Estrogen-responsive genes newly found to be modified by TCDD exposure in human cell lines and mouse systems

Morin hydrate ameliorates Di-2-ethylhexyl phthalate (DEHP) induced hepatotoxicity in a mouse model via TNF-α and NF-κβ signaling

Di-(2-ethylhexyl) phthalic acid (DEHP) pollutes the environment, and posing a significant risk to human and animal health. Consequently, a successful preventative strategy against DEHP-induced liver toxicity needs to be investigated. Morin hydrate (MH), a flavanol compound, possesses toxic preventive attributes against various environmental pollutants. However, the effects of MH have not been investigated against DEHP-induced liver toxicity. Female Swiss albino mice were divided into four groups: control, DEHP (orally administered with 500 mg/kg, DEHP plus MH 10 mg/kg, and DEHP plus MH 100 mg/kg for 14 days. The results showed that the MH treatment ameliorated the DEHP-induced liver dysfunctions by decreasing the alanine transaminase (ALT), aspartate aminotransferase (AST), total bilirubin, liver histoarchitecture, fibrosis, and markers of oxidative stress. Furthermore, DEHP increased apoptosis, increased active caspase 3 and decreased B cell lymphoma-2 (Bcl-2) expression. However, the MH treatment showed a differential effect on these proteins; a lower dose increased, and a higher dose decreased the expression. Thus, a lower dose of MH could be involved in the disposal of damaged hepatocytes. Expression of Estrogen receptors alpha (ERα) also showed a similar trend with active caspase 3. Furthermore, the expression of Tumor necrosis factor alpha (TNF-α) and Nuclear factor-κβ (NF-κβ) were up-regulated by DEHP treatment, and MH treatment down-regulated the expression of these two inflammatory markers. Since this down-regulation of TNF-α and NF-κβ coincides with improved liver functions against DEHP-induced toxicity, it can be concluded that MH-mediated liver function involves the singling of TNF-α and NF-κβ.

The insulin-like growth factor system: A target for endocrine disruptors?

The insulin-like growth factor (IGF) system is a critical regulator of growth, especially during fetal development, while also playing a central role in metabolic homeostasis. Endocrine disruptors (EDs) are ubiquitous compounds able to interfere with hormone action and impact human health. For example, exposure to EDs is associated with decreased birthweight and increased incidence of metabolic disorders. Therefore, the IGF system is a potential target for endocrine disruption. This review summarises the state of the science regarding effects of exposure to major classes of endocrine disruptors (dioxins and dioxin-like compounds, polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, phthalates, perfluoroalkyl substances and bisphenol A) on the IGF system. Evidence from both experimental models (in vitro and in vivo) and epidemiological studies is presented. In addition, possible molecular mechanisms of action and effects on methylation are discussed. There is a large body of evidence supporting the link between dioxins and dioxin-like compounds and IGF disruption, but mixed findings have been reported in human studies. On the other hand, although only a few animal studies have investigated the effects of phthalates on the IGF system, their negative association with IGF levels and methylation status has been more consistently reported in humans. For polybrominated diphenyl ethers, perfluoroalkyl substances and bisphenol A the evidence is still limited. Despite a lack of studies for some ED classes linking ED exposure to changes in IGF levels, and the need for further research to improve reproducibility and determine the degree of risk posed by EDs to the IGF system, this is clearly an area of concern.

Modulations of TCDD-mediated induction of zebrafish cyp1a1 and the AHR pathway by administering Cd2+in vivo

Trace metal ions such as cadmium (Cd²⁺) and trace organics typified by 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) are common co-contaminants in the environment and cause toxic effects in aquatic organisms that pose serious health risks. We studied the effects of Cd²⁺ on the regulation of cytochrome P450 1A1 (cyp1a1) gene-induction by TCDD using zebrafish embryos and larvae and adult zebrafish tissues. Our results showed that TCDD induced the cyp1a1 gene in all developmental stages and tissues of zebrafish, and the induction was higher in females than males. However, for the upstream genes (ahr2 and arnt2b) that mediate cyp1a1 gene induction in the zebrafish liver cell line was not induced by TCDD similar to the pattern of cyp1a1 in all investigated groups. After co-treatment with Cd²⁺, induction of the aryl hydrocarbon receptor pathway by TCDD was inhibited in the zebrafish larvae and the livers, intestines, kidneys and gills of adult zebrafish, but not in the embryos or brains of adult zebrafish, indicating that the toxicological effects of Cd²⁺ on TCDD are dependent on the developmental stages and tissue types. The present study confirms that Cd²⁺ blocks the TCDD-induced cyp1a1 gene in vivo but emphasizes that the effects are specific to the developmental stage, type of tissue and sex. The combined effects of Cd²⁺ and TCDD must be taken into consideration together with these parameters to accurately predict and assess cadmium and TCDD-induced toxicity in fish and carcinogenesis in animals in general.

