2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibition of coronary vasculogenesis is mediated, in part, by reduced responsiveness to endogenous angiogenic stimuli, including vascular endothelial growth factor A (VEGF-A)

Polycyclic aromatic hydrocarbons (PAHs) may explain the paradoxical effects of cigarette use on preeclampsia (PE)

Tobacco smoking and use of snus (smokeless tobacco) are associated with adverse effects on pregnancy and neonatal outcomes. Nicotine is considered a key toxicant involved in effects caused by both smoking and snus, while pyrolysis products including polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke represents the constituents most unequally divided between these two groups of tobacco products. The aim of this review was: i) to compare the impact, in terms of relative effect estimates, of cigarette smoking and use of Swedish snus on pregnancy outcomes using similar non-tobacco user controls, and ii) to examine whether exposure to PAHs from smoking could explain possible differences in impact on pregnancy outcomes. We systematically searched MEDLINE, Embase, PsycInfo, Web of Science and the Cochrane Database of Systematic Reviews up to October 2021 and identified studies reporting risks for adverse pregnancy and neonatal outcomes associated with snus use and with smoking relative to pregnant women with no use of tobacco. Both snus use and smoking were associated with increased risk of stillbirth, preterm birth, and oral cleft malformation, with comparable point estimates. These effects were likely due to comparable nicotine exposure. We also found striking differences. While both smoking and snus increased the risk of having small for gestational age (SGA) infants, risk from maternal smoking was markedly higher as was the reduction in birthweight. In contrast, the risk of preeclampsia (PE) was markedly lower in smokers than in controls, while snus use was associated with a slightly increased risk. We suggest that PAHs acting via AhR may explain the stronger effects of tobacco smoking on SGA and also to the apparent protective effect of cigarette smoking on PE. Possible mechanisms involved include: i) disrupted endocrine control of fetal development as well as placental development and function, and ii) stress adaption and immune suppression in placenta and mother.

Roles of aryl hydrocarbon receptor in endothelial angiogenic responses†

Aryl hydrocarbon receptor (AhR) is a transcription factor, which can be activated by a plethora of structure-diverse ligands. Historically, AhR is known for its involvements in regulation of metabolism of xenobiotics. However, normal physiological roles of AhR have been defined in other essential biological processes, including vascular growth and function, reproduction, and immunoresponses. In contrast, aberrant expression and activation of the AhR signaling pathway occur in a variety of human diseases, many of which (e.g., preeclampsia, atherosclerosis, and hypertension) could be associated with endothelial dysfunction. Indeed, emerging evidence has shown that either exogenous or endogenous AhR ligands can induce endothelial dysfunction in either an AhR-dependent or AhR-independent manner, possibly reliant on the blood vessel origin (artery and vein) of endothelial cells. Given that the AhR signaling pathway has broad impacts on endothelial and cardiovascular function, AhR ligands, AhR, and their downstream genes could be considered novel therapeutic targets for those endothelial-related diseases. This review will discuss the current knowledge of AhR's mediation on endothelial function and potential mechanisms underlying these actions with a focus on placental endothelial cells.

Air pollution and body burden of persistent organic pollutants at an electronic waste recycling area of China

This paper reviews the concentrations of persistent organic pollutants (POPs) in atmosphere of an electronic waste (e-waste) recycling town, Guiyu, in Southeast China, focusing on polybrominated diphenyl ethers (PBDEs), polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). We assess the evidence for the association between air pollution and human body burden, to provide an indication of the severity of respiratory exposure. Compared with standards and available existing data for other areas, it clearly shows that four typical POPs, derived from recycling processes, lead to serious atmospheric pollution and heavy body burden. From published data, the estimated respiratory exposure doses of Guiyu adults and children, varied between 2.48-10.37 and 3.25-13.6 ng kg^-1 body weight (bw) day^-1 for PBDEs, 2.31-7.6 and 4.09-13.58 pg World Health Organization-Toxic Equivalent Quantity (WHO-TEQ) kg^-1 bw day^-1 for PCDD/Fs, 5.57 and 20.52 ng kg^-1 bw day^-1 for PCBs, and 8.59-50.01 and 31.64-184.14 ng kg^-1 bw day^-1 for PAHs, respectively. These results show that air pollution is more harmful to children. Furthermore, except for PBDEs, the hazard quotient (HQ) of the other three pollutants was rated more than 1 by respiratory exposure only, and all of them are at risk of carcinogenesis. So we speculate these pollutants enter the body mainly through air inhalation, making respiratory exposure may be more important than dietary exposure in the Guiyu e-waste recycling area. Effective management policies and remediation techniques are urgently needed to prevent the deterioration of ambient air quality in the e-waste recycling area.

