Arg-Gly-Asp-containing peptides expose novel collagen receptors on fibroblasts: implications for wound healing

Porcine-derived collagen peptides promote re-epithelialisation through activation of integrin signalling

Chronic non-healing cutaneous wounds represent a major burden to patients and healthcare providers worldwide, emphasising the continued unmet need for credible and efficacious therapeutic approaches for wound healing. We have recently shown the potential for collagen peptides to promote proliferation and migration during cutaneous wound healing. In the present study, we demonstrate that the application of porcine-derived collagen peptides significantly increases keratinocyte and dermal fibroblast expression of integrin α2β1 and activation of an extracellular signal-related kinase (ERK)-focal adhesion kinase (FAK) signalling cascade during wound closure in vitro. SiRNA-mediated knockdown of integrin β1 impaired porcine-derived collagen peptide-induced wound closure and activation of ERK-FAK signalling in keratinocytes but did not impair ERK or FAK signalling in dermal fibroblasts, implying the activation of differing downstream signalling pathways. Studies in ex vivo human 3D skin equivalents subjected to punch biopsy-induced wounding confirmed the ability of porcine-derived collagen peptides to promote wound closure by enhancing re-epithelialisation. Collectively, these data highlight the translational and clinical potential for porcine-derived collagen peptides as a viable therapeutic approach to promote re-epithelialisation of superficial cutaneous wounds.

Elastin hydrolysate derived from fish enhances proliferation of human skin fibroblasts and elastin synthesis in human skin fibroblasts and improves the skin conditions

BACKGROUND

Recent studies have shown that certain peptides significantly improve skin conditions, such as skin elasticity and the moisture content of the skin of healthy woman. This study aimed to investigate the effects of elastin hydrolysate on human skin. Proliferation and elastin synthesis were evaluated in human skin fibroblasts exposed to elastin hydrolysate and proryl-glycine (Pro-Gly), which is present in human blood after elastin hydrolysate ingestion. We also performed an ingestion test with elastin hydrolysate in humans and evaluated skin condition.

RESULTS

Elastin hydrolysate and Pro-Gly enhanced the proliferation of fibroblasts and elastin synthesis. Maximal proliferation response was observed at 25 ng mL(-1) Pro-Gly. Ingestion of elastin hydrolysate improved skin condition, such as elasticity, number of wrinkles, and blood flow. Elasticity improved by 4% in the elastin hydrolysate group compared with 2% in the placebo group.

CONCLUSION

Therefore, elastin hydrolysate activates human skin fibroblasts and has beneficial effects on skin conditions.

An isolated cryptic peptide influences osteogenesis and bone remodeling in an adult mammalian model of digit amputation

Biologic scaffolds composed of extracellular matrix (ECM) have been used successfully in preclinical models and humans for constructive remodeling of functional, site-appropriate tissue after injury. The mechanisms underlying ECM-mediated constructive remodeling are not completely understood, but scaffold degradation and site-directed recruitment of progenitor cells are thought to play critical roles. Previous studies have identified a cryptic peptide derived from the C-terminal telopeptide of collagen IIIα that has chemotactic activity for progenitor cells. The present study characterized the osteogenic activity of the same peptide in vitro and in vivo in an adult murine model of digit amputation. The present study showed that the cryptic peptide increased calcium deposition, alkaline phosphatase activity, and osteogenic gene expression in human perivascular stem cells in vitro. Treatment with the cryptic peptide in a murine model of mid-second phalanx digit amputation led to the formation of a bone nodule at the site of amputation. In addition to potential therapeutic implications for the treatment of bone injuries and facilitation of reconstructive surgical procedures, cryptic peptides with the ability to alter stem cell recruitment and differentiation at a site of injury may serve as powerful new tools for influencing stem cell fate in the local injury microenvironment.

New assay to detect low-affinity interactions and characterization of leukocyte receptors for collagen including leukocyte-associated Ig-like receptor-1 (LAIR-1)

Leukocyte activity is controlled by numerous interactions between membrane receptors and ligands on the cell surface. These interactions are of low affinity making detection difficult. We developed a sensitive assay that could readily detect extremely weak interactions such as that between CD200 and the activating receptor CD200RLa (K(d)>500 microM) at the protein level. We used the new technology to screen for interactions of inhibitory receptors for collagens. We confirmed that both human and mouse leukocyte-associated Ig-like receptor-1, and in addition the related inhibitory leukocyte Ig-like receptor subfamily B member 4 (CD85K, Gp49B), bound collagen specifically, whereas other cell surface proteins gave no binding. The monomeric affinities of the interactions were then determined to allow comparison with other leukocyte interactions and indicate conditions when these interactions might lead to inhibitory signals.

