Physiological implications of estrogen receptor modulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Aromatic hydrocarbon receptors in the immune system: Review and hypotheses

Ah-receptors (AhRs) recognize and bind foreign environmental molecules as well as some target hormones of other nuclear receptors. As ligands activate transcription factors, they transmit the information on the presence of these molecules by binding to the DNA, which in turn activate xenobiotic metabolism genes. Cross talk with other nuclear receptors or some non-nuclear receptors also activates or inhibits endocrine processes. Immune cells have AhRs by which they are activated for physiological (immunity) or non-physiological (allergy and autoimmunity) processes. They can be imprinted by hormonal or pseudo-hormonal (environmental) factors, which could provoke pathological alterations for life (by faulty perinatal hormonal imprinting). The variety and amount of human-made new environmental molecules (endocrine disruptors) are enormously growing, so the importance of AhR functions is also expanding.

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.

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

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

Transcriptional activation of the membrane-bound progesterone receptor (mPR) by dioxin, in endocrine-responsive tissues

We originally identified the membrane-bound progesterone receptor (mPR) using a screening for genes differentially expressed in liver of rats exposed to dioxin. Recent findings have suggested a role for the mPR in sperm cells, ovary, and brain; however, its mechanisms of action are largely unknown. In this study, we examined the expression pattern of the mPR in liver of rats exposed to dioxin and identified possible mechanisms of its regulation. We observed that mPR expression was induced by dioxin, but was also dependent on the hormonal responsiveness of the tissue. In particular, in male, but not female liver, dioxin induced the expression of the mPR. However, in control, untreated female liver the level of mPR transcript was higher than in control males. Moreover, in breast cancer cells MCF-7 dioxin induced mPR expression. Promoter studies using the luciferase assay indicated that a fragment of approximately 350 bp of the mPR promoter was able to induce luciferase activity in the presence of dioxin, suggesting that the presumptive XREs sites contained in this mPR promoter region are responsive to dioxin. Analysis of mPR protein level confirmed the results observed at the RNA level, both in rat liver and MCF-7 cells. Taken together, these observations suggest the existence of a novel cross-talk between steroid and aromatic hydrocarbon receptors (AhR), and underline the importance of the mPR as a mediator of physiologic effects of the sex hormones.

Effects of steroids and dioxin (2,3,7,8-TCDD) on the developing wolffian ducts of the tiger salamander (Ambystoma tigrinum)

This study was undertaken to investigate effects of the prototypical dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on steroid-dependent development of the wolffian ducts of an amphibian, the tiger salamander (Ambystoma tigrinum). Larvae with immature gonads and undeveloped mullerian ducts were injected with the steroid hormones estradiol (E2), dihydrotestosterone (DHT), or vehicle alone. Additionally, steroid-treated and vehicle-control larvae were immersed in sub-lethal solutions of technical grade TCDD (0, 0.0003, 0.003, 0.03, 0.3, and 3.0 microg TCDD/L). Both steroid treatments stimulated hypertrophy of the wolffian duct epithelium and an increase in mean epithelial cell size. Only DHT treatment stimulated epithelial cell proliferation. TCDD stimulated wolffian duct hypertrophy through an increase in mean epithelial cell size. TCDD acted as an androgen agonist on wolffian duct epithelial area and epithelial cell size. TCDD had no effect on wolffian duct epithelium among E2-injected animals. Stimulatory effects on cell size were observed at 0.0003 microg/L TCDD in saline-injected animals and at 0.003 microg/L TCDD in DHT-injected animals. Both E2 and DHT stimulated growth of the wolffian ducts early in development. Technical grade TCDD alone mimics E2 and DHT action but exhibits an androgen-agonistic action in the presence of exogenously administered DHT. Implications of possible interactions between TCDD and xenosteroids are discussed.

Effects of TCDD in modulating the expression of Sertoli cell secretory products and markers for cell–cell interaction

Among different mammalian tissues, testis is found to be one of the most sensitive organs to TCDD exposure. In this study, primary Sertoli cell culture was established. The purity of the cultured cells was verified using 3beta-hydroxysteroid dehydrogenase, alkaline phosphatase as well as testosterone induction assays. Effects of TCDD in modulating the expression of CYP1A1, aromatase, secretory products (i.e. Mullerian inhibiting substance (MIS), 17beta-estradiol (E(2)) and lactate) and markers for cell-cell interaction (i.e. sertolin and testin) were then examined. Our data demonstrated that Sertoli cells exposed to 0.2-2000 pg/ml of TCDD showed a dose dependent induction of CYP1A1 mRNA. The minimal dose of activation was 2 pg/ml, which indicated that the cell was very sensitive to TCDD exposure. However, there was little or no detectable level CYP1A1 protein and EROD activities found. Dose-dependent inductions of aromatase transcript (200%) and E(2) (20%) secretion were measured. In addition there was a significant reduction (40%) of MIS mRNA. No detectable change in the level of secreted lactate was observed. Sertolin and testin, the gene makers for cell-cell interactions were differentially modulated upon TCDD treatment. Taken together, the results implicated that TCDD exposure might interfere with the normal Sertoli cell functions.

