Veronika Mlitz

Knockdown of Filaggrin Impairs Diffusion Barrier Function and Increases UV Sensitivity in a Human Skin Model

Loss-of-function mutations in the filaggrin gene are associated with ichthyosis vulgaris and atopic dermatitis. To investigate the impact of filaggrin deficiency on the skin barrier, filaggrin expression was knocked down by small interfering RNA (siRNA) technology in an organotypic skin model in vitro. Three different siRNAs each efficiently suppressed the expression of profilaggrin and the formation of mature filaggrin. Electron microscopy revealed that keratohyalin granules were reduced in number and size and lamellar body formation was disturbed. Expression of keratinocyte differentiation markers and the composition of lipids appeared normal in filaggrin-deficient models. The absence of filaggrin did not render keratins 1, 2, and 10 more susceptible to extraction by urea, arguing against a defect in aggregation. Despite grossly normal stratum corneum morphology, filaggrin-deficient skin models showed a disturbed diffusion barrier function in a dye penetration assay. Moreover, lack of filaggrin led to a reduction in the concentration of urocanic acid, and sensitized the organotypic skin to UVB-induced apoptosis. This study thus demonstrates that knockdown of filaggrin expression in an organotypic skin model reproduces epidermal alterations caused by filaggrin mutations in vivo. In addition, our results challenge the role of filaggrin in intermediate filament aggregation and establish a link between filaggrin and endogenous UVB protection.

Histamine suppresses epidermal keratinocyte differentiation and impairs skin barrier function in a human skin model

Defects in keratinocyte differentiation and skin barrier are important features of inflammatory skin diseases like atopic dermatitis. Mast cells and their main mediator histamine are abundant in inflamed skin and thus may contribute to disease pathogenesis.

Increased Sensitivity of Histidinemic Mice to UVB Radiation Suggests a Crucial Role of Endogenous Urocanic Acid in Photoprotection

Urocanic acid (UCA) is produced by the enzyme histidase and accumulates in the stratum corneum of the epidermis. In this study, we investigated the photoprotective role of endogenous UCA in the murine skin using histidinemic mice, in which the gene encoding histidase is mutated. Histidase was detected by immunohistochemistry in the stratum granulosum and stratum corneum of the normal murine skin but not in the histidinemic skin. The UCA content of the stratum corneum and the UVB absorption capacity of aqueous extracts from the stratum corneum were significantly reduced in histidinemic mice as compared with wild-type mice. When the shaved back skin of adult mice was irradiated with 250 mJ cm(-2) UVB, histidinemic mice accumulated significantly more DNA damage in the form of cyclobutane pyrimidine dimers than did wild-type mice. Furthermore, UVB irradiation induced significantly higher levels of markers of apoptosis in the epidermis of histidinemic mice. Topical application of UCA reversed the UVB-photosensitive phenotype of histidinemic mice and increased UVB photoprotection of wild-type mice. Taken together, these results provide strong evidence for an important contribution of endogenous UCA to the protection of the epidermis against the damaging effects of UVB radiation.

NF-E2-related factor 2 regulates the stress response to UVA-1-oxidized phospholipids in skin cells

Long‐wavelength ultraviolet (UVA‐1) radiation causes oxidative stress that modifies cellular molecules. To defend themselves against noxious oxidation products, skin cells produce detoxifying enzymes and antioxidants. We have recently shown that UVA‐1 oxidized the abundant membrane phospholipid 1‐palιrritoyl‐2‐arachidonoyl‐sn‐glycero‐3‐phosphorylcho‐line (PAPC), which then induced the stress‐response protein heme oxygenase 1 (HO‐1) in dermal fibro‐blasts. Here we examined the effects of UVA‐1‐ and UV‐oxidized phospholipids on global gene expression in human dermal fibroblasts and keratinocytes. We identified a cluster of genes that were coinduced by UVA‐1‐oxidized PAPC and UVA‐1 radiation. The cluster included HO‐1, glutamate‐cysteine ligase modifier subunit, aldo‐keto reductases‐1‐C1 and ‐C2, and IL‐8. These genes are members of the cellular stress response system termed “antioxidant response.” Accordingly, the regulatory regions of all of these genes contain binding sites for NF‐E2‐related factor 2 (NRF2), a major regulator of the antioxidant response. Both UVA‐1 irradiation and treatment with oxidized lipids led to increased nuclear accumulation and DNA binding of NRF2. Silencing and deficiency of NRF2 suppressed the antioxidant response. Taken together, our data show that UVA‐1‐mediated lipid oxidation induces expression of antioxidant response genes, which is dependent on the redox‐regulated transcription factor NRF2. Our findings suggest a different view on UV‐generated lipid mediators that were commonly regarded as detrimental.—Gruber, F., Mayer, H., Lengauer, B., Mlitz, V., Sanders, J. M., Kadl, A., Bilban, M., de Martin, R., Wagner, O., Kensler, T. W., Yamamoto, M., Leitinger, N., Tschachler, E. NF‐E2‐related factor 2 regulates the stress response to UVA‐1‐oxidized phospholipids in skin cells. FASEB J. 24, 39–48 (2010). www.fasebj.org

