Toxicology of chlorinated dibenzo-p-dioxins

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)

Relative toxicity and tumor-promoting ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) in hairless mice

2,3,7,8-Tetrachlorodibenzo-p-dixoin 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) are highly toxic members of a class of environmental contaminants, the polychlorinated aromatic hydrocarbons (PCAH), which exhibit a similar and highly characteristic spectrum of toxic effects. For purposes of risk assessment, it is important to be able to make accurate estimates of the relative potency of these and related compounds. Previous investigations have indicated that, in acute exposure or in vitro studies, PCDF is approximately 0.1 times as toxic and HCDF is approximately 0.01 times as toxic as TCDD. In this study, we compared the relative toxicity and tumor-promoting abilities of TCDD, PCDF, and HCDF in hairless mouse skin. Female hairless mice (HRS/J hr/hr) were treated dermally with the initiator MNNG, then dosed twice weekly for 20 weeks with acetone, TCDD (2.5-10 ng/mouse/dose), PCDF (25-100 ng/mouse/dose), or HCDF (250-1000 ng/mouse/dose) as promoter. TCDD, PCDF, and HCDF were all potent promoters for the induction of squamous cell papillomas. There was, however, no difference in the incidence or multiplicity of papilloma formation between groups. The same doses of the three PCAH, in the absence of initiator, induced no skin papillomas. TCDD produced a significant increase in liver:body weight ratio (p less than 0.001) at all doses and a decrease in thymus:body weight ratio at a dose of 10 ng (p less than 0.001). Mice treated with PCDF and HCDF had marked thymic and splenic involution, liver hypertrophy, mucous cell hyperplasia in the fundic portion of the glandular stomach, and loss of body weight. PCDF and HCDF produced a greater incidence and severity of dermatotoxic effects than TCDD. Based on data for dermal toxicity and changes in body weight and organ weights, PCDF is estimated to be 0.2 to 0.4 times, and HCDF 0.08 to 0.16 times, as toxic as TCDD following repeated dermal exposure. Therefore, toxic equivalence factors generated using data from acute and/or in vitro studies may underestimate the risk from repeated low-dose exposures to these compounds.

NAD(P)H: quinone oxidoreductase (DT-diaphorase) in chick embryo liver. Comparison to activity in rat and guinea pig liver and differences in co-induction with 7-ethoxyresorufin deethylase by 2,3,7,8-tetrachlorodibenzo-p-dioxin

NAD(P)H:quinone oxidoreductase (EC 1.6.99.2; DT-diaphorase) was present in the liver of 18- and 19-day-old chick embryos as assayed both by reduction of resorufin and by the more traditional assay, reduction of 2,6-dichlorophenolindophenol (DCPIP). Both reductions had the classic characteristics of DT-diaphorase: they were equally supported by NADPH and NADH and almost entirely inhibited by dicumarol. Chick embryo liver DT-diaphorase was entirely cytosolic. It was undetectable in the microsomal and mitochondrial fractions. Chick embryo liver cytosol and mitochondrial fractions contained an enzyme oxidizer of resorufin but not of DCPIP. The Km for NADPH for resorufin reductase was an order of magnitude higher in chick embryo than in rat or guinea pig cytosol (1 mM vs 0.1 mM). Resorufin reductase activity was higher for chick embryo than for rat or guinea pig cytosols: Vmax (nmol resorufin reduced per mg cytosolic protein per min +/- SEM) 355 +/- 28 for chick embryo, 159 +/- 10 for guinea pig and 68 +/- 28 for rat. The Vmax for DCPIP reduction was also twice as high in chick embryo as rat liver cytosol. In the chick embryo, 7 days after treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at 6.4 micrograms/kg egg (1 nmol/egg) mortality was increased 2.4-fold, hepatic DT-diaphorase 1.3-fold, and 7-ethoxyresorufin deethylase (7-EROD) 72-fold over control levels. At 32 micrograms/kg, mortality was increased 4.2-fold, DT-diaphorase 2.3-fold and 7-EROD 100-fold. In the guinea pig, 5 days after treatment with TCDD at 10 micrograms/kg, TCDD toxicity was also evident (loss of body weight and thymus weight); there was no change in DT-diaphorase as measured by resorufin reduction, confirming by a different assay the observation of Beatty and Neal (Biochem Pharmacol 27: 505-510, 1978) that TCDD does not induce DT-diaphorase in guinea pig liver, and 7-EROD was increased 8-fold. In contrast, in the rat, 7 days after exposure to TCDD at 10 micrograms/kg, there was no evidence of toxicity, DT-diaphorase was increased close to 7-fold and 7-EROD, 100-fold. The results demonstrate that avian liver contains DT-diaphorase and show that the extent to which DT-diaphorase is part of the pleiotypic response of the liver to an Ah (aryl hydrocarbon) receptor ligand is species dependent. They also suggest that DT-diaphorase induction and TCDD toxicity may be inversely related. The possibility that DT-diaphorase protects against TCDD toxicity and participates in species differences in sensitivity to TCDD toxicity warrants further investigation.

TCDD resistance is inherited as an autosomal dominant trait in the rat

In the present study, genetic crossings were performed between the most TCDD-susceptible (Long-Evans) and the most TCDD-resistant (Han/Wistar) rat strains. The F1 offspring were as resistant to TCDD as the Han/Wistar rats irrespective of the sex of their Han/Wistar parents. In test-cross and F2 progeny the distribution of resistant and susceptible phenotypes was consistent with inheritance regulated by 2 (possibly 3) autosomal genes displaying complete dominance, independent segregation, and an additive co-effect. These data show that, in contrast to earlier findings in mice, TCDD resistance seems to be the dominant trait in the rat. Moreover, the results challenge the current view that the Ah-locus is the exclusive determinant of TCDD sensitivity.

