Modulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated myelotoxicity by thyroid hormones

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.

The menace of endocrine disruptors on thyroid hormone physiology and their impact on intrauterine development

The delivery of the appropriate thyroid hormones quantity to target tissues in euthyroidism is the result of unopposed synthesis, transport, metabolism, and excretion of these hormones. Thyroid hormones homeostasis depends on the maintenance of the circulating 'free' thyroid hormone reserves and on the development of a dynamic balance between the 'free' hormones reserves and those of the 'bound' hormones with the transport proteins. Disturbance of this hormone system, which is in constant interaction with other hormone systems, leads to an adaptational counter-response targeting to re-establish a new homeostatic equilibrium. An excessive disturbance is likely to result, however, in hypo- or hyper- thyroid clinical states. Endocrine disruptors are chemical substances forming part of 'natural' contaminating agents found in most ecosystems. There is abundant evidence that several key components of the thyroid hormones homeostasis are susceptible to the action of endocrine disruptors. These chemicals include some chlorinated organic compounds, polycyclic aromatic hydrocarbons, herbicides, and pharmaceutical agents. Intrauterine exposure to endocrine disruptors that either mimic or antagonize thyroid hormones can produce permanent developmental disorders in the structure and functioning of the brain, leading to behavioral changes. Steroid receptors are important determinants of the consequences of endocrine disruptors. Their interaction with thyroid hormones complicates the effect of endocrine disruptors. The aim of this review is to present the effect of endocrine disruptors on thyroid hormones physiology and their potential impact on intrauterine development.

2,3,7,8-tetrachlorodibenzo-p-dioxin augments the modulation of gene expression mediated by the thyroid hormone receptor

Previously, we reported on genes whose expression was highly modulated by T3 in the HeLaTR cells that stably expressed the thyroid hormone receptor (TR). In this study, we examined the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on TR-mediated gene expression. In the HeLaTR cells, T3 induced the expression of the reporter gene in a thyroid hormone responsible element (TRE)-dependent manner. When the cells were cultured in the presence of T3, the addition of TCDD but not 4-hydroxy-2',3,4',5,6'-pentachlorobiphenyl (PCB-OH), bisphenol A (BPA), or di(2-ethylhexyl)phthalate (DEHP) to the culture media further enhanced the T3-induced expression of the reporter gene. RT-PCR revealed that mRNA levels of 4-1BB, fmfc, PSCA, PSG7, RANTES, and TRAF1, which were highly increased by T3, were further elevated in cells exposed to T3 and TCDD. Also, the mRNA level of BMP6, which was decreased by T3, further declined in the cells exposed to both T3 and TCDD. In contrast to the effect of TCDD, PCB-OH suppressed the modulation of these gene expressions by T3. Neither TCDD nor PCB-OH alone affected the expression of 4-1BB, fmfc, PSCA, PSG7, RANTES, TRAF1, or BMP6. These results indicate that TCDD augments the cellular responses to T3 by hyperactivating TR-mediated gene expression, whereas PCB-OH suppresses cellular responses to T3 by negatively regulating it. Based on these findings, enzyme-linked immunosorbent assay (ELISA) for the PSCA protein in the HeLaTR cells was established. Such assays will be useful to monitor the effects of endocrine disrupting chemicals (EDCs) on TR-mediated gene expression.

Effects of o,p'-DDT on the 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible CYP1A1 expression in murine Hepa-1c1c7 cells

Cultured mouse hepatoma Hepa-1c1c7 cells were treated with o,p'-DDT and/or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to assess the role of o,p'-DDT on CYP1A1 expression. o,p'-DDT alone did not affect CYP1A1-specific 7-ethoxyresorufin O-deethylase (EROD) activity. In contrast, TCDD-inducible EROD activities were markedly reduced on concomitant treatment with TCDD and o,p'-DDT in a dose-dependent manner. Treatment with ICI 182.780, an estrogen-receptor antagonist, did not affect the suppressive effects of o,p'-DDT on TCDD-inducible EROD activity. TCDD-inducible CYP1A1 mRNA levels were markedly suppressed on treatment with TCDD and o,p'-DDT, and this was consistent with their effects on EROD activity. A transient transfection assay using dioxin-response element (DRE)-linked luciferase and an electrophoretic mobility shift assay revealed that o,p'-DDT reduced the transformation of the aryl hydrocarbons (Ah) receptor to a form capable of specifically binding to the DRE sequence in the promoter region of the CYP1A1 gene. These results suggest that the downregulation of CYP1A1 gene expression by o,p'-DDT in Hepa-1c1c7 cells might be an antagonism of the DRE binding potential of the nuclear Ah receptor but is not mediated through the estradiol receptor.

Down-regulation of murine Cyp1a-1 in mouse hepatoma Hepa-1c1c7 cells by bisphenol A

Cultured mouse hepatoma Hepa-1c1c7 cells were treated with either bisphenol A or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or in combination to assess the role of bisphenol A in the process of Cyp1a-1 induction. Treatment of Hepa-1c1c7 cultures with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) induced Cyp1a-1, as determined by analysis of 7-ethoxyresorufin O-deethylase (EROD) activities. Bisphenol A alone did not affect the activity of Cyp1a-1-specific EROD; in contrast, TCDD-induced EROD activities were markedly reduced in the concomitant treatment of TCDD and bisphenol A in a dose-dependent manner. Treatment with tamoxifen, an antiestrogen that acts through the estrogen receptor, did not affect the suppressive effects of bisphenol A on TCDD-induced EROD activity. TCDD-induced Cyp1a-1 mRNA levels were markedly suppressed in the concomitant treatment of TCDD and bisphenol A consistent with their effects on EROD activity. Transient transfection assay using dioxin-response element (DRE)-linked luciferase revealed that bisphenol A reduced transformation of the aryl hydrocarbons (Ah) receptor to a form capable of specifically binding to the DRE sequence in the promoter of the Cyp1a-1 gene. These results suggest the down-regulation of the Cyp1a-1 gene expression by bisphenol A in Hepa-1c1c7 cells might be antagonism of the DRE binding potential of nuclear Ah receptor but not mediated through estradiol receptor.

