2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds as antioestrogens: characterization and mechanism of action

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.

Characterization of 2,3,7,8-tetrachlorodibenzofuran-dependent suppression and AH receptor pathway gene expression in the developing mouse mammary gland

The AH receptor (AHR) is a ligand-activated transcription factor and member of a growing family of homologous proteins implicated in development. In this study we have characterized the actions of 2,3, 7,8-tetrachlorodibenzofuran (TCDF), a well-studied AHR ligand, and the expression of AHR and selected AHR signal transduction pathway genes in the developing mouse mammary gland. High levels of AHR protein were observed in the mammary glands of C57Bl/6J (AHR +/+) mice during estrous-stimulated growth and branching of terminal end buds (TEBs). Comparative analysis of mammary gland development in AHR -/- and +/+ littermates revealed a 50% reduction in TEBs and an increase in blunt-ended terminal ducts in the AHR null animals. Treatment of mammary glands, removed from estrogen/progesterone-primed C57Bl/6J mice and maintained in organ culture, with TCDF suppressed lobule development (greater than twofold decreases in lobule number and size), with a concomitant suppression of DNA synthesis, as judged by a 35 to 45% decrease in [3H]thymidine incorporation in the TEBs. Immunohistochemical staining patterns for AHR, aryl hydrocarbon nuclear translocator (ARNT; the heterodimerization partner of AHR), and two AHR-regulated genes, Cyp1A1 and Cyp1B1, were similar and not altered by treatment of mammary glands in organ culture with TCDF. The observed differences in the development of mammary glands from AHR +/+ and -/- mice, associated expression of the AHR protein with hormone-dependent lobule development, and suppressive actions of TCDF support the position that, in C57Bl/6J mice, development of the mammary gland is at least in part AHR dependent. Development occurs in the absence of exogenous AHR ligand, suggesting that the unoccupied receptor may function to support the proliferative stages required for full lobule development.

Behavioural hyperactivity in rats following postnatal exposure to sub-toxic doses of polychlorinated biphenyl congeners 153 and 126

Rats were exposed through mother's milk either to the di-ortho-substituted polychlorinated biphenyl (PCB) congener 2,2',4,4',5,5'-CB (IUPAC no. 153) or to the non-ortho-substituted PCB congener 3,3',4,4',5-CB (IUPAC no. 126). The different groups of mothers were dosed via gavage with corn oil vehicle, 5 mg/kg b.w. of PCB 153 or 2 microg/kg b.w. of PCB 126 every second day from day 3 to 13 after delivery. The exposure did not affect the body weight (b.w.) of the dams or the physical development of the pups. A two-component schedule of reinforcement was used to study behavioural effects of the PCB exposures in male offspring. One component was operating when the house light was turned on. Then a reinforcer, a drop of water, was delivered every 2-min. This component is called a 2-min fixed interval (FI) schedule of reinforcement. The other component was in effect when the house light was turned off. Then no reinforcer was ever delivered. This is called an extinction (EXT) component. It was shown that the PCB-exposed offspring were hyperactive as they had an increased frequency of lever presses. In addition, the PCB 153-exposed male pups showed a behavioural pattern similar to that observed in spontaneously hypertensive rats (SHR), an animal model of attention-deficit hyperactivity disorder (ADHD). This behaviour is characterized by 'burst' of lever presses with short interresponse times (IRT) just before the next reinforcer is given. These results show that both PCBs 153 and 126 may produce significant neurotoxic effects following postnatal exposure through mother's milk.

Evaluation of reproductive outcomes in women inadvertently exposed to hexachlorobenzene in southeastern Turkey in the 1950s

In southeastern Turkey during the period of 1955-57, women were accidentally exposed to the fungicide hexachlorobenzene (HCB) after eating contaminated seed grain and developed porphyria cutanea tarda (PCT). While HCB has been shown to be a potent oocyte toxicant in primates and has been identified as an ovarian follicular fluid contaminant in women, its effect on human reproduction is poorly understood. This study was undertaken to evaluate the effects of HCB on women with a known high dose exposure. A retrospective controlled cohort comparison study of three groups was conducted. Group 1, those with confirmed PCT; Group 2, controls for the region and Group 3, controls for the country of Turkey, were followed-up after approximately 40 years (n=42/group). Blood samples were taken for analysis of serum HCB, estradiol, follicle-stimulating hormone (FSH) and inhibin. Frequency of HCB detection was greatest in Group 3, while number of cases with HCB values exceeding 1 ng/mL was significantly greater in Groups 1 and 2. There were no differences in the other biochemical measures. Interviews were completed for each patient regarding reproductive history (number of pregnancies, live births, spontaneous abortions, still births and sex of live babies). Multiple comparisons of the three groups, based on Fisher's test found the groups were inhomogeneous. When serum HCB was analyzed using correlated response logistic regression, there was a strong relationship between serum HCB levels and risk for spontaneous abortion but not sex ratio of children. The following findings were made. HCB is detectable and ubiquitous in serum samples from women in the country of Turkey with identified and unidentified exposure events. Spontaneous abortion risk is not restricted to women with identifiable exposure to HCB but to a surrogate marker of exposure (serum HCB sample). The risk of spontaneous abortion with HCB exposure requires further investigation.

Xenobiotics and xenoestrogens in fish: modulation of cytochrome P450 and carcinogenesis

As is the case with mammals, an ever-increasing number of cytochromes P450 (CYPs) are being characterized from fish. The focus of work on fish CYPs has been primarily on environmental induction of CYP1A by pollutants such as the polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and dibenzofurans. This response has been the basis for a sensitive biomonitoring tool of ecosystem health for a number of years. Studies have documented a correlation between CYP1A induction, pollutant levels and tumor incidence, especially in bottom-dwelling species. The rainbow trout has been utilized as a tumor model to document the role of CYP1A modulation in the inhibition or promotion of cancer. Fish are also very responsive to the class of chemicals known as xenoestrogens. Recent evidence is presented documenting the modulation of CYPs by xenoestrogens and their potential role as modulators of the tumor response. In this paper, we summarize the current knowledge concerning the occurrence of CYPs in fish and focus on the role of CYP1A induction in environmental monitoring of various genotoxic carcinogens and in the modulation of cancer in the trout model. Finally, the important class of aquatic pollutants known as xenoestrogens have now been shown to modulate CYP levels perhaps leading to alterations in tumor response or other adverse effects.

