The collagen receptor alpha 2 beta 1, from MG-63 and HT1080 cells, interacts with a cyclic RGD peptide

RGD cadherins and α2β1 integrin in cancer metastasis: A dangerous liaison

We propose a new cadherin family classification comprising epithelial cadherins (cadherin 17 [CDH17], cadherin 16, VE-cadherin, cadherin 6 and cadherin 20) containing RGD motifs within their sequences. Expression of some RGD cadherins is associated with aggressive forms of cancer during the late stages of metastasis, and CDH17 and VE-cadherin have emerged as critical actors in cancer metastasis. After binding to α2β1 integrin, these cadherins promote integrin β1 activation, and thereby cell adhesion, invasion and proliferation, in liver and lung metastasis. Activation of α2β1 integrin provokes an affinity increase for type IV collagen, a major component of the basement membrane and a critical partner for cell anchoring in liver and other metastatic organs. Activation of α2β1 integrin by RGD motifs breaks an old paradigm of integrin classification and supports an important role of this integrin in cancer metastasis. Recently, synthetic peptides containing the RGD motif of CDH17 elicited highly specific and selective antibodies that block the ability of CDH17 RGD to activate α2β1 integrin. These monoclonal antibodies inhibit metastatic colonization in orthotopic mouse models of liver and lung metastasis for colorectal cancer and melanoma, respectively. Hopefully, blocking the cadherin RGD ligand capacity will give us control over the integrin activity in solid tumors metastasis, paving the way for development of new agents of cancer treatment.

An RGD motif present in cadherin 17 induces integrin activation and tumor growth

Little is known about the mechanism of integrin activation by cadherin 17 (CDH17). Here we observed the presence of a tri-peptide motif, RGD, in domain 6 of the human CDH17 sequence and other cadherins such as cadherin 5 and cadherin 6. The use of CDH17 RAD mutants demonstrated a considerable decrease of proliferation and adhesion in RKO and KM12SM colon cancer cells. Furthermore, RGD peptides inhibited the adhesion of both cell lines to recombinant CDH17 domain 6. The RGD motif added exogenously to the cells provoked a change in β1 integrin to an active, high-affinity conformation and an increase in focal adhesion kinase and ERK1/2 activation. In vivo experiments with Swiss nude mice demonstrated that cancer cells expressing the CDH17 RAD mutant showed a considerable delay in tumor growth and liver homing. CDH17 RGD effects were also active in pancreatic cancer cells. Our results suggest that α2β1 integrin interacts with two different ligands, collagen IV and CDH17, using two different binding sites. In summary, the RGD binding motif constitutes a switch for integrin pathway activation and shows a novel capacity of CDH17 as an integrin ligand. This motif could be targeted to avoid metastatic dissemination in tumors overexpressing CDH17 and other RGD-containing cadherins.

Collagen XXII binds to collagen-binding integrins via the novel motifs GLQGER and GFKGER

Cell attachment to collagens is mediated by integrins. In the present study, we define two new integrin-binding motifs, GLQGER and GFKGER, within the collagen XXII triple helical domain. Mutation of the two motifs in collagen XXII abolishes the binding to HaCaT cells completely.

Isolation, characterization and biological evaluation of jellyfish collagen for use in biomedical applications

Fibrillar collagens are the more abundant extracellular proteins. They form a metazoan-specific family, and are highly conserved from sponge to human. Their structural and physiological properties have been successfully used in the food, cosmetic, and pharmaceutical industries. On the other hand, the increase of jellyfish has led us to consider this marine animal as a natural product for food and medicine. Here, we have tested different Mediterranean jellyfish species in order to investigate the economic potential of their collagens. We have studied different methods of collagen purification (tissues and experimental procedures). The best collagen yield was obtained using Rhizostoma pulmo oral arms and the pepsin extraction method (2–10 mg collagen/g of wet tissue). Although a significant yield was obtained with Cotylorhiza tuberculata (0.45 mg/g), R. pulmo was used for further experiments, this jellyfish being considered as harmless to humans and being an abundant source of material. Then, we compared the biological properties of R. pulmo collagen with mammalian fibrillar collagens in cell cytotoxicity assays and cell adhesion. There was no statistical difference in cytotoxicity (p > 0.05) between R. pulmo collagen and rat type I collagen. However, since heparin inhibits cell adhesion to jellyfish-native collagen by 55%, the main difference is that heparan sulfate proteoglycans could be preferentially involved in fibroblast and osteoblast adhesion to jellyfish collagens. Our data confirm the broad harmlessness of jellyfish collagens, and their biological effect on human cells that are similar to that of mammalian type I collagen. Given the bioavailability of jellyfish collagen and its biological properties, this marine material is thus a good candidate for replacing bovine or human collagens in selected biomedical applications.

Chondrocyte calcium signaling in response to fluid flow is regulated by matrix adhesion in 3-D alginate scaffolds

The interaction between chondrocytes and their surrounding extracellular matrix plays an important role in regulating cartilage metabolism in response to environmental cues. This study characterized the role of cell adhesion on the calcium signaling response of chondrocytes to fluid flow. Bovine chondrocytes were suspended in alginate hydrogels functionalized with RGD at concentrations of 0-400μM. The hydrogels were perfused and the calcium signaling response of the cells was measured over a range of fluid velocities from 0 to 68μm/s. Attachment to RGD-alginate doubled the sensitivity of chondrocytes to flows in the range of 8-13μm/s, but at higher fluid velocities, the contribution of cell adhesion to the observed calcium signaling response was no longer apparent. The enhanced sensitivity to flow was dependent on the density of RGD-ligand present in the scaffolds. The RGD-enhanced sensitivity to flow was completely inhibited by the addition of soluble RGD which acted as a competitive inhibitor. The results of this study indicate a role for matrix adhesion in regulating chondrocyte response to fluid flow through a calcium dependent mechanism.

Lebectin and lebecetin, two C-type lectins from snake venom, inhibit α5β1 and αv-containing integrins

Integrins are essential protagonists in the complex multistep process of cancer progression and metastasis. We recently reported that lebectin, a novel C-type lectin from Macrovipera lebetina venom, displays an anti-integrin activity. In this study, we extend this observation to lebecetin, a second C-type lectin isolated from the same venom and previously reported as a potent inhibitor of platelet aggregation. Both venom lectins appear to exert their effect on cell adhesion, migration, invasion and proliferation by inhibiting alpha5beta1 and alphav-containing integrins. Moreover, the inhibition of alpha5beta1 and alphav integrins is likely due to the binding of venom peptides, as both lebectin and lebecetin co-immunoprecipitate with these integrins. Lebectin and lebecetin are thus the first examples of venom C-type lectins inhibiting an integrin other than the collagen receptor alpha2beta1.