The aryl hydrocarbon receptor mediates degradation of estrogen receptor alpha through activation of proteasomes

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon receptor (AhR) ligands suppress 17beta-estradiol (E)-induced responses in the rodent uterus and mammary tumors and in human breast cancer cells. Treatment of ZR-75, T47D, and MCF-7 human breast cancer cells with TCDD induces proteasome-dependent degradation of endogenous estrogen receptor alpha (ERalpha). The proteasome inhibitors MG132, PSI, and PSII inhibit the proteasome-dependent effects induced by TCDD, whereas the protease inhibitors EST, calpain inhibitor II, and chloroquine do not affect this response. ERalpha levels in the mouse uterus and breast cancer cells were significantly lower after cotreatment with E plus TCDD than after treatment with E or TCDD alone, and our results indicate that AhR-mediated inhibition of E-induced transactivation is mainly due to limiting levels of ERalpha in cells cotreated with E plus TCDD. TCDD alone or in combination with E increases formation of ubiquitinated forms of ERalpha, and both coimmunoprecipitation and mammalian two-hybrid assays demonstrate that TCDD induces interaction of the AhR with ERalpha in the presence or absence of E. In contrast, E does not induce AhR-ERalpha interactions. Thus, inhibitory AhR-ERalpha cross talk is linked to a novel pathway for degradation of ERalpha in which TCDD initially induces formation of a nuclear AhR complex which coordinately recruits ERalpha and the proteasome complex, resulting in degradation of both receptors.

Thymic atrophy induced by methoxychlor in rat pups

The effect of Methoxychlor (MXC) on the thymus was examined in rat pups that were delivered from dams receiving MXC at a dietary concentration of 0 or 1500 ppm for a period from pregnancy to lactation. The pups of both sexes were euthanized on postnatal days (PNDs) 7, 14, and 21. Histologically, the thymus showed marked depletion of cortical lymphocytes on PND 7 and also had an increase in lymphophagocytosis in the cortical area on PNDs 14 and 21. Morphometrical analysis disclosed that both cortex and medulla of the thymus from treated pups were reduced in size, but the reduction was more evident in the cortex. A significant increase in transferase-mediated dUTP nick end labeling-positive cells was detected in the cortex area, corresponding to the presence of lymphophagocytosis. Flow cytometric analysis revealed a significant decrease in the double positive (CD3(int)CD4(+)CD8(+)) immature cells on PND 21. These results have suggested that MXC may impair maturation of thymic lymphocytes in rat pups, which results in enhancement of apoptosis leading to thymic atrophy during the postnatal period.

Effects of chronic dietary exposure to environmentally relevant concentrations to 2,3,7,8-tetrachlorodibenzo-p-dioxin on survival, growth, reproduction and biochemical responses of female rainbow trout (Oncorhynchus mykiss)

Adult female rainbow trout were exposed to dietary 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at concentrations of 1.8, 18 and 90 ng TCDD/kg (ww) food for up to 300 day. At the end of the exposure fish were spawned and the reproductive outcomes were assessed. TCDD was accumulated into tissues and eggs in a dose-dependent manner with steady state being achieved after 50-100 day of exposure. Biochemical and hematological parameters were monitored at 50, 100, 150, and 200 day after the beginning of exposure. The survival of adult female trout was reduced in a dose-dependent manner by exposure to TCDD in the diet. Fish fed 1.8 ng TCDD/kg, moist weight of diet, showed significantly reduced survival compared with those fed the control diet. TCDD also affected survival of fry from females fed 1.8 ng TCDD/kg. Observed adverse effects in adult fish were as sensitive as early life-stage endpoints. Liver EROD activity was only moderately increased in all exposure groups after 250+ day of exposure. Low rates of edema and deformities were observed in fry from all treatment groups including controls. This study has demonstrated adverse effects of TCDD to both adults and fry at concentrations comparable to current environmental concentrations. This suggests that direct adult toxicity as well as reproductive endpoints need to be incorporated in the current risk assessment paradigm for these compounds.

Dioxin exposure and porcine reproductive hormonal activity

To characterize the action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during both the follicular and luteal phases of the ovarian cycle, the direct effect of TCDD was investigated in vitro using a system of primary monolayer cell culture. Granulosa and theca cells were collected from the preovulatory follicles and cultured as a co-culture, thus resembling follicles in vivo. Luteal cells were isolated from the corpora lutea collected during the midluteal phase. In both cases cells were isolated from the ovaries of animals exhibiting natural estrus cycle. Results of these experiments suggest that TCDD decreases estradiol secretion by follicular cells and progesterone secretion by luteal cells in a dose-dependent manner. It was also shown that TCDD disrupts steroidogenesis through its influence on the activity of enzymes involved in the steroid biosynthesis cascade. In luteal cells, its action is mediated via the aryl hydrocarbon receptor (AhR) and is probably independent of estrogen receptor (ER) stimulation. Endocrine disruptors that interfere with estradiol production in the follicles can act as ovulatory disruptors, and while interfering with progesterone production by luteal cells they can act as abortifacients.

