Modulation of oestrogen receptor signalling by association with the activated dioxin receptor

An aryl hydrocarbon receptor agonist amplifies the mitogenic actions of estradiol in granulosa cells: evidence of involvement of the cognate receptors

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that, besides mediating toxic responses, may have a central role in ovarian physiology. Studying the actions of AHR ligands on granulosa cells function, we have found that beta-naphthoflavone amplifies the comitogenic actions of FSH and 17beta-estradiol in a dose-dependent manner. This amplification was even greater in cells that overexpress the AHR and was reversed by cotreatment with the AHR antagonist alpha-naphthoflavone, suggesting that this effect is mediated by the AHR. The estrogen receptor is likewise implicated in this phenomenon, because a pure antiestrogen abolished the described synergism. However, the more traditional inhibitory AHR-estrogen receptor interaction was observed on the estrogen response element-driven transcriptional activity. On the other hand, alpha-naphthoflavone inhibited dose-dependently the mitogenic actions of FSH and 17beta-estradiol. Beta-naphthoflavone induced the expression of Cyp1a1 and Cyp1b1 transcripts, two well-characterized AHR-inducible genes that code for hydroxylases that metabolize estradiol to catecholestrogens. Nevertheless, the positive effect of beta-naphthoflavone on proliferation was not caused by increased metabolism of estradiol to catecholestrogens, because these compounds inhibited the hormonally stimulated DNA synthesis. This latter inhibition exerted by catecholestrogens suggests that these hydroxylases would play a regulatory point in granulosa cell proliferation. Our study indicates that AHR ligands modulate the proliferation of rat granulosa cells, and demonstrates for the first time that an agonist of this receptor is able to amplify the comitogenic action of classical hormones through a mechanism that might implicate a positive cross-talk between the AHR and the estrogen receptor pathways.

Evidence that ligand binding is a key determinant of Ah receptor-mediated transcriptional activity

The aryl hydrocarbon receptor (AhR) mediates the biological activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Whether the AhR can mediate enhanced transcriptional activity in the absence of ligand binding has not been established. Hepatocytes from AhR-null (AhR-KO) and wild-type (AhR-WT) neonatal mice were immortalized with Simian virus 40. Two point mutants of the AhR, A375I and A375F, were generated to test the hypothesis that the AhR requires ligand binding to exhibit significant transcriptional activity, both mutants fail to bind ligand or exhibit enhanced activity in cells exposed to AhR ligands. Upon transient, co-expression of ARNT with AhR-A375I or AhR-A375F in AhR-KO cells, these mutants exhibited significant ligand-independent transcriptional activity. However, in CV-1 cells, which others have previously shown to contain relatively high levels of AhR ligand(s), these AhR mutants exhibit essentially no constitutive activity. These results indicate that while the AhR can potentially exhibit activity in the absence of ligand binding, the high constitutive receptor activity observed in many cell lines appears to be due to the presence of endogenous AhR ligands.

Associations between serum levels of selected organochlorine compounds and endometriosis in infertile Japanese women

Endocrine-disrupting chemicals (EDCs) have been proposed as risk factors for endometriosis. Persistent organochlorine compounds, a group of suspected EDCs, are present to some extent in almost all human adipose tissue and blood via the food chain. A few animal studies have confirmed that exposure to these compounds can increase the incidence of endometriosis. In this study, we examined the associations between endometriosis and exposure to selected organochlorine compounds, including 8 polychlorinated dibenzo-p-dioxins (PCDDs), 10 polychlorinated dibenzofurans (PCDFs), 4 coplanar polychlorinated biphenyls (cPCBs), 36 ortho-substituted polychlorinated biphenyls (PCBs), and 13 chlorinated pesticides or their metabolites. The participants were 139 infertile Japanese women who were examined by laparoscopy and diagnosed as either endometriosis cases (Stages II-IV) or controls (Stages 0-I). The serum levels (lipid adjusted) of the targeted organochlorine compounds were in both 58 cases and 81 controls. There were very few differences in the various levels between endometriosis cases and controls. The total serum toxic equivalency (TEQ) value of PCDDs was significantly higher in the controls than in the cases (P=0.02). No other total TEQ values differed between cases and controls. For PCDDs, PCDFs, cPCBs, and PCBs, the multivariate odds ratio was 0.38 [95% confidence interval (CI), 0.12-1.17] and 0.41 (95% CI, 0.14-1.27) for the third and highest quartiles, respectively, compared to the lowest quartile of total TEQ values. A weak, negative dose-response relationship was evident for total TEQs (P for trend of 0.06). The results of this study provide some evidence that serum levels of these organochlorine compounds are not associated with an increased risk of endometriosis in infertile Japanese women.

Preliminary molecular genetic analysis of the Receptor Interacting Protein 140 (RIP140) in women affected by endometriosis

Background

Endometriosis is a complex disease affecting 10–15% of women at reproductive age. Very few genes are known to be altered in this pathology. RIP140 protein is an important cofactor of oestrogen receptor and many other nuclear receptors. Targeting disruption experiments of nrip1 gene in mice have demonstrated that nuclear receptor interacting protein 1 gene (nrip1), the gene encoding for rip140 protein, is essential for female fertility. Specifically, mice null for nrip1 gene are viable, but females are infertile because of complete failure of mature follicles to release oocytes at ovulation stage. The ovarian phenotype observed in mice devoid of rip140 closely resembles the luteinized unruptured follicle (LUF) syndrome that is observed in a high proportion of women affected of endometriosis or idiopathic infertility. Here we present a preliminary work that analyses the role of NRIP1 gene in humans.

Methods

We have sequenced the complete coding region of NRIP1 gene in 20 unrelated patients affected by endometriosis. We have performed genetic association studies by using the DNA variants identified during the sequencing process.

Results

We identified six DNA variants within the coding sequence of NRIP1 gene, and five of them generated amino acid changes in the protein. We observed that three of twenty sequenced patients have specific combinations of amino-acid variants within the RIP140 protein that are poorly represented in the control population (p = 0.006). Moreover, we found that Arg448Gly, a common polymorphism located within NRIP1 gene, is associated with endometriosis in a case-control study (59 cases and 141 controls, p_{allele positivity test} = 0.027).

Conclusion

Our results suggest that NRIP1 gene variants, separately or in combinations, might act as predisposing factors for human endometriosis.

Molecular mechanisms of AhR functions in the regulation of cytochrome P450 genes

AhR, a ligand-activated transcription factor, mediates xenobiotic signaling to enhance the expression of target genes, including drug-metabolizing cytochrome P450s. The recent development of several new techniques, including chromatin immunoprecipitation and RNA interference, has expanded and deepened our knowledge of AhR function in the xenobiotic signal transduction. In this review, we briefly summarize our current understanding of the activation and inactivation of AhR activities and discuss the future directions of AhR research.

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

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

Aryl hydrocarbon receptor-mediated transcription: ligand-dependent recruitment of estrogen receptor alpha to 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive promoters

Using chromatin immunoprecipitation assays, we studied the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated recruitment of the aryl hydrocarbon receptor (AhR) and several co-regulators to the CYP1A1 promoter. AhR displayed a time-dependent recruitment, reaching a peak at 75 min and maintaining promoter occupancy for the remainder of the time course. Recruitment of AhR was followed by TIF2/SRC2, which preceded CBP, histone H3 acetylation, and RNA polymerase II (RNAPII). Simultaneous recruitment to the enhancer and the TATA box region suggests the formation of a large multiprotein complex bridging the two promoter regions. Interestingly, estrogen receptor alpha (ERalpha) displayed a TCDD- and time-dependent recruitment to the CYP1A1 promoter, which was increased by co-treatment with estradiol. Transfection in HuH7 human liver cells confirmed previously reported ERalpha enhancement of AhR activity. In contrast, TCDD did not induce the recruitment of ERalpha to the estrogen-responsive pS2 promoter, and after 120 min of co-treatment with estradiol, ERalpha is still present on the CYP1A1 promoter but no longer at pS2. RNA interference studies with T47D cells support a role for ERalpha in TCDD-dependent CYP1A1 expression. Our data suggest that ERalpha acts as a coregulator of AhR-mediated transcriptional activation and that the recruitment of ERalpha by AhR represents a novel mechanism AhR-ERalpha cross talk.