Aryl Hydrocarbon Receptor Ablation in Cardiomyocytes Protects Male Mice From Heart Dysfunction Induced by NKX2.5 Haploinsufficiency

Epidemiological studies in humans and research in vertebrates indicates that developmental exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a ubiquitous and biopersistent environmental toxicant, is associated with incidence of early congenital heart disease in the embryo and later in the adult. TCDD-mediated toxicity depends on the aryl hydrocarbon receptor (AHR) but the role of the TCDD-activated AHR in cardiac function is not well-defined. To characterize the mechanisms responsible for AHR-mediated disruption of heart function, we generated several mouse strains with cardiomyocyte-specific Ahr gene knockout. Here, we report results on one of these strains in which the Ahr gene was deleted by cre recombinase regulated by the promoter of the cardiomyocyte-specific Nkx2.5 gene. We crossed mice with loxP-targeted Ahrfx/fx alleles with Nkx2.5+/cre mice bearing a "knock-in" cre recombinase gene integrated into one of the Nkx2.5 alleles. In these mice, loss of one Nkx2.5 allele is associated with disrupted cardiac development. In males, Nkx2.5 hemizygosity resulted in cardiac haploinsufficiency characterized by hypertrophy, dilated cardiomyopathy, and impaired ejection fraction. Ahr ablation protected Nkx2.5+/cre haploinsufficient males from cardiac dysfunction while inducing a significant increase in body weight. These effects were absent or largely blunted in females. Starting at 3 months of age, mice were exposed by oral gavage to 1 μg/kg/week of TCDD or control vehicle for an additional 2 months. TCDD exposure restored cardiac physiology in aging males, appearing to compensate for the heart dysfunction caused by Nkx2.5 hemizygosity. Our findings underscore the conclusion that deletion of the Ahr gene in cardiomyocytes protects males from heart dysfunction due to NKX2.5 haploinsufficiency.

Ah Receptor Activation by Dioxin Disrupts Activin, BMP, and WNT Signals During the Early Differentiation of Mouse Embryonic Stem Cells and Inhibits Cardiomyocyte Functions

The AHR is a ligand-activated transcription factor that mediates gene-environment interactions. Genome-wide expression profiling during differentiation of mouse ES cells into cardiomyocytes showed that AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin; Dioxin (TCDD), its prototypical ligand, disrupted the expression of multiple homeobox transcription factors and inhibited cardiomyocyte contractility. Here we treated ES cells with TCDD at daily differentiation intervals to investigate whether TCDD-induced loss of contractility had a developmental window of sensitivity. Surprisingly, contractility was an AHR-dependent TCDD target solely between differentiation days 0 and 3 during the period of panmesoderm development, when TCDD also disrupted expression of genes in the TGFβ/BMP2/4 and wingless-type MMTV integration site (WNT)signaling pathways, suppressed the secretion of bone morphogenetic protein (BMP4), WNT3a, and WNT5a and elevated the secretion of Activin A, as determined by ELISA of the secreted proteins in the culture medium. Supplementing the culture medium with BMP4, WNT3a, or WNT5a during the first 3 days of differentiation successfully countered TCDD-induced impairment of contractility, while anti-WNT3a, or anti-WNT5a antibodies or continuous Noggin (a BMP4 antagonist) or Activin A treatment inhibited the contractile phenotype. In Ahr(+/+), but not in Ahr(-) (/) (-) ES cells, TCDD treatment significantly increased mitochondrial copy number, suggestive of mitochondrial stress and remodeling. Sustained AHR activation during ES cell differentiation appears to disrupt the expression of signals critical to the ontogeny of cardiac mesoderm and cause the loss of contractility in the resulting cardiomyocyte lineage.

