Effect of thyroidectomy and thyroxine on 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced immunotoxicity

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Carcinogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in experimental models

The contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a prototype compound of a whole class of halogenated aromatic hydrocarbons termed 'dioxinlike' contaminants present in food, human tissue, mothers milk, and environmental samples. Among the various adverse effects caused by TCDD in animal experiments, its carcinogenic effects caused particular concern. In rodents, long-term TCDD treatment leads to the development of tumors of the liver, thyroid, lung, skin, oral cavity and other sites. The occurrence of liver tumors mainly observed in female rats has been used as a basis for quantitative cancer risk assessment for TCDD. TCDD does not behave like a 'complete carcinogen', i. e. no DNA binding of the parent compound or metabolites thereof could be detected. However, enhanced oxidative damage of hepatic DNA was observed, probably resulting from a dramatic induction of cytochrome P450 enzymes, which are under the regulatory, transcriptional control of the TCDD-activated aryl hydrocarbon receptor. The marked enhancement of TCDD-related oxidative liver DNA damage in rats by estrogens warrants further mechanistic investigation. Furthermore, TCDD acts as a tumor promoter, i. e. it facilitates the growth of putative preneoplastic hepatic lesions after initiation with a complete carcinogen. The mechanisms underlying this effect may be related to altered intracellular signaling involving pronounced changes in the phosphorylation pattern of proteins regulating growth and apoptosis. These effects are thought to result in an enhanced survival of preneoplastic cells, some of which can undergo further steps on the way to malignancy. In summary, a better understanding of the mechanisms of the carcinogenicity of TCDD is mandatory to provide a rational basis for a better inter-species extrapolation. The final aim of these efforts is a more reliable risk assessment for the carcinogenic potency of the class of dioxinlike contaminants in humans.

Serum 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) levels and thyroid function in Air Force veterans of the Vietnam War

PURPOSE

We assessed potential health effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentration in serum on thyroid function in US Air Force veterans involved in Operation Ranch Hand, the unit responsible for the aerial spraying of herbicides, including TCDD-contaminated Agent Orange, during the Vietnam War from 1962 to 1971. Other Air Force veterans who were not involved with spraying herbicides were included as Comparisons.

METHODS

We analyzed thyroxine (total T4), thyroid stimulating hormone (TSH), triiodothyronin percent uptake (T3% uptake), the free thyroxine index (FTI), and thyroid diseases against serum TCDD levels. Data was available for 1,009 Ranch Hand and 1,429 Comparison veterans compliant to any of five examinations in 1982, 1985, 1987, 1992, and 1997. Each veteran was assigned to one of four exposure categories based on serum TCDD levels, named Comparison, Ranch Hand Background, Ranch Hand Low Elevated, and Ranch Hand High Elevated.

RESULTS

Cross-sectional analyses found statistically significantly increased TSH means at the 1985 and 1987 examinations in the High category and a significant increasing trend across the three Ranch Hand TCDD categories in 1982, 1985, 1987 and 1992. A repeated-measures analysis found significantly increased TSH means in the High TCDD category. We found no significant relation between the occurrence of thyroid disease and TCDD category.

CONCLUSIONS

These findings suggest that TCDD affects thyroid hormone metabolism and function in Ranch Hand veterans. Further follow-up will be necessary to understand the relation, if any, between thyroid disease and TCDD levels.

Effects of induced hypo- and hyperthyroidism on immune function and plasma biochemistry in mallards (Anas platyrhynchos)

Hypo- or hyperthyroid states were induced in adult male mallards (Anas platyrhynchos) by subchronic exposure to daily injections of methimazole or a 9:1 ratio of thyroxine (T4): triiodothyronine (T3). The levels of T4 given were 0, 125, 250, or 500 micrograms/kg/day and for methimazole; 10 mg/kg/day for 22 or 21 days. Plasma T3 showed a lasting decrease with T4:T3 treatment, despite the attempt to maintain the normal T4:T3 ratio. Antibody formation to sheep red blood cells was decreased only at the 125 micrograms/kg/day dose of T4, and was unaffected by methimazole treatment. Natural killer cell activity to RP-9 tumor cells and macrophage phagocytosis of killed, opsonized Saccaromyces cereviseae were unaffected by treatment throughout the study. However, lectin-dependent cellular cytotoxic activity to RP-9 tumor cells was significantly decreased after 21 days of methimazole treatment, indicating that hypothyroidism may have an influence on cell-mediated immunity. Hypo- and hyperthyroid conditions had opposing effects on plasma cholesterol levels.

