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

Contributions of Nitric Oxide to AHR-Ligand-Mediated Keratinocyte Differentiation

Activation of the aryl hydrocarbon receptor (AHR) in normal human epidermal keratinocytes (NHEKs) accelerates keratinocyte terminal differentiation through metabolic reprogramming and reactive oxygen species (ROS) production. Of the three NOS isoforms, NOS3 is significantly increased at both the RNA and protein levels by exposure to the very potent and selective ligand of the AHR, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Inhibition of NOS with the chemical N-nitro-l-arginine methyl ester (l-NAME) reversed TCDD-induced cornified envelope formation, an endpoint of terminal differentiation, as well as the expression of filaggrin (FLG), a marker of differentiation. Conversely, exposure to the NO-donor, S-nitroso-N-acetyl-DL-penicillamine (SNAP), increased the number of cornified envelopes above control levels and augmented the levels of cornified envelopes formed in response to TCDD treatment and increased the expression of FLG. This indicates that nitric oxide signaling can increase keratinocyte differentiation and that it is involved in the AHR-mediated acceleration of differentiation. As the nitrosylation of cysteines is a mechanism by which NO affects the structure and functions of proteins, the S-nitrosylation biotin switch technique was used to measure protein S-nitrosylation. Activation of the AHR increased the S-nitrosylation of two detected proteins of about 72 and 20 kD in size. These results provide new insights into the role of NO and protein nitrosylation in the process of epithelial cell differentiation, suggesting a role of NOS in metabolic reprogramming and the regulation of epithelial cell fate.

The Henna pigment Lawsone activates the Aryl Hydrocarbon Receptor and impacts skin homeostasis

As a first host barrier, the skin is constantly exposed to environmental insults that perturb its integrity. Tight regulation of skin homeostasis is largely controlled by the aryl hydrocarbon receptor (AhR). Here, we demonstrate that Henna and its major pigment, the naphthoquinone Lawsone activate AhR, both in vitro and in vivo. In human keratinocytes and epidermis equivalents, Lawsone exposure enhances the production of late epidermal proteins, impacts keratinocyte differentiation and proliferation, and regulates skin inflammation. To determine the potential use of Lawsone for therapeutic application, we harnessed human, murine and zebrafish models. In skin regeneration models, Lawsone interferes with physiological tissue regeneration and inhibits wound healing. Conversely, in a human acute dermatitis model, topical application of a Lawsone-containing cream ameliorates skin irritation. Altogether, our study reveals how a widely used natural plant pigment is sensed by the host receptor AhR, and how the physiopathological context determines beneficial and detrimental outcomes.

Hyperpigmentation and higher incidence of cutaneous malignancies in moderate-high PCB- and dioxin exposed individuals

Polychlorinated biphenyls (PCB) are well known persistent and toxic environmental pollutants. Our aim was to identify effects of moderate-high exposure to dioxin-like (dl) and non-dioxin-like (ndl)-PCBs on the skin in order to provide more insight in the pathophysiological effects of these compounds. We performed a dermatological examination on 92 former workers from a transformer recycling company with known elevated serum PCB and/or dioxin (polychlorinated dibenzo-p-dioxin/polychlorinated dibenzo-p-furan (PCDD/F)) levels. In addition, we performed a skin cancer screening over a period of seven years (2010-2016) on resp. 268, 271, 210, 149, 92, 129 and 79 participants. We found a higher incidence of acne and malignancies of the skin (malignant melanoma, basal cell carcinoma and mycosis fungoides) in the workers compared to normal population. The probability of having hyperpigmentation on the skin was statistically significantly higher in workers with higher sumPCBs- (OR:1.09(1.12-2.17)), dioxin-like (dl)-PCBs- (OR:1.56(1.12-2.17)) and dioxin (PCDD/Fs) (OR:1.09(1.02-1.16)) levels. Age was a confounding factor in this model. Formation of hyperpigmentation could be an indicator for (moderate-high) exposure to toxic compounds like PCBs. The higher incidence of cutaneous malignancies found in the workers might be associated with PCB- and dioxin exposure, warranting further investigation on larger cohorts.

Role of EGF receptor ligands in TCDD-induced EGFR down-regulation and cellular proliferation

In cultures of normal human epidermal keratinocytes (NHEKs), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces the expression of the epidermal growth factor receptor ligands transforming growth factor-α (TGF-α) and epiregulin (EREG). TCDD also down-regulates EGF receptors (EGFR), suggesting that decreases in signaling contribute to the effects of TCDD. In this study, we treated post-confluent NHEKs with 10 nM TCDD and assessed its effects on EGFR binding, EGFR ligand secretion, basal ERK activity, and proliferation. TCDD caused time-dependent deceases in [(125)I]-EGF binding to levels 78% of basal cell values at 72 h. Amphiregulin (AREG) levels increased with time in culture in basal and TCDD-treated cells, while TGF-α and epiregulin (EREG) secretion were stimulated by TCDD. Inhibiting EGFR ligand release with the metalloproteinase inhibitor batimastat prevented EGFR down-regulation and neutralizing antibodies for AREG and EREG relieved receptor down-regulation. In contrast, neutralizing TGF-α intensified EGFR down-regulation. Treating NHEKs with AREG or TGF-α caused rapid internalization of receptors with TGF-α promoting recycling within 90 min. EREG had limited effects on rapid internalization or recycling. TCDD treatment increased ERK activity, a response reduced by batimastat and the neutralization of all three ligands indicating that the EGFR and its ligands maintain ERK activity. All three EGFR ligands were required for the maintenance of total cell number in basal and TCDD-treated cultures. The EGFR inhibitor PD1530305 blocked basal and TCDD-induced increases in the number of cells labeled by 5-ethynyl-2'-deoxyuridine, identifying an EGFR-dependent pool of proliferating cells that is larger in TCDD-treated cultures. Overall, these data indicate that TCDD-induced EGFR down-regulation in NHEKs is caused by AREG, TGF-α, and EREG, while TGF-α enhances receptor recycling to maintain a pool of EGFR at the cell surface. These receptors are required for ERK activity, maintenance of total cell number, and stimulating the proliferation of a small subset cells.

The aryl hydrocarbon receptor: a review of its role in the physiology and pathology of the integument and its relationship to the tryptophan metabolism

The aryl hydrocarbon receptor (AHR) is a cytosolic receptor for low molecular weight molecules, of which the most widely recognized ligand is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and the most widely recognized effect, chloracne. Adverse effects of manipulation were most recently and graphically demonstrated by the poisoning of Viktor Yushchenko during the Ukrainian presidential elections of 2004. However, recent research has revealed a receptor with wide-ranging, and at times, paradoxical actions. It was arguably among the first biological receptors to be utilized by dermatologists, dating from the time of topical tar preparations as a therapeutic agent. I provide a review outlining the role AHR plays in the development, cellular oxidation/antioxidation, responses to ultraviolet light, melanogenesis, epidermal barrier function, and immune regulation and its relationship to tryptophan metabolism. Finally, I will review the role of AHR in diseases of the integument.

