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

Maternal serum dioxin-like activity and gestational age at birth and indices of foetal growth: The Aarhus birth cohort

Human exposure to lipophilic persistent organic pollutants (lipPOP) is ubiquitous and life-long, beginning during foetal development. Exposure to lipPOP elicits a number of species and tissue specific responses including dioxin-like activity which involve the activation of aryl hydrocarbon receptor (AhR). This study aims i) to describe the combined dioxin-like activity in serum from Danish pregnant women collected during 2011-2013; ii) to assess the association between maternal serum dioxin-like activity, gestational age at birth and foetal growth indices. The serum lipPOP fraction was extracted using Solid Phase Extraction and cleaned-up on Supelco multi-layer silica and florisil columns. The combined dioxin-like activity of the extract was determined using the AhR reporter gene bioassay, expressed as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxic equivalent (TEQ) [AhR-TEQ (pg/g lipid)]. The associations of AhR-TEQ and foetal growth indices (birth weight, birth length and head circumference) and gestational age were assessed by linear regression models. We detected AhR-TEQ in 93.9 % of maternal first trimester serum samples, with a median level of 185 pg/g lipid. Each ln-unit increase in AhR-TEQ was associated with an increase in birth weight of 36 g (95 % CI: 5; 68), birth length of 0.2 cm (95 % CI: 0.01; 0.3) and pregnancy duration of 1 day (95 % CI: 0; 1.5). In women who never smoked, higher AhR-TEQ values were associated with higher birth weight and longer duration of gestation, while in smokers the association was the opposite. Mediation analyses suggested that gestational age may mediate the association of AhR-TEQ with foetal growth indices. We conclude that AhR activating substances are present in the bloodstream of almost all pregnant women in Denmark and the AhR-TEQ level was around four times higher than previously reported. The AhR-TEQ was associated with slightly longer gestational duration and thereby higher birth weight and birth length.

Dioxin-like Activity in Pregnant Women and Indices of Fetal Growth: The ACCEPT Birth Cohort

Exposure to lipophilic persistent organic pollutants (lipPOPs) elicits a number of species- and tissue-specific toxic responses, many of which involve the aryl hydrocarbon receptor (AhR). This study aims to measure the combined serum dioxin-like activity of lipPOPs in Greenlandic Inuit pregnant women and the associations with fetal growth indices. The combined dioxin-like activity of serum lipPOPs extracts was determined using the AhR reporter gene bioassay and expressed as pico-gram (pg) TCDD equivalent (TEQ) per gram serum lipid [AhR-TEQ (pg/g lipid)]. Significant AhR-TEQ was found in >87% of serum samples with the median level of 86.2 pg TEQ/g lipid. The AhR-TEQ level positively correlated with the marine food intake biomarker n-3/n-6 polyunsaturated fatty acids ratio, while negatively correlated with body mass index and parity. Women giving birth to infants with low birth weight (<2500 g) and length (<50 cm) had higher AhR-TEQ level compared to those with normal weight and length infants. For previous smokers, we found significant inverse associations between maternal AhR-TEQ level and fetal growth indices. In conclusion, exposure of Greenlandic Inuit pregnant women to dioxin-like compounds through traditional marine food can adversely influence the fetal growth via induced AhR activity. Smoking might have modifying effects.

Kynurenine regulates NLRP2 inflammasome in astrocytes and its implications in depression

Increased kynurenine (Kyn) metabolized from tryptophan (Try) is a biomarker in the immune dysfunction of depression. However, the mechanism by which Kyn change promotes depression is poorly defined. Astrocytes are involved in the neuroinflammation of depression. Among the numerous inflammatory cytokines, interleukin-1β (IL-1β) produced by astrocytic Nod-like receptor protein (NLRP) inflammasome is crucial in the pathogenesis of depression. In the present study, Kyn was shown to be a proinflammatory metabolite in the neuroimmune signaling network mediating depressive-like behavior. First, in chronic mild stress (CMS)-induced depressive mice, the level of Kyn notably increased in the hippocampus, accompanied by the activation of astrocytic NLRP2 inflammasome. Kyn treatment specifically upregulated Nod-like receptor protein 2 (NLPR2) expression in primary mouse astrocytes. Kyn + ATP activated NLRP2 inflammasome, evidenced by increased caspase-1 expression and IL-1β release. After Kyn treatment, nuclear factor kappa-B (NF-κB) could translocate to the nucleus and bind the promoter of NLRP2, subsequently increased NLRP2 transcription in cultured astrocytes in vitro. Intraperitoneal injection of Kyn activated NLRP2 inflammasome in astrocytes of hippocampus in mice, while NLRP2 knockdown in astrocytes abolished depressive-like behaviors in mice induced by Kyn, suggesting the critical role of NLRP2 in Kyn-induced depression. These findings demonstrate a novel mechanism that Kyn upregulates NLRP2 in an NF-κB-dependent pathway and provide a new strategy for treatment of depression.

Heavy metal contamination in a highly consumed Brazilian fish: immunohistochemical and histopathological assessments

Due to industrial, rural, and domestic waste disposal, heavy metals such as cadmium (Cd), chromium (Cr), lead (Pb), zinc (Zn), copper (Cu), and iron (Fe) continually infiltrate aquatic environments. These pollutants do not degrade naturally and, thus, have a high capacity for bioaccumulation in tissues and organs. The present study uses histological and immunohistochemical analyses to evaluate the contamination status of Salminus franciscanus, a large and economically important fish. Levels of Cd, Cr, Pb, Zn, Cu, and Fe were evaluated by atomic absorption spectrometry in the liver and muscle of fish sampled from two tributaries of the upper São Francisco River Basin, Brazil: the Abaeté and Paraopeba Rivers. In addition, histopathological alterations and expressions of three environmental biomarkers were assessed: metallothionein (MT), heat shock protein-70 (HSP70), and cytochrome P450-1A (CYP1A). The results show that fish from the Paraopeba River are unsuitable for human consumption, with several metals being detected above the safe limits established by the World Health Organization. Histopathological alterations in the liver and spleen were also significantly more frequent in fish from the Paraopeba River than in those from the Abaeté River (P < 0.05). Significant differences in the expressions of environmental biomarkers were observed between the rivers. Fish from the Abaeté River presented significantly higher values of the gonadosomatic index (GSI) and lower levels of metal contamination in the liver and muscle.

Ochratoxin A-Induced Hepatotoxicity through Phase I and Phase II Reactions Regulated by AhR in Liver Cells

Ochratoxin A (OTA) is a widespread mycotoxin produced by several species of the genera Aspergillus and Penicillium. OTA exists in a variety of foods, including rice, oats, and coffee and is hepatotoxic, with a similar mode of action as aflatoxin B1. The precise mechanism of cytotoxicity is not yet known, but oxidative damage is suspected to contribute to its cytotoxic effects. In this study, human hepatocyte HepG2 cells were treated with various concentrations of OTA (5-500 nM) for 48 h. OTA triggered oxidative stress as demonstrated by glutathione depletion and increased reactive oxygen species, malondialdehyde level, and nitric oxide production. Apoptosis was observed with 500 nM OTA treatment. OTA increased both the mRNA and protein expression of phase I and II enzymes. The same results were observed in an in vivo study using ICR mice. Furthermore, the relationship between phase I and II enzymes was demonstrated by the knockdown of the aryl hydrocarbon receptor (AhR) and NF-E2-related factor 2 (Nrf2) with siRNA. Taken together, our results show that OTA induces oxidative stress through the phase I reaction regulated by AhR and induces apoptosis, and that the phase II reaction is activated by Nrf2 in the presence of oxidative stress.

Paclitaxel-mediated human aryl hydrocarbon receptor mRNA translation by an internal ribosomal entry site-dependent mechanism

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is best known in mediating the toxicities of dioxins and dioxin-like compounds. AHR is activated by a variety of endogenous ligands and participating in tumor development. Thus, it will provide a new approach for cancer prevention and treatment to study the translation mechanism of AHR in tumor cells. In this study, we show that the 5'-untranslated region (UTR) of AHR mRNA contains an internal ribosome entry site (IRES). After mapping the entire AHR 5'-UTR, we determined that the full-length 5'-UTR is indispensable for the highest IRES activity. Interestingly, we found that AHR expression is induced in ovarian (A2780), breast (MDA-MB231), hepatic (Bel7402) and colorectal cancer cells (SW620) by chemotherapeutic drug paclitaxel (PTX) through IRES-dependent translation mechanism. Moreover, IRES activity is increased in the PTX-resistant ovarian cancer cells in which AHR protein expression was also enhanced. These results strongly suggest an important role for AHR IRES-dependent translation mechanism in cancer cell response to paclitaxel treatment.

