Trans-dominant inhibition of integrin function

Fetal and neonatal alloimmune thrombocytopenia: Current pathophysiological insights and perspectives for future diagnostics and treatment

FNAIT is a pregnancy-associated condition caused by maternal alloantibodies against paternally-inherited platelet antigens, most frequently HPA-1a on integrin β3. The clinical effects range from no symptoms to fatal intracranial hemorrhage, but underlying pathophysiological determinants are poorly understood. Accumulating evidence suggests that differential antibody-Fc-glycosylation, activation of complement/effector cells, and integrin function-blocking effects contribute to clinical outcome. Furthermore, some antibodies preferentially bind platelet integrin αIIbβ3, but others bind αvβ3 on endothelial cells and trophoblasts. Defects in endothelial cells and angiogenesis may therefore contribute to severe anti-HPA-1a associated FNAIT. Moreover, anti-HPA-1a antibodies may cause placental damage, leading to intrauterine growth restriction. We discuss current insights into diversity and actions of HPA-1a antibodies, gathered from clinical studies, in vitro studies, and mouse models. Assessment of all factors determining severity and progression of anti-HPA-1a-associated FNAIT may importantly improve risk stratification and potentially reveal novel treatment strategies, both for FNAIT and other immunohematological disorders.

Regulation of Dynamic Cell Adhesion by Integrin-Integrin Crosstalk

Most cells express several integrins. The integrins are able to respond to various cellular functions and needs by modifying their own activation state, but in addition by their ability to regulate each other by activation or inhibition. This crosstalk or transdominant regulation is strictly controlled. The mechanisms resulting in integrin crosstalk are incompletely understood, but they often involve intracellular signalling routes also used by other cell surface receptors. Several studies show that the integrin cytoplasmic tails bind to a number of cytoskeletal and adaptor molecules in a regulated manner. Recent work has shown that phosphorylations of integrins and key intracellular molecules are of pivotal importance in integrin-cytoplasmic interactions, and these in turn affect integrin activity and crosstalk. The integrin β-chains play a central role in regulating crosstalk. In addition to Integrin-integrin crosstalk, crosstalk may also occur between integrins and related receptors, including other adhesion receptors, growth factor and SARS-CoV-2 receptors.

Integrin Crosstalk and Its Effect on the Biological Functions of the Trabecular Meshwork/Schlemm’s Canal

Integrins are a family of heterodimeric receptors composed of an α- and β-subunit that mediate cell-adhesion to a number of extracellular matrix (ECM) proteins in the Trabecular Meshwork/Schlemm’s canal (TM/SC) of the eye. Upon binding an ECM ligand, integrins transmit signals that activate a number of signaling pathways responsible for regulating actin-mediated processes (i.e phagocytosis, cell contractility, and fibronectin fibrillogenesis) that play an important role in regulating intraocular pressure (IOP) and may be involved in glaucoma. An important function of integrin-mediated signaling events is that the activity of one integrin can affect the activity of other integrins in the same cell. This creates a crosstalk that allows TM/SC cells to respond to changes in the ECM presumably induced by the mechanical forces on the TM/SC, aging and disease. In this review, we discuss how integrin crosstalk influences the function of the human TM/SC pathway. In particular, we will discuss how different crosstalk pathways mediated by either the αvβ3 or α4β1 integrins can play opposing roles in the TM when active and therefore act as on/off switches to modulate the cytoskeleton-mediated processes that regulate the outflow of aqueous humor through the TM/SC.

CDH6-activated αIIbβ3 crosstalks with α2β1 to trigger cellular adhesion and invasion in metastatic ovarian and renal cancers

Cadherin 6 (CDH6) is significantly overexpressed in advanced ovarian and renal cancers. However, the role of CDH6 in cancer metastasis is largely unclear. Here, we investigated the impact of CDH6 expression on integrin-mediated metastatic progression. CDH6 preferentially bound to αIIbβ3 integrin, a platelet receptor scarcely expressed in cancer cells, and this interaction was mediated through the cadherin Arginine-glycine-aspartic acid (RGD) motif. Furthermore, CDH6 and CDH17 were found to interact with α2β1 in αIIbβ3low cells. Transient silencing of CDH6, ITGA2B, or ITGB3 genes caused a significant loss of proliferation, adhesion, invasion, and lung colonization through the downregulation of SRC, FAK, AKT, and ERK signaling. In ovarian and renal cancer cells, integrin αIIbβ3 activation appears to be a prerequisite for proper α2β1 activation. Interaction of αIIbβ3 with CDH6, and subsequent αIIbβ3 activation, promoted activation of α2β1 and cell adhesion in ovarian and renal cancer cells. Additionally, monoclonal antibodies specific to the cadherin RGD motif and clinically approved αIIbβ3 inhibitors could block pro-metastatic activity in ovarian and renal tumors. In summary, the interaction between CDH6 and αIIbβ3 regulates α2β1-mediated adhesion and invasion of ovarian and renal cancer metastatic cells and constitutes a therapeutic target of broad potential for treating metastatic progression.

Integrin: Basement membrane adhesion by corneal epithelial and endothelial cells

Integrins mediate adhesion of cells to substrates and maintain tissue integrity by facilitating mechanotransduction between cells, the extracellular matrix, and gene expression in the nucleus. Changes in integrin expression in corneal epithelial cells and corneal endothelial cells impacts their adhesion to the epithelial basement membrane (EpBM) and Descemet's membrane, respectively. Integrins also play roles in assembly of basement membranes by both activating TGFβ1 and other growth factors. Over the past two decades, this knowledge has been translated into methods to grow corneal epithelial and endothelial cells in vitro for transplantation in the clinic thereby transforming clinical practice and quality of life for patients. Current knowledge on the expression and function of the integrins that mediate adhesion to the basement membrane expressed by corneal epithelial and endothelial cells in health and disease is summarized. This is the first review to discuss similarities and differences in the integrins expressed by both cell types.

Integrin Crosstalk Contributes to the Complexity of Signalling and Unpredictable Cancer Cell Fates

Integrins are heterodimeric cell surface receptors composed of α and β subunits that control adhesion, proliferation and gene expression. The integrin heterodimer binding to ligand reorganises the cytoskeletal networks and triggers multiple signalling pathways that can cause changes in cell cycle, proliferation, differentiation, survival and motility. In addition, integrins have been identified as targets for many different diseases, including cancer. Integrin crosstalk is a mechanism by which a change in the expression of a certain integrin subunit or the activation of an integrin heterodimer may interfere with the expression and/or activation of other integrin subunit(s) in the very same cell. Here, we review the evidence for integrin crosstalk in a range of cellular systems, with a particular emphasis on cancer. We describe the molecular mechanisms of integrin crosstalk, the effects of cell fate determination, and the contribution of crosstalk to therapeutic outcomes. Our intention is to raise awareness of integrin crosstalk events such that the contribution of the phenomenon can be taken into account when researching the biological or pathophysiological roles of integrins.