Indoor air contamination with polychlorinated dibenzo-p-dioxins and dibenzofurans

Pentachlorophenol (PCP), used extensively for wood preservative purposes, contains trace amounts of polychlorinated dibenzodioxins (PCDDs) and-dibenzofurans (PCDFs) as contaminants. Residues of these compounds are present on the surface and sub-surface of the treated wood. These contaminants have the potential to wear (or migrate) away or volatilize from the wood surface and become entrained in ambient air or dust particles, and thus becoming available for human contact. During the early sixties several day nursery facilities were built with PCP-treated wood in the northern part of West Germany. In this paper we describe the indoor air monitoring data in these kindergarten buildings and the associated possible long-term health risk. The indoor ambient air was found to be contaminated with highly toxic PCDDs/PCDFs at pg/m3 levels. HxCDDs, HpCFs and OCDDs/OCDFs congeners were the major contaminants.

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.

Species comparison of steroid UDP-glucuronyl transferase: correlation to TCDD sensitivity

Estradiol glucuronidation via steroid UDP-glucuronyl transferase (sUDPGT) was examined in 2,3,7,8-te trachlorodibenzo-p-dioxin (TCDD) sensitive and resistant species and strains. Steroid UDPGT was not induced by treatment with TCDD or estradiol. The most sensitive species to TCDD lethality, the guinea pig, had relatively high steroid UDPGT activity, while the hamster, the most resistant species, and rats had low levels of activity; no differences in sUDPGT activities were observed between mouse or rat strains differing in susceptibility to TCDD intoxication. These results suggest a role for differences in steroid physiology in the determination of species susceptibility to TCDD, but also demonstrate that other factors are involved.

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.

Toxicity of fenclor 42 in mice: effects on immunocompetent cells

The aim of this study is to evaluate the effects of Fenclor 42 (a mixture of trichlorobiphenyls) on the immune system. A prolonged administration of this compound to CD2F1 mice resulted in a reduction of relative spleen and thymus weight according to the dose. Furthermore, spleen weights, total number of splenocytes and relative spleen weights decreased significantly also following a single treatment with 0.5 g/kg or 1 g/kg of Fenclor 42. An analysis of the functional activity of splenocytes pointed out that proliferative response to mitogens was also inhibited. Splenic parameters returned to normal values within 5 days after a single treatment and between 8 and 15 days after a subchronic administration. The functional activity of splenocytes was restored between day +5 and day +8 according to the different schedules of treatment. On the contrary, natural killer cell (NK) activity was never affected by Fenclor 42. Studies are in progress to elucidate the intimate mechanism of the toxicity of Fenclor 42 on immunocompetent cells.

Effects of TCDD on the EGF receptor of XB mouse keratinizing epithelial cells

TCDD was found to cause a marked inhibition of 125I-epidermal growth factor (EGF) binding to its receptor on the cell surface of XB mouse keratinizing epithelial cells (XB cells) cultured in vitro. The EC50 concentration was estimated to be on the order of 3 x 10(-11) M 24 hours after TCDD administration. As early as 12 hours after the addition of 10(-9) M of TCDD, XB cells showed signs of a decline in 125I-EGF binding levels. The level of such EGF receptor downregulation reached a maximum at 24 hours, continued until day 2, but completely recovered by day 3. This was accompanied by a rise in protein kinase activities, particularly those of the protein tyrosine kinases during the initial period of 6-24 hours. To test the hypothesis that the EGF receptors of the cells, by showing TCDD-induced symptoms of downregulation, actually are being activated and triggering EGF-like signals, we examined the effects of both TCDD and exogenously added EGF on cell morphology, colony formation degree of keratinization, the pattern of activation of protein kinases and de novo protein synthesis, and EGF receptor phosphorylation. Based on the similarity of cell responses to these between TCDD- and EGF-treated cells, we concluded that TCDD, directly or indirectly, causes activation of the EGF receptor. In contrast, 12-O-tetradencanoylphorbol-13-acetate (TPA), which is known to downregulate EGF receptors by blocking their protein tyrosine kinase, produced dissimilar end results. The balance of evidence support the notion that the action of TCDD in this cell line is tightly coupled to the activation of the EGF receptor and that one of the key consequences of such a biochemical change is that it signals these cells to commit to terminal differentiation.

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.

Toxicity of Fenclor 42 on Immunocompetent Cells

Polychlorinated biphenyls (PCBs) are environmental contaminants, whose toxicity is related to the degree and the position of chlorine substitutes. Fenclor 42 is a mixture of trichlorobiphenyls employed for industrial use in Italy. In order to evaluate its effects on the immune system, CD2F1 mice were treated with Fenclor 42 at different doses and schedules. Spleen weights, total number of splenocytes and relative spleen weight decreased significatively following a single or a prolonged exposure to different doses of Fenclor 42.

An analysis of the functional activity of splenocytes showed that the proliferative response to Con A and PWM mitogens was also inhibited by Fenclor 42. Splenic parameters returned to normal values within 5 days after a single injection and between 8–15 days after a subchronic administration of the compound. The capability of splenocytes to proliferate, following mitogen stimulation, was restored between day 5 and day 8 according to the different schedules of treatment. By comparison, Fenclor 42 never affected NK activity.

These data clearly indicate damage at the splenic level by Fenclor 42. Studies are in progress to elucidate the mechanisms involved in the immunotoxic response.

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.

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.

