The proximal promoter of the mouse loricrin gene contains a functional AP-1 element and directs keratinocyte-specific but not differentiation-specific expression

Molecular evidence sterile tissue damage during pathogenesis of pododermatitis aseptica hemorrhagica circumscripta is associated with disturbed epidermal-dermal homeostasis

Podermatitis aseptica hemorrhagica circumscripta is associated with metalloproteinase 2 weakening of distal phalangeal suspensory structures and sinkage of the distal phalanx in the claw capsule. Pressure from the tuberculum flexorium on the sole epidermis and dermis produces hemorrhagic tissue injury and defective horn production appearing as yellow-red, softened claw horn in region 4 of the sole. A model of the MAPK/ERK signal cascade orchestrating epidermal-dermal homeostasis was employed to determine if sterile inflammatory responses are linked to disturbed signal transduction for epidermal homeostasis in sole epidermis and dermis. The objective was to assess shifts in target genes of inflammation, up- and downstream MAPK/ERK signal elements, and targeted genes supporting epidermal proliferation and differentiation. Sole epidermis and dermis was removed from lateral claws bearing lesions of podermatitis aseptica hemorrhagica circumscripta, medial claws from the same limb and lateral claws from completely normal limbs of multiparous, lactating Holstein cows. The abundance levels of targeted transcripts were evaluated by real-time QPCR. Lesion effects were assessed by ANOVA, and mean comparisons were performed with t-tests to assess variations between mean expression in ulcer-bearing or medial claw dermis and epidermis and completely normal lateral claw dermis and epidermis or between ulcer-bearing dermis and epidermis and medial claw dermis and epidermis. The lesions were sterile and showed losses across multiple growth factors, their receptors, several downstream AP1 transcription components, CMYC, multiple cell cycle and terminal differentiation elements conducted by MAPK/ERK signals and β 4, α 6 and collagen 17A hemidesmosome components. These losses coincided with increased cytokeratin 6, β 1 integrin, proinflammatory metalloproteinases 2 and 9, IL1B and physiologic inhibitors of IL1B, the decoy receptor and receptor antagonist. Medial claw epidermis and dermis from limbs with lateral claws bearing podermatitis aseptica hemorrhagica circumscripta showed reductions in upstream MAPK/ERK signal elements and downstream targets that paralleled those in hemorrhagic lesions. Inhibitors of IL1B increased in the absence of real increases in inflammatory targets in the medial claw dermis and epidermis. Losses across multiple signal path elements and downstream targets were associated with negative effects on targeted transcripts supporting claw horn production and wound repair across lesion-bearing lateral claws and lesion-free medial claw dermis and epidermis. It was unclear if the sterile inflammation was causative or a consequence of these perturbations.

Impact of systemic alitretinoin treatment on skin barrier gene and protein expression in patients with chronic hand eczema

BACKGROUND

Chronic hand eczema (CHE) is a common inflammatory skin disease that affects approximately 10% of the population. Systemic alitretinoin has been shown to be effective in patients with CHE who are refractory to topical corticosteroids.

OBJECTIVES

To analyse the impact of alitretinoin on the skin barrier genes and protein expression in the skin lesions of patients with CHE.

MATERIALS AND METHODS

Fifteen patients with CHE were treated with 30 mg daily of alitretinoin for up to 27 weeks. Disease severity was assessed using a clinical score. Skin biopsies from all the patients were evaluated before and after therapy for the expression of Ki-67, various skin barrier genes and thymic stromal lymphopoietin (TSLP) by real-time quantitative polymerase chain reaction and immunohistochemistry.

RESULTS

After alitretinoin application, an improvement in the clinical severity of CHE was observed in the majority of patients. Analysis of skin biopsies before treatment showed a significant increase in Ki-67-positive cells in the suprabasal layer and a dysregulated expression of various skin barrier genes, such as claudin 1, loricrin, filaggrin and cytokeratin 10, which were normalized after treatment. TSLP was significantly upregulated in patients with CHE and also normalized after alitretinoin treatment and negatively correlated with filaggrin.

CONCLUSIONS

Our data indicate that the expression of barrier genes and proteins was normalized following treatment with alitretinoin in patients with CHE. The change in expression levels of these genes correlated with the clinical efficacy, suggesting that alitretinoin exhibits a disease-modifying activity. TSLP is upregulated in CHE and seems to counteract filaggrin expression in the skin.

The E3 ligase Itch knockout mice show hyperproliferation and wound healing alteration

The HECT-type E3 ubiquitin ligase Itch is absent in the non-agouti-lethal 18H or Itchy mice, which develop a severe immunological disease. Several of the known Itch substrates are relevant for epidermal development and homeostasis, such as p63, Notch, c-Jun and JunB. By analysing Itchy mice before the onset of immunological alterations, we investigated the contribution of Itch in skin development and wound healing. Itchy newborn mice manifested hyperplastic epidermis, which is not present in adulthood. Itch(-/-) cultured keratinocytes showed overexpression of proliferating markers and increased capability to proliferate, migrate and to repair a scratch injury in vitro. These data correlated with improved in vivo wound healing in Itchy mice, at late time points of the repair process when Itch is physiologically upregulated. Despite healing acceleration, epidermal remodelling was delayed in the scars of Itch(-/-) mice, as indicated by enhanced epidermal thickening, keratinocyte proliferation and keratin 6 expression, and retarded keratin 14 polarization to the basal layer. Itch(-/-) keratinocyte prolonged activation was not associated with increased immune cell persistence in the scars. Our in vitro and in vivo results indicate that Itch plays a role in epidermal homeostasis and remodelling and this feature does not seem to depend on immunological alterations.

Unique Keratinization Process in Psoriasis: Late Differentiation Markers Are Abolished Because of the Premature Cell Death

The keratinization process in psoriasis is a unique phenomenon. We have proposed an organized system for keratinization in psoriasis based on the recognition of early and late differentiation markers combined with premature cell death. The early differentiation markers, such as involucrin, small proline-rich proteins (SPRR), cystatin A and transglutaminase l, are more conspicuously expressed in psoriasis, while the late differentiation markers, such as profilaggrin and loricrin, are abolished. Keratinization markers that are not observed in the normal epidermis are also detected; these include SKALP/elafin as well as K6 and K16. With a markedly diminished turnover time, the psoriatic epidermis rapidly synthesizes differentiation markers that are mostly under the control of the protein kinase C-AP1 transcriptional control system. Because of the premature cell death, however, the late differentiation markers are not expressed. During the improvement of the lesion and the therefore longer turnover time, the late differentiation markers rapidly catch up to reveal their expression. This explains the rapid appearance of keratohyalin granules (profilaggrin) in the healing lesion of psoriasis. Thus the keratinization process in psoriasis can be explained by the accelerated keratinization combined with premature cell death. The keratinization process in psoriasis is unique, because both accelerated keratinization and premature cell death co-exist, resulting in the disappearance of late differentiation markers such as profilaggrin and loricrin. It is interesting to note that the premature cell death is also under the control of protein kinase C signaling.

Amniotic fluid activates the nrf2/keap1 pathway to repair an epidermal barrier defect in utero

The loss of loricrin, a major component of the cornified envelope, results in a delay of epidermal barrier formation. Therefore, the living layers of the epidermis are aberrantly exposed to late-stage amniotic fluid, which may serve as the signal to upregulate genes that functionally compensate for the loss of loricrin. Consistent with this hypothesis, metabolomic studies revealed marked changes in amniotic fluid between E14.5 and E16.5 days postcoitum. In addition, we discovered that the Nrf2/Keap1 pathway detects these compositional changes and directly upregulates the expression of genes involved in the compensatory response, thus ensuring postnatal survival. In support of this finding, we demonstrate that genetically blocking the Nrf2 pathway abolishes the compensatory response and that preemptively activating Nrf2 pharmacologically rescues the delay in barrier formation in utero. Our findings reveal that the functions of Nrf2 and the composition of amniotic fluid have coevolved to ensure the formation of a functional barrier.

