Inhibition of estrogen-induced activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the MCF-7 human breast cancer and other cell lines transfected with vitellogenin A2 gene promoter constructs

Evaluation of estrogenic potency of a standardized hops extract on mammary gland biology and on MNU-induced mammary tumor growth in rats

Supplements with estrogenic activities are intensively investigated as potential alternatives for the treatment of menopausal symptoms. These investigations include studies on their safety regarding potential breast cancer risks. Therefore, the aim of this study was to assess whether or not a standardized hops (Humulus lupulus) extract, containing 0.42% of the estrogenic flavanone, 8-prenylnaringenin, would stimulate growth of methyl-nitrosourea (MNU) induced mammary cancer in ovariectomized (OVX) Sprague-Dawley (SD) rats or would impact on the proliferative activity within the normal mammary gland of Wistar rats. To induce tumorigenesis SD-rats received an intraperitoneal injection of 50mg/kg body weight of MNU on postnatal days PND 50 and 52. 28days later animals were OVX or were SHAM operated (positive control) and randomly allocated and maintained for 140days on either a phytoestrogen-free placebo diet (SHAM and negative control) or on the hops fortified diet. For the investigations in the normal mammary gland young adult Wistar rats were bilaterally OVX and randomly allocated to a control group fed to a phytoestrogen-free diet, or to a diet supplemented either with E₂-benzoate or the hops extract. As a major result, the tumor incidence was 15% (3 tumors totally) in OVX controls, whereas it was 85% (39 tumors totally) in SHAM operated positive controls. No tumors were detectable in the hops group. In addition, no estrogenic activity of the hops extract was detectable in uterus and liver of these animals. In investigations on the normal mammary gland, no impact of hops extract on the expression of estrogen dependent proliferation markers or of progesterone receptor became apparent. In conclusion, the lack of growth stimulation of MNU-induced breast cancer in OVX SD-rats and the lack of stimulation proliferative events in the normal mammary gland of OVX Wistar rats by standardized hops extracts provides an important piece of evidence regarding the safety of these extracts in the management of menopausal symptoms.

Aryl hydrocarbon receptor activation leads to impairment of estrogen-driven chicken vitellogenin promoter activity in LMH cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates most of the toxic effects of environmental contaminants. Among the multiple pleiotropic responses elicited by AHR agonists, the antiestrogenic and endocrine-disrupting action of the receptor activation is one of the most studied. It has been demonstrated that some AHR agonists disrupt estradiol-induced vitellogenin synthesis in the fish liver via a mechanism that involves crosstalk between the AHR and the estrogen receptor (ER). Chicken hepatocytes have become a model for the study of AHR action in birds and the induction of the signal and its effect in these cells are well established. However, the impact of AHR activation on estradiol-regulated responses in the chicken liver remains to be demonstrated. The aim of the present study was, therefore, to determine the effect of AHR action on ER-driven transcription in a convenient model of chicken liver cells. For this purpose, we designed a reporter construct bearing the 5' regulatory region of the chicken vitellogenin II gene and used it to transfect chicken hepatoma LMH cells. We found that β-naphthoflavone represses ER-driven vitellogenin promoter activity and that this action is mediated by the AHR. This inhibitory crosstalk between both pathways appears to be unidirectional, since estradiol did not alter the transcript levels of an AHR target gene. Besides, and highly relevant, we show that LMH cell line transfected with a reporter construct bearing the chicken vitellogenin promoter sequence is a useful and convenient model for the study of AHR-ER interaction in chicken liver-derived cells.

Differential expression of estrogen receptor alpha (ERalpha) protein in MCF-7 breast cancer cells chronically exposed to TCDD