Environmental endocrine modulators and human health: an assessment of the biological evidence

Recently, a great deal of attention and interest has been directed toward the hypothesis that exposure, particularly in utero exposure, to certain environmental chemicals might be capable of causing a spectrum of adverse effects as a result of endocrine modulation. In particular, the hypothesis has focused on the idea that certain organochlorine and other compounds acting as weak estrogens have the capability, either alone or in combination, to produce a variety of adverse effects, including breast, testicular and prostate cancer, adverse effects on male reproductive tract, endometriosis, fertility problems, alterations of sexual behavior, learning disability or delay, and adverse effects on immune and thyroid function. While hormones are potent modulators of biochemical and physiological function, the implication that exposure to environmental hormones (e.g., xenoestrogens) has this capability is uncertain. While it is reasonable to hypothesize that exposure to estrogen-like compounds, whatever their source, could adversely affect human health, biological plausibility alone is an insufficient basis for concluding that environmental endocrine modulators have adversely affected humans. Diethylstilbestrol (DES) is a potent, synthetic estrogen administered under a variety of dosing protocols to millions of women in the belief (now known to be mistaken) that it would prevent miscarriage. As a result of this use, substantial in utero exposure to large numbers of male and female offspring occurred. Numerous studies have been conducted on the health consequences of in utero DES exposure among the adult offspring of these women. There are also extensive animal data on the effects of DES and there is a high degree of concordance between effects observed in animals and humans. The extensive human data in DES-exposed cohorts provide a useful basis for assessing the biological plausibility that potential adverse effects might occur following in utero exposure to compounds identified as environmental estrogens. The effects observed in both animals and humans following in utero exposure to sufficient doses of DES are consistent with basic principles of dose response as well as the possibility of maternal dose levels below which potential non-cancer effects may not occur. Significant differences in estrogenic potency between DES and chemicals identified to date as environmental estrogens, as well as an even larger number of naturally occurring dietary phytoestrogens, must be taken into account when inferring potential effects from in utero exposure to any of these substances. The antiestrogenic properties of many of these same exogenous compounds might also diminish net estrogenic effects. Based on the extensive data on DES-exposed cohorts, it appears unlikely that in utero exposure to usual levels of environmental estrogenic substances, from whatever source, would be sufficient to produce many of the effects (i.e., endometriosis, adverse effects on the male reproductive tract, male and female fertility problems, alterations of sexual behavior, learning problems, immune system effects or thyroid effects) hypothesized as potentially resulting from exposure to chemicals identified to date as environmental estrogens.

Effects of some persistent halogenated environmental contaminants on aromatase (CYP19) activity in the human choriocarcinoma cell line JEG-3

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB126), a technical PCB mixture (Aroclor 1016), and a technical toxaphene mixture (Camphechlor) on aromatase (CYP19) activity were investigated in human choriocarcinoma JEG-3 cells. After 18 h incubation with TCDD, PCB126, Aroclor 1016 or toxaphene, ethoxyresorufin-O-deethylase (EROD), and aromatase activity were determined. To exclude serum effects, incubations were carried out with or without fetal calf serum in the medium. EROD activity was induced by both TCDD and PCB126 in the presence or absence of serum, which indicates that JEG-3 cells are responsive toward dioxin-like chemicals. Neither Aroclor 1016 nor toxaphene affected EROD activity in these cells. Calculated EC50 values for induction of EROD activity were 0.71 and 0.40 nM for TCDD, and 48 and 20 nM for PCB126 in presence or absence of serum, respectively. Incubation with TCDD or PCB126 with or without serum caused a concentration-dependent decrease in the aromatase activity of up to 4.9-fold. Calculated EC50 values for this effect were 52 pM and 13 nM for TCDD, and 75 and 48 nM for PCB126 in the presence and absence of serum, respectively. Aroclor 1016 and toxaphene had no effect on aromatase activity at concentrations up to 1.0 microM for Aroclor 1016 or 3.0 microM for toxaphene. These results show that aromatase activity can be decreased in a concentration dependent way within the same range where EROD activity is increased. In view of these results, possible effects of dioxin-like compounds on estrogen producing and androgen target cells should be studied in more detail.

Endosulfan elevates testosterone biotransformation and clearance in CD-1 mice

Toxicant-mediated induction of hepatic biotransformation enzymes is a mechanism by which endogenous steroid hormone metabolism and elimination may be altered. Endosulfan, an organochlorine insecticide that has been demonstrated to induce hepatic P450 biotransformation enzymes, was examined for its ability to alter the rate of steroid hormone metabolism in CD-1 mice. Our objective was to evaluate whether endosulfan-induced changes in the rate of testosterone metabolism were reflected in the rate of testosterone clearance and if those alterations were sufficient to disrupt steroid hormone homeostasis within the animal. Major pathways for testosterone metabolism in the liver, including hydroxylation, conjugation to glucuronic acid or sulfate, and reduction/dehydrogenation, were examined for changes due to endosulfan exposure. In female mice, endosulfan treatment elicited a dose-dependent increase in the rate of total testosterone hydroxyl metabolite formation by selectively increasing the rate of production of 16 beta-, 6 alpha, and 16 alpha-hydroxytestosterone metabolites. The hydroxylation of testosterone in the 16 beta position was most sensitive to endosulfan with a 3.3-fold increase in the rate of production of this metabolite observed following exposure to 7.5 mg/kg/day for 7 days. The rate of testosterone dehydrogenation to androstenedione was increased by 7.5 mg/kg/day of endosulfan, but the rate of direct glucuronic acid or sulfate conjugation to testosterone was not affected by any of the dosages investigated. Endosulfan was generally more toxic to male mice and did not significantly alter the rate of total hydroxytestosterone metabolite formation or glucuronic acid or sulfate conjugation. The ability of endosulfan to enhance the elimination of testosterone was, therefore, investigated in female mice. Exposure of mice to 7.5 mg/kg/day of endosulfan resulted in an approximately 3.6-fold increase in the rate of urinary elimination of [14C]androgen, but had no significant effect on the fecal elimination of [14C]androgen. The increase in androgen clearance was associated only with a small, nonsignificant decrease in serum testosterone levels. Results indicate that increases in testosterone biotransformation from endosulfan exposure can result in increases in the elimination of the steroid. However, homeostatic processes apparently compensate for the effect and minimize any consequences on serum hormone levels.

Modulation of ovarian follicle maturation and effects on apoptotic cell death in Holtzman rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in utero and lactationally

Recent reports have described the reproduction-modulating and endocrine-disrupting effects following exposure to toxic substances such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Herein, we set out (1) to determine whether TCDD exposure exerts detrimental effects on follicle maturation in the Holtzman rat ovary and (2) to determine whether the effects of TCDD are mediated in part via apoptotic cell death. In certain species, dioxin exposure is correlated with reduced fecundity, reduced ovulatory rate, an increased incidence of endometriosis, and various reproductive cancers. Although some of the effects of TCDD are mediated via the hypothalamic-pituitary axis, direct effects on the ovary have also been observed. In the present study, an oral dose of 1 microgram TCDD/kg maternal body weight was administered on Day 15 of gestation. Female pups were sacrificed on Postnatal Day 21/22, and the ovaries were excised, fixed for histologic analysis, and analyzed in a double-blind paradigm. The analysis included a count and measurement and classification of preantral and antral follicles throughout the entire ovary. The contralateral ovary from each animal was analyzed for DNA fragmentation indicative of apoptotic cell death. The results indicate that TCDD treatment significantly reduced the number of antral follicles in the size classes 50,000 to 74,999 microns2 and > 100,000 microns2. We also observed a reduction in the number of preantral follicles less than 50,000 microns2. No difference was observed in the degree of apoptotic cell death in antral (50,000 to > 100,000 microns2) and preantral follicles (50,000 microns2 to > 75,000 microns2) between TCDD-treated and control-treated tissues. These data support the hypothesis that TCDD results in a diminution in the number of antral and preantral follicles of certain size classes in animals exposed during critical periods of development, but that apoptosis does not appear to be the underlying mechanism in these particular follicles. This does not preclude apoptosis occurring in pools of smaller precursor follicles.