Hepatic Ah-receptor levels and the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on hepatic microsomal monooxygenase activities in a TCDD-susceptible and -resistant rat strain

Previous studies have shown that in two inbred strains of mice, straightforward correlations exist among the number of hepatic Ah-receptors, enzyme inducibility by TCDD, and lethality of TCDD. Here, studies were conducted in two strains of rats (Han/Wistar and Long-Evans) which differ widely in susceptibility to the lethal effects of TCDD, to determine if these are general phenomenona in TCDD toxicity. The total number of specific binding sites (Ah-receptors) for [3H]TCDD proved to be approximately equal in the livers of both rat strains. Likewise, no notable difference was detected in the effect of TCDD on the activities of 7-ethoxyresorufin O-deethylase, 7-ethoxycoumarin O-deethylase, and ethylmorphine N-demethylase or on the amount of cytochrome P-450 in hepatic microsomal fractions. Immunoblot analysis was carried out with monoclonal antibodies (Mabs). Mab 1-7-1 directed against rat liver 3-methylcholanthrene (MC)-inducible P-450 recognized forms P-450c and P-450d in TCDD-treated rats in a dose-dependent fashion and to a similar extent in both strains. In contrast, Mab 2-66-3 (against phenobarbital-inducible P-450) did not recognize any proteins in either strain, confirming the conclusion that TCDD elicits a MC-type induction of hepatic cytochrome P-450 in both strains of rats. Thus, it seems that the correlations observed in mice do not hold in rats and therefore should not be generalized. The parameters measured in the present study are causally unrelated to the mechanism of lethal action of TCDD in these rat strains.

Composition of diet modifies toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in cold-adapted rats

The effect of a high carbohydrate, high fat or high protein diet was studied on the acute toxicity of TCDD (125 micrograms/kg) in cold-adapted (4 +/- 1 degrees C) rats. Within 10 days after dosing, TCDD-treated rats fed a high carbohydrate or a high protein diet reduced their caloric intake by 25% whereas those fed a high fat diet consumed only 15% fewer kcal/MBS (metabolic body size). TCDD-treated rats fed a high protein diet lost body weight at the same rate as their pair-fed controls, whereas body weight loss in high fat-fed rats was significantly higher than in their pair-fed controls. In contrast, TCDD-treated rats fed a high carbohydrate diet effectively maintained their body weight in the 4 days immediately after TCDD dosage, whereas their pair-fed controls lost weight. Mortality in TCDD-treated animals was 100% irrespective of the diet; all pair-fed control rats (except one fed a high protein diet) were terminated on days corresponding to the spontaneous death of their TCDD-treated pairs. Mean time to 50% mortality and mean time to death were significantly longer in TCDD-treated rats fed a high carbohydrate diet in comparison with the other two TCDD-treated groups (p less than 0.05), although caloric intake was comparable. Serum triiodothyronine (T3) was reduced in TCDD-treated animals fed a high fat or a high carbohydrate diet but not in those fed a high protein diet; serum thyroxine (T4) was reduced in all the treated groups, irrespective of diet.(ABSTRACT TRUNCATED AT 250 WORDS)

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) causes an increase in protein tyrosine kinase activities at an early stage of poisoning in vivo in rat hepatocyte membranes

TCDD was found to cause a significant rise in protein tyrosine kinase levels at an early stage of poisoning in rat liver membrane preparations. The results of sephadex G-150 column chromatographic analysis on rat hepatocyte membranes indicate that there are at least three tyrosine kinases of which activities increase as a consequence of TCDD treatment in vivo. The TCDD-evoked rise in such protein tyrosine kinase activity precedes the down-regulation of the EGF (epidermal growth factor) receptor in the plasma membrane in vivo.

The influence of aging on intestinal absorption of TCDD in rats

The effects of age on intestinal absorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied using adult male Fischer-344 rats of 3 different age groups: 13 weeks old (young), 13 months old (mature), and 26 months old (senescent). Absorption was measured with an in situ intestinal recirculation perfusion procedure. Absorption expressed in terms of ng TCDD absorbed/g intestinal dry weight/h was 166, 149 and 143 ng/g/h in the young, mature and senescent groups, respectively. When absorption was calculated in terms of ng TCDD absorbed/g mucosal dry weight/h, the decrease between the senescent rats and the 2 younger age groups, from 544 ng/g/h (young) to 351 ng/g/h (senescent), was not statistically significant (P less than 0.05). It was demonstrated that absorption of TCDD was unaffected by the presence of 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) in the perfusate, but that HCB absorption was (P less than 0.01) enhanced by the presence of TCDD.

Health-risk estimates for 2,3,7,8-tetrachloro-dibenzodioxin: an overview

As has been abundantly noted by many investigators, the paucity of definitive knowledge of metabolism, pharmacokinetics and pathogenesis of the chlorinated dioxins, principally 2,3,7,8-TCDD, in human populations continues to severely limit our ability to evaluate the scope of the chronic and delayed effects of exposure to these agents. The marked qualitative and quantitative differences in the response of animals to TCDD plus the inability to validate a number of the critical assumptions and mathematical models relative to risk assessment make the extrapolation of data from intact animals to man particularly uncertain. Hence, it is vital to vigorously pursue the elaboration of the mechanism of action of TCDD at the molecular level. This should greatly contribute to our fundamental understanding of this agent and the potential danger that it may pose for man.

