Induction of normal and psoriatic phenotypes in submerged keratinocyte cultures

Effects of metal oxide nanoparticles on healthy and psoriasis-like human epidermal keratinocytes in vitro

The use of metal oxide nanoparticles (NPs) in skincare products has significantly increased human skin exposure, raising safety concerns. Whilst NP's ability to penetrate healthy skin is minimal, studies have demonstrated that metal oxide NPs can induce toxicity in keratinocytes through direct contact. Moreover, NP's effect on common skin disorders like psoriasis, where barrier impairments and underlying inflammation could potentially increase NP penetration and worsen nanotoxicity is largely unstudied. In this paper, we investigated whether psoriasis-like human keratinocytes (Pso HKs) would exhibit heightened toxic responses to titanium dioxide (TiO2), zinc oxide (ZnO), and/or silica (SiO2) NPs compared to healthy HKs. Cells were exposed to each NP at concentrations ranging between 0.5 and 500 µg/ml for 6, 24, and 48 h. Amongst the metal oxide NPs, ZnO NPs produced the most pronounced toxic effects in both cell types, affecting cell viability, inducing oxidative stress, and activating the inflammasome pathway. Notably, only in ZnO NPs-treated Pso HKs, trappin-2/pre-elafin was cleaved intracellularly through a non-canonical process. In addition, tissue remodelling-related cytokines were upregulated in ZnO NP-treated Pso HKs. The full impact of the observed outcomes on psoriatic symptoms will need further evaluation. Nonetheless, our findings indicate the importance of understanding the sub-lethal impacts of NP exposures on keratinocytes, even though direct exposure may be low, particularly in the context of skin disorders where repeated and long-term exposures are anticipated.

IL17A Blockade with Ixekizumab Suppresses MuvB Signaling in Clinical Psoriasis

Unbiased informatics approaches have the potential to generate insights into uncharacterized signaling pathways in human disease. In this study, we generated longitudinal transcriptomic profiles of plaque psoriasis lesions from patients enrolled in a clinical trial of the anti-IL17A antibody ixekizumab (IXE). This dataset was then computed against a curated matrix of over 700 million data points derived from published psoriasis and signaling node perturbation transcriptomic and chromatin immunoprecipitation-sequencing datasets. We observed substantive enrichment within both psoriasis-induced and IXE-repressed gene sets of transcriptional targets of members of the MuvB complex, a master regulator of the mitotic cell cycle. These gene sets were similarly enriched for pathways involved in the regulation of the G2/M transition of the cell cycle. Moreover, transcriptional targets for MuvB nodes were strongly enriched within IXE-repressed genes whose expression levels correlated strongly with the extent and severity of the psoriatic disease. In models of human keratinocyte proliferation, genes encoding MuvB nodes were transcriptionally repressed by IXE, and depletion of MuvB nodes reduced cell proliferation. Finally, we made the expression and regulatory networks that supported this study available as a freely accessible, cloud-based hypothesis generation platform. Our study positions inhibition of MuvB signaling as an important determinant of the therapeutic impact of IXE in psoriasis.

Interaction of the NRF2 and p63 transcription factors promotes keratinocyte proliferation in the epidermis

Epigenetic regulation of cell and tissue function requires the coordinated action of transcription factors. However, their combinatorial activities during regeneration remain largely unexplored. Here, we discover an unexpected interaction between the cytoprotective transcription factor NRF2 and p63- a key player in epithelial morphogenesis. Chromatin immunoprecipitation combined with sequencing and reporter assays identifies enhancers and promoters that are simultaneously activated by NRF2 and p63 in human keratinocytes. Modeling of p63 and NRF2 binding to nucleosomal DNA suggests their chromatin-assisted interaction. Pharmacological and genetic activation of NRF2 increases NRF2–p63 binding to enhancers and promotes keratinocyte proliferation, which involves the common NRF2–p63 target cyclin-dependent kinase 12. These results unravel a collaborative function of NRF2 and p63 in the control of epidermal renewal and suggest their combined activation as a strategy to promote repair of human skin and other stratified epithelia.

Advances in the Application of Natural Products and the Novel Drug Delivery Systems for Psoriasis

Psoriasis, an incurable autoimmune skin disease, is one of the most common immune-mediated disorders. Presently, numerous clinical research studies are underway, and treatment options are available. However, these treatments focus on improving symptoms of the disease and fail to achieve a radical cure; they also have certain toxic side effects. In recent years, natural products have increasingly gained attention because of their high efficiency and low toxicity. Despite their obvious therapeutic effects, natural products’ biological activity was limited by their instability, poor solubility, and low bioavailability. Novel drug delivery systems, including liposomes, lipospheres, nanostructured lipid carriers, niosomes, nanoemulsions, nanospheres, microneedles, ethosomes, nanocrystals, and foams could potentially overcome the limitations of poor water solubility and permeability in traditional drug delivery systems. Thus, to achieve a therapeutic effect, the drug can reach the epidermis and dermis in psoriatic lesions to interact with the immune cells and cytokines.

Human pluripotent stem cells: An alternative for 3D in vitro modelling of skin disease

To effectively study the skin and its pathology, various platforms have been used to date, with in vitro 3D skin models being considered the future gold standard. These models have generally been engineered from primary cell lines. However, their short life span leading to the use of various donors, imposes issues with genetic variation. Human pluripotent stem cell (hPSC)-technology holds great prospects as an alternative to the use of primary cell lines to study the pathophysiology of human skin diseases. This is due to their potential to generate an unlimited number of genetically identical skin models that closely mimic the complexity of in vivo human skin. During the past decade, researchers have therefore started to use human embryonic and induced pluripotent stem cells (hESC/iPSC) to derive skin resident-like cells and components. These have subsequently been used to engineer hPSC-derived 3D skin models. In this review, we focus on the advantages, recent developments, and future perspectives in using hPSCs as an alternative cell source for modelling human skin diseases in vitro.

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

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

Past, present and future of in vitro 3D reconstructed inflammatory skin models to study psoriasis

Psoriasis is a common chronic inflammatory skin disease with a significant socio-economic impact that can greatly affect the patients' quality of life. The prevailing dogma in the aetiology and pathophysiology of this complex disease is that skin cells, immune cells and environmental factors contribute to psoriatic skin inflammation. For a better understanding of the disease pathogenesis, models are required that mimic the disease and which can be used to develop therapeutics. Over the last decades, in vitro human reconstructed skin models have been widely used in dermatological research and have also been developed to mimic psoriatic skin. This viewpoint summarizes the most commonly used in vitro models and the latest accomplishments for the combination of the dermal and epidermal compartments with other cell types and factors that are important players in the psoriatic skin environment. We aim to critically list the most complete and best-validated models that include major psoriasis hallmarks with regard to gene and protein expression profile and epidermal morphology, but also discuss the shortcoming of the current models. This viewpoint intends to guide the development of in vitro 3D skin models that faithfully mimic all features of psoriatic skin. Such model will enable fundamental biological studies for a better understanding of the aetiology and pathophysiology of psoriasis and aid in novel therapeutic target identification and drug development studies.

