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

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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

Hexachlorobenzene, a dioxin-type compound, increases malic enzyme gene transcription through a mechanism involving the thyroid hormone response element

Hexachlorobenzene (HCB) is a dioxin-type chemical that acts mainly through the aryl hydrocarbon receptor. Chronic exposure of rats to HCB increases the activity of malic enzyme (ME). In this report, we show that this increase is correlated with an induction of ME messenger RNA (mRNA) levels, with the maximal HCB effect achieved after 9 days of intoxication. This effect is specific for ME, as other liver enzymes, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenol pyruvate carboxykinase, and mitochondrial alpha-glycerol-3-phosphate dehydrogenase, are not affected by HCB. The induction of ME mRNA levels is accompanied by an increase in ME promoter activity, as demonstrated by transient transfection experiments performed in rat hepatoma H35 cells. In an attempt to identify the cis-regulatory elements responsible for the HCB effect, different promoter deletions and mutations were used. The results obtained localize the responsive region between positions -315 and -177. This region does not contain either consensus xenobiotic response or activating protein-1 elements, the two main mediators of dioxin compounds described to date. In contrast, a thyroid hormone response element (TRE) is located between -281 to -261. Deletions and mutations of the TRE element do not respond to HCB, demonstrating that this element mediates the response of this dioxin-type compound. As ME gene expression is regulated mainly by thyroid hormones, we next investigated the role of T3 receptor (T3R) in the ME gene transcriptional induction mediated by HCB. Using Scatchard analysis, we show that neither T3R binding features for its ligand nor alpha1 or beta1T3R mRNA levels are changed with the toxic. In gel shift assays, however, we observed that protein/DNA complexes formed on TRE from the ME promoter were induced by HCB. Using an oligonucleotide with a mutation that eliminates the TRE function, we demonstrate a loss of the induced protein/DNA complexes. Together, these data suggest that the dioxin-type compound HCB increases ME gene transcription by modulating the levels of still unidentified nuclear proteins that bind to the TRE element of the ME promoter.

Effects of polychlorinated biphenyls on thyroid hormones and liver type I monodeiodinase in the chick embryo

Polychlorinated biphenyls (PCBs) are widespread environmental contaminants which can biomagnify to higher tropic level organisms including birds. Circulating thyroid hormones (TH) and growth are decreased by PCB exposure. The first set of studies investigated the effects of PCBs on an enzyme responsible for TH homeostasis, hepatic type I monodeiodinase (MDI) in chicken embryos. Fertile chicken eggs were injected with Aroclor 1242, Aroclor 1254, 2,2',6, 6'-tetrachlorobiphenyl (TCB), 3,3',4,4'-TCB, or 3,3',5,5'-TCB on Day 0 and studies were terminated on Incubation Day 21. Hepatic MDI activity was reduced in embryos treated with the Aroclor mixtures. No effects on MDI activities were observed after PCB isomer treatment. Liver weights from embryos treated with Aroclor 1242 were decreased. In the second study, chick embryos were exposed to these same PCBs in order to evaluate their effect on circulating THs and growth. Treatment with PCBs had no effect on body weight. Femur length were decreased with Arcolor 1242 treatment. A decrease in plasma concentration of thyroxine was observed after treatment with Aroclor 1242 and Aroclor 1254. Based on these findings, it is evident that PCBs alter the thyroid axis. Bird circulating TH levels, which are generally reported, may not be a good biomarker for low-dose exposure to PCBs. However, the reduction in MDI activity was more sensitive to PCB mixture exposure and may be a useful biomarker.

Effects of environmental synthetic chemicals on thyroid function

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

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.

Species-specific alteration of hepatic glucose 6-phosphate dehydrogenase activity with coplanar polychlorinated biphenyl: evidence for an Ah-receptor-linked mechanism

We examined the in vivo effect of a highly toxic coplanar polychlorinated biphenyl (PCB) on the hepatic activity of glucose 6-phosphate dehydrogenase (G6PDH) in aryl hydrocarbon (Ah)-responsive (C57/BL) and -less-responsive (DBA) strains of mice. The activity in the C57BL strain was moderately increased by 3,3',4,4',5-pentachlorobiphenyl (PCB 126) in a dose dependent manner. However, this was not observed in DBA mice although greater doses were injected. 2,2',5,5'-Tetrachlorobiphenyl (PCB 52) with a non-planar structure did not increase G6PDH activity. The increase in G6PDH activity with PCB 126 was also seen in rats, but not in guinea pigs. The activity in the latter species was decreased rather than increased. These results suggest that the induction of hepatic G6PDH by coplanar PCB is mediated by a mechanism involving the Ah receptor, and the response was highly species-specific.