The aryl hydrocarbon receptor: multitasking in the immune system

The aryl hydrocarbon receptor (AhR), for many years almost exclusively studied by the pharmacology/toxicology field for its role in mediating the toxicity of xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has more recently attracted the attention of immunologists. The evolutionary conservation of this transcription factor and its widespread expression in the immune system point to important physiological functions that are slowly being unraveled. In particular, the emphasis is now shifting from the role of AhR in the xenobiotic pathway toward its mode of action in response to physiological ligands. In this article, we review the current understanding of the molecular interactions and functions of AhR in the immune system in steady state and in the presence of infection and inflammation, with a focus on barrier organs such as the skin, the gut, and the lung.

TCDD induces dermal accumulation of keratinocyte-derived matrix metalloproteinase-10 in an organotypic model of human skin

The epidermis of skin is the first line of defense against the environment. A three dimensional model of human skin was used to investigate tissue-specific phenotypes induced by the environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Continuous treatment of organotypic cultures of human keratinocytes with TCDD resulted in intracellular spaces between keratinocytes of the basal and immediately suprabasal layers as well as thinning of the basement membrane, in addition to the previously reported hyperkeratinization. These tissue remodeling events were preceded temporally by changes in expression of the extracellular matrix degrading enzyme, matrix metalloproteinase-10 (MMP-10). In organotypic cultures MMP-10 mRNA and protein were highly induced following TCDD treatment. Q-PCR and immunoblot results from TCDD-treated monolayer cultures, as well as indirect immunofluorescence and immunoblot analysis of TCDD-treated organotypic cultures, showed that MMP-10 was specifically contributed by the epidermal keratinocytes but not the dermal fibroblasts. Keratinocyte-derived MMP-10 protein accumulated over time in the dermal compartment of organotypic cultures. TCDD-induced epidermal phenotypes in organotypic cultures were attenuated by the keratinocyte-specific expression of tissue inhibitor of metalloproteinase-1, a known inhibitor of MMP-10. These studies suggest that MMP-10 and possibly other MMP-10-activated MMPs are responsible for the phenotypes exhibited in the basement membrane, the basal keratinocyte layer, and the cornified layer of TCDD-treated organotypic cultures. Our studies reveal a novel mechanism by which the epithelial-stromal microenvironment is altered in a tissue-specific manner thereby inducing structural and functional pathology in the interfollicular epidermis of human skin.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

EGF receptor signaling blocks aryl hydrocarbon receptor-mediated transcription and cell differentiation in human epidermal keratinocytes

Dioxin is an extremely potent carcinogen. In highly exposed people, the most commonly observed toxicity is chloracne, a pathological response of the skin. Most of the effects of dioxin are attributed to its activation of the aryl hydrocarbon receptor (AHR), a transcription factor that binds to the Ah receptor nuclear translocator (ARNT) to regulate the transcription of numerous genes, including CYP1A1 and CYP1B1. In cultures of normal human epidermal keratinocytes dioxin accelerates cell differentiation, as measured by the formation of cornified envelopes. We show that this acceleration is mediated by the AHR; also, that dioxin increases the expression of several genes known to be regulated by ARNT, which have critical roles in the cornification and epidermal barrier function of the skin. Importantly, we demonstrate that all of these responses are opposed by ligand-activation of the EGF receptor (R), an important regulator of keratinocyte cell fate. In the CYP1A1 enhancer, EGFR activation prevents recruitment of the p300 coactivator, although not affecting the binding of the AHR or ARNT. The total cellular level of p300 protein does not decrease, and overexpression of p300 relieves EGFR-mediated repression of transcription, indicating that p300 is a critical target for the repression of the AHR complex by EGFR signaling. These results provide a mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin is able to disrupt epidermal homeostasis and identify EGFR signaling as a regulator of the AHR. This signaling may modulate the incidence and severity of chloracne and be of therapeutic relevance to human poisonings by dioxin.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure of normal human dermal fibroblasts results in AhR-dependent and -independent changes in gene expression

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) results in a variety of lesions in mammals including severe skin lesions. The majority of TCDD's biological effects are mediated through activation of the aryl hydrocarbon receptor (AhR). We have chosen to examine the effect of TCDD and the AhR pathway on dermal fibroblasts because this cell type plays an integral role in skin homeostasis through the production of cytokines and other factors that regulate epidermal proliferation and differentiation. Our data show that normal human dermal fibroblasts (NHDFs) are responsive to TCDD, as demonstrated by induction of cytochrome p450 1B1 (CYP1B1) expression. Further, our data demonstrate that TCDD treatment of NHDFs results in significant (75-90%) decrease in expression of Id-1 and Id-3, proteins that are involved in regulation of cell proliferation and differentiation. The Id (Inhibitor of DNA binding) proteins are transcriptional inhibitors that function by forming inactive heterodimers with other HLH proteins. TCDD-repression of Id-1 and -3 is independent of de novo protein synthesis; co-treatment with cycloheximide has no effect on TCDD inhibition of Id-1 and Id-3. Co-treatment with the AhR antagonist alpha-naphthoflavone also does not block inhibition of Id-1 and Id-3 by TCDD, suggesting that TCDD inhibition of Id-1 and Id-3 is, at least in part, mediated independently of the AhR pathway. Our data also show that TCDD inhibits expression of the cell cycle regulatory gene p16(ink4a), which is often linked to Id expression. TCDD-induced reduction of p16(ink4a) expression is also independent of protein synthesis and the AhR pathway.

Dioxin-induced chloracne--reconstructing the cellular and molecular mechanisms of a classic environmental disease

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is among the most toxic pollutants known to date that serves as a prototype for a group of halogenated hydrocarbon compounds characterized by extraordinary environmental persistence and unique ability to concentrate in animal and human tissues. TCDD can elicit a complex array of pleiotropic adverse effects in humans, although chloracne, a specific type of acne-like skin disease, is the only consistent manifestation of dioxin intoxication, thus representing a 'hallmark' of TCDD exposure. Chloracne is considered to be one of the most specific and sensitive biomarkers of TCDD intoxication that allows clinical and epidemiological evaluation of exposure level at threshold doses. The specific cellular and molecular mechanisms involved in pathogenesis of chloracne are still unknown. In this review, we summarize the available clinical data on chloracne and recent progress in understanding the role of the dioxin-dependent pathway in the control of gene transcription and discuss molecular and cellular events potentially involved in chloracne pathogenesis. We propose that the dioxin-induced activation of skin stem cells and a shift in differentiation commitment of their progeny may represent a major mechanism of chloracne development.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects tooth development in rhesus monkeys

We thought to validate the current tolerable daily intake (TDI) value for dioxin (4 pg/kg) in Japan. Pregnant rhesus monkeys received an initial dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 0, 30, or 300 ng/kg subcutaneously) on day 20 of gestation; the dams received additional injection of 5% of the initial dose every 30 days until day 90 after delivery. The teeth of stillborn, postnatally dead, and surviving offspring (now approximately 4 years old) were evaluated. None of the offspring in the 0 and 30 ng/kg groups (n=17 and 15, respectively) had tooth abnormalities, whereas 10 of 17 in the 300 ng/kg had them. These findings suggest the lowest-observed adverse-effect level (LOAEL) for TCDD in the rhesus monkey is between 30 and 300 ng/kg, and probably is close to that for rodents (86 ng/kg) on which the current TDI was based. It is reasonable to conclude that the current TDI needs no immediate modification.