Platelet-Derived Growth Factor BB–Mediated Normalization of Dermal Interstitial Fluid Pressure After Mast Cell Degranulation Depends on β3 but Not β1 Integrins

Interstitial fluid pressure (P IF ) is one of the determinants of transcapillary fluid flux and thereby interstitial fluid volume. Cell-mediated control of P IF regulates fluid content in the loose interstitial connective tissues that surround the capillary bed. To maintain a normal P IF in dermis, β1 integrins mediate the tensile strength applied by connective tissue cells on the extracellular matrix. Platelet-derived growth factor (PDGF)-BB normalizes anaphylaxis-induced reduction of P IF . Anti–β3 integrin IgG and a cyclic RGD peptide that inhibits the αVβ3 integrin blocked the ability of PDGF-BB to normalize the lowered P IF resulting from mast cell degranulation. PDGF-BB was unable to normalize P IF lowered as a result of mast cell degranulation in β3-negative mice. Monoclonal anti–β3 integrin IgG had no effect on P IF in normal mouse dermis. In contrast, administration of anti–β1 integrin IgM lowered P IF in normal dermis but had no effect on PDGF-BB–induced normalization of P IF after anaphylaxis. Furthermore, collagen gel contraction mediated by wild-type mouse embryonal fibroblasts were only marginally affected by function-blocking anti–β1 integrin antibodies, especially in the presence of PDGF-BB. In contrast, contraction mediated by αV-negative mouse embryonic fibroblasts was completely blocked by anti–β1 integrin antibodies, even after stimulation with PDGF-BB. These results show a previously unrecognized in vivo function for the αVβ3 integrin, as a participant in the control of P IF during inflammatory reactions. Furthermore, our data demonstrate that PDGF-BB induces connective tissue cells to generate tensile forces via αVβ3 during such reactions.

alpha 11beta 1 integrin recognizes the GFOGER sequence in interstitial collagens

The integrins alpha(1)beta(1), alpha(2)beta(1), alpha(10)beta(1), and alpha(11)beta(1) are referred to as a collagen receptor subgroup of the integrin family. Recently, both alpha(1)beta(1) and alpha(2)beta(1) integrins have been shown to recognize triple-helical GFOGER (where single letter amino acid nomenclature is used, O = hydroxyproline) or GFOGER-like motifs found in collagens, despite their distinct binding specificity for various collagen subtypes. In the present study we have investigated the mechanism whereby the latest member in the integrin family, alpha(11)beta(1), recognizes collagens using C2C12 cells transfected with alpha(11) cDNA and the bacterially expressed recombinant alpha(11) I domain. The ligand binding properties of alpha(11)beta(1) were compared with those of alpha(2)beta(1). Mg(2+)-dependent alpha(11)beta(1) binding to type I collagen required micromolar Ca(2+) but was inhibited by 1 mm Ca(2+), whereas alpha(2)beta(1)-mediated binding was refractory to millimolar concentrations of Ca(2+). The bacterially expressed recombinant alpha(11) I domain preference for fibrillar collagens over collagens IV and VI was the same as the alpha(2) I domain. Despite the difference in Ca(2+) sensitivity, alpha(11)beta(1)-expressing cells and the alpha(11) I domain bound to helical GFOGER sequences in a manner similar to alpha(2)beta(1)-expressing cells and the alpha(2) I domain. Modeling of the alpha I domain-collagen peptide complexes could partially explain the observed preference of different I domains for certain GFOGER sequence variations. In summary, our data indicate that the GFOGER sequence in fibrillar collagens is a common recognition motif used by alpha(1)beta(1), alpha(2)beta(1), and also alpha(11)beta(1) integrins. Although alpha(10) and alpha(11) chains show the highest sequence identity, alpha(2) and alpha(11) are more similar with regard to collagen specificity. Future studies will reveal whether alpha(2)beta(1) and alpha(11)beta(1) integrins also show overlapping biological functions.