The Ah receptor and the mechanism of dioxin toxicity

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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.

Tissue-specific alterations of de novo fatty acid synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

De novo fatty acid synthesis was determined by the 3H2O method in numerous tissues and organs of TCDD-treated (125 micrograms/kg), pair-fed and free-fed male Sprague-Dawley rats to investigate if this important pathway of intermediary metabolism is altered by TCDD. Of the 12 tissues and organs examined, liver showed an increased, and interscapular brown adipose tissue (IBAT) a decreased de novo fatty acid synthesis when comparing TCDD-treated to pair-fed or free-fed control rats. De novo fatty acid synthesis was unaffected in other organs and tissues examined, with the exception that the concentration of 3H-fatty acids in plasma reflected the increased rate of synthesis seen in the liver of TCDD-treated animals. Increased de novo fatty acid synthesis in liver coincided with increased plasma triiodothyronine (T3) concentrations, whereas decreased de novo fatty acid synthesis in IBAT parallelled decreased plasma thyroxine (T4) levels. Thyroidectomy decreased de novo fatty acid synthesis, as expected, in both liver and IBAT. However, TCDD elicited no response in either of these organs in thyroidectomized rats. This finding suggests that changes observed in non-thyroidectomized rats are probably secondary effects. Indeed, known tissue-specific effects of T3 on liver and T4 on IBAT provide a likely explanation for the altered de novo fatty acid synthesis of these organs. It is suggested that increased de novo fatty acid synthesis in the liver of TCDD-treated rats might be responsible for the additional wasting away observable in these animals as compared to pair-fed controls.

Activity of thyroid hormone-inducible enzymes following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) caused a depletion of serum thyroxine, but paradoxically did not change L-3,5,3'-triiodothyronine (T3) levels in serum of rats. The activities of the thyroid-regulated enzymes alpha-glycerol phosphate dehydrogenase (GPD) and malic enzyme (ME) were determined in livers of normal and thyroidectomized (THX) rats treated with 0.1 to 100 nmol TCDD/kg body weight. Mitochondrial GPD activity did not change significantly as a function of TCDD dose in either normal or THX rats. ME activity was induced by TCDD in a dose-dependent fashion, but only in non-THX animals. The absence of ME induction in THX rats treated with TCDD indicates that TCDD is not intrinsically thyromimetic. The dependence of ME induction on thyroid hormones is much like the thyroid-hormone-dependent, multihormonal induction of ME by insulin and glucocorticoids. However, TCDD had no additive or synergistic effects on induction of ME activity in THX rats fed T3. A 30% decrease in steady-state plasma T3 levels of T3-fed animals treated with TCDD relative to T3-fed controls suggested that T3 catabolism was more rapid in TCDD-treated rats than controls. Thus a thyroid-hormone-dependent, multihormonal interaction is suggested as the basis for induction of ME by TCDD, but a strictly T3-dependent process has not been ruled out.

Do residue levels of polychlorinated biphenyls (PCBs) in human blood produce mild hypothyroidism?

PCBs are nearly ubiquitous environmental contaminants, occurring in most human adipose tissue and blood samples. It has recently been recognized that PCBs and related compounds share important structural properties with thyroid hormones and can bind thyroid hormone binding proteins. It is reasonable that such specific binding interactions can modulate the distribution of these compounds in the body and alter hormone-protein interactions that are responsible for the maintenance of normal thyroid status. Most of the available evidence indicates that the levels of free thyroid hormones in plasma are a reflection of the maintenance of normal thyroid status in any individual. A theoretical model for the transport of thyroid hormones in blood has been extended to estimate the modulating effects of PCBs on free thyroid hormones. Using conservative assumptions based on experimental data, our calculations indicate that PCB concentrations normally found in humans can effect significant increases in free thyroxine levels in serum by competing with serum thyroid hormone binding proteins. Experimental data are discussed which support the proposal that antagonist binding of PCBs to thyroid hormone binding proteins in serum could produce varying degrees of hypothyroidism. The biological result is compatible with the "equilibrium hypothesis" in which thyroid hormone redistributes between specific and nonspecific binding proteins rather than emphasizing the importance of free hormone as the active moiety.