Functional down-regulation of alpha 5 beta 1 integrin in keratinocytes is reversible but commitment to terminal differentiation is not

Integrin beta 1 inhibition alleviates the chronic hyperproliferative dermatitis phenotype of SHARPIN-deficient mice

SHARPIN (Shank-Associated RH Domain-Interacting Protein) is a component of the linear ubiquitin chain assembly complex (LUBAC), which enhances TNF-induced NF-κB activity. SHARPIN-deficient (Sharpincpdm/cpdm) mice display multi-organ inflammation and chronic proliferative dermatitis (cpdm) due to TNF-induced keratinocyte apoptosis. In cells, SHARPIN also inhibits integrins independently of LUBAC, but it has remained enigmatic whether elevated integrin activity levels in the dermis of Sharpincpdm/cpdm mice is due to increased integrin activity or is secondary to inflammation. In addition, the functional contribution of increased integrin activation to the Sharpincpdm/cpdm phenotype has not been investigated. Here, we find increased integrin activity in keratinocytes from Tnfr1-/- Sharpincpdm/cpdm double knockout mice, which do not display chronic inflammation or proliferative dermatitis, thus suggesting that SHARPIN indeed acts as an integrin inhibitor in vivo. In addition, we present evidence for a functional contribution of integrin activity to the Sharpincpdm/cpdm skin phenotype. Treatment with an integrin beta 1 function blocking antibody reduced epidermal hyperproliferation and epidermal thickness in Sharpincpdm/cpdm mice. Our data indicate that, while TNF-induced cell death triggers the chronic inflammation and proliferative dermatitis, absence of SHARPIN-dependent integrin inhibition exacerbates the epidermal hyperproliferation in Sharpincpdm/cpdm mice.

Vimentin is necessary for colony growth of human diploid keratinocytes

The role of vimentin (Vim) in diploid epithelial cells is not well known. To understand its biological function, we cultured human epidermal keratinocytes under conditions that support migration, proliferation, stratification and terminal differentiation. We identified a keratinocyte subpopulation that shows a p63(+)/α5β1(bright) phenotype and displays Vim intermediate filaments (IFs) besides their keratin IF network. These cells were mainly located at the proliferative/migratory rim of the growing colonies; but also, they were scarce and scattered or formed small groups of basal cells in confluent stratified epithelia. Stimulation of cells with EGF and wounding experiments in confluent arrested epithelia increased the number of Vim(+) keratinocytes in an extent higher to the expected for a cell population doubling. BrdU labeling demonstrated that most of the proliferative cells located at the migratory border of the colony have Vim, in contrast with proliferative cells located at the basal layer at the center of big colonies which lacked of Vim IFs, suggesting that Vim expression was not solely linked to proliferation. Therefore, we silenced Vim mRNA in the cultured keratinocytes and observed an inhibition of colony growth. Such results, together with long-term cultivation assays which showed that Vim might be associated to pattern formation in cultured epithelia, suggest that Vim expression is essential for a highly motile phenotype, which is necessary for keratinocyte colony growth and possibly for development and wound healing. Vim(+)/p63(+)/α5β1(bright) epithelial cells may play a significant physiological role in embryonic morphogenetic movements; wound healing and other pathologies such as carcinomas and hyperproliferative diseases.

Development of microfabricated dermal epidermal regenerative matrices to evaluate the role of cellular microenvironments on epidermal morphogenesis

Topographic features at the dermal-epidermal junction (DEJ) provide instructive cues critical for modulating keratinocyte functions and enhancing the overall architecture and organization of skin. This interdigitated interface conforms to a series of rete ridges and papillary projections on the dermis that provides three-dimensional (3D) cellular microenvironments as well as structural stability between the dermal and epidermal layers during mechanical loading. The dimensions of these cellular microenvironments exhibit regional differences on the surface of the body, and quantitative histological analyses have shown that localization of highly proliferative keratinocytes also varies, according to the regional geometries of these microenvironments. In this study, we combined photolithography, collagen processing, and biochemical conjugation techniques to create microfabricated dermal epidermal regeneration matrices (μDERMs) with features that mimic the native 3D cellular microenvironment at the DEJ. We used this model system to study the effect of the 3D cellular microenvironment on epithelialization and basal keratinocyte interaction with the microenvironment on the surface of the μDERMs. We found that features closely mimicking those in high-friction areas of the body (deep, narrow channels) epithelialized faster than features mimicking low-friction areas. Additionally, when evaluating β1 expression, an integrin involved in epidermal morphogenesis, it was found that integrin-bright expression was localized in the depths of the features, suggesting that the μDERMs may play a role in defining cellular microenvironments as well as a protective environment for the regenerative population of keratinocytes. The outcomes of this study suggest that μDERMs can serve as a robust biomimetic model system to evaluate the roles of the 3D microenvironment on enhancing the regenerative capacity and structural stability of bioengineered skin substitutes.

Development of transgenic cloned pig models of skin inflammation by DNA transposon-directed ectopic expression of human β1 and α2 integrin

Integrins constitute a superfamily of transmembrane signaling receptors that play pivotal roles in cutaneous homeostasis by modulating cell growth and differentiation as well as inflammatory responses in the skin. Subrabasal expression of integrins α2 and/or β1 entails hyperproliferation and aberrant differentiation of keratinocytes and leads to dermal and epidermal influx of activated T-cells. The anatomical and physiological similarities between porcine and human skin make the pig a suitable model for human skin diseases. In efforts to generate a porcine model of cutaneous inflammation, we employed the Sleeping Beauty DNA transposon system for production of transgenic cloned Göttingen minipigs expressing human β1 or α2 integrin under the control of a promoter specific for subrabasal keratinocytes. Using pools of transgenic donor fibroblasts, cloning by somatic cell nuclear transfer was utilized to produce reconstructed embryos that were subsequently transferred to surrogate sows. The resulting pigs were all transgenic and harbored from one to six transgene integrants. Molecular analyses on skin biopsies and cultured keratinocytes showed ectopic expression of the human integrins and localization within the keratinocyte plasma membrane. Markers of perturbed skin homeostasis, including activation of the MAPK pathway, increased expression of the pro-inflammatory cytokine IL-1α, and enhanced expression of the transcription factor c-Fos, were identified in keratinocytes from β1 and α2 integrin-transgenic minipigs, suggesting the induction of a chronic inflammatory phenotype in the skin. Notably, cellular dysregulation obtained by overexpression of either β1 or α2 integrin occurred through different cellular signaling pathways. Our findings mark the creation of the first cloned pig models with molecular markers of skin inflammation. Despite the absence of an overt psoriatic phenotype, these animals may possess increased susceptibility to severe skin damage-induced inflammation and should be of great potential in studies aiming at the development and refinement of topical therapies for cutaneous inflammation including psoriasis.

