Signaling via beta1 integrins and mitogen-activated protein kinase determines human epidermal stem cell fate in vitro

Role of integrins in regulating epidermal adhesion, growth and differentiation

Mammalian epidermis is renewed throughout life by proliferation of a multipotential stem cell population and terminal differentiation of stem cell progeny. In recent years, extracellular matrix receptors of the integrin family have been identified as important regulators of epidermal homeostasis, influencing the balance between stem cell renewal and differentiation. Integrin expression is altered when the epidermis is damaged or diseased, and there is good evidence that specific integrins can contribute positively or negatively to pathogenesis. In this review I summarize what is known about the expression and function of epidermal integrins, and highlight the challenges for future research.

One for all and all for one: thymic epithelial stem cells and regeneration

It has long been believed that the thymic epithelial microenvironment originates from both the endodermal and ectodermal germ cell layers. However, a growing body of evidence indicates that such a dual origin is not the case, and that the diverse thymic epithelial populations all develop from a common epithelial stem cell. This article explores these data, investigates the identity of such cells and the signals that might control their expansion and differentiation, and considers the possibility of stem cell transplantation for thymic regeneration.

Decreased expression of alpha3 and beta1 integrin subunits is responsible for differentiation-associated changes in cells behavior in terminally differentiated human oral keratinocytes

Primary normal human oral keratinocytes (NHOKs) terminally differentiate in serial subculture. To investigate whether this subculture-induced differentiation of NHOKs affects integrin expression and cell-matrix interaction, we studied the expression levels of integrin subunits and cellular response to the extracellular matrix (ECM) proteins in NHOKs at different population doublings. The phosphorylation statuses of focal adhesion kinase (FAK), extracellular signal regulated kinase (ERK), p38, and c-Jun amino-terminal kinase (JNK) were also determined in NHOK cells cultured on ECM proteins, to evaluate the functions of integrins with respect to cellular responses to ECM proteins. The expression levels of alpha3 and beta1 integrin subunits progressively decreased in NHOKs undergoing terminal differentiation. The ability of NHOKs to spread upon laminin and type I collagen significantly decreased in terminally differentiated oral keratinocytes. Keratinocyte migration was significantly increased on type I collagen for terminally differentiated NHOKs. Similar results were seen following preincubation of rapidly proliferating NHOKs with function-blocking antibodies to alpha3 or beta1 integrin subunit. In contrast, fibronectin had no effect on cellular responses in NHOKs, which were almost negligible in the expression levels of alpha5 integrin subunits. The extent of FAK phosphorylation in terminally differentiated NHOKs was notably lower than that of rapidly proliferating cells, but was enhanced in terminally differentiated cells that were cultured on type I collagen. Our results indicate that decreased expression of alpha3 and beta1 integrin subunits is responsible for differentiation-associated changes in cells behavior in terminally differentiated oral keratinocytes. Our data also show that the abrogation of the alpha5beta1 integrin function caused by omitting alpha5 subunit is linked to the loss of a cell-fibronectin interaction in human oral keratinocytes.

Lung epithelial stem cells

This review concentrates on recent evidence about lung stem cell origins and plasticity. The range of potential cells which can repopulate the injured lung, classically the basal and mucous secretory cells of the trachea, the Clara cells of the bronchiole, and the type II pneumocyte of the alveolus, has been extended to include the mucus-gland duct cells of the trachea and bronchus. Some evidence suggests that there are variant Clara cells that lack cytochrome P-450 and so are spared toxic activation of xenobiotics, and may aid bronchiolar repopulation after injury, such as with naphthalene. There may even be involvement of the neuroepithelial bodies or cells in this, though the evidence is not yet conclusive. The search for a resident pulmonary multipotent cell for repopulating any lung epithelium has not yet been successful. The picture remains similar to earlier conclusions, in that the local stem or precursor cell is the most likely to contribute to local needs in times of tissue damage. There remains a major challenge for lung cancer treatment, where high-dose chemo- or radio-therapy may be hoped to promote the seeding and repair of lung parenchyma by circulating bone marrow stem cells, as seen in liver models. Patient survival rates do not yet suggest that this occurs to any great extent, but this remains to be shown formally. The effects of prior fibrosis and tumour necrosis are probably confounding factors in this lack of rescue.

The sunburn cell revisited: an update on mechanistic aspects

The sunburn cell (SBC), with its pyknotic nucleus and eosinophilic cytoplasm, is characteristic of mammalian epidermis after exposure to UVC and UVB radiation or UVA radiation in the presence of psoralens. SBC may be regarded as an example of apoptosis: controlled individual cell death. Since the discovery of apoptosis over thirty years ago, there has been a considerable increase in the knowledge of mechanisms involved in this process. DNA damage has been shown to be a major determinant of SBC production both in a p53-dependent and -independent manner. Extranuclear events such as activation of membrane bound death receptors also contribute to SBC formation. The development of new technologies and techniques has resulted in a better understanding of the mechanisms and machinery involved in apoptosis, triggered by various stimuli and in different cell types. Of particular importance has been the elucidation of regulatory molecules such as caspases, inhibitor of apoptosis proteins (IAP) and the role of mitochondria as key to the process of apoptosis and consequent production of SBC. This review attempts to give an update on those mechanisms involved and the occurrence and relevance of SBC in mammalian skin are discussed.