Overexpression of integrin alphav promotes human osteosarcoma cell populated collagen lattice contraction and cell migration

Cells attach and interact with the extracellular matrix (ECM) through heterodimeric alphabeta integrin receptors. Specifically, the promiscuous alphavbeta3 integrin and the alpha2beta1 integrin receptors engage numerous matrix components to influence cell adhesion, cell motility, and matrix organization. However, the role of alphav integrin mediating cell-collagen interactions is not clear. In the in vitro cell populated collagen lattice (PCL), a model of cell-matrix interaction, integrin receptors play a role in lattice contraction. To elucidate alphav integrins' effects on cell-collagen interactions, human osteosarcoma (HOS) cells were transfected with alphav integrin (alphav-pcDNA 3.1+). Control HOS cells were transfected with pcDNA 3.1+ vector alone. HOS-alphav cell PCLs contracted to a greater degree than control HOS cell PCLs (P < or = 0.0001). RT-PCR revealed that HOS-alphav cells express both beta1 and beta3 integrins, indicating that alphav has the potential to form a partnership with either beta1 or beta3 integrin. The alphavbeta3 specific inhibitory antibody LM609 significantly retarded HOS-alphav cell PCL contraction (P < or = 0.001), suggesting that alphavbeta3 promotes enhanced HOS-alphav cell PCL contraction. When plated on plastic, control HOS cells show greater elongation compared to HOS-alphav cells. In addition, HOS-alphav cells migrated faster and to a greater degree than control HOS cells (P < or = 0.0001). The possibility that enhanced HOS-alphav cell migration and HOS-alphav cell PCL contraction was caused by increased myosin ATPase activity was examined. HOS-alphav cells showed less myosin ATPase activity than control HOS cells, by an ATP cell contraction bioassay. The enhancement of HOS-alphav cell migration and lattice contraction appears unrelated to increased myosin ATPase activity.

Collagen-binding I domain integrins--what do they do?

Collagens are the most abundant proteins in the mammalian body and it is well recognized that collagens fulfill an important structural role in the extracellular matrix in a number of tissues. Inactivation of the collagen alpha 1(I) gene in mice results in embryonic lethality and collagen mutations in humans cause defects leading to disease. Integrins constitute a major group of receptors for extracellular matrix components, including collagens. Currently four collagen-binding I domain-containing integrins are known, namely alpha 1 beta 1, alpha 2 beta 1, alpha 10 beta 1 and alpha 11 beta 1. Unlike the undisputed role of collagens as structural elements, the biological importance of integrin mediated cell-collagen interactions is far from clear. This is in part due to the limited information available on the most recent additions of the integrin family, alpha 10 beta 1 and alpha 11 beta 1. Future studies using gene inactivation of individual and multiple integrin genes will allow testing of the hypothesis that collagen-binding integrins have redundant functions but will also shed light on their importance in pathological conditions. In this review we will describe what is currently known about the collagen-binding integrins and discuss their biological functions.

alpha11beta1 integrin is a receptor for interstitial collagens involved in cell migration and collagen reorganization on mesenchymal nonmuscle cells

alpha11beta1 integrin constitutes a recent addition to the integrin family. Here, we present the first in vivo analysis of alpha11 protein and mRNA distribution during human embryonic development. alpha11 protein and mRNA were present in various mesenchymal cells around the cartilage anlage in the developing skeleton in a pattern similar to that described for the transcription factor scleraxis. alpha11 was also expressed by mesenchymal cells in intervertebral discs and in keratocytes in cornea, two sites with highly organized collagen networks. Neither alpha11 mRNA nor alpha11 protein could be detected in myogenic cells in human embryos. The described expression pattern is compatible with alpha11beta1 functioning as a receptor for interstitial collagens in vivo. To test this hypothesis in vitro, full-length human alpha11 cDNA was stably transfected into the mouse satellite cell line C2C12, lacking endogenous collagen receptors. alpha11beta1 mediated cell adhesion to collagens I and IV (with a preference for collagen I) and formed focal contacts on collagens. In addition, alpha11beta1 mediated contraction of fibrillar collagen gels in a manner similar to alpha2beta1, and supported migration on collagen I in response to chemotactic stimuli. Our data support a role for alpha11beta1 as a receptor for interstitial collagens on mesenchymally derived cells and suggest a multifunctional role of alpha11beta1 in the recognition and organization of interstitial collagen matrices during development.