New approaches to blockade of alpha4-integrins, proven therapeutic targets in chronic inflammation

The recruitment of leukocytes into tissue is a pivotal step in inflammation. alpha4-Integrins are adhesion receptors on circulating leukocytes that mediate attachment to the endothelium and facilitate their migration into the inflamed tissue. This multistep process is mediated by the interaction of alpha4-integrins with their counter receptors VCAM-1 and MadCAM-1 that are expressed on endothelial cells. alpha4-Integrins act as both adhesive and signaling receptors. Paxillin, a signaling adaptor molecule, binds directly to the alpha4 cytoplasmic tail and its binding is important for cell migration. Blocking the adhesive functions of alpha4-integrins has been shown to be an effective therapeutic approach in the treatment of autoimmune diseases, but also carries the risk of defects in development, hematopoiesis and immune surveillance. Interfering with alpha4 signaling by inhibiting the alpha4-paxillin interaction decreases alpha4-mediated cell migration and adhesion to VCAM-1 and MadCAM under shear flow. These in vitro effects are accompanied by a selective impairment of leukocyte migration into inflammatory sites when the alpha4-paxillin interaction is blocked in vivo. Thus, blockade of alpha4-integrin signaling may offer a novel strategy for interfering with the functions of these receptors in pathological events while sparing important physiological functions.

Novel peptide ligands for integrin α4β1 overexpressed in cancer cells

Using the “one-bead one-peptide” combinatorial technology, a library of random cyclic octapeptides and nonapeptides, consisting of natural and unnatural amino acids, was synthesized on polystyrene beads. This library was used to screen for peptides that promoted attachment and proliferation of bronchioloalveolar carcinoma cells (H1650), employing a “cell growth on bead” assay. Consensus peptide sequences of cNleDXXXXc and cXNleDXXXXc (where Nle is norleucine) were identified. With alanine scanning and site-directed deletion, a typical ligand consisted of a motif of -NleDI/V/Nle- with two flanking cysteines. These peptide ligands were specific for promoting cell attachment of the H1650 cells and the cells of lymphoid cancers (Jurkat and Raji) but not other selected human cell lines of lung cancer and fibroblast. In an antibody blocking assay, integrin α4β1, which was overexpressed in H1650, Jurkat, and Raji, was identified as a putative receptor for these peptide ligands. Using Chinese hamster ovary cells transfected with either wild-type or mutant integrin α4, a critical binding site for these peptides was localized to the glycine residue at position 190 of integrin α4.

Lebectin, a novel C-type lectin from Macrovipera lebetina venom, inhibits integrin-mediated adhesion, migration and invasion of human tumour cells

The adhesion receptors of the integrin family play an essential role during tumour progression and thus represent interesting potential targets for the development of new therapeutic agents. The snake venom contains natural inhibitors of integrin-ligand interactions called disintegrins. It also contains C-type lectin proteins mainly known as modulators of platelet aggregation. In this study, we demonstrate that lebectin, a novel C-type lectin isolated from Macrovipera lebetina venom, displayed an anti-integrin activity. Lebectin inhibited the integrin-mediated attachment of various tumour cell lines to different adhesion substrata. The C-type lectin also completely blocked cell migration towards fibronectin in haptotaxis assays and prevented invasion of fibrin gels by tumour cells. In addition, lebectin proved to be a potent inhibitor of tumour cell proliferation. Although the specific integrins affected by lebectin are not identified in this study, the integrin alpha 5 beta 1 might be involved.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.

Prospects and limitations of the rational engineering of fibrillar collagens

Recombinant collagens are attractive proteins for a number of biomedical applications. To date, significant progress was made in the large-scale production of nonmodified recombinant collagens; however, engineering of novel collagen-like proteins according to customized specifications has not been addressed. Herein we investigated the possibility of rational engineering of collagen-like proteins with specifically assigned characteristics. We have genetically engineered two DNA constructs encoding multi-D4 collagens defined as collagen-like proteins, consisting primarily of a tandem of the collagen II D4 periods that correspond to the biologically active region. We have also attempted to decrease enzymatic degradation of novel collagen by mutating a matrix metalloproteinase 1 cleavage site present in the D4 period. We demonstrated that the recombinant collagen alpha-chains consisting predominantly of the D4 period but lacking most of the other D periods found in native collagen fold into a typical collagen triple helix, and the novel procollagens are correctly processed by procollagen N-proteinase and procollagen C-proteinase. The nonmutated multi-D4 collagen had a normal melting point of 41 degrees C and a similar carbohydrate content as that of control. In contrast, the mutant multi-D4 collagen had a markedly lower thermostability of 36 degrees C and a significantly higher carbohydrate content. Both collagens were cleaved at multiple sites by matrix metalloproteinase 1, but the rate of hydrolysis of the mutant multi-D4 collagen was lower. These results provide a basis for the rational engineering of collagenous proteins and identifying any undesirable consequences of altering the collagenous amino acid sequences.

Engineering integrin-specific surfaces with a triple-helical collagen-mimetic peptide

Integrin-mediated cell adhesion to extracellular matrix proteins anchors cells and triggers signals that direct cell function. The integrin alpha(2)beta(1) recognizes the glycine-phenylalanine-hydroxyproline-glycine-glutamate-arginine (GFOGER) motif in residues 502-507 of the alpha(1)(I) chain of type I collagen. Integrin recognition is entirely dependent on the triple-helical conformation of the ligand similar to that of native collagen. This study focuses on engineering alpha(2)beta(1)-specific bioadhesive surfaces by immobilizing a triple-helical collagen-mimetic peptide incorporating the GFOGER binding sequence onto model nonadhesive substrates. Circular dichroism spectroscopy verified that this peptide adopts a stable triple-helical conformation in solution. Passively adsorbed GFOGER-peptide exhibited dose-dependent HT1080 cell adhesion and spreading comparable to that observed on type I collagen. Subsequent antibody blocking conditions verified the involvement of integrin alpha(2)beta(1) in these adhesion events. Focal adhesion formation was observed by immunofluorescent staining for alpha(2)beta(1) and vinculin on MC3T3-E1 cells. Model functionalized surfaces then were engineered using three complementary peptide-tethering schemes. These peptide-functionalized substrates supported alpha(2)beta(1)-mediated cell adhesion and focal adhesion assembly. Our results suggest that this peptide is active in an immobilized conformation and may be applied as a surface modification agent to promote alpha(2)beta(1)-specific cell adhesion. Engineering surfaces that specifically target certain integrin-ligand interactions and signaling cascades provides a biomolecular strategy for optimizing cellular responses in biomaterials and tissue engineering applications.

