Synergistic interaction of 2,3,7,8,-tetrachlorodibenzo-p-dioxin and hydrocortisone in the induction of cleft palate in mice

AhR promotes phosphorylation of ARNT isoform 1 in human T cell malignancies as a switch for optimal AhR activity

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a transcription factor present in immune cells as a long and short isoform, referred to as isoforms 1 and 3, respectively. However, investigation into potential ARNT isoform–specific immune functions is lacking despite the well-established heterodimerization requirement of ARNT with, and for the activity of, the aryl hydrocarbon receptor (AhR), a critical mediator of immune homeostasis. Here, using global and targeted transcriptomics analyses, we show that the relative ARNT isoform 1:3 ratio in human T cell lymphoma cells dictates the amplitude and direction of AhR target gene regulation. Specifically, shifting the ARNT isoform 1:3 ratio lower by suppressing isoform 1 enhances, or higher by suppressing isoform 3 abrogates, AhR responsiveness to ligand activation through preprograming a cellular genetic background that directs explicit gene expression patterns. Moreover, the fluctuations in gene expression patterns that accompany a decrease or increase in the ARNT isoform 1:3 ratio are associated with inflammation or immunosuppression, respectively. Molecular studies identified the unique casein kinase 2 (CK2) phosphorylation site within isoform 1 as an essential parameter to the mechanism of ARNT isoform–specific regulation of AhR signaling. Notably, CK2-mediated phosphorylation of ARNT isoform 1 is dependent on ligand-induced AhR nuclear translocation and is required for optimal AhR target gene regulation. These observations reveal ARNT as a central modulator of AhR activity predicated on the status of the ARNT isoform ratio and suggest that ARNT-based therapies are a viable option for tuning the immune system to target immune disorders.

Environmental mechanisms of orofacial clefts

Orofacial clefts (OFCs) are among the most common birth defects and impart a significant burden on afflicted individuals and their families. It is increasingly understood that many nonsyndromic OFCs are a consequence of extrinsic factors, genetic susceptibilities, and interactions of the two. Therefore, understanding the environmental mechanisms of OFCs is important in the prevention of future cases. This review examines the molecular mechanisms associated with environmental factors that either protect against or increase the risk of OFCs. We focus on essential metabolic pathways, environmental signaling mechanisms, detoxification pathways, behavioral risk factors, and biological hazards that may disrupt orofacial development.

My Winding Road: From Microbiology to Toxicology and Environmental Health

I would certainly never have predicted that I would become the director of the National Institute of Environmental Health Sciences (NIEHS) and the National Toxicology Program (NTP) when I was a Jewish girl growing up in Teaneck, New Jersey. My family stressed the importance of education. Yet for a girl there were many not-so-subtle suggestions that the appropriate careers were in teaching or nursing, and the most important thing was to be a wife and mother. Well, I can't disagree with the latter, although I would have to add grandmother to that list of achievements. My parents were both college graduates, but my mom only taught high school English for one year before leaving the field to start our family. My dad returned from World War II and joined his brother in accounting. After my first sister was born, my father joined my mother's family jewelry business and helped to open a second retail store. My mother helped my dad out during the busy times—Christmas and wedding season—but otherwise focused on our growing family of three girls and one boy. This became increasingly challenging when it became clear that my little brother was severely retarded and would require extra care.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

Review of the toxicity of chemical mixtures containing at least one organochlorine

An analysis of current research on mixture toxicity was conducted by critically reviewing published journal articles. The scope was limited to complex mixtures (more than two components) where at least one component was a chlorinated organic chemical. Although the basics of dose-response are widely accepted for mixtures, a number of technical issues, including dose metrics and the unquantified influence of toxicity modifying factors, confound data interpretation and restrict the ability to establish reliable determinations of the presence, nature, and extent of additivity. Lack of knowledge about dose level influences and species-specific variations contribute further interpretational limitations. Within this context, available data indicates that most tested mixtures are near or below simple dose/concentration additivity. Exceptions (both positive and negative) tend to occur when tested mixtures have only a few components or where sensitive whole organism or sub-organismal changes are used as the response metric. Available information does not routinely identify the presence of chlorine as a marker either of a particular type of toxicity or consistently greater potency. The most profound difficulty is the problem of clearly defining when and why similarity and dissimilarity of toxic action is expected for a particular mixture. This impediment largely results from the lack of a generally accepted, technical classification for mode/mechanism of toxic action coupled with the lack of a generally accepted classification scheme for mode/mechanism of toxicity interactions.

