2,3,7,8-Tetrachlorodibenzo-p-dioxin toxicity in yellow perch (Perca flavescens)

Assessing the Toxicity of Benzotriazole Ultraviolet Stabilizers to Fishes: Insights into Aryl Hydrocarbon Receptor-Mediated Effects

Benzotriazole ultraviolet stabilizers (BUVSs) are chemicals used to mitigate UV-induced damage to manufactured goods. Their presence in aquatic environments and biota raises concerns, as certain BUVSs activate the aryl hydrocarbon receptor (AhR), which is linked to adverse effects in fish. However, potencies of BUVSs as AhR agonists and species sensitivities to AhR activation are poorly understood. This study evaluated the toxicity of three BUVSs using embryotoxicity assays. Zebrafish (Danio rerio) embryos exposed to BUVSs by microinjection suffered dose-dependent increases in mortality, with LD50 values of 4772, 11 608, and 56 292 ng/g-egg for UV-P, UV-9, and UV-090, respectively. The potencies and species sensitivities to AhR2 activation by BUVSs were assessed using a luciferase reporter gene assay with COS-7 cells transfected with the AhR2 of zebrafish and eight other fishes. The rank order of potency for activation of the AhR2 from all nine species was UV-P > UV-9 > UV-090. However, AhR2s among species differed in sensitivities to activation by up to 100-fold. An approximate reversed rank order of species sensitivity was observed compared to the rank order of sensitivity to 2,3,7,8-tetrachlorodibenzo[p]dioxin, the prototypical AhR agonist. Despite this, a pre-existing quantitative adverse outcome pathway linking AhR activation to embryo lethality could predict embryotoxicities of BUVSs in zebrafish.

Immunotoxicity of Xenobiotics in Fish: A Role for the Aryl Hydrocarbon Receptor (AhR)?

The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.

Aryl Hydrocarbon Receptor Signaling Is Functional in Immune Cells of Rainbow Trout (Oncorhynchus mykiss)

The arylhydrocarbon receptor (AhR) is an important signaling pathway in the immune system of mammals. In addition to its physiological functions, the receptor mediates the immunotoxic actions of a diverse range of environmental contaminants that bind to and activate the AhR, including planar halogenated aromatic hydrocarbons (PHAHs or dioxin-like compounds) and polynuclear aromatic hydrocarbons (PAHs). AhR-binding xenobiotics are immunotoxic not only to mammals but to teleost fish as well. To date, however, it is unknown if the AhR pathway is active in the immune system of fish and thus may act as molecular initiating event in the immunotoxicity of AhR-binding xenobiotics to fish. The present study aims to examine the presence of functional AhR signaling in immune cells of rainbow trout (Oncorhynchus mykiss). Focus is given to the toxicologically relevant AhR2 clade. By means of RT-qPCR and in situ hybdridization, we show that immune cells of rainbow trout express ahr 2α and ahr 2β mRNA; this applies for immune cells isolated from the head kidney and from the peripheral blood. Furthermore, we show that in vivo as well as in vitro exposure to the AhR ligand, benzo(a)pyrene (BaP), causes upregulation of the AhR-regulated gene, cytochrome p4501a, in rainbow trout immune cells, and that this induction is inhibited by co-treatment with an AhR antagonist. Taken together, these findings provide evidence that functional AhR signaling exists in the immune cells of the teleost species, rainbow trout.

Aryl hydrocarbon receptor nuclear translocators (ARNT1, ARNT2, and ARNT3) of white sturgeon (Acipenser transmontanus): Sequences, tissue-specific expressions, and response to β-naphthoflavone

Sturgeons (Acipenseridae) are ancient fishes that have tissue-specific profiles of transcriptional responses to dioxin-like compounds (DLCs) that are unique from those generally measured in teleost fishes. Because DLCs exert their critical toxicities through activation of the aryl hydrocarbon receptor (AHR), this transcription factor has been the subject of intensive study. However, less attention has focused on the aryl hydrocarbon receptor nuclear translocator (ARNT), which is the dimerization partner of the AHR and required for AHR-mediated transcription. The present study sequenced ARNT1, ARNT2, and ARNT3 in a representative species of sturgeon, the white sturgeon (Acipenser transmontanus), and quantified tissue-specific basal transcript abundance for each ARNT and the response following exposure to the model agonist of the AHR, β-naphthoflavone. In common with other proteins in sturgeons, the amino acid sequences of ARNTs are more similar to those of tetrapods than are ARNTs of other fishes. Transcripts of ARNT1, ARNT2, and ARNT3 were detected in all tissues investigated. Expression of ARNTs are tightly regulated in vertebrates, but β-naphthoflavone caused down-regulation in liver and up-regulation in gill, while an upward trend was measured in intestine. ARNTs are dimeric partners for multiple proteins, including the hypoxia inducible factor 1α (HIF1α), which mediates response to hypoxia. A downward trend in abundance of HIF1α transcript was measured in liver of white sturgeon exposed to β-naphthoflavone. Altered expression of ARNTs and HIF1α caused by activation of the AHR might affect the ability of certain tissues in sturgeons to respond to hypoxia when co-exposed to DLCs or other agonists.