Estrogens play a key role in the development and evolution of breast cancer tumors. Estrogen receptor alpha (ERalpha) mediates many of the biological activities of estrogens, and its expression is associated with low invasiveness and good prognosis. Recent epidemiological reports suggest that long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is implicated in the increased incidence of breast cancer in exposed women. TCDD interferes with the expression of some ERalpha-dependent genes and inhibits estradiol (E2)- dependent growth of breast cancer cells in vitro. However, E2-dependent xenographs of MCF-7 human breast cancer cells resumed growth after a 2-week exposure to TCDD. The mechanisms involved in the resumption of cell growth are not completely understood. In this study, we show that short term-exposure (16 days) to 1 nM TCDD results in the suppression of ERalpha protein expression, while chronic exposure for more than 1 year (LTDX cells) results in the partial re-expression of the receptor. Immunocytochemistry studies showed that re-expression of ERalpha in LTDX cells occurred in some of the cells. Analysis by Western immunoblots indicated that four out of five LTDX clones expressed ERalpha at levels comparable to those in unexposed MCF-7 cells. Removal of TCDD treatment for 16 days restored the expression of ERalpha in the ERalpha-negative clonal cells. These results suggest that MCF-7 cells chronically exposed to TCDD contain at least two cell subpopulations that may respond differently to the ERalpha-mediated effects of TCDD.

Interactions between estrogen- and Ah-receptor signalling pathways in primary culture of salmon hepatocytes exposed to nonylphenol and 3,3',4,4'-tetrachlorobiphenyl (congener 77)

BACKGROUND

The estrogenic and xenobiotic biotransformation gene expressions are receptor-mediated processes that are ligand structure-dependent interactions with estrogen-receptor (ER) and aryl hydrocarbon receptor (AhR), probably involving all subtypes and other co-factors. The anti-estrogenic activities of AhR agonists have been reported. In teleost fish, exposure to AhR agonists has been associated with reduced Vtg synthesis or impaired gonadal development in both in vivo- and in vitro studies. Inhibitory AhR and ER cross-talk have also been demonstrated in breast cancer cells, rodent uterus and mammary tumors. Previous studies have shown that AhR-agonists potentiate xenoestrogen-induced responses in fish in vivo system. Recently, several studies have shown that AhR-agonists directly activate ER alpha and induce estrogenic responses in mammalian in vitro systems. In this study, two separate experiments were performed to study the molecular interactions between ER and AhR signalling pathways using different concentration of PCB-77 (an AhR-agonist) and time factor, respectively. Firstly, primary Atlantic salmon hepatocytes were exposed to nonylphenol (NP: 5 microM--an ER agonist) singly or in combination with 0.001, 0.01 and 1 microM PCB-77 and sampled at 48 h post-exposure. Secondly, hepatocytes were exposed to NP (5 microM) or PCB-77 (1 microM) singly or in combination for 12, 24, 48 and 72 h. Samples were analyzed using a validated real-time PCR for genes in the ER pathway or known to be NP-responsive and AhR pathway or known to be PCB-77 responsive.

RESULTS

Our data showed a reciprocal inhibitory interaction between NP and PCB-77. PCB-77 produced anti-NP-mediated effect by decreasing the mRNA expression of ER-responsive genes. NP produced anti-AhR mediated effect or as inhibitor of AhR alpha, AhRR, ARNT, CYP1A1 and UDPGT expression. A novel aspect of the present study is that low (0.001 microM) and medium (0.01 microM) PCB-77 concentrations increased ER alpha mRNA expression above control and NP exposed levels, and at 12 h post-exposure, PCB-77 exposure alone produced significant elevation of ER alpha, ER beta and Zr-protein expressions above control levels.

CONCLUSION

The findings in the present study demonstrate a complex mode of ER-AhR interactions that were dependent on time of exposure and concentration of individual chemicals (NP and PCB-77). This complex mode of interaction is further supported by the effect of PCB-77 on ER alpha and ER beta (shown as increase in transcription) with no concurrent activation of Vtg (but Zr-protein) response. These complex interactions between two different classes of ligand-activated receptors provide novel mechanistic insights on signalling pathways. Therefore, the degree of simultaneous interactions between the ER and AhR gene transcripts demonstrated in this study supports the concept of cross-talk between these signalling pathways.

Essiac and Flor-Essence herbal tonics stimulate the in vitro growth of human breast cancer cells

BACKGROUND

People diagnosed with cancer often self-administer complementary and alternative medicines (CAMs) to supplement their conventional treatments, improve health, or prevent recurrence. Flor-Essence and Essiac Herbal Tonics are commercially available complex mixtures of herbal extracts sold as dietary supplements and used by cancer patients based on anecdotal evidence that they can treat or prevent disease. In this study, we evaluated Flor-Essence and Essiac for their effects on the growth of human tumor cells in culture.