First trimester exposure to corticosteroids and oral clefts

BACKGROUND

The possible association between oral cleft in the newborn and maternal exposure to corticoids during pregnancy is still controversial. The aim of this study was to test this association by a case-control analysis using the large multicentric MADRE database.

METHODS

The MADRE database is a collection of information on malformed infants with a history of maternal first-trimester drug exposure. Nine malformation registries participate in the data collection. Cases were defined as infants presenting with a cleft palate or cleft lip, and exposure was defined by the use of corticosteroids during the first trimester of pregnancy.

RESULTS

After 12 years of data collection, the database includes data on 11,150 malformed infants. A slight association is observed between exposure to corticoids for systemic use and the occurrence of cleft lip with or without cleft palate (OR, 2.59; 95% CI, 1.18-5.67).

CONCLUSIONS

If the observed association is real, an interpretation is suggested, based on a likely interaction between corticosteroids and environmental dioxins. It is indeed possible that human fetuses may become sensitive to the teratogenic effect of corticosteroids when they are exposed in utero to environmental pesticides as well.

The blockade of mineralocorticoid hormone signaling provokes dramatic teratogenesis in cultured rat embryos

Although the administration of adrenocortical hormones to pregnant rats provokes only limited effect on the growth and development of the fetus, the direct influence of these steroids on cultured embryos has never been studied. The disruption of cell signaling by ZK 91587, which specifically occupies the mineralocorticoid receptor, resulted within 2 days in significant and pronounced adverse effects on the total length, the somite number, the embryo curvature, the communication between vitelline and umbilical blood vessels in the allantoid, and the vascularization of the vitelline sac, in 244-hour Wistar rat embryos in culture. The average score of 16 organs declined in a dose-dependent manner, following exposure to ZK 91587, and this was totally reversed by 10 microM aldosterone which, by itself, did not at all influence the embryonic development. The organogenesis was inhibited in the order: hind limb > fore limb > optic stalk > brain > olfactory pit > otic vesicle. ZK 91587 was completely ineffective in embryos that had attained the age of 260 hours. Similar, but less dramatic, results were obtained with the mineralocorticoid antagonist RU 26752, and with the antiglucocorticoid RU 38486. Sprague-Dawley rat embryos responded in a manner similar to the Wistar conceptuses. Thus, steroid receptor-mediated cell signaling is of critical importance to the growth and development of cultured rat embryos, which form a new model system to unravel adrenocortical hormone action.

Glucocorticoid receptor regulation in the rat embryo: a potential site for developmental toxicity?

Glucocorticoids play a key role in controlling numerous cellular processes during embryogenesis and fetal development. Excess glucocorticoids during development have been linked to dysmorphogenesis and/or intrauterine growth impairment in rodents. The actions of glucocorticoids are mediated by interaction with their receptors. Negative feedback regulation of glucocorticoid receptor (GR) is important for limiting cellular sensitivity to the hormones. Hence, acute exposure of the adult rat to the synthetic glucocorticoid dexamethasone (DEX) reduced both GR mRNA and protein in a variety of tissues that include hippocampus and liver, in a dose- and time-dependent fashion. Reduction in GR mRNA and protein were observable when DEX was given repeatedly at doses as low as 0. 05 mg/kg. In the control whole rat embryo, GR mRNA was low but measurable at as early as gestational day (GD) 10, but underwent rapid ontogenetic increase in the ensuring days. In contrast to the adult, neither GR mRNA nor protein in the whole rat embryo was affected by acute or repeated DEX administration to pregnant rats on GD10-13, even at doses as high as 0.8 mg/kg. Similar results were obtained in embryonic palate and liver, tissues known to be glucocorticoid targets. These data suggest that GR autoregulation does not occur during organogenesis in the rat. Accordingly, hormonal elevations from stress or chemical insults can be transduced unrestrictedly, ultimately leading to aberrant cell function and development. The unique mode of GR regulation seen in the embryonic cells may provide a potential common mechanism for developmental perturbation and toxicity for a variety of insults.