Novel cyclic and linear oligopeptides that bind to integrin beta1 chain and either inhibit or costimulate T lymphocytes

There is a redundancy of cellular beta1 integrin (very late antigen or VLA) receptors that mediate interactions between different extracellular matrix proteins (ECMP) and T lymphocytes. This suggests that antagonists targeted at individual VLA receptors may be of limited therapeutic efficacy in T cell-mediated diseases and that agents such as monoclonal antibody 4B4, which bind to the common integrin beta1 chain and inhibit interactions between effector T cells and a range of ECMP, may be of greater therapeutic interest if toxicity can be avoided. We have therefore sought proof of principle as to whether small molecules that interact with the integrin beta1 chain at or near the 4B4 binding site can modulate T cell costimulation and adhesion in the presence of type I collagen or fibronectin (FN). Two phage display libraries, each expressing more than 10(9) independent cyclic or linear 7-mer peptides, were used to identify molecules of interest by an enrichment process involving specific recovery of phage bound to a human T cell line by elution with a large excess of 4B4 antibody. Novel cyclic and linear peptides have thus been identified and found to inhibit interactions between T cells and both type I collagen and fibronectin. A separate cyclic peptide was found to costimulate T cells in a beta1 integrin-dependent manner. These findings form a basis for the development of small molecules that interact in inhibitory or stimulatory capacities with the common integrin beta1 chain, and may be of interest as therapeutic antagonists or immunologic adjuvants.

Differentiation of human bone-derived cells grown on GRGDSP-peptide bound titanium surfaces

Various surface modifications have been applied to titanium alloy (Ti-6Al-4V) implants, in an attempt to enhance osseointegration; crucial for ideal prosthetic fixation. Despite the numerous studies demonstrating that peptide-modified surfaces influence in vitro cellular behavior, there is relatively little data reporting their effects on bone remodeling. The objective of this article was to examine the effects of chemically modifying Ti-6Al-4V surfaces with a common RGD sequence, a 15-residue peptide containing GRGDSP (glycine-arginine-glycine-aspartate-serine-proline), on the modulation of bone remodeling. The expression of proteins known to be associated with osseous matrix and bone resorption were studied during the growth of human bone-derived cells (HBDC) on these peptide-modified surfaces. HBDC grown for 7 days on RGD surfaces displayed significantly increased levels of osteocalcin, and pro-collagen Ialpha1 mRNAs, compared with the production by HBDC grown on the native Ti-6Al-4V. A pattern that was also reflected at the protein levels for osteocalcin, type I collagen, and bone sialoprotein. Moreover, HBDC grown for 7 and 14 days on RGD-modified Ti-6Al-4V expressed significantly higher level of osteoclast differentiation factors and lower levels of osteoprotegerin and IL-6 proteins compared with other surfaces tested. These results suggest that different chemical treatments of implant material (Ti-6Al-4V) surface result in differential bone responses, not only their ability to form bone but also to stimulate osteoclastic formation.

GRGDSP peptide-bound silicone membranes withstand mechanical flexing in vitro and display enhanced fibroblast adhesion

Mechanobiological studies of cardiac tissue require devices that allow forces to be exerted on cells in vitro. Silicone elastomer is often used in these devices because it is flexible and transparent, permitting optical imaging of the cells. However, native untreated silicone is hydrophobic and is unsuitable for cell culture. Peptides covalently bound to silicone surfaces are examined here for the enhancement of cellular adhesion during in vitro dynamic flexing. A procedure is described for the chemical modification of medical grade silicone membranes with covalently bound GRGDSP peptides. The conditions for mechanical studies of cardiac cell cultures are then duplicated and it is demonstrated that the peptide layers survive 48 h of mechanical flexing in vitro. Specifically, mechanical flexing in vitro of the 30 pmol/cm2 peptide-modified silicone membranes has no significant effect on the amount of peptides that remains bound to the surface. Cardiac fibroblasts display enhanced adhesion to these peptide-bound silicone membranes for at least 24 h of growth, compared with native silicone or tissue culture polystyrene. The effects of serum versus serum-free media on fibroblast growth are also examined.

The involvement of beta1 integrin in the modulation by collagen of chondrocyte-response to transforming growth factor-beta1

The physiologic response of chondrocytes to maintenance of the matrix and response to injury likely involves signaling from multiple sources including soluble cytokines, mechanical stimulation, and signaling from the extracellular matrix. The signaling from the extracellular matrix may serve to effect cell differentiation and to modulate the response to cytokines. We have previously reported that type II collagen modulates the response of bovine articular chondrocytes to TGF-beta1. The molecular nature of the signaling mechanism has not been elucidated but presumably involves a similar mechanism by which the cell attaches to the surrounding matrix. An alginate bead culture system is utilized to which exogenous type II collagen is added. The inclusion of type II collagen results in an alteration of integrin expression with a down regulation of alpha2. The response of the chondrocyte to TGF-beta1 can be modulated by the inclusion of exogenous type II collagen. The modulation of DNA and proteoglycan synthesis was blocked by the treatment of anti-beta1 integrin antibody (4B4) or by cyclic RGD containing peptides. These events occur at concentrations that block cell adhesion to type II collagen. Linear RGD containing peptides and anti-anchorin antibodies had no effect on the modulation by type II collagen. These results suggest that type II collagen binding by chondrocytes at least in part occurs through the beta1 integrin. This binding results in modulation of the cell response to TGF-beta1. This modulation may serve to provide physiologic specificity to the cytokine-signaling cascade. An understanding of the regulatory milieu of the chondrocyte may permit the stimulation of an intrinsic repair of articular cartilage in the future. A near term application of this understanding can be made to tissue engineering attempts at articular cartilage repair.

Mapping critical sites in collagen II for rational design of gene-engineered proteins for cell-supporting materials

Collagen II is the most abundant protein of cartilage and forms a network of fibrils extended by proteoglycans that enables cartilage to resist pressure. The surface of the collagen fibril serves as a platform for the attachment of collagen IX, growth factors, and cells. In this study we examined the mechanism of the interaction of chondrocytes with recombinant versions of procollagen II, in which one of the four blocks of 234 amino acids that define repeating D periods of the collagen triple helix has been deleted. Analysis of the attachment of chondrocytes to collagen II variants with deleted D periods indicated that the collagen II monomer contains randomly distributed sites critical for cell binding. However, as was shown by spreading and migration assays, the D4 period, which is between residues 703 to 936, contains amino acids critical for cell motility. We also showed that binding, spreading, and migration of chondrocytes through three-dimensional nanofibrillar collagenous matrices are controlled by an interaction of the collagen triple helix with beta1 integrins. The results of this study provide a basis for the rational design of a scaffold containing genetically engineered collagen with a high density of specific sites of interaction.

