Hairless mice as models for chloracne: a study of cutaneous changes induced by topical application of established chloracnegens

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.

Dibutyl Phthalate Exposure Disrupts Evolutionarily Conserved Insulin and Glucagon-Like Signaling in Drosophila Males

Phthalate diesters are commonly used as industrial plasticisers, as well as in cosmetics and skin care products, as a result people are constantly exposed to these xenobiotics. Recent epidemiological studies have found a correlation between circulating phthalate levels and type 2 diabetes, whereas animal studies indicate that phthalates are capable of disrupting endocrine signaling. Nonetheless, how phthalates interfere with metabolic function is still unclear. Here, we show that feeding Drosophila males the xenobiotic dibutyl phthalate (DBP) affects conserved insulin- and glucagon-like signaling. We report that raising flies on food containing DBP leads to starvation resistance, increased lipid storage, hyperglycemia, and hyperphagia. We go on to show that the starvation-resistance phenotype can be rescued by overexpression of the glucagon analogue adipokinetic hormone (Akh). Furthermore, although acute DBP exposure in adult flies is able to affect insulin levels, only chronic feeding influences Akh expression. We establish that raising flies on DBP-containing food or feeding adults DBP food affects the expression of homologous genes involved in xenobiotic and lipid metabolism (AHR [Drosophila ss], NR1I2 [Hr96], ABCB1 [MDR50], ABCC3 [MRP], and CYP3A4 [Cyp9f2]). Finally, we determined that the expression of these genes is also influenced by Akh. Our results provide comprehensive evidence that DBP can disrupt metabolism in Drosophila males, by regulating genes involved in glucose, lipid, and xenobiotic metabolism.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

The hairless mouse in skin research

The hairless (Hr) gene encodes a transcriptional co-repressor highly expressed in the mammalian skin. In the mouse, several null and hypomorphic Hr alleles have been identified resulting in hairlessness in homozygous animals, characterized by alopecia developing after a single cycle of relatively normal hair growth. Mutations in the human ortholog have also been associated with congenital alopecia. Although a variety of hairless strains have been developed, outbred SKH1 mice are the most widely used in dermatologic research. These unpigmented and immunocompetent mice allow for ready manipulation of the skin, application of topical agents, and exposure to UVR, as well as easy visualization of the cutaneous response. Wound healing, acute photobiologic responses, and skin carcinogenesis have been extensively studied in SKH1 mice and are well characterized. In addition, tumors induced in these mice resemble, both at the morphologic and molecular levels, UVR-induced skin malignancies in man. Two limitations of the SKH1 mouse in dermatologic research are the relatively uncharacterized genetic background and its outbred status, which precludes inter-individual transplantation studies.

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.

Polychlorinated biphenyls affect gene expression in the rabbit preimplantation embryo

Polychlorinated biphenyls (PCBs) have been shown to be embryotoxic. The mechanism(s) of action is not clearly understood. The toxic effects could be either direct or indirect. Furthermore, PCB congeners vary in their toxic potential. They can be classified in coplanar PCBs binding to the transcription factor aryl hydrocarbon receptor (AhR), which induce subsequent changes in gene expression, and noncoplanar PCBs exhibiting AhR-independent effects. In order to investigate possible mechanisms, 5 and 6 days old preimplantation rabbit embryos were exposed in vitro to low levels of coplanar (PCB 77, 126, and 169) or noncoplanar PCBs (PCB 28, 52, 101, 118, 138, 153, and 180). The PCB effects were studied by semiquantitative RT-PCR analysis of AhR target genes (cytochrome P450 (CYP) 1A1, 1A2, UDP-glucuronosyl transferase 1, glutathione S-transferase pi1 and aldehyde dehydrogenase) and dioxin-responsive genes (IL 1beta, PAI 2, Cox 2, TGFalpha, EGF, erbB 1-4, c-fos, c-jun, HSP 90, cyclophilin 40), and by differential display (DD) RT-PCR. CYP 1B1 mRNA and AhR protein were localized by in situ hybridization and immunohistochemistry, respectively. From the AhR target genes studied only CYP 1B1, and cyclooxygenase 2 showed an increase in mRNA levels after coplanar and noncoplanar PCB. Interleukin 1beta and plasminogen activator inhibitor 2 were downregulated. CYP 1B1 mRNA showed a stage specific inducibility at day 6, but not at day 5. By DD RT-PCR we identified six new genes previously not reported to be regulated by PCBs. The mRNAs encoding the subunits 1, 2, 4, and 5 of the NADH ubiquinone oxidoreductase and beta-globin showed a decrease, whereas trichohyalin mRNA was increased after PCB exposure. Coplanar and noncoplanar PCB congeners elicited similar responses on the mRNA levels of the studied genes. Exposure to coplanar PCBs did not result in the AhR being translocated to the nucleus. Our results show that (i). PCBs induce changes in gene expression in rabbit day 5 and 6 preimplantation embryos and imply (ii). that the transcriptional changes observed were not mediated by the nuclear AhR.

