Changes in keratinocyte adhesion during terminal differentiation: reduction in fibronectin binding precedes alpha 5 beta 1 integrin loss from the cell surface

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.

Modulation of osteosarcoma cell growth and differentiation by silane-modified surfaces

The effects of growing the Saos-2 human osteosarcoma cell line onto surfaces containing -CH(3), -OH, -COOH, -NH(2), and C6H5 groups obtained by silane modification were examined. These cells were used because of the great importance of bone cells in many aspects of biomaterials research. Silane-modified surfaces were characterized by contact angle measurements and, subsequently, surface energies were calculated. Cells grown on clean glass, as well as those grown on glass surfaces containing the functional groups cited above, were examined by light and scanning electron microscopy and assessed for their growth characteristics (i.e., determination of cell number and Ki67 antigen expression). The data presented seemed to indicate that if Saos-2 cells are grown on silane-modifed surfaces containing the methyl (CH(3)), hydroxyl (OH), and phenyl (C6H5) functional groups, their proliferation is slowed down while growth of these cells on glass surfaces modified with amino (NH(2)) and carboxyl (COOH) groups did not significantly affect growth. Once it was demonstrated that these three functional groups induce significant variations in proliferation, cells grown on these surfaces were also tested for apoptosis and expression of important markers of bone cell differentiation (i.e., osteonectin and osteopontin) by flow cytometry and eventual rearrangement of these markers by fluorescence microscopy. The data suggested that growth of Saos-2 cells on CH(3) induces the most evident morphological changes while growth of these cells on OH and C6H5 brings about the greater variations in osteonectin and osteopontin. We hypothesized that these changes are indicative of an increase in differentiation of Saos-2 cells when grown on the OH and C6H5 groups.

Serum-free cultured keratinocytes fail to organize fibronectin matrix and possess different distribution of beta-1 integrins

The development of serum free medium formulation for culturing keratinocytes was a breakthrough in achieving a high number of epidermal cells for experimental and therapeutic studies, in particular to support the wound healing process. It is not clear, however, if switching the cells to highly proliferative phenotype may reflect change in other cellular functions important for the wound repair as their adhesive interactions with the extracellular matrix components. Remodelling of the extracellular matrix, particularly of fibronectin plays an essential role for guiding the cells during wound healing. The molecular mechanisms for organization of this provisional fibronectin matrix, however, are still not clear. We found that keratinocytes in serum containing medium, although in fewer numbers than fibroblasts, were able to remove adsorbed fluorescent labelled fibronectin from the substratum and reorganize it in a fibrilar pattern along the cell periphery. After 3 days the secreted fibronectin had also been organized as matrix-like fibers and as clusters deposited on the substratum after migrating cells. In contrast, serum free cultured keratinocytes fail to organize pre-adsorbed fluorescent labelled fibronectin, as well as the secreted fibronectin, although they grow very well under these conditions. Switching the cells to serum containing medium initiates the removal of fluorescent labelled fibronectin from the substratum, however without reorganization in fibrillar pattern. Most likely, these keratinocytes remove fluorescent labelled fibronectin by the expression of proteolytic activity, rather than with the mechanical function of beta(1) integrins. The latter were diffusely dispersed in serum containing conditions and tend to organize in focal adhesions in serum free cultured cells. We assumed their transient expression and different affinity state might be important for the keratinocyte migration and matrix assembly mechanism.

Keratinocyte growth factor induces hyperproliferation and delays differentiation in a skin equivalent model system

Keratinocyte growth factor (KGF) is a paracrine mediator of epithelial cell growth. To examine the direct effects of KGF on the morphogenesis of the epidermis, we generated skin equivalents in vitro by seeding human keratinocytes on the papillary surface of acellular dermis and raising them up to the air-liquid interface. KGF was either added exogenously or expressed by keratinocytes via a recombinant retrovirus encoding KGF. KGF induced dramatic changes to the 3-dimensional organization of the epidermis including pronounced hyperthickening, crowding, and elongation of the basal cells, flattening of the rete ridges, and a ripple-like pattern in the junction of stratum corneum and granular layers. Quantitative immunostaining for the proliferation antigen, Ki67, revealed that in addition to increasing basal proliferation, KGF extended the proliferative compartment by inducing suprabasal cell proliferation. KGF also induced expression of the integrin alpha 5 beta 1 and delayed expression of keratin 10 and transglutaminase. However, barrier formation of the epidermis was not disrupted. These results demonstrate for the first time that a single growth factor can alter the 3-dimensional organization and proliferative function of an in vitro epidermis. In addition to new strategies for tissue engineering, such a well-defined system will be useful for analyzing growth factor effects on the complex links between cell proliferation, cell movement and differentiation within a stratified tissue.

Basement membrane induced differentiation of HEC-1B(L) endometrial adenocarcinoma cells affects both morphology and gene expression

In vitro studies of endometrial carcinogenesis have been hampered by dedifferentiation of the cells in culture. Using the endometrial carcinoma cell line HEC-1B(L), we aimed to establish and characterize culture conditions that preserve a more differentiated state of the tumor cells. HEC-1B(L) cells grown in a serum-free defined medium on plastic (PL/SFDM) on top of a reconstituted basement membrane (Matrigel, MG/SFDM) or in a thick layer of Matrigel showed pronounced morphological differentiation as compared with HEC-1B(L) cells cultured on plastic in a medium containing serum (PL/10% FCS). Features of differentiation included cuboidal to columnar cell shape and an increase of rough endoplastic reticulum in Matrigel cultures. Gene expression of HEC-1B(L) cells was studied by metabolic [35S]methionine labeling and SDS-gel electrophoresis. HEC-1B(L) cells cultured in the presence of Matrigel showed two additional secretory proteins approximately 31 kD and 77 kD in size. rt-PCR was used to screen cell cultures for the presence of estrogen receptor, progesterone receptor, and lactoferrin-mRNA, genes typically expressed by normal endometrial epithelium. We found no expression of the estrogen receptor or progesterone receptor. Lactoferrin-mRNA was present under all culture conditions tested. Our results suggest a regulatory role of the extracellular matrix for the differentiation of the HEC-1B(L) cell line.

