Integrin receptors and RGD sequences in human keratinocyte migration: unique anti-migratory function of alpha 3 beta 1 epiligrin receptor

Specific association of CD63 with the VLA-3 and VLA-6 integrins

We screened monoclonal antibodies to cell-surface proteins and selected an antibody, called 6H1, that recognizes a putative integrin-associated protein. The 6H1 monoclonal antibody (mAb) indirectly coprecipitated alpha 3 beta 1 and/or alpha 6 beta 1, but not alpha 2 beta 1, or alpha 5 beta 1 from Brij 96 detergent lysates of multiple cell lines. Large scale purification using the 6H1 mAb yielded a single protein of 45-60 kDa with an amino-terminal sequence that exactly matched CD63. Confirming that the 6H1 mAb recognized the CD63 protein, 6H1 and a known anti-CD63 mAb yielded identical coprecipitation results and identical colocalization into lysosomal granules containing cathepsin D. Furthermore, we used an established anti-CD63 mAb to detect this protein in an alpha 3 beta 1 immunoprecipitate, and also we observed VLA-3 and CD63 colocalization in cellular "footprints." Notably, the cytoplasmic domain of alpha 3 was neither required nor sufficient for CD63 association, suggesting that it occurred elsewhere within the alpha 3 beta 1 complex. Knowledge of these specific CD63-alpha 3 beta 1 and CD63-alpha 6 beta 1 biochemical associations should lead to critical insights into the specialized functions of alpha 3 beta 1, alpha 6 beta 1, and CD63.

Comparative study of hepatocyte growth factor/scatter factor and keratinocyte growth factor effects on human keratinocytes

Hepatocyte growth factor/scatter factor (HGF/SF) and keratinocyte growth factor (KGF, also designated FGF-7) are paracrine growth factors secreted by mesenchymal cells and active on a variety of epithelial cell types. In this study, the biologic responses of keratinocytes to these paracrine growth factors were compared. Stimulation of mitogenesis, migration, plasminogen activator (PA) activity, and fibronectin production were examined using human foreskin keratinocytes cultured in serum-free MCDB 153 medium. Although the two factors stimulated a similar level of proliferation when cells were maintained for 5 d in 1.8 mM Ca++, the peak effect of KGF, observed at 10 ng/ml, was approximately threefold higher than that of HGF/SF when cells were in medium containing 0.15 mM Ca++. Both agents promoted the migration of cells in low-calcium medium (0.08 mM Ca++). However, the magnitude of the response was approximately twofold greater for HGF/SF at 10 ng/ml than KGF at the same concentration. None of the matrix proteins such as type I collagen, type IV collagen, laminin, or fibronectin either stimulated or suppressed HGF/SF- or KGF-stimulated keratinocyte migration. Both factors stimulated PA activity of the cell extracts, especially urokinase-type, with similar potencies. Promoted PA activity was maximal with the addition of 10 ng/ml of either factor. Neither factor increased the production of fibronectin under conditions in which transforming growth factor-beta 1 was active. These results indicate that HGF/SF and KGF, both recognized as paracrine growth factors, elicit distinctive patterns of response by keratinocytes, implying that they have different roles in epidermal physiology.

Divalent cations (Mg2+, Ca2+) differentially influence the beta 1 integrin-mediated migration of human fibroblasts and keratinocytes to different extracellular matrix proteins

Directed migration of keratinocytes and fibroblasts is a fundamental prerequisite in wound healing. Cation-dependent affinity changes of integrins are responsible for cell adhesion to and deadhesion from extracellular matrix proteins and have been implicated in driving cell migration. The specific requirements for divalent cations in the integrin-dependent migration of human dermal fibroblasts and human epidermal keratinocytes to various extracellular matrix proteins have been studied in vitro using blindwell Boyden chambers. The migration of the tested cells to collagen type I was mediated by the alpha 2 beta 1 integrins, to fibronectin by the combined action of the alpha 3 beta 1 and the alpha 5 beta 1 integrin, and the migration of fibroblasts to laminin dependent both on the alpha 2 beta 1 and the alpha 6 beta 1 integrins. No migration of keratinocytes to laminin was detected. Mg2+ alone induced cell migration with an optimum at 2 mM for fibroblasts and at 10 mM for keratinocytes. Ca2+ alone at 2 mM only marginally enhanced fibroblast and keratinocyte migration. At higher concentrations Ca2+ suppressed the stimulatory Mg2+ effect. 2 mM Ca2+ combined with 2 mM Mg2+ showed an additive stimulatory effect on the migration of fibroblasts to fibronectin. These data suggest that extracellular divalent cations differentially influence the integrin-mediated cell migration. A concentration gradient of Mg2+/Ca2+, as reported in tissue injury, thus may play a regulatory role in cell migration required for tissue remodelling.