Extrathyroidal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on thyroid hormone turnover in male Sprague-Dawley rats

Treatment of rats with different polyhalogenated aromatic hydrocarbons strongly decreases plasma T4, with little or no decrease in plasma T3. The extrathyroidal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thyroid hormone turnover were studied by i.p. administration of a single dose of 10 microg TCDD/kg BW or vehicle (corn oil) to euthyroid (Eu) rats, thyroidectomized (Tx) rats, and Tx rats infused with 1 microg T4 (Tx+T4) or 0.4 microg T3 (Tx+T3)/100 g BW x day by osmotic minipumps. Tx rats showed decreased plasma T4 and T3 and increased plasma TSH levels, decreased hepatic type I deiodinase (D1) and malic enzyme activities, and increased brain type II deiodinase (D2) activities. All parameters were largely restored to Eu levels in Tx+T4 rats and, except for plasma T4 and brain D2 activity, in Tx+T3 rats, validating the thyroid hormone-replaced Tx rats as models to study the peripheral effects of TCDD. Three days after TCDD administration, plasma T4 and free T4 levels were significantly reduced in Eu and Tx+T4 rats, and plasma T3 was significantly reduced in Tx+T3, but not in Eu or Tx+T4 rats. Plasma TSH was not affected by TCDD in any group. Hepatic T4 UDP-glucuronyltransferase (UGT) activity was induced approximately 5-fold by TCDD, whereas T3 UGT activity was only increased by about 20% (P = NS) in the different groups. TCDD produced an insignificant decrease in liver D1 activity in Tx rats and an insignificant increase in brain D2 activity in Tx rats and hormone-replaced Tx rats. Hepatic malic enzyme activity was significantly increased by TCDD in all groups, except Tx rats. These results strongly suggest that the thyroid hormone-decreasing effects of TCDD are predominantly extrathyroidal and mediated by the marked induction of hepatic T4 UGT activity.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on humoral and cell-mediated immunity in Sprague-Dawley rats

There is much discussion about the occurrence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced effects on the human immune system. Extensive studies have been conducted in mice, but those results cannot explain some of the epidemiological data obtained in exposed humans. Therefore, studies in other laboratory animal species are needed. The aim of these experiments was to examine effects of TCDD on cell- and humoral-mediated immunity in male Sprague-Dawley (SD) rats. A delayed-type hypersensitivity (DTH) assay was used to examine cell-mediated immunity. A time-course study demonstrated that TCDD treatment on day -5 relative to immunization (day 0) produced the greatest effect on cell-mediated immunity. In a dose-response experiment, rats were treated with 1, 3, 10, 20, 30, 40 and 90 micrograms TCDD/kg The effect of TCDD on cell-mediated immunity displayed an inverted U-shaped dose-response curve, in that low doses enhanced and high doses suppressed this immune function. This is the first study to demonstrate an U-shaped dose-response curve of TCDD on the immune system. Primary antibody response to sheep red blood cells (SRBC) was used as endpoint to study the effect of TCDD on humoral immunity. Serum anti-SRBC IgM and IgG levels were measured using an enzyme-linked immunosorbent assay (ELISA). In the dose range examined (10, 20 and 40 micrograms TCDD/kg), serum IgM levels were not affected by TCDD compared to controls at 7 and 14 days after immunization. In contrast, serum IgG levels were dose-dependently elevated both 7 and 14 days after immunization, with a maximum increase of 59% over controls.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

Effect of thyroidectomy and thyroxine on hexachlorobenzene induced porphyria

The role of thyroid status in hexachlorobenzene (HBC) induced porphyria was studied in normal, thyroidectomized and thyroxine (T4) treated rats. Female Wistar rats were treated with HCB for different periods of time. Serum T4 levels were depressed after 8 days of HCB administration whereas levels of triiodothyronine (T3) were not altered. T4 administered simultaneously with HCB resulted in hyperthyroxinemia. The time course of porphyrinogen carboxy-lyase (PCL) activity in the three HBC treated groups was studied. A rapid and significant decrease of PCL activity was found after 8 days of HCB treatment in T4 treated rats with respect to untreated animals. In contrast, HCB treatment of normal and thyroidectomized rats elicited a significant decrease of PCL activity after 21 and 30 days, respectively. This study shows that thyroid hormone plays an important role in the early establishment of HCB induced porphyria.