Activation of the Ahr signalling pathway by polychlorobiphenyls causes a marked induction of cytochrome P450 only after depletion of vitellogenin in Sparus aurata

The effects of polychlorinated biphenyl congeners 126 and 153 (PCB-126 and PCB-153) on vitellogenin (Vtg) and cytochrome (CYP1A1) expression were evaluated in 60 juvenile Sparus aurata. Fish were divided into four groups and the control group (Group A) was compared to fish exposed to PCB-126 (10-8M) (Group B), PCB-153 (10-6M) (Group C) singly and also in combination (Group D) for 12, 24 and 72 h. Hepatic expression of Vtg and CYP1A1 were analyzed using histological examinations and by immunochemical (Western blotting and immunohistochemistry) methods. Vtg increased in juvenile fishes of Groups B, C and D after 12h respect to Group A and decreased after 24 and 72 h respect to 12h in each group. CYP1A1 increased after 12 and 24h in all groups vs control group and increased in Group B only at 72 h vs in control group. The results showed that chemical interaction and endocrine disruption in fish might produce deleterious consequences not only for fish but also for human.

Protective role of lycopene against Aroclor 1254-induced changes on GLUT4 in the skeletal muscles of adult male rat

Aroclor 1254 is the commercial mixture of highly toxic environmental pollutant, polychlorinated biphenyls (PCBs). Being immensely durable, it is extensively used and widely distributed. Studies show that Aroclor 1254 causes a variety of adverse health effects through free radical generation. The present investigation was designed to check the effect of Aroclor 1254 on the glucose transporter protein, GLUT4, which plays a key role in glucose homeostasis. The protective role of lycopene against the adverse effect of Aroclor 1254 was also tested. Group 1 rats received corn oil as vehicle and served as control. Groups 2, 3, and 4 were administered with Aroclor 1254 [2 mg kg(-1) body weight (b.w.) day(-1)] intraperitoneally for 30 days. Groups 3 and 4 received lycopene (2 and 4 mg kg(-1) b.w. day(-1), respectively) orally in addition to Aroclor 1254. After 30 days, animals were euthanized and the skeletal muscles were dissected to determine the following parameters: GLUT4 messenger RNA (mRNA), GLUT4 protein (both plasma membrane and cytosolic fractions), and (14)C-2-deoxyglucose uptake. Though there was no change in GLUT4 mRNA and fasting plasma glucose levels, Aroclor 1254 significantly decreased the GLUT4 protein level in both the subcellular fractions of the gracilis and triceps muscles. Most important, (14)C-2-deoxyglucose uptake showed a significant decrease in Aroclor 1254 alone treated rats, and Aroclor 1254 plus 4 mg lycopene supplementation treatment maintained the same at par with control. Thus, Aroclor 1254 has adverse effects on GLUT4 translocation and (14)C-2-deoxyglucose uptake, and lycopene administered along with Aroclor 1254 has a protective role over it.

Polycyclic aromatic hydrocarbons (PAHs) mediate transcriptional activation of the ATP binding cassette transporter ABCB6 gene via the aryl hydrocarbon receptor (AhR)

Liver is endowed with a mechanism to induce hepatic cytochromes P450 (CYP450s) in response to therapeutic drugs and environmental contaminants, leading to increased detoxification and elimination of the xenobiotics. Each CYP450 is composed of an apoprotein moiety and a heme prosthetic group, which is required for CYP450 activity. Thus, under conditions of CYP450 induction, there is a coordinate increase in heme biosynthesis to compensate for the increased expression of CYP450s. ABCB6, a mitochondrial ATP binding cassette transporter, which regulates coproporphyrinogen transport from the cytoplasm into the mitochondria to complete heme biosynthesis, represents a previously unrecognized rate-limiting step in heme biosynthesis. However, it is not known if exposure to drugs and environmental contaminants induces ABCB6 expression, to assure an adequate and apparently coordinated supply of heme for the generation of functional cytochrome holoprotein. In the present study, we demonstrate that polycyclic aromatic hydrocarbons (PAHs), the widely distributed environmental toxicants shown to induce porphyrin accumulation causing hepatic porphyria, up-regulate ABCB6 expression in both mice and humans. Using siRNA technology and Abcb6 knock-out mice, we demonstrate that PAH-mediated increase in hepatic porphyrins is compromised in the absence of ABCB6. Moreover, in vivo studies in aryl hydrocarbon receptor (AhR) knock-out mice demonstrate that PAH induction of ABCB6 is mediated by AhR. Promoter activation studies combined with electrophoretic mobility shift assay and chromatin immunoprecipitation assay demonstrate direct interactions between the AhR binding sites in the ABCB6 promoter and the AhR receptor, implicating drug activation mechanisms for ABCB6 similar to those found in inducible cytochrome P450s. These studies are the first to describe direct transcriptional activation of both mouse and human ABCB6 by xenobiotics.

Gene expression changes as markers of early lapatinib response in a panel of breast cancer cell lines

Background

Lapatinib, a tyrosine kinase inhibitor of HER2 and EGFR and is approved, in combination with capecitabine, for the treatment of trastuzumab-refractory metastatic breast cancer. In order to establish a possible gene expression response to lapatinib, a panel of breast cancer cell lines with varying sensitivity to lapatinib were analysed using a combination of microarray and qPCR profiling.

Methods

Co-inertia analysis (CIA), a data integration technique, was used to identify transcription factors associated with the lapatinib response on a previously published dataset of 96 microarrays. RNA was extracted from BT474, SKBR3, EFM192A, HCC1954, MDAMB453 and MDAMB231 breast cancer cell lines displaying a range of lapatinib sensitivities and HER2 expression treated with 1 μM of lapatinib for 12 hours and quantified using Taqman RT-PCR. A fold change ≥ ± 2 was considered significant.

Results

A list of 421 differentially-expressed genes and 8 transcription factors (TFs) whose potential regulatory impact was inferred in silico, were identified as associated with lapatinib response. From this group, a panel of 27 genes (including the 8 TFs) were selected for qPCR validation. 5 genes were determined to be significantly differentially expressed following the 12 hr treatment of 1 μM lapatinib across all six cell lines. Furthermore, the expression of 4 of these genes (RB1CC1, FOXO3A, NR3C1 and ERBB3) was directly correlated with the degree of sensitivity of the cell line to lapatinib and their expression was observed to “switch” from up-regulated to down-regulated when the cell lines were arranged in a lapatinib-sensitive to insensitive order. These included the novel lapatinib response-associated genes RB1CC1 and NR3C1. Additionally, Cyclin D1 (CCND1), a common regulator of the other four proteins, was also demonstrated to observe a proportional response to lapatinib exposure.

Conclusions

A panel of 5 genes were determined to be differentially expressed in response to lapatinib at the 12 hour time point examined. The expression of these 5 genes correlated directly with lapatinib sensitivity. We propose that the gene expression profile may represent both an early measure of the likelihood of sensitivity and the level of response to lapatinib and may therefore have application in early response detection.

Mechanistic insights into the events that lead to synergistic induction of interleukin 6 transcription upon activation of the aryl hydrocarbon receptor and inflammatory signaling

The aryl hydrocarbon receptor (AHR) is the ligand-activated transcription factor responsible for mediating the toxicological effects of dioxin and xenobiotic metabolism. However, recent evidence has implicated the AHR in additional, nonmetabolic physiological processes, including immune regulation. Certain tumor cells are largely nonresponsive to cytokine-mediated induction of the pro-survival cytokine interleukin (IL) 6. We have demonstrated that multiple nonresponsive tumor lines are able to undergo synergistic induction of IL6 following combinatorial treatment with IL1beta and the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin. Such data implicate the AHR in tumor expansion, although the mechanistic basis for the AHR-dependent synergistic induction of IL6 has not been determined. Here, we demonstrate that ligand-activated AHR is involved in priming the IL6 promoter through binding to nonconsensus dioxin response elements located upstream of the IL6 start site. Such binding appears to render the promoter more permissive to IL1beta-induced binding of NF-kappaB components. The nature of the AHR-dependent increases in IL6 promoter transcriptional potential has been shown to involve a reorganization of repressive complexes as exemplified by the presence of HDAC1 and HDAC3. Dismissal of these HDACs correlates with post-translational modifications of promoter-bound NF-kappaB components in a time-dependent manner. Thus the AHR plays a role in derepressing the IL6 promoter, leading to synergistic IL6 expression in the presence of inflammatory signals. These observations may explain the association between enhanced expression of AHR and tumor aggressiveness. It is likely that AHR-mediated priming is not restricted to the IL6 promoter and may contribute to the expression of a variety of genes, which do not have consensus dioxin response elements.

