Transcriptional and post-translational regulation of beta 1 integrin expression during keratinocyte terminal differentiation

Elucidating the role of plexin D1 in body fat distribution and susceptibility to metabolic disease using a zebrafish model system

Non-communicable diseases (NCDs) such as cardiovascular disease, diabetes and cancer were responsible for 68% of all deaths worldwide in 2012. The regional distribution of lipid deposited within adipose tissue (AT) - so called body fat distribution (BFD) - is a strong risk factor for NCDs. BFD is highly heritable; however, the genetic basis of BFD is almost entirely unknown. Genome-wide association studies have identified several loci associated with BFD, including at Plexin D1 (PLXND1) - a gene known to modulate angiogenesis. We recently demonstrated that zebrafish homozygous for a null mutation in plxnd1 had a reduced capacity to store lipid in visceral AT (VAT) leading to altered BFD. Moreover, we found that type V collagens were upregulated in plxnd1 mutants, and mediated the inhibitory effect of Plxnd1 on VAT growth. These results strengthen evidence that Plxnd1 influences BFD in human populations, and validate zebrafish as a model to study BFD. However, many pertinent questions remain unanswered. Here we outline potential Plxnd1 mechanisms of action in AT, and describe the genetic architecture at human PLXND1 that is associated with BFD and NCD susceptibility.

Skin and skin substitutes in burn management

Early burn excision has reduced the mortality from major burns. This practice presents the problem of wound coverage after excision, since the availability of autologous donor sites is limited in very large burns. This article reviews the methods available for covering burn wounds. Methods of expanding autologous skin are discussed as well as techniques using allogeneic tissue and xenograft. Newer synthetic skin substitutes have become an important advance and are also described. Cultured skin replacements are also discussed along with their shortfalls. The treatment of a patient with major burns may require the use of many different skin substitutes, as none is entirely satisfactory on its own.

Islet Endothelial Cells Induce Glycosylation and Increase Cell-surface Expression of Integrin β1 in β Cells

The co-culturing of insulinoma and islet-derived endothelial cell (iEC) lines results in the spontaneous formation of free-floating pseudoislets (PIs). We previously showed that iEC-induced PIs display improved insulin expression and secretion in response to glucose stimulation. This improvement was associated with a de novo deposition of extracellular matrix (ECM) proteins by iECs in and around the PIs. Here, iEC-induced PIs were used to study the expression and posttranslational modification of the ECM receptor integrin β1. A wide array of integrin β subunits was detected in βTC3 and NIT-1 insulinomas as well as in primary islets, with integrin β1 mRNA and protein detected in all three cell types. Interestingly, the formation of iEC-induced PIs altered the glycosylation patterns of integrin β1, resulting in a higher molecular weight form of the receptor. This form was found in native pancreas but was completely absent in monolayer β-cells. Fluorescence-activated cell sorting analysis of monolayers and PIs revealed a higher expression of integrin β1 in PIs. Antibody-mediated blocking of integrin β1 led to alterations in β-cell morphology, reduced insulin gene expression, and enhanced glucose secretion under baseline conditions. These results suggest that iEC-induced PI formation may alter integrin β1 expression and posttranslational modification by enhancing glycosylation, thereby providing a more physiological culture system for studying integrin-ECM interactions in β cells.

Beta-1 integrin is important for the structural maintenance and homeostasis of differentiating fiber cells

β1-Integrin is a heterodimeric transmembrane protein that has roles in both cell-extra-cellular matrix and cell-cell interactions. Conditional deletion of β1-integrin from all lens cells during embryonic development results in profound lens defects, however, it is less clear whether this reflects functions in the lens epithelium alone or whether this protein plays a role in lens fibers. Thus, a conditional approach was used to delete β1-integrin solely from the lens fiber cells. This deletion resulted in two distinct phenotypes with some lenses exhibiting cataracts while others were clear, albeit with refractive defects. Analysis of "clear" conditional knockout lenses revealed that they had profound defects in fiber cell morphology associated with the loss of the F-actin network. Physiological measurements found that the lens fiber cells had a twofold increase in gap junctional coupling, perhaps due to differential localization of connexins 46 and 50, as well as increased water permeability. This would presumably facilitate transport of ions and nutrients through the lens, and may partially explain how lenses with profound structural abnormalities can maintain transparency. In summary, β1-integrin plays a role in maintaining the cellular morphology and homeostasis of the lens fiber cells.

N-ethylmaleimide-sensitive factor attachment protein α (αSNAP) regulates matrix adhesion and integrin processing in human epithelial cells

Integrin-based adhesion to the extracellular matrix (ECM) plays critical roles in controlling differentiation, survival, and motility of epithelial cells. Cells attach to the ECM via dynamic structures called focal adhesions (FA). FA undergo constant remodeling mediated by vesicle trafficking and fusion. A soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein α (αSNAP) is an essential mediator of membrane fusion; however, its roles in regulating ECM adhesion and cell motility remain unexplored. In this study, we found that siRNA-mediated knockdown of αSNAP induced detachment of intestinal epithelial cells, whereas overexpression of αSNAP increased ECM adhesion and inhibited cell invasion. Loss of αSNAP impaired Golgi-dependent glycosylation and trafficking of β1 integrin and decreased phosphorylation of focal adhesion kinase (FAK) and paxillin resulting in FA disassembly. These effects of αSNAP depletion on ECM adhesion were independent of apoptosis and NSF. In agreement with our previous reports that Golgi fragmentation mediates cellular effects of αSNAP knockdown, we found that either pharmacologic or genetic disruption of the Golgi recapitulated all the effects of αSNAP depletion on ECM adhesion. Furthermore, our data implicates β1 integrin, FAK, and paxillin in mediating the observed pro-adhesive effects of αSNAP. These results reveal novel roles for αSNAP in regulating ECM adhesion and motility of epithelial cells.

Interactions between the discoidin domain receptor 1 and β1 integrin regulate attachment to collagen

Collagen degradation by phagocytosis is essential for physiological collagen turnover and connective tissue homeostasis. The rate limiting step of phagocytosis is the binding of specific adhesion receptors, which include the integrins and discoidin domain receptors (DDR), to fibrillar collagen. While previous data suggest that these two receptors interact, the functional nature of these interactions is not defined. In mouse and human fibroblasts we examined the effects of DDR1 knockdown and over-expression on β1 integrin subunit function. DDR1 expression levels were positively associated with enhanced contraction of floating and attached collagen gels, increased collagen binding and increased collagen remodeling. In DDR1 over-expressing cells compared with control cells, there were increased numbers, area and length of focal adhesions immunostained for talin, paxillin, vinculin and activated β1 integrin. After treatment with the integrin-cleaving protease jararhagin, in comparison to controls, DDR1 over-expressing cells exhibited increased β1 integrin cleavage at the cell membrane, indicating that DDR1 over-expression affected the access and susceptibility of cell-surface β1 integrin to the protease. DDR1 over-expression was associated with increased glycosylation of the β1 integrin subunit, which when blocked by deoxymannojirimycin, reduced collagen binding. Collectively these data indicate that DDR1 regulates β1 integrin interactions with fibrillar collagen, which positively impacts the binding step of collagen phagocytosis and collagen remodeling.