METHODS

The effect of Flor-Essence and Essiac((R)) herbal tonics on cell proliferation was tested in MCF-7, MDA-MB-436, MDA-MB-231, and T47D cancer cells isolated from human breast tumors. Estrogen receptor (ER) dependent activation of a luciferase reporter construct was tested in MCF-7 cells. Specific binding to the ER was tested using an ICI 182,780 competition assay.

RESULTS

Flor-Essence and Essiac herbal tonics at 1%, 2%, 4% and 8% stimulated cell proliferation relative to untreated controls in both estrogen receptor positive (MCF-7 and T47D) and estrogen receptor negative (MDA-MB-231 and MDA-MB-436) cell lines. Exposure to the tonics also produced a dose-dependent increase in ER dependent luciferase activity in MCF-7 cells. A 10(-7) M concentration of ICI 182,780 inhibited the induction of ER dependent luciferase activity by Flor-Essence and Essiac, but did not affect cell proliferation.

CONCLUSION

Flor-Essence and Essiac Herbal Tonics can stimulate the growth of human breast cancer cells through ER mediated as well as ER independent mechanisms of action.

PhIP Carcinogenicity in Breast Cancer: Computational and Experimental Evidence for Competitive Interactions with Human Estrogen Receptor

Many carcinogens have been shown to cause tissue specific tumors in animal models. The mechanism for this specificity has not been fully elucidated and is usually attributed to differences in organ metabolism. For heterocyclic amines, potent carcinogens that are formed in well-done meat, the ability to either bind to the estrogen receptor and activate or inhibit an estrogenic response will have a major impact on carcinogenicity. Here, we describe our work with the human estrogen receptor alpha (ERalpha), the mutagenic/carcinogenic heterocyclic amines PhIP, MeIQx, and IFP, and the hydroxylated metabolite of PhIP, N2-hydroxy-PhIP. We demonstrate both by computational docking and NMR analysis that PhIP binds with the ligand binding domain (LBD). This binding competes with estradiol (E2) in the native E2 binding cavity of the receptor. In vitro assays show that PhIP, in contrast to the other heterocyclic amines, increases cell proliferation in MCF-7 human breast cancer cells and activates the ERalpha receptor. We also find that other heterocyclic amines and N2-hydroxy-PhIP inhibit ERalpha activation. We propose that the mechanism for the tissue-specific carcinogenicity seen in the rat breast tumors and the presumptive human breast cancer associated with the consumption of well-done meat maybe mediated by this receptor activation.

Bone resorption by aryl hydrocarbon receptor-expressing osteoclasts is not disturbed by TCDD in short-term cultures

Polychlorinated dibenzo-p-dioxins (PCDDs) are highly toxic environmental contaminants, and 2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD) is the most potent dioxin. Dioxins bind specifically to the cytosolic aryl hydrocarbon receptor (AHR), which is a ligand-activated transcription factor, and a majority of toxic effects of dioxins are mediated via AHR. We have recently demonstrated that TCDD disrupts bone modeling and decreases bone mechanical strength, and that partial resistance to these effects is related to an altered transactivation domain in AHR structure. In order to better understand the effects of dioxins on bone, we studied the presence and precise localization of AHR and also the number and activity of osteoclasts after TCDD treatments. Total RNA was extracted from mixed bone cell population cultures and expression of AHR mRNA was studied using RT-PCR. Bone cells expressed a considerable amount of AHR mRNA. To see which bone cells express AHR, immunostainings were performed in primary rat bone cell cultures, pure human osteoclast cultures and histological sections from AHR knockout and wild type bones. Immunostaining revealed a strong expression of AHR both in osteoclasts and osteoblasts with an especially prominent stain in bone resorbing osteoclasts. Effects of dioxin on primary bone cells were evaluated after TCDD treatment in the pit formation assay. The activity of osteoclasts was not affected measured as the percentage of active osteoclasts and the actual area of resorbed bone. These data indicate that even though TCDD-treated bones show decreased mechanical strength and size, this is not a direct result from increased osteoclastic bone resorption.