Analyzing Integrin-Dependent Adhesion

In this unit, methods for the analysis of integrin-dependent adhesion are described. Two major types of assays are commonly used for this analysis. The first are cell adhesion assays. A key application of this type of assay is to identify which integrin(s) mediate cell-substrate interactions; a comprehensive list of antibodies suitable for this purpose is detailed. The second are solid-phase assays in which purified integrins and integrin ligands are used. These assays can be used, e.g., to measure apparent affinities of integrins for different ligands and IC₅₀ values of pharmacological inhibitors. Curr. Protoc. Cell Biol. 53:9.4.1-9.4.17. © 2018 by John Wiley & Sons, Inc.

Beyond adhesion: emerging roles for integrins in control of the tumor microenvironment

While integrins were originally discovered as cell adhesion receptors, recent studies have reinforced the concept that integrins have central roles in cancer that extend far beyond controlling cell adhesion and migration. Indeed, as transmembrane cell surface receptors that occupy a critical position at the interface of cellular and extracellular interactions and are capable of both "inside-out" and "outside-in" signaling, integrins are uniquely poised to regulate the cell's ability to promote, sense, and react to changes in the tumor microenvironment. Moreover, integrins are present on all cell types in the tumor microenvironment, and they have important roles in regulating intercellular communication. Decades of promising pre-clinical studies have implicated certain integrins as attractive therapeutic targets in the cancer clinic. Nevertheless, results of the few clinical trials that target integrins in cancer have thus far been disappointing. Importantly, these clinical failures likely reflect the emerging complexity of individual and combinatorial integrin function within both tumor cells and other cell types of the tumor microenvironment, together with a need to explore integrin-targeting agents not just as monotherapies but also as adjuvants to more conventional radiotherapies or chemotherapies. In this review, we will examine recent advances toward understanding how integrins regulate cancer progression, including their roles in intercellular communication and modulation of the tumor microenvironment. Additionally, we will discuss factors that underlie the limited efficacy of current efforts to target integrins in the cancer clinic as well as potential strategies to overcome these challenges.

Suppression of integrin α3β1 by α9β1 in the epidermis controls the paracrine resolution of wound angiogenesis

The development of novel therapies to promote wound healing is hindered by our poor understanding of how different integrins function together in the epidermis. Longmate et al. show that cross-suppression by integrins within the epidermis controls paracrine signals that regulate wound angiogenesis. Integrin α9β1 suppresses the proangiogenic functions of α3β1 during late-stage wound healing, leading to the normalization of blood vessel density in the wound bed.

Development of wound therapies is hindered by poor understanding of combinatorial integrin function in the epidermis. In this study, we generated mice with epidermis-specific deletion of α3β1, α9β1, or both integrins as well as keratinocyte lines expressing these integrin combinations. Consistent with proangiogenic roles for α3β1, α3-null keratinocytes showed reduced paracrine stimulation of endothelial cell migration and survival, and wounds of epidermis-specific α3 knockout mice displayed impaired angiogenesis. Interestingly, α9β1 in keratinocytes suppressed α3β1-mediated stimulation of endothelial cells, and wounds of epidermis-specific α9 knockout mice displayed delayed vascular normalization and reduced endothelial apoptosis, indicating that α9β1 cross-suppresses α3β1 proangiogenic functions. Moreover, α9β1 inhibited α3β1 signaling downstream of focal adhesion kinase (FAK) autoactivation at the point of Src-mediated phosphorylation of FAK Y861/Y925. Finally, α9β1 cross-suppressed many α3β1-dependent genes, including the gene that encodes MMP-9, which we implicated as a regulator of integrin-dependent cross talk to endothelial cells. Our findings identify a novel physiological context for combinatorial integrin signaling, laying the foundation for therapeutic strategies that manipulate α9β1 and/or α3β1 during wound healing.

Neutrophil Integrins and Matrix Ligands and NET Release

Neutrophils are motile and responsive to tissue injury and infection. As neutrophils emigrate from the bloodstream and migrate toward a site of affliction, they encounter the tissue extracellular matrix (ECM) and thereby engage integrins. Our laboratory studies the neutrophilic response to the fungal pathogen Candida albicans either in the filamentous state of the microbe or to the purified pathogen-associated molecular pattern, β-glucan. We have gained an appreciation for the role of integrins in regulating the neutrophil anti-Candida response and how the presence or absence of ECM can drive experimental outcome. The β2 integrin CR3 (complement receptor 3; αMβ2; Mac-1; CD11b/CD18) plays an important role in fungal recognition by its ability to bind β-glucan at a unique lectin-like domain. The presence of ECM differentially regulates essential neutrophil anti-fungal functions, including chemotaxis, respiratory burst, homotypic aggregation, and the release of neutrophil extracellular traps (NETs). We have shown that NET release to C. albicans hyphae or immobilized β-glucan occurs rapidly and without the requirement for respiratory burst on ECM. This is in contrast to the more frequently reported mechanisms of NETosis to other pathogens without the context of ECM, which occur after a prolonged lag period and require respiratory burst. As expected for an ECM-dependent phenotype, NETosis and other neutrophil functions are dependent on specific integrins. The focus of this review is the role of ECM ligation by neutrophil integrins as it pertains to host defense functions with an emphasis on lessons we have learned studying the anti-Candida response of human neutrophils.

Targeted inactivation of β1 integrin induces β3 integrin switching, which drives breast cancer metastasis by TGF-β

Chemotherapeutic targeting of β1 integrin has been proposed as a way to alleviate breast cancer metastasis. It is shown here that inactivation of β1 integrin elicits compensatory expression of β3 integrin, which rescues mammary tumor growth and metastasis, as well as promoting oncogenic TGF-β signaling in late-stage breast cancer.