Effects of binary mixtures of six xenobiotics on hormone concentrations and morphometric endpoints of northern bobwhite quail (Colinus virginianus)

This study investigated the effects of six endocrine disrupters in five different doses (0.1, 0.3, 1, 3, 10 mg/kg or microg/kg) in ethanol administered by oral gavage to bobwhite quail eggs. Six eggs each were in each dose group of coumestrol, ethynyl estradiol, indole-3-carbinol, o,p'-DDE, p,p'-DDE, or TCDD. Eggs were also dosed in two sets. One set was ethynyl estradiol (0, 0.03, 0.1, 0.3, 1.0, 3.0, 10.0 microg/kg) and TCDD (0, 0.003, 0.01, 0.03, 0.1, 0.3 microg/kg). This set was dosed below the air cell with corn oil as vehicle. Also, northern bobwhite quail eggs were injected in ovo with nine binary mixtures of six xenobiotics prior to incubation (coumestrol (0.3 mg/kg), ethynyl estradiol (3.0 microg/kg), indole-3-carbinol (3.0 mg/kg), o,p'-DDE (1.0 mg/kg), p,p'-DDE (1.0 mg/kg), TCDD (0.1 microg/kg)). The mixtures injected were p,p'-DDE+indole-3-carbinol, coumestrol+indole-3-carbinol, TCDD+indole-3-carbinol, p,p'-DDE+o,p'-DDE, p,p'-DDE+ethynyl estradiol, coumestrol+ethynyl estradiol, coumestrol+TCDD, o,p'-DDE+ethynyl estradiol, TCDD+ethynyl estradiol. Eggs were dosed once prior to initiating incubation. Quail were allowed to hatch and were sacrificed at 21 days of age. Blood, measurements, and tissues were collected. Survival was significantly affected by increasing concentrations of TCDD in ethanol as revealed by trend analysis. Survival was also affected significantly by o,p'-DDE in ethanol but not by trend. Survival results of mixtures indicate significant differences among mixture, mixture components, and controls for coumestrol+TCDD, ethynyl estradiol+TCDD, and indole-3-carbinol+TCDD. Some trends from doses of single compounds that are supported by results in the literature were observed for hatchling weight of ethynyl estradiol dosed females, weight gain of indole-3-carbinol dosed males, weight gain and liver somatic index of o,p'-DDE dosed males, spleen somatic index of TCDD dosed males, and weight gain, gonad somatic index and egg gland somatic index of TCDD dosed females. In conclusion, the dose response treatments appeared to have effects beyond effects on survival of in ovo dosed quail. For mixtures, plasma estradiol concentrations were significantly different among coumestrol+ethynyl estradiol, ethynyl estradiol, coumestrol, and vehicle treatments. Liver somatic index among the same treatments was also significantly different. Kidney somatic index among ethynyl estradiol+p,p'-DDE, ethynyl estradiol, p,p'-DDE, and vehicle treatments was significantly different. Plasma estradiol and plasma testosterone ratios were very different among o,p'-DDE+p,p'-DDE, o,p'-DDE, p,p'-DDE, and vehicle treatments. Coumestrol and ethynyl estradiol appear antagonistic for plasma estradiol concentrations and liver somatic index when both chemicals are present together. Ethynyl estradiol and p,p'-DDE appear to act additively on kidney somatic index when combined together. Mixtures of compounds, used in this study indicate effects very different from either or both mixture components, indicating the lack of predictability of chemicals when combined in mixtures.

Interactions of nuclear receptor coactivator/corepressor proteins with the aryl hydrocarbon receptor complex

MCF-7 human breast cancer cells express the aryl hydrocarbon receptor (AhR), and treatment with AhR agonists such as 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits estrogen receptor (ER)-mediated responses. This study investigates physical and functional interactions of the AhR complex with a prototypical coactivator (estrogen receptor associating protein 140, ERAP 140) and corepressor (silencing mediator for retinoic acid and thyroid hormone receptor, SMRT) for ER and other members of the nuclear receptor superfamily. The AhR, AhR nuclear translocator (Arnt), and AhR/Arnt proteins were coimmunoprecipitated with 35S-ERAP 140 and 35S-SMRT and, in gel mobility shift assays, AhR/Arnt binding to 32P-dioxin response element (DRE) was enhanced by ERAP-140 and inhibited by SMRT; supershifted bands were not observed. In transactivation assays, coactivator and corepressor proteins enhanced or inhibited AhR-mediated gene expression; however, these responses varied with the amount of coactivator/corepressor expression. These results confirmed functional and physical interactions of AhR/Arnt with ERAP 140 and SMRT in breast cancer cells.