Expression of integrins in human cultured mesothelial cells: the roles in cell-to-extracellular matrix adhesion and inhibition by RGD-containing peptide

Integrins play key roles in cell-to-cell and cell-to-extracellular matrix (ECM) adhesion. We investigated integrin expression on pleural mesothelial cells (PMCs) and the inhibitory effect of arginine-glycine-asparate (RGD)-containing peptide on the adhesion of PMCs to fibronectin and collagen. Using flow cytometry and immunostaining, PMCs freshly isolated from pleural effusions and one mesothelial cell line were screened for different integrins. Intact pleural tissue was evaluated by immunohistochemistry. The adhesion of Met-5A cells to fibronectin and collagen types I, III and IV was assayed with prior treatment of various concentrations of glycine-arginine-glycine aspartate-serine (GRGDS). On primary PMCs, alpha2, alpha3, alpha5, beta1, beta3 and alphavbeta3 were highly expressed (>70%); alpha1 expression was intermediate (30-70%); and alpha4 and alpha6 expressions were low (< 30%). On Met-SA cells, alpha3, alpha5, alpha6 and beta1 were highly expressed (>70%); alpha1 was intermediate (30-70%); and alpha2, alpha4, beta3 and alphavbeta3 were low (<30%). The patterns of immunostaining on pleural tissues were similar to the results of flow cytometry for primary PMCs except for beta3. There was no statistically different expression in various disease states (transudate vs. exudate, benign vs. malignant). The inhibitory effect of GRGDS peptide on Met-5A cell adhesion to all four matrix proteins was dose-dependent.

Colon cancer cells adhesion and spreading on autocrine laminin-10 is mediated by multiple integrin receptors and modulated by EGF receptor stimulation

Epidermal growth factor (EGF) receptor ligands such as EGF and transforming growth factor-alpha (TGF-alpha) play an important role in controlling the proliferation, survival, morphology, and motility of colonic epithelial cells. There is also increasing evidence that growth factors and extracellular matrix (ECM) proteins cooperate to regulate these cellular processes. We have reported previously that autocrine TGF-alpha and an unidentified ECM protein in the serum-free conditioned medium of the human colon carcinoma cell line LIM1215 synergize to induce spreading of these cells in low-density cultures. We have now purified the ECM protein secreted by LIM1215 cells and show that it synergizes with EGF to induce spreading of LIM1215 cells and other human cell lines from the colon and other tissues. The purified ECM migrated as a single protein band with an apparent molecular mass of approximately 800 kDa on SDS-PAGE under nonreducing conditions and, under reducing conditions, as three protein bands of approximately 360, 210, and 200 kDa. Immunoblotting experiments and mass spectrometry analysis of tryptic digests on the purified protein identified the 360-, 210-, and 200-kDa protein bands as laminin alpha5, beta1, and gamma1 chains, respectively, indicating that LIM1215 cells secrete laminin-10 (alpha5 beta1 gamma1). In serum-free medium, LIM1215 cells adhere to laminin-10 primarily via alpha2 beta1 and alpha3 beta1 integrin receptors. EGF-induced spreading of LIM1215 cells on laminin-10 is partially inhibited by pretreatment of the cells with blocking antibodies directed against integrin alpha3 or beta1 but not alpha2, alpha6, or beta4 subunits. Spreading is almost completely inhibited by blocking alpha3 + alpha2, alpha3 + alpha6, or beta1 + beta4 integrin chains and results in cell death. Increased spreading in the presence of EGF correlates with up-regulation of alpha6 beta4 integrins in these cells after exposure to EGF. These results indicate that colon cancer cells attach and spread on laminin-10 via multiple integrin receptors and suggest a critical role for alpha3 beta1 integrins in the spreading response. Together, our results support the concept that the adhesive properties of colon cancer cells are modulated by autocrine production of TGF-alpha and laminin-10 and autocrine induction of appropriate integrins.

Peptides from the PKD repeats of polycystin, the PKD1 gene product, modulate pattern formation in the developing kidney

Mutations in the PKD1 gene cause the majority of cases of autosomal dominant polycystic kidney disease. The PKD1 gene codes for a protein of unknown function, polycystin-1, that is predicted to be a receptor. Its large extracellular domain contains 16 copies of novel motif, the PKD repeat, that is likely to be a ligand binding domain based on its similarity to immunoglobulin domains. These observations suggested that soluble fragments of the extracellular domain of polycystin-1 could be used as competitive inhibitors of polycystin function in a suitable model system. Polycystin-1 is highly expressed in the ureteric bud and other branching epithelia during development and interacts with beta-catenin, a molecule known to play a role in branching morphogenesis. These data suggested that polycystin-1 might play a role in branching morphogenesis. I show here that peptides derived from the PKD repeats of polycystin-1 caused an asymmetric pattern of ureteric bud branching in cultured kidney rudiments. Treatment of kidney rudiments with experimental but not control peptides reduced both the number of ureteric bud branches and the number of nephrons. Experimental peptides produced significant morphogenetic effects at concentrations < or = 0.1 mM. These data suggest that polycystin-1 plays a role in branching morphogenesis by the ureteric bud.

Extracellular matrix cell adhesion peptides: functional applications in orthopedic materials

This review describes research on selected peptide sequences that affect cell adhesion as it applies in orthopedic applications. Of particular interest are the integrin-binding RGD peptides and heparin-binding peptides. The influence of these peptides on cell adhesion is described. Cell adhesion is defined as a sequence of four steps: cell attachment, cell spreading, organization of an actin cytoskeleton, and formation of focal adhesions. RGD sequences clearly influence cell attachment and spreading, whereas heparin-binding sequences appear to be less efficient. Collectively, these sequences appear to promote all steps of cell adhesion in certain cell types. This review also addresses issues related to peptide immobilization, as well as potential complexities that may develop as a result of using these versatile cell-binding sequences. Also described are future directions in the field concerning use of existing and more sophisticated peptide substrata.

Inhibition of platelet aggregation by the recombinant cysteine-rich domain of the hemorrhagic snake venom metalloproteinase, atrolysin A

The P-III class of venom metalloproteinases has, in addition to the proteinase domain, a disintegrin-like domain and a cysteine-rich domain. Recent evidence has shown that the nonproteinase domains of the P-III class of hemorrhagic metalloproteinases function in the inhibition of platelet aggregation by blocking essential procoagulant integrins on platelets. A specific role for the highly conserved cysteine-rich domain has yet to be described. In this study, we expressed the cysteine-rich domain from the hemorrhagic metalloproteinase atrolysin A and demonstrated its ability to inhibit collagen-stimulated platelet aggregation. Additionally, the cysteine-rich domain was shown to interact with MG-63 cells to inhibit adhesion to collagen I. These data suggest a functional role for the cysteine-rich domain of the P-III toxins in the observed coagulopathy by targeting the toxin to platelets and inhibiting collagen-stimulated platelet aggregation. These characteristics may function to synergistically increase the hemorrhagic effect of the toxins.