Effects of early prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on postnatal reproduction in the laying hen (Gallus gallus)

This study demonstrates the long-term effects of very early embryonic exposure to a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0, 10 and 20 ng/egg), administered before the beginning of embryonic development, on growth and reproductive performance in laying hens. Hatchability and body weight gain from 11 weeks onwards were significantly depressed in 20 ng treated hens. All hens started laying egg at around the same age and the laying performance of TCDD-treated hens was normal. No disturbances in the age-related pattern and concentrations of oestradiol, LH or FSH in plasma could be found but mean progesterone concentrations were significantly lower in 20 ng treated hens. Moreover, follicular distribution was changed with less small white follicles and smaller yellow follicles, which probably resulted in the lower egg weight of the 20 ng treated hens. At 43 weeks of age, hens treated in ovo with TCDD showed a retained right oviduct, mostly filled with clear fluid. From these results, it seems that in ovo exposure to TCDD interferes in the right oviduct regression during embryonic development and induces some changes in follicular distribution but without impairment of reproductive performance in the adult laying hen.

Analysis of the AhR, ARNT, and AhRR gene polymorphisms: genetic contribution to endometriosis susceptibility and severity

OBJECTIVE

To explore whether polymorphisms in AhR, ARNT, and AhRR contribute to endometriosis susceptibility and severity.

DESIGN

Case control study.

SETTING

Hospital.

PATIENT(S)

One hundred thirty-eight Japanese women with or without endometriosis, diagnosed endoscopically.

INTERVENTION(S)

Endoscopic laparoscopy, with blood samples for genotyping obtained before the laparoscopic examination for genomic DNA extraction from peripheral leukocytes.

MAIN OUTCOME MEASURE(S)

AhR, ARNT, and AhRR polymorphisms were genotyped using real-time polymerase chain reaction (PCR) analysis. Odds ratios and 95% confidence intervals were calculated for AhR, ARNT, and AhRR genotypes to evaluate the risk of endometriosis. Associations between these polymorphisms and stage of endometriosis were also examined.

RESULT(S)

The C/G + G/G genotypes at codon 185 of AhRR showed a statistically significant association with risk of endometriosis (adjusted odds ratio, 2.53; 95% confidence interval, 1.16-5.55). Furthermore, a statistically significant trend associated the C/G + G/G genotypes with the clinical stage of endometriosis. No statistically significant association was observed between AhR codon 554 or ARNT codon 189 polymorphisms and endometriosis.

CONCLUSION(S)

AhRR codon 185 polymorphism was associated with susceptibility to and severity of endometriosis in Japanese women.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats disrupts brain sexual differentiation

The effects of in utero and lactational exposure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on brain sexual differentiation were investigated. TCDD was orally administered to pregnant Holtzman rats on gestation day (GD) 15, and the activity of brain aromatase, a key enzyme for sexual differentiation, was measured in offspring on postnatal day (PND) 2. Changes in sexual dimorphisms of saccharin preference and the volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) were examined in adult offspring. In controls, litter means of brain aromatase activity were higher in males than in females. In utero exposure to 200 ng/kg TCDD significantly decreased the sex ratio of aromatase activity (male/female) on PND 2. Offspring were weaned on PND28 and the saccharin test was started on PND84. In controls, saccharin (0.25%) intake (g/kg body weight) was significantly higher in female offspring than in males. In utero exposure to 200 ng/kg TCDD significantly increased saccharin intake in male offspring compared with control males, whereas 800 ng/kg TCDD had no effect. Neither dose of TCDD influenced saccharin intake of female offspring. In controls, SDN-POA volume was significantly greater in males than in females at 14 weeks of age. Exposure to 200 ng/kg TCDD significantly decreased SDN-POA volume in males, whereas 800 ng/kg TCDD had no effect. Neither doses of TCDD influenced the SDN-POA volume in female offspring. These results suggest that in utero and lactational TCDD exposure dose-dependently induces demasculinization in male offspring by inhibiting brain aromatase activity in the hypothalamus-preoptic area during central nervous system development.

Targets for indole-3-carbinol in cancer prevention

Mounting preclinical and clinical evidence indicate that indole-3-carbinol (I3C), a key bioactive food component in cruciferous vegetables, has multiple anticarcinogenic and antitumorigenic properties. Evidence that p21, p27, cyclin-dependent kinases, retinoblastoma, Bax/Bcl-2, cytochrome P-450 1A1 and GADD153 are targets for I3C already exists. Modification of nuclear transcription factors including Sp1, estrogen receptor, nuclear factor kappaB and aryl hydrocarbon receptor may represent a common site of action to help explain downstream cellular responses to dietary I3C and, ultimately, to its anticancer properties. While the current information is intriguing, future I3C research needs to focus on why these changes in nuclear transcription factors occur and how they relate to phenotypic responses and the quantity and duration of exposure to I3C and its dimer 3,3'-diindolylmethane.

Evaluation of estrogenic activities of hydroxylated polycyclic aromatic hydrocarbons in cigarette smoke condensate

Estrogenic activities of cigarette smoke condensates obtained from the extraction of particulate matters from mainstream and sidestream cigarette smoke with benzene/ethanol were evaluated by using a yeast two-hybrid assay system expressing human estrogen receptor alpha (hERalpha). To identify the constituents of the cigarette smoke condensate which are responsible for the estrogenic activity, the condensate was fractionated into eleven fractions by liquid-liquid extractions. Among these fractions, the neutral fractions of mainstream and sidestream smoke showed the strongest estrogen receptor-mediated activity by the yeast two-hybrid assay. Then the neutral fractions were fractionated by medium-pressure liquid chromatography with silica gel column. In the fractions that showed strong estrogenic activity, 2-hydroxyfluorene (2-OHFle), 2- and 3-hydroxyphenanthrene (2- and 3-OHPhe), 1-hydroxypyrene (1-OHPyr) and n-propyl-p-hydroxybenzoate (n-PHB) were identified by LC- and GC-MS and HPLC with fluorescence detection. 2-OHFle, 2-OHPhe and n-PHB exhibited estrogenic activity, whereas weak activity was observed with 3-OHPhe and 1-OHPyr. Several other hydroxylated polycyclic aromatic hydrocarbons having no activity were also identified. This is a first study to identify estrogenic hydroxylated PAHs in cigarette smoke condensate. The present findings points out the necessity for detailed investigation of exposure to aerosols containing apparently estrogenic compounds.

Influence of cAMP on reporter bioassays for dioxin and dioxin-like compounds

In reporter assays for detection of dioxins, the dioxin-responsive element (DRE) is generally used as a sensor sequence. In several systems, the CYP1A1 promoter containing DREs (DRE(cyp)) is inserted into a part of the long terminal repeat of mouse mammary tumor virus (LTR(MMTV)) to improve sensitivity of assays. We found that DRE(cyp)-LTR(MMTV) responds not only to dioxins and dioxin-like compounds but also to forskolin, a cAMP-elevating agent. This effect was dose-dependent and reproduced by other cAMP-elevating agents including 8-bromo-cAMP and 3-isobutyl-methylxanthine. The cAMP response element (CRE) and CRE-like sequences were absent in DRE(cyp)-LTR(MMTV) and not involved in this process. In contrast to the effect of dioxin, the activation of DRE(cyp)-LTR(MMTV) by cAMP was independent of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor for DRE. Furthermore, neither DRE(cyp), LTR(MMTV) nor the consensus sequence of DRE alone was activated in response to cAMP. These data elucidated for the first time that the combination of DRE(cyp) with LTR(MMTV) causes a peculiar response to cAMP and suggested that use of AhR antagonists is essential to exclude false-positive responses of DRE(cyp)-LTR(MMTV)-based bioassays for detection and quantification of dioxins and dioxin-like compounds.