3,3'4,4'-Tetrachlorobiphenyl exhibits antiestrogenic and antitumorigenic activity in the rodent uterus and mammary cells and in human breast cancer cells

3,3',4,4'-Tetrachlorobiphenyl (tetraCB) binds to the aryl hydrocarbon receptor (AhR), and several reports have demonstrated that AhR agonists exhibit antiestrogenic and antitumorigenic activities in human breast cancer cells, the rodent uterus and breast. In contrast, a recent study showed that 3,3',4,4'-tetraCB bound the estrogen receptor (ER) and exhibited ER agonist activities, and we therefore have reinvestigated the estrogenic and antiestrogenic activities of 3,3',4,4'-tetraCB. Our results showed that 3,3',4,4'tetraCB and a structurally related analog, 3,3',4,4',5-pentaCB, did not bind the mouse uterine or human ER, did not induce proliferation of MCF-7 or T47D human breast cancer cells or induce reporter gene activity in cells transfected with E2-responsive constructs derived from the creatine kinase B (pCKB) or cathepsin D (pCD) gene promoters. Moreover, 3,3',4,4'-tetraCB and 3,3',4,4',5-pentaCB did not induce an increase in uterine wet weight, peroxidase activity or progesterone receptor binding in the 21-25-day-old female B6C3F1 mouse uterus. In contrast, both compounds inhibited 17beta-estradiol (E2)-induced cell proliferation and transactivation in MCF-7/T47D cells and uterine responses in B6C3F1 mice; surprisingly inhibition of E2-induced reporter gene activity was not observed in T47D cells transfected with pCKB, and this was observed as a cell-specific response with other AhR agonists. Additionally, 3,3',4,4'-tetraCB significantly inhibited mammary tumor growth in female Sprague-Dawley rats initiated with 7,12-dimethylbenzanthracene. Our results indicate that 3,3',4,4'-tetraCB does not exhibit ER agonist activity but exhibits a broad spectrum of antiestrogenic responses consistent with ligand-mediated AhR-ER crosstalk.

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.

Effect of combined neonatal imprinting by vitamin A, vitamin D3, benzpyrene and allylestrenol on adult rat thymus glucocorticoid and uterine estrogen receptors

1. Combined neonatal imprinting with allylestrenol, vitamins A and D3 and benzpyrene significantly increased thymic glucocorticoid receptor capacity in male and female animals and decreased receptors affinity in adult females only. 2. Uterine estrogen receptor affinity or density was not influenced. 3. Considering that perinatal treatment with allylestrenol or vitamin D3 decreased glucocorticoid receptor capacity, the dominance of the positive effect of retinol should be surmised. 4. The experiments call attention to the interrelation of different materials acting simultaneously in the perinatal period.

Vulnerability of the endocrine system to xenobiotic influence

Natural sex hormones are most important factors guaranteeing the homeostasis of male and female sexual functions, including sexual differentiation and reproduction. Main target tissues include bone and skin, cardiovascular system, and possibly central nervous and immune systems. In medicine, synthetic hormonal substances with agonistic and antagonistic properties have been widely used for decades. Therapeutic benefit is the aim, and the many possibilities to interfere with normal or pathological hormonal situations are rather well understood. Synthetic hormonal agonists or (partial) antagonists may exhibit specific affinities to special receptors resulting in a spectrum of organotropies, or they may even induce opposite actions on different targets. Although not a new issue, environmental substances mimicking potentials of sex hormones have recently gained increased attention. There is not need to reinvent the wheel, since most (adverse) effects may be revealed with today's routine procedures used for testing medicinal substances, but some additional testing strategies should be included. Adverse effects of ecohormones may preferentially affect systems other than the human organism (assuming lower exposure and possibly lower susceptibility). Nevertheless, this survey is confined to possible alterations in the mammalian organism, since such effects are best understood from numerous experimental studies and clinical trials.

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.

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.

Estrogen receptor and aromatic hydrocarbon receptor in the primate ovary

We have previously shown by immunocytochemistry and autoradiography the presence of estrogen receptors (ER) in rhesus monkey ovary. Intense chromogen staining showed specific binding for ER in nuclei of germinal epithelium and granulosa cells of antral follicles; and radiolabeled ligand bound specifically to functional corpora lutea (CL). Although it is accepted that the germinal epithelium of the primate ovary contains ER, some controversy still persists regarding the intraovarian localization of this molecule. In addition, no data exist that localize the aromatic hydrocarbon (dioxin) receptor (AHR), which is known to modulate ER, to the primate ovary. In the present study, we show the presence of ER using Western blot analysis, and ER capable of binding DNA within intraovarian compartments in two species of the genusMacaca (rhesus macaque,Macaca mulatta and stumptail macaque,Macaca arctoides); extend these findings to human ovarian granulosa cells (GC) using Western blot, reverse transcription-polymerase chain reaction (RT-PCR), and gel mobility-shift analysis; and localize the AHR to intraovarian compartments of the macaque ovary by Western blots and gel-shift assays. These experiments strongly suggest that estrogens can exert effects on follicle development directly at the ovary, and provide the first direct evidence that AHR-mediated toxicity may be manifested at the ovary to induce possible antifertility effects.

Modulation of growth axis gene expression by in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the weaning Holtzman rat

While thein utero and lactational effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on both male and female reproductive systems appear to be severe, little is known about its effects on the developing growth axis. The objective of this study was to describe changes in growth axis gene expression that accompany exposure to TCDD duringin utero and lactational development. Pregnant Holtzman rats were administered 1 μg TCDD/kg maternal body weight or vehicle control on gestational day 15 by gavage. Using ribonuclease protection assays, we compared mRNA levels measured in 21-d-old female pups exposed to TCDD with levels measured in control animals for the following genes: somatostatin, growth hormone-releasing hormone (GHRH), hypothalamic and pituitary galanin (GAL), growth hormone (GH), and insulin-like growth factor-I (IGF-I). Serum GH concentrations measured by radio-immunoassay were significantly increased, although GH mRNA levels were unchanged from controls by TCDD exposure. Hypothalamic GAL mRNA was decreased in TCDD-treated animals, whereas pituitary GAL mRNA in TCDD-treated animals was not altered. GHRH mRNA was increased in hypothalami from TCDD-exposed animals. IGF-I mRNA in the liver was decreased to 67% of controls. These data indicate that the growth axis is sensitive to the effects of TCDD delivered during critical periods of development. The alterations observed in growth axis gene expression with exposure to TCDD add to the body of data demonstrating a potent effect of this compound on the fetal and neonatal endocrine system.