Soluble collagen VI drives serum-starved fibroblasts through S phase and prevents apoptosis via down-regulation of Bax

We previously showed that soluble, pepsin-solubilized collagen VI increases de novo DNA synthesis in serum-starved HT1080 and 3T3 fibroblasts up to 100-fold compared with soluble collagen I, reaching 80% of the stimulation caused by 10% fetal calf serum. Here we show that collagen VI also inhibits apoptotic cell death in serum-starved cells as evidenced by morphological criteria, DNA laddering, complementary apoptosis assays (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, enzyme-linked immunosorbent assay, and fluorescence-activated cell sorting), and quantification of apoptosis-regulating proteins. In the presence of starving medium alone or collagen I, the proapoptotic Bax was up-regulated 2-2.5-fold, compared with soluble collagen VI and fetal calf serum, whereas levels of the antiapoptotic Bcl-2 protein remained unaffected. In accordance with its potent stimulation of DNA synthesis, soluble collagen VI carries serum-starved HT1080 and Balb 3T3 fibroblasts through G(2) as shown by fluorescence-activated cell sorting analysis, whereas cells exposed to medium and collagen I where arrested at G(1)-S. This was accompanied by a 2-3-fold increase in cyclin A, B, and D1 protein expression. Collagen VI-induced inhibition of apoptotic cell death may be operative during embryogenesis, wound healing, and fibrosis when elevated tissue and blood levels of collagen VI are observed, thus initiating a feedback loop of mesenchymal cell activation and proliferation.

A peptide inhibiting the collagen binding function of integrin alpha2I domain

Integrin alpha2 subunit forms in the complex with the beta1 subunit a cell surface receptor binding extracellular matrix molecules, such as collagens and laminin-1. It is a receptor for echovirus-1, as well. Ligands are recognized by the special "inserted" domain (I domain) in the integrin alpha2 subunit. Venom from a pit viper, Bothrops jararaca, has been shown to inhibit the interaction of platelet alpha2beta1 integrin with collagen because of the action of a disintegrin/metalloproteinase named jararhagin. The finding that crude B. jararaca venom could prevent the binding of human recombinant ralpha2I domain to type I collagen led us to study jararhagin further. Synthetic peptides representing hydrophilic and charged sequences of jararhagin, including the RSECD sequence replacing the well known RGD motif in the disintegrin-like domain, were synthesized. Although the disintegrin-like domain derived peptides failed to inhibit ralpha2I domain binding to collagen, a basic peptide from the metalloproteinase domain proved to be functional. In an in vitro assay, the cyclic peptide, CTRKKHDNAQC, was shown to bind strongly to human recombinant alpha2I domain and to prevent its binding to type I and IV collagens and to laminin-1. Mutational analysis indicated that a sequence of three amino acids, arginine-lysine-lysine (RKK), is essential for ralpha2I domain binding, whereas the mutation of the other amino acids in the peptide had little if any effect on its binding function. Importantly, the peptide was functional only in the cyclic conformation and its affinity was strictly dependent on the size of the cysteine-constrained loop. Furthermore, the peptide could not bind to alpha2I domain in the absence of Mg2+, suggesting that the conformation of the I domain was critical, as well. Cells could attach to the peptide only if they expressed alpha2beta1 integrin, and the attachment was inhibited by anti-integrin antibodies.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

How platelet aggregation affects B16BL6 melanoma cell trafficking

In blood-borne metastasis, intravasated metastatic tumor cells are thought to localize at the target site via a series of processes involving platelet aggregation, adhesion to endothelium, and invasion through the basal membrane. In the present study, we examined how platelet aggregation contributes to the trafficking of metastatic tumor cells in vivo by use of an inhibitor of platelet aggregation. Highly invasive B16BL6 melanoma cells were labeled with [2-18F]2-fluoro-2-deoxy-D-glucose and injected into mice to determine cell trafficking non-invasively by positron emission tomography. Both platelet aggregation inhibitor cyclo(RSarDPhg), which could not inhibit metastasis, and metastatic inhibitor cyclo(GRGDSPA) suppressed the accumulation of B16BL6 cells in the lung by about 12%, suggesting that platelet aggregation partly affects cell trafficking but not to a great extent, and that platelet aggregation is not the essential step for B16BL6 cell arrest in targets.

Therapeutic effect of arginine-glycine-aspartic acid peptides in acute renal injury

1. Previous studies from our laboratory have suggested that arginine-glycine-aspartic acid (RGD) peptides, serving as a decoy, may prevent tubular obstruction in the ischaemic model of acute renal failure. Specifically, we have demonstrated that: (i) stressed tubular epithelial cells reverse the polarity of integrin receptors from the predominantly basolateral location to the apical cell membrane as a part of a more generalized process of the loss of epithelial cell polarity; (ii) depletion of integrins expressed on the basal cell surface leads to the loss of anchorage to the basement membrane and cell desquamation; (iii) expression of integrin receptors on the apical cell membrane leads to indiscriminate interactions (e.g. the adhesion of desquamated cells to the cells remaining in situ), thus initiating the process of tubular obstruction; and (iv) conglomeration of the desquamated cells via integrin receptors further aggravates tubular obstruction. 2. Importantly, these integrin-based interactions can be blocked by synthetic RGD peptides. The linear RGD peptide injected into the renal artery upon release of the renal artery clamp prevented the elevation of proximal tubular hydrostatic pressure characteristically seen in animals with renal ischaemia that received injection of the vehicle of an inactive peptide. 3. In vivo study of RGD peptides in ischaemic acute renal failure in rats demonstrated attenuation of renal injury and accelerated recovery of renal function. 4. Using linear RGD peptide labelled with 99mTc; we have shown that this probe was retained in ischaemic kidneys. 5. To visualize RGD binding sites at the cellular level, we performed a mapping using fluorescent derivatives of two RGD peptides, a cyclic biotinylated (Bt)-RGD peptide and a linear Rhodamine green-labelled (RhoG)-RGD peptide. 6. The findings suggest that the binding sites for RGD peptide are represented by the alphaVbeta3 integrin in the vasculature and some desquamated cells, whereas the majority of the desquamated cells bind Bt-RGD via beta1 integrins. 7. These findings were further tested using cultured endothelial cells co-incubated with leucocytes. When co-incubation experiments were performed in the presence of cyclic RGD pentapeptide, the adhesion of HL-60 cells to both control and hypoxic endothelial monolayers was significantly reduced.

Microinjection of protein tyrosine phosphatases into fibroblasts disrupts focal adhesions and stress fibers

Microinjection and scrape-loading have been used to load cells in culture with soluble protein tyrosine phosphatases (PTPs). The introduction of protein tyrosine phosphatases into cells caused a rapid (within 5 minutes) decrease in tyrosine phosphorylation of major tyrosine phosphorylated substrates, including the focal adhesion kinase and paxillin. This decrease was detected both by blotting whole cell lysates with anti-phosphotyrosine antibodies and visualizing the phosphotyrosine in focal adhesions by immunofluorescence microscopy. After 30 minutes, many of the cells injected with tyrosine phosphatases revealed disruption of focal adhesions and stress fibers. To determine whether this disruption was due to the dephosphorylation of FAK and its substrates in focal adhesions, we have compared the effects of protein tyrosine phosphatase microinjection with the effects of displacing FAK from focal adhesions by microinjection of a dominant negative FAK construct. Although both procedures resulted in a marked decrease in the level of phosphotyrosine in focal adhesions, disruption of focal adhesions and stress fibers only occurred in cells loaded with exogenous protein tyrosine phosphatases. These results lead us to conclude that although tyrosine phosphorylation regulates focal adhesion and stress fiber stability, this does not involve FAK nor does it appear to involve tyrosine-phosphorylated proteins within focal adhesions. The critical tyrosine phosphorylation event is upstream of focal adhesions, a likely target being in the Rho pathway that regulates the formation of stress fibers and focal adhesions.