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.

SPBP is a phosphoserine-specific repressor of estrogen receptor alpha

Multiple signaling pathways stimulate the activity of estrogen receptor alpha (ERalpha) by direct phosphorylation within its N-terminal activation function 1 (AF1). How phosphorylation affects AF1 activity remains poorly understood. We performed a phage display screen for human proteins that are exclusively recruited to the phosphorylated form of AF1 and found the stromelysin-1 platelet-derived growth factor-responsive element-binding protein (SPBP). In a purified system, SPBP bound only the in vitro-phosphorylated form of the ERalpha AF1 or the phosphoserine mimic S118E, and the interaction domain could be mapped to a 42-amino-acid fragment of SPBP. In cells, SPBP preferentially interacted with liganded and phosphorylated ERalpha. Functionally, SPBP behaved as a repressor of activated ERalpha, which extends its previously demonstrated roles as a DNA binding transactivation factor and coactivator of other transcription factors. By targeting the phosphorylated form of AF1, SPBP may contribute to attenuating and fine-tuning ERalpha activity. A functional consequence is that SPBP inhibits the proliferation of ERalpha-dependent but not ERalpha-independent breast cancer cell lines, mirroring a reported negative correlation with the ERalpha status of breast tumors.

ERα-AHR-ARNT Protein-Protein Interactions Mediate Estradiol-dependent Transrepression of Dioxin-inducible Gene Transcription

The aryl hydrocarbon receptor (AHR) and the aryl hydrocarbon receptor nuclear translocator (ARNT) form a heterodimeric transcription factor upon binding a wide variety of environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). AHR target gene activation can be repressed by estrogen and estrogen-like compounds. In this study, we demonstrate that a significant component of TCDD-inducible Cyp1a1 transcription is the result of recruitment of estrogen receptor (ER)-alpha by AHR/ARNT as a transcriptional co-repressor. Both AHR and ARNT were capable of interacting directly with ER alpha, as ascertained by glutathione S-transferase pull-down. 17Beta-estradiol repressed TCDD-activated Cyp1a1 and Cyp1b1 gene transcription in MCF-7 cells in the presence of cycloheximide, as determined by reverse transcription/real-time PCR. Furthermore, chromatin immunoprecipitation (ChIP) assays have shown that ER alpha is present at the Cyp1a1 enhancer only after co-treatment with E2 and TCDD, in MCF-7 cells. Sequential two-step ChIP assays were performed which demonstrate that AHR and ER alpha are present together at the same time on the Cyp1a1 enhancer during transrepression. Taken together these data support a role for ER-mediated transrepression of AHR-dependent gene regulation.

Splicing potentiation by growth factor signals via estrogen receptor phosphorylation

Mitogen-activated protein kinase-mediated growth factor signals are known to augment the ligand-induced transactivation function of nuclear estrogen receptor alpha (ERalpha) through phosphorylation of Ser-118 within the ERalpha N-terminal transactivation (activation function-1) domain. We identified the spliceosome component splicing factor (SF)3a p120 as a coactivator specific for human ERalpha (hERalpha) activation function-1 that physically associated with ERalpha dependent on the phosphorylation state of Ser-118. SF3a p120 potentiated hERalpha-mediated RNA splicing, and notably, the potentiation of RNA splicing by SF3a p120 depended on hER Ser-118 phosphorylation. Thus, our findings suggest a mechanism by which growth factor signaling can regulate gene expression through the modulation of RNA splicing efficiency via phosphorylation of sequence-specific activators, after association between such activators and the spliceosome.

Comparisons of brain, uterus, and liver mRNA expression for cytochrome p450s, DNA methyltransferase-1, and catechol-o-methyltransferase in prepubertal female Sprague-Dawley rats exposed to a mixture of aryl hydrocarbon receptor agonists

Non-ortho polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) are ubiquitous environmental contaminants that exert their toxicity mostly through activation of the aryl-hydrocarbon receptor (AhR), and are referred to as AhR agonists. The objective was to study, by real time reverse-transcriptase-polymerase chain reaction (RT-PCR), the effects of postnatal exposure to a reconstituted mixture of AhR agonists present in breast milk (3 non-ortho PCBs, 6 PCDDs, and 7 PCDFs, referred to here-in-after as AhRM) on mRNA expression of estrogen receptor (ERalpha), enzymes involved with the metabolism of estrogens [catechol-o-methyltransferase (Comt), cytochrome P450 (Cyp)1A1, 1B1 and 2B1], and DNA methyltransferase-1 (Dnmt1), in brain areas, liver and uterus of immature female rats. Neonates were exposed by gavage during postnatal day (PND) 1-20 with dosages equivalent to 1, 10, 100, and 1000 times the estimated average human exposure level, and were sacrificed at PND 21. None of the end points were affected in uterine cross-sections, or in samples of uterine tissue layers collected by laser capture microdissection. At 1000x, the AhRM reduced Dnmt1 mRNA abundance to 28% and 32% of control in the liver and hypothalamus, respectively. In the brain, Cyp1A1 was increased (409%) but ERalpha was reduced (66%). Similarly, mRNA abundance for Comt isoforms was reduced in the liver (45%) and brain areas (55-70%). AhRM at 100x, the lowest effective dose, exerted a 220% increase in brain cortex Comt [membrane bound (Mb)], a 219% increase in hepatic Cyp1B1, and a 63% decrease in hepatic Comt (soluble (S)+Mb). These results support the possibility that early exposure to environmental contaminants could lead to effects mediated by changes in DNA methylation and/or estrogen metabolism and signaling.

Molecular interactions of the aryl hydrocarbon receptor and its biological and toxicological relevance for reproduction

The dioxin/aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor responsive to both natural and man-made environmental compounds. AhR and its nuclear partner ARNT are expressed in the female reproductive tract in a variety of species and several indications suggest that the AhR might play a pivotal role in the physiology of reproduction. Furthermore, it appears to be the mediator of most, if not all, the adverse effects on reproduction of a group of highly potent environmental pollutants collectively called aryl hydrocarbons (AHs), including the highly toxic compound 2,3,7,8-tetrachlor-odibenzo-p-dioxin (TCDD). Although a large body of recent literature has implicated AhR in multiple signal transduction pathways, the mechanisms of action resulting in a wide spectrum of effects on female reproduction are largely unknown. Here we summarize the major types of molecular cross-talks that have been identified for the AhR and linked cell signaling pathways and that are relevant for the understanding of the role of this transcription factor in female reproduction.