Carbohydrate-carbohydrate binding of ganglioside to integrin alpha(5) modulates alpha(5)beta(1) function

Gangliosides GT1b and GD3, components of keratinocyte membranes, inhibit keratinocyte adhesion to fibronectin. Although ganglioside sialylation is known to be important, the mechanism of inhibition is unknown. Using purified insect recombinant alpha(5) and beta(1) proteins and alpha(5)beta(1) integrin from lysed keratinocyte-derived SCC12 cells, we have shown that GT1b and GD3 inhibit the binding of alpha(5)beta(1) to fibronectin. Co-immunoprecipitation of GT1b and alpha(5)beta(1) from SCC12 cells and direct binding of GT1b and GD3 to affinity-purified alpha(5)beta(1) from SCC12 cells and insect recombinant alpha(5)beta(1), particularly the alpha(5) subunit, further suggest interaction between ganglioside and alpha(5)beta(1). The carbohydrate moieties of integrin appear to be critical since gangliosides are unable to bind deglycosylated forms of alpha(5)beta(1) from SCC12 and insect cells or poorly glycosylated recombinant alpha(5)beta(1) from Escherichia coli cells. The GT1b-alpha(5)beta(1) interaction is inhibited by concanavalin A, suggesting that GT1b binds to mannose structures in alpha(5)beta(1). The preferential binding of GT1b to high mannose rather than reduced mannose ovalbumin further implicates the binding of GT1b to mannose structures. These data provide evidence that highly sialylated gangliosides regulate alpha(5)beta(1)-mediated adhesion of epithelial cells to fibronectin through carbohydrate-carbohydrate interactions between GT1b and the alpha(5) subunit of alpha(5)beta(1) integrin.

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.

Keratinocyte migration requires alpha2beta1 integrin-mediated interaction with the laminin 5 gamma2 chain

Keratinocyte migration is an absolute requirement for correct epithelialization during the process of wound healing. This process requires changes in extracellular matrix ligand expression as well as changes in ligand-binding affinity of the corresponding cellular integrins. In this study, we attempt to understand the role of laminin 5 in migration by investigating the integrin-mediated interactions of migrating keratinocytes with their newly synthesized laminin 5. We chose to induce migration of freshly isolated NHK in vitro by exposing them to TGF-beta1 which, in addition to promoting epithelial cell migration, is also known to prevent cell proliferation. This important feature allowed the study to be focused on cell migration without interfering with cell proliferation. We confirm that keratinocyte migration on plastic, fibronectin or collagen IV substrates requires endogenous laminin 5 deposition, which is predominantly detected under its unprocessed form. Despite a crucial role for laminin 5 in migration, we show that this process is accompanied by a significant decrease in adhesion to purified laminin 5. Moreover, we provide evidence that the alpha2beta1 integrin interaction with newly synthesized laminin 5 renders the cells more adherent and retards migration. Conversely, we provide evidence that the alpha2beta1 integrin-laminin 5 interaction is absolutely required for keratinocyte migration and that the alpha2beta1 integrin is responsible for cell spreading on laminin 5. Finally, we demonstrate that the alpha2beta1 integrin binding to laminin 5 occurs within the short arm of the gamma2 subunit.

Assessment of adhesion assays for use with keratinocytes

Cell attachment, as a biological process, is an important aspect with respect to graft survival and "take". With the ever-increasing use of cultured epithelial autografts for coverage and re-epithelialization of wounds, the assessment of keratinocyte adhesion in vitro has become a more common requirement in studies involving extracellular matrix proteins and their receptor molecules. Cell adhesion has been well-documented in immunological research, however keratinocyte adhesion has been investigated by manual counting (using methylene blue) or other less sensitive colorimetric methods. With the increase in number of fluorogenic probes available, their use as a sensitive alternative to radioactive labelling has been promoted in the literature. This study was carried out to investigate the possibility of using fluorescent probe 5,6-carboxyfluorescein diacetate succidimyl ester to achieve a more standardized assay in the assessment of keratinocyte adhesion. Adhesion was assessed on extracellular matrix proteins such as fibronectin, collagen types I & IV and laminin. We concluded that the fluorescent probe might provide greater sensitivity in measuring adhesion, however it may be cytotoxic to keratinocytes. Pre-labelling of keratinocytes may affect cellular functions such as adhesion and even proliferation and consequently the probe must be chosen with care.

The expression of syndecan-1 is preferentially reduced compared with that of E-cadherin in acantholytic squamous cell carcinoma

BACKGROUND

Syndecan-1 and E-cadherin are cell adhesion molecules which are expressed primarily on the surface of adult epithelial cells. They appear to be co-regulated and may act in concert to stabilize the epithelium. Loss of expression of both E-cadherin and syndecan-1 is seen in malignant transformation and invasion.

METHODS

Thirteen cutaneous biopsies of acantholytic squamous cell carcinoma (SCC) were examined for coexpression of E-cadherin and syndecan-1.

RESULTS

Interestingly, immunoreactivity for E-cadherin was increased in the in situ component while immunoreactivity for syndecan-1 was similar to that seen in normal skin. Conversely, in invasive SCC the expression of these two adhesion molecules was very similar. Both diminished with decreasing cell differentiation, as well as in the acantholytic areas where both molecules exhibited increasing cytosolic staining rather than cell membrane staining.

CONCLUSIONS

Our results suggest that it is likely E-cadherin and syndecan-1 act in concert to stabilize the epithelium and that the loss or decreased expression of both of these adhesion molecules is associated with malignant transformation.

Vero cell growth and differentiation on poly(L-lactic acid) membranes of different pore diameters

In the last few years, the demand has increased for research on polymeric materials, which can be used as substitutes for injured tissues and organs or to improve their regeneration. In this work, we studied poly(L-lactic acid) (PLLA) membranes, a resorbable biomaterial, which were either dense or had different pore diameters (less than 45 microm, between 180 and 250 microm, and between 250 and 350 microm), in relation to stimulation of cell adhesion, growth, and differentiation in vitro. We used Vero cells, a fibroblastic cell line, as the biological model of investigation. We found that cells attached slowly to all PLLA membranes studied. On the other hand, once the adhesion occurs, the cells are able to grow and differentiate on the different polymers. The cells grew to form a confluent monolayer and were capable of producing collagen Type IV and fibronectin on different PLLA membranes. This behavior indicates that cells try to create a better environment to stimulate their growth. This also indicates that Vero cells alter their differentiation pattern once they are producing extracellular matrix molecules related to epithelial differentiation.

Ras-independent activation of the Raf/MEK/ERK pathway upon calcium-induced differentiation of keratinocytes

MAPKs are crucially involved in the regulation of growth and differentiation of a variety of cells. To elucidate the role of MAPKs in keratinocyte differentiation, activation of ERK, JNK, and p38 in response to stimulation with extracellular calcium was analyzed. We provide evidence that calcium-induced differentiation of keratinocytes is associated with rapid and transient activation of the Raf/MEK/ERK pathway. Stimulation of keratinocytes with extracellular calcium resulted in activation of Raf isozymes and their downstream effector ERK within 10-15 min, but did not increase JNK or p38 activity. Calcium-induced ERK activation differed in kinetics from mitogenic ERK activation by epidermal growth factor and could be modulated by alterations of intracellular calcium levels. Interestingly, calcium stimulation led to down-regulation of Ras activity at the same time that ERK activation was initiated. Expression of a dominant-negative mutant of Ras also did not significantly impair calcium-induced ERK activation, indicating that calcium-mediated ERK activation does not require active Ras. Despite the transient nature of ERK activation, calcium-induced expression of the cyclin-dependent kinase inhibitor p21/Cip1 and the differentiation marker involucrin was sensitive to MEK inhibition, which suggests a role for the Raf/MEK/ERK pathway in early stages of keratinocyte differentiation.

