Discoidin Domain Receptors Promote α1β1- and α2β1-Integrin Mediated Cell Adhesion to Collagen by Enhancing Integrin Activation

The discoidin domain receptors, DDR1 and DDR2, are receptor tyrosine kinases that bind to and are activated by collagens. Similar to collagen-binding β1 integrins, the DDRs bind to specific motifs within the collagen triple helix. However, these two types of collagen receptors recognize distinct collagen sequences. While GVMGFO (O is hydroxyproline) functions as a major DDR binding motif in fibrillar collagens, integrins bind to sequences containing Gxx’GEx”. The DDRs are thought to regulate cell adhesion, but their roles have hitherto only been studied indirectly. In this study we used synthetic triple-helical collagen-derived peptides that incorporate either the DDR-selective GVMGFO motif or integrin-selective motifs, such as GxOGER and GLOGEN, in order to selectively target either type of receptor and resolve their contributions to cell adhesion. Our data using HEK293 cells show that while cell adhesion to collagen I was completely inhibited by anti-integrin blocking antibodies, the DDRs could mediate cell attachment to the GVMGFO motif in an integrin-independent manner. Cell binding to GVMGFO was independent of DDR receptor signalling and occurred with limited cell spreading, indicating that the DDRs do not mediate firm adhesion. However, blocking the interaction of DDR-expressing cells with collagen I via the GVMGFO site diminished cell adhesion, suggesting that the DDRs positively modulate integrin-mediated cell adhesion. Indeed, overexpression of the DDRs or activation of the DDRs by the GVMGFO ligand promoted α1β1 and α2β1 integrin-mediated cell adhesion to medium- and low-affinity integrin ligands without regulating the cell surface expression levels of α1β1 or α2β1. Our data thus demonstrate an adhesion-promoting role of the DDRs, whereby overexpression and/or activation of the DDRs leads to enhanced integrin-mediated cell adhesion as a result of higher integrin activation state.     

Integrin α9β1

Integrins are transmembrane heterodimeric receptors responsible for transducing and modulating signals between the extracellular matrix and cytoskeleton, ultimately influencing cell functions such as adhesion and migration. Integrin α9β1 is classified within a two member sub-family of integrins highlighted in part by its specialized role in cell migration. The importance of this role is demonstrated by its regulation of numerous biological functions including lymphatic valve morphogenesis, lymphangiogenesis, angiogenesis and hematopoietic homeostasis. Compared to other integrins the signaling mechanisms that transduce α9β1-induced cell migration are not well described. We have recently shown that Src tyrosine kinase plays a key proximal role to control α9β1 signaling. Specifically it activates inducible nitric oxide synthase (iNOS) and in turn nitric oxide (NO) production as a means to transduce cell migration. Furthermore, we have also described a role for FAK, Erk and Rac1 in α9β1 signal transduction. Here we provide an over view of known integrin α9β1 signaling pathways and highlight its roles in diverse biological conditions.     

Syndecan-1-Induced ECM Fiber Alignment Requires Integrin αvβ3 and Syndecan-1 Ectodomain and Heparan Sulfate Chains

Expression of the cell surface proteoglycan syndecan-1 (Sdc1) is frequently induced in stromal fibroblasts of invasive breast carcinomas. We have recently identified a correlation between stromal Sdc1 expression and extracellular matrix (ECM) fiber alignment, both in vitro and in vivo. ECMs derived from Sdc1-positive human mammary fibroblasts (HMF) showed an aligned fiber architecture, which contrasted markedly with the more random fiber arrangement in the ECM produced by Sdc1-negative HMFs. We further demonstrated that aligned fiber architecture promotes the directional migration and invasion of breast carcinoma cells. To decipher the molecular mechanisms governing the formation of an aligned, invasion-permissive ECM, a series of Sdc1 mutants was introduced into HMF. We found that both the ectodomain and heparan sulfate chains of Sdc1 were required for full activity of Sdc1 in regulating ECM alignment, while transmembrane and cytoplasmic domains were dispensable. Sdc1 regulates the activities of several integrins via its ectodomain. Integrins are key players in the assembly of fibronectin-rich ECM. In addition, integrins are capable of regulating cell morphology and cell shape and orientation may affect ECM architecture. Therefore, we investigated the role of integrins in Sdc1-mediated ECM fiber alignment. Sdc1-overexpressing HMF gained an enhanced spindle-shaped morphology when cultured in an overconfluent state under conditions permissive for ECM production, which was partially reversed by siRNA-mediated silencing of β3 integrin expression. Moreover, suppression of αvβ3 integrin activity by a function-blocking antibody or β3 knockdown largely abolished the aligned ECM fiber architecture and consequently the invasion-permissive properties of the ECM induced by Sdc1. The results suggest that Sdc1 may modulate fibronectin fibrillogenesis and/or alter cell morphology during ECM production through αvβ3 integrin, thereby mediating ECM fiber alignment. Understanding the mechanisms governing ECM organization may lead to the development of novel stroma-targeted therapy for breast cancer, aiming at converting an invasion-permissive to an invasion-restrictive microenvironment.     

