Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the golden Syrian hamster

Long-term metabolic consequences of acute dioxin exposure differ between male and female mice

Epidemiological studies have consistently shown an association between exposure to environmental pollutants and diabetes risk in humans. We have previously shown that direct exposure of mouse and human islets (endocrine pancreas) to the highly persistent pollutant TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) causes reduced insulin secretion ex vivo. Furthermore, a single high-dose of TCDD (200 µg/kg) suppressed both fasting and glucose-induced plasma insulin levels and promoted beta-cell apoptosis after 7 days in male mice. The current study investigated the longer-term effects of a single high-dose TCDD injection (20 µg/kg) on glucose metabolism and beta cell function in male and female C57Bl/6 mice. TCDD-exposed males displayed modest fasting hypoglycemia for ~4 weeks post-injection, reduced fasting insulin levels for up to 6 weeks, increased insulin sensitivity, decreased beta cell area, and increased delta cell area. TCDD-exposed females also had long-term suppressed basal plasma insulin levels, and abnormal insulin secretion for up to 6 weeks. Unlike males, TCDD did not impact insulin sensitivity or islet composition in females, but did cause transient glucose intolerance 4 weeks post-exposure. Our results show that a single exposure to dioxin can suppress basal insulin levels long-term in both sexes, but effects on glucose homeostasis are sex-dependent.

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.

Association of dioxin and other persistent organic pollutants (POPs) with diabetes: epidemiological evidence and new mechanisms of beta cell dysfunction

The worldwide explosion of the rates of diabetes and other metabolic diseases in the last few decades cannot be fully explained only by changes in the prevalence of classical lifestyle-related risk factors, such as physical inactivity and poor diet. For this reason, it has been recently proposed that other "nontraditional" risk factors could contribute to the diabetes epidemics. In particular, an increasing number of reports indicate that chronic exposure to and accumulation of a low concentration of environmental pollutants (especially the so-called persistent organic pollutants (POPs)) within the body might be associated with diabetogenesis. In this review, the epidemiological evidence suggesting a relationship between dioxin and other POPs exposure and diabetes incidence will be summarized, and some recent developments on the possible underlying mechanisms, with particular reference to dioxin, will be presented and discussed.

Knockout of the aryl hydrocarbon receptor results in distinct hepatic and renal phenotypes in rats and mice

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor which plays a role in the development of multiple tissues and is activated by a large number of ligands, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to examine the roles of the AHR in both normal biological development and response to environmental chemicals, an AHR knockout (AHR-KO) rat model was created and compared with an existing AHR-KO mouse. AHR-KO rats harboring either 2-bp or 29-bp deletion mutation in exon 2 of the AHR were created on the Sprague-Dawley genetic background using zinc-finger nuclease (ZFN) technology. Rats harboring either mutation type lacked expression of AHR protein in the liver. AHR-KO rats were also insensitive to thymic involution, increased hepatic weight and the induction of AHR-responsive genes (Cyp1a1, Cyp1a2, Cyp1b1, Ahrr) following acute exposure to 25 μg/kg TCDD. AHR-KO rats had lower basal expression of transcripts for these genes and also accumulated ~30-45-fold less TCDD in the liver at 7 days post-exposure. In untreated animals, AHR-KO mice, but not AHR-KO rats, had alterations in serum analytes indicative of compromised hepatic function, patent ductus venosus of the liver and persistent hyaloid arteries in the eye. AHR-KO rats, but not AHR-KO mice, displayed pathological alterations to the urinary tract: bilateral renal dilation (hydronephrosis), secondary medullary tubular and uroepithelial degenerative changes and bilateral ureter dilation (hydroureter). The present data indicate that the AHR may play significantly different roles in tissue development and homeostasis and toxicity across rodent species.

Comparative analysis of temporal and dose-dependent TCDD-elicited gene expression in human, mouse, and rat primary hepatocytes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-elicited time- and dose-dependent differential gene expression was compared in human, mouse, and rat primary hepatocytes. Comprehensive time course (10 nM TCDD or dimethyl sulfoxide vehicle control for 1, 2, 4, 8, 12, 24, and 48h) studies identified 495, 2305, and 711 differentially expressed orthologous genes in human, mouse, and rat hepatocytes, respectively. However, only 16 orthologs were differentially expressed across all three species, with the majority of orthologs exhibiting species-specific expression (399 human, 2097 mouse, and 533 rat), consistent with species-specific expression reported in other in vitro and in vivo comparative studies. TCDD also elicited the dose-dependent induction of 397 human, 100 mouse, and 443 rat genes at 12h and 615 human, 426 mouse, and 314 rat genes at 24h. Comparable EC50 values were obtained for AhR battery genes including Cyp1a1 (0.1 nM human, 0.05 nM mouse, 0.08 nM rat at 24h) and Tiparp (0.97 nM human, 0.63 nM mouse, 0.14 nM rat at 12h). Overrepresented functions and pathways included amino acid metabolism in humans, immune response in mice, and energy homeostasis in rats. Differentially expressed genes functionally associated with lipid transport, processing, and metabolism were overrepresented in all three species but exhibited species-specific expression consistent with the induction of hepatic steatosis in mice but not in rats following a single oral gavage of TCDD. Furthermore, human primary hepatocytes showed lipid accumulation following 48h of treatment with TCDD, suggesting that AhR-mediated steatosis in mice more closely resembles human hepatic fat accumulation compared with that in rats. Collectively, these results suggest that species-specific gene expression profiles mediate the species-specific effects of TCDD despite the conservation of the AhR and its signaling mechanism.

Differences in TCDD-elicited gene expression profiles in human HepG2, mouse Hepa1c1c7 and rat H4IIE hepatoma cells

Background

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that elicits a broad spectrum of toxic effects in a species-specific manner. Current risk assessment practices routinely extrapolate results from in vivo and in vitro rodent models to assess human risk. In order to further investigate the species-specific responses elicited by TCDD, temporal gene expression responses in human HepG2, mouse Hepa1c1c7 and rat H4IIE cells were compared.

Results

Microarray analysis identified a core set of conserved gene expression responses across species consistent with the role of AhR in mediating adaptive metabolic responses. However, significant species-specific as well as species-divergent responses were identified. Computational analysis of the regulatory regions of species-specific and -divergent responses suggests that dioxin response elements (DREs) are involved. These results are consistent with in vivo rat vs. mouse species-specific differential gene expression, and more comprehensive comparative DRE searches.