Evidence for a role of mitogen-activated protein kinases in proliferating and differentiating odontogenic epithelia of inflammatory and developmental cysts

OBJECTIVE

The activation of intracellular signaling cascades involving serine/threonine kinases ERK1/2 has been variably reported either to stimulate or inhibit epithelial cell differentiation in response to extracellular signals. The purpose of our study was to determine the distribution of the signaling molecule ERK1 and its activated form pERK1/2 in the epithelial components of developmental and inflammatory odontogenic cysts in relation to parameters of differentiation and proliferation.

STUDY DESIGN

Thirty samples of dental follicles, dentigerous cysts, and radicular cysts were immunostained with antibodies to ERK1, pERK1/2, and proliferating cell nuclear antigen (a marker for proliferation). The tissues were subclassified according to the pattern of histomorphological differentiation (ie, squamous differentiation) and the proliferation rate of their epithelial components. The significance of differences in the proportion of ERK1- and pERK1/2-expressing cells among the tissue groups was determined by chi-square analysis or Fisher's exact test.

RESULTS

ERK1 and pERK1/2 were found to be expressed in a significantly higher proportion of cells with differentiated and highly proliferating epithelial components, as compared with those of nondifferentiated, quiescent epithelial rests. The epithelium of radicular cysts exhibited the highest proportion of pERK1/2-positive cells. In both dentigerous and radicular cyst samples, pERK1/2 expression was significantly higher in the inflamed tissues.

CONCLUSIONS

These data demonstrate that ERK1 and its active form pERK1/2 are associated with differentiating and actively proliferating epithelia of odontogenic cysts, and are consistent with pERK1/2 involvement in the activation of odontogenic epithelia in response to inflammation.

A crucial role of β1 integrins for keratinocyte migration in vitro and during cutaneous wound repair

Integrins are ubiquitous transmembrane receptors that play crucial roles in cell-cell and cell-matrix interactions. In this study, we have determined the effects of the loss of β1 integrins in keratinocytes in vitro and during cutaneous wound repair. Flow cytometry of cultured β1-deficient keratinocytes confirmed the absence of β1 integrins and showed downregulation of α6β4 but not of αv integrins. β1-null keratinocytes were characterised by poor adhesion to various substrates, by a reduced proliferation rate and by a strongly impaired migratory capacity. In vivo, the loss of β1 integrins in keratinocytes caused a severe defect in wound healing. β1-null keratinocytes showed impaired migration and were more densely packed in the hyperproliferative epithelium. Surprisingly, their proliferation rate was not reduced in early wounds and even increased in late wounds. The failure in re-epithelialisation resulted in a prolonged inflammatory response, leading to dramatic alterations in the expression of important wound-regulated genes. Ultimately, β1-deficient epidermis did cover the wound bed, but the epithelial architecture was abnormal. These findings demonstrate a crucial role of β1 integrins in keratinocyte migration and wound re-epithelialisation.

Movies available on-line

The stem cell compartment in human interfollicular epidermis

In this review, I summarise recent work from my laboratory in which we have been examining the distribution of stem cells in human interfollicular epidermis and the factors that regulate stem cell fate in vitro. The non-random distribution of stem cells is emphasised and beta1 integrins and Delta1 are suggested to play a role in stem cell patterning. beta1 integrins, Notch, c-Myc and beta-catenin all regulate the size of the stem cell compartment in vitro and recent evidence from transgenic mice suggests that they are also important in vivo.

Estimation of size of clonal unit for keratinocytes in normal human skin

OBJECTIVE

It has been suggested that keratinocyte (KC) stem cells reside at the epicenter of a clonal population of cells. To estimate the territory or surface area covered by a single stem-cell-derived KC population in human skin, clonal skin maps were created from 3 healthy adult women and from normal skin of a psoriatic patient.

DESIGN

Two hundred fifty-eight punch biopsy samples of various sizes (ranging from 2 to 8 mm in diameter) were analyzed for clonality employing X chromosome inactivation patterns at the human androgen receptor gene (HUMARA) locus. DNA was isolated and clonality established by significant decrease of either maternal or paternal X chromosome band patterns following restriction enzyme digestion, polymerase chain reaction amplification, and gel electrophoresis.

RESULTS

Fifty-three (41%) of 128 two-mm biopsies were clonal, whereas only 6 (14%) of 43 three-mm, 5 (14%) of 36 four-mm, and 3 (8%) of 35 five-mm biopsies revealed a clonal population of KCs. By contrast, in 5 different biopsies from a psoriatic patient, including 4- or 5-mm sizes, all but 1 were clonal; even an 8-mm biopsy contained a clonal population of KCs. Mantel-Haenszel chi(2) analysis revealed a P value of.001, reflecting a strong trend in probability for presence of a single clone of KCs as related to size of the biopsy sample. By sequentially analyzing 30 contiguous 2-mm biopsy samples within a given strip of skin, 10 clonal domain changes, as reflected in maternal versus paternal switches, were observed.

CONCLUSIONS

These results provide direct evidence of a clonal population of KCs in normal and psoriatic lesion-free skin, and indicate that a clonal epidermal unit of KCs frequently can be detected in small biopsies (2 mm), but that in normal skin sampling, overlapping clones are apparently present in larger (ie, 4-5-mm) biopsies, producing nonclonal patterns. The clonal domain of progeny in normal skin has a rather limited territorial boundary (2 mm in diameter). However, in lesion-free skin from a psoriatic patient, there may be clonal expansion of KCs due to perturbation in epidermopoiesis and/or stem cell distribution.