Male and female mice show significant differences in hepatic transcriptomic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Background

2,3,7,8–tetrachlorodibenzo-p-dixion (TCDD) is the most potent of the dioxin congeners, capable of causing a wide range of toxic effects across numerous animal models. Previous studies have demonstrated that males and females of the same species can display divergent sensitivity phenotypes to TCDD toxicities. Although it is now clear that most TCDD-induced toxic outcomes are mediated by the aryl hydrocarbon receptor (AHR), the mechanism of differential responses to TCDD exposure between sexes remains largely unknown. To investigate the differential sensitivities in male and female mice, we profiled the hepatic transcriptomic responses 4 days following exposure to various amounts of TCDD (125, 250, 500 or 1000 μg/kg) in adult male and female C57BL/6Kuo mice.

Results

Several key findings were revealed by our study. 1) Hepatic transcriptomes varied significantly between the sexes at all doses examined. 2) The liver transcriptome of males was more dysregulated by TCDD than that of females. 3) The alteration of “AHR-core” genes was consistent in magnitude, regardless of sex. 4) A subset of genes demonstrated sex-dependent TCDD-induced transcriptional changes, including Fmo3 and Nr1i3, which were significantly induced in livers of male mice only. In addition, a meta-analysis was performed to contrast transcriptomic profiles of various organisms and tissues following exposure to equitoxic doses of TCDD. Minimal overlap was observed in the differences between TCDD-sensitive or TCDD-resistant models.

Conclusions

Sex-dependent sensitivities to TCDD exposure are associated with a set of sex-specific TCDD-responsive genes. In addition, complex interactions between the aryl hydrocarbon and sex hormone receptors may affect the observable differences in sensitivity phenotypes between the sexes. Further work is necessary to better understand the roles of those genes altered by TCDD in a sex-dependent manner, and their association with changes to sex hormones and receptors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1840-6) contains supplementary material, which is available to authorized users.

Identification of genomic features in environmentally induced epigenetic transgenerational inherited sperm epimutations

A variety of environmental toxicants have been shown to induce the epigenetic transgenerational inheritance of disease and phenotypic variation. The process involves exposure of a gestating female and the developing fetus to environmental factors that promote permanent alterations in the epigenetic programming of the germline. The molecular aspects of the phenomenon involve epigenetic modifications (epimutations) in the germline (e.g. sperm) that are transmitted to subsequent generations. The current study integrates previously described experimental epigenomic transgenerational data and web-based bioinformatic analyses to identify genomic features associated with these transgenerationally transmitted epimutations. A previously identified genomic feature associated with these epimutations is a low CpG density (<12/100bp). The current observations suggest the transgenerational differential DNA methylation regions (DMR) in sperm contain unique consensus DNA sequence motifs, zinc finger motifs and G-quadruplex sequences. Interaction of molecular factors with these sequences could alter chromatin structure and accessibility of proteins with DNA methyltransferases to alter de novo DNA methylation patterns. G-quadruplex regions can promote the opening of the chromatin that may influence the action of DNA methyltransferases, or factors interacting with them, for the establishment of epigenetic marks. Zinc finger binding factors can also promote this chromatin remodeling and influence the expression of non-coding RNA. The current study identified genomic features associated with sperm epimutations that may explain in part how these sites become susceptible for transgenerational programming.

