2,3,7,8-Tetrachlorodibenzo-p-dioxin tissue distribution, excretion, and effects on clinical chemical parameters in guinea pigs

Chronic exposure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces an obesogenic effect in C57BL/6J mice fed a high fat diet

Contaminant involvement in the pathophysiology of obesity is widely recognized. It has been shown that low dose and chronic exposure to endocrine disruptor compounds (EDCs) potentiated diet- induced obesity. High and acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a persistent organic pollutant (POP) and an EDC with anti-estrogenic property, causes wasting syndrome . However at lower doses, the TCDD metabolic effects remain poorly understood. We investigated the obesogenic effect during chronic exposure of TCDD at 1μg/kg body weight (bw)/week in adult C57BL/6J mice fed with a high fat diet (HFD) and exposed from 10 to 42 weeks old to TCDD or equal volume of vehicle by intragastric gavage. Under these conditions, TCDD was obesogenic in adult mice (7% in males and 8% in females), which was linked to fat mass. A sex effect was observed in the fat mass distribution in adipose tissue and in the hepatic triglyceride content evolution. In visceral fat pad weight, we observed a decrease (11%) in males and an increase (14%) in females. The hepatic triglyceride content increase (41%) in females only. TCDD failed to induce any change in plasma parameters regarding glucose and lipid homeostasis. Messenger ribonucleic acid (mRNA) levels involved in adipose tissue and hepatic metabolism, inflammation, xenobiotic metabolism and endocrine disruption were differently regulated between males and females. In conclusion, these results provide new evidence that dioxin, a POP and EDC can be obesogenic for adult mice with multi-organ effects.

Evaluation of serum markers of blood redox homeostasis and inflammation in PCB naturally contaminated heifers undergoing decontamination

Dioxins and polychlorinated biphenyls (PCBs) are widely spread and long persistent contaminants. The aim of this study was to evaluate physiological changes associated with the decontamination of animals previously exposed to environmental pollutants. Eight Limousine heifers were removed from a polluted area and fed a standard ration for six months. The extent of contamination was defined by measuring total toxic equivalents (TEQ) values of dioxin like-PCBs (DL-PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs), and NDL-PCBs amount in pericaudal fat two weeks after the removal from the contaminated area (day 0) and then bimonthly for six months during the decontamination (days 59, 125, and 188). The concentrations of both DL-PCBs + PCDD/Fs and NDL-PCBs at the start of decontamination (day 0) were higher than those legally admitted, and they were strongly decreased at the end of the experimental period. Specific indices of blood redox homeostasis and inflammation were also measured at each time. Serum concentrations of Retinol, Tocopherol and Ascorbate, the total antioxidant capacity (TAC) and the activities of superoxide dismutase and glutathione peroxidase were lower at day 0 than after 59, 125 or 188 days of decontamination. Protein-bound carbonyls (PC), nitro-tyrosine (N-Tyr), and lipid hydroperoxides concentrations were higher at day 0 than during decontamination. In addition, TAC, PC and N-Tyr levels correlated with both DL-PCB and NDL-PCB concentrations only at day 0. Serum concentrations of TNF-alpha and Haptoglobin were higher in samples collected at day 0 than in those obtained during decontamination. As Haptoglobin and TNF-alpha levels correlated with both DL-PCB and NDL-PCB concentrations at day 0 and at day 59 (when these concentrations are still over legal limit), they might represent easily measurable parameters for assessing acute exposure to pollutants. Further both N-Tyr and TNF-alpha concentrations could be used as bio-monitoring markers of the decontamination procedure.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on hormones of energy balance in a TCDD-sensitive and a TCDD-resistant rat strain

One of the hallmarks of the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a drastically reduced feed intake by an unknown mechanism. To further elucidate this wasting syndrome, we followed the effects of a single large dose (100 μg/kg) of TCDD on the serum levels of several energy balance-influencing hormones, clinical chemistry variables, and hepatic aryl hydrocarbon receptor (AHR) expression in two rat strains that differ widely in their TCDD sensitivities, for up to 10 days. TCDD affected most of the analytes in sensitive Long-Evans rats, while there were few alterations in the resistant Han/Wistar strain. However, analyses of feed-restricted unexposed Long-Evans rats indicated several of the perturbations to be secondary to energy deficiency. Notable increases in ghrelin and glucagon occurred in TCDD-treated Long-Evans rats alone, which links these hormones to the wasting syndrome. The newly found energy balance regulators, insulin-like growth factor 1 and fibroblast growth factor 21 (FGF-21), appeared to function in concert in body weight loss-induced metabolic state, and FGF-21 was putatively linked to increased lipolysis induced by TCDD. Finally, we demonstrate a reverse set of changes in the AHR protein and mRNA response to TCDD and feed restriction, suggesting that AHR might function also as a physiological regulator, possibly involved in the maintenance of energy balance.

Toxicogenomic evaluation of long-term hepatic effects of TCDD in immature, ovariectomized C57BL/6 mice

Acute exposure to hepatotoxic doses of 2,3,7,8-tetrachloro- dibenzo-p-dioxin (TCDD) in mice is characterized by differential gene expression that can be phenotypically anchored to elevated levels of serum alanine aminotransferase, increased relative liver weights, hepatic steatosis, inflammation, and hepatocellular necrosis. Unlike most studies that focus on acute exposure effects, this study evaluated the long-term effects of a single oral gavage of 30 μg/kg TCDD at 1, 4, 12, 24, 36, and 72 weeks postdose in ovariectomized C57BL/6 mice. Hepatic TCDD levels were almost completely eliminated by 24 weeks with a calculated half-life of 12 days. Hepatic gene expression analysis identified 395 unique differentially expressed genes between 1 and 12 weeks that decreased to ≤ 8 by 72 weeks, consistent with the minimal hepatic TCDD levels. Hepatic vacuolization, characteristic of short-term exposure, subsided by 4 weeks. Similarly, TCDD-elicited hepatic necrosis and inflammation dissipated by 1 week. Collectively, these results suggest that TCDD-elicited histologic and gene expression responses can be correlated to elevated hepatic TCDD levels, which, once eliminated, elicit minimal hepatic gene expression and histologic alterations.

TCDD-elicited effects on liver, serum, and adipose lipid composition in C57BL/6 mice

The aryl hydrocarbon receptor (AhR) mediates alterations in hepatic lipid composition elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to further investigate the effects of TCDD, liver, serum, and gonadal white adipose tissue (gWAT) fatty acid methyl esters (FAMEs) and lipids were examined in fasted 4-week-old female mice orally gavaged with 30 µg/kg TCDD at 24, 72, and 168 h postdose. Mean hepatic FAME levels increased (236.7 µmol/g in controls compared with 392.2 µmol/g in TCDD treated) with minimal changes in gWAT and serum. In the liver, TCDD decreased saturated fatty acids (SFAs 16:0, 18:0, 20:0, and 22:0) and increased monounsaturated fatty acids (MUFAs 16:1n7, 18:1n9, and 20:1n9). Hepatic polyunsaturated fatty acids (PUFAs) 20:2n6, 20:3n6, 18:3n3, and 22:5n3 also increased, whereas 20:4n6 and 22:6n3 levels decreased. gWAT PUFAs 20:2n6 and 20:3n6 exhibited modest increases, whereas serum 18:0 decreased and 18:1n9 increased. Serum analyses also identified a ~25% decrease in total cholesterol (CHOL), low-density lipoprotein (LDL), and high-density lipoprotein following TCDD treatment. The decrease in serum CHOL was consistent with the induction of hepatic reverse CHOL transport genes Lcat (2.0-fold), Apoa1 (1.7-fold), and Ldlr (3.6-fold), and the repression of CHOL biosynthesis genes Hmgcs1 (-2.1-fold) and Hmgcr (-2.3-fold). In addition, TCDD decreased serum Apob100 (4.4-fold) and Apob48 (2.2-fold) protein levels, suggesting serum lipid clearance and decreased hepatic efflux. Collectively, the TCDD-elicited decreases in serum lipid levels are consistent with AhR-mediated enhancement of dietary fat distribution to the liver.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipogenic differentiation and insulin-induced glucose uptake in 3T3-L1 cells