The expression of keratinocyte growth factor receptor (FGFR2-IIIb) correlates with the high proliferative rate of HaCaT keratinocytes

Keratinocyte growth factor receptor (KGFR = FGFR2-IIIb) is a tyrosine kinase receptor expressed by keratinocytes, which mediates the effects of fibroblast growth factors (FGF). There are contradictory data in the literature regarding the role of FGFR2-IIIb during the proliferation/differentiation programme of keratinocytes. In this study, we aimed to investigate whether overexpression of FGFR2-IIIb may have a role in the regulation of keratinocyte proliferation. We analysed the expression of FGFR2-IIIb in an in vitro HaCaT model system representing different stages of proliferation and differentiation of keratinocytes. Real-time RT-PCR and Western blot analyses demonstrated a correlation between FGFR2-IIIb mRNA and protein expression and the proportion of cells in S/G2/M phase in synchronized HaCaT keratinocytes and thus with proliferation activity (r = 0.96). After treatment with the antipsoriatic drug, dithranol, FGFR2-IIIb is downregulated dose dependently both at mRNA and protein levels. Moreover, when the rate of proliferation is decreased by the lack of cell attachment to the culturing surface, FGFR2-IIIb mRNA (P = 0.0315) and protein expressions were also reduced (P = 0.0242), while a differentiation marker, keratin 10, mRNA (P = 0.0003) and protein levels (P = 0.001) were increased (r = -0.92). Based on our results we conclude that FGFR2-IIIb expression in HaCaT keratinocytes corresponds with the proliferative activation of the cells and is not related to the differentiation programme.

Morphological evidence for the role of suprabasal keratinocytes in wound reepithelialization

The process by which wounds reepithelialize remains controversial. Two models have been proposed to describe reepithelialization: the "sliding" model and the "rolling" model. In the "sliding" model, basal keratinocytes are the principal cells responsible for migration and wound closure. In this model, basal and suprabasal keratinocytes remain strongly attached to leading edge basal keratinocytes and are then passively dragged along as a sheet. The "rolling" model postulates that basal keratinocytes remain strongly attached to the basement membrane zone while suprabasal keratinocytes at the wound margin are activated to roll into the wound site. The purpose of this study was to determine which populations of keratinocytes are actively involved in reepithelialization. We evaluated expression of keratins K14, K15, K10, K2e, and K16 as well as the proliferation marker Ki67 in the migrating tongue of normal human incisional 1-hour to 28-day wounds and normal human 3 mm diameter excisional 1- to 7-day wounds. Our results show dramatic changes in phenotype and protein expression of keratins K10, K2e, K14, K15, and K16 in suprabasal keratinocytes in response to injury. We conclude that this large population of suprabasal keratinocytes actively participates in wound closure.

Phenotypical and Functional Differences in Germinative Subpopulations Derived from Normal and Psoriatic Epidermis

A model that explains how maintenance of normal homeostasis in human epidermis is achieved describes a heterogeneous cell population of stem cells (SC) and transit amplifying cells (TAC). There must be a tightly regulated balance between SC self-renewal and the generation of TAC that undergo a limited number of divisions before giving rise to postmitotic, terminally differentiated cells. To investigate whether this balance is disturbed in psoriatic epidermis, we have characterized flow sorted enriched SC and TAC using immunocytochemistry, flow cytometry, and real-time quantitative PCR. Our data demonstrate phenotypical and functional differences in SC (beta(1)-integrin bright) and TAC (beta(1)-integrin dim) enriched fractions between normal and psoriatic keratinocytes. Some of these were expected, such as mRNA levels of keratins 6 and 10 and of the Ki-67 antigen. Most remarkable were differences in phenotype of the psoriatic TAC compared with TAC from normal skin. These subpopulations also displayed striking differences following culture. Downregulation of markers associated with the regenerative phenotype (psoriasin, elafin, psoriasis-associated fatty acid binding protein) in cultured psoriatic dim cells in the absence of hyperproliferation suggests that proliferation and regenerative maturation are coupled. From these results, in combination with our earlier findings, we propose a model for epidermal growth control in which TAC play a crucial role.

Epithelial stem cells in human prostate growth and disease

Benign prostatic hyperplasia and prostate cancer arise as a consequence of changes in the balance between cell division and differentiation. Little, however, is known about the control of this process. Stem cells are a small population of cells that divide occasionally to produce transit-amplifying cells that in turn produce the differentiated cell types of the tissue. It is believed that cancer cell proliferation is also driven by stem cells. We have shown that around one in 200 prostate epithelial cells have characteristics of stem cells and that these cells are contained within a population with a distinct keratin expression pattern. Work is now ongoing to identify markers for these cells that will allow us to study the role they play in prostatic disease.