2,3,7,8-Tetrachlorodibenzo-p-dioxin differentially suppresses angiogenic responses in human placental vein and artery endothelial cells

Placental angiogenesis is dramatically increased during pregnancy in association with the elevated placental blood flows to support the rapidly growing fetus. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental toxicant and a ligand of aryl hydrocarbon receptor (AhR). Herein, we investigated the effects of TCDD on proliferation, migration, and viability of fetoplacental endothelial cells in response to a complete growth medium which contained serum and growth supplement using human umbilical cord vein (HUVECs) and artery (HUAECs) cells as models. We found that TCDD dose- and time-dependently inhibited (p < 0.05) proliferation of HUVECs and HUAECs. Treatment with TCDD at 10 nM for 6 days inhibited (p < 0.05) migration (by ∼ 30%) of HUAECs, but not HUVECs. TCDD at 10nM also decreased (p < 0.05) viability of HUVECs and HUAECs. Interestingly, specific AhR siRNA blocked (p < 0.05) the TCDD-inhibited cellular responses in HUAECs, but not HUVECs. Nonetheless, TCDD at 10nM neither affected the cell cycle progression, nor did it induce cell apoptosis in HUVECs and HUAECs. In addition, TCDD at 10 nM also did not alter activation of ERK1/2 and AKT1 in HUVECs and HUAECs. Collectively, TCDD suppresses proliferation and/or migration (two key steps of angiogenesis) of HUVECs and HUAECs independent and dependent of AhR, respectively. These data suggest that TCDD inhibited growth of HUVECs and HUAECs via decreasing cell viability. Thus, TCDD may inhibit fetoplacental angiogenesis, leading to negative pregnancy outcomes.

Altering HIF-1α through 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure affects coronary vessel development

Differential tissue hypoxia drives normal cardiogenic events including coronary vessel development. This requirement renders cardiogenic processes potentially susceptible to teratogens that activate a transcriptional pathway that intersects with the hypoxia-inducible factor (HIF-1) pathway. The potent toxin 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to cause cardiovascular defects by way of reduced myocardial hypoxia, inhibition of angiogenic stimuli, and alterations in responsiveness of endothelial cells to those stimuli. Our working hypothesis is that HIF-1 levels and thus HIF-1 signaling in the developing myocardium will be reduced by TCDD treatment in vivo during a critical stage and in particularly sensitive sites during heart morphogenesis. This inadequate HIF-1 signaling will subsequently result in outflow tract (OFT) and coronary vasculature defects. Our current data using the chicken embryo model showed a marked decrease in the intensity of immunostaining for HIF-1α nuclear expression in the OFT myocardium of TCDD-treated embryos. This area at the base of the OFT is particularly hypoxic during normal development; where endothelial cells initially form a concentrated anastomosing network known as the peritruncal ring; and where the left and right coronary arteries eventually connect to the aortic lumen. Consistent with this finding, anomalies of the proximal coronaries were detected after TCDD treatment and HIF-1α protein levels decreased in a TCDD dose-dependent manner.

Sox9b is required for epicardium formation and plays a role in TCDD-induced heart malformation in zebrafish

Activation of the transcription factor aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prevents the formation of the epicardium and leads to severe heart malformations in developing zebrafish (Danio rerio). The downstream genes that cause heart malformation are not known. Because TCDD causes craniofacial malformations in zebrafish by downregulating the sox9b gene, we hypothesized that cardiotoxicity might also result from sox9b downregulation. We found that sox9b is expressed in the developing zebrafish heart ventricle and that TCDD exposure markedly reduces this expression. Furthermore, we found that manipulation of sox9b expression could phenocopy many but not all of the effects of TCDD at the heart. Loss of sox9b prevented the formation of epicardium progenitors comprising the proepicardium on the pericardial wall, and prevented the formation and migration of the epicardial layer around the heart. Zebrafish lacking sox9b showed pericardial edema, an elongated heart, and reduced blood circulation. Fish lacking sox9b failed to form valve cushions and leaflets. Sox9b is one of two mammalian Sox9 homologs, sox9b and sox9a. Knock down of sox9a expression did not cause cardiac malformations, or defects in epicardium development. We conclude that the decrease in sox9b expression in the heart caused by TCDD plays a role in many of the observed signs of cardiotoxicity. We find that while sox9b is expressed in myocardial cells, it is not normally expressed in the affected epicardial cells or progenitors. We therefore speculate that sox9b is involved in signals between the cardiomyocytes and the nascent epicardial cells.