Regulation of the Cyp2a5 gene involves an aryl hydrocarbon receptor-dependent pathway

We have investigated the role of the aryl hydrocarbon receptor (AHR) in the regulation of the Cyp2a5 gene. The C57BL/6 and DBA/2 mouse strains with a genetically determined difference in AHR function were used to study the CYP2A5 induction by typical AHR ligands, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene. The CYP2A5 mRNA up-regulation in these mouse strains showed a difference in response, typical for AHR-regulated genes, both by TCDD in cultured primary hepatocytes and by 3-methylcholanthrene in vivo. In primary hepatocytes, TCDD caused a 3-fold elevation of the CYP2A5 protein level and a similar induction of the CYP2A5-catalyzed coumarin 7-hydroxylation activity. In reporter gene assays, the Cyp2a5 promoter region -3033 to +10 mediated a 2- to 5-fold induction of luciferase activity by TCDD treatment in primary hepatocytes and in Hepa-1 hepatoma cells with an intact AHR/AHR nuclear translocator (ARNT) complex. In Hepa-1 variant cell lines with deficiencies in the AHR/ARNT complex, the absence of ARNT abolished the induction. A putative AHR response element (XRE) was identified in the Cyp2a5 promoter at the position -2514 to -2492 and found to interact with the AHR/ARNT heterodimer. Transfection experiments combined with mutation of the XRE site indicated that the site partly mediates the TCDD induction of Cyp2a5. An additional AHR-dependent mechanism also regulates the proximal promoter of the Cyp2a5 gene. In conclusion, our studies showed that AHR ligands up-regulate Cyp2a5 transcriptionally by an AHR/ARNT-dependent mechanism and established Cyp2a5 as a novel AHR-regulated gene.

Identification of the Tryptophan Photoproduct 6-Formylindolo[3,2-b]carbazole, in Cell Culture Medium, as a Factor That Controls the Background Aryl Hydrocarbon Receptor Activity

The presence of high affinity ligands for the aryl hydrocarbon receptor (AhR) in cell culture medium has generally been overlooked. Such compounds may confound mechanistic studies of the important AhR regulatory network. Numerous reports have described that light exposed cell culture medium induces AhR-dependent activity. In this study, we aimed at identifying the causative substance(s). A three-dimensional factorial design was used to study how the background activity of CYP1A1 in a rat hepatoma cell line (MH1C1) was controlled by photoproducts formed in the medium exposed to normal laboratory light. The light induced activity was found to be tryptophan dependent, but independent of riboflavin and other components in the medium. The light exposed medium showed the same transient enzyme inducing activity in vitro as the AhR ligand 6-formylindolo[3,2-b]carbazole (FICZ). This substance, which we have previously identified as being formed in UV-exposed tryptophan solutions, is a substrate for CYP1A1 and it has a higher AhR binding affinity than TCDD. Several tryptophan related photoproducts were detected in the light-exposed medium. For the first time one of the formed photoproducts was identified as FICZ with bioassay driven fractionation coupled with HPLC/MS. These results clearly show that tryptophan derived AhR ligands, which have been suggested to be endogenous AhR ligands, influence the background levels of CYP1A1 activity in cells in culture.

Association of estrogen receptor 1 intron 1 C/T polymorphism in Korean vitiligo patients

BACKGROUND

Vitiligo is a common disease characterized by cutaneous white maculae due to loss of melanocytes. It is a polygenic disease, however, the exact pathogenesis of vitiligo is not yet known. The estrogen receptor (ESR) 1 gene was selected as a candidate gene because some researchers treated vitiligo successfully with the steroid-thyroid hormone mixture containing estrogen. Furthermore ESR was expressed in the melanocytes which have an important role in the pigmentation. The polymorphisms of ESR1 gene in vitiligo patients was not reported yet.

OBJECTIVE

To determine whether polymorphisms of ESR1 gene were associated with susceptibility to vitiligo patients in Korean population.

METHODS

We conducted case-control association study of vitiligo patients (120) and healthy controls (254). Genotypes of ESR1 gene (intron 1 C/T, exon 4 C/G, and exon 8 A/G) were determined by polymerase chain reaction followed by restriction enzyme digestion.

RESULTS

Intron 1 T/C allele frequency was significantly different between patients and controls (P = 0.034). Intron 1 T/C genotype distribution (P = 0.021) and allele frequency (P = 0.013) were different between female vitiligo patients and female controls. Intron 1 T/C allele frequency showed significantly difference between generalized type of vitiligo patients and controls (P = 0.044). Genotype distributions and allele frequencies of exon 4 C/G and exon 8 A/G polymorphisms were not different between patients and controls.

CONCLUSION

The present study suggests that ESR1 may be a possible risk factor for female or generalized type of vitiligo patients.

Isolation and characterization of two cytochrome P450 aromatase forms in killifish (Fundulus heteroclitus): differential expression in fish from polluted and unpolluted environments

Populations of killifish (Fundulus heteroclitus) persist in many different highly polluted environment indicative of adaptation or tolerance. In this study, we sought to determine whether long term, multigenerational exposures to environmental contaminants has affected reproductively relevant genes and biological processes. A homology cloning strategy was used to isolate the killifish cytochrome P450 aromatase (P450arom, estrogen synthetase) cDNAs. Consistent with previous fish studies, killifish were found to have two P450arom forms, which segregated into A- and B-gene clades and were differentially expressed in brain (B >> A) and ovary (A >> B). Comparison of killifish from highly polluted (New Bedford Harbor, NBH) and unpolluted (Scorton Creek, SC) environments revealed no site-related differences in P450arom coding sequences or in overall tissue distribution patterns. As measured by real-time quantitative PCR (QPCR) analysis, however, P450arormB (a known marker of estrogen effect) was approximately two-fold higher in the brain of NBH than of SC fish, a difference seen in reproductively active and inactive males and females. Providing further evidence of exposure to estrogen-like pollutants or metabolites in NBH, vitellogenin (vtg) mRNA and protein were elevated in seasonally active and inactive males, and in reproductively inactive females, when compared to SC fish. By contrast, during the period of reproductive activity, NBH females had a lower gonadosomatic index, lower plasma estrogen, a decreased hepatosomatic index, and reduced vtg expression as compared to SC females, indicating that the female hypothalamic-pituitary-gonadal (HPG)-liver axis is impaired in the polluted environment. As measured by a decrease in plasma androgen (but not GSI), the male HPG axis was impaired in reproductively active NBH versus SC fish. In agreement with reports that NBH killifish are resistant to dioxin-like chemicals (DLC) that activate arylhydrocarbon receptor (AhR) signaling, ovarian P450aromA (a marker of dioxin-like effect in zebrafish embryos) did not differ in SC and NBH fish. In conclusion, the killifish population at the NBH Superfund site maintains a level of reproductive competence in the face of evidence of exposure to estrogen-like pollutants and endocrine disruption.

Effects of In Utero and Lactational TCDD Exposure on Bone Development in Differentially Sensitive Rat Lines

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a notorious model compound of highly toxic environmental pollutants, polychlorinated dibenzo-p-dioxins (PCDDs). Their toxic effects are mediated via cytosolic aryl hydrocarbon receptor (AHR). We studied the effects of several dose levels of TCDD on developing rat bone after maternal exposure at different times of gestation and lactation in three differentially sensitive rat lines. Rat lines A, B, and C differ in their sensitivity to TCDD due to mutated AHR (Ahr(hw)) in line A and another TCDD-resistance allele (B(hw)) in line B. Line C rats have no resistance alleles. Offspring were analyzed for bone mineral density and geometry by peripheral quantitative computed tomography (pQCT) and for bone biomechanics by three-point bending at mid-diaphysis of tibia and femur and by axial loading at femoral neck. TCDD treatment resulted in bone defects, mainly in offspring of the most sensitive line C at a maternal dose of 1 microg/kg. They included decreased bone length, cross-sectional area of cortex, and bone mineral density. Mechanical testing revealed significantly reduced bending breaking force and stiffness of tibia, femur, and femoral neck. The effects were exposure time-dependent, and earlier exposure caused more severe defects. Gestational exposure alone was not sufficient, but lactational exposure was required to cause the bone defects. Most of the defects were recovered at the age of 1 year. The results indicate that dioxins affect developing bone by interfering with bone growth and mechanical strength and that the effects are mainly reversible. The dioxin-resistance alleles, Ahr(hw) and B(hw) increase the resistance to these defects.