Dioxin exposure has been positively associated with human type II diabetes. Because lipophilic dioxins accumulate mainly in adipose tissue, this study aimed to determine if dioxins induce metabolic dysfunction in fat cells. Using 3T3-L1 cells as an in vitro model, we analyzed the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a model dioxin, on adipogenic differentiation, glucose uptake, and lipolysis. TCDD inhibited adipogenic differentiation, as determined by using oil droplet formation and adipogenic marker gene expression, including PPARgamma (peroxisome proliferator-activated receptor gamma), C/EBPalpha (CCAAT/enhancer-binding protein alpha), and Glut4 (glucose transporter type 4). Effects of TCDD on glucose uptake were evaluated using fully differentiated 3T3-L1 adipocytes, revealing that TCDD significantly attenuated insulin-induced glucose uptake dose dependently. Inhibition of aryl hydrocarbon receptor (AhR) by alpha-naphthoflavone (alpha-NF), an AhR inhibitor, did not prevent the inhibitory effect of TCDD on glucose uptake, suggesting that TCDD attenuates insulin-induced glucose uptake in an AhR-independent manner. Effects of TCDD on lipolysis were determined using glycerol release assay. We found that TCDD had no marked effect on isoproterenol-induced glycerol release in fully differentiated 3T3-L1 adipocytes. These results provide in vitro evidence of TCDD's effects on fat cell metabolism, suggesting dioxin exposure in development of insulin resistance and type II diabetes.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Growth factors, cytokines and their receptors as downstream targets of arylhydrocarbon receptor (AhR) signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant, which causes a variety of severe health effects, e.g. immunosuppression, hepatotoxicity, and carcinogenesis. The main mediator of TCDD toxicity is the arylhydrocarbon receptor (AhR), which, upon activation, translocates into the nucleus and enforces gene expression. Since most of the pleiotropic effects caused by TCDD are associated with alterations in cell growth and differentiation, the analysis of the interference of the AhR with factors controlling these cellular functions seems to be a promising target regarding the prevention and treatment of chemical-provoked diseases. Cell growth and differentiation are regulated by numerous growth factors and cytokines. These multifunctional peptides promote or inhibit cell growth and regulate differentiation and other cellular processes, depending on cell-type and developmental stage. They are involved in the regulation of a broad range of physiological processes, including immune response, hematopoiesis, neurogenesis, and tissue remodeling. The complex network of growth factors and cytokines is accurately regulated and disturbances of this system are associated with adverse health effects. The molecular mechanisms by which the AhR interferes with this signaling network are multifaceted and the physiological consequences of this cross-talk are quite enigmatic. The investigation of this complex interaction is an exciting task, especially with respect to the recently described non-genomic and/or ligand-independent activities of AhR. Therefore, we summarize the current knowledge about the interaction of the AhR with three cytokine-/growth factor-related signal transducers -- the epidermal growth factor (EGF) family, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta) -- with regard to pathophysiological findings.

Dioxin-induced chloracne--reconstructing the cellular and molecular mechanisms of a classic environmental disease

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is among the most toxic pollutants known to date that serves as a prototype for a group of halogenated hydrocarbon compounds characterized by extraordinary environmental persistence and unique ability to concentrate in animal and human tissues. TCDD can elicit a complex array of pleiotropic adverse effects in humans, although chloracne, a specific type of acne-like skin disease, is the only consistent manifestation of dioxin intoxication, thus representing a 'hallmark' of TCDD exposure. Chloracne is considered to be one of the most specific and sensitive biomarkers of TCDD intoxication that allows clinical and epidemiological evaluation of exposure level at threshold doses. The specific cellular and molecular mechanisms involved in pathogenesis of chloracne are still unknown. In this review, we summarize the available clinical data on chloracne and recent progress in understanding the role of the dioxin-dependent pathway in the control of gene transcription and discuss molecular and cellular events potentially involved in chloracne pathogenesis. We propose that the dioxin-induced activation of skin stem cells and a shift in differentiation commitment of their progeny may represent a major mechanism of chloracne development.

Aryl hydrocarbon receptor polymorphisms and dioxin resistance in Atlantic killifish (Fundulus heteroclitus)

The aryl hydrocarbon receptor (AHR) gene encodes a ligand-activated transcription factor through which planar halogenated aromatic hydrocarbons (HAHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as polynuclear aromatic hydrocarbons (PAHs) cause altered gene expression and toxicity. To understand the role of AHR genetic variability in differential sensitivity to HAHs and PAHs, we are currently studying a population of the teleost Fundulus heteroclitus (Atlantic killifish) that has evolved genetic resistance to the toxic and biochemical effects of these compounds. Here, we report that the killifish AHR1 locus is highly polymorphic and that the frequencies of the major allele types differ between dioxin-sensitive and dioxin-resistant populations. Twenty-five single nucleotide polymorphisms (SNPs), nine of which are non-synonymous, were identified in the AHR1 coding sequence. Seven identified alleles were assigned to three groups, designated AHR1*1, AHR1*2 and AHR1*3. AHR1*1 alleles were under-represented in a population of dioxin- and polychlorinated biphenyl (PCB)-resistant fish from a PCB-contaminated Superfund site (New Bedford Harbor, Massachusetts, USA) compared to dioxin-sensitive fish from a less contaminated reference site (Scorton Creek, Massachusetts, USA). To determine the possible role of these AHR1 variants in differential HAH sensitivity, we expressed representative variant proteins from the two most divergent allelic groups (AHR1*1 and AHR1*3) by in-vitro transcription and translation and assessed their functional properties. AHR1*1A and AHR1*3A proteins displayed similar binding capacities and affinities for [H]TCDD. In transient transfection assays using mammalian cells, AHR1*1A and AHR1*3A exhibited similar abilities to support TCDD-dependent transactivation of a luciferase reporter gene under control of AHR-responsive enhancer elements. We discuss the possibility of other functional differences in AHR1 variants or their interaction with other killifish loci (AHR2, AHRR) that may contribute to differences in dioxin sensitivity.

Basis for a proposed reference dose (RfD) for dioxin of 1-10 pg/kg-day: a weight of evidence evaluation of the human and animal studies

The dioxins have been perhaps the most studied of all chemicals to which humans are routinely exposed. It has been reported that more than 5,000 scientific papers have been published that have evaluated the toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Although the cancer hazard posed by this chemical has probably received the bulk of attention over the past 20 years, the U.S. Environmental Protection Agency (EPA) and the recent U.S. EPA Science Advisory Board (SAB) that reviewed the "Reassessment" have suggested that the noncancer hazard may well be more important than the cancer hazard at current background doses to the general public. The World Health Organization (WHO) and U.K. Food Standards Agency (FAO) committee (JECFA) on dioxins has reached similar conclusions. This article reviews the published studies involving laboratory animals and humans that address the noncancer effects. Based on our review, developmental toxicity is the most sensitive effect of TCDD consistently seen in mice and rats. Specifically, of the various studies, a no-observed-adverse-effects level (NOAEL) of 13 ng/kg (maternal body burden) was identified as the most pertinent for deriving a reference dose (RfD) for humans. Although more than a dozen different adverse effects have been reported in various studies of humans over the past 25 years, the most consistent clinically important adverse effect of human exposure appears to be chloracne. Following a review of all published studies, we concluded that the best estimate of a LOAEL for production of chloracne is approximately 160 ng/kg (body burden). Based on our analysis, an RfD of between 1 and 10 pg/kg-d (TCDD TEQ) is consistent with the objectives of this risk criterion. Maintaining a lifetime average daily dose below this concentration, based on what is known today, should prevent noncancer effects in virtually all persons. This value is consistent with the JECFA recommendation of 70 pg/kg-mo.

