Toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin Research Articles

2,3,7,8-Tetrachlorodibenzo-p-dioxin Induces Gimap5 Expression to Ameliorate Concanavalin-Induced T Cell-Mediated Liver Injury

Abstract GTPase of the immune-associated protein 5 (Gimap5) has been implicated in immune cell maintenance and development. A mouse model comparable to autoimmune hepatitis in humans was used to study the alterations induced in the gene expression profile of immune cells upon administration of environmental toxin 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD). In this study, we tested the hypothesis that TCDD is capable of modulating gene expression to ameliorate autoimmune hepatitis severity. Mice were injected intravenously with 12.5 mg/kg concanavalin A (ConA) and treated intraperitoneally one hour after challenge with vehicle or 10 μg/kg TCDD. TCDD-treated mice showed lower levels of liver injury enzymes, alanine aminotransferase (ALT) and aspartate aminotransferase (AST), as compared to vehicle-treated mice. Histopathological analysis revealed a reduction in liver damage in the TCDD-treated group as compared to the vehicle-treated control. Furthermore, flow cytometric analysis of liver-infiltrating mononuclear cells demonstrated that following treatment with TCDD, there was an increase in FoxP3+CD4+ Tregs as well as a decrease in proinflammatory Tbet+ Th1 and RORγt+ Th17 T helper cell subtypes after induction of the disease. Single cell RNA-sequencing was conducted on liver mononuclear cells to further assess the changes induced by TCDD. The gene expression profile showed that TCDD treatment significantly upregulated the expression of vital genes such as Gimap5, a maintenance gene for normal liver function and survival of T cells. In summary, our data suggests that TCDD is capable of inducing Gimap5 expression in autoimmune hepatitis, which in turn modulates the progression of this disease.

Specific Caleosin/Peroxygenase and Lipoxygenase Activities Are Tissue-Differentially Expressed in Date Palm (Phoenix dactylifera L.) Seedlings and Are Further Induced Following Exposure to the Toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Two caleosin/peroxygenase isoforms from date palm, Phoenix dactylifera L., PdCLO2 and PdCLO4, were characterized with respect to their tissue expression, subcellular localization, and oxylipin pathway substrate specificities in developing seedlings. Both PdCLO2 and PdCLO4 had peroxygenase activities that peaked at the mid-stage (radicle length of 2.5 cm) of seedling growth and were associated with the lipid droplet (LD) and microsomal fractions. Recombinant PdCLO2 and PdCLO4 proteins heterologously expressed in yeast cells were localized in both LD and microsomal fractions. Each of the purified recombinant proteins exhibited peroxygenase activity but they were catalytically distinct with respect to their specificity and product formation from fatty acid epoxide and hydroxide substrates. We recently showed that date palm CLO genes were upregulated following exposure to the potent toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Hanano et al., 2016), and we show here that transcripts of 9- and 13-lipoxygenase (LOX) genes were also induced by TCDD exposure. At the enzyme level, 9-LOX and 13-LOX activities were present in a range of seedling tissues and responded differently to TCDD exposure, as did the 9- and 13-fatty acid hydroperoxide reductase activities. This demonstrates that at least two branches of the oxylipin pathway are involved in responses to the environmental organic toxin, TCDD in date palm.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) enhances placental inflammation