Physiological and Pathological Roles of α3β1 Integrin

α3β1 integrin has been considered to be a mysterious adhesion molecule due to the pleiotropy in its ligand-binding specificity. However, recent studies have identified laminin isoforms as high-affinity ligands for this integrin, and demonstrated that α3β1 integrin plays a number of essential roles in development and differentiation, mainly by mediating the establishment and maintenance of epithelial tissues. Furthermore, α3β1 integrin is also implicated in many other biological phenomena, including cell growth and apoptosis, angiogenesis and neural functions. This integrin receptor forms complexes with various other membrane proteins, such as the transmembrane-4 superfamily proteins (tetraspanins), cytoskeletal proteins and signaling molecules. Recently, lines of evidence have been reported showing that complex formation regulates integrin functions in cell adhesion and migration, signal transduction across cell membranes, and cytoskeletal organization. In addition to these roles in physiological processes, α3β1 integrin performs crucial functions in various pathological processes, especially in wound healing, tumor invasion and metastasis, and infection by pathogenic microorganisms.This revised version was published online in June 2005 with a corrected cover date.     

The Integrin-Linked Kinase-PINCH-Parvin Complex Supports Integrin αIIbβ3 Activation

Integrin-linked kinase (ILK) is an important signaling regulator that assembles into the heteroternary complex with adaptor proteins PINCH and parvin (termed the IPP complex). We recently reported that ILK is important for integrin activation in a Chinese hamster ovary (CHO) cell system. We previously established parental CHO cells expressing a constitutively active chimeric integrin (αIIbα6Bβ3) and mutant CHO cells expressing inactive αIIbα6Bβ3 due to ILK deficiency. In this study, we further investigated the underlying mechanisms for ILK-dependent integrin activation. ILK-deficient mutant cells had trace levels of PINCH and α-parvin, and transfection of ILK cDNA into the mutant cells increased not only ILK but also PINCH and α-parvin, resulting in the restoration of αIIbα6Bβ3 activation. In the parental cells expressing active αIIbα6Bβ3, ILK, PINCH, and α-parvin were co-immunoprecipitated, indicating the formation of the IPP complex. Moreover, short interfering RNA (siRNA) experiments targeting PINCH-1 or both α- and β-parvin mRNA in the parent cells impaired the αIIbα6Bβ3 activation as well as the expression of the other components of the IPP complex. In addition, ILK mutants possessing defects in either PINCH or parvin binding failed to restore αIIbα6Bβ3 activation in the mutant cells. Kindlin-2 siRNA in the parental cells impaired αIIbα6Bβ3 activation without disturbing the expression of ILK. For CHO cells stably expressing wild-type αIIbβ3 that is an inactive form, overexpression of a talin head domain (THD) induced αIIbβ3 activation and the THD-induced αIIbβ3 activation was impaired by ILK siRNA through a significant reduction in the expression of the IPP complex. In contrast, overexpression of all IPP components in the αIIbβ3-expressing CHO cells further augmented THD-induced αIIbβ3 activation, whereas they did not induce αIIbβ3 activation without THD. These data suggest that the IPP complex rather than ILK plays an important role and supports integrin activation probably through stabilization of the active conformation.     

