Effect of oestradiol benzoate, tamoxifen and monohydroxytamoxifen on immature rat uterine progesterone receptor synthesis and endometrial cell division

Turning scientific serendipity into discoveries in breast cancer research and treatment: a tale of PhD students and a 50-year roaming tamoxifen team

PURPOSE

This retrospective, about a single "mobile" laboratory in six locations on two continents, is intended as a case study in discovery for trainees and junior faculty in the medical sciences. Your knowledge of your topic is necessary to expect the unexpected.

HISTORICAL METHOD

In 1972, there was no tamoxifen, only ICI 46, 474, a non-steroidal anti-estrogen with little chance of clinical development. No one would ever be foolish enough to predict that the medicine, 20 years later, would achieve legendary status as the first targeted treatment for breast cancer, and millions of women would benefit from long-term adjuvant tamoxifen therapy. The secret of tamoxifen's success was a translational research strategy proposed in the mid 1970's. This strategy was to treat only patients with estrogen receptor (ER)-positive breast cancer and deploy 5 or more years of adjuvant tamoxifen therapy to prevent recurrence. Additionally, tamoxifen prevented mammary cancer in animals. Could the medicine prevent breast cancer in women?

RESULTS

Tamoxifen and the failed breast cancer drug raloxifene became the first selective estrogen receptor modulators (SERMs): a new drug group, discovered at the University of Wisconsin, Comprehensive Cancer Center. Serendipity can play a fundamental role in discovery, but there must be a rigorous preparation for the investigator to appreciate the possibility of a pending discovery. This article follows the unanticipated discoveries when PhD students "get the wrong answer." The secret of success of my six Tamoxifen Teams was their technical excellence to create models, to decipher mechanisms, that drove the development of new medicines. Discoveries are listed that either changed women's health or allowed an understanding of originally opaque mechanisms of action of potential therapies. These advances in women's health were supported entirely by government-sponsored peer-reviewed funding and major philanthropy from the Lynn Sage Breast Cancer Foundation, the Avon Foundation, and the Susan G. Komen Breast Cancer Foundation. The resulting lives saved or extended, families aided in a time of crisis and the injection of billions of dollars into national economies by drug development, is proof of the value of Federal or philanthropic investment into unencumbered research aimed at saving millions of lives.

Long-range transcriptional control of progesterone receptor gene expression

Estrogen receptor alpha (ERalpha) binds to specific target DNA sequences, estrogen response elements (EREs), to regulate estrogen-responsive gene expression. The progesterone receptor (PR) gene has been used extensively as a marker of estrogen responsiveness. Although we previously identified cis elements within 1 kb of the PR-B transcription start site that are associated with ERalpha and help to confer estrogen responsiveness, the identification of ERalpha binding sites far removed from the transcription start site suggested that long-range regulation of this gene may occur. We now show that eight regions of the PR gene from 311 kb upstream to 4 kb downstream of the PR-B transcription start site interact with ERalpha and that coactivator proteins and acetylated histones are selectively associated with these gene regions. Specific PR gene regions confer estrogen responsiveness to a heterologous reporter plasmid, and mutation of EREs within these regions diminishes estrogen-induced transactivation. Importantly, chromosome conformation capture assays reveal ERalpha- and ligand-dependent interactions between proximal and distal PR gene regions. Taken together, our studies suggest that distal regions of the PR gene participate in the dynamic regulation of this gene and that the coordinated action of proximal and distal PR gene regions allows cells to respond to changes in hormone levels with extraordinary versatility and sensitivity.

Comparative temporal and dose-dependent morphological and transcriptional uterine effects elicited by tamoxifen and ethynylestradiol in immature, ovariectomized mice

BACKGROUND

Uterine temporal and dose-dependent histopathologic, morphometric and gene expression responses to the selective estrogen receptor modulator tamoxifen (TAM) were comprehensively examined to further elucidate its estrogen receptor-mediated effects. These results were systematically compared to the effects elicited by the potent estrogen receptor ligand 17alpha-ethynylestradiol (EE) to identify pathways similarly and uniquely modified by each compound.