alpha3beta1 and alpha6beta4 integrin receptors for laminin-5 are not essential for epidermal morphogenesis and homeostasis during skin development

Continuous regeneration and homeostasis of the stratified epidermis requires coordinated regulation of cell proliferation, cell differentiation, and cell survival. Integrin-mediated cell adhesion to the extracellular matrix has important roles in regulating each of these processes. Integrins alpha3beta1 and alpha6beta4 are both receptors on epidermal keratinocytes for the basement membrane protein laminin-5, the major ligand for epidermal adhesion in mature skin. Ablation in mice of either alpha3beta1 or alpha6beta4, through null mutation of the gene encoding the alpha3, alpha6, or beta4 integrin subunit, results in epidermal blistering of varying severity. Our previous studies showed that, despite blistering, differentiation and stratification of the epidermis appeared essentially normal in mice that lacked either alpha3beta1 or alpha6beta4. However, these studies did not definitively address the specific developmental importance of each integrin, since they may have overlapping and/or compensatory functions. Given the individual importance of alpha3beta1 or alpha6beta4 in maintaining the dermo-epidermal junction in mature skin, we sought to determine the importance of these integrins for embryonic skin development and epidermal morphogenesis. In the current study, we analyzed skin development in mutant embryos that completely lack both integrins alpha3beta1 and alpha6beta4. Although alpha3beta1/alpha6beta4-deficient embryos displayed epidermal blistering by stage E15.5 of development, they also retained regions of extensive epidermal adhesion to the basement membrane through stage E16.5, indicating alternative adhesion mechanisms. Apoptosis was induced in detached epidermis of alpha3beta1/alpha6beta4-deficient embryos, exemplifying vividly the importance of epithelial attachment to the basement membrane for cell survival. However, apoptotic cells were completely absent from attached epidermis of alpha3beta1/alpha6beta4-deficient embryos, showing that epithelial adhesion that occurred independently of alpha3beta1 and alpha6beta4 also protected cells from apoptosis. Remarkably, in the absence of the known laminin-5 binding integrins (alpha3beta1, alpha6beta4, and alpha6beta1), keratinocytes retained the capacity to proliferate in the epidermis, and epidermal stratification and skin morphogenesis appeared normal prior to blister formation. These findings show that while alpha3beta1 and alpha6beta4 are both required for integrity of the dermo-epidermal junction, neither one is essential for epidermal morphogenesis during skin development.

Ras-transfection up-regulated HaCaT cell migration: inhibition by Marimastat

Cell migration is an essential process in physiological and pathological conditions such as wound healing and tumor invasion. This phenomenon involves cell adhesion on the extracellular matrix mediated by integrins, and cell detachment promoted in part by metalloproteinases (MMPs). In the present study, the migration of two HaCaT-ras clones (metastatic or not), was compared with HaCaT cells, and normal human primary cultured keratinocytes. Using colloidal gold migration assay, the migration index on type I and type IV collagen was similar for primary cultured keratinocytes and HaCaT, whereas it was markedly higher for the HaCaT-ras clones. High motility of ras-transfected cells was confirmed from an in vitro wound healing assay. It was not correlated with changes in integrin expression or related to a different adhesion on extracellular matrix. The Marismastat (BB-2516), a MMP inhibitor, inhibited in a dose-dependent effect the migration in both assays, demonstrating the important role of MMPs in the migration process. Under our experimental conditions, MMP-1 activity was not detected in HaCaT and MMP-9 activity was secreted by these cells only after their stimulation by EGF. Here, MMP-2 was the major gelatinolytic activity secreted by all the cells and its secretion was markedly higher for HaCaT-nis clones compared with HaCaT. In addition, Western blotting results confirmed a higher expression of MMP-2 associated with a lower expression of TIMP-2 in HaCaT-ras compared with HaCaT. These results suggest that Ha-ras oncogene could be a stimulating factor of migration and might modified the balance between MMP-2 and TIMP-2 in keratinocyte cell lines.

Targeted inhibition of wound-induced PAI-1 expression alters migration and differentiation in human epidermal keratinocytes

In the adult epidermis, keratinocytes do not normally express the type-1 inhibitor of plasminogen activator (PAI-1). Basal epithelial cell-specific PAI-1 synthesis, however, accompanies epidermal wound repair in vivo in which PAI-1 transcripts and immunoreactive protein are confined to epithelial cells in the migrating tongue and the hyperproliferative zone. A model system using human keratinocytes (HaCaT cells) was developed to assess functional relationships between epithelial growth state transitions and PAI-1 expression. PAI-1 synthesis was maximal in low population density, exponentially growing HaCaT cultures; relative PAI-1 mRNA and protein levels progressively declined as cells attained, and were maintained in, a postconfluent condition. While the fraction of PAI-1(+) keratinocytes remained stable (at approximately 85-90% of the population) throughout the culture period, both PAI-1 mRNA abundance and mean cell-associated PAI-1 protein declined by >90% during prolonged (i.e., 8-day) growth arrest. Similar to epidermal trauma in vivo, scrape wounding of HaCaT monolayers resulted in the rapid and location-specific induction of PAI-1 protein (an increase of 11- to 16-fold relative to unwounded cultures) in cells immediately bordering the injury site. PAI-1 expression was evident in keratinocytes that comprised the opposed migrating fronts and remained elevated until wound closure. Down-regulation of PAI-1 synthesis in HaCaT cells transfected with an inducible LacSwitch-based antisense vector system markedly impaired both the rate and the extent of wound closure. All injuries created in antisense PAI-1 monolayers remained unhealed at day 8 postinjury compared to the 3-day complete repair typical of control cultures. Vector-driven modulation of PAI-1 synthesis was also associated with an increase in the percentage of suprabasal-type keratinocytes within the wound field. PAI-1 expression by migrating HaCaT cells appears necessary to maintain the basal epidermal phenotype and/or appropriate cell-to-substrate adhesion during injury repair.