Conclusions

Comparative analysis of human HepG2, mouse Hepa1c1c7 and rat H4IIE TCDD-elicited gene expression responses is consistent with in vivo rat-mouse comparative gene expression studies, and more comprehensive comparative DRE searches, suggesting that AhR-mediated gene expression is species-specific.

Dioxin toxicity, aryl hydrocarbon receptor signaling, and apoptosis-persistent pollutants affect programmed cell death

Exogenous ligands of the aryl hydrocarbon receptor (AhR) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related substances are highly toxic pollutants ubiquitously present in the environment. They cause a variety of toxic effects to different organs and tissues. Among other effects, TCDD exposure to laboratory animals leads to thymus atrophy and immunosuppression on the one hand, and to tumor formation on the other. Apoptosis appears to be involved in both these toxic effects: AhR activation by TCDD was discussed to induce apoptosis of immune cells, leading to the depletion of thymocytes and ultimately immunosuppression. This mechanism could help to explain the highly immunotoxic actions of TCDD but it is nevertheless under debate whether this is the mode of action for immunosuppression by this class of chemical substances. In other cell types, especially liver cells, TCDD inhibits apoptosis induced by genotoxic treatment. In initiation-promotion studies, TCDD was shown to be a potent liver tumor promoter. Among other theories it was hypothesized that TCDD acts as a tumor promoter by preventing initiated cells from undergoing apoptosis. The exact mechanisms of apoptosis inhibition by TCDD are not fully understood, but both in vivo and in vitro studies consistently showed an involvement of the tumor suppressor p53 in this effect. Various strings of evidence have been established linking apoptosis to the detrimental effects of exogenous activation of the AhR. Within this article, studies elucidating the effects of TCDD and related substances on apoptosis signaling, be it inducing or repressing, is to be reviewed.

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.

Dioxin-induced toxicity on vascular remodeling of the placenta

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

Xenobiotic-activated receptors: from transcription to drug metabolism to disease

Xenobiotic-activated receptors (XARs) are a group of ligand-activated transcription factors that are evolutionally specialized to regulate genomic programs to protect the body against innumerable chemicals from the environment. XARs share unique properties, such as promiscuous ligand binding, conserved structural motifs, common protein partners, and overlapping target genes. These unique features of XARs clearly distinguish them from receptors that are activated by endogenous chemicals to regulate energy metabolism, reproduction, and growth and differentiation. XARs regulate xenobiotic metabolism and disposition by controlling the expression and induction of drug-metabolizing enzymes and transporters. Furthermore, XARs integrate a broad range of protective mechanisms, such as antioxidative response and immune/inflammatory functions, to antagonize foreign chemicals. As the primary means of xenobiotic sensing and defense, XARs are intimately involved in drug disposition, polymorphic drug clearance, drug-drug interaction, and pathogenesis of some chemically induced cancers and chronic diseases. As a consequence, some XAR characteristics have been exploited in drug development and safety evaluation of drugs and environmental carcinogens and toxicants. In this perspective, common features and recent advances in the structures, modes of action, and implications in disease and drug development of XARs are discussed.

The role of exposure versus body burden data in deriving health guidance values

The Agency for Toxic Substances and Disease Registry (ATSDR) derives health-based guidance values to estimate daily human exposure to hazardous substances that are likely to be without appreciable risk of adverse noncancer effects for specific routes and durations of exposure. Most of these guidance values are derived from data showing external dose/health effect relationships. However, for chemicals that persist in the body, information on body burdens may provide more accurate understanding of their toxicity. This article evaluates the exposure versus body burden approaches using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and lead as examples.

Cell death and impairment of glucose-stimulated insulin secretion induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the beta-cell line INS-1E

The aim of this research was to characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity on the insulin-secreting beta-cell line INS-1E. A sharp decline of cell survival (below 20%) was observed after 1 h exposure to TCDD concentrations between 12.5 and 25 nM. Ultrastructurally, beta-cell death was characterized by extensive degranulation, appearance of autophagic vacuoles, and peripheral nuclear condensation. Cytotoxic concentrations of TCDD rapidly induced a dose-dependent increase in intracellular calcium concentration. Blocking calcium entry by EGTA significantly decreased TCDD cytotoxicity. TCDD was also able to rapidly induce mitochondrial depolarization. Interestingly, 1 h exposition of INS-1E cells to very low TCDD concentrations (0.05-1 nM) dramatically impaired glucose-stimulated but not KCl-stimulated insulin secretion. In conclusion, our results clearly show that TCDD exerts a direct beta-cell cytotoxic effect at concentrations of 15-25 nM, but also markedly impairs glucose-stimulated insulin secretion at concentrations 20 times lower than these. On the basis of this latter observation we suggest that pancreatic beta-cells could be considered a specific and sensitive target for dioxin toxicity.

DOSE- AND TIME-DEPENDENT EFFECTS OF 2,3,7,8-TETRABROMODIBENZO-P-DIOXIN ON RAT LIVER

Dose- and time-dependent effects of 2,3,7,8-tetrabromodibenzo-p-dioxin (TBDD) on the liver were examined by single administration of TBDD by gavage to male and female rats. Fifteen Wistar rats of each sex per group received 0, 10, 30, 100 or 300 microg TBDD/kg body weight. Rats surviving to scheduled necropsy on Day 2, 7 or 36 after the TBDD administration were examined for hepatic histopathology, activities of hepatic microsomal enzymes and serum levels of lipids, total cholesterol and transaminases and hepatic concentrations of TBDD. Tigroid basophilic cytoplasm and hepatocellular hypertrophy were observed at 10 microg/kg on Day 2 or 7 through 36, whereas degenerative and aggressive lesions such as necrosis, fibrosis, multinucleated hepatocytes and disarrangement of hepatocytes occurred later at higher dose levels. Persistently increased activities of hepatic aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECOD) and ethoxyresorufin O-deethylase (EROD), increased serum levels of total cholesterol and phospholipid and increased relative liver weight were observed in all groups dosed 10 mug/kg and above, suggesting that hepatic microsomal monooxygenases and basophilic cytoplasm of hepatocytes were early and sensitive indicators among those TBDD-induced effects. A dose-dependent increase in liver concentrations of TBDD on Day 2 was followed by logarithmic decreases in TBDD concentrations against the days elapsed after the TBDD administration. An elimination half-life (t(1/2)) of TBDD from the liver was estimated to range from 12 to 16 days. It was suggested that females were more susceptible to TBDD than males, and that acute hepatotoxicity of TBDD was as potent as that of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Comparative toxicogenomic analysis of the hepatotoxic effects of TCDD in Sprague Dawley rats and C57BL/6 mice