Genetic mouse models for skin research: strategies and resources

A number of features contributed to establishing the mouse as the favorite model organism for skin research: the genetic and pathophysiological similarities to humans, the small size and relatively short reproductive period, meaning low maintenance costs, and the availability of sophisticated tools for manipulating the genome, gametes, and embryos. While initial studies depended on strains displaying skin abnormalities due to spontaneous genetic mutations, the availability of the transgenic and knockout technologies and their astonishing perfection during the last decades allowed the development of mouse lines permitting any imaginable genetic modification including gene inactivation, substitution, modification, or overexpression. While these technologies have already contributed to the functional analysis of several genes and processes related to skin research, continued progress requires understanding, awareness, and access to these mouse resources. This review will identify the strategies currently employed for the genetic manipulation of mice in skin research, and outline current resources and their limitations.

The human epidermal differentiation complex: cornified envelope precursors, S100 proteins and the 'fused genes' family

  The skin is essential for survival and protects our body against biological attacks, physical stress, chemical injury, water loss, ultraviolet radiation and immunological impairment. The epidermal barrier constitutes the primordial frontline of this defense established during terminal differentiation. During this complex process proliferating basal keratinocytes become suprabasally mitotically inactive and move through four epidermal layers (basal, spinous, granular and layer, stratum corneum) constantly adapting to the needs of the respective cell layer. As a result, squamous keratinocytes contain polymerized keratin intermediate filament bundles and a water-retaining matrix surrounded by the cross-linked cornified cell envelope (CE) with ceramide lipids attached on the outer surface. These cells are concomitantly insulated by intercellular lipid lamellae and hold together by corneodesmosmes. Many proteins essential for epidermal differentiation are encoded by genes clustered on chromosomal human region 1q21. These genes constitute the 'epidermal differentiation complex' (EDC), which is divided on the basis of common gene and protein structures, in three gene families: (i) CE precursors, (ii) S100A and (iii) S100 fused genes. EDC protein expression is regulated in a gene and tissue-specific manner by a pool of transcription factors. Among them, Klf4, Grhl3 and Arnt are essential, and their deletion in mice is lethal. The importance of the EDC is further reflected by human diseases: FLG mutations are the strongest risk factor for atopic dermatitis (AD) and for AD-associated asthma, and faulty CE formation caused by TG1 deficiency causes life-threatening lamellar ichthyosis. Here, we review the EDC genes and the progress in this field.

Activation of vascular endothelial growth factor receptor 2 in a cellular model of loricrin keratoderma

Loricrin is a major constituent of the epidermal cornified cell envelope. Recently, heterozygous loricrin gene mutations have been identified in two dominantly inherited skin diseases, Vohwinkel syndrome with ichthyosis and progressive symmetric erythrokeratoderma, collectively termed loricrin keratoderma. We generated stable HaCaT cell lines that express wild-type (WT) loricrin and a mutant form found in Vohwinkel syndrome with ichthyosis, using an ecdysone-inducible promoter system. The cells expressing the mutant loricrin grew more rapidly than those expressing WT loricrin after induction for 5 days. Confocal immunofluorescence microscopy revealed that phospho-Akt occurred in the nucleolus where the mutant loricrin was also located. The level of activity of Akt kinase was about nine times higher in cells with the mutant than in those with WT loricrin. ERK1/2, the epidermal growth factor receptor, vascular endothelial growth factor (VEGF) receptor 2 and Stat3 were all phosphorylated in cells with the mutant loricrin. The docking proteins, Gab1 and c-Cbl, were also tyrosine-phosphorylated in these cells. Furthermore, chromatin immunoprecipitation assays showed that Stat3 protein bound to the VEGF promoter in cells with the mutant. Thus, this study suggests that VEGF release and the subsequent activation of VEGF receptor 2 link loricrin gene mutations to rapid cell proliferation in a cellular model of loricrin keratoderma.

Ezh2 orchestrates gene expression for the stepwise differentiation of tissue-specific stem cells

Although in vitro studies of embryonic stem cells have identified polycomb repressor complexes (PRCs) as key regulators of differentiation, it remains unclear as to how PRC-mediated mechanisms control fates of multipotent progenitors in developing tissues. Here, we show that an essential PRC component, Ezh2, is expressed in epidermal progenitors but diminishes concomitant with embryonic differentiation and with postnatal decline in proliferative activity. We show that Ezh2 controls proliferative potential of basal progenitors by repressing the Ink4A-Ink4B locus and tempers the developmental rate of differentiation by preventing premature recruitment of AP1 transcriptional activator to the structural genes that are required for epidermal differentiation. Together, our studies reveal that PRCs control epigenetic modifications temporally and spatially in tissue-restricted stem cells. They maintain their proliferative potential and globally repressing undesirable differentiation programs while selectively establishing a specific terminal differentiation program in a stepwise fashion.

Regulation of gene transcription and keratinocyte differentiation by anandamide

Anandamide (AEA) is a member of an endogenous class of lipid mediators, known as endocannabinoids, which are involved in various biological processes. In particular, AEA regulates cell growth, differentiation, and death. Accumulating evidence demonstrates that AEA controls also epidermal differentiation, one of the best characterized mechanisms of cell specialization. Indeed, the epidermis is a keratinized multistratified epithelium that functions as a barrier to protect the organism from dehydration, mechanical trauma, and microbial insults. Its function is established during embryogenesis and is maintained during the whole life span of the organism, through a complex and tightly controlled program, termed epidermal terminal differentiation (or cornification). Whereas the morphological changes that occur during cornification have been extensively studied, the molecular mechanisms that underlie this process remain poorly understood. In this chapter, we summarize current knowledge about the molecular regulation of proliferation and terminal differentiation in mammalian epidermis. In this context, we show that endocannabinoids are finely regulated by, and can interfere with, the differentiation program. In addition, we review the role of AEA in the control of cornification, and show that it occurs by maintaining a transcriptional repression of gene expression through increased DNA methylation.

The immunoglobulin-like cell adhesion molecule nectin and its associated protein afadin

Nectins are immunoglobulin-like cell adhesion molecules (CAMs) that compose a family of four members. Nectins homophilically and heterophilically interact in trans with each other to form cell-cell adhesions. In addition, they heterophilically interact in trans with other immunoglobulin-like CAMs. Nectins bind afadin, an actin filament (F-actin)-binding protein, at its cytoplasmic tail and associate with the actin cytoskeleton. Afadin additionally serves as an adaptor protein by further binding many scaffolding proteins and F-actin-binding proteins and contributes to the association of nectins with other cell-cell adhesion and intracellular signaling systems. Nectins and afadin play roles in the formation of a variety of cell-cell junctions cooperatively with, or independently of, cadherins. Cooperation between nectins and cadherins is required for the formation of cell-cell junctions; cadherins alone are not sufficient. Additionally, nectins regulate many other cellular activities (such as movement, proliferation, survival, differentiation, polarization, and the entry of viruses) in cooperation with other CAMs and cell surface membrane receptors.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Up-regulation of loricrin expression by cell adhesion molecule nectin-1 through Rap1-ERK signaling in keratinocytes

Nectin is an immunoglobulin-like cell-cell adhesion molecule, which plays essential roles in the initial step of formation of adherens junctions and tight junctions. We demonstrate here the role of nectin-1 in the epidermis using nectin-1-/- mice. Newborn nectin-1-/- pups showed shiny and slightly reddish skin; the amount of loricrin, one of the differentiation markers and also a major component of cornified cell envelopes, was markedly reduced in the epidermis of nectin-1-/- mice. The amounts of repetin and SPRRP, other components of cornified cell envelopes, were markedly elevated probably due to a compensatory mechanism to overcome the impaired expression of loricrin. However, cornified cells from nectin-1-/- mice were sensitive to mechanical stress. Moreover, Ca2+-induced activation of ERK through Rap1 and expression of loricrin were reduced in primary cultured nectin-1-/- keratinocytes; in turn, the inhibition of ERK activation reduced the amount of loricrin in wild-type keratinocytes. These results indicate that nectin-1 plays a key role in the expression of loricrin in the epidermis.