Multiple chemical exposures: synergism vs. individual exposure levels

Exposure to single chemicals is known to produce congenital malformations in both pregnant animals and humans exposed at sufficiently high intensity. However, real life involves multiple, simultaneous exposures. Using as a database the 43 multiple chemical exposure studies located by Nelson (Teratology 49:33-71; 1994) where synergism was reported, we explored the degree to which such concerns may be realistic from the viewpoint of the current standard developmental toxicity safety evaluation process. Focusing on the assessment of the lowest tested dose of a given agent participating in synergistic activity as compared to its threshold level for eliciting toxicity when administered alone, we found that while the availability of adequate data was limited, all cases, with the possible exception of one, demonstrated synergistic toxic expression only when at least one, and usually both, compounds were used at or above their individual threshold for toxicity. These findings suggest that in animals such phenomena of synergistic chemical interactions are likely to occur only when at least one and more likely both agents are administered at or above their individual threshold for toxicity. To the extent animal studies are predictive of human developmental hazards due to single chemical exposures, available data do not establish multiple chemical exposures as a major human developmental concern.

Review of the interaction between TCDD and glucocorticoids in embryonic palate

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that produces adverse biological effects including developmental toxicity and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. The synthetic glucocorticoid, hydrocortisone (HC), induces cleft palate and a potent, synergistic interaction has been observed between TCDD and HC in C57BL/6N embryonic mice. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different with formation of small palatal shelves following HC exposure and failure of normally-sized shelves to fuse after TCDD treatment. Each exposure also alters expression of several growth factors. When EGF, TGF alpha, EGF receptor, and the TGF beta's are considered as a combinatorial, interacting set of regulators, TCDD and HC each produce a unique pattern of increased and/or decreased expression across the set. The interaction of HC and TCDD results in a cleft palate whose etiology most closely resembles that observed after HC exposure, i.e. small palatal shelves. HC+TCDD-exposure also produces a pattern of growth factor expression which closely resembles that seen after HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor. The Ah receptor (AhR) binds TCDD and the glucocorticoid receptor (GR) binds HC. On gestation day (GD) 14, in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC+TCDD produced increased expression of both receptors and this pattern would be predicted to produce HC-like clefts as the GR-mediated responses would result in small palatal shelves. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Developmental expression of two members of a new class of transcription factors: II. Expression of aryl hydrocarbon receptor nuclear translocator in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix (bHLH) proteins involved in transcriptional regulation. The AhR is a ligand-activated partner of the ARNT protein. Both proteins are required to transcriptionally regulate gene expression. ARNT must be complexed to AhR to permit binding to the regulatory DNA sequence. The AhR-ligand complex is known to mediate a range of biological responses, such as developmental toxicity, induction of cleft palate, and hydronephrosis. AhR and ARNT are expressed in human embryonic palatal cells and AhR was recently shown to have a specific developmental pattern of expression in the mouse embryo. In the present study, expression of ARNT is characterized in C57Bl/6N mouse embryos from gestation day (GD) 10-16 using immunohistochemistry and in situ hybridization. Af affinity purified antibody against human ARNT (1.1 micrograms/ml) was detected with an avidin-biotin-peroxidase complex. ARNT mRNA was localized with a 35S-RNA probe from pBM5/NEO-M1-1. Specific spatial and temporal patterns of ARNT expression emerged and mRNA and protein expression correlated. The GD 10-11 embryos showed highest levels of ARNT in neuroepithelial cells of the neural tube, visceral arches, otic and optic placodes, and preganglionic complexes. The heart also had significant expression of ARNT with strong nuclear localization. After GD11, expression in heart and brain declined. In GD 12-13 embryos expression was highest in the liver where expression increased from GD 12-16. At GD 15-16 the highest levels of ARNT occurred in adrenal gland and liver, although ARNT was also detected in submandibular gland, ectoderm, tongue, bone, and muscle. In all of these tissues ARNT was cytoplasmic as well as nuclear, except in some of the cortical adrenal cells in which ARNT was strongly cytoplasmic with little or no nuclear localization. These specific patterns of ARNT expression, which differ in certain tissues from the expression of AhR, suggest that ARNT may have additional roles in normal embryonic development.