Influence of anti-CD49f and anti-CD29 monoclonal antibodies on mitotic activity of epithelial cells (HaCaT) and gingival fibroblasts in vitro

A major complication in the treatment of periodontitis marginalis is the reepithelization of periodontal defects inhibiting collagen fiber attachment and periodontal regeneration. In this study we investigated the possibility of a molecular blockade of epithelial mitosis in vitro. Monoclonal antibodies against the VLA-6 laminin receptor subunit alpha6 interrupted interactions between epithelial cells (HaCaT cells) and their extracellular matrix and thus resulted in reduction of proliferation rates by more than 50%. The same effect was observed with anti alpha1-antibodies. In contrast, collagen-producing and -secreting gingival fibroblasts, which play an important role in periodontal regeneration, remained unaffected by the applied anti alpha6 antibodies. Correspondingly, these cells were found to lack VLA-6 laminin receptors. Selective molecular inhibition of epithelial proliferation and apical migration by monoclonal anti alpha6 antibody application may provide an adjuvant periodontitis therapy resulting in an enhanced periodontal regeneration.

RAC1 regulates adherens junctions through endocytosis of E-cadherin

The establishment of cadherin-dependent cell-cell contacts in human epidermal keratinocytes are known to be regulated by the Rac1 small GTP-binding protein, although the mechanisms by which Rac1 participates in the assembly or disruption of cell-cell adhesion are not well understood. In this study we utilized green fluorescent protein (GFP)-tagged Rac1 expression vectors to examine the subcellular distribution of Rac1 and its effects on E-cadherin-mediated cell-cell adhesion. Microinjection of keratinocytes with constitutively active Rac1 resulted in cell spreading and disruption of cell-cell contacts. The ability of Rac1 to disrupt cell-cell adhesion was dependent on colony size, with large established colonies being resistant to the effects of active Rac1. Disruption of cell-cell contacts in small preconfluent colonies was achieved through the selective recruitment of E-cadherin-catenin complexes to the perimeter of multiple large intracellular vesicles, which were bounded by GFP-tagged L61Rac1. Similar vesicles were observed in noninjected keratinocytes when cell-cell adhesion was disrupted by removal of extracellular calcium or with the use of an E-cadherin blocking antibody. Moreover, formation of these structures in noninjected keratinocytes was dependent on endogenous Rac1 activity. Expression of GFP-tagged effector mutants of Rac1 in keratinocytes demonstrated that reorganization of the actin cytoskeleton was important for vesicle formation. Characterization of these Rac1-induced vesicles revealed that they were endosomal in nature and tightly colocalized with the transferrin receptor, a marker for recycling endosomes. Expression of GFP-L61Rac1 inhibited uptake of transferrin-biotin, suggesting that the endocytosis of E-cadherin was a clathrin-independent mechanism. This was supported by the observation that caveolin, but not clathrin, localized around these structures. Furthermore, an inhibitory form of dynamin, known to inhibit internalization of caveolae, inhibited formation of cadherin vesicles. Our data suggest that Rac1 regulates adherens junctions via clathrin independent endocytosis of E-cadherin.

Co-localization of Rac1 and E-cadherin in human epidermal keratinocytes

The Rac1 small GTP-binding protein is known to be involved in reorganization of the actin cytoskeleton and in regulation of intracellular signal transduction. The assembly and maintenance of cadherin-based cell cell junctions in epidermal keratinocytes is thought to be dependent on activity of Rac1. In this study we have generated green fluorescent protein (GFP)-tagged wild type, dominant negative and constitutively active Rac1 expression vectors and analyzed distribution of Rac1 following microinjection of human SCC12F epidermal keratinocytes. Wild type, dominant negative and constitutively active GFP Rac1 proteins distribute to sites of cell cell adhesion and co-localize with E-cadherin and the catenins. Disruption of cadherin-based junctions by reduction in extracellular calcium concentrations, or by use of antibodies to E-cadherin, results in redistribution of Rac1 away from sites of cell cell interaction but the co-localization with E-cadherin is maintained. In addition, expression of constitutively active GFP Rac1 results in formation of membrane ruffles on the apical surface of cells and intracellular vesicles. Interestingly, co-localization of Rac1 with E-cadherin is maintained in these structures. In contrast to previously published work we find that expression of dominant negative Rac1 neither disrupts cell cell adhesion nor prevents assembly of new cadherin-based adhesion structures.

Effects of BisGMA and TEGDMA on proliferation, migration, and tenascin expression of human fibroblasts and keratinocytes

Previous studies have documented a marked cytotoxic potency of BisGMA and TEGDMA. The purpose of this investigation was to determine if these substances also affect proliferation, migration, and tenascin expression of primary human gingival fibroblasts (HGF) and immortalized human keratinocytes (HaCaT). These parameters play an important role in healing wounds. HGF and HaCaT cultures were incubated with TEGDMA and BisGMA. Cell proliferation (BrdU-assay) and migration (Boyden method) were determined 24 h after incubation. Tenascin expression was investigated four and seven days after treatment. Results were statistically evaluated by ANOVA using the Wilcoxon-Mann-Whitney test (p < 0.05). Proliferation of both cell types was significantly inhibited at concentrations > or = 0.25 mM (TEGDMA) or > or = 0.01 mM (BisGMA). Migration of HaCaT was significantly increased after incubation with BisGMA for 24 h. TEGDMA did not alter migration of HGF and HaCaT. In addition, TEGDMA had no effect on tenascin expression of both cell cultures. After 4 days of incubation, BisGMA (at a concentration of 0.01 mM) significantly reduced tenascin production of HaCaT cultures related to cell number. However, 7 days after treatment, BisGMA significantly increased tenascin expression of HGF and HaCaT cultures. Altogether, our results indicate that BisGMA can affect migration of keratinocytes and alters the expression of the extracellular matrix component tenascin. Thus, BisGMA may significantly influence the healing of injured oral tissues.