A critical review of histopathological findings associated with endocrine and non-endocrine hepatic toxicity in fish models

Although frequently examined as a target organ for non-endocrine toxicity, histopathological evaluation of the liver is becoming a routine component of endocrine disruption studies that utilize various fish species as test subjects. However, the interpretation of microscopic liver findings can be challenging, especially when attempting to distinguish adverse changes associated with endocrine disrupting substances from those caused by systemic or direct hepatic toxicity. The purpose of this project was to conduct a critical assessment of the available peer-reviewed and grey literature concerning the histopathologic effects of reproductive endocrine active substances (EAS) and non-endocrine acting substances in the livers of fish models, and to determine if liver histopathology can be used to reliably distinguish endocrine from non-endocrine etiologies. The results of this review suggest that few compound-specific histopathologic liver effects have been identified, among which are estrogen agonist-induced increases in hepatocyte basophilia and proteinaceous intravascular fluid in adult male teleosts, and potentially, decreased hepatocyte basophilia in female fish exposed to substances that possess androgenic, anti-estrogenic, or aromatase inhibitory activity. This review also used published standardized methodology to assess the credibility of the histopathology data in each of the 117 articles that reported liver effects of treatment, and consequently it was determined that in only 37% of those papers were the data considered either highly credible or credible. The outcome of this work highlights the value of histopathologic liver evaluation as an investigative tool for EAS studies, and provides information that may have implications for EAS hazard assessment.

The development of cellular immune defence in marine medaka Oryzias melastigma

Environmentally induced alterations of the immune system during sensitive developmental stages may manifest as abnormalities in immune organ configuration and/or immune cell differentiation. These not only render the early life stages more vulnerable to pathogens, but may also affect the adult immune competence. Knowledge of these sensitive periods in fish would provide an important prognostic/diagnostic tool for aquatic risk assessment of immunotoxicants. The marine medaka Oryzias melastigma is an emerging seawater fish model for immunotoxicology. Here, the presence and onset of four potentially sensitive periods during the development of innate and adaptive cellular immune defence were revealed in O. melastigma: 1.) initiation of phagocyte differentiation, 2.) migration and expansion of lymphoid progenitor cells, 3.) colonization of immune organs through lymphocyte progenitors and 4.) establishment of immune competence in the thymus. By using an established bacterial resistance assay for O. melastigma, larval immune competence (from newly hatched 1dph to 14dph) was found concomitantly increased with advanced thymus development and the presence of mature T-lymphocytes. A comparison between the marine O. melastigma and the freshwater counterpart Oryzias latipes disclosed a disparity in the T-lymphocyte maturation pattern, resulting in differences in the length of T-lymphocyte maturation. The results shed light on a potential difference between seawater and freshwater medaka in their sensitivity to environmental immunotoxicants. Further, medaka immune system development was compared and contrasted to economically important fish. The present study has provided a strong scientific basis for advanced investigation of critical windows for immune system development in fish.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) role in hematopoiesis and in hematologic diseases: A critical review

Dioxin exposure and its effect on hematopoiesis and cancer have been largely investigated in both human and non-human settings. Here we systematically reviewed literature to address the question of what we know about TCDD biology and exposure. Most effects are due to TCDD interaction with a receptor of xenobiotics called AHR, which is ubiquitously represented and also works on hematopoietic myeloid and lymphoid stem cells, inducing proliferation and stem cell release from bone marrow to peripheral circulation. Epidemiologic studies on TCDD exposure demonstrated an association with onco-hematologic diseases, particularly with non Hodgkin lymphomas and multiple myeloma, and non hematologic cancers, such as sarcomas, although these relationships are affected by multiple confounding factors.