In vitro psoriasis models with focus on reconstructed skin models as promising tools in psoriasis research

Psoriasis is a complex chronic immune-mediated inflammatory cutaneous disease associated with the development of inflammatory plaques on the skin. Studies proved that the disease results from a deregulated interplay between skin keratinocytes, immune cells and the environment leading to a persisting inflammatory process modulated by pro-inflammatory cytokines and activation of T cells. However, a major hindrance to study the pathogenesis of psoriasis more in depth and subsequent development of novel therapies is the lack of suitable pre-clinical models mimicking the complex phenotype of this skin disorder. Recent advances in and optimization of three-dimensional skin equivalent models have made them attractive and promising alternatives to the simplistic monolayer cultures, immunological different in vivo models and scarce ex vivo skin explants. Moreover, human skin equivalents are increasing in complexity level to match human biology as closely as possible. Here, we critically review the different types of three-dimensional skin models of psoriasis with relevance to their application potential and advantages over other models. This will guide researchers in choosing the most suitable psoriasis skin model for therapeutic drug testing (including gene therapy via siRNA molecules), or to examine biological features contributing to the pathology of psoriasis. However, the addition of T cells (as recently applied to a de-epidermized dermis-based psoriatic skin model) or other immune cells would make them even more attractive models and broaden their application potential. Eventually, the ultimate goal would be to substitute animal models by three-dimensional psoriatic skin models in the pre-clinical phases of anti-psoriasis candidate drugs. Impact statement The continuous development of novel in vitro models mimicking the psoriasis phenotype is important in the field of psoriasis research, as currently no model exists that completely matches the in vivo psoriasis skin or the disease pathology. This work provides a complete overview of the different available in vitro psoriasis models and suggests improvements for future models. Moreover, a focus was given to psoriatic skin equivalent models, as they offer several advantages over the other models, including commercial availability and validity. The potential and reported applicability of these models in psoriasis pre-clinical research is extensively discussed. As such, this work offers a guide to researchers in their choice of pre-clinical psoriasis model depending on their type of research question.

Cathepsin B as a potential cystatin M/E target in the mouse hair follicle

Deficiency of the cysteine protease inhibitor cystatin M/E (Cst6) in mice leads to disturbed epidermal cornification, impaired barrier function, and neonatal lethality. We report the rescue of the lethal skin phenotype of ichq (Cst6-deficient; Cst6^−/−) mice by transgenic, epidermis-specific, reexpression of Cst6 under control of the human involucrin (INV) promoter. Rescued Tg(INV-Cst6)Cst6^ichq/ichq mice survive the neonatal phase, but display severe eye pathology and alopecia after 4 mo. We observed keratitis and squamous metaplasia of the corneal epithelium, comparable to Cst6^−/−Ctsl^+/− mice, as we have reported in other studies. We found the INV promoter to be active in the hair follicle infundibulum; however, we did not observe Cst6 protein expression in the lower regions of the hair follicle in Tg(INV-Cst6)Cst6^ichq/ichq mice. This result suggests that unrestricted activity of proteases is involved in disturbance of hair follicle biology, eventually leading to baldness. Using quenched activity-based probes, we identified mouse cathepsin B (CtsB), which is expressed in the lower regions of the hair follicle, as an additional target of mouse Cst6. These data suggest that Cst6 is necessary to control CtsB activity in hair follicle morphogenesis and highlight Cst6-controlled proteolytic pathways as targets for preventing hair loss.—Oortveld, M. A. W., van Vlijmen-Willems, I. M. J. J., Kersten, F. F. J., Cheng, T., Verdoes, M., van Erp, P. E. J., Verbeek, S., Reinheckel, T., Hendriks, W. J. A. J., Schalkwijk, J., Zeeuwen, P. L. J. M. Cathepsin B as a potential cystatin M/E target in the mouse hair follicle.

Dual Role of Act1 in Keratinocyte Differentiation and Host Defense: TRAF3IP2 Silencing Alters Keratinocyte Differentiation and Inhibits IL-17 Responses

TRAF3IP2 is a candidate psoriasis susceptibility gene encoding Act1, an adaptor protein with ubiquitin ligase activity that couples the IL-17 receptor to downstream signaling pathways. We investigated the role of Act1 in keratinocyte responses to IL-17 using a tetracycline inducible short hairpin RNA targeting TRAF3IP2. Tetracycline exposure for 7 days effectively silenced TRAF3IP2 mRNA and Act1 protein, resulting in 761 genes with significant changes in expression (495 down, 266 up; >1.5-fold, P < 0.05). Gene ontology analysis showed that genes affected by TRAF3IP2 silencing are involved in epidermal differentiation, with early differentiation genes (KRT1, KRT10, DSC1, DSG1) being down-regulated and late differentiation genes (SPRR2, SPRR3, LCE3) being up-regulated. AP1 binding sites were enriched upstream of genes up-regulated by TRAF3IP2 silencing. Correspondingly, nuclear expression of FosB and Fra1 was increased in TRAF3IP2-silenced cells. Many genes involved in host defense were induced by IL-17 in a TRAF3IP2-dependent fashion. Inflammatory differentiation conditions (serum addition for 4 days postconfluence) markedly amplified these IL-17 responses and increased basal levels and TRAF3IP2 silencing-dependent up-regulation of multiple late differentiation genes. These findings suggest that TRAF3IP2 may alter both epidermal homeostasis and keratinocyte defense responses to influence psoriasis risk.

Genome-wide p63-regulated gene expression in differentiating epidermal keratinocytes

The transcription factor p63 is a key regulator in epidermal keratinocyte proliferation and differentiation. However, the role of p63 in gene regulation during these processes is not well understood. To investigate this, we recently generated genome-wide profiles of gene expression, p63 binding sites and active regulatory regions with the H3K27ac histone mark (Kouwenhoven et al., 2015). We showed that only a subset of p63 binding sites are active in keratinocytes, and that differentiation-associated gene expression dynamics correlate with the activity of p63 binding sites rather than with their occurrence per se. Here we describe in detail the generation and processing of the ChIP-seq and RNA-seq datasets used in this study. These data sets are deposited in the Gene Expression Omnibus (GEO) repository under the accession number GSE59827.

Transcription factor p63 bookmarks and regulates dynamic enhancers during epidermal differentiation

The transcription factor p63 plays a pivotal role in keratinocyte proliferation and differentiation in the epidermis. However, how p63 regulates epidermal genes during differentiation is not yet clear. Using epigenome profiling of differentiating human primary epidermal keratinocytes, we characterized a catalog of dynamically regulated genes and p63-bound regulatory elements that are relevant for epithelial development and related diseases. p63-bound regulatory elements occur as single or clustered enhancers, and remarkably, only a subset is active as defined by the co-presence of the active enhancer mark histone modification H3K27ac in epidermal keratinocytes. We show that the dynamics of gene expression correlates with the activity of p63-bound enhancers rather than with p63 binding itself. The activity of p63-bound enhancers is likely determined by other transcription factors that cooperate with p63. Our data show that inactive p63-bound enhancers in epidermal keratinocytes may be active during the development of other epithelial-related structures such as limbs and suggest that p63 bookmarks genomic loci during the commitment of the epithelial lineage and regulates genes through temporal- and spatial-specific active enhancers.

S100A7 acts as a dual regulator in promoting proliferation and suppressing squamous differentiation through GATA-3/caspase-14 pathway in A431 cells

S100A7 is expressed in many squamous cell carcinomas (SCCs), such as SCC of the skin, and well-differentiated SCC always displays stronger staining of this protein. A431 cells, an epidermal cancer cell line, were selected as a cell model to investigate the roles and mechanism of S100A7 in SCC of the skin. In this study, we demonstrated that the overexpression of S100A7 in A431 cells significantly promoted cell proliferation in vitro and tumor growth in vivo, whereas it suppressed the expression of GATA-3, caspase-14 and three squamous differentiation markers, keratin-1, TG-1 and involucrin. Conversely, the overexpression of caspase-14 not only significantly decreased cell proliferation and delayed tumor growth but also markedly induced the expression of three squamous differentiation markers, whereas S100A7 and GATA-3 were not influenced. Further evidence showed that silencing GATA-3 greatly inhibited the expression of caspase-14 and three differentiation markers, while the expression of S100A7 was not changed; contrary results were obtained when overexpressing GATA-3. Importantly, restoring the expression of GATA-3 and caspase-14 in A431-S100A7 cells could bypass the ability of S100A7 to increase cell viability and repress squamous differentiation. These data suggested that S100A7 expression in SCC may play an important role in the maintenance of SCC cell dedifferentiation, at least in SCC of the skin.

