Studies on the uterine, cytoplasmic estrogen binding protein. Thermal stability and ligand dissociation rate. An assay of empty and filled sites by exchange

Nonsteroidal antiestrogens: their biological effects and potential mechanisms of action

The uterotropic and antiuterotrapic effects of a variety of structural derivatives of the nonsteroidal antiestrogen tamoxifen have been determined in the rat and the mouse. One derivative, monohydroxytamoxifen, was found to be a potent antiestrogen in the rat, with a high affinity for the estrogen receptor. Various techniques of sucrose density gradient analysis were used to demonstrate that estradiol and tamoxifen bind to the rat uterine cytoplasmic estrogen receptor. Estrogens and antiestrogens provoke the translocation of estrogen receptors to the nucleus and deplete the cytoplasmic estrogen receptor pool for short or long periods depending on the dose administered. Estradiol stimulates endometrial hyperplasia with an increase in total uterine DNA content, whereas tamoxifen stimulates endometrial hypertrophy with only a slight increase in uterine DNA content. It is concluded that the molecular shape of the ligand that binds to the estrogen receptor determines antiestrogenic activity.

[Estrogen and progestogen cytosol receptors in human breast carcinoma (author's transl)]

The authors have studied Estrogen (RE) and Progestin (RP) cytosol receptors in 379 human breast carcinomas: 281 tumors suitable for surgery, 26 pseudo-inflammatory tumors, 52 metastases and 20 recurrences. They have used an exchange technique with estradiol for RE and a synthetic compound, R 5020, for RP. The results point out that high rates of RE correlate with post-menopausal women and high rates of RP with pre-menopausal women. Tumours are considered receptor-positive when the binding sites concentration exceeds 100 fmoles/g tissue. Using this as a base 32% of the tumors are RE and RP negative. Considering only the positive tumors, 54% contain both receptors, 31% only RE and 15% only RP. The percentages are also given in terms of the hormonal state of the patients. The results are discussed in terms of McGuire's hypothesis that both receptors are necessary to obtain a positive response to hormonal therapy. Correlation between histology and receptors is also discussed.

Estradiol-17beta receptors in the immature rat ovary

Estradiol binding components in the cytosol and nuclear fractions of the ovary from immature rats (22-28 days old) were characterized by in vitro methods. Several of the biochemical characteristics of the estradiol binding components in the ovarian tissue were compared with the estradiol receptor from the uterus. The results suggest that the ovarian estradiol binding components are similar to the specific high affinity estradiol receptors in the uterus. In the cytosol of intact rat ovary a significant fraction of the total binding sites was found to be occupied, presumably by the endogenous estrogen. Following hypophysectomy there was a significant increase in the available cytosol binding sites. Evidence for translocation of cytosol receptor-estrogen (RE) complex to the nucleus was obtained for the ovary. The sedimentation properties of the RE complex of the ovary and the uterus are similar. The ovarian cytosol RE complex sediments at 7-8S in glycerol gradients at low ionic strength and at 4S in sucrose gradients at high ionic strength. Following extraction with 0.4 M KCl the ovarain nuclear RE complex sediments at 5S in sucrose gradients which is identical to that of the uterine nuclear receptor.

Oestrogen receptors in relation to human breast cancer

Work leading to the concept of hormone responsiveness of human breast cancer and current views of the mode of action of oestrogens in target tissues are reviewed. Results of oestrogen receptor assays are considered in relation to clinical response to tumours to endocrine therapy and difficulties encountered in the interpretation of receptor determinations are discussed. Recent work on the distribution of different forms of the oestrogen receptors in human tumours and their relation to clinical response may lead to better selection of patients for endocrine therapy. Results of receptor assays in male patients with breast cancer, in benign breast tumours and in malignant tumours at sites other than breast are reviewed, and work on receptors for hormones other than oestrogens is discussed briefly. Further work is needed on the integrity of the receptor mechanism in tumour cells and on the effects of hormone treatment. Methods for the separation and estimation of oestrogen receptors in human tumour tissue are reviewed and discussed. Errors arising during collection, storage and analysis are considered. Different ways of calculating and expressing results of these assays are mentioned. It is suggested that more accurate determinations of oestrogen receptors may lead to better discrimination between hormone responsive and unresponsive human breast tumours.