Preterm birth is a leading cause of perinatal morbidity and mortality that is often associated with ascending infections from the lower genital tract. Recent studies with animal models have suggested that developmental exposure to the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can increase the risk of preterm birth in the offspring. How TCDD may modify placental immunity to ascending infections is unclear. Therefore, we studied the effects of TCDD treatment on basal and Escherichia coli-stimulated cytokine production by placental explants. Cultures of second-trimester placentas were treated with up to 40nM TCDD for 72h and then stimulated with 107CFU/ml E. coli for an additional 24h. Concentrations of cytokines and PGE2 were measured in conditioned medium by immunoassay. TCDD exposure increased mRNA levels of IL-1β by unstimulated cultures, but no effects on protein levels of this cytokine were detected. TNF-α production was unaffected by TCDD for unstimulated cultures, but pre-treatment with 40nM TCDD significantly increased E. coli-stimulated TNF-α production. Both basal and bacteria-stimulated PGE2 and COX-2 gene expression were enhanced by TCDD pretreatment. In contrast, production of the anti-inflammatory cytokine, IL-10, was reduced by TCDD pretreatment for both unstimulated and E. coli-stimulated cultures. No effect of TCDD on the viability of the cultures was detected. These results suggest that TCDD exposure may shift immunity to enhance a proinflammatory phenotype at the maternal–fetal interface that could increase the risk of infection-mediated preterm birth.

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.

Estrogen and TCDD Influence RLN2 Gene Activity in Estrogen Receptor‐Positive Human Breast Cancer Cells

Expression of relaxin is increased in human breast cancer, and relaxin was shown to increase in vitro invasiveness through increased production and secretion of matrix metalloproteinases in human breast cancer cells. The role of estrogen in the promotion of breast cancer is well-known. The environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a known carcinogen but has been shown to have antiestrogenic effects in human breast cancer cells. In this study, we have employed real-time PCR and chromatin immunoprecipitation (ChIP) assays to investigate the influence of estrogen and TCDD on relaxin-1 (RLN1) and relaxin-2 (RLN2) gene expression in MCF-7 and T47D human breast cancer cells. Estrogen increased RLN2 transcripts in T47D and MCF-7 cells after just 4 h of exposure, whereas TCDD did not. RLN1 transcripts were only induced after 24 h of estrogen exposure. TCDD did have antiestrogenic activity and reduced the estrogen-mediated increase in RLN2 and RLN1 mRNA. The estrogen-mediated increase in RLN2 mRNA levels was not caused by changes in the mRNA stability. ChIP analysis revealed binding of estrogen receptor-alpha (ERalpha) to promoter sequences of the RLN2 gene. Thus, we provide evidence that RLN2 gene activity is directly regulated by activated ERalpha in human breast cancer cells and we show that activation of the arylhydrocarbon receptor by TCDD inhibits this regulation by estrogen.

Mitochondrial P450-dependent arachidonic acid metabolism by TCDD-induced hepatic CYP1A5; conversion of EETs to DHETs by mitochondrial soluble epoxide hydrolase

Several P450 enzymes localized in the endoplasmic reticulum and thought to be involved primarily in xenobiotic metabolism, including mouse and rat CYP1A1 and mouse CYP1A2, have also been found to translocate to mitochondria. We report here that the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces enzymatically active CYP1A4/1A5, the avian orthologs of mammalian CYP1A1/1A2, in chick embryo liver mitochondria as well as in microsomes. P450 proteins and activity levels (CYP1A4-dependent 7-ethoxyresorufin-O-deethylase and CYP1A5-dependent arachidonic acid epoxygenation) in mitochondria were 23–40% of those in microsomes. DHET formation by mitochondria was twice that of microsomes and was attributable to a mitochondrial soluble epoxide hydrolase as confirmed by Western blotting with antiEPHX2, conversion by mitochondria of pure 11,12 and 14,15-EET to the corresponding DHETs and inhibition of DHET formation by the soluble epoxide hydrolase inhibitor, 12(-3-adamantan-1-yl-ureido)-dodecanoic acid (AUDA). TCDD also suppressed formation of mitochondrial and microsomal 20-HETE. The findings newly identify mitochondria as a site of P450-dependent arachidonic acid metabolism and as a potential target for TCDD effects. They also demonstrate that mitochondria contain soluble epoxide hydrolase and underscore a role for CYP1A in endobiotic metabolism.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and long term immunologic memory.