Genetic and pharmacological analysis identifies a physiological role for the AHR in epidermal differentiation

Stimulation of the aryl hydrocarbon receptor (AHR) by xenobiotics is known to affect epidermal differentiation and skin barrier formation. The physiological role of endogenous AHR signaling in keratinocyte differentiation is not known. We used murine and human skin models to address the hypothesis that AHR activation is required for normal keratinocyte differentiation. Using transcriptome analysis of Ahr(-/-) and Ahr(+/+) murine keratinocytes, we found significant enrichment of differentially expressed genes linked to epidermal differentiation. Primary Ahr(-/-) keratinocytes showed a significant reduction in terminal differentiation gene and protein expression, similar to Ahr(+/+) keratinocytes treated with AHR antagonists GNF351 and CH223191, or the selective AHR modulator (SAhRM) SGA360. In vitro keratinocyte differentiation led to increased AHR levels and subsequent nuclear translocation, followed by induced CYP1A1 gene expression. Monolayer cultured primary human keratinocytes treated with AHR antagonists also showed an impaired terminal differentiation program. Inactivation of AHR activity during human skin equivalent development severely impaired epidermal stratification, terminal differentiation protein expression, and stratum corneum formation. As disturbed epidermal differentiation is a main feature of many skin diseases, pharmacological agents targeting AHR signaling or future identification of endogenous keratinocyte-derived AHR ligands should be considered as potential new drugs in dermatology.

The changes in the expression levels of follicular markers in keratoacanthoma depend on the stage: keratoacanthoma is a follicular neoplasm exhibiting infundibular/isthmic differentiation without expression of CK15

BACKGROUND

Although the precise etiology of keratoacanthoma (KA) is unknown, KA is generally assumed to differentiate toward hair follicles based on previous studies of experimental carcinogenesis.

METHODS

We performed a comprehensive immunohistochemical study of various follicular markers in all stages of KA. A total of 67 tumors, including 16 early or proliferative stage lesions, 43 well-developed stage lesions, five regressing stage lesions and three regressed stage lesions, were subjected to the analysis.

RESULTS

CK15 (clone C8/144B), CK19 and CD34 were not expressed at any stage. CK1, CK10, CK16, CK17, CK15 (clone LHK15) and calretinin showed dynamic changes in their expression in KA depending on the stage.

CONCLUSIONS

KA is a follicular neoplasm with infundibular/isthmic (upper segmental region of hair follicles) differentiation. It is considered that early or proliferative stage tumors show keratin-filled invaginations with infundibular differentiation and gradual isthmic differentiation. Well-developed examples of KA generally show isthmic differentiation in the whole lesions. The regressed stage KAs lose the features of this type of follicular differentiation and show epidermal characteristics. No expression of CK15 (clone C8/144B) was observed in KAs, although this finding is insufficient to completely rule out the correlation between the regression of KA and the hair follicle cycle.

Identifying targets for topical RNAi therapeutics in psoriasis: assessment of a new in vitro psoriasis model

Diseases of the skin are amenable to RNAi-based therapies and targeting key components in the pathophysiology of psoriasis using RNAi may represent a successful new therapeutic strategy. We aimed to develop a straightforward and highly reproducible in vitro psoriasis model useful to study the effects of gene knockdown by RNAi and to identify new targets for topical RNAi therapeutics. We evaluated the use of keratinocytes derived from psoriatic plaques and normal human keratinocytes (NHKs). To induce a psoriatic phenotype in NHKs, combinations of pro-inflammatory cytokines (IL-1α, IL-17A, IL-6 and TNF-α) were tested. The model based on NHK met our needs of a reliable and predictive preclinical model, and this model was further selected for gene expression analyses, comprising a panel of 55 psoriasis-associated genes and five micro-RNAs (miRNAs). Gene silencing studies were conducted by using small interfering RNAs (siRNAs) and miRNA inhibitors directed against potential target genes such as CAMP and DEFB4 and miRNAs such as miR-203. We describe a robust and highly reproducible in vitro psoriasis model that recapitulates expression of a large panel of genes and miRNAs relevant to the pathogenesis of psoriasis. Furthermore, we show that our model is a powerful first step model system for testing and screening RNAi-based therapeutics.

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

Topical application of coal tar is one of the oldest therapies for atopic dermatitis (AD), a T helper 2 (Th2) lymphocyte-mediated skin disease associated with loss-of-function mutations in the skin barrier gene, filaggrin (FLG). Despite its longstanding clinical use and efficacy, the molecular mechanism of coal tar therapy is unknown. Using organotypic skin models with primary keratinocytes from AD patients and controls, we found that coal tar activated the aryl hydrocarbon receptor (AHR), resulting in induction of epidermal differentiation. AHR knockdown by siRNA completely abrogated this effect. Coal tar restored filaggrin expression in FLG-haploinsufficient keratinocytes to wild-type levels, and counteracted Th2 cytokine-mediated downregulation of skin barrier proteins. In AD patients, coal tar completely restored expression of major skin barrier proteins, including filaggrin. Using organotypic skin models stimulated with Th2 cytokines IL-4 and IL-13, we found coal tar to diminish spongiosis, apoptosis, and CCL26 expression, all AD hallmarks. Coal tar interfered with Th2 cytokine signaling via dephosphorylation of STAT6, most likely due to AHR-regulated activation of the NRF2 antioxidative stress pathway. The therapeutic effect of AHR activation herein described opens a new avenue to reconsider AHR as a pharmacological target and could lead to the development of mechanism-based drugs for AD.

Study of the genetic factors predisposing to the development of psoriasis

Background papers on psoriasis epidemiology, pathogenesis and genetics are presented. Special attention is given to genetic factors of the aptitude to psoriasis development. Were analysed researches, dedicated to the genome-wide screening of associations of polymorphic genetic locus with psoriasis development. Obtained results allow to reveal pathogenic psoriasis mechanisms, to forecast the character of the clinical course of the disease, as well as the efficiency of therapy and forecast the risk of psoriasis origination at patient’s relatives.

Rho kinase inhibitor Y-27632 prolongs the life span of adult human keratinocytes, enhances skin equivalent development, and facilitates lentiviral transduction

The use of tissue-engineered human skin equivalents (HSE) for fundamental research and industrial application requires the expansion of keratinocytes from a limited number of skin biopsies donated by adult healthy volunteers or patients. A pharmacological inhibitor of Rho-associated protein kinases, Y-27632, was recently reported to immortalize neonatal human foreskin keratinocytes. Here, we investigated the potential use of Y-27632 to expand human adult keratinocytes and evaluated its effects on HSE development and in vitro gene delivery assays. Y-27632 was found to significantly increase the life span of human adult keratinocytes (up to five to eight passages). The epidermal morphology of HSEs generated from high-passage, Y-27632-treated keratinocytes resembled the native epidermis and was improved by supplementing Y-27632 during the submerged phase of HSE development. In addition, Y-27632-treated keratinocytes responded normally to inflammatory stimuli, and could be used to generate HSEs with a psoriatic phenotype, upon stimulation with relevant cytokines. Furthermore, Y-27632 significantly enhanced both lentiviral transduction efficiency of primary adult keratinocytes and epidermal morphology of HSEs generated thereof. Our study indicates that Y-27632 is a potentially powerful tool that is used for a variety of applications of adult human keratinocytes.