Mammary tumorigenesis and epithelial–mesenchymal transition (EMT) programs cooperate in converting transforming growth factor-β (TGF-β) from a suppressor to a promoter of breast cancer metastasis. Although previous reports associated β1 and β3 integrins with TGF-β stimulation of EMT and metastasis, the functional interplay and plasticity exhibited by these adhesion molecules in shaping the oncogenic activities of TGF-β remain unknown. We demonstrate that inactivation of β1 integrin impairs TGF-β from stimulating the motility of normal and malignant mammary epithelial cells (MECs) and elicits robust compensatory expression of β3 integrin solely in malignant MECs, but not in their normal counterparts. Compensatory β3 integrin expression also 1) enhances the growth of malignant MECs in rigid and compliant three-dimensional organotypic cultures and 2) restores the induction of the EMT phenotypes by TGF-β. Of importance, compensatory expression of β3 integrin rescues the growth and pulmonary metastasis of β1 integrin–deficient 4T1 tumors in mice, a process that is prevented by genetic depletion or functional inactivation of β3 integrin. Collectively our findings demonstrate that inactivation of β1 integrin elicits metastatic progression via a β3 integrin–specific mechanism, indicating that dual β1 and β3 integrin targeting is necessary to alleviate metastatic disease in breast cancer patients.

Mechanisms of talin-dependent integrin signaling and crosstalk

Cells undergo dynamic remodeling of the cytoskeleton during adhesion and migration on various extracellular matrix (ECM) substrates in response to physiological and pathological cues. The major mediators of such cellular responses are the heterodimeric adhesion receptors, the integrins. Extracellular or intracellular signals emanating from different signaling cascades cause inside-out signaling of integrins via talin, a cystokeletal protein that links integrins to the actin cytoskeleton. Various integrin subfamilies communicate with each other and growth factor receptors under diverse cellular contexts to facilitate or inhibit various integrin-mediated functions. Since talin is an essential mediator of integrin activation, much of the integrin crosstalk would therefore be influenced by talin. However, despite the existence of an extensive body of knowledge on the role of talin in integrin activation and as a stabilizer of ECM-actin linkage, information on its role in regulating inter-integrin communication is limited. This review will focus on the structure of talin, its regulation of integrin activation and discuss its potential role in integrin crosstalk. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Loss of epithelial hypoxia-inducible factor prolyl hydroxylase 2 accelerates skin wound healing in mice

Skin wound healing in mammals is a complex, multicellular process that depends on the precise supply of oxygen. Hypoxia-inducible factor (HIF) prolyl hydroxylase 2 (PHD2) serves as a crucial oxygen sensor and may therefore play an important role during reepithelialization. Hence, this study was aimed at understanding the role of PHD2 in cutaneous wound healing using different lines of conditionally deficient mice specifically lacking PHD2 in inflammatory, vascular, or epidermal cells. Interestingly, PHD2 deficiency only in keratinocytes and not in myeloid or endothelial cells was found to lead to faster wound closure, which involved enhanced migration of the hyperproliferating epithelium. We demonstrate that this effect relies on the unique expression of β3-integrin in the keratinocytes around the tip of the migrating tongue in an HIF1α-dependent manner. Furthermore, we show enhanced proliferation of these cells in the stratum basale, which is directly related to their attenuated transforming growth factor β signaling. Thus, loss of the central oxygen sensor PHD2 in keratinocytes stimulates wound closure by prompting skin epithelial cells to migrate and proliferate. Inhibition of PHD2 could therefore offer novel therapeutic opportunities for the local treatment of cutaneous wounds.

Integrin-mediated cell-matrix interaction in physiological and pathological blood vessel formation

Physiological as well as pathological blood vessel formation are fundamentally dependent on cell-matrix interaction. Integrins, a family of major cell adhesion receptors, play a pivotal role in development, maintenance, and remodeling of the vasculature. Cell migration, invasion, and remodeling of the extracellular matrix (ECM) are integrin-regulated processes, and the expression of certain integrins also correlates with tumor progression. Recent advances in the understanding of how integrins are involved in the regulation of blood vessel formation and remodeling during tumor progression are highlighted. The increasing knowledge of integrin function at the molecular level, together with the growing repertoire of integrin inhibitors which allow their selective pharmacological manipulation, makes integrins suited as potential diagnostic markers and therapeutic targets.

Why integrin as a primary target for imaging and therapy

Integrin-mediated cell adhesion is involved in many essential normal cellular and pathological functions including cell survival, growth, differentiation, migration, inflammatory responses, platelet aggregation, tissue repair and tumor invasion. 24 different heterodimerized transmembrane integrin receptors are combined from 18 different α and 8 different β subunits. Each integrin subunit contains a large extracellular domain, a single transmembrane domain and a usually short cytoplasmic domain. Integrins bind extracellular matrix (ECM) proteins through their large extracellular domain, and engage the cytoskeleton via their short cytoplasmic tails. These integrin-mediated linkages on either side of the plasma membrane are dynamically linked. Thus, integrins communicate over the plasma membrane in both directions, i.e., outside-in and inside-out signaling. In outside-in signaling through integrins, conformational changes of integrin induced by ligand binding on the extracellular domain altered the cytoplasmic domain structures to elicit various intracellular signaling pathways. Inside-out signaling originates from non-integrin cell surface receptors or cytoplasmic molecules and it activates signaling pathways inside the cells, ultimately resulting in the activation/deactivation of integrins. Integrins are one of key family proteins for cell adhesion regulation through binding to a large number of ECM molecules and cell membrane proteins. Lack of expression of integrins may result in a wide variety of effects ranging from blockage in pre-implantation to embryonic or perinatal lethality and developmental defects. Based on both the key role they played in angiogenesis, leukocytes function and tumor development and easy accessibility as cell surface receptors interacting with extracellular ligands, the integrin superfamily represents the best opportunity of targeting both antibodies and small-molecule antagonists for both therapeutic and diagnostic utility in various key diseases so far.

Regulation of integrin activation

Regulation of cell-cell and cell-matrix interaction is essential for the normal physiology of metazoans and is important in many diseases. Integrin adhesion receptors can rapidly increase their affinity (integrin activation) in response to intracellular signaling events in a process termed inside-out signaling. The transmembrane domains of integrins and their interactions with the membrane are important in inside-out signaling. Moreover, integrin activation is tightly regulated by a complex network of signaling pathways. Here, we review recent progress in understanding how the membrane environment can, in cooperation with integrin-binding proteins, regulate integrin activation.

Molecular architecture and function of matrix adhesions

Cell adhesions mediate important bidirectional interactions between cells and the extracellular matrix. They provide an interactive interface between the extracellular chemical and physical environment and the cellular scaffolding and signaling machinery. This dynamic, reciprocal regulation of intracellular processes and the matrix is mediated by membrane receptors such as the integrins, as well as many other components that comprise the adhesome. Adhesome constituents assemble themselves into different types of cell adhesion structures that vary in molecular complexity and change over time. These cell adhesions play crucial roles in cell migration, proliferation, and determination of cell fate.