Differences between scar and dermal cultured fibroblasts derived from a patient with recurrent abdominal incision wound herniation

Fibroblasts were derived from dermis and scar of a 47-year-old white man with a recurrent incisional hernia as a result of fractured ribs. The scar was thin and stretched, suggesting a defect in the maturation of granulation tissue. After surgical repair, biopsy specimens of discarded scar and skin were used to generate fibroblast cell lines. Fibroblasts maintained in medium containing 10% fetal bovine serum and antibiotic were studied between their third and eighth passage. By phase contrast microscopy, no structural differences were obvious, but it was noted that to pass scar fibroblasts, a more aggressive trypsin regimen was required. Immunohistologic and Western blot analysis of patient scar fibroblasts showed (1) more a smooth muscle actin within stress fibers, (2) increased expression of the vitronectin integrin receptor alpha(v) (CD 51), and (3) reduced expression of the collagen integrin receptor alpha2 (CD 49b). The expression of vinculin from focal adhesions or a tubulin from microtubules was the same among cell lines. Contractions of scar and dermal fibroblast-populated collagen lattice were compared. At 24 hours, contractions were 69 percent with newborn fibroblasts (normal); 68 percent for patient dermal fibroblasts; and only 48 percent for patient scar fibroblasts. The retarded contraction of scar fibroblast-populated collagen lattice was significant (p > or = 0.002). Myosin ATPase activity, critical for lattice contraction, and cell migration were equivalent among all cell lines. A plausible mechanism for the retardation of scar lattice contraction is disruption of fibroblasts and collagen interactions, for which the attachment of cells to collagen is altered. It is proposed that either the decrease in the expression of collagen integrin receptor alpha2 (CD 49b), an increase in the expression of the vitronectin receptor alpha(v) (CD 51), or a combination of both is responsible for disruption of collagen fibroblast interactions.

Engagement of variant CD44 confers resistance to anti-integrin antibody-mediated apoptosis in a colon carcinoma cell line

The LIM 1863 colon carcinoma cell line grows as structured organoids around a central lumen, and we have previously demonstrated that the three-dimensional arrangement protects the individual cells from apoptosis induced by an anti-alpha v integrin antibody, 23C6 (Bates et al., 1994). Here we show that the intercellular forces which drive spheroid formation can be overcome by exposure of the cells to a collagen substrate, or more specifically through ligation of the CD44 receptor by a monoclonal antibody. Binding to immobilized anti-CD44 antibody induced a monolayer morphology which is accompanied by fibronectin production and secretion, and expression of the integrin alpha v beta 6. Significantly, the cells of the monolayer acquired resistance to 23C6 antibody-mediated apoptosis over time and this property was sustained even after removal from the monolayer. We provide data to show that this resistance is not dependent on monolayer morphology, constant engagement of the CD44 receptor, loss of the 23C6 antigen, or elevation of Bcl-2 or Bcl-XL protein. The CD44 expressed by LIM 1863 is shown to be the metastatic variant of the molecule therefore these results provide a possible explanation for the selective advantages conferred by expression of this variant for metastasizing colon cancer cells. Overall, the findings of this study support a model for the development of malignancy through the production of specific survival and growth signals as a direct consequence of a signaling event induced by stimulation of an epithelial variant of CD44.

Integrin receptor involvement in actin cable formation in an in vitro model of events associated with wound contraction

Actin cables have been reported to act in vivo as contractile 'purse strings' capable of closing embryonic wounds through generation of circumferential tension. Furthermore, their involvement in wounds within in vitro model systems suggests that actin cable contraction may be an important mechanism involved in the process of wound closure. The aim of this study therefore, was to investigate the appearance of actin cables in a contracting fibroblast populated collagen lattice, an in vitro model of events associated with wound contraction. Utilising this in vitro model, the time-course of actin cable production was investigated and the involvement of integrin receptors analysed using immunofluorescent labelling techniques. Over a period of hours distinct cellular cable-like structures developed at the edges of collagen lattices coinciding with the onset of contraction. Cellular organisation within the cable was evident as was polymerisation of actin microfilaments into elongated stress fibres forming a continuous cell-cell 'actin cable' around the circumference of the lattice. Immunolocalisation demonstrated that integrin receptor subunits beta 1 and alpha 2 but not alpha 5 were involved in apparent intimate cell-cell contact between juxtaposed fibroblasts within this actin cable. This study demonstrates the involvement of integrin receptors in actin cable formation within collagen lattice systems undergoing reorganisation. Such integrin involvement may enable participating cells to respond to the tensional status of their surrounding environment and via cell-cell communication, to permit a co-ordinated contraction of the cable. It is concluded that integrin receptor involvement in active actin cable contraction may be involved in the process of wound contraction.

