Regulation of estrogen receptor mRNA by 2,3,7,8-tetrachlorodibenzo-p-dioxin as measured by competitive RT-PCR

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Effects of PCB 126 and PCB 153 on secretion of steroid hormones and mRNA expression of steroidogenic genes (STAR, HSD3B, CYP19A1) and estrogen receptors (ERα, ERβ) in prehierarchical chicken ovarian follicles

The objective of this study was to assess the in vitro effects of dioxin-like PCB 126 and non-dioxin-like PCB 153 on basal and ovine LH (oLH)-stimulated testosterone (T) and estradiol (E2) secretion and expression of steroidogenic genes (STAR, HSD3B and CYP19A1) and estrogen receptors α (ERα) and β (ERβ) in white (WF) and yellowish (YF) prehierarchical follicles of the hen ovary. Steroid concentrations in a medium and gene expression in follicles following 6h of exposition were determined by RIA and real-time qPCR, respectively. Both PCBs increased basal and oLH-stimulated T secretion by the WF follicles. PCB 126 reduced basal E2 secretion by the WF follicles. PCB 153 elevated but PCB 126 reduced oLH-stimulated E2 secretion by the prehierarchical follicles. PCB 126 increased basal STAR and HSD3B and reduced CYP19A1 mRNA expression in these follicles. PCB 153 increased basal expression of STAR and HSD3B in YF follicles, but diminished HSD3B mRNA levels in the WF. The studied PCBs had an opposite effect on basal and oLH-stimulated CYP19A1 mRNA expression in prehierarchical follicles. Both PCBs modulated basal and inhibited oLH-stimulated ERα and ERβ gene expression in the prehierarchical follicles. In conclusion, data of the current study demonstrate the congener-specific effects of PCBs on sex steroid secretion by prehierarchical follicles of the chicken ovary, which are at least partly related to STAR, HSD3B and CYP19A1 gene expression. It is suggested that PCBs, by influencing follicular steroidogenesis and expression of estrogen receptors, may impair development and selection of yellowish follicles to the preovulatory hierarchy.

Halowax 1051 affects steroidogenesis by down-regulation of aryl hydrocarbon and estrogen receptors and up-regulation of androgen receptor in porcine ovarian follicles

Polychlorinated naphthalenes (PCNs) are thought to interact with the aryl hydrocarbon receptor (AHR) and to have enzyme-inducing properties comparable to polychlorinated dibenzo-p-dioxins, therefore activation of steroid hormone receptors in endocrine tissues is also possible. The aim of the present study was to examine the effects of PCNs mixture, Halowax 1051 on gene and protein expression of receptors: estradiol (ERα/β), androgen (AR) and AHRGene expression was evaluated by real-time PCR after 6 h of exposition and protein expression by Western blot after 24 h. Levels of sex steroids: androstenedione (A4), estradiol (E2) and testosterone (T) were measured by enzyme immunoassays. Results of the data show down-regulation of AHR gene expression after 6 h in parallel with an inhibition in AHR protein expression at doses 10 pg-10 ng/mL, down-regulation of ER at all doses used, and up-regulation of AR gene expression at doses 1 and 10 ng/mL without affecting their protein expression. To indicate the involvement of AHR, ERs and AR in the impact of PCNs on steroidogenesis, we used their specific blockers. Blocker of AHR reversed the inhibitory effect of Halowax 1051 on A4 secretion, and strengthened its effect on T secretion. Blockers of both ER and AR had no effect on Halowax 1051 action on steroids secretion. The results of this study suggest that AHR is involved in the effect of PCNs on steroidogenesis in the ovary. Additionally, we propose that cross-talk between AHR-ER and AHR-AR receptors mediates the effects of Halowax 1051 on ovarian follicles.

Estrogenic endocrine disruptors: Molecular mechanisms of action

A comprehensive summary of more than 450 estrogenic chemicals including estrogenic endocrine disruptors is provided here to understand the complex and profound impact of estrogen action. First, estrogenic chemicals are categorized by structure as well as their applications, usage and effects. Second, estrogenic signaling is examined by the molecular mechanism based on the receptors, signaling pathways, crosstalk/bypassing and autocrine/paracrine/homeostatic networks involved in the signaling. Third, evaluation of estrogen action is discussed by focusing on the technologies and protocols of the assays for assessing estrogenicity. Understanding the molecular mechanisms of estrogen action is important to assess the action of endocrine disruptors and will be used for risk management based on pathway-based toxicity testing.

