2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibition of coronary development is preceded by a decrease in myocyte proliferation and an increase in cardiac apoptosis

Species-Specific Differences in Aryl Hydrocarbon Receptor Responses: How and Why?

The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates a wide range of biological and toxicological effects by binding to specific ligands. AhR ligands exist in various internal and external ecological systems, such as in a wide variety of hydrophobic environmental contaminants and naturally occurring chemicals. Most of these ligands have shown differential responses among different species. Understanding the differences and their mechanisms helps in designing better experimental animal models, improves our understanding of the environmental toxicants related to AhR, and helps to screen and develop new drugs. This review systematically discusses the species differences in AhR activation effects and their modes of action. We focus on the species differences following AhR activation from two aspects: (1) the molecular configuration and activation of AhR and (2) the contrast of cis-acting elements corresponding to AhR. The variations in the responses seen in humans and other species following the activation of the AhR signaling pathway can be attributed to both factors.

Environment-Sensing Aryl Hydrocarbon Receptor Inhibits the Chondrogenic Fate of Modulated Smooth Muscle Cells in Atherosclerotic Lesions

BACKGROUND

Smooth muscle cells (SMC) play a critical role in atherosclerosis. The Aryl hydrocarbon receptor (AHR) is an environment-sensing transcription factor that contributes to vascular development, and has been implicated in coronary artery disease risk. We hypothesized that AHR can affect atherosclerosis by regulating phenotypic modulation of SMC.

METHODS

We combined RNA-sequencing, chromatin immunoprecipitation followed by sequencing, assay for transposase-accessible chromatin using sequencing, and in vitro assays in human coronary artery SMCs, with single-cell RNA-sequencing, histology, and RNAscope in an SMC-specific lineage-tracing Ahr knockout mouse model of atherosclerosis to better understand the role of AHR in vascular disease.

RESULTS

Genomic studies coupled with functional assays in cultured human coronary artery SMCs revealed that AHR modulates the human coronary artery SMC phenotype and suppresses ossification in these cells. Lineage-tracing and activity-tracing studies in the mouse aortic sinus showed that the Ahr pathway is active in modulated SMCs in the atherosclerotic lesion cap. Furthermore, single-cell RNA-sequencing studies of the SMC-specific Ahr knockout mice showed a significant increase in the proportion of modulated SMCs expressing chondrocyte markers such as Col2a1 and Alpl, which localized to the lesion neointima. These cells, which we term "chondromyocytes," were also identified in the neointima of human coronary arteries. In histological analyses, these changes manifested as larger lesion size, increased lineage-traced SMC participation in the lesion, decreased lineage-traced SMCs in the lesion cap, and increased alkaline phosphatase activity in lesions in the Ahr knockout in comparison with wild-type mice. We propose that AHR is likely protective based on these data and inference from human genetic analyses.

CONCLUSIONS

Overall, we conclude that AHR promotes the maintenance of lesion cap integrity and diminishes the disease-related SMC-to-chondromyocyte transition in atherosclerotic tissues.

Polyaromatic hydrocarbons in pollution: a heart-breaking matter

Air pollution is associated with detrimental effects on human health, including decreased cardiovascular function. However, the causative mechanisms behind these effects have yet to be fully elucidated. Here we review the current epidemiological, clinical and experimental evidence linking pollution with cardiovascular dysfunction. Our focus is on particulate matter (PM) and the associated low molecular weight polycyclic aromatic hydrocarbons (PAHs) as key mediators of cardiotoxicity. We begin by reviewing the growing epidemiological evidence linking air pollution to cardiovascular dysfunction in humans. We next address the pollution-based cardiotoxic mechanisms first identified in fish following the release of large quantities of PAHs into the marine environment from point oil spills (e.g. Deepwater Horizon). We finish by discussing the current state of mechanistic knowledge linking PM and PAH exposure to mammalian cardiovascular patho-physiologies such as atherosclerosis, cardiac hypertrophy, arrhythmias, contractile dysfunction and the underlying alterations in gene regulation. Our aim is to show conservation of toxicant pathways and cellular targets across vertebrate hearts to allow a broad framework of the global problem of cardiotoxic pollution to be established. AhR; Aryl hydrocarbon receptor. Dark lines indicate topics discussed in this review. Grey lines indicate topics reviewed elsewhere.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

MALDI imaging reveals NCOA7 as a potential biomarker in oral squamous cell carcinoma arising from oral submucous fibrosis

Oral squamous cell carcinoma (OSCC) ranks among the most common cancer worldwide, and is associated with severe morbidity and high mortality. Oral submucous fibrosis (OSF), characterized by fibrosis of the mucosa of the upper digestive tract, is a pre-malignant lesion, but the molecular mechanisms underlying this malignant transformation remains to be elucidated. In this study, matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI-IMS)-based proteomic strategy was employed to profile the differentially expressed peptides/proteins between OSCC tissues and the corresponding adjacent non-cancerous OSF tissues. Sixty-five unique peptide peaks and nine proteins were identified with altered expression levels. Of them, expression of NCOA7 was found to be up-regulated in OSCC tissues by immunohistochemistry staining and western blotting, and correlated with a pan of clinicopathologic parameters, including lesion site, tumor differentiation status and lymph node metastasis. Further, we show that overexpression of NCOA7 promotes OSCC cell proliferation in either in vitro or in vivo models. Mechanistic study demonstrates that NCOA7 induces OSCC cell proliferation probably by activating aryl hydrocarbon receptor (AHR). The present study suggests that NCOA7 is a potential biomarker for early diagnosis of OSF malignant transformation, and leads to a better understanding of the molecular mechanisms responsible for OSCC development.

Protective effects of levamisole, acetylsalicylic acid, and α-tocopherol against dioxin toxicity measured as the expression of AhR and COX-2 in a chicken embryo model

Polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxins) are classed as persistent organic pollutants and have adverse effects on multiple functions within the body. Dioxins are known carcinogens, immunotoxins, and teratogens. Dioxins are transformed in vivo, and interactions between the products and the aryl hydrocarbon receptor (AhR) lead to the formation of proinflammatory and toxic metabolites. The aim of this study was to determine whether α-tocopherol (vitamin E), acetylsalicylic acid (ASA), and levamisole can decrease the amount of damage caused by dioxins. Fertile Hubbard Flex commercial line chicken eggs were injected with solutions containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or containing TCDD and the test compounds. The chicken embryos and organs were analyzed after 7 and 13 days. The levels at which AhR and cyclooxygenase-2 (COX-2) proteins (which are induced during inflammation) were expressed were evaluated by performing immunohistochemical analyses on embryos treated with TCDD alone or with TCDD and the test compounds. TCDD caused developmental disorders and increased AhR and COX-2 expression in the chicken embryo tissues. Vitamin E, levamisole, ASA, and ASA plus vitamin E inhibited AhR and COX-2 expression in embryos after 7 days and decreased AhR and COX-2 expression in embryos after 13 days. ASA, levamisole, and ASA plus vitamin E weakened the immune response and prevented multiple organ changes. Vitamin E was not fully protective against developmental changes in the embryos.