The effect of ether on formation and dissociation of rat uterine estradiol-receptor complex

The addition of 2% v/v ethyl ether to uterine cytosol incubation mixtures produced a 25% increase in the rat of association of 0 degrees with 3H-estradiol and almost a 3-fold increase in the rate of exchange of estradiol-receptor complex with 3H-estradiol at 20 degrees. Uterine cytosol saturated with non-radioactive estradiol required over 8 hours for complete exchange with 3H-estradiol at 20 degrees while in the presence of 2% ether exchange was complete in 2 hours and revealed about 15% more specific binding sites. The presence of ether in reaction mixtures had no effect on dextran coated charcoal separation on bound from free 3H-estradiol nor on the stability of the receptor on uterine cytosol.

Affinity of estradiol mustard for estrogen receptors and its enzymatic degradation in uterine and breast cancer cytosols

Estradiol mustard (EM) is the 3,17beta-diester of estradiol-17beta (E2) with the nitrogen mustard derivative chlorphenacyl. The ability of EM to bind to cytoplasmic estrogen receptors was tested by inhibition of the binding of 3H-E2 to rat uterine cytosol at 18 degrees C and 30 degrees C. At both temperatures an inhibition curve was observed in the presence of a large excess of drug, suggesting that the latter has a very weak binding affinity (100,000 times lower than E2). Incubation of uterine cytosol with increasing amounts of 3H-E2 in the presence and absence of an excess of EM indicated that the drug interacted with the receptors at the same sites as E2 (competitive inhibition). Preincubation of uterine cytosol at 18 degrees C with EM induced a progressive reduction of 3H-E2 binding capacity. This reduction also occurred, although to a lesser extent, when long-term incubation of the cytosol with EM was performed in the presence of labelled E2 from the start. The process was faster at 18 degrees C than at 4 degrees C and did not occur with EM preincubated in homogenization buffer. Exchange assays by 3H-E2 of uterine receptors preincubated with labelled E2 and excess EM indicated that the drug-induced inhibition of binding capacity was reversible and produced no apparent alteration of the receptors. Furthermore, the rate of exchange was similar to that observed with receptors previously filled with unlabelled E2. In 9 "receptor-positive" cytosols from human breast cancers, time-course study of the binding of 3H-E2 in the presence of excess of EM yielded similar results as those obtained with rat uterine cytosol. These results show that EM has a very low binding affinity for the extrogen receptors and that it is metabolized into one or several compounds of higher binding affinity. They suggest that EM is probably not significantly concentrated by the estrogen target tissues such as mammary cancers. Therefore, the drug is unlikely to be very valuable in the treatment of breast cancer through a specific mechanism involving concentration by the estrogen receptors.

Quantitative estimates of cytoplasmic and nuclear oestrogen receptors in chick oviduct. Effect of oestrogen on receptor concentration and subcellular distribution