Interactions of persistent environmental organohalogens with the thyroid hormone system: mechanisms and possible consequences for animal and human health

Several classes of environmental contaminants have been claimed or suggested to possess endocrine-disrupting potency, which may result in reproductive problems and developmental disorders. In this paper the focus is on the multiple and interactive mechanisms of interference of persistent polyhalogenated aromatic hydrocarbons (PHAHs) and their metabolites with the thyroid hormone system. Evidence suggests that pure congeners or mixtures of PHAHs directly interfere with the thyroid gland; with thyroid hormone metabolizing enzymes, such as uridine-diphosphate-glucuronyl transferases (UGTs), iodothyronine deiodinases (IDs), and sulfotransferases (SULTs) in liver and brain; and with the plasma transport system of thyroid hormones in experimental animals and their offspring. Changes in thyroid hormone levels in conjunction with high PHAH exposure was also observed in captive as well as free ranging wildlife species and in humans. Maternal exposure to PHAHs during pregnancy resulted in a considerable fetal transfer of hydroxylated PHAHs, which are known to compete with thyroxine (T4) for plasma transthyretin (TTR) binding sites, and thus may be transported to the fetus with those carrier proteins that normally mediate the delivery of T4 to the fetus. Concomitant changes in thyroid hormone concentrations in plasma and in brain tissue were observed in fetal and neonatal stages of development, when sufficient thyroid hormone levels are essential for normal brain development. Alterations in structural and functional neurochemical parameters, such as glial fibrillary acidic protein (GFAP), synaptophysin, calcineurin, and serotonergic neurotransmitters, were observed in the same offspring up to postnatal day 90. In addition, some changes in locomotor and cognitive indices of behavior were observed in rat offspring, following in utero and lactational exposure to PHAHs. Alterations in thyroid hormone levels and subtle changes in neurobehavioral performance were also observed in human infants exposed in utero and through lactation to relatively high levels of PHAHs. Overall these studies indicate that persistent PHAHs can disrupt the thyroid hormone system at a multitude of interaction sites, which may have a profound impact on normal brain development in experimental animals, wildlife species, and human infants.

Hormonal interactions in the effects of halogenated aromatic hydrocarbons on the developing brain

Halogenated arylhydrocarbons (HAHs) exert a wide range of effects on the developing brain. These effects result in altered patterns of neuroendocrine function and behavior in adulthood, as well as changes in cognitive function. The underlying mechanisms have not yet been clearly defined. This paper briefly reviews the effects of HAHs on brain development, and proposes the hypothesis that interactions between different hormone-sensitive systems may contribute to the broad spectrum of responses observed after fetal or early postnatal HAH exposure. Physiological interactions between the effects of sex steroids, corticosteroids, and thyroid hormone are known to influence the development of the central nervous system (CNS). Since the biosynthesis and/or action of each of these hormones is sensitive to developmental HAH exposure, it is suggested that convergent effects of HAHs on different endocrine pathways may underlie some of the disruptive effects of these chemicals on CNS differentiation. Data are presented indicating that the disruptive effects of low dose dioxin exposure on sexual differentiation of the rat brain are probably not mediated through blockade of estrogen responses, but may instead involve subtle developmental changes in other endocrine systems, perhaps also affecting the feedback control of adrenocortical function. The potential for interactive endocrine effects illustrates the need for a fuller understanding of the range of biological activities of HAHs in the brain, so that the potential risks of low dose developmental exposure to these environmental toxicants can be predicted with greater certainty.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases mRNA levels for interleukin-1beta, urokinase plasminogen activator, and tumor necrosis factor-alpha in human uterine endometrial adenocarcinoma RL95-2 cells

This study investigated the potential role of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in uterine growth utilizing a human endometrial adenocarcinoma cell line (RL95-2). Western immunoblot analysis showed a maximal induction of cytochrome P4501A1 (CYP1A1) at 1 nM TCDD, but no change in epidermal growth factor receptor (EGFR) protein level. Northern blot analysis showed that TCDD significantly increased the steady state mRNA level of CYP1A1 and CYP1B1 which was maximal at 1 nM. TCDD significantly increased mRNA levels for interleukin-1beta (IL-1beta) by 6h, and for urokinase plasminogen activator (uPA) and tumor necrosis factor-alpha (TNF-alpha) by 36h. Nuclear runoff analysis showed that transcription of CYP1A1 was significantly increased by TCDD with no effect on CYP1B1, uPA or IL-1beta. These results indicate that TCDD can differentially alter the expression of growth factor and cytokine gene products in uterine cells which may contribute to the promotion of uterine disease.

Effects of in utero and lactational exposure of the laboratory rat to 2,4,2',4'- and 3,4,3',4'-tetrachlorobiphenyl on dopamine function

Offspring of Sprague-Dawley derived dams were exposed to either 2,4, 2',4'-tetrachlorobiphenyl (TCB) (1, 10, or 20 mg/(kg.day)) or 3,4,3', 4'-TCB (0.1 or 1 mg/(kg.day)) from gestational Day 6 through weaning by providing the dams with cookies adulterated with the appropriate amount and type of PCB. Male and female offspring were sacrificed on postnatal Days 35, 60, and 90, and brain concentrations of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, were determined in the frontal cortex, caudate nucleus and substantia nigra by high-performance liquid chromatography with electrochemical detection. In utero and lactational exposure to 3,4,3',4'-TCB resulted in significant elevations in concentrations of dopamine in the frontal cortex, and of dopamine and its metabolites in the substantia nigra that persisted into adulthood. In contrast, in utero and lactational exposure to 2,4,2',4'-TCB resulted in significant decreases in concentrations of dopamine in the frontal cortex and caudate nucleus that also persisted into adulthood. We suggest that the reductions in brain dopamine concentrations are a consequence of ortho-substituted PCB congener-induced inhibition of the synthesis of dopamine during critical periods of development acting, perhaps, in concert with PCB-induced changes in cholinergic receptor function. On the other hand, the persistent elevations in brain dopamine and metabolite concentrations following perinatal exposure to 3,4,3',4'-TCB may be mediated by alterations in steroid hormone function during key developmental periods.

Environmental toxins and endometriosis

The effect of natural, environmental, or manufactured substances on endocrine organs is a current topic of discussion. Recently, some toxicants have been suggested to promote endometriosis; therefore, attention has turned to the role of environmental factors as a stimulant for endometriosis growth and maintenance. This article reviews the influence of various toxicants on endometriosis.