Modulating effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on skin carcinogenesis initiated by the weak inducer 7,12-dimethylbenz(a)anthracene

The effects of topical pretreatment of CF1-Swiss mice with TCDD on the carcinogenesis induced by DMBA were studied. We also determined the intrinsic features of DMBA as an aryl hydrocarbon hydroxylase (AHH) inducer through either its binding ability to Ah receptor or its inducing effects on benzo(a)pyrene (BP) hydroxylase or DMBA hydroxylase. DMBA is a poor ligand of the Ah receptor (26-fold and 4.3-fold weaker than 3-methylcholanthrene and BP respectively) and a very weak AHH inducer (ten million-fold weaker than TCDD). Nevertheless, DMBA induces a specific isozyme of cytochrome P-450 1A1 since, for an equal dose administered to C57BL/6 mice (200 mg/kg body weight), the DMBA-hydroxylase activity was 1.72-fold increased by DMBA while it remained unchanged after BP treatment. In contrast, the BP-hydroxylase activity was 1.91-fold increased by BP and only 1.47-fold by DMBA. A dose-dependent relationship exists between the increasing dose of TCDD (from 0.001 to 1 microg per mouse) applied to mouse skin and the induction of AHH activity of skin microsomes (from 1 to 60-fold increase). For carcinogenesis experiments, mice were either untreated or pretreated with single different doses of TCDD and, after 24h, DMBA (10 or 25 microg per mouse) was applied to the skin. The average number of papillomas per mouse was dependent on 1) the dose of DMBA and 2) the metabolic capacity of the skin. For 10 microg DMBA, the TCDD only exerts an anticarcinogenic effect (from 5.5 to 0.6 tumor per mouse) whereas for 25 microg DMBA, TCDD exerts a dual effect: first, a cocarcinogenic effect (from 6.2 to 9 and 11.5 tumors per mouse for 0.001 and 0.01 microg TCDD respectively) then an anticarcinogenic effect (2.3 and 1.5 tumors per mouse for 0.1 and 1 microg TCDD respectively). The discussion underlines the decisive importance of two factors: 1) the effective dose of the ultimate carcinogen in contact with cellular targets during a sensitive step of the cell cycle and 2) the time-persistence of a high steady state level of the carcinogen.

The zebrafish (Danio rerio) aryl hydrocarbon receptor type 1 is a novel vertebrate receptor

Fish are known to have two distinct classes of aryl hydrocarbon receptors, and their roles in mediating xenobiotic toxicity remain unclear. In this study, we have identified and characterized a cDNA tentatively named zebrafish AHR1 (zfAHR1). Analysis of the deduced amino acid sequence reveals that the protein is distinct from zfAHR2 and is more closely related to the mammalian aryl hydrocarbon receptor (AHR). zfAHR1 and zfAHR2 share 40% amino acid identity overall and 58% in the N-terminal half. The zfAHR1 gene maps to linkage group 16 in a region that shares conserved synteny with human chromosome 7 containing the human AHR, suggesting that the zfAHR1 is the ortholog of the human AHR. zfAHR2 maps to a separate linkage group (LG22). Both zfAHR mRNAs are expressed in early development, but they are differentially expressed in adult tissues. zfAHR2 can dimerize with zfARNT2b and binds with specificity to dioxin-responsive elements (DREs). Under identical conditions, zfAHR1/zfARNT2b/DRE complexes are formed; however, the interactions are considerably weaker. In COS-7 cells expressing zfARNT2b and zfAHR2, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure leads to a significant induction of dioxin-responsive reporter genes. In identical experiments, TCDD exposure fails to induce the reporter gene in zfAHR1-expressing cells. Ligand-binding experiments suggested that the differential zfAHR activities are attributable to differences in TCDD binding because only zfAHR2 exhibits high-affinity binding to [(3)H]TCDD or beta-naphthoflavone. Finally, using chimeric zfAHR1/zfAHR2 constructs, the lack of TCDD-mediated transcriptional activity was localized to the ligand-binding and C-terminal domains of zfAHR1.

Regulatory interactions among three members of the vertebrate aryl hydrocarbon receptor family: AHR repressor, AHR1, and AHR2

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds occur via the aryl hydrocarbon receptor (AHR), a member of the basic helix-loop-helix-Per-ARNT-Sim homology (bHLH-PAS) protein superfamily. A single AHR gene has been identified in mammals, whereas many fish species, including the Atlantic killifish (Fundulus heteroclitus) possess two distinct AHR genes (AHR1 and a novel form, AHR2). A mouse bHLH-PAS protein closely related to AHR and designated AHR repressor (AHRR) is induced by 3-methylcholanthrene and represses the transcriptional activity of the AHR. To determine whether AHRR is the mammalian ortholog of fish AHR2 and to investigate the mechanisms by which AHRR regulates AHR function, we cloned an AHRR ortholog in F. heteroclitus with high sequence identity to the mouse and human AHRRs. Killifish AHRR encodes a 680-residue protein with a predicted molecular mass of 75.2 kDa. We show that in vitro expressed AHRR proteins from human, mouse, and killifish all fail to bind [(3)H]TCDD or [(3)H]beta-naphthoflavone. In transient transfection experiments using a luciferase reporter gene under control of AHR response elements, killifish AHRR inhibited the TCDD-dependent transactivation function of both AHR1 and AHR2. AHRR mRNA is widely expressed in killifish tissues and is inducible by TCDD or polychlorinated biphenyls, but its expression is not altered in a population of fish exhibiting genetic resistance to these compounds. The F. heteroclitus AHRR promoter contains three putative AHR response elements. Both AHR1 and AHR2 activated transcription of luciferase driven by the AHRR promoter, and AHRR could repress its own promoter. Thus, AHRR is an evolutionarily conserved, TCDD-inducible repressor of AHR1 and AHR2 function. Phylogenetic analysis shows that AHRR, AHR1, and AHR2 are distinct genes, members of an AHR gene family; these three vertebrate AHR-like genes descended from a single invertebrate AHR.

cDNA cloning and characterization of a high affinity aryl hydrocarbon receptor in a cetacean, the beluga, Delphinapterus leucas

Some cetaceans bioaccumulate substantial concentrations of planar halogenated aromatic hydrocarbons (PHAHs) in their tissues, but little is known about the effects of such burdens on cetacean health. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related PHAHs cause toxicity via activation of the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS family of transcription factors. Differences in AHR structure and function are known to contribute to species-specific differences in susceptibility to PHAH toxicity. To ascertain the potential for PHAH effects in a cetacean, we characterized an AHR from the beluga whale, Delphinapterus leucas. The 3.2 kb cDNA encodes an 845-amino acid protein with a predicted size of 95.5 kDa. Overall, the beluga AHR shares 85% amino acid sequence identity with the human AHR and 75% identity with the mouse AHR Ah(b-1) allele. Beluga AHR protein synthesized in a rabbit reticulocyte lysate system demonstrated specific, high-affinity [(3)H]TCDD binding. Saturation binding analysis was used to compare the [(3)H]TCDD binding affinity of the in vitro-expressed beluga AHR with affinities of in vitro-expressed AHRs from a dioxin-sensitive mouse strain (Ah(b-1) allele) and humans. The beluga AHR bound [(3)H]TCDD with an affinity (K(d)= 0.43 +/- 0.16 nM) that was at least as high as that of the mouse AHR (K(d)= 0.68 +/- 0.23 nM), and significantly greater than that of the human AHR (K(d)= 1.63 +/- 0.64 nM). In electrophoretic mobility shift assays, the beluga AHR exhibited sequence-specific, Arnt-dependent binding to a dioxin responsive enhancer (DRE). Upon transient transfection into mammalian cells, the beluga AHR activated transcription of a luciferase reporter under control of a DRE-containing fragment of the mouse Cyp1a1 promoter. These results show that in an in vitro system, the beluga AHR possesses characteristics similar to those of AHRs from other mammals that are considered sensitive to toxic effects of PHAHs. Together, these results demonstrate that the use of in vitro-expressed proteins is a promising approach for addressing molecular and biochemical questions concerning PHAH toxicity in endangered or protected species.