The disintegrin eristostatin interferes with integrin alpha 4 beta 1 function and with experimental metastasis of human melanoma cells

Peptides containing the integrin recognition sequence, RGD, can inhibit experimental metastasis of mouse melanoma cells, but the integrin(s) affected in these experiments is unknown. Besides "classical" RGD-binding integrins such as alpha 5 beta 1 and alpha v beta 3, RGD has been reported to bind alpha 4 beta 1, and mAbs to alpha 4 beta 1 can inhibit melanoma metastasis. We investigated the mode of action of the disintegrin eristostatin, an RGD-containing peptide isolated from snake venom, in a human melanoma experimental metastasis model. Lung colonization following i.v. injection of MV3 cells in nude mice was strongly inhibited by eristostatin. MV3 cells bound FITC-eristostatin and adhered to eristostatin-coated wells. This adhesion was partially inhibited by a GRGDSP peptide and by alpha 4 mAb. Binding of FITC-eristostatin to Jurkat cells and adhesion of Jurkat (but not K562) cells to eristostatin-coated wells further suggested that eristostatin binds alpha 4 beta 1, even though, again, alpha 4 mAb only partially inhibited adhesion. Expression of alpha 4 beta 1 was enhanced in metastatic melanoma cells compared to normal melanocytes and nonmetastatic melanoma cells. Finally, eristostatin inhibited adhesion of both MV3 and CHO alpha 4 cells to the alpha 4 beta 1-ligand VCAM-1, while adhesion to other ligands via other integrins was not affected. These findings demonstrate that inhibition of melanoma cell metastasis by RGD-containing peptides such as eristostatin, may be due to interference with alpha 4 beta 1-VCAM binding, in addition to inhibition of the classical RGD-binding integrins.

A collagen peptide motif activates tyrosine kinase-dependent calcium signalling pathways in human osteoblast-like cells

A collagen peptide motif (DGEA) which is a putative alpha 2 beta 1 integrin binding site was examined for its ability to activate Ca2+ signalling pathways in the human osteoblast-like cell line SaOS-2. We show that these cells express both alpha 2 beta 1 integrin subunits (by immunocytochemistry) and that an anti-beta 1 monoclonal antibody (DF5) mobilizes Ca2+ in these cells. DGEA elevated intracellular Ca2+ in fura-2-loaded cells, in a concentration- and sequence-dependent fashion, with an EC50 of 250 microM. The tyrosine kinase inhibitor herbimycin A reduced the number of cells responding to DGEA and to transforming growth factor alpha. Thrombin also stimulated a rise in intracellular Ca2+, but the number of cells responding was not reduced by herbimycin A. The DGEA response was dependent on extracellular Ca2+, but was not due to Ca2+ influx, since it was blocked by thapsigargin and not by lanthanum. Using three different anti-alpha 2 monoclonal antibodies, we were unable to show that the DGEA-induced Ca2+ signal was mediated by the alpha 2 beta 1 integrin. In summary, the DGEA collagen motif does appear to activate receptor-mediated Ca2+ signalling events in SaOS-2 cells, in a divalent cation-dependent manner, but we were unable to demonstrate a role for alpha 2 beta 1 integrin in this response.

Potent alpha 4 beta 1 peptide antagonists as potential anti-inflammatory agents

The migration, adhesion, and subsequent extravasation of leukocytes into inflamed tissues contribute to the pathogenesis of a variety of inflammatory diseases including asthma, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. The integrin adhesion receptor alpha 4 beta 1 expressed on leukocytes binds to the extracellular matrix protein fibronectin and to the cytokine inducible vascular cell adhesion molecule-1 (VCAM-1) at inflamed sites. Binding of alpha 4 beta 1 to VCAM-1 initiates firm adhesion of the leukocyte to the vascular endothelium followed by extravasation into the tissue. Monoclonal antibodies generated against either alpha 4 beta 1 or VCAM-1 can moderate this inflammatory response in a variety of animal models. Recently peptides containing a consensus LDV sequence based on the connecting segment-1 (CS-1) of fibronectin and cyclic peptides containing an RCD motif have shown promise in modulating leukocyte migration and inflammation presumably by blocking the interaction of alpha 4 beta 1 with VCAM-1. Here we describe novel, highly potent, cyclic peptides that competitively inhibit alpha 4 beta 1 binding to VCAM-1 and fibronectin at sub nanomolar concentrations. The structure of a representative analog was determined via NMR spectroscopy and used to facilitate optimization of peptide leads. The peptides discussed here utilize similar functional groups as the binding epitope of VCAM-1, inhibit lymphocyte migration in vivo, and are highly selective for alpha 4 beta 1. Furthermore the structure--activity relationships described here have provided a template for the structure-based design of small molecule antagonists of alpha 4 beta 1-mediated cell adhesion processes.

Two novel probes reveal tubular and vascular Arg-Gly-Asp (RGD) binding sites in the ischemic rat kidney

We have previously demonstrated that RGD peptides prevent tubular obstruction in ischemic acute renal failure (ARF) suggested that exposed unoccupied integrin receptors represent the target for such therapy. The present study investigated the topography of RGD binding sites and integrin receptors in ischemic rat kidneys. Two RGD peptides were synthesized: a cyclic biotinylated (Bt) RGD peptide and a linear RGD peptide (GRGDSP) labeled with rhodamine green (RhoG). Rats were subjected to 45 minutes of renal artery occlusion kidneys were harvested at different times post-ischemia, and stained with RGD peptides and a panel of antibodies to integrins. In control, Bt-RGD staining was undetectable in alkaline phosphatase histochemistry, whereas immunofluorescence detection with Rho-streptavidin conjugate as well as RhoG-GRGDSP staining faintly decorated the basolateral aspect of the proximal tubular cells in a punctate fashion. In contrast, ischemic kidneys showed binding to the basolateral and apical aspects of proximal tubules, peritubular capillaries, and desquamated cells within tubular lumen. The most conspicuous staining of ischemic kidneys was obtained with antibodies to the beta 1 (labeling of the apical aspect of proximal and distal tubules, as well as desquamated cells obstructing tubular lumen) and the alpha V (glomeruli, tubular epithelia, intima of blood vessels stained faintly, while the obstructing cellular conglomerates showed intense staining) subunits. Double staining with Bt-RGD and antibodies against the beta 1 and alpha V beta 3 integrins showed co-localization of staining within the tubules and vasculature, respectively. In vitro attachment of HL-60 leukocytes to the endothelial cells was inhibited by the cyclic RGD peptide. In conclusion, expression of RGD binding sites and beta 1 integrin subunits along the apical aspect of tubular epithelia and on the surface of desquamated cells is in concert with the hypothesis on the pathogenetic role of RGD-recognizing integrins in tubular obstruction. The expression of RGD binding sites along the intimal surface of blood vessels in ischemic kidneys suggests an additional target for RGD peptides in vascular endothelial cells.