Development of selective aryl hydrocarbon receptor modulators for treatment of breast cancer

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix DNA-binding protein that forms a transcriptionally-active heterodimer with the AhR nuclear translocator (Arnt) protein. The nuclear AhR complex is a ligand-induced transcription factor and the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a high affinity ligand for the AhR. TCDD induces a diverse spectrum of tissue-, sex- and species-specific biochemical and toxic responses in Ah-responsive cells/tissues including the inhibition of 17beta-oestradiol (E2)-induced gene expression in the rodent uterus and mammary and in human breast cancer cell lines. TCDD also inhibits spontaneous and carcinogen-induced mammary tumour formation and growth in rodent models. Research in this laboratory has utilised the AhR as a target for developing anticancer drugs for treatment of breast cancer and two different structural classes of selective AhR modulators (SAhRMs) have been developed. Alternate-substituted (1,3,6,8- and 2,4,6,8-) alkyl polychlorinated dibenzofurans (PCDFs) and substituted diindolylmethanes (DIMs) bind the AhR and induce a pattern of AhR-oestrogen receptor (ER) inhibitory cross-talk similar to that observed for TCDD including inhibition of mammary tumour growth at doses < 1.0 mg/kg/day. In contrast, effective doses of these compounds do not induce hepatic CYP1A1-dependent activity or other AhR-mediated toxic responses induced by TCDD. These results indicate that SAhRMs may be an important new class of drugs for clinical treatment of breast cancer via AhR-ER inhibitory cross-talk.

The aryl hydrocarbon receptor-mediated disruption of vitellogenin synthesis in the fish liver: Cross-talk between AHR- and ERalpha-signalling pathways

BACKGROUND: In the fish liver, the synthesis of egg yolk protein precursor vitellogenin (VTG) is under control of the estrogen receptor alpha (ERalpha). Environmental contaminants such as 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) are suspected to have antiestrogenic effects. The aryl hydrocarbon receptor (AHR) is the initial cellular target for TCDD and related compounds. The AHR is a ligand-activated transcription factor that stimulates the expression of the genes encoding xenobiotic metabolizing enzymes, such as cytochrome P450 1A (CYP1A). In this study, the effects of activation of AHR on the hepatic expression of VTG and ERalpha genes, in primary cultured salmon hepatocytes, have been investigated. RESULTS: The expression of the genes encoding VTG and ERalpha were strongly induced by 17beta-estradiol (E2). However, the expression of VTG was disrupted by exposure of the cells to TCDD while CYP1A expression was enhanced. The effect of TCDD on VTG and CYP1A expression was annulled by the AHR-inhibitor alpha-naphthoflavone. Furthermore, exposure of the cells to TCDD abolished E2-induced accumulation of ERalpha mRNA. The AHR-mediated inhibitory effects on the expression of the VTG and ERalpha genes may occur at transcriptional and/or post-transcriptional levels. Nuclear run-off experiments revealed that simultaneous exposure of the cells to E2 and TCDD strongly inhibited the initiation of transcription of the VTG and ERalpha genes. In addition, inhibition of RNA synthesis by actinomycin D treatment showed that post-transcriptional levels of VTG and ERalpha mRNAs were not significantly altered upon treatment of the cells with TCDD. These results suggested that activation of AHR may inhibit the transactivation capacity of the ERalpha. Further, electrophoretic mobility shift assays using nuclear extracts prepared from cells treated for one or two hours with E2, alone or in mixture with TCDD, showed a strong reduction in the DNA binding activities upon TCDD treatment. These results also suggested that activation of the AHR signalling pathway caused a marked decrease in the number of the nuclear ERalpha or that activated AHR blocked the ability of ERalpha to bind to its target DNA sequence. Finally, our results from Northern hybridizations indicated that E2 treatment of the cells did not cause any significant effect on the TCDD-induced levels of CYP1A mRNA. CONCLUSION: In fish hepatocytes E2 induces ERalpha and VTG gene expression. The presence of dioxin (TCDD) abolishes this induction, probably through the action of AHR in complex with AHR nuclear translocator, and possibly by direct interference with the auto-regulatory transcriptional loop of ERalpha. Furthermore, E2 does not interfere with TCDD induced CYP1A gene expression, suggesting that cross-talk between the ERalpha- and AHR-signalling pathways is unidirectional.