Perspectives on the potential involvement of the AH receptor-dioxin axis in cardiovascular disease

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the induction of the CYP1 family of cytochrome P450s and of several phase II detoxification enzymes. Although induction of these genes is the best characterized AHR function, it does not adequately explain the diversity of AHR-mediated effects. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the prototypical AHR ligand and dioxin congener and a model for many environmentally relevant organochlorinated compounds. Research over the course of the last 30 years has made it evident that AHR activation in response to TCDD and other xenobiotic agonists directly affects multiple metabolic pathways, leading to the identification of many AHR-directed effects of dioxin involved in regulation of growth factor signaling, cell cycle proliferation, differentiation, arrest, and apoptosis. There is ample evidence that TCDD causes persistent cardiac defects in zebrafish, chickens, mice, and likely humans and is associated with human cardiovascular disease. The question that I address here is whether exposure to TCDD during early development perturbs the concerted differentiation patterns of cardiovascular cell lineages and tissues and leads to cardiac malformations and long-term cardiovascular disease. Research to define the mechanisms responsible for the lifelong cardiovascular malformations resulting from TCDD exposure during embryonic development will be highly significant to the prevention of environmental cardiovascular injury.

Benzo[a]pyrene inhibits angiogenic factors-induced alphavbeta3 integrin expression, neovasculogenesis, and angiogenesis in human umbilical vein endothelial cells

New blood vessel formation is necessary for the repair of ischemia-damaged tissues. Endothelial cells produce exogenous and endogenous angiogenic factors in the mediation of angiogenesis and neovasculogenesis during neovascularization. Exposure to environmental pollutants may alter proangiogenic capacity or desensitize the responses of endothelial cells to stimulation by basic fibroblast growth factor and vascular endothelial growth factor. Human umbilical vein endothelial cells (HUVECs) were pretreated with benzo[a]pyrene (B[a]P), the major carcinogenic constituent found in tobacco smoke, for 24 h. Neovasculogenesis, migration, and proliferation were evaluated in solvent-treated and B[a]P-treated HUVECs. Endothelial capillary-like tube formation, cell migration, mitogen-activated protein kinase (MAPK) phosphorylation, and integrin expression were reduced in B[a]P-treated HUVECs with angiogenic factor stimulation, in comparison to solvent-treated HUVECs, although cell proliferation and Akt activation remained unaffected. Inhibition of B[a]P-mediated MAPK and neovasculogenesis was significantly rescued by pretreatment with α-naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist. The B[a]P-mediated inhibition of neovasculogenesis was also rescued in AhR-silenced HUVECs, suggesting the requirement for AhR in B[a]P-associated effects. B[a]P also inhibited angiogenesis in a chorioallantoic membrane assay. We conclude that B[a]P is a potent inhibitor of angiogenesis, and its effects are mediated via AhR-dependent phenotypic changes in B[a]P-treated HUVECs. These findings contribute to an understanding of the involvement of AhR agonists in vasculotoxicity.

2,3,7,8-tetrachlorodibenzo-p-dioxin increases reactive oxygen species production in human endothelial cells via induction of cytochrome P4501A1

Studies in our laboratory have demonstrated that subchronic 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD) exposure of adult mice results in hypertension, cardiac hypertrophy, and reduced nitric oxide (NO)-mediated vasodilation. Moreover, increased superoxide anion production was observed in cardiovascular organs of TCDD-exposed mice and this increase contributed to the reduced NO-mediated vasodilation. Since cytochrome P4501A1 (CYP1A1) can contribute to some TCDD-induced toxicity, we tested the hypothesis that TCDD increases reactive oxygen species (ROS) in endothelial cells by the induction of CYP1A1. A concentration-response to 24h TCDD exposure (10pM-10nM) was performed in confluent primary human aortic endothelial cells (HAECs). Oxidant-sensitive fluorescent probes dihydroethidium (DHE) and 2',7'-dichlorofluorescin diacetate (DCFH-DA), were used to measure superoxide anion, and hydrogen peroxide and hydroxyl radical, respectively. NO was also measured using the fluorescent probe diaminofluorescein-2 diacetate (DAF-2DA). These assessments were conducted in HAECs transfected with siRNA targeting the aryl hydrocarbon receptor (AhR), CYP1A1, or CYP1B1. TCDD concentration-dependently increased CYP1A1 and CYP1B1 mRNA, protein, and enzyme activity. Moreover, 1nM TCDD maximally increased DHE (Cont=1.0+/-0.3; TCDD=5.1+/-1.0; p=0.002) and DCFH-DA (Cont=1.0+/-0.2; TCDD=4.1+/-0.5; p=0.002) fluorescence and maximally decreased DAF-2DA fluorescence (Cont=1.0+/-0.4; TCDD=0.68+/-0.1). siRNA targeting AhR and CYP1A1 significantly decreased TCDD-induced DHE (siAhR: Cont=1.0+/-0.1; TCDD=1.3+/-0.2; p=0.093) (siCYP1A1: Cont=1.0+/-0.1; TCDD=1.1+/-0.1; p=0.454) and DCFH-DA (siAhR: Cont=1.0+/-0.2; TCDD=1.3+/-0.3; p=0.370) (siCYP1A1: Cont=1.0+/-0.1; TCDD=1.3+/-0.2; p=0.114) fluorescence and increased DAF-2DA fluorescence (siAhR: Cont=1.00+/-0.03; TCDD=0.97+/-0.03; p=0.481) (siCYP1A1: Cont=1.00+/-0.03; TCDD=0.92+/-0.03; p=0.034), while siRNA targeting CYP1B1 did not. These data suggest that TCDD-induced increase in ROS is AhR-dependent and may be mediated, in part, by CYP1A1 induction.