TCDD‑induced chick cardiotoxicity is abolished by a selective cyclooxygenase‑2 (COX‑2) inhibitor NS398

Halogenated aromatic hydrocarbons, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), are known to cause severe heart defects in avian species. However, the mechanism of TCDD-induced chick cardiovascular toxicity is unclear. In this study, we investigated cyclooxygenase-2 (COX-2) as a possible mechanism of TCDD-induced cardiotoxicity. Fertile chicken eggs were injected with TCDD and a COX-2 selective inhibitor, NS398, and we investigated chick heart failure on day 10. We found that the chick heart to body weight ratio and atrial natriuretic factor mRNA expression were increased, but this increase was abolished with treatment of NS398. In addition, the morphological abnormality of an enlarged ventricle resulting from TCDD exposure was also abolished with co-treatment of TCDD and NS398. Our results suggested that TCDD-induced chick heart defects are mediated via the nongenomic pathway and that they do not require the genomic pathway.

Aryl hydrocarbon receptor deletion in cerebellar granule neuron precursors impairs neurogenesis

The aryl hydrocarbon receptor (AhR) is a ligand-activated member of the basic-helix-loop-helix/PER-ARNT-SIM(PAS) transcription factor superfamily that also mediates the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increasing evidence suggests that AhR influences the development of many tissues, including the central nervous system. Our previous studies suggest that sustained AhR activation by TCDD and/or AhR deletion disrupts cerebellar granule neuron precursor (GNP) development. In the current study, to determine whether endogenous AhR controls GNP development in a cell-autonomous manner, we created a GNP-specific AhR deletion mouse, AhR(fx/fx) /Math1(CRE/+) (AhR CKO). Selective AhR deletion in GNPs produced abnormalities in proliferation and differentiation. Specifically, fewer GNPs were engaged in S-phase, as demonstrated by ∼25% reductions in thymidine (in vitro) and Bromodeoxyuridine (in vivo) incorporation. Furthermore, total granule neuron numbers in the internal granule layer at PND21 and PND60 were diminished in AhR conditional knockout (CKO) mice compared with controls. Conversely, differentiation was enhanced, including ∼40% increase in neurite outgrowth and 50% increase in GABARα6 receptor expression in deletion mutants. Our results suggest that AhR activity plays a role in regulating granule neuron number and differentiation, possibly by coordinating this GNP developmental transition. These studies provide novel insights for understanding the normal roles of AhR signaling during cerebellar granule cell neurogenesis and may have important implications for the effects of environmental factors in cerebellar dysgenesis.

Cardiac toxicity by sublethal 2,3,7,8-tetrachlorodibenzo-p-dioxin correlates with its anti-proliferation effect on cardiomyocytes in zebrafish embryos

The cardiac toxicity of zebrafish embryos in response to the lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been well characterized. Dioxin contamination levels in nature are usually lower, however, and sublethal TCDD toxicity is less investigated. The present study found that the nonlethal doses of TCDD for 72-h-postfertilization (hpf) zebrafish embryos were 25 pg/mL and lower. For the present study, sublethal TCDD concentrations of 10 pg/mL and 25 pg/mL were selected, and their toxicity was then characterized. The results showed that embryos still exhibited acute and subchronic cardiac toxicity at these 2 dosages. The stroke volume and cardiac output of these embryos significantly declined early until 8 d postexposure. Embryos' heart size became smaller, and the hearts contained fewer cardiomyocytes per heart, with decreased cardiomyocyte proliferation. Apoptosis was not detected either in the TCDD-treated or the control hearts. Real-time polymerase chain reaction (PCR) revealed that the transcription of a battery of cell-cycle-related genes was suppressed within the sublethal TCDD-treated heart. In contrast, embryonic jaw development seemed not to be affected. The present study suggests that dioxin contamination, even at lower levels, might lead to cardiac toxicity in fish embryos. Such cardiac toxicity presents as disrupted normal heart function, originating from the anti-proliferative effect of sublethal TCDD on cardiomyocytes.

Cardiac myocyte-specific AHR activation phenocopies TCDD-induced toxicity in zebrafish

Exposure of zebrafish embryos to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activates the zebrafish aryl hydrocarbon receptor 2 (AHR) to produce developmental and cardiovascular toxicity. AHR is found in the heart; however, AHR activation by TCDD is not confined to the heart and occurs throughout the organism. In order to understand the cause of cardiotoxicity, we constructed a constitutively active AHR (caAHR) based on the zebrafish AHR2 and expressed it specifically in cardiomyocytes. We show that AHR activation within the cardiomyocytes can account for the heart failure induced by TCDD. Expression of the caAHR within the heart produced cardiac malformations, loss of circulation, and pericardial edema. The heart-specific activation of AHR reproduced several other well-characterized endpoints of TCDD toxicity outside of the cardiovascular system, including defects in swim bladder and craniofacial development. This work identifies a single cellular site of TCDD action, the myocardial cell, that can account for the severe cardiovascular collapse observed following early life stage exposure to TCDD, and contributes to other forms of toxicity.

Histopathologic alterations associated with global gene expression due to chronic dietary TCDD exposure in juvenile zebrafish

The goal of this project was to investigate the effects and possible developmental disease implication of chronic dietary TCDD exposure on global gene expression anchored to histopathologic analysis in juvenile zebrafish by functional genomic, histopathologic and analytic chemistry methods. Specifically, juvenile zebrafish were fed Biodiet starter with TCDD added at 0, 0.1, 1, 10 and 100 ppb, and fish were sampled following 0, 7, 14, 28 and 42 d after initiation of the exposure. TCDD accumulated in a dose- and time-dependent manner and 100 ppb TCDD caused TCDD accumulation in female (15.49 ppb) and male (18.04 ppb) fish at 28 d post exposure. Dietary TCDD caused multiple lesions in liver, kidney, intestine and ovary of zebrafish and functional dysregulation such as depletion of glycogen in liver, retrobulbar edema, degeneration of nasal neurosensory epithelium, underdevelopment of intestine, and diminution in the fraction of ovarian follicles containing vitellogenic oocytes. Importantly, lesions in nasal epithelium and evidence of endocrine disruption based on alternatively spliced vasa transcripts are two novel and significant results of this study. Microarray gene expression analysis comparing vehicle control to dietary TCDD revealed dysregulated genes involved in pathways associated with cardiac necrosis/cell death, cardiac fibrosis, renal necrosis/cell death and liver necrosis/cell death. These baseline toxicological effects provide evidence for the potential mechanisms of developmental dysfunctions induced by TCDD and vasa as a biomarker for ovarian developmental disruption.

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.