Clustering of Syndecan-4 and Integrin β1 by Laminin α3 Chain–derived Peptide Promotes Keratinocyte Migration

Syndecans function as receptors for extracellular matrix (ECM) with integrins in cell spreading. However, the molecular mechanism of their specific involvement in cell migration or in wound healing has not been elucidated yet. Here, we report that a synthetic peptide, PEP75, which contains the syndecan-binding sequence of the laminin α3LG4 module, induces keratinocyte migration in in vitro and in vivo. Soluble PEP75 induced the clustering of syndecan-4 and conformation-modified integrin β1 colocalized with syndecan-4 in soluble PEP75-induced clusters. Treatment of cells in solution with PEP75 resulted in the exposure of the P4G11 antibody epitope of integrin β1 in immunostaining as well as in flow cytometry and augmented integrin β1–dependent cell adhesion to ECM. Pulldown assays demonstrated that PEP75 bound to syndecan-4, but not to integrin β1. A siRNA study revealed a role for syndecan-4 in PEP75-induced up-regulation of P4G11 antibody binding and migration of HaCaT cells. We conclude that binding of soluble PEP75 to syndecan-4 induces the coupling of integrin β1, which is associated with integrin β1-conformational changes and activation, and leads to keratinocyte migration. To activate integrin function through syndecans could be a novel therapeutic approach for chronic wound.     

On the Activation of Integrin αIIbβ3: Outside-in and Inside-out Pathways

Ataxin-1 is a human protein responsible for spinocerebellar ataxia type 1, a hereditary disease associated with protein aggregation and misfolding. Essential for ataxin-1 aggregation is the anomalous expansion of a polyglutamine tract near the protein N-terminus, but the sequence-wise distant AXH domain modulates and contributes to the process. The AXH domain is also involved in the nonpathologic functions of the protein, including a variety of intermolecular interactions with other cellular partners. The domain forms a globular dimer in solution and displays a dimer of dimers arrangement in the crystal asymmetric unit. Here, we have characterized the domain further by studying its behavior in the crystal and in solution. We solved two new structures of the domain crystallized under different conditions that confirm an inherent plasticity of the AXH fold. In solution, the domain is present as a complex equilibrium mixture of monomeric, dimeric, and higher molecular weight species. This behavior, together with the tendency of the AXH fold to be trapped in local conformations, and the multiplicity of protomer interfaces, makes the AXH domain an unusual example of a chameleon protein whose properties bear potential relevance for the aggregation properties of ataxin-1 and thus for disease.     

The Structure of the N-Terminus of Kindlin-1: A Domain Important for αIIbβ3 Integrin Activation

The integrin family of heterodimeric cell adhesion molecules exists in both low- and high-affinity states, and integrin activation requires binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to membrane-proximal sequences in the β-integrin cytoplasmic domain. However, it has recently become apparent that the kindlin family of FERM domain proteins is also essential for talin-induced integrin activation. FERM domains are typically composed of F1, F2, and F3 domains, but the talin FERM domain is atypical in that it contains a large insert in F1 and is preceded by a previously unrecognized domain, F0. Initial sequence alignments showed that the kindlin FERM domain was most similar to the talin FERM domain, but the homology appeared to be restricted to the F2 and F3 domains. Based on a detailed characterization of the talin FERM domain, we have reinvestigated the sequence relationship with kindlins and now show that kindlins do indeed contain the same domain structure as the talin FERM domain. However, the kindlin F1 domain contains an even larger insert than that in talin F1 that disrupts the sequence alignment. The insert, which varies in length between different kindlins, is not conserved and, as in talin, is largely unstructured. We have determined the structure of the kindlin-1 F0 domain by NMR, which shows that it adopts the same ubiquitin-like fold as the talin F0 and F1 domains. Comparison of the kindlin-1 and talin F0 domains identifies the probable interface with the kindlin-1 F1 domain. Potential sites of interaction of kindlin F0 with other proteins are discussed, including sites that differ between kindlin-1, kindlin-2, and kindlin-3. We also demonstrate that F0 is required for the ability of kindlin-1 to support talin-induced αIIbβ3 integrin activation and for the localization of kindlin-1 to focal adhesions.     

Dual Control by Divalent Cations and Mitogenic Cytokines of α4β1 and α5β1 Integrin Avidity Expressed by Human Hemopoietic Cells