Molecular basis for the rhino Yurlovo (hr(rhY)) phenotype: severe skin abnormalities and female reproductive defects associated with an insertion in the hairless gene

In 1989, mice bearing mutations at the hr (hairless) locus were first proposed as a model for the human hair growth disorder papular atrichia, since in both these mice and in corresponding patients, a complete hair loss develops due to disintegration of the normal follicle structure into dermal cysts and so-called utriculi. Recently, the human hairless gene was characterized, and pathogenetic mutations were found to be associated with a recessively inherited form atrichia with papular lesions; however, the functions of hr gene remain unclear. Allelic mutations in the murine hairless gene represent a potentially powerful tool to elucidate the role of the hairless gene protein product in hair follicle physiology. In 1980, several naked animals were discovered in a breeding colony of B10.R109/Y mice maintained in the Laboratory of Experimental Biological Models (L.E.B.M., Yurlovo, Moscow District, Russia). By cross breeding with hairless HRS/J hr/hr mice, this mutation was shown to be allelic with hairless. Here, we describe the molecular basis of the hr(rhY) mutation in mice, which consists of a 13 bp insertion in exon 16 of the hr gene. Histological evaluation of Yurlovo mouse skin revealed some differences as compared to the hairless and rhino mutations, with the formation of dermal megacysts being the most specific peculiarity of the Yurlovo mutation. These results, together with previous studies of hr(rhY)/hr(rhY) mutant mice, suggest that the rhino Yurlovo (hr(rhY)) mutation represents a third and potentially more severe variation of the hairless phenotype.

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.

Serum dioxin, chloracne, and acne in veterans of Operation Ranch Hand

The possible relationship between exposure to Agent Orange and its contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin), during the Vietnam War and chloracne was investigated. The index subjects were veterans of Operation Ranch Hand, the unit responsible for aerial herbicide spraying in Vietnam from 1962 to 1971. Other Air Force veterans who served in Southeast Asia during the same period, but who were not involved with spraying herbicides, served as comparisons. None of the Ranch Hand veterans were diagnosed with chloracne; therefore, we restricted our analyses to acne. We found no meaningful or consistent association between dioxin exposure and prevalence of acne without or with regard to anatomical location. These results suggested that exposure of Ranch Hand veterans to dioxin was insufficient for the production of chloracne or perhaps the exposure may have caused chloracne that resolved and was currently undetectable.

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.

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.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

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.

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans: the risks to human health. A review

1 PCDDs and PCDFs are ubiquitous and persistent in the environment. They are to be found in body tissues of both humans and animals. 2 The most extensively studied PCDD is 2,3,7,8-TCDD. It has been shown to produce a wide range of effects and is considered to be a (non-genotoxic) carcinogen in animals. 3 Studies into the mechanisms of toxicity so far reveal that there is involvement of a specific receptor (Ah), however further work is required to elucidate the mechanisms of the various effects. 4 Reports on a number of human exposures to PCDDs and PCDFs are described. Results from human epidemiological studies are difficult to interpret: there have been problems in methodology; there has been inadequate information on intake, and exposures have often been to mixtures of PCDDs and/or PCDFs together with other related compounds. 5 Many regulatory authorities faced with the problem of providing an index of risk from exposure to mixtures of PCDDs and PCDFs have employed the concept of 'TCDD equivalents'. 6 Whether or not PCDDs and PCDFs pose a significant human health risk at current levels of exposure they remain of considerable interest to the toxicologist.