Dioxin exposure disrupts the differentiation of mouse embryonic stem cells into cardiomyocytes

Experimental exposure of fish, birds, and rodents to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) causes multiple Ah receptor-mediated developmental abnormalities, an observation consistent with compelling evidence in human populations that TCDD exposure is responsible for a significant incidence of birth defects. To characterize molecular mechanisms that might explain the developmental effects of dioxin, we have studied the consequences of TCDD exposure on the differentiation of mouse embryonic stem (ES) cells in culture and on the expression of genes, including those coding for homeodomain containing transcription factors, with a role in progression of tissue differentiation and embryonic identity during development. We find that TCDD treatment causes expression changes in a number of homeobox genes concomitant with Ah receptor recruitment to the promoters of many of these genes, whether under naïve or dioxin-activated conditions. TCDD exposure also derails temporal expression trajectories of developmentally regulated genes in a wide diversity of differentiation pathways, including genes with functions in neural and cardiovascular development, self-renewal, hematopoiesis and mesenchymal lineage specification, and Notch and Wnt pathways. Among these, we find that TCDD represses the expression of the cardiac development-specific Nkx2.5 homeobox transcription factor, of cardiac troponin-T and of alpha- and beta-myosin heavy chains, inhibiting the formation of beating cardiomyocytes, a characteristic phenotype of differentiating mouse ES cells in culture. These data identify potential pathways for dioxin to act as a developmental teratogen, possibly critical to cardiovascular development and disease, and provide molecular targets that may help us understand the molecular basis of Ah receptor-mediated developmental toxicity.

Expression of active Notch1 in avian coronary development

Notch1 is an important regulator of intercellular interactions in cardiovascular development. We show that the nuclear-localized, cleaved and active form of Notch1, the Notch1 intracellular domain (N1ICD), appeared in mesothelial cells of the pro-epicardium during epicardial formation at looped heart stages. N1ICD was also present in mesothelial cells and mesenchymal cells specifically within the epicardium at sulcus regions. N1ICD-positive endothelial cells were detected within the nascent vessel plexus at the atrioventricular junction and within the compact myocardium (Hamburger and Hamilton stage [HH] 25-HH30). The endothelial cells expressing N1ICD were surrounded by N1ICD-positive smooth muscle cells after coronary orifice formation (HH32-HH35), while N1ICD expression was absent in the mesenchymal and mesothelial cells surrounding mature coronary vessels. We propose that differential activation of the hypoxia/HIF1-VEGF-Notch pathway may play a role in epicardial cell interactions that promote epicardial epithelial/mesenchymal transition and coronary progenitor cell differentiation during epicardial development and coronary vasculogenesis in particularly hypoxic sulcus regions.

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the mortality and growth of two amphibian species (Xenopus laevis and Pseudacris triseriata)

We observed a slight drop in the growth of Xenopus laevis and Pseudacris triseriata larvae following acute exposure (24-48 h) during egg development to three concentrations of TCDD (0.3, 3.0, 30.0 microg/l). Our exposure protocol was modeled on a previous investigation that was designed to mimic the effects of maternal deposition of TCDD. The doses selected were consistent with known rates of maternal transfer between mother and egg using actual adult body burdens from contaminated habitats. Egg and embryonic mortality immediately following exposure increased only among 48 h X. laevis treatments. Control P. triseriata and X. laevis completed metamorphosis more quickly than TCDDtreated animals. The snout-vent length of recently transformed P. triseriata did not differ between treatments although controls were heavier than high-dosed animals. Likewise, the snout-vent length and weight of transformed X. laevis did not differ between control and TCDD treatments. These findings provide additional evidence that amphibians, including P. triseriata and X. laevis are relatively insensitive to acute exposure to TCDD during egg and embryonic development. Although the concentrations selected for this study were relatively high, they were not inconsistent with our current understanding of bioaccumulation via maternal transfer.