A comprehensive analysis of pattern recognition receptors in normal and inflamed human epidermis: upregulation of dectin-1 in psoriasis

Human epidermis plays an important role in host defense by acting as a physical barrier and signaling interface between the environment and the immune system. Pattern recognition receptors (PRRs) are crucial to maintain homeostasis and provide protection during infection, but are also causally involved in monogenic auto-inflammatory diseases. This study aimed to investigate the epidermal expression of PRRs and several associated host defense molecules in healthy human skin, psoriasis, and atopic dermatitis (AD). Using microarray analysis and real-time quantitative PCR, we found that many of these genes are transcribed in normal human epidermis. Only a few genes were differentially induced in psoriasis (CLEC7A (dectin-1), Toll-like receptor (TLR) 4, and mannose receptor C type 1 (MRC1)) or AD (MRC1, IL1RN, and IL1β) compared with normal epidermis. A remarkably high expression of dectin-1 mRNA was observed in psoriatic epidermis and this was corroborated by immunohistochemistry. In cultured primary human keratinocytes, dectin-1 expression was induced by IFN-γ, IFN-α, and Th17 cytokines. Keratinocytes were unresponsive, however, to dectin-1 ligands such as β-glucan or heat-killed Candida albicans, nor did we observe synergy with TLR2/TLR5 ligands. In conclusion, upregulation of dectin-1 in psoriatic lesions seems to be under control of psoriasis-associated cytokines. Its role in the biology of skin inflammation and infection remains to be explored.

Psoriasis genetics: breaking the barrier

Psoriasis is a common incurable inflammatory skin disease affecting 2-3% of the European population. Psoriatic skin contains large numbers of immune cells which produce many cytokines, chemokines and inflammatory molecules. The epidermis divides much faster than normal and has a defective outer layer or barrier which under normal circumstances protects from infection and dehydration. Psoriatic skin is characterized by a distinct set of inflammation and epidermal proliferation and differentiation markers, and it has been unclear whether the genetic basis of psoriasis reflects defects of the immune system or of the skin. One genetic determinant lies within the major histocompatibility complex class 1 region. Genome-wide association studies have revealed genetic susceptibility factors that play a role in the formation of immune cells found in psoriasis lesions. Others affect epidermal proliferation and skin barrier formation. Hence, genetic components of both the immune system and the epidermis can predispose to disease.

Potential skin antiinflammatory effects of 4-methylthiobutylisothiocyanate (MTBI) isolated from rocket (Eruca sativa) seeds

Isothiocyanates (ITCs), which are organosulfur compounds present in cruciferous vegetables, have anticarcinogenic, antiinflammatory, and antiproliferative activities. These biological activities, and the knowledge that rocket seed (Eruca sativa) extract is used in skin disorders in traditional Middle Eastern medicine, led to the isolation and assessment of 4-methylthiobutylisothiocyanate (MTBI), the major ITC in rocket seeds, for its potential in the prevention of inflammatory skin diseases, such as psoriasis. MTBI was found to depress the growth of activated keratinocytes and to arrest the activated THP-1 monocytes in the G2 stage. Both MTBI and its oxidized derivative sulforaphane (SFN), which was found in the rocket seed at a low concentration, downregulated the expression of the proinflammatory genes, tumor necrosis factor (TNF)-alpha and interleukin (IL)-12/23 p40, as well as that of intercellular adhesion molecule-1, in activated THP-1 cells. These results demonstrate that MTBI may deter the inflammation process, as has been reported for SFN. Furthermore, pretreatment with MTBI hindered the induction of the inflammatory state in the THP-1 cells, as shown by the inhibition of cytokine mRNA expression of IL-1beta, IL-12/23 p40, and TNF-alpha. Overall, our results imply that MTBI may represent a new family of natural compounds possessing significant skin inflammation-preventive activities.

Expression of the vanin gene family in normal and inflamed human skin: induction by proinflammatory cytokines

The vanin gene family encodes secreted and membrane-bound ectoenzymes that convert pantetheine into pantothenic acid and cysteamine. Recent studies in a mouse colitis model indicated that vanin-1 has proinflammatory activity and suggest that pantetheinases are potential therapeutic targets in inflammatory diseases. In a microarray analysis of epidermal gene expression of psoriasis and atopic dermatitis lesions, we identified vanin-3 as the gene showing the highest differential expression of all annotated genes that we studied (19-fold upregulation in psoriasis). Quantitative real-time PCR analysis confirmed the microarray data on vanin-3 and showed similar induction of vanin-1, but not of vanin-2, in psoriatic epidermis. Immunohistochemistry showed that vanin-3 is expressed in the differentiated epidermal layers. Using submerged and organotypic keratinocyte cultures, we found that vanin-1 and vanin-3 are induced at the mRNA and protein level by psoriasis-associated proinflammatory cytokines (Th17/Th1) but not by Th2 cytokines. We hypothesize that increased levels of pantetheinase activity are part of the inflammatory-regenerative epidermal differentiation program, and may contribute to the phenotype observed in psoriasis.

Development and validation of human psoriatic skin equivalents

Psoriasis is an inflammatory skin disease driven by aberrant interactions between the epithelium and the immune system. Anti-psoriatic drugs can therefore target either the keratinocytes or the immunocytes. Here we sought to develop an in vitro reconstructed skin model that would display the molecular characteristics of psoriatic epidermis in a controlled manner, allowing the screening of anti-psoriatic drugs and providing a model in which to study the biology of this disease. Human skin equivalents generated from normal human adult keratinocytes after air exposure and stimulation by keratinocyte growth factor and epidermal growth factor displayed the correct morphological and molecular characteristics of normal human epidermis whereas the psoriasis-associated proteins, hBD-2, SKALP/elafin, and CK16, were absent. Skin equivalents generated from foreskin keratinocytes were clearly abnormal both morphologically and with respect to gene expression. When normal skin equivalents derived from adult keratinocytes were stimulated with psoriasis-associated cytokines [tumor necrosis factor-alpha, interleukin (IL)-1alpha, IL-6, and IL-22] or combinations thereof, strong expression of hBD-2, SKALP/elafin, CK16, IL-8, and tumor necrosis factor-alpha was induced as shown by quantitative polymerase chain reaction and immunohistochemistry. Retinoic acid but not cyclosporin A was found to inhibit cytokine-induced gene expression at both the mRNA and protein levels. These results illustrate the potential of this disease model to study the molecular pathology and pharmacological intervention in vitro.

Genetically programmed differences in epidermal host defense between psoriasis and atopic dermatitis patients

In the past decades, chronic inflammatory diseases such as psoriasis, atopic dermatitis, asthma, Crohn’s disease and celiac disease were generally regarded as immune-mediated conditions involving activated T-cells and proinflammatory cytokines produced by these cells. This paradigm has recently been challenged by the finding that mutations and polymorphisms in epithelium-expressed genes involved in physical barrier function or innate immunity, are risk factors of these conditions. We used a functional genomics approach to analyze cultured keratinocytes from patients with psoriasis or atopic dermatitis and healthy controls. First passage primary cells derived from non-lesional skin were stimulated with pro-inflammatory cytokines, and expression of a panel of 55 genes associated with epidermal differentiation and cutaneous inflammation was measured by quantitative PCR. A subset of these genes was analyzed at the protein level. Using cluster analysis and multivariate analysis of variance we identified groups of genes that were differentially expressed, and could, depending on the stimulus, provide a disease-specific gene expression signature. We found particularly large differences in expression levels of innate immunity genes between keratinocytes from psoriasis patients and atopic dermatitis patients. Our findings indicate that cell-autonomous differences exist between cultured keratinocytes of psoriasis and atopic dermatitis patients, which we interpret to be genetically determined. We hypothesize that polymorphisms of innate immunity genes both with signaling and effector functions are coadapted, each with balancing advantages and disadvantages. In the case of psoriasis, high expression levels of antimicrobial proteins genes putatively confer increased protection against microbial infection, but the biological cost could be a beneficial system gone awry, leading to overt inflammatory disease.