Integrin-linked kinase regulates integrin signaling in human trabecular meshwork cells

PURPOSE

To determine whether integrin-linked kinase (ILK) controls the organization of the actin cytoskeleton in the trabecular meshwork (TM) by regulating integrin co-signaling.

METHODS

The cell binding domain and the Heparin II (Hep II) domain of fibronectin were used to activate α5β1 and α4β1 integrin signaling, respectively, in differentiated human TM (HTM) cells. The role of ILK was determined using either ILK small interfering RNA (siRNA) to knockout ILK expression or the ILK inhibitors, KP392 and QLT0267. The knockdown of ILK expression was verified by Western blot analysis. The presence of actin stress fibers and focal adhesions was determined by labeling cultures with phalloidin and anti-talin or ILK antibodies, respectively.

RESULTS

Cell spreading in differentiated HTM cells required ILK, since ILK siRNA and the ILK inhibitors significantly reduced cell spreading, actin polymerization, and the localization of talin and ILK in focal adhesions (FAs). Both cell spreading and the localization of talin and ILK to FAs in differentiated HTM cells could be rescued by inducing α4β1 integrin signaling with a recombinant Hep II domain of fibronectin, even though α4β1 integrins were not found in FAs. In the absence of ILK inhibition, the Hep II domain had minimal effect on α5β1 integrin-mediated spreading.

CONCLUSIONS

This study suggests that cooperative α5β1/α4β1 integrin signaling may be regulated by ILK trans-dominantly and that alterations in ILK activity may affect actin cytoskeleton organization and contractility in the TM.

Stimulated single-cell force spectroscopy to quantify cell adhesion receptor crosstalk

To control their attachment to substrates and other cells, cells regulate their adhesion receptors. One regulatory process is receptor crosstalk, where the binding of one type of cell adhesion molecule influences the activity of another type. To identify such crosstalk and gain insight into their mechanisms, we developed the stimulated single-cell force spectroscopy assay. In this assay, the influence of a cells adhesion to one substrate on the strength of its adhesion to a second substrate is examined. The assay quantifies the adhesion of the cell and the contributions of specific adhesion receptors. This allows mechanisms by which the adhesion is regulated to be determined. Using the assay we identified crosstalk between collagen-binding integrin alpha(1)beta(1) and fibronectin-binding integrin alpha(5)beta(1) in HeLa cells. This crosstalk was unidirectional, from integrin alpha(1)beta(1) to integrin alpha(5)beta(1), and functioned by regulating the endocytosis of integrin alpha(5)beta(1). The single-cell assay should be expandable for the screening and quantification of crosstalk between various cell adhesion molecules and other cell surface receptors.

Structure of a double ubiquitin-like domain in the talin head: a role in integrin activation

Talin is a 270-kDa protein that activates integrins and couples them to cytoskeletal actin. Talin contains an N-terminal FERM domain comprised of F1, F2 and F3 domains, but it is atypical in that F1 contains a large insert and is preceded by an extra domain F0. Although F3 contains the binding site for beta-integrin tails, F0 and F1 are also required for activation of beta1-integrins. Here, we report the solution structures of F0, F1 and of the F0F1 double domain. Both F0 and F1 have ubiquitin-like folds joined in a novel fixed orientation by an extensive charged interface. The F1 insert forms a loop with helical propensity, and basic residues predicted to reside on one surface of the helix are required for binding to acidic phospholipids and for talin-mediated activation of beta1-integrins. This and the fact that basic residues on F2 and F3 are also essential for integrin activation suggest that extensive interactions between the talin FERM domain and acidic membrane phospholipids are required to orientate the FERM domain such that it can activate integrins.

Transdominant regulation of integrin function: mechanisms of crosstalk

Transdominant inhibition of integrins or integrin-integrin crosstalk is an important regulator of integrin ligand binding and subsequent signaling events that control a variety of cell functions in many tissues. Here we discuss examples of integrin crosstalk and detail our current understanding of the molecular mechanisms that are involved in this receptor phenomenon. The cytoskeleton associated protein talin is a key regulator of integrin crosstalk. We describe how the interaction of talin and the cytoplasmic tail of beta integrin is controlled and how competitive inhibitors of this binding play a role in integrin crosstalk. We conclude with a discussion of how integrin crosstalk impacts the interpretation of integrin inhibitor and knockdown studies in both the laboratory and clinical setting.

Accessibility to the Fibronectin Synergy Site in a 3D Matrix Regulates Engagement of α 5β 1 versus α vβ 3 Integrin Receptors

Cell adhesion and migration on fibronectin (FN) extracellular matrix are mediated by integrin receptors. Integrins alpha5beta1 and alphavbeta3 require the RGD cell-binding sequence in FN, but alpha5beta1 also requires the nearby synergy site for maximal binding. In this study, we investigated how differences in the numbers of RGD or synergy sites within a three-dimensional (3D) FN-rich matrix influence cell adhesion and migration. CHO cell adhesion, spreading, and migration were reduced on 3D chimeric matrix containing FN lacking RGD (FN(RGD-)). Incorporation of FN with mutation of the synergy site (FN(syn-)), however, resulted in selective usage of integrins. CHO cells expressing alpha5beta1 showed decreased interactions with FN(syn-) chimeric matrix. In contrast, the presence of FN(syn-) had no effect on CHOalphavbeta3 cell migration. Interestingly, CHOalpha5/alphavbeta3 cells expressing both integrins selectively used alpha5beta1 for migration on wild type FN matrix but preferred alphavbeta3 for migration on FN(syn-) chimeric matrix. Thus sequestration or exposure of the FN synergy site within a 3D matrix may represent a novel mechanism for regulating cell functions through differential usage of integrin receptors. [Supplementary materials are available for this article. Go to the publisher's online edition of Cell Communication and Adhesion for the following free supplemental resource: a video recording shows migration of HT1080 cells on 3D matrix. HT1080 cells were allowed to attach to the matrix in serum-free DMEM for 2 h. FBS was then added to the medium to a final concentration of 10% and video recording was started. Images were taken every 5 min for 2 h. The video plays at 6 frames/s.].