The platelet reactivity of synthetic peptides based on the collagen III fragment alpha1(III)CB4. Evidence for an integrin alpha2beta1 recognition site involving residues 522-528 of the alpha1(III) collagen chain

The platelet-reactive collagen III-derived fragment alpha1(III)CB4 has been synthesized as seven overlapping peptides, each as a homotrimeric triple-helical species covalently linked at the C terminus. Additional Gly-Pro-Hyp triplets were introduced at each end of the peptide sequence to ensure a stable triple-helical conformation at 20 degrees C, the temperature at which cell reactivity was measured. A Cys-containing triplet was included at each end to allow intermolecular cross-linking. All seven peptides in triple-helical, cross-linked form were able to cause platelet aggregation. Peptide 6, the most reactive species, was more aggregatory than collagen fibers. Platelet adhesion occurred to all peptides immobilized on plastic in monomeric form. Adhesion was integrin alpha2beta1-independent except in the case of peptide 6, adhesion to which was partially reduced by anti-integrin alpha2beta1 monoclonal antibodies. The presence of an alpha2beta1 recognition site in peptide 6 was confirmed using HT 1080 cells, which express alpha2beta1 as their major or sole collagen receptor. HT 1080 adhesion to both peptide 6 and collagen was strongly inhibited by anti-integrin alpha2beta1 monoclonal antibodies. These cells did not adhere to any of the other peptides. Comparison of the structure of peptide 6 with that of adjacent peptides indicates that the sequence Gly-Gly-Pro-Hyp-Gly-Pro-Arg, residues 522-528 of the collagen alpha1(III) chain, represents the minimum structure required for the recognition of alpha2beta1. Our findings support the view that the collagen triple helix possesses an intrinsic platelet reactivity that can be expressed independently of integrin alpha2beta1 and the precise level of which is governed by the exact nature of the primary sequence. Sequences such as those recognizing alpha2beta1 may potentiate the activity, whereas others may have the opposite effect.

Beta 1 integrins and osteoclast function: involvement in collagen recognition and bone resorption

The extracellular matrix of bone is composed mainly of type I collagen. In this report we studied the role and collagen-binding properties of osteoclast integrins (alpha v, alpha 2, beta 1, and beta 3). Cell adhesion assays with rat osteoclasts and affinity chromatography/SDS-PAGE analysis with purified human osteoclast membranes demonstrated adhesion of osteoclasts to native type I collagen in a divalent cation and Arg-Gly-Asp (RGD)-dependent way via alpha 2 beta 1 integrin, whereas osteoclast adhesion to denatured collagen predominantly involved alpha v beta 3. In receptor-binding assays, the involvement of human recombinant alpha v beta 3 in adhesion to denatured collagen was confirmed. Additionally, osteoclasts adhered to type I collagen fibers and to monomeric types II-V collagen with characteristics similar to those on native monomeric type I collagen. Osteoclastic bone resorption in vitro was inhibited (> 40%) in the presence of alpha 2 and beta 1 antibodies. Using scanning laser confocal microscopy, alpha v beta 3, alpha 2, and beta 1 integrin were detected within podosomes in nonresorbing osteoclasts and in the ruffled border area and basolateral membrane in resorbing osteoclasts, but not in the sealing zone of resorbing osteoclasts. These results demonstrate that alpha 2 beta 1, in addition to alpha v beta 3, has an important role in osteoclast function and acts as a receptor for native, but not denatured, collagen.

Structural requirements for alpha 1 beta 1 and alpha 2 beta 1 integrin mediated cell adhesion to collagen V

A large variety of cells adhere to and spread on specific regions within the triple helix of collagens, mainly via alpha 1 beta 1 and alpha 2 beta 1 integrins. Disruption of collagen triple helical integrity generally affects the efficiency of cell adhesion on different collagens including collagen V. This report addresses the question of the importance of the linear sequence of the constitutive alpha-chains versus the triple helical conformation in the recognition of collagen V binding sites. To investigate this question, in vitro renaturation of the isolated alpha 1 (V) and alpha 2 (V) chains was performed according to the annealing procedure and formation of the triple helix was monitored by rotary shadowing and by mild trypsin digestion followed by electrophoretic analysis. The results indicate that the alpha 1 (V) and alpha 2 (V) homotrimeric reassociation can occur up to a full-length triple helix but intermediate forms of 50–200 nm long rod-like segments are also observed. We have previously shown that alpha 1 beta 1 and alpha 2 beta 1 integrins, the major collagen receptors, are also involved in cell adhesion to native collagen V. Therefore we chose the following two different cell lines for this study: HT1080 (a human fibrosarcoma cell line) expressing alpha 2 beta 1 and HBL100 (a human mammary epithelial cell line) containing significant amounts of alpha 1 beta 1 and alpha 2 beta 1 integrins. We showed that both alpha 1 (V) and alpha 2(V) homotrimers induced cell adhesion but refolded alpha2(V) chains were more efficient and promoted cell adhesion as well as native collagen V. Thermal stability of refolded alpha-chains was monitored by adhesion promoting activity and showed that cell adhesion was dependent on triple helical conformation of the substrates. Adhesion in all cases was strongly Mg2+ and Mn(2+)-dependent and Ca2+ ions alone were ineffective. Antibodies against alpha 2 and beta 1 integrin subunits completely inhibited HT1080 cell adhesion to all substrates. Moreover, addition of cyclic RGD peptides, which had been shown to interact with alpha 2 beta 1, dramatically affected HT1080 cell adhesion to native collagen V and to the refolded alpha-chains. Antibody to beta 1 subunits abolished HBL100 cell adhesion to all substrates. A complete inhibition of HBL100 cell adhesion to native collagen V was achieved only by simultaneous addition of function-blocking specific monoclonal antibodies against alpha 1 and alpha 2 integrin subunits. However, only alpha 2 beta 1 was engaged obviously in HBL100 cell adhesion to refolded alpha-chains. These data indicate that triple helical conformation is particularly critical for alpha 2 beta 1- and alpha 1 beta 1-dependent adhesion and that the integrin alpha 2 beta 1 is a dominant functional receptor for refolded alpha-chains. We conclude that alpha 2 beta 1-dependent adhesion seems to involve multiple different conformational binding sites while alpha 1 beta 1-dependent adhesion is more restricted to the heterotrimeric native form of the molecule.