Depression of rat testicular 17-hydroxylase and 17,20-lyase after administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), such as chloracne, hirsutism, and skin hyperpigmentation, suggest endocrine involvement, however, little is known about the effects of TCDD on steroidogenic organs. It is known that TCDD can cause decreases in testicular heme, testicular microsomal cytochrome P-450, and serum testosterone in the rat. This study was designed to examine the activities of the testicular hemoprotein microsomal cytochrome P-450-dependent enzymes, 17-hydroxylase and 17,20-lyase, following a single, oral dose of either 12.5, 25, or 50 micrograms/kg TCDD. TCDD caused dose- and time-dependent decreases in the activity of the 17-hydroxylase enzyme. Significant decreases were observed at 3, 7 and 14 days at the lowest dose of 12.5 micrograms/kg TCDD. The 17,20-lyase enzyme seemed to be less sensitive to the toxic effects of TCDD with significant decreases in enzyme activity being observed at days 3, 7 and 14 only after treatment with 50 micrograms/kg TCDD. Both microsomal cytochrome P-450 and serum testosterone levels decreased in a dose- and time-dependent manner following 12.5, 25 and 50 micrograms/kg doses of TCDD. These results indicate that decreased testosterone production following treatment with TCDD is related to decreased activities of the testicular microsomal cytochrome P-450-dependent enzymes 17-hydroxylase and 17,20-lyase.

Han/Wistar rats are exceptionally resistant to TCDD. I

Adult male Han/Wistar rats were administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) intraperitoneally at doses ranging from 125 to 1400 micrograms/kg and monitored for 39 to 48 days. Two rats succumbed in the course of the experiment: one in the group receiving 625 and one dosed 1000 micrograms/kg. Body weights of the animals decreased by 20 to 30% during the first 10 to 14 days and became stable thereafter. Feed consumption decreased to 1/3-1/2 of control levels by Day 4 (calculated per metabolic body mass) and returned gradually to starting values by about 4 weeks after dosing. Water intake displayed a triphasic pattern: at first it was slightly increased (Days 1 to 3), then reduced (on Days 4 to 12) and finally increased again throughout the remainder of the test period. The absolute and/or relative weights of thymus, testicles, ventral prostate and interscapular brown fat were significantly decreased at termination. These results indicate that the LD50-value for TCDD in the male, adult Han/Wistar rat is substantially above 1400 micrograms/kg, and that suppression of appetite is the principal phenomenon responsible for TCDD-induced body weight reduction.

Induction of immunotoxicity in mice by polyhalogenated biphenyls

Acute administration of Aroclor-1254 (500 mg/kg) or 3,4,5,3',4',5'-hexabromobiphenyl (HBB) (2-6 mg/kg) IP, profoundly inhibited the plaque forming response to subsequent challenge with sheep erythrocytes in Ah locus positive (C57Bl/6N or B6C3F1N) mice. These studies showed: the immunotoxicity results paralleled enzyme induction results insofar as HBB was approximately 100 times more potent than Aroclor 1254; neither Aroclor nor HBB treatment caused significant induction in the Ah locus negative DBA/2N mice; when B6C3F1 mice were challenged with sheep red blood cells (SRBC) 6 or 16 weeks post Aroclor 1254 treatment, substantial recovery of a PFC response was observed; when these compounds were administered to older (76-week-old) (B6C3F1 mice, severe depression of a PFC response was observed. In contrast to its profound depression of a PFC response, Aroclor-1254 (up to 1250 mg/kg) caused slight increases in lymphocyte proliferation induced by either T or B cell mitogens. A single 500 mg/kg dose of Aroclor-1254 also suppressed the ability of recipient B6C3F1 animals to reject a challenge with either the syngenic fibrosarcoma (PYB6) or the gram negative pathogen (Listeria monocytogenes).

Effects of adenine and its isomer 4-aminopyrazolo-[3,4-d]-pyrimidine on 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced mortality in rats

Adult male Fischer rats were given a single po dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equal to 2 times the LD50 to increase the serum and liver lipid concentrations and to induce mortality. In addition, animals were given 4-aminopyrazolo-[3,4-d]-pyrimidine (4APP), an agent that decreases serum lipids, or adenine (Ad), an agent that prevents the formation of fatty liver, to examine the relationship between changes in lipids and TCDD-induced mortality. The principal effect of 4APP on TCDD-induced mortality (325 micrograms TCDD/kg body wt) was that it shortened the mean time to death. In contrast, Ad stimulated feed consumption and decreased body weight loss, but the mean times to death were similar for TCDD and TCDD + Ad animals. Based on these mortality studies, 4APP, but not Ad, affects the TCDD-induced mortality in Fischer rats. The TCDD-induced sensitivity to 4APP, based on decreased mean time to death, implies that blocking the release and/or synthesis of triglyceride-rich lipoproteins by the liver, and the subsequent decrease in serum lipids, may play an important role in the TCDD-induced mortality. The increase in serum triglyceride associated with TCDD exposure appears to be essential in providing metabolic energy under circumstances where lipoprotein retrieval is reduced.

Effects of thyroid hormones on the induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6N mice

The induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administered with thyroid hormones triiodothyronine (T3) or thyroxine (T4) was investigated in C57BL/6N mice. Timed-pregnant mice were treated with vehicle, TCDD, T3, T4, TCDD plus T3, or TCDD plus T4 on Days 10 to 13 of gestation. No cleft palates were observed in any control fetuses in this study, nor have there been any cleft palates in 1193 fetuses or 154 control litters in the past 24 months. TCDD (3 micrograms/kg/day) caused about 8% cleft palates per litter, while T3 (120, 240, 480 micrograms/kg/day) and T4 (625, 1250, 2500 micrograms/kg/day) resulted in no more than 1.2% cleft palates per litter in any of the treatment groups and the incidence was not dose related. The combination of TCDD (3 micrograms/kg/day) plus T3 at 120, 240, and 480 micrograms/kg/day resulted in 15.9, 20.6, and 31.4% cleft palates per litter, respectively. TCDD plus T4 at 625, 1250 and 2500 micrograms/kg/day caused 15.1, 22.9, and 27.2% cleft palates per litter. No cleft palates were observed when large doses of T3 were given in combination with T4. These data demonstrated that coadministration of T3 or T4 with TCDD increased the incidence of cleft palate to incidences greater than expected from the separate administration of the hormones plus TCDD.