A tale of two collagen receptors, integrin β1 and discoidin domain receptor 1, in epithelial cell differentiation

As increase in collagen deposition is no longer taken as simply a consequence but, rather, an inducer of disease progression; therefore, the understanding of collagen signal transduction is fundamentally important. Cells contain at least two types of collagen receptors: integrins and discoidin domain receptors (DDRs). The integrin heterodimers α1β1, α2β1, α10β1, and α11β1 are recognized as the non-tyrosine kinase collagen receptors. DDR1 and 2, the tyrosine kinase receptors of collagen, are specifically expressed in epithelium and mesenchyme, respectively. While integrin β1 and DDR1 are both required for cell adhesion on collagen, their roles in epithelial cell differentiation during development and disease progression seem to counteract each other, with integrin β1 favoring epithelium mesenchyme transition (EMT) and DDR1 inducing epithelial cell differentiation. The in vitro evidence shows that the integrin β1 and DDR1 exert opposing actions in regulation of membrane stability of E-cadherin, which itself is a critical regulator of epithelial cell differentiation. Here, we review the functional roles of integrin β1 and DDR1 in regulation of epithelial cell differentiation during development and disease progression, and explore the underlining mechanisms regarding to the regulation of membrane stability of E-cadherin.

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.

Differential regulation of adhesion complex turnover by ROCK1 and ROCK2

BACKGROUND

ROCK1 and ROCK2 are serine/threonine kinases that function downstream of the small GTP-binding protein RhoA. Rho signalling via ROCK regulates a number of cellular functions including organisation of the actin cytoskeleton, cell adhesion and cell migration.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we use RNAi to specifically knockdown ROCK1 and ROCK2 and analyse their role in assembly of adhesion complexes in human epidermal keratinocytes. We observe that loss of ROCK1 inhibits signalling via focal adhesion kinase resulting in a failure of immature adhesion complexes to form mature stable focal adhesions. In contrast, loss of ROCK2 expression results in a significant reduction in adhesion complex turnover leading to formation of large, stable focal adhesions. Interestingly, loss of either ROCK1 or ROCK2 expression significantly impairs cell migration indicating both ROCK isoforms are required for normal keratinocyte migration.

CONCLUSIONS

ROCK1 and ROCK2 have distinct and separate roles in adhesion complex assembly and turnover in human epidermal keratinocytes.

An activating beta1 integrin mutation increases the conversion of benign to malignant skin tumors

Identifying the physiologic relevance of cancer-associated genetic polymorphisms is a major challenge. Several changes in the coding sequence of beta integrin subunits have now been described in human tumors. One of these, T188Ibeta1, was identified as a heterozygous mutation in a poorly differentiated squamous cell carcinoma (SCC) and shown to activate extracellular matrix adhesion and inhibit keratinocyte differentiation in vitro. To study its contribution to tumor development, we overexpressed the mutant or wild-type (WT) human beta1 subunit in the basal layer of mouse epidermis using the keratin 14 promoter. The transgenic integrins were expressed at the cell surface and were functional, with the T188Ibeta1 subunit promoting cell spreading to a greater extent than WTbeta1. Epidermal proliferation and differentiation were unaffected and no expansion of the stem cell compartment was detected. During chemical carcinogenesis, both transgenes increased papilloma formation, but only the T188Ibeta1 transgene stimulated the conversion of papillomas to SCCs. Papillomas bearing the mutation showed increased Erk activity and reduced differentiation. SCCs expressing T188Ibeta1 were less well-differentiated than those expressing WTbeta1. These observations establish that the expression of a genetic variant in the I-like domain of beta1 integrins does not affect normal epidermal homeostasis, but increases tumor susceptibility and influences tumor type.

Isolation and culture of different epidermal and dermal cell types from human scalp suitable for the development of a therapeutical cell spray

BACKGROUND

Previous studies demonstrated, that cultured epithelial autografts (CEA) can be isolated and skin cell sprays can be produced for application on different types of wounds. The purpose of the present study was to determine which cell types can be isolated from the human scalp and whether these cells can be used for spray transplantation.

METHODS

Outer root sheath cells (ORS), keratinocytes, melanocytes, dermal papilla cells (DP), and dermal sheath cells (DSC) were isolated from human scalp tissue. Isolated cells were characterized, expanded and sprayed in an in vitro model. Growth behaviour, morphology and cell counts were compared with non-sprayed cells.

RESULTS

With acceptable time, equipment and laboratory personnel a sufficient amount of keratinocytes, ORS, melanocytes, DP cells and DSC cells could be achieved. The cells are sufficient for application as a cell spray. Cells, positive for Integrin alpha6, Cytokeratin 19, CD73 and CD105 were identified within the cultures.

CONCLUSIONS

Human scalp is suitable to gain epidermal and dermal cells for the development of therapeutic cell spray transplantation. Further studies have to determine, whether these cells can be combined to produce wound specific skin substitutes.

Novel quinazoline-based compounds impair prostate tumorigenesis by targeting tumor vascularity

Previous evidence showed the ability of the quinazoline-based alpha(1)-adrenoreceptor antagonist doxazosin to suppress prostate tumor growth via apoptosis. In this study, we carried out structural optimization of the chemical nucleus of doxazosin and a subsequent structure-function analysis toward the development of a novel class of apoptosis-inducing and angiogenesis-targeting agents. Our lead compound, DZ-50, was effective at reducing endothelial cell viability via a nonapoptotic mechanism. Treatment with DZ-50 effectively prevented in vitro tube formation and in vivo chorioallantoic membrane vessel development. Confocal microscopy revealed a significantly reduced ability of tumor cells to attach to extracellular matrix and migrate through endothelial cells in the presence of DZ-50. In vivo tumorigenicty studies using two androgen-independent human prostate cancer xenografts, PC-3 and DU-145, showed that DZ-50 treatment leads to significant suppression of tumorigenic growth. Exposure to the drug at the time of tumor cell inoculation led to prevention of prostate cancer initiation. Furthermore, DZ-50 resulted in a reduced formation of prostate-tumor derived metastatic lesions to the lungs in an in vivo spontaneous metastasis assay. Thus, our drug discovery approach led to the development of a class of lead (quinazoline-based) compounds with higher potency than doxazosin in suppressing prostate growth by targeting tissue vascularity. This new class of quinazoline-based compounds provides considerable promise as antitumor drugs for the treatment of advanced prostate cancer.

Characterization of the Epstein–Barr virus glycoprotein BMRF-2

Epstein-Barr virus (EBV) BMRF-2 protein interaction with the beta1 family of integrins plays an important role in EBV infection of polarized oral epithelial cells. In this work, we characterized BMRF-2 protein expression in EBV-infected B lymphoblastoid and polarized oral epithelial cells, and in hairy leukoplakia (HL) epithelium. BMRF-2 expression in B cells and polarized oral epithelial cells was associated with the EBV lytic infection. In these cells, BMRF-2 is efficiently transported to the cell membrane and its integrin binding Arg-Gly-Asp (RGD) motif is exposed on the cell surface. BMRF-2 is highly expressed in HL epithelium and accumulates at the lateral border of oral keratinocytes. In EBV-infected polarized oral epithelial cells, this protein is transported to the basolateral membranes and co-localized with beta1 integrin. These data suggest that BMRF-2 may play an important role in cell-to-cell spread of EBV within the oral epithelium. BMRF-2 is glycosylated through O-linked oligosaccharides; it forms oligomers and is associated with the virion envelope. Its C-terminal tail is localized in the cytoplasm. We found that beta1, alpha5, and alpha3 integrins are present in purified EBV virions. We show that BMRF-2 is a ligand for beta1, alpha5, alpha3, and alphav integrins and our data are consistent with a role for BMRF-2 in viral lytic infection.

