Anti-integrin monoclonal antibodies

Cytoplasmic domain interactions of syndecan-1 and syndecan-4 with α6β4 integrin mediate human epidermal growth factor receptor (HER1 and HER2)-dependent motility and survival

Epithelial cells are highly dependent during wound healing and tumorigenesis on the α6β4 integrin and its association with receptor tyrosine kinases. Previous work showed that phosphorylation of the β4 subunit upon matrix engagement depends on the matrix receptor syndecan (Sdc)-1 engaging the cytoplasmic domain of the β4 integrin and coupling of the integrin to human epidermal growth factor receptor-2 (HER2). In this study, HER2-dependent migration activated by matrix engagement is compared with migration stimulated by EGF. We find that whereas HER2-dependent migration depends on Sdc1, EGF-dependent migration depends on a complex consisting of human epidermal growth factor receptor-1 (HER1, commonly known as EGFR), α6β4, and Sdc4. The two syndecans recognize distinct sites at the extreme C terminus of the β4 integrin cytoplasmic domain. The binding motif in Sdc1 is QEEXYX, composed in part by its syndecan-specific variable (V) region and in part by the second conserved (C2) region that it shares with other syndecans. A cell-penetrating peptide containing this sequence competes for HER2-dependent epithelial migration and carcinoma survival, although it is without effect on the EGFR-stimulated mechanism. β4 mutants bearing mutations specific for Sdc1 and Sdc4 recognition act as dominant negative mutants to block cell spreading or cell migration that depends on HER2 or EGFR, respectively. The interaction of the α6β4 integrin with the syndecans appears critical for it to be utilized as a signaling platform; migration depends on α3β1 integrin binding to laminin 332 (LN332; also known as laminin 5), whereas antibodies that block α6β4 binding are without effect. These findings indicate that specific syndecan family members are likely to have key roles in α6β4 integrin activation by receptor tyrosine kinases.

OSBP-related protein 3 (ORP3) coupling with VAMP-associated protein A regulates R-Ras activity

ORP3 is an R-Ras interacting oxysterol-binding protein homolog that regulates cell adhesion and is overexpressed in several cancers. We investigated here a novel function of ORP3 dependent on its targeting to both the endoplasmic reticulum (ER) and the plasma membrane (PM). Using biochemical and cell imaging techniques we demonstrate the mechanistic requirements for the subcellular targeting and function of ORP3 in control of R-Ras activity. We show that hyperphosphorylated ORP3 (ORP3-P) selectively interacts with the ER membrane protein VAPA, and ORP3-VAPA complexes are targeted to PM sites via the ORP3 pleckstrin homology (PH) domain. A novel FFAT (two phenylalanines in an acidic tract)-like motif was identified in ORP3; only disruption of both the FFAT-like and canonical FFAT motif abolished the phorbol-12-myristate-13-acetate (PMA) stimulated interaction of ORP3-P with VAPA. Co-expression of ORP3 and VAPA induced R-Ras activation, dependent on the interactions of ORP3 with VAPA and the PM. Consistently, downstream AktS473 phosphorylation and β1-integrin activity were enhanced by ORP3-VAPA. To conclude, phosphorylation of ORP3 controls its association with VAPA. Furthermore, we present evidence that ORP3-VAPA complexes stimulate R-Ras signaling.

Modulating the stem cell niche for tissue regeneration

The field of regenerative medicine holds considerable promise for treating diseases that are currently intractable. Although many researchers are adopting the strategy of cell transplantation for tissue repair, an alternative approach to therapy is to manipulate the stem cell microenvironment, or niche, to facilitate repair by endogenous stem cells. The niche is highly dynamic, with multiple opportunities for intervention. These include administration of small molecules, biologics or biomaterials that target specific aspects of the niche, such as cell-cell and cell-extracellular matrix interactions, to stimulate expansion or differentiation of stem cells, or to cause reversion of differentiated cells to stem cells. Nevertheless, there are several challenges in targeting the niche therapeutically, not least that of achieving specificity of delivery and responses. We envisage that successful treatments in regenerative medicine will involve different combinations of factors to target stem cells and niche cells, applied at different times to effect recovery according to the dynamics of stem cell-niche interactions.

Expression and purification of bioactive, low-endotoxin recombinant human vitronectin

The secreted adhesive glycoprotein vitronectin (VTN) is a multifunctional component of plasma and the extracellular matrix. A high-yielding, inexpensive, low endotoxin source of bioactive recombinant human vitronectin (rhVTN) is highly desirable for in vitro use in diverse cell culture systems ranging from basic research settings to clinical-grade production of human cells. We describe modifications to a previously reported heparin-based affinity chromatography procedure that improve yield and achieve efficient removal of endotoxin from washed and urea-solubilized human VTN inclusion bodies following standard autoinduction of expression in Escherichia coli. This simple procedure makes accessible the low-cost expression and purification of large quantities of bioactive rhVTN using basic equipment and facilitates its use in a spectrum of endotoxin-sensitive applications.