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.

Chronic toxicity of 3,4,3',4'-tetrachlorobiphenyl in the marmoset monkey (Callithrix jacchus)

Cotton top marmoset monkeys (Callithrix jacchus) were orally dosed with 3, 1, 0.1 or 0 mg 3,4,3',4'-tetrachlorobiphenyl (TCB)/kg body weight twice per week for 18-23 weeks. Severe toxicity occurred in the highest dose group. Clinical signs of toxicity were a rapid decrease in body weight, alopecia, abnormal nail growth, nodular enlargement of the nipple area and scaly skin. Haematological analysis of peripheral blood revealed mild leukocytosis and anemia. Biochemical alterations observed were elevated triglyceride levels and cholesterol levels. Histopathology revealed dose dependent changes in a variety of tissues. Squamous metaplasia was found in skin and adnexa as well as in salivary glands. In the stomach, parietal cells were decreased and mucus producing cells were increased. The duodenal mucosa was hyperplastic. Ovaries showed an absence of corpora lutea. In the thyroid follicular cell hyperplasia and hypertrophy were noted. Toxicity was less severe in marmoset monkeys dosed with 1 mg TCB/kg, while minor toxic effects were observed in the animals dosed with 0.1 mg TCB/kg. The marmoset monkey appears to be less sensitive to the toxic action of TCB than the rhesus monkey. The pattern of histological and biochemical changes induced by TCB in marmoset monkeys is comparable to that described in humans and in other primate species exposed to PCBs. The marmoset monkey model may be valuable for investigations on human-related toxicity of PCBs.

Pleiotropic effect of the gene hairless on hepatotoxicity of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin in mice

Treatment of mice of the A2G-hr/+ congenic line with 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in the development of hepatic porphyria over a period of 4 weeks. Female mice responded to a lesser extent than did males. The degree of porphyria in haired heterozygotes (hr/+) was less than in the corresponding hairless homozygotes (hr/hr) and the haired mice had lower resting metabolic rates than hairless mice. Adaptation of mice of either genotype to a 32-33 degrees C environment resulted in a decrease in resting metabolic rate and a reduction in hepatic porphyrin levels. Histologically-demonstrated necrotic changes in livers were accompanied by increased activity of alanine aminotransferase and sorbitol dehydrogenase in the plasma; however, there was no clear temporal trend in plasma enzyme levels. Elevated environmental temperature reduced the plasma alanine aminotransferase activity. The study provided evidence for a pleiotropic effect of variation at the hr locus being expressed in TCDD hepatotoxicity. Suggestions for mechanisms whereby the effect can be mediated through alterations in resting metabolic rate are made.

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.

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

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and isosteric halogenated analogs produce a spectrum of pathologic changes in the epidermis of humans. In this study, the actions of TCDD on cultured human epidermal cells were characterized to determine whether these cells are an appropriate in vitro model to examine the mechanisms of TCDD toxicity to human skin. The differential staining properties of TCDD-treated cultures indicated that TCDD decreased basal cell numbers and increased the degree of keratinization. Histologic examination of cross-sections of the cultures confirmed a loss of small nucleated cells and increased cell layering in response to TCDD. TCDD produced no change in total cell number or cell protein, but decreased the number of small (basal) cells and DNA synthesis. TCDD increased the number of cells containing spontaneous envelopes, as well as the number of envelope-competent cells. The quantitative changes observed in these parameters were consistent with a TCDD-induced commitment of proliferating cells to terminal differentiation. TCDD also decreased epidermal growth factor (EGF) specific binding. Maximal changes in EGF binding occurred after 4 days, and in small cell number after 5 days. The decreases in EGF binding and small cell number were stereospecific and concentration dependent (EC50, 1 to 2 nM), implicating the human Ah receptor in mediating these responses to TCDD. These data indicate that TCDD treatment produces hyperkeratinization in cultured human epidermal cells. It is proposed that TCDD acts on epidermal basal cells to enhance terminal differentiation through mechanisms regulated at least in part by the Ah receptor.