Neutrophils are a key component of the antitumor efficacy of topical chemotherapy with ingenol-3-angelate

Harnessing neutrophils for the eradication of cancer cells remains an attractive but still controversial notion. In this study, we provide evidence that neutrophils are required to prevent relapse of skin tumors following topical treatment with a new anticancer agent, ingenol-3-angelate (PEP005). Topical PEP005 treatment induces primary necrosis of tumor cells, potently activates protein kinase C, and was associated with an acute T cell-independent inflammatory response characterized by a pronounced neutrophil infiltrate. In Foxn1(nu) mice depleted of neutrophils and in CD18-deficient mice (in which neutrophil extravasation is severely impaired) PEP005 treatment was associated with a >70% increase in tumor relapse rates. NK cell or monocyte/macrophage deficiency had no effect on relapse rates. Both in vitro and in mice, PEP005 induced MIP-2/IL-8, TNF-alpha, and IL-1beta, all mediators of neutrophil recruitment and activation. In vitro, PEP005 activated human endothelial cells resulting in neutrophil adhesion and also induced human neutrophils to generate tumoricidal-reactive oxygen intermediates. Treatment of tumors with PEP005 significantly elevated the level of anticancer Abs, which were able to promote neutrophil-mediated Ab-dependent cellular cytotoxicity (ADCC) in vitro. PEP005 treatment of tumors grown in SCID mice was also associated with >70% increase in tumor relapse rates. Taken together, these data suggest a central role for neutrophil-mediated ADCC in preventing relapse. PEP005-mediated cure of tumors therefore appears to involve initial chemoablation followed by a neutrophil-dependent ADCC-mediated eradication of residual disease, illustrating that neutrophils can be induced to mediate important anticancer activity with specific chemotherapeutic agents.

Different consequences of beta1 integrin deletion in neonatal and adult mouse epidermis reveal a context-dependent role of integrins in regulating proliferation, differentiation, and intercellular communication

There are conflicting reports of the consequences of deleting beta1 integrins from the epidermis of transgenic mice. Epidermal thinning with normal differentiation and lack of inflammation has been observed; conversely, epidermal thickening, abnormal differentiation, and dermal fibrosis can occur. beta1 integrin deletion results in decreased epidermal proliferation, yet on wounding the proliferative defect is overcome. To distinguish primary from secondary consequences of beta1 integrin loss, we compared epidermal beta1 deletion at E14.5 via K5Cre and 4-hydroxy-tamoxifen induced deletion in adulthood via K14CreER. As reported previously, there was dermo-epidermal splitting, inflammation, reduced proliferation, and hair follicle and sebaceous gland loss in 30-d-old K5Cre beta1-null mice. These changes were not observed 30 d after beta1 integrin deletion in adult epidermis, however, and there were no changes in the hair follicle stem cell compartment. Deletion in adult epidermis revealed a previously unreported correlation between the level of beta1 integrins and proliferation in the interfollicular epidermis that was remarkably consistent with human epidermis. In addition, the number of melanocytes in interfollicular epidermis was greatly increased. Our results highlight the context-dependent effects of beta1 integrin deletion and suggest that inflammation may be responsible for some of the K5Cre beta1-null phenotype.

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.

Golgi fragmentation is associated with ceramide-induced cellular effects

Ceramide has been shown to cause anoikis, a subtype of apoptosis due to inadequate cell adhesion. However, the underlying mechanism is unclear. Herein, we report that D-e-C6-ceramide (D-e-Cer), via generating sphingosine, disrupts the Golgi complex (GC), which is associated with various cellular effects, including anoikis. Treatment of HeLa cells with D-e-Cer caused cell elongation, spreading inhibition, rounding, and detachment before apoptosis (anoikis). In D-e-Cer-treated cells, glycosylation of beta1 integrin in the GC was inhibited, thus its associated integrin receptors failed to translocate to the cell surface. Ceramide treatment also inhibited the reorganization of both microtubule and F-actin cytoskeletons, focal adhesions, and filopodia. These cellular effects were preceded by fragmentation of the Golgi complex. In contrast, L-e-C6-ceramide (L-e-Cer), the enantiomer of D-e-Cer, failed to induce these cellular effects. Mass spectrometric analysis revealed that treatment HeLa cells with D-e-Cer but not L-e-Cer caused a >50-fold increase in the levels of sphingosine, a product of hydrolysis of ceramide. Treatment with D-e-sphingosine and its enantiomer, L-e-sphingosine, caused massive perinuclear vacuolization, Golgi fragmentation, and cell rounding. Together, these results suggest that sphingosine generated from hydrolysis of ceramide causes the GC disruption, leading to various cellular effects.

A review of keratinocyte delivery to the wound bed

Over the last 20 years, confluent sheets of cultured epithelial autograft have been used for patients with major burns. Problems with the lack of "take" and long-term durability, as well as the time delay to produce such grafts, have led to the development of delivery systems to transfer keratinocytes to the wound bed. This review article describes the problems of using cultured epithelial autograft and the advantages of using preconfluent keratinocytes. Despite the numerous delivery systems that have been reported, most studies are limited to animal wound bed models. There are a few small clinical studies that have demonstrated enhanced healing using mainly subjective methods. There is a need for controlled, randomized clinical trials to prove the efficacy of keratinocyte delivery systems. Proposals for the use of this technology are made.

Variant glycosylation: an underappreciated regulatory mechanism for β1 integrins

Although it has been known for many years that beta1 integrins undergo differential glycosylation in accordance with changes in cell phenotype, the potential role of N-glycosylation as a modulator of integrin function has received little attention. One reason for the relatively limited interest in this topic likely relates to the fact that much of the prior research was correlative in nature. However, new results now bolster the hypothesis that there is a causal relationship between variant glycosylation and altered integrin activity. In this review, the evidence for variant glycosylation as a regulatory mechanism for beta1 integrins are summarized, with particular emphasis on: (1). outlining the instances in which cell phenotypic variation is associated with differential beta1 glycosylation, (2). describing the specific alterations in glycan structure that accompany phenotypic changes and (3). presenting potential mechanisms by which variant glycosylation might regulate integrin function.

Enterophilin-1 Interacts with Focal Adhesion Kinase and Decreases β1 Integrins in Intestinal Caco-2 Cells

Intestinal cell growth and differentiation are tightly regulated by growth factors and extracellular matrix components along the crypt-villus axis. We previously described enterophilin-1 (Ent-1) as a new intestinal protein associated with growth arrest and enterocyte differentiation. Ent-1 interacted with sorting nexin 1 and decreased cell surface epidermal growth factor receptor. Because beta(1) integrins are mostly found in vivo in the proliferative crypt cells, we investigated the role of Ent-1 in the fate of beta(1) integrin subunits. In undifferentiated intestinal Caco-2 cells, overexpression of Ent-1 induces a marked decrease of alpha(5)beta(1) integrin pools, whereas alpha(2)beta(1) integrin is weakly affected. Conversely, overexpression of sorting nexin 1 has no effect on integrin levels despite its ability to interact with Ent-1. Interestingly, we identified focal adhesion kinase as a new Ent-1 partner using yeast two-hybrid screening and co-precipitation experiments. Furthermore by confocal microscopy, we observed that Ent-1 and beta(1) integrins partly co-localize on vesicular structures, suggesting a role for Ent-1 in integrin trafficking. Because focal adhesion kinase is able to bind both Ent-1 and beta(1) integrins, the kinase might act as a molecular bridge between the two proteins. Altogether, these results support a role of Ent-1 in regulating beta(1) integrin expression that could favor intestinal differentiation.