Photocontact dermatitis and chloracne: two major occupational and environmental skin diseases induced by different actions of halogenated chemicals

Among occupational and environmental disorders, contact or photocontact dermatitis and an acneiform eruption are two major skin disorders. Photocontact dermatitis was historically caused by various halogenated salicylanilides, while the acne is induced by halogenated aromatic hydrocarbons and thus called chloracne. Therefore, it should be noted that halogenated chemical compounds are important causative agents in the occupational and environmental medicine. In photocontact dermatitis, photoconjugation of epidermal cells with a photohaptenic halogenated chemical is the initial step. Langerhans cells serve as antigen-presenting cells and T cells sensitized by photoantigen-bearing Langerhans cells induce this photosensitivity. On the other hand, in chloracne, halogeneted hydrocarbons render keratinocytes of the outer root sheath and sebaceous duct hyperplastic. The dilated infundibulum of most hair follicles is then filled with comedone that consist of many accumulated layers of keratinized cells and sebum. Therefore, halogenated chemicals exhibit different actions, i.e. the induction of an immunologic consequence and the modulation of keratinocyte biology. These two conditions also provide good experimental models for investigating dermatology.

Treatment of normal human keratinocytes with 2,3,7,8-tetrachlorodibenzo-p-dioxin causes a reduction in cell number, but no increase in apoptosis

We have examined the effect of TCDD on the growth of normal human keratinocytes. TCDD is a ubiquitous environmental toxicant that causes a severe dermatopathology in humans, which is known as chloracne. The cell biological basis of this pathology remains unknown. We conducted growth experiments in preconfluent normal human keratinocytes with both low (0.05 mM) and standard (0.66 mM) extracellular calcium concentrations in the media. TCDD treatment reduced the number of adherent keratinocytes relative to controls in media containing 0.05 or 0.66 mM calcium. Based on these observations, we speculated that the decrease in the cell number of TCDD-treated cultures might be the result of increased apoptosis. Analysis of nucleosomal fragmentation, nuclear morphology, and caspase-3 activity in keratinocytes reveals no increase in the characteristics of apoptosis in response to TCDD treatment. We therefore conclude that TCDD impacts on keratinocyte homeostasis independent of apoptosis.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters the differentiation pattern of human keratinocytes in organotypic culture

Human exposure to the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces a severe skin pathology known as chloracne. In these studies we employed a three-dimensional, organotypic model system to study the effects of TCDD on human skin. This model uses the spontaneously immortalized human keratinocyte cell line NIKS and recapitulates both the three-dimensional microenvironment and epithelial-mesenchymal interactions found in intact human skin. Treatment of the organotypic cultures with TCDD causes alterations in the pattern of keratinocyte terminal differentiation. Analysis at both the light and electron microscope levels reveals a fully differentiated cornified layer in TCDD-treated organotypic cultures at earlier time points than observed in vehicle (dimethyl sulfoxide)-treated controls. Furthermore, TCDD-treated organotypic cultures exhibit aberrant distribution of several differentiation-specific protein markers. Basal cells in TCDD- and DMSO-treated organotypic cultures show no differences in proliferation as measured by quantification of 5-bromo-2'-deoxyuridine (BrdU)-positive nuclei. No aberrant BrdU uptake was detected outside of the basal layer. Neither TUNEL labeling nor immunohistochemical staining with an antibody to active caspase-3 revealed increased apoptosis in TCDD-treated organotypic cultures relative to controls. These data clearly indicate that TCDD modulates homeostasis in a model of human stratifying epithelium independent of changes in proliferation and apoptosis, exclusively by impacting keratinocyte terminal differentiation. This TCDD-induced effect on differentiation-specific proteins results in profound changes in the tissue architecture.

Molecular basis of dioxin actions: evidence supporting chemoprotection

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin), a highly publicized environmental contaminant, was shown to be chemoprotective against breast cancer in both rats and mice in bioassays conducted in the late 1970s. This finding went largely unnoticed as investigators focused on animal tumors that were increased by dioxin. The position that dioxin causes human tumors remains a subject for debate; however, recent epidemiological studies of a population highly exposed to dioxin in 1976 as a result of an industrial accident suggest that women with higher dioxin body burdens may have a lower incidence of breast cancer. With the growth of new knowledge about the molecular basis of dioxin actions in humans and animals, it is clear that most of the responses produced by this agent are initiated by a specific recognition protein (designated the Ah receptor) found in almost all animal and human tissues and organs. The recognition event between the Ah receptor and environmental agents like dioxin is due to the formation of a complex. We have observed that in the presence of dioxin, the Ah receptor turns off proliferation in tumor cells and suppresses the ability of these cells to invade normal tissue. We believe that these findings provide a molecular and biochemical basis for understanding the chemoprotective mechanisms suggested by the findings of rodent bioassays and could lead to the development of novel therapeutic agents targeting the Ah receptor.

Characterization of 2,3,7,8-tetrachlorodibenzofuran-dependent suppression and AH receptor pathway gene expression in the developing mouse mammary gland

The AH receptor (AHR) is a ligand-activated transcription factor and member of a growing family of homologous proteins implicated in development. In this study we have characterized the actions of 2,3, 7,8-tetrachlorodibenzofuran (TCDF), a well-studied AHR ligand, and the expression of AHR and selected AHR signal transduction pathway genes in the developing mouse mammary gland. High levels of AHR protein were observed in the mammary glands of C57Bl/6J (AHR +/+) mice during estrous-stimulated growth and branching of terminal end buds (TEBs). Comparative analysis of mammary gland development in AHR -/- and +/+ littermates revealed a 50% reduction in TEBs and an increase in blunt-ended terminal ducts in the AHR null animals. Treatment of mammary glands, removed from estrogen/progesterone-primed C57Bl/6J mice and maintained in organ culture, with TCDF suppressed lobule development (greater than twofold decreases in lobule number and size), with a concomitant suppression of DNA synthesis, as judged by a 35 to 45% decrease in [3H]thymidine incorporation in the TEBs. Immunohistochemical staining patterns for AHR, aryl hydrocarbon nuclear translocator (ARNT; the heterodimerization partner of AHR), and two AHR-regulated genes, Cyp1A1 and Cyp1B1, were similar and not altered by treatment of mammary glands in organ culture with TCDF. The observed differences in the development of mammary glands from AHR +/+ and -/- mice, associated expression of the AHR protein with hormone-dependent lobule development, and suppressive actions of TCDF support the position that, in C57Bl/6J mice, development of the mammary gland is at least in part AHR dependent. Development occurs in the absence of exogenous AHR ligand, suggesting that the unoccupied receptor may function to support the proliferative stages required for full lobule development.