High-resolution genome-wide mapping of AHR and ARNT binding sites by ChIP-Seq

The aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) activated complex regulates genes in response to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). AHR has also emerged as a potential therapeutic target for the treatment of human diseases and different cancers, including breast cancer. To better understand AHR and ARNT signaling in breast cancer cells, we used chromatin immunoprecipitation linked to high-throughput sequencing to identify AHR- and ARNT-binding sites across the genome in TCDD-treated MCF-7 cells. We identified 2594 AHR-bound, 1352 ARNT-bound, and 882 AHR/ARNT cobound regions. No significant differences in the genomic distribution of AHR and ARNT were observed. Approximately 60% of the cobound regions contained at least one core an aryl hydrocarbon response element (AHRE), 5'-GCGTG-3'. AHR/ARNT peak density was the highest within 1 kb of transcription start sites (TSS); however, a number of AHR/ARNT cobound regions were located as far as 100 kb from TSS. De novo motif discovery identified a symmetrical variation of the AHRE (5'-GTGCGTG-3'), as well as FOXA1 and SP1 binding motifs. Microarray analysis identified 104 TCDD-responsive genes where 98 genes were upregulated by TCDD. Of the 104 regulated genes, 69 (66.3%) were associated with an AHR- or ARNT-bound region within 100 kb of their TSS. Overall our study identified AHR/ARNT cobound regions across the genome, revealed the importance but not absolute requirement for an AHRE in AHR/ARNT interactions with DNA, and identified a modified AHRE motif, thereby increasing our understanding of AHR/ARNT signaling pathway.

Aryl hydrocarbon receptor modulation of estrogen receptor α-mediated gene regulation by a multimeric chromatin complex involving the two receptors and the coregulator RIP140

Although crosstalk between aryl hydrocarbon receptor (AhR) and estrogen receptor α (ERα) is well established, the mechanistic basis and involvement of other proteins in this process are not known. Because we observed an enrichment of AhR-binding motifs in ERα-binding sites of many estradiol (E2)-regulated genes, we investigated how AhR might modulate ERα-mediated gene transcription in breast cancer cells. Gene regulations were categorized based on their pattern of stimulation by E2 and/or dioxin and were denoted E2-responsive, dioxin-responsive, or responsive to either ligand. ERα, AhR, aryl hydrocarbon receptor translocator, and receptor interacting protein 140 (RIP140) were recruited to gene regulatory regions in a gene-specific and E2/dioxin ligand-specific manner. Knockdown of AhR markedly increased the expression of ERα-mediated genes upon E2 treatment. This was not attributable to a change in ERα level, or recruitment of ERα, phosphoSer5-RNA Pol II, or several coregulators but rather was associated with greatly diminished recruitment of the coregulator RIP140 to gene regulatory sites. Changing the cellular level of RIP140 revealed coactivator or corepressor roles for this coregulator in E2- and dioxin-mediated gene regulation, the choice of which was determined by the presence or absence of ERα at gene regulatory sites. Coimmunoprecipitation and chromatin immunoprecipitation (ChIP)-reChIP studies documented that E2- or dioxin-promoted formation of a multimeric complex of ERα, AhR, and RIP140 at ERα-binding sites of genes regulated by either E2 or dioxin. Our findings highlight the importance of cross-regulation between AhR and ERα and a novel mechanism by which AhR controls, through modulating the recruitment of RIP140 to ERα-binding sites, the kinetics and magnitude of ERα-mediated gene stimulation.

Application of Fluolid-Orange-labeled probes for DNA microarray and immunological assays

The usefulness of Fluolid-Orange, a novel fluorescent dye, for DNA microarray and immunological assays has been examined. Fluolid-Orange-labeled probes (DNA and IgG) were stable as examined by laser-photo-bleaching and under heat and dry conditions. Statistical analyses were performed to evaluate the reproducibility of the microarray assay, while stage-specific immunostaining of marker proteins, Kank1 and calretinin, was performed for renal cancers, both giving satisfactory results. The stability of the dye should provide advantages for storing fluorescently labeled probes and re-examining the specimens later in genetic and pathological diagnostics.

Epigenetics: prenatal exposure to genistein leaves a permanent signature on the hematopoietic lineage

Recent studies demonstrate that maternal diet during pregnancy results in long-lasting effects on the progeny. Supplementation of maternal diet with genistein, a phytoestrogen ubiquitous in the daily diet, altered coat color of agouti mice due to epigenetic changes. We studied hematopoiesis of mice prenatally exposed to genistein (270 mg/kg feed) compared with that of mice prenatally exposed to phytoestrogen-poor feed and observed a significant increase in granulopoiesis, erythropoiesis, and mild macrocytosis at the adult age of 12 wk. Genistein exposure was associated with hypermethylation of certain repetitive elements, which coincided with a significant down-regulation of estrogen-responsive genes and genes involved in hematopoiesis in bone marrow cells of genistein-exposed mice, as assessed by microarray technology. Although genistein exposure did not affect global methylation in fetal liver of fetuses at embryonic day 14.5, it accelerated the switch from primitive to definitive erythroid lineage. Taken together, our data demonstrate that prenatal exposure to genistein affects fetal erythropoiesis and exerts lifelong alterations in gene expression and DNA methylation of hematopoietic cells.

