds to BPH tissue with a similar specificity. These data suggest the presence of progestin binding components or of an atypical androgen receptor in human BPH cytosol.