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

The structure of the integrin alphaIIbbeta3 transmembrane complex explains integrin transmembrane signalling

Heterodimeric integrin adhesion receptors regulate cell migration, survival and differentiation in metazoa by communicating signals bi-directionally across the plasma membrane. Protein engineering and mutagenesis studies have suggested that the dissociation of a complex formed by the single-pass transmembrane (TM) segments of the alpha and beta subunits is central to these signalling events. Here, we report the structure of the integrin alphaIIbbeta3 TM complex, structure-based site-directed mutagenesis and lipid embedding estimates to reveal the structural event that underlies the transition from associated to dissociated states, that is, TM signalling. The complex is stabilized by glycine-packing mediated TM helix crossing within the extracellular membrane leaflet, and by unique hydrophobic and electrostatic bridges in the intracellular leaflet that mediate an unusual, asymmetric association of the 24- and 29-residue alphaIIb and beta3 TM helices. The structurally unique, highly conserved integrin alphaIIbbeta3 TM complex rationalizes bi-directional signalling and represents the first structure of a heterodimeric TM receptor complex.

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Clustering and occupancy of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin alpha and beta transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, we develop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin alpha(IIb)beta(3) TMD tails via specific TMD interactions in mammalian cell membranes in lipid bicelles. Furthermore, charge reversal mutations at alpha(IIb)(R995)beta(3)(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the beta(3) tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin alpha(IIb)beta(3) TMD tails in mammalian cell membranes and document their role in integrin signaling.

The integrin antagonist cilengitide activates alphaVbeta3, disrupts VE-cadherin localization at cell junctions and enhances permeability in endothelial cells

Cilengitide is a high-affinity cyclic pentapeptdic αV integrin antagonist previously reported to suppress angiogenesis by inducing anoikis of endothelial cells adhering through αVβ3/αVβ5 integrins. Angiogenic endothelial cells express multiple integrins, in particular those of the β1 family, and little is known on the effect of cilengitide on endothelial cells expressing αVβ3 but adhering through β1 integrins. Through morphological, biochemical, pharmacological and functional approaches we investigated the effect of cilengitide on αVβ3-expressing human umbilical vein endothelial cells (HUVEC) cultured on the β1 ligands fibronectin and collagen I. We show that cilengitide activated cell surface αVβ3, stimulated phosphorylation of FAK (Y³⁹⁷ and Y^576/577), Src (S⁴¹⁸) and VE-cadherin (Y⁶⁵⁸ and Y⁷³¹), redistributed αVβ3 at the cell periphery, caused disappearance of VE-cadherin from cellular junctions, increased the permeability of HUVEC monolayers and detached HUVEC adhering on low-density β1 integrin ligands. Pharmacological inhibition of Src kinase activity fully prevented cilengitide-induced phosphorylation of Src, FAK and VE-cadherin, and redistribution of αVβ3 and VE-cadherin and partially prevented increased permeability, but did not prevent HUVEC detachment from low-density matrices. Taken together, these observations reveal a previously unreported effect of cilengitide on endothelial cells namely its ability to elicit signaling events disrupting VE-cadherin localization at cellular contacts and to increase endothelial monolayer permeability. These effects are potentially relevant to the clinical use of cilengitide as anticancer agent.

Integrin alpha1beta1 is involved in the differentiation into myofibroblasts in adult reactive tissues in vivo

Connective tissue cell activation is of importance during reactive conditions such as solid tumour growth, wound healing and pannus formation in rheumatoid arthritis. Here, we have compared connective tissue cells of mesenchymal origin in human tissues from these conditions and their normal counterparts using a panel of cell-type-specific markers. In particular, we investigated variations of integrin expression among connective tissue cell phenotypes. Connective tissue cell populations were defined based on their association with the microvasculature and their expression of activation markers. The phenotype of these cells varied according to the type of pathological connective tissue examined. Our morphological data from human tissues suggested that the alpha(1)beta(1) integrin, a collagen/laminin receptor, is involved in the differentiation of precursor cells into myofibroblasts. To mechanistically investigate this hypothesis, we employed experimental models for carcinoma growth and wound healing utilizing alpha(1) integrin-deficient mice. The data confirmed that the alpha(1)beta(1) integrin is of importance not only for the differentiation of mesenchymal cells into myofibroblasts but also for the neovascularization and connective tissue organization and emphasize the importance of myofibroblasts in the pathophysiology of tissue repair, inflammation and tumour growth.

Integrins

Angiogenesis, the formation of new blood vessels from preexisting vasculature, contributes to the pathogenesis of many disorders, including ischemic diseases and cancer. Integrins are cell adhesion molecules that are expressed on the surface of endothelial cells and pericytes, making them potential targets for antiangiogenic therapy. Here we review the contribution of endothelial and mural cell integrins to angiogenesis and highlight their potential as antiangiogenesis targets.

Integrin cross-talk in endothelial cells is regulated by protein kinase A and protein phosphatase 1

In endothelial cells (ECs) beta1 integrin function-blocking antibodies inhibit alphavbeta3 integrin-mediated adhesion to a recombinant alpha4-laminin fragment (ralpha4LN fragment). beta1 integrin sequestration of talin is not the mechanism by which beta1 integrin modulates alphavbeta3 integrin ligand binding. Rather, treatment of the ECs with beta1 integrin function-blocking antibodies enhances cAMP-dependent protein kinase (PKA) activity and increases beta3 integrin serine phosphorylation. The PKA inhibitor H-89 abrogates the effect of beta1 integrin function-blocking antibodies on beta3 integrin serine phosphorylation and EC-ralpha4LN fragment binding. beta3 integrin contains a serine residue at position 752. To confirm the importance of this residue in alphavbeta3 integrin-ralpha4LN fragment binding, we mutated it to alanine (beta3S752A) or aspartic acid (beta3S752D). Chinese hamster ovary (CHO) cells expressing wild type or beta3S752A integrin attach robustly to ligand. CHO cells expressing beta3S752D integrin do not. Because the beta3 cytoplasmic tail lacks a PKA consensus site, it is unlikely that PKA acts directly on beta3 integrin. Instead, we have tested an hypothesis that PKA regulates beta3 integrin serine phosphorylation indirectly through phosphorylation of inhibitor-1, which, when phosphorylated, inhibits protein phosphatase 1 (PP1). Treatment of ECs with beta1 integrin function-blocking antibodies significantly increases phosphorylation of inhibitor-1. Furthermore, blocking PP1 activity pharmacologically inhibits alphavbeta3-mediated cell adhesion to the ralpha4LN fragment when both PKA and beta1 integrin function are inhibited. Concomitantly, there is an increase in serine phosphorylation of the beta3 integrin cytoplasmic tail. These results indicate a novel mechanism by which beta1 integrin negatively modulates alphavbeta3 integrin-ligand binding via activation of PKA and inhibition of PP1 activity.

alphavbeta3 integrin and angiogenesis: a moody integrin in a changing environment

In the adult, angiogenesis, the formation of new blood vessels from pre-existing vasculature contributes to the pathogenesis of many disorders including cancer. The role of adhesion molecules, especially integrins, in pathological angiogenesis has long been the subject of investigation, mostly because of their potential as anti-angiogenic targets. Recent studies have highlighted the complexities connected with understanding the roles of one particular integrin, alphavbeta3, in neovascularization. This integrin is notoriously promiscuous and its precise functions in angiogenesis are unclear. Here, I have firstly summarized some of the salient features of the roles played by alphavbeta3 during angiogenesis; secondly attempted to address the apparently conflicting issues surrounding this topic; and finally raised some questions that appear to be unanswered.