Estrogen-independent actions of environmentally relevant AhR-agonists in human endometrial epithelial cells

The human endometrium is a cyclically regenerating organ under the influence of ovarian steroid hormones. Disturbances in this highly coordinated regulation of endometrial proliferation and differentiation may result in infertility and diseases such as endometriosis and endometrial cancer. Environmental toxins belonging to the group of polyhalogenated aromatic hydrocarbons (PAHs) are lipophilic xenobiotics, which accumulate in biological systems. PAHs have been implicated in the etiology of uterine pathologies, including infertility, endometriosis and endometrial cancer. However, suitable cellular models of the endometrium are lacking and the molecular mechanism of PAH action in the endometrium is not fully understood. In this study, we have characterized a previously established immortalized human telomerase reverse transcriptase (hTERT) endometrial epithelial cell (hTERT-EEC) model as a responsive in vitro cell model to investigate the cellular and molecular mechanisms of selected environmentally relevant PAH in human EECs. We show that dioxin-type PAHs activate the endogenous arylhydrocarbon receptor (AhR) signaling pathway in hTERT-EEC in a time-, concentration- and congener-specific manner and that the induction of AhR target genes is modulated by estrogen. Strikingly, AhR activation did not interfere with estrogenic actions in these EECs. Independent of their ability to bind to AhR, the PAHs investigated here increased cell migration by hTERT-EEC. Furthermore, we have identified several candidates by proteomic analysis, which are involved in heat shock responses and protein modification and turnover. Our data suggest that AhR-activating environmental pollutants directly alter endometrial cell stress responses and metabolism independent of estrogenic actions.

Activation of the aryl hydrocarbon receptor during different critical windows in pregnancy alters mammary epithelial cell proliferation and differentiation

Exposure to the aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during pregnancy causes severe defects in mammary gland development and function; however, the underlying mechanism remains unclear. Alterations in epithelial cell proliferation, differentiation, and apoptosis during pregnancy-related mammary development can lead to failed lactogenesis. To determine which of these processes are affected and at what time periods, we examined proliferation, differentiation and apoptosis in mammary glands following exposure to TCDD during early, mid or throughout pregnancy. Although AhR activation throughout pregnancy did not cause early involution, there was a 50% decrease in cell proliferation, which was observed as early as the sixth day of pregnancy (DP). TCDD treatment on the day of impregnation only reduced development and proliferation in early and mid-pregnancy, followed by partial recovery by DP17. However, when AhR activation was delayed to DP7, developmental impairment was not observed in mid-pregnancy, but became evident by DP17, whereas proliferation was reduced at all times. Thus, early exposure to TCDD was neither necessary nor sufficient to cause persistent defects in lactogenesis. These varying outcomes in mammary development due to exposure at different times in pregnancy suggest there are critical windows during which AhR activation impairs mammary epithelial cell proliferation and differentiation.

High-throughput evaluation of aryl hydrocarbon receptor-binding sites selected via chromatin immunoprecipitation-based screening in Hepa-1c1c7 cells stimulated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Upon binding to ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR) is activated to form a heterodimer with an aryl hydrocarbon receptor nuclear translocator (Arnt) and binds to DNA. It has been shown that the binding of AhR to DNA depends on the dioxin response element (DRE) and controls xenobiotic-response genes. AhR-binding DNA fragments from mouse hepatoma Hepa-1c1c7 cells stimulated with TCDD were once enriched in a chromatin immunoprecipitation (ChIP) DNA library and screened through a high-throughput southwestern chemistry-based enzyme-linked immunosorbent assay (SW-ELISA). After screening 1700 fragments, the ChIP-SW-ELISA screening strategy allowed us to isolate 77 fragments tightly interacting with AhR in the presence of TCDD. Only 39 of the 77 fragments appeared to contain a typical DRE, indicating that in some cases the DRE was dispensable for AhR-binding, while 75 fragments were located within promoter-distal regions. Genomic mapping of the 77 fragments enabled us to estimate 121 potential AhR targets including known targets such as Cyp1A1 and Cyp1B1, but only a limited number exhibited an altered expression dependent on TCDD. This study revealed the fact that TCDD-activated AhR frequently binds to promoter-distal regions even without a DRE and is not always involved in transcriptional regulation, suggesting that within the genome DNA-binding of AhR could take place often in many regions without cis-regulatory elements and might not be a key determinant to establish its regulatory function.