Beta-1 integrins have essential functions in hemopoietic and immune systems by controlling phenomenons such as cell homing and cell activation. The function α4β1 and α5β1 integrins is regulated by divalent cations and, as demonstrated more recently, by mitogenic cytokines which activate them by “inside-out” mechanisms. Using the adhesive interaction of a cytokine-dependent human hemopoietic cell line to immobilized fibronectin, we have analyzed the requirements in divalent cations Mn²⁺, Mg²⁺ and Ca²⁺ for α4β1 and α5β1 activation by “inside-out” mechanisms triggered by cytokines such as granulocyte-macrophage colony stimulating factor or KIT ligand, or by external conformational constraints with the function-activating anti-β₁ integrin monoclonal antibody 8A2. The intrinsic difference between these two modes of β₁ integrin activation was revealed by their different requirements in divalent cations. We found that in the absence of any divalent cations, α4β1 and α5β1 were non-functional even after further stimulation by cytokines or 8A2. However, whilst either Ca²⁺, Mg²⁺ or Mn²⁺ were able to restore adhesive functions of α4β1 and α5β1 when activated by 8A2, only Mg²⁺ and Mn²⁺ were able to support activation of α5β1 and α5β1 by cytokines. Furthermore, high concentrations of Ca²⁺ exceeding 20 mM dramatically inhibited cell adhesion to fibronectin induced by Mn²⁺ and cytokines but not by 8A2. On the contrary, in the presence of both Ca²⁺ and Mg²⁺, Mn²⁺ had an additive effect on the activation of α5β1 and α5β1 by mitogenic cytokines. The presence of the absence of these divalent cations did not inhibit early tyrosine phosphorylation induced by the binding of KIT ligand to its tyrosine-kinase receptor KIT. Therefore, we propose that in hemopoietic cells, Ca²⁺, Mg²⁺ and Mn²⁺ may modulate in vivo α4β1 and α5β1 regulation by mitogenic cytokines, a phenomenon involved in the regulation of hemopoietic progenitor cell homing within the bone marrow.     

Integrin α2β1 Mediates Tyrosine Phosphorylation of Vascular Endothelial Cadherin Induced by Invasive Breast Cancer Cells

The molecular mechanisms that regulate the endothelial response during transendothelial migration (TEM) of invasive cancer cells remain elusive. Tyrosine phosphorylation of vascular endothelial cadherin (VE-cad) has been implicated in the disruption of endothelial cell adherens junctions and in the diapedesis of metastatic cancer cells. We sought to determine the signaling mechanisms underlying the disruption of endothelial adherens junctions after the attachment of invasive breast cancer cells. Attachment of invasive breast cancer cells (MDA-MB-231) to human umbilical vein endothelial cells induced tyrosine phosphorylation of VE-cad, dissociation of β-catenin from VE-cad, and retraction of endothelial cells. Breast cancer cell-induced tyrosine phosphorylation of VE-cad was mediated by activation of the H-Ras/Raf/MEK/ERK signaling cascade and depended on the phosphorylation of endothelial myosin light chain (MLC). The inhibition of H-Ras or MLC in endothelial cells inhibited TEM of MDA-MB-231 cells. VE-cad tyrosine phosphorylation in endothelial cells induced by the attachment of MDA-MB-231 cells was mediated by MDA-MB-231 α₂β₁ integrin. Compared with highly invasive MDA-MB-231 breast cancer cells, weakly invasive MCF-7 breast cancer cells expressed lower levels of α₂β₁ integrin. TEM of MCF-7 as well as induction of VE-cad tyrosine phosphorylation and dissociation of β-catenin from the VE-cad complex by MCF-7 cells were lower than in MDA-MB-231 cells. These processes were restored when MCF-7 cells were treated with β₁-activating antibody. Moreover, the response of endothelial cells to the attachment of prostatic (PC-3) and ovarian (SKOV3) invasive cancer cells resembled the response to MDA-MB-231 cells. Our study showed that the MDA-MB-231 cell-induced disruption of endothelial adherens junction integrity is triggered by MDA-MB-231 cell α₂β₁ integrin and is mediated by H-Ras/MLC-induced tyrosine phosphorylation of VE-cad.     

Integrin α1β1 participates in chondrocyte transduction of osmotic stress

Background/purpose

The goal of this study was to determine the role of the collagen binding receptor integrin α1β1 in regulating osmotically induced [Ca²⁺]_i transients in chondrocytes.

Method

The [Ca²⁺]_i transient response of chondrocytes to osmotic stress was measured using real-time confocal microscopy. Chondrocytes from wildtype and integrin α1-null mice were imaged ex vivo (in the cartilage of intact murine femora) and in vitro (isolated from the matrix, attached to glass coverslips). Immunocytochemistry was performed to detect the presence of the osmosensor, transient receptor potential vanilloid-4 (TRPV4), and the agonist GSK1016790A (GSK101) was used to test for its functionality on chondrocytes from wildtype and integrin α1-null mice.