High Conservation in Transcriptomic and Proteomic Response of White Sturgeon to Equipotent Concentrations of 2,3,7,8-TCDD, PCB 77, and Benzo[a]pyrene

Adverse effects associated with exposure to dioxin-like compounds (DLCs) are mediated primarily through activation of the aryl hydrocarbon receptor (AHR). However, little is known about the cascades of events that link activation of the AHR to apical adverse effects. Therefore, this study used high-throughput, next-generation molecular tools to investigate similarities and differences in whole transcriptome and whole proteome responses to equipotent concentrations of three agonists of the AHR, 2,3,7,8-TCDD, PCB 77, and benzo[a]pyrene, in livers of a nonmodel fish, the white sturgeon (Acipenser transmontanus). A total of 926 and 658 unique transcripts were up- and down-regulated, respectively, by one or more of the three chemicals. Of the transcripts shared by responses to all three chemicals, 85% of up-regulated transcripts and 75% of down-regulated transcripts had the same magnitude of response. A total of 290 and 110 unique proteins were up- and down-regulated, respectively, by one or more of the three chemicals. Of the proteins shared by responses to all three chemicals, 70% of up-regulated proteins and 48% of down-regulated proteins had the same magnitude of response. Among treatments there was 68% similarity between the global transcriptome and global proteome. Pathway analysis revealed that perturbed physiological processes were indistinguishable between equipotent concentrations of the three chemicals. The results of this study contribute toward more completely describing adverse outcome pathways associated with activation of the AHR.

Developmental toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in artificially fertilized crucian carp (Carassius auratus) embryo

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent bioaccumulative environmental contaminant that is an endocrine disruptor. Embryos of various fish species are responsive to TCDD and have been used as an alternative method to the acute toxicity test with juvenile and adult fish. The TCDD test has similar endpoints of developmental toxicity. However, their sensitivity and signs of TCDD-induced toxicity are different depending on fish species and its habit. Crucian carp (Carassius auratus) - the sentinel species for persistent organic pollutants and a common foodfish in China, Japan, and Korea - was used to identify the developmental toxicity of TCDD. We obtained the fertilized eggs from the artificial fertilization of crucian carp (97.45% success rate). Embryos at 3h post fertilization (hpf) were exposed to no vehicle, vehicle (dimethylsulfoxide, 0.1% v/v) or TCDD (0.128, 0.32, 0.8, 2 and 5 μg/L) for 1h and then fresh water was changed and aerated. Embryonic development and toxicity were monitored until 150 hpf. TCDD-exposed group showed no effects on embryo mortality and hatching rate from 6 to 126 hpf. On the other hand, the post-hatching mortality rate in TCDD-exposed group was increased in a dose-dependent manner, especially at high doses (0.8, 2 and 5 μg/L). The LD50 for larval mortality was calculated to 0.24 ng TCDD/g embryo. Pericardial edema was continuously observed in larvae of TCDD-exposed groups from hatching complete time (78 hpf), followed by the onset of yolk sac edema. Hemorrhage and edema showed a significant increase depending on exposure concentration and time. Expression of TCDD-related CYP1A genes was evaluated quantitatively. Embryo and larvae in TCDD-exposed groups displayed a significant increase of CYP1A gene expression. Overall, we defined TCDD-induced toxicity in artificially fertilized crucian carp embryo and these results suggest that crucian carp can be applied as an early life stage model of TCDD-induced 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.

Gene expression and pathologic alterations in juvenile rainbow trout due to chronic dietary TCDD exposure