The combination of ubiquitous transcription factors AP-1 and Sp1 directs keratinocyte-specific and differentiation-specific gene expression in vitro

Previous studies of epidermal-specific gene promoters suggested that a limited set of transcription factors regulate keratinocyte-specific and differentiation-specific gene expression in the epidermis. In the present study, we investigated the functional importance of AP-1- and Sp1-binding elements in the determination of cell type-specific and differentiation-specific gene expression by transient transfection into undifferentiated and differentiated keratinocytes as well as into various non-epidermal cell lines. Synthesized short AP-1- and/or Sp1-binding elements were inserted into a minimal reporter vector, and the artificial promoter containing both AP-1 and Sp1 elements showed high levels of transcriptional activity only when transfected into differentiated keratinocytes. Promoters containing either the AP-1 or the Sp1 motif alone showed little activity in any of the cells examined. We also found that close proximity of the Sp1 and AP-1 sites is essential for transcriptional activity, suggesting that the physical interaction between Sp1 and AP-1 factors is important for functional activity. These results clearly demonstrate that the combination of ubiquitously expressed transcription factors AP-1 and Sp1 confers keratinocyte specificity and differentiation specificity on the gene expression. Our findings also provide a simple model of the mechanisms underlying regulation of cell type-specific and cell differentiation-specific gene expression by ubiquitously expressed transcription factors.

Identification of regulatory elements by gene family footprinting and in vivo analysis

Gene families of recently duplicated but subsequently diverged genes provide an unique opportunity for comparative analysis of regulatory elements. We have studied the human SPRR gene family of small proline rich proteins involved in barrier function of stratified squamous epithelia. These genes are all expressed in normal human keratinocytes, but respond differently to environmental insults. Comparisons of the functional promoter regions allows the rapid identification of both conserved and of novel regulatory elements that appeared after gene duplication. Competitive electrophoretic mobility shift assays can be used to confirm their presence. Here we show the power of gene family footprinting by the identification of two novel elements in the SPRR3 promoter, not present in SPRR1A and SPRR2A. One of these elements binds a protein similar to GAAP-1, a pro-apoptotic activator of IRF-1 and p53. In vivo analysis shows that this element functions as an inhibitor of SPRR3 transcription. The second novel element functions as an activator of promoter activity and is characterized by its A/T rich sequence. The latter interacting protein indeed binds through contacts in the minor groove, and strikingly, depends on the presence of calcium for DNA interaction.

Differentiation-specific transcriptional regulation of the ESE-2 gene by a novel keratinocyte-restricted factor

Epithelium specific Ets-2 (ESE-2), an epithelium-specific ETS-domain transcription factor, is highly expressed in differentiated keratinocytes. To understand the molecular mechanisms that govern the cell-type and differentiation-specific expression of ESE-2 in keratinocytes, we have focused our studies on the identification and characterization of its cis-regulatory elements. We first performed DNase I hypersensitive site mapping and demonstrated that the promoter region of ESE-2 is in open chromatin conformation in differentiated keratinocytes. Next, we performed transient transfection assays with several 5' serially deleted constructs containing segments of the ESE-2 promoter. These experiments have led to the identification of a short fragment that shows remarkable sequence conservation between several species and harbors most of the transcriptional activity. Interestingly, a high level of transcriptional activity was only observed when the transfected keratinocytes were induced to differentiate by increasing the calcium concentration in the cell-culture medium. To identify the factors that mediate the transcriptional activity, we analyzed this segment by mutational and electrophoretic mobility shift assays (EMSA) experiments. Our studies have identified a critical stretch of nucleotides that is important for both basal as well as calcium responsive reporter activity and that binds to a nuclear factor, keratinocyte restricted factor (KRF). KRF is a novel transcription factor that is restricted to nuclear extracts isolated from keratinocytes and that binds to unique DNA sequences, which do not resemble any known consensus binding motif for transcription factors. Our preliminary experiments shed light on the biochemical nature of KRF and set the stage for future studies in identification of KRF and testing its role in governing ESE-2 gene expression in vivo.

Ovol1 regulates the growth arrest of embryonic epidermal progenitor cells and represses c-myc transcription

Transcriptional control plays a key role in regulating epidermal proliferation and differentiation. Although ample information has been obtained on how epidermal homeostasis is controlled in adult skin, less is known about the control of proliferation/differentiation of epidermal stem/progenitor cells in the developing embryo. Ovol1, encoding a zinc finger protein homologous to Drosophila melanogaster Ovo, is expressed in embryonic epidermal progenitor cells that are transiting from proliferation to terminal differentiation. In this study, we demonstrate a function for Ovol1 in interfollicular epidermal development. In its absence, developing epidermis fails to properly restrict the proliferative potential of progenitor cells, and cultured keratinocytes fail to efficiently undergo growth arrest in response to extrinsic growth-inhibitory signals. We present molecular evidence that c-myc expression is up-regulated in Ovol1-deficient suprabasal cells and that Ovol1 represses c-myc transcription by directly binding to its promoter. Collectively, our findings indicate that Ovol1 is required for proliferation exit of committed epidermal progenitor cells and identify c-myc as an Ovol1 target.

Proliferation and cornification during development of the mammalian epidermis

The skin is the largest organ of the body and consists of the underlying dermis and outer epidermis. Proper embryonic development and continual renewal of the adult epidermis are essential to provide an impenetrable barrier against fluid loss and serve as our most important defense against insults from the external environment. During mammalian embryogenesis the epidermis develops from the surface ectoderm, which initially consists of a multipotent single-layer epithelium. Once these epithelial cells receive the appropriate developmental cues, they become committed to stratify, initiate a massive expansion program, and finally embark on a journey of terminal differentiation to produce the morphologically distinct layers of the epidermis. The culmination of this journey is the formation of an impervious cornified envelope via a highly specialized form of programmed cell death, termed "cornification" (reviewed in Candi et al.), which is distinct in many ways from the classic apoptotic pathways. The epidermal developmental program that is first seen in the fetus is recapitulated for the entire life of the organism. The basal layer of adult skin harbors stem cells, which can divide to produce daughter stem cells and transit amplifying (TA) cells that go on to differentiate and cornify (reviewed in Fuchs and Raghavan). In this review we summarize current knowledge about the molecular regulation of proliferation and cornification in the developing mammalian epidermis. We focus on events in the interfollicular epidermis, with special emphasis on transcriptional regulation by p63, Notch, NF-kappaB/IKK, Hox, AP-1, AP-2, and C/EBP factors. We end with a discussion about perturbations in epidermal proliferation and cornification as they pertain to human skin pathologies.

Targeted deletion of the sciellin gene resulted in normal development and maturation

Sciellin, together with other precursor proteins, was cross-linked by transglutaminase 1 to form the cornified envelope, an essential component of the physical barrier of the epidermis and stratified squamous epithelia. To more fully understand the function of sciellin in cornified envelope formation, we generated sciellin null mice. The mice appeared normal in their development and maturation and there were no structural features that distinguished them from littermate controls. Isolated cornified envelopes appeared normal in structure and were not more fragile to mechanical stress. There was no evidence of decreased barrier function or altered expression of other cornified envelope components. Transgenic mice expressing the repeat domain appeared to have a normal phenotype, like the null, and did not alter endogenous sciellin expression. We conclude that sciellin null mice had no structural anomalies and the transgenic mice did not act as a dominant-negative mutation.