Hypoxia induces proteasome-dependent degradation of estrogen receptor alpha in ZR-75 breast cancer cells

Regulation of estrogen receptor alpha (ERalpha) plays an important role in hormone responsiveness and growth of ER-positive breast cancer cells and tumors. ZR-75 breast cancer cells were grown under conditions of normoxia (21% O(2)) or hypoxia (1% O(2) or cobaltous chloride), and hypoxia significantly increased hypoxia-inducible factor 1alpha protein within 3 h after treatment, whereas ERalpha protein levels were dramatically decreased within 6-12 h, and this response was blocked by the proteasome inhibitor MG-132. In contrast, hypoxia induced only minimal decreases in cellular Sp1 protein and did not affect ERalpha mRNA; however, hypoxic conditions decreased basal and 17beta-estradiol-induced pS2 gene expression (mRNA levels) and estrogen response element-dependent reporter gene activity in ZR-75 cells. Although 17beta-estradiol and hypoxia induce proteasome-dependent degradation of ERalpha, their effects on transactivation are different, and this may have implications for clinical treatment of mammary tumors.

Analysis of the antiestrogenic activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in human ovarian carcinoma BG-1 cells

We have used human ovarian carcinoma BG-1 cells to determine which steps in the pathway of estrogen signaling are disrupted by the aryl hydrocarbon receptor (AhR) ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We report that inhibition of estrogen signaling occurs between 7 and 18 h after TCDD treatment and that this effect is not caused by a decrease in estradiol concentration. TCDD decreased estrogen receptor (ER) levels in cells grown in standard medium; however, in estrogen-stripped medium, ER (but not AhR) levels were dramatically reduced (approximately 7-fold) but were not decreased further by TCDD. Because the absolute level of estradiol inducibility and inhibition by TCDD was similar in either medium, decreases in ER are not responsible for the antiestrogenic effect. The AhR also did not bind to the estrogen-responsive element (ERE) in vitro, and ERE binding by nuclear ER complexes was not decreased by TCDD, indicating that the effect of TCDD does not involve direct competition between the AhR and ER for DNA binding. However, inhibition of protein synthesis by cycloheximide blocked the TCDD-induced inhibition of ER-dependent gene expression. Overall, our results are consistent with the action of a TCDD-induced protein at a step(s) after ER-DNA binding, most likely at the level of gene transcription.

Gestational and lactational exposure to TCDD or coplanar PCBs alters adult expression of saccharin preference behavior in female rats

Previous studies have shown that maternal doses of 1 microg/kg or less of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in late gestation can demasculinize and feminize reproductive behavior in male rats. However, it was not known whether coplanar polychlorinated biphenyls (PCBs) had similar effects, or whether non-reproductive sexually dimorphic behaviors such as saccharin preference behavior were also altered. We determined the effects of TCDD or coplanar PCBs on saccharin consumption and saccharin preference in male and female rats. Sprague-Dawley rats were dosed with 3,3',4, 4'-tetrachlorobiphenyl (PCB 77; 2 or 8 mg/kg/day), 3,3',4,4', 5-pentachlorobiphenyl (PCB 126; 0.25 or 1.0 microg/kg/day), TCDD (0. 025 or 0.10 microg/kg/day), or corn oil vehicle on days 10-16 of gestation. Maternal exposure to TCDD or coplanar PCBs did not change saccharin consumption or saccharin preference in male rats. However, TCDD and coplanar PCB-exposed females showed decreased saccharin consumption and saccharin preference. The results indicate that saccharin consumption is masculinized in female rats exposed to TCDD or coplanar PCBs during perinatal development. This effect could be related to the anti-estrogenic actions of these chemicals.