Assessing the ecological risk of soil irrigated with wastewater using in vitro cell bioassays

In the most recent research work, the accumulation of toxicants in soil was always assessed through concentration level of the target contaminants. However, assessments based on chemical analysis were limited in numbers and in their unpredictable bioavailability. An alternative assessment could be based on toxicity assessment. It means that a screening bioassay is a necessary tool for identifying and defining contaminants at the sites, which should warrant further attention. In the present study, three in vitro cell bioassays, including the SOS/umu bioassay for genotoxic effects, human estrogen receptor recombinant yeast bioassay for estrogenic effects, and ethoxyresorfin O-deethylase (EROD) with H4IIE rat hepatoma cells bioassay for Ah-receptor agonistic effects, were used for the evaluation of the accumulation of toxicants in soils irrigated with wastewater in the suburb of Beijing, China. The results indicated that there were significant increases of genotoxic, estrogenic, and Ah-receptor agonistic effects in soils irrigated with wastewater, as compared with soils irrigated with groundwater. There was the decreased effect gradient following the increase of the distances from the inlet of the wastewater. It was concluded that wastewater irrigation could cause accumulation of genotoxic, estrogenic, and Ah-receptor agonistic chemicals in soil.

Targeting cytochrome P450 CYP1B1 with a therapeutic cancer vaccine

The role that the immune system plays in limiting tumor formation and growth is becoming increasingly clear and passive immunotherapeutic approaches, such as the use of monoclonal antibodies, are now being successfully applied in clinical practice. Active immunization against tumors, however, has not yet been shown to have the same level of clinical efficacy. Two important reasons for this lack of efficacy have to do with the antigens being targeted, as well as the immunization approaches that have been tested. This review will highlight some of the requirements thought to be important for the successful development of an active immunization approach, with a focus on the ongoing development efforts for a novel agent targeting the cytochrome P450 family member, CYP1B1.

NF-kappaB and epithelial to mesenchymal transition of cancer

During progression of an in situ to an invasive cancer, epithelial cells lose expression of proteins that promote cell-cell contact, and acquire mesenchymal markers, which promote cell migration and invasion. These events bear extensive similarities to the process of epithelial to mesenchymal transition (EMT), which has been recognized for several decades as critical feature of embryogenesis. The NF-kappaB family of transcription factors plays pivotal roles in both promoting and maintaining an invasive phenotype. After briefly describing the NF-kappaB family and its role in cancer, in this review we will first describe studies elucidating the functions of NF-kappaB in transcription of master regulator genes that repress an epithelial phenotype. In the second half, we discuss the roles of NF-kappaB in control of mesenchymal genes critical for promoting and maintaining an invasive phenotype. Overall, NF-kappaB is identified as a key target in prevention and in the treatment of invasive carcinomas.

Coplanar polychlorinated biphenyl-induced CYP1A1 is regulated through caveolae signaling in vascular endothelial cells

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that can induce inflammatory processes in the vascular endothelium. We hypothesize that the plasma membrane microdomains called caveolae are critical in endothelial activation and toxicity induced by PCBs. Caveolae are particularly abundant in endothelial cells and play a major role in endothelial trafficking and the regulation of signaling pathways associated with the pathology of vascular diseases. We focused on the role of caveolae and their major protein component, caveolin-1 (Cav-1), on aryl hydrocarbon receptor (AhR)-mediated induction of cytochrome P450 1A1 (CYP1A1) by coplanar PCBs. Endothelial cell exposure to PCB77 increased both caveolin-1 and CYP1A1 levels in a time-dependent manner in total cell lysates, with a maximum increase at 6h. Furthermore, PCB77 accumulated mainly in the caveolae-rich fraction, as determined by gas chromatograph-mass spectrometry. Immunoprecipitation analysis revealed that PCB77 increased AhR binding to caveolin-1. Silencing of caveolin-1 significantly attenuated PCB77-mediated induction of CYP1A1 and oxidative stress. Similar effects were observed in caveolin-1 null mice treated with PCB77. These data suggest that caveolae may play a role in regulating vascular toxicity induced by persistent environmental pollutants such as coplanar PCBs. This may have implications in understanding mechanisms of inflammatory diseases induced by environmental pollutants.

Key developments in endocrine disrupter research and human health

Environmental etiologies involving exposures to chemicals that mimic endogenous hormones are proposed for a number of adverse human health effects, including infertility, abnormal prenatal and childhood development, and reproductive cancers (National Research Council, 1999; World Health Organization, 2002). Endocrine disrupters represent a significant area of environmental research with important implications for human health. This article provides an overview of some of the key developments in this field that may enhance our ability to assess the human health risks posed by exposure to endocrine disrupters. Advances in methodologies of hazard identification (toxicogenomics, transcriptomics, proteomics, metabolomics, bioinformatics) are discussed, as well as epigenetics and emerging biological endpoints.

Pcbs and tight junction expression

Polychlorinated biphenyl (PCB) congeners exhibit a broad range of adverse biological effects including neurotoxicity. The mechanisms by which PCBs cause neurotoxic effects are still not completely understood. The blood-brain barrier (BBB) is a physical and metabolic barrier separating brain microenvironment from the peripheral circulation and is mainly composed of endothelial cells connected by tight junctions. We examined the effects of several highly-chlorinated PCB congeners on expression of tight junction proteins in human brain endothelial cells. Treatment for 24 h with selective PCB congeners disrupted expression of the cytosolic scaffold proteins of tight junctions, such as zonula occludens (ZO)-1, ZO-2, and AF6. In contrast, PCB exposure did not alter expression of integral membrane proteins, junctional adhesion molecule-A (JAM-A), and claudin-1. Based on these data, we suggest that PCB-mediated selective alterations of tight junction protein expression may contribute to their neurotoxic effects in the central nervous system.

ORAL ADMINISTRATION OF PCBs INDUCES PROINFLAMMATORY AND PROMETASTATIC RESPONSES

Exposure to specific congeners of polychlorinated biphenyls (PCBs) can induce proinflammatory alterations, which may contribute to the formation of blood-borne tumor metastasis. The main aim of the present study was to establish an experimental model of PCB exposure in which PCBs are administered by oral gavage, which resembles the human exposure through the food chain. To determine structure-function relationship, we studied induction of inflammatory responses in the livers, lungs and brains of mice treated with PCB77 (a major coplanar PCB), PCB104 (a non-coplanar PCB with multiple ortho-chlorine substituents), and PCB153 (a major non-coplanar PCB) after a single gavage dose (150 µmol/kg body weight). The strongest expression of proinflammatory proteins occurred 24 h following the PCB administration independent of the class of PCB congeners. These data indicate that food-chain exposure to PCBs can induce proinflammatory mediators in organs that are potential targets for PCB-induced toxicity.

A role for the aryl hydrocarbon receptor in mammary gland tumorigenesis

The aryl hydrocarbon receptor (AhR) is an evolutionarily conserved transcription factor bound and activated by ubiquitous environmental pollutants. Historically, the AhR has been studied for its transcriptional regulation of genes encoding cytochrome P450 enzymes, which metabolize many of these chemicals into mutagenic and toxic intermediates. However, recent studies demonstrate that the AhR plays an important role in the biology of several cell types in the absence of environmental chemicals. Here, this paradigm shift is discussed in the context of a putative role for the AhR in mammary gland tumorigenesis. Data demonstrating high levels of constitutively active AhR in mammary tumors are summarized. Particular focus is placed on the likelihood that the AhR contributes to ongoing mammary tumor cell growth and on the possibility that the AhR inhibits apoptosis while promoting transition to an invasive, metastatic phenotype. A working model is proposed that may help explain the sometimes contradictory outcomes observed after AhR manipulation and that serves as a blueprint for the design of therapeutics which target the AhR in breast cancer. The theme that malignant cells reveal the functions for which the AhR has been evolutionarily conserved is presented throughout this discussion.