Urinary phthalate metabolite concentrations in relation to history of infertility and use of assisted reproductive technology

OBJECTIVE

To examine urinary phthalate metabolite concentrations in pregnant women with planned pregnancies in relation to history of infertility and use of assisted reproductive technology (ART).

DESIGN

Phthalate metabolite concentrations were measured in first-trimester urine samples collected from women participating in a prospective pregnancy cohort study.

SETTING

Prenatal clinics.

PATIENT(S)

A total of 750 women, of whom 86 had a history of infertility. Forty-one women used ART to conceive.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Primary outcomes were concentrations of four metabolites of diethylhexyl phthalate (DEHP) and their molar sum (∑DEHP). Multivariable analyses compared phthalate metabolite levels in [1] women reporting a history of infertility vs. those who did not (comparison group); and [2] those who used ART to conceive the index pregnancy vs. women with a history of infertility who did not use ART.

RESULT(S)

Among women with a history of infertility, ∑DEHP was significantly lower in women who conceived after ART compared with those who did not (geometric mean ratio: 0.83; 95% confidence interval 0.71-0.98). Similar significant associations were observed for all of the individual DEHP metabolites. There were no differences in DEHP metabolite concentrations between women with a history of infertility and the comparison group.

CONCLUSION(S)

Women who used ART to conceive had lower first-trimester phthalate metabolite concentrations than women with a history of infertility who did not use ART. Further research is needed to explore whether those pursuing fertility treatments take precautions to avoid exposure to environmental toxins, to improve treatment outcomes.

Aryl hydrocarbon receptor activation in lactotropes and gonadotropes interferes with estradiol-dependent and -independent preprolactin, glycoprotein alpha and luteinizing hormone beta gene expression

Arylhydrocarbon receptor (Ahr) activation by 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) interferes with female reproductive functions, but there is little information on the specific targets of TCDD in the hypothalamic-pituitary-gonadal (HPG) axis. In these studies, we found that TCDD upregulated known AhR target genes, cytochrome p450 1a1 (Cyp1a1), Cyp1a2 and Cyp1b1 in the rat pituitary gland. Moreover, 75% of pituitary lactotropes and 45% of gonadotropes contained Ahr mRNA, and most Ahr-containing cells were estrogen receptor 1 (Esr1)-positive. TCDD abrogated estradiol (E(2))-induced prolactin (Prl) expression in vivo and in vitro; conversely, E(2) blocked TCDD upregulation of luteinizing hormone beta (Lhb) and glycoprotein hormone alpha polypeptide (Cga) expression. TCDD had no effect on levels of Ahr mRNA, but upregulated Esr1 mRNA. E(2) independently repressed Ahr and Esr1 expression and blocked TCDD upregulation of Esr1. Thus, complex interactions between Ahr and Esr alter Prl and luteinizing hormone (LH) synthesis by direct actions in lactotropes and gonadotropes. These findings provide important insights into how TCDD disrupts female reproductive functions.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Estrogen responses in killifish (Fundulus heteroclitus) from polluted and unpolluted environments are site- and gene-specific

Epidemiological, ecological, and laboratory-based studies support the hypothesis that endocrine disrupting chemicals (EDCs) in the environment are responsible for developmental and reproductive abnormalities. We have previously described a killifish population resident in a highly polluted Superfund site (New Bedford Harbor, NBH) that shows evidence of exposure to an estrogenic environment and endocrine disruption. Here, we compare NBH with a local reference population (Scorton Creek, SC) for developmental patterns and direct effects of exogenous estradiol on the estrogenic markers, brain cytochrome P450 aromatase (CYP19A2 or AroB), hepatic vitellogenin (Vtg), and hepatic estrogen receptor alpha (ER alpha). In contrast to our previous observation of elevated ER alpha in NBH embryos, developmental levels of AroB and Vtg mRNAs did not differ between the two sites, demonstrating that not all estrogen-responsive genes are upregulated in NBH embryos. A dose-response experiment showed that NBH larvae are less responsive (lower maximum induction, as measured by ER alpha) and less sensitive (higher EC(50) for induction, as measured by AroB) to estradiol than SC larvae, changes that would be adaptive in an estrogenic environment. In contrast, induction of Vtg mRNA is similar in the two populations, indicating that the adaptive mechanism is target gene-specific. Based on the lower basal levels of ER alpha mRNA in several tissues from adult NBH fish vs SC fish (Greytak and Callard, 2007), we predicted estrogen hyporesponsiveness; however, induction of ER alpha by estradiol exposure in reproductively inactive males did not differ between the two sites. Moreover, AroB was more responsive and Vtg induction was greater (2d) or similar (5d) in NBH as compared to SC males. Worth noting is the high inter-individual variability in estrogen responses of gene targets, especially in NBH killifish, which may indicate evolving preadaptive or adaptive mechanisms. In conclusion, although multi-generational exposure to a highly polluted environment is associated with changes in basal levels of ER alpha mRNA, this is not a simple predictor of estrogen responsiveness. We hypothesize that adaptation of killifish to the estrogenic and polluted environment may be occurring through diverse mechanisms that are gene-, tissue type- and life-stage-specific.