Cornulin, a New Member of the “Fused Gene” Family, Is Expressed During Epidermal Differentiation

The protein encoded by the C1orf10 gene was described to be esophageal-specific and a marker for cancer development. This protein, however, has the previously unreported structural features of the "fused gene" family combining sequences and structural similarities of both the S100 proteins and precursor proteins of the cornified cell envelope as in profilaggrin, trichohyalin, and repetin. Since all members of this family are expressed in keratinocytes, we suspected a role in epidermal differentiation and named the protein cornulin. Here, we report that human cornulin mRNA is expressed primarily in the upper layers of differentiated squamous tissues including the epidermis. Using polyclonal peptide antibodies, we show that cornulin is expressed in the granular and lower cornified cell layers of scalp epidermis and foreskin, as well as in calcium-induced differentiated cultured keratinocytes. Ca(2+)-overlay assay indicated that EF-hand domains of cornulin are functional and bind calcium. In HeLa cells, cornulin, co-transfected with transglutaminase 1, was diffusely distributed throughout the cytoplasm in contrast to small proline-rich 4, which localized to the cell periphery. We conclude that cornulin is a new member of the "fused gene" family, does not appear to be a precursor of the cornified cell envelope by itself, and is a marker of late epidermal differentiation.

Transglutaminase function in epidermis

Surface epithelial cells, such as the epidermal keratinocyte, undergo a process of terminal cell differentiation that results in the construction of a multilayered epithelium. This epithelium functions to protect the organism from the environment. Transglutaminases, enzymes that catalyze the formation of isopeptide protein-protein cross-links, are key enzymes involved in the construction of this structure. This brief review will focus on the role of these enzymes in constructing the epidermal surface.

Fos and Jun Proteins Are Specifically Expressed During Differentiation of Human Keratinocytes

Activator protein 1 (AP-1) proteins play key roles in the regulation of cell proliferation and differentiation. In this study we investigated the expression of Fos and Jun proteins in different models of terminal differentiation of human keratinocytes and in skin from psoriasis patients. All Jun and Fos proteins, with the exception of FosB, were efficiently expressed in keratinocytes in monolayer cultures. In contrast, in normal epidermis as well as in organotypic epidermal cultures, the expression pattern of AP-1 proteins was dependent on the differentiation stage. Fos proteins were readily detected in nuclei of keratinocytes of basal and suprabasal layers. JunB and JunD were expressed in all layers of normal epidermis. Interestingly, expression of c-Jun started suprabasally, then disappeared and became detectable again in distinct cells of the outermost granular layer directly at the transition zone to the stratum corneum. In psoriatic epidermis, c-Jun expression was prominent in both hyperproliferating basal and suprabasal keratinocytes, whereas c-Fos expression was unchanged. These data indicate that AP-1 proteins are expressed in a highly specific manner during terminal differentiation of keratinocytes and that the enhanced expression of c-Jun in basal and suprabasal keratinocytes might contribute to the pathogenesis of psoriasis.

Activator protein 1 DNA binding activity is decreased in lesional psoriatic skin compared with nonlesional psoriatic skin

BACKGROUND

Psoriasis is a common benign skin disease characterized by hyperproliferation and abnormal differentiation of keratinocytes. The transcription factor activator protein 1 (AP-1) is known to play an important role in cell proliferation and differentiation.

OBJECTIVES

To investigate AP-1 DNA binding activity in psoriatic skin.

METHODS

Keratome biopsies were taken from patients with plaque-type psoriasis. Electrophoretic mobility shift assays were used to determine the AP-1 DNA binding activity, whereas Western and Northern blotting was used to determine Jun and Fos protein and mRNA expression.

RESULTS

We found that AP-1 DNA binding activity was almost completely abolished in lesional psoriatic skin compared with nonlesional psoriatic skin. Furthermore, experiments revealed that the protein and mRNA expression of the AP-1 subunits c-Fos, Fra-1 and c-Jun was reduced in lesional psoriatic skin compared with nonlesional psoriatic skin, whereas the protein and mRNA expression of the subunit JunB was increased. Topical application of the vitamin D analogue calcipotriol under occlusion to involved psoriatic skin for 4 days resulted in an increase in AP-1 DNA binding activity, and an increase in the protein and mRNA expression of c-Fos, Fra-1 and c-Jun, together with a decrease in JunB protein and mRNA expression.

CONCLUSIONS

Together, these results suggest that the activity of the transcription factor AP-1 is impaired in lesional psoriatic skin and that this impairment may be important for the disturbed epidermal growth observed in psoriasis.

Yin-Yang 1 Negatively Regulates the Differentiation-Specific Transcription of Mouse Loricrin Gene in Undifferentiated Keratinocytes

Loricrin is a major component of the epidermal cornified cell envelope, and is expressed only in terminally differentiated keratinocytes. This cell differentiation-specific expression pattern suggests specific suppression of loricrin gene expression in undifferentiated keratinocytes as well as its activation in differentiated keratinocytes. We identified a negative regulatory sequence element in the first intron of the mouse loricrin gene involved in suppression of loricrin gene expression in undifferentiated keratinocytes. A database search indicated that this sequence contained the putative inverted Yin-Yang 1 (YY1)-binding motif. Constructs with point mutations in the putative YY1-binding motif showed increased reporter activity, indicating that YY1 negatively regulates loricrin gene transcription. Co-transfection experiments using a YY1 expression vector revealed that YY1 represses loricrin promoter activity. Western blotting and immunohistochemical analyses indicated that YY1 is more abundant in undifferentiated than in differentiated keratinocytes. These findings suggest that YY1 contributes to specific loricrin gene expression in differentiated keratinocytes by suppression of its transcription in undifferentiated keratinocytes. Furthermore, we demonstrated that forced expression of YY1 in differentiated keratinocytes results in specific downregulation of expression of other early and late differentiation markers.

The Effect of LXR Activators on AP-1 Proteins in Keratinocytes

Oxysterols, via activation of liver X receptor (LXR), regulate keratinocyte differentiation by stimulating transglutaminase cross-linking of several constituent proteins leading to the formation of the cornified envelope. We previously reported that oxysterols increase the expression of one of these cross-linked proteins, involucrin, and that this effect can be abolished by mutations of the distal activator protein (AP)-1 response element in the involucrin promoter. Furthermore, oxysterols increase AP-1 binding in an electrophoretic gel mobility shift assay and increase the expression of an AP-1 reporter. In this study, we describe the individual components of the AP-1 complex that are involved in the oxysterol-mediated AP-1 activation and stimulation of keratinocyte differentiation. We identified Fra-1 within the AP-1 DNA binding complex by supershift analysis of nuclear extracts from oxysterol-treated, cultured keratinocytes and confirmed that oxysterol treatment increased the levels of Fra-1 by western blot analysis. Additionally, on Western and Northern analysis, oxysterol treatment increased two other AP-1 proteins, Jun-D and c-Fos, whereas Fra-2, Jun-B, and c-Jun were not changed. Similar alterations in AP-1 proteins occurred when 25-OH-cholesterol or non-steroidal LXR agonists (GW3965, TO-901317) were used. These results indicate that oxysterols induce specific AP-1 proteins, thereby activating involucrin, one of the genes required for epidermal differentiation.

Ultraviolet Irradiation Represses PATCHED Gene Transcription in Human Epidermal Keratinocytes through an Activator Protein-1-Dependent Process

Basal cell carcinoma (BCC) is one of the major types of skin cancer arising from keratinocytes. The SONIC HEDGEHOG pathway is deregulated in 100% of sporadic BCCs, as indicated by the overexpression of PATCHED, whose product encodes the receptor of SONIC HEDGEHOG, in 100% of analyzed BCCs. Reverse transcription-PCR analysis revealed that exposure to UVB irradiation, which is a risk factor known to contribute to BCC development, induces a strong and sharp decrease of PATCHED mRNA level both in vitro and ex vivo. Transcription of a reporter gene driven by the 4.4-kb 5'-regulatory region of the human PATCHED gene was shown to be down-regulated after UVB irradiation. Furthermore, overexpression of c-JUN, a member of the activator protein (AP)-1 family, induced repression of the PATCHED promoter. The role of AP-1 in UVB-induced PATCHED repression was confirmed in mouse embryonic fibroblasts knocked out for c-JUN NH(2)-terminal protein kinase. This study thus provides the first evidence of UV-induced down-regulation at the transcriptional level of the BCC-associated tumor suppressor PATCHED relying on activation of the AP-1 oncogenic pathway.