TCDD inhibits heart regeneration in adult zebrafish

Normal adult zebrafish can completely regenerate lost myocardium following partial amputation of the ventricle apex. We report that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) significantly impairs this regeneration. Adult male zebrafish were injected with vehicle (control) or TCDD (70ng/g, ip) 1 day prior to partial amputation of the ventricle apex. Gross observation and histological analysis of the amputated heart at 21 days postamputation revealed that TCDD-exposed fish had not progressed beyond the initial clot formation stage, whereas the vehicle control fish showed substantial recovery and almost complete resolution of the formed clot. In contrast, hearts that were not surgically wounded showed no signs of TCDD toxicity. Striking features in the TCDD-exposed hearts were the absence of the normal sheath of new tissue enveloping the wound and the absence of intense cell proliferation at the site of the wound. In addition, the patterns of collagen deposition at the wound site were different between the TCDD and vehicle groups. Because the receptor for TCDD is the aryl hydrocarbon receptor ligand-activated transcriptional regulator, we examined the effects of TCDD exposure on gene expression in the ventricle using DNA microarrays. Samples were collected just prior to amputation and at 6h and 7 days postamputation. TCDD-pretreated hearts had dysregulated expression of genes involved in heart function, tissue regeneration, cell growth, and extracellular matrix. Because embryonic, but not adult, hearts are major targets for TCDD-induced cardiotoxicity, we speculate that the need for embryonic-like cells in regeneration is connected with the effects of TCDD in inhibiting the response to wounding.

Effects of short-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on microRNA expression in zebrafish embryos

Although many drugs and environmental chemicals are teratogenic, the mechanisms by which most toxicants disrupt embryonic development are not well understood. MicroRNAs, single-stranded RNA molecules of ~22 nt that regulate protein expression by inhibiting mRNA translation and promoting mRNA sequestration or degradation, are important regulators of a variety of cellular processes including embryonic development and cellular differentiation. Recent studies have demonstrated that exposure to xenobiotics can alter microRNA expression and contribute to the mechanisms by which environmental chemicals disrupt embryonic development. In this study we tested the hypothesis that developmental exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a well-known teratogen, alters microRNA expression during zebrafish development. We exposed zebrafish embryos to DMSO (0.1%) or TCDD (5nM) for 1h at 30hours post fertilization (hpf) and measured microRNA expression using several methods at 36 and 60hpf. TCDD caused strong induction of CYP1A at 36hpf (62-fold) and 60hpf (135-fold) as determined by real-time RT-PCR, verifying the effectiveness of the exposure. MicroRNA expression profiles were determined using microarrays (Agilent and Exiqon), next-generation sequencing (SOLiD), and real-time RT-PCR. The two microarray platforms yielded results that were similar but not identical; both showed significant changes in expression of miR-451, 23a, 23b, 24 and 27e at 60hpf. Multiple analyses were performed on the SOLiD sequences yielding a total of 16 microRNAs as differentially expressed by TCDD in zebrafish embryos. However, miR-27e was the only microRNA to be identified as differentially expressed by all three methods (both microarrays, SOLiD sequencing, and real-time RT-PCR). These results suggest that TCDD exposure causes modest changes in expression of microRNAs, including some (miR-451, 23a, 23b, 24 and 27e) that are critical for hematopoiesis and cardiovascular development.

Reproductive and developmental toxicity of dioxin in fish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is a global environmental contaminant and the prototypical ligand for investigating aryl hydrocarbon receptor (AHR)-mediated toxicity. Environmental exposure to TCDD results in developmental and reproductive toxicity in fish, birds and mammals. To resolve the ecotoxicological relevance and human health risks posed by exposure to dioxin-like AHR agonists, a vertebrate model is needed that allows for toxicity studies at various levels of biological organization, assesses adverse reproductive and developmental effects and establishes appropriate integrative correlations between different levels of effects. Here we describe the reproductive and developmental toxicity of TCDD in feral fish species and summarize how using the zebrafish model to investigate TCDD toxicity has enabled us to characterize the AHR signaling in fish and to better understand how dioxin-like chemicals induce toxicity. We propose that such studies can be used to predict the risks that AHR ligands pose to feral fish populations and provide a platform for integrating risk assessments for both ecologically relevant organisms and humans.

Genetic architecture of susceptibility to PCB126-induced developmental cardiotoxicity in zebrafish

Variability in risk of developmental defects caused by dioxin-like compounds (DLCs) has been demonstrated within and among several vertebrate species. Beyond our knowledge of the aryl hydrocarbon receptor (AHR) and its role in mediating toxicity for this class of compounds, little else is known concerning precise downstream targets influencing this vulnerability. In the present study, zebrafish with divergent genetic backgrounds were screened for susceptibility to developmental cardiotoxicity caused by the prototypical DLC, 3,3',4,4',5-pentachlorobiphenyl (PCB126); a range up to ∼40-fold differences was observed. Differentially sensitive zebrafish were chosen for a genetic cross, and the recombinant generation was used for genome-wide quantitative trait loci (QTL) mapping. Multiple QTLs were identified--several acting alone, one additively, and two others via epistatic interaction. Together, these QTLs account for 24% of the phenotypic variance observed in cardioteratogenicity resulting from PCB126 exposure (logarithm of the odds = 13.55, p = 1.89 × 10⁻¹⁰). Candidate genes in these QTL regions include the following: ahr2, bcor, and capn1 (Chr 22); e2f1 and pdyn (Chr 23); ctnnt2, plcg1, eno3, tgm1, and tgm2 (interacting on Chr 23); and vezf1 (Chr 15). These data demonstrate that DLC-induced cardiac teratogenicity is a multifactorial complex trait influenced by gene × gene and gene × environment interactions. The identified QTLs harbor many DLC-responsive genes critical to cardiovascular development and provide insight into the genetic basis of susceptibility to AHR-mediated developmental toxicity.

The differentiation of cardiomyocytes from mouse embryonic stem cells is altered by dioxin

2,3,7,8-Tetrachlorodibenzo-para-dioxin (TCDD) causes abnormalities during heart development. Cardiomyocytes derived from embryonic stem (ES) cells are a robust model for the study of early cardiomyogenesis. Here, we evaluated the effects of TCDD at key stages during the differentiation of mouse ES cells into cardiomyocytes analysing: (i) the transcription of lineage differentiation (Brachyury, Nkx-2.5, Actc-1), cardiac-specific (Alpk3, cTnT, cTnI, cTnC) and detoxification phase I (Cyp1A1, Cyp1A2 and Cyp1B1) and phase II (Nqo1, Gsta1 and Ugt1a6) genes; (ii) the global gene expression; (iii) the ultrastructure of ES-derived cardiomyocytes; (iv) level of ATP production and (v) the immunolocalisation of sarcomeric α-actinin, β-myosin heavy chain and cTnT proteins. We show that TCDD affects the differentiation of ES cells into cardiomyocytes at several levels: (1) induces the expression of phase I genes; (2) down-regulates a group of heart-specific genes, some involved in the oxidative phosphorylation pathway; (3) reduces the efficiency of differentiation; (4) alters the arrangement of mitochondria, that show twisted and disrupted cristae, and of some sarcomeres, with misalignement or disarrangement of the myofibrillar organisation and (5) reduces ATP production. This study provides novel evidences that TCDD impairs cardiomyocyte differentiation. Sarcomeres and mitochondria could be a target for dioxin toxicity, their disruption representing a possible mechanism developing cardiac injury.