Aryl hydrocarbon receptor-mediated signal transduction

The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bHLH protein ARNT (aryl hydrocarbon nuclear translocator) forming a complex that binds to xenobiotic regulatory elements in target gene enhancers. Genetic, biochemical, and molecular biology studies have revealed that the AhR mediates the toxic and biological effects of environmentally persistent dioxins and related compounds. Cloning of the receptor and its DNA-binding partner, ARNT, has facilitated detailed efforts to understand the mechanisms of AhR-mediated signal transduction. These studies have determined that this unique receptor consists of several functional domains and belongs to a subfamily of bHLH proteins that share a conserved motif termed the PAS domain. In addition, recent genetic studies have revealed that expression of the AhR is a requirement for proper embryonal development, which appears to be a common function shared by many other bHLH proteins. This review is a summary of recent molecular studies of AhR-mediated gene regulation.

Assessment of PCB estrogenicity in a human breast cancer cell line

Lower chlorinated, ortho substituted, non coplanar polychlorinated biphenyls (PCBs) are weakly estrogenic in rodents and in some in vitro assays. The estrogenic potency of six PCB congeners and one of their para-hydroxylated metabolites have been tested in an estrogen-responsive MCF-7 human breast-cancer cell-culture system, to evaluate the utility of this system for assessment of PCB and hydroxylated PCB estrogenic activity. This assay is based on the estrogen receptor-mediated induction of postconfluent cell proliferation. The results of the limited test series were generally consistent with, but not absolute in the requirement for ortho-chlorine substitution and para-hydroxylation for estrogenic potency, demonstrating the usefulness of the MCF-7 focus assay for estrogenic structure-activity evaluation of PCBs.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) increases release of luteinizing hormone and follicle-stimulating hormone from the pituitary of immature female rats in vivo and in vitro

Recent findings that serum levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were dramatically increased in weanling female Sprague-Dawley (S-D) rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prompted a more detailed study to define the effect and to investigate its mechanism(s). Dose-response and time-course studies were performed in vivo. Single doses (0.03-30 micrograms/kg) of TCDD were administered orally by gastric intubation to 22-day-old female rats. Control animals received vehicle (corn oil) only, whereas naive controls were treated with an empty intubation syringe. Trunk blood was collected after decapitation at various time points during the subsequent 72 hr. Concentrations of LH and FSH were determined in serum by radioimmunoassays (RIA). Two distinct peaks of both hormones were detected. The first, at 1 hr, appeared to be a non-specific response to vehicle as it was present in both vehicle control and TCDD-treated animals, but not in naive control animals. The second peak, at 24 hr, occurred only in animals dosed with TCDD. Gonadotropin levels in these animals were dose-dependently elevated. The ED50 was about 5 micrograms/kg with a maximum elevation of 15- and 20-fold for LH and FSH, respectively. Subsequently, in vitro studies were conducted in cultured pituitary halves and in primary pituitary cell cultures exposed to gonadotropin releasing hormone (GnRH) and/or TCDD. The amount of LH released into the media was measured by RIA. TCDD caused a dose-dependent release of LH from pituitary halves with an ED50 of about 0.1 nM. This effect was abolished in calcium-free medium but was not attenuated by an GnRH antagonist. Further in vitro studies were conducted in primary pituitary cell cultures. Although the cells responded to GnRH very well, no effect of up to 100 nM TCDD on the release of gonadotropins was detected. The results suggest that TCDD induces dose-dependently a brief release of gonadotropins in immature female rats. This effect is at least partially due to an effect of TCDD in the pituitary. Increased release of gonadotropins as a result of TCDD treatment depends on the action of calcium but does not occur via activation of GnRH receptors. However, cells in a primary pituitary culture do not respond to TCDD with increased release of gonadotropins, suggesting that the effect of TCDD in the pituitary is mediated by a factor present in pituitary halves but not in primary cell culture.

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

The antiestrogenic activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was investigated in several cell lines using transient transfection assays and constructs containing 5'-regulatory sequences from the estrogen (E2)-responsive vitellogenin (Vit) A2 gene linked to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene. TCDD significantly inhibited CAT activity induced by E2 in MCF-7 human breast cancer cells transiently transfected with 5'-deletion plasmids containing the homologous promoter [(-821/+14)- and (-482/+14)-CAT] or the heterologous thymidine kinase (tk) promoter [(-821/-87)tk-, (-482/-87)tk-, (-397/-87)tk-, and (-331/-87)tk-CAT]. In parallel experiments using wild-type mouse Hepa 1c1c7 and human HeLa cells cotransfected with a human estrogen receptor expression plasmid, TCDD also inhibited E2-induced CAT activity. The role of the nuclear Ah receptor complex was confirmed by results of the following studies using MCF-7 or mouse Hepa 1c1c7 cells transiently transfected with E2-responsive Vit A2 gene 5'-promoter constructs: (i) for a series of Ah receptor ligands, there was a correlation between their antiestrogenic activity in MCF-7 cells and their rank order binding affinity for the Ah receptor; (ii) alpha-naphthoflavone, an Ah receptor antagonist, inhibited the antiestrogenic activity of TCDD in MCF-7 cells; and (iii) TCDD inhibited E2-induced CAT activity in Ah-responsive wild-type but not in Ah-nonresponsive class 2 mutant Hepa 1c1c7 cells. The antiestrogenic activity of TCDD was also observed in cells which transiently overexpressed the human estrogen receptor (ER), suggesting that the mechanism does not involve downregulation of the ER by TCDD.

Continuous exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin inhibits the growth of surgically induced endometriosis in the ovariectomized mouse treated with high dose estradiol

The objectives of this study were to evaluate the effects of repeated injection of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the growth of endometriosis and the formation of adhesions in the mouse. Sexually mature female B6C3F1 mice (n = 19) were ovariectomized and subcutaneously implanted with a silastic capsule containing estradiol (E2). Uterine tissues in full thickness (I x I mm2, 5 pieces) from the left uterine horn were surgically autotransplanted to 5 different locations in the peritoneal cavity. One week later the animals were randomly assigned to four groups and treated with: vehicle alone (corn oil). 10.50 or 100 ng/kg/day TCDD for 28 days. Two days after last dose a necropsy was performed to measure the diameter of endometrial implants and to score the adhesions. Endometrial implants were harvested for histology and blood was collected for assessment of E2 levels. The diameter of endometrial implants was significantly smaller in the TCDD treated groups compared to the controls (p < 0.05). Survival of endometrial implants, the formation and scores of adhesions, and serum E2 levels were not significantly different among the dosing groups. Histology of implants showed a significant regression of endometrial tissues in the animals receiving 50 and 100 ng/kg/day TCDD compared to the controls. We conclude that continuous exposure to TCDD resulted in regression of the endometriotic implants in the ovariectomized mice treated with a high dose exogenous F2.