Results/interpretation

Deletion of the integrin α1 subunit inhibited the ability of chondrocytes to respond to a hypo-osmotic stress with [Ca²⁺]_i transients ex vivo and in vitro. The percentage of chondrocytes responding ex vivo was smaller than in vitro and of the cells that responded, more single [Ca²⁺]_i transients were observed ex vivo compared to in vitro. Immunocytochemistry confirmed the presence of TRPV4 on wildtype and integrin α1-null chondrocytes, however application of GSK101 revealed that TRPV4 could be activated on wildtype but not integrin α1-null chondrocytes. Integrin α1β1 is a key participant in chondrocyte transduction of a hypo-osmotic stress. Furthermore, the mechanism by which integrin α1β1 influences osmotransduction is independent of matrix binding, but likely dependent on the chondrocyte osmosensor TRPV4.     

Activation of α-secretase by curcumin-aminoacid conjugates

The extracellular senile plaques observed in Alzheimer's disease (AD) patients are mainly composed of amyloid peptides produced from the β-amyloid precursor protein (βAPP) by β- and γ-secretases. A third non-amyloidogenic α-secretase activity performed by the disintegrins ADAM10 and ADAM17 occurs in the middle of the amyloid-β peptide Aβ and liberates the large sAPPα neuroprotective fragment. Since the activation of α-secretase recently emerged as a promising therapeutic approach to treat AD, the identification of natural compounds able to trigger this cleavage is highly required. Here we describe new curcumin-based modified compounds as α-secretase activators. We established that the aminoacid conjugates curcumin-isoleucine, curcumin-phenylalanine and curcumin-valine promote the constitutive α-secretase activity and increase ADAM10 immunoreactivity. Strickingly, experiments carried out under conditions mimicking the PKC/muscarinic receptor-regulated pathway display different patterns of activation by these compounds. Altogether, our data identified new lead natural compounds for the future development of powerful and stable α-secretase activators and established that some of these molecules are able to discriminate between the constitutive and regulated α-secretase pathways.     

Regulator of G-Protein Signalling-14 (RGS14) Regulates the Activation of αMβ2 Integrin during Phagocytosis

Integrin-mediated phagocytosis, an important physiological activity undertaken by professional phagocytes, requires bidirectional signalling to/from αMβ2 integrin and involves Rap1 and Rho GTPases. The action of Rap1 and the cytoskeletal protein talin in activating αMβ2 integrins, in a RIAM-independent manner, has been previously shown to be critical during phagocytosis in mammalian phagocytes. However, the events downstream of Rap1 are not clearly understood. Our data demonstrate that one potential Rap1 effector, Regulator of G-Protein Signalling-14 (RGS14), is involved in activating αMβ2. Exogenous expression of RGS14 in COS-7 cells expressing αMβ2 results in increased binding of C3bi-opsonised sheep red blood cells. Consistent with this, knock-down of RGS14 in J774.A1 macrophages results in decreased association with C3bi-opsonised sheep red blood cells. Regulation of αMβ2 function occurs through the R333 residue of the RGS14 Ras/Rap binding domain (RBD) and the F754 residue of β2, residues previously shown to be involved in binding of H-Ras and talin1 head binding prior to αMβ2 activation, respectively. Surprisingly, overexpression of talin2 or RAPL had no effect on αMβ2 regulation. Our results establish for the first time a role for RGS14 in the mechanism of Rap1/talin1 activation of αMβ2 during phagocytosis.     

SHP2 Mediates the Localized Activation of Fyn Downstream of the α6β4 Integrin To Promote Carcinoma Invasion