In an effort to further characterize conserved and species-specific mechanisms of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated toxicity, comparative temporal and dose-response microarray analyses were performed on hepatic tissue from immature, ovariectomized Sprague Dawley rats and C57BL/6 mice. For temporal studies, rats and mice were gavaged with 10 or 30 microg/kg of TCDD, respectively, and sacrificed after 2, 4, 8, 12, 18, 24, 72, or 168 h while dose-response studies were performed at 24 h. Hepatic gene expression profiles were monitored using custom cDNA microarrays containing 8567 (rat) or 13,361 (mouse) cDNA clones. Affymetrix data from male rats treated with 40 microg/kg TCDD were also included to expand the species comparison. In total, 3087 orthologous genes were represented in the cross-species comparison. Comparative analysis identified 33 orthologous genes that were commonly regulated by TCDD as well as 185 rat-specific and 225 mouse-specific responses. Functional annotation using Gene Ontology identified conserved gene responses associated with xenobiotic/chemical stress and amino acid and lipid metabolism. Rat-specific gene expression responses were associated with cellular growth and lipid metabolism while mouse-specific responses were associated with lipid uptake/metabolism and immune responses. The common and species-specific gene expression responses were also consistent with complementary histopathology, clinical chemistry, hepatic lipid analyses, and reports in the literature. These data expand our understanding of TCDD-mediated gene expression responses and indicate that species-specific toxicity may be mediated by differences in gene expression which may help explain the wide range of species sensitivities and will have important implications in risk assessment strategies.

Interactive effects between CYP1A1 genotypes and environmental polychlorinated dibenzo-p-dioxins and dibenzofurans exposures on liver function profile

Previous studies reported that polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) induced hepatic cytochrome P-4501A1 (CYP1A1). The aim of this study was to examine the interactive influence of CYP1A1 genotypes and PCDD/Fs exposure on liver function profile. PCDD/Fs levels and liver function parameters were determined in serum and correlated with genetic polymorphism of CYP1A1/Msp 1 in 225 human volunteers who had no or minimal occupational exposure to PCDD/F. The results showed that the highest glutamate pyruvate transaminase (GPT) activity levels were found in subjects with homozygous variant CYP1A1/Msp 1, followed by heterozygous variant, and finally homozygous wild type for those individuals whose serum PCDD/Fs levels were higher than 17.4 pg WHO-TEQ/g lipid. Data suggest that GPT activity levels may be modified by interaction of CYP1A1/Msp 1 genotype with dioxin after adjustment for age, alcohol consumption, and history of liver illness. Further studies are needed to characterize the variation in other related genes to verify whether a correlation exists between serum PCDD/Fs levels and adverse health effects.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced impairment of glucose-stimulated insulin secretion in isolated rat pancreatic islets

We have explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administration on the secretory function of isolated rat pancreatic islets. Twenty-four hours after TCDD administration (1 microg/kg b.w., i.p.), rats showed no significant differences in plasma glucose, insulin, triglycerides and leptin levels whereas plasma-free fatty acids were significantly increased with respect to untreated controls. In isolated islets, DNA and protein content were unchanged, whereas insulin content was significantly decreased in TCDD-treated rats. Incubation with different concentrations of glucose demonstrated a significant impairment of glucose-stimulated insulin secretion in islets isolated from TCDD-treated rats, whereas insulin release was better preserved upon alpha-ketoisocaproate stimulation. A significant reduction of [3H]-2-deoxy-glucose uptake was observed in pancreatic tissue of TCDD-treated rats, whereas no significant reduction in GLUT-2 protein levels was detectable by immunoblotting in islets from TCDD-treated rats. We concluded that low-dose TCDD could rapidly induce significant alterations of the pancreatic endocrine function in the rat.

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

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

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.

The retinoid signaling system--a target in dioxin toxicity

This review summarizes the available data on the effects of dioxins on retinoid levels, retinoid-related enzyme activities, and toxicological endpoints that have been correlated to retinoid effects. Similarities between dioxin toxicity and retinoid deficiency as well as retinoid excess are pointed out. Several possible levels of interaction between the dioxin and the retinoid signaling pathways are discussed, including the involvement of the Ah receptor, altered retinoic acid homeostasis, and an altered set point for retinoid storage. A hypothesis for the effect of dioxins on retinoids is suggested. In this hypothesis, comprising two cascades of effects on the molecular level, the effect of dioxins on retinoic acid levels is central.

Toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

Dioxin-type chemicals produce a variety of toxic and biochemical changes, some of which occur at very low doses and last for long periods of time. The most consistent toxin effect is body weight loss. In animals, the histopathologic changes, which are hyperplastic and hypertrophic, affect the gastrointestinal mucosa and the urinary track epithelium. In contrast, atrophic responses are seen in the thymus. Both hyperplasia and necrosis are observed in the liver. The administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin also produces endocrine effects and changes in carbohydrate and lipid metabolism. The most serious toxic effects and the biochemical background of the lesions are reviewed.

Hepatic vitamin a depletion is a sensitive marker of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in four rodent species

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-treated animals show altered retinoid homeostasis and exhibit signs of toxicity similar to those of vitamin A-deficient animals. In this study we established dose-response curves for sublethal oral doses of TCDD and hepatic vitamin A gain in four rodent species. This was done to evaluate any potential correlation between decreased hepatic vitamin A gain and other TCDD-induced effects, particularly depressed body weight gain and hepatic CYP1A induction. Young Hartley guinea pigs, Sprague-Dawley rats, C57BL/6 mice, and Golden Syrian hamsters were given single oral doses of TCDD at up to 2.5, 100, 1000, and 1000 microg/kg bw, respectively, and killed 28 days after treatment. Hepatic vitamin A gain was decreased 25% compared to controls at estimated doses of 0.1, 0.9, 1.1 and 3.6 microg/kg bw in guinea pigs, hamsters, rats, and mice, respectively. CYP1A induction and hepatic vitamin A gain were affected at similar dose levels and showed similar, but inverse dose-response curves in each of the four species, consistent with the hypothesis that altered vitamin A homeostasis is Ah-receptor mediated. In addition, there was an apparent correlation between the dose-response curves for decreased hepatic vitamin A gain and decreased body weight gain in all species. Taken together with the known importance of vitamin A in body weight regulation, this result was consistent with a contributing role for altered retinoid homeostasis in the wasting syndrome induced by TCDD.