Differentiation status of cultured murine keratinocytes modulates induction of genes responsive to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Primary murine keratinocytes were cultured in a chemically defined, serum-free medium which facilitated manipulation of their differentiation status. Exposure of basal cell and differentiating cultures to >/= 0.1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) preferentially elevated 7-ethoxyresorufin O-deethylase specific activities in differentiating cultures (28-fold versus 4-fold increases after 36 h of exposure). Semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analyses demonstrated the presence of constitutive mRNA transcripts corresponding to four known TCDD-inducible genes (e.g., Cyp1a1, Cyp1b1, Ahd4, and Nmo1) in both differentiating and proliferating cultures of murine keratinocytes. All four genes were induced in differentiating cultures following exposure to TCDD. No induction occurred in comparably treated basal cell cultures. Indirect immunofluorescence analyses demonstrated the presence of aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) proteins in both basal and differentiating keratinocytes. Both proteins appeared to be associated with the nucleus and their nuclear association was independent of prior exposure to TCDD. These studies suggest that AHR activation in murine skin is regulated as a function of the keratinocyte differentiation program.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD) affects keratin 1 and keratin 17 gene expression and differentially induces keratinization in hairless mouse skin

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes chloracne in humans by mechanisms that are as yet poorly understood. Because TCDD is known to affect keratinocyte differentiation in vitro, we have studied TCDD-dependent morphologic changes and the expression of murine keratin 1 (MK1; differentiation associated) and keratin 17 (MK17; presumably hyperproliferation associated) in HRS/J hr/hr hairless mouse skin. TCDD (0.2 microg in acetone) applied topically to the dorsal skin caused epidermal acanthosis and hyperkeratosis of the dermal cysts as well as an involution of the utricles and the sebaceous glands. By means of in situ hybridization with digoxigenin-labeled riboprobes of sections from untreated and vehicle (control)-treated skin, we localized MK1 mRNA to the epidermal spinous cell compartment. MK17 transcripts were detected only in the derivatives of the hair follicle-utricle epithelium and dermal cysts. No spatial overlap was observed between MK1 and MK17 expression. After TCDD application, MK17 was newly expressed in the upper spinous cell layers of the interfollicular epidermis, although it was suppressed in the involuting utricles. In contrast, MK1 expression in the interfollicular epidermis was not affected by TCDD. Furthermore, MK1 expression was induced in the epithelium of the utricle remnants and in some dermal cysts. These data suggest that increased keratinization of the part of the follicular epithelium corresponding to the dermal cyst epithelium of hairless mice most probably explains the pathogenesis of TCDD-induced chloracne. The results demonstrate, furthermore, that TCDD can differentially affect keratinocyte differentiation in vivo as well as in vitro.

Retinoic acid affects the expression rate of the differentiation-related genes aryl hydrocarbon receptor, ARNT and keratin 4 in proliferative keratinocytes only

The environmental contaminant dioxin exerts most of its effects by activating the aryl hydrocarbon receptor (AhR). The AhR is considered to play not only a role in the regulation of xenobiotic metabolism, but also for development, growth, and differentiation. The transcript levels of the AhR and its associated translocator protein (ARNT) were found to increase with ongoing differentiation in the human keratinocyte cell line HaCaT. Correspondingly, in situ hybridization studies in normal human skin revealed an absence of AhR-expression in proliferating basal cells and increasing transcript levels in upper cell layers, in dependence of keratinocyte differentiation. AhR expression in differentiation-deficient hyperproliferative psoriatic skin was markedly decreased. When keratinocytes were continuously treated with 1 microM retinoic acid (RA), the upregulation of AhR- and ARNT-mRNA levels was inhibited as was keratin 4-expression, a marker of HaCaT-keratinocyte differentiation. In contrast, treatment of already differentiated cells with RA did not down-regulate these transcript levels. The mRNA levels of the prevalent retinoic acid receptors in keratinocytes, RAR gamma and RXR alpha, were not influenced by the process of differentiation or by addition of RA. Our data suggest that the regulation of AhR-, ARNT- and keratin 4-expression by RA is indirect and mediated by a yet to be identified factor.

Dioxin-receptor ligands in urban air and vehicle exhaust

The ability of extracts of urban air and vehicle exhaust particulates to bind to the dioxin receptor has been determined. It was shown that such extracts do contain significant amounts of dioxin-receptor binding activity. The level of dioxin-receptor binding found in ambient air reflects its pollution level as determined by mutagenic activity. Furthermore, it was shown that the extracts of both urban air and vehicle exhaust particulates could provoke the induction of cytochrome P450IA1 in cultured rat hepatoma cells. Chemical fractionation of the extracts revealed that the majority of the dioxin-receptor binding activity from urban air and gasoline vehicle samples fractionated with the polycyclic aromatic compounds. However, unknown polycyclic aromatic compounds were responsible for the majority of the binding activity measured. In the case of diesel vehicle exhausts, the majority of the dioxin-receptor binding activity was found to be associated with nitro-polycyclic aromatic compounds. Studies with a variety of diesel fuels showed that the amount of dioxin-receptor ligands present in exhaust emissions are fuel-dependent and that substantial amounts of dioxin-receptor ligands are present in the semivolatile phase of exhaust emissions.