A novel role for alpha 3 beta 1 integrins in extracellular matrix assembly

To study the biological role of alpha 3 beta 1 integrins in cell adhesion, migration, and in the deposition of extracellular matrix, we stably expressed the human alpha 3 integrin subunit in the alpha 4, alpha 5 integrin deficient CHO cell line B2. The expression of alpha 3 beta 1 integrins enhanced cell adhesion on entactin (also known as nidogen), but not on fibronectin. Using recombinant GST-fusion proteins that span the entire length of the entactin molecule, we located cell adhesive activity to the G2 domain of entactin. These results suggest that the alpha 3 beta 1 integrin functions as an adhesion receptor interacting with the G2 domain of entactin. On the other hand, the expression of alpha 3 beta 1 integrins did not confer the ability to migrate on entactin. Strikingly, the expression of alpha 3 beta 1 dramatically increased the deposition of entactin and fibronectin into the pericellular matrix. This was accompanied by increased binding activity of the 29 kDa amino-terminal domain of fibronectin. Thus, similar to alpha 5 beta 1 integrins, alpha 3 beta 1 integrins can play an important role in modulating the assembly of pericellular matrices. However, unlike fibronectin deposition supported by alpha 5 beta 1, alpha 3 beta 1 supported fibronectin deposition into pericellular matrix was not inhibited by antibodies binding to the RGD containing cell adhesion domain of fibronectin, demonstrating that the two processes are mechanistically distinct. The role of alpha 3 beta 1 in pericellular matrix assembly potentially implicates this receptor in the assembly and/or recognition of entactin-containing pericellular matrices, an observation consistent with its apparent role in the renal glomerulus of the mammalian kidney.

Interleukin-1 alpha stimulates keratinocyte migration through an epidermal growth factor/transforming growth factor-alpha-independent pathway

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) stimulate keratinocyte migration on collagen by up-regulating the alpha 2 subunit of the collagen integrin, alpha 2 beta 1. Interleukin-1 (IL-1) is an autocrine factor, produced by keratinocytes themselves, that is modulated by ultraviolet light and increases the proliferative potential of keratinocytes in culture. The autocrine nature of keratinocyte-derived IL-1 alpha is emphasized by the fact that it induces the keratinocyte to synthesize IL-1 alpha and TGF-alpha, a cytokine known to induce keratinocyte motility. Further, topical application of IL-1 alpha has been shown to promote wound healing in animals. In this study, we used a well-defined keratinocyte migration assay to assess the effect of IL-1 alpha on keratinocyte motility and to examine whether the IL-1 alpha/TGF alpha pathway is involved. The addition of recombinant human IL-1 alpha to keratinocytes produced a statistically significant and concentration-dependent increase in migration on matrices of collagen types I and IV, but not on laminin. Maximal levels of keratinocyte migration obtained on these matrices with IL-1 alpha were comparable to those obtained with stimulation by EGF and TGF-alpha. The effects of TGF-alpha and IL-1 alpha on keratinocyte migration are additive; however, the maximal level of migration achieved by using IL-1 alpha and TGF-alpha in combination never exceeds the maximal level of migration found by using either cytokine alone. The time course of keratinocyte migration induced by IL-1 alpha is delayed (onset of migration 9-12 h after addition) as compared with that induced by TGF-alpha (onset of migration 6-9 h after addition) even if the cells are preincubated in IL-1 alpha. Flow cytometry analysis demonstrated no change in surface expression of integrin subunits, specifically that of integrin subunit alpha 2, previously shown to be up-regulated by EGF/TGF-alpha. These results suggest that IL-1 alpha stimulates keratinocyte migration on collagen via a mechanism distinct from that of EGF/TGF-alpha.

