Histologic changes produced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in the skin of mice carrying mutations that affect the integument

Cutaneous Effects of In Utero and Lactational Exposure of C57BL/6J Mice to 2,3,7,8-Tetrachlorodibenzo-p-dioxin

To determine the cutaneous effects of in utero and lactational exposure to the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), pregnant C57BL/6J mice were exposed by gavage to a vehicle or 5 μg TCDD/kg body weight at embryonic day 12 and epidermal barrier formation and function were studied in their offspring from postnatal day 1 (P1) through adulthood. TCDD-exposed pups were born with acanthosis. This effect was AHR-dependent and subsided by P6 with no evidence of subsequent inflammatory dermatitis. The challenge of adult mice with MC903 showed similar inflammatory responses in control and treated animals, indicating no long-term immunosuppression to this chemical. Chloracne-like sebaceous gland hypoplasia and cyst formation were observed in TCDD-exposed P21 mice, with concomitant microbiome dysbiosis. These effects were reversed by P35. CYP1A1 and CYP1B1 expression in the skin was increased in the exposed mice until P21, then declined. Both CYP proteins co-localized with LRIG1-expressing progenitor cells at the infundibulum. CYP1B1 protein also co-localized with a second stem cell niche in the isthmus. These results indicate that this exposure to TCDD causes a chloracne-like effect without inflammation. Transient activation of the AhR, due to the shorter half-life of TCDD in mice, likely contributes to the reversibility of these effects.

Allelic variants of the aryl hydrocarbon receptor differentially influence UVB-mediated skin inflammatory responses in SKH1 mice

The mouse strain SKH1 is widely used in skin research due to its hairless phenotype and intact immune system. Due to the complex nature of aryl hydrocarbon receptor (AHR) function in the skin, the development of additional in vivo models is necessary to study its role in cutaneous homeostasis and pathology. Variants of the Ah allele, exist among different mouse strains. The Ah^b-1 and Ah^d alleles express high and low affinity ligand binding forms of the AHR, respectively. The outbred SKH1 mice express the Ah^b-2 and/or Ah^d alleles. SKH1 mice were crossed with C57BL/6J mice, which harbor the Ah^b-1 allele, to create useful models for studying endogenous AHR function. SKH1 mice were bred to be homozygous for either the Ah^b-1 or Ah^d allele to establish strains for use in comparative studies of the effects of differential ligand-mediated activation through gene expression changes upon UVB exposure. Ah^b-1 or Ah^d allelic status was confirmed by DNA sequence analysis. We tested the hypothesis that SKH1-Ah^b-1 mice would display enhanced inflammatory signaling upon UVB exposure compared to SKH1-Ah^d mice. Differential basal AHR activation between the strains was determined by assessing Cyp1a1 expression levels in the small intestine, liver, and skin of the SKH1-Ah^b-1 mice compared to SKH1-Ah^d mice. To determine whether SKH1-Ah^b-1 mice are more prone to a pro-inflammatory phenotype in response to UVB, gene expression of inflammatory mediators was analyzed. SKH1-Ah^b-1 mice expressed enhanced gene expression of the chemotactic factors Cxcl5, Cxcl1, and Ccl20, as well as the inflammatory signaling factors S100a9 and Ptgs2, compared to SKH1-Ah^d mice in skin. These data supports a role for AHR activation and enhanced inflammatory signaling in skin.

Inflammatory and chloracne-like skin lesions in B6C3F1 mice exposed to 3,3',4,4'-tetrachloroazobenzene for 2 years

Exposure to dioxin and dioxin-like compounds (DLCs) has been connected to the induction of chloracne in humans and animals. 3,3',4,4'-Tetrachloroazobenzene (TCAB) is an environmental contaminant that induces chloracne in humans. TCAB has been studied only to a limited extent in laboratory animals. While performing a 2-year gavage study in B6C3F1 mice to evaluate the toxic and carcinogenic effects of TCAB, we also explored potential chloracnegenic properties. Groups of 50 male and 50 female B6C3F1 mice were exposed by gavage to TCAB at dose levels of 0, 3, 10 and 30 mg/kg for 5 days a week for 2 years. The animals developed treatment-related gross inflammatory skin lesions, which were characterized histologically by inflammation, fibrosis, hyperplasia, and ulcers. Additionally, many of the animals developed follicular dilatation and sebaceous gland atrophy, consistent with chloracne-like lesions. This current 2-year study supports recently published papers showing susceptibility to chloracne in mouse strains other than hairless mice. The chloracne-like lesions were not clinically evident; therefore, our study highlights the need for careful examination of the skin in order to identify subtle lesions consistent with chloracne-like changes in rodents exposed to dioxin and DLCs. Since previous short-term studies did not demonstrate any skin lesions, we suggest that reliable assessment of all safety issues involving dioxin and DLCs requires evaluation following chronic exposure. Such studies in animal models will help to elucidate the mechanisms of dioxin-related health hazards.