The human health effects of DDT (dichlorodiphenyltrichloroethane) and PCBS (polychlorinated biphenyls) and an overview of organochlorines in public health

Organochlorines are a diverse group of persistent synthetic compounds, some of which are detectable in nearly everyone. Many organochlorines are endocrine disruptors or carcinogens in experimental assays. p,p'-DDE (dichlorodiphenyl-dichloroethene) and PCBs (polychlorinated biphenyls) comprise the bulk of organochlorine residues in human tissues. We reviewed relevant human data cited in the 1991-1995 Medline database and elsewhere. High-level exposure to selected organochlorines appears to cause abnormalities of liver function, skin (chloracne), and the nervous system. Of more general interest, however, is evidence suggesting insidious effects of background exposure. Of particular concern is the finding of neonatal hypotonia or hyporeflexia in relation to PCB exposure. The epidemiologic data reviewed, considered in isolation, provide no convincing evidence that organochlorines cause a large excess number of cancers. A recent risk assessment that considered animal data, however, gives a cancer risk estimate for background exposure to dioxin and dioxin-like compounds (e.g. some PCBs) with an upper bound in the range of 10(-4) per year.

Selected lesions of dioxin in laboratory rodents

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) has been the subject of intensive investigations in laboratory animals during the last 2 decades. Toxicity studies have been conducted in several species of rodents and include several carcinogenicity studies as well as numerous mechanistic studies initiated to elucidate dioxin's mode of action, as both a carcinogen and a toxicant. Hepatotoxicity is a primary effect of dioxin. There has been an increase in hepatocellular tumors reported in both rats and mice exposed to dioxin. In addition to neoplastic changes, dioxin causes a spectrum of toxic changes in the liver. Additional neoplastic changes include subcutaneous fibrosarcomas and thyroid follicular cell tumors in both rats and mice and histiocytic sarcomas in mice. Dioxin causes developmental effects in the palate and kidney of mice. Changes in the female reproductive tract include ovarian atrophy, sertoliform hyperplasia, and Sertoli cell tumors. Dosing in utero results in gross malformations of the external genetalia. The effects of dioxin on the rodent model of endometriosis are described. In males, there are lowered sperm counts in the epididymis and minor testicular effects following gestational administration of dioxin. Both estrogenic and antiestrogenic-like effects have been ascribed to dioxin in laboratory animals; these activities are the result of dioxin-specific pathways resulting in the same end points as classic reproductive toxicants.

Retinoic acid affects the expression rate of the differentiation-related genes aryl hydrocarbon receptor, ARNT and keratin 4 in proliferative keratinocytes only

The environmental contaminant dioxin exerts most of its effects by activating the aryl hydrocarbon receptor (AhR). The AhR is considered to play not only a role in the regulation of xenobiotic metabolism, but also for development, growth, and differentiation. The transcript levels of the AhR and its associated translocator protein (ARNT) were found to increase with ongoing differentiation in the human keratinocyte cell line HaCaT. Correspondingly, in situ hybridization studies in normal human skin revealed an absence of AhR-expression in proliferating basal cells and increasing transcript levels in upper cell layers, in dependence of keratinocyte differentiation. AhR expression in differentiation-deficient hyperproliferative psoriatic skin was markedly decreased. When keratinocytes were continuously treated with 1 microM retinoic acid (RA), the upregulation of AhR- and ARNT-mRNA levels was inhibited as was keratin 4-expression, a marker of HaCaT-keratinocyte differentiation. In contrast, treatment of already differentiated cells with RA did not down-regulate these transcript levels. The mRNA levels of the prevalent retinoic acid receptors in keratinocytes, RAR gamma and RXR alpha, were not influenced by the process of differentiation or by addition of RA. Our data suggest that the regulation of AhR-, ARNT- and keratin 4-expression by RA is indirect and mediated by a yet to be identified factor.

Epidemiological and laboratory evidence of PCB-induced neurotoxicity

The purpose of this review is to provide a selective, but critical, assessment of important findings derived from both epidemiological and laboratory studies suggesting that: (1) exposure to polychlorinated biphenyls (PCBs) and related halogenated aromatic hydrocarbons induces significant neurological and behavioral dysfunctions in humans and laboratory animals, particularly following exposure during gestation and lactation; (2) the neurochemical actions of PCBs depend on their structure and the developmental status of the animal at the time of exposure; and (3) the mechanisms responsible for these changes may involve alterations in basic cellular signaling processes and endocrine function that influence the synthesis and activity of important central nervous system neurotransmitters, the organization of the developing brain, and the behavioral responses to these environmental contaminants.

Dioxinlike properties of a trichloroethylene combustion-generated aerosol

Conventional chemical analyses of incineration by-products identify compounds of known toxicity but often fail to indicate the presence of other chemicals that may pose health risks. In a previous report, extracts from soot aerosols formed during incomplete combustion of trichloroethylene (TCE) and pyrolysis of plastics exhibited a dioxinlike response when subjected to a keratinocyte assay. To verify this dioxinlike effect, the complete extract, its polar and nonpolar fractions, some containing primarily halogenated aromatic hydrocarbons, were evaluated for toxicity using an embryo assay, for antiestrogenicity using primary liver cell cultures, and for the ability to transform the aryl hydrocarbon receptor into its DNA binding form using liver cytosol in a gel retardation assay. Each of these assays detect dioxinlike effects. Medaka (Oryzias latipes) embryos and primary liver cell cultures of rainbow trout (Oncorhynchus mykiss) were exposed to concentrations of extract ranging from 0.05 to 45 micrograms/l. Cardiotoxicity with pericardial, yolk sac, and adjacent peritoneal edema occurred after exposure of embryos to concentrations of 7 micrograms/l or greater. These same exposure levels were associated with abnormal embryo development and, at the higher concentrations, death. Some of the fractions were toxic but none was as toxic as the whole extract. In liver cells, total cellular protein and cellular lactate dehydrogenase activity were not altered by in vitro exposure to whole extract (0.05-25 micrograms/l). However, induction of cytochrome P4501A1 protein and ethoxyresorufin O-deethylase activity occurred. In the presence of whole extract, estradiol-dependent vitellogenin synthesis was reduced. Of the fractions, only fraction 1 (nonpolar) showed a similar trend, although vitellogenin synthesis inhibition was not significant. The soot extract and fractions bound to the Ah receptor and showed a significantly positive result in the gel retardation/DNA binding test. Chemical analyses using GC-MS with detection limits for 2,3,7,8-tetrachlorodibenzo-p-dioxin and dibenzofuran in the picomole range did not show presence of these compounds. Our results indicate that other chemicals associated with TCE combustion and not originally targeted for analysis may also pose health risks through dioxinlike mechanisms.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, 12-O-tetradecanoylphorbol-13-acetate and 17 beta-estradiol on estrogen receptor regulation in MCF-7 human breast cancer cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits remarkably potent antiestrogenic activity. To further elucidate the role of estrogen receptor (ER) regulation in this response, we examined the effects of exposure to TCDD in MCF-7 human breast cancer cells on ER mRNA levels by using an RNase protection assay, on ER accumulation by using an ER immunocytochemical essay (ER-ICA), and on ER function by competitive binding assays under conditions of saturating 17 beta-estradiol (E2). Comparative studies were conducted with E2 and 12-O-tetradecanoylphorbol-13-acetate (TPA), as both compounds are known to suppress ER expression. Our results indicate that 1 nM E2 and 100 nM TPA both suppress ER mRNA levels as early as 4 h after exposure and to 33.6% and 16.5% of control levels, respectively, after 72 h. In contrast, no significant effect on ER mRNA levels was attributed to exposure to 10 nM TCDD. A greater than 50% reduction in positive staining was observed by ER-ICA after 72 h exposure to 1 nM E2 and to 100 nM TPA, while only an 11% reduction in positive staining was observed with 10 nM TCDD. Specific binding of [3H]E2 under saturating conditions (10 nM E2) in whole cells was reduced by 50% in cultures exposed to 100 nM TPA, although no effect on binding was observed with exposure to 10 nM TCDD. In contrast, specific binding using subsaturating 1 nM [3H]E2 was depressed by 49% in MCF-7 cells exposed to 10 nM TCDD for 72 h. This depression was inhibited by a 1-h treatment with 5 microM alpha-naphthoflavone, which inhibits TCDD-induced, P450-mediated, E2 metabolism, and subsequent E2 depletion. In conclusion, while TPA and E2 effectively down-regulate ER expression, TCDD, under antiestrogenic conditions, has little if any effect on total ER levels in MCF-7 cells, and thus ER modulation is probably not necessary for the suppression of estrogenic activity in MCF-7 cells by TCDD.