The goal of this project was to use functional genomic methods to identify molecular biomarkers as indicators of the impact of TCDD exposure in rainbow trout. Specifically, we investigated the effects of chronic dietary TCDD exposure on whole juvenile rainbow trout global gene expression associated with histopathological analysis. Juvenile rainbow trout were fed Biodiet starter with TCDD added at 0, 0.1, 1, 10 and 100 ppb (ngTCDD/g food), and fish were sampled from each group at 7, 14, 28 and 42 days after initiation of feeding. 100 ppb TCDD caused 100% mortality at 39 days. Fish fed with 100 ppb TCDD food had TCDD accumulation of 47.37 ppb (ngTCDD/g fish) in whole fish at 28 days. Histological analysis from TCDD-treated trout sampled from 28 and 42 days revealed that obvious lesions were found in skin, oropharynx, liver, gas bladder, intestine, pancreas, nose and kidney. In addition, TCDD caused anemia in peripheral blood, decreases in abdominal fat, increases of remodeling of fin rays, edema in pericardium and retrobulbar hemorrhage in the 100 ppb TCDD-treated rainbow trout compared to the control group at 28 days. Dose- and time-dependent global gene expression analyses were performed using the cGRASP 16,000 (16K) cDNA microarray. TCDD-responsive whole body transcripts identified in the microarray experiments have putative functions involved in various biological processes including growth, cell proliferation, metabolic process, and immune system processes. Nine microarray-identified genes were selected for QPCR validation. CYP1A3 and CYP1A1 were common up-regulated genes and HBB1 was a common down-regulated gene among each group based on microarray data, and their QPCR validations are consistent with microarray data for the 10 and 100 ppb TCDD treatment groups after 28 days exposure (p<0.05). In addition, in the 100 ppb group at 28 days, expression of complement component C3-1 and trypsin-1 precursor have a more than 10-fold induction from the microarray experiments, and their QPCR validations are consistent and showed significant induction in the 100 ppb group at 28 days (p<0.05). Overall, lesion in nasal epithelium is a novel and significant result in this study, and TCDD-responsive rainbow trout transcripts identified in the present study may lead to the development of new molecular biomarkers for assessing the potential impacts of environmental TCDD on rainbow trout populations.

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.

Persistent adverse effects on health and reproduction caused by exposure of zebrafish to 2,3,7,8-tetrachlorodibenzo-p-dioxin during early development and gonad differentiation

Little is understood regarding the impacts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure during early development on the health, survival, and reproductive capability of adults. Here we use zebrafish to determine whether early life stage exposure to TCDD induces toxicity in adult zebrafish and their offspring. Zebrafish were exposed to graded concentrations of TCDD (0-400 pg/ml) via waterborne exposure for 1 h/week from 0 to 7 weeks of age. The heart and swim bladder were identified as being most sensitive to TCDD exposure during early development. Subtle developmental toxic responses collectively impaired survival, and only zebrafish in the 0, 25, and 50 pg TCDD/ml groups survived to adulthood. Surviving fish exhibited TCDD toxicity in craniofacial structures (i.e., operculum and jaw), heart, swim bladder, and ovary. Exposure to 25 pg TCDD/ml impaired egg production (40% of control), fertility (90% of control), and gamete quality. TCDD-treated males contributed more than females to impaired reproductive capacity. Transgenerational effects were also discovered in that offspring from parents exposed to TCDD during early life stages showed a 25% increase in mortality compared with the F1 of dimethyl sulfoxide fish, reduced egg production (30-50% of control) and fertility (96% of control). Thus, adverse effects resulting from TCDD exposure during early life stages for one generation of zebrafish were sufficient to cause adverse health and reproductive effects on a second generation of zebrafish. In the environment, transgenerational effects such as these may contribute to population declines for the most TCDD sensitive fish species.

Inhibition of follicular development, vitellogenesis, and serum 17beta-estradiol concentrations in zebrafish following chronic, sublethal dietary exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent endocrine disruptor with the ability to affect several biologic processes, including reproduction. In fish, sublethal exposure to TCDD is known to modulate overall reproductive capacity, but impacts on follicular development and vitellogenesis are unknown. Here we show that chronic, dietary exposure to 0.08, 0.32, or 0.80 ng TCDD female(-1) day(-1) decreased egg production by more than 50% and that spawning success was reduced by as much as 96%. Serum estradiol concentrations were decreased more than twofold, accounting, in part, for observed decreases in serum vitellogenin concentrations by as much as 29%. Our data suggest that decreased egg production is likely the result of TCDD-mediated inhibition of the transition from pre-vitellogenic stage follicles to vitellogenic stage follicles, as well as the induction of follicular atresia. The majority of reproductive toxicity of TCDD is likely due to direct impacts on the ovary, yet histopathologic observations suggest liver toxicity could also contribute to observed impacts on vitellogenesis. Importantly, even when overall egg production is not significantly affected, our data show that subtle physiologic changes induced by TCDD can lead to altered gonadogenesis. This suggests that long-term exposure to very low concentrations of TCDD could greatly affect fecundity and reproductive success in fishes.

