Identification of immunoassayable estrogen receptor lacking hormone binding ability in tamoxifen-treated rat uterus

A two-site model for antiestrogen action

Evidence is presented for a unified model for the reaction of antiestrogens with estrogen receptors that explains much of the unusual pharmacology of these clinically important agents. Agonist activity results from occupancy of the estradiol-binding (primary) site in the receptor and antagonism from the additional interaction with a secondary locus not recognized by hormone. In the case of type I antiestrogens, such as tamoxifen, this is weaker than primary site binding, so these substances are agonists at low concentrations and antagonists at higher levels. With type II antiestrogens, affinities for both sites are comparable, so one never has the agonist situation (only primary site occupied), and these agents are pure antagonists. Reproductive tissues of the mouse and guinea pig contain a small macromolecule that binds hydroxytamoxifen, but not estradiol, keeping the free antiestrogen concentration below that required for secondary binding. Thus, in these species, tamoxifen is a pure agonist.

Evidence of an estrogen receptor form devoid of estrogen binding ability in MCF-7 cells

In MCF-7 breast cancer cells, hydroxytamoxifen (OH-Tam) up-regulates the estrogen receptor (ER) in a form unable to bind [(3)H]estradiol (E(2)). We show here that this property is not restricted to this antiestrogen. [(3)H]E(2) binding assays (whole cell assays, DCC assays on cell extracts) and enzyme immunoassays (Abbott) performed in parallel, establish the permanent presence of such unusual ERs in the absence of any exposure of the cells to a ligand. E(2) and the pure antiestrogen RU 58 668, which down-regulate ER, also decrease [(3)H]E(2) binding. In control cells, these ERs represent about the half of the whole receptor population; they also display a tendency to stabilize within the cell nucleus. Loss of E(2) binding ability appears irreversible, since we failed to label receptor accumulated under OH-Tam with [(3)H]E(2) or [(3)H]tamoxifen aziridine (TAZ). Cycloheximide (CHX), which blocks E(2)-induced down regulation of ER, failed to stabilize [(3)H]E(2) binding (whole cell assay) after an [(3)H]E(2) pulse (1 h), confirming that regulation of E(2) binding and peptide level are related to different regulatory mechanisms. Loss of binding ability could not be ascribed to any ER cleavage as demonstrated by Western blotting with a panel of ER antibodies raised against its various domains (67 kDa ER solely detected). We propose that loss of E(2) binding ability is related to the aging process of the receptor, i.e. it is progressively converted to a form devoted to degradation after it has accomplished its physiological role. Ligands may favor (E(2), RU 58 668) or impede (OH-Tam) this elimination process.

A new interpretation of antiestrogen action

Despite differences in their pharmacological behavior, type I and type II antiestrogens have certain important properties in common. Both differ from estradiol in that they enhance the immunoreactivity of estrogen receptors, apparently by inducing conformational change that exposes an additional epitope for a particular monoclonal antibody. Moreover, both types of antihormones not only compete with estradiol for its binding to the receptor but they also react with another domain not recognized by the hormone. The binding capacity for either type of antiestrogen is nearly twice that for estradiol, providing definitive evidence for the existence of specific antiestrogen-binding sites that are postulated to be important in antagonist action. These findings suggest a unified two-site model which helps explain how the same substance can be both an agonist and an antagonist; why there may be species variations in the agonist/antagonist relationship of type I compounds; and why type II agents show only antagonistic properties. It is suggested that interaction with secondary, antagonist-specific binding sites may provide a useful screen in the search for new and improved antihormonal agents.

Agonist-antagonist activity of anti-estrogens in the human breast cancer cell line MCF-7: an hypothesis for the interaction with a site distinct from the estrogen binding site

Non-steroidal anti-estrogens exhibit an extremely complex pharmacology because of their estrogenic and anti-estrogenic effects in different species. Recently, we have reported evidence for an immunochemical difference in the estrogen receptor (ER) when it is occupied with anti-estrogens as compared to estrogens (Martin et al., 1988). In this study, we have compared immunoreactivity of MCF-7 cell estrogen receptor when bound to anti-estrogen versus estrogen. We show that the occupation of ER with antiproliferative concentrations of various anti-estrogens leads to the appearance of additional antigenic determinants for the H222 monoclonal anti-estrogen receptor antibody. When performing ER immunoassay after sedimentation of estrogen receptors on sucrose gradients, we show that exposure of new epitopes induced by anti-estrogens can occur on a 4 s molecular form related to the 66 kDa monomeric estrogen receptor. Also, when ER are previously occupied by estradiol, the addition of low anti-estrogen concentrations, which are unable to displace estradiol from the estrogen receptor, leads to a significant increase of H222 epitopes. Our results led us to propose a molecular model for anti-estrogen-receptor interaction in which their dual agonist/antagonist activity may be due to the occupation of distinct binding sites on the estrogen receptor.