Characterization of protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in murine lymphocytes: indirect evidence for a role in the suppression of humoral immunity

Studies were undertaken to more thoroughly characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced stimulation of kinase activity in murine lymphocytes. In female B6C3F1 mice, TCDD-induced phosphorylation of 29, 45, 52 and 63 KDa proteins was selective for B cells, with little or no enhancement observed in T cells. When B cells were purified and separated by density on a percoll gradient, phosphorylation was only observed in the band composed of activated B cells, and was not enhanced in the band composed of resting B cells. TCDD-stimulated phosphorylation was associated with both the cytosol (45 and 52 KDa species) and membrane (52 KDa species) fractions. Purified B cells from both DBA/2 (Ahdd) and C57B16 (Ahbb) mice demonstrated equivalent enhancement of phosphorylation in response to TCDD. Administration of human gamma interferon (Hu-IFNg) at concentrations from 0.5 to 500 Units/ml produced a dose-related reversal of TCDD-induced suppression of in vitro antibody responses to both the polyclonal B cell activator, LPS, and the T-dependent antigen, sRBC in whole splenocytes isolated from female B6C3F1 mice. These concentrations of Hu-IFNg did not affect the magnitude of either response in the absence of TCDD, and did not reverse dexamethasone-induced suppression of either in vitro antibody response. TCDD-induced suppression of the T-dependent response was reversed only when Hu-IFNg was added to culture within the first 18 hours after treatment with TCDD and sRBC. These studies demonstrate that Hu-IFNg can reverse TCDD-induced in vitro Ab response suppression if it is administered during the period of susceptibility to TCDD. TCDD-induced phosphorylation in isolated B cells was also antagonized following co-incubation with Hu-IFNg. The profile of TCDD-induced increases in protein phosphorylation, including the selective effect on activated B cells, the general involvement of both cytosolic and membrane proteins, the lack of segregation with the Ah-dependent processes, and the ability of Hu-IFNg to reverse both the suppression of the Ab response and the increase in phosphorylation, supports the interpretation that such phosphorylation is involved in TCDD-induced suppression of the Ab response.

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.

Induction of terminal differentiation in cultured human keratinocytes by polychlorinated aromatic hydrocarbons as measured by cell size analysis

Polychlorinated aromatic hydrocarbons modulate the proliferation and differentiation of human epidermal cells in vivo and in culture. One of the earliest events in the process of terminal differentiation is the increase in cell size. In this report the usefulness of morphometric cell size analysis as a quantifiable marker for chemical-induced differentiation was examined. Concentration-related increases in cell size distribution were induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and 2,3,4,7,8-pentachlorodibenzofuran in normal human keratinocytes and cells from an SV40-transformed keratinocyte cell line (SVK14) whereas the analog 1,2,3,4-tetrachlorodibenzo-p-dioxin did not affect the cell size distribution up to a concentration of 100 nM. The minimal effective concentrations of five 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins/dibenzofurans and a coplanar polychlorbiphenyl necessary to induce an increase in cell size distribution were determined in SVK14 cells. It was found that the potency of these compounds relative to that of 2,3,7,8-TCDD correlated well with the toxicity equivalency factors observed in other test systems. This indicates that the keratinocyte cell assay is a useful method for establishing the relative potency of various "dioxins" and their mixtures.

Response of the antral mucosa of the rat stomach to 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) produces a striking hypergastrinemia in rats that is thought to mediate the antiatrophy effect of TCDD on the oxyntic gland mucosa of the stomach. However, effects of TCDD on the antral mucosa, which is the origin of most physiologically released gastrin and is not a target for the trophic action of gastrin, has yet to be thoroughly investigated. Also gastrin release from gastrin-containing cells (i.e., G-cells) in the antral mucosa is inhibited by the paracrine secretion of somatostatin from D-cells in the antrum. Our purpose was to determine if the antral mucosa is affected by the trophic influence of TCDD and if alterations in antral mucosa levels of gastrin or somatostatin cause the hypergastrinemia. TCDD (100 micrograms/kg, Day 14 post-treatment) had a trophic effect on the antral mucosa. This was demonstrated histologically and by significant increases in antral wet weight and antral mucosa height. In contrast, pair-fed control rats that lost the same amount of body weight developed antral mucosa atrophy. With respect to serum and antral levels of gastrointestinal hormones, TCDD produced a 7- to 10-fold increase in serum gastrin concentrations that was not detected until Day 14 post-treatment. In contrast, serum gastrin concentrations in pair-fed control rats were comparable to those of control rats. The number of G-cells in the antral mucosa was not affected by either TCDD treatment or paired-feed restriction. These findings demonstrate that hypergastrinemia in TCDD-treated rats is not caused by reduced feed intake or antral G-cell hyperplasia. A major finding was that antral mucosa levels of both gastrin and somatostatin were decreased significantly in TCDD-treated rats. However, the temporal development and dose-dependence of these TCDD effects on antral hormone levels were quite different than those for hypergastrinemia. TCDD-induced decreases in antral levels of gastrin and somatostatin were detected 1 week earlier than hypergastrinemia. Also, the ED50 of TCDD on Day 14 post-treatment for the decrease in antral mucosa content and concentration of gastrin (29 and 22 micrograms/kg, respectively) and somatostatin (24 and 19 micrograms/kg, respectively) was less than that for hypergastrinemia (46 micrograms/kg). These time- and dose-dependent differences demonstrate that hypergastrinemia in TCDD-treated rats is not a consequence of reduced antral levels of gastrin or somatostatin. We conclude that the antral mucosa, an epithelial tissue not responsive to the proliferative effect of gastrin, is nevertheless a target for the trophic influence and gastrointestinal hormone-altering effects of TCDD.

Reproductive hazards of fire fighting. II. Chemical hazards

Studies of the health of fire fighters have historically focused on non-malignant respiratory disease and cancer. More recently, concerns have surfaced about reproductive health effects in many areas of the workforce, including fire fighting. These concerns prompted this review of chemical exposures that may contribute to adverse reproductive health outcomes in male as well as female fire fighters. A review of the industrial hygiene literature was undertaken to identify agents commonly found in fire smoke. These agents were then examined for evidence of reproductive toxicity or mutagenicity/carcinogenicity. This profile of chemical agents and their reproductive toxicities permits a qualitative determination that fire fighters are exposed to potential reproductive toxicants as a part of their normal fire fighting duties. Considerations for mitigating these risks are also discussed.

Effects of TCDD on embryonic ureteric epithelial EGF receptor expression and cell proliferation