Suppression of prolactin and cytotoxic T-lymphocyte activity in PCB-treated mice

Halogenated aromatic hydrocarbons (HAH) are ubiquitous environmental contaminants. Studies in rats have shown that HAH treatment can lead to dysregulation of circulating hormone levels, including prolactin. Reduction of prolactin levels in both rats and mice is inhibitory to immune function. Previous studies have reported suppression of alloantigen-specific cytotoxic T-lymphocyte (CTL) activity in mice treated with 3,3', 4,4', 5,5'-hexachlorobiphenyl (HxCB). Here we report that treatment of mice with HxCB (10 mg/kg body weight) leads to a significant reduction of serum prolactin levels (by 89% to 3.7 ng/ml) on day 10 post alloantigen injection (P815 mastocytoma), the day of peak alloantigen-specific CTL activity. Prolactin levels were not altered on day 3 post alloantigen injection. Treatment with bromocriptine (5 mg/kg/day) reduced serum prolactin levels slightly on day 3 and significantly (94% to 2.1 ng/ml) on day 10 post alloantigen injection. Splenic CTL activity was not altered by treatment with bromocriptine. The data presented here suggest that reduction of prolactin levels alone, to the extent observed in HxCB-treated mice, is not causative for CTL suppression.

Immunologic and endocrine effects of the flame-retardant pentabromodiphenyl ether (DE-71) in C57BL/6J mice

Polybrominated diphenyl ethers are manufactured for use as flame retardants in commercial plastics and textiles in Europe and North America. These studies investigated the acute and subchronic immunotoxicity and endocrine effects of a commercial pentabromodiphenyl either mixture, DE-71, in female C57BL/6 mice. Mice were orally exposed to acute single doses of DE-71 of 0, 0.8, 4.0, 20, 100, or 500 mg/kg, or to subchronic daily doses totaling 0, 250, 500, or 1000 mg/kg over a 14 day period. Immunotoxicity was assessed by measuring the plaque-forming cell response to sheep erythrocytes (SRBC) and natural killer cell (NKC) activity (basal and poly I:C stimulated) to YAC-1 target cells. Liver cytochrome P450 content and activities (ethoxyresorufin-o-deethylase (EROD) and pentoxyresorufin-o-deethylase (PROD)) as well as corticosterone (CS) and thyroxine (T4) concentrations were also measured. PROD activity was induced 3-5-fold in mice exposed acutely or subchronically to DE-71 at doses > 250 mg/kg. EROD activity and total microsomal cytochrome P450 content were significantly induced only in mice treated subchronically with DE-71; maximum induction of EROD was 3.3-fold. Total serum T4 concentrations were significantly lower in mice treated acutely with DE-71 at all doses except the 100 mg/kg dose. Total and free T4 concentrations were dose-dependently decreased in DE-71-treated mice following subchronic exposure. Plasma CS levels were elevated following subchronic exposure to DE-71. The elevation of CS was correlated with order of capture at necropsy, suggesting an interactive effect of DE-71 and stress. In regard to immunotoxicity, significant suppression of the anti-SRBC response was seen only in mice exposed subchronically to 1000 mg DE-71/kg, an exposure that also resulted in decreased thymus weight. NKC activity was not altered by exposure to DE-71.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Biochemical and functional effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the heart of female rats

Biochemical, functional and morphologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the hearts of female rats were examined. Six days after the treatment of rats with TCDD, the blood pressures and resting heart rates were significantly less than in control animals. Treated animals were also less responsive to the effects of the beta-1 agonist, (-)isoproterenol. No histopathologic changes were observed in the heart although extensive centrilobular necrosis occurred in the liver after TCDD administration. Serum levels of thyroxine were 66% less than in control animals. Marked lipid peroxidation was produced in the liver with small but significant increases occurring in the heart. TCDD administration had no effect on catalase activity in the heart, but produced a 20% decrease in superoxide dismutase activity relative to control animals. The effects of TCDD on cardiac function do not appear to be due to a direct action of the xenobiotic on the heart but possibly to a down-regulation of beta-receptors in the heart as a result of the hypothyroid state.