Enhancement of hypoxia-induced gene expression in fish liver by the aryl hydrocarbon receptor (AhR) ligand, benzo[a]pyrene (BaP)

Fish in polluted coastal habitats commonly suffer simultaneous exposure to both hypoxia and xenobiotics. Although the adaptive molecular responses to each stress have been described, little is known about the interaction between the signaling pathways mediating these responses. Previous studies in mammalian hepatoma cell lines have shown that hypoxia-inducible factor (HIF)- and/or aryl hydrocarbon receptor (AhR)-activated gene expression is suppressed following co-exposure to hypoxia and the hallmark AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, whether similar crosstalk exists in the non-tumor liver tissues of fish and whether other non-TCDD ligands also play the same inhibitory role in this crosstalk remain unknown. Here, the in vivo hepatic mRNA expression profiles of multiple hypoxia- and AhR-responsive genes (later gene expression=mRNA expression of the gene) were examined in the orange-spotted grouper (Epinephelus coioides) upon single and combined exposures to hypoxia and benzo[a]pyrene (BaP). Combined exposure enhanced hypoxia-induced gene expression but did not significantly alter BaP-induced gene expression. Protein carbonyl content was markedly elevated in fish subjected to combined exposure, indicating accumulation of reactive oxygen species (ROS). Application of diethyldithiocarbamate (DDC) to hypoxia-treated grouper liver explants similarly exaggerated hypoxia-induced gene expression as in the combined stress tissues in vivo. These observations suggest that ROS derived from the combined hypoxia and BaP stress have a role in enhancing hypoxia-induced gene expression.

Dioxin-induced toxicity on vascular remodeling of the placenta

Arylhydrocarbon receptor (AhR) activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) triggers its downstream signaling pathway to exert adverse effects on vasculature development, which can be initiated by vasculogenesis, followed by angiogenesis, or vascular remodeling, in a variety of animals including avians, piscines and mammals. The placenta, a mammalian organ rich in vasculature, consists of endothelial and trophoblast cells of fetal origin, which proliferate and differentiate under hypoxic condition in the uterine horn. Our studies demonstrated that vascular remodeling occurs prominently in the placenta of the control Holtzman rat strain during the late period of gestation, and induces changes in cell shape and elimination by apoptosis of trophoblasts. As a result, the net volumes of both maternal and fetal blood in the placenta increase to cope with the essential requirements of oxygen and nutrients in the late period of gestation. On the other hand, in utero exposure to TCDD markedly suppressed the development of sinusoids and trophoblast cells and the apoptosis of trophoblast cells with a concomitant increase in the incidence of fetal death under hypoxic condition. A crosstalk between the hypoxia-inducible factor (HIF)-mediated pathway and AhR-mediated pathway is considered to play an important role in this physiological process. No such changes were observed in the Sprague-Dawley rat strain that turned out to have an AhR conformation identical to that of the Holtzman rat strain. In this commentary, we will discuss a possible link of the TCDD toxicities with the AhR signaling pathway and gestation-related diseases.

Molecular Mechanisms of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Cardiovascular Embryotoxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons are widespread environmental contaminants and potent developmental toxicants. Hallmarks of embryonic exposure include edema, hemorrhage, and mortality. Recent studies in zebrafish and chicken have revealed direct impairment of cardiac muscle growth that may underlie these overt symptoms. TCDD toxicity is mediated by the aryl hydrocarbon receptor, but downstream targets remain unclear. Oxidative stress and growth factor modulation have been implicated in TCDD cardiovascular toxicity. Gene expression profiling is elucidating additional pathways by which TCDD might act. We review our understanding of the mechanism of TCDD embryotoxicity at morphological and molecular levels.

The impact of aryl hydrocarbon receptor signaling on matrix metabolism: implications for development and disease

The aryl hydrocarbon receptor (AhR) was identified as the receptor for polycyclic aromatic hydrocarbons and related compounds. However, novel data indicate that the AhR binds a variety of unrelated endogenous and exogenous compounds. Although AhR knockout mice demonstrate that this receptor has a role in normal development and physiology, the function of this receptor is still unclear. Recent evidence suggests that AhR signaling also alters the expression of genes involved in matrix metabolism, specifically the matrix metalloproteinases (MMPs). MMP expression and activity is critical to normal physiological processes that require tissue remodeling, as well as in mediating the progression of a variety of diseases. MMPs not only degrade structural proteins, but are also important mediators of cell signaling near or at the cell membrane through exposure of cryptic sites, release of growth factors, and cleavage of receptors. Therefore, AhR modulation of MMP expression and activity may be critical, not only in pathogenesis, but also in understanding the endogenous function of the AhR. In this review we will examine the data indicating a role for the AhR-signaling pathway in the regulation of matrix remodeling, and discuss potential molecular mechanisms.