Contact with fibronectin enhances preservation of normal but not chronic myelogenous leukemia primitive hematopoietic progenitors

OBJECTIVE

Coculture with stromal cells enhances preservation and self-renewal of primitive progenitor potential in hematopoietic cells during ex vivo culture with growth factors (GF). However, the respective roles of growth factors, stromal contact, and extracellular matrix (ECM) ligands in this effect are not clear. Here we investigated the role of direct contact with stroma and the ECM protein fibronectin (FN) in these effects, and investigated whether abnormal integrin receptor function in chronic myelogenous leukemia (CML) progenitors was associated with perturbation in these responses.

METHODS

Normal bone marrow CD34+ cells were cultured in GF-containing medium with or without contact with stromal layers, glutaraldehyde-fixed stromal layers (stroma-contact), or integrin-binding FN fragments for 7 days. Progeny cells were assayed for primitive progenitors in week-6 long-term culture-initiating cell (LTC-IC) and week-10 extended LTC-IC (ELTC-IC) assays.

RESULTS

Increased LTC-IC and ELTC-IC preservation was seen following coculture with stroma, and was also observed after culture in contact with fixed stromal layers and FN. Both alpha4beta1 and alpha5beta1-integrin binding FN fragments enhanced LTC-IC preservation. Analysis of single CD34+CD38- cells showed that coculture with FN resulted in significantly reduced cell division, but enhanced retention of LTC-IC capacity in divided cells. FN also increased LTC-IC frequency in undivided cells. CML progenitors demonstrate deficient integrin-mediated adhesion, migration, and signaling. Coculture of CML CD34+ cells with stroma and FN failed to enhance LTC-IC preservation.

CONCLUSION

We conclude that beta1 integrin-FN interactions enhance normal primitive progenitor preservation with or without cell division, and that these mechanisms are impaired in CML primitive progenitors.

E2F-1 is essential for normal epidermal wound repair

E2F factors are involved in proliferation and apoptosis. To understand the role of E2F-1 in the epidermis, we screened wild type and E2F-1(-/-) keratinocyte mRNA for genes differentially expressed in the two cell populations. We demonstrate the reduced expression of integrins alpha(5), alpha(6), beta(1), and beta(4) in E2F-1(-/-) keratinocytes associated with reduced activation of Jun terminal kinase and Erk upon integrin stimulation. As a consequence of altered integrin expression and function, E2F-1(-/-) keratinocytes also show impaired migration, adhesion to extracellular matrix proteins, and a blunted chemotactic response to transforming growth factor-gamma1. E2F-1(-/-) keratinocytes, but not dermal fibroblasts, exhibit altered patterns of proliferation, including significant delays in transit through both G(1) and S phases of the cell cycle. Recognizing that proliferation and migration are key for proper wound healing in vivo, we postulated that E2F-1(-/-) mice may exhibit abnormal epidermal repair upon injury. Consistent with our hypothesis, E2F-1(-/-) mice exhibited impaired cutaneous wound healing. This defect is associated with substantially reduced local inflammatory responses and rates of re-epithelialization. Thus, we demonstrate that E2F-1 is indispensable for a hitherto unidentified cell type-specific and unique role in keratinocyte proliferation, adhesion, and migration as well as in proper wound repair and epidermal regeneration in vivo.

Epidermal Ras blockade demonstrates spatially localized Ras promotion of proliferation and inhibition of differentiation

While important in carcinogenesis, the role of Ras in normal self-renewing tissues such as epidermis is unclear. To address this, we altered Ras function in undifferentiated and differentiating epidermal layers. Ras blockade within undifferentiated basal epidermal cells leads to decreased integrin expression, diminished growth capacity and induction of differentiation. Ras blockade in post-mitotic suprabasal epidermis exerts no effect. In contrast, regulated Ras and Raf activation inhibits differentiation. These findings indicate that spatially restricted Ras/Raf signaling divides epidermis into an undifferentiated proliferative compartment and a differentiating post-mitotic compartment and suggest a new role for Ras in tissue homeostasis.

Stem cell bioengineering

Tissue engineering and cellular therapies, either on their own or in combination with therapeutic gene delivery, have the potential to significantly impact medicine. Implementation of technologies based on these approaches requires a readily available source of cells for the generation of cells and tissues outside a living body. Because of their unique capacity to regenerate functional tissue for the lifetime of an organism, stem cells are an attractive "raw material" for multiple biotechnological applications. By definition they are self-renewing because on cell division they can generate daughter stem cells. They are also multipotent because they can differentiate into numerous specialized, functional cells. Recent findings have shown that stem cells exist in most, if not all, tissues, and that stem cell tissue specificity may be more flexible than originally thought. Although the potential for producing novel cell-based products from stem cells is large, currently there are no effective technologically relevant methodologies for culturing stem cells outside the body, or for reproducibly stimulating them to differentiate into functional cells. A mechanistic understanding of the parameters important in the control of stem cell self-renewal and lineage commitment is thus necessary to guide the development of bioprocesses for the ex vivo culture of stem cells and their derivates.

Role of the nude gene in epithelial terminal differentiation

Loss-of-function mutations in Whn (Hfh11, Foxn1), a winged-helix/forkhead transcription factor, cause the nude phenotype, which is characterized by the abnormal morphogenesis of the epidermis, hair follicles, and thymus. To delineate the biochemical pathway of Whn, we investigated its upstream regulation and downstream effects using primary keratinocytes from wild-type and transgenic mice. The transgenic animals express whn from the involucrin promoter, which is active in keratinocytes undergoing terminal differentiation. In wild-type cultures, as in the epidermis, Whn was induced during the early stages of terminal differentiation and declined during later stages. In transgenic keratinocytes, whn overexpression altered the terminal differentiation program, stimulating an early differentiation marker (keratin 1) and suppressing later markers (profilaggrin, loricrin, and involucrin). These results suggest a role for Whn in the stepwise or temporal regulation of differentiation, as Whn can ensure that the differentiation program is carried out in proper sequence. Before the start of differentiation, Whn levels were suppressed by the p42/p44 mitogen-activated protein kinase cascade, and this signaling pathway was rapidly inactivated as differentiation began. Thus, as keratinocytes commit to terminal differentiation, mitogen-activated protein kinase signaling decreases, which permits the induction of Whn; Whn then activates early features of the differentiation program.