3,3'-Diindolylmethane suppresses the inflammatory response to lipopolysaccharide in murine macrophages

3,3'-Diindolylmethane (DIM), a major acid-condensation product of indole-3-carbinol, has been shown to have multiple anticancer effects in experimental models. Because recurrent or chronic inflammation has been implicated in the development of a variety of human cancers, this study examined the antiinflammatory effects of DIM and the underlying mechanisms using lipopolysaccharide (LPS)-stimulated RAW264.7 murine macrophages. DIM significantly decreased the release of nitric oxide (NO), prostaglandin (PG)E2, tumor necrosis factor alpha, interleukin (IL)-6, and IL-1beta by RAW264.7 cells treated with LPS. DIM inhibited LPS-induced increases in protein levels of inducible NO synthase (iNOS), which were accompanied by decreased iNOS mRNA levels and transcriptional activity. The mRNA levels of phospholipase A2 decreased, whereas neither cyclooxygenases-2 protein nor transcript was altered by DIM. In addition, DIM suppressed LPS-induced nuclear factor-kappaB (NF-kappaB) transcriptional activity, NF-kappaB DNA-binding activity, translocation of p65 (RelA) to the nucleus, and degradation of inhibitor of kappaB alpha. Furthermore, DIM decreased LPS-induced transcriptional activity of activator protein (AP)-1, AP-1 DNA-binding activity, and phosphorylation of stress-activated protein kinase/Jun-N-terminal kinase and c-Jun. We demonstrate that DIM inhibits LPS-induced release of proinflammatory mediators in murine macrophages. Downregulation of NF-kappaB and AP-1 signaling may be one of the mechanisms by which DIM inhibits inflammatory responses.

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

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

3-Methylcholanthrene displays dual effects on estrogen receptor (ER) alpha and ER beta signaling in a cell-type specific fashion

The biological effects of 17beta-estradiol (E(2)) are mediated by the two estrogen receptor (ER) isoforms ERalpha and ERbeta. These receptors are ligand-inducible transcription factors that belong to the nuclear receptor superfamily. These receptors are also targets for a broad range of natural and synthetic compounds that induce ER activity, including dietary compounds, pharmaceuticals, and various types of environmental pollutants such as bisphenols and polychlorinated hydroxy-biphenyls. Here, we study the effect of the combustion byproduct 3-methylcholanthrene (3-MC) on ERalpha and ERbeta. 3-MC is a compound identified previously as an activator of the aryl hydrocarbon receptor (AhR). Activation of AhR is traditionally associated with an inhibition of the E(2) signaling network. In this study, we demonstrate that 3-MC is a cell-specific activator or inhibitor of E(2) signaling pathways. We show that 3-MC acts as a repressor in some cells, presumably via the AhR, whereas it is a potent activator of ER activity in other cells. It is interesting that we demonstrate that the estrogenic effects of 3-MC are dependent on the ability of cells to metabolize parental 3-MC to alternative compounds. In summary, our results suggest that exposure to AhR ligands like 3-MC can lead to either activation or repression of E(2) signaling, depending on the cellular context.

Activation of estrogen receptor signaling by the dioxin-like aryl hydrocarbon receptor agonist, 3,3',4,4',5-pentachlorobiphenyl (PCB126) in salmon in vitro system

Available toxicological evidence indicates that environmental contaminants with strong affinity to the aryl hydrocarbon receptor (AhR) have anti-estrogenic properties in both mammalian and non-mammalian in vivo and in vitro studies. The primary objective of the present study was to investigate the interactions between the AhR and estrogen receptor (ER) in salmon in vitro system. Two separate experiments were performed and gene expression patterns were analyzed using real-time PCR, while protein analysis was done by immunoblotting. Firstly, salmon primary hepatocytes were exposed to the dioxin-like PCB126 at 1, 10 and 50 nM [corrected] and ER agonist nonylphenol (NP) at 5 and 10 microM, singly or in combination. Our data showed increased levels of ER-mediated gene expression (vitellogenin: Vtg, zona radiata protein: Zr-protein, ERalpha, ERbeta and vigilin) as well as increased cellular ERalpha protein levels after treatment with NP and PCB126, singly or in combination. PCB126 treatment alone produced, as expected, increased transcription of AhR nuclear translocator (Arnt), CYP1A1 and AhR repressor (AhRR) mRNA, and these responses were reduced in the presence of NP concentrations. PCB126 exposure alone did not produce significant effect on AhR2alpha mRNA but increased (at 1 and 50 pM) and decreased (at 10 pM) AhR2beta mRNA below control level. For AhR2delta and AhR2gamma isotypes, PCB126 (at 1 nM) [corrected] produced significant decreases (total inhibition for AhR2gamma) of mRNA levels but was indifferent at 10 and 50 pM, compared to control. NP exposure alone produced concentration-dependent significant decrease of AhR2beta mRNA. In contrast, while 5 microM NP produced an indifferent effect on AhR2delta and AhR2gamma, 10 microM NP produced significant decrease (total inhibition for AhR2gamma) and the presence of NP produced apparent PCB126 concentration-specific modulation of all AhR isotypes. A second experiment was performed to evaluate the involvement of ER isoforms in PCB126 mediated estrogenicity. Here, cells were treated with the different concentrations of PCB126, alone or in combination with ICI182,780 (ICI) and sampled at 12, 24 and 48 h post-exposure. Our data showed that PCB126 produced a time- and concentration-specific increase of ERalpha and Vtg expressions and these responses were decreased in the presence of ICI. In general, these responses show a direct PCB126 induced transcriptional activation of ERalpha and estrogenic responses in the absence of ER agonists. Although not conclusive, our findings represent the first study showing the activation of estrogenic responses by a dioxin-like PCB in fish in vitro system and resemble the "ER-hijacking" hypothesis that was recently proposed. Thus, the direct estrogenic actions of PCB126 observed in the present study add new insight on the mechanisms of ER-AhR cross-talk, prompting a new wave of discussion on whether AhR-mediated anti-estrogenicity is an exception rather than rule of action.

AhR transcriptional activity in serum of Inuits across Greenlandic districts

BACKGROUND

Human exposure to lipophilic persistent organic pollutants (POPs) including polychlorinated dibenzo-p-dioxins/furans (PCDDs/PCDFs), polychlorinated biphenyls (PCBs) and organochlorine pesticide is ubiquitous. The individual is exposed to a complex mixture of POPs being life-long beginning during critical developmental windows. Exposure to POPs elicits a number of species- and tissue-specific toxic responses, many of which involve the aryl hydrocarbon receptor (AhR). The aim of this study was to compare the actual level of integrated AhR transcriptional activity in the lipophilic serum fraction containing the actual POP mixture among Inuits from different districts in Greenland, and to evaluate whether the AhR transactivity is correlated to the bio-accumulated POPs and/or lifestyle factors.

METHODS

The study included 357 serum samples from the Greenlandic districts: Nuuk and Sisimiut (South West Coast), Qaanaaq (North Coast) and Tasiilaq (East Coast). The bio-accumulated serum POPs were extracted by ethanol: hexane and clean-up on Florisil columns. Effects of the serum extract on the AhR transactivity was determined using the Hepa 1.12cR mouse hepatoma cell line carrying an AhR-luciferase reporter gene, and the data was evaluated for possible association to the serum levels of 14 PCB congeners, 10 organochlorine pesticide residues and/or lifestyle factors.

RESULTS

In total 85% of the Inuit samples elicited agonistic AhR transactivity in a district dependent pattern. The median level of the AhR-TCDD equivalent (AhR-TEQ) of the separate genders was similar in the different districts. For the combined data the order of the median AhR-TEQ was Tasiilaq > Nuuk > or = Sisimiut > Qaanaaq possibly being related to the different composition of POPs. In overall, the AhR transactivity was inversely correlated to the levels of sum POPs, age and/or intake of marine food.

CONCLUSION

i) We observed that the proportion of dioxin like (DL) compounds in the POP mixture was the dominating factor affecting the level of serum AhR transcriptional activity even at very high level of non DL-PCBs; ii) The inverse association between the integrated serum AhR transactivity and sum of POPs might be explained by the higher level of compounds antagonizing the AhR function probably due to selective POP bioaccumulation in the food chain.