Nonadditive effects of PAHs on Early Vertebrate Development: mechanisms and implications for risk assessment

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants. Traditionally, much of the research has focused on the carcinogenic potential of specific PAHs, such as benzo(a)pyrene, but recent studies using sensitive fish models have shown that exposure to PAHs alters normal fish development. Some PAHs can induce a teratogenic phenotype similar to that caused by planar halogenated aromatic hydrocarbons, such as dioxin. Consequently, mechanism of action is often equated between the two classes of compounds. Unlike dioxins, however, the developmental toxicity of PAH mixtures is not necessarily additive. This is likely related to their multiple mechanisms of toxicity and their rapid biotransformation by CYP1 enzymes to metabolites with a wide array of structures and potential toxicities. This has important implications for risk assessment and management as the current approach for complex mixtures of PAHs usually assumes concentration addition. In this review we discuss our current knowledge of teratogenicity caused by single PAH compounds and by mixtures and the importance of these latest findings for adequately assessing risk of PAHs to humans and wildlife. Throughout, we place particular emphasis on research on the early life stages of fish, which has proven to be a sensitive and rapid developmental model to elucidate effects of hydrocarbon mixtures.

Dioxin affects glucose transport via the arylhydrocarbon receptor signal cascade in pluripotent embryonic carcinoma cells

Intoxication by dioxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads, among other damages, to early embryo loss, fetal malformations, and cardiovascular toxicity. Apart from binding to the arylhydrocarbon receptor (AhR), the mechanism of TCDD-mediated embryo toxicity is still unclear. We investigated possible modes of a TCDD-mediated toxicity, particularly in glucose metabolism, in pluripotent P19 mouse embryonic carcinoma cells. Undifferentiated P19 cells were exposed to 1-100 nM TCDD and characterized for AhR signaling. For studying cell differentiation, P19 cells were exposed to 10 nM TCDD at stage of embryoid body formation, and analyzed on glucose metabolism and cardiac differentiation during the next 3 wk. TCDD treatment activated the AhR-signaling cascade within 1 h, confirmed by AhR translocation, induction of cytochrome P450 1A1 expression, and activation of the xenobiotic response element. Although cell viability and transcription of the cardiac marker protein alpha-myosin heavy chain were affected, TCDD did not inhibit the differentiation of P19 cells to pulsating cardiomyocytes. TCDD significantly down-regulated the expression levels of the glucose transporter (GLUT) isoforms 1 and 3. After 24-h TCDD treatment, GLUT1 was no longer localized in the plasma membrane of P19 cells. The impaired GLUT expression correlated with a lower glucose uptake in 5-d-old embryoid bodies. The TCDD effects were mediated by AhR, as shown by preculture with the AhR antagonist alpha-naphthoflavone. Our data demonstrate that an AhR-mediated disturbance in GLUT expression and insufficient glucose uptake may be major mechanisms in TCDD embryo toxicity.

The role of the aryl hydrocarbon receptor pathway in mediating synergistic developmental toxicity of polycyclic aromatic hydrocarbons to zebrafish

Planar halogenated aromatic hydrocarbons (pHAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin), show strong binding affinity for the aryl hydrocarbon receptor (AHR) and are potent inducers of cytochrome P4501A (CYP1A). It is widely accepted that dioxin toxicity is largely AHR mediated; however, the role of CYP1A activity in causing that toxicity is less clear. Another class of AHR agonists of increasing concern because of their known toxicity and ubiquity in the environment is the polycyclic aromatic hydrocarbons (PAHs). Like dioxin, some PAHs also cause toxicity to early life stages of vertebrates. Symptoms include increased cardiovascular dysfunction, pericardial and yolk sac edemas, subcutaneous hemorrhages, craniofacial deformities, reduced growth, and increased mortality rates. Although developmental effects are comparable between these two types of AHR agonists, the roles of both the AHR and CYP1A activity in PAH toxicity are unknown. As observed in previous studies with killifish (Fundulus heteroclitus), we demonstrate here that coexposure of zebrafish (Danio rerio) embryos to the PAH-type AHR agonist beta-naphthoflavone (BNF) and the CYP1A inhibitor alpha-naphthoflavone (ANF) significantly enhanced toxicity above that observed for single-compound exposures. In order to elucidate the role of the AHR pathway in mediating synergistic toxicity of PAH mixtures to early life stages, we used a morpholino approach to knock down expression of zebrafish AHR2 and CYP1A proteins during development. We observed that while knock down of AHR2 reduces cardiac toxicity of BNF combined with ANF to zebrafish embryos, CYP1A knockdown markedly enhanced toxicity of BNF alone and BNF + ANF coexposures. These data support earlier chemical inducer/inhibitor studies and also suggest that mechanisms underlying developmental toxicity of PAH-type AHR agonists are different from those of pHAHs. Identifying the pathways involved in PAH toxicity will provide for more robust, mechanistic-based tools for risk assessment of single compounds and complex environmental mixtures.