Effect of combined treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin and phototherapy on bilirubin metabolism in the jaundiced Gunn rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin, a potent inducer of microsomal cytochrome P448-dependent monoxygenases, and phototherapy both accelerate bilirubin metabolism and decrease jaundice in Gunn rats. The effects of combined treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin and light were studied in these rats by applying phototherapy for 65 hr, beginning 5 days after induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin. 2,3,7,8-Tetrachlorodibenzo-p-dioxin pretreatment caused a 75% decline in plasma bilirubin in 5 days, with no change thereafter, whether or not the rats were exposed subsequently to phototherapy. In the uninduced rats, plasma bilirubin levels declined by 55% after 40 hr of phototherapy. As determined by [14C]bilirubin kinetics, both 2,3,7,8-tetrachlorodibenzo-p-dioxin and phototherapy increased fractional bilirubin turnover and decreased the total bilirubin pool. In the 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced rats, the contracted bilirubin pool shifted from skin to liver, but these tissue pools did not change further during phototherapy. By contrast, in uninduced rats, phototherapy decreased the cutaneous bilirubin pool, which is the main target of phototherapy. 2,3,7,8-Tetrachlorodibenzo-p-dioxin was more effective than phototherapy in diminishing plasma bilirubin levels and the total bilirubin pool, but the combined treatment (2,3,7,8-tetrachlorodibenzo-p-dioxin followed by phototherapy) was no more effective than 2,3,7,8-tetrachlorodibenzo-p-dioxin alone.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on lipid peroxidation in microsomal systems in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) when added to suspensions of rat hepatic microsomes in the presence of NADPH has little influence on the peroxidation of microsomal lipids unless the system also contains complexed ferric ion, in which case TCDD stimulates. This stimulation does not appear to require metabolism of the TCDD. Peroxidation was monitored by production of thiobarbiturate-reactive substances (malondialdehyde and dienals), production of conjugated dienes, and disappearance of polyunsaturated fatty acids. Stimulation of lipid peroxidation by TCDD in a mixed lysosome-microsome preparation resulted in significantly decreased 'leakage' of acid phosphatase into the medium, implying an effect on lysosomal membranes. Consideration both of the present results and data in the literature leads to the conclusion that it is premature to attempt to define the relationship between enzyme induction, lipid peroxidation and TCDD lethality.

Estimation of exposure to Agent Orange and other defoliants among American troops in Vietnam: a methodological approach

Two pivotal problems in determining whether exposure to herbicides has caused disease in Vietnam veterans or their offspring are definition of which troops were exposed and extent of exposure. The DoD HERBS tape is the most complete publicly available record of herbicide spraying in Vietnam. It contains about 17,000 records consisting of coordinates of spray missions, dates, chemical agent, quantity and area sprayed, and mission purpose. We have developed a set of discrete and continuous indexes of probability of exposure to herbicides for individual veterans. These probability indexes are based on HERBS tape spray data and on locations and dates of service derived from a place-and-date matrix completed by the veteran. They can take into account a conservative estimate of environmental persistence of herbicide by using first-order exponential decay kinetics with an estimated half-life of dioxin. Mean values for the continuous exposure probability indexes were significantly greater among veterans judged to be exposed according to self-reported job titles and specific military experiences than among men judged unlikely to have been exposed. Probabilistic exposure indexes based on HERBS tape for classification of likely exposure to herbicides in South Vietnam during 1965-1971 appear to be well suited for comparative classification of veterans and hence for use in epidemiologic studies.

Development of an acceptable factor to estimate chronic end points from acute toxicity data

Acceptable daily intake (ADI) values are routinely developed for threshold toxicants from NOAELs determined from human or animal chronic or subchronic data. These NOAELs are then divided by appropriate uncertainty factors ranging from 10 to 1000 depending on the quality of the data. However, for the vast majority of chemicals used industrially, adequate toxicity data needed to use this process are not available. Thus, a procedure to estimate a chronic toxicity endpoint from acute toxicity data, such as an oral rat LD50, becomes necessary. An acute-to-chronic application factor of 0.0001 was developed, which when multiplied by an oral LD50 for an individual chemical, yields a surrogate chronic NOAEL. This figure can then be used to estimate an acceptable daily exposure for humans. The process used to estimate this application factor is detailed.

Health hazard assessment for chlorinated dioxins and dibenzofurans other than 2, 3, 7, 8-TCDD

EPA is faced with the need to perform health risk assessments of environmental materials containing mixtures of chlorinated dioxins and -dibenzofurans. Preferably, such an assessment must be based on the direct evaluation of chronic health effects of the mixture or on a knowledge of the toxic effects of each of the components. In the absence of such data, the authors have developed two pragmatic approaches that can reasonably be applied to mixtures such as flyash or contaminated soil. The details of these approaches, viewed as interim procedures, are compared and contrasted with those used by other regulatory and public health authorities.