Phytoestrogen content of foods--a compendium of literature values

Plant compounds with estrogenic activity may play a role in cancer prevention, moderation of menopausal symptoms, and other health effects. To facilitate research on these possible actions, the literature was reviewed for quantitative data on the levels of known phytoestrogens (daidzein, genistein, coumestrol, formononetin, and biochanin A) in food plants. For comparative purposes, all phytoestrogen levels were recalculated on a wet weight basis. Details on analytic procedures are given as well. High-performance liquid chromatography was the method most often used to analyze these compounds in foods. Most significant sources of isoflavone and coumestan phytoestrogens include soybeans, soy flour, soy flakes, isolated soy protein, traditional soy foods such as tofu and soy drinks, second-generation say foods, sprouts, and other legumes. Finally, medians among reported values of phytoestrogen content are provided for some of the most commonly eaten foods with quantitative data available. These may be used to calculate dietary intake of daidzein, genistein, coumestrol, formononetin, and biochanin A.

Molecular mechanism of inhibition of estrogen-induced cathepsin D gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in MCF-7 cells

17 beta-Estradiol (E2) induces cathepsin D mRNA levels and intracellular levels of immunoreactive protein in MCF-7 human breast cancer cells. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alone does not affect cathepsin D gene expression in this cell line; however, in cells cotreated with TCDD and E2, TCDD inhibited E2-induced cathepsin D mRNA levels, the rate of gene transcription, and levels of immunoreactive protein. The inhibitory responses were observed within 30 to 120 min after the cells were treated with TCDD. TCDD also inhibited E2-induced secreted alkaline phosphatase activity in aryl hydrocarbon (Ah)-responsive MCF-7 and wild-type mouse Hepa 1c1c7 cells cotransfected with the human estrogen receptor (hER) and the pBC12/S1/pac plasmid, which contains the 5' promoter region (-296/+57) of the cathepsin D gene and an alkaline phosphatase reporter gene. The E2-responsive ER/Sp1 sequence (-199 to -165) in the cathepsin D 5' region contains an imperfect GTGCGTG (-175/-181) xenobiotic responsive element (XRE); the role of this sequence in Ah responsiveness was investigated in gel electrophoretic mobility shift assays and with plasmid constructs containing a wild-type ER/Sp1 oligonucleotide or a mutant ER/Sp1-"XRE" oligonucleotide containing two C-->A mutations in the XRE sequence (antisense strand). In plasmid constructs which contained a chloramphenicol acetyltransferase reporter gene and the wild-type ER/Sp1 promoter sequence, E2-induced chloramphenicol acetyltransferase activity and mRNA levels were inhibited by TCDD whereas no inhibition was observed with the mutant ER/Sp1-"XRE" plasmids. Electrophoretic mobility shift assays showed that the nuclear or transformed cytosolic Ah receptor complex blocked formation of the ER-Sp1 complex with the wild-type but not the ER/Sp1 mutant oligonucleotide. Moreover, incubation of the wild-type bromodeoxyuridine-substituted ER/Sp1 oligonucleotide with the nuclear Ah receptor complex gave a specifically bound cross-linked 200-kDa band. These data demonstrate that Ah receptor-mediated inhibition of E2-induced cathepsin D gene expression is due to disruption of the ER-Sp1 complex by targeted interaction with an overlapping XRE.

Chlorinated hydrocarbons: estrogens and antiestrogens

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds bind to the intracellular aryl hydrocarbon (Ah) receptor and induce a diverse spectrum of biochemical and toxic responses. Ah receptor agonists also modulate several endocrine pathways, and research in several laboratories has shown that TCDD and related compounds inhibit estrogen (E2)-induced responses in the rodent mammary and uterus and in human breast cancer cell lines. The mechanisms of interaction between the TCDD- and E2-induced signaling pathways are complex and some of the inhibitory effects may be related to 5'-flanking inhibitory-dioxin responsive elements (i-DREs) in target genes. The antiestrogenic and antitumorigenic activity of Ah receptor agonists has been used to prepare a series of relatively non-toxic alkyl polychlorinated dibenzofurans which have clinical potential for treatment of mammary cancer.

Organochlorine compounds and estrogen-related cancers in women

The organochlorines, a diverse group of some 15,000 compounds, have been implicated increasingly as being harmful to humans. Some congeners of DDT and PCB elicit very weak estrogenic responses in animals, while the dioxin TCDD and related compounds have antiestrogenic properties. This review summarizes the evidence regarding whether certain organochlorine compounds, usually as persistent food-chain contaminants, increase the risk of breast and endometrial cancers through their estrogenic potential. In humans, neither ecologic data nor occupational studies provide clear support for an association between organochlorine exposure and the occurrence of these cancers. In our summary analysis of occupational exposure, the rate ratio of breast cancer for exposed cf unexposed women was 0.84 (95 percent confidence interval [CI] = 0.50-1.33) for PCBs and 1.08 (CI = 0.68-1.58) for TCDD. Similarly, effect estimates close to unity were found in summary analysis of breast cancer case-control studies regarding levels of DDE and PCB in adipose tissue or serum. In two recent nested case-control studies using stored specimens, the odds ratio per standard deviation increase in serum p,p'-DDE was 1.27 (CI = 0.95-1.69). Although estrogenic effects of certain organochlorine compounds should be easier to detect on the endometrium, we know of no analytic epidemiologic studies of endometrial cancer published to data. We conclude that available data do not indicate that organochlorines will affect the risk of these two cancers in any but the most unusual situation.