The potent toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, producing hydronephrosis and cleft palate. Because of the long half-life of TCDD, the urinary tract is exposed throughout development after a single dose on gestation day (GD) 10 or earlier. TCDD-induced hydronephrosis is a consequence of occlusion of the ureter by epithelial cells. Since embryonic growth factors and the epidermal growth factor (EGF) receptor are probably involved in regulation of embryonic cell proliferation, this study examines the effects of TCDD on expression of EGF receptors and proliferation of ureteric epithelial cells in vivo and in culture. After exposure to TCDD by gavage (12, 24, or 30 micrograms/kg on GD 10; 6 or 24 micrograms/kg on GD 12) the mean cell depth of the ureteric and bladder epithelia was increased. EGF receptors were detected immunohistochemically in sectioned urinary tracts. The expression of receptors decreased with advancing development in control ureteric epithelia. However, after TCDD exposure the level of EGF receptors failed to decline. The incorporation of 3H-TdR was observed in sections by autoradiography, and after exposure to TCDD more epithelial cells showed incorporation than was apparent in controls. Transmission electron microscopy (TEM) of embryonic ureters from fetuses exposed to TCDD in vivo showed no cytotoxicity in basal cells and the cells remained undifferentiated, as in controls. Ureters taken from GD 12 embryos and cultured with 1 x 10(-10)M TCDD showed ureteric epithelial hyperplasia without cytotoxicity, but at 1 x 10(-8)M TCDD evidence of cytotoxicity was observed by TEM. The levels of TCDD found in fetuses after in vivo exposure (204-307 pg/fetus, with 1-2 pg in the urinary tract) compare well with the in vitro level (32 pg/ml), which was most effective in producing hyperplasia of the epithelial cells. The present study correlates a TCDD-induced increase in cell depth with altered regulation of EGF receptors and excessive proliferation, both in vivo and in cultured embryonic ureters.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters embryonic palatal medial epithelial cell differentiation in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, inducing cleft palate and hydronephrosis. After exposure in vivo, TCDD specifically alters differentiation of embryonic palatal medial epithelial cells. In this study, the palatal epithelial cell response to TCDD is determined in vitro. C57BL/6N palatal shelves were placed in organ culture on gestation day (GD) 12 in Richter's improved modified Eagle's medium:Ham's F12 medium (1:1) with 1% fetal bovine serum for 3 or 4 days. Medium contained 0.1% dimethylsulfoxide and TCDD at 0, 10(-13), 10(-12), 10(-11), 10(-10), and 10(-9) M, with some doses at 5 x 10(-11), 7.5 x 10(-11), and 5 x 10(-12) M. Epithelial cell responses to TCDD occurred over a narrow range of concentrations, with maximal response at 5 x 10(-11) M. Cytotoxicity was detected at 1 x 10(-10) M. At a stage when control medial cells ceased proliferation and EGF receptors were not detected immunohistochemically. TCDD-exposed medial cells incorporated [3H]thymidine and high levels of epidermal growth factor receptors were detected. TCDD prevented programmed cell death of medial peridermal cells, and induced a shift in the differentiation of medial cells toward an oral-like phenotype. The responses to TCDD observed after exposure in vitro were indistinguishable from previously reported effects observed after exposure in vivo. In the present study, the distribution of TCDD in the fetus after exposure in vivo was examined. The levels of exposure to TCDD are similar for in vitro and in vivo exposure routes. The levels of TCDD in 1 x 10(-11) to 1 x 10(-10) M solutions (3 to 32 pg/ml) were comparable to levels observed in fetal tissues after in vivo exposure on GD 11 to 30 microns/kg [3H]TCDD, where the palatal shelf contained 1.4 to 3.5. pg TCDD, representing 0.0003% of the total dose. In vivo, TCDD was detected in the GD 11 embryo 3 hr postexposure and the TCDD was equally distributed between the embryonic head and body. At 72 hr postexposure, 0.035% of the total dose was in fetal tissues, and 1% of the TCDD in the fetus was found in the palatal shelf. The present study shows that the palatal epithelium responds to TCDD in vitro in a manner comparable to that observed after in vivo exposure, and that the response occurs at a concentration comparable to in vivo levels in the fetus. The availability of an in vitro system will facilitate studies of TCDD toxicity that are difficult or impossible to perform in vivo, such as comparisons of TCDD effects between species, including human tissues.

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

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

Dioxins and the Ah receptor

Despite continuing controversies related to public policy, information on the molecular biology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has advanced significantly over the past decade. Current understanding of the biological mechanisms of TCDD action is based upon the interactions of TCDD with a genetically expressed cytosolic macromolecule that functions as a receptor in many cells across many species. The Ah receptor recognizes TCDD and structurally similar molecules and serves as the transducing step whereby TCDD alters gene expression through the association of the TCDD:receptor complex with specific TCDD-responsive elements on the genome. Understanding these molecular events and their relevance to the organ-level manifestations of TCDD toxicity may be critical to formulating scientifically based assessments of the risk of TCDD exposure.

Applications of the in vitro aryl hydrocarbon hydroxylase induction assay for determining "2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents": pyrolyzed brominated flame retardants

The pyrolysis of brominated flame retardants FR 300 BA (decabromobiphenyl) ether, FireMaster BP-6 (polybrominated biphenyls), Bromkal 70-5-DE (primarily pentabromodiphenylether), Bromkal 70-DE (primarily penta and tetrabromodiphenylether) and Bromkal G1 (pentabromodiphenylether) resulted in the formation of relatively high levels of polybrominated dibenzofurans (PBDFs) and dibenzo-p-dioxins (PBDDs as determined by gas chromatography-mass spectrometric analysis. The dose response EC50 values for the induction of aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) by the flame retardant pyrolysates was determined in rat hepatoma H-4-II E cells and compared to the relative induction activities of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the concentrations of "2,3,7,8-TCDD equivalents" were calculated. The range of "2,3,7,8-TCDD equivalents" levels (micrograms/g or ppm) derived from values obtained from the AHH and EROD bioassays for each of the pyrolyzed flame retardant samples was: 174-194, 480-1400, 2140-4680, 6740-8780 and 3920-5260 ppm for FR 300 BA, FireMaster BP-6, Bromkal 70 DE, Bromkal 70-5 DE and Bromkal G1, respectively. The in vivo dose-response effects of 2 pyrolyzed flame retardants were determined in immature male Wistar rats and compared to the dose-response activities of 2,3,7,8-TCDD. The in vivo responses which were measured included hepatic microsomal AHH and EROD induction, body weight loss and thymic atrophy. For the pyrolyzed FireMaster BP-6 and Bromkal 70-5 DE samples, the range of calculated in vivo "2,3,7,8-TCDD equivalents" (ppm in sample) for the 4 in vivo bioassays was 520-1780 ppm and 3860-8960 ppm, respectively. The excellent overlap between the in vivo and in vitro 2,3,7,8-TCDD equivalents for the 2 flame retardant pyrolysate extracts supports the utility of the in vitro induction bioassay for quantitatively determining "2,3,7,8-TCDD equivalents" for mixtures containing toxic halogenated aryl hydrocarbons.