Environmental estrogenic effects of alkylphenol ethoxylates

Alkylphenol ethoxylates (APEs) and related compounds recently have been reported to be estrogenic because it has been demonstrated in laboratory studies that they mimic the effects of estradiol both in vitro and in vivo. Chemicals referred to as "environmental estrogens" are suspected of causing health effects in both humans and wildlife through disruption of the endocrine system. In this review, the occurrence, environmental fate, and biological effects of APEs are presented. To provide understanding of the potential for endocrine disruption due to environmental estrogens, the physiology of estrogens in mammals and fish is also reviewed. The estrogenic potency of other environmental estrogens is compared to the potency of APE degradation products. The reproductive effects of estrogenic compounds are considered when evaluating the potential health effects of APEs. Given the reported environmental concentrations and bioconcentration factors of APE products, the potential for these compounds to produce estrogenic effects in the environment appears low. Although questions concerning the physiological effects of APEs and other environmental estrogens remain unanswered, there are indications that research is in progress that will lead to better understanding of the risks to humans and wildlife.

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.

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.

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.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on interleukin-4-mediated mechanisms of immunity

Because of similarities in the independent actions of the pleiotropic cytokine, interleukin-4 (IL-4), and the environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), on murine B-lymphocytes suggested in earlier studies, we have investigated whether the immunosuppression mediated by direct exposure to TCDD in vitro is due to an IL-4-like biological activity. In particular, the ability of TCDD to mimic hallmark responses of B-cells to IL-4, such as upregulation of major histocompatibility complex (MHC) antigens of the class II type, increases in cell surface expression of the low affinity form of the Fc receptor for IgE (CD23) and induction of immunoglobulin class switching, was tested. At concentrations that readily suppress B-cell proliferative and antibody-forming cell responses, TCDD failed to demonstrate any of the activities of IL-4 observed in parallel cultures. Further, in experiments in which TCDD was preincubated with B-cells before addition of IL-4, no evidence of increased IL-4 activity was observed. Rather, TCDD preincubation resulted in decreased secretion of IgG1 and IgE in B-cell cultures stimulated to undergo immunoglobulin class switching by incubation with bacterial lipopolysaccharide (LPS) and IL-4. Because TCDD produced comparable suppression of IgM secretion induced by LPS alone (i.e., no IL-4), it appears that TCDD inhibits the formation of fully differentiated B-cells capable of secreting antibody and has no effects on class switching events per se. Coupled with previous reports from this and other laboratories, these observations indicate that TCDD is able to suppress secretion of several classes of immunoglobulin.

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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits differentiation of normal diploid rat osteoblasts in vitro

The influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent halogenated aromatic hydrocarbon, on the development of bone tissue-like organization in primary cultures of normal diploid calvarial-derived rat osteoblasts was examined. Initially, when placed in culture, these cells actively proliferate while expressing genes associated with biosynthesis of the bone extracellular matrix. Then, post-proliferatively, genes are expressed that render the osteoblast competent for extracellular matrix mineralization and maintenance of structural as well as functional properties of the mature bone-cell phenotype. Our results indicate that, in the presence of TCDD, proliferation of osteoblasts was not inhibited but post-confluent formation of multicellular nodules that develop bone tissue-like organization was dramatically suppressed. Consistent with TCDD-mediated abrogation of bone nodule formation, expression of alkaline phosphatase and osteocalcin was not upregulated post-proliferatively. These findings are discussed within the context of TCDD effects on estrogens and vitamin D-responsive developmental gene expression during osteoblast differentiation and, from a broader biological perspective, on steroid hormone control of differentiation.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Inhibition of acute TCDD toxicity by treatment with anti-tumor necrosis factor antibody or dexamethasone

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acute toxicity is characterized in part by a wasting syndrome with depletion of adipose tissue. Tumor necrosis factor (TNF) induces a similar response during chronic infection. The similarities of these toxic effects led to a hypothesis that TNF plays a role in TCDD acute toxicity. To test this hypothesis pharmacologic doses of an antibody specific for murine TNF and the potent anti-inflammatory agent Dexamethasone (DEX) were used to inhibit TCDD toxicity in mice. TNF antibody treatment resulted in a 54% reduction in TCDD-mediated mortality while DEX treatment, a glucocorticoid agonist that inhibits transcription of TNF, reduced mortality by 92%. Cyp 1A1 induction, the most commonly measured TCDD-mediated response, was not blocked by DEX, demonstrating separation of this biochemical effect from acute toxic responses to TCDD. These data suggest that TCDD-mediated changes in the TNF pathway may be an important mechanism for acute TCDD toxicity.