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

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

Effects of vitamin A and/or thyroidectomy on liver microsomal enzymes and their induction in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Vitamin A and thyroid hormone status have been shown previously to alter the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats. In the present study, we have examined the effects of a vitamin A-excess and a vitamin A-deficient diet on thyroid hormone levels, on selected drug-metabolizing enzymes in liver microsomes, and on their inducibility by TCDD in male Sprague-Dawley rats. Except for a slight increase in serum T3 levels, none of these end points was affected by feeding rats the vitamin A-deficient diet. In contrast, excess dietary vitamin A caused a decrease in serum thyroxine (T4) and triiodothyronine (T3) levels, although the levels of T3 remained in the euthyroid range (60-80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b5 and the basal activity of benzo[a]pyrene hydroxylase and 7-ethoxyresorufin O-de-ethylase were unaffected by excess dietary vitamin A. This result is consistent with our previous observation that the basal activity of these enzymes is dependent more on T3 than on T4 levels. Vitamin A excess markedly suppressed the activity of liver microsomal UDP-glucuronosyl transferase toward 1-naphthol. However, no such enzyme suppression was observed in thyroidectomized rats. This suggests that the suppressive effect of vitamin A on UDP-glucuronosyl transferase activity may be dependent on T3. Neither vitamin A nor thyroid status had any major effect on the inducibility of UDP-glucuronosyl transferase and cytochrome P-450-dependent enzyme activities by TCDD. However, vitamin A and TCDD had a nearly additive effect on suppression of serum T4. It is concluded that liver microsomal enzyme induction is not associated with the modulatory effect of vitamin A and thyroid hormones on the toxicity of TCDD.

Evidence that 2,3,7,8-tetrachlorodibenzo-p-dioxin and thyroid hormones act through different mechanisms in human keratinocytes

It has been proposed [J. D. McKinney, J. Fawkes, S. Jordan, K. Chae, S. Oatley, R. E. Coleman, and W. Briner (1985). Environ. Health Perspect. 61, 41-53] that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces toxic responses through persistent occupancy of nuclear thyroxine (T4) receptors, and that maintenance of receptor occupancy by supraphysiologic concentrations of thyroid hormones mimics TCDD toxicity [L. H. Hong, J. D. McKinney, and M. I. Luster (1987). Biochem. Pharmacol., 36, 1361-1365]. TCDD induces hyperkeratinization in cultured normal human epidermal cells and the human keratinocyte line, SCC-12F. This response is associated with a decrease in high-affinity epidermal growth factor (EGF) receptors. These cell systems were used as models to compare the actions of TCDD with those of triiodothyronine (T3) and T4 on human target cells. Keratinocytes were treated simultaneously with T3 and T4 in a 4:1 molar ratio (T3/T4; Hong et al., 1987) and levels of EGF binding and 7-ethoxycoumarin O-deethylase activity (a marker for cytochrome P1-450 induction) were measured. T3/T4 (at concentrations up to 10 microM T3/2.5 microM T4) and T3 or T4 alone (0.1 to 10 microM) did not produce the hyperkeratinization, the decrease in EGF binding, or the increase in ECOD activity that are characteristic of TCDD exposure. Nonresponsiveness to T3/T4 was not due to metabolism of these hormones by the keratinocytes. T3 and T4 did not compete with [3H]TCDD for binding to cytosolic Ah receptor from C57BL6 mouse liver, SCC-12F, or normal human epidermal cells. TCDD and an active stereoisomer, 2,3,7,8-tetrachlorodibenzofuran, did not compete with [125I]T3 or [125I]T4 for binding to nuclear receptors from SCC-12F cells or C57BL6 mouse liver. Taken together, these data demonstrate that the actions of TCDD and thyroid hormones are mediated by distinct mechanisms in human keratinocytes.