Heterotropic modulation of selectin affinity by allosteric antibodies affects leukocyte rolling

Selectins are a family of adhesion receptors designed for efficient leukocyte tethering to the endothelium under shear. As a key property to resist premature bond disruption, selectin adhesiveness is enhanced by tensile forces that promote the conversion of a bent into an extended conformation of the N-terminal lectin and epidermal growth factor-like domains. Conformation-specific Abs have been invaluable in deciphering the activation mechanism of integrins, but similar reagents are not available for selectins. In this study, we show that the anti-human L-selectin mAbs DREG-55 and LAM1-5 but not DREG-56, DREG-200, or LAM1-1 heterotropically modulate adhesion presumably by stabilizing the extended receptor conformation. Force-free affinity assays, flow chamber, and microkinetic studies reveal a ligand-specific modulation of L-selectin affinity by DREG-55 mAb, resulting in a dramatic decrease of rolling velocity under flow. Furthermore, secondary tethering of polymorphonuclear cells was blocked by DREG-200 but significantly boosted by DREG-55 mAb. The results emphasize the need for a new classification for selectin Abs and introduce the new concept of heterotropic modulation of receptor function.

Reduction in oxidative stress during cellular responses to chemically functionalised graphene

Nitrogen-functionalisation of chemically driven graphene (CDG) thin films effectively reduced the ROS generated from cells on the f-CDG films. The results indicate that N₂plasma treatment of CDG is very useful in improving biocompatibility for the bio-application of graphene materials.

Eosinophil major basic protein activates human cord blood mast cells primed with fibroblast membranes by integrin-β1

BACKGROUND

Mast cell (MC) - eosinophil (Eos) activating cross-talk might be critical for the severity and chronicity of allergy. Among soluble mediators, eosinophil major basic protein (MBP), a hallmark of allergy, is particularly important because it was shown to activate specific MC subtypes. We previously demonstrated that MBP activates IgE-desensitized rat MC and human lung and cord blood-derived MC (CBMC) after priming with fibroblast membranal stem cell factor. However, a distinct mechanism for this activation was missing. Therefore, we aimed to investigate it.

METHODS

Major basic protein-1 activation of CBMC primed with fibroblast-derived membranes (FBM) was measured by β-hexosaminidase and tryptase release. Chemical cross-linking followed by micrometric flow cytometry probed direct interactions. Antibodies neutralized integrin-β1 and recognized its active form. Pertussis toxin (Ptx) was used to decrease integrin-β1 active form expression. Hematopoietic cell kinase (Hck) was identified by immunoprecipitation (IP) and silenced by siRNA.

RESULTS

Major basic protein-1-induced CBMC activation is mediated partly by MBP1-integrin-β1 interaction on the MC surface. FBM prime CBMC via a G protein, as confirmed by Ptx, to shift integrin-β1 to its active form. Following MBP1 binding, integrin-β1 binds Hck that further transduces the activation signal. MC priming with FBM leads to up-regulation in Hck protein level. MC integrin-β1 neutralization inhibits MBP1-induced activation and uptake. Hck silencing results with reduced MBP1-induced activation.

CONCLUSIONS

Fibroblast-derived membranes, integrin-β1, and Hck are involved in MBP1-induced activation of CBMC and therefore represent a distinct mechanism for this activation. This finding might implicate integrin-β1 and Hck as targets for decreasing MC - Eos activating cross-talk in allergy.

Novel insights into the biology of interleukin-32

Interleukin (IL)-32 is known as a proinflammatory cytokine that is likely involved in several diseases, including infections, chronic inflammation, and cancer. Since the first report in 2005, IL-32 has been the subject of numerous studies to unravel the biological function of this molecule. For example, silencing of endogenous IL-32 in primary or cell lines of human origin consistently suppressed responses to Toll-like receptors. The protein folding structure of the six isoforms of IL-32 does not resemble that of any classical cytokine and as of this writing, a specific IL-32 receptor has not been identified. Instead, we propose a mechanism by which exposure to extracellular IL-32 or overexpression of the molecule results in binding to intracellular partners that influences functions such as gene expression, cell death, or survival. As such, this review offers insights into the role of IL-32 in several diseases, host defense, inflammation, immune function, and cancer. Finally, possibilities to target IL-32 in several diseases are proposed.