Pathology reevaluation of the Kociba et al. (1978) bioassay of 2,3,7,8-TCDD: implications for risk assessment

The chronic bioassay of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) reported in 1978 by Kociba et al. has been considered to be the primary evidence supporting its carcinogenicity, and is the basis for most dioxin regulations in North America and Western Europe. Because the histopathological criteria for proliferative lesions in the rat liver have changed significantly since 1978, a reevaluation of the liver slides was conducted recently by an independent panel of pathologists. Using current National Toxicology Program criteria, their study showed, in contrast to the original findings, that about two-thirds fewer tumors were present in the livers of female Sprague-Dawley rats. The no-observed-adverse-effect level (NOAEL) for hepatocellular carcinomas was 0.01 micrograms/kg/d rather than 0.001 micrograms/kg/d, which had been reported in 1978. In light of these significant findings, a quantitative dose-response assessment of 2,3,7,8-TCDD was undertaken to predict the potential carcinogenic risks to humans. Risk-specific doses (RsDs) and cancer potency factors (CPFs) were calculated by applying the linearized multistage (LMS) model to the combined incidences of hepatocellular carcinomas and adenomas, classified in accordance with the 1990 histopathological criteria. Based on the weight of evidence regarding the mechanism of action of 2,3,7,8-TCDD, body weight rather than surface area was selected as the appropriate means for scaling rodent data to predict the human response. Using the survival-adjusted data, the RsD for a 1 in 1,000,000 (10(-6)) plausible upper bound (95%) lifetime incremental cancer risk was 370 fg/kg/d based only on the incidence of hepatocellular carcinomas, and 100 fg/kg/d when hepatocellular carcinomas and adenomas were combined. The corresponding upper-bound (95%) CPFs were 2700 and 9700 (mg/kg/d)-1, respectively. These results indicate that the carcinogenic risk to humans from exposure to 2,3,7,8-TCDD is at least 16-fold lower than previous estimates derived from the Kociba et al. (1978) bioassay.

Epidemiology of Great Lakes bald eagles

Historical data are provided to support the hypothesis that organochlorine chemicals introduced into the Great Lakes ecosystem following World War II are the cause of reproductive loss among bald eagles (Haliaeetus leucocephalus) in the basin. This is supported with data on concurrent population fluxes of extrabasin North American bald eagle populations and the European white-tailed sea eagle (Haliaeetus albicillus) where the same chemicals were produced and released. Organochlorine chemicals appear as a unique stress on Great Lakes bald eagle populations when compared with stresses on successful populations of bald eagles continentwide. Shoreline birds bear significantly higher concentrations of these persistent toxics than inland birds. Association between contaminated prey and elevated concentrations of PCBs, DDT, and DDE in Great Lakes bald eagles are presented. A fledging ratio is used to support the hypothesis that maternal prezygotic exposure affects the viability of embryos and chicks. The ratio of the mean number of fledglings per successful territory to the mean number of fledglings per active territory, when the numerator is greater than 1.4, provides an index of exposure to contaminants by parental animals and affected offspring. When the ratio is greater than 2, parental exposure to organochlorine chemicals should be considered. The adverse effects of prezygotic exposure to the same contaminants in other animal species dependent upon Great Lakes fish, and extrabasin bald eagle populations dependent upon contaminated fish, provide consistency to the argument. The mechanism of action of the organochlorine chemicals further strengthens the causal argument indicting DDT, DDE, and PCBs. A strong association between DDT/DDE and bald eagle reproductive success is provided. However, the role of PCBs is not ruled out. Only data for total PCB concentrations in bald eagle tissue are available, and until specific PCB congeners are quantified there will be uncertainty concerning PCB's role in the Great Lakes bald eagle's lack of success.

Great Lakes embryo mortality, edema, and deformities syndrome (GLEMEDS) in colonial fish-eating birds: similarity to chick-edema disease

Several species of colonial fish-eating birds nesting in the Great Lakes basin, including herring gulls, common terns and double-crested cormorants, have exhibited chronic impairment of reproduction. In addition to eggshell thinning caused by high levels of DDT and metabolites, the reproductive impairment is characterized by high embryonic and chick mortality, edema, growth retardation, and deformities, hence the name Great Lakes embryo mortality, edema, and deformities syndrome (GLEMEDS). The hypothesis has been advanced that GLEMEDS in colonial fish-eating birds resembles chick-edema disease of poultry and has been caused by exposure to chick-edema active compounds that have a common mode of action through the cytochrome P-448 system. Detailed evidence has been collected from the following three groups of studies on herring gulls in the lower Great Lakes during the early 1970s; Forster's terns in Green Bay, Wisconsin in 1983; and double-crested cormorants and Caspian terns in various locations in the upper Great Lakes from 1986 onwards. It has proved difficult to establish not only the onset of the disease in the various species at various locations but also the period in which chick-edema active compounds were released. Anecdotal evidence suggested that serious egg mortality in Lake Ontario herring gulls first occurred in 1966, through the signs of chick-edema disease were not looked for until 1974. Only indirect evidence is available on the date of the release of one of the presumed causal agents, 2,3,7,8-tetrachlorodibenzo-p-dioxin, but highest levels may have occurred in the early to mid 1960s. More reliable data show that the onset of the improvement of reproduction of Lake Ontario herring gulls coincided with the declines in organochlorine compounds and particularly 2,3,7,8-TCDD and PCB. Similarly, information on the onset of the disease and exposures in the Forster's tern and double-crested cormorants in Green Bay is uncertain but bird banders did not observe deformities until the 1970s, which corresponds with the onset of high levels of PCB. If the disappearance of the Caspian tern from Saginaw Bay in the mid 1960s corresponds with the onset of GLEMEDS at that location, then there is a close temporal relationship to the onset of high PCB levels. Chick-edema disease is difficult to diagnose because there is no specific lesion, but rather there is a suite of lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