Receptor-dependent mechanisms of glucocorticoid and dioxin-induced cleft palate

Glucocorticoids (triamcinolone) and dioxins (TCDD) are highly specific teratogens in the mouse, in that cleft palate is the major malformation observed. Glucocorticoids and TCDD both readily cross the yolk sac and placenta and appear in the developing secondary palate. Structure-activity relationships for glucocorticoid- and TCDD-induced cleft palate suggest a receptor involvement. Receptors for glucocorticoids and TCDD are present in the palate and their levels in various mouse strains are highly correlated with their sensitivity to cleft palate induction. Receptors for glucocorticoids appear to be more prevalent in the palatal mesenchymal cells whereas those for TCDD are probably located in the palatal epithelial cells. Glucocorticoids exert their teratogenic effect on the palate by inhibiting the growth of the palatal mesenchymal cells whereas TCDD alters the terminal cell differentiation of the medial palatal epithelial cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) in pregnant C57BL/6N mice: distribution to the embryo and excretion

The distribution and excretion of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlordibenzofuran (TCDF) were studied in pregnant C57BL/6N mice following an oral dose of 30 micrograms/kg 14C-TCDD and 800 micrograms/kg 14C-TCDF on gestation day 11. The distribution in maternal blood and liver and excretion in urine and feces was similar to that previously reported in males of the same strain. However, the rates of elimination were more rapid in pregnant females for both chemicals. This was more pronounced for TCDD than for TCDF. At all time points examined, the levels of radioactivity in the individual embryos were below 0.5% of the total TCDD dose and below 0.05% of the total TCDF dose. Assuming that all radioactive material found in embryos was unmetabolized compound, no more than 2.6 ng (8 pmoles) TCDD and 6.4 ng (21 pmoles) TCDF per g tissue were detected. In light of recent findings which strongly suggest a direct effect of TCDD and related compounds on embryonic palatal tissue, our data clearly support the potent teratogenic effect of TCDD and TCDF on the development of the secondary palate.

Teratogenic potency of TCDD, TCDF and TCDD-TCDF combinations in C57BL/6N mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) cause the same spectrum of fetal anomalies in C57BL/6N mice. Pregnant dams were treated with TCDD, TCDF and combinations of the 2 compounds on gestation day 10, and examined for maternal and fetal effects on day 18. The fetal kidneys were the most sensitive target for teratogenicity. The dose response for cleft palate induction fit the probit model for both compounds, suggesting that TCDD was approximately 30 times more potent than TCDF. The interaction between these 2 compounds was consistent with a model for additive toxicity.

Toxicity of chlorinated aromatic compounds in animals and humans: in vitro approaches to toxic mechanisms and risk assessment

Human exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and chlorinated analogs commonly results in pathological changes in the skin and its appendages characterized by thickening of the epidermis (acanthosis), hyperkeratosis and squamous metaplasia of the epithelial lining of the sebaceous glands. Acneform lesions (chloracne) develop as hair follicles dilate and fill with keratin and sebaceous glands become cystic. In animal models it has been found that the chloracneogenic potential of the halogenated aromatic compounds examined corresponds with the relative affinity of these same compounds for the cytosolic TCDD receptor. This receptor controls the coordinate expression of a number of inducible enzyme activities and in certain cell targets can alter normal programs of proliferation and differentiation. In this report we describe some of our ongoing studies on the mechanisms of action of TCDD in normal human epidermal cells and squamous cell carcinoma (SCC) lines. These systems permit detailed investigation of the molecular and biochemical events underlying pathologic changes in the skin and offer the potential of establishing a risk assessment model for halogenated aromatic compounds by using human target cells.

Mechanisms of the biological effects of PCBs, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in experimental animals

Polychlorinated biphenyls, certain polychlorinated dibenzo-p-dioxins and certain polychlorinated dibenzofurans cause a variety of biological effects in experimental animals. The mechanism of the induction of certain enzymes is perhaps best understood. That is, there is binding of certain chlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans to a receptor, translocation of the compound-receptor complex into the nucleus followed by an increased activity of a number of enzymes in the cell. Although the concentration of this receptor in various tissues of some mouse strains correlates well with the intensity of some of the biological effects observed in the mouse strains exposed to these compounds, this correlation apparently does not extend across various species. The current evidence suggests that the acute toxic effects of TCDD in various species is in some way associated with binding of TCDD to the receptor. However, biological effects of TCDD in addition to those resulting from binding to the receptor may be required to produce acute toxicity and, perhaps, other effects. The acute toxic effects of TCDD are probably caused by the parent compound rather than metabolites; however, this conclusion must be viewed as tentative. Also, it cannot be excluded at this time that biological effects other than acute toxicity may be caused by metabolites of TCDD. Finally, the acute toxic effects of TCDD appear not to be related, at least not directly, to the rate of metabolism of TCDD in experimental animals nor to the half-life of excretion.