The binding of type I collagen to lymphocyte function-associated antigen (LFA) 1 integrin triggers the respiratory burst of human polymorphonuclear neutrophils. Role of calcium signaling and tyrosine phosphorylation of LFA 1

Monoclonal antibodies to the alpha L beta 2 integrin inhibit the binding of type I collagen to PMN (polymorphonuclear neutrophil leukocytes) as well as the subsequent stimulation of superoxide production and enzyme secretion-elicited by this collagen. Pepsinized collagen still binds PMN but no longer stimulates them. The I domain of the alpha chain of the integrin is involved in the binding. Two sequences of the alpha 1(I) polypeptide chain of collagen participate in the process. Experiments of competitive inhibition by synthetic peptides showed that the sequence RGD (915-917) is used for binding to the cells and DGGRYY (1034-1039) serves to stimulate PMN. Experiments of radioactive labeling of the cells and affinity chromatography on Sepharose-collagen confirmed the presence in PMN extracts of two proteins, 95 and 185 kDa, respectively, corresponding to the molecular weights of the beta 2 and alpha L chains of the integrin and recognized by their specific monoclonal antibodies. The transduction pathways depending on the alpha L beta 2 integrin do not involve a G protein (ruled out by the use of cholera and pertussis toxins), whereas the cytoskeleton was found to participate in the process, as evidenced by inhibition by cytochalasin B. After collagen stimulation, cytoplasmic inositol trisphosphate and calcium ion increased sharply for less than 2 min. The use of the inhibitors staurosporine and calphostin C demonstrated that protein kinase C was involved. Evaluation of the activity of this enzyme showed that, upon stimulation of PMN with collagen I, it was translocated to plasma membrane. Acrylamide gel electrophoresis of the protein bands corresponding to the integrin alpha L beta 2, followed by immunoblotting using monoclonal antibodies to phosphotyrosine, permitted us to demonstrate that, prior to stimulation by type I collagen, there was no phosphorylation, whereas after stimulation, both alpha L and beta 2 chains were stained by anti-phosphotyrosine antibodies. The adhesion of PMN to pepsinized type I collagen triggered tyrosine phosphorylation of the beta 2 chain of the integrin, without stimulating O2-. production by these cells, whereas their stimulation by complete type I collagen induced the tyrosine phosphorylation of both alpha L and beta 2 subunits. The tyrosine phosphorylation of both integrin subunits during transduction of stimuli is a heretofore undescribed phenomenon that may correspond to a new system of transmembrane communication.

The exchange of Arg-Gly-Asp (RGD) and Arg-Tyr-Asp (RYD) binding sequences in a recombinant murine Fab fragment specific for the integrin alpha IIb beta 3 does not alter integrin recognition

The murine monoclonal antibody OPG2 is an excellent paradigm of natural RGD ligands and binds specifically to alpha IIb beta 3 integrin. A reactive Arg103-Tyr104-Asp105 (RYD) tripeptide is located in an extended loop, the third complementarity-determining region of the heavy chain (H3). When compared to other RGD ligands, the RYD tripeptide of OPG2 is unique, in that the side chains are fixed in a stable orientation that we have defined by x-ray crystallography. In this study, we express OPG2 H chain segments (Fd) and kappa chains as components of active, Fab heterodimers by coinfection of Spodoptera frugiperda cell lines with recombinant baculoviruses containing cDNA specific for each protein. Recombinant AP7 Fd segments are generated from the parent OPG2 Fd segments by replacement of Tyr104 with Gly, while recombinant AP7E Fd segments are produced from AP7 Fd segments, by exchange of Asp105 with Glu. Neither the free Fd segments nor the free kappa chains of OPG2 or AP7 can bind to alpha IIb beta 3. The AP7 Fab fragment, like the parent OPG2 Fab, binds strongly to purified alpha IIb beta 3 but weakly, if at all, to purified alpha V beta 3. The affinity of OPG2 and AP7 Fab fragments for gel-filtered platelets, whether nonstimulated or activated by 0.2 microM phorbol 12-myristate 13-acetate, is identical. As with other natural RGD ligands, the binding of recombinant OPG2 Fab or AP7 Fab fragments to purified alpha IIb beta 3 or to gel-filtered platelets is completely inhibited by the peptide RGDW or by addition of EDTA, AP7E Fab fragments do not bind at all to either purified alpha IIb beta 3 or platelets. Our results demonstrate, for the first time within a natural protein ligand, that the tripeptides RGD and RYD exhibit equivalent binding capacity and specificity for the integrin alpha IIb beta 3.

Another look at collagen V and XI molecules

The fibrillar collagens are the most abundant proteins of extracellular matrices. Among them, collagens V and XI are quantitatively minor components which participate in the formation of the fibrillar collagen network. Since these collagens were discovered, studies have demonstrated that they may play a fundamental role in the control of fibrillogenesis, probably by forming a core within the fibrils. Another characteristic of these collagens is the partial retention of their N-propeptide extensions in tissue forms, an unusual observation in comparison to the other known fibrillar collagens. The tissue locations of collagens V and XI are different, but their structural and biological properties seem to be closely related. It has been shown that their primary structures are highly conserved at both the gene and protein levels, and that these conserved features are the bases of their similar biological properties. In particular, they are both resistant to mammalian collagenases, and surprisingly sensitive to trypsin treatment. Collagens V and XI are usually buried within the major collagen fibrils, although they have both cell adhesion and heparin binding sites which could be of crucial importance in physiological processes such as development and wound healing. It has became evident that several molecules are in fact heterotypic associations of chains from both collagens V and XI, demonstrating that these two collagens are not distinct types but a single type which can be called collagen V/XI.

Attachment of articular cartilage chondrocytes to the tissue form of type VI collagen

Type VI collagen is composed of a short triple helix rich in RGD sequences with globular domains at each extremity of the helix. Disulfide-bonded tetramers of the monomeric molecule associate non-covalently to form networks of microfibrils in connective tissues, including cartilage. The disulfide-bonded tetramer can be extracted with 6 M guanidine HCl and purified without pepsin digestion and is referred to here as the tissue form of type VI collagen. Type VI collagen in mature articular cartilage appears to be concentrated pericellularly. We undertook a systematic investigation using solid phase assays to establish the nature of the attachment of bovine articular cartilage chondrocytes to the intact, tissue form of bovine type VI collagen. The tissue form of type VI collagen was extracted from bovine meniscus cartilage with 6 M guanidine HCl and purified by polyethylene glycol precipitation. When equal molar quantities were coated on microwells, the tissue form of type VI collagen attached more cells than the pepsin-digested form of the molecule that lacked the globular domains. The attachment to the intact, tissue form was dose-dependent and saturable and was not inhibited by heparin or type II collagen. A linear GRGDSP peptide failed to inhibit attachment of the chondrocytes to the intact, tissue or pepsin-digested forms of type VI collagen, but totally inhibited the interaction when the intact molecule was reduced and alkylated. In contrast, a cyclic C*GRGDSPC* peptide inhibited attachment to the tissue form of type VI collagen, but not to fibronectin. The attachment had a metal ion dependence that could be satisfied by MnCl2, slightly less by MgCl2, but not at all by CaCl2. A direct interaction between the tissue form of type VI collagen and a chondrocyte cell surface receptor or receptors is a structural feature of the pericellular matrix in cartilage.