Methoxychlor suppresses the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible CYP1A1 expression in murine Hepa-1c1c7 cells

Methoxychlor (MXC) is a pesticide that was developed as a replacement for dichlorodiphenyltrichloroethane (DDT). The influence of MXC on CYP1A1 expression or the functions of mouse hepatoma Hepa-1clc7 remain unclear. Cultured Hepa-1c1c7 cells were treated with MXC with or without 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to assess the role of MXC on CYP1A1 expression. MXC alone did not affect CYP1A1-specific 7-ethoxyresorufin O-deethylase (EROD) activity. In contrast, TCDD-inducible EROD activities were markedly reduced upon concomitant treatment with TCDD and MXC in a concentration-dependent manner. Treatment with ICI 182.780, an estrogen-receptor antagonist, did not affect the suppressive effects of MXC on TCDD-inducible EROD activity. TCDD-inducible CYP1A1 mRNA levels were markedly suppressed upon treatment with TCDD and MXC, and this is consistent with their effects on EROD activity. A transient transfection assay using dioxin-response element (DRE)-linked luciferase and an electrophoretic mobility shift assay revealed that MXC reduced the transformation of the aryl hydrocarbons (Ah) receptor to a form capable of specifically binding to the DRE sequence in the promoter region of the CYP1A1 gene. These results suggest that the downregulation of CYP1A1 gene expression by MXC in Hepa-1c1c7 cells might be an antagonism of the DRE binding potential of the nuclear Ah receptor but is not mediated through the estradiol receptor.

Interplay of genes regulated by estrogen and diindolylmethane in breast cancer cell lines

Diindolylmethane (DIM), a biologically active congener of indole-3-carbinol (I3C) derived from cruciferous vegetables, is a promising agent for the prevention of estrogen-sensitive cancers. Both DIM and estrogen affect transcription of genes by binding receptors, such as aryl hydrocarbon receptor (AhR) or estrogen receptors (ER). Gene regulation by DIM and estradiol (E2) can be very complex. While DIM typically binds the AhR, this complex can directly associate with the ER, recruit co-activators that bind to estrogen-responsive promoters, and activate transcription. Alternately, DIM can bind the ER directly. In this study, we have analyzed gene expression using microarray profiling and quantitative real time-polymerase chain reaction in MCF7 breast cancer cells treated with E2 (1 nM) or DIM (25 microM) alone or in combination for 16 h. The interplay of E2 and DIM was reflected in the expression of a subset of genes (<90) in which the combination of E2 and DIM acted either additively or antagonistically to alter gene expression.

Evaluation of PCDD/F and dioxin-like PCB serum concentration data from the 2001-2002 National Health and Nutrition Examination Survey of the United States population

We analyzed the weighted 2001-2002 National Health and Nutrition Examination Survey data to assess potential differences in mean total 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) TEQ between various groups of individuals and to determine serum reference concentrations for polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (PCBs) in the general US population. Differences appeared to exist between female smokers and non-smokers and between non-Hispanic groups and all other races. Potential differences were also observed among the four age groups with an increasing trend in mean total TCDD TEQ with increasing age. Both age and gender appeared to confound the association between smoking status and total TCDD TEQ, dictating the need for further analysis. As anticipated, PCBs contributed appreciably to the total TCDD TEQ levels in the referent population and accounted for 38% to 41% of the total TEQ depending on age. Nearly 80% of the mean total TCDD TEQ was attributable to four PCDD/F congeners and three PCB congeners. In this analysis, two methods were used to assess samples where the concentrations were below the limits of detection (LODs), and this did not have significant impact on the mean total TCDD TEQ at the higher percentiles and for older individuals. Comparison of our results to those from a recent PCDD/F biomonitoring study indicates that the mean TCDD TEQ serum concentration of the individuals studied does not appear to be different from typical levels found in the general US population. Additionally, an assessment of data from the National Human Adipose Tissue Survey using our referent statistics shows that levels of these chemicals have been declining in the general population for at least two decades. The reference TEQs presented in this paper provide relevant, current data that can be used to evaluate biomonitoring results of individuals or groups exposed or potentially exposed to PCDD/Fs and PCBs above referent levels.

Dioxin-like activities in serum across European and Inuit populations

BACKGROUND

Persistent organic pollutants (POPs) such as polychlorinated dibenzo-p-dioxins/furans, polychlorinated biphenyls (PCBs) and organochlorine pesticides can cause a series of adverse effects on e.g. reproduction in animals and humans, many of which involve the aryl hydrocarbon receptor (AhR). The aim of the present study was to compare the integrated serum level of AhR mediated activity among European and Inuit populations, and evaluate whether the activity was associated to the selected POP markers, 2,2',4,4',5,5'-hexachlorobiphenyl (CB-153) and 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene (p,p'-DDE).

METHODS

The study included 338 males from Greenland (Inuit's), Sweden, Warsaw (Poland) and Kharkiv (Ukraine). The AhR transactivity of serum extracts alone (AhRag) and competitive AhR activity (AhRcomp) upon co-exposure with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were determined in the lipophilic serum fraction containing the POPs using the AhR mediated luciferase reporter Hepa1.12cR cell assay.

RESULTS

The European groups showed higher median level of AhR-TEQ (TCDD toxic equivalents) compared to the Inuit's, whereas higher incidence of Inuits sample further induced AhRcomp activity. Neither AhRag nor AhR-TEQ were correlated to CB-153 or p,p'-DDE for any of the study groups. Multiple regressions showed a significant heterogeneity of association between the CB-153 and the AhRcomp across the study groups, and accordingly a negative association between AhRcomp and CB-153 was found for the Kharkiv group.

CONCLUSION

No consistent correlation between AhR activities and two POP markers was found. Although the difference of AhRag between European and Inuit men could not be explained by CB-153 or p,p'-DDE levels alone, we believe that the variation of AhR serum activity reflects different pattern of POP exposure, genetics and/or life style factors.

Prenatal 3,3′,4,4′,5-pentachlorobiphenyl exposure modulates induction of rat hepatic CYP 1A1, 1B1, and AhR by 7,12-dimethylbenz[a]anthracene

We previously reported the finding that prenatal exposure to a relatively low dose of PCB126 increases the rate of DMBA-induced rat mammary carcinoma, while a high dose decreased it. One of the most important factors determining the sensitivity to mammary carcinogenesis is the metabolic stage at administration of the carcinogenic agent. DMBA is a procarcinogen that recruits the host metabolism to yield its ultimate carcinogenic form, and CYP1A1 and CYP1B1 (CYP1) conduct this metabolism. We investigated the hepatic expression of CYP1 and AhR following oral administration of DMBA (100 mg/kg b.w.) (i.g.) to 50-day-old female Sprague-Dawley rats whose dams had been treated (i.g.) with 2.5 ng, 250 ng, 7.5 microg of PCB126/kg or the vehicle on days 13 to 19 post-conception. Real-time quantitative RT-PCR analysis revealed that the prenatal exposure to a relatively low dose of PCB126 (the 250 ng group) prolonged the higher expression of CYP1A1, CYP1B1, and AhR mRNA, while prenatal exposure to a high dose of PCB126 (the 7.5 microg group) prolonged the higher expression of CYP1A1 and AhR mRNA. Western blotting and immunohistochemical analyses were consistent with mRNAs changes. Because DMBA oxidation produces a highly mutagenic metabolite and is finally catalyzed by CYP1B1, a relatively low PCB126 dose might produce the biological character to potentially increase the risk of DMBA-induced mammary carcinoma.

Fertility in four regions spanning large contrasts in serum levels of widespread persistent organochlorines: a cross-sectional study

BACKGROUND

Persistent organochlorine pollutants (POPs) may interfere with reproductive function but direct evidence in humans is very limited.

METHODS

Fertility was examined in four regions with contrasting blood levels of POPs. Pregnant women and their partners in Warsaw (Poland), Kharkiv (Ukraine) and Greenland were consecutively enrolled during antenatal visits. Swedish fishermen and their spouses were recruited separately and independently of current pregnancy. Lipid adjusted serum concentrations of 2,2',4,4',5,5'-hexachlorobiphenyl (CB-153) and 1,1-dichloro-2,2-bis (p-chlorophenyl)-ethylene (DDE) were available for both partners. Time to pregnancy interviews were obtained among 2269 women and 798 men provided a semen sample.

RESULTS

Inuits had high levels of both POP markers, Swedish fishermen were high in CB-153 but low in DDE, men from Kharkiv were high in DDE and low in CB-153 while men from Warsaw were low in CB-153 and had intermediate DDE levels. Compared to Warsaw couples, fecundability was reduced among couples from Kharkiv [adjusted fecundability ratio (FR) 0.64 (95% CI 0.5-0.8)] and elevated in Swedish fishermen families [FR 1.26 (95% CI 1.0-1.6)]. Adjusted geometric means of sperm counts and morphology did not differ between regions while sperm motility was higher in men living in Warsaw.