Suppressive effects of estradiol on 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated transcriptional activation of murine Cyp1a-1 in mouse hepatoma Hepa 1c1c7 cells

Cultured mouse hepatoma Hepa lclc7 cells were treated with either estradiol or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or in combination to assess the role of estradiol in the process of Cypla-1 induction. Estradiol at a concentration as high as 1 microM slightly increased the activity of Cypla-1-specific 7-ethoxyresorufin O-deethylase (EROD); in contrast, TCDD-induced EROD activity and Cypla-1 mRNA levels were markedly reduced in the concomitant treatment of TCDD and estradiol in a dose-dependent manner. Treatment with tamoxifen, an anti-estrogen which acts through the estrogen receptor, did not affect the suppressive effects of estradiol on TCDD-induced EROD activity. Electrophoretic mobility shift assay using nuclear extract of cells revealed that estradiol reduced transformation of the Ah receptor to the form capable of specifically binding to an oligonucleotide containing dioxin-response element (DRE) sequence. Consistent with this, estradiol decreased TCDD-induced increased chloramphenicol acetyltransferase (CAT) activity from a DRE-containing CAT reporter plasmid after transient transfection into the cells. The levels of the cytosolic [3H]TCDD-Ah receptor complex were reduced by estradiol in competitive Ah receptor binding assay using [3H]TCDD. This study demonstrated that estradiol acts as an antagonist to TCDD and can regulate Cyp1a-1 expression in an Ah receptor-dependent manner but not through estradiol receptor in Hepa 1c1c7 cells.

Environmental exposures that affect the endocrine system: public health implications

In recent years much attention has been focused on the potential for a wide range of xenobiotic chemicals to interact with and disrupt the endocrine systems of animal and human populations. An overview of the chemicals that have been implicated as endocrine disruptors is presented. The ubiquity in the environment and associated body burdens of these chemicals in human populations are described. Potential mechanisms of action are reviewed, including the role of specific intracellular receptors and their interactions with endogenous and exogenous materials. The subsequent upregulation or downregulation of physiological processes at critical stages of development is discussed. The potential for joint toxic action and interaction of chemical mixtures is also discussed. The acknowledged role of wildlife populations as sentinels of potential human health effects is reviewed, and the weight of evidence for the role and impact of endocrine disruptors is presented. The implications of exposure to endocrine-disrupting chemicals for human health are reviewed, with special emphasis on the potential for transgenerational effects in at-risk populations. Recommendations for future research include the development of (1) structural activity and in vivo and in vitro functional toxicology methods to screen chemicals for their endocrine-disrupting ability, (2) biomarkers of exposure and effect, and (3) in situ sentinel systems.

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.

Role of genetics and drug metabolism in human cancer risk

The research field concerning responses to drugs having a hereditary basis is called 'pharmacogenetics'. At least 5 dozen pharmacogenetic polymorphisms have been described in clinical medicine; many are responsible for marked differences in genetic predisposition toward toxicity or cancer. Three are detailed here: the acetylation, the debrisoquine, and the AH locus polymorphism. All 3 are very common among the United States' population: 1 in 2 is a 'slow acetylator', 1 in 12 is a 'poor metabolizer' for more than 2 dozen commonly prescribed drugs in the debrisoquine panel, and the CYP1A1 and CYP1A2 (cytochromes P(1)450 and P(3)450) genes are highly inducible by cigarette smoke in 1 of 10 patients. Differences in xenobiotic metabolism between individuals in the same family can be greater than 200-fold, suggesting that occupationally hazardous chemicals, as well as prescribed drugs having a narrow therapeutic window, might cause strikingly dissimilar effects between patients of differing genotypes. Our ultimate goal is 'preventive toxicology', i.e. the development of simple, inexpensive, unequivocal and sensitive assays to predict individual risk of toxicity or cancer. These tests could help the individual in choosing a safer life style or place of work and might aid the physician in deciding which drug to prescribe.

Bioavailability and cytochrome P-450 induction from 2,3,7,8-tetrachlorodibenzo-P-dioxin contaminated soils from Times Beach, Missouri, and Newark, New Jersey

Bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from contaminated soils from Times Beach, Missouri and Newark, New Jersey, was examined using liver concentrations and toxicity in guinea pigs observed up to 60 days following a single oral administration, and induction of cytochrome P-450 in rats sacrificed 24 hours after a single oral dose as endpoints. Both soils are contaminated with several chlorinated dioxins and numerous other compounds. Times Beach soil resulted in greater TCDD concentration in liver and TCDD was considerably more bioavailable from Times Beach soil than from Newark soil. However, both soils induced cytochrome P-450 activity to approximately the same extent. Moreover, similar banding patterns of microsomal proteins were seen on polyacrylamide electrophoretic gels. The many other compounds present in the soils, particularly in Newark, may account for the similar protein bands and levels of cytochrome P-450 observed.

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.

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.