Developmental expression of two members of a new class of transcription factors: I. Expression of aryl hydrocarbon receptor in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a basic region/helix-loop-helix (bHLH) motif. AhR has been sequenced and the functional domains defined and there is information on the formation of complexes with other peptides and interactions with DNA, although these areas continue to be investigated. AhR mediates many biological effects such as developmental toxicity, including induction of cleft palate and hydronephrosis. This regulatory protein is expressed in embryonic liver and has been immunohistochemically localized in cells of human and mouse secondary palate. The expression of AhR in embryonic tissues and its ability to disrupt development suggests a significant role for this protein in development. The present study examines the pattern of AhR expression in the C57BL/6N mouse embryo from gestation days (GD) 10-16, using in situ hybridization and immunohistochemical analysis. AhR mRNA was localized with 35S-RNA antisense riboprobe (cAh1 probe, 1.8 Kb amino terminal DNA). AhR protein was localized with purified monoclonal antibody (RPT-9) raised against the N-terminal peptide sequence. AhR mRNA and protein were expressed in GD 10-13 neuroepithelium, and as development progressed the levels in brain decreased. GD 10-12 embryos also showed AhR in branchial arches, heart, somites, and liver. AhR protein and mRNA in heart were highest at GD 10-11 and decreased with age. In liver, AhR mRNA and protein levels increased and nuclear localization became more pronounced with gestational age. In GD 14-16 embryos levels in liver and adrenal were highest, but AhR was present in ectoderm, bone, and muscle. AhR expression was specific for both cell type, organ/tissue, and developmental stage, suggesting that this novel ligand-activated transcriptional regulator may be important in normal embryonic development.

Immunohistochemical double-staining for Ah receptor and ARNT in human embryonic palatal shelves

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix-PAS (HLH) proteins involved in transcriptional regulation. Polycyclic aromatic halogenated chemicals, of which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent, bind to the AhR. In the cellular cytoplasm, the AhR exists as a complex with the heat shock protein HSP90 and other small peptides. This complex dissociates following ligand binding and then the ligand-bound AhR binds ARNT. The ligand-AhR-ARNT complex interacts with a specific, nuclear DNA sequence, the dioxin response element (DRE), altering transcription of a regulated gene. Studies in hepatoma cell lines indicate that both proteins are required for regulation of transcription. In this study, AhR and ARNT were localized immunohistochemically in human embryonic palatal cells and specific patterns of expression were seen for each protein. A double-staining protocol revealed that epithelial cells expressed both AhR and ARNT, but in mesenchyme and nasal spine cartilage individual cells were identified which expressed either AhR or ARNT. This heterogeneous pattern may be a means of suppressing transcriptional regulation and also suggests the existence of other, unidentified basic-helix-loop-helix partner(s). The heterogeneous expression pattern may also reflect a complex role for these HLH proteins as transcriptional regulators of embryonic development.

Induction of cleft palate by gamma-irradiation of prenatal CD-1 mice

Fetuses of pregnant mice exposed to whole body gamma-irradiation (400 rads) on the 12th gestational day were compared with control fetuses to assess the effect of radiation on palatogenesis. Experimental and control groups were sacrificed on day 18 of gestation. The fetuses were removed via laparotomy and counted to determine the litter size. Animals were decapitated and the maximum head height, width, and circumference recorded. For light microscopic examination of palatogenesis, heads were fixed in Bouin's solution and prepared for paraffin sectioning and staining with hematoxylin and eosin. A significant reduction in litter size (p < .0025) and head measurements (p < .005) of irradiated fetuses was observed when compared to those of the control group. Histologic examination of serial coronal sections demonstrated that 100% of the experimental fetuses had palatal clefts. It was concluded that gamma-rays have adverse effects on the litter size, head measurements, and palatal closure of mouse fetuses.