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.

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.

Molecular basis for the rhino (hrrh-8J) phenotype: a nonsense mutation in the mouse hairless gene

The hairless (hr) and rhino (hrrh) mutations are autosomal recessive allelic mutations that map to mouse Chromosome 14. Both hairless and rhino mice have a number of skin and nail abnormalities and develop a striking form of total alopecia at approximately 3-4 weeks of age. The molecular basis of the hairless mouse phenotype was previously found to be the result of a murine leukemia proviral insertion in intron 6 of the hr gene that resulted in aberrant splicing. In this study, we report a 2-bp substitution in exon 4 of the hr gene in a second allele of hr, rhino 8J (hrrh-8J), leading to a nonsense mutation. These findings document the molecular basis of the rhino phenotype for the first time and suggest that rhino is a functional knock-out of the hr gene.

Molecular and functional aspects of the hairless (hr) gene in laboratory rodents and humans

For many years, hairless and rhino mouse mutants have provided a useful and extensively exploited model for studying different aspects of skin physiology, including skin aging, pharmacokinetic evaluation of drug activity and cutaneous absorption, skin carcinogenesis, and skin toxicology. Interestingly, however, hairless and rhino mice have rarely been studied for their primary cellular defect - hairlessness - and thus, the hairless gene itself and its physiological functions have been largely overlooked for decades. The recent identification of the human homolog of the hairless gene on human Chromosome 8p12 confirmed the clinical significance of the phenomenon of "hairlessness" in humans, which was predicted on the basis of similarities between hairless mice and a congenital hair disorder characterized by atrichia with papules. Mutations in the hairless gene of mice provide instructive models for further studies of hr gene function, and may facilitate insights into the pathophysiology of different human disorders associated with the disruption of hr gene activity. We provide an overview of current data on the structure and expression patterns of the hr gene, and of mutations at the hairless locus in mice and humans, including the genetic basis of different alleles, the pathology of hairlessness, reproductive and immunological defects, and susceptibility to dioxin toxicity. On the basis of our current understanding of hairlessness, we speculate on the putative functions of the hr gene product in skin physiology, and particularly, in hair follicle biology.

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.

Effects of polychlorinated dibenzo-p-dioxin and dibenzofuran congeners in human lymphoblastoid cells on aryl hydrocarbon hydroxylase activity

The separate and concurrent effects of polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) congeners on aryl hydrocarbon hydroxylase (AHH) activity in human lymphoblastoid cells were examined. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8-tetraCDD) induced AHH activity about 9-fold acetone-treated AHH activity, while octachlorodibenzo-p-dioxin (octaCDD) did not affect the AHH activity and 2,4,6,8-tetrachlorodibenzofuran (2,4,6,8-tetraCDF) reduced the AHH activity by 34%. The concurrent effect of 2,3,7,8-tetraCDD with 1,3,6,8-tetraCDD on AHH inducibility was much smaller than expected. The concurrent effect of 2,3,7,8-tetraCDD with 2,4,6,8-tetraCDF, octaCDD or 3,3',4,4',5,5'-hexaCB was as much as the effect of 2,3,7,8-tetraCDD alone. There was a difference between 3-methylcholanthrene (MC) and 2,3,7,8-tetraCDD in the concurrent effects of 3-methylsulfone-3',4,4',5-tetraCB (3-MSF-3',4,4',5-tetraCB) on AHH activity. The concurrent effect of MC with 3-MSF-3',4,4',5-tetraCB on AHH activity was equal to the effect of MC alone while that of 2,3,7,8-tetraCDD and 3-MSF-3',4,4',5-tetraCB on AHH activity was inhibitory. These results imply that congeners of PCDD and PCDF are considered to be more or less effective and isozymes of cytochrome P450, key enzymes for the AHH, induced by them are differently inhibited.

Structure and expression of the hairless gene of mice

The hairless mutation of mice was caused by insertion of a murine leukemia virus. Starting with sequences flanking the provirus, a series of overlapping clones surrounding the viral integration site were obtained. By using a combination of sequencing, PCR, and exon-trapping techniques, the hairless gene was identified. It encodes a predicted protein of 1182 amino acids, including a potential zinc-finger domain. The expression patterns of the gene closely reflect the phenotype of animals carrying the hairless mutation.