Dioxin-induced perturbations in tryptophan homeostasis in laboratory animals

Polychlorinated dioxins (PCDD) are widespread environmental contaminants. The most potent and the general model compound for dioxins is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Our laboratory has developed a new model for studies of dioxin toxicity based on totally disparate sensitivity to the lethal action of TCDD between Long-Evans (L-E, Turku AB; LD50 ca. 10 micrograms/kg) and Han/Wistar (H/W, Kuopio; LD50 over 10,000 micrograms/kg) rat strains. We have shown that body weight regulation is differentially regulated by TCDD in these rat strains: body weight gain is permanently reduced in the sensitive L-E but not in the resistant H/W strain. In concert with reduced body weight, TCDD increased brain TRP concentration, 5-HT synthesis and its metabolism to 5-HIAA at lethal doses in TCDD-susceptible L-E rats, and almost not at all in resistant H/W rats in which lethal dose levels were not reached. Further studies showed that TCDD indirectly increases free TRP concentration in the circulation in TCDD-susceptible L-E rats. Blood free fatty acids seem to be involved in the latter phenomenon. It is not likely that the enhanced serotonergic tone in the CNS is a causative factor in TCDD-induced anorexia. However, the present results may open up an interesting avenue to better understand physiology of TRP and the complex regulation of energy balance.

Interspecies differences in cancer susceptibility and toxicity

One of the most complex challenges to the toxicologist represents extrapolation from laboratory animals to humans. In this article, we review interspecies differences in metabolism and toxicity of heterocyclic amines, aflatoxin B1, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and related compounds, endocrine disrupters, polycyclic aromatic hydrocarbons, tamoxifen, and digitoxin. As far as possible, extrapolations to human toxicity and carcinogenicity are performed. Humans may be more susceptible to the carcinogenic effect of heterocyclic amines than monkeys, rats, and mice. Especially, individuals with high CYP1A2 and 3A4 activities and the rapid acetylator phenotype may be expected to have an increased risk. Striking interspecies variation in susceptibility to aflatoxin B1 carcinogenesis is known, with rats representing the most sensitive and mice the most resistant species, refractory to dietary levels three orders of magnitude higher than rats. An efficient conjugation with glutathione, catalyzed by glutathione S-transferase mYc, confers aflatoxin B1 resistance to mice. Extremely large interspecies differences in TCDD-induced toxicity are known. The guinea pig is the most susceptible mammal known, with an LD50 in the range 1-2 micrograms TCDD/kg, whereas the hamster is the most resistant species with an LD50 greater than 3000 micrograms/kg. A number of experts have pointed out to the fact that humans appear to be less sensitive to TCDD than most laboratory animals. Human exposure to background levels of TCDD is not likely to cause an incremental cancer risk. A clear cause--effect relationship has been shown between environmental endocrine-disrupting contaminants and adverse health effects in wildlife, whereas the effects seem to be less critical for humans. Studies on DNA adduct formation and metabolism of the nonsteroidal antiestrogen tamoxifen indicate that rats and mice are orders of magnitude more susceptible than humans.

Panax ginseng protects the testis against 2,3,7, 8-tetrachlorodibenzo-p-dioxin induced testicular damage in guinea pigs

OBJECTIVES

To investigate histopathologically the beneficial effects of Panax ginseng extract on guinea pig testes damaged by 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD).

MATERIALS AND METHODS

Ninety guinea pigs were divided into six equal groups. The normal controls (group 1) received vehicle and saline; group 2 received TCDD (1 microgram/kg) intraperitoneally; group 3 and 4 received 100 or 200 mg/kg per day of Panax ginseng water extract (PG-WE) intraperitoneally for 28 days from 1 week before TCDD injection; groups 5 and 6 received PG-WE for 14 days from 1 week after TCDD treatment.

RESULTS

The gain in body weight was less in groups treated with TCDD than in controls. Moreover, the body weight of group 2 decreased from 14 days after TCDD exposure, while that of groups 3 and 4 increased; there was no decrease in body weight in groups 3-6. The decrease in testicular weight caused by TCDD was prevented by PG-WE. Light microscopy showed smaller tubules and late maturation arrest in group 2; electron microscopy showed a dissolution of the germinal epithelium, disrupted tight junctions between adjacent Sertoli cells, and altered germ cells at all developmental stages. The maturation arrest in germ cells caused by TCDD was ameliorated in groups 3-6. The testes almost completely recovered in groups 3 and 4 and there was some therapeutic effect of PG-WE in groups 5 and 6.

CONCLUSIONS

These results confirm the protective and therapeutic effects of Panax ginseng on atrophy and testicular damage induced by TCDD, providing evidence that ginseng might be a useful agent in preventing and treating testicular damage induced by environmental pollutants.

Considerations on genetic and environmental factors that contribute to resistance or sensitivity of mammals including humans to toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Part 1: Genetic factors affecting the toxicity of TCDD

The marked species differences in short-term toxicity (30-day LD50) of ca. 10,000 (LD50: guinea pigs ca. 1 microgram/kg body wt and Han/Wistar Kuopio rats more than 9600 micrograms/kg body wt) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is one of the central issues of the controversies that have developed on the validity of risk assessment strategies for TCDD and related compounds. One of the most challenging issues that toxicologists face today is the identification of genes that contribute to or are responsible for increased resistance or sensitivity to TCDD and related compounds. It is assumed that most, if not all, toxic effects of TCDD are mediated more or less through the binding affinity to the Ah receptor. This hypothesis was extended and tries to explain the differences in sensitivity/resistance of animals including humans to TCDD by their total fat (lipid) content. In this respect the gene or genes which is or are responsible for obesity of mammals including humans are of great interest. An obvious linear positive logarithmic relationship between the oral 30-day LD50 (microgram/kg) of TCDD in different species and strains of mammals and their total body fat content (TBF%) was found: log LD50 = 5.30 x log (TBF)-3.22, or LD50 = 0.000603 x (TBF)5.30. By means of this regression the toxicity of TCDD in mammals including humans of different age and/or body weight can be predicted if their total body fat content is known. Examples of single-gene and polygenic disease models in different mammals, such as nonobese diabetic, diabetic, viable yellow, obese, and fat mice, as well as transgenic mice, and other suitable animal models, such as fatty Zucker rats, Han/Wistar (Kuopio) rats, and minipigs, are discussed, and predicted LD50 values of TCDD in these animals and humans are presented.