Changes in integrin expression are associated with altered homing properties of Lin(-/lo)Thy1.1(lo)Sca-1(+)c-kit(+) hematopoietic stem cells following mobilization by cyclophosphamide/granulocyte colony-stimulating factor

OBJECTIVE

Although migration of hematopoietic stem cells (HSC) is essential for normal hematopoiesis and successful hematopoietic cell transplantation, little is known about the mechanisms that underlie this movement. We have sought to characterize the factors that regulate HSC migration by analyzing changes in expression of particular adhesion receptors associated with cyclophosphamide/granulocyte colony-stimulating factor (Cy/G-CSF)-induced HSC mobilization.

METHODS

Expression by Lineage(-/lo)Thy1.1(lo)Sca-1(+)c-kit(+) HSC of members of the beta1 integrin family of adhesion molecules was assessed in untreated or Cy/G-CSF-treated mice by multiparameter flow cytometry. In parallel, the in vivo homing properties of normal and mobilized HSC were compared following intravenous transfer of fluorescently marked HSC.

RESULTS

Normal adult HSC express high levels of several beta1 integrin family members. Following Cy/G treatment, bone marrow HSC selectively downregulate alpha 2 integrin expression and upregulate alpha 5 expression. HSC found in the blood following Cy/G-CSF treatment express significantly lower levels of multiple integrins than their bone marrow and/or splenic counterparts. Changes in integrin expression by blood-borne HSC correlate with a 50% decrease in their ability to home to the bone marrow in short-term assays, and with previously observed defects in competitive engraftment by these HSC. Similar reductions in bone marrow (BM) homing are observed for BM HSC treated with alpha 4 integrin function blocking mAb prior to injection. Modulation of integrin expression induced by mobilization was not associated with cell-cycle progression.

CONCLUSION

Changes in integrin expression and function are associated with HSC mobilization and likely significantly affect the engraftment potential of hematopoietic stem cells.

Regulation of human beta-defensin-2 in gingival epithelial cells: the involvement of mitogen-activated protein kinase pathways, but not the NF-kappaB transcription factor family

Stratified epithelia of the oral cavity are continually exposed to bacterial challenge that is initially resisted by neutrophils and epithelial factors, including antimicrobial peptides of the beta-defensin family. Previous work has shown that multiple signaling pathways are involved in human beta-defensin (hBD)-2 mRNA regulation in human gingival epithelial cells stimulated with a periodontal bacterium, Fusobacterium nucleatum, and other stimulants. The goal of this study was to further characterize these pathways. The role of NF-kappaB in hBD-2 regulation was investigated initially due to its importance in inflammation and infection. Nuclear translocation of p65 and NF-kappaB activation was seen in human gingival epithelial cells stimulated with F. nucleatum cell wall extract, indicating possible involvement of NF-kappaB in hBD-2 regulation. However, hBD-2 induction by F. nucleatum was not blocked by pretreatment with two NF-kappaB inhibitors, pyrrolidine dithiocarbamate and the proteasome inhibitor, MG132. To investigate alternative modes of hBD-2 regulation, we explored involvement of mitogen-activated protein kinase pathways. F. nucleatum activated p38 and c-Jun NH(2)-terminal kinase (JNK) pathways, whereas it had little effect on p44/42. Furthermore, inhibition of p38 and JNK partially blocked hBD-2 mRNA induction by F. nucleatum, and the combination of two inhibitors completely blocked expression. Our results suggest that NF-kappaB is neither essential nor sufficient for hBD-2 induction, and that hBD-2 regulation by F. nucleatum is via p38 and JNK, while phorbol ester induces hBD-2 via the p44/42 extracellular signal-regulated kinase pathway. Studies of hBD-2 regulation provide insight into how its expression may be enhanced to control infection locally within the mucosa and thereby reduce microbial invasion into the underlying tissue.

Identification and isolation of human prostate epithelial stem cells based on α2β1-integrin expression

A major impediment to our understanding of the biology of stem cells is the inability to distinguish them from their differentiating progeny. We made use of the known association of stem cells with basement membranes to isolate prostate epithelial stem cells. We show that, in vivo, putative stem cells express higher levels of the α2-integrin subunit than other cells within the basal layer. Approximately 1% of basal cells examined by confocal microscopy were integrin ‘bright’, and these cells can be selected directly from the tissue on the basis of rapid adhesion to type I collagen. This selected population has a basal phenotype, as determined by expression of CK5 and CK14 and lack of expression of the differentiation-specific markers prostate specific antigen (PSA) and prostatic acid phosphatase (PAP), and has a fourfold greater ability to form colonies in vitro than the total basal population. These putative stem cells are distinguished from other basal cells by their ability to generate prostate-like glands in vivo with morphologic and immuno-histochemical evidence of prostate-specific differentiation. These properties are consistent with a stem cell origin. Furthermore, the presence of surface integrins on prostate stem cells suggests that these cells share common pathways with stem cells in other tissues.