Regulation of beta1C and beta1A integrin expression in prostate carcinoma cells

beta(1C) and beta(1A) integrins are two splice variants of the human beta(1) integrin subfamily that act as an inhibitor and a stimulator of cell proliferation, respectively. In neoplastic prostate epithelium, both these variants are down-regulated at the mRNA level, but only beta(1C) protein levels are reduced. We used an experimental model consisting of PNT1A, a normal immortalized prostate cell line, and LNCaP and PC-3, two prostate carcinoma cell lines, to investigate both the transcription/post-transcription and translation/post-translation processes of beta(1C) and beta(1A). Transcriptional regulation played the key role for the reduction in beta(1C) and beta(1A) mRNA expression in cancer cells, as beta(1C) and beta(1A) mRNA half-lives were comparable in normal and cancer cells. beta(1C) translation rate decreased in cancer cells in agreement with the decrease in mRNA levels, whereas beta(1A) translation rate increased more than 2-fold, despite the reduction in mRNA levels. Both beta(1C) and beta(1A) proteins were degraded more rapidly in cancer than in normal cells, and pulse-chase experiments showed that intermediates and/or rates of beta(1C) and beta(1A) protein maturation differ in cancer versus normal cells. Inhibition of either calpain- or lysosomal-mediated proteolysis increased both beta(1C) and beta(1A) protein levels, the former in normal but not in cancer cells and the latter in both cell types, albeit at a higher extent in cancer than in normal cells. Interestingly, inhibition of the ubiquitin proteolytic pathway increased expression of ubiquitinated beta(1C) protein without affecting beta(1A) protein levels in cancer cells. These results show that transcriptional, translational, and post-translational processes, the last involving the ubiquitin proteolytic pathway, contribute to the selective loss of beta(1C) integrin, a very efficient inhibitor of cell proliferation, in prostate malignant transformation.

Modulation in vitro of keratinocyte integrins by interferon-alpha and interferon-gamma

BACKGROUND

Interferon-alpha and -gamma are glycoproteins with antiviral and immunoregulatory properties. In vitro studies have shown a role for these cytokines in the regulation of epidermal keratinocyte growth and differentiation. In the same way, integrins are adhesion molecules which regulate keratinocyte proliferation and differentiation.

AIM

To determine whether the regulatory activity of interferons on keratinocyte proliferation and differentiation is related to a modulation of keratinocyte integrins.

METHODS

Two different methods were used: monolayers and reconstituted skin, incubated either with 1,200 U/mL interferon-alpha or 500 U/mL interferon-gamma or control medium for 48 h. The integrin expression was assessed by flow cytometry and immunohistochemistry.

RESULTS

In monolayers, only the alpha3 subunit was significantly inhibited by interferon-gamma. In reconstituted skin, where keratinocytes are differentiated, both interferons had an inductive effect on beta1 expression and interferon-alpha had an inhibitory effect on alpha6 expression.

CONCLUSION

Interferon-alpha and -gamma induce a modulatory effect on alpha3, alpha6 and beta1 which appears to be related to the state of differentiation. Moreover, the decreased expression of alpha6 and alpha3 could be one of the mechanisms involved in the formation of bullous lesions during long-term interferon therapy.

A signaling role for the uncleaved form of alpha 6 integrin in differentiating lens fiber cells

Many alpha integrin subunits are cleaved during their processing to yield heavy and light chains, which remain associated by disulfide bonds. While uncleaved alpha integrin subunits can form functional receptors that sometimes have distinct signaling roles from their better-characterized endoproteolytically cleaved counterparts, their expression at the cell surface and their association with signaling complexes have yet to be determined in vivo. In this study, we demonstrate that, in differentiating lens fiber cells, the uncleaved form of alpha 6 integrin was expressed at the cell surface. This form of alpha 6 integrin coimmunoprecipitated with both the signaling adaptor molecule Shc and its downstream effector Grb2, suggesting that, in lens fiber cells, uncleaved alpha 6 integrin was associated with a Shc-mediated signaling complex. We show that expression of the cleaved form of alpha 6 integrin progressively decreased relative to uncleaved alpha 6 integrin as the state of lens cell differentiation increased, resulting in the predominance of uncleaved alpha 6 integrin in the lens fiber cell zones. Interestingly, we previously have shown that alpha 6 integrin is localized principally along the extensive cell-cell interfaces of these lens fiber cells, in the absence of its extracellular matrix ligand laminin. While we found that the cleaved form of alpha 6 integrin contained both high mannose and complex sugars, the uncleaved form of alpha 6 integrin contained only high mannose sugars. These properties suggest that the uncleaved form of alpha 6 integrin may have a unique role in the embryonic lens. Its high association with Shc and Grb2 in the differentiating cortical fiber cell zone indicates that alpha 6 integrin may provide a cell survival signal in the presence of the apoptotic-like processes that are initiated in this region of the embryonic lens to clear the lens cells of their organelles.

Phosphatidylinositol 3-kinase is a key regulator of early phase differentiation in keratinocytes

The survival and growth of epithelial cells depend on adhesion to the extracellular matrix. Because epidermal keratinocytes differentiate as they leave the basement membrane, an adhesion signal may regulate the initiation of differentiation. Phosphatidylinositol 3-kinase (PI3K) is a fundamental signaling molecule that regulates the adhesion signal. Transfection of a dominant negative form of PI3K into keratinocytes using an adenovirus vector resulted in significant morphological changes comparable to differentiation and the induction of differentiation markers, keratin (K) 1 and K10. In turn, transfection with the constitutively active form of PI3K almost completely abolished the induction of K1 and K10 by differentiation in suspension cultures using polyhydroxyethylmethacrylate-coated dishes. PI3K activity was lost in suspension culture, except by cells bearing the constitutively active form of PI3K. These data demonstrate that blockade of PI3K results in differentiation and that activation of PI3K prevents differentiation. Furthermore, expression of the dominant negative form of PI3K significantly inhibited keratinocyte adhesion to the extracellular matrix and reduced the surface expression of alpha(6) and beta(1) integrins in suspension culture. Moreover, expression of the active form of PI3K restored the mRNA levels of adhesion molecules that were reduced in suspension culture, including alpha(3), alpha(6), and beta(1) integrins, BP180, and BP230. In conclusion, loss of PI3K activity results in keratinocytes leaving the basement membrane and the initiation of a "default" differentiation mechanism.

DR-nm23 expression affects neuroblastoma cell differentiation, integrin expression, and adhesion characteristics

BACKGROUND AND PROCEDURE

Nm23 gene family has been associated with metastasis suppression and differentiation. We studied DR-nm23 during neuroblastoma cells differentiation. DR-nm23 expression increased after retinoic acid induction of differentiation in human cell lines SK-N-SH and LAN-5.