Regulation of epidermal growth factor binding in a human keratinocyte cell line by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) decreased the binding of epidermal growth factor (EGF) by the human keratinocyte cell line SCC-12F. This response was concentration dependent (half-maximal effective concentration, EC50 = 1.8 nM) and stereospecific. Scatchard analysis of EGF binding indicated that treatment with TCDD resulted in a loss of high-affinity (Kd = 0.28 nM) binding sites. This loss was accompanied by a concomitant inhibition of EGF-stimulated DNA synthesis. The kinetics for the decrease of EGF binding by TCDD and benzo[a]pyrene (BP) were compared. Inhibition of EGF binding by BP was maximal by 24 hr, with 90% recovery of EGF binding apparent by 48 hr. In contrast, TCDD treatment for 72 hr was required to produce maximal inhibition, and no recovery was evident up to 10 day after removal of TCDD from the growth medium. The data indicate that modulation of EGF binding by TCDD was mediated by the Ah receptor. Subsequent cellular responses, for example, inhibition of EGF-stimulated DNA synthesis, may be important in the expression of altered differentiation patterns observed in human epidermal keratinocytes exposed to TCDD.

Assessment of the chloracnegenic response induced by 3,4,3',4'-tetrachloroazoxybenzene in mice

Chloracne is a follicular hyperkeratosis produced by exposure to certain halogenated aromatic compounds. The rabbit ear bioassay has been used successfully for testing the acnegenic activity of compounds, but the lack of reference data in this species limits its usefulness in correlating chloracne to other toxic effects such as skin carcinogenesis. In this study, a prototype chloracnegen, 3,4,3',4'-tetrachloroazoxybenzene (TCAOB), was used. Five strains of mice (hairless, rhino, rhino+, DBA/2J, and C57BL/6) were treated topically with 100 microliters of 0.001, 0.01, or 0.1% TCAOB daily for 3-9 wk. Skin and liver histology were performed and hepatic enzyme activities measured. At the 0.001% TCAOB level, induction of hepatic aniline hydroxylase and cytochrome P-450 occurred in the C57BL/6 mice and induction of cytochrome c reductase occurred in the rhino mice. Dose-dependent gross and histologic skin lesions, characteristic of follicular hyperkeratosis, were observed in the rhino and hairless strains at the 0.01% and 0.1% levels. These two strains also had induction of hepatic cytochrome c reductase, cytochrome P-450, and aniline hydroxylase at TCAOB concentrations of 0.01 or 0.1%. These results suggest that the rhino and hairless strains of mice may be useful in the study of chloracne.

Development of a nude mouse model to study human sebaceous gland physiology and pathophysiology