Mouse keratinocytes immortalized with large T antigen acquire alpha3beta1 integrin-dependent secretion of MMP-9/gelatinase B

Remodeling of the extracellular matrix during tissue development, wound repair and tumor cell invasion depends on the coordinated regulation of cell adhesion receptors, matrix proteins and enzymes that proteolyse the extracellular matrix. Integrin alpha3beta1 is a major receptor on epidermal keratinocytes for laminin-5 in the cutaneous basement membrane and is required for normal basement membrane organization during skin development. alpha3beta1 is also expressed at high levels in the majority of adherent transformed cells and in most tumors, and it could have similar roles in extracellular matrix remodeling during tumorigenesis and cell invasion. In the present study, we show that alpha3beta1 expression is required in immortalized mouse keratinocytes (MK) for the production of the matrix metalloproteinase MMP-9/gelatinase B, an MMP that is coexpressed with alpha3beta1 in epithelial cell carcinomas and during wound healing, and contributes to the invasive potential of some tumor cells. MMP-9 was expressed in MK cells derived from wild-type mice, but not in MK cells derived from alpha3-null mice. Reconstitution of alpha3beta1 expression in alpha3-null MK cells through transfection with the alpha3 subunit restored MMP-9 secretion, indicating an alpha3beta1-dependent pathway for MMP-9 production. alpha3beta1-dependent expression of MMP-9 was associated with the immortalized phenotype, since nonimmortalized, primary keratinocytes required soluble growth factors, but not alpha3beta1, for efficient expression of MMP-9. Our results suggest that an alpha3beta1-independent pathway(s) for MMP-9 production is suppressed in keratinocytes immortalized with large T antigen, and that an alpha3beta1-dependent pathway is required for sustained production of MMP-9 in the absence of other pathways.

Vinculin, Talin, Integrin alpha6beta1 and laminin can serve as components of attachment complex mediating contraction force transmission from cardiomyocytes to extracellular matrix

Recently, we reported that cardiomyocytes adhere to extracellular matrix at costameres, the striated distribution of vinculin between Z-lines and the sarcolemma, where transmission of contraction forces from myofibrils to the extracellular matrix occurs. To identify other molecules involved in force transmission at costameres, we examined adult rat and embryonic chick cardiomyocytes cultured on coverslips or flexible thin silicone rubber substrata. Immunolocalization of talin showed a costameric, striated distribution, which corresponded to dark contacts with interference reflection microscopy. The molecules involved in substrate adhesion were cross-linked with the non-penetrating cross-linking agent Bis(sulfosuccinimidyl)-suberate and detected by immunohistochemical staining with anti-alpha6, alpha3, alphav, or beta1 integrin antibodies. Both alpha6 and beta1 showed costameric distributions, but alpha3 and alpha(v) did not. The distribution of laminin after cross-linking and extraction also showed a costameric distribution. When anti-integrin beta1 antibody was added to live cardiomyocytes grown on the silicone rubber substratum, the transmission of contraction forces was inhibited. These findings suggest that vinculin, talin, integrin alpha6beta1 and laminin system can be involved in transmission of contraction force to the extracellular matrix.

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.

The cytoplasmic domain of the alpha1 integrin subunit influences stress fiber formation via the conserved GFFKR motif

Integrins are heterodimeric transmembrane proteins that mediate substrate adhesion and migration but also the bidirectional transfer of information across the plasma membrane via their cytoplasmic domains. We addressed the question of whether the very short cytoplasmic tail of the alpha1 integrin subunit of alpha1beta1 integrin is required for alpha1beta1-specific adhesion, spreading, and migration. For this purpose we transfected the alpha1 integrin subunit and two cytoplasmically truncated alpha1 subunits into Chinese hamster ovary (CHO) cells. Elimination of the entire cytoplasmic domain of the alpha1 subunit does not affect adhesion but leads to inhibition of spreading and stress fiber formation. The defect in spreading could not be rescued by lysophosphatidic acid, which has been reported to stimulate actin stress fiber formation via Rho. Additionally, deletion of the entire cytoplasmic domain of the alpha1 subunit abolishes migration toward alpha1beta1-specific substrates. Migration and stress fiber formation are similar in CHO-alpha1 cells and CHO cells carrying an alpha1 subunit still containing the conserved GFFKR motif. So, the GFFKR motif of the alpha1 subunit is essential and sufficient for these processes.

beta1B integrin subunit contains a double lysine motif that can cause accumulation within the endoplasmic reticulum

Human epidermal keratinocytes are one of the few cell types that express the beta1B splice variant of the beta1 integrin subunit. Although in transfection experiments beta1B acts as a dominant negative inhibitor of cell adhesion, we found that beta1B was expressed at very low levels in keratinocytes, both in vivo and in culture, and had a predominantly cytoplasmic distribution, concentrated within the endoplasmic reticulum. To examine why beta1B accumulated in the cytoplasm, we prepared chimeras between CD8alpha and the beta1A and beta1B integrin cytoplasmic domains. In transfected HeLa cells, both constructs reached the cell surface but the rate of maturation of the beta1B chimera was considerably retarded relative to beta1A. The beta1B cytoplasmic domain contains two lysine residues that resemble the double lysine motif characteristic of many proteins that are resident within the endoplasmic reticulum. Mutation of each lysine individually to serine had no effect on CD8beta1B maturation, but when both residues were mutated the rate of CD8beta1B maturation increased to that of CD8beta1A. Further analysis of beta1B function in keratinocytes must, therefore, take into account the low abundance of the isoform relative to beta1A and the potential for beta1B to accumulate in the endoplasmic reticulum.

E-cadherin at the cell periphery is a determinant of keratinocyte differentiation in vitro

The origin of the signal for keratinocyte differentiation is still unknown. Here, we show that Ca(2+)- and density-induced translocation of E-cadherin, but not P-cadherin, is accompanied by induction of differentiation-specific proteins in cultured keratinocytes. Antibodies that artificially cluster cell-surface E-cadherin in low extracellular Ca(2+) also induce differentiation-specific proteins, implicating E-cadherin as a determinant of keratinocyte differentiation in vitro.

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.

Fibronectin fragments and their role in inflammatory arthritis

OBJECTIVES

To identify potential immunopathogenic links between fibronectin (Fn) fragmentation and the inflammatory response in chronic joint disease.

METHODS

Scientific papers involving studies of Fn fragments and inflammatory processes important in the pathogenesis of arthritis, including chondrolysis, synoviocyte growth and adhesion, polymorphonuclear leukocyte (PMN) and monocyte function, proteolysis, and immune complex activation were reviewed. In addition, reports identifying Fn fragments in synovial fluid (SF) were assessed.

RESULTS

A series of Fn fragments have been identified in arthritic SF by several investigators. Fn and fragments ranging from 30 to 200 kd are present in elevated concentrations in inflammatory SF. SF Fn fragments display reduced affinity for fibrin and collagen. The 29- and 50-kd amino terminal fragments mediate release of proteoglycan from articular cartilage by RGD-independent mechanisms. Fn fragments can induce fibroblast gene expression of metalloproteinases or can act as proteinases themselves. A 90-kd plasmin generated fragment possesses homology with streptokinase. Fragments mediate PMN chemotaxis and enhance proliferation of CD4+ lymphocytes as well as binding to the C1q component of complement and influencing the behavior of immune complexes.

CONCLUSIONS

Fn fragments can be functionally and biochemically characterized in diseased SF. Modification of fragment formation and inhibition of fragment function may have potential therapeutic value in the interruption of chronic synovial inflammation.

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.