Changes in thyroid hormones and thyroxine glucuronidation in hamsters compared with rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin

In rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, serum thyroxine (T4) is depressed. Since hamsters are relatively insensitive to TCDD-induced lethality, the effects of TCDD on several parameters of thyroid status were measured in hamsters as a comparison with the more sensitive rat. At 7 days after ip injection of TCDD, there was a dose-dependent increase in serum 3,5,3'-triiodothyronine (T3) and T4 in hamsters to a maximum level 200% of control; the ED50 was approximately 10 micrograms/kg. Hamsters receiving 100 micrograms/kg lost up to 4% of their body weight but began to recover after about 3 weeks. Serum T4 in these animals was elevated compared to pair-fed and ad libitum controls throughout the 53-day experiment, although it also began to recover after Day 21. This was in direct contrast to the marked reduction of T4 in rats exposed to lower doses of TCDD. T3 was significantly higher in TCDD-treated hamsters than in pair-fed controls on Days 2-7, and TSH was also elevated on Days 2-21. Reverse T3, like T4, was increased by TCDD in hamsters whereas it was decreased in rats. Hepatic microsomal UDP-glucuronosyltransferase (GT) activity was measured using T4 as substrate (T4-GT). On a whole liver basis, T4-GT was induced by TCDD by the same proportion in both rats and hamsters (170-180% of controls) although absolute activities in rats were 3- to 4-fold higher than in hamsters. This similarity in T4-GT inducibility by TCDD suggests that there are likely mechanisms in addition to T4-GT induction which account for the species-specific alterations in T4. Thus, while the response of thyroid hormones to TCDD differed qualitatively, effective doses in hamsters were higher than in rats, suggesting that these changes, although secondary, may correlate more directly with toxicity than does enzyme induction (whose ED50s are similar in both species). An understanding of the mechanism of this species difference may be helpful in unravelling the primary mechanisms of TCDD toxicity.

Circadian alterations in prolactin, corticosterone, and thyroid hormone levels and down-regulation of prolactin receptor activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Studies were initiated to determine whether 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects circadian rhythms of serum prolactin (PRL), corticosterone, thyroxine (T4), and triiodothyronine (T3) in male Sprague-Dawley rats. In addition, the effects of TCDD on PRL receptor activity, as assessed by the ability of PRL to induce ornithine decarboxylase (ODC), were determined. The earliest effect detected following TCDD administration was a significant decrease in the serum PRL concentration compared with that of pair-fed controls within 4 hr (p less than 0.05). This was followed by a significant decrease in serum T4 by 6 hr (p less than 0.05). By 8 hr the serum peak of corticosterone was shifted to 2 hr later in the TCDD-treated rats. This temporal sequence of hormonal changes suggests that the earlier alteration in PRL may be involved in the later alterations in the concentrations of serum T4 and corticosterone. The serum PRL concentration 7 days after TCDD administration was significantly higher (p less than 0.05) in TCDD-treated animals compared with that in pair-fed controls (mean of 20.5 +/- 3.7 vs 13.6 +/- 1.8 ng/ml serum, p less than 0.05, respectively). The elevation of ODC activity in response to PRL, 2 days after TCDD, was decreased in the order of thymus greater than adrenal greater than spleen greater than heart greater than kidney greater than liver. By 7 days, liver ODC activity in response to PRL was only 12% that detected in pair-fed controls. Liver ODC activity in response to dexamethasone and aminophylline was decreased to 25 and 22% of pair-fed controls, respectively, by 7 days after TCDD administration. However, in kidney, TCDD-treated rats had an increased ODC response to aminophylline to 191% of pair-fed controls by Day 7. These results suggest that the ability of TCDD to alter receptor coupling or the receptor number for diverse hormones may play a role in TCDD toxicity.

Hepatic indices of thyroid status in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The functional thyroid status of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is unknown. Therefore, activities of certain thyroid-responsive enzymes were examined in the livers of adult male Sprague-Dawley rats 1 week after treatment with TCDD (6.25, 25 or 100 micrograms/kg). Activity of the thyroid-responsive flavin L-glycerol-3-phosphate dehydrogenase (per mg mitochondrial protein) was decreased slightly in livers of TCDD-treated rats, while that of succinate dehydrogenase remained unchanged. In contrast, activities (per mg supernatant protein) of three thyroid-responsive NADP-dependent cytosolic enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were increased by TCDD treatment in a dose-dependent manner. Lactate dehydrogenase (activity per mg supernatant protein) was also augmented slightly 1 week after TCDD administration. Liver mass was increased by TCDD treatment in a dose-dependent manner, but DNA content per liver was similar at all doses examined. Total hepatic protein, expressed per liver or mg hepatic DNA, was increased in TCDD-treated rats when compared to their pair-fed counterparts. The decreased activity of the mitochondrial L-glycerol-3-phosphate dehydrogenase, in contrast to the increased activities of the supernatant enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, is not consistent with a shift in functional thyroid status following TCDD treatment.