The effects of ageing on cutaneous wound healing in mammals

The dogma that cutaneous wound healing is impaired as a function of age is largely unsubstantiated. This can be attributed to poor experimental design of human studies, the lack of subject characterisation with the exclusion of disease processes, and the study of inappropriate animal models. Structural and functional changes in skin with age have been reported, such as a decrease in dermal thickness, decline in collagen content, a subtle alteration in the glycosaminoglycan profile, and a loss of elasticity, but these reports are subject to the above criticisms in addition to the often-neglected requirement for site specificity. Wound repair can be thought of as a culmination of three major overlapping phases: inflammation, proliferation and remodelling. The inflammatory process has not been studied systematically with respect to age, and despite a reported decline in cellular function and number, there is a confounding increase in the production of specific cytokines involved in the process of repair. The proliferative phase is associated with a loss of cellular responsiveness to specific cytokines with a decline in motility and proliferation; however caution in interpreting these findings is important as, for example, the definition of 'ageing' is used rather loosely with the result that neonatal versus young adult cells are compared instead of young versus old adults. During remodelling, fibronectin and collagen production may increase with age, as may wound contraction; the deposition of elastin has not been assessed and the resulting mechanical properties of the scar are controversial, not least because human in vivo studies have been ignored. The absence of a critical review on the effects of advancing age on wound healing has conspired to permit the perpetuation of the belief that well defined tenets exist. This review aims to redress this imbalance and to highlight the need for well designed research into an increasingly important field.

Effect of TGF-beta on differentiated organoids of the colon carcinoma cell line LIM 1863

The LIM 1863 colon carcinoma cell line grows in suspension as morphologically and functionally organized organoids in serum-containing medium. Addition of TGF-beta caused the organoids to adhere and inhibited DNA synthesis. A 20 min incubation with TGF-beta was sufficient to induce adherence and this could be inhibited by cycloheximide. The adhesion and DNA synthesis inhibition were demonstrated to be separate events. We were not able to detect any changes in matrix or cell membrane antigens. Similarly there were no changes in synthesized proteins (by two-dimensional gel electrophoresis), and no upregulation of proteoglycan. When adhered organoids were lysed from the tissue culture plastic surface, untreated organoids adhered to this surface. This 'conditioned' surface was destroyed by trypsin but not collagenase or medium from normal LIM 1863 cultures. However, the adherent phenotype was prevented when organoids were treated with transforming growth factor-beta (TGF-beta) in the presence of medium conditioned by normal LIM 1863 cultures rather than in fresh medium. The adhesion process was inhibited by an antibody (QE2E5) against beta 1 integrin although no quantitative changes in integrins were observed (by immunoprecipitation or RNA analysis). A second anti-beta 1 integrin antibody (61.2C4) inhibited LIM 1863 adhesion to collagen but not TGF-beta induced adhesion, implying that TGF-beta induced a specific conformational change or interaction of a beta 1 integrin. In this morphologically structured system TGF-beta induced a number of subtle effects including formation of new extracellular matrix and conformational change of a beta 1 integrin, rather than the major quantitative changes in cell/matrix molecules reported previously.

Collagen and stretch modulate autocrine secretion of insulin-like growth factor-1 and insulin-like growth factor binding proteins from differentiated skeletal muscle cells