Increased recycling of (alpha)5(beta)1 integrins by lung endothelial cells in response to tumor necrosis factor

Tumor necrosis factor (alpha) (TNF-(alpha) can change the interaction of lung endothelial cell monolayers with their extracellular matrix in association with an increase in endothelial monolayer protein permeability. Using immunofluorescence microscopy and flow cytometry, we determined if exposure of calf pulmonary artery endothelial monolayers to TNF-(alpha) may influence cell-matrix interactions by altering the clustering as well as internalization of the (α)5(beta)1 integrins (or fibronectin receptors) on the surface of endothelial cells. Immunofluorescence microscopy revealed that TNF-(alpha) caused an increase in the intracellular staining of (alpha)5(alpha)1 integrins within structures similar to endocytic vesicles as well as an increase in antibody-induced clustering of the integrins at the cell periphery. Flow cytometric analysis of endothelial cells incubated at 37 degrees C after antibody-labeling of their surface (alpha)5(beta)1 integrins at 4 degrees C confirmed an increase in the rate of (alpha)5(beta)1 integrin internalization which was at least 3 times greater after TNF-(α) exposure, based on the half-life for antibody-labeled surface integrins to reach equilibrium with non-labeled integrins within the intracellular pool. Interestingly, the total cell surface expression of (alpha)5(beta)1 integrins was relatively constant after TNF-(alpha) exposure despite the enhanced rate of internalization, suggesting an accelerated recycling of the internalized (alpha)5(beta)1 integrins back to the cell surface. This response was confirmed by the measurement of labeled integrin recycling, which showed a significant (P<0.01) increase in the rate of recycling of the internalized integrins in TNF-treated endothelial cells. Enhanced internalization and subsequent recycling of (alpha)5(beta)1 integrins by endothelial monolayers exposed to TNF-(alpha) may facilitate the redistribution of cell-surface integrins in response to this inflammatory cytokine and may also modify cell-matrix interactions leading to reduced integrity and increased protein permeability of the lung endothelial monolayers.

Keratinocyte growth factor (KGF) promotes keratinocyte cell attachment and migration on collagen and fibronectin

Keratinocyte growth factor (KGF) induction of keratinocyte attachment and migration on provisional and basement membrane proteins was examined. KGF-treated keratinocytes showed increased attachment to collagen types I and IV and fibronectin, but, not to laminin-1, vitronectin, or tenascin. This increase was time- and dose-dependent. Increase in attachment occurred with 2 10 microg/ml of ECM proteins. This KGF-stimulated cell attachment was beta1 integrin-dependent but was not associated with stimulation of the cell surface expression nor affinity (activity) of the collagen integrin receptor (alpha2beta1) nor the fibronectin integrin receptors (alpha5beta1 or alphav). At the basal layer of KGF-treated cells significant accumulation of beta1 integrins was found at the leading edges, and actin stress fibers colocalized with beta1. KGF also induced migratory phenotype and stimulated keratinocyte migration on both fibronectin and collagen types I and IV but not on laminin-1, vitronectin nor tenascin. The results suggest that in addition to its proliferation promoting activity. KGF is able to modulate keratinocyte adhesion and migration on collagen and fibronectin. Our data suggest that KGF induced integrin avidity (clustering), a signaling event, which is not dependent on the alteration of cell surface integrin numbers.

The tetraspanin CD9 associates with the integrin alpha6beta4 in cultured human epidermal keratinocytes and is involved in cell motility

Integrins are involved in several ways in keratinocyte physiology, including cell motility. CD9 is a member of the tetraspanin protein family which is found in association with other transmembrane proteins like the integrins. CD9 is expressed in the epidermal tissue, but this expression is not regulated by differentiation. The present work focuses on association of CD9 with the integrin alpha6beta4 in keratinocytes. In vivo, CD9 does not co-localize with alpha6beta4, and is not internalized with the integrin upon basal detachment with dispase. In vitro, CD9 is found partly in co-localization with alpha6beta4 and is internalized with the integrin after keratinocyte detachment with dispase. Using blocking antibodies in a phagokinetic tracks assay, we show that CD9, and to a lesser extent alpha6beta4, but not the tetraspanin CD82, promote motility of subconfluent keratinocytes on collagen I. Our observations also suggest that CD9 is involved in the formation of lamellipodia. We also report that the phorbol ester TPA has no effect on CD9 expression and association with alpha6beta4, but increases keratinocyte motility, possibly through modulation of integrin subunits expression, or through upregulation of collagenase-1 expression. Together, these results confirm that CD9 associates with alpha6beta4 in cultured keratinocytes, possibly in order to regulate the function of the integrin, and that CD9 is involved in keratinocyte motility on collagen. The data suggest that regulation of adhesion characteristics by CD9 in keratinocytes may play a role in epidermal repair.

Adhesion and migration of avian neural crest cells on fibronectin require the cooperating activities of multiple integrins of the (beta)1 and (beta)3 families

Based on genetic, functional and histological studies, the extracellular matrix molecule fibronectin has been proposed to play a key role in the migration of neural crest cells in the vertebrate embryo. In the present study, we have analyzed in vitro the repertoire and function of integrin receptors involved in the adhesive and locomotory responses of avian truncal neural crest cells to fibronectin. Immunoprecipitation experiments showed that neural crest cells express multiple integrins, namely (alpha)3(beta)1, (alpha)4(beta)1, (alpha)5(beta)1, (alpha)8(beta)1, (alpha)v(beta)1, (alpha)v(beta)3 and a (beta)8 integrin, as potential fibronectin receptors, and flow cytometry analyses revealed no major heterogeneity among the cell population for expression of integrin subunits. In addition, the integrin repertoire expressed by neural crest cells was found not to change dramatically during migration. At the cellular level, only (alpha)v(beta)1 and (alpha)v(beta)3 were concentrated in focal adhesion sites in connection with the actin microfilaments, whereas the other integrins were predominantly diffuse over the cell surface. In inhibition assays with function-perturbing antibodies, it appeared that complete abolition of cell spreading and migration could be achieved only by blocking multiple integrins of the (beta)1 and (beta)3 families, suggesting possible functional compensations between different integrins. In addition, these studies provided evidence for functional partitioning of integrins in cell adhesion and migration. While spreading was essentially mediated by (alpha)v(beta)1 and (alpha)8(beta)1, migration involved primarily (alpha)4(beta)1, (alpha)v(beta)3 and (alpha)8(beta)1 and, more indirectly, (alpha)3(beta)1. (alpha)5(beta)1 and the (beta)8 integrin were not found to play any major role in either adhesion or migration. Finally, consistent with the results of inhibition experiments, recruitment of (alpha)4(beta)1 and (alpha)v(beta)3, individually or in combination using antibodies or recombinant VCAM-1 and PECAM-1 molecules as a substratum, was required for migration but was not sufficient to produce migration of the cell population as efficiently as with fibronectin. In conclusion, our study indicates that neural crest cells express a multiplicity of fibronectin-binding integrins and suggests that dispersion of the cell population requires cooperation between distinct integrins regulating different events of cell adhesion, locomotion and, possibly, proliferation and survival.