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in cold-adapted rats

The toxicity of 60 micrograms/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) given IP in corn oil/5% acetone was examined in male Sprague-Dawley rats adapted to 25 degrees C or 4 degrees C ambient temperature. Cold exposure significantly reduced mean time to death and tended to increase mortality. Body weight at the time of death was reduced at both ambient temperatures to about the same extent. Thus, the rate of body weight loss was about twice as fast in non-survivors at 4 degrees C than at 25 degrees C. There was a continuous decrease in feed intake of the non-survivors at 25 degrees C until death. However, no reduction in feed intake occurred in any of the rats at 4 degrees C ambient temperature. At 14 days after dosing all TCDD-dosed animals were hypothyroid in terms of T4 but essentially euthyroid in terms of T3. Oxygen consumption at 10 days after dosing was reduced to the same extent in all TCDD-dosed rats without regard to survival status. By day 20 after TCDD dosage, survivors increased their oxygen consumption at both ambient temperatures to nearly control levels whereas non-survivors were unable to do so. Body temperature of all animals remained within normal range except for the non-survivors, which showed reduced rectal temperature shortly before death.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyroidectomy on the Ah receptor and enzyme inducibility by 2,3,7,8-tetrachlorodibenzo-p-dioxin in the rat liver

Thyroidectomy of rats confers some protection, by an unknown mechanism, from the weight loss, immunotoxicity, and mortality induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Since at least some of the many effects of TCDD appear to be mediated by the Ah receptor, perhaps the thyroid plays a role in regulation of this receptor, thereby modifying the toxicity of TCDD. We tested this hypothesis by comparing TCDD-binding characteristics of the receptor and hepatic enzyme inducibility by TCDD (a receptor-mediated response) in thyroidectomized (ThX) and euthyroid rats. There were no significant differences in levels of TCDD binding in vitro in hepatic cytosol, in receptor affinity, nor in the molecular size of the TCDD-bound receptor in untreated ThX rats compared to controls fed ad libitum or pair-fed. Total hepatic cytochrome P-450 (P-450) levels and NADPH-menadione oxidoreductase (NMOR) activity were unaffected by thyroid status, whereas 7-ethoxycoumarin O-deethylase (ECOD) activity was approx. 50% lower in ThX animals than in ad libitum or pair-fed controls. At 3 and 10 days after TCDD administration (10 micrograms/kg, i.p.), P-450 concentrations and NMOR and ECOD activities were induced by approximately the same proportions in ThX and pair-fed intact rats; however, the absolute levels of the induced activities were lower in ThX than in pair-fed controls. It was concluded that hypothyroidism does not regulate Ah receptor concentration or function in the liver. Therefore, the modulation of TCDD toxicity by hypothyroidism appears not to involve changes in the hepatic Ah receptor.

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.

Thyroid status and thermogenesis in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Several key aspects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity resemble the effects of hypothyroidism, while in other ways the toxic responses are characteristics of hyperthyroidism. Whether thyroid dysfunction plays a role in TCDD toxicity remained unknown, however. We therefore determined the dose-related effects of TCDD treatment on plasma concentrations of L-thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), and thyroid-stimulating hormone (TSH), and compared these changes with signs of TCDD toxicity. We also determined whether indices of functional thyroid status (and thermogenesis) were altered in response to TCDD treatment. Young adult male Sprague-Dawley rats were given single oral doses of TCDD (6.25-100 micrograms/kg) and evaluated 1 week later. Toxicity, measured by decreases in feed intake and body weight, ranged from minimal to severe. Plasma concentrations of T4 were greatly reduced at all doses tested, while T3 was increased in a dose-related fashion (up to 35%). TSH was elevated but was inversely proportional to dose. Thyroid histology was unremarkable, and TCCD treatment had little effect on the ability of rats to raise serum T4, T3, and TSH concentrations in response to acute cold stress. TCDD treatment caused a slight (8%) decrease in basal metabolic rate, yet comparable decreases were seen in pair-fed control animals. Thermogenesis, as measured by O2 consumption and colonic temperatures in rats exposed to various ambient temperatures, was only marginally affected. In summary, although thyroid hormone concentrations were markedly altered, rats given doses of TCDD sufficient to cause overt toxicity appeared to be essentially euthyroid. These results do not support proposals by other researchers that altered thyroid status is a major contributor to TCDD toxicity and/or a key response to TCDD exposure.