The Ah receptor and the mechanism of dioxin toxicity

Images

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the hepatic estrogen and glucocorticoid receptors in congenic strains of Ah responsive and Ah nonresponsive C57BL/6J mice

The present study examines the role of the Ah receptor in the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the binding capacity of the hepatic glucocorticoid (GC) and estrogen (E) receptors in female congenic C57BL/6J mice differing only at the Ah (aromatic hydrocarbon responsiveness) locus. The Ah locus is thought to encode the Ah receptor, which regulates the effects of TCDD and related compounds on cytochrome P450IA1 and appears to mediate most of the toxic effects of TCDD. The differences between Ah responsive (Ahb/b) and nonresponsive (Ahd/d) mice appear to reflect differences in the affinity of the Ah receptor in the two strains for ligands such as TCDD. Administration of a single oral dose of TCDD (30 micrograms/kg) to Ahb/b mice produced approximately a 30% decrease in the maximum binding capacities of both the hepatic GC and E receptors, as well as 50-fold induction of a P450IA1-mediated enzymatic activity, ethoxyresorufin-O-deethylase (EROD). Tyrosine aminotransferase (TAT) activity, which is mediated by the GC receptor, was also decreased approximately 30% by TCDD. Dose-response curves indicated that Ah responsive mice are 10-fold more sensitive to induction of EROD than Ah nonresponsive mice (ED50 1.6 vs 15 micrograms/kg), as would be expected for an effect mediated by the Ah receptor. Dose-response curves also indicated that there was a statistical difference in the responsiveness of the hepatic E receptor to TCDD in the two congenic strains of mice (p less than 0.01). Surprisingly, no significant differences in the dose-response curves for the effect of TCDD on hepatic GC receptor binding or TAT activity were observed in the two strains of mice in two separate experiments. These results indicate that the Ah receptor regulates the effects of TCDD on the binding of estrogen to the hepatic estrogen receptor, but suggest that the decrease in the binding capacity of the hepatic GC receptor does not appear to be mediated directly by the Ah locus.

2,3,7,8-tetrachlorodibenzo-p-dioxin increases the potency of androgens and estrogens as feedback inhibitors of luteinizing hormone secretion in male rats

Tetrachlorodibenzo-p-dioxin (TCDD) decreases plasma androgen concentrations in male rats, without increasing plasma luteinizing hormone (LH) concentrations. If plasma LH concentrations had increased appropriately, plasma androgen concentrations in these animals would have returned to normal. The mechanism by which TCDD prevents the compensatory increase in plasma LH concentrations was therefore investigated. TCDD was found to have no effect on the plasma disappearance of iv administered LH. Therefore, the failure of plasma LH concentrations to rise was not due to increased clearance of LH from the circulation, but rather to an effect of TCDD on LH synthesis and/or secretion by the pituitary. In the absence of gonadal steroids (i.e., in castrated rats) TCDD did not prevent the compensatory increase in plasma LH concentrations from occurring. This was shown by 20-fold increases in plasma LH concentrations in both control and TCDD-treated rats 1 week after castration. Thus, (1) the presence of gonadal steroids is required for TCDD to prevent the compensatory increase in plasma LH concentrations, and (2) TCDD does not impair LH secretion by acting, itself, as an androgen or estrogen. TCDD treatment also did not affect pituitary LH content in castrated, testosterone-implanted rats. The above findings demonstrate that TCDD does not decrease the maximum rate at which the pituitary can synthesize and secrete LH. Rather, TCDD alters the feedback regulation of LH secretion when gonadal steroids are present. To determine if TCDD affects the potency of testosterone and its metabolites 5 alpha-dihydrotestosterone and 17 beta-estradiol as feedback inhibitors of LH secretion, rats were dosed with TCDD, castrated, and implanted with sustained-release capsules containing graded amounts of each steroid. Seven days later, the potencies of all three hormones as feedback inhibitors of LH secretion were increased by TCDD, with little effect on their plasma concentrations. The TCDD dose dependence for the increased effectiveness of testosterone as a feedback inhibitor of LH secretion (ED50 10 micrograms/kg) was similar to that reported for the imbalance between plasma LH and androgen concentrations (ED50 15 micrograms/kg). Also, time courses for both responses were similar; each was detected within 1 day of TCDD dosing and each was fully developed after 7 days. We conclude that the mechanism by which TCDD prevents the compensatory increase in plasma LH concentrations in male rats is by increasing the potencies of androgens (and estrogens) as feedback inhibitors of LH secretion.