Expression of beta 1 integrin receptors in transformed mouse epidermal keratinocytes: upregulation of alpha 5 beta 1 in spindle carcinoma cells

The adhesive properties and the expression of extracellular matrix receptors of the beta 1-integrin subfamily were analyzed in transformed epidermal keratinocyte cell lines of different stages of mouse skin carcinogenesis. One- and two-dimensional analyses of the immunoprecipitates obtained with anti-beta 1- and specific anti-alpha-integrin subunits showed qualitative and quantitative changes in the expression of beta 1 integrins by the different cell lines. The polyvalent alpha 3 beta 1 integrin was expressed by all analyzed cell lines, although the levels detected in undifferentiated spindle CarC cells were lower than those present in the rest of keratinocyte cell lines. In contrast, spindle cells expressed high levels of the specific fibronectin receptor alpha 5 beta 1, whereas this integrin was absent or expressed at very reduced levels in the other epithelial cell lines. Expression of alpha 5 beta 1 integrin in spindle cells appeared organized in cell-substratum contact areas on spread cells. In addition, high and homogenous expression of alpha 5 beta 1 was detected in fully undifferentiated tumors induced in nude mice by three independent spindle cell lines. These results suggest that the expression of alpha 5 beta 1 integrin is upregulated during the development of spindle cell carcinomas that occur in the last stages of mouse skin carcinogenesis and can be associated with the acquisition of the fibroblastoid phenotype of spindle cells. On the other hand, expression of the collagen receptor alpha 2 beta 1 was demonstrated in a transformed cell line (PDV), and it was apparently also expressed in two other malignant keratinocyte cell lines (PDVC57 and HaCa4). The expression of alpha 2 beta 1 was correlated with the increased adhesion to collagen type I and collagen type IV exhibited by the tumorigenic cell lines.

Polarized expression and basic fibroblast growth factor-induced down-regulation of the alpha 6 beta 4 integrin complex on human microvascular endothelial cells

Endothelial cells rest on a basement membrane that anchors them to the vessel wall. The alpha 6 beta 4 integrin complex has been described on epithelial cells, frequently localizes to basement-membrane structures, and appears to play a role in binding epithelial cells to laminin. We have determined that human microvascular endothelial cells express the beta 4 integrin chain in vivo and that it preferentially localizes to the endothelial basement membrane. Human microvascular endothelial cells and human umbilical vein endothelial cells also express cell-surface beta 4 in vitro. In addition, the expression of beta 4 appears to be polarized to the undersurface of endothelial cell monolayers in vitro, mimicking its in vivo localization. Stimulation of microvascular endothelial cells with basic fibroblast growth factor or phorbol 12-myristate 13-acetate, agents previously shown to induce endothelial cell migration in vitro, resulted in a marked decrease in cell-surface expression of the beta 4 integrin chain, associated with a decrease in beta 4 mRNA. These data demonstrate that human endothelial cells express the beta 4 integrin chain in vivo and in vitro, the expression of this integrin chain is polarized, and its expression is regulated on microvascular endothelial cells by factors important in wound healing and vascular regeneration.

Integrin and phosphotyrosine expression in normal and migrating newt keratinocytes

BACKGROUND

Cells interact with the extracellular matrix through a family of cell surface receptors known as integrins. Ligand specificity of a given integrin is determined in part by the type of alpha and the type of beta subunit comprising it. Accumulating evidence suggests that integrin-ligand binding in some systems influences cell behavior through tyrosine phosphorylation of intracellular proteins.

METHODS

In this study, we utilized immunohistochemistry to examine the expression of beta 1 and beta 4 integrin subunits as well as tyrosine phosphorylation in normal keratinocytes and in keratinocytes migrating to form a wound epithelium. An adhesion assay was used to determine if freshly isolated keratinocytes could interact with fibronectin and collagen. Polyacrylamide gel electrophoresis followed by immunoblotting was employed to compare beta 1 integrins in migrating and nonmigrating keratinocytes.