Morphologic lesions and acute toxicity in rainbow trout (Salmo gairdneri) treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

To determine effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth, mortality, and morphologic lesions in rainbow trout, juvenile Shasta or Wytheville strain fish, obtained from 4 hatcheries, were administered graded single doses of TCDD, 0.1-125 micrograms/kg, ip. TCDD doses of 25 and 125 micrograms/kg caused 85% lethality 2-4 wk after treatment. At these high doses, death occurred before body weight loss could be detected. A lower dose of 5 micrograms/kg caused decreased growth and cumulative mortality of 20% after 11 wk. Stress associated with netting and weighing the fish at weekly intervals significantly shortened the delay period prior to TCDD-induced lethality. Gross and microscopic lesions were evident in rainbow trout treated with 10 micrograms TCDD/kg, but not in fish treated with 1 or 0.1 microgram/kg. Morphologic lesions occurred consistently in epithelial and lymphomyeloid tissues of TCDD-treated fish. Lymphomyeloid lesions included thymic involution, splenic lymphoid depletion, and hypocellularity of hematopoietic tissues in the head kidney and trunk kidney. In association with decreased hematopoiesis, peripheral leukopenia and thrombocytopenia occurred in Shasta strain yearling trout treated with 1 microgram/kg or more TCDD. Regarding epithelial lesions, all 4 hatchery strains treated with 10 micrograms/kg or more TCDD showed multifocal necrosis of gastric cardiac glandular mucosa, 3 of 4 hatchery strains showed vacuolar inclusions in exocrine pancreatic cells, and 2 of 4 hatchery strains showed fin necrosis. The severity and character of lesions in the liver and gastric mucosa varied markedly between hatchery strains of trout. One hatchery strain showed no hepatic lesions, two showed mild hepatocyte lesions, and one exhibited severe diffuse hepatopathy. In this severely affected hatchery strain, hyaline intracytoplasmic inclusions occurred in hepatocytes at 14 and 34 d after TCDD exposure, and bile-duct hyperplasia occurred at 34 d following TCDD exposure. One of 4 hatchery strains showed atrophy of serous gastric glands and 1 of 4 hatchery strains showed hyperplasia of these same glands at 25 and 34 d, respectively, following TCDD treatment. Thus, lymphomyeloid and epithelial tissues are the primary targets for TCDD-induced pathologic lesions in rainbow trout, and the incidence and severity of these lesions is influenced by the strain of trout used and the hatchery from which the trout were obtained.

2,3,7,8-Tetrachlorodibenzo-p-dioxin toxicity in yellow perch (Perca flavescens)

Growth, mortality and morphologic lesions in juvenile, hatchery-reared yellow perch, Perca flavescens, were studied after treatment with graded single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 1-125 micrograms/kg, intraperitoneally). TCDD doses of 25 and 125 micrograms/kg caused 95% mortality by 28 d after treatment, without decreasing body weight. A TCDD dose of 5 micrograms/kg resulted in progressive loss of body weight with cumulative mortality of 80% by 80 d posttreatment. Periodic handling stress did not affect the time course of mortality or cumulative percent lethality in TCDD-treated perch. Fin necrosis, petechial cutaneous hemorrhage, and ascites occurred in perch treated with 5 micrograms/kg or more of TCDD. Thymic atrophy, decreased hematopoiesis in the head kidney, fibrinous pericarditis, focal myocardial necrosis, submucosal gastric edema, and hyperplasia of the epithelium of gill filaments and lamellae occurred in perch dosed with 25 or 125 micrograms/kg. Dose-related splenic lymphoid depletion occurred in perch receiving 5 micrograms/kg or more TCDD, and hepatocyte lipidosis occurred in groups treated with doses of 1 microgram/kg or more TCDD. Thus yellow perch are as responsive to the acute toxic effects of TCDD as some of the more sensitive mammalian species, and neither loss of body weight nor histologic lesions in TCDD-treated perch are sufficient to explain mortality.

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.

Regulation of cytochrome P-450c in differentiated and dedifferentiated rat hepatoma cells: role of the Ah receptor

The induction of cytochrome P 450c mRNA and associated aryl hydrocarbon hydroxylase (AHH) activity is mediated by the Ah receptor in rodent liver and hepatic cells in vitro. In the present study we have investigated the underlying mechanisms responsible for the regulation of AHH activity in differentiated and dedifferentiated variants of the rat hepatoma cell line H4IIEC3. All of the dedifferentiated variants expressed inducible cytochrome P-450c mRNA and AHH activity following treatment with polycyclic aromatic hydrocarbons or the compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Most of the differentiated derivatives, however, were not inducible for either of these functions. Somatic cell hybridization studies revealed that the differentiated cells were AHH negative due to a defect that corresponded to the Ah receptor D gene product. 5-Azacytidine and sodium butyrate, but not mutagens, reactivated a functional Ah receptor in the differentiated line Fao, indicating that a requisite gene had been silenced by an epigenetic mechanism in this strain. Since many of the 5-azacytidine-induced revertant clones resembled dedifferentiated derivatives with respect to morphology and/or diminished expression of hepatic traits, our results support a correlation between coexpression of the dedifferentiated phenotype and AHH inducibility in these hepatoma cells.

Polybrominated dibenzo-p-dioxins and related compounds: quantitative in vivo and in vitro structure-activity relationships

The effects of structure on the in vitro receptor binding affinities, aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) induction potencies in rat hepatoma cells were determined for the following compounds: 2-bromo-, 2,7/2,8-dibromo-, 2,3,7-tribromo-, 2,4,6,8/1,3,7,9-tetrabromo-, 2,3,7,8-tetrabromo-, 1,3,7,8-tetrabromo-, 1,2,3,7,8-pentabromo-, 1,2,4,7,8-pentabromo-, 2,3-dibromo-7,8-dichloro-, 2,8-dibromo-3,7-dichloro- and 2-bromo-3,7,8-trichlorodibenzo-p-dioxin. The structure-activity relationships (SARs) for the polybrominated dibenzo-p-dioxins (PBDDs) were comparable for both in vitro responses: the most active compounds were substituted only in the lateral 2,3,7 and 8 position and the addition of non-lateral or removal of lateral halogen substituents reduced the activity of the resultant compound. The biologic and toxic effects of 2,3,7,8-tetrabromo-, 1,3,7,8-tetrabromo-, 1,2,4,7,8-pentabromo-1,2,3,7,8-pentabromo-, 2-bromo-3,7,8-trichloro- and 2,3-dibromo-7,8-dichlorodibenzo-p-dioxin on several receptor-mediated responses (thymic atrophy, body weight loss, hepatic microsomal AHH and EROD induction) were determined in a dose-response fashion in immature male Wistar rats. A comparison of the ED50 values for the in vivo responses demonstrated that the SARs for the PBDDs and brominated polychlorinated dibenzo-p-dioxins were comparable to those observed for in vitro receptor binding and AHH induction. Moreover, there was an excellent linear correlation between the -log EC50 (in vitro AHH induction) vs. the in vivo -log ED50 (thymic atrophy) and -log ED50 (body wt loss) correlation coefficient, r = 0.97 for all 2 correlations).