A novel gene expressed in human keratinocytes with long-term in vitro growth potential is required for cell growth

The herpes simplex virus large subunit of ribonucleotide reductase differs from its counterparts in eukaryotic and prokaryotic cells and in other viruses in that it contains a unique domain that codes for a distinct serine-threonine protein kinase that activates the Ras/MEK/MAPK mitogenic pathway and is required for virus growth. Previous studies suggested that ribonucleotide reductase protein kinase was co-opted from a cellular gene. Cellular genes similar to ribonucleotide reductase protein kinase were not cloned, however, and their function is unknown. Here we report that a novel gene (H11) that codes for a protein similar to herpes simplex virus 2 ribonucleotide reductase protein kinase, is expressed in skin tissues, cultured keratinocytes, and the keratinocyte cell line A431. The protein is phosphorylated and it associates with the plasma membrane. H11 is expressed in keratinocytes with long-term in vitro growth potential and is coexpressed with high levels of adhesion molecules involved in signal transduction, such as beta1 integrin. Antisense oligonucleotides that inhibit H11 expression inhibit DNA synthesis and keratinocyte proliferation, suggesting that H11 expression is required for cell growth.

Switch from p53 to MDM2 as differentiating human keratinocytes lose their proliferative potential and increase in cellular size

p53 transcription factor is mutated in most skin cell carcinomas and in more than 50% of all human malignancies. One of its transcriptional targets is MDM2, which in turn down-regulates p53. The role of the p53/MDM2 regulatory loop upon genotoxic stress is well documented, but less is known about its role in normal tissue homeostasis. We have explored this pathway during the different transitions of the human epidermal differentiation programme and after isolating stem cells, transit amplifying cells or differentiating cells from epidermis. Maximum expression of p53 was found in proliferating keratinocytes. A striking and transient induction of MDM2 and a down-modulation of p53 characterized the transition from proliferation to differentiation in primary human keratinocytes. These changes were delayed in late differentiating carcinoma cells, and were clearly different in suspended primary fibroblasts. Interestingly, these changes correlated with an increase in cell size, at the time of irreversible commitment to differentiation. Induction of MDM2 was also associated with suppression of proliferation in normal, or hyperproliferative, psoriatic epidermis. Moreover, both proteins were induced as keratinocytes were driven to leave the stem cell compartment by c-Myc activation. Overall, our results show a critical regulation of the p53/MDM2 pathway at the epidermal transition from proliferation to differentiation.

beta1 integrins regulate keratinocyte adhesion and differentiation by distinct mechanisms

In keratinocytes, the beta1 integrins mediate adhesion to the extracellular matrix and also regulate the initiation of terminal differentiation. To explore the relationship between these functions, we stably infected primary human epidermal keratinocytes and an undifferentiated squamous cell carcinoma line, SCC4, with retroviruses encoding wild-type and mutant chick beta1 integrin subunits. We examined the ability of adhesion-blocking chick beta1-specific antibodies to inhibit suspension-induced terminal differentiation of primary human keratinocytes and the ability of the chick beta1 subunit to promote spontaneous differentiation of SCC4. A D154A point mutant clustered in focal adhesions but was inactive in the differentiation assays, showing that differentiation regulation required a functional ligand-binding domain. The signal transduced by beta1 integrins in normal keratinocytes was "do not differentiate" (transduced by ligand-occupied receptors) as opposed to "do differentiate" (transduced by unoccupied receptors), and the signal depended on the absolute number, rather than on the proportion, of occupied receptors. Single and double point mutations in cyto-2 and -3, the NPXY motifs, prevented focal adhesion targeting without inhibiting differentiation control. However, deletions in the proximal part of the cytoplasmic domain, affecting cyto-1, abolished the differentiation-regulatory ability of the beta1 subunit. We conclude that distinct signaling pathways are involved in beta1 integrin-mediated adhesion and differentiation control in keratinocytes.

Role of interactions between integrins and extracellular matrix components in healthy epithelial tissue and establishment of a long junctional epithelium during periodontal wound healing: a review

Following the surgical treatment of adult periodontitis, the epithelial regeneration of the periodontal attachment is non-physiological and thus unsatisfactory, if membranes or artificial bone material are not used. Re-epithelialization is based on the proliferation, migration, and differentiation of basal epithelial cells which are in contact with a wound matrix and whose molecular makeup differs from the extracellular matrix of intact regions. Interactions between basal epithelial cells and the extracellular matrix are mediated by special receptors on the cell surface which are known as integrins and belong to the family of cellular adhesion molecules (CAM). Various studies indicate that integrin-mediated interaction plays a decisive role in regulating the proliferation, migration, and differentiation of the epithelial cells. This review provides an overview of the in vivo and in vitro expression of integrins by epithelial cells and the interaction between integrins and extracellular matrix (ECM) in the case of a stationary epithelium and during wound healing. The regulation of these cell-cell and cell-matrix interactions may represent a method for inhibiting non-physiological epithelial regeneration on the molecular level.

Cultivation and transplantation of epidermal keratinocytes

Transplantation of autologous cultured keratinocytes is the most advanced area of tissue engineering which has clinical application in restoration of skin lesions. In vitro, disaggregated keratinocytes undergo activation and after adhesion and histogenic aggregation form three-dimensional epithelial sheets suitable for grafting on prepared wounds that provide a reparative environment. Epidermal stem cells survive and proliferate in culture, retaining their potential to differentiate and to produce neoepidermis. Reconstructed skin is physiologically compatible to split-thickness autografts. Autotransplantation of cultured keratinocytes is a promising technique for gene therapy. In many cases allografting of cultured keratinocytes promotes wound healing by stimulation of epithelialization. Banking of cryopreserved keratinocytes is a significant improvement in usage of cultured keratinocytes for wound healing. Skin substitutes reconstructed in vitro that have morphological, biochemical, and functional features of the native tissue are of interest as model systems that enable extrapolation to situations in vivo.

Monoclonal antibodies against integrin subunits alpha6 and beta1 inhibit migration of gingival epithelium in organ culture

BACKGROUND

The main problem in the posterior instrumental treatment of periodontitis is the re-epithelization of the periodontal defects, leading to the formation of an unphysiological, long junctional epithelium inhibiting the regeneration of periodontal attachment. A precondition for the re-epithelization is the interaction between epithelial cells and their extracellular matrix mediated by integrins. Integrins are cellular adhesion molecules (CAM), binding to different structures of the extracellular matrix, e.g., collagen, laminin, or fibronectin.