Molecular targets of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) within the zebrafish ovary: insights into TCDD-induced endocrine disruption and reproductive toxicity

TCDD is a reproductive toxicant and endocrine disruptor, yet the mechanisms by which it causes these reproductive alterations are not fully understood. In order to provide additional insight into the molecular mechanisms that underlie TCDD's reproductive toxicity, we assessed TCDD-induced transcriptional changes in the ovary as they relate to previously described impacts on serum estradiol concentrations and altered follicular development in zebrafish. In silico computational approaches were used to correlate candidate regulatory motifs with observed changes in gene expression. Our data suggest that TCDD inhibits follicle maturation via attenuated gonadotropin responsiveness and/or depressed estradiol biosynthesis, and that interference of estrogen-regulated signal transduction may also contribute to TCDD's impacts on follicular development. TCDD may also alter ovarian function by disrupting various signaling pathways such as glucose and lipid metabolism, and regulation of transcription. Furthermore, events downstream from initial TCDD molecular-targets likely contribute to ovarian toxicity following chronic exposure to TCDD. Data presented here provide further insight into the mechanisms by which TCDD disrupts follicular development and reproduction in fish, and can be used to formulate new hypotheses regarding previously documented ovarian toxicity.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin administration and high-fat diet on the body weight and hepatic estrogen metabolism in female C3H/HeN mice

We studied the effect of administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by i.p. injection once every 2 weeks in combination with a high-fat (HF) diet for 8 or 16 weeks on the body and organ weight changes as well as on the hepatic enzyme activity for estrogen metabolism in C3H/HeN female mice. Administration of TCDD at 100 microg/kg b.w. once every 2 weeks for 8 weeks increased the body weight by 46% in the HF diet-fed animals, but not in the regular diet-fed animals. This is the first observation suggesting that TCDD at a high dose (100 microg/kg b.w.), but not at lower doses (1 or 10 microg/kg b.w.), may have a strong obesity-inducing effect in C3H/HeN mice fed an HF diet. While TCDD increased liver weight and decreased thymus weight in animals, these effects were enhanced by feeding animals an HF diet. Metabolism studies showed that TCDD administration for 8 or 16 weeks increased the liver microsomal activity for the 2- and 4-hydroxylation of 17 beta-estradiol in animals fed a control diet, but surprisingly not in animals fed an HF diet. Treatment with TCDD dose-dependently increased the hepatic activity for the O-methylation of catechol estrogens in both control and HF diet-fed animals, and it also decreased the levels of liver microsomal sulfatase activity for hydrolysis of estrone-3-sulfate. TCDD did not significantly affect the hepatic enzyme activity for the glucuronidation or esterification of endogenous estrogens. It is suggested that enhanced metabolic inactivation of endogenous estrogens by hepatic estrogen-metabolizing enzymes in TCDD-treated, control diet-fed animals contributes importantly to the reduced incidence of estrogen-associated tumors in animals treated with TCDD.