Histopathology of interscapular brown adipose tissue, thyroid, and pancreas in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

The time course of histological changes was studied in rats lethally intoxicated (150 micrograms/kg) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition to TCDD-caused tissue damage described by others, the thyroid, pancreas, and interscapular brown adipose tissue (IBAT) were identified as tissues affected by TCDD. Because histological changes in the thyroid and pancreas occurred late (7 days after dosing), these effects are viewed as secondary due to altered hormonal homeostases. Both light and electron microscopic examination of IBAT identified this tissue as a target in TCDD toxicity. Histological changes in IBAT are characterized by three phases: (1) "fatty" IBAT (Days 1 to 3 after dosing); (2) fat depletion accompanied by glycogen accumulation (Days 4 to 7 after dosing); and (3) complete fat and glycogen depletion together with massive cellular damage (Days 8 to 14), particularly affecting the mitochondria. It is concluded that brown adipose tissue is a primary target in TCDD toxicity. It seems that destruction of brown adipose tissue by TCDD leads to an energy imbalance resulting in reduced oxygen consumption which forces animals to contribute a greater proportion of energy to the maintenance of their body temperature by anaerobic pathways. It is suggested that this less efficient energy utilization is the cause of a wasting syndrome.

Reduced serum thyroid hormone levels in hexachlorobenzene-induced porphyria

The effect of feeding 0.1% hexachlorobenzene (HCB) for 55 days on mortality, body weight, urinary porphyrin excretion, serum thyroid hormones and induction of liver microsomal enzymes was studied in female Sprague-Dawley rats. This dosage regimen, followed by 42 days of a regular diet, resulted in 33% mortality with a mean time to death of 67 +/- 4 days. Body weight of survivors was not affected by dietary HCB, whereas non-survivors underwent a rapid weight loss (wasting) prior to death. At the end of the dosing period (day 55), rats fed the HCB diet exhibited an increase in the excretion of urinary porphyrins (4-fold) and a significant decrease in the levels of serum thyroxine (T4) and triiodothyronine (T3). When rats were returned to a regular diet the excretion of urinary porphyrins continued to rise (approx. 100 times higher than controls) and serum thyroid hormones remained suppressed. At the end of the experiment (day 97), the concentration of liver microsomal cytochrome P-450 and cytochrome bs and the activity of ethoxyresorufin-O-deethylase, pentoxyresorufin-O-dealkylase, aminopyrine-N-demethylase and UDP-glucuronosyl transferase were significantly induced, whereas the activity of NADPH-cytochrome c reductase and benzo[a]pyrene hydroxylase was not. Results demonstrate that HCB-induced lethality and porphyria occur by different mechanisms, reduced T4 and T3 serum levels accompany induction of porphyria by HCB, and induction of aryl hydrocarbon hydroxylase (with benzo[a]pyrene as substrate) is not a sensitive indicator of HCB exposure.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity

The selective toxicity of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) for the thymus, consisting primarily of immature T-cells, led us to search for an analogous selective toxicity for the immature B-lymphocytes in the bone marrow. In the dose-response study C57B1/6 male mice were injected with either vehicle alone (corn oil), 30, 60, or 120 micrograms/kg of TCDD i.p. The mice were killed by cervical dislocation 7 days later. In the time-response study, mice were injected with either saline or 120 micrograms/kg i.p. TCDD, 3, 7, 14, or 21 days before killing. In both studies, the following were analyzed: change in body weight, thymus weight, spleen and bone marrow cellularity, and spleen and marrow B-lymphocyte function, measured using the in vitro B-lymphocyte colony forming unit in culture assay, with the mitogen lipopolysaccharide (LPS) from Salmonella typhosa, and the in vitro plaque forming cell assay, with the thymus independent antigen, TNP-LPS. In the dose-response study there was a reduction in thymic weight, spleen B-cell functional response (per spleen), and bone marrow B-cell functional response to 14%, 35-54%, and 20-32% of control, respectively, at a dosage of 120 micrograms/kg. In the time-response study, thymic weight and bone marrow B-cell functional response (per femur) were reduced to 6% and 18% of control, respectively, at day 21. The results indicate that TCDD was selectively more toxic to the immature B-cells in the bone marrow than the more mature B-cells in the spleen. This immunotoxicity was dose-dependent.