Evaluation of serum lipid concentrations among U.S. workers exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters lipid metabolism in animals; however, evidence for such an effect in humans is conflicting. This conflict was addressed using data from a cross-sectional medical study conducted between 1987 and 1988. The exposed participants had been employed at least 15 y earlier in the manufacture of 2,4,5-trichlorophenol or one of its derivatives at two chemical plants in the United States. A total of 281 workers and 260 unexposed referents participated. Workers had substantial exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin, evidenced by a median serum 2,3,7,8-tetrachlorodibenzo-p-dioxin concentration of 406.6 femtograms/gram of serum (fg/g serum), compared with 36.9 fg/g serum among the referents. A slight association between triglyceride concentration and serum 2,3,7,8-tetrachlorodibenzo-p-dioxin concentration was found (p = .05). Over the range of observed 2,3,7,8-tetrachlorodibenzo-p-dioxin values (i.e., 37-19000 fg/g serum), triglyceride concentration increased only about 0.4 mmol/I. No association was found between an abnormally elevated triglyceride (i.e., > 2.82 mmol/I) concentration and serum 2,3,7,8-tetrachlorodibenzo-p-dioxin concentration. An association was also found between serum 2,3,7,8-tetrachlorodibenzo-p-dioxin concentration and an abnormal high-density lipoprotein concentration (p = .09). in summary, there was evidence of an effect on lipid metabolism in a group of workers with high serum 2,3,7,8-tetrachlorodibenzo-p-dioxin concentrations. The influence of serum 2,3,7,8-tetrachlorodibenzo-p-dioxin on lipid concentrations, however, was small, compared with the influence of other factors.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on tryptophan and glucose homeostasis in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters

We have previously reported that in rats 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) lethality is associated (although not necessarily causally) with changes in brain serotonin (5-HT) metabolism. In the present study, we have examined whether this holds for other species by comparing the effect of TCDD in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters, respectively. Body weight gain of guinea pigs exposed to TCDD (0.3-2.7 micrograms/kg) diminished dose dependently, while the effect was marginal in hamsters (900-4600 micrograms/kg). Brain 5-hydroxyindoleacetic acid (the main metabolite of brain 5-HT), brain tryptophan (the precursor amino acid of 5-HT), and plasma free and total tryptophan were not affected at any dose in guinea pigs. In contrast, 4 days after exposure, the levels of plasma free and total tryptophan were consistently increased in hamsters. These, as well as brain tryptophan, were still elevated 10 days after exposure. TCDD did not affect plasma glucose level in either species. Liver glycogen was decreased in a dose-dependent manner in TCDD-treated guinea pigs as well as in their pair-fed controls on day 10. There was no change in liver glycogen in hamsters. The activity of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase was only depressed in hamsters by all doses of TCDD. We conclude that changes in tryptophan metabolism or in carbohydrate homeostasis cannot explain the wide interspecies differences in susceptibility to the acute lethality of TCDD, although they may correlate with some aspects of its toxicity in certain species.

Dioxins: model chemicals for assessing receptor-mediated toxicity

Dioxins and related compounds are chlorinated aromatic hydrocarbons that are persistent in both environmental and biological samples. Many members of this class of compounds produce a similar spectrum of toxicity which is mediated by interaction with the Ah receptor. The toxic effects of these chemicals can best be described by their actions as growth dysregulators. Dioxins disrupt normal homeostatic processes that tightly regulate cellular growth and differentiation. Disruption in these processes produce a variety of toxicities and pathologies. The available data indicate that humans are sensitive to the toxic effects of these chemicals. Clearer definition of human responses and the body burdens associated with such effects requires more research. Comprehensive risk assessments of dioxins should include all Ah receptor ligands such as the halogenated dibenzofurans and biphenyls.

Biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds on the central nervous system

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds are an important class of environmental contaminants which induce several types of biochemical alterations. Their effects have been most thoroughly characterized in the liver, especially regarding the Ah receptor-mediated induction of xenobiotic metabolizing enzymes. The behavioral signs exhibited by animals exposed to TCDD (progressive anorexia and body weight loss) suggest a role for the central nervous system (CNS) in TCDD toxicity. At lethal doses, TCDD affects the metabolism of serotonin, a neurotransmitter able to modulate food intake in the brain. This effect is associated with an elevated concentration of free tryptophan in the plasma. There does not appear to be any major changes in catecholaminergic neurotransmitter systems in TCDD-treated rats. Cytochrome P-450 related enzyme activities are induced by TCDD in the brain. As is the case in the liver, this induction does not correlate with susceptibility to TCDD lethality in rats. The involvement of the CNS in TCDD toxicity is still obscure. Elucidation of this role as well as the mechanism of TCDD-induced wasting may well advance our understanding of the regulation of food intake and body weight.

Effect of 2,3,7,8-tetrachlorodibenzoy-p-dioxin on the hepatic 7-ethoxyresorufin O-deethylase activity in four rodent species

Temporal and dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on hepatic 7-ethoxyresorufin O-deethylase (EROD) activity were investigated in young male Hartley guinea pigs, Sprague-Dawley rats, C57Bl/6 mice, DBA/2 mice and Golden Syrian hamsters. Animals were terminated 1, 7, 14, 28, 56 and 112 days after the administration of a single i.p. dose of TCDD. The maximal induction of EROD activity, at doses producing a similar toxic and limited lethal effect in all species/strains, was 42-, 18-, 7- and 3-fold in rats, DBA/2 mice, C57Bl/6 mice and guinea pigs, respectively. No treatment-related induction of EROD activity was observed in hamsters. Generally, maximal induction occurred 1-4 weeks after injection in all species. The guinea pig alone maintained the same magnitude of induction of EROD activity throughout the study. Observed toxic effects, i.e., lethality, loss of body weight gain, liver enlargement and thymic atrophy, did not correlate with the TCDD-induced hepatic EROD activity. The obtained results suggest that the fold induction of cytochrome P450 1A1 activity is not a critical event for the expression of the lethal effect of TCDD in rodents.

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.