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) enhances antibody production and protein kinase activity in murine B cells

Treatment of murine spleen cells with 30 nM TCDD resulted in an approximately 3 fold increase in unstimulated antibody production after 3 days in culture. This response was not accompanied by increased cellular proliferation and may represent an effect of TCDD on B cell activation or differentiation. Since PMA is capable of activating B cells, presumably via PKC, we have compared the effects of PMA and TCDD on protein kinase activation and phosphorylation of endogenous proteins in a highly purified preparation of B cells. In contrast to a reduction of cytosolic PKC activity, the expected effect of PMA, TCDD caused an increase in basal kinase activity with no effect on PKC activity. Addition of either PMA or TCDD resulted in enhanced phosphorylation of a similar profile of proteins, including proteins of Mr 12.2, 14.6, 29.2, 52.3 and 62.7 KDa. Addition of TCDD also resulted in the increased phosphorylation of a protein of Mr 45.2, which was unaffected by PMA. Combined treatment with PMA and TCDD resulted in additive responses. The additive effects of PMA and TCDD suggest an interaction at the level of protein phosphorylation which is mediated by different kinases. Therefore, TCDD may be stimulating B cells via an early effect on an unidentified protein kinase.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic and uterine estrogen receptor levels in rats

Administration of a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 20 or 80 micrograms/kg) resulted in significantly decreased hepatic and uterine estrogen receptor (ER) levels in 25-day-old female Long-Evans rats. By contrast, estradiol (5 and 15 micrograms/kg) administration increased hepatic and uterine ER levels, while a combination of 2,3,7,8-TCDD plus estradiol resulted in uterine and hepatic ER levels which were similar or lower than those observed after treatment with only 2,3,7,8-TCDD. In addition, 2,3,7,8-TCDD significantly decreased the effects of estradiol on uterine wet weight increase. Competitive binding studies showed that estradiol did not bind to the aryl hydrocarbon (Ah) receptor and that 2,3,7,8-TCDD did not bind to the ER. The effects of structure on the activity of polychlorinated dibenzo-p-dioxin congeners on their activity to down-regulate hepatic and uterine ER levels were determined by using 2,3,7,8-TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,3,7,8-TCDD, and 1,2,4,7,8-PeCDD. Both 2,3,7,8-TCDD and 1,2,3,7,8-PeCDD exhibit high affinities for the Ah receptor and at dose levels of 80 micrograms/kg the hepatic ER levels were decreased 42 and 41%, respectively, and uterine ER levels were decreased 53 and 49%, respectively. 1,3,7,8-TCDD and 1,2,4,7,8-PeCDD bind less avidly to the Ah receptor and at dose levels of 400 micrograms/kg these compounds decreased hepatic ER levels 36 and 40%, respectively, and uterine ER levels 21 and 24%, respectively. These results support a role for the Ah receptor in the down-regulation of uterine and hepatic ER levels in the female rat by 2,3,7,8-TCDD and this effect may be associated with the decrease in spontaneous mammary and uterine tumors observed in female rats treated with 2,3,7,8-TCDD.

Direct suppression of antibody responses by chlorinated dibenzodioxins in cultured spleen cells from (C57BL/6 x C3H)F1 and DBA/2 mice

Direct addition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 5-20 nM) to cultures of spleen cells from (C57BL/6 x C3H)F1 (B6C3F1) mice produced a suppression of the number of antibody-producing cells which developed in response to lipopolysaccharide, dinitrophenyl-Ficoll and sheep erythrocytes. The suppression of all three parameters was dose-related and parallel. This parallelism and the observation that the magnitude of the suppression was comparable in all three models suggested that the B-lymphocyte was the primary target. The defect was attributed to an effect on early activation or impaired differentiation because direct addition of TCDD had no effect on mitogen-induced proliferation. Temporal studies showed that TCDD produced the greatest suppression of the polyclonal antibody response to lipopolysaccharide when added at the beginning of the culture and that there was no suppression when TCDD was added as soon as 3 h after 200 micrograms/ml lipopolysaccharide. The observation that TCDD could directly suppress the antibody response by spleen cells from DBA/2 mice, at concentrations comparable to those required to suppress the B6C3F1 mice, suggested that the effect on the B-lymphocyte was atypical of the profile of activity (i.e., dependence on the Ah locus) previously reported to characterize the effects of dioxin in other systems. Similar results were demonstrated with congenic mice, as Ahd/d homozygotes were suppressed comparably to Ahb/d heterozygotes. The direct suppression by 2,7-dichlorodibenzo-p-dioxin, a congener previously demonstrated to be devoid of affinity for the Ah locus, further suggests a dissociation from the traditional profile of activity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) modulates epidermal growth factor (EGF) binding to basal cells from a human keratinocyte cell line

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) modulates the proliferation, differentiation, or both, of epidermal keratinocytes in vivo and in culture. The growth of epidermal cells in culture is regulated by several biochemical mediators including epidermal growth factor (EGF). In this report the actions of TCDD on EGF binding in a basal cell population from a human keratinocyte cell line were examined. TCDD decreased the specific binding of 125I-EGF to basal cells by 40% within 96 hr. This reduction in EGF binding could not be attributed to changes in the state of differentiation as assessed by cell size and morphology, and cornified envelope competence, a marker of terminal differentiation. Modulation of EGF binding by TCDD was concentration-dependent (EC50 = 1 nM) and stereospecific, suggesting involvement of the Ah receptor. Scatchard analysis of EGF binding to the basal cells indicated a single class of high-affinity sites in both control (Kd = 0.14 nM) and treated (Kd = 0.11 nM) cultures and confirmed a decrease in the number of these sites in response to TCDD. The reduction in EGF binding correlated with a decrease in EGF-stimulated DNA synthesis and cell proliferation. Comparison of differentiation-competent squamous cell carcinoma (SCC) lines treated with TCDD supported an association between modulation of EGF binding and enhanced differentiation. The data indicate that basal cells are a target for TCDD. We propose that the modulation of EGF binding in basal keratinocytes by TCDD is one of the critical regulatory events resulting in enhanced differentiation.

Actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on human epidermal keratinocytes in culture

In humans, the skin is a particularly sensitive target for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and certain halogenated analogs. Reported lesions include a thickening of the epidermis (acanthosis), hyperkeratosis, and squamous metaplasia of the epithelial lining of the sebaceous glands. In this report we describe ongoing studies on the actions of TCDD on cultured human epidermal cells. This system has been established as an in vitro model for interfollicular epidermal hyperkeratinization. Treatment of newly confluent cultures with TCDD results in enhanced differentiation as judged by histologic examination of the cultures, a decrease in the number of basal proliferating cells, and an increase in the number of envelope competent (differentiating) cells and terminally differentiated cells with highly cross-linked cornified envelopes. Changes in the differentiation program are preceded by a decrease in epidermal growth factor (EGF) binding. The concentration dependence and stereospecificity for these responses suggest the involvement of the Ah receptor. We propose that TCDD modulates normal patterns of epidermal differentiation through direct actions on proliferating basal cells, modulating the responsiveness of these cells to growth factors such as EGF.