Primary sources and immunological prerequisites for sST2 secretion in humans

AIMS Serum levels of the soluble growth stimulation gene-2 (sST2) are elevated in heart and pulmonary diseases. However, the relationship of the sST2/interleukin (IL)-33 axis and its triggers as well as its organ distribution is still not known. This study was thus designed to investigate the cellular origin and regulation of sST2 and IL-33 in vitro and in vivo. METHODS AND RESULTS sST2 and IL-33 gene expression and protein secretion were analysed in pooled organ-specific cDNAs and in primary cell cultures, respectively, by RT-PCR and ELISA technology. The strongest sST2 mRNA expression was detected in heart and lung tissues, which correlated with spontaneous secretion of sST2 protein in vitro. The inflammatory cytokines IL-1alpha, IL-1beta, and tumour necrosis factor alpha as well as supernatants of lipopolysaccharide (LPS)-stimulated peripheral blood mononuclear cells led to an enhanced secretion of sST2 in cultured cardiac myocytes and lung alveolar epithelial cells. These cytokines enhanced sST2 secretion via an NFkappaB-dependent mechanism. In addition, LPS stimulation in humans in vivo induced a short-term inflammatory response that was followed by a massive enhancement of sST2 secretion. CONCLUSION These results identify the primary sources and inflammatory triggers for the enhancement of sST2 secretion and demonstrate a relationship between inflammation and the secretion of a bioactive member of the IL-1R family, both in vitro and in vivo.

Photooxidation Generates Biologically Active Phospholipids That Induce Heme Oxygenase-1 in Skin Cells

Heme oxygenase-1 (HO-1) is a key enzyme in the cellular response to tissue injury and oxidative stress. HO-1 enzymatic activity results in the formation of the cytoprotective metabolites CO and biliverdin. In the skin, HO-1 is strongly induced after long wave ultraviolet radiation (UVA-1). Here we show that UVA-1 irradiation generates oxidized phospholipids derived from 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) that mediate the expression of HO-1 in skin cells. Using EO6 antibodies that recognize oxidized phospholipids, we show that UVA-1 irradiation of dermal fibroblasts generates oxidation-specific epitopes. Irradiation of arachidonate-containing phospholipids with UVA-1 led to formation of defined lipid oxidation products including epoxyisoprostane-phosphatidylcholine that induced HO-1 expression in dermal fibroblasts, in keratinocytes, and in a three-dimensional epidermal equivalent model. In addition, we demonstrate that the oxidation of PAPC by UVA-1 is a singlet oxygen-dependent mechanism. Together, we present a novel mechanism of UVA-1-induced HO-1 expression that is mediated by the generation of biologically active phospholipid oxidation products. Because UVA-1 irradiation is a mainstay treatment of several inflammatory skin diseases, structural identification of UVA-1-generated biomolecules with HO-1-inducing capacity should lead to the development of drugs that could substitute for irradiation.