An overview of TCDD half-life in mammals and its correlation to body weight

Major determinants of TCDD half-life in organisms are lipophilicity, metabolism, and hepatic binding sites. In addition, half-life seems to be empirically correlated to organism body weight. In this paper, this correlation is evaluated by a regression analysis of half-life measures and body weight data selected from the literature. Single exposure studies on laboratory mammals and human half-life data were specifically taken in consideration. The analysis outcome appears to be highly significant probably owing to the stability and generally slow metabolism of the substance in the organisms considered. The effect on half-life of factors other than body weight does not seem to influence significantly data dispersion around the regression line. The potential effects of a dose-dependent excretion cannot be excluded as toxicokinetic studies have been usually carried out at high exposure doses.

Identification and functional characterization of two highly divergent aryl hydrocarbon receptors (AHR1 and AHR2) in the teleost Fundulus heteroclitus. Evidence for a novel subfamily of ligand-binding basic helix loop helix-Per-ARNT-Sim (bHLH-PAS) factors

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor through which 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds cause altered gene expression and toxicity. The AHR belongs to an emerging multigene family of transcription factors possessing basic helix loop helix (bHLH) and Per-ARNT-Sim (PAS) domains. Most bHLH-PAS proteins occur as duplicates or "paralog groups" in mammals, but only a single mammalian AHR has been identified. Here we report the cDNA cloning of two distinct AHRs, designated FhAHR1 and FhAHR2, from a single vertebrate species, the teleost Fundulus heteroclitus (Atlantic killifish). Both Fundulus AHR proteins possess bHLH and PAS domains that are closely related to those of the mammalian AHR. FhAHR1 and FhAHR2 are highly divergent (40% overall amino acid identity; 61% identity in the N-terminal half), suggesting that they arose from a gene duplication predating the divergence of mammals and fish. Photoaffinity labeling with 2-azido-3-[(125)I]iodo-7, 8-dibromodibenzo-p-dioxin and velocity sedimentation analysis using 2,3,7,8-[1,6-(3)H]TCDD showed that both FhAHR1 and FhAHR2 exhibit specific, high-affinity binding of dioxins. Both AHRs also showed specific, TCDD- and ARNT-dependent interactions with a mammalian xenobiotic response element. The two Fundulus AHR genes displayed different tissue-specific patterns of expression; FhAHR1 transcripts were primarily expressed in brain, heart, ovary, and testis, while FhAHR2 transcripts were equally abundant in many tissues. Phylogenetic analysis demonstrated that Fundulus AHR1 is an ortholog of mammalian AHRs, while AHR2 forms in Fundulus and other fish are paralogous to Fundulus AHR1 and the mammalian AHRs and thus represent a novel vertebrate subfamily of ligand-binding AHRs.

Two forms of aryl hydrocarbon receptor type 2 in rainbow trout (Oncorhynchus mykiss). Evidence for differential expression and enhancer specificity

Two aryl hydrocarbon receptors (AhRs), rtAhR2alpha and rtAhR2beta, were cloned from rainbow trout (rt) cDNA libraries. The distribution of sequence differences, genomic Southern blot analysis, and the presence of both transcripts in all individual rainbow trout examined suggest that the two forms of rtAhR2 are derived from separate genes. The two rtAhR2s have significant sequence similarity with AhRs cloned from mammalian species, especially in the basic helix-loop-helix and PAS functional domains located in the amino-terminal 400 amino acids of the protein. In contrast, the Gln-rich transactivation domain found in the carboxyl-terminal half of mammalian AhRs is absent from both rtAhR2s. Both clones were expressed by in vitro transcription/translation and proteins of approximately 125 kDa were produced. These proteins bind 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and are able to bind dioxin response elements in gel shift assays. rtAhR2alpha and rtAhR2beta are expressed in a tissue-specific manner with the highest expression of rtAhR2beta in the heart. Expression of rtAhR2alpha and rtAhR2beta mRNAs is positively regulated by TCDD. Both rtAhR2alpha and rtAhR2beta produced TCDD-dependent activation of a reporter gene driven by dioxin response elements. Surprisingly, the two receptors showed distinct preferences for different enhancer sequences. These results suggest that the two receptor forms may regulate different sets of genes, and may play different roles in the toxic responses produced by AhR agonists such as TCDD.

Dioxin exposure and non-malignant health effects: a mortality study

OBJECTIVE

To investigate, in a population heavily exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the possible unusual occurrence of diseases other than cancer.

METHODS

Five year extension of the follow up of the cohort involved in the Seveso accident. Soil measurements identified three exposure zones: (A) highest contamination, (B) substantial, and (R) low but higher than background contamination. Blood TCDD measurements, although limited in number, confirmed zone exposure ranking. The 15 year mortality in the exposed cohort was compared with that of a large population in the surrounding non-contaminated territory. Relative risks (RRs) and 95% confidence intervals (95% CIs) were estimated with Poisson regression techniques.

RESULTS

The already noted increased occurrence of cardiovascular deaths was confirmed, in particular in zone A, among males for chronic ischaemic heart disease (five deaths, RR 3.0, 95% CI 1.2 to 7.3), and among females for hypertensive disease (three deaths, RR 3.6, 95% CI 1.2 to 11.4) and chronic rheumatic heart disease. Novel findings were the increase of chronic obstructive pulmonary disease, most notably among males in zone A (four deaths, RR 3.7, 95% CI 1.4 to 9.9) and females in zone B (seven deaths, RR 2.4, 95% CI 1.1 to 5.1); and from diabetes, which was significantly increased in females in zone B (13 deaths, RR 1.9, 95% CI 1.1 to 3.2). In zone R, chronic ischaemic heart disease (males and females), hypertension (females), and diabetes (females) showed less pronounced, although significant excesses.

CONCLUSIONS

As well as high TCDD exposure, the accident caused a severe burden of strain in the population. Both these factors might have contributed to the noted increased risks (in particular, circulatory and respiratory). The cardiovascular and immune toxicity of TCDD, as well as its complex interaction with the endocrine system, might be relevant to the explanations of these findings. These results, although not conclusive, concur with previous data in suggesting cardiopulmonary and endocrine effects in humans highly exposed to TCDD.

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on tumor necrosis factor (TNF) production by peritoneal cells

Recent studies in mice have demonstrated that TNF plays a critical role in mediating the TCDD-induced enhanced inflammatory response to intraperitoneal (i.p.) sheep red blood cells. The current studies were designed to evaluate the effects of TCDD on TNF production by ex-vivo peritoneal cells and a peritoneal macrophage cell line (IC-21) stimulated with LPS. In support of the hypothesis that TCDD can act directly on the peritoneal macrophage to increase TNF production, following pretreatment with TCDD, both ex-vivo peritoneal cells and IC-21 cells produced increased levels of bioactive TNF when stimulated with LPS. Flow cytometric analyses of IC-21 cells indicate that TCDD exposure increases intracellular production and secretion of TNF but does not alter levels of membrane associated TNF.

Regulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) of the DNA binding activity of transcriptional factors via nuclear protein phosphorylation in guinea pig adipose tissue

2,3,7,8-Tetrachloro-p-dioxin (TCDD) induced a modest stimulation of nuclear protein phosphorylation in explant tissue cultures in 10 min, followed by a substantial decrease in the level of total protein phosphorylation activity in the nucleus. Curiously, this TCDD-induced decline in nuclear protein phosphorylation was accompanied by an increase in cytosolic and extranuclear protein phosphorylation activity. One of the main causes for such a decrease in the protein phosphorylation activity in the nucleus appears to be related to some increase in protein phosphatase activities as judged by the counteractions of okadaic acid and Na3VO4 to the above effect. In addition, TCDD induced changes in nuclear protein kinase activities as well. Manganese-stimulated protein kinase was found to be the predominant type of nuclear protein phosphorylating activity affected by TCDD, with 60% of the total activity due to heparin-sensitive casein kinase II (CK II), a major nuclear protein kinase. The level of CK II activity in the nuclear protein preparation from adipose tissue of TCDD-treated guinea pigs (1 microgram/kg) in the presence of 100 nM heparin was only 35% of the control value after 24 hr. In addition, TCDD was found to increase the protein kinase C and microtubule-associated protein 2 kinase activities as early as 15 min after treatment in isolated adipose tissues in culture. Under in situ incubation conditions with explant tissues in culture, TCDD rapidly enhanced the DNA binding activity of the transcriptional factor AP-1, whereas the same treatment reduced c-Myc DNA binding activity. Genistein, a specific protein tyrosine kinase inhibitor, abolished the stimulatory effect of TCDD on AP-1 binding activity, but not on DNA binding activity of c-Myc. Phorbol ester (TPA) increased the binding activity of AP-1 and c-Myc, as expected. However, TCDD in combination with TPA caused a slight reduction in binding activity of both transcriptional factors. On the other hand, in the presence of forskolin, the stimulatory effect of TCDD on AP-1 binding activity and the inhibitory effect on c-Myc were still apparent. Okadaic acid almost abolished the binding activity of c-Myc, whereas in combination with TCDD a stimulatory effect was found. These observations are consistent with the idea that TCDD regulates the DNA binding activity of AP-1 and c-Myc mainly through modulating their states of phosphorylation by altering protein kinase and phosphatase activities.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced changes in glucose transporting activity in guinea pigs, mice, and rats in vivo and in vitro

The toxic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on glucose uptake was studied on different species, sexes, and strains of animals by using a nonmetabolizable glucose analog, 3-0-methyl D [1-3H] glucose (3H-Me-glc). We have found a drastic reduction in glucose uptake in different organs from male guinea pigs in vivo as well as in vitro. Female guinea pigs were less responsive to this effect of TCDD compared to males. Highly TCDD-responsive mice strains responded in the same way as the male guinea pigs, while less TCDD-responsive mice strains did not. A reduction in glucose uptake was found in explant adipose tissue cultures (in vitro) of male mice (highly TCDD-responsive) after a 4 hour incubation with 10(-8) M TCDD. In less responsive mice strains, stimulation was observed using the same model (in vitro). In the case of male rats, as well as male guinea pigs, glucose uptake was highly reduced by 10(-8) M TCDD using an in vitro system of explant adipose tissue. The Ah-receptor blocker, 4,7-phenanthroline, abolished the effect of TCDD on inhibition of glucose uptake in adipose tissue of male guinea pigs both in vivo and in vitro. Transcription and translation inhibitors (actinomycin D and cycloheximide, respectively) were tested separately or in combination with TCDD using the guinea pig adipose tissue culture model. It was found that actinomycin D could block the effect of TCDD on glucose uptake throughout the experiment time (360 min) while cycloheximide blocked TCDD action for about 60 minutes. Protein kinase inhibitors (e.g., genistein, neomycin) did not change the effect of TCDD on glucose uptake in male guinea pig adipose tissue. Based on these observations, we conclude that TCDD inhibits Me-glc uptake in various organs in guinea pigs, mice, and rats, and this response is mediated by the Ah-receptor.

Lipid peroxidation in the adrenal glands of male rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

This study was performed to determine whether TCDD (50 micrograms/kg; single oral dose) could induce adrenal microsomal lipid peroxidation, which might be correlated to decreased levels of cytochrome P-450 and 21-hydroxylase activity. The amount of malondialdehyde (MDA) formed was significantly higher than controls at days 1 through 5 following TCDD treatment. Microsomal cytochrome P-450 levels were depressed after lipid peroxidation at days 1, 3, and 5, and 21-hydroxylase activity decreased at day 5 after TCDD treatment. This study shows that TCDD stimulates adrenal microsomal lipid peroxidation which is associated with decreased cytochrome P-450 levels and 21-hydroxylase activity.

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.

TCDD alters pituitary-adrenal function. I: Adrenal responsiveness to exogenous ACTH

Plasma ACTH concentrations in 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats (50 micrograms/kg; single, oral dose) were 2.1-, 2.1-, 2.9-, 1.7-, 1.5-, 2.0- and 3.0-fold greater than control values, respectively, at days 1, 3, 5, 7, 10, and 14. At days 1 and 5 plasma corticosterone concentrations were increased 5.1- and 8.0-fold, respectively; whereas, at days 10 and 14 they were depressed to values of 50% and 39% of controls, respectively. Adrenal glands were excised from rats treated with TCDD and corticosterone production was assessed. Basal corticosterone concentrations produced by treated adrenals were depressed to 81%, 72%, and 71% of control values at days 5, 7, and 14, respectively. Corticosterone secretion by ACTH stimulated adrenals was equivalent to controls. These findings suggest that TCDD exposure decreases the bioactivity of the ACTH secreted by the anterior pituitary.

Modeling receptor-mediated processes with dioxin: implications for pharmacokinetics and risk assessment

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD), a widespread polychlorinated aromatic hydrocarbon, caused tumors in the liver and other sites when administered chronically to rats at doses as low as 0.01 microgram/kg/day. It functions in combination with a cellular protein, the Ah receptor, to alter gene regulation, and this resulting modulation of gene expression is believed to be obligatory for both dioxin toxicity and carcinogenicity. The U.S. EPA is reevaluating its dioxin risk assessment and, as part of this process, will be developing risk assessment approaches for chemicals, such as dioxin, whose toxicity is receptor-mediated. This paper describes a receptor-mediated physiologically based pharmacokinetic (PB-PK) model for the tissue distribution and enzyme-inducing properties of dioxin and discusses the potential role of these models in a biologically motivated risk assessment. In this model, ternary interactions among the Ah receptor, dioxin, and DNA binding sites lead to enhanced production of specific hepatic proteins. The model was used to examine the tissue disposition of dioxin and the induction of both a dioxin-binding protein (presumably, cytochrome P4501A2), and cytochrome P4501A1. Tumor promotion correlated more closely with predicted induction of P4501A1 than with induction of hepatic binding proteins. Although increased induction of these proteins is not expected to be causally related to tumor formation, these physiological dosimetry and gene-induction response models will be important for biologically motivated dioxin risk assessments in determining both target tissue dose of dioxin and gene products and in examining the relationship between these gene products and the cellular events more directly involved in tumor promotion.