Regulation of MAPK Activation, AP-1 Transcription Factor Expression and Keratinocyte Differentiation in Wounded Fetal Skin

Fetal epithelium retains the ability to re-epithelialize a wound in organotypic culture in a manner not dependent on the presence of underlying dermal substrata. This capacity is lost late in the third trimester of gestation or after embryonic day 17 (E(17)) in the rat such that embryonic day 19 (E(19)) wounds do not re-epithelialize. Moreover, wounds created in E(17) fetuses in utero heal in a regenerative, scar-free fashion. To investigate the molecular events regulating re-epithelialization in fetal skin, the wound-induced expression profile and tissue localization of activator protein 1 (AP-1) transcription factors c-Fos and c-Jun was characterised in E(17) and E(19) skin using organotypic fetal cultures. The involvement of mitogen-activated protein kinase (MAPK) signaling in mediating wound-induced transcription factor expression and wound re-epithelialization was assessed, with the effect of wounding on the expression of keratinocyte differentiation markers determined. Our results show that expression of AP-1 transcription factors was induced immediately by wounding and localized predominantly to the epidermis in E(17) and E(19) skin. c-fos and c-jun induction was transient in E(17) skin with MAPK-dependent c-fos expression necessary for the re-epithelialization of an excisional wound in organotypic culture. In E(19) skin, AP-1 expression persisted beyond 12 h post-wounding, and marked upregulation of the keratinocyte differentiation markers keratin 10 and loricrin was observed. No such changes in the expression of keratin 10 or loricrin occurred in E(17) skin. These findings indicate that re-epithelialization in fetal skin is regulated by wound-induced AP-1 transcription factor expression via MAPK and the differentiation status of keratinocytes.

1alpha,25-dihydroxyvitamin D3 stimulates activator protein 1 DNA-binding activity by a phosphatidylinositol 3-kinase/Ras/MEK/extracellular signal regulated kinase 1/2 and c-Jun N-terminal kinase 1-dependent increase in c-Fos, Fra1, and c-Jun expression in human keratinocytes

1alpha,25-Dihydroxyvitamin D3 added to human keratinocytes increases differentiation through an activation of the transcription factor activator protein 1. We have previously reported that the 1alpha,25-dihydroxyvitamin D3-induced increase of activator protein 1 DNA binding activity is mediated by a protein kinase C-independent mechanism. The purpose of this study was to investigate further the mechanisms by which 1alpha,25-dihydroxyvitamin D3 modulates activator protein 1 DNA binding activity in cultured normal human keratinocytes. Western blotting experiments revealed that 1alpha,25-dihydroxyvitamin D3 caused a rapid and transient activation of the mitogen-activated protein kinases, extracellular signal regulated kinase 1/2 and c-Jun N-terminal kinase 1. 1alpha,25-Dihydroxyvitamin D3 also enhanced the expression of the activator protein 1 subunits, c-Fos, Fra1, and c-Jun as determined by northern and western blotting. The 1alpha,25-dihydroxyvitamin D3-induced activator protein 1 DNA binding activity was completely blocked by the MEK inhibitor PD 98059 indicating that the MEK/extracellular signal regulated kinase pathway is involved in the activation of activator protein 1. Transfection experiments showed that 1alpha,25-dihydroxyvitamin D3 also increased the activator protein 1-dependent transactivation, which was completely blocked by expression of a dominant negative Ras, suggesting that the 1alpha,25-dihydroxyvitamin D3-induced activator protein 1 activity involves Ras-dependent signaling. Furthermore, preincubation of the keratinocytes with the specific phosphatidylinositol 3-kinase inhibitors, Wortmannin and LY294002, demonstrated that the 1alpha,25-dihydroxyvitamin D3-induced activation of extracellular signal regulated kinase 1/2 and c-Jun N-terminal kinase 1 required phosphatidylinositol 3-kinase activity. Finally, preincubation of keratinocytes with a polyclonal antibody against the membrane receptor annexin II, blocked the 1alpha,25-dihydroxyvitamin D3-induced activation of extracellular signal regulated kinase 1/2 and c-Jun N-terminal kinase 1. Taken together, our results indicate that 1alpha,25-dihydroxyvitamin D3, via binding to the membrane receptor annexin II, induces activation of the phos-phatidylinositol 3-kinase/Ras/MEK/extracellular signal regulated kinase 1/2 and c-Jun N-terminal kinase 1 signal transduction pathway resulting in increased expression of c-Fos, Fra1, and c-Jun, and subsequently increased activator protein 1 DNA binding activity and gene transcription.

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.

Loricrin expression in cultured human keratinocytes is controlled by a complex interplay between transcription factors of the Sp1, CREB, AP1, and AP2 families

The major protein component of the cornified cell envelope barrier structure of the epidermis is loricrin, and it is expressed late during terminal differentiation in epidermal keratinocytes. We have previously shown that an AP1 site located in the proximal promoter region (position -55) is essential for human loricrin promoter activity (Rossi, A., Jang, S-I., Ceci, R., Steinert, P. M., and Markova, N. G. (1998) J. Invest. Dermatol. 110, 34-40). In this study we show that its regulation requires complex cooperative and competitive interactions between multiple transcription factors in keratinocytes located in different compartments of the epidermis. We show that as few as 154 base pairs of 5'-upstream sequences from the cap site can direct the keratinocyte-specific expression in cultured keratinocytes. Mutation and DNA-protein analyses show that Sp1, c-Jun, an unidentified regulator, and the co-activator p300/CREB-binding protein up-regulate whereas Sp3, CREB-1/CREMalpha/ATF-1, Jun B, and an AP2-like protein (termed the keratinocyte-specific repressor-1 (KSR-1)) suppress loricrin promoter activity. We show that CREB protein can compete with c-Jun for the AP1 site and repress loricrin promoter activity. We show here that the protein kinase A pathway can activate loricrin expression by manipulation of the Sp1, Sp3, and KSR-1 levels in the nucleus. Thus, in undifferentiated cells, loricrin expression is suppressed by Jun B, Sp3, and KSR-1 proteins. But in advanced differentiated cells, levels of Sp3, KSR-1, and CREB proteins are lower; the unidentified regulator protein can bind; Sp1 and c-Jun are increased; and then p300/CBP is recruited. Together, these events allow loricrin transcription to proceed. Indeed, the synergistic effects of the Sp1, c-Jun, and p300 factors indicate that p300/CBP might act as bridge to form an active transcription complex.

Getting under the skin of epidermal morphogenesis

At the surface of the skin, the epidermis serves as the armour for the body. Scientists are now closer than ever to understanding how the epidermis accomplishes this extraordinary feat, and is able to survive and replenish itself under the harshest conditions that face any tissue. By combining genetic engineering with cell-biological studies and with human genome data analyses, skin biologists are discovering the mechanisms that underlie the development and differentiation of the epidermis and hair follicles of the skin. This explosion of knowledge paves the way for new discoveries into the genetic bases of human skin disorders and for developing new therapeutics.