Environmental toxicants and effects on female reproductive function

One of the most toxic substances known to humans, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin), is also highly pervasive in the environment. It is created naturally in volcanic eruptions and forest fires, and anthropogenically in waste incineration, chlorination processes and certain plastics manufacture. From reports of large industrial and other accidents, or from experimental studies, dioxin exposure has been correlated in animal models and/or humans with chloracne of the skin, organ cancers, hepatotoxicity, gonadal and immune changes, pulmonary and other diseases such as diabetes, skewing of the sex ratio, and infertility. We have demonstrated that the aromatic hydrocarbon receptor (AHR) that binds dioxin in tissues is localized in zebrafish, rat and rhesus monkey (Macaca mulatta) ovaries and in rat and human luteinizing granulosa cells (GC) (among other tissues), that labeled dioxin is specifically localized to granulosa cells of the ovarian follicle as observed by autoradiography, and that incubations of GC or ovarian fragments with environmentally relevant concentrations (fM to nM) of dioxin inhibit estradiol secretion significantly. Our experiments show that in human, non-human primate, rat, trout, and zebrafish ovarian tissues, dioxin inhibits estrogen synthesis at some level of the steroid biosynthetic pathway, most likely by inhibiting transcription of mRNAs for or activity of side-chain cleavage (Cyp11a1 gene) and/or aromatase (Cyp19a1 gene) enzymes, or conceivably other steroidogenic enzymes/factors. Such an untoward effect on estrogen synthesis in females exposed to dioxin environmentally may predispose them to defects in aspects of their fertility.

Identification of estrogen receptor beta expression in Chinese hamster ovary (CHO) cells and comparison of estrogen-responsive gene transcription in cells adapted to serum-free media

Most cultured cell lines require addition of serum to the medium to maintain their proliferative capacity. For studies examining the cellular effects of estrogens serum is charcoal-stripped to remove steroids. Nonetheless, addition of the selective estrogen receptor modulator (SERM) 4-hydroxytamoxifen (4-OHT) inhibits the basal transcriptional activity of estrogen receptors alpha or beta (ERalpha or ERbeta) in transfected cells. We tested the hypothesis that elimination of serum from the culture medium will block 4-OHT's repression of basal activity. Chinese hamster ovary (CHO-K1) cells adapted to serum-free medium exhibited estrogen responsiveness that was identical with that of the cells grown in serum-containing media. 4-OHT-suppressed basal transcription of an estrogen response element (ERE)-reporter in ERalpha-transfected cells even in the absence of serum, indicating that the 4-OHT suppressive activity is not mediated by blocking ER interaction with serum estrogens. We speculate that 4-OHT-ER recruits co-repressors to suppress basal transcription. We discovered that CHO-K1 cells express ERalpha and ERbeta mRNA. However only ERbeta protein was expressed and use of ERbeta-selective 2,3-bis(4-hydroxy-phenyl)propionitrile (DPN) and ERalpha-selective 4-propyl-1,3,5-tris(4-hydroxy-phenyl)pyrazole) (PPT) revealed that only ERbeta was transcriptionally active. In conclusion, growing CHO-K1 in serum-free medium does not impact the estrogen responsiveness and this cell line expresses functional ERbeta.

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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases steady-state estrogen receptor-beta mRNA levels after CYP1A1 and CYP1B1 induction in rat granulosa cells in vitro

Previous in-vitro investigations of rat granulosa cells (GC) have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits estrogen secretion and FSH-induced aromatase activity. Although TCDD exerted no effect on basal aromatase enzyme activity, TCDD did reduce steady-state aromatase mRNA levels in GC using competitive RT-PCR. TCDD is hypothesized to induce these changes through aromatic hydrocarbon receptor(AHR)-mediated gene transcription and the modulation of the estrogen receptor (ER)-signaling pathway. In this study we show that rat GC express mRNA for AHR and the AHR nuclear translocator (ARNT) as well as biomarkers of TCDD action, CYP1A1 and CYP1B1 mRNA. Basal CYP1A1 and ER-alpha mRNAs were present only in trace amounts. By relative RT-PCR analysis we showed that CYP1A1 and CYP1B1 mRNA were induced significantly by TCDD at 6 h and that induction of CYP1A1 was maintained throughout the experiment. Using competitive RT-PCR, we observed no significant change in the mRNA levels of ARNT between control and TCDD-treated GC. Both AHR and ER-beta mRNA levels increased significantly at 48 h with TCDD compared with controls. Since ER-beta mRNA was not increased significantly until 48 h in culture, we suggest that in rat GC, the observed ER-beta mRNA increase by TCDD might be a result of CYP1A1/CYP1B1 catalyzed estrogen metabolism and aromatase mRNA inhibition via AHR.