RESULTS

In several cell lines, overexpression of DR-nm23 was associated with more differentiated phenotypes. SK-N-SH cells increased vimentin expression, increased deposition of collagen type IV, modulated integrin expression, and underwent growth arrest; the murine neuroblastoma cell line N1E-115 showed neurite outgrowth and a striking enhancement of beta1 integrin expression. Up-regulation of beta1 integrin was specifically responsible for the increase in the adhesion to collagen type I-coated plates. Finally, cells overexpressing DR-nm23 were unable to growth in soft agar.

CONCLUSIONS

In conclusion, DR-nm23 expression is directly involved in differentiation of neuroblastoma cells, and its ability to affects the adhesion to extracellular substrates and to inhibit growth in soft agar suggests an involvement in the metastatic potential of neuroblastoma.

Low asialoglycoprotein receptor expression as markers for highly proliferative potential hepatocytes

Development of a reliable method to isolate highly proliferative potential hepatocytes will provide insight into the molecular mechanisms of liver regeneration, as well as proving crucial for the development of a biohybrid artificial liver. The aim of this study is to isolate highly proliferative, e.g., progenitor-like, hepatocytes. To this end, we fractionated hepatocytes expressing low and high levels of the asialoglycoprotein receptor (ASGP-R) based on the difference in their adhesion to poly[N-p-vinylbenzyl-O-beta-d-galactopyranosyl-(1-->4)-d-gluconamide] (PVLA), and examined the proliferative activity and gene expression of these fractionated hepatocytes. The results showed that approximately 0.5 to 1% of the total number of hepatocytes, which showed low adhesion to PVLA, expressed low levels of the ASGP-R, while the rest of hepatocyte population with high adhesion to PVLA expressed high levels of the ASGP-R. Interestingly hepatocytes with low ASGP-R expression levels had much higher DNA synthesizing activity (i.e., are much more proliferative) than those with high ASGP-R expression levels. Moreover, hepatocytes with low ASGP-R expression levels expressed higher levels of epidermal growth factor receptor (EGF-R), CD29 (beta1 integrin) and CD49f (alpha6 integrin) and lower levels of glutamine synthetase than those with high ASGP-R expression. These findings suggested that hepatocytes with low adhesion to PVLA due to their low ASGP-R expression could be potential candidates for progenitor-like hepatocytes due to their high proliferative capacity; hence, the low expression of the ASGP-R could be a unique marker for progenitor hepatocytes. The isolation of hepatocytes with different functional phenotypes using PVLA may provide a new research tool for a better understanding of the biology of hepatocytes and the mechanisms regulating their proliferation and differentiation in health and disease.

Activity of proximal promoter of the human beta(1)-integrin gene was increased in Sézary syndrome

Changes in beta1-integrin expression have been involved in abnormal cellular interactions between malignant lymphocytes from Sézary (Sz) patients and keratinocytes. In this paper, we compare the activity of both distal and proximal promoters of the beta1-integrin gene in malignant lymphocytes from Sz patients with human normal lymphocytes. Activity of both beta1-integrin promoters was also analysed in human normal keratinocytes. Northern blot analysis shows that beta1-integrin mRNA expression is higher in malignant Sz lymphocytes than in normal lymphocytes. CAT assays show that the activity of proximal beta1-integrin promoter is markedly increased (up to 6-fold) in malignant lymphocytes from Sz patients, in comparison to normal lymphocytes. These results suggest that changes in activity of the proximal promoter of beta1-integrin subunit could be, in part, responsible for the abnormal cellular interactions between malignant lymphocytes and keratinocytes observed in Sz syndrome.

DR-nm23 expression affects neuroblastoma cell differentiation, integrin expression, and adhesion characteristics

BACKGROUND AND PROCEDURE

Nm23 gene family has been associated with metastasis suppression and differentiation. We studied DR-nm23 during neuroblastoma cells differentiation. DR-nm23 expression increased after retinoic acid induction of differentiation in human cell lines SK-N-SH and LAN-5.

RESULTS

In several cell lines, overexpression of DR-nm23 was associated with more differentiated phenotypes. SK-N-SH cells increased vimentin expression, increased deposition of collagen type IV, modulated integrin expression, and underwent growth arrest; the murine neuroblastoma cell line N1E-115 showed neurite outgrowth and a striking enhancement of beta1 integrin expression. Up-regulation of beta1 integrin was specifically responsible for the increase in the adhesion to collagen type I-coated plates. Finally, cells overexpressing DR-nm23 were unable to growth in soft agar.

CONCLUSIONS

In conclusion, DR-nm23 expression is directly involved in differentiation of neuroblastoma cells, and its ability to affects the adhesion to extracellular substrates and to inhibit growth in soft agar suggests an involvement in the metastatic potential of neuroblastoma.

Synthesis of extracellular matrix and adhesion through beta(1) integrins are critical for fetal ventricular myocyte proliferation

Extracellular matrix (ECM) regulates vascular smooth muscle cell proliferation. The role of ECM in myocardial growth is unexplored. We sought to determine whether human fetal ventricular myocytes (HFVMs) produce ECM and whether synthesis and attachment to ECM are necessary for their epidermal growth factor (EGF)-dependent and -independent proliferation. Cultured HFVMs proliferate in the presence but not absence of serum and EGF, as determined by increase in cell number and [(3)H]thymidine and [(14)C]leucine incorporation (measures of DNA and protein synthesis, respectively). Using a cyanogen bromide digestion technique to measure collagen and elastin and using affinity chromatography for fibronectin, we found that HFVMs synthesized collagen and fibronectin but not elastin. HFVMs grown on exogenous ECM (including fibronectin and type I collagen and laminin) demonstrated no change in proliferation or DNA and protein synthesis with or without EGF. However, inhibition of collagen synthesis using cis-4-hydroxyproline resulted in a decrease in EGF-related HFVM proliferation and DNA and protein synthesis, which was reversed by exposure to L-proline but not by growth on type I collagen. Use of beta(1) but not beta(3) integrin antibody to inhibit cell interaction with ECM resulted in a decrease in HFVM proliferation and DNA and protein synthesis in response to EGF. Furthermore, EGF-dependent proliferation was enhanced by alpha(1)beta(1) and alpha(5)beta(1) antibodies that act as functional ligands, but not alpha(3)beta(1), the only beta(1) subtype expressed in adult myocytes. In conclusion, proliferating HFVMs synthesize collagen and fibronectin. The proliferative response of HFVMs to EGF requires the synthesis of collagen as well as attachment to specific alpha/beta(1) integrin heterodimers.

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.

The alpha2 and alpha5 integrin genes: identification of transcription factors that regulate promoter activity in epidermal keratinocytes

We analysed the activity of the proximal promoters of the alpha2 and alpha5 integrin genes in human keratinocytes. An AP-1 site, found in the alpha5 but not the alpha2 promoter, bound c-Jun/c-Fos dimers and contributed strongly to promoter activity. Both promoters had a CCAAT/enhancer binding protein (C/EBP) binding site: the alpha5 C/EBP element enhanced activity, while the alpha2 site was a negative regulatory element. C/EBP overexpression repressed the activity of both promoters, but the effect was independent of occupancy of the identified C/EBP binding sites, suggesting interactions with additional transcription factors. We propose that upregulation of C/EBPs contributes to the inhibition of integrin transcription during keratinocyte terminal differentiation, while AP-1 factors play a role in the selective induction of the alpha5 gene during wound healing.