Identification of avarol derivatives as potential antipsoriatic drugs using an in vitro model for keratinocyte growth and differentiation

Avarol, a marine sesquiterpenoid hydroquinone, and 14 avarol derivatives have shown interesting anti-inflammatory properties in previous studies. In this study, avarol and derivatives were evaluated in high-throughput keratinocyte culture models using cytokeratin 10 and SKALP/Elafin expression as markers for respectively normal and psoriatic differentiation. Avarol and five of its derivatives (5, 10, 13, 14 and 15) were selected for further study. Only 10, 13, 14 and 15 were able to inhibit keratinocyte cell growth. Changes in expression levels of 22 genes were assessed by quantitative real time PCR (qPCR). From these genes, TNFalpha mRNA levels showed the strongest changes. For compound 13, 15 and dithranol (used as a model antipsoriatic drug), a dose-dependent downregulation of TNFalpha mRNA was found. The changes in TNFalpha mRNA were confirmed at the protein level for compound 13. Additionally, this compound was able to reduce also IL-8 and COX-2 mRNA levels and this effect was correlated with a reduction in COX-2 protein expression. The mechanism of action of this compound involves at least the inhibition of NF-kappaB-DNA binding activity. In conclusion, our high-throughput screening models in combination with quantitative assessment of changes in gene expression profiles identified the avarol derivative 13, a benzylamine derivative of avarol at the 4' position of benzoquinone ring, as an interesting anti-psoriatic drug candidate that inhibits keratinocyte cell growth and TNFalpha and COX-2 expression.

Transcription promoter activity of the human S100A7 gene in oral squamous cell carcinoma cell lines

The S100A7 (psoriasin) gene has been shown to be markedly over-expressed in squamous cell carcinomas (SCCs) as well as in psoriasis. We herein examined the S100A7 gene promoter activity in human oral SCC cell lines to identify the putative SCC-specific regulatory regions for the S100A7 transcription. Functional deletion assays of 5'-flanking region demonstrated that the segments, (-1513 to -988), (-1954 to -1513) and (-3040 to -2578), play important roles in the transcription activity in the oral SCCs. The internal deletion of the short segments, (-1248 to -1110), (-1109 to -988) and (-1248 to -988), decreased this activity. These segments cloned upstream of the heterologous promoter increased the promoter activity in oral SCC cell line. Electrophoretic mobility shift assays, using the sequence segmental probes, (-1248 to -1110) and (-1109 to -988), showed different DNA-protein complex patterns depending on the types of used cell lines. One of the complexes was only observed in the oral SCCs. These data suggested that the segment from -1513 to -988 contains up-regulatory elements for the transcription activity of the S100A7 gene in oral SCCs.

A molecular signature of epithelial host defense: comparative gene expression analysis of cultured bronchial epithelial cells and keratinocytes

BACKGROUND

Epithelia are barrier-forming tissues that protect the organism against external noxious stimuli. Despite the similarity in function of epithelia, only few common protective mechanisms that are employed by these tissues have been systematically studied. Comparative analysis of genome-wide expression profiles generated by means of Serial Analysis of Gene Expression (SAGE) is a powerful approach to yield further insight into epithelial host defense mechanisms. We performed an extensive comparative analysis of previously published SAGE data sets of two types of epithelial cells, namely bronchial epithelial cells and keratinocytes, in which the response to pro-inflammatory cytokines was assessed. These data sets were used to elucidate a common denominator in epithelial host defense.

RESULTS

Bronchial epithelial cells and keratinocytes were found to have a high degree of overlap in gene expression. Using an in silico approach, an epithelial-specific molecular signature of gene expression was identified in bronchial epithelial cells and keratinocytes comprising of family members of keratins, small proline-rich proteins and proteinase inhibitors. Whereas some of the identified genes were known to be involved in inflammation, the majority of the signature represented genes that were previously not associated with host defense. Using polymerase chain reaction, presence of expression of selected tissue-specific genes was validated.

CONCLUSION

Our comparative analysis of gene transcription reveals that bronchial epithelial cells and keratinocytes both express a subset of genes that is likely to be essential in epithelial barrier formation in these cell types. The expression of these genes is specific for bronchial epithelial cells and keratinocytes and is not seen in non-epithelial cells. We show that bronchial epithelial cells, similar to keratinocytes, express components that are able to form a cross-linked protein envelope that may contribute to an effective barrier against noxious stimuli and pathogens.

Hydrogen peroxide-induced cell death in normal human keratinocytes is differentiation dependent

More than other tissues, skin is exposed to numerous external stresses generating ROS that, in addition to endogenous oxygen radicals, cause keratinocyte alterations and contribute in part to photocarcinogenesis and aging. Recent evidence suggests a differentiation-dependent susceptibility of keratinocytes to apoptosis. We explored hydrogen peroxide-induced cell death in normal human keratinocytes according to their differentiation. On H(2)O(2)-exposed skin explants, caspase-3 was strongly activated in basal keratinocytes double stained with beta(1) integrin, whereas DNA fragmentation occurred in suprabasal cells only without caspase-3 activation. In addition, isolated basal keratinocytes, selected by adhesion to type IV collagen, were more sensitive than nonadherent cells to H(2)O(2)-induced apoptosis with regard to mitochondrial transmembrane potential (Deltapsi(mt)) collapse and membrane integrity. Similarly, necrotic/late apoptotic cells were present at low levels only in the adherent epidermal population. Furthermore, in primary cultures of undifferentiated keratinocytes H(2)O(2)-induced cell death appeared via a mitochondrial failure. Deltapsi(mt) collapse was associated with a strong early activation of the initiatory caspase-8, then the executive caspase-3, and, to a lesser extent, the inflammatory caspase-1. Finally, undifferentiated basal cells possess a higher sensitivity than differentiated suprabasal cells to H(2)O(2)-induced cell death, and apoptosis in human keratinocytes occurs via different pathways depending on the cell's differentiation state.

The Development and Characterization of an In Vitro Model of Psoriasis

In this study, the phenotype of psoriatic keratinocytes and fibroblasts in reconstructed skin models was compared to those constructed from normal cells. Characterization of this model by immunohistochemistry showed that classical markers of keratinocyte differentiation exhibited similar patterns of distribution in the psoriatic models to those derived from normal cells and generally reflected in vivo observations. Some crucial differences, however, were observed between normal and psoriatic models when pro-inflammatory gene expression and keratinocyte proliferation were investigated. Notably, the chemokine receptor CXCR2 was overexpressed in the psoriatic models, and, moreover, was localized to the granular layer of keratinocytes as seen in psoriasis in vivo. Pro-inflammatory genes (tumor necrosis factor alpha [TNF-alpha], interferon gamma [IFN-gamma], and interleukin 8 [IL-8]) were expressed at high levels in the psoriatic models, but were only minimally expressed in the normal models. Models derived from uninvolved psoriatic skin showed the same gene expression profile as those derived from involved skin along with an increased proliferation rate when compared to normal models. These results suggest that psoriatic individuals possess an inherent predisposition to develop the disease phenotype even in the absence of T cells. This study represents a comprehensive characterization of psoriatic human skin reconstructed in vitro, and demonstrates the potential of this model as a valuable tool in drug discovery.

Normalized proliferation of normal and psoriatic keratinocytes by suppression of sAPPalpha-release

The soluble form of the beta-amyloid precursor protein (sAPPalpha) is known to function in the autocrine regulation of epidermal growth and repair. Here we show that its proteolytic release by alpha-secretase in normal human keratinocytes is susceptible to hydroxamic-acid-based zinc metalloproteinase inhibitors and suppressed by these inhibitors by 80%-90%. As various other growth factors participate in regulating epidermal growth we investigated whether the inhibitor-induced sAPPalpha-deficiency would affect keratinocyte proliferation. At optimal inhibitor concentrations the suppression of sAPPalpha-release was followed by a decline in proliferation by 50%-60%, indicating that sAPPalpha is a major growth factor that cannot be compensated for by other growth factors. This finding was the basis for the treatment of human lesional psoriatic keratinocytes with these inhibitors, which resulted in the normalization of their increased proliferation rates. The reversibility of these effects and the lack of toxicity underline the value of these inhibitors and suggest their therapeutic application in psoriatic skin diseases.