CONCLUSION

We observed regional differences in time to pregnancy and sperm motility that may be related to regional differences in POP blood levels, but other interpretations are also plausible. In particular, differences in access to safe contraception and in the prevalence of contraceptive failures are most likely to bias comparisons of time to pregnancy.

CYP1 and AhR expression in 7,12-dimethylbenz[a]anthracene-induced mammary carcinoma of rats prenatally exposed to 3,3′,4,4′,5-pentachlorobiphenyl

We previously reported the finding that prenatal exposure to a relatively low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) acted as an enhancing agent for 17-beta-estradiol (E2)-dependent 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary carcinoma, while a high dose decreased it. E2 is a known risk factor for mammary carcinoma, and CYP1A1 and 1B1 (CYP1) are the major enzymes catalyzing 2- and 4-hydroxylation of E2, respectively. We investigated the induction of CYP1 and aryl hydrocarbon receptor (AhR) in DMBA-induced mammary carcinoma using female Sprague-Dawley rats whose dams had been treated (i.g.) with 2.5 ng, 250 ng, 7.5 microg of PCB126/kg or the vehicle on days 13-19 post-conception. Immunohistochemical analysis revealed that the mammary carcinoma of the 250 ng group showed a significantly higher number of nuclei expressing estrogen receptor alpha (ER) and proliferating cell nuclear antigen (PCNA) compared to those of the other groups. Quantitative real-time RT-PCR analysis revealed that the 7.5 microg group showed a significantly higher level of CYP1A1 mRNA, and that the 250 ng group showed significantly higher levels of CYP1B1 mRNA. The level of AhR mRNA was significantly higher in both the 7.5 microg and 250 ng groups. Western blotting analysis was consistent with mRNA changes. It has been revealed that CYP1B1 catalyzes a step in the formation of 4-hydroxylated E2 metabolites, which show quite high mammary carcinogenicity. This study indicates that the enhancement of DMBA-induced mammary carcinogenicity in a relatively low PCB126 dose group might partially involve the higher expression of CYP1B1 and AhR in these carcinomas.

Fluorinated 2-(4-amino-3-methylphenyl)benzothiazoles induce CYP1A1 expression, become metabolized, and bind to macromolecules in sensitive human cancer cells

Fluorinated 2-(4-amino-3-methylphenyl)benzothiazoles possess potent antiproliferative activity against certain cancer cells, similar to the unfluorinated 2-(4-amino-3-methylphenyl)benzothiazole (DF 203, NSC 674495). In "sensitive" cancer cells, DF 203 is metabolized by, can induce expression of, and binds covalently to CYP1A1. Metabolism appears to be essential for its antiproliferative activity through DNA adduct formation. However, a biphasic dose-response relationship compromises its straightforward development as a chemotherapeutic agent. We investigated whether fluorinated benzothiazoles inhibit cancer cell growth without the biphasic dose-response, and whether the fluorinated benzothiazoles are also metabolized into reactive species, with binding to macromolecules in sensitive cancer cells. One fluorinated benzothiazole, 2-(4-amino-methylphenyl)-5-fluorobenzothiazole (5F 203, NSC 703786) did exhibit potent, antiproliferative activity without a biphasic dose-response. The fluorinated benzothiazoles were also metabolized only in cells, which subsequently showed evidence of cell death. We used microsomes from genetically engineered human B-lymphoblastoid cells expressing cytochromes P450 (CYP1A1, CYP1A2, or CYP1B1) to clarify the basis for fluorinated benzothiazole metabolism. 5F 203 induced CYP1A1 and CYP1B1 mRNA expression in sensitive breast and renal cancer cells, whereas 5F 203 induced CYP1A1 mRNA but not CYP1B1 mRNA expression in sensitive ovarian cancer cells. 5F 203 did not induce CYP1A1 or CYP1B1 mRNA expression in any "resistant" cancer cells. The fluorinated benzothiazoles induced CYP1A1 protein expression exclusively in sensitive cells. [14C]5F 203 bound substantially to subcellular fractions in sensitive cells but only minimally in resistant cells. These data are concordant with the antiproliferative activity of fluorinated benzothiazoles deriving from their ability to become metabolized and bind to macromolecules within sensitive cells.

Eggshell and egg yolk proteins in fish: hepatic proteins for the next generation: oogenetic, population, and evolutionary implications of endocrine disruption

The oocyte is the starting point for a new generation. Most of the machinery for DNA and protein synthesis needed for the developing embryo is made autonomously by the fertilized oocyte. However, in fish and in many other oviparous vertebrates, the major constituents of the egg, i.e. yolk and eggshell proteins, are synthesized in the liver and transported to the oocyte for uptake. Vitellogenesis, the process of yolk protein (vitellogenin) synthesis, transport, and uptake into the oocyte, and zonagenesis, the synthesis of eggshell zona radiata proteins, their transport and deposition by the maturing oocyte, are important aspects of oogenesis. The many molecular events involved in these processes require tight, coordinated regulation that is under strict endocrine control, with the female sex steroid hormone estradiol-17beta in a central role. The ability of many synthetic chemical compounds to mimic this estrogen can lead to unscheduled hepatic synthesis of vitellogenin and zona radiata proteins, with potentially detrimental effects to the adult, the egg, the developing embryo and, hence, to the recruitment to the fish population. This has led to the development of specific and sensitive assays for these proteins in fish, and the application of vitellogenin and zona radiata proteins as informative biomarkers for endocrine disrupting effects of chemicals and effluents using fish as test organisms. The genes encoding these important reproductive proteins are conserved in the animal kingdom and are products of several hundred million years of evolution.

Proinflammatory properties of coplanar PCBs: in vitro and in vivo evidence

So-called coplanar polychlorinated biphenyls (PCBs), as well as other environmental contaminants that are aryl hydrocarbon receptor (AhR) agonists, may compromise the normal functions of vascular endothelial cells by activating oxidative stress-sensitive signaling pathways and subsequent proinflammatory events critical in the pathology of atherosclerosis and cardiovascular disease. To test this hypothesis, porcine endothelial cells were exposed to PCB 153 and to three coplanar PCBs (PCB 77, PCB 126, or PCB 169). In contrast to PCB 153, which is not a ligand for the Ah receptor (AhR), all coplanar PCBs disrupted endothelial barrier function. All coplanar PCBs increased expression of the CYP1A1 gene, oxidative stress (DCF fluorescence), and the DNA-binding activity of nuclear factor kappaB (NF-kappaB). PCB-induced oxidative stress was concentration-dependent, with PCB 126 exhibiting a maximal response at the lowest concentration (0.5 microM) tested. The increase in NF-kappaB-dependent transcriptional activity was confirmed in endothelial cells by a luciferase reporter gene assay. In contrast to PCB 153, coplanar PCBs that are AhR ligands increased endothelial production of interleukin-6. At 3.4 microM, expression of the adhesion molecule VCAM-1 was most sensitive to PCB 77 and 169. We also provide in vivo evidence, suggesting that binding to the AhR is critical for the proinflammatory properties of PCBs. Twenty hours after a single administration of PCB 77, VCAM-1 expression was increased only in wild-type mice, while mice lacking the AhR gene showed no increased staining for VCAM-1. These data provide evidence that coplanar PCBs, agonists for the AhR, and inducers of cytochrome P450 1A1, produce oxidative stress and an inflammatory response in vascular endothelial cells. An intact AhR may be necessary for the observed PCB-induced responses. These findings suggest that activation of the AhR can be an underlying mechanism of atherosclerosis mediated by certain environmental contaminants.