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

Genomic expression analysis implicates Wnt signaling pathway and extracellular matrix alterations in hepatic specification and differentiation of murine hepatic stem cells

HBC-3 hepatic stem cells maintained in the undifferentiated state can be induced to differentiate along the hepatocyte lineage in response to DMSO (Rogler, 1997). In order to understand the complex transcriptional regulatory mechanisms associated with the differentiation of these somatic stem cells and to identify novel candidate stem cell and differentiation associated genes, we have begun to characterize the transcriptome of HBC-3 cells during a 7-day differentiation protocol. This analysis showed that differentiating HBC-3 cells undergo biphasic bursts of gene regulation peaking at 3 hours and 120 hours of DMSO treatment. In the undifferentiated state, HBC-3 cells express muscle, neuron, myeloid, and lymphoid specific genes that are rapidly downregulated during hepatocytic differentiation. Cluster analysis has revealed large groups of genes with different temporal regulation profiles demonstrating complex and widespread transcriptional changes. Specifically, we discovered a multifaceted downregulation of the Wnt/beta-catenin pathway accompanied by the repression of TCF target genes during HBC-3 differentiation. In addition, there is downregulation of cellular receptors for fibronectin and laminin and other extracellular matrix molecules indicative of widespread cell surface alterations. DMSO induces cell cycle arrest, and this is reflected in upregulation of growth inhibitory proteins such as cyclin I and p18 and downregulation of cyclins B1 and D. Genes needed for hepatocytic functions, such as apolipoprotein C-IV, phosphoenolpyruvate carboxykinase, alcohol dehydrogenase, and asialoglycoprotein receptor were upregulated. Finally, transcriptional regulators including Twist, Snail, HNF1a, and GATA6 were upregulated during differentiation of HBC-3 cells. The significance of these findings is that our genome-based approach has allowed the parallel identification of multiple regulatory pathways that is needed to begin to fully understand the complex differentiation process.

Growth defects induced by perturbation of beta1-integrin function in the mammary gland epithelium result from a lack of MAPK activation via the Shc and Akt pathways

Adhesion to extracellular matrix (ECM) induces intracellular signals that modulate cell proliferation, survival and differentiation. To study signalling events triggered by cell-ECM interactions in vivo we used transgenic mice exhibiting reduced mammary epithelial cell proliferation and increased apoptosis rates during the growth phase in pregnancy and lactation due to expression of a beta1-integrin dominant-negative mutant in the mammary gland epithelium. Here we show that ERK and JNK MAPKs were markedly less activated in lactating transgenic glands thereby accounting for the growth defects. The FAK pathway was not affected suggesting a mechanism of activation additional to the ECM signal. On the contrary, the significant decrease of Shc phosphorylation, Grb2 recruitment and the reduced phosphorylation level of Akt Thr308 and Akt substrates FKHR and Bad detected in transgenic glands show that activation of the Shc and the Akt pathways require intact cell-ECM interactions. These results provide an insight into the mechanisms of growth control by integrin-mediated adhesion that operate in vivo.

Delta regulates keratinocyte spreading and motility independently of differentiation

In human interfollicular epidermis stem cells lie in clusters surrounded by their differentiated daughters, transit amplifying cells, an arrangement that reflects differences in cell cohesiveness and motility. Keratinocytes expressing a dominant negative Delta1 mutant, Delta(T), lacking most of the cytoplasmic domain, acquired the motile behaviour of transit cells while retaining their stem cell identity. Conversely, overexpression of Delta1 promoted keratinocyte cohesiveness. The adhesive effects of Delta1 and Delta(T) were independent of SuH-dependent Notch signalling. Delta(T) increased motility and spreading of individual keratinocytes and stimulated lamellipodia formation. Delta and Delta(T) colocalised with cortical actin and redistributed on Latrunculin treatment. We propose that Delta promotes keratinocyte cohesiveness by restricting motility and discuss potential mechanisms by which Delta could interact with the actin cytoskeleton.

A role for mitogen-activated protein kinase activation by integrins in the pathogenesis of psoriasis

In normal epidermis, beta1 integrin expression is confined to the basal layer, whereas in hyperproliferative epidermis, integrins are also expressed in the suprabasal layers. Transgenic mice in which integrins are expressed suprabasally via the involucrin promoter have a sporadic psoriatic phenotype; however, the mechanism by which integrins contribute to the pathogenesis of psoriasis is unknown. We observed activation of mitogen-activated protein kinase (MAPK) in basal and suprabasal keratinocytes of human and transgenic mouse psoriatic lesions and healing mouse skin wounds, correlating in each case with suprabasal integrin expression. Phenotypically normal human and transgenic mouse epidermis did not contain activated MAPK. Transgene-positive keratinocytes produced more IL-1alpha than controls did, and keratinocyte MAPK could be activated by ligation of suprabasal integrins or treatment with IL-1alpha. Constitutive activation of MAPK increased the growth rate of human keratinocytes and delayed the onset of terminal differentiation, recreating many of the histological features of psoriatic epidermis. We propose that activation of MAPK by integrins, either directly or through increased IL-1alpha production, is responsible for epidermal hyperproliferation in psoriasis and wound healing, and that the sporadic phenotype of the transgenic mice may reflect the complex mechanisms by which IL-1 release and responsiveness are controlled in skin.

Stem cell fate and patterning in mammalian epidermis

Recent studies highlight characteristics of epidermal stem cells that were not fully appreciated before. Stem cells are multipotential and signals exchanged with their neighbours help to regulate exit from the stem cell compartment and differentiation along specific lineages. Stem cells exhibit a high degree of spatial organisation, and cell clustering and motility contribute to the assembly and maintenance of the epidermis.