Tumor necrosis factor related apoptosis inducing ligand triggers apoptosis in dividing but not in differentiating human epidermal keratinocytes

Using serial analysis of gene expression we have previously identified the expression of several pro-apoptotic and anti-apoptotic genes in cultured human primary epidermal keratinocytes, including tumor necrosis factor related apoptosis inducing ligand (TRAIL). TRAIL is a potent inducer of apoptosis in transformed and tumor cell lines, but usually not in other cells. Here we present a study on the effect of TRAIL on cultured keratinocytes. It is shown that differentiated and undifferentiated keratinocytes undergo apoptosis after addition of TRAIL to the medium as determined by morphologic and biochemical criteria, such as cellular shrinkage and activation of caspases. The sensitivity for TRAIL differs greatly between undifferentiated and differentiating keratinocytes, however, with undifferentiated cells being much more susceptible to apoptosis. Commitment to terminal differentiation in the absence of TRAIL does not in itself induce apoptosis. In contrast to the promyelocytic cell line HL60, internucleosomal DNA fragmentation is not observed in keratinocytes, as assessed by flow cytometric analysis and agarose gel electrophoresis. Interestingly, the prime effector of DNA fragmentation, DNA fragmentation factor of 40 kDa (DFF40), is expressed in keratinocytes, yet internucleosomal cleavage fails to occur. Our data indicate that programmed cell death during keratinocyte differentiation is distinct from receptor-mediated apoptosis in response to a death ligand.

Psoriasin (S100A7) expression is altered during skin tumorigenesis

BACKGROUND

Psoriasin (S100A7) expression has previously been associated with psoriasiform hyperplasia as well as with tumor progression in breast cancer. Its expression profile for different stages of skin lesions is unknown. The aim of this study was to determine the relationship between psoriasin (S100A7) and tumor progression in skin.

METHODS

Psoriasin was assessed by immunohistochemistry and levels of expression determined by semi-quantitative scoring in skin biopsies from 50 patients. The cohort included normal skin, actinic keratosis, squamous carcinoma in-situ, invasive squamous cell carcinoma, and basal cell carcinoma.

RESULTS

In normal skin, psoriasin was rarely detected in epidermis but was expressed in underlying adnexae. In abnormal epidermis psoriasin was frequently expressed in abnormal keratinocytes in actinic keratosis, in-situ and invasive squamous cell carcinoma, but was rarely observed in the basal epidermal layer or in superficial or invasive basal cell carcinoma. The highest levels of expression were seen within squamous carcinoma in-situ. Significantly reduced levels of expression were observed in both unmatched (p = 0.0001) and matched (p < 0.004) invasive squamous cell carcinoma. Psoriasin expression within abnormal squamous lesions correlated with mitotic count (r = 0.54, p = 0.0036), however no significant relation was found with the intensity of dermal inflammatory cell infiltrates assessed within each pathology.

CONCLUSION

These results suggest that altered psoriasin expression occurs in abnormal epidermis and that downregulation may be related to the onset of invasion in squamous cell carcinoma in skin.

Transcriptional regulation of the elafin gene in human keratinocytes

Elafin (also known as skin-derived anti-leukoproteinase/trappin-2) is an epithelial host-defense protein that is absent in normal skin but highly induced in keratinocytes of inflamed skin (e.g., psoriasis), in epidermal skin tumors, and after wounding. Previously, it was shown that in cultured keratinocytes, elafin expression is induced by serum or tumor necrosis factor-alpha, and that expression is suppressed by retinoids, dithranol, and p38 mitogen-activated protein kinase inhibitors. Here we have studied the regulation of elafin gene expression in epidermal keratinocytes at the molecular level. First we determined the transcription start site of the elafin gene and found that the elafin mRNA possesses an unusually short 5'-untranslated region. Using transient transfection of luciferase reporter constructs of the elafin promoter, we mapped a 440 bp region upstream of the translation start site that conferred high-level expression in keratinocytes, but not in A431 cells or cells of mesenchymal origin. We observed that the promoter constructs were not subjected to the same regulation as the endogenous elafin gene as these constructs were highly active independent of keratinocyte stimulation. When elafin promoter constructs were stably transfected in the HaCaT keratinocyte cell line, tumor necrosis factor-alpha inducible expression of both the endogenous elafin gene and the transgene was observed, suggesting that regulation of the elafin gene is also dependent on chromatin structure. We found, however, that a stably transfected 4 kb elafin promoter fragment did not confer retinoid sensitivity indicating that additional sequences are required for proper regulation. This study reveals the complex regulation of a gene that can be used as a paradigm for the specific differentiation program of activated epidermal keratinocytes.

Comparison of antiproliferative effects of experimental and established antipsoriatic drugs on human keratinocytes, using a simple 96-well-plate assay

Pharmacological treatments for psoriasis are generally based on antiproliferative, anti-inflammatory, or differentiation-modifying activity, or a combination of two or more of these actions. Potentially new drugs for treatment of psoriasis, which act on proliferation, can be identified by screening large compound libraries in a cell proliferation model that allows for characterization of drug effects on in vitro growth of normal human keratinocytes. High-throughput programs based on biological testing of diverse collections of compounds can rapidly identify leads for potential drug candidates in the treatment of psoriasis. In this study, we describe nonradioactive measurement of keratinocyte proliferation in the exponential growth phase in a 96-well format, using a sensitive deoxyribonucleic acid-binding dye to analyze drugs that are pharmacologically active in growth inhibition. Release of lactate dehydrogenase was used to exclude cytotoxic effects. We examined a number of compounds in a test range of 10(-7) to 10(-5) M, including known antipsoriatic drugs, and experimental drugs that are potentially useful in the treatment of psoriasis. We found strong concentration-dependent growth inhibition by dithranol, an antipsoriatic compound that is presumed to target the epidermal compartment. Methotrexate, cyclosporin A, and all-trans retinoic acid did not significantly affect proliferation at therapeutically relevant concentrations. The p38 mitogen-activated protein kinase inhibitor, SB220025, and curcumin, a natural phytochemical, inhibited keratinocyte proliferation at 10(-5) M. We conclude that this assay, in combination with the previously developed assays for psoriatic differentiation, provides a useful tool for identification of antipsoriatic drugs.

Evidence for local control of gene expression in the epidermal differentiation complex

The epidermal differentiation complex (EDC), located on chromosomal band 1q21, consists of at least 43 genes that are expressed during keratinocyte differentiation. Indicative of a role for chromatin structure in tissue specificity of EDC gene expression, we identified an inverse correlation between expression and DNA methylation for two EDC genes (S100A2 and S00A6) in human keratinocytes and fibroblasts. 5-azacytidine (5AC) and sodium butyrate (NaB) are two agents known to promote 'open' chromatin structure. To explore the relationship between chromatin structure and keratinocyte differentiation, we treated normal human keratinocytes (NHK) with 5AC or NaB, or with protocols known to promote their terminal differentiation. We then measured the steady-state mRNA levels for several S100 genes, small proline rich region-1, -2, and -3, loricrin, and involucrin by Northern blotting. 5AC and NaB each markedly increased expression of SPRR1/2 and involucrin in NHK. In contrast, expression of S100A2 was reduced by both agents, and by induction of keratinocyte differentiation. Moreover, while the clustered EDC genes displayed a general tendency to be expressed in epithelial cells, they displayed different patterns of cell type-specific expression. These results indicate that local, gene-specific factors play an important role in the regulation of EDC gene expression in the keratinocyte lineage and during keratinocyte terminal differentiation.