RESULTS

In normal epidermis, beta 1 and beta 4 localized primarily to basal cells, where both subunits were generally distributed over all parts of the cell periphery. Except for a modest presence in suprabasal cells and a minimal presence adjacent to the epidermal basement membrane, phosphotyrosine (ptyr) had a similar distribution. In migrating keratinocytes, beta 1, beta 4, and ptyr localized most heavily at the interface between the forming wound epithelium and the wound bed. Adhesion assays using keratinocytes from normal epidermis revealed a population of cells that could specifically adhere and spread on fibronectin and type I collagen. Immunoblots of beta 1 subunits from normal and migrating keratinocytes showed no increase in amount of beta 1, nor did the apparent size of beta 1 change in migrating compared to normal cells.

CONCLUSIONS

The heavy accumulation of beta 1 and beta 4 at the wound bed interface in migrating cells suggests that these subunits may be involved in attachments of migrating cells to extracellular matrix proteins in the wound. The accumulation of ptyr in the same region further suggests that integrin-ligand interaction in keratinocytes modulates cell behavior through phosphorylated proteins. The fact that freshly isolated newt keratinocytes could adhere and spread on fibronectin or collagen shows that these cells are constitutively activated. This view is supported by the absence of any evidence that the beta 1 in migrating keratinocytes is larger and therefore more mature than beta 1 in normal keratinocytes. By comparison, beta 1 integrins on human keratinocytes are not constitutively activated (Takashima and Grinnell, 1985; Toda et al., 1987; Guo et al., 1990, 1991), a difference that may explain why epidermal wound healing is faster in newts than in humans.

TGF-beta 1 stimulates expression of keratinocyte integrins during re-epithelialization of cutaneous wounds

Epidermal keratinocytes migrate over a provisional matrix during the re-epithelialization of cutaneous wounds. We have investigated the expression of integrins and of transforming growth factor-beta 1 (TGF-beta 1) during re-epithelialization in a porcine model. Tissue specimens were collected at different times after injury and stained with antibodies against subunits of the fibronectin receptor, integrin alpha 5 beta 1, and the vitronectin receptor, integrin alpha v beta 5. Intense staining was observed in the migrating keratinocytes of 5-d wounds; basal and suprabasal cells were stained around the entire cell periphery. Staining returned toward normal levels in 14-d wounds. The appearance of the extracellular form of TGF-beta 1 seemed to be coordinated with the increased expression of integrin subunits: it was detected in migrating keratinocytes and in the adjacent epidermis of early wounds at 5 and 7 d. We also investigated the effect of TGF-beta 1 on cultured epidermal cells. Treating human keratinocytes with TGF-beta 1 increased the levels of mRNA for the integrin subunits alpha 5, alpha v, and beta 5, but had little effect on beta 1. The corresponding cell-surface expression of alpha 5 and alpha v was also increased after treatment. Thus, during wound repair, TGF-beta 1 may induce epidermal keratinocytes to express integrins that facilitate the migratory component of re-epithelialization.

Dibutyryl cyclic AMP modulates keratinocyte migration without alteration of integrin expression

Cyclic adenosine monophosphate (cAMP) has long been regarded as a second messenger and a regulator of human keratinocyte proliferation. It has been demonstrated that cAMP inhibits keratinocyte proliferation when used at high concentrations. Nevertheless, new recent reports have demonstrated that cAMP may stimulate or inhibit keratinocyte growth depending upon the concentration used. Studies to examine the influence of cAMP upon the migration of other cell types have been contradictory. To determine the direct effect of dibutyryl cAMP (DBcAMP) upon human keratinocyte migration, we used a quantitative locomotion assay using a wide range of DBcAMP concentrations. We found a bi-phasic effect of DBcAMP on keratinocyte migration across connective tissue matrices. Keratinocyte locomotion on the matrices was promoted at 10(-5) M and 10(-6) M of DBcAMP, but not at higher or lower concentrations. Time-course experiments demonstrated that the effect of DBcAMP on keratinocyte locomotion and proliferation occurred independently. Fluorescence-activated cell sorter analysis demonstrated that the effect of DBcAMP on the migration of human keratinocytes was independent from the modulation of integrin receptors. Although the cellular mechanisms by which DBcAMP promotes keratinocyte migration is unclear, the addition of DBcAMP or TPA to keratinocyte cultures enhanced the synthesis of a 92-kDa metalloproteinase in association with enhanced cellular migration. These observations suggest a possible link between metalloproteinase expression and cellular migration.