RESULTS

Three daily doses of 100 microg/kg TAM elicited a dose-dependent increase in uterine wet weight (UWW) in immature, ovariectomized C57BL/6 mice at 72 hrs with concurrent increases in luminal epithelial cell height (LECH), luminal circumference and glandular epithelial tubule number. Significant UWW and LECH increases were detected at 24 hrs after a single dose of 100 microg/kg TAM. cDNA microarray analysis identified 2235 differentially expressed genes following a single dose of 100 microg/kg TAM at 2, 4, 8, 12, 18 and 24 hrs, and at 72 hrs after three daily doses (3 x 24 hrs). Functional annotation of differentially expressed genes was associated with cell growth and proliferation, cytoskeletal organization, extracellular matrix modification, nucleotide synthesis, DNA replication, protein synthesis and turnover, lipid metabolism, glycolysis and immunological responses as is expected from the uterotrophic response. Comparative analysis of TAM and EE treatments identified 1209 common, differentially expressed genes, the majority of which exhibited similar profiles despite a temporal delay in TAM elicited responses. However, several conserved and treatment specific responses were identified that are consistent with proliferation (Fos, Cdkn1a, Anapc1), and water imbibition (Slc30a3, Slc30a5) responses elicited by EE.

CONCLUSION

Overall, TAM and EE share similar gene expression profiles. However, TAM responses exhibit lower efficacy, while responses unique to EE are consistent with the physiological differences elicited between compounds.

Cell- and Ligand-specific Regulation of Promoters Containing Activator Protein-1 and Sp1 Sites by Estrogen Receptors α and β

Estrogen plays a critical role in development and maintenance of female reproductive and mammary tissues, but is also involved in maintenance of cardiovascular, skeletal, and neural function. Although it is widely accepted that the estrogen-occupied receptor mediates its effects by interacting with estrogen response elements (EREs) residing in target genes, a number of estrogen-responsive genes contain no identifiable ERE. To understand how estrogen-responsive genes lacking EREs but containing activator protein 1 (AP-1) and Sp1 sites respond to hormone treatment, we have identified four discrete regions of the human progesterone receptor gene that contain AP-1 or Sp1 sites and examined their abilities to modulate transcription in the presence of 17 beta-estradiol, ICI 182,780, tamoxifen, raloxifene, genistein, or daidzein. Transient cotransfection assays demonstrated that ER alpha was a more potent activator of transcription than ER beta in bone, uterine, and mammary cells. The Sp1-containing promoters were substantially more potent transcriptional enhancers than the AP-1-containing promoters, but a 1.5-kb region of the human progesterone receptor gene containing both AP-1 and Sp1 sites was the most hormone-responsive promoter tested. The ability of ligands to modulate transcription of AP-1- or Sp1-containing promoters was dependent on cell context, but the expression of AP-1 or Sp1 proteins was not necessarily related to transcriptional response. Taken together, these studies have helped to delineate the roles of ER alpha and ER beta in modulating transcription of genes containing AP-1 and Sp1 sites and define the effects of widely used, pharmacologic agents in target cells with distinct cellular environments.

AIB1/SRC-3 deficiency affects insulin-like growth factor I signaling pathway and suppresses v-Ha-ras-induced breast cancer initiation and progression in mice

Although the amplified in breast cancer 1 (AIB1) coactivator is amplified and overexpressed in breast cancers, its role in mammary carcinogenesis remains unknown. We demonstrate that during mammary development and tumorigenesis, the elevation of AIB1 level and its nuclear localization correlate with normal and transformed mammary epithelial proliferation, whereas its lower expression and cytoplasmic localization correlate with mammary epithelial quiescence and differentiation. In this study, the role of AIB1 in breast tumor initiation, progression, and metastasis was studied by generating AIB1(+/+), AIB1(+/-), and AIB1(-/-) mice harboring the mouse mammary tumor virus/v-Ha-ras (ras) transgene that induces breast tumors. Breast tumor incidence was reduced dramatically in the intact AIB1(-/-)-ras virgin mice and inhibited completely in the ovariectomized AIB1(-/-)-ras mice. Breast tumor latency was delayed significantly in AIB1(-/-)-ras virgin mice with natural estrous cycles, multiparous mice with cyclically elevated reproductive hormones, and virgin mice bearing pituitary isografts with persistently elevated hormones. Although AIB1 deficiency significantly suppressed mammary tumorigenesis under all of the concentrations of ovarian hormones, it did not affect the promotional role of ovarian hormones on mammary tumorigenesis, suggesting that AIB1 and ovarian hormones contribute to mammary carcinogenesis through different pathways. AIB1 deficiency did not alter the expression of estrogen and progesterone-responsive genes in the mammary gland, but it caused partial resistance to the insulin-like growth factor I because of a significant reduction in the insulin receptor substrates. The impaired insulin-like growth factor I pathway in AIB1(-/-)-ras mammary epithelium and tumor cells was responsible in part for the suppression of mammary tumorigenesis and metastasis caused by inhibition of cell proliferation and migration. These results suggest that a more effective strategy to control breast cancer is to target AIB1-mediated and ovarian hormone-initiated pathways.