PCB-induced oxidative stress in endothelial cells: modulation by nutrients

There is an increasing body of evidence suggesting that exposure to Superfund chemicals may have adverse consequences on many organ systems, as well as carcinogenic and atherogenic effects. This is particularly true for polyhalogenated aromatic hydrocarbons such as the polychlorinated biphenyls (PCBs). The vascular endothelium, which is constantly exposed to blood components including environmental contaminants, is extremely vulnerable to chemical insult as well as necrotic and apoptotic injury. Our recent studies suggest that certain PCBs, especially coplanar PCBs, can compromise normal functions of vascular endothelial cells by activating oxidative stress-sensitive signaling pathways and subsequent proinflammatory events critical in the pathology of atherosclerosis and cardiovascular disease. Our findings suggest that an increase in the level of cellular oxidative stress is a significant event in PCB-mediated endothelial cell dysfunction and that nutrients can modulate PCB-induced oxidative stress and endothelial toxicity. We have demonstrated that the dietary fat linoleic acid, the parent unsaturated fatty acid of the omega-6 family, can increase endothelial dysfunction induced by selected PCBs, probably by contributing to oxidative stress and as the result of the production of toxic metabolites called leukotoxins. The subsequent imbalance in the overall cellular oxidant/antioxidant status can activate oxidative stress- or redoxsensitive transcription factors, which in turn promote gene expression for inflammatory cytokines and adhesion molecules, intensifying the inflammatory response and endothelial cell dysfunction. Our data also suggest that antioxidant nutrients such as vitamin E can protect against endothelial cell damage mediated by PCBs or polyunsaturated dietary fats by interfering with oxidative stress-sensitive and proinflammatory signaling pathways. The concept that nutrition can modify or ameliorate the toxicity of Superfund chemicals is provocative and warrants further study as the implications for human health are significant. The information from such studies could be used to develop dietary recommendations and nutritional interventions for populations at high risk for exposure to PCBs, including communities living near Superfund sites and those exposed via occupation or diet.

Cytochrome P4501A induction and testosterone hydroxylation in cultured hepatocytes of four fish species

Primary hepatocytes of the rainbow trout (Oncorhynchus mykiss), flounder (Platychthis flesus), dab (Limanda limanda) and lemon sole (Microstomus kitt) were exposed to 3,3'4,4'5 pentachlorobiphenyl (PCB 126) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for two days. This resulted in a dose-dependent induction of cytochrome P4501A (CYP1A) activity, measured as ethoxyresorufin O-deethylase (EROD), or methoxyresorufin O-deethylase (MROD) activity. In all species, a linear relationship was observed between EROD and MROD activities, suggesting that the same CYP1A enzyme metabolizes the two alkoxy-resorufin substrates. Exposures of hepatocytes of flounder or dab to TCDD, resulted in a 59-fold and 8.2-fold induction of EROD activity, respectively. This did not concur with a change in the in vitro testosterone hydroxylation profiles of both species. These and other in vitro data indicate that TCDD exposure does not influence monooxygenase activities involved in testosterone hydroxylation. Furthermore, CYP1A is of minor importance for testosterone hydroxylation in these fish species.

Cell bioassays for detection of aryl hydrocarbon (AhR) and estrogen receptor (ER) mediated activity in environmental samples

In vitro cell bioassays are useful techniques for the determination of receptor-mediated activities in environmental samples containing complex mixtures of contaminants. The cell bioassays determine contamination by pollutants that act through specific modes of action. This article presents strategies for the evaluation of aryl hydrocarbon receptor (hereafter referred as dioxin-like) or estrogen receptor mediated activities of potential endocrine disrupting compounds in complex environmental mixtures. Extracts from various types of environmental or food matrices can be tested by this technique to evaluate their 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents or estrogenic equivalents and to identify contaminated samples that need further investigation using resource-intensive instrumental analyses. Fractionation of sample extracts exhibiting significant activities, and subsequent reanalysis with the bioassays can identify important classes of contaminants that are responsible for the observed activity. Effect-directed chemical analysis is performed only for the active fractions to determine the responsible compounds. Potency-balance estimates of all major compounds contributing to the observed effects can be calculated to determine if all of the activity has been identified, and to assess the potential for interactions such as synergism or antagonism among contaminants present in the complex mixtures. The bioassay approach is an efficient (fast and cost effective) screening system to identify the samples of interest and to provide basic information for further analysis and risk evaluation.

Induction profile of rat organic anion transporting polypeptide 2 (oatp2) by prototypical drug-metabolizing enzyme inducers that activate gene expression through ligand-activated transcription factor pathways

Knowledge of regulation of transporters would aid in predicting pharmacokinetics and drug-drug interactions. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN) and phenobarbital increases hepatic uptake of cardiac glycosides. Rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) transports cardiac glycosides with high affinity. Levels of rat hepatic oatp2 protein and mRNA are regulated by PCN and phenobarbital treatment; however, the effects of other microsomal enzyme inducers on oatp2 have not been investigated. Therefore, the purpose of this study was to further determine whether oatp2 is regulated by a broader scale of drug-metabolizing enzyme inducers that are ligands or activators for the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor (PPAR), and antioxidant/electrophile response element (ARE/EpRE). Oatp2 protein levels determined by Western blot were decreased 56 to 72% by the AhR ligands, increased 84 to 132% by the CAR ligands, and increased 230 to 360% by PXR ligands. The PPAR ligands and ARE/EpRE activators generally had minimal effects on oatp2 protein levels. Oatp2 mRNA levels, determined by the bDNA technique, generally did not show a correlation with the altered oatp2 protein levels, e.g., among PXR ligands, only PCN increased oatp2 mRNA levels, but spironolactone and dexamethasone did not. Furthermore, only PCN, but not spironolactone and dexamethasone, increased the transcription of the oatp2 gene as the amount of hnRNA was increased when determined by reverse transcription-polymerase chain reaction. In conclusion, some drug-metabolizing enzyme inducers regulate oatp2 protein levels, especially the CYP3A inducers. However, there is no correlation between their ability to increase levels of oatp2 protein and mRNA, suggesting that regulation of oatp2 by drug-metabolizing enzyme inducers occurs at both the transcriptional and post-translational levels.

Short heterodimer partner (SHP) orphan nuclear receptor inhibits the transcriptional activity of aryl hydrocarbon receptor (AHR)/AHR nuclear translocator (ARNT)

SHP (short heterodimer partner) is an orphan nuclear receptor lacking a DNA binding domain that interacts with nuclear receptors (NR) including thyroid receptor (TR), retinoic acid receptors (RAR and RXR), and estrogen receptors alpha and beta (ERalpha and ERbeta). SHP acts as a negative regulator of these receptors by inhibiting DNA binding and transcriptional activation. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) binds to arylhydrocarbon receptor (AHR), activating the AHR/AHR nuclear translocator (ARNT) heterodimer. We investigated the physical and functional interaction of SHP with AHR/ARNT. In RL95-2 human endometrial carcinoma cells, SHP inhibited TCDD-stimulated reporter activity from the AHR-responsive CYP1A1 and UGT1A6 gene promoters in a concentration-dependent manner. In GST pull-down assays, ARNT interacted directly with SHP in vitro, but AHR did not interact with GST-SHP. SHP inhibited AHR/ARNT-DNA binding in vitro. These results identify ARNT as a novel SHP target. We speculate a role for SHP in the suppression of agonist-activated AHR/ARNT activity.

In vivo modulation of nonylphenol-induced zonagenesis and vitellogenesis by the antiestrogen, 3,3'4,4'-tetrachlorobiphenyl (PCB-77) in juvenile fish

Zonagenesis and vitellogenesis (eggshell zona radiata protein (Zrp) and vitellogenin (Vtg) production, respectively), are two estrogen-regulated processes in oviparous vertebrates that are crucial for oocyte maturation. Treatment of juvenile Atlantic salmon (Salmo salar) with nonylphenol (NP; 25 mg kg(-1)) alone or in combination with 3,3',4,4'-tetrachlorobiphenyl (TCB; 0.1 mg kg(-1)) resulted in pronounced elevations of plasma eggshell Zrp and Vtg and their respective liver mRNA levels in two separate experiments. TCB treatment alone caused the elevation of CYP1A mRNA, protein and enzyme levels (7-ethoxyresorufin O-deethylase (EROD)). In experiment 3, which also included the time factor, exposure of juvenile salmon to 10 and 25 mg NP per kg in combination with TCB generally resulted in reduced plasma Zrp and Vtg levels, compared with NP treatments alone. In a fourth experiment, juvenile salmon were exposed to different doses of TCB either 2 days before or 2 days after a single dose (25 mg kg(-1)) of NP. Samples were always collected 5 days after the NP exposure and analyzed for mRNA and protein levels. Generally, TCB doses given 2 days after NP exposure resulted in the elevation of Vtg and Zrp protein and mRNA levels. Vtg and Zrp mRNA levels were also elevated in the groups treated with 0.1 mg TCB 2 days before NP exposure. In all experiments, TCB injection resulted in the induction of liver CYP1A mRNA, CYP1A protein and EROD activity, but no Zrp or Vtg protein/mRNA inductions were observed when given alone. The present study documents for the first time the apparent stimulation of xenoestrogen-induced responses by an antiestrogenic CYP1A-inducer, in fish or any other lower vertebrate. However, the stimulatory or inhibitory effect of TCB on NP-induced responses appear to be dependent on the ratio of NP and TCB doses, and temporal sequence of exposure. Fish hepatic zonagenesis and vitellogenesis continue to provide interesting models for further studies on the mechanisms and possible interactions between endocrine disruptors and CYP1A-inducers, their antiestrogenic and/or estrogen potentiating effects.