METHODS

Biopsy specimens of marginal gingiva, composed of epithelium and subepithelial connective tissue, were cultivated on microporous membranes in tissue culture plates. The culture medium was supplemented with monoclonal antibodies directed against human integrin subunits. The period of cultivation was 6 days and the medium was replaced daily. After cultivation, the tissue development was analyzed by histological and immunohistochemical methods.

RESULTS

We found that the combination of antibodies directed against the integrin subunits alpha6 and beta1 inhibited the migration of epithelium completely in 9 out of 10 cultures, whereas control cultures containing none or only irrelevant antibodies demonstrated connective tissues completely covered by epithelium.

CONCLUSIONS

Influencing the regeneration of periodontal tissue on a molecular basis by using antibodies directed against integrin subunits may be of future interest in postsurgical treatment of periodontal diseases.

Keratin 15 expression in stratified epithelia: downregulation in activated keratinocytes

Keratin 15 (K15) is a type I keratin without a defined type II partner whose expression in epidermal diseases has not been investigated. In this study we have used LHK15, a monoclonal antibody raised against the last 17 amino acids of the K15 polypeptide, to show that K15 is expressed primarily in the basal keratinocytes of stratified tissues, including the fetal epidermis and fetal nail. Although K15 in normal hair follicles was virtually absent from hair bulbs, it was expressed by a subset of keratinocytes in the outer root sheath. By comparison, K14 expression was found throughout the outer root sheath of hair follicles; however, when both K14 alleles were naturally ablated, the expression of K15 was also observed throughout the outer root sheath of the follicles. Expression of K15 mRNA was assessed by in situ hybridization and corroborated the data from immunostaining. An increase in K15 mRNA and protein expression in hair follicles from the K14 ablated epidermis suggested an upregulation of the K15 gene in the absence of the K14 protein. In organotypical cultures where differentiating keratinocytes expressed markers of activated phenotype, i.e., K6 and K16, expression of K15 was undetectable. The expression of K15 mRNA and protein was also downregulated in two hyperproliferating situations, psoriasis and hypertrophic scars. Because keratinocytes in psoriasis and hypertrophic scars are activated, we conclude that K15 expression is not compatible with keratinocyte activation and the K15 gene is downregulated to maintain the activated phenotype.

Analysis of cultured keratinocytes from a transgenic mouse model of psoriasis: effects of suprabasal integrin expression on keratinocyte adhesion, proliferation and terminal differentiation

Many important transgenic mouse models of benign and neoplastic skin diseases have been generated through the use of promoters that target transgene expression to the different epidermal layers. However, more mechanistic studies of the specific effects of the transgenes on keratinocytes have been hampered by difficulties in culturing keratinocytes from adult mouse epidermis and by the low differentiation potential of many established mouse keratinocyte lines. We have used the Rheinwald & Green technique to cultivate primary adult keratinocytes and to generate keratinocyte lines from transgenic mice which have a sporadic psoriatic phenotype due to expression of human integrin subunits under the control of the involucrin promoter. We show that the transgenes are induced when keratinocytes are placed in suspension and that the transgenic integrins are capable of clustering in focal adhesions and mediating cell adhesion and spreading. We also show that suprabasal integrin expression has no direct effect on proliferation of cells in the underlying basal layer, ruling this out as a possible explanation for the epidermal hyperproliferation observed in the transgenic mice.

Lack of intrinsic polarity in the ligand-binding ability of keratinocyte beta1 integrins

Within the basal layer of the epidermis the beta1 integrins have a pericellular distribution. Two monoclonal antibodies, 15/7 and 12G10, that detect a conformation of the beta1 integrin subunit that is induced following cation or ligand occupancy selectively recognized beta1 integrins at the basement membrane zone in vivo and in focal adhesions of cultured keratinocytes; they did not recognize integrins on the apical and upper lateral membranes of basal keratinocytes nor integrins on the suprabasal keratinocytes of hyperproliferative epidermis. Inhibition of intercellular adhesion did not induce the 15/7 epitope on the lateral and apical membrane domains. The surface distribution of the epitopes was consistent with the antibodies acting as reporters of ligand-binding; in addition, the 15/7 epitope was exposed on unglycosylated, immature beta1 integrins. Although the apical membrane of basal keratinocytes is not normally in contact with extracellular matrix proteins, we found that it was capable of binding fibronectin-coated beads and that the 15/7 epitope was exposed on plasma membrane in contact with the beads. When a chimeric molecule consisting of the extracellular domain of CD8 and the cytoplasmic domain of the beta1 integrin subunit, used to mimic a constitutively active beta1 heterodimer, was introduced into keratinocytes it localized to the basal, lateral and apical membrane domains. We conclude that although the conformation of the keratinocyte beta1 integrins differs between the basal and the lateral/apical membrane domains there is no intrinsic polarity in the ligand binding potential of the receptors.

Differential expression of activation epitopes of beta1 integrins in psoriasis and normal skin

In the epidermis, beta1 integrin expression is normally confined to the basal layer; however, suprabasal expression of beta1 integrins in keratinocytes has been found in psoriasis, and it has been suggested that it could be a pathogenic factor of the disease. We have investigated herein the functional state of beta1 integrins of human keratinocytes in normal skin and psoriasis. The expression of beta1-activation-reporter epitopes was monitored with two monoclonal antibodies, HUTS-21 and MG5A7, that recognize epitopes whose expression parallels functional activity of beta1 integrins and correlates with the ligand binding activity of these heterodimeric glycoproteins. We have found that keratinocytes express activation epitopes of beta1 integrins, and that these epitopes can be modulated by manganese. The expression of activation epitopes of beta1 integrins was related to an enhanced adhesion to fibronectin and collagen. Immunohistochemical studies of normal and psoriatic skin with HUTS-21 and other monoclonal antibodies indicate that, although there is suprabasal expression of beta1 integrins in psoriasis, these molecules seem to be in an inactive state. Moreover, most beta1 integrins in lateral and apical surfaces of basal keratinocytes of psoriasis are also in a nonactive conformation, implying a decrease of activity compared with normal skin, in which active beta1 integrins are distributed all over the basal keratinocytes.