Development of a keratinocyte-based screening model for antipsoriatic drugs using green fluorescent protein under the control of an endogenous promoter

Inflamed epidermis (psoriasis, wound healing, ultraviolet-irradiated skin) harbors keratinocytes that are hyperproliferative and display an abnormal differentiation program. A distinct feature of this so-called regenerative maturation pathway is the expression of proteins such as the cytokeratins CK6, CK16, and CK17 and the antiinflammatory protein SKALP/elafin. These proteins are absent in normal skin but highly induced in lesional psoriatic skin. Expression of these genes can be used as a surrogate marker for psoriasis in drug-screening procedures of large compound libraries. The aim of this study was to develop a keratinocyte cell line that contained a reporter gene under the control of a psoriasis-associated endogenous promoter and demonstrate its use in an assay suitable for screening. We generated a stably transfected keratinocyte cell line that expresses enhanced green fluorescent protein (EGFP), under the control of a 0.8-kb fragment derived from the promoter of the SKALP/elafin gene, which confers high levels of tissue-specific expression at the mRNA level. Induction of the SKALP promoter by tumor necrosis factor-alpha resulted in increased expression levels of the secreted SKALP-EGFP fusion protein as assessed by direct readout of fluorescence and fluorescence polarization in 96-well cell culture plates. The fold stimulation of the reporter gene was comparable to that of the endogenous SKALP gene as assessed by enzyme-linked immunosorbent assay. Although the dynamic range of the screening system is limited, the small standard deviation yields a Z factor of 0.49. This indicates that the assay is suitable as a high-throughput screen, and provides proof of the concept that a secreted EGFP fusion protein under the control of a physiologically relevant endogenous promoter can be used as a fluorescence-based high-throughput screen for differentiation-modifying or antiinflammatory compounds that act via the keratinocyte.

A simple technique for high-throughput screening of drugs that modulate normal and psoriasis-like differentiation in cultured human keratinocytes

Established treatments for psoriasis act ei-ther on hyperproliferation, inflammation, aberrant epidermal differentiation or a combination of these aspects of the disease. Potential new drugs for treatment of psoriasis or other disorders with abnormalities in epidermal differentiation can be identified by high-throughput screening of large compound libraries using surrogate markers for the disease. Here we describe a screening model to detect pharmacologically active drugs in two keratinocyte-based, 96-well culture models that use expression of cytokeratin 10 (CK10) and skin-derived antileucoprotease (SKALP)/elafin as markers for normal and psoriatic differentiation, respectively, and allow multiple parameters to be determined from a single well. In this model we tested a number of compounds in a pharmacological range from 10(-7) to 10(-5) M, including known antipsoriatic drugs, and experimental drugs that are potentially useful in the treatment of psoriasis. All-trans-retinoic acid, dithranol and the p38 mitogen-activated protein (MAP) kinase inhibitor SB220025 displayed a strong inhibitory effect on SKALP expression while cyclosporin A, dexamethasone, the vitamin D(3) derivative calcipotriol and the p38 MAP kinase inhibitor SB203580 showed only moderate inhibition. Methotrexate and dimethylfumarate did not affect the expression of SKALP. With respect to CK10 expression, all-trans-retinoic acid, calcipotriol, SB203580 and SB220025 exhibited strong inhibition while dithranol showed only moderate suppression of this normal differentiation marker. Expression levels of CK10 were not significantly affected by dexamethasone, methotrexate, cyclosporin A or dimethylfumarate. This model system parallels most, but not all, findings on the in vitro effect of known antipsoriatic drugs on keratinocytes. In addition, the model identifies p38 MAP kinase inhibitors as potent suppressors of differentiation-associated gene expression. Although further delineation and validation of this model is required, we conclude that the system is amenable to down-scaling and application as a high-throughput screen for differentiation-modifying compounds.

A partial transcriptome of human epidermis

Serial analysis of gene expression (SAGE) is a powerful technique for global expression profiling without prior knowledge of the genes of interest. We carried out SAGE analysis of purified keratinocytes derived from human skin biopsy specimens, resulting in a partial transcriptome of human epidermis. We identified 7645 unique SAGE tags with quantitative information from 15,131 collected SAGE tags obtained from approximately 3 x 10(6) epidermal cells. This catalog contains a large number of genes that were not previously known to be expressed by human epidermis. Comparison with the databases of all known human SAGE tags allowed us to identify a number of keratinocyte-specific tags that putatively correspond to formerly unknown genes. Surprisingly, human epidermal keratinocytes in vivo show relatively low expression levels of genes typically associated with epidermal differentiation, whereas the expression levels of housekeeping genes are considerably higher than in cultured keratinocytes. This study provides a first step toward a transcriptome of human epidermis and, as such, harbors a wealth of information to identify genes involved in skin function, and candidate genes for genetic skin disorders.

Differential gene expression in premalignant human epidermis revealed by cluster analysis of serial analysis of gene expression (SAGE) libraries

Serial analysis of gene expression (SAGE) has been used for quantitative analysis of gene expression. We applied cluster analysis on multiple SAGE libraries derived from premalignant epidermal tissue (actinic keratosis), normal human epidermis, and cultured keratinocytes. The samples were obtained from skin biopsies without contamination by dermal tissue or blood. A total of 60,000 transcripts (tags) were analyzed. Two-way cluster analysis was applied to both the transcripts and the tissues, resulting in separation of the cultured cells from the epidermal samples, and clustering of many, presumably coregulated, genes. Two clusters of genes, strongly up-regulated in the tumor tissue compared with normal epidermis, were investigated in more detail. The differential expression of genes could be confirmed in actinic keratosis from four patients. Several of these genes have been previously associated with carcinogenesis or are likely to be important on the basis of their presumed function. Automated literature search tools show that a subgroup of these genes is coexpressed in other tissues and is part of an epidermal differentiation gene cluster on chromosome 1q21. We conclude that cluster analysis on large data sets uncovers clear partitions and correlations that could be confirmed by independent methods. We predict that these partitions will lead to biological interpretations that can be relevant for understanding the processes of carcinogenesis and tumor progression.

Cystatin M/E Expression is Restricted to Differentiated Epidermal Keratinocytes and Sweat Glands: a New Skin-Specific Proteinase Inhibitor that is a Target for Cross-Linking by Transglutaminase

Using serial analysis of gene expression on cultured human keratinocytes we found high expression levels of genes putatively involved in host protection and defense, such as proteinase inhibitors and antimicrobial proteins. One of these expressed genes was the recently discovered cysteine proteinase inhibitor cystatin M/E that has not been characterized so far at the protein level with respect to tissue distribution and additional biologic properties. Here we report that cystatin M/E has a tissue-specific expression pattern in which high expression levels are restricted to the stratum granulosum of normal human skin, the stratum granulosum/spinosum of psoriatic skin, and the secretory coils of eccrine sweat glands. Low expression levels were found in the nasal cavity. The presence of cystatin M/E in skin and the lack of expression in a variety of other tissues was verified both at the protein level by immunohistochemistry or western blotting, and at the mRNA level by reverse transcriptase polymerase chain reaction or northern blotting. Using biotinylated hexapeptide probes we found that cystatin M/E is an efficient substrate for tissue type transglutaminase and for transglutaminases extracted from stratum corneum, and that it acts as an acyl acceptor but not as an acyl donor. Western blot analysis showed that recombinant cystatin M/E could be cross-linked to a variety of proteins extracted from stratum corneum. In vitro, we found that cystatin M/E expression in cultured keratinocytes is upregulated at the mRNA and protein level, upon induction of differentiation. We demonstrate that cystatin M/E, which has a putative signal peptide, is indeed a secreted protein and is found in vitro in culture supernatant and in vivo in human sweat by enzyme-linked immunosorbent assay or western blotting. Cystatin M/E showed moderate inhibition of cathepsin B but was not active against cathepsin C. We speculate that cystatin M/E is unlikely to be a physiologically relevant inhibitor of intracellular lysosomal cysteine proteinases but rather functions as an inhibitor of self and nonself cysteine proteinases that remain to be identified.