Urinary D-glucaric acid excretion in the Seveso area, polluted by tetrachloro-dibenzo-p-dioxin (TCDD): five years of experience

On July 10, 1976, an explosion in a factory in Seveso, Italy, located 30 km north of Milan, producing trichlorophenol caused the release of TCDD-containing compounds in the surrounding area. Since extremely small doses of TCDD have been shown to induce hepatic microsomal enzymes in animals, urinary D-glucaric acid excretion (a measurable index of enzyme induction), has been investigated in Seveso in adults and children 6 to 8 years old, in order to clarify whether levels of environmental exposure to TCDD were sufficient to produce an induction in man. Urine samples were collected from 1976 to 1981. As a control group, people living in Cannero (a nonindustrialized village on lake Magiore), in Busto Arsizio (a small industrial town near Milan) and in Lentate (a noncontaminated zone near Seveso) were chosen. In the first period of collection, children with chloracne (which is considered to be a characteristic manifestation of intoxication with chlorinated products) showed significantly increased levels of D-glucaric acid excretion compared to children without chloracne living in the same zone. As far as chronic exposure is concerned, up to 3 years after the accident both adults and children living in the Seveso area showed a statistically significant enhancement of D-glucaric acid elimination compared to the control groups. This study demonstrates that adults and children living in the polluted zones had an increased activity of hepatic microsomal enzymes for some years, since, although the urinary excretion of D-glucaric acid is only an indirect measure of enzyme activity, studies in man have indicated that it is, however, sensitive and quantitative.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6J and DBA/2J mice

The dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in B6D2F1/J (B6D), C57BL/6J (C57), and DBA/2J (DBA) mice. A 14-fold difference in lethality was observed in C57 and DBA mice, based upon 30-day LD50 values of 182 and 2570 micrograms TCDD/kg body wt, respectively. The 30-day LD50 for B6D mice was 296 micrograms TCDD/kg body wt. A progressive loss of body weight in all strains of mice was observed during the 30-day LD50 studies, with maximal weight losses of 24.7, 34.0, and 33.4% prior to death of C57, B6D, and DBA mice, respectively. In separate experiments, it was found that decreased feed consumption did not contribute to weight loss in C57 mice exposed to lethal or sublethal doses of TCDD until the animals were moribund. Time-course studies in C57 mice treated with 200 micrograms TCDD/kg body wt indicated that decreases in serum glucose and triglyceride concentrations and increases in hepatic triglyceride content occurred within 4 to 8 days of exposure, and were maximally altered within 17 to 21 days postexposure, concomitant with a 25% body weight loss. C57 mice fasted for 24 to 96 hr lost 18% of body weight and also exhibited alterations in glucose and lipid parameters; however, these changes were substantially different than the effects of TCDD exposure. In concert, these observations demonstrate that decreased feed consumption (hypophagia) does not account for weight loss and changes in carbohydrate and lipid metabolism in TCDD-treated C57 mice. Dose-response experiments resulted in comparable changes in glucose and lipid parameters when DBA mice were exposed to 10-fold higher doses of TCDD than C57 mice. Parallel LD50 responses and parallel changes in carbohydrate and lipid metabolism, at 10- to 15-fold differences in dose range, are indicative of a common mechanism of toxicity in TCDD-treated C57 and DBA mice.

Comparative toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in four (sub)strains of adult male rats

Four (sub)strains of adult male rats were given single oral doses of various concentrations of TCDD to establish and compare the oral 30-day LD50 values. The strains of rats were Fischer (F/334N) supplied by Harlan Industries, Frederick Cancer Research Center, and Charles River Breeding Laboratories; and CD supplied by Charles River Breeding Laboratories. The Charles River/Fischer rats were most sensitive to TCDD (LD50 = 164, 95% confidence limits 104-217 micrograms/kg), the Frederick/Fischer and Charles River/CD rats were moderately sensitive to TCDD (LD50 = 303, 250-360; and 297, 240-360 micrograms/kg, respectively), and the Harlan/Fischer rats were most resistant to TCDD (LD50 = 340, 281-409 micrograms/kg). The mean times of death were from 24.5 +/- 1.0 to 28.3 +/- 0.5 days and the percentage body weight loss at death was 37.4 +/- 1.2 to 42.7 +/- 1.3%. One week after exposure of the Charles River/Fischer animal to 45 micrograms TCDD/kg (1/4 the established 30-day LD50 dose), the same serum profile was induced as previously observed in the Harlan/Fischer rat, which includes hypoglycemia, hypertriglyceridemia, and hypercholesterolemia. These results emphasize the importance of indicating the precise dose, strain of rat, and time after dosing before termination in reporting the effects of TCDD on a particular biological response.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) enhances terminal differentiation of cultured human epidermal cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and isosteric halogenated analogs produce a spectrum of pathologic changes in the epidermis of humans. In this study, the actions of TCDD on cultured human epidermal cells were characterized to determine whether these cells are an appropriate in vitro model to examine the mechanisms of TCDD toxicity to human skin. The differential staining properties of TCDD-treated cultures indicated that TCDD decreased basal cell numbers and increased the degree of keratinization. Histologic examination of cross-sections of the cultures confirmed a loss of small nucleated cells and increased cell layering in response to TCDD. TCDD produced no change in total cell number or cell protein, but decreased the number of small (basal) cells and DNA synthesis. TCDD increased the number of cells containing spontaneous envelopes, as well as the number of envelope-competent cells. The quantitative changes observed in these parameters were consistent with a TCDD-induced commitment of proliferating cells to terminal differentiation. TCDD also decreased epidermal growth factor (EGF) specific binding. Maximal changes in EGF binding occurred after 4 days, and in small cell number after 5 days. The decreases in EGF binding and small cell number were stereospecific and concentration dependent (EC50, 1 to 2 nM), implicating the human Ah receptor in mediating these responses to TCDD. These data indicate that TCDD treatment produces hyperkeratinization in cultured human epidermal cells. It is proposed that TCDD acts on epidermal basal cells to enhance terminal differentiation through mechanisms regulated at least in part by the Ah receptor.