Structure of the integrin beta3 transmembrane segment in phospholipid bicelles and detergent micelles

Integrin adhesion receptors transduce bidirectional signals across the plasma membrane, with the integrin transmembrane domains acting as conduits in this process. Here, we report the first high-resolution structure of an integrin transmembrane domain. To assess the influence of the membrane model system, structure determinations of the beta3 integrin transmembrane segment and flanking sequences were carried out in both phospholipid bicelles and detergent micelles. In bicelles, a 30-residue linear alpha-helix, encompassing residues I693-H772, is adopted, of which I693-I721 appear embedded in the hydrophobic bicelle core. This relatively long transmembrane helix implies a pronounced helix tilt within a typical lipid bilayer, which facilitates the snorkeling of K716's charged side chain out of the lipid core while simultaneously immersing hydrophobic L717-I721 in the membrane. A shortening of bicelle lipid hydrocarbon tails does not lead to the transfer of L717-I721 into the aqueous phase, suggesting that the reported embedding represents the preferred beta3 state. The nature of the lipid headgroup affected only the intracellular part of the transmembrane helix, indicating that an asymmetric lipid distribution is not required for studying the beta3 transmembrane segment. In the micelle, residues L717-I721 are also embedded but deviate from linear alpha-helical conformation in contrast to I693-K716, which closely resemble the bicelle structure.

Analyzing integrin-dependent adhesion

This unit describes methods for the analysis of integrin-ligand binding in both cell-based assays and solid-phase assays. A major application of cell adhesion assays is in investigating whether a certain cell type can adhere to a specific adhesive substrate, and, if so, which receptors are involved. Particularly if the substrate is a matrix component (e.g., fibronectin), members of the integrin family are likely to play a dominant role in adhesion. Procedures are described here for assessing which integrins are involved in this process. A detailed analysis of ligand recognition by individual integrins can be performed using a solid-phase receptor-ligand binding assay. The unit also contains support protocols for integrin purification and coupling of antibodies to Sepharose for use in this purification, as well as for biotinylation of integrin ligands to be used in the solid-phase assay.

The N-terminal domains of talin cooperate with the phosphotyrosine binding-like domain to activate beta1 and beta3 integrins

The activation of integrin adhesion receptors from low to high affinity in response to intracellular cues controls cell adhesion and signaling. Binding of the cytoskeletal protein talin to the beta3 integrin cytoplasmic tail is required for beta3 activation, and the integrin-binding PTB-like F3 domain of talin is sufficient to activate beta3 integrins. Here we report that, whereas the conserved talin-integrin interaction is also required for beta1 activation, and talin F3 binds beta1 and beta3 integrins with comparable affinity, expression of the talin F3 domain is not sufficient to activate beta1 integrins. beta1 integrin activation could, however, be detected following expression of larger talin fragments that included the N-terminal and F1 domains, and mutagenesis indicates that these domains cooperate with talin F3 to mediate beta1 activation. This effect is not due to increased affinity for the integrin beta tail and we hypothesize that the N-terminal domains function by targeting or orienting talin in such a way as to optimize the interaction with the integrin tail. Analysis of beta3 integrin activation indicates that inclusion of the N-terminal and F1 domains also enhances F3-mediated beta3 activation. Our results therefore reveal a role for the N-terminal and F1 domains of talin during integrin activation and highlight differences in talin-mediated activation of beta1 and beta3 integrins.

Three-dimensional microenvironments modulate fibroblast signaling responses

Modes of signaling in fibroblasts can differ substantially depending on whether these cells are in their natural three-dimensional environment compared to artificial two-dimensional culture conditions. Although studying cell behavior in two-dimensional environments has been valuable for understanding biological processes, questions can be raised about their in vivo physiological relevance. This review focuses on some of our research involving fibroblast behavior in cell-derived three-dimensional matrices. Specifically, we examine how these matrices affect cell morphology, adhesion, proliferation, and signaling compared to two-dimensional substrates. We stress the importance of controls for three-dimensional matrix studies and discuss cancer as an example in which altered three-dimensional matrices can influence fibroblast signaling. Studying cells in three-dimensional microenvironments can lead to the design of more physiologically relevant conditions for assaying drug responses and deciphering biological mechanisms.

A co-clustering model involving alpha5beta1 integrin for the biological effects of GPI-anchored human carcinoembryonic antigen (CEA)

CEA functions as an intercellular adhesion molecule and is up-regulated in a wide variety of human cancers, including colon, breast and lung. Its over-expression inhibits cellular differentiation, blocks cell polarization, distorts tissue architecture, and inhibits anoikis of many different cell types. Here we report results concerning the molecular mechanism involved in these biological effects, where relatively rapid molecular changes not requiring alterations in gene expression were emphasized. Confocal microscopy experiments showed that antibody-mediated clustering of a deletion mutant of CEA (DeltaNCEA), normally incapable of self binding and clustering, led to the co-localization of integrin alpha5beta1 with patches of DeltaNCEA on the cell surface. Activation of alpha5, as defined by an anti-alpha5 mAb-sensitive increase in cell adhesion to immobilized fibronectin, and an increased binding of soluble fibronectin to cells, was also observed. This was accompanied by the recruitment of integrin-linked kinase (ILK), protein kinase B (PKB/Akt), and the mitogen-activated protein kinase (MAPK) to membrane microdomains and the phosphorylation of Akt and MAPK. Inhibition of PI3-K and ILK, but not MAPK, prevented the alpha5beta1 integrin activation. Conversely, anti-alpha5 antibody inhibited the PI3-K-mediated activation of Akt, implying the involvement of outside-in and inside-out signaling in integrin activation. Therefore we propose that CEA-mediated signaling involves clustering of CEA and co-clustering and activation of the alpha5beta1 and associated specific signaling elements on the internal surfaces of membrane microdomains. These changes may represent a molecular mechanism for the biological effects of CEA.