Expression of estrogen receptor coactivators in the rat uterus

Nuclear receptor coactivators associate in a ligand-dependent manner with estrogen receptors (ER) and other nuclear receptors, and they enhance ligand-dependent transcriptional activation. This study examined basal coactivator expression in rat uterus to investigate if expression of these genes is regulated by estradiol-17 beta or tamoxifen. Ovariectomized mature and immature rats were injected with estradiol-17 beta, tamoxifen, or vehicle (i.e., sesame oil) alone. Uteri were collected and analyzed for changes in coactivator mRNA expression using Northern blot and in situ hybridization analyses. Constitutive uterine mRNA expression of switch protein for antagonist (SPA), SRC-1, GRIP1, RAC3, RIP140, and p300 mRNAs was observed in control uteri, and treatment with ER ligands did not alter coactivator mRNA levels. The data suggest that expression of these coactivator genes is not sensitive to estradiol or tamoxifen in the rat uterus. No cell type-specific pattern of expression was apparent in uterine sections from mature and immature rats; however, silver grains were more abundant in luminal and glandular epithelial cells compared with the stroma and myometrium, indicating that coactivator mRNA levels vary among the uterine compartments. Thus, to our knowledge, we show for the first time that there is constitutive expression of several uterine nuclear receptor coactivators in a physiological setting that remains insensitive to estrogenic regulation. Furthermore, we speculate that higher constitutive levels of coactivator expression in glandular and luminal epithelial cells may be associated with increased hormonal responsiveness by these uterine compartments.

Long-term inhibitory effects of a novel anti-estrogen on the growth of ZR-75-1 and MCF-7 human breast cancer tumors in nude mice

The effects of the novel anti-estrogen EM-343 on the growth of 2 hormone-responsive human breast cancer tumors have been examined in athymic nude mice. At the low daily dose of 5 microg, EM-343 administered subcutaneously for 6 months completely blocked the stimulatory effect of endogenous estrogens on the growth of ZR-75-1 and MCF-7 tumors implanted in nude mice. In addition, uterine weight decreased by 60% while ovarian weight increased by 37%. Estrogen receptor (ER) levels measured by [3H]-labeled estrogen binding were markedly reduced (by 96%, 96% and 92%) in ZR-75-1 and MCF-7 tumors, and in the mouse uterus, respectively. Accompanying the decrease in ER, progesterone receptor levels were reduced by 79%, 87% and 76%, respectively, in the above-mentioned tissues following EM-343 treatment. Our data show the pure anti-estrogenic properties of EM-343 and its high potency as an inhibitor of growth of human ZR-75-1 and MCF-7 breast tumors in nude mice.

Estrogen receptor function as revealed by knockout studies: neuroendocrine and behavioral aspects

Estrogens are an important class of steroid hormones, involved in the development of brain, skeletal, and soft tissues. These hormones influence adult behaviors, endocrine state, and a host of other physiological functions. Given the recent cloning of a second estrogen receptor (ER) cDNA (the ER beta), work on alternate spliced forms of ER alpha, and the potential for membrane estrogen receptors, an animal with a null background for ER alpha function is invaluable for distinguishing biological responses of estrogens working via the ER alpha protein and those working via another ER protein. Data generated to date, and reviewed here, indicate that there are profound ramifications of the ER alpha disruption on behavior and neuroendocrine function. First, data on plasma levels of estradiol (E2), testosterone (T), and luteinizing hormone (LH) in wild-type (WT) versus ER alpha- mice confirm that ER alpha is essential in females for normal regulation of the hypothalamic-pituitary gonadal axis. Second, ovariectomized female ER alpha- mice do not display sexual receptivity when treated with a hormonal regime of estrogen and progesterone that induces receptivity in WT littermates. Finally, male sexual behaviors are disrupted in ER alpha- animals. Given decades of data on these topics our findings may seem self-evident. However, these data represent the most direct test currently possible of the specific role of the ER alpha protein on behavior and neuroendocrinology. The ER alpha- mouse can be used to ascertain the specific functions of ER alpha, to suggest functions for the other estrogen receptors, and to study indirect effects of ER alpha on behavior via actions on other receptors, neurotransmitters, and neuropeptides.