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

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

Retention of cell adhesion and growth capability in human cervical cancer cells deprived of cell anchorage

Cell adhesion is linked to various regulatory processes of growth as well as apoptotic cell death in normal and transformed epithelial cells. We investigated changes of cellular responses to the deprivation of cell anchorage associated with immortalization or malignant transformation. Normal human ectocervical keratinocytes (NCE cells) deprived of cell anchorage become susceptible to apoptosis, and in parallel they lose their adhesion to the culture substratum. The loss of cell adhesion is not directly due to apoptosis. NCE16 cells, an immortalized but not malignantly transformed subline of NCE, underwent apoptosis and lost cell adhesion in suspension, as the NCE cells did. By contrast, apoptosis was not inducible in human cervical cancer-derived C33A cells in suspension. Of other cell lines derived from human cervical cancer, SiHa cells showed a weak apoptotic response and Caski cells were highly sensitive to apoptosis in the absence of cell anchorage. Unlike NCE or NCE16 cells, all these cancer cells retained cell adhesion as well as growth capability in suspension cultures. These results indicate that retention of cell adhesion and growth capability in the absence of cell anchorage is more closely associated with cancer cell lines than resistance to apoptosis upon the deprivation of cell anchorage.

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

Human epidermal stem cells express higher levels of beta1 integrins and are more adhesive than keratinocytes that are destined to differentiate. To investigate whether high beta1 integrin expression and adhesiveness are essential for maintaining keratinocytes in the stem cell compartment, we introduced a dominant-negative beta1 integrin mutant, CD8beta1, into cultured human keratinocytes, thereby interfering with beta1 integrin function. Surface beta1 integrin levels, adhesiveness, and mitogen-activated protein (MAP) kinase activation on fibronectin were reduced, and exit from the stem cell compartment was stimulated. Adhesiveness and proliferative potential were restored by overexpressing wild-type beta1 integrin or by constitutive MAP kinase activation. Conversely, a dominant-negative MAP kinase kinase 1 mutant decreased adhesiveness and stem cell number in the absence of CD8beta1. MAP kinase activation by alpha6beta4-mediated adhesion and mitogens was normal in CD8beta1 cells, and constitutive MAP kinase activation did not affect adhesion and proliferation of control keratinocytes. We conclude that beta1 integrins and MAP kinase cooperate to maintain the epidermal stem cell compartment in vitro.

Characterisation of membrane oligonucleotide-binding proteins and oligonucleotide uptake in keratinocytes

Inadequate cellular compartmentalisation of plasmid DNA and antisense oligodeoxynucleotides (ODNs) is generally considered as a major limitation in their use. In this study, an approach combining in situ visual-isation of rhodamine-labelled ODNs and affinity modification of proteins by radiolabelled-alkylating ODN derivatives has been used to investigate the uptake of ODNs into keratinocytes. We confirm here that unmodified ODNs are efficiently taken up and accumulate in cell nuclei in primary keratinocytes as well as in HaCaT and A431 keratinocyte cell lines. Uptake is fast, irreversible, saturable and not significantly altered by incubation at low temperature. Affinity modification studies in keratinocyte cell lines has revealed two high-affinity, cell-specific interactions between ODNs and proteins of 61-63 kDa and 35 kDa. Trypsin pre-treatment of A431 cells and pre-incubation with polyanions, or with unlabelled nucleic acid competitors, inhibited the accumulation of rhodamine-labelled ODNs in nuclei as well as the affinity labelling of the 61-63 kDa doublet and 35 kDa ODN-binding proteins by reactive ODN derivatives. Finally, cell fractionation studies indicated that these ODN-binding proteins were essentially localised in the plasma membrane. Our results suggest that these ODN-binding proteins might be involved in the recognition and transport of ODNs into keratinocytes.

The spatial relationship between stem cells and their progeny in the basal layer of human epidermis: a new view based on whole-mount labelling and lineage analysis

In order to examine the spatial organisation of stem cells and their progeny in human epidermis, we developed a method for whole-mount epidermal immunofluorescence labelling using high surface beta1 integrin expression as a stem cell marker. We confirmed that there are clusters of high beta1 integrin-expressing cells at the tips of the dermal papillae in epidermis from several body sites, whereas alpha6 integrin expression is more uniform. The majority of actively cycling cells detected by Ki67 or bromodeoxyuridine labelling were found in the beta1 integrin-dull, transit amplifying population and integrin-negative, keratin 10-positive cells left the basal layer exclusively from this compartment. When we examined p53-positive clones in sun-exposed epidermis, we found two types of clone that differed in size and position in a way that was consistent with the founder cell being a stem or transit amplifying cell. The patterning of the basal layer implies that transit amplifying cells migrate over the basement membrane away from the stem cell clusters. In support of this, isolated beta1 integrin-dull keratinocytes were more motile on type IV collagen than beta1 integrin-bright keratinocytes and EGFP-labelled stem cell clones in confluent cultured sheets were compact, whereas transit amplifying clones were dispersed. The combination of whole-mount labelling and lineage marking thus reveals features of epidermal organisation that were previously unrecognised.

Epithelial cell adhesion to extracellular matrix proteins induces tyrosine phosphorylation of the Epstein-Barr virus latent membrane protein 2: a role for C-terminal Src kinase

The Epstein-Barr virus (EBV) latent membrane protein 2 (LMP2) is expressed in latently EBV-infected B cells, where it forms patches in the plasma membrane and interferes with B-cell receptor signal transduction through dominant-negative effects on protein kinases. LMP2 transcripts are detected in nasopharyngeal carcinoma, an epithelial-cell malignancy. In this study the function of LMP2A in epithelial cells was investigated. LMP2A was found to coprecipitate with protein kinase activities and to become phosphorylated in in vitro kinase assays. Analysis of LMP2A deletion mutants demonstrated that tyrosines implicated in interacting with Src family kinase SH2 domains and the SH2 domain of Csk, as well as the LMP2A immunoreceptor tyrosine-based activation motif, are important for its phosphorylation in epithelial cells. LMP2A tyrosine phosphorylation was triggered by cell adhesion to extracellular-matrix (ECM) proteins. Src family kinases, whose involvement in cell-ECM signaling and LMP2A phosphorylation in B lymphocytes has been well established, were found not to be responsible for LMP2A phosphorylation in epithelial cells. Instead, coexpression of Csk, a negative Src regulator, and LMP2A led to an increase in LMP2A phosphorylation both in nonadherent cells and upon cell adhesion. Csk also phosphorylated LMP2A in vitro. These results suggest that LMP2A has a different role in epithelial cells, where it interacts with cell adhesion-initiated signaling pathways. Although tyrosine phosphorylation of LMP2A occurs in both cell types, different protein kinases seem to be used: Src family kinases in B lymphocytes and Csk in epithelial cells.