The relationship between hyperproliferation and epidermal thickening in a mouse model for BCIE

Epidermal thickening is a phenomenon common to many genodermatoses but little is known about the underlying causes. We have recently created a mouse model for the human skin disease bullous congenital ichthyosiform erythroderma by gene targeting. Mice heterozygous for a truncated keratin 10 gene exhibit acanthosis and hyperkeratosis as seen in the human disease. The degree of epidermal thickening is highly variable, offering a novel opportunity to investigate how epidermal homeostasis is modulated in keratin disorders by comparing epidermis from different body regions. We have performed bromodeoxyuridine labeling experiments and detected proliferation antigens by immunohistochemical means to compare proliferation in the epidermis of wild-type and heterozygous mice. These results have been compared with the expression of epidermal differentiation markers and of the "hyperproliferation associated" keratins K6 and K16. These experiments indicated that hyperproliferation is only partly responsible for the morphologic changes and that other mechanisms such as decreased desquamation are likely to be involved.

Up-regulation of p27Kip1, p21WAF1/Cip1 and p16Ink4a is associated with, but not sufficient for, induction of squamous differentiation

Irreversible growth arrest is an early and integral part of squamous cell differentiation in normal human epidermal keratinocytes (NHEKs) and is assumed to be linked to the control of expression of differentiation-specific genes. In this study, we examine the link between the molecular events associated with growth arrest and the expression of differentiation genes. NHEKs that have been induced to undergo growth arrest and differentiation by suspension culture contain populations in both G1 and G2/M of the cell cycle. The irreversible growth arrest state in NHEKs is characterized by an accumulation of the hypophosphorylated forms of Rb and p130, with subsequent down-regulation of levels of Rb, up-regulation of p130 and associated down-regulation of E2F-regulated genes such as cyclin A. These events correlate with an inhibition of G1 cdk activity, mediated in part by an increase in the cdk inhibitors p21(WAF1/Cip1), p27(Kip1) and p16(Ink4a). Flow cytometric and immunoblot analysis demonstrated that the timing of the up-regulation of p27, p16 and p130 corresponds closely with the induction of the squamous-specific genes cornifin alpha (SPRR-1) and transglutaminase type I, suggesting a close link between control of growth arrest and differentiation. However, growth arrest induced by over-expression of p27, p21 or p16 by recombinant adenovirus is not sufficient to induce expression of the differentiation genes, or to invoke the pattern of cell cycle regulatory protein expression characteristic of the differentiation-specific irreversible growth arrest. We conclude that growth arrest mediated by activation of the Rb pathway is not sufficient to trigger terminal squamous differentiation and additional signals which can be generated during suspension culture are required to promote the complete differentiation program.

Evidence that beta1 integrins in keratinocyte cell-cell junctions are not in the ligand-occupied conformation

Integrins are a family of heterodimeric cell surface molecules that function as adhesion receptors in cell-cell and cell-extracellular matrix contact. Integrins of the beta1 family are found on keratinocytes clustered at sites of cell-cell junctions both in culture and in normal skin. The possibility that these integrins function in cell-cell adhesion has been both supported and refuted in recent conflicting publications. Rather than testing further for the presence or absence of an interaction, we present evidence to show that beta1 integrins in keratinocyte cell-cell junctions are in the non-ligand-occupied conformation. We transfected keratinocytes with a construct that expresses a chimeric cell surface molecule containing the integrin beta1 cytoplasmic tail. This chimera is thought to mimic the ligand-occupied receptor and has previously been shown to be actively localized to focal adhesions in fibroblasts. We find that keratinocytes are also able to localize this chimera in focal adhesions but do not localize it to areas of cell-cell junctions. A monoclonal anti-beta1 antibody that has been previously shown to preferentially recognize ligand-occupied beta1 receptors was used to stain keratinocytes. This antibody showed staining of focal adhesions, with little or no staining of cell-cell junctions. In contrast, four other anti-beta1 antibodies showed strong, preferential staining at cell-celljunctions. Double staining confirmed that both the conformation-specific monoclonal antibody and a pan-beta1 antibody were capable of recognizing the same focal adhesions. Taken together, these data indicate that integrins in cell- cell junctions of keratinocytes are in the non-ligand-occupied conformation. Although we do not directly prove the absence of an integrin-integrin interaction at this site, we show that any such interaction does not induce the ligand-occupied conformation and, therefore, is less likely to play a major role in cytoskeletal re-organization or signal transduction.

Rapid normalization of epidermal integrin expression after allografting of human keratinocytes

Allogeneic keratinocyte grafts have beneficial effects on skin wounds, but the underlying interactions between graft and woundbed remain to be explored in detail. The epidermal integrins play a pivotal role in mediating cell-to-cell and cell-to-matrix interactions. In unwounded epidermis, alpha 2 beta 1-, alpha 3 beta 1-, alpha 6 beta 4-, alpha 5 beta 1-, and alpha v beta 5-integrins are confined to basal cells. During healing of incisional wounds, these integrins are also expressed in suprabasal cells, where they remain detectable even after epidermal integrity is fully reestablished. We examined the integrin subunits alpha 2, alpha 3, alpha 6, alpha 5, and alpha v in partial thickness burn wounds grafted with allogeneic keratinocytes and asked whether the effect of allogeneic keratinocyte grafts, i.e., fast reepithelialization, is reflected by an accelerated reversion to a normal integrin pattern. Biopsies were taken after wound debridement before grafting and 10 d after transplantation. After 10 d, a stratified epidermis had developed in all cases and integrins were mainly restricted to the basal cell layer of the neo-epidermis. alpha 2-, alpha 3-, alpha 6-, and alpha v-subunits were present at basal and/or lateral cell borders, duplicating the integrin pattern in normal epidermis. The findings indicate that grafting accelerates the shift of the epidermis from an inflammatory to a regenerative state, as reflected by the reversion of the integrin pattern from a "spread-and-migrate" to the "steady-state" phenotype.

