2,3,7,8-Tetrachlorodibenzo-p-dioxin Increases the Expression of Genes in the Human Epidermal Differentiation Complex and Accelerates Epidermal Barrier Formation

Abstract

Chloracne is commonly observed in people exposed to dioxins, yet the mechanism of toxicity is not well understood. The pathology of chloracne is characterized by hyperkeratinization of the interfollicular squamous epithelium, hyperproliferation and hyperkeratinization of hair follicle cells as well as a metaplastic response of the ductular sebum secreting sebaceous glands. In vitro studies using normal human epidermal keratinocytes to model interfollicular human epidermis demonstrate a 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated acceleration of differentiation and increase in gene expression of several prodifferentiation genes, including filaggrin (FLG). Here, we demonstrated that the TCDD-activated aryl hydrocarbon receptor (AHR) bound a small fragment of DNA upstream of the transcriptional start sites of the FLG gene, containing one of two candidate xenobiotic response elements (XREs). Reporter assays using the promoter region of FLG containing the two putative XREs indicated that the increase in this messenger RNA (mRNA) was due to TCDD-mediated enhanced transcription, which was lost when both XREs were mutated. As FLG is part of the human epidermal differentiation complex (EDC) found on chromosome 1, we measured mRNAs from an additional 18 EDC genes for their regulation by TCDD. Of these genes, 14 were increased by TCDD. Immunoblot assays demonstrated that the proteins of FLG as well as that of another prodifferentiation gene, small proline rich protein 2, were increased by TCDD. In utero exposure to TCDD accelerated the formation of the epidermal barrier in the developing mouse fetus by approximately 1 day. These results indicate that the epidermal permeability barrier is a functional target of the TCDD-activated AHR.

Coal tar induces AHR-dependent skin barrier repair in atopic dermatitis

“…These new insights are strongly guiding current efforts in AD research and therapy development. The accelerated filaggrin expression and barrier function in fetal mouse skin due to dioxin-mediated AHR activation (17,39) suggested to us that there is a potential role for AHR in AD treatment. Here, we show that coal tar activates AHR signaling and found that AHR regulates and induces epidermal differentiation and stimulates filaggrin expression in keratinocytes harboring a monoallelic FLG loss-of-function mutation.…”

“…AHR knockdown also caused a similar decrease in coal tar-induced FLG and HRNR gene expression ( Figure 1I) and other terminal differentiation genes (Supplemental Table 1). The induction rate after coal tar treatment and subsequent inhibition by AHR knockdown was most pronounced in terminal differentiation genes with a reported xenobiotic response element in their promoter region (17), such as FLG, HRNR, and filaggrin family member 2 (FLG2). In contrast, the early differentiation marker keratin 10, or the basal keratinocyte markers keratin 5 and keratin 14, were not affected by coal tar stimulation or AHR knockdown.…”

“…Keratinocytes were isolated from human abdominal skin derived from donors who underwent surgery for abdominal wall correction, as previously described (17). For submerged culture, keratinocytes were cultured in keratinocyte growth medium (Lonza) and differentiated by depletion of growth factors, as described previously (58).…”

“…Human skin equivalents were generated using de-epidermized dermis, as described previously, and seeded with 10 5 human primary keratinocytes (17). Skin equivalents were cultured submerged for 3 days, after which the medium level was decreased and equivalents were cultured at the air-liquid interface at indicated time points in the presence of coal tar.…”

“…AHR was found to be crucial in regulating the development of lymphoid cells (14) and the induction of regulatory T cells (15,16). Recently, the environmental pollutant and AHR ligand, tetrachloro-dibenzodioxin (TCDD), has been described to induce epidermal differentiation, thereby providing an explanation for the occurrence of chloracne (17). Thus far, no endogenous AHR ligands affecting epidermal gene expression programs have been described.…”