Characterization of the 4 S polycyclic aromatic hydrocarbon-binding protein in human liver and cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein (PBP) is a soluble protein that binds PAHs with high affinity in mouse, rat, and rabbit. Until now, this protein had not been detected in human placenta or human cells in culture by cytosol labeling and gradient centrifugation assay. Thanks to a preliminary fractionation of cytosol by sedimentation on sucrose gradient or/and gel permeation chromatography, we found that PBP was present in liver, MCF-7 cell line, and hepatocytes of human. To accurately quantitate PBP binding and determine specific binding parameters, a reduction in the amount of charcoal used to adsorb nonspecifically bound benzo[a]pyrene was required. By saturation analysis, the concentration of specific binding sites for [3H]BP in PBP fraction from human liver was 4.6 pmol/mg of protein compared with 14.7 +/- 1.4 pmol/mg in the same fraction from DBA/2J mouse liver. Kinetic studies analyzed by Scatchard and Woolf plots indicate that human liver and MCF-7 cells contain a low-affinity PBP form: the Kd derived from Woolf plot analysis were 14.2 +/- 1.4 and 26.2 +/- 1.8 nM, respectively. DBA/2J mouse possesses a higher-affinity PBP form, the same analysis indicating a Kd of 6.1 +/- 0.3 nM. These data demonstrate that, by comparison to the mouse liver, a lower-affinity form of PBP is present in reduced concentration in human liver, explaining the impossibility of detecting this protein by sedimentation of human cytosol in sucrose gradient.

Inhibition of acute TCDD toxicity by treatment with anti-tumor necrosis factor antibody or dexamethasone

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acute toxicity is characterized in part by a wasting syndrome with depletion of adipose tissue. Tumor necrosis factor (TNF) induces a similar response during chronic infection. The similarities of these toxic effects led to a hypothesis that TNF plays a role in TCDD acute toxicity. To test this hypothesis pharmacologic doses of an antibody specific for murine TNF and the potent anti-inflammatory agent Dexamethasone (DEX) were used to inhibit TCDD toxicity in mice. TNF antibody treatment resulted in a 54% reduction in TCDD-mediated mortality while DEX treatment, a glucocorticoid agonist that inhibits transcription of TNF, reduced mortality by 92%. Cyp 1A1 induction, the most commonly measured TCDD-mediated response, was not blocked by DEX, demonstrating separation of this biochemical effect from acute toxic responses to TCDD. These data suggest that TCDD-mediated changes in the TNF pathway may be an important mechanism for acute TCDD toxicity.

Mechanism of antimutagenicity of wheat sprout extracts

In this paper we have demonstrated that wheat sprout extract, which has been shown to be antimutagenic towards benzo[a]pyrene (BP), reduced formation of BP metabolites by hepatic microsomes of either benzo[a]pyrene- or phenobarbital-treated rats as analyzed in high-pressure liquid chromatography (HPLC). Comparing the time dependence of profiles and values of BP metabolites, formed in experiments in which the same dose of wheat sprout extract was added to the incubation medium, it has been observed that the later this extract was added the higher the percent of BP that was metabolized. In a bacterial test (cytochrome P450 induction assay) high inhibition of mutagenic activity of cyclophosphamide and ethidium bromide, in the presence of wheat sprout extract, reflected decreased levels of cytochromes P4502B1 and P4501A1 respectively. Decreased levels of both cytochromes P4501A1 and P4502B1 were also observed in either wheat sprout extract- or wheat sprout extract plus benzo[a]pyrene-treated rats. In all of these studies it has been observed that wheat sprout extract displays much more affinity for cytochrome P4501A1 than for the P4502B1 form. On the other hand the wheat sprout extract had higher affinity for carcinogen binding protein (4S protein) than for the aryl hydrocarbon receptor. The strong inhibition of BP mutagenicity and BP metabolism with non-chlorophyllic wheat sprout extract suggests that chlorophyll is not the main compound responsible for the antimutagenic activity of wheat sprout extract. The similar chromatographic behavior of both the main inhibitory fraction, obtained from wheat sprout extract, and two pure glycosides of apigenin--shaftoside, purified from wheat sprout extract and synthetic swertisine--suggests that antimutagenic compound(s) contained in the wheat sprout extract belong(s) to this family of flavonoids.

Metabolism and disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin in ring-necked pheasant hens, chicks, and eggs

The T 1/2 for whole-body elimination of [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) derived radioactivity in ring-necked pheasant hatchlings was 13 d, whereas in adult hen pheasants that were not producing eggs it was 378 d. All TCDD-derived radioactivity in hen tissues was from the parent compound. The oral bioavailability of TCDD in the adult hen pheasant varied with the environmental matrix, with 30% of the dose absorbed from a suspension of earthworms, 33% absorbed from a soil suspension, 41% absorbed from a suspension of paper mill sludge, and 58% absorbed from a suspension of crickets. A cumulative dose of 1.0 micrograms TCDD/kg body weight, administered as weekly doses of 0.1 micrograms/kg for 10 wk, did not adversely affect hen condition or egg production. Under these exposure conditions, hens translocated about 1% of their cumulative TCDD dose to each of the first 15 eggs laid. All of the TCDD-derived radioactivity in the eggs was the parent compound and was confined entirely to the yolk; no TCDD was detected in egg albumin. We conclude that TCDD was more persistent in pheasant hens than in chicks and that egg laying was an important route of elimination in the hen.

The Ah receptor and the mechanism of dioxin toxicity

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Pharmacokinetics of [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin in mice: analysis with a physiological modeling approach

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent inducer of hepatic microsomal enzymes. The influence of an inducing dose of TCDD on tissue distribution and other pharmacokinetic behavior of a TCDD analog in the mice was examined by employing a high specific activity radioligand. [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITCDD). Female C57BL/6J mice were pretreated with 0.1 mumol/kg of TCDD or the vehicle only, followed by 0.1 nmol ITCDD/kg 3 days later. The control animals had the highest concentration of ITCDD-derived radioactivity in the fat, but the TCDD-pretreated animals had the highest concentration in their livers. Whole-body elimination of ITCDD approximated first-order behavior, and induction by pretreatment with the inducing dose of TCDD almost doubled the rate of excretion (control mice, t1/2 = 14.2 days; pretreated mice, t1/2 = 8.0 days). All disposition results in naive and pretreated mice were satisfactorily described by a consistent physiologically based pharmacokinetic model (Leung et al., 1988a) in which induction increased the amount of microsomal ITCDD-binding protein from 1.75 to 20 nmol/liver and increased the rate constant for metabolism of free ITCDD from 1 to 3/hr/kg liver. The binding affinity of the microsomal ITCDD-binding protein was the same (20 nM) in both induced and noninduced mice. Model simulations indicated a time delay in the elimination of nonparent ITCDD metabolites from the body and a more rapid absorption of the parent ligand in the pretreated mice. Consistent with previous physiological modeling with TCDD in different mouse strains, the primary factor influencing the liver/fat concentration ratio appears to be the affinity and capacity of the microsomal TCDD-binding proteins, which are altered by induction. These dose-dependent pharmacokinetic differences with ITCDD are important considerations for TCDD risk assessment in which data from high dose rodent experiments are extrapolated to predict behavior at much lower environmental concentrations in exposed humans.