Interactions in developmental toxicology: a literature review and terminology proposal

Developmental toxicologists have investigated the interactive effects from concurrent exposures to a variety of chemical and physical agents, including therapeutic drugs, industrial agents, and some biological organisms or their toxins. Of approximately 160 reports of concurrent exposures reviewed in this paper, about one third report no interactive effects (including additive effects--usually referring to response--as opposed to dose-additivity); another one third report antagonistic effects, and the final third report potentiative or synergistic effects. The quality of the studies is highly variable. Frequently, only small numbers of animals were included, and very few dose levels were evaluated. Maternal toxicity was rarely discussed. Time-effect relationships were examined infrequently. In addition, these studies are also inconsistent in the use of terms to describe interactive effects, and more than 90% of the terms were not in harmony with currently accepted definitions in toxicology. Because interaction studies will continue to be important in the future, this paper proposes uniform usage of terms for additivity and interactions in developmental toxicology: additivity (the combined effect of two or more developmental toxicants approximates the sum of the effects of the agents administered separately); antagonism (the combined effect of two or more agents, one or more of which are present at doses that would be developmentally toxic if given individually, is significantly less than the sum of the effects of the agents administered separately); potentiation (the increased effect of a developmental toxicant by concurrent action of another agent at a dose that is not developmentally toxic); synergism (the combined effect of two or more developmental toxicants is significantly greater than the sum of the effects of each agent administered alone); and, interaction if more precise terminology does not apply.

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

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

Comparisons of the effects of TCDD and hydrocortisone on growth factor expression provide insight into their interaction in the embryonic mouse palate

Cleft palate (CP) can be induced in embryonic mice by a wide range of compounds, including glucocorticoids and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Hydrocortisone (HC), a glucocorticoid, retards embryonic growth producing small palatal shelves, while TCDD exposure blocks the fusion of normally sized shelves. TCDD induction of CP involves altered differentiation of the medial epithelial cells. Recent studies indicate that growth factors such as EGF, TGF-alpha, TGF-beta 1, and TGF-beta 2 are involved in palatogenesis, regulating proliferation, differentiation, and extracellular matrix production. A synergism has been observed between HC and TCDD in which doses too low to induce CP alone are able to produce greater than 90% incidence when coadministered. In the present study a standard teratology protocol was performed in C57BL/6N mice to examine the synergism at doses lower than those previously published. Data from this study indicate synergistic interactions at doses as low as 3 micrograms TCDD/kg + 1 mg HC/kg. This extreme sensitivity suggests the involvement of a receptor-mediated mechanism possibly resulting in altered regulation of gene expression. Mechanisms of interaction were further studied by comparing growth of the shelves, fate of the medial epithelium, and expression of growth factor mRNAs and peptides. Pregnant mice were dosed on GDs 10-13 with HC (100 mg/kg sc) or with HC (25 mg/kg sc) + TCDD (3 micrograms/kg orally), doses producing 30% and 99% CP, respectively. The interaction between HC and TCDD results in a small HC-like palate, rather than the morphology typical of TCDD-induced clefting. Both compounds inhibited programmed cell death of the medial epithelium, which instead differentiated into an oral-like epithelium. The alterations in growth factor expression after HC or HC + TCDD were similar. Expression of EGF, TGF-beta 1, TGF-beta 2, and EGF receptor increased in specific palatal regions. Increased levels of mRNA were observed only for TGF-beta 1. The effect of TCDD alone on growth factor expression differ from those seen with HC or HC + TCDD. These divergent effects on growth factor expression may contribute to the differences in shelf size and thus to the different mechanisms of HC and TCDD clefting. Thus the synergism between HC and TCDD may involve similar and potentially additive effects on regulators of proliferation and differentiation in the palate, but additional contributing factors cannot be excluded.