Immunoresponsiveness in endometriosis: implications of estrogenic toxicants

Endometriosis is a reproductive disease characterized by the growth of endometrial cells at sites outside the uterus. This disease is a serious disorder associated with chronic pain and infertility, which may be present in 6 million women in this country. Traditional medical therapy has consisted of hormonal regimens that limit the action of endogenous estrogen. The etiology of endometriosis is unknown, but studies suggest that soluble factors known as cytokines play a role in disease pathogenesis. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is an environmental toxicant that alters the action of estrogen in reproductive organs and adversely affects immunocompetence. The incidence of endometriosis was determined in rhesus monkeys that were chronically exposed to dioxin for a period of approximately 4 years. Ten years after termination of dioxin treatment, the presence and severity of endometriosis was assessed by surgical laparoscopy. The incidence of endometriosis correlated with dioxin exposure and disease severity was dependent upon the dose administered. Moderate to severe endometriosis was not found in control animals but was documented in three of seven animals exposed to 5 ppt dioxin (43%) and in five of seven animals exposed to 25 ppt dioxin (71%). The frequency of spontaneous disease in the control group was 33%, similar to an overall prevalence of 30% in 304 rhesus monkeys with no history of dioxin exposure. This study indicates that endometriosis may be associated with dioxin exposure in the rhesus. In view of overwhelming evidence that cytokines participate in the mediation of reproductive-endocrine phenomena and regulation of endometrial growth, future assessment of the effects of environmental toxicants on reproductive health may depend upon our understanding of the bidirectional cytokine network between the immune and endocrine systems.

Functional aspects of developmental toxicity of polyhalogenated aromatic hydrocarbons in experimental animals and human infants

A scientific evaluation was made of functional aspects of developmental toxicity of polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in experimental animals and in human infants. Persistent neurobehavioral, reproductive and endocrine alterations were observed in experimental animals, following in utero and lactational exposure to PCBs, PCDDs and PCDFs. The lowest observable adverse effect levels (LOAELs) for developmental neurobehavioral and reproduction endpoints, based on body burden of TCDD-toxic equivalents (TEQs) in animals, are within the range of current background human body burdens. Relatively subtle adverse effects on neurobehavioral development and thyroid hormone alterations have also been observed in infants and children exposed to background levels. Exclusive use of the toxic equivalency factor (TEF) approach may underestimate the risk of neurodevelopmental effects, because both Ah receptor dependent and independent mechanisms may be involved in these effects. The use of marker congeners and/or bioassays based on Ah receptor mediated mechanisms are rapid, low cost pre-screening alternatives for expensive and time consuming gas chromatographic-mass spectrometric analysis.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

Cellular and molecular biology of aryl hydrocarbon (Ah) receptor-mediated gene expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds elicit diverse toxic and biochemical responses in laboratory animals and mammalian cells in culture. TCDD induces CYP1A1 gene expression and results of extensive research have delineated the molecular mechanism of this response. In target cells, TCDD initially binds to the aryl hydrocarbon (Ah) receptor which accumulates in the nucleus as an Ah-receptor:aryl hydrocarbon nuclear translocator (Arnt) protein heterodimeric complex. The nuclear Ah receptor complex acts as a ligand-induced transcription factor which binds to transacting genomic dioxin/xenobiotic responsive elements (DREs/XREs) located in the 5'-regulatory region upstream from the initiation start site and this interaction results in transactivation of gene transcription. DREs have been identified in several other genes which are induced by TCDD, including CYP1A2, aldehyde-3-dehydrogenase, NAD(P)H quinone oxidoreductase, and glutathione S transferase Ya and similar induction response pathways have been observed or proposed. However, TCDD and other Ah receptor agonists also inhibit expression of several genes and research in this laboratory has investigated inhibition of estrogen (E2)-induced genes including uterine epidermal growth factor, c-fos protooncogene, and the progesterone receptor, estrogen receptor (ER) and cathepsin D genes in human breast cancer cell lines. In MCF-7 human breast cancer cells, E2 induces cathepsin D gene expression and this is associated with formation of an ER/Sp1 complex at the sequence in the promoter region (-199/-165) of this gene. Within 30 min TCDD causes a rapid inhibition of E2-induced cathepsin D gene expression in MCF-7 cells. Moreover, using a series of synthetic oligonucleotides which include the wild-type ER/Sp1 and various mutants, it was shown by gel electromobility shift and transient transfection assays that the nuclear Ah receptor complex binds to an imperfect DRE located between the ER and Sp1 binding sequences. This interaction results in disruption of the ER/Sp1 complex and inhibition of E2-induced gene expression. These results illustrate that the nuclear Ah receptor complex also exhibits activity as a negative transcription factor via a mechanism which is similar to that reported for Ah receptor-mediated induction of gene expression.

Organochlorine compounds in relation to breast cancer, endometrial cancer, and endometriosis: an assessment of the biological and epidemiological evidence

There is an increasing public and scientific concern that certain chlorinated compounds, recognized as environmental pollutants, may cause estrogen-related neoplastic disease in humans. The main hypothesis has been that certain organochlorines, through their estrogenic actions, might cause breast cancer. From experimental studies, both in vitro and in vivo, there is evidence that certain organochlorine compounds may cause estrogenic effects, whereas others may cause antiestrogenic effects. In limited studies, some of these compounds in high doses have also been shown to increase and reduce the frequency of estrogen-related tumors in animals. The epidemiological findings regarding the association between organochlorines and breast cancer are inconclusive. However, the largest and best designed study has been interpreted as negative with respect to DDT and polychlorinated biphenyls (PCB) in relation to breast cancer. Associations between organochlorine exposure and endometrial cancer or endometriosis have even more limited empirical basis. The hypothesis that human exposure to environmental levels or organochlorines would favor an estrogenic overactivity leading to an increase in estrogen-dependent formation of mammary or endometrial tumors is not supported by the existing in vitro, animal and epidemiological evidence. It can, however, not be conclusively rejected on the basis of available data.

Peri- and postnatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: effects on physiological development, reflexes, locomotor activity and learning behaviour in Wistar rats

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the development of rat offspring were studied after administration of a loading dose of 300 or 1000 ng TCDD/kg body wt on day 19 of pregnancy, followed by weekly maintenance doses of 120 or 400 ng TCDD/kg body wt. The dose regimens led to a fluctuation of average TCDD concentrations in the liver of the offspring of 4.9-14.9 ng/g (TCDD1000/400 group) or 1.4-6.3 ng/g (TCDD300/120 group) during the course of the experiment. In both TCDD-exposed groups the body weight of the offspring was significantly lower on postnatal day 7 (PND 7); in the high dose group from PND 7 to PND 31. Some landmarks of postnatal development were retarded in the exposed groups; in particular, the vaginal opening was delayed for several days in both TCDD-exposed groups. The TCDD-exposed animals revealed a reduced ability to remain on a rotating rod. During reflex testing, the rate of successfully responding animals was higher in the exposed groups. No statistically significant differences in the locomotor activity between controls and TCDD-exposed offspring were detectable under our experimental conditions. In a discrimination learning test no effects on the learning ability were found. However, TCDD-exposed offspring showed an increase in unanswered trials during critical phases of the task. They also exhibited increased locomotor activity in a novel environment; prior to an amphetamine challenge dose of 1 mg/kg body weight. Amphetamine-induced activity was decreased in a dose-dependent manner.