Developmental regulation of alphav integrins produces functional changes in astrocyte behavior

To examine the role of the extracellular matrix in regulating astrocyte behavior we previously characterized alphav integrin expression on postnatal astrocytes in vitro and found that they express alphavbeta5 and alphavbeta8. Here we show that differentiation of immature cells into astrocytes is accompanied by developmental regulation of alphav integrins, downregulation of alphavbeta1 and alphavbeta8, and upregulation of alphavbeta5. In addition, using two previously described astrocyte cell lines, we found that the neurite-permissive A7 cell line expressed high levels of alphavbeta1 in addition to alphavbeta5 and alphavbeta8, while the neurite-inhibitory Neu7 cell line expressed only alphavbeta5. To examine integrin function we generated clones of the Neu7 cell line expressing alphavbeta1 or alphavbeta3 in addition to alphavbeta5. This showed that the parent Neu7 cells migrated more slowly than the A7 cells on fibronectin and vitronectin, but that Neu7 cells expressing alphavbeta1 or alphavbeta3 integrins showed enhanced migration on fibronectin and vitronectin, respectively. These results show that alphav integrin expression is regulated during astrocyte development and confirm an instructive role in cell migration for alphavbeta1 in embryonic cells and alphavbeta3 in astroglial tumors.

Modulation of cell-substrate adhesion by arachidonic acid: lipoxygenase regulates cell spreading and ERK1/2-inducible cyclooxygenase regulates cell migration in NIH-3T3 fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.

Collagen IV synthesis is restricted to the enteroendocrine pathway during multilineage differentiation of human colorectal epithelial stem cells

The human large intestine is lined by a rapidly renewing epithelial monolayer where cell loss is precisely balanced with cell production. The continuous supply of new cells is produced by undifferentiated multipotent stem cells via a coordinated program of proliferation and differentiation yielding three epithelial lineages: absorptive, goblet and enteroendocrine. Cell-matrix interactions have been suggested to be regulators of the multilineage differentiation program of the colorectal crypt but the expression of matrix proteins or their receptors does not appear to have the subtlety expected for this task.

We have developed an in vitro model system of intestinal epithelial stem cells to facilitate the direct analysis of stem cells undergoing lineage commitment and differentiation. Using this culture system, we can now directly investigate the role of cell-matrix signalling in stem-cell decisions. In this study, collagen-IV synthesis has been followed in monolayers of multipotent cells that have been induced to differentiate into absorptive, goblet and enteroendocrine cells. Our experiments demonstrate that commitment to the enteroendocrine lineage is specifically accompanied by the expression of type-IV collagen that remains enteroendocrine-cell associated. Undifferentiated cells, absorptive cells and goblet cells do not express collagen IV. To confirm that the differential lineage-specific expression of collagen IV observed in the model system was representative of the in vivo situation, collagen-IV synthesis was analysed in isolated human colorectal crypts and tissue sections using immunocytochemistry and in situ hybridisation. These studies confirmed the in vitro findings, in that implementation of the enteroendocrine differentiation program involves synthesis and accumulation of a collagen-IV matrix. Thus, human colorectal enteroendocrine cells are unique in the colorectal crypt in that they assemble a cell-associated collagen-IV-rich matrix not observed on other colorectal epithelial cells.

This study provides the first evidence for differential matrix synthesis between colorectal epithelial lineages in human colorectal epithelium. The specialised pericellular environment of the enteroendocrine cells might explain some of the unique phenotypic characteristics of this cell lineage. Furthermore, these findings suggest a potential mechanism whereby individual epithelial cells could modulate their cell-matrix signalling even while rapidly migrating in heterogeneous sheets over a shared basement membrane.

Dermal fibroblast-derived growth factors restore the ability of beta(1) integrin-deficient embryonal stem cells to differentiate into keratinocytes

Embryonal stem (ES) cells that are homozygous null for the beta(1) integrin subunit fail to differentiate into keratinocytes in vitro but do differentiate in teratomas and wild-type/beta(1)-null chimeric mice. The failure of beta(1)-null ES cells to differentiate in culture might be the result of defective extracellular matrix assembly or reduced sensitivity to soluble inducing factors. By culturing embryoid bodies on dead, deepidermized human dermis (DED) we showed that epidermal basement membrane did not induce beta(1)-null ES cells to undergo keratinocyte differentiation and did not stimulate the differentiation of wild-type ES cells. Coculture with epidermal keratinocytes also had no effect. However, when human dermal fibroblasts were incorporated into DED, the number of epidermal cysts formed by wild-type ES cells increased dramatically, and small groups of keratin 14-positive cells differentiated from beta(1)-null ES cells. Fibroblast-conditioned medium stimulated differentiation of K14-positive cells in wild-type and beta(1)-null embryoid bodies. Of a range of growth factors tested, KGF, FGF10, and TGFalpha all stimulated differentiation of keratin 14-positive beta(1)-null cells, and KGF and FGF10 were shown to be produced by the fibroblasts used in coculture experiments. The effects of the growth factors on wild-type ES cells were much less pronounced, suggesting that the concentrations of inducing factors already present in the medium were not limiting for wild-type cells. We conclude that the lack of beta(1) integrins decreases the sensitivity of ES cells to soluble factors that induce keratinocyte differentiation.

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.