Induction of ethoxyresorufin-O-deethylase and inhibition of glucocorticoid receptor binding in skin and liver of haired and hairless HRS/J mice by topically applied 2,3,7,8-tetrachlorodibenzo-p-dioxin

The biochemical changes associated with the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been reported to include alterations in glucocorticoid and epidermal growth factor receptors and mixed function oxidase (MFO) induction. TCDD induces MFO activity in skin of both haired and hairless HRS/J mice. However, epidermal hyperplasia and hyperkeratosis are produced only in the skin of hairless mice. Therefore, since steroid and growth factor responses are implicated in cell proliferation and differentiation, these mice constitute a model system for assessing the possible roles of glucocorticoid and epidermal growth factor receptors in the toxicity of TCDD. The effect of dermal TCDD application (12 micrograms/kg in 100 microliters acetone) on ethoxyresorufin-O-deethylase (EROD) activity, glucocorticoid receptor binding and epidermal growth factor receptors in liver and skin of hairless and haired mice was determined. No differences existed in the basal number of cytosolic glucocorticoid receptors (Bmax) or the apparent equilibrium binding constants (Kd) in control liver, dorsal skin and abdominal skin of male and female hairless mice and haired male mice. Seven days after topical application of TCDD, decreases of approximately 38% were observed in the hepatic Bmax of the glucocorticoid receptors in both haired and hairless mice. However, in dorsal skin, TCDD decreased Bmax by approximately 40% in hairless mice but only 18% in haired mice. The dexamethasone-glucocorticoid receptor complex from both liver and skin of control and TCDD treated mice had similar sedimentation co-efficients in sucrose density gradients. TCDD had no effect on the Kd of glucocorticoid receptors of skin or liver in haired and hairless mice. No difference was observed in the time-dependent increases in hepatic EROD activity between haired and hairless mice after dermal application of TCDD. However, the maximum induction of EROD activity in microsomes from the skin of haired mice was only 60% of the activity observed in hairless animals. The induction of EROD by TCDD did not correlate temporally with the decrease in glucocorticoid receptor binding. The application of TCDD to the skin of hairless mice resulted in epidermal hyperplasia and dermal keratinization, while little change was observed in the general morphology of the skin of haired mice following dermal application. The application of TCDD had no effect on the incidence and distribution of epidermal growth factor receptors in skin of haired and hairless mice as determined immunohistochemically. Thus, the biochemical effects of TCDD are not only strain dependent, but tissue specific. Furthermore, decreases in glucocorticoid and epidermal growth factor receptors do not appear to be general markers of TCDD toxicity.

A critical review of the developmental toxicity and teratogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin: recent advances toward understanding the mechanism

A specific teratogenic response is elicited in the mouse as a result of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin). The characteristic spectrum of structural malformations induced in mice following exposure to TCDD and structurally related congeners is highly reproducible and includes both hydronephrosis and cleft palate. In addition, prenatal exposure to TCDD has been shown to induce thymic hypoplasia. These three abnormalities occur at doses well below those producing maternal or embryo/fetal toxicity and are thus among the most sensitive indicators of dioxin toxicity. In all other laboratory species tested, TCDD causes maternal and embryo/fetal toxicity but does not induce a significant increase in the incidence of structural abnormalities even at toxic dose levels. Developmental toxicity occurs in a similar dose range across species; however, mice are particularly susceptible to development of TCDD-induced terata. Recent experiments using an organ culture were an attempt to address the issue of species and organ differences in sensitivity to TCDD. Human palatal shelves examined in this in vitro system were found to approximate the rat in terms of sensitivity for induction of cleft palate. Investigators have suggested that altered regulation of growth factors and their receptors may involve inappropriate proliferation and differentiation of target cells, ultimately producing TCDD-induced terata. Why the teratogenic effects of TCDD are so highly species and tissue specific, and which animal species most accurately predicts the response of the human embryo/fetus, at the levels of exposure experienced by humans, still remains to be clarified.

Retinoic acid and 2,3,7,8-tetrachlorodibenzo-p-dioxin selectively enhance teratogenesis in C57BL/6N mice