Characterization of the antigens recognized by two monoclonal antibodies reactive with basal-layer keratinocytes of human epidermis

Two monoclonal antibodies, GR3 and GR4, reactive with the basal-layer keratinocytes of human epidermis, were derived by immunization of Balb/c mice with glycoproteins isolated from cultured keratinocytes by lectin-affinity chromatography. Immunoprecipitation of Triton X-100 extracts from human keratinocytes metabolically labelled with D-[1-14C]glucosamine revealed that GR3 recognized a major glycoprotein with migration properties identical with those of a glycoprotein (reduced form M(r) 126,000) which was previously shown to be implicated in intercellular adhesion [Roberts and Brunt (1985) Biochem J. 232, 67-70]. In their unreduced forms the antigens recognized by GR3 and GR4 both migrated as two bands with M(r) values of 118,000 and 147,000. Comparison of 125I-labelled glycoproteins immunoprecipitated by GR3, GR4 and integrin antibodies revealed that, under reducing conditions, the major band immunoprecipitated by both GR3 and GR4 co-migrated with the alpha 3 and beta 1 integrin chains. In addition the immunoprecipitate obtained with GR4 contained an additional band co-migrating with the alpha 2 integrin chain. Sequential immunoprecipitation studies with GR3, GR4 and integrin antibodies confirmed that GR3 is directed against the alpha 3 integrin chain, whereas GR4 is directed against the beta 1 chain. These studies also indicate that some of the alpha 2 integrin chains on keratinocytes may be associated with a beta-chain not recognized by the antisera against the beta 1 integrin chain used in this study.

Kalinin is more efficient than laminin in promoting adhesion of primary keratinocytes and some other epithelial cells and has a different requirement for integrin receptors

Kalinin was purified from squamous cell carcinoma (SCC25) spent culture media using an immunoaffinity column prepared from the mAb BM165. The affinity-purified material was separated by SDS-PAGE into three bands of 165-155, 140, and 105 kD identical to those obtained from normal human keratinocyte cultures and previously identified as kalinin. Kalinin promoted adhesion of a large number of normal cells and established cell lines with an activity similar to other adhesion molecules such as the laminin-nidogen complex, fibronectin, or collagen IV. However, kalinin was a much better substrate than laminin-nidogen complex for adhesion of cells of epithelial origin including primary human keratinocytes. Adhesion to kalinin was followed by cell shape changes ranging from rounded to fully spread cells depending on the cell types. The adhesion-promoting activity of kalinin was conformation dependent and was abolished by heat denaturation. mAb BM165 prevented cell adhesion to kalinin but not to other extracellular matrix substrates. However, either complete or partial inhibition was observed with different cells suggesting the existence of at least two cell-binding sites on the kalinin molecule. Experiments inhibiting cell adhesion with function-blocking anti-integrin subunit antibodies indicated that both alpha 3 beta 1 and alpha 6 beta 1 integrins are involved in the cellular interactions with kalinin, while for cell adhesion to classical mouse Engelbreth-Holm-Swarm laminin only alpha 6 beta 1 integrins, and not alpha 3 beta 1, appeared to be functional. Altogether, these results suggest that kalinin may fulfill additional functions than laminin, particularly for epithelial cells.

Basement membrane invasion of glioma cells mediated by integrin receptors

Basement membrane invasion precedes meningeal dissemination and systemic metastasis of glioma cells. In order to investigate the invasive ability of glioblastomas and the functional role of extracellular matrix receptors, the authors performed in vitro invasion assays where the number of cells was determined from freshly resected tumors (primary cultures and fifth passages) and from cell lines (U-138 MG, U-373 MG, and GaMg) that had migrated through a filter coated with a reconstituted basement membrane (Matrigel). The involvement of integrin adhesion molecules was examined by preincubation of glioma cells with blocking antibodies to specific integrin chains. Cells from all of the glioblastomas had migrated through the Matrigel after 4 to 24 hours; the number of invasive cells was highest in the cell lines. Invasion of U-138 MG cells was reduced with antibodies to alpha 7, alpha v, beta 1, and beta 3 integrin chains and markedly increased by anti-alpha 5, while invasion of U-373 MG cells was reduced by antibodies to alpha 3, alpha v, beta 1, and beta 3 and increased by anti-alpha 6. It is concluded that: 1) glioma cells are able to penetrate Matrigel, indicating that the basement membrane is not a resistant barrier for infiltrating cells; and 2) basement membrane invasion is mediated by integrins in a complex manner. Some integrins promote while others inhibit basement membrane invasion. Furthermore, the integrins involved may differ between various glioma cells.