TCDD induced pericardial edema and relative COX-2 expression in medaka (Oryzias Latipes) embryos

Exposure to dioxin and other aryl hydrocarbon receptor (AhR) ligands results in multiple, specific developmental cardiovascular phenotypes including pericardial edema and circulatory failure in small aquarium fish models. Although phenotypes are well described, mechanistic underpinnings for such toxicities remain elusive. Here we suggest that AhR activation results in stimulation of inflammation and "eicosanoid" pathways, which contribute to the observed developmental, cardiovascular phenotypes. We demonstrate that medaka embryos exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0.05-1 ppb) during early development result in a dose-related increase in the prevalence of pericardial edema and that this phenotype correlates with an increase in cyclooxygenase-2 (COX-2) gene expression. Those individuals exhibiting the edema phenotype had significantly greater COX-2 mRNA than their nonedematous cohort. Selective pharmacological inhibition of COX-2, with NS-398, and genetic knock down of COX-2 with a translation initiation morpholino significantly attenuated prevalence and severity of edema phenotype. Subsequently, exposures of medaka embryos to arachidonic acid (AA) resulted in recapitulation of the pericardial edema phenotype and significantly increased COX-2 expression only in those individuals exhibiting the edema phenotype compared with their nonedematous cohort. AA exposure does not result in significant induction of cytochrome P450 1A expression, suggesting that pericardial edema can be induced independent of AhR/aryl hydrocarbon receptor nuclear translocator/dioxin response element interactions. Results from this study demonstrate that developmental exposure to TCDD results in an induction of inflammatory mediators including COX-2, which contribute to the onset, and progression of heart dysmorphogenesis in the medaka model.

Dynamic zebrafish interactome reveals transcriptional mechanisms of dioxin toxicity

Background

In order to generate hypotheses regarding the mechanisms by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) causes toxicity, we analyzed global gene expression changes in developing zebrafish embryos exposed to this potent toxicant in the context of a dynamic gene network. For this purpose, we also computationally inferred a zebrafish (Danio rerio) interactome based on orthologs and interaction data from other eukaryotes.

Methodology/Principal Findings

Using novel computational tools to analyze this interactome, we distinguished between dioxin-dependent and dioxin-independent interactions between proteins, and tracked the temporal propagation of dioxin-dependent transcriptional changes from a few genes that were altered initially, to large groups of biologically coherent genes at later times. The most notable processes altered at later developmental stages were calcium and iron metabolism, embryonic morphogenesis including neuronal and retinal development, a variety of mitochondria-related functions, and generalized stress response (not including induction of antioxidant genes). Within the interactome, many of these responses were connected to cytochrome P4501A (cyp1a) as well as other genes that were dioxin-regulated one day after exposure. This suggests that cyp1a may play a key role initiating the toxic dysregulation of those processes, rather than serving simply as a passive marker of dioxin exposure, as suggested by earlier research.

Conclusions/Significance

Thus, a powerful microarray experiment coupled with a flexible interactome and multi-pronged interactome tools (which are now made publicly available for microarray analysis and related work) suggest the hypothesis that dioxin, best known in fish as a potent cardioteratogen, has many other targets. Many of these types of toxicity have been observed in mammalian species and are potentially caused by alterations to cyp1a.

Overview of developmental heart defects by dioxins, PCBs, and pesticides

The developing cardiovascular system is a sensitive target of many environmental pollutants, including dioxins, dioxin-like polychlorinated biphenyls (PCBs), and some pesticides such as methyl parathion. Laboratory research has utilized a variety of vertebrate models to elucidate potential mechanisms that mediate this cardioteratogenicity and to establish the sensitivity of different species for predicting potential risk to environmental and human health. Studies of dioxin and dioxin-like PCBs have illustrated that piscine, avian, and mammalian embryos exhibit cardiovascular structural changes and functional deficits, although the specific characteristics vary among the individual models. Piscine models typically exhibit reduced blood flow, altered heart looping, and reduced heart size and contraction rate. The chick embryo exhibits extensive cardiac dilation, thinner ventricle walls, and reduced responsiveness to chronotropic stimuli, while the murine embryo exhibits reduced heart size. It is notable that in all models the dioxin-associated cardioteratogenicity is associated with increases in cardiovascular apoptosis and decreases in cardiocyte proliferation. While the cardiotertogenicity in piscine and avian species is associated with overt morbidity and mortality, that is not the case for the murine embryo. However, murine offspring exposed during development to dioxin exhibit cardiac hypertrophy and an increased sensitivity to a second cardiovascular insult in adulthood. Thus, although the mammalian embryo is less sensitive to cardiovascular defects by dioxin and dioxin-like compounds, developmental exposure increases the risk of cardiovascular disease later in life. The impact of developmental exposure to dioxin-like chemicals on human cardiovascular disease susceptibility is not known. However, recent animal research has confirmed human epidemiology studies that dioxin exposure in adulthood is associated with hypertension and cardiovascular disease.

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.

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 aryl hydrocarbon receptor (AhR) pathway as a regulatory pathway for cell adhesion and matrix metabolism

The aryl hydrocarbon receptor (AhR) is an orphan receptor in the basic helix-loop-helix PAS family of transcriptional regulators. Although the endogenous regulator of this pathway has not been identified, the AhR is known to bind and be activated by a variety of compounds ranging from environmental contaminants to flavanoids. The function of this receptor is still unclear; however, animal models indicate that the AhR is important for normal development. One hypothesis is that the AhR senses cellular stress and initiates the cellular response by altering gene expression and inhibiting cell cycle progression and that activation of the AhR by exogenous environmental chemicals results in the dysregulation of this normal function. In this review we will examine the role of the AhR in the regulation of genes and proteins involved in cell adhesion and matrix remodeling, and discuss the implications of these changes in development and disease. In addition, we will discuss evidence suggesting that the AhR pathway is responsive to changes in matrix composition as well as cell-cell and cell-matrix interactions.

PCB126 exposure disrupts zebrafish ventricular and branchial but not early neural crest development

We have used zebrafish and 3,3',4,4',5-pentachlorobiphenyl (PCB126) to investigate the developmental toxicity of polychlorinated biphenyls (PCBs) that exert their effects through the aryl hydrocarbon receptor (AHR). We found that cardiac and neural crest (NC)-derived jaw and branchial cartilages are specifically targeted early in development. The suite of malformations, which ultimately leads to circulatory failure, includes a severely dysmorphic heart with a reduced bulbus arteriosus and abnormal atrioventricular and outflow valve formation. Early NC migration and patterning of the jaw and branchial cartilages was normal. However, the jaw and branchial cartilages failed to grow to normal size. In the heart, the ventricular myocardium showed a reduction in cell number and size. The heart and jaw/branchial phenotype could be rescued by pifithrin-alpha, a blocker of p53. However, the function of pifithrin-alpha in this model may act as a competitive inhibitor of PCB at the AHR and is likely independent of p53. Morpholinos against p53 did not rescue the phenotype, nor were zebrafish with a mutant p53-null allele resistant to PCB126 toxicity. Morpholino knockdown of cardiac troponin T, which blocks the onset of cardiac function, prevented the PCB126-induced cardiac dysmorphogenesis but not the jaw/branchial phenotype. The cardiovascular characteristics appear to be similar to hypoplastic left heart syndrome (HLHS) and introduce the potential of zebrafish as a model to study this environmentally induced cardiovascular malformation. HLHS is a severe congenital cardiovascular malformation that has previously been linked to industrial releases of dioxins and PCBs.