Study of human sebaceous gland physiology and pathophysiology is limited by lack of an adequate animal model. This study was designed to develop an animal model using human face skin grafted onto the nude mouse to study human sebaceous glands. Full-thickness human face skin was grafted onto 60 adult male nude mice. 4 wk after grafting, androgens, which are known to stimulate sebaceous glands, were administered to test the system. Androgens were administered to 21 animals by implanted catheters that were filled with testosterone (T) or dihydrotestosterone (DHT). Empty catheters were implanted in 15 control animals. Graft biopsies and blood for androgen levels were obtained at time 1 (pre-catheter) and time 2 (26 d after catheter implantation). Three assessments were made on each biopsy: sebaceous gland volume, using an image analyzing computer; sebaceous cell size; and sebaceous gland labeling index. 29 mice completed the study through time 2. In the androgen-treated group, T levels (nanogram per milliliter) five times increased to 4.92 +/- 0.35, and DHT levels (nanogram per milliliter) increased 50 times to 16.70. In the androgen-treated group, sebaceous gland volume (micron 3 X 10(-3) increased from 896 +/- 194 to 3,233 +/- 754 (P less than 0.001), sebaceous cell area (micron 2) increased from 167 +/- 12 to 243 +/- 19 (P less than 0.001), and labeling index (percentage) increased from 2.7 +/- 0.7 to 6.4 +/- 0.9 (P less than 0.01). In the control group, sebaceous gland volume fell from 1,070 +/- 393 to 417 +/- 99 (NS), sebaceous cell size remained the same, and the labeling index fell from 5.1 +/- 1.9 to 3.2 +/- 1.1. After androgen administration, Halowax N-34, a known comedogen, or its vehicle, was applied to 15 grafts for 2-6 wk. Twice as many microcomedones were seen in the Halowax-treated grafts, compared with vehicle-treated grafts at the end of this time period. No visible comedones were produced. This study demonstrated that: (a) human sebaceous glands can be successfully transplanted and studied on the nude mouse; (b) after androgen stimulation, sebaceous gland volume, cell size, and labeling index increase; (c) microcomedones can be produced in the human skin grafts by the application of a comedogenic substance. Thus, this model demonstrates significant potential for the future study of human sebaceous gland physiology and pathology.

Separation of wasting syndrome and lethality caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Twenty rats were administered LD50 doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) i.p. (60 micrograms/kg). Two days later 10 rats were started and maintained on a diet containing 5% hexadecane, whereas the other 10 rats continued receiving basal diet. Dietary hexadecane increased the mortality rate of rats from 60 to 100%. All animals destined to die showed a biphasic body weight and food intake pattern. The first 16 days after dosing were characterized by progressive weight loss which went hand in hand with increasing appetite suppression. No animal died during this initial phase. After day 16, rats overcame their loss of appetite, started eating and stabilized their body weight. During this second phase, lasting from day 16 to day 40, mortality progressed to 60 and 100% in the controls and hexadecane-treated rats, respectively. No body weight loss or change in food intake was observed in any of the surviving animals. Data indicate that TCDD-induced wasting syndrome is due to appetite suppression. However, wasting syndrome does not seem to be the ultimate cause of lethality.

Increased epidermal transglutaminase activity following 2,3,7,8-tetrachlorodibenzo-p-dioxin: in vivo and in vitro studies with mouse skin

In previous studies it has been shown that topical treatment of hairless mice with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) induces hyperproliferation and hyperkeratinization in the epidermis of hairless mice. The present investigation demonstrated that such TCDD-induced morphological changes in skin in vivo are accompanied by increased levels in activity of epidermal transglutaminase (ETG), the enzyme associated with terminal epidermal differentiation. Exposure of mouse epidermal cells in tissue culture to 10(-9) M TCDD also resulted in a significant increase in ETG activity, despite the fact that morphologically these cultures (grown at 0.07 mM ionic calcium concentrations) exhibited no signs of terminal differentiation. Thus one mechanism of action of TCDD in inducing cutaneous changes appears to relate to the stimulation of increased ETG levels.

Decreased induction of aryl hydrocarbon hydroxylase activity in hyperproliferative hairless mouse epidermis

Hairless mice were treated topically with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to induce experimental chloracne. The aryl hydrocarbon hydroxylase (AHH) activity in the epidermis was monitored. TCDD is a potent inducer of AHH activity in normal skin. However, as continued TCDD application induced hyperproliferative and hyperkeratotic changes in the epidermis, the AHH activity decreased. Similar suppression in AHH activity was demonstrated in epidermis made hyperplastic by (a) tape stripping of stratum corneum and (b) repeated application of 50% oleic acid. This suggests that the epidermal AHH response varies with the state of epidermal differentiation. In the hairless mouse, AHH activity in hyperplastic epidermis is lower than that in normal epidermis.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.