Antiproliferative and antiangiogenic effects of 3-methylcholanthrene, an aryl-hydrocarbon receptor agonist, in human umbilical vascular endothelial cells

There is increasing interest in the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and polycyclic aromatic hydrocarbons on cardiovascular diseases. Their chemical structures are similar, although polycyclic aromatic hydrocarbons contain no chlorine as does TCDD. The biochemical mechanism of their action is mainly mediated by the aryl hydrocarbon receptor. In addition, oxidative stress also plays a role in the biological and toxic effects of these chemicals. In this study, we used an aryl hydrocarbon receptor agonist, 3-methylcholanthrene (3-MC), to investigate its effect on the proliferation and angiogenesis of human umbilical vascular endothelial cells. 3-MC suppressed DNA synthesis of human umbilical vascular endothelial cells as determined by [(3)H]thymidine incorporation in a concentration-dependent fashion and arrested cells at the G0/G1 phase of the cell cycle. Interestingly, the inhibition of DNA synthesis by 3-MC was eliminated to a greater extent by aryl hydrocarbon receptor antagonists, alpha-NF (0.5 and 1 microM) and resveratrol (5 and 10 microM), than by the antioxidant, N-acetylcysteine (5 and 10 mM). Cell permeability, adhesion, and tube formation in human umbilical vascular endothelial cells exposed to 3-MC decreased in concentration-dependent manners. We also demonstrated that cell adhesion signaling (phosphorylated focal adhesion kinase (FAK)) decreased upon 3-MC treatment, suggesting that cell adhesion inhibited by 3-MC might be due to inhibition of cell adhesion signaling. Additionally, alpha-naphthoflavon (alpha-NF) ameliorated the effects of 3-MC on cell permeability, adhesion and tube formation, indicating the involvement of the aryl hydrocarbon receptor in angiogenesis. The results suggest that the adverse effects of 3-MC are mainly mediated by the aryl hydrocarbon receptor and not via increased oxidative stress.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on murine heart development: alteration in fetal and postnatal cardiac growth, and postnatal cardiac chronotropy

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals are potent cardiovascular teratogens in developing piscine and avian species. In the present study we investigated the effects of TCDD on murine cardiovascular development. Pregnant mice (C57Bl6N) were dosed with 1.5-24 microg TCDD/kg on gestation day (GD) 14.5. At GD 17.5, fetal mice exhibited a dose-related decrease in heart-to-body weight ratio that was significantly reduced at a maternal dose as low as 3.0 microg TCDD/kg. In addition, cardiocyte proliferation was reduced in GD 17.5 fetal hearts at the 6.0-microg TCDD/kg maternal dose. To determine if this reduction in cardiac weight was transient, or if it continued after birth, dams treated with control or 6.0 microg TCDD/kg were allowed to deliver, and heart weight of offspring was determined on postnatal days (P) 7 and 21. While no difference was seen on P 7, on P 21 pups from TCDD-treated litters showed an increase in heart-to-body weight ratio and in expression of the cardiac hypertrophy marker atrial natriuretic factor. Additionally, electrocardiograms of P 21 offspring showed that the combination of in utero and lactational TCDD exposure reduced postnatal heart rate but did not alter cardiac responsiveness to isoproterenol stimulation of heart rate. These results demonstrate that the fetal murine heart is a sensitive target of TCDD-induced teratogenicity, resembling many of TCDD-induced effects observed in fish and avian embryos, including reduced cardiocyte proliferation and altered fetal heart size. Furthermore, the combination of in utero and lactational TCDD exposure can induce cardiac hypertrophy and bradycardia postnatally, which could increase the risk of cardiovascular disease development.