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.

Relative toxicity and tumor-promoting ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) in hairless mice

2,3,7,8-Tetrachlorodibenzo-p-dixoin 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) are highly toxic members of a class of environmental contaminants, the polychlorinated aromatic hydrocarbons (PCAH), which exhibit a similar and highly characteristic spectrum of toxic effects. For purposes of risk assessment, it is important to be able to make accurate estimates of the relative potency of these and related compounds. Previous investigations have indicated that, in acute exposure or in vitro studies, PCDF is approximately 0.1 times as toxic and HCDF is approximately 0.01 times as toxic as TCDD. In this study, we compared the relative toxicity and tumor-promoting abilities of TCDD, PCDF, and HCDF in hairless mouse skin. Female hairless mice (HRS/J hr/hr) were treated dermally with the initiator MNNG, then dosed twice weekly for 20 weeks with acetone, TCDD (2.5-10 ng/mouse/dose), PCDF (25-100 ng/mouse/dose), or HCDF (250-1000 ng/mouse/dose) as promoter. TCDD, PCDF, and HCDF were all potent promoters for the induction of squamous cell papillomas. There was, however, no difference in the incidence or multiplicity of papilloma formation between groups. The same doses of the three PCAH, in the absence of initiator, induced no skin papillomas. TCDD produced a significant increase in liver:body weight ratio (p less than 0.001) at all doses and a decrease in thymus:body weight ratio at a dose of 10 ng (p less than 0.001). Mice treated with PCDF and HCDF had marked thymic and splenic involution, liver hypertrophy, mucous cell hyperplasia in the fundic portion of the glandular stomach, and loss of body weight. PCDF and HCDF produced a greater incidence and severity of dermatotoxic effects than TCDD. Based on data for dermal toxicity and changes in body weight and organ weights, PCDF is estimated to be 0.2 to 0.4 times, and HCDF 0.08 to 0.16 times, as toxic as TCDD following repeated dermal exposure. Therefore, toxic equivalence factors generated using data from acute and/or in vitro studies may underestimate the risk from repeated low-dose exposures to these compounds.

The murine retinoblastoma homolog maps to chromosome 14 near Es-10

Restriction fragment length variants have been exploited to map genetically Rb-1, the murine homolog of the human retinoblastoma gene. Rb-1 localized to mouse chromosome 14 on the basis of results from analysis of somatic cell hybrids. In an interspecific backcross involving Mus spretus, Rb-1 and the murine homolog of the human esterase D gene (ESD), which we refer to here as Esd, were inseparable. Furthermore, the strain distribution patterns of Rb-1 and Es-10 are the same in 31 of 32 recombinant inbred strains. Close linkage of the chromosome 14 morphological marker hairless (hr) to Rb-1 is also implied. These results localize Rb-1 on the mouse linkage map and provide close genetic markers to follow Rb-1 in somatic as well as in germline genetic experiments. Additionally, the results suggest that Es-10 is the murine homolog of ESD and provide further evidence for linkage conservation during mammalian evolution.

Effect of TCDD on the density of Langerhans cells in murine skin

Tetrachlorodibenzo-p-dioxin (TCDD) is a prototype for a group of toxic polyhalogenated aromatic hydrocarbons. We have studied the effect of TCDD on skin, specifically the difference in cutaneous response of congenic haired (hr/+) and hairless (hr/hr) mice. Topical application of 0.6 microgram of TCDD induces epidermal hyperplasia/hyperkeratinization in the skin of hr/hr mice, but does not affect the epidermis of congenic hr/+ littermates. Suppression of various parameters of the immune response has been found to be another effect of TCDD exposure in experimental animals. In the present study, we investigated the effect of topical treatment with TCDD on the density of epidermal immune cells, the Langerhans cells (LC), in the skin of hr/hr and hr/+ mice. Results showed that TCDD-induced epidermal hyperplasia/hyperkeratinization in skin of hr/hr mice is accompanied by an increase in the density of LC. In the skin of hr/+ mice, in which TCDD exposure does not induce hyperplastic changes, LC densities are not affected. The increase in LC densities in TCDD-treated hr/hr mouse skin did not result in increased sensitivity of the skin to contact hypersensitization with dinitrofluorobenzene, as measured by changes in ear thickness. When hr/hr murine skin was grafted into skin of hr/+ mice and the entire dorsal skin (including the graft) treated with TCDD, LC were increased in the grafted skin, but not in the surrounding hr/+ skin. Conversly, when hr/+ murine skin was grafted into hr/hr mice and both treated with TCDD, there was no increase in the density of LC in the grafted hr/+ skin. Concomitant treatment of hairless mice with TCDD and with indomethacin did not affect the increase in the density of LC induced by TCDD treatment alone. These findings suggest that TCDD-induced epidermal changes in hr/hr murine skin involve production of factors which mediate the increase in epidermal LC.