Reproductive phenotpes of the progesterone receptor null mutant mouse

Although progesterone has been traditionally associated with the establishment and maintenance of mammalian pregnancy, a number of studies have implicated physiological roles of this steroid hormone in other reproductive events. At present most of the downstream molecular and cellular mechanisms by which progesterone exerts its effects are unclear; however, the progesterone signal is known to be mediated initially by the progesterone receptor (PR), a member of the nuclear receptor superfamily of transcription factors. In most tissues studied, the PR is induced by ovarian estrogen via the estrogen receptor (ER), thereby implying that many of the observed reproductive physiological responses attributed to PR could conceivably be due to the combined effects of progesterone and estrogen. Therefore, to define clearly the distinct roles of progesterone and estrogen in vivo and to understand better progesterone function in a physiological context, we recently have generated a novel mouse strain in which both forms of the PR were ablated using gene targeting/embryonic stem cell techniques. Surprisingly, both male and female embryos, homozygous for the PR null mutation, developed to adulthood at the normal Mendelian frequency with no deviation in the sex ratio. Although developmental defects have yet to be detected in the adult male PR homozygote, extensive reproductive abnormalities were observed in the female. The reproductive phenotypes consisted of an inability to ovulate, uterine hyperplasia and inflammation, severely limited mammary gland development and an impairment in the induction of a sexual behavioral response. Collectively, these results provide direct in vivo evidence for progesterone's role as a pleiotropic coordinator of diverse reproductive events that together ensure female fertility. Finally, we believe that this animal model will be an invaluable tool in exploring the effects of progesterone in physiological systems other than reproduction and may, in the future, help to redefine progesterone not just as a sex steroid hormone but also as a key regulator of diverse physiological processes.

Mice lacking progesterone receptor exhibit pleiotropic reproductive abnormalities

Although progesterone has been recognized as essential for the establishment and maintenance of pregnancy, this steroid hormone has been recently implicated to have a functional role in a number of other reproductive events. The physiological effects of progesterone are mediated by the progesterone receptor (PR), a member of the nuclear receptor superfamily of transcription factors. In most cases the PR is induced by estrogen, implying that many of the in vivo effects attributed to progesterone could also be the result of concomitantly administered estrogen. Therefore, to clearly define those physiological events that are specifically attributable to progesterone in vivo, we have generated a mouse model carrying a null mutation of the PR gene using embryonic stem cell/gene targeting techniques. Male and female embryos homozygous for the PR mutation developed normally to adulthood. However, the adult female PR mutant displayed significant defects in all reproductive tissues. These included an inability to ovulate, uterine hyperplasia and inflammation, severely limited mammary gland development, and an inability to exhibit sexual behavior. Collectively, these results provide direct support for progesterone's role as a pleiotropic coordinator of diverse reproductive events that together ensure species survival.

Estrogenic effects of the antiprogestin onapristone (ZK98.299) in the rodent uterus

OBJECTIVE

Our purpose was to assess the estrogenic action of onapristone (ZK299).

STUDY DESIGN

Three rodent models of estrogen action in the uterus were used. Deoxyribonucleic acid synthesis in the uterine epithelium of neonatal mice was determined by thymidine autoradiography. In adult ovariectomized mice uterine wet weight and progesterone receptor and estrogen receptor concentrations were determined. In immature rats uterine deoxyribonucleic acid synthetic activity was determined by thymidine autoradiography, epithelial hypertrophy and stromal edema were assessed histomorphometrically, and complement C3 protein synthesis was assessed by metabolic labeling in vitro. The effects of ZK299 were challenged with the antiestrogens ICI164,384 and tamoxifen. The ability of ZK299 to displace tritiated estradiol from the estrogen receptor was assessed in cytosolic preparations from mouse uterus.