The highlighted article by B. Paige Lawrence and Beth Vorderstrasse addresses an oft forgotten aspect of immunotoxicity, the effects of environmental toxins on immunologic memory. Here, the authors take a step towards filling that information gap by evaluating the effects of a prototypic environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), on memory responses to a real-world pathogen, influenza A virus, presented to an animal model in a physiologically relevant manner. Multiple outcomes are evaluated, the vast majority of which suggest important and long-term TCDD-induced changes in the immune system after both primary and secondary exposure to this pathogen. The implications of these studies with regard to the immuno-competence of TCDD-exposed individuals are far reaching.

2,3,7,8-Tetrachlorodibenzo-p-dioxin Alters the Differentiation Pattern of Human Keratinocytes in Organotypic Culture

Human exposure to the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces a severe skin pathology known as chloracne. In these studies we employed a three-dimensional, organotypic model system to study the effects of TCDD on human skin. This model uses the spontaneously immortalized human keratinocyte cell line NIKS and recapitulates both the three-dimensional microenvironment and epithelial–mesenchymal interactions found in intact human skin. Treatment of the organotypic cultures with TCDD causes alterations in the pattern of keratinocyte terminal differentiation. Analysis at both the light and electron microscope levels reveals a fully differentiated cornified layer in TCDD-treated organotypic cultures at earlier time points than observed in vehicle (dimethyl sulfoxide)-treated controls. Furthermore, TCDD-treated organotypic cultures exhibit aberrant distribution of several differentiation-specific protein markers. Basal cells in TCDD- and DMSO-treated organotypic cultures show no differences in proliferation as measured by quantification of 5-bromo-2′-deoxyuridine (BrdU)-positive nuclei. No aberrant BrdU uptake was detected outside of the basal layer. Neither TUNEL labeling nor immunohistochemical staining with an antibody to active caspase-3 revealed increased apoptosis in TCDD-treated organotypic cultures relative to controls. These data clearly indicate that TCDD modulates homeostasis in a model of human stratifying epithelium independent of changes in proliferation and apoptosis, exclusively by impacting keratinocyte terminal differentiation. This TCDD-induced effect on differentiation-specific proteins results in profound changes in the tissue architecture.

Ventricular Preexcitation Sensitive to Flecainide in Late Stage Chick Embryo ECGs: 2,3,7,8-Tetrachlorodibenzo-p-dioxin Impairs Inotropic but Not Chronotropic or Dromotropic Responses to Isoproterenol and Confers Resistance to Flecainide

ECGs free of movement artefacts were obtained without anesthesia in 16- to 18-day-old chick embryos close to hatching and used to study the effect of the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on cardiac rhythm and conduction. The ECGs of normal late stage chick embryos exhibited short PR intervals, frequent nonisoelectric PR segments, delta waves, and inverted T waves. Those ECG characteristics are found in patients with the Wolff–Parkinson–White syndrome (WPW) in which they reflect ventricular preexcitation associated with the use of accessory conduction pathways and arrhythmias. Isoproterenol (30 μg/egg) did not alter the ECG preexcitation characteristics. Flecainide, a sodium channel blocker used clinically to suppress WPW accessory pathway activity, at 0.5 to 5 mg per egg diminished the preexcitation and caused atrioventricular (AV) block, supporting the use of accessory pathways together with AV-nodal conduction in normal late stage chick embryos. The findings challenge the dogma that accessory pathways are entirely replaced by AV conduction pathways in late fetal development. TCDD, at 1–2 nmol per egg for 48 h, did not affect heart rate, the increase in heart rate by isoproterenol, or the ECG characteristics, suggesting that short-term TCDD treatment did not affect sinus node function or cardiac conduction. The latter results taken together with prior findings indicate that TCDD differentially impairs the inotropic and lusitropic effects but not the chronotropic or dromotropic effects of isoproterenol. In TCDD-treated embryos, flecainide, tested at 5 mg per egg, caused much less inhibition of preexcitation or production of AV block than in the untreated or solvent-treated controls. The resistance to flecainide represents a new TCDD effect consistent with the reported increase of cardiac myocyte [Ca2+]i by TCDD treatment.