Chemical modulation of VLA integrin affinity in human breast cancer cells

The fact that disruption of integrin-extracellular matrix contacts leads to cell death, has converted cell adhesion into a potential target for the control of invasive cancer. In this work, we studied the functional consequences of the interference with the activity of the very late activation antigen (VLA) family of integrins in human breast cancer cell lines of distinct malignancy. The alpha2beta1-mediated adhesion reduced the entry of highly malignant, hormone-independent breast cancer cells into apoptosis. Adhesion of breast cancer cells through the VLA integrins alpha2beta1 and alpha5beta1 was significantly reduced by an apoptosis-inducing natural triterpenoid, dehydrothyrsiferol (DT), when studied on low amounts of extracellular matrix. This effect was dose-dependent, not related to cell toxicity and not shared with apoptosis-inducing standard chemotherapeutics, such as doxorubicin and taxol. The compound did not affect either the cell surface expression level of VLA integrins or cell distribution of vinculin and actin during cell spreading. In addition, neither phosphorylation of the focal adhesion kinase pp125FAK on Tyr397 nor the protein kinase B (Akt/PKB) on Ser473 was significantly altered by DT. The integrin activation level, assessed by binding of soluble collagen to the alpha2beta1 integrin, was reduced upon cell treatment with DT. Importantly, the TS2/16, an anti-beta1 activating monoclonal antibody was able to rescue DT-treated cells from apoptosis. Since the activation state of integrins is increasingly recognized as an essential factor in metastasis formation, findings presented herein reveal that the chemical regulation of integrin affinity may be a potential therapeutic strategy in cancer therapy.

Integrin α2β1 Is Required for Regulation of Murine Wound Angiogenesis but Is Dispensable for Reepithelialization

The alpha2beta1 integrin functions as the major receptor for collagen type I on a large number of different cell types, including keratinocytes, fibroblasts, endothelial cells, and a variety of inflammatory cells. Recently, we demonstrated that adhesion of keratinocytes to collagen critically depends on alpha2beta1, whereas fibroblasts can partly compensate for loss of alpha2beta1 in simple adhesion to collagen. However, in three-dimensional collagen matrices, alpha2beta1-null fibroblasts are hampered in generating mechanical forces. These data suggested a pivotal role for alpha2beta1 during wound healing in vivo. Unexpectedly, reepithelialization of excisional wounds of alpha2beta1-null mice was not impaired, indicating that keratinocytes do not require adhesion to or migration on collagen for wound closure. Whereas wound contraction and myofibroblast differentiation were similar, wound tensile strain was reduced in alpha2beta1-null mice, suggesting subtle changes in organization of the extracellular matrix. In addition, we observed reduced influx of mast cells into the granulation tissue, whereas infiltration of other inflammatory cells was not impaired. Interestingly, ablation of alpha2beta1 resulted in strong enhancement of neovascularization of granulation tissue and sponge implants. Both ultrastructurally and functionally, these new blood vessels appeared intact. In conclusion, our data show unique and overlapping functions of alpha2beta1 integrin during murine cutaneous wound healing.

Beta3 integrin and Src facilitate transforming growth factor-beta mediated induction of epithelial-mesenchymal transition in mammary epithelial cells

Introduction

Transforming growth factor (TGF)-β suppresses breast cancer formation by preventing cell cycle progression in mammary epithelial cells (MECs). During the course of mammary tumorigenesis, genetic and epigenetic changes negate the cytostatic actions of TGF-β, thus enabling TGF-β to promote the acquisition and development of metastatic phenotypes. The molecular mechanisms underlying this conversion of TGF-β function remain poorly understood but may involve signaling inputs from integrins.

Methods

β₃ Integrin expression or function in MECs was manipulated by retroviral transduction of active or inactive β₃ integrins, or by transient transfection of small interfering RNA (siRNA) against β₃ integrin. Altered proliferation, invasion, and epithelial-mesenchymal transition (EMT) stimulated by TGF-β in control and β₃ integrin manipulated MECs was determined. Src involvement in β₃ integrin mediated alterations in TGF-β signaling was assessed by performing Src protein kinase assays, and by interdicting Src function pharmacologically and genetically.

Results

TGF-β stimulation induced α_vβ₃ integrin expression in a manner that coincided with EMT in MECs. Introduction of siRNA against β₃ integrin blocked its induction by TGF-β and prevented TGF-β stimulation of EMT in MECs. β₃ integrin interacted physically with the TGF-β receptor (TβR) type II, thereby enhancing TGF-β stimulation of mitogen-activated protein kinases (MAPKs), and of Smad2/3-mediated gene transcription in MECs. Formation of β₃ integrin:TβR-II complexes blocked TGF-β mediated growth arrest and increased TGF-β mediated invasion and EMT. Dual β₃ integrin:TβR-II activation induced tyrosine phosphorylation of TβR-II, a phosphotransferase reaction mediated by Src in vitro. Inhibiting Src activity in MECs prevented the ability of β₃ integrin to induce TβR-II tyrosine phosphorylation, MAPK activation, and EMT stimulated by TGF-β. Lastly, wild-type and D119A β₃ integrin expression enhanced and abolished, respectively, TGF-β stimulation of invasion in human breast cancer cells.

Conclusion

We show that β₃ integrin alters TGF-β signaling in MECs via Src-mediated TβR-II tyrosine phosphorylation, which significantly enhanced the ability of TGF-β to induce EMT and invasion. Our findings suggest that β₃ integrin interdiction strategies may represent an innovative approach to re-establishing TGF-β mediated tumor suppression in progressing human breast cancers.

Platelet integrin adhesive functions and signaling

Integrin-mediated cellular events affect all cell types and functions, in physiological as well as pathological settings. Blood platelets, because of their unique nature, have proven to be a powerful cell model with which to study the adhesive and signaling properties of integrins. The characterization of the structural and molecular mechanisms regulating the main platelet integrin, alphaIIbbeta3, has provided some essential clues as to how integrins are regulated in general. The present chapter details the various protocols and reagents currently in use in our laboratory to study alphaIIbbeta3 adhesive responses and signaling in both human and murine cell models.