RESULTS

In the neonatal mouse ZK299 stimulated epithelial deoxyribonucleic acid synthesis; two other antiprogestins, RU486 and ZK98.734, had no effect. Three daily injections of ZK299 at 10 micrograms/gm body weight to 6-week-old ovariectomized mice increased uterine progesterone receptor 42%; this effect was blocked by ICI164,384. In another experiment three daily doses of ZK299 (20 micrograms/gm) to 10-week-old ovariectomized mice increased progesterone receptor concentration by 63% and uterine wet weight by 42%. In 21-day-old rats a single injection of ZK299 increased uterine epithelial deoxyribonucleic acid synthesis; this effect was blocked by tamoxifen. Both ZK299 and tamoxifen increased epithelial cell height and thymidine labeling in the stroma. ZK98.734 had no effect on epithelium or stroma. ZK299 also stimulated synthesis of complement C3 by uteri of immature rats. In competitive binding assays ZK299 exhibited weak relative binding affinity (0.05%) for mouse uterine estrogen receptor.

CONCLUSIONS

ZK299 can act as a weak estrogen in the rodent uterus, most likely through a direct, low-affinity interaction with the estrogen receptor. Because estrogens may increase the risk for endometrial, breast, and liver cancer, caution is warranted in long-term administration of this drug to women.

Inhibition of estradiol-induced early osteoarthritic changes by tamoxifen

Previous studies on osteoarthritic changes induced by intraarticular injections of estradiol benzoate (EB) suggest estrogen as a risk factor in the development of knee osteoarthritis (OA). The present study observed the anti-arthritic effects of tamoxifen (TMX). Oophorectomized rabbits were subjected to intraarticular injections of vehicle control, EB, TMX, or EB/TMX for 3 weeks. The cartilage changes were assessed by light and scanning electron microscopic examination, enzyme histochemical analysis, and the amount of alcian blue stain binding to glycosaminoglycans. EB injections resulted in cell necrosis, chondrocyte clonings, and pittings, whereas the vehicle control, TMX, and EB/TMX-injected groups showed no histologic abnormalities. Histochemical analysis showed that the numbers of lactate dehydrogenase (LDH)-reactive chondrocytes in the EB-injected group were significantly reduced when compared to other groups (p less than 0.001). The injections of EB/TMX significantly reduced the chondrocyte numbers in the lateral superficial layer (p less than 0.05), compared with the vehicle injection. TMX-injected group revealed slight although insignificant decreases in chondrocyte numbers. The amount of alcian blue stains, representing the relative amount of proteoglycans, significantly decreased only in the superficial layer of the EB- and EB/TMX-injected groups (p less than 0.05). TMX, when concurrently injected with EB, antagonized the chondrodestructive effects of estradiol at the early stage of knee OA in rabbits. The results suggest the potential therapeutic use of TMX at the early stage of OA.

Alterations in developing rat uterine cell populations after neonatal exposure to estrogens and antiestrogens

Exposure of rats to either estrogens or antiestrogens during early postnatal development reduces subsequent uterine growth as measured by uterine weight. However, individual uterine cell types respond differently to these agents and uterine weight alone cannot discern subtle or even large alterations in individual cell populations. Using a computerized planimetric technique, we estimated the prepubertal growth of the uterine luminal epithelium, endometrial stroma, glands, and circular and longitudinal muscle after exposure of neonatal rats (postnatal days 1-5) to the estrogens 17 beta-estradiol (E2), diethylstilbestrol (DES), or ethynylestradiol (EE), and the antiestrogens tamoxifen or clomiphene citrate. On postnatal day 26, the cross-sectional areas of the luminal epithelium, endometrial stroma, and circular muscle were reduced after estrogen exposure, compared to untreated controls, while longitudinal muscle cross-sectional area was not affected. Since cell densities (cell number/unit area) were increased, these estrogen-induced area reductions demonstrate a decrease in cell size. Total cell numbers, estimated as the product of cell type areas and their respective cell densities, were also reduced by neonatal estrogen exposure. The synthetic estrogens DES and EE were more potent than E2 with respect to reduction of uterine growth. Neonatal antiestrogen exposure caused large area reductions only in the uterine glands and luminal epithelium. Little change in cell density occurred in any cell population exposed to antiestrogen. These data demonstrate that the decreased uterine growth resulting from estrogen exposure during early postnatal development is a consequence of combined hypotrophy and hypoplasia in all cell types except longitudinal muscle while antiestrogen-induced morphological alterations were limited to hypoplasia having epithelial cell specificity.