Mechanisms of inhibitory aryl hydrocarbon receptor-estrogen receptor crosstalk in human breast cancer cells.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that forms a functional heterodimeric complex with the AhR nuclear translocator (Arnt) protein. The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a high affinity ligand for the AhR and has been extensively used to investigate AhR-mediated biochemical and toxic responses. TCDD modulates several endocrine pathways including inhibition of 17beta-estradiol-induced responses in the immature and ovariectomized rodent uterus and mammary gland and in human breast cancer cell lines. TCDD inhibits formation and growth of mammary tumors in carcinogen-induced rodent models and relatively nontoxic selective AhR modulators (SAhRMs) are being developed for treatment of breast cancer. The mechanisms of inhibitory AhR-estrogen receptor (ER) crosstalk have been investigated in MCF-7 breast cancer cells by analysis of promoter regions of genes induced by E2 and inhibited by TCDD. AhR-mediated inhibition of E2-induced cathepsin D, pS2, c-fos, and heat shock protein 27 gene expression involves direct interaction of the AhR complex with inhibitory pentanucleotide (GCGTG) dioxin responsive elements (iDREs) resulting in disruption of interactions between proteins binding DNA elements required for ER action and the basal transcription machinery. Mechanisms of inhibitory AhR-ER crosstalk indicate that functional iDREs are required for inhibition of some genes; however, results indicate that other interaction pathways are important including AhR-mediated proteasome-dependent degradation of the ER.

Molecular Cloning and Expression of Two Novel Avian Cytochrome P450 1A Enzymes Induced by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Transcriptional regulation by the aryl hydrocarbon receptor, for which the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent ligand, leads in mammalian liver to the induction of genes for two distinct cytochrome P450 (CYP)1A enzymes, CYP1A1 and -1A2. Fish seem to have only one CYP1A enzyme. CYP1A enzymes have been regarded as injurious largely because of their ability to activate chemical carcinogens. We report here the cloning and sequencing of cDNAs for two catalytically distinct TCDD-induced CYP enzymes in chick embryo liver. One mediates classic CYP1A1 activities. The other has some -1A2-like activities and is also responsible for TCDD-induced arachidonic acid epoxygenation, a much more conspicuous effect in liver of chicks than of mammalian species. Amino acid sequence analysis shows that although each chick enzyme can be classified in the CYP1A family, both are more like CYP1A1 than -1A2, and neither can be said to be directly orthologous to CYP1A1 or -1A2. Phylogenetic analysis shows that the two chick enzymes form a separate branch in the CYP1A family tree distinct from mammalian CYP1A1 and -1A2 and from fish CYP1A enzymes. The findings suggest that CYP1A progenitors split into two CYP enzymes with some parallel functions independently in two evolutionary lines, evidence for convergent evolution in the CYP1A family. Northern analysis shows that the chick enzymes have a different tissue distribution from CYP1A1 and -1A2. Polymerase chain reaction and in situ hybridization data show that both chick enzymes are expressed in response to TCDD even before organ morphogenesis. The findings further suggest that beyond their role in activating carcinogens, CYP1A enzymes have conferred evolutionary and developmental advantages, perhaps as defenses in maintaining homeostatic responses to toxic chemicals.

Detection and characterization of the Ah receptor for 2,3,7,8-tetrachlorodibenzo- p-dioxin in the human colon adenocarcinoma cell line LS180