Cell vacuolation induced by the VacA cytotoxin of Helicobacter pylori is regulated by the Rac1 GTPase

Chronic gastric infection with the Gram-negative bacterium Helicobacter pylori is a major contributing factor in the development of duodenal ulcers and is believed to be a significant risk factor in the development of gastric tumors. The VacA cytotoxin of H. pylori is a 90-kDa secreted protein that forms trans-membrane ion channels. In epithelial cells, VacA activity is associated with the rapid formation of acidic vacuoles enriched for late endosomal and lysosomal markers. Rac1 is a member of the Rho family of small GTP-binding proteins that regulate reorganization of the actin cytoskeleton and intracellular signal transduction and are being shown increasingly to play a role in membrane trafficking events. In this study we report that: (i) green fluorescent-tagged Rac1 localizes around the perimeter of the vacuoles induced by VacA; (ii) expression of dominant negative Rac1 in epithelial cells inhibits vacuole formation; (iii) expression of constitutively active Rac1 potentiates the activity of VacA. Taken together, these data demonstrate a role for Rac1 in the regulation of VacA activity.

Adhesive properties of human basal epidermal cells: an analysis of keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells

The basal layer of human epidermis is a heterogeneous population of proliferative and differentiating cells that can be divided into at least three functionally discrete compartments: keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells. Basal cells adhere to the underlying basement membrane via integrins, and although decreased adhesion is a key event in epidermal differentiation, the specific role of particular integrins is poorly understood. We report here on the comparative expression and function of the beta1 versus alpha6beta4 integrins in keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells of neonatal human foreskin epidermis. Adhesion assays demonstrate that both keratinocyte stem cells and transit amplifying cells comprise rapidly adhering cells that exhibit high levels of functional beta1 and alpha6beta4 integrins. Interestingly, a proportion of basal cells that have begun to differentiate in vivo within the basal layer as determined by their expression of the differentiation-specific markers K10 and involucrin also retain high levels of activated beta1 integrin, but downregulate alpha6beta4 expression selectively (termed alpha6dimbeta1bri). These cells also retain their adhesive capacity, indicating that induction of differentiation in vivo does not correlate with decreased beta1 integrin expression or function. We have previously reported on the use of alpha6 integrin in conjunction with a proliferation associated marker (10G7 ag) to separate keratinocyte stem cells (phenotype alpha6bri10G7dim) from other basal cells (Li et al. Proc Natl Acad Sci 95:3902-3907 1998). A comparison of the long-term proliferative potential of beta1bri10G7dim cells with alpha6bri10G7dim showed that selection of alpha6bri10G7dim allows the isolation of a purer fraction of keratinocyte stem cells.

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.

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.

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.

Modulation of integrin activity is vital for morphogenesis

Cells can vary their adhesive properties by modulating the affinity of integrin receptors. The activation and inactivation of integrins by inside-out mechanisms acting on the cytoplasmic domains of the integrin subunits has been demonstrated in platelets, lymphocytes, and keratinocytes. We show that in the embryo, normal morphogenesis requires the alpha subunit cytoplasmic domain to control integrin adhesion at the right times and places. PS2 integrin (alphaPS2betaPS) adhesion is normally restricted to the muscle termini, where it is required for attaching the muscles to the ends of other muscles and to specialized epidermal cells. Replacing the wild-type alphaPS2 with mutant forms containing cytoplasmic domain deletions results in the rescue of the majority of defects associated with the absence of the alphaPS2 subunit, however, the mutant PS2 integrins are excessively active. Muscles containing these mutant integrins make extra muscle attachments at aberrant positions on the muscle surface, disrupting the muscle pattern and causing embryonic lethality. A gain- of-function phenotype is not observed in the visceral mesoderm, showing that regulation of integrin activity is tissue-specific. These results suggest that the alphaPS2 subunit cytoplasmic domain is required for inside-out regulation of integrin affinity, as has been seen with the integrin alphaIIbbeta3.

Calcium-induced intercellular adhesion of keratinocytes does not involve accumulation of beta 1 integrins at cell-cell contacts and does not involve changes in the levels or phosphorylation of catenins

On initiation of terminal differentiation human epidermal keratinocytes detach from the underlying basement membrane as a result of inactivation and subsequent loss of integrins from the cell surface. Assembly of keratinocytes into multilayered sheets requires functional E- and P-cadherin and when stratification is inhibited in low calcium medium differentiating keratinocytes continue to express functional integrins. Using immunofluorescence microscopy, we found that on addition of calcium ions to keratinocyte monolayers there was colocalisation of the beta 1 integrins and E-cadherin along the lateral membranes except for a zone close to the substratum which exclusively contained integrins. Quantitative immunoelectron microscopy showed that on induction of stable cell-cell contacts the density of beta 1 integrins was the same on the apical and lateral membranes, suggesting that the accumulation of integrins on the lateral membranes observed by immunofluorescence microscopy is due to the increased area of contact between adjacent cells and not to an increase in receptor density. There were no changes in the levels of catenins and their degree of phosphorylation after induction of cell-cell contacts. These observations provide new sights into the mechanism of calcium-dependent intercellular adhesion of keratinocytes.

Oncogenic Ki-ras but not oncogenic Ha-ras blocks integrin beta1-chain maturation in colon epithelial cells

Human colorectal tumors commonly contain mutations in Ki-ras but rarely, if ever, in Ha-ras. The selectivity for Ki-ras mutations in this tumor was explored using the HD6-4 colon epithelial cell line which contains no ras mutations. After adhesion to an extracellular matrix, HD6-4 cells polarize into columnar goblet cells with distinct apical and basal regions. Stable HD6-4 transfectants were made with mini-gene constructs of the oncogenic cellular Ki-ras4BG12V gene, the oncogenic Ha-rasG12V gene, or mini-gene constructs of wild-type Ki-ras4B as a control. Ki-ras mutations, but not Ha-ras mutations, disrupted colon epithelial cell apicobasal polarity and adhesion to collagen I and laminin. Three Ha-ras transfectants and three Ki-ras transfectants exhibited Ras proteins expressing the Val-12 mutation by Western blotting with pan-rasG12V antibody. Only wild-type Ki-ras transfectant cells and oncogenic Ha-ras transfectant cells synthesized the mature, fully glycosylated forms of beta1 integrin. Instead of the mature integrin beta1-chain, a faster migrating beta1-chain intermediate was detected on the cell surface and in the cytoplasm of the oncogenic Ki-ras transfectants. Expression of the oncogenic Ki-ras gene caused the altered beta1 integrin maturation because phosphorothiolated antisense oligonucleotides to Ki-ras reduced expression of both the mutant Ki-Ras protein and the aberrant integrin beta1-chain and increased expression of the mature integrin beta1-chain. Altered glycosylation generated the new beta1 integrin form since integrin core beta1-chain proteins of the same molecular weight were yielded in Ki-ras, Ha-ras, and control transfectants after removal of sugar residues with endoglycosidase F or following tunicamycin treatment to inhibit glycosylation. The selective effect of oncogenic Ki-ras on beta1 integrin glycosylation was not due to selective activation of mitogen-activated protein kinases because both mutated Ki- and Ha-ras genes activated this pathway and increased cell proliferation. Since blocking the glycosylation of integrin beta1-chain inhibited the adherence, polarization, and subsequent differentiation of colon epithelial cells, the selective effects of the oncogenic cellular Ki-ras gene on integrin beta1-chain glycosylation may account, at least in part, for the selection of Ki-ras mutations in human colon tumors.