The human involucrin gene contains spatially distinct regulatory elements that regulate expression during early versus late epidermal differentiation

Human involucrin (hINV) is a keratinocyte protein that is expressed in the suprabasal compartment of the epidermis and other stratifying surface epithelia. Involucrin gene expression is initiated early in the differentiation process and is maintained until terminal cell death. The distal regulatory region (DRR) is a segment of the hINV promoter (nucleotides -2473/-1953) that accurately recapitulates the normal pattern of suprabasal (spinous and granular layer) expression in transgenic mouse epithelia. To identify sequences that mediate expression at specific stages of differentiation, we divided the DRR into two segments, a 376 nucleotide upstream region (DRR(-2473/-2100)) and a 147 nucleotide downstream region (DRR(-2100/-1953)), and evaluated the ability of these sequences to drive expression in transgenic mice. The DRR(-2473/-2100) segment drives expression at a level comparable to that observed for the DRR, but expression is restricted to the upper granular layers (i.e., no spinous layer expression). In contrast, the DRR(-2100/-1953) segment does not drive expression. However, reassembling the DRR restores the complete range of expression. These results suggest that two distinct, spatially-separate elements are required to specify the complete differentiation-dependent program of involucrin gene expression. To identify specific transcription factor binding sites involved in this regulation, we mutated an activator protein-1 binding site, AP1-5, located within DRR(-2473/-2100) segment. This site binds AP1 transcription factors present in mouse epidermal extracts, and its mutation eliminates appropriate hINV expression. This result suggests that AP1 factors participate as components of a multi-component transcription factor complex that is required for regulation.

Transcription factor activation in response to cutaneous injury: role of AP-1 in reepithelialization

Reepithelialization is the process responsible for restoring an intact epidermis following cutaneous injury. A change in the activity of keratinocytes is required for reepithelialization to occur, and this is likely to be regulated by the altered expression of effector genes, mediated by transcription factors. The injury itself provides a stimulus for transcription factor activation either directly due to mechanical stress, or via paracrine mechanisms such as the release of growth factors from damaged cells. Members of the activator protein-1 family, in particular c-fos and c-jun, have been the most widely studied wound-induced transcription factors. The signal transduction pathways linking cellular injury to activator protein-1 stimulation appear to involve an increase in intracellular Ca2+ and activation of mitogen-activated protein kinases. Given that a number of genes involved in the reepithelialization of wounds are regulated by activator protein-1, a distinct role for this transcription factor in reepithelialization is beginning to emerge. This article reviews the evidence for activator protein-1 involvement in reepithelialization, with particular focus on the activation of this transcription factor in response to wounding, the second messenger/kinase pathways involved, and the modulation of downstream genes that have the capacity to regulate keratinocyte function.

Regulation of human profilaggrin promoter activity in cultured epithelial cells by retinoic acid and glucocorticoids

Vitamin A and other retinoids profoundly inhibit both morphological and biochemical aspects of epidermal differentiation in vitro. Profilaggrin, like most other markers of keratinocyte differentiation, is negatively regulated by retinoic acid in vitro, both at the level of mRNA synthesis and by inhibiting the activity of endoproteases that convert profilaggrin to filaggrin. Profilaggrin is an abundant component of keratohyalin granules and forms the precursor of filaggrin, the keratin associated protein of the stratum corneum. In this report, we identify a region of the human profilaggrin promoter that is involved in the transcriptional regulation of expression by retinoic acid (RA). A series of promoter deletions linked to the chloramphenicol acetyl transferase (CAT) reporter gene were prepared and analyzed by transfection into Hela cells and keratinocytes. We also cotransfected vectors expressing retinoic acid receptor and cultured the transfected cells in the presence and absence of ligand. The region responsive to retinoic acid was localized to a 53 bp sequence between -1109 and -1056 (relative to the mRNA start site at +1) that contains a cluster of five retinoic acid response elements with variable spacing and orientation. In vitro gel shift analysis demonstrated that nuclear retinoid receptors do not bind directly to the identified sequence, suggesting that the mode of regulation by RA may be indirect or that binding requires another cofactor in addition to retinoid receptors. Whereas in keratin genes retinoic acid and glucocorticoid responsive sequences frequently coincide, the glucocorticoid response element in the profilaggrin promoter was located downstream of the RARE cluster between -965 and -951. These studies demonstrate that RA and glucocorticoids regulate profilaggrin expression at least in part by transcriptional mechanisms, via a region of the promoter that contains both retinoid and glucocorticoid responsive elements.

The POU domain factor Skin-1a represses the keratin 14 promoter independent of DNA binding. A possible role for interactions between Skn-1a and CREB-binding protein/p300

The genes encoding keratin 5 and 14 are highly expressed in the basal cell layer keratinocytes of the epidermis, but both genes are silenced when keratinocytes move into the suprabasal compartment. The POU homeodomain factors Skn-1a and Tst-1, which are expressed in epidermis, may play a role in the suprabasal repression of the keratin 5 and 14 genes because keratin 14 mRNA expression persists in suprabasal cells in Skn-1/Tst-1 double knockout mice. In transfection experiments, both Skn-1a and Tst-1 repress the keratin 14 promoter, with the POU domain being sufficient for repression. The region of the keratin 14 gene sufficient and required for repression by Skn-1a is a 100-base pair sequence lacking POU-binding sites adjacent to the transcription start site. DNA-binding defective mutants of Skn-1a and Tst-1 are as effective at mediating repression as the wild type proteins, suggesting that protein-protein interactions rather than direct DNA binding are important for repression. We also show that CREB-binding protein (CBP)/p300 co-activators are strong activators of keratin 14 gene expression, acting through sequences close to the keratin 14 promoter. Further, CBP interacts directly with the POU domain of Skn-1a, and increasing concentrations of CBP can overcome Skn-1a-mediated repression, suggesting that POU domain factors may repress keratin 14 gene expression by interfering with the activity of co-activators such as CBP/p300.

Cross talk among calcineurin, Sp1/Sp3, and NFAT in control of p21(WAF1/CIP1) expression in keratinocyte differentiation

Calcium functions as a trigger for the switch between epithelial cell growth and differentiation. We report here that the calcium/calmodulin-dependent phosphatase calcineurin is involved in this process. Treatment of primary mouse keratinocytes with cyclosporin A, an inhibitor of calcineurin activity, suppresses the expression of terminal differentiation markers and of p21(WAF1/Cip1) and p27(KIP1), two cyclin-dependent kinase inhibitors that are usually induced with differentiation. In parallel with down-modulation of the endogenous genes, suppression of calcineurin function blocks induction of the promoters for the p21(WAF1/Cip1) and loricrin differentiation marker genes, whereas activity of these promoters is enhanced by calcineurin overexpression. The calcineurin- responsive region of the p21 promoter maps to a 78-bp Sp1/Sp3-binding sequence next to the TATA box, and calcineurin induces activity of the p21 promoter through Sp1/Sp3-dependent transcription. We find that the endogenous NFAT-1 and -2 transcription factors, major downstream targets of calcineurin, associate with Sp1 in keratinocytes in a calcineurin-dependent manner, and calcineurin up-regulates Sp1/Sp3-dependent transcription and p21 promoter activity in synergism with NFAT1/2. Thus, our study reveals an important role for calcineurin in control of keratinocyte differentiation and p21 expression, and points to a so-far-unsuspected interconnection among this phosphatase, NFATs, and Sp1/Sp3-dependent transcription.

Analysis of mouse keratin 6a regulatory sequences in transgenic mice reveals constitutive, tissue-specific expression by a keratin 6a minigene

The analysis of keratin 6 expression is complicated by the presence of multiple isoforms that are expressed constitutively in a number of internal stratified epithelia, in palmoplantar epidermis, and in the companion cell layer of the hair follicle. In addition, keratin 6 expression is inducible in interfollicular epidermis and the outer root sheath of the follicle, in response to wounding stimuli, phorbol esters, or retinoic acid. In order to establish the critical regions involved in the regulation of keratin 6a (the dominant isoform in mice), we generated transgenic mice with two different-sized mouse keratin 6a constructs containing either 1.3 kb or 0.12 kb of 5' flanking sequence linked to the lacZ reporter gene. Both constructs also contained the first intron and the 3' flanking sequence of mouse keratin 6a. Ectopic expression of either transgene was not observed. Double-label immunofluorescence analyses demonstrated expression of the reporter gene in keratin 6 expressing tissues, including the hair follicle, tongue, footpad, and nail bed, showing that both transgenes retained keratinocyte-specific expression. Quantitative analysis of beta-galactosidase activity verified that both the 1.3 and 0.12 kb keratin 6a promoter constructs produced similar levels of the reporter. Notably, both constructs were constitutively expressed in the outer root sheath and interfollicular epidermis in the absence of any activating stimulus, suggesting that they lack the regulatory elements that normally silence transcription in these cells. This study has revealed that a keratin 6a minigene contains critical cis elements that mediate tissue-specific expression and that the elements regulating keratin 6 induction lie distal to the 1.3 kb promoter region.