Alteration of the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by estradiol and tamoxifen

We have hypothesized that part of the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is mediated by interaction with the estrogen receptor complex. The experiments reported here investigate the interactions of TCDD with agonists and antagonists of the estrogen receptor. CD-1 female mice were observed for 2 months after treatment with various combinations of corn oil, estradiol, or tamoxifen, and/or TCDD in corn oil on 3 consecutive days. Estradiol had little effect on acute TCDD lethality but increased severity of TCDD-induced ascites and antagonized TCDD-induced uterine suppression. Severe liver damage did occur in TCDD and estradiol:TCDD treatment groups. Tamoxifen, a competitive inhibitor and a mixed agonist of the mouse estrogen receptor, antagonized the estrogenic effects of estradiol and estradiol:TCDD. Tamoxifen or tamoxifen:TCDD treatment greatly slowed body weight gain in comparison to controls and estrogen-treated animals. While the dose of tamoxifen used was otherwise non-toxic, tamoxifen greatly increased toxicity of TCDD as measured by time to death and percent lethality while having no effect on relative liver weight or relative uterine weight changes induced by TCDD. These findings are consistent with the hypothesis that a portion of the toxicity of TCDD is manifest through activity of the estrogen receptor complex.

The central nervous system may be involved in TCDD toxicity

In the present study, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was administered to both the most TCDD-susceptible (Long-Evans) and the most TCDD-resistant rat strain (Han/Wistar) as a constant 1-week infusion either centrally (intracerebroventricularly; i.c.v.) or peripherally (s.c.). Lethality, feed and water consumption as well as weight gain were observed. For both strains of rat, feed intake was most severely affected in the groups given TCDD i.c.v., while the s.c. infusion of TCDD did not markedly depress eating. The same pattern of responsiveness was discernible in the reduction of water consumption and of weight gain. Two out of 7 i.c.v.-treated rats of the TCDD-susceptible strain died after TCDD exposure, whereas all s.c.-dosed animals survived. A statistically significant strain difference was manifest in the magnitude of response between the i.c.v.-TCDD groups in feed intake and body weight change. Moreover, no deaths occurred among the TCDD-resistant Han/Wistar rats. An additional experiment did not disclose any difference in TCDD toxicity between 2 peripheral routes (s.c. and i.p.). Further, lethality tended to have a shorter latent period with the readily absorbable dimethyl sulphoxide (DMSO) as the solvent than with the potentially slowly absorbed corn oil. These findings suggest an important role for the central nervous system in TCDD toxicity.

Species differences in estrogen receptors and in the response to 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure

The acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibits marked interspecies variability, with the guinea-pig, rat and hamster representing the species most sensitive, intermediate and most resistant to acute toxicity. Prepubertal guinea-pigs, rats and hamsters were treated with a single intraperitoneal injection of TCDD in olive oil at doses of 4, 50 and 1500 micrograms/kg, respectively. These exposures were chosen to produce acute toxicity and all 3 species exhibited a decrease in the rate of body weight gain during the 7 days following TCDD exposure when compared with control (olive oil-treated) animals. On the 7th day after exposure, the density and affinity of 17 beta-estradiol receptors were determined in the uterus and liver of TCDD-treated and control animals. The treatment with TCDD did not alter the affinity of the receptors in these 3 species. The density of hepatic 17 beta-estradiol receptors was decreased 65% in the guinea pig and 92% in the rat following exposure to TCDD. In contrast, TCDD-treated hamsters exhibited no change in the density of hepatic 17 beta-estradiol receptors. The uterine 17 beta-estradiol receptors were increased in density by TCDD treatment in the hamster and in the rat when expressed per mg protein. Uterine wet weights in the guinea-pig and rat were also significantly decreased by TCDD treatment but were not changed in the hamster. When the Bmax for uterine 17 beta-estradiol receptors was expressed as pmol/g tissue wet weight. TCDD exposure was found to produce an 11% decrease in density in the rat, while producing a 44% increase in the hamster. In control animals, the density of uterine 17 beta-estradiol receptors correlated inversely with the lethal dose of TCDD in these 3 species (i.e., the guinea-pig has the lowest LD50 and highest density of uterine 17 beta-estradiol receptors). The different responses to TCDD in the 3 species suggest that the changes in 17 beta-estradiol receptors may be related to species-specific toxic responses associated with TCDD exposure.

Species comparison of steroid UDP-glucuronyl transferase: correlation to TCDD sensitivity

Estradiol glucuronidation via steroid UDP-glucuronyl transferase (sUDPGT) was examined in 2,3,7,8-te trachlorodibenzo-p-dioxin (TCDD) sensitive and resistant species and strains. Steroid UDPGT was not induced by treatment with TCDD or estradiol. The most sensitive species to TCDD lethality, the guinea pig, had relatively high steroid UDPGT activity, while the hamster, the most resistant species, and rats had low levels of activity; no differences in sUDPGT activities were observed between mouse or rat strains differing in susceptibility to TCDD intoxication. These results suggest a role for differences in steroid physiology in the determination of species susceptibility to TCDD, but also demonstrate that other factors are involved.