Suprabasal integrin expression in the epidermis of transgenic mice results in developmental defects and a phenotype resembling psoriasis

Integrin expression is normally confined to the basal layer of the epidermis, but when epidermal homeostasis is perturbed, the receptors are also expressed by suprabasal, differentiating keratinocytes. We have used the involucrin promoter to express functional human integrin subunits alpha 2, alpha 5, and beta 1 in the suprabasal epidermal layers of transgenic mice. In mice expressing alpha 5 or beta 1 alone or alpha 2 beta 1 or alpha 5 beta 1 heterodimers, there were hair and whisker abnormalities and a failure of eyelid fusion. In addition, mice expressing beta 1 alone or in combination with alpha 2 or alpha 5 exhibited epidermal hyper-proliferation, perturbed keratinocyte differentiation, and skin inflammation, all of which are features of a common human skin disease, psoriasis.

Extracellular matrix components in intestinal development

Intestinal morphogenesis and differentiation are dependent on heterotypic cell interactions between embryonic epithelial cells (endoderm) and stromal cells (mesenchyme). Extracellular matrix molecules represent attractive candidates for regulators of these interactions. The structural and functional diversity of the extracellular matrix as intestinal development proceeds is demonstrated by 1) spatio-temporal specific expression of the classically described constituents, 2) the finding of laminin and collagen IV variants, 3) changes in the ratio of individual constituent chains, and 4) a stage-specific regulation of basement membrane molecule production, in particular by glucocorticoids. The orientation/assembly of these extracellular matrix molecules could direct precise cellular functions through interactions via integrin molecules. The involvement of extracellular matrix, and in particular basement membrane molecules in heterotypic cell interactions leading to epithelial cell differentiation, has been highlighted by the use of experimental models such as cocultures, hybrid intestines and antisense approaches. These models allowed us to conclude that a correct elaboration and assembly of the basement membrane, following close contacts between epithelial and fibroblastic cells, is necessary for the expression of differentiation markers such as digestive enzymes.

Ganglioside GT1b inhibits keratinocyte adhesion and migration on a fibronectin matrix

Highly sialylated gangliosides have been shown to alter cellular adhesion to a fibronectin matrix. The effect of these gangliosides on the adhesion, spreading, and migration of cultured keratinocytes on a fibronectin matrix has not been explored. Ganglioside GT1b significantly prevented attachment of keratinocytes to fibronectin and also detached previously adherent keratinocytes in a concentration-dependent manner without cell toxicity. GT1b did not affect adhesion of keratinocytes to wells coated with laminin, type I or type IV collagen, 804G extracellular matrix, or albumin. GT1b also inhibited keratinocyte migration on fibronectin in a concentration-dependent manner at concentrations as low as 5 nM GT1b, but had no effect on migration of keratinocytes plated on other matrices. GT1b binds to intact fibronectin and to the 120-kD RGDS-containing cell-binding fibronectin fragment, but not to the heparin- or gelatin-binding fragments of fibronectin. Although RGDS competes with GT1b in inhibiting adhesion, GT1b does not diminish binding of keratinocytes to a derivatized RGDS substratum, suggesting that the GT1b effect involves a non-RGDS site in the cell-binding region that modulates RGDS/alpha 5 beta 1 integrin receptor interaction. Through a specific effect on keratinocyte interaction with fibronectin, GT1b may participate in the regulation of cell adhesion and migration on a fibronectin substratum, which are important events during wound healing and the spreading of cutaneous neoplasia.

Calcium-induced changes in distribution and solubility of cadherins, integrins and their associated cytoplasmic proteins in human keratinocytes

Studies with cultured human epidermal keratinocytes have shown that stratification, the movement of differentiating cells out of the basal layer, involves changes in cell-extracellular matrix and cell-cell adhesiveness mediated by receptors of the integrin and cadherin families, respectively. Keratinocytes normally lose their integrins when they initiate terminal differentiation. However, when stratification is inhibited by a low concentration of calcium ions in the medium (0.1 mM) or by addition of antibodies to P- and E-cadherin in standard medium (1.8 mM calcium ions), differentiating, involucrin-positive, cells continue to express functional integrins. In order to investigate the mechanism by which cadherins may regulate integrin expression, we have examined the distribution and detergent solubility of the receptors and associated cytoplasmic proteins in keratinocytes grown as a monolayer in low calcium medium or transferred to standard medium to induce stratification. Within 1 hour of raising the concentration of calcium ions, integrins, cadherins, alpha-catenin, beta-catenin, plakoglobin, vinculin and alpha-actinin appeared to accumulate at cell-cell borders, whereas the focal contact proteins, paxillin and talin, did not. The change in distribution was correlated with decreased solubility in 0.5% Triton X-100 of some of the proteins examined, but the integrins, alpha-actinin, paxillin and talin remained completely soluble. Addition of cytochalasin D inhibited both the redistribution of proteins and subsequent stratification of involucrin-positive cells. Cycloheximide treatment allowed protein redistribution and stratification, but involucrin-positive cells continued to express integrins. These results suggest that stratification requires the interactions of cadherins and integrins with the actin cytoskeleton and that the selective loss of integrins from differentiating cells requires de novo protein synthesis.

Identification of an 80kD protein associated with the alpha 3 beta 1 integrin as a proteolytic fragment of the alpha 3 subunit: studies with human keratinocytes

We have characterised a protein of approximately 80kD previously observed to co-immunoprecipitate with the alpha 3 beta 1 integrin in lysates of surface labelled human epidermalkeratinocytes. The 80kD protein only appeared when keratinocytes were harvested with trypsin/EDTA prior to lysis and a protein of similar molecular mass could be immunoprecipitated from human dermal fibroblasts following treatment of the cells with trypsin/EDTA. N terminal sequencing established that the 80kD protein had homology with the alpha 3 integrin subunit. Peptide-mass fingerprinting was used to confirm that the protein comprised the amino terminus of alpha 3 and established that the site of cleavage was after amino acid 629. The 80kD fragment could be coimmunoprecipitated with alpha 3 beta 1 using an antibody to the cytoplasmic domain of the alpha 3 subunit, showing that the fragment remained complexed with intact alpha 3 beta 1. When antibodies to the cytoplasmic and extracellular domains of alpha 3 were used to label human epidermis by immunofluorescence, the staining patterns were indistinguishable and there is therefore no evidence that proteolysis of alpha 3 plays a role in keratinocyte detachment from the basement membrane during terminal differentiation. Whether the 80kD fragment has any effects, positive or negative, on alpha 3 beta 1-mediated adhesion remains to be determined.