Signaling from E-cadherins to the MAPK pathway by the recruitment and activation of epidermal growth factor receptors upon cell-cell contact formation

E-cadherins are well characterized cell surface molecules expressed in epithelial cells, which play a major role in cell adhesion through the establishment of calcium-dependent homophilic interactions at sites of cell-cell contacts. They are also integral components of morphogenetic programs controlling the maintenance of the structural and functional integrity of epithelia. Accumulated evidence indicates that the E-cadherin-mediated cell adhesion system is highly regulated from inside the cells by a number of intracellular signaling pathways. Recently available information suggests that E-cadherins may also play a role in the transduction of signals from the outside of the cell to the cytoplasm. However, the nature of the biochemical routes regulated by E-cadherins is still largely unknown. In this study, we set out to explore the possibility that E-cadherins may regulate the activity of MAPK, a key signaling pathway involved in cell fate decisions, upon the formation of cell-cell contacts among neighboring cells. By using an immortalized non-tumorigenic keratinocyte cell line, HaCat, as a model system, we provide evidence that the assembly of calcium-dependent adherens junctions leads to a rapid and remarkable increase in the state of activation of MAPK and that this event is mediated by E-cadherins. Furthermore, we found that E-cadherins stimulate the MAPK pathway through the ligand-independent activation of epidermal growth factor receptors and the consequent activation of a biochemical route leading to the stimulation of MAPKs. These findings suggest that E-cadherins can initiate outside-in signal transducing pathways through the engagement of tyrosine kinase receptors for epidermal growth factor, thus providing a novel molecular mechanism whereby these cell adhesion molecules may ultimately control the fate of normal and transformed epithelial cells.

PA-FABP, a novel marker of human epidermal transit amplifying cells revealed by 2D protein gel electrophoresis and cDNA array hybridisation

Human epidermal stem cells express higher levels of beta1 integrins than their more differentiated daughters, transit amplifying cells. In a search for additional stem and transit cell markers we used proteomics and differential cDNA hybridisation to compare keratinocytes fractionated on the basis of beta1 integrin expression. There were remarkably few differences between the two populations and none of the RNAs differed in abundance by more than 2-fold. Nevertheless, proteomics revealed upregulated expression of epidermal fatty acid binding protein (PA-FABP, also known as E-FABP), Annexin II and two keratin related proteins in the transit population. An unknown high molecular mass protein was upregulated in the stem cell population. The upregulation of PA-FABP was confirmed by Northern blotting and conventional and whole mount labelling of human epidermis. We conclude that PA-FABP is a novel marker of epidermal transit amplifying cells.

Asymmetric stem-cell divisions define the architecture of human oesophageal epithelium

In spite of its clinical importance, little is known about the stem-cell compartment of the human oesophageal epithelium [1,2]. The epithelial basal layer consists of two distinct zones, one overlying the papillae of the supporting connective tissue (PBL) and the other covering the interpapillary zone (IBL) [3]. In examining the oesophageal basal layer, we found that proliferating cells were rare in the IBL and a high proportion of mitoses were asymmetrical, giving rise to one basal daughter and one suprabasal, differentiating daughter. In the PBL, mitoses were more frequent and predominantly symmetrical. The IBL was characterised by low expression of ?1 integrins and high expression of the beta2 laminin chain. By combining fluorescence-activated cell sorting (FACS) with in vitro clonal analysis, we obtained evidence that the IBL is enriched for stem cells. A normal oesophageal epithelium with asymmetric divisions was reconstituted on denuded oesophageal connective tissue. In contrast, asymmetric divisions were not sustained on skin connective tissue, and the epithelium formed resembled epidermis. We propose that stem cells located in the IBL give rise to differentiating daughters through asymmetric divisions in response to cues from the underlying basement membrane. Until now, stem-cell fate in stratified squamous epithelia was believed to be achieved largely through populational asymmetry [4-6].

Epidermal stem cells as targets for gene transfer

One of the characteristics of the epidermis that makes it an attractive tissue for gene therapy is that it is renewed through proliferation of stem cells. If the stem cells can be transduced with the gene of interest then expression of that gene should continue throughout adult life. This article discusses current research on epidermal stem cells, highlighting progress in their identification and in discovering the mechanisms that regulate exit from the stem cell compartment.

Conditional ablation of beta1 integrin in skin. Severe defects in epidermal proliferation, basement membrane formation, and hair follicle invagination

The major epidermal integrins are alpha3beta1 and hemidesmosome-specific alpha6beta4; both share laminin 5 as ligand. Keratinocyte culture studies implicate both integrins in adhesion, proliferation, and stem cell maintenance and suggest unique roles for alphabeta1 integrins in migration and terminal differentiation. In mice, however, whereas ablation of alpha6 or beta4 results in loss of hemidesmosomes, epidermal polarity, and basement membrane (BM) attachment, ablation of alpha3 only generates microblistering due to localized internal shearing of BM. Using conditional knockout technology to ablate beta1 in skin epithelium, we have uncovered biological roles for alphabeta1 integrins not predicted from either the alpha3 knockout or from in vitro studies. In contrast to alpha3 null mice, beta1 mutant mice exhibit severe skin blistering and hair defects, accompanied by massive failure of BM assembly/organization, hemidesmosome instability, and a failure of hair follicle keratinocytes to remodel BM and invaginate into the dermis. Although epidermal proliferation is impaired, a spatial and temporal program of terminal differentiation is executed. These results indicate that beta1's minor partners in skin are important, and together, alphabeta1 integrins are required not only for extracellular matrix assembly but also for BM formation. This, in turn, is required for hemidesmosome stability, epidermal proliferation, and hair follicle morphogenesis. However, beta1 downregulation does not provide the trigger to terminally differentiate.