In vivo targeting of integrin receptors in human skin xenografts by intravenously applied antibodies

We examined whether systemically injected monoclonal antibodies (mAbs) directed to cell-surface glycoproteins of human keratinocytes reach their target antigens in xenograft transplants of normal human skin on SCID mice. The integrins alpha 6 beta 4, expressed in the basal cell layer of human epidermis, and glycoprotein T16 (gp 40/50), expressed in terminally differentiating keratinocytes of the stratum spinosum, were selected as targets. It was found that all injected mAbs selectively localized to their antigens and bound and saturated their targets even in the uppermost layers of the stratum malpighii. This could easily be monitored by direct immunofluorescence staining since SCID mice lack endogenous production of significant amounts of immunoglobulins. After a single injection, mAbs could still be detected at the target site after 14 days. Our results proved that heterologous immunoglobulins pass systemic capillary filters in this xenograft model and specifically bind to their target molecules. Thus, xenografted SCID mice provide a versatile model for studying cell-surface glycoprotein-mediated interactions by the use of functionally interfering antibodies under in vivo conditions in human skin.

Cell-matrix interactions modulate interstitial collagenase expression by human keratinocytes actively involved in wound healing

We reported that interstitial collagenase is produced by keratinocytes at the edge of ulcers in pyogenic granuloma, and in this report, we assessed if production of this metalloproteinase is a common feature of the epidermal response in a variety of wounds. In all samples of chronic ulcers, regardless of etiology, and in incision wounds, collagenase mRNA, localized by in situ hybridization, was prominently expressed by basal keratinocytes bordering the sites of active re-epithelialization indicating that collagenolytic activity is a characteristic response of the epidermis to wounding. No expression of mRNAs for 72- and 92-kD gelatinases or matrilysin was seen in keratinocytes, and no signal for any metalloproteinase was detected in normal epidermis. Immunostaining for type IV collagen showed that collagenase-positive keratinocytes were not in contact with an intact basement membrane and, unlike normal keratinocytes, expressed alpha 5 beta 1 receptors. These observations suggest that cell-matrix interactions influence collagenase expression by epidermal cells. Indeed, as determined by ELISA, primary cultures of human keratinocytes grown on basement membrane proteins (Matrigel; Collaborative Research Inc., Bedford, MA) did not express significant levels of collagenase, whereas cells grown on type I collagen produced markedly increased levels. These results suggest that migrating keratinocytes actively involved in re-epithelialization acquire a collagenolytic phenotype upon contact with the dermal matrix.

Melanocyte movement in vitro: role of matrix proteins and integrin receptors

During the repigmentation of vitiliginous skin, melanocytes migrate from the outer root sheath of the hair follicle into the depigmented skin. We hypothesize that this requires changes in the local microenvironment that are conductive to melanocyte migration. One important change in the microenvironment could be the localized production of matrix proteins. We have previously employed time-lapse photography to evaluate the effect of inflammatory mediators and cytokines on melanocyte movement. We have adapted this system to study the effect of matrix proteins on melanocyte movement in vitro. Type IV collagen significantly increases melanocyte migration, whereas laminin and fibronectin have no effect. Cell/matrix interactions are in part controlled by cell-surface integrins. Integrins have been demonstrated to be important in controlling the migration of many cell types. We demonstrate that melanocytes express cell-membrane alpha 2, alpha 3, and alpha 5 integrins and that the enhanced melanocyte migration on type IV collagen is inhibited by specific function-blocking antibodies to integrins alpha 2 and alpha 3, but not to alpha 5 integrins.