Transgenic mice expressing a mutant form of loricrin reveal the molecular basis of the skin diseases, Vohwinkel syndrome and progressive symmetric erythrokeratoderma

Mutations in the cornified cell envelope protein loricrin have been reported recently in some patients with Vohwinkel syndrome (VS) and progressive symmetric erythrokeratoderma (PSEK). To establish a causative relationship between loricrin mutations and these diseases, we have generated transgenic mice expressing a COOH-terminal truncated form of loricrin that is similar to the protein expressed in VS and PSEK patients. At birth, transgenic mice (ML.VS) exhibited erythrokeratoderma with an epidermal barrier dysfunction. 4 d after birth, high-expressing transgenic animals showed a generalized scaling of the skin, as well as a constricting band encircling the tail and, by day 7, a thickening of the footpads. Histologically, ML. VS transgenic mice also showed retention of nuclei in the stratum corneum, a characteristic feature of VS and PSEK. Immunofluorescence and immunoelectron microscopy showed the mutant loricrin protein in the nucleus and cytoplasm of epidermal keratinocytes, but did not detect the protein in the cornified cell envelope. Transfection experiments indicated that the COOH-terminal domain of the mutant loricrin contains a nuclear localization signal. To determine whether the ML.VS phenotype resulted from dominant-negative interference of the transgene with endogenous loricrin, we mated the ML.VS transgenics with loricrin knockout mice. A severe phenotype was observed in mice that lacked expression of wild-type loricrin. Since loricrin knockout mice are largely asymptomatic (Koch, P.K., P. A. de Viragh, E. Scharer, D. Bundman, M.A. Longley, J. Bickenbach, Y. Kawachi, Y. Suga, Z. Zhou, M. Huber, et al., J. Cell Biol. 151:389-400, this issue), this phenotype may be attributed to expression of the mutant form of loricrin. Thus, deposition of the mutant protein in the nucleus appears to interfere with late stages of epidermal differentiation, resulting in a VS-like phenotype.

Nitric oxide inhibits cornified envelope formation in human keratinocytes by inactivating transglutaminases and activating protein 1

Epidermal keratinocytes undergo terminal differentiation to form the stratum corneum, which consists of many layers of flat dead cells. These cells assemble an insoluble cornified envelope composed of specific proteins deposited on the intracellular surface of the cell membrane. The proteins are crosslinked by the action of transglutaminases, which catalyze the formation of isodipeptide bonds between the epsilon-NH2 side chain of a lysine residue and the gamma-amide side chain of a glutamine residue. Transglutaminases share a conserved, highly reactive cysteine in their active site. In this study, we found that nitric-oxide-releasing compounds inhibited cornified envelope formation in cultured keratinocytes and the in vitro crosslinking of loricrin, a natural substrate of transglutaminases. The NO donors inhibited transglutaminase catalytic activity in a dose-dependent manner, in both purified enzymes and keratinocyte extracts. Titration of thiol groups of transglutaminases indicated that NO regulates their enzymatic activity by chemically modifying a cysteine residue, possibly by S-nitrosylation. NO was also found to inhibit DNA-binding activity of activating protein 1 in keratinocyte nuclear extracts, and to interfere with the transactivation of activating protein 1 responsive genes such as transglutaminase 1, involucrin, and loricrin, whose expression is regulated during epidermal differentiation. In conclusion, we propose that NO may inhibit keratinocyte differentiation, acting both at transcriptional level (inactivation of activating protein 1) and at post-translational level (inhibition of transglutaminase activity).

Chemiluminescent reporter gene assays with 1,2-dioxetane enzyme substrates

1,2-Dioxetane chemiluminescent substrates provide highly sensitive, quantitative detection with simple, rapid assay formats for the detection of reporter enzymes that are widely used in gene expression studies. Chemiluminescent detection methodologies typically provide up to 100-1000x higher sensitivities than can be achieved with the corresponding fluorescent or colorimetric enzyme substrates. The varieties of 1,2-dioxetane substrates available provides assay versatility, allowing optimization of assay formats with the available instrumentation, and are ideal for use in gene expression assays performed in both biomedical and pharmaceutical research. These assays are amenable to automation with a broad range of instrumentation for high throughput compound screening.

Structure and regulation of the envoplakin gene

Envoplakin, a member of the plakin family of proteins, is a component of desmosomes and the epidermal cornified envelope. To understand how envoplakin expression is regulated, we have analyzed the structure of the mouse envoplakin gene and characterized the promoters of both the human and mouse genes. The mouse gene consists of 22 exons and maps to chromosome 11E1, syntenic to the location of the human gene on 17q25. The exon-intron structure of the mouse envoplakin gene is common to all members of the plakin family: the N-terminal protein domain is encoded by 21 small exons, and the central rod domain and the C-terminal globular domain are coded by a single large exon. The C terminus shows the highest sequence conservation between mouse and human envoplakins and between envoplakin and the other family members. The N terminus is also conserved, with sequence homology extending to Drosophila Kakapo. A region between nucleotides -101 and 288 was necessary for promoter activity in transiently transfected primary keratinocytes. This region is highly conserved between the human and mouse genes and contains at least two different positively acting elements identified by site-directed mutagenesis and electrophoretic mobility shift assays. Mutation of a GC box binding Sp1 and Sp3 proteins or a combined E box and Krüppel-like element interacting with unidentified nuclear proteins virtually abolished promoter activity. 600 base pairs of the mouse upstream sequence was sufficient to drive expression of a beta-galactosidase reporter gene in the suprabasal layers of epidermis, esophagus, and forestomach of transgenic mice. Thus, we have identified a regulatory region in the envoplakin gene that can account for the expression pattern of the endogenous protein in stratified squamous epithelia.

E2F-1 induces proliferation-specific genes and suppresses squamous differentiation-specific genes in human epidermal keratinocytes

Squamous differentiation of keratinocytes is associated with decreases in E2F-1 mRNA expression and E2F activity, and these processes are disrupted in squamous cell carcinoma cell lines. We now show that E2F-1 mRNA expression is increased in primary squamous cell carcinomas of the skin relative to normal epidermis. To explore the relationship between E2F-1 and squamous differentiation further, we examined the effect of altering E2F activity in primary human keratinocytes induced to differentiate. Promoter activity for the proliferation-associated genes, cdc2 and keratin 14, are inhibited during squamous differentiation. This inhibition can be inhibited by overexpression of E2F-1 in keratinocytes. Overexpression of E2F-1 also suppressed the expression of differentiation markers (transglutaminase type 1 and keratin 10) in differentiated keratinocytes. Blocking E2F activity by transfecting proliferating keratinocytes with dominant negative E2F-1 constructs inhibited the expression of cdc2 and E2F-1, but did not induce differentiation. Furthermore, expression of the dominant negative construct in epithelial carcinoma cell lines and normal keratinocytes decreased expression from the cdc2 promoter. These data indicate that E2F-1 promotes keratinocyte proliferation-specific marker genes and suppresses squamous differentiation-specific marker genes. Moreover, these data indicate that targeted disruption of E2F-1 activity may have therapeutic potential for the treatment of squamous carcinomas. Oncogene (2000).