Stretch-induced skeletal muscle growth may involve increased autocrine secretion of insulin-like growth factor-1 (IGF-1) since IGF-1 is a potent growth factor for skeletal muscle hypertrophy, and stretch elevates IGF-1 mRNA levels in vivo. In tissue cultures of differentiated avian pectoralis skeletal muscle cells, nanomolar concentrations of exogenous IGF-1 stimulated growth in mechanically stretched but not static cultures. These cultures released up to 100 pg of endogenously produced IGF-1/micrograms of protein/day, as well as three major IGF binding proteins of 31, 36, and 43 kilodaltons (kDa). IGF-1 was secreted from both myofibers and fibroblasts coexisting in the muscle cultures. Repetitive stretch/relaxation of the differentiated skeletal muscle cells stimulated the acute release of IGF-1 during the first 4 h after initiating mechanical activity, but caused no increase in the long-term secretion over 24-72 h of IGF-1, or its binding proteins. Varying the intensity and frequency of stretch had no effect on the long-term efflux of IGF-1. In contrast to stretch, embedding the differentiated muscle cells in a three-dimensional collagen (Type I) matrix resulted in a 2-5-fold increase in long-term IGF-1 efflux over 24-72 h. Collagen also caused a 2-5-fold increase in the release of the IGF binding proteins. Thus, both the extracellular matrix protein type I collagen and stretch stimulate the autocrine secretion of IGF-1, but with different time kinetics. This endogenously produced growth factor may be important for the growth response of skeletal myofibers to both types of external stimuli.

Expression of beta 1 integrins changes during transformation of avian lens epithelium to mesenchyme in collagen gels

Remarkably, a number of definitive epithelia, such as that of the anterior lens, give rise when suspended within 3D gels of type I collagen, to elongate, bipolar shaped cells that exhibit the ultrastructure, polarity, and migratory ability of mesenchymal cells. They begin producing type I collagen and stop producing crystallins, type IV collagen, and laminin. Here, we investigated changes in beta 1 integrins and their extracellular matrix (ECM) ligands during this transdifferentiation. The former free surface of the lens epithelium that is now in contact with collagen begins within a day to stain intensely for beta 1 and it is this surface rather than the surface facing the basement membrane that gives rise to mesenchymal cells. Immunoprecipitation experiments reveal a large increase in the beta 1 integrin subunit on mesenchymal cells as compared to the epithelium of origin. The alpha 5 integrin subunit, which is barely detectable in the lens, increases in the mesenchymal cells and alpha 3 continues to be expressed at about the same level as in the epithelium. alpha 6, the epithelial integrin subunit, and laminin, its ECM ligand, are not detected immunohistochemically or biochemically in the mesenchyme. Rather, the mesenchymal cells secrete abundant fibronectin, the major ECM ligand for alpha 5 beta 1. RGD peptides do not inhibit the transformation but antibodies to beta 1 do perturb the emigration of mesenchymal cells from the lens apical surface. We conclude that the beta 1 integrins newly expressed on the apical epithelial surface interact with the surrounding 3D collagen gel to help bring about this unusual epithelial-mesenchymal transition.

Apoptosis induced by inhibition of intercellular contact

The LIM 1863 colon carcinoma cell line grows as structural organoids of goblet and columnar cells around a central lumen and provides a model for the development of stem cells in the normal colon. The organoid structure can be disrupted by removal of calcium from the medium, resulting in a suspension of single cells. Upon readdition of calcium, the cells reform the organoid structure over a period of 24 h, and ultrastructural examination of the reforming cells reveals that this involves a complex process that we have termed clutching. To determine the adhesion molecules involved in organoid formation we attempted to block this process by single cell suspensions of LIM 1863 reseeded in the presence of monoclonal antibodies. An anti-integrin antibody directed against a conformational epitope on the alpha v subunit totally inhibited organoid reformation. As a consequence of this inhibition of cell contact the colon carcinoma cells rapidly underwent apoptosis. Investigations of the apoptotic pathway involved suggested an induction mechanism since the onset of apoptosis in the contact-inhibited cells showed specific increased synthesis of 68- and 72-kD proteins. In addition, immunoblotting of cytosolic and nuclear extracts of the cells revealed the rapid translocation of the tumor suppressor gene product, p53 to the cell nucleus upon induction of apoptosis. These results suggest that cell-cell adhesion may be a vital regulator of colon development overcome in tumor cells by loss of adhesion molecules or of functional p53 protein.

Colorectal cancer and the integrin family of cell adhesion receptors: current status and future directions

Tumour progression is thought to be determined, at least in part, by the balance between available cell surface receptors and the nature of the surrounding extracellular matrix. The integrin family of transmembrane adhesion receptors involved in tumour cell-matrix interactions mediates cell adhesion, migration, and differentiation. Certain patterns of integrin receptor expression on normal and malignant colon epithelial cells are emerging, and it is now clear that integrins can also regulate such divergent processes as cell proliferation and programmed cell death in this tumour type. This implies that integrins are involved in signal transduction events within colon carcinoma cells consequent upon their adhesive interaction with matrix molecules. A better understanding of the mechanisms involved in these events may lead to useful therapeutic strategies in the management of this disease.