Epithelial stem cells

New molecular markers for epidermal stem cells have enabled their isolation both in vitro and from the epidermis lying between hair follicles. Micro-dissection experiments have localised a second population of stem cells within hair follicles. Epidermal stem cells have a patterned distribution in vivo. The patterning can be reconstituted in vitro, showing that it is generated by interactions between keratinocytes and that the differentiation of epidermal stem cells is regulated by signals from other keratinocytes. Recent evidence from transgenic mice suggests that stem cell behaviour in the gut may be regulated by similar cell-cell interactions in vivo. Candidate genes for mediating these interactions are the homologues of Drosophila cell fate patterning genes such as Notch and Wingless and the Cadherin family of cell-cell adhesion molecules. The roles of stem cells and of mutations of the Patched gene in epithelial carcinogenesis are discussed.

Possible role of coexpression of CD9 with membrane-anchored heparin-binding EGF-like growth factor and amphiregulin in cultured human keratinocyte growth

CD9 is a protein with 4 transmembrane domains, and functions as a cell surface antigen. We have previously reported that CD9 functions as an up-regulator of membrane-anchored heparin-binding EGF-like growth factor (proHB-EGF) activity, which is a potent mitogen as well as a soluble HB-EGF. Anti-CD9 antibodies can neutralize the juxtacrine activity of proHB-EGF when both CD9 and proHB-EGF are coexpressed. We demonstrated here: (1) the CD9 gene was transcribed and translated in the cultured human keratinocytes; (2) anti-CD9 antibody inhibited the approximately 50% growth of human keratinocytes in culture; (3) CD9 was coprecipitated with proHB-EGF and membrane-anchored amphiregulin (proAR), and (4) the transient coexpression of CD9 with proHB-EGF or proAR in mouse L cells up-regulated their juxtacrine growth factor activities. These results suggest that CD9 would make a heterodimer and/or trimer complex with proHB-EGF and proAR, and might cooperate with proHB-EGF and proAR for human keratinocyte growth in a juxtacrine manner.

alpha3beta1 Integrin is required for normal development of the epidermal basement membrane

Integrins alpha3beta1 and alpha6beta4 are abundant receptors on keratinocytes for laminin-5, a major component of the basement membrane between the epidermis and the dermis in skin. These integrins are recruited to distinct adhesion structures within keratinocytes; alpha6beta4 is present in hemidesmosomes, while alpha3beta1 is recruited into focal contacts in cultured cells. To determine whether differences in localization reflect distinct functions of these integrins in the epidermis, we studied skin development in alpha3beta1-deficient mice. Examination of extracellular matrix by immunofluorescence microscopy and electron microscopy revealed regions of disorganized basement membrane in alpha3beta1-deficient skin. Disorganized matrix was first detected by day 15.5 of embryonic development and became progressively more extensive as development proceeded. In neonatal skin, matrix disorganization was frequently accompanied by blistering at the dermal-epidermal junction. Laminin-5 and other matrix proteins remained associated with both the dermal and epidermal sides of blisters, suggesting rupture of the basement membrane itself, rather than detachment of the epidermis from the basement membrane as occurs in some blistering disorders such as epidermolysis bullosa. Consistent with this notion, primary keratinocytes from alpha3beta1-deficient skin adhered to laminin-5 through alpha6 integrins. However, alpha3beta1-deficient keratinocytes spread poorly compared with wild-type cells on laminin-5, demonstrating a postattachment requirement for alpha3beta1 and indicating distinct roles for alpha3beta1 and alpha6beta4. Our findings support a novel role for alpha3beta1 in establishment and/or maintenance of basement membrane integrity, while alpha6beta4 is required for stable adhesion of the epidermis to the basement membrane through hemidesmosomes.

Defective expression of beta 1-integrins in cells with constitutively active alpha L beta 2-integrins

We have investigated a potential relationship between expression of beta 1-integrins and adhesiveness of the beta 2-integrin LFA-1 (alpha L beta 2, CD11a/CD18). By an approach of random mutagenesis and selection we established clones from the human acute lymphatic leukemia cell line HPB-ALL with (i) constitutively active LFA-1 and (ii) with no apparent integrin-beta 1 cell surface expression. Thirty-seven of 42 clones selected for activated LFA-1 were found to have lost apparent integrin-beta 1 expression. Conversely, 7 of 21 clones selected for lack of beta 1 expression were found to have activated LFA-1. Since this pointed toward a possible coupling between beta 1 expression and LFA-1 activity, we further analyzed at which level beta 1 expression was blocked. We focused on one clone, HAP4, with activated LFA-1 and no detectable beta 1 cell surface expression and found, surprisingly, that it expressed wild-type levels of beta 1 mRNA and, in Western blots of whole cell lysates, apparently normal levels of beta 1 protein. However, in addition to beta 1 of the expected molecular weight, HAP4 expressed a unique 48-kDa band recognized by the polyclonal anti-beta 1 antiserum. Immunoprecipitation experiments revealed that the epitope recognized by the anti-beta 1 antibody 4B4 was hidden or lost. The alpha 4-chain was found in its precursor form but it did not associate with any beta-chain, and it was not processed to its mature form. Instead alpha 4-chains were eventually degraded. Taken together this showed that beta 1-chains were produced but not properly processed in HAP4. From this we propose that HAP4 is deficient in a gene product required both for proper beta 1 folding and for repression of LFA-1 adhesiveness.

Modulation of alpha 6/beta 1 integrin expression during differentiation of F9 murine embryonal carcinoma cells to parietal endoderm

In vitro differentiation of the murine embryonal carcinoma (EC) cell line F9 parallels that of the early blastocyst, where visceral (VE) and parietal endoderm (PE) diverge from a common precursor, the primitive endoderm. This differentiation pathway is induced by retinoic acid (RA) and dibutyryl cyclicAMP (dcAMP) and is accompanied by progressive and dramatic changes in cell morphology and functions. Within 7 days of treatment the cells organize their cytoskeleton and synthesize large amounts of extracellular matrix proteins, becoming fully differentiated migratory cells; all these changes are likely to involve integrins expression and organization. We have investigated the changes in beta 1 integrin expression, its maturation, and organization on the cell surface in association with alpha 6, during the transition from undifferentiated F9 stem cells to migrating PE cells. By Western blotting and immunoprecipitation we showed a gradual decrease in the amount of the beta 1 subunit on the cell surface and a parallel progressive accumulation of immature protein, indicating that the control of beta 1 expression during F9 cells differentiation occurs first at post-translational level and then at the level of transcription. Moreover, the induction of differentiation produces a marked decrease of alpha 6B and its association to a high molecular weight protein, while alpha 6A level increases. By immunofluorescence we found that upon differentiation there is a relocation of the beta 1 and alpha 6B integrin subunits from cell-cell contacts to focal contacts where they colocalize with vinculin. On the contrary alpha 6A, weakly present in F9 stem cells, is present in the focal contacts of PE cells and along the stress fibers. We suggest different roles for the two alpha 6 isoforms.