Comparative acute lethality of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin in the most TCDD-susceptible and the most TCDD-resistant rat strain

We have previously demonstrated a more than 300-fold difference in acute LD50 values for 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) between male Long-Evans (Turku AB; L-E) and Han/Wistar (Kuopio; H/W) rats after intraperitoneal exposure. In the present study, we compared the acute lethality of TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HCDD) in these strains by intragastric administration. In agreement with previous data, H/W rats proved to be strikingly resistant to TCDD, since even the highest dose tested, 7200 micrograms/kg, was below the LD50 level for both genders. The corresponding LD50 values for female and male L-E rats were 9.8 and 17.7 micrograms/kg, respectively. A similar strain difference was discovered for PCDD: the LD50 value was > 1620 micrograms/kg for female H/W rats and between 20 and 60 micrograms/kg for female L-E rats. Surprisingly, the acute lethality of HCDD did not follow the same pattern. Female H/W rats turned out to be only about 10 times less susceptible to that congener than female L-E rats (LD50 values 1871 and between 120 and 360 micrograms/kg, respectively). These findings do not support the widely accepted concept that sufficiently high doses of all dioxin congeners will produce the same effects. Either the higher chlorinated dioxins have toxic effects distinct from those of TCDD or the relative contribution of toxic impacts varies among these compounds.

Developmental and reproductive toxicity of dioxins and related compounds: cross-species comparisons

Developmental toxicity to TCDD-like congeners in fish, birds, and mammals, and reproductive toxicity in mammals are reviewed. In fish and bird species, the developmental lesions observed are species dependent, but any given species responds similarly to different TCDD-like congeners. Developmental toxicity in fish resembles "blue sac disease," whereas structural malformations can occur in at least one bird species. In mammals, developmental toxicity includes decreased growth, structural malformations, functional alterations, and prenatal mortality. At relatively low exposure levels, structural malformations are not common in mammalian species. In contrast, functional alterations are the most sensitive signs of developmental toxicity. These include effects on the male reproductive system and male reproductive behavior in rats, and neurobehavioral effects in monkeys. Human infants exposed during the Yusho and Yu-Cheng episodes, and monkeys and mice exposed perinatally to TCDD developed an ectodermal dysplasia syndrome that includes toxicity to the skin and teeth. Toxicity to the central nervous system in monkey and human infants is a potential part of the ectodermal dysplasia syndrome. Decreases in spermatogenesis and the ability to conceive and carry a pregnancy to term are the most sensitive signs of reproductive toxicity in male and female mammals, respectively.

Aryl hydrocarbon or dioxin receptor: biologic and toxic responses

1. The AhR represents a ligand-activated transcription factor. Receptor agonists include planar aromatic compounds, a variety of heterocyclic plant constituents, and PCDD/PCDF. The latter lead to persistent activation of the receptor due to their strong binding affinity and long biologic half-life of over 10 years in human blood and fat. Practically every person on earth is exposed to these compounds via the diet (> 90%) and by high concentrations in mother's milk. PCDD/PCDF produced toxic responses in exposed people (primarily chloracne and immunosuppression) in the past. However, the present PCDD/PCDF levels (basal levels) in the general population are below those warranting toxicologic concern. 2. The AhR has been characterized as a helix-loop-helix transcription factor related to the Drosophila developmental genes sim and per. The cytosolic form of the receptor is present as an inactive complex with two subunits of HSP90. After ligand binding HSP90 is released and the receptor enters the nucleus as a heterodimer together with a related protein ARNT. It binds with high affinity to certain enhancer elements in the upstream region of several genes such as cytochrome P4501A1 (CYP1A1). The AhR transcriptionally activates several drug-metabolizing enzymes and proteins involved in growth/differentiation, such as the plasminogen activator inhibitor PAI-2 and IL-1 beta. In addition, it modulates the action of a number of other nuclear transcription factors such as receptors of the steroid hormone receptor superfamily and of cell surface receptors such as EGF. With the exception of CYP1A1 induction, little is known about the mechanism of transcriptional activation of the AhR-controlled genes. Many AhR-modulated biologic responses (such as modulation of the estrogen and EGF receptor) appear to be indirect. 3. Persistent activation of the AhR is probably responsible for toxic responses in experimental animals and humans. They are markedly tissue and species specific. In rodents a wasting syndrome, immunosuppression, teratogenicity, chloracne, and carcinogenicity/tumor promotion have been well studied. There is good evidence for an involvement for the AhR in these responses. However, the chain of events from receptor activation to the diverse toxic endpoints is largely unknown. Alteration of growth and differentiation of epithelial tissues may underlie most of the toxic responses. A lot has already been achieved, mostly by characterizing the AhR and transcriptional activation of CYP1A1. Still more work lies ahead of us, for example, elucidation of the physiologic roles of the AhR and of the chains of events from receptor activation to the various biologic and toxic endpoints.

Toxicity of 2,3,7,8-tetrabromodibenzo-p-dioxin in rats after single oral administration

Five male and female rats per dose-group received 2,3,7,8-tetrabromodibenzo-p-dioxin (2,3,7,8-TBDD) once on the first day of the study. Doses of 10, 33, 100, or 300 micrograms 2,3,7,8-TBDD/kg body wt. and the vehicle control were administered by gavage. About 20% of 2,3,7,8-TBDD was excreted via feces. Severe body weight retardation was observed in the 100 and 300 micrograms/kg dose-groups. Most animals in the 300 micrograms/kg dose-group and the females receiving 100 micrograms/kg showed emaciation, rough coat and a poor health (wasting syndrome). Of the animals dosed with 300 micrograms/kg, 3 males and all females died. After 100 micrograms 2,3,7,8-TBDD/kg 3 females died. Measured 4 weeks after dosing, triiodothyronine (T3) was increased and thyroxin (T4) was reduced dose dependently in serum. A dose-dependent decrease in thymus weights was observed at necropsy and histological examinations showed that thymus and spleen were depleted of mature lymphocytes. An increase in liver-to-body weight ratio was observed in all dose-groups. The histological examination revealed hypertrophy of centrilobular hepatocytes in the liver of animals treated with 100 micrograms/kg, which was less severe at the 33 micrograms/kg dose. Hypertrophic hepatocytes were also detected in some animals at the lowest dose. Induction of enzyme activities of the mixed function oxidases ethoxycoumarin O-deethylase (ECOD), ethoxyresorufin O-deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH) in liver tissue differed for each of the three enzymes. Two days after administration, enzyme activities were increased but did not differ substantially between dose-groups. Twenty-eight days after dosing the increase in activity after 10 micrograms/kg was largest and the EROD of the 100 micrograms/kg dose-group in females was close to that of the control. This inverse dose-response relationship may be due to impaired liver cell function at higher doses.