Src family kinase (SFK) activity is elevated in many cancers, and this activity correlates with aggressive tumor behavior. The α6β4 integrin, which is also associated with a poor prognosis in many tumor types, can stimulate SFK activation; however, the mechanism by which it does so is not known. In the current study, we provide novel mechanistic insight into how the α6β4 integrin selectively activates the Src family member Fyn in response to receptor engagement. Both catalytic and noncatalytic functions of SHP2 are required for Fyn activation by α6β4. Specifically, the tyrosine phosphatase SHP2 is recruited to α6β4 and its catalytic activity is stimulated through a specific interaction of its N-terminal SH2 domain with pY1494 in the β4 subunit. Fyn is recruited to the α6β4/SHP2 complex through an interaction with phospho-Y580 in the C terminus of SHP2. In addition to activating Fyn, this interaction with Y580-SHP2 localizes Fyn to sites of receptor engagement, which is required for α6β4-dependent invasion. Of significance for tumor progression, phosphorylation of Y580-SHP2 and SFK activation are increased in orthotopic human breast tumors that express α6β4 and activation of this pathway is dependent upon Y1494.Expression of the α6β4 integrin, a laminin receptor, is associated with poor patient prognosis and reduced survival in many human cancers (32). For this reason, there is considerable interest in understanding how this integrin is regulated and how it functions to promote tumor progression. In normal tissues, the α6β4 integrin plays a major role in maintaining the integrity of epithelia by binding to laminins in the basement membrane and regulating the assembly of hemidesmosomes on the basal epithelial cell surface (7, 17). In pathophysiological conditions such as wound healing and cancer, the stable adhesive interactions of the α6β4 receptor are disrupted by phosphorylation of the β4 cytoplasmic domain, converting α6β4 to a signaling-competent receptor that promotes dynamic adhesion and invasion (18). Phosphorylation of the β4 subunit cytoplasmic domain on serine residues contributes to the dynamic adhesive functions of the receptor by disrupting interactions with hemidesmosomal proteins that regulate stable adhesion (33, 37). Phosphorylation of the β4 cytoplasmic domain on tyrosine residues may also contribute to the regulation of hemidesmosomes, but it is likely that the major contribution of tyrosyl phosphorylation is to mediate interactions that stimulate downstream signaling from the receptor (22).In transformed cells, engagement of the α6β4 integrin stimulates the activation of several signaling molecules, including phosphatidylinositol-3 kinase (PI3K), mitogen-activated protein kinases (MAPK), NFκB, and Src family kinases (SFKs) (10, 12, 21, 40). In previous studies, we identified Y1494 in the β4 subunit cytoplasmic domain as an important mediator of α6β4-dependent signaling by demonstrating that mutation of Y1494 inhibits the ability of α6β4 to stimulate PI3K, MAPK, and SFK activation (10, 39). Restoration of both PI3K and SFK signaling, but not MAPK signaling, rescues invasion in tumor cells expressing Y1494F-β4, indicating that PI3K and SFK signaling pathways cooperate downstream of Y1494 to promote α6β4-dependent invasion (10). Y1494 is localized within an immunoreceptor tyrosine-based inhibition motif (ITIM), a canonical binding site for Src-homology-2 (SH2) domain-containing protein-tyrosine phosphatase 1 (SHP1) and SHP2 (44). Examination of a chimeric receptor containing the extracellular domain of TrkB and the transmembrane and cytoplasmic domains of the β4 subunit demonstrated that SHP2 binds to and is activated by sequences in the β4 cytoplasmic domain in response to dimerization (23). Moreover, Y1494 is one of three tyrosine residues, along with Y1257 and Y1440, that mediate the interaction of SHP2 with the β4 subunit cytoplasmic domain in response to c-Met signaling (6). Importantly, SHP2 is essential for the activation of SFKs both by the chimeric TrkB/β4 receptor and when the β4 subunit functions as a signaling adaptor for c-Met (6, 23). However, the mechanism by which SHP2 activates SFKs in response to α6β4 engagement has not been established.Elevated SFK activity correlates strongly with breast cancer invasion and metastasis, and these kinases are frequently activated in human cancers (15). Given the parallels between α6β4 expression and SFK activation in cancer, investigation of how α6β4 contributes to the activation of this invasion-promoting pathway is warranted. In the current study, we sought to elucidate the mechanism by which engagement of α6β4 activates SFKs and the significance of the β4/SHP2/SFK signaling axis for tumor progression. Our results reveal a novel mechanism for SHP2-dependent activation of the SFK family member Fyn which involves Y580 in the C terminus of SHP2.     

Integrin α4β7 Co-Stimulation of Human Peripheral Blood T Cell Proliferation

The Integrin α4β7 mediates lymphocyte adhesion to VCAM-1 on activated endothelium, fibronectin in the extracellular matrix, and the mucosal vascular addressin MAdCAM-1. It is unclear whether α4β7 performs any function beyond directing specific adhesion reactions. We addressed the possibility that triggering of α4β7 with a specific monoclonal antibody was capable of delivering an accessory stimulus that would coactivate T cells and lead to proliferation. At submitogenic levels of anti-CD3 stimulation, triggering of α4β7 by immobilized mAb ACT-1 resulted in T cell blastogenesis, IL-2 production, expression of the IL-2 receptor α chain CD25, and ultimately DNA synthesis. These results indicate that the integrin α4β7 is involved in more than lymphocyte adhesion and homing but also plays a role in cell signaling.