Deficiency of the integrin beta 4 subunit in junctional epidermolysis bullosa with pyloric atresia: consequences for hemidesmosome formation and adhesion properties

Junctional epidermolysis bullosa (JEB) comprises a group of inherited autosomal recessive blistering disorders characterized by dermo-epidermal separation through the lamina lucida of the basement membrane. We identified a patient with JEB associated with pyloric atresia (PA), in whom the integrin beta 4 subunit was completely absent. At the ultrastructural level, the hemidesmosomes were reduced in number, appeared rudimentary and lacked a subbasal dense plate and frequently an inner attachment plaque. However, keratin filaments were still anchored to the cytoplasmic plaque of the hemidesmosome. Immunofluorescence analysis showed that the beta 4 subunit was absent in the skin of the PA-JEB patient, whereas the alpha 6 subunit appeared to be normally distributed along the basement membrane zone, as were the other hemidesmosomal components BP230, BP180 and HD1. Furthermore, the alpha 3 and beta 1 subunits were not only detected at the lateral membranes of basal cells in PA-JEB skin, as in normal skin, but also along the basement membrane zone. The few hemidesmosome-like structures found in cultured keratinocytes from the PA-JEB patient contained the hemidesmosomal components BP230, BP180 and HD1, but not the integrin alpha 6 subunit. Like alpha 3, this subunit was colocalized with vinculin in focal contacts at the ends of actin stress fibers. Immunoprecipitation analysis revealed that alpha 6 was associated with beta 1 on PA-JEB keratinocytes, whereas normal human keratinocytes (NHKs) exclusively express alpha 6 beta 4 on their cell surface. The initial adhesion of PA-JEB and normal keratinocytes to laminin-1 and laminin-5, both ligands for alpha 6 beta 1 and alpha 6 beta 4, was similar. In migration assays, the PA-JEB keratinocytes were more motile on laminin-5 than normal keratinocytes. Our observations indicate that the integrin alpha 6 beta 4 plays a crucial role in the proper assembly of hemidesmosomes and in the stabilization of the dermal-epidermal junction. The fragility of the skin and the blistering in this patient appear to have been due to the deficiency of the integrin beta 4 subunit, which results in the formation of too few and structurally abnormal hemidesmosomes.

Altered glycosylation of alpha(s)beta 1 integrins from rat colon carcinoma cells decreases their interaction with fibronectin

Malignant cell transformation is generally accompanied by changes in their interactions with environing matrix proteins in a way to facilitate their migration and generate invasion. Our results show the binding of rat colon adenocarcinoma PROb cells to fibronectin strongly reduced when compared to normal rat intestine epithelial cells. This decrease was not due to the level of alpha(s)beta 1 integrins expressed at the surface of the cell line. However, beta 1- and alpha(s)-associated subunits appeared to be structurally altered as shown by immunoprecipitation followed by electrophoresis. Pulse chase experiments using 35S methionine evidenced differences in the biosynthesis of beta 1- and alpha(s)associated integrins: normal epithelial IEC18 cells required 16 h for maximal biosynthesis of the completely mature beta 1 subunit, while PROb cells did it within 4-6 h. Studies using endoglycosidases O, H, D, and N glycanase confirmed that the molecular weight alterations were due to abnormal glycosylation and suggested that alpha(s)beta 1 integrins of PROb cells could bear both mature complex and immature high mannose types while IEC18 cells borne only mature complex type oligosaccharidic chains. Treatment of both cell types with castanospermine, an inhibitor of N-glycosylation, reduced the differences observed in their adhesion to the fibronectin without significantly affecting beta 1 receptors expression at the cell surface. These results strongly suggest a role of the glycosylation of beta 1 receptors in the adhesion of rat colon adenocarcinoma PROb cells to fibronectin substrata.

Wound closure with human keratinocytes cultured on a polyurethane dressing overlaid on a cultured human dermal replacement

BACKGROUND

Burn excision followed by immediate wound coverage has become the clinical standard for managing extensive burn injuries in much of the world. When sufficient autograft skin to achieve permanent wound closure is unavailable, cell culture technology has made the use of cultured human keratinocyte (HK) sheets clinically feasible. Whereas previous techniques have focused on development of multilayered, differentiated HK sheets, our attention has been drawn to using HK in a highly proliferative, less differentiated state. Time requirements for preparation of multistratified cultured HK are high, and preparatory steps may destroy important integrin adhesion molecules.

METHODS

We describe the use of HK cultured to single layer confluence on a polyurethane membrane(HD), with serum-free medium. HK-HD grafts were transplanted to full-thickness wounds on athymic mice (n = 31). A second group of mice (DG-HK-HD), n = 28) received a living human dermal replacement containing cultured fibroblasts before placement of HK-HD. Control mice received HD alone (n = 4). Basement membrane proteins on healed wounds and surface integrins on cultured HK were identified by means of immunostaining and direct microscopic visualization.

RESULTS

HK cultured just to the confluent state on polyurethane membrane were positive for integrins alpha(5) and alpha(6), major integrins on proliferating HK. Histologic analysis showed epithelialized wounds in all groups after 21 days. Using an anti-human involucrin antibody we demonstrated the presence of HK in 64.5% of the HK-HD group, 61% of the DG-HK-HD group, and 0% in the HD group. Mice that received the living human dermal replacement containing cultured fibroblasts in combination with HK-HD grafts developed a thick, well-vascularized neodermis. Strong laminin and collagen IV staining was observed in wound areas covered with HK.

CONCLUSIONS

These data show that full-thickness wounds can be closed by application of a single layer of proliferating HK cultured on a biocompatible polyurethane membrane. This technique is an alternative to the use of multilayered, differentiated HK sheets. Preparation times for HK-HD grafts should be significantly shorter than required for multilayered HK sheets, technical efforts should be less, and more extensive wound areas could be covered.

Adhesive and migratory behaviors of nevus cells differ from those of epidermal melanocytes and are not linked to the histological type of nevus

It has been postulated that acquired nevi undergo life span continuous evolution from junctional, presumably in radial expanding phase at the dermal epidermal junction, to compound and then to dermal nested nevi. In an attempt to correlate the morphology of nevi with biological data, we have investigated whether migratory and adhesive phenotypes of nevus cells could account for histological patterns and possible spatiotemporal changes in nevi. Nevus cells were cultured from compound and dermal nevi and compared to normal epidermal cultured melanocytes from children and adults. AR nevus cells showed similar in vitro adhesive and migratory indexes on laminin-1, laminin-5/nicein, fibronectin, or collagen IV substrates, suggesting that these intrinsic characteristics do not account for the tendency to dermal nesting and/or to radial growth along the dermal-epidermal junction. The cells from epidermal and dermal parts of compound nevi migrated similarly across a reconstituted basement membrane. The results show that intrinsic adhesive and migratory behaviors of nevus cells were not associated with a histological type of nevus. Interestingly, differences in migratory phenotype and intercellular adhesion capacities between nevus cells and normal melanocytes indicated that they could represent different melanocytic cell subpopulations. Finally, melanocytes from adults and children expressed similar levels of the same integrins as all nevus cells but showed differences in function of both alpha3 and alpha6 integrin subunits and in migratory/adhesive behaviors, which may suggest different states of melanocyte maturation.