(3H)Oestradiol exchange techniques were developed for the determination of specific oestrogen receptor site concentrations in the cytoplasm and nuclei of chick oviduct cells. Non-labelled, receptor-bound oestrogens were exchanged with (3H)oestradiol during a 24-h incubation at 20 degrees C, 2 h at 30 degrees C or 45 min at 3 degrees C. Both "soluble" and "insoluble" nuclear receptors were stable for at least 6 h at 30 degrees C and 3 degrees C but a proportion (approx. 30%) of cytoplasmic sites from withdrawn chickens were inactivated after 2 h at 20 degrees C. The magnum of 4-week-old immature chickens (weight = 15 mg) contained 0.20 pmol of oestrogen receptor which corresponds to 4275 receptor sites/cell, when it is assumed that all magnum cells have equal concentrations of receptor. In primarily stimulated chickens of similar age which had received 10x1 mg of oestradiol benzoate/day, the magnum weighed approximately 800 mg and contained 8.65 pmol of oestrogen receptor (4610 sites/cell). Withdrawal from primary oestrogenic stimulation for 3-6 weeks resulted in a 110 mg magnum which contained 1.20 pmol of receptor (2225 sites/cell). Oviducts from immature and withdrawn chickens had the majority (73-77%) of their oestrogen receptors sites in the receptor sites in the cytoplasmic fraction, while in primary stimulated chicken oviducts the majority (82%) of receptor sites were located in the nucleus. A single secondary injection of oestradiol, to oestrogen-withdrawn chickens, resulted in apparent translocation of cytoplasmic receptors to the nucleus during the first hour after injection. The magnitude of the decline in cytoplasmic receptor, and the concurrent increase in nuclear receptor concentration, was dose-dependent between 2 and 100 mug oestradiol/kg body weight. Larger doses of oestradiol up to 1 mg/kg did not increase the concentration of nuclear receptor above the maximum level seen at 100 mug oestradiol/kg. The initial rapid accumulation of nuclear receptor sites was followed by a period of progressive decline. The initial rapid accumulation of nuclear receptor sites was followed by a period of progressive decline. By 15 h after an injection of 100 mug oestradiol/kg, the concentration of nuclear sites had reached pre-injection levels. During the same time period, the depleted cytoplasmic receptor levels were replenished such that they reached control values by 12 h and were about 150% of the pre-injection level at 24 h.

Nuclear estradiol receptor in the adult rat uterus: a new exchange assay

A protamine exchange assay has been developed to measure uterine nuclear estrogen receptor in mature rats exposed to estradiol (E). After ovariectomized-adrenalectomized mature rats are injected with E, estrogen receptor (RnE) is extracted from uterine nuclei with 0.6 M potassium chloride, diluted, and quantitatively precipitated with protamine sulfate. The precipitate is subjected to a ligand exchange with radiolabeled estradiol (E), with or without unlabeled diethylstilbesterol, to determine nonspecific binding. At 37 degrees C complete exchange of E for E in RnE is observed at 2.5 h; virtually no receptor degradation occurs up to at least 5 h. Exchange does not occur at 4 degrees C. Using the protamine assay, the depletion of cytoplasmic estrogen receptor (Rc) and the accumulation of RnE were studied at various doses of E at specific time points. Increasing doses of E result in a decrease of Rc with an equal increase of RnE. At the highest dose of E (10 mug) Rc is completely depleted within 10 min, by 6 h it is 25% replenished, and by 24 h returns to slightly above control levels. Within 10 min after the injection, RnE increases to 80-90% of the original cytoplasmic level of receptor (approximately 2-3 pmol/mg of DNA or approximately 1.5 pmol/100 mg of uterus). At 6 h RnE is 75% depleted and it is completely absent at 24 h. The protamine assay permits precise quantitative studies of nuclear estrogen receptor and avoids the problems of receptor degradation and excessive nonspecific binding often found in exchange reactions at elevated temperatures.

Effect of endogenous corticosterone on the determination of dexamethasone receptor levels in rat liver cytosol

The effect of endogenous corticosterone on the quantitative measurement of dexamethasone receptors in liver cytosols from developing rats has been studied. Liver cytosols from adrenalectomized rats were preincubated with increasing concentrations of nonlabeled corticosterone and the levels of detectable dexamethasone receptors were subsequently determined either directly or after removal of unbound corticosterone. Corticosterone concentrations of 50 nM or lower had no significant effect on the specific binding of labeled dexamethasone. Higher concentrations of corticosterone resulted in under-estimation of dexamethasone receptor levels. The mean levels of endogenous corticosterone in liver cytosols from 19.5- to 21.5- day fetuses, 22-day fetuses, 6-day-old immature rats and adult rats were 27.40, 11.91, 0.81 and 4.05 nM, respectively. It is concluded that variations in the levels of circulating corticosterone in the rat under normal physiological conditions have no significant effect on the quantitative measurement of total (occupied and unoccupied) receptor sites for dexamethasone in liver cytosol. This is supported by the finding that prior treatment of liver cytosols, from rats at different stages of development, with charcoal to remove unbound steroids has no effect on the amount of detectable dexamethasone receptors.