Expression of integrins and basement membrane components by wound keratinocytes

Extracellular matrix proteins and their cellular receptors, integrins, play a fundamental role in keratinocyte adhesion and migration. During wound healing, keratinocytes detach, migrate until the two epithelial sheets confront, and then regenerate the basement membrane. We examined the expression of different integrins and their putative ligands in keratinocytes during human mucosal wound healing. Migrating keratinocytes continuously expressed kalinin but not the other typical components of the basement membrane zone: type IV collagen, laminin, and type VII collagen. When the epithelial sheets confronted each other, these missing basement membrane components started to appear gradually through the entire wound area. The expression of integrin beta 1 subunit was increased in keratinocytes during migration. The beta 1-associated alpha 2 and alpha 3 subunits were expressed constantly by wound keratinocytes whereas the alpha 5 subunit was present only in keratinocytes during reepithelialization. Furthermore, migrating cells started to express alpha v-integrins which were not present in the nonaffected epithelium. All keratinocytes also expressed the alpha 6 beta 4 integrin during migration. In the migrating cells, the distribution of integrins was altered. In normal mucosa, beta 1-integrins were located mainly on the lateral plasma membrane and alpha 6 beta 4 at the basal surface of basal keratinocytes in the nonaffected tissue. In wounds, integrins were found in filopodia of migrating keratinocytes, and also surrounding cells in several cell layers of the migrating sheet. The results indicate that migrating keratinocytes, in deep human wounds enlarge their integrin repertoire. The changes in integrin expression take place concomitantly with changes in the basement membrane composition, suggesting a close interplay of these two groups of molecules during wound healing.

Regulation of keratinocyte terminal differentiation by integrin-extracellular matrix interactions

Suspension-induced terminal differentiation of human epidermal keratinocytes can be inhibited by fibronectin through binding to the alpha 5 beta 1 integrin. We have investigated the effect of fibronectin on expression of integrins and proteins of the actin cytoskeleton and have explored the nature of the differentiation stimulus by testing different combinations of anti-integrin monoclonal antibodies or extracellular matrix proteins in the suspension assay. Fibronectin prolonged cell surface expression of beta 1 integrins but did not overcome the inhibition of intracellular transport of integrins that occurs when keratinocytes are placed in suspension. Fibronectin did not prevent the suspension-induced decline in the level of mRNAs encoding the beta 1 integrin subunit, actin, filamin and alpha-actinin; furthermore, the inhibition of terminal differentiation did not depend on the state of assembly of microfilaments or microtubules. Terminal differentiation could be partially inhibited by an adhesion-blocking monoclonal antibody to the beta 1 integrin subunit or by a combination of adhesion blocking antibodies recognising the alpha subunits that associate with beta 1 (alpha 2, alpha 3 and alpha 5). Although laminin and type IV collagen do not inhibit terminal differentiation individually, they were inhibitory when added to cells in combination with a low concentration of fibronectin. We conclude that the proportion of keratinocyte beta 1 integrins occupied by ligand can regulate the initiation of terminal differentiation independently of the state of assembly of the actin cytoskeleton.

Keratinocyte growth factor (FGF-7) stimulates migration and plasminogen activator activity of normal human keratinocytes

Keratinocyte growth factor (KGF), a member of the fibroblast growth factor (FGF) family (and alternatively designated FGF-7), is a paracrine growth factor produced by mesenchymal cells and mitogenic specifically for epithelial cells. The potential effect of KGF on wound healing was assessed in vitro by measuring randomized migration and plasminogen activator (PA) activity of keratinocytes in response to the growth factor. Incubation of normal human keratinocytes with KGF in modified MCDB 153 medium significantly stimulated cell migration and PA activity compared with control (p < 0.001 and p < 0.01, respectively). When tested in these assays on an equimolar basis, 1 nM KGF was at least as potent as transforming growth factor alpha and more active than basic FGF. None of these effects were observed when KGF was administered to fibroblasts or endothelial cells. Stimulation of keratinocyte migration by KGF was dose dependent, and a neutralizing monoclonal antibody against KGF reduced KGF-stimulated migration and cell growth. Zymographic analyses of cell extracts and conditioned medium from KGF-treated keratinocytes revealed increased PA activity, which was mainly attributable to an elevated level of urokinase-type PA. These in vitro results suggest that KGF may have an important role in stimulating reepithelialization during the process of wound repair.