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.

Binding of estrogenic compounds to recombinant estrogen receptor-alpha: application to environmental analysis

Estrogenic activity in environmental samples could be mediated through a wide variety of compounds and by various mechanisms. High-affinity compounds for estrogen receptors (ERs), such as natural or synthetic estrogens, as well as low-affinity compounds such as alkylphenols, phthalates, and polychlorinated biphenyls are present in water and sediment samples. Furthermore, compounds such as polycyclic aromatic hydrocarbons, which do not bind ERs, modulate estrogen activity by means of the aryl hydrocarbon receptor (AhR). In order to characterize compounds that mediate estrogenic activity in river water and sediment samples, we developed a tool based on the ER-alphaligand-binding domain, which permitted us to estimate contaminating estrogenic compound affinities. We designed a simple transactivation assay in which compounds of high affinity were captured by limited amounts of recombinant ER-alpha and whose capture led to a selective inhibition of transactivation. This approach allowed us to bring to light that water samples contain estrogenic compounds that display a high affinity for ERs but are present at low concentrations. In sediment samples, on the contrary, we showed that estrogenic compounds possess a low affinity and are present at high concentration. Finally, we used immobilized recombinant ER-alpha to separate ligands for ER and AhR that are present in river sediments. Immobilized ER-alpha, which does not retain dioxin-like compounds, enabled us to isolate and concentrate ER ligands to facilitate their further analysis.

TCDD and PCBs inhibit breast cancer cell proliferation in vitro

The effects on cell proliferation of arylhydrocarbon receptor (AhR) agonists in estrogen-responsive T47D and ZR-75-1 cells were investigated. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and the non-ortho-substituted polychlorinated biphenyl (PCB) congeners, PCB 77, PCB 81, PCB 126, and PCB 169 each inhibited 17beta-estradiol (E(2))-stimulated cell proliferation in a dose-responsive manner. In the absence of added E(2), TCDD, PCB 77, PCB 81, and PCB 169 had no significant effect on cell proliferation, while PCB 126 at high concentrations caused slight elevations. The order of effective inhibition of E(2)-stimulated cell proliferation by the PCB congeners was: PCB 81>PCB 126 approximately = PCB 169>PCB 77. In the comparative literature, mammalian TEFs for these congeners toxic potency are in the order: PCB 126>PCB 169>PCB 81 approximately = PCB 77 [Organohalogen Compd. 34 (1997) 237]. Our results thus show an unexpected different pattern for the inhibitory effects of PCBs congeners on E(2)-mediated cell proliferation.

Constitutively Active Aryl Hydrocarbon Receptor Expressed Specifically in T-Lineage Cells Causes Thymus Involution and Suppresses the Immunization-Induced Increase in Splenocytes

The aryl hydrocarbon receptor (AhR) is a transcription factor belonging to the basic helix-loop-helix-PER-ARNT-SIM superfamily. Xenobiotics, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, bind the receptor and trigger diverse biological reactions. Thymocyte development and T cell-dependent immune reactions are sensitive targets of AhR-dependent 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity. However, the exact role of the AhR in T cells in animals exposed to exogenous ligands has not been clarified because indirect effects of activated AhR in other cell types cannot be excluded. In this study, we generated transgenic (Tg) mice expressing a constitutively active mutant of AhR under the regulation of a T cell-specific CD2 promoter to examine AhR function in T cells. The mRNAs of the constitutively active mutant of AhR and an AhR-induced gene, CYP1A1, were expressed in the thymus and spleen of the Tg mice. The transgene expression was clearly detected in the thymocytes, CD4, and CD8 T cells, but not in the B cells or thymus stromal cells. These Tg mice had a decreased number of thymocytes and an increased percentage of CD8 single-positive thymocytes, but their splenocytes were much less affected. By contrast, the increase in number of T cells and B cells taking place in the spleen after immunization was significantly suppressed in the Tg mice. These results clearly show that AhR activation in the T-lineage cells is directly involved in thymocyte loss and skewed differentiation. They also indicate that AhR activation in T cells and not in B cells suppresses the immunization-induced increase in both T cells and B cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces insulin-like growth factor binding protein-1 gene expression and counteracts the negative effect of insulin

Recent epidemiological studies have revealed a possible correlation between exposure to high levels of dioxins or dioxin-like compounds and diabetes. Yet the interaction between insulin and dioxin actions remains elusive. We studied the regulation of insulin-like growth factor binding protein-1 (IGFBP-1), a protein involved in glucose homeostasis and whose expression is down-regulated by insulin. We showed that 2,3,7,8-tetrachorodibenzo-p-dioxin (TCDD) specifically induced IGFBP-1 mRNA in human hepatocytes and HepG2 human hepatoma cells (2.5- and 8-fold, respectively). Cellular and secreted IGFBP-1 protein levels were also up-regulated. Transfection and reporter assays showed that the IGFBP-1 promoter was activated by TCDD and that this activation was dependent on the integrity of a proximal xenobiotic-responsive element (XRE). This XRE, located near the insulin-glucocorticoid regulatory region, binds the aryl-hydrocarbon receptor. In agreement with previous studies, the IGFBP-1 promoter was down-regulated by insulin (50%); we show here that although TCDD activated the IGFBP-1 promoter 5- to 6-fold, the combination of TCDD and insulin led to an expression level of IGFBP-1 that was higher than basal level (2- to 3-fold activation). Similar regulations were observed for the endogenous IGFBP-1 mRNA. These data suggest that the xenobiotic-hormonal regulatory region of the IGFBP-1 promoter mediates an up-regulation of IGFBP-1 expression by TCDD even in the presence of insulin. Because IGFBP-1 modulates blood glucose levels, the up-regulation of IGFBP-1 by dioxins might account for the disruptive effects of these pollutants on glucose metabolism.

Association of micropenis with Pro185Ala polymorphism of the gene for aryl hydrocarbon receptor repressor involved in dioxin signaling

The prevalence of undermasculinized external genitalia has increased in several countries including Japan, and this phenomenon has primarily been ascribed to the deleterious effects of environmental endocrine disruptors such as dioxins. To examine a possible role of the genetic susceptibility to dioxins in the development of micropenis (MP), we studied the Arg554Lys polymorphism of the gene for aryl hydrocarbon receptor (AHR) and the Pro185Ala polymorphism of the gene for aryl hydrocarbon receptor repressor (AHRR), in 73 boys with MP (34 boys with mild MP from -2.1 to -2.5 SD and 39 boys with severe MP below -2.5 SD) and 80 control males (50 boys and 30 fertile adult males). The allele and genotype frequencies of the AHR polymorphism were comparable between the two groups of males, but those of the AHRR polymorphism were significantly different, with the Pro allele and the Pro/Pro genotype being more frequent in boys with MP than in control males (P-value: 0.0029 for the allele frequency and 0.011 for the genotype frequency). In addition, both polymorphisms were comparable in the allele and genotype frequencies between boys with mild MP and those with severe MP and between control boys and control fertile adult males. The results suggest that the AHRR Pro185Ala polymorphism may constitute a susceptibility locus for the development of MP in response to dioxins.