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.

An echistatin C-terminal peptide activates GPIIbIIIa binding to fibrinogen, fibronectin, vitronectin and collagen type I and type IV

Integrin binding to proteins often involves recognition of domains containing the arginine-glycine-aspartate (RGD) motif. Different binding affinities and specificities of the integrin-ligand protein interactions involve additional protein domains. The n.m.r. structure of the snake-venom protein echistatin suggested that the C-terminal portion of the molecule might be important, in addition to the RGD domain, in binding to the integrin glycoprotein IIbIIIa (GPIIbIIIa) [Saudek, Atkinson and Pelton (1991) Biochem. 30, 7369-7372]. The synthetic C-terminal peptide, echistatin-(40-49), PRNPHKGPAT, (1) inhibited binding of GPIIbIIIa to immobilized echistatin (IC50 3-6 mM), but did not inhibit binding of GPIIbIIIa to immobilized fibrinogen (up to 5 mM peptide), (2) activated GPIIbIIIa binding to fibronectin and vitronectin, usual ligands for the activated integrin, (3) activated binding of GPIIbIIIa to collagen type I and type IV, proteins not usually regarded as ligands for the integrin, and (4) stimulated 125I-fibrinogen binding by human platelets. These findings argue for an interaction of this non-RGD domain in echistatin with GPIIbIIIa, leading to activation of the integrin and extension of the ligand specificity to include immobilized collagen.

Protein kinase C-dependent effects of 12(S)-HETE on endothelial cell vitronectin receptor and fibronectin receptor

12(S)-HETE, a lipoxygenase metabolite of arachidonic acid induced a nondestructive and reversible endothelial cell (EC) retraction. 12(S)-HETE induced EC retraction was inhibited by protein kinase C inhibitors calphostin C and staurosporine but not by the protein kinase A inhibitor H8. The role of EC integrins alpha v beta 3 and alpha 5 beta 1 in 12(S)-HETE induced EC retraction was investigated. In confluent EC cultures, alpha v beta 3 is localized to focal adhesions at both the cell body and cell-cell borders and is colocalized with vinculin-containing focal adhesions. In contrast, alpha 5 beta 1 is primarily enriched at the cell-cell borders, demonstrating codistribution with cell cortical microfilaments and extracellular fibronectin. Both receptors were functional in mediating cell-cell or cell-matrix interactions based on the observations that specific antibodies inhibited EC adhesion to intact subendothelial matrix and disrupted the monolayer integrity. 12(S)-HETE induced a multistep, temporally defined redistribution of the alpha v beta 3-containing focal adhesions, leading to an eventual decrease in alpha v beta 3 plaques in the retracted ECs. This effect of 12(S)-HETE was inhibited by calphostin C but not by H8. The alterations of alpha v beta 3-containing focal adhesions preceded the development of EC retraction. 12(S)-HETE also enhanced EC alpha v beta 3 surface expression as revealed by immunofluorescence, flow cytometry, and digitized image analysis. 12(S)-HETE-induced alpha v beta 3 rearrangement (i.e., decreased focal adhesion localization and enhanced surface expression) did not result from altered mRNA transcription (as revealed by semi-quantitative RT-PCR analysis) or protein translation (as revealed by Western blotting). In contrast to its effect on alpha v beta 3, 12(S)-HETE did not demonstrate a temporally related, well-defined effect on the distribution pattern and the surface expression of alpha 5 beta 1, although the cell-cell border staining pattern of alpha 5 beta 1 was disrupted due to EC retraction. It is concluded that 12(S)-HETE-induced decrease of alpha v beta 3 localization to focal adhesions may contribute to the development of EC retraction and that 12(S)-HETE induced increase in alpha v beta 3 surface expression may promote adhesion of inflammatory leukocytes as well as tumor cells to endothelium.

Functional domains of cell adhesion molecules

A number of molecules involved in cell adhesion (e.g. fibronectin, laminin, collagens I and IV, thrombospondin, entactin) have now been identified and the consequent roles that they play in the processes of growth, migration, differentiation and tumor spread have been described. Active sequences of the molecules have been identified using synthetic peptides derived from specific domains. Several adhesive molecules contain multiple active domains with different biological activities.