Up-regulation of estrogen receptor by tamoxifen in human breast cancer

BACKGROUND

Influence of tamoxifen treatment on estrogen receptor (ER) and progesterone receptor (PR) levels in human breast cancer has not been fully elucidated in vivo. This problem was studied in 20 postmenopausal patients with ER-positive and PR-positive primary breast cancer.

METHODS

Each patient underwent two fine-needle aspiration (FNA) biopsies. Between two FNA, 10 patients received no treatment (control group) and the other 10 patients were given tamoxifen (20 mg/day) for an average of 8 days (range, 6-10 days) (TAM group). Total ER and PR values (cytosolic plus nuclear fraction) in FNA samples were determined by enzyme immunoassay (EIA) for detecting ER and PR regardless of their occupancy with corresponding ligands.

RESULTS

In the control group, no significant difference was found in ER and PR values between the first and second FNA samples. In the TAM group, ER and PR values (mean +/- standard error of the mean [SEM] fmol/mg DNA) in the second FNA samples were 605 +/- 186 and 1130 +/- 344, respectively, and were significantly higher (P < 0.05) than ER and PR values in the first FNA samples, which were 312 +/- 74 and 639 +/- 159, respectively. ER and PR values increased by 201 +/- 27% and 163 +/- 23%, respectively, on an individual basis after tamoxifen treatment.

CONCLUSIONS

These results demonstrated that tamoxifen up-regulates ER and PR in human breast cancer.

Expression of estrogen receptor and its messenger ribonucleic acid in the MCF-7 cell line: multiparametric analysis of its processing and regulation by estrogen

Experimentally, a portion of the detectable cellular estrogen receptor (ER) is seen to disappear in human breast cancer cells submitted to estradiol treatment. In this study, we have applied several detection methods to analyze the loss (processing) then the replenishment of ER in the MCF-7 cell line. Radioligand exchange assay and enzyme immunoassay revealed an accumulation of ER in the nuclei with a concomitant depletion in cytosol shortly after the addition of estradiol in cell culture. Then, a time-dependent decrease of ER level in the nuclear compartment without rescue in the cytosol was observed. When an immunocytochemical assay was performed on whole cells treated with estradiol, a similar decrease of ER number was shown, indicating that a decrease in the extractability of estradiol-filled ER was not involved in the processing. Analysis of ER mRNA also indicated that the estrogen treatment induces a time-dependent decrease of its expression. Measurement of [35S]methionine-labeled ER following the arrest of the hormone treatment suggested that ER replenishment was due to newly synthesized receptors. Sucrose gradient experiments confirmed the generation of small molecular forms of ER, following its binding with estradiol. All these data are indicative of estrogen-receptor complex degradation. We also confirm that estrogen regulates ER level through the decrease of its mRNA expression.

Identification of unoccupied but transformed nuclear estrogen receptor in cultured mouse Leydig cell

The molecular forms of estrogen receptor (ER) in estrogen-responsive mouse Leydig cell line (B-1) have been examined in relation to their biological activity. ER was predominantly recovered in the nuclear fraction upon homogenization even after cells were precultured in the absence of E2 and Phenol Red. This unoccupied nuclear ER (ERn) whose hormone binding ability was extremely thermostable could be extracted with 0.4 M KCl. This stability enabled us to determine hydrodynamic parameters in the ligand-free condition. The Stokes radius and sedimentation constant of this ERn in high salt condition were 5.5 nm and 6.0S, respectively, resulting in its molecular weight of 140,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of ER labeled with [3H]tamoxifen aziridine gave a single band of 65,000 Da, indicating that this ERn had a oligomer structure similar to that of transformed nuclear ER complexed with estrogen in the putative target cells. Therefore, we further examined the possibility that this ERn in B-1 cells can activate estrogen-responsive genes without any aid from estrogen. Estrogen responsive element-thymidine kinase promoter-chloramphenicol acetyltransferase fusion gene (ERE-tk-CAT) was transfected into B-1 cells. CAT activity was enhanced only in cells stimulated with estrogen. It may be concluded from these results that transformed ERn can be formed in the absence of estrogen but that binding to estrogen may be required in order to exert its biological activity.