Mechanism of action of estrogens and selective estrogen receptor modulators

Estrogen, one of several sex steroid hormones, mediates its actions through the estrogen receptor. The estrogen receptor (ER) has two subtypes, ER alpha and ER beta, each of which predominates in specific tissues and organs. Cofactor proteins interact with the ER to maximize ligand-dependent transactivation of target-gene promoters. The estrogen response element is the final step in estrogen-mediated gene regulation, and current research is focused on alternate response elements. The resulting biologic action can vary according to the specific type of ER, cofactor milieu, response element, and ligand. Selective estrogen receptor modulators (SERMs) exhibit tissue-specific estrogen agonist or antagonist activity. The SERM raloxifene, which binds to ER and targets a distinct DNA element, may distinguish agonist vs antagonist activity by ER subtype and has unique activity among other SERMs because of its molecular conformation. Phytoestrogens, a potential alternative to hormone replacement therapy and for cancer prevention, do not consistently mimic estrogen's activity. Different types of phytoestrogens have different potencies, and taking high-dose supplements after menopause may not emulate the apparent benefits of lifelong consumption of phytoestrogen-rich diets. In conclusion, the complexity of estrogen action--through different ER subtypes, with various cofactors, on alternate response element--is further enhanced by ligands with selective estrogen activity. Additional research is needed to elucidate these pathways and the resulting biological effects.

Cellular thiols and redox-regulated signal transduction

In contrast to the conventional notion that reactive oxygen is mostly a trigger for oxidative damage of biological structures, now we know that low physiologically relevant concentrations of ROS can regulate a variety of key molecular mechanisms that may be linked with important cell functions (Fig. 4). Redox-based regulation of gene expression has emerged as a fundamental regulatory mechanism in cell biology. Several proteins, with apparent redox-sensing activity, have been described. Electron flow through side-chain functional CH2-SH groups of conserved cysteinyl residues in these proteins account for the redox-sensing properties. Protein thiol groups with high thiol-disulfide oxidation potentials are likely to be redox-sensitive. The ubiquitous endogenous thiols thioredoxin and glutathione are of central importance in redox signaling. Signals are transduced from the cell surface to the nucleus through phosphorylation and dephosphorylation chain reactions of cellular proteins at tyrosine and serine/threonine. Protein phosphorylation, one of the most fundamental mediators of cell signaling, is redox-sensitive. DNA-binding proteins are involved in the regulation of cellular processes such as replication, recombination, viral integration and transcription. Several studies show that the interaction of certain transcription regulatory proteins with their respective cognate DNA sites is also redox-regulated. Changes in the concentration of Ca2+i control a wide variety of cellular functions, including transcription and gene expression; Ca(2+)-driven protein phosphorylation and proteolytic processing of proteins are two major intracellular events that are implicated in signal transduction from the cell surface to the nucleus. Intracellular calcium homeostasis is regulated by the redox state of cellular thiols, and it is evident that cell calcium may play a critical role in the activation of the redox-sensitive transcription factor NF-kappa B. Among the several thiol agents tested for their efficacy in modulating cellular redox status, N-acetyl-L-cysteine and alpha-lipoic acid hold most promise for human use. A strong therapeutic potential of strategies that would modulate the cellular thioredoxin system has been also evident.

Expression of the aryl hydrocarbon receptor/transcription factor (AhR) and AhR-regulated CYP1 gene transcripts in a rat model of mammary tumorigenesis

Exposure to ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAH), may contribute to human breast cancer. In animals, PAH induce tumors in part by activating the aryl hydrocarbon receptor (AhR)/transcription factor. Historically, investigations into AhR-regulated carcinogenesis have focused on AhR-dependent transcriptional regulation of cytochrome P450 (CYP) enzymes which oxidize PAH to mutagenic intermediates. However, recent studies suggest that the AhR directly regulates cell growth. Given the postulated role of the AhR in carcinogenesis, we predicted that: (1) tissue predisposed to PAH tumorigenesis would express the AhR and (2) aberrant AhR and/or AhR-regulated gene expression would accompany malignant transformation. To test these hypotheses, AhR and CYP1 protein and/or mRNA levels were evaluated in rat mammary tumors induced with 7, 12-dimethylbenz[a]anthracene (DMBA), a prototypic PAH and AhR ligand. Results indicate modest AhR expression in normal mammary myoepithelial and ductal epithelial cells. In contrast, high AhR levels were detected in DMBA-induced tumors. Nuclear AhR localization in tumors suggested constitutive AhR activation. In situ hybridization and quantitative RT-PCR assays indicated high AhR mRNA levels in neoplastic epithelial cells. While both AhR-regulated CYP1A1 and CYP1B1 mRNAs were induced in breast tissue within 6 h of DMBA gavage, only CYP1B1 mRNA remained elevated in tumors. These results: (1) help explain targeting of breast tissue by carcinogenic PAH, (2) imply that AhR and CYP1B1 hyper-expression represent molecular biomarkers for, at least, PAH-induced mammary cell transformation, and (3) suggest mechanisms through which the AhR may contribute to carcinogenesis well after exogenous AhR ligands have been eliminated.

The bioflavonoid galangin blocks aryl hydrocarbon receptor activation and polycyclic aromatic hydrocarbon-induced pre-B cell apoptosis

Bioflavonoids are plant compounds touted for their potential to treat or prevent several diseases including cancers induced by common environmental chemicals. Much of the biologic activity of one such class of pollutants, polycyclic aromatic hydrocarbons (PAH), is mediated by the aryl hydrocarbon receptor/transcription factor (AhR). For example, the AhR regulates PAH immunotoxicity that manifests as pre-B cell apoptosis in models of B cell development. Because bioflavonoids block PAH-induced cell transformation and are structurally similar to AhR ligands, it was postulated that some of them would suppress PAH-induced, AhR-dependent immunotoxicity, possibly through a direct AhR blockade. This hypothesis was tested using a model of B cell development in which pre-B cells are cultured with and are dependent on bone marrow stromal or hepatic parenchymal cell monolayers. Of seven bioflavonoids screened, galangin (3,5,7-trihydroxyflavone) blocked PAH-induced but not C(2)-ceramide- or H(2)O(2)-induced pre-B cell apoptosis. Because galangin blocked AhR-dependent reporter gene expression, AhR complex-DNA binding, and AhR nuclear translocation, inhibition of a relatively early step in AhR signaling was implicated. This hypothesis was supported by the ability of galangin to bind the AhR and stabilize AhR-90-kDa heat shock protein complexes in the presence of AhR agonists. These studies demonstrate the utility of pre-B cell culture systems in identifying compounds capable of blocking PAH immunotoxicity, define at least one mechanism of galangin activity (i.e., repression of AhR activation), and motivate the use of this and similar dietary bioflavonoids as relatively nontoxic inhibitors of AhR agonist activity and as pharmacologic agents with which to dissect AhR signaling pathways.

Xenobiotic metabolism, genetic polymorphisms and male infertility

Male reproductive function may be impaired by various occupational and environmental chemical agents. The majority of these xenobiotics, however, require metabolic activation in order to exert adverse effects via covalent interactions between intermediate metabolites and cellular macromolecules such as DNA or protein. In addition, metabolization may alter endocrine-disrupting properties of xenobiotics. Thus tissue-specific expression and regulation of multiple xenobiotic-metabolizing enzymes are likely to play an important role in chemically induced disorders of male reproductive organs. Recent studies suggest that genetic polymorphisms underlying inter-individual and inter-ethnic variability of xenobiotic metabolism modulate susceptibility to male reproductive disorders. For cytochrome P450 1A1 (CYP1A1), a key enzyme in extra-hepatic metabolic activation of lipophilic xenobiotics, increased frequencies of two genetically linked polymorphisms have been found among infertile men.