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.

Evidence for a novel glutamate-mediated signaling pathway in keratinocytes

Phenotypic alterations in keratinocyte behavior are essential for maintaining epidermal integrity during growth and wound repair and rely on co-ordinated cell signaling events. Numerous growth factors and cytokines have been shown to be instrumental in guiding such changes in keratinocyte activity; here we provide evidence which proposes a novel epidermal signaling pathway mediated by the excitatory amino acid glutamate. Glutamate is the major excitatory neurotransmitter at synaptic junctions within the central nervous system; however, we have identified expression in vivo of several regulatory molecules associated with glutamate signaling in keratinocytes. In resting rat skin epidermis, different classes of glutamate receptors, transporters, and a recently described clustering protein were shown to display distinct distribution patterns, supportive of a multifunctional cellular communication pathway. Immunoreactive N-methyl-D-aspartate-type, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type, and metabotropic-type glutamate receptors were colocalized with the specific glutamate transporter EAAC1 in basal layer keratinocytes, and GLT-1, a related transporter, was expressed suprabasally. In full-thickness rat skin wounds, marked modifications in the distribution of N-methyl-D-aspartate receptors and EAAC1 were observed during re-epithelialization, and alterations in N-methyl-D-aspartate receptor expression accompanied embryonic epidermal development, implicating glutamate signaling in these important biologic events. Furthermore, we provide evidence that these receptors are functional in vitro. These data provide strong evidence to support a role for glutamate in the control of epidermal renewal, and therefore suggest potentially novel therapeutic targets for the treatment of skin disease and enhancement of wound healing.

Modulation of cell proliferation and differentiation through substrate-dependent changes in fibronectin conformation

Integrin-mediated cell adhesion to extracellular matrices provides signals essential for cell cycle progression and differentiation. We demonstrate that substrate-dependent changes in the conformation of adsorbed fibronectin (Fn) modulated integrin binding and controlled switching between proliferation and differentiation. Adsorption of Fn onto bacterial polystyrene (B), tissue culture polystyrene (T), and collagen (C) resulted in differences in Fn conformation as indicated by antibody binding. Using a biochemical method to quantify bound integrins in cultured cells, we found that differences in Fn conformation altered the quantity of bound alpha5 and beta1 integrin subunits but not alphav or beta3. C2C12 myoblasts grown on these Fn-coated substrates proliferated to different levels (B > T > C). Immunostaining for muscle-specific myosin revealed minimal differentiation on B, significant levels on T, and extensive differentiation on C. Differentiation required binding to the RGD cell binding site in Fn and was blocked by antibodies specific for this site. Switching between proliferation and differentiation was controlled by the levels of alpha5beta1 integrin bound to Fn, and differentiation was inhibited by anti-alpha5, but not anti-alphav, antibodies, suggesting distinct integrin-mediated signaling pathways. Control of cell proliferation and differentiation through conformational changes in extracellular matrix proteins represents a versatile mechanism to elicit specific cellular responses for biological and biotechnological applications.

Localization and expression of integrin subunits in the embryoid bodies of F9 teratocarcinoma cells

F9 embryonal carcinoma cells can differentiate in vitro into either parietal (PE) or visceral (VE) endoderm, depending upon specific retinoic acid (RA) treatment and growth conditions. In differentiated aggregates of F9 cells (EB), the VE is a polarized monolayer surrounding a core of undifferentiated cells. Within 7 days of treatment the cells organize their cytoskeleton and synthesize large amounts of extracellular matrix proteins to form a basal lamina under the newly formed epithelium. All these changes are likely to involve integrin expression and organization. In this study we have analyzed the spatio-temporal changes in the pattern and level of expression of beta1, beta4, alpha5, alpha6A, and alpha6B integrin subunits. We found that the organization of the VE monolayer in F9 aggregates involves both qualitative and quantitative changes in integrin expression. beta1 is downregulated and accumulates in the forming epithelium. The same occurs for alpha5, although its location on the surface of the aggregate appears to be transient as in fully differentiated EB its distribution is uniform. beta4 and alpha6A are also mainly localized in the VE but they are undetectable in undifferentiated aggregates and their expression is induced by RA treatment. An important exception is represented by alpha6B whose distribution and expression remain almost unchanged throughout treatment.

The muscle-specific laminin receptor alpha7 beta1 integrin negatively regulates alpha5 beta1 fibronectin receptor function

alpha7 beta1 is the major integrin complex expressed in differentiated muscle cells where it functions as a laminin receptor. In this work we have expressed the alpha7 integrin subunit in CHO cells to investigate the functional properties of this receptor. After transfection with alpha7 CHO cells acquired the ability to adhere and spread on laminin 1 consistent with the laminin receptor activity of the alpha7 beta1. alpha7 transfectants, however, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was inversely related to the level of alpha7 expression. To define the mechanisms underlying this adhesive defect we analyzed surface expression and functional properties of the alpha5 beta1 fibronectin receptor. Although cell surface expression of alpha5 beta1 was reduced by a factor of 20-25% in alpha7 transfectants compared to control untransfected cells, this slight reduction was not sufficient to explain the dramatic reduction in cell adhesion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of 125I-fibronectin for its surface receptor was decreased by 50% in alpha7 transfectants, indicating that the alpha5 beta1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn2+, a cation known to increase integrin affinity for their ligands. In fact, incubation of cells with Mn2+ restored fibronectin binding affinity, adhesion to fibronectin, and assembly of fibronectin matrix in alpha7 transfectants. These data indicate that alpha7 expression leads to the functional down regulation of alpha5beta1 integrin by decreasing ligand binding affinity and surface expression. In conclusion, the data reported establish the existence of a negative cooperativity between alpha7 and alpha5 integrins that may be important in determining functional regulation of integrins during myogenic differentiation.

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.

In vitro modulation of keratinocyte wound healing integrins by zinc, copper and manganese

Although the trace elements zinc, copper and manganese are used in vivo for their healing properties, their mechanism of action is still only partially known. Some integrins expressed by basal layer keratinocytes play an essential part in healing, notably alpha2beta1, alpha3beta1, alpha6beta4 and alphaVbeta5, whose expression and distribution in epidermis are modified during the re-epithelialization phase. This study demonstrates how the expression of these integrins are modulated in vitro by trace elements. Integrin expression was studied in proliferating keratinocytes in monolayer cultures and in reconstituted skin that included a differentiation state. After 48 h incubation with zinc gluconate (0.9, 1.8 and 3.6 microg/mL), copper gluconate (1, 2 and 4 microg/mL), manganese gluconate (0.5, 1 and 2 microg/mL) and control medium, integrin expression was evaluated by FACScan and immunohistochemistry. Induction of alpha2, alpha3, alphaV and alpha6 was produced by zinc gluconate 1.8 microg/mL in monolayers, of alpha2, alpha6 and beta1 by copper gluconate 2 and 4 microg/mL and of all the integrins studied except alpha3 by manganese gluconate 1 microg/mL. Thus, alpha6 expression was induced by all three trace elements. The inductive effect of zinc was particularly notable on integrins affecting cellular mobility in the proliferation phase of wound healing (alpha3, alpha6, alphaV) and that of copper on integrins expressed by suprabasally differentiated keratinocytes during the final healing phase (alpha2, beta1 and alpha6), while manganese had a mixed effect.