Activation of αIIbβ3 is a sufficient but also an imperative prerequisite for activation of α2β1 on platelets

Platelet integrins α2β1 and αIIbβ3 play critical roles in platelet adhesion and thrombus formation after vascular injury. On resting platelets, both integrins are in a low-affinity state. However, agonist stimulation results in conformational changes that enable ligand binding that can be detected with conformation dependent monoclonal antibodies (mAbs). By using such conformation-dependent mAbs, we could demonstrate that activation of integrin αIIbβ3 is not only sufficient, but also a prerequisite for α2β1 activation. Compared with platelets in plasma, stimulation of washed platelets resulted in only a minor activation of α2β1, as detected with the activation-sensitive mAb IAC-1. Addition of fibrinogen to stimulated washed platelets greatly potentiated activation of this integrin. Also, treatment of αIIbβ3 with the ligand-mimetic peptide RGDS, resulting in outside-in signaling, led to a powerful α2β1 activation, even in the absence of overall platelet activation, involving tyrosine kinase activity but no protein kinase C activation. The absolute necessity of αIIbβ3 for proper α2β1 activation on platelets was demonstrated by using the αIIbβ3 antagonist aggrastat, which was able to completely abolish α2β1 activation, both under static and flow conditions. In addition, analogous experiments with Glanzmann platelets lacking αIIbβ3 confirmed the indispensability of αIIbβ3 for α2β1 activation.

Matrix-specific suppression of integrin activation in shear stress signaling

Atherosclerotic plaque develops at sites of disturbed flow. We previously showed that flow activates endothelial cell integrins, which then bind to the subendothelial extracellular matrix (ECM), and, in cells on fibronectin or fibrinogen, trigger nuclear factor-kappaB activation. Additionally, fibronectin and fibrinogen are deposited into the subendothelial ECM at atherosclerosis-prone sites at early times. We now show that flow activates ECM-specific signals that establish patterns of integrin dominance. Flow induced alpha2beta1 activation in cells on collagen, but not on fibronectin or fibrinogen. Conversely, alpha5beta1 and alphavbeta3 are activated on fibronectin and fibrinogen, but not collagen. Failure of these integrins to be activated on nonpermissive ECM is because of active suppression by the integrins that are ligated. Protein kinase A is activated specifically on collagen and suppresses flow-induced alphavbeta3 activation. Alternatively, protein kinase Calpha is activated on fibronectin and mediates alpha2beta1 suppression. Thus, integrins actively cross-inhibit through specific kinase pathways. These mechanisms may determine cellular responses to complex extracellular matrices.

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.

Modulation of chondrocyte adhesion to collagen by echistatin

Primary chondrocytes from quail embryo epiphysis (quail epiphyseal chondrocytes, QEC) can grow either in suspension or in monolayer. In this study, the adhesion of QEC to collagen II was used as a model to study the regulation of the ligand-binding activity of integrin receptors that allows these cells to undergo a rapid transition from suspension to an adherent state. Preincubation of suspension QEC (QECSP) with the disintegrin echistatin increased by 40% their adhesion to collagen II. An inverse relationship between immobilized collagen density and echistatin-induced increase of chondrocyte adhesion was observed, thus suggesting that the disintegrin acts by increasing the ligand-binding affinity of collagen receptor(s). Further, echistatin activity does not appear to depend upon a direct binding of the disintegrin to collagen receptor(s). In fact, immobilized anti-beta1 antibodies, but not immobilized echistatin, served as effective binding sites for QECSP. Echistatin failed to stimulate chondrocyte adhesion to collagen in the presence of metabolic inhibitors, while an activating anti-beta1 antibody was still effective. Thus, echistatin may promote cell adhesion by interfering with energy-dependent signals that keep the collagen receptor(s) in a low-affinity state. Adhesion experiments performed in the presence of pharmacological inhibitors indicate that phosphatidyl inositol 3-kinase (PI3-K)/protein kinase C (PKC) and protein kinase A (PKA) pathways may transmit opposing signals on chondrocyte adhesion, and that collagen receptors are kept in a low-affinity state by PI3-kinase/PKC signalling. Since echistatin is a high-affinity ligand for alphavbeta3 integrin, the effect of the function-blocking anti-alphavbeta3 antibody LM609 was investigated. Like echistatin, LM609 stimulated chondrocyte adhesion to collagen and failed to support their attachment. Therefore, our data suggest that alphavbeta3-antagonists might regulate the binding activity of the beta1 collagen receptor, which in turn leads to the rapid transition of chondrocytes from suspension to an adherent state.

Expression of integrins in cerebral arteriovenous and cavernous malformations

OBJECTIVE

To assess and compare levels and patterns of expression for integrins alphavbeta1, alphavbeta3, and alphavbeta5 in arteriovenous malformations (AVMs) and cavernous malformations (CCMs) of the brain.

MATERIALS AND METHODS

Specimens from 10 AVM and 10 CCM lesions were selected from 112 patients with AVMs and 97 patients with CCMs who were treated microsurgically in the Department of Neurosurgery, Marmara University, Istanbul, Turkey. Sections were immunohistochemically stained with antibodies for integrins alphavbeta1, alphavbeta3, and alphavbeta5. Separate histological layers of the vascular wall were evaluated, and levels of expression were graded using a four-tier system.

RESULTS

Integrin alphavbeta1 was more strongly expressed in AVMs than in CCMs. This difference was most pronounced in the endothelium and subendothelium/media. Integrin alphavbeta3 was more strongly expressed in CCM endothelium than in AVM endothelium (average grades, 0.9 and 0.4, respectively). All 10 of the CCM lesions expressed integrin alphavbeta5 in the endothelium, whereas only five of the AVMs showed minimal expression of this molecule in the endothelium.

CONCLUSION

Current scientific understanding of the roles integrins play in angiogenesis is far from complete. The levels and patterns of expression for these molecules in the histological layers of the vascular walls of AVMs and CCMs provide some clues about the complex biological activities of integrins in these lesions. If one accepts the premise that immunohistochemistry has its inherent methodological problems, integrins alphavbeta1, alphavbeta3, and alphavbeta5 are expressed in AVMs and CCMs in different ways that may be linked to stages of angiogenic maturation. Integrin alphavbeta1 is expressed more strongly in endothelium and subendothelium/media of AVMs than in the corresponding layers of CCMs. Integrins alphavbeta3 and alphavbeta5 are expressed more strongly in CCM endothelium than in AVM endothelium. In addition, integrin alphavbeta5 staining was stronger in CCM subendothelium than AVM subendothelium/media.