Skin and hair follicle integrity is crucially dependent on beta 1 integrin expression on keratinocytes

beta 1 integrins are ubiquitously expressed receptors that mediate cell-cell and cell-extracellular matrix interactions. To analyze the function of beta1 integrin in skin we generated mice with a keratinocyte-restricted deletion of the beta 1 integrin gene using the cre-loxP system. Mutant mice developed severe hair loss due to a reduced proliferation of hair matrix cells and severe hair follicle abnormalities. Eventually, the malformed hair follicles were removed by infiltrating macrophages. The epidermis of the back skin became hyperthickened, the basal keratinocytes showed reduced expression of alpha 6 beta 4 integrin, and the number of hemidesmosomes decreased. Basement membrane components were atypically deposited and, at least in the case of laminin-5, improperly processed, leading to disruption of the basement membrane and blister formation at the dermal-epidermal junction. In contrast, the integrity of the basement membrane surrounding the beta 1-deficient hair follicle was not affected. Finally, the dermis became fibrotic. These results demonstrate an important role of beta 1 integrins in hair follicle morphogenesis, in the processing of basement membrane components, in the maintenance of some, but not all basement membranes, in keratinocyte differentiation and proliferation, and in the formation and/or maintenance of hemidesmosomes.

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.

alpha5beta1 integrin protects intestinal epithelial cells from apoptosis through a phosphatidylinositol 3-kinase and protein kinase B-dependent pathway

Renewal of the gastrointestinal epithelium involves a coordinated process of terminal differentiation and programmed cell death. Integrins have been implicated in the control of apoptotic processes in various cell types. Here we examine the role of integrins in the regulation of apoptosis in gastrointestinal epithelial cells with the use of a rat small intestinal epithelial cell line (RIE1) as a model. Overexpression of the integrin alpha5 subunit in RIE1 cells conferred protection against several proapoptotic stimuli. In contrast, overexpression of the integrin alpha2 subunit had no effect on cell survival. The antiapoptotic effect of the alpha5 subunit was partially retained by a mutated version that had a truncation of the cytoplasmic domain. The antiapoptotic effects of the full-length or truncated alpha5 subunit were reversed upon treatment with inhibitors of phosphatidylinositol 3-kinase (PI-3-kinase), suggesting that the alpha5beta1 integrin might interact with the PI-3-kinase/Akt survival pathway. When cells overexpressing alpha5 were allowed to adhere to fibronectin, there was a moderate activation of protein kinase B (PKB)/Akt, whereas no such effect was seen in alpha2-overexpressing cells adhering to collagen. Furthermore, in cells overexpressing alpha5 and adhering to fibronectin, there was a dramatic enhancement of the ability of growth factors to stimulate PKB/Akt; again, this was not seen in cells overexpressing alpha2 subunit and adhering to collagen or fibronectin. Expression of a dominant negative version of PKB/Akt in RIE cells blocked to ability of alpha5 to enhance cell survival. Thus, the alpha5beta1 integrin seems to protect intestinal epithelial cells against proapoptotic stimuli by selectively enhancing the activity of the PI-3-kinase/Akt survival pathway.

Stimulation of human epidermal differentiation by delta-notch signalling at the boundaries of stem-cell clusters

BACKGROUND

Human epidermis is renewed throughout life from stem cells in the basal layer of the epidermis. Signals from the surrounding keratinocytes influence the differentiation of the stem cells, but the nature of the signals is unknown. In many developing tissues, signalling mediated by the transmembrane protein Delta1 and its receptor Notch1 inhibits differentiation. Here, we investigated the role of Delta-Notch signalling in postnatal human epidermis.

RESULTS

Notch1 expression was found in all living epidermal layers, but Delta1 expression was confined to the basal layer of the epidermis, with highest expression in those regions where stem cells reside. By overexpressing Delta1 or Delta(T), a truncated form of Delta1, in primary human keratinocytes and reconstituting epidermal sheets containing mixtures of Delta-overexpressing cells and wild-type cells, we found that cells expressing high levels of Delta1 or Delta(T) failed to respond to Delta signals from their neighbours. In contrast, wild-type keratinocytes that were in contact with neighbouring cells expressing Delta1 were stimulated to leave the stem-cell compartment and initiate terminal differentiation after a few rounds of division. Delta1 promoted keratinocyte cohesiveness, whereas Delta(T) did not.

CONCLUSIONS

We propose that high Delta1 expression by epidermal stem cells has three effects: a protective effect on stem cells by blocking Notch signalling; enhanced cohesiveness of stem-cell clusters, which may discourage intermingling with neighbouring cells; and signalling to cells at the edges of the clusters to differentiate. Notch signalling in epidermal stem cells thus differs from other progenitor cell populations in promoting, rather than suppressing, differentiation.

Out of Eden: stem cells and their niches

Stem cells are currently in the news for two reasons: the successful cultivation of human embryonic stem cell lines and reports that adult stem cells can differentiate into developmentally unrelated cell types, such as nerve cells into blood cells. Both intrinsic and extrinsic signals regulate stem cell fate and some of these signals have now been identified. Certain aspects of the stem cell microenvironment, or niche, are conserved between tissues, and this can be exploited in the application of stem cells to tissue replacement therapy.

Why stem cells?

Stem cells are viewed from the perspectives of their function, evolution, development, and cause. Counterintuitively, most stem cells may arise late in development, to act principally in tissue renewal, thus ensuring an organism's long-term survival. Surprisingly, recent reports suggest that tissue-specific adult stem cells have the potential to contribute to replenishment of multiple adult tissues.

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.