Stage-specific toxicity of cypermethrin to medaka (Oryzias latipes) eggs and embryos using a refined methodology for an in vitro fertilization bioassay

Using original artificial fertilization methods with medaka (Oryzias latipes), the effects of exposure to cypermethrin on gametes, fertilization, and embryonic development were investigated. The relative sensitivity was studied with 96-hour duration, 24-hour renewal exposures to six nominal concentrations of cypermethrin ranging from 3.1 100.0 microg cypermethrin/L. Tests were initiated at different developmental stages: unfertilized egg (stage 0), late morula (stage 9), eminence of swim bladder (stage 29), and maximum flexion of the atrioventricular region (stage 34). Cypermethrin did not affect the fertilization process. Predominant sublethal effects in embryos included transient visceral edemas intimately associated to the gall bladder with subsequent pericardial edemas. Other sublethal effects were observed in surviving larvae and included spastic movements with or without ability to respond to stimulus (>/=6.3 microg cypermethrin/L), spinal curvatures, and delayed or absence of swim bladder inflation (>/=12.5 microg cypermethrin/L). The exposure of the gametes (stage 0) or animals during earlier embryonic development (stages 9 or 29) was not a critical window for cypermethrin exposure. Although the incidence of edemas in embryos occurred mainly during exposure of these early developmental stages, embryo and larva lethality and the incidence of transient sublethal effects in hatchlings showed that the later exposure window (stage 34) was the most sensitive. The stage 34 group involved advanced organogenetic stages in which the chorion partially degraded before hatching. Our studies reinforced the idea that a combination of morphologic and functional impairment evaluation is a more sensitive response to developmental toxicants than morphologic defects alone.

Histological analysis of acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in zebrafish

Previous studies have demonstrated that acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by injection leads to inhibition of caudal fin regeneration in zebrafish. Since the TCDD exposure in these studies is systemic, it is possible that pathology in organs other than the fin could result in inhibition of fin regeneration. Therefore, histopathology of adult zebrafish (Danio rerio) organs was characterized following abdominal cavity injection of a TCDD dose (70ng/g). The most pronounced histopathologic changes 5 days post-injection included lipidosis and hypertrophy of liver hepatocytes and hypertrophy of gill lamellae. Effects of TCDD exposure on immunolocalization of the zebrafish aryl hydrocarbon receptor nuclear translocator (ARNT2), the heterodimer partner of the aryl hydrocarbon receptor (AHR2), and an AHR regulated gene cytochrome P450 1A (CYP1A) was also determined. ARNT2 was immunolocalized to the gastrointestinal tract, gill lamellae, kidney, ventricle of the heart, caudal fin, brain and liver of zebrafish. TCDD exposure had no measurable effect on ARNT2 abundance or localization. CYP1A was immunolocalized in TCDD exposed fish as a biomarker for cells with an activated AHR pathway. CYP1A was not detected in any tissue from vehicle exposed fish. Significant TCDD-dependent induction of CYP1A was detected in the proximal tubules of the kidney, in liver hepatocytes and in the gastrointestinal tract of TCDD exposed fish. Significant but lower TCDD-dependent CYP1A expression was evident in the gill, caudal fin and ventricle of the heart. Overall, TCDD exposure in adult zebrafish leads to histopathology similar to that reported in other fish species, and it appears unlikely that the histopathology in these organs completely explains the inhibition of fin regeneration.

Sequential morphological changes of erythrocyte apoptosis in Xenopus larvae exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

We previously demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of Xenopus laevis during the early stages of life induces apoptosis in larval erythrocytes (Sakamoto et al., 1997). In the present study, an examination of these cells at the ultrastructural level was undertaken to elucidate the sequential morphological changes that occur during apoptosis. Xenopus embryos were exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin for 5 days shortly after fertilization. The circulating erythrocytes in larvae 12 days after fertilization were examined. Ultramicroscopic studies revealed four roughly defined stages of apoptosis. During the first stage, many small roundish vacuoles begin to appear in the cytoplasm. No noticeable changes can be found in the nucleus. In the second stage, the perinuclear cisterna become dilated, and huge cisternae can be seen in some erythrocytes. The roundish cytoplasmic vacuoles also become larger. Condensation of nuclear chromatin is not yet evident and the erythrocytes still maintain their elliptical shape. During the third stage, chromatin condensation and margination along the nuclear membrane becomes apparent. The nuclear pores gather in the diffuse chromatin region where the perinuclear cisterna is not dilated. The cytoplasm of some erythrocytes also becomes condensed and electron-dense. The normal arrangement of microtubules is disorderly and the erythrocytes deform into a roundish shape. Also, macrophages usually contact some part of the cell. In the final stage, those erythrocytes which show typical nuclear condensation, where neither nuclear or cytoplasmic fragmentation have occurred, are almost or completely phagocytosed by macrophages.