Tissue distribution, metabolism, and excretion of 14C-TCDD in a TCDD-susceptible and a TCDD-resistant rat strain

A comparative study was carried out in the most TCDD-resistant [Han/Wistar (H/W), LD50 greater than 3000 micrograms/kg] and the most TCDD-susceptible [Long-Evans (L-E), LD50 about 10 micrograms/kg] rat strain to assess the significance of kinetic factors in TCDD toxicity. Young adult males of both strains were administered 5 micrograms/kg (1.9 microCi/kg) 14C-TCDD intraperitoneally. Four rats per strain were killed at 4 hr, 1, 4, 8, 16, and 32 days after exposure. A total of 22 tissues along with blood and serum were sampled for liquid scintillation counting. From half of the animals, daily urine and faeces were also analyzed. In addition, 3 rats per strain were given 50 micrograms/kg (19 microCi/kg) 14C-TCDD and prepared for whole-body autoradiography after 1, 4 or 8 days. The livers of two rats per strain killed at 4 hr, 4 or 16 days, and the excreta from two rats of both strains collected on days 1-4, 5-8, 13-16, and 29-32 after exposure were analyzed for metabolites of TCDD by high pressure liquid chromatography. The label was mainly excreted in faeces as metabolites of TCDD, and the half-life of elimination was 20.8 (L-E) or 21.9 (H/W) days. A very similar overall distribution pattern was observed in both strains irrespective of dose, and the liver was the major site of accumulation. Practically all liver 14C-activity was found as the parent compound. Moderate strain-related differences were observed in the thyroid, thymus, prostate, adrenals, and brown and white fat, where lower values were recorded in H/W rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular lipoproteins as carriers for 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene in rat and mouse liver

The possible role of hepatic lipoproteins as intracellular carriers in the transport of 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene was assessed by in vitro and in vivo studies. Following administration of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin or unlabelled 2,3,7,8-tetrachlorodibenzofuran to C57 BL/6 mice or Sprague-Dawley rats these compounds were bound to lipoproteins which subsequently underwent rapid and pronounced degradative processing, possibly catalysed by lipoprotein lipase, to heavier entities. At the highest doses of xenobiotics administered, an almost complete disappearance of lipoprotein particles was observed. The in vitro incubation of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin-lipoprotein and [3H]benzo(a)pyrene-lipoprotein complexes with separated Ah receptor and 4S protein, respectively, demonstrated that a passive transfer occurred; the latter was likely dependent on both the relative affinities of the ligands towards the different cellular binding components as well as on their quantitative binding capacity. Taken together, these findings support the idea of a carrier-role for lipoproteins in the intracellular transport of hydrophobic xenobiotics and it may be asked whether the widespread modulators of lipoprotein level such as fibrates or others affect drug transfer or action.

Separation of the different classes of intrahepatic lipoproteins from various animal species. Their binding with 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene

Using several analytical methods, including sucrose density gradient and potassium bromide density gradient ultracentrifugations, we have demonstrated that liver cells contain a range of lipoproteins somewhat distinct from those found in plasma. In addition to very low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL), many heavier entities have been found in the cytosol of various animal species. These heavier entities might represent either anabolic or catabolic intermediates of lipoproteins. Labelled hydrophobic xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin or benzo(a)pyrene strongly bind to the various classes of lipoproteins and may be used as radioactive tracers in the analysis and possibly in the metabolic studies of intracellular lipoproteins. Moreover, this binding may be a prerequisite for a storage or/and a carrier--roles of lipoproteins in the intracellular distribution of lipophilic xenobiotics within the cells.

Characteristics of 2,3,7,8-tetrachlorodibenzo-p-dioxin induced endotoxin hypersensitivity: association with hepatotoxicity

Hepatic clearance is a major route of endotoxin detoxification. In the present study, the potential relationship between TCDD-induced endotoxin hypersensitivity and hepatotoxicity was examined. Acute doses of 50, 100, or 200 micrograms TCDD/kg body weight induced an endotoxin hypersensitive state in B6C3F1 mice as demonstrated by increased mortality 24-48 h following i.v. injection of endotoxin. This hypersensitive state occurred when endotoxin was administered 7 days following TCDD exposure, but not 1 day post-TCDD exposure. TCDD did not affect endogenous serum endotoxin levels. However, clearance of injected endotoxin was significantly inhibited following exposure to TCDD. Six hours post endotoxin treatment serum triglycerides were significantly increased in TCDD/endotoxin-treated mice compared to either treatment alone. Methylprednisolone and uridine were both examined in this model due to their roles in inflammation and RNA synthesis, respectively. Both compounds significantly reversed the mortality associated with the combined exposure. [3H]Uridine incorporation into liver was decreased following TCDD treatment alone, further suggesting impaired RNA synthesis. Studies performed on congenic mice indicate that the observed effects segregate with the Ah locus. The ability of methylprednisolone and uridine to reverse the mortality associated with TCDD/endotoxin treatment is consistent with an inflammatory response and impaired hepatic detoxification mechanisms. Thus, changes in hepatic handling of endotoxin, caused by progressive TCDD-induced liver dysfunction, may be responsible for the endotoxin hypersensitivity.

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans: the risks to human health. A review

1 PCDDs and PCDFs are ubiquitous and persistent in the environment. They are to be found in body tissues of both humans and animals. 2 The most extensively studied PCDD is 2,3,7,8-TCDD. It has been shown to produce a wide range of effects and is considered to be a (non-genotoxic) carcinogen in animals. 3 Studies into the mechanisms of toxicity so far reveal that there is involvement of a specific receptor (Ah), however further work is required to elucidate the mechanisms of the various effects. 4 Reports on a number of human exposures to PCDDs and PCDFs are described. Results from human epidemiological studies are difficult to interpret: there have been problems in methodology; there has been inadequate information on intake, and exposures have often been to mixtures of PCDDs and/or PCDFs together with other related compounds. 5 Many regulatory authorities faced with the problem of providing an index of risk from exposure to mixtures of PCDDs and PCDFs have employed the concept of 'TCDD equivalents'. 6 Whether or not PCDDs and PCDFs pose a significant human health risk at current levels of exposure they remain of considerable interest to the toxicologist.

Interaction of hexachlorobenzene with the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in vitro and in vivo. Evidence that hexachlorobenzene is a weak Ah receptor agonist

Hexachlorobenzene (HCB) produces hepatic porphyria and induces the hepatic cytochrome P450 isozymes P450c (P450IA1) and P450d (P450IA2) in rodents. These and other effects of HCB resemble those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which acts via its binding to the aromatic hydrocarbon (Ah) receptor. We therefore examined the ability of HCB to interact with this receptor in vitro and in vivo. HCB, at concentrations of 1 microM or higher, inhibited the specific binding of [3H]TCDD (0.3 nM) to the Ah receptor in vitro, whereas the solubility of [3H]TCDD was affected only at 100 microM HCB. The inhibition was competitive, with a KI of approximately 2.1 microM. In rats fed a diet containing 3000 ppm HCB for varying times (4 h to 7 days), the specific binding of [3H]TCDD in hepatic cytosol was reduced by up to 40%, as observed previously for known Ah receptor agonists. The decrease in [3H]TCDD specific binding in cytosol of HCB-treated rats was due principally to a decrease in the number of binding sites for [3H]TCDD rather than competition from residual HCB. As shown by immunoblotting and radioimmunoassay, HCB induced the cytochrome P450 isozymes P450c and P450d, which are regulated by the Ah receptor, as well as the phenobarbital-inducible isozymes P450b and P450e. Together these results indicate that HCB is a weak agonist for the Ah receptor, and suggest that some of its effects may be mediated by its interaction with this gene-regulatory protein.

Differential effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipose tissue lipoprotein lipase activity in the guinea pig, rat, hamster, rabbit, and mink

1. Adipose tissue lipoprotein lipase (LPL) activity of several different animal species was determined after i.p. administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). 2. TCDD caused a significant reduction in LPL activity and an increase in serum triglyceride concentration in guinea pigs, rabbits, and hamsters but not rats. 3. TCDD increased adipose tissue LPL activity of mink and lowered their serum triglyceride concentration. 4. Results of this study indicate that profound differences occur in lipid metabolism between various species in response to TCDD and these changes do not appear to be related to generalized toxicity such as wasting.