Estrogen- and antiestrogen-induced ornithine decarboxylase activity and uterine growth in the rat

The estrogen antagonists tamoxifen and monohydroxytamoxifen are also classified as partial estrogen agonists. In infantile rats, estradiol induced a single peak of uterine ODC activity at 6h following injection regardless of the extent of induction by various estradiol doses. By contrast, the timing of the ODC activity peak induced by tamoxifen and monohydroxytamoxifen was highly dependent upon the dosing conditions and was delayed to 18 h at lower tamoxifen doses. In immature rats, tamoxifen and monohydroxytamoxifen induced two peaks of uterine ODC activity resembling those induced by estradiol. Both ODC activity peaks were delayed by 9 h, without decreases in peak heights, by a 50-fold tamoxifen dose reduction. In all experiments the initial appearance of antiestrogen- and estradiol-induced ODC activity corresponded to initial uterine wet weight gain regardless of dosing condition. Thus, when dose-related temporal shifts are taken into account, tamoxifen and monohydroxytamoxifen are complete agonists with respect to induction of uterine weight gain and ODC activity.

Hormonal regulation of rat uterine secretory protein synthesis

Treatment of the rat uterus with estradiol results in many morphological and biochemical changes. In order to examine the biochemical mechanisms of these changes, we are interested in finding a protein which can be used as an end-point indicator or marker. Estradiol administration results in the increased synthesis and release of a 115,000 and 65,000 dalton protein into the incubation media. In this paper, we demonstrate that these proteins are actually subunits of a larger protein having a molecular weight of 180,000 daltons. This protein appears to be specific to the uterus and may be produced mainly or solely by the epithelial cells. In the normal estrous cycle, maximal production is seen at estrus with essentially no production at diestrus. In the immature rat, synthesis of the 180K dalton protein is blocked by the interaction of progesterone and estradiol. However, unlike some uterine responses, the 180K protein is increased by treatment with several antiestrogens. This protein is also present in sufficient quantities to allow for the production of monoclonal antibodies. Thus this protein has many attributes of an excellent "marker protein" and will be of value in future studies aimed at elucidating the molecular mechanism(s) of steroid hormone action in the uterus.

Cellular distribution and hormonal regulation of h-SBP in human hepatoma cells

The cellular distribution of human Sex Steroid Binding Plasma Protein (h-SBP) was studied in human cells and tissues by indirect immunofluorescence. h-SBP was detected in the cytoplasm of hepatocytes, of prostate and epididymis epithelial cells and in endometrium. Sexual and non-sexual skin, intestine epithelium, striated muscle and some rodent organs were not labelled. The intracellular localization of h-SBP indicate that h-SBP could be taken up from the extracellular compartment or synthesized in situ in sex steroid target organs, where it may play a role in hormone uptake. The hormonal regulation of h-SBP secretion by a human hepatoma cell line, H5A, showed that tri-iodothyronine was more potent than estradiol or tamoxifen, which acted as estrogen agonist, in increasing secreted h-SBP and the combined effect of both thyroid and estrogen hormones resulted in an additive stimulation of h-SBP secretion. As shown by Northern blot analysis, oligonucleotides synthesized from the known sequence of h-SBP hybridized with a RNA of approximately 2 kb which was more represented in H5A cells than in normal human liver, and was increased 2-3 times after hormonal stimulation of the cells. The presence of a poly(A+)RNA coding for h-SBP in the human liver indicated the hepatic synthesis of this protein.

Hormonal control of SBP in human hepatoma cells

Since it is currently believed that the biosynthesis of human sex steroid binding plasma protein (SBP) takes place in the liver, the secretion of this protein and its hormonal control were studied in a human hepatoma cell line. The human hepatoma-derived cell line, Hep G2, and a clone, H5A, isolated from Hep G2, were both found to secrete SBP-like protein. This protein had the same dihydrotestosterone binding parameters as plasma SBP, with a Kd ranging from 0.3 to 1 nM at 4 degrees C, and it cross-reacted with a monospecific goat anti-human SBP antiserum. In a chemically defined medium, SBP-like protein secretion was stimulated approx 2-fold by estradiol (1 microM) whereas a smaller concentration of estrogen (100 nM) has only a slight effect. A combined incubation with estradiol (100 nM) and triiodothyronine (10 nM) increased SBP-like protein secretion more than estradiol (1 microM) alone. In response to dexamethasone (100 nM) or tamoxifen (100 nM) treatment, a 3-fold increase is obtained. Therefore, these human parenchymal cells should provide a potent material for investigation of the hormonal regulation of SBP gene.