Interaction of estrogen and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in immature male chickens (Gallus domesticus)

The interaction of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and estrogen was studied in chickens to more clearly define this relationship in an avian species and its role in the enhanced sensitivity of female chickens to TCDD-induced wasting syndrome. Twenty male chickens (7-9 weeks old) were divided evenly into four groups: control (CTL, received the same volume of vehicle); estrogen-treated (E2, 1 mg/kg estradiol cypionate injections on days 1, 2 and 3); TCDD-treated (TCDD, single 50 microg/kg injection on day 4); and estrogen plus TCDD (E2+TCDD, as above), with measurements taken on day 14. The E2 group compared with the CTL group had decreased comb height (24%), comb length (26%) and adipose tissue (AT) lipoprotein lipase (LPL) activity relative to AT mass (51%), while liver mass and body weight gain were each increased by 28%. The TCDD group had increased liver mass (62%), reduced comb length (17%), and reduced AT LPL activity indexed to AT mass (70%) compared with the CTL group. Finally, the E2+TCDD group had 37% lower body weight gain and 30% larger livers relative to body mass compared with the E2 group, but were not significantly different from the TCDD group. These data show that TCDD antagonized several effects of exogenous estrogen in male chickens, while estrogen enhanced TCDD toxicity in a tissue-specific manner.

Demonstration of 2,3,7,8-tetrachlorodibenzo-p-dioxin attenuation of P450 steroidogenic enzyme mRNAs in rat granulosa cell in vitro by competitive reverse transcriptase-polymerase chain reaction assay

We investigated the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), in prepubertal (PP) and adult (A) rat granulosa cells (GC) in vitro by examining the changes in estrogen secretion, aromatase enzyme activity and mRNAs for steroidogenic enzymes P450scc, 3beta-HSDI, P450arom; and for components of the AHR signaling pathway-CYP1A1, aromatic hydrocarbon receptor (AHR), and the AHR nuclear translocator protein (ARNT). In PP and A rat GC, TCDD (3.1 nM) reduced estrogen secretion at 48 h without altering aromatase enzyme activity. Addition of FSH (50 ng/ml) increased aromatase activity in GC with or without TCDD. FSH-induced aromatase activity was significantly reduced by TCDD (3.1 nM) at 48 h. Semi-quantitative RT-PCR showed a significant increase in CYP1A1 mRNA both at 24 and 48 h with TCAP, while a significant reduction in P450scc and P450arom mRNA was observed with competitive RT-PCR. All steroidogenic enzyme mRNAs were significantly lower in adults than in PP GC. We conclude that in rat GC, TCDD modulates the level of cytochrome P450 enzymes involved in the steroid biosynthetic cascade. This effect may be attributable to AHR interaction with dioxin-responsive elements present in the genes encoding these enzymes.

The aryl hydrocarbon receptor interacts with estrogen receptor alpha and orphan receptors COUP-TFI and ERRalpha1

The molecular mechanisms underlying the apparent "cross-talk" between estrogen receptor (ER)- and arylhydrocarbon receptor (AHR)-mediated activities are unknown. To determine how AHR ligand 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) may inhibit ER action and, conversely, to examine how 17-beta-estradiol (E(2)) affects AHR activity, we examined discrete activities of each receptor, i.e., protein-protein interactions, DNA binding, and transcriptional activation. We report that AHR interacts directly with ERalpha, COUP-TF, and ERRalpha1, in a ligand-specific manner in vitro. Unoccupied or beta-napthoflavone (beta-NF)-occupied AHR showed stronger interaction with ERalpha, COUP-TF, and ERRalpha1 than when AHR was occupied by the partial antagonist alpha-naphthoflavone (alpha-NF), indicating a role for ligand in AHR interaction with these proteins. We also report that AHR interacts with COUP-TF in transfected CV-1 cells. In contrast, the AHR nuclear translocator protein (ARNT) did not interact with COUP-TF, ERRalpha1, or ERalpha. We next examined the interaction of either ERalpha or COUP-TF with a consensus xenobiotic response element (XRE). Purified ERalpha did not bind the consensus XRE, but COUP-TFI bound the consensus XRE, suggesting a role for COUP-TF as a AHR/ARNT competitor for XRE binding. In transiently transfected MCF-7 human breast cancer cells, overexpression of COUP-TFI inhibited TCDD-activated reporter gene activity from the CYP1A1 promoter. TCDD inhibited estradiol (E(2))-activated reporter gene activity from a consensus ERE and from the EREs in the pS2 and Fos genes, and COUP-TFI did not block the antiestrogenic activity of TCDD. The specific interaction of COUP-TF with XREs and AHR together with the inhibition of TCDD-induced gene expression by COUP-TF suggests that COUP-TF may regulate AHR action both by direct DNA binding competition and through protein-protein interactions.