Two-stage activation for alpha5beta1 integrin binding to surface-adsorbed fibronectin

By analyzing the functional binding of alpha5beta1 integrin to adsorbed fibronectin in intact cells, we demonstrate that integrin activation results in linear increases in adhesion strength as a function of ligand density, suggesting that modulation of the receptor-ligand interaction is the dominant mechanism for adhesion during the initial stages of adhesion and that cooperative binding contributes little to initial adhesion strength. Using this experimental framework, we show the existence of three distinct activation states for alpha5beta1 integrin binding to adsorbed fibronectin for both passive, antibody-induced and active, cell-controlled activation. During the initial phase of adhesion, alpha5beta1 integrin is activated in an energy-dependent process from the nonbinding ground state to an intermediate state in which the receptor binds fibronectin and provides significant mechanical coupling. In later stages of adhesion maturation, alpha5beta1 integrin is activated to a higher binding state, which provides significant increases in adhesion strength compared with the intermediate state. These multiple binding states most likely result from different integrin conformations and reflect distinct interactions between alpha5beta1 and sites on adsorbed fibronectin. Multiple activation states for alpha5beta1 suggest the existence of distinct stages in adhesion signaling and strengthening and can provide a versatile mechanism for the regulation of adhesive interactions.

Induction of bone morphogenetic protein-6 in skin wounds. Delayed reepitheliazation and scar formation in BMP-6 overexpressing transgenic mice

Growth factors of the transforming growth factor-beta superfamily are involved in cutaneous wound healing. In this study we analyze the expression of the bone morphogenetic protein-6 (BMP-6) gene, a transforming growth factor-beta related gene, in skin wounds. In normal mouse skin high levels of BMP-6 mRNA and protein are expressed by postmitotic keratinocytes of stratified epidermis until day 6 after birth. BMP-6 expression is strongly reduced in adult epidermis with diminished mitotic activity. After skin injury we found large induction of BMP-6-specific RNA and protein in keratinocytes at the wound edge and keratinocytes of the newly formed epithelium as well as in fibroblast shaped cells in the wound bed. BMP-6-specific RNA was induced within 24 h after injury, whereas significant upregulation of BMP-6 on the protein level was detected only 2-3 d after injury. Protein was confined to outermost suprabasal epidermal layers, whereas BMP-6-specific RNA was distributed throughout all epidermal layers including basal keratinocytes and the leading edge of the migrating keratinocytes. We also detected high levels of BMP-6-specific RNA and protein in chronic human wounds of different etiology. In contrast to the overall distribution pattern of BMP-6-specific RNA, the protein was not detected in keratinocytes directly bordering the wound. In order to test the influence of BMP-6 abundance on the progress of wound healing, we analyzed the wound response of transgenic mice overexpressing BMP-6 in the epidermis. In these mice, reepitheliazation of skin wounds was significantly delayed, suggesting that strict spatial and temporal regulation of BMP-6 expression is necessary not only for formation but also for reestablishment of a fully differentiated epidermis.

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.

In vitro expression of soluble and cell surface-associated CD44 on endometrial cells from women with and without endometriosis

BACKGROUND

Endometriosis is one of the most common benign gynaecological diseases, and attachment of retrogradely shed viable endometrial cells is considered to be important in its development. CD44 is a multifunctional adhesion molecule that undergoes alternative splicing, giving rise to different isoforms.

METHODS

The expression of cell surface-associated CD44 std, v4, v5, v6 and v10 variants before and after cytokine treatment was investigated in endometrial cultures derived from 10 endometriosis patients and 22 women without the disease using immunocytochemistry. The immunoreactivity of soluble CD44 std, v5 and v6 variants was measured in culture medium using an enzyme immunoassay kit.

RESULTS

We report on the presence of soluble CD44 in endometrial culture supernatants. In particular, circulating CD44 standard form levels were significantly higher than levels of splice variants. We also found that both epithelial and stromal cells express surface-associated CD44 molecules in a distinct pattern and that this expression is not modulated by tumour necrosis factor (TNF)-alpha or/and interleukin 1 (IL-1) alpha/beta. Finally, cell surface-associated as well as soluble CD44 expression was similar in the two groups.

CONCLUSION

Our results indicate that endometrial cells can serve as a source of circulating CD44, but a direct role in the pathogenesis of endometriosis is rather improbable.

Cultivation of human keratinocyte stem cells: current and future clinical applications

Cultured human keratinocytes have a wide spectrum of clinical applications. Clinical results reported by several investigators are, however, contradictory. In this review, the authors discuss the biological and surgical issues which play a key role in the clinical outcome of cultured epidermal autografts used for the treatment of massive full-thickness burns. The importance of cultivation of epidermal stem cells and of their transplantation onto a wound bed prepared with donor dermis is emphasised. The paper also reviews recent data showing that: (i) cultured epidermal autografts bearing melanocytes can be used for the treatment of stable vitiligo; (ii) keratinocytes isolated from other lining epithelia, such as oral, urethral and corneal epithelia, can be cultivated and grafted onto patients suffering from disabling epithelial defects; (iii) keratinocyte stem cells can be stably transduced with retroviral vectors and are therefore attractive targets for the gene therapy of genodermatoses.

Integrins of the beta1 family influence keratinocyte-lymphocyte interaction

Data from the literature indicate that ICAM-1 molecules play an important role in keratinocyte interactions with lymphocytes via the lymphocyte function-associated-1 lymphocyte-adhesion molecule. We examined the role of beta1 integrins in keratinocyte-lymphocyte adhesion under different activation conditions. Among the beta1 integrins expressed on keratinocytes and lymphocytes detected by indirect immunofluorescence microscopy and flow cytofluorometry, primarily the alpha2 and the alpha3 subunits on both cell types were involved in keratinocyte-lymphocyte adhesion. Moreover, the highest adhesion level was observed when both cell types were activated by IFN-gamma for keratinocytes and phorbol 12-myristate 13-acetate for lymphocytes, suggesting that the former involved the protein kinase C pathway. Keratinocyte activation, characterized by the expression of ICAM-1, a decrease of beta1 integrins, and the absence of alpha5beta1 integrin, was required for optimal lymphocyte adhesion. Thus, beta1 integrins remaining at the surface of IFN-gamma-treated keratinocytes could be activated by this cytokine, and could synergize with ICAM-1 and lymphocyte function-associated-1 molecules to consolidate keratinocyte-lymphocyte adhesion.