In situ demonstration of phosphorylated c-jun and p38 MAP kinase in epidermal keratinocytes following ultraviolet B irradiation of human skin

Ultraviolet B (UVB) irradiation is known to induce activation of cellular stress response pathways in cultured cells or intact human skin, as demonstrated by phosphorylation of MAP kinase family members and up- or down-stream targets, using biochemical assays. This study demonstrates by immunohistochemistry that low-dose UVB irradiation of normal human skin induces rapid and reversible phosphorylation of c-jun (a target of c-jun N-terminal kinase) and p38 mitogen activated protein kinase (p38 MAP kinase). Phosphorylation was maximal at 4-8 h and returned to normal levels at 48 h after irradiation. Nuclear localization of these phosphorylated substrates was found using antisera against the epitope containing the phosphorylated serine-73 of c-jun, and the dually phosphorylated epitope (threonine-180 and tyrosine-182) of p38 MAP kinase. Nearly all epidermal cells were positive for c-jun phosphorylation, whereas p38 phosphorylation was seen predominantly in the differentiated layers. In contrast to the massive activation of c-jun and p38, only a small population of the suprabasal cells showed nuclear translocation of nuclear factor kappa B (NFkappaB), and a few scattered cells became apoptotic, as determined by TUNEL (TdT mediated dUTP nick end labelling) staining. The expression of involucrin and skin-derived anti-leukoproteinase (SKALP)/elafin, two genes putatively under control of the c-jun and p38 pathways, was found to be increased. These findings establish the first cellular localization of UVB-induced protein phosphorylation of stress response proteins in human epidermis, thereby providing a link between cellular activation and gene expression in defined cell populations.

Serial analysis of gene expression in differentiated cultures of human epidermal keratinocytes

Keratinocyte gene expression was surveyed more comprehensively than before, by means of serial analysis of gene expression. A total of 25,694 tags derived from expressed mRNA, were analyzed in a model for normal differentiation and in a model where cultured keratinocytes were stimulated for a prolonged period of time with tumor necrosis factor-alpha, thus mimicking aberrant differentiation in the context of cutaneous inflammation. Serial analysis of gene expression revealed many transcripts derived from unknown genes and a large number of genes that are not known to be expressed in keratinocytes; furthermore, these data provide quantitative information about the relative abundance of transcripts, allowing the identification of differentially expressed genes. A major part of the identified transcripts accounted for genes involved in energy metabolism and protein synthesis. A large proportion of all transcripts (6%) corresponded to genes associated with terminal differentiation and barrier formation. Another highly expressed functional group of genes (2% of all transcripts) corresponded to proteins involved in host protection such as antimicrobial proteins and proteinase inhibitors. Three of these genes were not known to be expressed in keratinocytes, and some were upregulated after prolonged tumor necrosis factor-alpha exposure. Our data on expressed genes in keratinocytes are consistent with the known function of human epidermis, and provide a first step to generate a transcriptome of human keratinocytes.

Induction of SLPI (ALP/HUSI-I) in epidermal keratinocytes

Secretory leukocyte protease inhibitor (SLPI) is a small, cationic protein that is known to be constitutively expressed by several glandular epithelia. SLPI inhibits leukocyte-derived proteinases, has anti-HIV-1, antibacterial, and anti-fungal properties, and interferes with the induction of synthesis of proinflammatory mediators in monocytes and macrophages. We now report that at both the mRNA and the protein level, SLPI shows inducible expression in a nonglandular epithelium. A weak expression of SLPI was found in the stratum granulosum of adult normal human epidermis; however, in lesional psoriatic epidermis and in migrating keratinocytes of healing wounds, a strong cytoplasmic staining was seen in the suprabasal keratinocytes. Remarkably, in the dermis adjacent to SLPI-expressing keratinocytes, SLPI was found extracellularly associated with elastin fibers, whereas the dermis in normal skin was negative. In cell culture, SLPI was hardly expressed in monolayers of proliferating keratinocytes. Differentiating cultures with a phenotype of normal skin expressed low levels of SLPI, whereas cultures with a regenerative/psoriatic phenotype expressed high levels. Functional studies with recombinant SLPI indicated that its antibacterial spectrum and potency are distinct from other anti-microbial peptides such as lysozyme and defensins. In view of the multiple functions of SLPI and the inducibility, we propose that it acts as an important first line defence mechanism in cutaneous injury.

Up-regulation of glucosylceramide synthase expression and activity during human keratinocyte differentiation

During keratinocyte differentiation, the glycolipid, glucosylceramide (GlcCer), is thought to be synthesized, stored in intracellular lamellar granules and eventually extruded into the intercellular space where GlcCer is hydrolyzed to ceramide, a major component of the epidermal permeability barrier. Previous studies showed that GlcCer synthase (GCS) activity increases during keratinocyte differentiation; however, the mechanism by which GCS activity is regulated was not established. In the present study, we prepared anti-peptide antibodies and amplified cDNA probes based on the cDNA sequence for human GCS (Ichikawa, S., Sakiyama, H., Suzuki, G., Hidari, K. I.-P. J., and Hirabayashi, Y. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 4638-4643) in order to study GCS expression during keratinocyte differentiation. Confluent human keratinocytes in culture were induced to terminally differentiate by elevation of Ca+2 in the medium without exogenous hormones or growth factors. GlcCer synthesis assayed in situ using a fluorescent ceramide analog increased approximately 5-fold during keratinocyte differentiation, peaking at day 6. Fluorescence microscopy studies of living keratinocytes showed that fluorescent ceramide and/or its metabolites accumulated in the Golgi in undifferentiated cells but targeted to unique vesicular structures that may be derived from the trans-Golgi region. Expression of both GCS mRNA, a approximately 3. 8-kilobase transcript on Northern blots, and GCS protein, a approximately 38-kDa polypeptide detected by Western blotting, increased dramatically (approximately 5-fold) during differentiation, reaching a maximum at about day 8. These results suggest that GCS is up-regulated at the transcriptional level during keratinocyte differentiation and provide the first direct evidence for GCS up-regulation in any cell type.

Differential effects of detergents on keratinocyte gene expression

We have studied the effect of various detergents on keratinocyte gene expression in vitro, using an anionic detergent (sodium dodecyl sulfate), a cationic detergent cetyltrimethylammoniumbromide (CTAB), and two nonionic detergents, Nonidet P-40 and Tween-20. We measured the effect of these detergents on direct cellular toxicity (lactate dehydrogenase release), on the expression of markers for normal differentiation (cytokeratin 1 and involucrin expression), and on disturbed keratinocyte differentiation (SKALP) by northern blot analysis. As reported in other studies, large differences were noted in direct cellular toxicity. In a culture model that mimics normal epidermal differentiation we found that low concentrations of sodium dodecyl sulfate could induce the expression of SKALP, a proteinase inhibitor that is not normally expressed in human epidermis but is found in hyperproliferative skin. Sodium dodecyl sulfate caused upregulation of involucrin and downregulation of cytokeratin 1 expression, which is associated with the hyperproliferative/inflammatory epidermal phenotype found in psoriasis, wound healing, and skin irritation. These changes were not induced after treatment of cultures with CTAB, Triton X-100, and Nonidet-P40. This effect appeared to be specific for the class of anionic detergents because sodium dodecyl benzene sulfonate and sodium laurate also induced SKALP expression. These in vitro findings showed only a partial correlation with the potential of different detergents to induce clinical, biophysical, and cell biologic changes in vivo in human skin. Both sodium dodecyl sulfate and CTAB were found to cause induction and upregulation of SKALP and involucrin at low doses following a 24 h patch test, whereas high concentrations of Triton X-100 did not. Sodium dodecyl sulfate induced higher rates of transepidermal water loss, whereas CTAB treated skin showed more signs of cellular toxicity. We conclude that the action of anionic detergents on epidermal keratinocytes is qualitatively different from the other detergents tested, which might have implications for in vitro toxicology studies that use cell biologic parameters as a read-out. We would hypothesize that detergents cause skin injury by several mechanisms that include direct cellular toxicity, disruption of barrier function, and detergent specific effects on cellular differentiation, as demonstrated here for sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, and sodium laurate.