The aryl hydrocarbon receptor (AHR)/AHR nuclear translocator (ARNT) heterodimer interacts with naturally occurring estrogen response elements

To determine the molecular mechanisms underlying the "cross talk" between the activity of 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), which binds to arylhydrocarbon receptor (AHR) and estradiol (E2)-liganded estrogen receptor (ER), we first examined the initial step of estrogen action, ligand binding to ER. None of the AHR ligands tested, i.e. TCDD, benzo[a]pyrene, 3,3',4,4',5-pentachlorobiphenyl, beta-naphthoflavone, or alpha-naphthoflavone, bound to ER alpha. We report the first examination of TCDD interaction with ER beta: TCDD did not displace E2 from ER beta. We then examined a second possible mechanism, i.e. direct inhibition of ER alpha binding to estrogen response elements (EREs) by the AHR/AHR nuclear translocator (ARNT) complex. The AHR/ARNT heterodimer did not bind either a full or half-site ERE. However, AHR/ARNT bound specifically to oligomers containing naturally occurring EREs derived from the human c-fos, pS2, and progesterone receptor (PR) gene promoters that include xenobiotic response element (XRE)-like sequences. In contrast, neither purified E2-liganded-ER from calf uterus or recombinant human ER alpha bound a consensus XRE. TCDD inhibited E2-activated reporter gene activity from a consensus ERE and from EREs in the pS2, PR, and Fos genes in transiently transfected MCF-7 human breast cancer cells. However, this inhibition was not reciprocal since E2 did not inhibit TCDD-stimulated luciferase activity from the CYP1A1 promoter in transiently transfected MCF-7 or human endometrial carcinoma HEC-1A cells. We propose that at least part of the mechanism by which the AHR/ARNT complex inhibits estrogen action is by competitively inhibiting ER alpha binding to imperfect ERE sites, adjacent to or overlapping XREs.

Transcriptional suppression of estrogen receptor gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

TCDD, the most potent congener of the polychlorinated dioxins, has been shown to be an antiestrogen. The mechanisms of TCDD-induced antiestrogenicity are still under investigation. In this study, we investigated the effects of TCDD on the expression of the estrogen receptor (ER) gene. We studied the levels of un-spliced ER transcript (hnRNA) as well as the ER mRNA in ovary, uterus and liver of TCDD-treated mice with different genetic backgrounds. To quantitate the ER hnRNA levels, the intron and exon boundary of ER hnRNA was amplified by competitive RT-PCR. The ER mRNA from these mice was quantitated by competitive RT-PCR amplifying exons separated by an intron. ER hnRNA and ER mRNA levels were quantitated 4 days after a single i.p. dose of TCDD (5 microg/kg) in female C57BL/6J (B6) mice, which carry the responsive allele to TCDD. TCDD treatment significantly (p < 0.05) suppressed the levels of ER hnRNA in the ovary (27.4%) and uterus (21.9%). The decreases in ER hnRNA were coordinated with significant (p < 0.01) decreases in ER mRNA in ovary (57.7%) and uterus (37.6%). There was a significant decrease (20.3%, p < 0.05) in liver ER mRNA, however, the changes of ER hnRNA in liver were not significant. The coordinated decreases in ER hnRNA and mRNA in TCDD-treated mice suggest a suppression of transcription of the ER gene. We performed the same study on DBA/2J (D2) mice, which possess the "non-responsive" allele of the aryl hydrocarbon receptor (AhR). These mice demonstrated no significant decrease in either the ER mRNA or hnRNA after TCDD treatment. Overall, these results suggest that TCDD suppresses the gene expression of the ER receptor by decreasing its transcription, and the AhR plays an important role in mediating this response.