2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure prevents cardiac valve formation in developing zebrafish

Cardiovascular malformations are one of the most common congenital birth defects observed in humans. Defects in cardiac valves disrupt normal blood flow. Zebrafish are an outstanding experimental model for studying the effects that environmental contaminants have on developmental processes. Previous research has shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes blood regurgitation in the heart and reduces peripheral blood flow in embryonic zebrafish, suggesting some form of valve failure. To test this we used video microscopy to examine valve function and structure in developing zebrafish exposed to TCDD. TCDD exposure produced blood regurgitation at both the atrioventricular (AV) and bulboventricular (BV) junctions. In marked contrast to control embryos exposed to the vehicle dimethyl sulfoxide, embryos exposed to TCDD failed to form valve leaflets as the heart matured. In addition, whereas TCDD did not block initial formation of the bulbus arteriosus, we found that TCDD exposure prevented the normal growth and development of this portion of the outflow tract. TCDD altered the localization of endothelial cells at the AV and BV junctions and altered the localized expression of mRNAs bmp4 and notch1b normally associated with the nascent valves. Taken together, our results demonstrate that although TCDD does not prevent the initial specification of the presumptive valve locations, TCDD exposure produces severe alterations in valve development, leading to blood regurgitation and failing circulation in the developing zebrafish.

An aryl hydrocarbon receptor repressor from Xenopus laevis: function, expression, and role in dioxin responsiveness during frog development

Xenopus laevis and other frogs are extremely insensitive to the toxicity of xenobiotic ligands of the aryl hydrocarbon receptor (AHR), including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Premetamorphic life stages are especially insensitive, and they are reported to be refractory to induction of Cytochrome P4501As, which are readily induced in older animals. The AHR repressor (AHRR) is a member of the AHR gene family. AHRR expression is induced by TCDD; it then represses AHR in an apparent negative feedback loop. In this study, we sought to test the hypothesis that constitutive AHRR expression underlies the lack of TCDD responsiveness in frog early life stages. We determined the sequence of an AHRR complimentary DNA encoding an 85.3-kDa protein sharing 52-55% identity with the bHLH/PAS domains of other AHRRs. In transient transfection assays, X. laevis AHRR inhibited TCDD-induced reporter gene expression mediated by either X. laevis AHR paralog, AHR1alpha or AHR1beta. AHRR messenger RNA was expressed at low levels in embryos (Nieuwkoop-Faber stage 33-38; approximately 52 h.p.f.) and was induced approximately twofold following TCDD exposure (42 ng/g wet weight). In contrast, AHRR exhibited higher constitutive expression and was induced more than threefold in tadpoles at stage 52-55 (prometamorphic; approximately 4 weeks postfertilization) and in isolated viscera of stage 62 tadpoles (in the metamorphic climax; approximately 7 weeks postfertilization). Although the magnitude of induction was smaller, the temporal pattern of AHRR expression and inducibility resembled that of CYP1A6. Thus, attenuated transcriptional activation of AHR target genes and low TCDD toxicity in X. laevis embryos cannot be explained by constitutive, high-level expression of AHRR.

2,3,7,8-Tetracholorodibenzo-p-dioxin exposure disrupts granule neuron precursor maturation in the developing mouse cerebellum

The widespread environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been linked to developmental neurotoxicity associated with abnormal cerebellar maturation in both humans and rodents. TCDD mediates toxicity via binding to the aryl hydrocarbon receptor (AhR), a transcription factor that regulates the expression of xenobiotic metabolizing enzymes and growth regulatory molecules. Our previous studies demonstrated that cerebellar granule neuron precursor cells (GNPs) express transcriptionally active AhR during critical developmental periods. TCDD exposure also impaired GNP proliferation and survival in vitro. Therefore, this study tested the hypothesis that TCDD exposure disrupts cerebellar development by interfering with GNP differentiation. In vivo experiments indicated that TCDD exposure on postnatal day (PND) 6 resulted in increased expression of a mitotic marker and increased thickness of the external granule layer (EGL) on PND10. Expression of the early differentiation marker TAG-1 was also more pronounced in postmitotic, premigratory granule neurons of the EGL, and increased apoptosis of GNPs was observed. On PND21, expression of the late GNP differentiation marker GABA(A alpha 6) receptor (GABAR(A alpha 6)) and total estimated cell numbers were both reduced following exposure on PND6. Studies in unexposed adult AhR(-/-) mice revealed lower GABAR(A alpha 6) levels and DNA content. In vitro studies showed elevated expression of the early differentiation marker p27/Kip1 and the GABAR(A alpha 6) in GNPs following TCDD exposure, and the expression patterns of proteins related to granule cell neurite outgrowth, beta III-tubulin and polysialic acid neural cell adhesion molecule, were consistent with enhanced neuroblast differentiation. Together, our data suggest that TCDD disrupts a normal physiological role of AhR, resulting in compromised GNP maturation and neuroblast survival, which impacts final cell number in the cerebellum.

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.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

Aryl hydrocarbon receptor activation produces heart-specific transcriptional and toxic responses in developing zebrafish

Proper regulation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, is required for normal vertebrate cardiovascular development. AHR hyperactivation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during zebrafish (Danio rerio) development results in altered heart morphology and function, culminating in death. To identify genes that may cause cardiac toxicity, we analyzed the transcriptional response to TCDD in zebrafish hearts. Zebrafish larvae were exposed to TCDD for 1 h at 72 h after fertilization (hpf), and the hearts were extracted for microarray analysis at 1, 2, 4, and 12 h after exposure (73, 74, 76, and 84 h postfertilization). The remaining body tissue was also collected at each time for comparison. TCDD rapidly induced expression in 42 genes within 1 to2hof exposure. These genes function in xenobiotic metabolism, proliferation, heart contractility, and pathways that regulate heart development. Furthermore, these expression changes preceded signs of cardiovascular toxicity, characterized by decreased stroke volume, peripheral blood flow, and a halt in heart growth. This identifies strong candidates for important AHR target genes. It is noteworthy that the TCDD-induced transcriptional response in the hearts of zebrafish larvae was substantially different from that induced in the rest of the body tissues. One of the biggest differences included a cluster of genes that were down-regulated 12 h after exposure in heart tissue, but not in the body samples. More than 70% of the transcripts in this heart-specific cluster promote cellular growth and proliferation. Thus, the developing heart stands out as being responsive to TCDD at both the level of toxicity and gene expression.