Rabbit serum hypertriglyceridemia after administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) caused a dose-dependent decrease of adipose tissue lipoprotein lipase (LPL) activity and caused a concomitant increase in serum triglyceride concentration in the rabbit 10 d after single ip administration of either 1 or 50 micrograms/kg. Hepatic low-density lipoprotein (LDL) binding was markedly depressed and serum cholesterol concentrations were modestly increased relative to pair-fed control animals. Serum glucose concentrations were significantly lower in the rabbit administered TCDD compared to ad libitum or pair-fed control animals, although little change was observed in serum insulin concentration. Electron microscopic examination of aortic arches 20 d after a single ip administration of 50 micrograms TCDD/kg revealed ruffling, denudation, and sloughing off of the cell surface and the appearance of macrophage-like structures in the intima and media of the endothelial cells. These alterations resemble preatherosclerotic lesions typical in animals with hyperlipidemia. It is proposed that TCDD causes hyperlipidemia in the rabbit through suppression of LPL activity and LDL receptor binding.

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.

A physiologically based pharmacokinetic model for 2,3,7,8-tetrachlorodibenzo-p-dioxin in C57BL/6J and DBA/2J mice

A five-compartment physiologically based pharmacokinetic (PB-PK) model was developed to describe the time course of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the tissues of both C57BL/6J and DBA/2J mice. The PB-PK model included binding in blood and two hepatic binding sites, one in the cytosol and the other in the microsomes. First-order metabolism occurred in the liver. Model simulations were compared to literature results for the disposition of a single intraperitoneal dose of 10 micrograms/kg of [3H]TCDD, reported by Gasiewicz et al. [Drug Metab. Dispos. 11 (1983) 397-403]. In contrast to previous speculation, the greater accumulation of TCDD in the liver of the C57BL/6J mouse, as compared to the DBA/2J mouse, was not attributable to the higher fat content in the DBA/2J mouse. Instead, the disposition of TCDD in these mice was more dependent on the affinity of the microsomal binding proteins than on fat content. The microsomal dissociation constant in the C57BL/6J mouse estimated by the PB-PK model was about one-third its value in the DBA/2J mouse (20 versus 75 nM), i.e. there is more avid microsomal binding in the liver of the C57BL/6J mouse. In the concentration range covered in these time-course studies, the cytosolic receptor, with its low capacity and very high affinity binding characteristics, does not play a major role in determining the overall tissue distribution pattern. The concentration and affinity of the microsomal binding protein in the liver appear to be primarily responsible for explaining the differences in the liver/fat concentration ratios between various strains and species of laboratory animals.

Physiological implications of estrogen receptor modulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The interactions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with hormones and hormone receptors have important implications for TCDD toxicity. Evidence suggests that TCDD modulates receptors for glucocorticoids, prolactin, thyroxine, low density lipids, epidermal growth factor, and estrogens. Estrogen receptor modulation and the animal's physiological responses to this modulation appear to be particularly important effects and can explain much of the toxicity observed in TCDD-treated animals. Susceptibility of different species to TCDD correlates with their steroid glucuronidation capacity. Because of the close interactions and interdependent regulation of hormonal systems, other hormones may have a similar role in TCDD toxicity.

Hypothalamic site of action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Administration of TCDD produced a significant decrease in the serum concentration of prolactin (PRL) detected in rats after 4 hr compared to pair-fed vehicle controls and noninjected controls. This effect of TCDD was reversed by pimozide, a dopamine receptor antagonist. These data suggest that TCDD decreased the release of PRL from the adenohypophysis either by a direct effect on the gland or by altering the dopamine concentration in the median eminence (ME). Concentrations of TCDD from 5 to 500 ng/ml had no direct effect on the ability of the adenohypophysis to secrete PRL in vitro. However, the dopamine concentration increased to 3.24 +/- 0.07 ng per ME in TCDD-treated rats compared to 2.81 +/- 0.08 ng in vehicle controls. This is a dramatic alteration in the dopamine concentration, since the dopamine is being measured in the portal circulation which exhibits a rapid turnover. The rate constant of dopamine depletion after alpha-methyl-p-tyrosine and the turnover rate were also significantly elevated in the ME of TCDD-treated rats. These data provide the first biochemical evidence for a hypothalamic site of action of TCDD. Since dopamine is inhibitory to PRL release from the adenohypophysis, increased ME steady-state concentrations and turnover of this catecholamine may be responsible for the decreased concentration of serum PRL detected within 4 hr of TCDD injection. Thus, one of the early modes and sites of action of TCDD is to elevate the dopaminergic activity of the tuberoinfundibular nucleus. A hypothalamic site of action for TCDD may result in a number of the endocrinological effects known to be produced by exposure to TCDD.

Reduction of adipose tissue lipoprotein lipase activity as a result of in vivo administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin to the guinea pig

Within 1 hr of intraperitoneal administration of 1 microgram 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg, lipoprotein lipase (LPL) activity was reduced 38% from initial levels in the adipose tissue of the guinea pig. Maximal depression was observed after 2 days and persisted throughout the 10-day observation period. Oral administration of glucose restored LPL activity in TCDD-treated animals after 1 day but only partially after 2 and 5 days, and had no effect after 10 days of exposure. Although initial (2-day) serum insulin levels were depressed, the inability of glucose to restore LPL activity after prolonged exposure was not due to malabsorption of glucose nor to changes in serum thyroxine or insulin concentration. TCDD also inhibited the lipolytic pathway in the adipocyte, but had no effect on hormone sensitive lipase (HSL). Since HSL and LPL are reciprocally regulated, it was concluded that TCDD acts on the adipocyte to uncouple HSL-LPL reciprocity as well as to reduce LPL production.

Effects of SH-modifying reagents on the rat hepatic Ah receptor: inhibition of ligand binding and transformation, and disruption of the ligand-receptor complex

The importance of sulfhydryl (SH) groups in maintenance of physicochemical properties of the rat hepatic Ah receptor was demonstrated using a variety of sulfhydryl (SH)-modifying reagents. Inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) specific binding was approximately equivalent by 5,5'-dithiobis(2-nitrobenzoate), mersalyl, N-ethylmaleimide, and p-chloromercuriphenylsulfonate, whose inhibition curves were steep in the concentration range close to that of nonprotein SH groups in cytosol (ED50 values 50-200 microM or 13-48 nmol/mg cytosolic protein). Inhibition by p-hydroxymercuribenzoate (PHMB), although exhibiting a lower ED50, was more gradual over this range; iodoacetamide was an order of magnitude less potent. The ability of dithiothreitol to reverse binding inhibition induced by 150 microM (approximately 60 nmol/mg protein) mersalyl diminished with time; it decreased more rapidly in the simultaneous presence of TCDD and mersalyl than when mersalyl was present alone, consistent with increased accessibility of key SH group(s) due to conformational changes attending TCDD-receptor complex formation. Brief exposure of unoccupied receptor to mersalyl prior to TCDD binding caused slower sedimentation of the complex in 0-KCl sucrose gradients and alterations in its elution profiles on DEAE- and DNA-Sepharose suggestive of some impairment of the transformation process. When reagents were added to the transformed TCDD-receptor complex, loss of binding was observed only at concentrations which were an order of magnitude higher than those inhibiting TCDD binding. Loss of binding by each reagent was biphasic, and except for that caused by mersalyl, was not complete even after 6-8 h. Dithiothreitol was able to reverse the effects of mersalyl or PHMB only partially and only if added during the early phase (10-30 min) of binding loss. Mersalyl was much more potent in disrupting the untransformed than the transformed TCDD receptor complex. Physical alteration of the mersalyl-treated TCDD-receptor complex was evident from gel filtration, sucrose gradients, and DNA- and DEAE-Sepharose chromatography. Our results are in striking contrast to the effects of these reagents on steroid receptors, whose bound steroid hormone ligand is rapidly and reversibly displaced by lower concentrations of reagent.(ABSTRACT TRUNCATED AT 400 WORDS)