Assay of androgen binding sites by exchange with methyltrienolone (R 1881)

Methyltrienolone (R 1881 - 17beta-hydroxy-17alpha-methyl-estra-4,9,11-trien-3-one) binds specifically to androgen receptor in rat prostate cytosol where, unlike androstanolone, it is not metabolized. By exchanging bound endogenous hormone in rat prostate cytosol with labelled R 1881, it is possible to measure total (free anc occupied) binding sites. This assay method has also been applied to the measurement of androgen receptor sites in human benign prostatic hypertrophy where R 1881 has the added advantage of not being bound by any contaminating plasma protein (sex hormone binding protein).

Existence of receptors bound to endogenous estradiol in breast cancers of premenopausal and postmenopausal women

The amounts of free and occupied estrogen receptors were determined in cytosols and cellular (cytoplasmic + nuclear) extracts of 23 human breast cancers. In vivo undersaturation of the receptors was observed in all tumors. A positive correlation was found between the degree of saturation and the concentration of circulating estrogens in premenopausal women. The saturation of the sites was higher in cellular extracts than in cytosols, pointing to the existence of receptor-estradiol complexes bound to nuclei. Our results suggest that the absolute number of occupied receptors depends upon the level of available cytoplasmic receptors as well as upon the level of tissue estradiol.

A simple method for the determination of oestrogen receptor concentrations in breast tumours and other tissues

A simple method for the determination of oestrogen receptor activity in breast tumours and other tissues is described. By competition between radioactive and non-radioactive oestradiol-17beta for binding to a tissue extract prepared with centrifugation at low speed, the presence or absence of receptors is decided from the unmanipulated data. The concentration of receptors (P0) and dissociation constant of binding (Kd) are calculated by Scatchard analysis. In two relatively homogeneous tissues, assay precision was of the order of 16% for receptor concentration and 21-33% for the dissociation constant of binding: in heterogeneous breast tumors, much poorer precision can be expected. Sensitivity determined empirically is approximately 0.1 fmol/mg tissue. The method has been applied to the determination of receptor concentrations in human breast cancers, benign breast tumours, non-malignant breast, gynaecomastic breast, and in various tissues of the rat. Receptor activity was detected in 70% of the human breast cancers examined, and in lower amounts, in 21% of the benign human breast tissues. Activity was also detected in rat uterus and mammary gland, and in low amounts in 82% of the rat mammary tumours analysed. Dissociation constant of binding was generally of the order of 0.5 x 10(-10) M. The advantages of the method are discussed.

In vivo effect of anti-estrogens on the localisation and replenishment of estrogen receptor

Interactions between estrogen recpetors and triaryl ethylene anti-estrogens (U-11100A, MER 25 and CI 628) were tested on immature rat uteri. The accessible and the total (accessible and occupied) estrogen receptor sites were assayed in the cytosol and nuclear extracts using charcoal adsorption. After in vivo administration of anti-estrogens, the estradiol receptor sites were occupied and subsequently transferred to the nuclear compartment. The nuclear localisation of the receptor induced by the antagonist lasted for several days, during which replenishment of the cytosol receptor occurred. The nuclear receptors transferred by anti-estrogens and labelled in vitro with [3H]estradiol, were similar to the nuclear receptor-estradiol complex formed in vivo as far as their sedimentation constants and extractability from nuclei were concerned. Although these anti-estrogens are capable to translocate the estrogen receptor to the nucleus and to induce the replenishment of the cytosol receptor, the mechanism of their antagonism and of their weak estrogenic activity is still not clear.

Ultraviolet photosensitivity of the estrogen binding protein from rat uterus. Wavelength and ligand dependence. Photocovalent attachment of estrogens to proteins