Role of HSP90 in mediating cross-talk between the estrogen receptor and the Ah receptor signal transduction pathways

Tetrachlorodibenzo-p-dioxin (TCDD)-mediated gene transactivation via the Ah receptor (AhR) has been shown to be dependent upon estrogen receptor (ER) expression in human breast cancer cells. We have investigated the 90-kDa heat shock protein (HSP90) as a mediator of cross-talk between the AhR and the ER signal transduction pathways. The effect of HSP90 overexpression on receptor activity was determined by transient transfection assays using a HSP90 expression vector. Ligand-inducible gene expression was inhibited when the HSP90 expression vector was cotransfected with a TCDD-responsive reporter plasmid. However, overexpression of HSP90 did not block induction of an estrogen-responsive reporter plasmid. To determine whether ER facilitates AhR signaling through its ability to squelch HSP90, two vectors expressing protein products that bind HSP90 were transfected into MDA-MB-231 cells. Introduction of (i) He11, an ER deletion mutant that does not bind DNA, and (ii) the ligand-binding domain of human AhR, both led to increased basal and TCDD-inducible CYP1A1 expression. Finally, the subcellular distribution of HSP90 was investigated in human breast cancer cell lines. These studies showed HSP90 to be primarily cytoplasmic in ER-positive cell lines, whereas in matched ER-negative cell lines HSP90 was distributed equally between the cytoplasm and nucleus. Taken together, these results demonstrate that HSP90 can regulate AhR activity in vivo, and that Ah-responsiveness is dependent upon cellular ER content through a mechanism that involves HSP90.

ECC-1 human endometrial cells as a model system to study dioxin disruption of steroid hormone function

ECC-1, an established epithelial cell line derived from an adenocarcinoma of human endometrial lining, was examined for growth optimization, steroid hormone receptor- and Ah receptor content, and dioxin modulation of estrogen receptor function. Proliferation of ECC-1 cells was accelerated by growth on a lethally irradiated feeder layer of murine 3T3 fibroblasts. Immunoblot analysis demonstrated the presence of Ah receptor an intracellular protein that binds and regulates the toxic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The Ah receptor was functional in these cells as assessed by concentration and kinetic patterns of CYP1A1-mediated 7-ethoxycoumarin O-deethylase (ECOD) induction. The half-maximal effective concentration (EC50) for TCDD was 0.2 nM, and maximal activity appeared after 24-h exposure. A limited structure-activity examination of ECOD activity provided additional evidence for Ah receptor involvement. Competitive binding assays were performed to examine kinetic parameters for estrogen, progesterone, and glucocorticoid receptors. Binding parameters of dissociation constant (Kd) and number of binding sites (Bmax) derived from Scatchard analysis were: estrogen, Kd = 0.67 nM; Bmax = 321 fmol/mg cytosolic protein; progesterone, Kd = 1.31 nM; Bmax = 258 fmol/mg cytosolic protein; dexamethasone, Kd = 1.75 nM, Bmax = 128 fmol/mg cytosolic protein. Exposure of ECC-1 cells to TCDD reduced the estrogen receptor level by 40% without affecting the Kd value, and reduced estrogen receptor-mediated transcription by 50% assessed by transient transfection of an estrogen-responsive reporter plasmid. These data suggest that the ECC-1 cell line is a useful model system for examining the action of dioxin in human endometrial tissue. Both the estrogen receptor and Ah receptor have been implicated in diseases of the endometrium, and examining their interactions may elucidate mechanisms of uterine disease etiology, as well as potential targets for disease prevention.

Estrogen- and antiestrogen-responsiveness of HEC1A endometrial adenocarcinoma cells in culture

HEC1A endometrial cancer cells express the wild-type form of the estrogen receptor (ER) and 17beta-estradiol (E2) induces proliferation of these cells. In contrast, tamoxifen only causes a minimal increase (<20%) in cell proliferation. In HEC1A cells transiently transfected with the C3-Luc plasmid derived from the complement C3 gene, both E2 and tamoxifen exhibited ER agonist activity and tamoxifen was also a partial antagonist for this response. The relative ER agonist/antagonist activities of E2, tamoxifen and ICI 182,780 were also investigated in HEC1A1 cells transiently transfected with two E2-responsive plasmids, pCATHD-CAT and pCKB-CAT which contain 5'-promoter inserts from the cathepsin D and creatine kinase B genes, respectively. The results showed that E2 and tamoxifen induced reporter gene activity in cells transiently transfected with both constructs. ICI 182,780 exhibited partial ER agonist activity only in cells transiently transfected with pCKB-CAT and antagonized E2-induced reporter gene activity using both the CKB- and CATHD-derived constructs. These results demonstrate that HEC1A endometrial cancer cells are E2-responsive and represent a useful cell culture model for understanding hormone/antihormone-induced endometrial cell responses.