Inactivation of the uterine estrogen receptor binding of estradiol during P-450 catalyzed metabolism of chlorotrianisene (TACE). Speculation that TACE antiestrogenic activity involves covalent binding to the estrogen receptor

Chlorotrianisene (TACE) exhibits in vitro little or no binding to the uterine estrogen receptor (ER) but demonstrates potent estrogenic activity in vivo, indicating that TACE is a proestrogen/proantiestrogen. Our earlier studies demonstrated that the incubation of TACE with rat liver microsomes and NADPH generates a reactive intermediate (T*) which binds covalently to proteins. The current study examined the possibility that T* may inactivate the uterine ER. The incubation of TACE with rat liver microsomes and NADPH in the presence of rat uteri, under conditions which generate T*, markedly decreased the binding capacity of the ER for [3H]estradiol (E2). The evidence indicates that ER inactivation was probably due to irreversible (covalent) binding of T* to the E2 binding site. The possibility that the antiestrogenic action of TACE and of other triphenylethylenes involves such a novel mechanism is discussed.

Immunological and physiological analysis of human breast cancer progesterone receptor heterogeneity, following KCl dissociation and size exclusion chromatography

Progesterone receptor (PR) levels were determined in 69 human breast cancer specimens by both radioligand assay (RLA) and enzyme immunoassay (EIA). These methods did not detect the same number of sites, and for each tumor there was a constant ratio between epitopes and PR-binding sites corresponding to 1/4, 2/4, ... 8/4. High performance size exclusion chromatography was performed to separate the various PR isoforms, and the ability of these isoforms to interact with the monoclonal antibodies was assessed. Determination of PR in the chromatographic fractions by EIA and RLA showed that the various isoforms isolated by chromatography presented variable quantities of steroid-binding sites and epitopes, thus confirming the differences observed in the cytosol assays. The dissociation of molybdate-stabilized PR by KCl and measurement by RLA and EIA of the isoforms obtained showed two different types of chromatographic patterns, particularly in the 8S polymeric form where the monoclonal antibodies appeared to detect mainly the heavier 8S fraction, which may correspond to the 8S-B form of the progesterone receptor. The monoclonal antibodies also detected an intermediate PR polymeric form (236 kDa) which was not always detected by the tritiated ligand. Our results suggest that breast cancer PR exhibit a marked molecular heterogeneity which may partially explain the differences in response to hormonal therapy, particularly for tumors with high receptor levels which nonetheless fail to respond to treatment.

Inhibitory effect of tamoxifen on diethylstilbestrol-promoted hepatic tumorigenesis in male rats and its possible mechanism of action

Male Sprague-Dawley rats were given a single intraperitoneal injection of diethylnitrosamine (DEN, 200 mg/kg body weight). Two weeks later the rats were divided into 4 groups; DEN-C group rats were given no further treatment; DEN-DES group rats were fed diethylstilbestrol (DES, 0.5 mg/day); DEN-TMX group rats were given tamoxifen (TMX, 1.0 mg/day) orally; DEN-DES TMX group rats were fed both DES and TMX for 8 months. Rats of the DEN-DES group developed grossly visible hepatic tumors. On the other hand, tumor development was significantly inhibited in rats of the DEN-DES TMX group. Total area of gamma-glutamyl transpeptidase-positive lesions and the mean area per lesion were significantly larger in rats of the DEN-DES group than those of the DEN-C, DEN-TMX or DEN-DES-TMX group. Estrogen receptor (ER) content of liver cytosol assayed by enzyme immunoassay (EIA) was significantly greater in rats of the DEN-DES group than in those of the DEN-C group and smaller in rats of the DEN-TMX and DEN-DES TMX group than in the DEN-C group. On the contrary, ER content of liver nuclei was significantly greater in rats of the DEN-TMX and DEN-DES TMX group than in those of the DEN-C or DEN-DES group. These results suggest that the promotive action of DES and the inhibitory action of TMX on DES-promoted hepatic tumorigenesis are, at least in part, mediated by ER in the rat.

Use of an enzymeimmunoassay (EIA) for quantitation of cytosolic and nuclear estrogen receptor, and correlation with progesterone receptor in human breast cancer

We have adapted a commercially available enzyme immunoassay for ER (ER-EIA) for use with nuclear extracts of breast carcinoma specimens, and have examined the data obtained in relation to the results from cytosolic ER-EIA and radioligand (DCC) assays and from DCC assays for PgR. In a series of 139 carcinoma specimens, we observed a very significant correlation between the cytosolic ER concentration as measured by the EIA and DCC methods, and also between the log10 of the concentration of ER in the cytosolic and nuclear fractions assayed by the ER-EIA method. The correlation between nuclear ER and cytosolic PgR was also highly significant, but not as close as for cytosolic ER. If 130 fmol ER/mg DNA was used as a "cut-off" point to discriminate between specimens "positive" and "negative" for ERN, there was 87% concordance in receptor status between ERN and cytosolic ER, and 79% concordance between ERN and cytosolic PgR. Forty-one percent of specimens were positive or borderline for both ERN and cytosolic PgR, and it is suggested that this receptor combination may be a valuable predictive factor in prognosis and response to hormonal treatment.