Translocation of PKC-betaII is mediated via RACK-1 in the neuronal cells following dioxin exposure

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to induce neurotoxic effects. However, the mechanism of TCDD-mediated signaling pathways and its possible molecular targets in neurons remains unknown. In this study, we analyzed effects of TCDD on neurofilament subunits, receptor for activated C kinase-1 (RACK-1), and PKC-betaII activity in developing neuronal cells. TCDD induced a significant increase of RACK-1, an adaptor protein for protein kinase C (PKC), in cerebellar granule cells in both dose- and time-dependent manner, indicating that RACK-1 is a sensitive molecular target in neuronal cells for TCDD exposure. TCDD induced a dose-dependent translocation of PKC-betaII from cytosol to membrane fractions. However, when RACK-1 induction was blocked by antisense oligonucleotide or alpha-naphthoflavone, Ah receptor (AhR) inhibitor, the translocation of PKC-betaII was inhibited. Our data suggests that TCDD activates PKC-betaII via RACK-1 in an AhR-dependent manner. This is the first report identifying RACK-1 as a target molecule involved in TCDD-mediated signaling pathways. TCDD exposure also increased the level of neurofilament-H mRNA. These results suggest that identification of target molecules may contribute to improve our understanding of TCDD-mediated signaling pathway and the risk assessment of TCDD-induced neurotoxicities.

Understanding dioxin developmental toxicity using the zebrafish model

Zebrafish (Danio rerio) have advantages over mammals as an animal model for investigating developmental toxicity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a persistent global contaminant, is the most comprehensively studied developmental toxicant in zebrafish. The hallmark responses of TCDD developmental toxicity manifested in zebrafish larvae include edema, anemia, hemorrhage, and ischemia associated with arrested growth and development. Heart and vasculature development and function are severely impaired, and jaw malformations occur secondary to inhibited chondrogenesis. The swim bladder fails to inflate, and the switch from embryonic to adult erythropoiesis is blocked. This profile of developmental toxicity responses, commonly referred to as "blue sac syndrome" because the edematous yolk sac appears blue, is observed in the larval form of all freshwater fish species exposed to TCDD at the embryonic stage of development. Components of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AHR/ARNT) signaling pathway in zebrafish have been identified and functionally characterized. Their role in mediating TCDD toxicity has been determined using morpholinos to specifically knockdown the translation of zfAHR1, zfAHR2, zfARNT1, and zfARNT2 mRNAs, respectively, and a line of zfARNT2 null mutant zebrafish has provided further insight. These studies have shown that zfAHR2 and zfARNT1 mediate TCDD developmental toxicity. In addition, the growing use of molecular and genomic tools for research on zebrafish have led to advances in our understanding of the mechanism of TCDD developmental toxicity at the molecular level, including the recent finding that toxicity is not mediated by increased cytochrome P4501A (zfCYP1A) expression.

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)

BACKGROUND

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure prior to chick embryo incubation (GD 0) induces dilated cardiomyopathy, and reduces myocardial hypoxia, vascular endothelial growth factor A (VEGF-A) expression, and coronary vascularization. We investigated whether reduced coronary vascularization 1) occurs in the absence of changes in cardiac morphology and 2) is associated with altered secretion of VEGF-A and/or an antivasculogenic factor.

METHODS

Chicken eggs were treated with control (corn oil) or TCDD (0.075-0.3 pmol of TCDD/gm) on GD 5. In vivo cardiac morphology and artery number were determined on GD 10, while in vitro vascular outgrowth and VEGF-A secretion were determined from cardiac explants on GD 6. Effects of recombinant VEGF-A (rcVEGF-A), soluble flt-1 (sFlt-1) receptor plus rcVEGF-A, and control conditioned media were assessed in TCDD explants, while effects of TCDD-conditioned media was assessed in control explants.

RESULTS

TCDD reduced coronary artery number in vivo by 53 +/- 8% and induced a dose-related reduction in tube outgrowth in vitro, but had no effect on cardiac morphology. All TCDD doses reduced explant VEGF-A secretion equally (43 +/- 3%), compared to control. sFlt-1 blocked outgrowth in control cultures and blocked rcVEGF-A-mediated rescue of outgrowth in TCDD explants. Control conditioned media partially rescued outgrowth from TCDD explants, while conditioned media from TCDD explants had no effect on controls.

CONCLUSIONS

TCDD inhibition of coronary vascularization can occur in the absence of changes in cardiac morphology and is associated with reduced VEGF-A secretion but not an antivasculogenic factor. Since control media only partly rescues TCDD's inhibitory effect, we suggest that TCDD-exposed endothelial cells are less responsive to vasculogenic stimuli.

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.

Toxicogenomic profile of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the murine fetal heart: modulation of cell cycle and extracellular matrix genes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and similar environmental contaminants have been demonstrated to be potent cardiovascular teratogens in developing piscine and avian species. In the present study, we investigated the effects of TCDD on gene expression during murine cardiovascular development. C57Bl6N pregnant mice were dosed with 1.5, 3.0, or 6.0 microg TCDD/kg on gestational day (GD) 14.5, and microarray analysis was used to characterize the global changes in fetal cardiac gene expression on GD 17.5. TCDD significantly altered expression of a number of genes involved in xenobiotic metabolism, cardiac homeostasis, extracellular matrix production/remodeling, and cell cycle regulation. Interestingly, while the AhR-responsive genes Cyp1A1, Cyp1B1, Ugt1a6, and Ahrr, were all induced by TCDD in the fetal murine heart, other AhR-responsive genes, Cyp1a2, Nqo1, and Gsta1, were not. Quantitative real-time polymerase chain reactions confirmed the changes in expression of several G1/S-type cyclins and extracellular matrix-related genes. These results demonstrate the global changes in cardiac gene expression that result from TCDD exposure of the fetal murine heart and implicate genes involved in cell cycle and extracellular matrix regulation in TCDD-induced cardiac teratogenicity and functional deficits.

Aryl hydrocarbon receptors in the frog Xenopus laevis: two AhR1 paralogs exhibit low affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent developmental toxicant in most vertebrates. However, frogs are relatively insensitive to TCDD toxicity, especially during early life stages. Toxicity of TCDD and related halogenated aromatic hydrocarbons is mediated by the aryl hydrocarbon receptor (AhR), and specific differences in properties of the AhR signaling pathway can underlie in TCDD toxicity in different species. This study investigated the role of AhR in frog TCDD insensitivity, using Xenopus laevis as a model system. X. laevis, a pseudotetraploid species, expresses two distinct AhR1 genes, AhR1alpha and AhR1beta. Sharing 86% amino acid identity, these likely represent distinct genes, both orthologous to mammalian AhR and paralogous to the AhR2 gene(s) in most fish. Both AhR1alpha and AhR1beta exhibit TCDD-dependent binding of cognate DNA sequences, but they bind TCDD with at least 20-fold lower affinity than the mouse AhR(b-1) protein, and they are similarly less responsive in TCDD-induced reporter gene induction in conjunction with the mouse CYP1A1 promoter. Furthermore, CYP1A6 and CYP1A7 induction by TCDD in cultured X. laevis A6 cells appears much less responsive than CYP1A induction in cell lines derived from more sensitive animals. Taken together, these data suggest that low affinity binding by X. laevis AhRs plays an important mechanistic role in the insensitivity of frogs to TCDD. An understanding of these molecular mechanisms should aid amphibian ecotoxicology and refine the use of frog embryos as a model [e.g. in FETAX (Frog Embryo Teratogenesis Assay-Xenopus)] for determining developmental toxicity of samples containing dioxin-like compounds.