Search for the target genes involved in the suppression of antibody production by TCDD in C57BL/6 mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppresses antibody production through activation of a transcription factor, the aryl hydrocarbon receptor (AhR). To explore the genes that are involved in the suppression of antibody production by TCDD, we investigated TCDD-induced changes in gene expression in the CD4 T cells and B cells of C57BL/6 mice immunized with ovalbumin (OVA) plus alum as an adjuvant. Changes in gene expression were analyzed with Affymetrix oligonucleotide microarrays. The results showed that OVA-immunization alone up-regulated expression levels of many genes in the CD4 T cells as early as 3 h after immunization, with 55 up-regulated and 5 down-regulated. At 24 h, 42 genes were found to be up-regulated and 30 down-regulated. Fewer genes were affected in the B cells than in the CD4 T cells. In contrast to the up-regulation of genes induced by immunization in the CD4 T cells, administration of TCDD to mice 3 h prior to the immunization mainly caused down-regulation of genes in the CD4 T cells when compared with immunization alone, with 1 being up-regulated and 4 down-regulated at 3 h after immunization and 3 up-regulated and 34 down-regulated at 24 h. In particular, at 3 and 24 h, TCDD suppressed expression of three and seven genes, respectively, that were up-regulated by immunization. Another characteristic of the TCDD-induced changes in gene expression was the suppression of many genes encoding proteins that are involved in GTP-binding protein-linked signaling in CD4 T cells. These results suggest that the inhibition of immunization-induced gene expression and modulation of G-protein-linked signaling in CD4 T cells are responsible for the TCDD-induced suppression of antibody production.

Role of coactivators in transcriptional activation by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) mediates the carcinogenic and other toxic effects of a variety of environmental pollutants, including 2,37,8-tetrachlorodibenzo-p-dioxin (TCDD), and some polycyclic aromatic hydrocarbons (PAHs). In most if not all cases, these deleterious effects depend upon modulation of gene transcription effected by the ligand-bound AHR. The responsive genes required for toxicity of TCDD have yet to be defined. However, induction of Cyp1a1 is known to represent a significant event in the toxicity of PAHs. Furthermore, the Cyp1a1 gene provides a model system for studying the mechanism of gene transcription by AHR. This review discusses the roles of transcriptional coactivator proteins in induction of Cyp1a1 by AHR ligands. Coactivators physically associate with the gene upon induction, and provide a bridge between AHR molecules, located at 5'enhancer elements, and general transcription factors, located at the promoter of the gene. Studies on the endogenous Cyp1a1 gene in its natural chromosomal setting are emphasized. The recent development of several new experimental techniques including the chromatin immunoprecipitation (ChIP) assay, RNA interference, and real-time PCR has provided a major boost to such studies. Future directions for research are also discussed. Since variations in coactivator expression or activity may result in inter-individual differences in response to AHR ligands, and may also underlie tissue-specific differences in sensitivity to such ligands during development, and in adulthood, the role of coactivators in transcriptional activation by AHR constitutes a very important area of research.

Formation of estrogenic metabolites of benzo[a]pyrene and chrysene by cytochrome P450 activity and their combined and supra-maximal estrogenic activity

Metabolism of polycyclic aromatic hydrocarbons (PAHs) has been studied intensively, and potential metabolites with estrogenic activity have been identified previously. However, little attention has been paid to the metabolic pathways in mammalians and to the combined effect of individual metabolites. Several hydroxylated metabolites of benzo[a]pyrene (BaP) and chrysene (CHN) were formed by rat liver microsomal cytochrome P450 (CYP) activity, some of which possess estrogenic activity. All mono- and several dihydroxylated metabolites of BaP and CHN were tested for ER affinity and estrogenic activity in a proliferation assay (E-screen) and in a reporter-gene assay (ER-CALUX). Twelve estrogenic metabolites were identified with EC50 values ranging from 40nM to 0.15mM. The combined effect of a mixture of seven PAH-metabolites was also studied in the ER binding assay. At concentrations that show little activity themselves, their joint action clearly exhibited significant estrogenic activity. BaP itself exhibited estrogenicity in the ER-CALUX assay due to bio-activation into estrogenic metabolites, probably via aryl hydrocarbon receptor (AhR) induced CYP activity. Furthermore, 2-hydroxy-CHN (2-OHCHN) induced supra-maximal (400%) estrogenic effects in the ER-CALUX assay. This effect was entirely ER-mediated, since the response was completely blocked with the ER-antagonist ICI182,780. We showed that 2-OHCHN increased ER-concentration, using ELISA techniques, which may explain the observed supra-maximal effects. Co-treatment with the AhR-antagonist 3',4'-dimethoxyflavone (DMF) enhanced ER-signaling, possibly via blockage of AhR-ER inhibitory cross-talk.

Principles for modulation of the nuclear receptor superfamily

Nuclear receptors are major targets for drug discovery and have key roles in development and homeostasis, as well as in many diseases such as obesity, diabetes and cancer. This review provides a general overview of the mechanism of action of nuclear receptors and explores the various factors that are instrumental in modulating their pharmacology. In most cases, the response of a given receptor to a particular ligand in a specific tissue will be dictated by the set of proteins with which the receptor is able to interact. One of the most promising aspects of nuclear receptor pharmacology is that it is now possible to develop ligands with a large spectrum of full, partial or inverse agonist or antagonist activities, but also compounds, called selective nuclear receptor modulators, that activate only a subset of the functions induced by the cognate ligand or that act in a cell-type-selective manner.

Long-term exposure to environmental concentrations of the pharmaceutical ethynylestradiol causes reproductive failure in fish

Heightened concern over endocrine-disrupting chemicals is driven by the hypothesis that they could reduce reproductive success and affect wildlife populations, but there is little evidence for this expectation. The pharmaceutical ethynylestradiol (EE2) is a potent endocrine modulator and is present in the aquatic environment at biologically active concentrations. To investigate impacts on reproductive success and mechanisms of disruption, we exposed breeding populations (n = 12) of zebrafish (Danio rerio) over multiple generations to environmentally relevant concentrations of EE2. Life-long exposure to 5 ng/L EE2 in the F1 generation caused a 56% reduction in fecundity and complete population failure with no fertilization. Conversely, the same level of exposure for up to 40 days in mature adults in the parental F0 generation had no impact on reproductive success. Infertility in the F1 generation after life-long exposure to 5 ng/L EE2 was due to disturbed sexual differentiation, with males having no functional testes and either undifferentiated or intersex gonads. These F1 males also showed a reduced vitellogenic response when compared with F0 males, indicating an acclimation to EE2 exposure. Depuration studies found only a partial recovery in reproductive capacity after 5 months. Significantly, even though the F1 males lacked functional testes, they showed male-pattern reproductive behavior, inducing the spawning act and competing with healthy males to disrupt fertilization. Endocrine disruption is therefore likely to affect breeding dynamics and reproductive success in group-spawning fish. Our findings raise major concerns about the population-level impacts for wildlife of long-term exposure to low concentrations of estrogenic endocrine disruptors.

Modulation of the hepatic CYP1A1 system in the marine fish Gobius niger, exposed to xenobiotic compounds

Anthropogenic chemicals in the aquatic environment are known to cause reproductive disturbances in vertebrate and invertebrate organisms by interfering with the endocrine systems. Large efforts have recently been devoted to dissect the mechanisms of action of xenobiotics in aquatic species, with the ultimate aim of detecting and controlling the effects of chemical exposure on the aquatic ecosystem and humans. In the present paper, males of a marine species, the black goby (Gobius niger), were treated with estrogenic and dioxin-like compounds commonly discharged into the environment from industry, agriculture, and urban waste such as nonylphenol (NP) and beta-naphthoflavone (beta-NF). Their effects were compared with those induced by estradiol (E2), analyzing the expression of biomarkers commonly used in ecotoxicological studies such as vitellogenin (VTG) and cytochrome P4501A1. The treatment with NP induced the synthesis of the female specific protein VTG in males, showing its estrogenic activity. NP and E2 lowered cytochrome P4501A1 basal levels while beta-NF determined a significant rise of its expression. The detoxification pathway was investigated, and the most relevant finding of this paper was the evidence that cytochrome P4501A1 inhibition by estrogen and estrogen-like compounds is mediated through the activation of the aryl hydrocarbon receptor repressor.