Evolutionary Origin and Diversification of Epidermal Barrier Proteins in Amniotes

The evolution of amniotes has involved major molecular innovations in the epidermis. In particular, distinct structural proteins that undergo covalent cross-linking during cornification of keratinocytes facilitate the formation of mechanically resilient superficial cell layers and help to limit water loss to the environment. Special modes of cornification generate amniote-specific skin appendages such as claws, feathers, and hair. In mammals, many protein substrates of cornification are encoded by a cluster of genes, termed the epidermal differentiation complex (EDC). To provide a basis for hypotheses about the evolution of cornification proteins, we screened for homologs of the EDC in non-mammalian vertebrates. By comparative genomics, de novo gene prediction and gene expression analyses, we show that, in contrast to fish and amphibians, the chicken and the green anole lizard have EDC homologs comprising genes that are specifically expressed in the epidermis and in skin appendages. Our data suggest that an important component of the cornified protein envelope of mammalian keratinocytes, that is, loricrin, has originated in a common ancestor of modern amniotes, perhaps during the acquisition of a fully terrestrial lifestyle. Moreover, we provide evidence that the sauropsid-specific beta-keratins have evolved as a subclass of EDC genes. Based on the comprehensive characterization of the arrangement, exon–intron structures and conserved sequence elements of EDC genes, we propose new scenarios for the evolutionary origin of epidermal barrier proteins via fusion of neighboring S100A and peptidoglycan recognition protein genes, subsequent loss of exons and highly divergent sequence evolution.

Trichohyalin-Like Proteins Have Evolutionarily Conserved Roles in the Morphogenesis of Skin Appendages

S100 fused-type proteins (SFTPs) such as filaggrin, trichohyalin, and cornulin are differentially expressed in cornifying keratinocytes of the epidermis and various skin appendages. To determine evolutionarily conserved, and thus presumably important, features of SFTPs, we characterized nonmammalian SFTPs and compared their amino acid sequences and expression patterns with those of mammalian SFTPs. We identified an ortholog of cornulin and a previously unknown SFTP, termed scaffoldin, in reptiles and birds, whereas filaggrin was confined to mammals. In contrast to mammalian SFTPs, both cornulin and scaffoldin of the chicken are expressed in the embryonic periderm. However, scaffoldin resembles mammalian trichohyalin with regard to its expression in the filiform papillae of the tongue and in the epithelium underneath the forming tips of the claws. Furthermore, scaffoldin is expressed in the epithelial sheath around growing feathers, reminiscent of trichohyalin expression in the inner root sheath of hair. The results of this study show that SFTP-positive epithelia function as scaffolds for the growth of diverse skin appendages such as claws, nails, hair, and feathers, indicating a common evolutionary origin.

Epidermal keratinocytes form a functional skin barrier in the absence of Atg7 dependent autophagy

BACKGROUND Cornification of keratinocytes involves the degradation of intracellular constituents which has led to the hypothesis that autophagy plays a role in this process. Mice, in which essential autophagy-related genes such as Atg7 are deleted systemically, die after birth and have not been characterized for potential epidermal defects. OBJECTIVE This study tested whether autophagy is essential for epidermal barrier formation and function. METHODS Atg7 was inactivated in epidermal keratinocytes by the Cre-loxP system under the control of the keratin K14 promoter (Atg7Δepi mice). Autophagic activity was detected using the GFP-microtubule-associated protein light chain 3 (GFP-LC3) reporter construct and Western blot analysis of LC3. Epidermal morphology was examined by histological and ultrastructural analyses, and barrier functions were assessed by dye diffusion and water loss assays. RESULTS Suprabasal epidermal cells of normal mice contained GFP-LC3-labeled autophagosomes and epidermal lysates of these mice showed an excess of lipidated over non-lipidated LC3. These features of active autophagy were efficiently suppressed in Atg7Δepi epidermis. Atg7Δepi mice survived the perinatal period and were apparently healthy. Histologically, their epidermis was inconspicuous and ultrastructural analysis revealed no significant defect in cornification. There was however, an increase in the thickness of corneocytes in the back skin of mutant mice. Nevertheless, resistance to dye penetration into the skin and transepidermal water loss were normal in Atg7Δepi mice. CONCLUSION This study demonstrates that autophagy is constitutively active in the epidermis but not essential for the barrier function of the skin.

Impact of filaggrin mutations on Raman spectra and biophysical properties of the stratum corneum in mild to moderate atopic dermatitis

Background  Atopic dermatitis (AD) is associated with null mutations in the filaggrin (FLG) gene.