Cardiovascular gene expression profiles of dioxin exposure in zebrafish embryos

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental contaminant that causes altered heart morphology, circulatory impairment, edema, hemorrhage, and early life stage mortality in fish. TCDD toxicity is dependent, in large part, on the aryl hydrocarbon receptor (AHR), but understanding of the molecular mechanism of cardiovascular embryotoxicity remains incomplete. To identify genes potentially involved in cardiovascular effects, we constructed custom cDNA microarrays consisting of 4896 zebrafish adult heart cDNA clones and over 200 genes with known developmental, toxicological and housekeeping roles. Gene expression profiles were obtained for 3-day-old zebrafish after early embryonic exposure to either 0.5 or 5.0 nM TCDD. In all, 516 clones were significantly differentially expressed (p < 0.005) under at least one treatment condition; 123 high-priority clones were selected for further investigation. Cytochromes P450 1A and 1B1, and other members of the AHR gene battery, were strongly and dose-dependently induced by TCDD. Importantly, altered expression of cardiac sarcomere components, including cardiac troponin T2 and multiple myosin isoforms, was consistent with the hypothesis that TCDD causes dilated cardiomyopathy. Observed increases in expression levels of mitochondrial energy transfer genes also may be related to cardiomyopathy. Other TCDD-responsive genes included fatty acid and steroid metabolism enzymes, ribosomal and signal-transduction proteins, and 18 expressed sequence tags (ESTs) with no known protein homologs. As the first broad-scale study of TCDD-modulated gene expression in a non-mammalian system, this work provides an important perspective on mechanisms of TCDD toxicity.

Zebrafish as a Model Vertebrate for Investigating Chemical Toxicity

Zebrafish (Danio rerio) has been a prominent model vertebrate in a variety of biological disciplines. Substantial information gathered from developmental and genetic research, together with near-completion of the zebrafish genome project, has placed zebrafish in an attractive position for use as a toxicological model. Although still in its infancy, there is a clear potential for zebrafish to provide valuable new insights into chemical toxicity, drug discovery, and human disease using recent advances in forward and reverse genetic techniques coupled with large-scale, high-throughput screening. Here we present an overview of the rapidly increasing use of zebrafish in toxicology. Advantages of the zebrafish both in identifying endpoints of toxicity and in elucidating mechanisms of toxicity are highlighted.

Heart malformation is an early response to TCDD in embryonic zebrafish

The zebrafish (Danio rerio) has become an attractive vertebrate model for studying developmental processes, and is emerging as a model system for studying the mechanisms by which toxic compounds perturb normal development. When exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) shortly after fertilization, zebrafish embryos exhibit pericardial edema and reduced blood flow by 72 h post fertilization (hpf). To better understand the progression of dioxin toxicity in zebrafish, we have examined the effects of TCDD on heart development. At 72 hpf, TCDD-treated embryos exhibited altered looping, with the atria positioned distinctly posterior to the ventricles, contrary to the looping of control hearts, where the two chambers lied side by side. Moreover, the ventricles in dioxin-exposed hearts became more compact, and the atria elongated in comparison to controls. These defects are not secondary to pericardial edema because they were observed when edema formation was suppressed with osmotic support. In addition to morphological changes, TCDD produced functional deficits in the developing hearts, including blood regurgitation and a striking ventricular standstill that became prevalent by 120 hpf. We also assessed the effect of TCDD on the heart size using stereological measurements, which demonstrated significant reduction in heart tissue volume at 72 hpf. Perhaps our most significant finding was a decrease in the total number of cardiomyocytes in TCDD-exposed embryos by 48 hpf, one day prior to observable effects on peripheral blood flow. We conclude that the developing heart is an important target for TCDD in zebrafish.

Increase in cardiovascular pathology in female Sprague-Dawley rats following chronic treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3,3',4,4',5-pentachlorobiphenyl

The effects of chronic exposure to dioxin (2,3,7,8,-tetrachlorodibenzo-pdioxin [TCDD]) and a dioxin-like compound (3,3',4,4',5-pentachlorobiphenyl [PCB126]) on the cardiovascular system were evaluated in female Harlan Sprague-Dawley rats as part of an ongoing National Toxicology Program investigation. The animals were gavage treated 5 d per week with up to 1000 ng of PCB126 per kilogram of body weight per day or up to 100 ng of TCDD per kilogram of body weight per day for up to 2 yr. The control animals received only a corn oil/acetone vehicle (99:1 mixture). The corresponding stop-study groups received the highest doses for 31 wk and then received only the vehicle for the remainder of the study. After a full necropsy of all animals, a complete set of tissues was examined microscopically. Administration of each compound was associated with treatment-related increases in the incidences of degenerative cardiovascular lesions. Cardiomyopathy and chronic active arteritis increased in a dose-related manner in all groups treated with PCB126 or with TCDD. Increased incidences were also observed in the stop-study groups, indicating that a shorter term exposure may produce some effects. The average severity of cardiomyopathy was minimal or slightly greater in all dose groups, including the controls. Chronic active arteritis occurred primarily in the mesentery and pancreas, although the rectum, liver, heart, ovary, uterus, and glandular stomach in the PCB126 study and the liver and ovary in the TCDD study were affected in a few of the dosed animals. The authors' investigations indicate that the rat cardiovascular system is a target for dioxin toxicity, which increases the incidence of spontaneous cardiomyopathy and arteritis.

Vascular endothelial growth factor rescues 2,3,7,8-tetrachlorodibenzo-p-dioxin inhibition of coronary vasculogenesis

BACKGROUND

We previously demonstrated that the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reduces coronary vascular development in chick embryos in vivo. In the current study, we assessed whether TCDD inhibits early events in coronary endothelial tube formation and outgrowth, and whether this inhibition occurs through a vascular endothelial growth factor (VEGF)-dependent mechanism.

METHODS

Fertile chicken eggs were treated with control (corn oil) or TCDD (0.3 pmol TCDD/g) on incubation day 0. On embryonic day 6, cardiac ventricle explants were cultured on a three-dimensional collagen gel, when coronary angioblasts are present, but prior to their assembly into endothelial tubes. Endothelial cells migrating out from explants were identified by immunohistochemistry, and endothelial tube number and length were quantitated. In addition, on incubation days 6 and 8, cardiac VEGF mRNA and protein were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively.

RESULTS

Endothelial tube length and number were significantly reduced (40% +/- 1.7% and 36% +/- 3%, respectively) in TCDD explants, compared to controls. Recombinant exogenous VEGF, as well as hypoxic stimulation with CoCl2 or 10% O2, significantly increased the length and number of outgrowing tubes in TCDD cultures, and this stimulation was prevented by a VEGF neutralizing antibody. In contrast, VEGF neutralizing antibody reduced the length and number of tubes only in control cultures, and had no inhibitory effect on tube outgrowth from TCDD explants. Finally, hearts from TCDD-treated embryos exhibited a significant reduction in both VEGF mRNA and protein, compared to controls.