The Ah (aromatic hydrocarbon) receptor mediates induction of aryl hydrocarbon hydroxylase (AHH; an enzyme activity associated with cytochrome P450IA1) by polycyclic aromatic hydrocarbon carcinogens such as 3-methylcholanthrene (MC) and benzo[ a]pyrene (BP) and the halogenated toxin 2,3,7,8-tetrachlorodibenzo- pdioxin (TCDD). Until recently the AhR seemed to be present only at very low levels in human cells and tissue. With a modified assay (the presence of sodium molybdate and a reduction in the amount of charcoal used to adsorb “excess” ligand) we found that cytosol from LS180 cells contains a high concentration of AhR (400–500 fmol/mg cytosolic protein) when detected by [ 3H]TCDD or [ 3H]MC. Cytosolic receptor also was detected with [ 3H]BP but at a level that was 35% of that detected with [ 3H]TCDD or [ 3H]MC. These levels are similar to those found in mouse Hepa-1 hepatoma cells in which AhR has been extensively characterized. The apparent binding affinity ( K d ) of the cytosolic receptor for [ 3H]TCDD and for [ 3H]MC was about 5 n M. As with Hepa-1, the human LS180 cytosolic AhR sedimented at about 9 S on sucrose gradients when detected with [ 3H]TCDD, [ 3H]BP or [ 3H]MC. The nuclear-associated ligand · receptor complex recovered from cells incubated in culture with [ 3H]TCDD sedimented at about 6.2 S. The 9.8 S cytosolic form corresponds to a multimeric protein of a relative molecular mass ( M r) of about 285,000 whereas the 6.2 S nuclear receptor corresponds to a multimeric protein of M r 175,000. The smallest specific ligand-binding subunit (detected by sodium dodecyl sulfate-polyacrylamide electrophoresis under denaturing conditions of receptor photoaffinity labeled with [ 3H]TCDD) was about M r 110,000. AHH activity was induced in cells exposed in culture to TCDD or benz[ a]anthracene (BA). The EC 50 was 4 × 10 −10 10 m for TCDD and 1.5 × 10 −5 m for BA. For both inducers the EC 50 in LS180 cells was shifted about one log unit to the right as compared to the EC 50 for AHH induction in mouse Hepa-1 cells. The lower sensitivity of the LS180 cells to induction of AHH activity by TCDD or BA is consistent with the lower affinity of TCDD and MC for binding to human AhR. The ligand-binding properties, physicochemical properties, and mode of action of the AhR in this human cell line are therefore very similar to those of the extensively characterized AhR in rodent cells and tissues.

2,3,7,8-Tetrachlorodibenzo- p-dioxin increases reliance on fats as a fuel source independently of diet: Evidence that diminished carbohydrate supply contributes to dioxin lethality

The environmental toxin, 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) causes a wasting syndrome marked by hypophagia, loss of body fat, changes in intermediary metabolism and death. Use of conventional laboratory animals has not resolved whether or not TCDD affects intermediary metabolism independently of hypophagia. We used the chick embryo, which does not require an exogenous food supply for energy, to answer this question. Our results show that TCDD treatment increases dependence on fats as a fuel source independently of changes in food intake and therefore can affect intermediary metabolism independently of hypophagia. Results of experiments using aminocarnitine to inhibit fatty acid oxidation suggest that TCDD treatment impairs carbohydrate production rather than its utilization and that the former effect contributes to TCDD lethality.

Effects of TCDD on embryonic ureteric epithelial EGF receptor expression and cell proliferation

The potent toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, producing hydronephrosis and cleft palate. Because of the long half-life of TCDD, the urinary tract is exposed throughout development after a single dose on gestation day (GD) 10 or earlier. TCDD-induced hydronephrosis is a consequence of occlusion of the ureter by epithelial cells. Since embryonic growth factors and the epidermal growth factor (EGF) receptor are probably involved in regulation of embryonic cell proliferation, this study examines the effects of TCDD on expression of EGF receptors and proliferation of ureteric epithelial cells in vivo and in culture. After exposure to TCDD by gavage (12, 24, or 30 micrograms/kg on GD 10; 6 or 24 micrograms/kg on GD 12) the mean cell depth of the ureteric and bladder epithelia was increased. EGF receptors were detected immunohistochemically in sectioned urinary tracts. The expression of receptors decreased with advancing development in control ureteric epithelia. However, after TCDD exposure the level of EGF receptors failed to decline. The incorporation of 3H-TdR was observed in sections by autoradiography, and after exposure to TCDD more epithelial cells showed incorporation than was apparent in controls. Transmission electron microscopy (TEM) of embryonic ureters from fetuses exposed to TCDD in vivo showed no cytotoxicity in basal cells and the cells remained undifferentiated, as in controls. Ureters taken from GD 12 embryos and cultured with 1 x 10(-10)M TCDD showed ureteric epithelial hyperplasia without cytotoxicity, but at 1 x 10(-8)M TCDD evidence of cytotoxicity was observed by TEM. The levels of TCDD found in fetuses after in vivo exposure (204-307 pg/fetus, with 1-2 pg in the urinary tract) compare well with the in vitro level (32 pg/ml), which was most effective in producing hyperplasia of the epithelial cells. The present study correlates a TCDD-induced increase in cell depth with altered regulation of EGF receptors and excessive proliferation, both in vivo and in cultured embryonic ureters.