Disease and gill lesions in yellow perch (Perca flavescens) exposed to oil sands mining-associated waters

Adult yellow perch were stocked into experimental ponds designed to test the biological effects of aquatic reclamation alternatives currently being pursued by the oil sands mining industry. Water-quality characteristics of oil sands-influenced water in the experimental ponds included increased salinity and elevated trace organics associated with raw oil sands (bitumen). After 3 and 10 months of exposure to affected waters, perch gross pathologies including severe fin erosion and virally induced tumors were observed in exposed individuals. Gill histopathology revealed large aneurysms accompanied by a proliferation of chloride and epithelial cells in the interlamellar spaces. Gill pathologies were not paralleled by a decrease in plasma sodium, calcium, or chloride. The frequencies of gross pathologies and gill changes were correlated to the concentrations of the oil sands-related compounds. As inorganic and organic compounds associated with oil sands activities are highly intercorrelated, and the observed lesions and changes are not diagnostic of particular toxicants, it was not possible to isolate the causative chemical factor(s) responsible. The incidence of observed lesions and gill pathologies could not be conclusively linked to increased mortality rates observed in the exposed populations. Evidence of recovery in the pathologies was observed between 3 and 10 months of exposure, coincident with a stabilization in population numbers.

Apoptotic cell death of erythrocytes in Xenopus larvae exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Xenopus embryos were exposed to 200 ppb 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 5 days from the 2- to 8-cell stage of cleavage to the early larval stage. Larvae that developed generalized edema were collected at 7 days after the end of TCDD exposure for light and electron microscopic studies. Erythrocytes in the peripheral blood of the edematous larvae were examined. Between 0.3 and 33.9% of identifiable erythrocytes of exposed larvae had dilated perinuclear cisternae. Furthermore, some had extremely condensed nuclear chromatin usually coalesced against 1 pole of the nuclear membrane and overall compacted cytoplasm. The erythrocytes showing nuclear condensation were phagocytosed by macrophages. These features are typical of cells undergoing apoptosis. Anemia is 1 symptom of TCDD toxicity in various animal species, including mammals. In this study, we demonstrate that TCDD induces apoptotic cell death in circulating erythrocytes of Xenopus larvae, which may be 1 cause of anemia in this species.

Dioxinlike properties of a trichloroethylene combustion-generated aerosol

Conventional chemical analyses of incineration by-products identify compounds of known toxicity but often fail to indicate the presence of other chemicals that may pose health risks. In a previous report, extracts from soot aerosols formed during incomplete combustion of trichloroethylene (TCE) and pyrolysis of plastics exhibited a dioxinlike response when subjected to a keratinocyte assay. To verify this dioxinlike effect, the complete extract, its polar and nonpolar fractions, some containing primarily halogenated aromatic hydrocarbons, were evaluated for toxicity using an embryo assay, for antiestrogenicity using primary liver cell cultures, and for the ability to transform the aryl hydrocarbon receptor into its DNA binding form using liver cytosol in a gel retardation assay. Each of these assays detect dioxinlike effects. Medaka (Oryzias latipes) embryos and primary liver cell cultures of rainbow trout (Oncorhynchus mykiss) were exposed to concentrations of extract ranging from 0.05 to 45 micrograms/l. Cardiotoxicity with pericardial, yolk sac, and adjacent peritoneal edema occurred after exposure of embryos to concentrations of 7 micrograms/l or greater. These same exposure levels were associated with abnormal embryo development and, at the higher concentrations, death. Some of the fractions were toxic but none was as toxic as the whole extract. In liver cells, total cellular protein and cellular lactate dehydrogenase activity were not altered by in vitro exposure to whole extract (0.05-25 micrograms/l). However, induction of cytochrome P4501A1 protein and ethoxyresorufin O-deethylase activity occurred. In the presence of whole extract, estradiol-dependent vitellogenin synthesis was reduced. Of the fractions, only fraction 1 (nonpolar) showed a similar trend, although vitellogenin synthesis inhibition was not significant. The soot extract and fractions bound to the Ah receptor and showed a significantly positive result in the gel retardation/DNA binding test. Chemical analyses using GC-MS with detection limits for 2,3,7,8-tetrachlorodibenzo-p-dioxin and dibenzofuran in the picomole range did not show presence of these compounds. Our results indicate that other chemicals associated with TCE combustion and not originally targeted for analysis may also pose health risks through dioxinlike mechanisms.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.