CONCLUSIONS

These data suggest that TCDD inhibits early coronary vascular outgrowth via a VEGF-dependent mechanism.

2,3,7,8-tetrachlorodibenzo-p-dioxin reduces myocardial hypoxia and vascular endothelial growth factor expression during chick embryo development

BACKGROUND

Previous research has demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces cardiomyocyte growth arrest, thinner ventricle walls, and reduced number and size of coronary arteries during chick embryogenesis. Coronary vascular development is believed to be mediated, in part, by myocardial oxygen gradients and a subsequent increase in hypoxia-inducible factor 1alpha (HIF-1alpha) and vascular endothelial growth factor-A (VEGF-A) expression. We investigated whether TCDD inhibition of coronary development was associated with altered myocardial oxygen status and reduced cardiac HIF-1alpha and VEGF-A.

METHODS

Chick embryos were exposed to 15% or 20% O2 for 24 hr from incubation days 9-10 or were injected with control (corn oil) or 0.24 pmol TCDD/gm egg on day 0. On day 9, embryos were injected with control (0.9% NaCl) or EF5, a tissue hypoxia marker, and cardiac binding of EF5 was determined by immunohistochemistry on day 10. In addition, embryo hearts were analyzed for VEGF-A mRNA by in situ hybridization and quantitative RT-PCR, and for HIF-1alpha mRNA by quantitative RT-PCR.

RESULTS

Cardiac binding of EF5 was significantly increased in embryos exposed to 15% O2, compared to embryos exposed to 20% O2. In contrast, TCDD-exposed embryos exhibited significantly reduced binding of EF5 in the heart, compared to controls. Similarly, cardiac expression of HIF-1alpha and VEGF-A were increased following hypoxia and tended to be decreased following TCDD exposure.

CONCLUSIONS

These results suggest that the myocardium may be a target of TCDD toxicity, resulting in reduced myocardial hypoxia, and HIF-1alpha and VEGF-A expression believed necessary for normal coronary development.

Developmental toxicity of dioxin to mouse embryonic teeth in vitro: arrest of tooth morphogenesis involves stimulation of apoptotic program in the dental epithelium

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can arrest molar tooth development in rats after in utero and lactational exposure, and that the sensitive stage is temporally restricted. To define the stage in which TCDD is able to arrest tooth development and the cellular background of the effect, mouse embryonic molar tooth explants including various early developmental stages from initiation to late cap stage were exposed to TCDD in organ culture. TCDD did not inhibit morphogenesis of the first molar teeth including the early bud-staged E12 first molars, but the teeth were smaller than in control cultures. Accordingly, the second molars underwent morphogenesis in the presence of TCDD when explanted at E15 when they were at the bud stage. TCDD arrested their development when explanted at E14 when they had not yet reached the early bud stage. Immunohistochemical localization of incorporated bromodeoxyuridine in cultured E14 teeth showed that TCDD did not affect cell proliferation. Localization of apoptosis by terminal deoxynucleotidyl transferase (TdT)-mediated nick end labeling (TUNEL) method revealed that TCDD enhanced apoptosis of dental epithelial cells, especially in the dental lamina of both the first and second molars, and in the inner dental epithelium at the cusp tips of the first molars. Thus, TCDD can arrest tooth development in vitro if the exposure starts at the initiation stage, whereas exposure at later stages leads to smaller tooth size and deformation of cuspal morphology. TCDD interferes with tooth development by stimulating apoptosis in those cells of the dental epithelium, which are predetermined to undergo apoptosis during normal development.

Mercury, cadmium, and arsenite enhance heat shock protein synthesis in chick embryos prior to embryotoxicity

BACKGROUND

Cells respond to adverse environmental stimuli by enhancing the expression of specific genes, the products of which include a suite of proteins known as heat shock proteins (hsps), a response often attributed to cellular protection.

METHODS

In this study, we characterized alterations in hsp expression in chick embryos (Hamburger-Hamilton stage 17, 72 h) exposed in ovo to arsenite (As), mercury (Hg), and cadmium (Cd), known developmental toxicants. Embryos were incubated for 2 h following exposure to 3, 10, 30, or 100 nmol metal, or for 2, 4, 12, or 24 h following treatment with 10 nmol metal.

RESULTS

An enhanced de novo synthesis of 24-, 70-, and 90-kD, 70- and 90-kD, and 70-kD proteins was observed with As, Hg, and Cd treatments, respectively. These responses were transient; apparent rates of protein synthesis were maximal 2-4 h after exposure and returned to control rates by 24 h. Actinomycin D experiments demonstrated that arsenite-induced expression of these proteins is transcriptionally regulated. Immunoblotting experiments identified the 24-, 70-, and 90-kD proteins as the heat shock proteins hsp24, hsp70, and hsp90, respectively. Exposure duration-related abnormalities were noted in the neural tube with all metals and in the ganglia and somites with Cd and As. Retina, allantois, and limb defects were specific to Cd-treated embryos, and branchial arch defects were specific to As-treated embryos.

CONCLUSIONS

The data support metal-induced developmental abnormalities, which are preceded by synthesis of stress proteins.

Cytochrome P450: major player in reperfusion injury

While attention has historically focused on mitochondria as the primary source of ROS in myocardial ischemia/reperfusion injury, recent evidence has implicated cytochrome P450 monooxygenases (CYPs) as a significant factor. CYPs represent a large family of enzymes that catalyze the oxidation of endogenous and exogenous compounds. They catalyze arachidonic acid oxidation to a variety of biologically active eicosanoids that regulate ion channels and protein kinases, with effects on vasomotor tone and cardiac inotropy. They also represent a significant source of reactive oxygen species that may target cellular homeostatic mechanisms and mitochondria. In this review, we will consider the contribution of cytochrome P450 enzymes to reperfusion injury and will speculate on whether the mechanism of injury is due to CYP-mediated ROS production or arachidonic acid metabolites.

Identification of a putative calcium-binding protein as a dioxin-responsive gene in zebrafish and rainbow trout

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a widespread environmental contaminant that causes multiple effects in vertebrates. TCDD elicits its toxicity through aryl hydrocarbon receptor (AhR)-mediated modulation of gene regulation, increasing intracellular free calcium, and inducing calcium-mediated apoptosis in cell culture. Two TCDD-responsive cDNAs, which encode putative calcium-binding proteins, have been isolated from zebrafish and rainbow trout. The zebrafish and rainbow trout sequences are 88% similar to each other at the amino acid level and are orthologs of the human S100A4 calcium-binding protein. In zebrafish liver cell culture, treatment with TCDD increases S100A4a mRNA abundance. In juvenile rainbow trout, S100A4 mRNA was constitutively expressed in the heart, kidney, intestine, and spleen, but not in the liver. Exposure to TCDD significantly increased rainbow trout S100A4 mRNA abundance in the rainbow trout kidney. Taken together, these findings demonstrate in zebrafish and rainbow trout that dioxin increases expression of this EF-hand calcium-binding protein gene in a tissue-dependent fashion. However, demonstration that the encoded S100A4 proteins actually bind calcium and play a role in dioxin toxicity will require further study.