In silico tools to aid risk assessment of endocrine disrupting chemicals

In silico or computational tools could be used more effectively in endocrine disruptor risk assessment for prescreening potential endocrine disruptors, improving experimental in vitro screening assay design and facilitating more thorough data analyses. The in silico tools reviewed here are three-fold and include the use of: (1) nuclear receptor (NR) crystal structures and homology models to examine potential modes of ligand binding by different representative compounds; (2) multivariate principal component analyses (PCA) techniques to select best predicted cell lines for endocrine disrupting chemicals (EDC) risk assessment purposes; (3) NR quantitative structure-activity relationships (QSARs) that can be constructed from varied biological data sources, using multivariate partial least squares (PLS) techniques and specific descriptors. The cytosolic and NR examples discussed here include the Ah receptor, (AhR), the human oestrogen receptor alpha (hERalpha) and the human pregnane X receptor (PXR). The varied biological data sets can be compared to give a more integrated dimension to receptor cross talk mechanisms, with further support from molecular modelling studies.

Rooster feathering, androgenic alopecia, and hormone-dependent tumor growth: what is in common?

Different epithelial organs form as a result of epithelial-mesenchymal interactions and share a common theme modulated by variations (Chuong ed. In Molecular Basis of Epithelial Appendage Morphogenesis, 1998). One of the major modulators is the sex hormone pathway that acts on the prototype signaling pathway to alter organ phenotypes. Here, we focus on how the sex hormone pathway may interface with epithelia morphogenesis-related signaling pathways. We first survey these sex hormone-regulated morphogenetic processes in various epithelial organs. Sexual dimorphism of hairs and feathers has implications in sexual selection. Diseases of these pathways result in androgenic alopecia, hirsutism, henny feathering, etc. The growth and development of mammary glands, prostate glands, and external genitalia essential for reproductive function are also dependent on sex hormones. Diseases affecting these organs include congenital anomalies and hormone-dependent breast and prostate cancers. To study the role of sex hormones in new growth in the context of system biology/pathology, an in vivo model in which organ formation starts from stem cells is essential. With recent developments (Yu et al. (2002) The morphogenesis of feathers. Nature 420:308-312), the growth of tail feathers in roosters and hens has become a testable model in which experimental manipulations are possible. We show exemplary data of differences in their growth rate, proliferative cell population, and signaling molecule expression. Working hypotheses are proposed on how the sex hormone pathways may interact with growth pathways. It is now possible to test these hypotheses using the chicken model to learn fundamental mechanisms on how sex hormones affect organogenesis, epithelial organ cycling, and growth-related tumorigenesis.

Lack of antagonism of 2,3,7,8-tetrachlorodibenzo-p-dioxin's (TCDDs) induction of cytochrome P4501A1 (CYP1A1) by the putative selective aryl hydrocarbon receptor modulator 6-alkyl-1,3,8-trichlorodibenzofuran (6-MCDF) in the mouse hepatoma cell line Hepa-1c1c7

Regulation of gene expression by the aryl hydrocarbon (AHR) receptor is a much-studied pathway of molecular toxicology. Activation of AHR by the xenobiotic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is hypothesized as the mechanism by which TCDD exerts its toxic and carcinogenic effects. Paradoxically, some studies have shown that TCDD acts as an antiestrogen. This has led to the hypothesis that so-called selective aryl hydrocarbon receptor modulators (SAhRMs), AHR ligands that retain the antiestrogenic effects but lack the transcriptional effects of TCDD associated with toxicity, may be utilized as cancer chemotherapeutics in conjunction with other antiestrogenic compounds such as tamoxifen. The present study attempts to further define the molecular mechanism of action of the putative SAhRMs, 6-alkyl-1,3,8-trichlorodibenzofuran (6-MCDF), and diindolylmethane (DIM), focusing particularly on the former. We tested 6-MCDF and DIM for the recruitment of AHR and RNA polymerase II (pol II) to the regulatory region of the AHR responsive gene, cytochrome P4501A1 (CYP1A1), using the chromatin immunoprecipitation (ChIP) assay in the mouse hepatoma cell line Hepa-1c1c7 (Hepa-1). We also tested the level of CYP1A1 induction in Hepa-1 cells using quantitative real-time PCR. We show no difference in the recruitment of AHR or pol II to the regulatory region of CYP1A1 in response to TCDD, 6-MCDF, or co-treatment with both TCDD and 6-MCDF. Our results also show no antagonism of CYP1A1 induction with co-treatment of Hepa-1 cells with TCDD and 6-MCDF. These data suggest that 6-MCDF exhibits agonist activity with respect to induction of CYP1A1 in the Hepa-1 cell line.

Dioxin-responsive AHRE-II gene battery: identification by phylogenetic footprinting

We identified a set of genes that respond to dioxins through the recently discovered AHRE-II ("XRE-II") enhancer element. A total of 36 genes containing AHRE-II motifs conserved across human, mouse, and rat gene orthologs were identified by genome-wide transcription-factor binding-site searches and phylogenetic footprinting. Microarray experiments on liver from rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin revealed statistically significant changes in mRNA levels for 13 of these 36 genes after three hours and 15 genes after 19h. The set of responsive genes was functionally characterized by ontological analysis and found to be enriched in ion-channels and transporters. Our identification of 36 putatively AHRE-II-regulated genes highlights the regulatory versatility of the aryl hydrocarbon receptor (AHR) and the ability of the AHR and its dimerization partner, ARNT, to act both as a ligand-activated transcription-factor (on AHRE-I) and as a ligand-activated coactivator (on AHRE-II). Collectively, these results demonstrate that the AHRE-II induction mechanism is employed by multiple genes and provide the first categorization of the gene battery of a ligand-activated coactivator.

Environmental PCB exposure and risk of endometriosis

BACKGROUND

Hormonally active environmental agents have recently been associated with the development of endometriosis.

METHODS

We undertook a study to assess the relationship between endometriosis, an estrogen-dependent gynaecological disease, and 62 individual polychlorinated biphenyl (PCBs) congeners. We enrolled 84 eligible women aged 18-40 years undergoing laparoscopy for study, which included an interview and blood specimen (n=79; 94%). Thirty-two women had visually confirmed endometriosis at laparoscopy while 52 did not. Blood specimens were run in batches of 14 including four quality control samples for toxicological analysis. Each PCB congener was adjusted for recovery; batch-specific reagent blanks were subtracted. All PCB concentrations were log transformed and expressed in ng/g serum first as a sum and then as tertiles by purported estrogenic or anti-estrogenic activity of PCB congeners.

RESULTS

Using unconditional logistic regression analysis, a significantly elevated odds ratio (OR) was observed for women in the third tertile of anti-estrogenic PCBs [OR 3.77; 95% confidence interval (CI) 1.12-12.68]. Risk remained elevated after controlling for gravidity, current cigarette smoking and serum lipids (OR 3.30; 95% CI 0.87-12.46).

CONCLUSIONS

These data suggest that anti-estrogenic PCBs may be associated with the development of endometriosis.

Coregulators and chromatin remodeling in transcriptional control

Despite many years of investigation by numerous investigators, transcriptional regulatory control remains an intensely investigated and continuously evolving field of research. Transcriptional regulation is dependent not only on transcription factor activation and chromatin remodeling, but also on a host of transcription factor coregulators-coactivators and corepressors. In addition to transcription factor activation and chromatin changes, there is an expanding array of additional modifications that titrate transcriptional regulation for the specific conditions of a particular cell type, organ system, and developmental stage, and such events are likely to be greatly influenced by upstream signaling cascades. Here, we will briefly review the highlights and perspectives of chromatin remodeling and transcription controls as affected by cofactor availability, cellular energy state, relative ratios of reducing equivalents, and upstream signaling. We also present the C-terminal binding protein (CtBP) as a novel nuclear receptor (NR) coregulator, which exemplifies the integration of a number of transcriptional regulatory controls.