Ultraviolet irradiation of the estrogen binding protein in rat uterine cytosol results in a progressive photoinactivation which is rapid at 254 nm and slower at greater than 315 nm. Both unfilled and estradiol-filled sites are inactivated at approimately the same rates at 254 nm (t 1/2 equals 8 min and 11 min, respectively), but at 315 nm, empty sites are consumed much more rapidly (t 1/2 equals 3.4 hr) than filled ones (t 1/2 equals 24 hr). The protective effect of the estrogen ligand at this wavelength appears to depend on its binding to the estrogen-specific binding site, as inactivation rate studies at different concentrations of estrone, estradiol, and estriol show a good correlation between the extent of protection and the fractional saturation of the high affinity estrogen binding sites. Scatchard analysis indicates that inactivation is the result of a loss of binding sites and not a decrease in their affinity, and sedimentation analysis shows that increased heterogeneity and aggregation of the estrogen binding species accompanies the photoinactivation process. Photoinactivation appears to be the result of direct irradiative damage of the animo acid residues, as the inactivation rate is the same under air and nitrogen atmospheres, and is unaffected by nucleophiles, reductants, and radical scavengers. When photoinactivation is measureed by irradiation of cytosol containing [3-H]estradiol, a concurrent photocovalent attachment process is noted; the steroid becomes linked to protein in a solvent-extractable manner (boiling ethanol inextractable). This attachment, however, does not appear to be related to the steroid binding at the estrogen binding site. Its rate is affected by reductants and scavengers. A similar photocovalent attachment reaction occurs when bovine serum albumin or ovalbumin is irradiated in the presence of [3-H]estradiol or [3-H]diethylstilbestrol. The detailed reactions involved in this photocovalent attachment process have not been defined at present.

Protamine-precipitated estrogen receptor: a solid-phase ligand exchange assay

Cytoplasmic estrogen receptor can exist either free (R) or bound to estradiol-17beta (RE). Both forms can be precipitated from cytosols by protamine sulfate. After protamine precipitation R binds 3-H-estradiol-17beta quantitatively at either 0 degrees or 30 degrees, while precipitated RE binds 3-H-estradiol-17beta only at 30 degrees by exchanging with previously bound hormone. Using these observations, we have developed a method for separate determination of both cytoplasmic R and RE. This method should also be applicable for assay of other steroid receptors, especially in cases where interfering components are present in the whole cytosol.

Iodohexestrols. I. Synthesis and photoreactivity of iodinated hexestrol derivatives

The five possible ortho-iodinated hexestrols, containing from 1-4 iodines per molecule, have been synthesized as potential iodine-containing estrogenic ligands. After chromatographic separation and purification, each derivative has been fully characterized by spectroscopic methods. [3-H]-3-Iodohexestrol, [131-I]- and [125-I]-3-iodohexestrol, and [125-IA1-3,5-diiodohexestrol have also been prepared. Photolysis of 3,5-diiodohexestrol in methanol results in rapid deiodination to 3-iodohexestrol; further reduction to hexestrol is slower. Photolysis of 3-iodohexestrol in benzene gives 3-phenylhexestrol.

Iodohexestrols. II. Characterization of the binding and estrogenic activity of iodinated hexestrol derivatives, in vitro and in vivo

The affinity of ortho-iodinated hexestrols for the estrogen binding protein from rat uterus, determined by competitive binding assay, decreases with progressive iodine substitution; 3-iodohexestrol (I-Hex) has a binding affinity 42% that of estradiol. Analysis of [3-H]-I-Hex binding in rat uterine cytosol by sucrose density gradient centrifugation shows both an estrogen-specific binding component (8 S) and a more abundant component (4 S) that is not estrogen specific. Scatchard analysis indicates that this latter binding is of high affinity (Kd equals to 3.7-8.3 times 10- minus-9 M) but is not uterine specific. Polyacrylamide gel electrophoresis shows that most of the [3-H]-I-Hex binding activity in serum and uterine cytosol is distinct from and anodic to the principal protein component (albumin), and that is comigrates with [14-C]thyroxine binding activity. In in vitro incubation of rat uteri, I-Hex can block the specific uptake of [3-H]estradiol into the nuclear fraction; it itself causes a translocation of estrogen-specific binding capacity (as measured by exchange) from cytoplasm to nuclei, and can induce the synthesis of an estrogen-specific uterine protein, all under conditions where it is not metabolically deiodinated to hexestrol. The uterotrophic activities of the iodohexestrols are in most cases comparable to that expected on the basis of their competitive binding affinities. However, selective, estrogen-specific uptake of [3-H]-I-Hex into rat uterus, either in vitro or in vivo, cannot be demonstrated.