Changes in the distribution of actin-associated proteins during epidermal wound healing

We have examined the distribution of actin filaments and a number of actin-associated proteins during human epidermal wound healing, using a suction blister model in which the epidermis is detached from the dermis, leaving the basement membrane intact. Filamentous actin was found in all the living epidermal layers before, during and after wound healing. alpha-actinin was also present in all the living layers of normal epidermis, but diffuse cytoplasmic staining was observed at the leading edge of migrating epidermis. Vinculin and talin were concentrated at the basement membrane prior to wounding, but were absent from the leading edge during wound healing. In normal epidermis, filamin and gelsolin showed a complementary distribution, with filamin most abundant in the basal layer and gelsolin most abundant suprabasally. The abundance of both proteins was reduced at the leading edge of migrating epidermis. All of the changes were transient, as the expression patterns returned to normal by 1 week after wounding, when the epidermis had reformed. The relevance of these changes to the process of keratinocyte migration is discussed.

From the Journals

Enhancing wound healing in animals with interferon

Kératinocyte allografts for chronic venous leg ulcers

Integrin receptors and RGD sequences in healing

Nutrition in patients with spinal cord injuries

Vitamin C deficiency and pressure sores

Epidermal growth factor and insulin-like growth factor I enhance keratinocyte migration

Although their mechanisms of action are unclear, a number of growth factors has been shown to promote cutaneous wound repair. Keratinocyte migration and proliferation are required for re-epithelialization, and there is evidence to suggest that these processes may be regulated by one or more growth factors that promote wound repair. Using the phagokinetic assay, which allows direct observation of migration path as a gold-particle-free area, we examined the effects of epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) on human keratinocyte migration. Addition of EGF to defined medium in the absence of any other growth factor induced an increase in migration of 2.5-4.5 fold after overnight incubation; the effect of EGF on migration was concentration dependent, with a maximum at 10 to 50 ng/ml EGF. Concentration-dependent enhancement of keratinocyte migration was similarly observed with IGF-I as well as insulin. With all factors, migration was observed on colloidal gold plates coated with collagen IV or with fibronectin but not in the absence of matrix coating. To examine further the involvement of the EGF receptor in keratinocyte migration, we tested the effect of a monoclonal antibody to the EGF receptor that acts as an antagonist. EGF-induced migration was completely prevented by this antibody; however, the enhancement by insulin or IGF-I was not blocked. These results suggest that IGF-I and insulin enhance keratinocyte migration by a mechanism distinct from that of EGF.

Altered glycosylation and cell surface expression of beta 1 integrin receptors during keratinocyte activation

We studied the mechanism by which cell adhesiveness becomes activated when keratinocytes are removed from skin and placed into cell culture. Our results suggest that activation involves altered beta 1 integrin subunit glycosylation accompanied by an increase in cell surface beta 1 integrin receptors. Activated keratinocytes contained two forms of the beta 1 integrin subunit, approximately 93 kDa and approximately 113 kDa. As shown by pulse-chase experiments, the smaller represented the cytoplasmic precursor of the larger, and only the 113 kDa mature form was detected in integrin receptors expressed at the cell surface. Pre-activated keratinocytes contained beta 1 integrin subunits ranging from approximately 97 to 110 kDa. These beta 1 subunits had been processed through the Golgi, based on resistance to endoglycosidase-H treatment, and were not converted to 113 kDa subunits during subsequent cell culture. Experiments with endoglycosidase-F showed that differences in the apparent sizes of beta 1 integrin subunits observed in pre-activated and activated keratinocytes could be attributed to differences in subunit glycosylation. Smaller beta 1 subunits found in pre-activated keratinocytes, like the precursor beta 1 subunits of activated cells, appeared to be less efficient in reaching the cell surface. Overall, a approximately 10-fold increase in the level of cell surface integrin receptors occurred concomitant with the increased proportion of 113 kDa beta 1 subunits found in activated cells. Endoglycosidase-F experiments also indicated that there were changes in keratinocyte alpha subunits associated with beta 1. In related experiments, keratinocytes cultured in low Ca2+, serum-free MCDB medium for 4 days proliferated but their adhesiveness did not become activated. Therefore, keratinocyte proliferation and activation of adhesion are regulated separately. Finally, substantial activation of keratinocytes was observed when serum was added to cells cultured in MCDB with serum, indicating a role for serum factors in the activation process.