Chlorocatechol detection based on a clc operon/reporter gene system

A sensitive and selective sensing system for chlorocatechols (3-chlorocatechol and 4-chlorocatechol) was developed based on Pseudomonas putida bacteria harboring the plasmid pSMM50R-B'. In this plasmid, the regulatory protein of the clc operon, ClcR, controls the expression of the reporter enzyme beta-galactosidase. When bacteria containing components of the clc operon are grown in the presence of chlorocatechols, ClcR activates the clcA promoter, which is located upstream from the beta-galactosidase gene. Thus, the concentration of chlorocatechols can be related to the production of beta-galactosidase in the bacteria. The concentration of beta-galactosidase expressed in the bacteria was determined by measuring the chemiluminescence signal emitted with the use of a 1,2-dioxetane substrate. ClcR has a high specificity for chlorocatechols and provides the sensing system with high selectivity. This was demonstrated by evaluating several structurally related organic compounds as potential interfering agents. Both 3-chlorocatechol and 4-chlorocatechol can be detected with this sensing system at concentrations as low as 8 x 10(-10) and 2 x 10(-9) M, respectively, using a 2-h induction period. In the case of 3-chlorocatechol, a highly selective sensing system was developed that can detect this species at concentrations as low as 6 x 10(-8) M after a 5-min induction period; the presence of 4-chlorocatechol at concentrations as high as 2 x 10(-4) M did not interfere with this system.

1alpha,25-dihydroxyvitamin D3 induced differentiation of cultured human keratinocytes is accompanied by a PKC-independent regulation of AP-1 DNA binding activity

1alpha,25(OH)2D3 and its analogs are potent mediators of keratinocyte differentiation in vitro. The precise mechanism of this action is still unknown. The nuclear transcription factor activator protein 1 seems to play an important role in keratinocyte differentiation. The purpose of this study was to investigate the effect of 1alpha,25(OH)2D3 on activator protein 1 DNA binding activity in cultured human keratinocytes. In a time-course study of human keratinocytes incubated with 1alpha,25(OH)2D3 (10-7-10-11 M) a significant dose-dependent increase in activator protein 1 DNA binding activity as determined by electrophoretic mobility shift assay was seen after 36 h. This increase was followed by a significant dose-dependent decrease in activator protein 1 DNA binding activity after 72 h. When differentiation was induced by raising the calcium concentration in the culture medium from 0.09 to 0.3 mM a similar increase in activator protein 1 DNA binding activity was observed after incubation for 48 h. Pharmacologic down-modulation of the protein kinase C activity with GF 109203X reversed the calcium-induced increase in activator protein 1 DNA binding activity and abolished keratinocyte differentiation as determined by a transglutaminase assay. In contrast, activator protein 1 DNA binding activity and keratinocyte differentiation were not affected when protein kinase C activity was down-modulated in the experiments with 1alpha,25(OH)2D3. The activator protein 1 complex in human keratinocytes consists of dimers of Fra-1, Fra-2, c-Jun, JunD, and c-Fos. Our results demonstrate that 1alpha, 25(OH)2D3- and calcium-induced keratinocyte differentiation are accompanied by changes in activator protein 1 DNA binding activity. Protein kinase C activation appears to be essential for the calcium-dependent induction of keratinocyte differentiation, whereas a protein-kinase-C-independent activation of activator protein 1 DNA binding and keratinocyte differentiation is responsible for the 1alpha,25(OH)2D3-induced effects.

Conditional cell transformation by doxycycline-controlled expression of the ASV17 v-jun allele

To investigate the molecular basis of oncogenesis induced by the v-jun oncogene of avian sarcoma virus 17 (ASV17), we developed a conditional cell transformation system in which transcription of the ASV17 v-jun allele is controlled by a doxycycline-sensitive transactivator (tTA) or a reverse (doxycycline-dependent) transactivator (rtTA), respectively. Permanent cell lines of quail embryo fibroblasts conditionally transformed by a doxycycline-controlled v-jun allele revert to the normal phenotype within 3 days and lose their ability to grow in soft agar, strictly dependent on the addition or removal of the drug, respectively. The reverted cells are rapidly retransformed on conditional activation of v-jun. While full-level synthesis of v-jun mRNA and v-Jun protein in these cells is established within 2 and 14 h, respectively, after switching to the permissive conditions, the first morphological alterations are observed after 24 h, and as early as 2 days later the morphology has changed entirely from flat cells resembling normal fibroblasts to spindle-shaped fusiform cells showing a typical jun-transformed phenotype. Kinetic expression analysis revealed that transcriptional activation of the direct jun target gene BKJ precisely coincides with the establishment of full-level v-Jun protein synthesis. Furthermore, we have analyzed the expression of a novel candidate v-jun target gene, termed JAC, which shows no sequence homology to known genes. Similar to BKJ, JAC is specifically activated in jun-transformed fibroblasts, and induction of JAC is tightly linked to the conditional expression of oncogenic v-Jun. These results demonstrate the high stringency of the doxycycline-controlled v-jun expression system, and they also indicate that expression of v-jun in these cells is indispensable for enhanced proliferation, cell transformation, and the induction of specific expression patterns of downstream target genes.

Defining the regulatory factors required for epidermal gene expression

Keratins K5 and K14 are the hallmarks of mitotically active keratinocytes of stratified epithelia. They are transcribed at a high level and in a tissue-specific manner, enabling us to use the K14 gene to elucidate the regulatory mechanism underlying epidermis-specific transcription. We have identified four DNase I-hypersensitive sites (HSs) present in the 5' regulatory sequences of the K14 gene under specific conditions where the gene is actively expressed. Two of these sites (HSsII and -III) are conserved in position and sequence within the human and mouse K14 genes. Using an in vivo transgenic approach and an in vitro keratinocyte culture approach, we have discovered that most of K14's transcriptional activity is restricted to a novel 700-bp regulatory domain encompassing these HSs. This enhancer is sufficient to confer epidermis-specific activity to a heterologous promoter in transfection assays in culture and in transgenic mice in vivo. A 125-bp DNA fragment encompassing HSsII harbors the majority of the transactivation activity in vitro, and electrophoretic mobility shift and mutational assays reveal a role for AP-1, ets, and AP-2 family members in orchestrating the keratinocyte-preferred expression of HSsII. The HSsII element also confers epidermal expressivity to a heterologous promoter in transgenic mice, although it is not sufficient on its own to fully restrict activity to keratinocytes. Within the HSsII element, the ets and AP-2 sites appear to be most critical in collaborating to regulate epidermal specificity in vivo.

Expression of MK6a dominant-negative and C-terminal mutant transgenes in mice has distinct phenotypic consequences in the epidermis and hair follicle

Mouse keratin 6a (MK6a) is constitutively expressed in a single cell layer of the outer root sheath (ORS) of hair follicles, but its synthesis can be induced in interfollicular epidermis including the basal cell layer in response to perturbing stimuli. A basally inducible human K6 (HK6) isoform has not been described, and it is not clear which of the known HK6 isoforms is expressed in the ORS. In this study we show that expression of a dominant-negative MK6a construct (Delta2B-P) in the interfollicular epidermis caused severe blistering and neonatal lethality, suggesting that mutations in a yet to be identified basally expressed HK6 isoform might result in a severe blistering phenotype. Surviving Delta2B-P animals showed transgene expression only in isolated epidermal cells and not in all cells of the ORS, but nevertheless developed severe alopecia. Expression of two different C-terminal mutant transgenes also caused alopecia while a third C-terminal mutant had no phenotypic conse- quences. Electron microscopy revealed that Delta2B-P expression resulted in the collapse of keratin filaments, while destruction of hair follicles in the two phenotypic C-terminal mutant lines occurred in the absence of filament abnormalities. The latter finding indicates that the innermost ORS cells are uniquely sensitive to expression of even slightly altered K6 proteins, suggesting that mutations affecting an HK6 isoform expressed in this cell layer could result in alopecia in humans as well.