Alterations in the expression of specific epidermal keratin markers in the hairless mouse by the topical application of the tumor promoters 2,3,7,8-tetrachlorodibenzo-p-dioxin and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate

The potent toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes chloracne and acts as a tumor promoter in the hairless HRS/J mouse model. In the present study we characterized changes in mouse epidermal keratin expression following a single topical application of TCDD to the skin of hairless (hr/hr) and haired (hr/+) HRS/J mice. Morphologic changes and alterations in keratin biosynthesis following TCDD treatment were compared with those induced by the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Both TPA and TCDD induced dose-dependent epidermal hyperplasia in hr/hr and hr/+ mice and this was associated with altered keratin subunit expression. In hr/hr mice TCDD caused a pattern of keratin expression that was similar to TPA, characterized by a marked decrease in the synthesis of the Mr 67,000 (basic) and 59,000 (acidic) keratins that are specific markers for suprabasal differentiation in the epidermis. In addition, the synthesis of an acidic keratin of Mr 48,000 and a basic keratin of Mr 62,000 was also decreased. Concomitantly, TCDD caused an increase in the synthesis of a basic keratin of Mr 60,000 and acidic keratins of Mr 54,000, 52,000 and 49,000 that are normally observed in proliferating basal cells and primary epidermal cell cultures. In contrast, while TPA induced similar changes in keratinization in both the hr/+ and hr/hr mice, TCDD-induced hyperplasia in hr/+ mice was only associated with changes in keratin synthesis reflecting increased basal cell proliferation. These results demonstrate that a single application of TCDD to the skin alters the normal pattern of epidermal differentiation in the hr/hr mouse. Molecular events influencing the expression of the keratin genes associated with this process may be linked to the strain- and/or species-specific toxicity of TCDD.

The absence of hepatic cellular anomalies in TCDD-exposed beach mice - a field study

Quantitative ultrastructural studies were conducted on liver tissue from beach mice, Peromyscus polionotus, exposed to the toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a field environment. Hepatic tissue from 30 animals was examined for changes in smooth endoplasmic reticulum, rough endoplasmic reticulum and mitochondria. Fifteen animals were collected from a unique military test site in northwest Florida, where they had been continuously exposed to soil levels of < 10 to 710 parts per trillion (ppt) TCDD. The remaining animals were from a control field not exposed to TCDD. All tissue was examined both histopathologically and by an ultrastructural stereological technique. The mean levels of TCDD in composite liver samples from exposed mice collected in the field varied from 960 ppt for females to 1,300 ppt for males. The levels of TCDD in the livers of the beach mice collected from the field substantiated the bioaccumulation of TCDD. Although the levels of TCDD in the livers were greater than those found in the soil, TCDD was not detected in the portion of the food chain consisting of seeds. No significant histopathological or ultrastructural changes were found in hepatic parenchymal cells after long-term, low-level exposure to TCDD in the field. The levels of TCDD encountered in this study, though much lower than those normally found in most laboratory experiments, are far greater than the estimated concentrations in the environment, or in animal tissues after normal application of the herbicide 2,4,5-T.