Estrogen-like effects of combined dexamethasone and tamoxifen in the chick oviduct

The effects of dexamethasone alone on withdrawn chick oviduct weight, DNA, protein content and progesterone receptor concentration were barely detectable, whereas ovalbumin and conalbumin synthesis were increased. When dexamethasone and tamoxifen were combined, a marked increase of total proteins, including egg white proteins, DNA and wet weight occurred. Progesterone receptor also was increased. The most striking result was the stimulation of DNA polymerase-alpha activity by combined dexamethasone and tamoxifen, whereas either compound was completely ineffective.

An evaluation of tamoxifen as a partial agonist by classical receptor theory--an explanation of the dual action of tamoxifen

Tamoxifen is known to have both agonist and antagonist properties. Classical receptor theory predicts that given the relative concentrations of a partial agonist and full agonist acting on the same receptor, the partial agonist may reduce the effect of the full agonist. The immature rat uterine model is an excellent system to evaluate the interactions of estradiol and tamoxifen by application of receptor theory. Using this model, tamoxifen demonstrates both additive and antagonistic effects to estradiol in the fashion predicted by theory. The effects of tamoxifen are additive at low doses of estradiol and antagonistic over higher estradiol doses. It is possible that the dualism of agonism and antagonism seen in other target organs and species is a function of these basic characteristics of a partial agonist.

High affinity binding of anti-oestrogen to the chick liver nuclear oestrogen receptor

Tamoxifen is a potent inhibitor of specific oestrogen-induced yolk protein synthesis by chicken liver. The oestradiol receptor in salt extracts of liver nuclei from oestrogen-treated chicks has a K(d) for oestradiol of 0.7+/-0.2nm. Tamoxifen and its metabolite, monohydroxytamoxifen, compete for binding to the salt-soluble nuclear receptor with K(i) values of 2.6 and 0.1nm respectively. The anti-oestrogens show much less inhibition of [(3)H]oestradiol binding when assays are carried out using intact nuclei. The competition by unlabelled oestradiol for [(3)H]oestradiol binding to receptor is identical in both salt extracts and intact nuclei. This suggests that intact nuclei contain components which bind anti-oestrogens, but not oestradiol. While tamoxifen and desmethyltamoxifen will readily dissociate from the salt-soluble nuclear oestrogen receptor, monohydroxytamoxifen does not dissociate under the conditions generally used for exchange assays. A modified assay was developed in which 60-70% of monohydroxytamoxifen-bound sites were shown to be exchangeable for [(3)H]oestradiol. Soluble receptor preparations were first incubated in a 1.7% charcoal suspension at 37 degrees C for 15min before assay of specific oestradiol binding. This technique was used in examining the effects of tamoxifen and monohydroxytamoxifen given in vivo on the nuclear oestrogen receptor concentration. Despite their 30-fold difference in binding affinity for the receptor, both anti-oestrogens increase nuclear receptor levels to about the same degree. When given with oestradiol, both compounds have the same apparent partial inhibitory effect on the oestrogen-induced increase in nuclear receptor. These data are consistent with the metabolic hydroxylation of tamoxifen before binding to the hepatic oestrogen receptor.

Nuclear progesterone and estradiol receptors in deciduomata induced in ovariectomised rats

The levels of nuclear progesterone and estradiol receptors have been followed during the development and decline of deciduomata induced in the uteri of ovariectomised rats. There were broadly 2 phases of deciduomal growth, the first phase being associated with high estrogen nuclear receptor levels per unit DNA. Progesterone nuclear receptor levels did not greatly change (in relation to DNA content) during both phases of full deciduomal development but peak levels were observed on day 5 after decidualization. Alkaline phosphatase activity was maximal by day 3, before peak hormone receptor levels were reached in the nuclei.