Comparison of 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated hepatotoxicity in C57BL/6J and DBA/2J mice

The toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was examined by clinical chemistry and liver histopathology in Ah-responsive C57BL/6J (C57) and Ah-nonresponsive DBA/2J (DBA) mice. Hepatotoxicity was assessed at 1, 3, and 7 d following a single ip injection of TCDD at doses that maximally induce hepatic aryl hydrocarbon hydroxylase (AHH) activity (3 micrograms/kg for C57 and 30 micrograms/kg for DBA mice) and at doses approaching the LD50 (150 micrograms/kg for C57 and 600 micrograms/kg for DBA mice). Histological examination of liver sections was found to be a more sensitive detection method for TCDD-induced hepatic changes than clinical chemistry analyses. Dramatic differences in the development and type of liver injury were observed between TCDD-treated C57 and DBA mice. C57 mice given 3 micrograms TCDD/kg developed mild to moderate hepatic lipid accumulation in the absence of both inflammation and necrosis. Severe fatty change and mild inflammation and necrosis occurred in C57 mice that received 150 micrograms TCDD/kg. In contrast, DBA mice exposed to 30 micrograms TCDD/kg developed hepatocellular necrosis and inflammation without any fatty change. Only slight hepatic lipid accumulation occurred with some necrosis and inflammation in DBA mice given 600 micrograms TCDD/kg. The Ah locus may play a role in determining the sensitivity of C57 mice to the steatotic effects of TCDD.

Response of the antral mucosa of the rat stomach to 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) produces a striking hypergastrinemia in rats that is thought to mediate the antiatrophy effect of TCDD on the oxyntic gland mucosa of the stomach. However, effects of TCDD on the antral mucosa, which is the origin of most physiologically released gastrin and is not a target for the trophic action of gastrin, has yet to be thoroughly investigated. Also gastrin release from gastrin-containing cells (i.e., G-cells) in the antral mucosa is inhibited by the paracrine secretion of somatostatin from D-cells in the antrum. Our purpose was to determine if the antral mucosa is affected by the trophic influence of TCDD and if alterations in antral mucosa levels of gastrin or somatostatin cause the hypergastrinemia. TCDD (100 micrograms/kg, Day 14 post-treatment) had a trophic effect on the antral mucosa. This was demonstrated histologically and by significant increases in antral wet weight and antral mucosa height. In contrast, pair-fed control rats that lost the same amount of body weight developed antral mucosa atrophy. With respect to serum and antral levels of gastrointestinal hormones, TCDD produced a 7- to 10-fold increase in serum gastrin concentrations that was not detected until Day 14 post-treatment. In contrast, serum gastrin concentrations in pair-fed control rats were comparable to those of control rats. The number of G-cells in the antral mucosa was not affected by either TCDD treatment or paired-feed restriction. These findings demonstrate that hypergastrinemia in TCDD-treated rats is not caused by reduced feed intake or antral G-cell hyperplasia. A major finding was that antral mucosa levels of both gastrin and somatostatin were decreased significantly in TCDD-treated rats. However, the temporal development and dose-dependence of these TCDD effects on antral hormone levels were quite different than those for hypergastrinemia. TCDD-induced decreases in antral levels of gastrin and somatostatin were detected 1 week earlier than hypergastrinemia. Also, the ED50 of TCDD on Day 14 post-treatment for the decrease in antral mucosa content and concentration of gastrin (29 and 22 micrograms/kg, respectively) and somatostatin (24 and 19 micrograms/kg, respectively) was less than that for hypergastrinemia (46 micrograms/kg). These time- and dose-dependent differences demonstrate that hypergastrinemia in TCDD-treated rats is not a consequence of reduced antral levels of gastrin or somatostatin. We conclude that the antral mucosa, an epithelial tissue not responsive to the proliferative effect of gastrin, is nevertheless a target for the trophic influence and gastrointestinal hormone-altering effects of TCDD.

The tissue distribution of 2,3,7,8-chlorine substituted dibenzo-p-dioxins in humans who died of cancer

The tissue distribution of 2,3,7,8-chlorine substituted dibenzo-p-dioxins was conducted in 11 patients who died of cancer. The concentration of octachlorodibenzo-p-dioxin (octa-CDD) was the highest in each organ and tissue and hepta-CDD was also found at relatively high levels, second only to OCDD. The levels of 1,2,3, 7,8-penta-CDD and 1,2,3,6,7,8-hexa-CDD in the spleen were the highest, respectively. 2,3,7,8-Tetra-CDD was also detected and its concentration was the highest in the gonad (0.8-3.2 pg/g-range). From the 2,3,7,8-TCDD toxic equivalent calculations, the highest equivalent value was obtained from a 54-year-old female who died of cancerous goiter. This individual had the highest concentrations of 2,3,7,8-substituted penta- and hexa-CDDs among the 11 patients.

Correlation between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and total body fat content in mammals

Single oral 30-day LD50s of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were correlated with total body fat (TBF) content in various species and strains of laboratory mammals. LD50 values and TBF contents were either obtained from the literature or determined by experiments. A log (LD50) vs. log (TBF) plot yielded a highly significant linear regression equation (r2 = 0.834, P less than 0.001, n = 20). It is suggested that this correlation exists for at least two reasons: (1) increasing TBF content in organisms represents an enhanced capacity to remove TCDD from the systemic circulation and (2) different TBF content reflects a differential role and regulation of fat metabolism for various organisms. Extrapolation of this correlation to man suggests that adult humans are among the less sensitive species to the acute toxicity of TCDD.