TCDD is one of the most toxic man-made compounds and an extremely potent teratogen in mice. Many of its toxic symptoms resemble those seen during vitamin A deficiency. Vitamin A and its derivatives, such as alltrans-retinoic acid (RA), are also teratogenic in mice, as well as many other species. Both TCDD and RA produce cleft palate in susceptible strains of mice. However, while TCDD produces hydronephrosis, RA does not, and TCDD does not produce limb bud defects while RA does. To determine whether TCDD and RA would enhance or antagonize the teratogenic effects of the other compound, C57BL/6N dams were treated po on Gestation Day (gd) 10 or 12 with 10 ml corn oil/kg containing TCDD (0-18 micrograms/kg), RA (0-200 mg/kg), or combinations of the two chemicals. Dams were killed on gd 18 and toxicity and teratogenicity assessed. Coadministration of TCDD and RA had no effect on maternal or fetal toxicity beyond what would be expected by either compound alone. Cleft palate was induced by RA at lower doses on gd 10 than on gd 12, but by TCDD at lower doses on gd 12 than on gd 10. Sensitivity to TCDD-induced hydronephrosis was similar on both gd 10 and 12. The limb bud defects were only observed when RA was administered on gd 10, not when given on gd 12. No other soft tissue or skeletal malformations were related to administration of TCDD or RA. No effect of TCDD was observed on the incidence or severity of limb bud defects induced by RA, nor did RA influence the incidence or severity of hydronephrosis induced by TCDD. However, the incidence of cleft palate was dramatically enhanced by coadministration of the xenobiotic and vitamin. On both gd 10 and 12, the dose-response curves for cleft palate induction were parallel, suggesting some similarities in mechanism between the two compounds. However, combination treatment resulted in a synergistic response that varied with the stage of development and was tissue specific.

Teratogenic effects of polychlorinated dibenzofurans in combination in C57BL/6N mice

Polychlorinated dibenzofurans (PCDFs) are highly toxic environmental contaminants which have been involved in several incidents of human poisoning. Two congeners, 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF), have been shown to persist in the tissues of victims of accidental ingestion from Japan and Taiwan. The teratogenicity of these compounds, both alone and in combination, was assessed in C57BL/6N mice. Pregnant mice were treated with 10 ml/kg corn oil containing no PCDFs, 4-PeCDF (0-30 micrograms/kg), HCDF (0-300 micrograms/kg), or a combination of the two on gestation Days 10-13, followed by necropsy on gestation Day 18. Maternal and fetal toxicity were assessed and selected soft tissues were examined for abnormalities. Both chemicals caused hydronephrosis and cleft palate in the absence of any overt toxicity. Hydronephrosis occurred at doses approximately fivefold lower than those causing cleft palate. The combination of 4-PeCDF and HCDF was additive for terata based on responses predicted by probit analysis. In addition, the combination of 2,3,4,5,3',4'-hexachlorobiphenyl (0-60 mg/kg), a structurally related compound also present in PCDF poisoning victims, and 4-PeCDF appears additive. Thus, these chemicals, which cause toxic effects similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin, are additive in the induction of fetal anomalies in the mouse.

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

Although binding by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the Ah receptor is a prerequisite for toxicity, the events responsible for subsequent TCDD effects are essentially unknown. Several lines of evidence have indicated that thyroid hormones share common molecular properties with TCDD and can modulate its toxicity. In the present studies we employed suppression of murine bone marrow hematopoiesis by TCDD as an in vitro model to study the relationship between thyroid hormones and TCDD toxicity. Supraphysiological levels of thyroid hormone mimicked TCDD myelotoxicity, in that both were inhibited by a common antagonist, 1-NH2-3,7,8-trichlorodibenzo-p-dioxin. Furthermore, myelotoxicity by both TCDD and thyroid hormone segregated with the Ah locus in congenic mice. These data provide evidence of a relationship between TCDD and thyroid hormones in that hormonal activity may help regulate TCDD toxicity.

Skeletal muscle glucocorticoid receptor and glutamine synthetase activity in the wasting syndrome in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

This study demonstrated specific changes in rat skeletal muscles after a single oral dose (100 micrograms/kg) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The development of the wasting syndrome was characterized by marked body weight loss, as well as atrophy of plantaris and gastrocnemius muscles. Fourteen days after administration of TCDD, gastrocnemius muscle cytosolic glucocorticoid receptor binding, measured at a single saturating concentration of [3H]triamcinolone acetonide, was significantly diminished, while plantaris muscle glutamine synthetase activity was strikingly elevated, indicating that specific biochemical alterations occur in skeletal muscle in the wasting syndrome.

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.