Lack of effects of postnatal exposure to a mixture of aryl hydrocarbon-receptor agonists on the development of methylnitrosourea-induced mammary tumors in sprague-dawley rats

There are concerns that early life exposure to organochlorines, including aryl hydrocarbon receptor (AhR) agonists, may lead to long-term effects and increase the risk of developing breast cancer. Our objective was to test if postnatal exposure to a mixture of 2,3,7,8-tetrachlorodibenzodioxin (TCDD)-like chemicals would modulate the development of methylnitrosourea (MNU)-induced mammary tumors. Females received by gavage a mixture containing 3 non-ortho-polychlorinated biphenyls (PCBs), 6 polychlorinated dibenzodioxins (PCDDs), and 7 polychlorinated dibenzofurans (PCDFs), at 1, 5, 10, 15, and 20d of age. The doses were equivalent to 0, 1, 10, 100, or 1000 times the amount ingested through breast milk by a human infant during its first 24 d of life. Subgroups of 1000 x reated rats and controls were sacrificed at 21 d of age for assessment of mammary-gland development, cell death, and proliferation. Mammary-tumor development was assessed in MNU (30 mg/kg body weight ip at 50 days of age)-induced rats pre-exposed to the mixture (MNU-0, MNU-1, MNU-10, MNU-100, MNU-1000). Rats were sacrificed when their mammary tumors reached 1 cm in diameter, or when the rats reached > or = 32 wk of age. Mammary-gland whole mounts were analyzed with all palpable and microscopic lesions (n = 1563) histologically classified and grouped as benign, intraductal proliferations, or malignant. There were no marked effects on age at onset of puberty (vaginal opening) and estrous cyclicity. Despite a significant decrease in proliferating cell nuclear antigen (PCNA)-positive mammary cells in 1000 x treated 21-d-old rats, there were no long-term dose-response effects on mammary-gland morphology and tumor development. In conclusion, postnatal exposure to the mixture of AhR agonists had no significant effects on the development of MNU-initiated mammary tumors.

Role of estrogen receptor in regulation of polycyclic aromatic hydrocarbon metabolic activation in lung

Epidemiological and biochemical studies have indicated that females may be at greater risk of smoking associated lung cancer compared with males. Among lung cancer patients, female smokers have been found to have higher levels of PAH-related DNA adducts and CYP1A1 gene expression in their normal lung tissue compared to male smokers. A possible role of steroid hormones in these sex differences via interactions between aryl hydrocarbon receptor and estrogen receptor mediated cellular effects has been suggested. In the present study the impact of the estrogen receptor (ERalpha) on CYP1A1 and CYP1B1 gene expression was studied in vitro in human bronchial epithelial cells. Transient transfection of the BEP2D cell line with ERalpha influenced neither constitutive expression of CYP1A1 or CYP1B1 nor induction of these genes by TCDD as measured by real-time RT-PCR. ERalpha had no effect on the constitutive or TCDD-induced enzymatic activity of CYP1A1 (EROD). We also studied the effect of steroid hormones on lung PAH metabolic activation in A/J mice. Intact and ovariectomized female mice were orally exposed to a single dose of benzo[a]pyrene. Ovariectomy did not influence the levels of either benzo[a]pyrene-derived protein or DNA adducts in the lung tissue measured by HPLC and 32P-postlabeling, respectively. In conclusion, the present data do not support the hypothesis of a role of estrogen or the ERalpha in regulating the metabolic activation of polycyclic aromatic hydrocarbons in lung.

2,3,7,8-Tetrachlorodibenzo-p-dioxin blocks androgen-dependent cell proliferation of LNCaP cells through modulation of pRB phosphorylation

Cell-cycle regulatory events associated with inhibition of androgen-dependent cell proliferation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in the human-derived LNCaP cell line. TCDD blocked the G(1) to S transition of LNCaP cells synchronized in G(0)/G(1) when these cells were induced to reinitiate cell-cycle progression by dihydrotestosterone (DHT). Western blot analyses of these cells revealed altered expression levels of G(1) regulatory proteins, including increases in hypophosphorylated retinoblastoma protein and concomitant decreases in cyclin D1. p21(WAF1/CIP1), which is involved in the assembly of cyclin D1/cyclin-dependent kinase-4 complexes, was increased by DHT or TCDD when each compound was administered singly but was reduced to background levels in cells simultaneously treated with DHT and TCDD. Reporter gene assays revealed the presence of several Ah receptor response-element motifs in the promoter and first intron of the p21(WAF1/CIP1) gene that respond to TCDD-mediated Ah receptor activation independently of p53. Transcription studies showed that activation of aryl hydrocarbon receptor blocks androgen-dependent gene induction in LNCaP cells as well as in African green monkey CV-1 cells. These data point to at least two mechanisms whereby TCDD blocks androgen receptor function: 1) by blocking androgen-induced cell proliferation through modulation of the expression and activities of regulatory proteins controlling cell-cycle progression; and 2) by squelching androgen receptor-mediated gene transcription through receptor cross-talk, possibly involving competition for coregulators or by direct protein interaction.

BRCA1 function mediates a TRAP/DRIP complex through direct interaction with TRAP220

Breast cancer susceptibility gene 1 (BRCA1) is a tumor suppressor gene mutated in a high percentage of hereditary breast and ovarian cancers. The multifunctional BRCA1 protein acts on cell cycle control, exerting several highly specialized DNA repair processes through diverse domains. Gene regulation through its C-terminal domain (BRCT) is indispensable for BRCA1-mediated tumor suppression, suggesting the possibility that the BRCT domain interacts with co-regulator complexes. Using a biochemical approach with HeLa S3 nuclear extracts, we isolated BRCT-associated complexes and identified one of the purified components as TRAP220. We then performed interaction studies in vivo (co-immunoprecipitation) and in vitro (glutathione S-transferase pull-down assays) and showed that BRCT directly interacted with TRAP220. This in vitro interaction was completely abolished by BRCT point mutations typical of those found in patients with BRCA1 that lack transactivation function. BRCA1 transactivation function was dependent on TRAP220 expression level in a transient expression assay. Moreover, a cell survival assay showed that antisense TRAP220 expression to disrupt endogenous TRAP220 expression significantly reduced the survival rate potentiated by BRCA1 after DNA damage. These results suggested that a TRAP220 complex play an important role as putative co-activator complexes in BRCA1-mediated tumor suppression.

Ecdysone receptor-dependent gene regulation mediates histone poly(ADP-ribosyl)ation

While the ecdysone dependency of puff formation in giant polytene chromosomes from fly salivary glands has been well documented, the molecular mechanisms underlying this process remain unknown. However, it does appear to involve chromatin remodeling and modification mediated by ecdysone receptor (EcR). As Drosophila poly(ADP-ribose) polymerase (dPARP) has recently been reported to be involved in ecdysone-induced puff formation, we decided to test the possible role of dPARP in ligand-induced dEcR transactivation in an insect system. dPARP co-activated the ligand-induced transactivation function of EcR in the insect cell line S2, and appeared to physically interact with EcR in a ligand-dependent manner. ChIP analysis of an EcR target gene promoter revealed ligand-dependent recruitment of dPARP with poly(ADP-ribosyl)ation of histones in the EcR binding site and, surprisingly, also in a distal region of the promoter. Our results indicated that EcR-mediated gene regulation may be coupled with chromatin modification through poly(ADP-ribosyl)ation.