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)

Role of hypergastrinemia in the antiatrophy effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on oxyntic gland mucosa of the rat stomach

Atrophy of the gastrointestinal mucosa that occurs in pair-fed control rats is not observed in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats (1). Our objective was to determine if the gastrointestinal trophic hormone, gastrin, is involved in the antiatrophy effect of TCDD on the gut mucosa. Adult male Sprague-Dawley rats treated with 100 micrograms/kg of TCDD were slightly hypergastrinemic 7 days after dosing and markedly hypergastrinemic 14 days after treatment whereas pair-fed control rats were normogastrinemic. After 14 days of feed restriction, atrophy of the oxyntic gland and ileum mucosa occurred in pair-fed control rats but only atrophy of the ileum mucosa developed in TCDD-treated animals. The oxyntic gland mucosa of TCDD-treated rats was protected from mucosa atrophy as well as from mucosa erosions. The protection against feed restriction-induced atrophy was demonstrated by measurements of oxyntic gland mucosal height and DNA and protein content. Since hypergastrinemia stimulates growth of oxyntic gland mucosa, but not ileum mucosa, the antiatrophy effect of TCDD on mucosa of the oxyntic gland might in part be due to hypergastrinemia. In support of this interpretation, TCDD treatment exerted an antiatrophy effect on the oxyntic gland mucosa only when TCDD-treated animals were hypergastrinemic. For example, hypergastrinemia does not develop within the first 48 hr after TCDD administration, and TCDD treatment affords no protection against fasting-induced atrophy of the oxyntic gland mucosa during this time. On the other hand, the ability of TCDD treatment to protect against feed restriction-induced erosions of the oxyntic gland mucosa might be mediated by hypergastrinemia since these events occur at a later time.(ABSTRACT TRUNCATED AT 250 WORDS)

Species differences in estrogen receptors and in the response to 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure

The acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibits marked interspecies variability, with the guinea-pig, rat and hamster representing the species most sensitive, intermediate and most resistant to acute toxicity. Prepubertal guinea-pigs, rats and hamsters were treated with a single intraperitoneal injection of TCDD in olive oil at doses of 4, 50 and 1500 micrograms/kg, respectively. These exposures were chosen to produce acute toxicity and all 3 species exhibited a decrease in the rate of body weight gain during the 7 days following TCDD exposure when compared with control (olive oil-treated) animals. On the 7th day after exposure, the density and affinity of 17 beta-estradiol receptors were determined in the uterus and liver of TCDD-treated and control animals. The treatment with TCDD did not alter the affinity of the receptors in these 3 species. The density of hepatic 17 beta-estradiol receptors was decreased 65% in the guinea pig and 92% in the rat following exposure to TCDD. In contrast, TCDD-treated hamsters exhibited no change in the density of hepatic 17 beta-estradiol receptors. The uterine 17 beta-estradiol receptors were increased in density by TCDD treatment in the hamster and in the rat when expressed per mg protein. Uterine wet weights in the guinea-pig and rat were also significantly decreased by TCDD treatment but were not changed in the hamster. When the Bmax for uterine 17 beta-estradiol receptors was expressed as pmol/g tissue wet weight. TCDD exposure was found to produce an 11% decrease in density in the rat, while producing a 44% increase in the hamster. In control animals, the density of uterine 17 beta-estradiol receptors correlated inversely with the lethal dose of TCDD in these 3 species (i.e., the guinea-pig has the lowest LD50 and highest density of uterine 17 beta-estradiol receptors). The different responses to TCDD in the 3 species suggest that the changes in 17 beta-estradiol receptors may be related to species-specific toxic responses associated with TCDD exposure.

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.

In vivo kinetics and DNA-binding properties of the Ah receptor in the golden Syrian hamster

The in vivo long-term cytosolic-nuclear kinetics and DNA-binding properties of the Ah receptor were examined in liver from the golden Syrian hamster. For the kinetic studies, a dose of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin ([3H]TCDD) that has been previously shown to produce maximal and sustained hepatic enzyme induction without substantial toxicity was used. Following an intraperitoneal dose of 10 micrograms/kg of [3H]TCDD, occupied cytosolic receptor levels reached a peak within 8 h and then decreased rapidly to a level that was approximately 2% of the total receptor. Throughout the 35-day period, unoccupied cytosolic receptor represented from 65 to 80% of the total receptor content. At 8 h following dosing, less than 30% of the total amount of receptor was associated with the nuclear fraction; this percentage declined slowly to less than 5% of the total at Day 35. The half-life for the decline in detectable nuclear receptor levels was 13 days and was similar to the half-life for the decline in [3H]TCDD content of the whole liver, cytosol, and nuclear extract. The Ah receptor contained in hamster hepatic cytosol underwent a ligand-dependent transformation in vitro to two forms having affinity for DNA-Sepharose, one of which was isolated from nuclei of animals treated with [3H]TCDD in vivo. A comparison of the specific binding recovered following various analytical procedures revealed that the binding of [3H]TCDD to the form not found in nuclear extracts was more labile under certain experimental conditions. These studies indicate the heterogeneity of the Ah receptor in hamster hepatic cytosol and suggest that DNA binding in vitro and nuclear uptake in vivo occur through a ligand-dependent transformation process. The maintenance of maximal hepatic enzyme induction is, in part, a consequence of the sustained presence in the nucleus of only a small percentage of the total receptor content. The whole-tissue kinetics of TCDD appears to be a major factor regulating the long-term retention of the TCDD-receptor complex in the nucleus.

Investigations on the role of free radical processes in hexachlorobenzene-induced porphyria in mice

Male C57Bl/10 mice were chronically fed hexachlorobenzene (HCB) (0.02% of the diet) alone or in combination with a single subcutaneous dose of iron (12.5 mg iron per mouse). After eight weeks the group of mice pretreated with the iron overload was highly sensitized to the porphyrogenic effect of HCB, as shown by liver porphyrin accumulation. A synergistic effect of iron was evident on other parameters too, such as HCB-induced hepatic damage, activation of type O of xanthine oxidase, and decreased activity of copper zinc superoxide dismutase and glutathione peroxidase(s). None of these parameters was affected by iron alone. Iron alone and in association with HCB markedly raised the level of lipid peroxides, the increase in the HCB group being smaller. The combined treatment resulted in a significant reduction of HCB's inductive effects on microsomal heme and cytochromes P-450 and b5 and on the activity of aryl hydrocarbon hydroxylase. The content of nonprotein sulfhydryl groups was reduced to the same extent in mice treated with HCB or HCB plus iron. The results suggest that reactive intermediates such as are formed by lipid peroxidation are not sufficient on their own to create the conditions for uroporphyrinogen decarboxylase impairment, as evident in the group of mice receiving iron overload alone. Conversely, HCB administration induced a specific condition of imbalance in the liver between formation and inactivation of reactive intermediates which was associated with hepatic porphyrin accumulation and was potentiated by concomitant administration of iron.