Toxic interaction of specific polychlorinated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin: increased incidence of cleft palate in mice

The induction of cleft palate in C57BL/6N mice is an extremely reproducible and sensitive indicator of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity. This endpoint was used to look for potential interactions between two polychlorinated biphenyl (PCB) congeners and TCDD. Both 2,3,4,5,3',4'-hexachlorobiphenyl (HCB) and 2,4,5,2',4',5'-HCB are of relatively low toxic potency, but their biological properties differ. Pregnant mice were treated with TCDD and either HCB on gestation Days 10 through 13, and the fetuses examined for the presence of cleft palate and renal abnormalities on gestation Day 18. At a dose of TCDD which caused a low level of cleft palate, moderate hydronephrosis was observed. No renal or palatal anomalies were detected after 2,4,5,2',4',5'-HCB treatment, and the combination of this isomer with TCDD had no effect on the incidence of TCDD-induced cleft palate. 2,3,4,5,3',4'-HCB caused mild renal toxicity, but no cleft palate. However, treatment of pregnant mice with a combination of TCDD and 2,3,4,5,3',4'-HCB resulted in a 10-fold increase in the incidence of cleft palate. Thus, the toxicity of compounds such as TCDD may be enhanced by compounds of relatively low acute toxicity such as selected PCBs. The widespread environmental occurrence of such combinations suggests a need for further evaluation of the mechanism of this interaction.

Mechanism of action of toxic halogenated aromatics

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons are a highly toxic class of environmental contaminants, as evidenced by numerous cases of accidental poisonings of human and animal populations and their extreme toxic potency in laboratory animals. The proposed model for the mechanism of action of TCDD and related compounds is analogous to that of the steroid hormones, which modulate gene expression through a receptor mechanism. In the steroid receptor model, the compound enters the cell cytoplasm where it acts as a specific ligand, binding selectively to a high affinity receptor protein. Bound to the appropriate ligand, the receptor concentrates in the nucleus where its increased association with chromatin leads to altered gene expression. This model has been useful in characterizing the Ah receptor; however, it does not provide a unifying hypothesis for all biochemical and toxic effects associated with exposure to halogenated aromatic hydrocarbons. Several findings suggest that a primary factor in determining TCDD toxicity might be tissue and species specific factors that control the actions of Ah receptor(s) in target tissues. Furthermore, numerous mechanisms might be involved. Clarifying the mechanism(s) for TCDD toxicity would enhance our ability to predict human health consequences to toxic halogenated aromatic hydrocarbons and would provide a more rational basis for risk analysis.

Effects of in vivo-administered 2,3,7,8-tetrachlorodibenzo-p-dioxin on receptor binding of epidermal growth factor in the hepatic plasma membrane of rat, guinea pig, mouse, and hamster

The effect of in vivo-administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on epidermal growth factor (EGF) receptor activity of the rat hepatic plasma membrane was studied. TCDD causes a significant reduction in EGF binding at an early stage of toxicity (day 2) and at very low doses (1 microgram/kg, single i.p., rat). This reduction appears to be due to a decline in the number of receptors. There is a good correlation between levels of decline in EGF binding and loss of body weight among TCDD-treated rats. The reduction in EGF binding occurs at a relatively low dose in the guinea pig (a very sensitive species) and at high doses in the hamster (a tolerant species). Among three mice strains, TCDD (115 micrograms/kg, single i.p.) caused 98% reduction in EGF binding in the sensitive strains (C57BL/6J and CBA/J) but only a 50% reduction in the tolerant strain (AKR/J). To relate the above biochemical changes to in vivo effects, TCDD was postnatally administered (through mother's milk) to mouse neonates. The most prominent toxic manifestations were early eye opening and incisor eruption, loss in body weight gain, and retardation of hair growth. All of these symptoms have been ascribed to EGF effects. TCDD was also found to stimulate phosphorylation of the EGF receptor in the rat hepatic plasma membrane. This phosphorylation effect was observed at day 1 and persisted until the end of the test (day 10). It has long been recognized that agents causing reduction in number of EGF receptors (e.g., phorbol esters) elicit in vivo cellular responses that are similar to those caused by exposure to excess doses of growth factors. Accordingly, a hypothesis has been proposed to ascribe some of the EGF-like effects of TCDD, such as fatty infiltration of the liver and hyperplastic proliferation of gastric epithelia and epidermal cells to its action on the EGF receptor.

Prevention of TCDD toxicity in laboratory rodents by addition of charcoal or cholic acids to chow

The mortalities of mice, rats and guinea-pigs given a single lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were substantially reduced by feeding them with chows containing 2.5 or 5% activated charcoal, which presumably enhances elimination of the toxic agent from the organism. Chows with 0.25 or 0.5% cholic acid and dehydrocholic acid added had a similar protective action on mice intoxicated with TCDD. Since no treatment is currently available for this type of poisoning, charcoal-whose safe clinical use is well established-is worth considering in view of its potential benefit to humans acutely exposed to TCDD.

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) reduces lipoprotein lipase activity in the adipose tissue of the guinea pig

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) administered to young male guinea pigs at a dose of 1 microgram/kg (single intraperitoneal injection) caused a large reduction in adipose tissue lipoprotein lipase (LPL) activity. This effect occurred rapidly; a 70% decrease was noticed after 24 hour and 80% of LPL activity was lost by 48 hours when the serum triglyceride levels increased to 175% of control levels. LPL is known to play an important role in controlling the amount of free fatty acids supplied to adipose tissues. Administration of a large dose of glucose to fasted guinea pigs, which have shown a similar weight loss, but less LPL loss than TCDD-treated animals, had the effect of elevating their adipose LPL levels back to a near normal level, whereas the same treatment caused no significant increase in the LPL levels of TCDD-treated animals. Evidence indicates that the TCDD-caused decline in LPL activity is irreversible. As a consequence, the affected guinea pigs are incapable of responding to changes in nutritional status.