Alterations in retinoid concentrations in several extrahepatic organs of rats by 3,4,3',4'-tetrachlorobiphenyl

In this study, the effect of polychlorinated biphenyls on retinoid homeostasis was investigated in Sprague-Dawley rats, by analysing [3H]retinoid concentrations in peripheral organs, following exposure to 3,4,3',4'-tetrachlorobiphenyl (TCB). The rats were rendered retinoid-deficient through dietary restriction, followed by dietary supplementation with [3H]retinol for 14 days, in order to facilitate determination of retinoid concentrations in various tissues. At day 7 of [3H]retinol supplementation the rats were exposed to a single i.p. dose of 15 mg TCB dissolved in corn oil/kg body weight. In corn oil-treated control rats, the highest concentrations of [3H]retinoid radioactivity, consisting mainly of retinol and several retinylesters, were obtained in the liver (greater than 10(6) cpm/g,), followed by the kidney and the lung, while only minor concentrations were found in skin and heart. Exposure to TCB resulted in a significant reduction of both retinol and retinylester concentrations in the liver (to 25% of controls) and the lung (to 44% of controls), while in the heart a reduction of retinol to 35% of controls was observed. No significant alterations in retinoid concentrations were observed in the skin and kidney. It is suggested that the reductions in retinoid concentrations might contribute to the toxicological alterations reported in these organs upon exposure to TCB.

Role of endogenous retroviruses as mutagens: the hairless mutation of mice

We have developed an experimental approach to distinguish the 40-60 endogenous C-type proviruses of mice and to determine their association with well characterized developmental and physiological mutations. The hairless (hr) mutation causes a variety of pleiotropic effects. Using oligonucleotide probes specific for different classes of murine leukemia virus, we have identified and cloned a provirus present in HRS/J hr/hr animals but absent in HRS/J +/+. Genetic analyses showed perfect concordance between the hr phenotype and the presence of the provirus in a number of inbred and congenic strains of mice. Molecular analysis of a haired revertant established the causal relationship since it revealed the excision of most of the proviral genome leaving behind one long terminal repeat. These findings show that virus integration caused the hairless mutation and point to the utility of naturally occurring retroviral integrations for accessing the genome of the mouse.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on murine skin

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on skin of congenic haired and hairless newborn and adult HRS/J mice was studied. In all adult animals topical application of TCDD caused an involution of sebaceous glands. Epidermal/epithelial hyperplasia and hyperkeratinization was induced in the hairless, but not the haired mice. Trans-glutaminase (TG) activity was stimulated in both haired and hairless animals. A single application of 1 microgram of TCDD did not stimulate significant ornithine decarboxylase activity in the skin in either strain. Other than a reduction in the density of the inflammatory cell infiltrate in the dermis, topical treatment with antiinflammatory agents fluocinolone acetonide and indomethacin did not affect the cutaneous response to TCDD. Skin of newborn mice treated topically with TCDD over a 2-wk period reacted much the same as adult skin in that sebaceous glands were reduced in size and TG activity was stimulated in both haired and hairless neonates; but epidermal hyperplasia occurred only in the hairless, not the haired newborns.

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.

Response of murine epidermal keratinocyte cultures to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): comparison of haired and hairless genotypes

Based on the observation that congenic haired and hairless mice differ in the hyperkeratinizing/hyperproliferative epidermal response following topical exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vivo, it has been proposed that this response in mice segregates with the Ah locus (which controls for the Ah receptors) and the hr locus (for hairlessness) (Knutson and Poland, 1982). In the present studies this hypothesis was tested by comparing the response of epidermal keratinocytes derived from genotypically segregated newborn haired and hairless HRS/J mice to TCDD exposure in in vitro cultures. the parameters monitored were stimulation of cell proliferation (as measured by protein content and rate of tritiated thymidine incorporation in cultures), epidermal transglutaminase activity, cornified cell envelope formation, and keratin staining with Rhodanile blue. Results suggested that the sensitivity of HRS/J haired and hairless mouse epidermal keratinocyte cultures to TCDD exposure in vitro was very similar. In both cell cultures all parameters monitored were stimulated by TCDD exposure in a similar dose-dependent manner. This suggests that physiologic factors beyond the epidermal cells may be involved in expression of the different responses seen in the skin of mice to TCDD in vivo.