CD44 is the major peanut lectin-binding glycoprotein of human epidermal keratinocytes and plays a role in intercellular adhesion

Although binding of peanut agglutinin (PNA) to keratinocytes is often used as a marker of terminal differentiation, the identity of the PNA-binding glycoproteins has been unclear. We now show that an antiserum raised against the glycoproteins recognises isoforms of CD44, the most abundant of which could be labelled with [35S]sulphate, indicating the presence of glycosaminoglycan side chains. RT-PCR analysis showed that keratinocytes expressed at least 5 forms of CD44 containing different numbers of exons from the variable region of the extracellular domain and also expressed the standard ‘haemopoietic’ form of CD44 which lacks the variable exons. Standard and variant isoforms of CD44 were expressed both by proliferating keratinocytes and cells undergoing terminal differentiation, although the level of CD44 mRNAs decreased when keratinocytes were placed in suspension to induce differentiation. The role of CD44 in intercellular adhesion was investigated by plating keratinocytes onto a rat pancreatic carcinoma line transfected with different CD44 isoforms. Keratinocyte adhesion to transfectants expressing variant exons 4–7 was greater than to cells expressing standard CD44 and could be inhibited with hyaluronan or digestion with hyaluronidase. These observations confirm earlier predictions that the PNA-binding glycoproteins of keratinocytes play a role in intercellular adhesion.

Integrin expression by human epidermal keratinocytes can be modulated by interferon-gamma, transforming growth factor-beta, tumor necrosis factor-alpha, and culture on a dermal equivalent

Receptors of the integrin family are largely confined to the basal layer of keratinocytes, both in human epidermis and in stratified cultures of human keratinocytes. However, suprabasal integrin expression is observed during epidermal wound healing and in psoriatic lesions. We have investigated potential stimuli of suprabasal expression. Addition of transforming growth factor-beta (TGF-beta), interferon-gamma (IFN-gamma), or tumor necrosis factor-alpha (TNF-alpha) to keratinocytes cultured with a 3T3 feeder layer did not induce suprabasal expression. The cytokines caused small changes in the levels of alpha 2 beta 1 or alpha 3 beta 1 on the surface of basal keratinocytes but had no significant effect on the proportion of cells adhering to fibronectin, type IV collagen, and laminin, and did not cause changes in the mobility of integrin subunits on polyacrylamide gels. Injection of TNF-alpha or IFN-gamma intradermally into healthy human volunteers induced an inflammatory response but did not induce suprabasal integrin expression. However, we did observe transient suprabasal integrin expression when keratinocytes were grown on a dermal equivalent consisting of fibroblasts in a collagen gel. One week after raising the cultures to the air-liquid interface, beta 1 integrins were found in all the viable cell layers, with suprabasal cells co-expressing integrins and involucrin; 1 week later integrins were confined to the basal layer. Addition of TGF-beta, IFN-gamma, or TNF-alpha to the dermal equivalents neither induced nor inhibited suprabasal integrin expression. We conclude that suprabasal integrin expression is not induced by the inflammatory cytokines tested, and instead may reflect the proliferation/differentiation status of the epidermis.

Stem cell patterning and fate in human epidermis

Within human epidermis there are two types of proliferating keratinocyte: stem cells, which have high proliferative potential, and transit-amplifying cells, which are destined to undergo terminal differentiation after a few rounds of division. We show that, in vivo, stem cells express higher levels of the alpha 2 beta 1 and alpha 3 beta 1 integrins than transit-amplifying cells and that this can be used both to determine the location of stem cells within the epidermis and to isolate them directly from the tissue. The distribution of stem cells and transit-amplifying cells is not random: patches of integrin-bright and integrin-dull cells have a specific location with respect to the epidermal-dermal junction that varies between body sites and that correlates with the distribution of S phase cells. Stem cell patterning can be recreated in culture, in the absence of dermis, and appears to be subject to autoregulation.

Adhesion of human epidermal keratinocytes to laminin

We have examined the mechanism by which human epidermal keratinocytes adhere to the A/B1/B2 (alpha 1 beta 1 gamma 1) form of laminin. Adhesion could be completely inhibited with an antibody to the beta 1 integrin subunit or a combination of antibodies recognising the alpha 2 beta 1, alpha 3 beta 1 and alpha 6 beta 4 integrins. Keratinocytes adhered in the presence of magnesium and manganese ions, but calcium ions did not support adhesion and inhibited adhesion when combined with magnesium and manganese. The effects of anti-integrin antibodies (including a stimulatory antibody to the beta 1 subunit) were not influenced by specific cations, with the exception that inhibition by an antibody to alpha 2 beta 1 was abrogated by the presence of manganese ions. The E3 and E8 proteolytic fragments of laminin did not support keratinocyte adhesion and heat inactivation of the E8 site in intact laminin did not reduce adhesion. Three laminin fragments that did support adhesion were P1, E4 and E1X-Nd, P1 activity being attributable at least in part to the RGD site; antibody blocking experiments suggested that adhesion to these fragments was primarily via alpha 3 beta 1. The synthetic peptide GD-6, derived from the carboxy terminus of the laminin A chain (included within E3) did support adhesion, but the significance of this observation is unclear, since a scrambled control peptide could also support adhesion. In conclusion, keratinocyte adhesion to A/B1/B2 laminin involves three integrins and multiple binding sites that are different from those defined previously.