Concentrations and profiles of PCDDs and PCDFs in human mammary carcinoma tissue

PCDD/PCDF concentrations in eight mammary carcinoma tissue samples obtained after surgical excision were similar to those found in two healthy breast glandular tissue samples from autopsy material. These levels agree well with mean concentrations in human adipose tissue from German adults. An analogous consistency was found for the congener profiles of the normalized concentrations, also in comparison with mothers' milk from Germany. In spite of similar congener profiles the concentrations in four axillary adipose tissue samples corresponding to the carcinoma samples were about 40% lower. This discrepancy was not found in one tissue pair from a healthy breast.

Early development of Yu-Cheng children born seven to twelve years after the Taiwan PCB outbreak

In Taiwanese children born seven to twelve years after their mothers' or fathers' intoxication of heat-degraded polychlorinated biphenyls (PCBs) (Yu-Cheng mothers or fathers), we measured the development and behavioral problems in 1991-2. The development was assessed with the Chinese Child Developmental Inventory (CCDI), and the behavioral problems with Rutter's Child Behavior Scale A. Children of Yu-Cheng mothers had delayed development on several subscales of CCDI and on the combined general development score. Girls were more affected than boys in their development. Children of Yu-Cheng fathers did not score differently from unexposed controls on this measurement. In Rutter's scale, the scores in Yu-Cheng mothers' children and Yu-Cheng fathers' children were not different from those of their unexposed controls. We conclude that children born seven to twelve years after mothers' exposure to degraded PCBs had delayed development, but they did not show more behavior problems when compared with unexposed controls.

Inhibition of estrogen-induced progesterone receptor in MCF-7 human breast cancer cells by aryl hydrocarbon (Ah) receptor agonists

17 beta-Estradiol (E2) induces progesterone receptor (PR) binding, immunoreactive protein, nuclear PR formation and PR mRNA levels in MCF-7 human breast cancer cells. Gel mobility shift analysis of nuclear extracts from E2-treated cells also exhibited a higher intensity retarded band associated with formation of a PR complex with a consensus [32P]progesterone/glucocorticoid responsive element. In contrast, 1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone did not alter or decrease these same responses in MCF-7 cells; however, in cells co-treated with 1 nM TCDD plus 1 nM E2, TCDD significantly inhibited all the E2-induced responses. Scatchard analysis of PR binding demonstrated that TCDD decreased the number of E2-induced PR cellular binding sites but not the binding affinity of the PR for a radiolabeled promegestrone. In parallel studies, 3-methylcholanthrene, a prototypical polynuclear aromatic hydrocarbon, also inhibited E2-induced PR binding and immunoreactive protein. For a series of halogenated aromatics including 2,3,7,8- and 1,2,7,8-tetrachlorodibenzofuran, 1,3,7,8-TCDD and 6-methyl-1,3,8-trichlorodibenzofuran, their rank order potency for inhibiting E2-induced PR binding paralleled their rank order binding to the aryl hydrocarbon (Ah) receptor. These results support a role for the Ah receptor in mediating the antiestrogenic activity of polynuclear and halogenated aromatic hydrocarbons and illustrate cross-talk between the Ah and estrogen receptor signal transduction pathways.

The aromatic hydrocarbon receptor, dioxin action, and endocrine homeostasis

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin acts through the aromatic hydrocarbon (AH) receptor to enhance CYP1A1 gene transcription. There are notable mechanistic analogies between dioxin action and lipophilic hormone action. In addition, some of dioxin's adverse effects are associated with disruptions in endocrine homeostasis. Thus, dioxin biology intersects several disciplines, including toxicology, endocrinology, molecular biology, and public health.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced thymic atrophy and lymphocyte stem cell alterations by mechanisms independent of the estrogen receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has both agonist and antagonist effects on estrogen-mediated activities and estrogen receptor (ER) levels in epithelial tissues following exposure. We previously demonstrated that TCDD alters bone marrow lymphocyte stem cells, including prothymocytes, as measured by functional assays and alterations in the lymphocyte stem cell-specific markers terminal deoxynucleotidyl transferase (TdT) and recombinase activating gene-1 (RAG-1). We have also shown that 17 beta-estradiol valerate (E2V) affects lymphocyte stem cells by reducing TdT and RAG-1 mRNA. It has been suggested that the effect of TCDD on these lymphocyte stem cells may be mediated directly or indirectly through estrogenic action and/or the ER. Studies were designed to evaluate whether endogenous estrogens or the ER mediate TCDD-elicited bone marrow alterations and thymic atrophy. Ovariectomy did not alter the sensitivity of mice to TCDD-induced thymic atrophy or to a reduction in TdT biosynthesis in bone marrow cells compared with either intact or sham-operated mice. The pure estrogen antagonist ICI 164,384 blocked E2V-induced uterine hypertrophy, thymic atrophy and reductions in lymphocyte stem cell markers. However, the antiestrogen failed to protect against TCDD-elicited thymic atrophy or bone marrow alterations in intact animals. The results are consistent with the hypothesis that the effects of TCDD on the thymus and/or bone marrow are mediated by mechanisms independent of estrogens or the ER.

Stimulation of 17 beta-estradiol metabolism in MCF-7 cells by bromochloro- and chloromethyl-substituted dibenzo-p-dioxins and dibenzofurans: correlations with antiestrogenic activity

Mixed halo- and haloalkyl-substituted dibenzo-p-dioxins (DD) and dibenzofurans (DF) are known environmental contaminants, although there is limited information on the toxic effects of these compounds in human cells. In this study antiestrogenicity, a property of 2,3,7,8-Cl4-DD, was investigated with a series of bromochloro- and chloromethyl-substituted DDs and DFs. The effects of these compounds on the metabolism of 17 beta-estradiol (E2) and on the estrogen-dependent formation of multicellular foci in cultures of MCF-7 human breast cells were examined. Pretreatment of MCF-7 cells with 2,3,7,8-Cl4-DD induced pathways of E2 metabolism involving cytochrome P-450-catalyzed hydroxylation, methylation of the catechol estrogens, and conjugation. Several Br-Cl3-DD and Br2-Cl2-DD congeners with halogen substitution at the 2, 3, 7, and 8 positions also stimulated E2 metabolism with similar potency to that of 2,3,7,8-Cl4-DD; however, compounds with substitution of a methyl group for a halogen at any of these positions did not stimulate the metabolism of E2. For the series of compounds tested in MCF-7 cultures, a close correlation was observed between the antiestrogenicity as measured by the inhibition of estrogen-dependent postconfluent growth that results in focus formation and the efficacy with which the compounds stimulated the metabolism of E2. 2,3,7,8-TetrahaloDDs with one or two bromine atoms at these positions were highly antiestrogenic as determined by their inhibition of estrogen-dependent focus formation, whereas the methyl-substituted polychlorinated DDs and DFs investigated did not inhibit focus formation. These results indicate that the 2,3,7,8-substituted mixed halo-substituted DDs and DFs are of importance when the biologic effects of halogenated DD and DF congeners are considered, and provide additional evidence for the role of increased metabolism of E2 in the antiestrogenic effects of halogenated DDs and DFs.