A ZO-1/α5β1-integrin complex regulates cytokinesis downstream of PKCε in NCI-H460 cells plated on fibronectin

Recently, we demonstrated that integrin adhesion to the extracellular matrix at the cleavage furrow is essential for cytokinesis of adherent cells. Here, we report that tight junction protein ZO-1 (Zonula Occludens-1) is required for successful cytokinesis in NCI-H460 cells plated on fibronectin. This function of ZO-1 involves interaction with the cytoplasmic domain of α5-integrin to facilitate recruitment of active fibronectin-binding integrins to the base of the cleavage furrow. In the absence of ZO-1, or a functional ZO-1/α5β1-integrin complex, proper actin-dependent constriction between daughter cells is impaired and cells fail cytokinesis. Super-resolution microscopy reveals that in ZO-1 depleted cells the furrow becomes delocalized from the matrix. We also show that PKCε-dependent phosphorylation at Serine168 is required for ZO-1 localization to the furrow and successful cell division. Altogether, our results identify a novel regulatory pathway involving the interplay between ZO-1, α5-integrin and PKCε in the late stages of mammalian cell division.

Anti-IL-5 attenuates activation and surface density of β(2) -integrins on circulating eosinophils after segmental antigen challenge

BACKGROUND

IL-5 activates α(M) β(2) integrin on blood eosinophils in vitro. Eosinophils in bronchoalveolar lavage (BAL) following segmental antigen challenge have activated β(2) -integrins.

OBJECTIVE

To identify roles for IL-5 in regulating human eosinophil integrins in vivo.

METHODS

Blood and BAL eosinophils were analysed by flow cytometry in ten subjects with allergic asthma who underwent a segmental antigen challenge protocol before and after anti-IL-5 administration.

RESULTS

Blood eosinophil reactivity with monoclonal antibody (mAb) KIM-127, which recognizes partially activated β(2) -integrins, was decreased after anti-IL-5. Before anti-IL-5, surface densities of blood eosinophil β(2) , α(M) and α(L) integrin subunits increased modestly post challenge. After anti-IL-5, such increases did not occur. Before or after anti-IL-5, surface densities of β(2) , α(M) , α(L) and α(D) and reactivity with KIM-127 and mAb CBRM1/5, which recognizes high-activity α(M) β(2) , were similarly high on BAL eosinophils 48 h post-challenge. Density and activation state of β(1) -integrins on blood and BAL eosinophils were not impacted by anti-IL-5, even though anti-IL-5 ablated a modest post-challenge increase on blood or BAL eosinophils of P-selectin glycoprotein ligand-1 (PSGL-1), a receptor for P-selectin that causes activation of β(1) -integrins. Forward scatter of blood eosinophils post-challenge was less heterogeneous and on the average decreased after anti-IL-5; however, anti-IL-5 had no effect on the decreased forward scatter of eosinophils in post-challenge BAL compared with eosinophils in blood. Blood eosinophil KIM-127 reactivity at the time of challenge correlated with the percentage of eosinophils in BAL post-challenge.

CONCLUSION AND CLINICAL RELEVANCE

IL-5 supports a heterogeneous population of circulating eosinophils with partially activated β(2) -integrins and is responsible for up-regulation of β(2) -integrins and PSGL-1 on circulating eosinophils following segmental antigen challenge but has minimal effects on properties of eosinophils in BAL. Dampening of β(2) -integrin function of eosinophils in transit to inflamed airway may contribute to the decrease in lung inflammation caused by anti-IL-5.

Rac1 is deactivated at integrin activation sites through an IQGAP1-filamin-A-RacGAP1 pathway

Cell migration makes a fundamental contribution to both normal physiology and disease pathogenesis. Integrin engagement with extracellular ligands spatially controls, via the cyclical activation and deactivation of the small GTPase Rac1, the dynamic membrane protrusion and cytoskeletal reorganization events that are required for directional migration. Although the pathways that control integrin-mediated Rac1 activation are reasonably well defined, the mechanisms that are responsible for switching off activity are poorly understood. Here, proteomic analysis of activated integrin-associated complexes suggests filamin-A and IQ-motif-containing GTPase-activating protein 1 (IQGAP1) as candidates that link β1 integrin to Rac1. siRNA-mediated knockdown of either filamin-A or IQGAP1 induced high, dysregulated Rac1 activity during cell spreading on fibronectin. Using immunoprecipitation and immunocytochemistry, filamin-A and IQGAP1 were shown to be part of a complex that is recruited to active β1 integrin. Mass spectrometric analysis of individual filamin-A, IQGAP1 and Rac1 pull-downs and biochemical analysis, identified RacGAP1 as a novel IQGAP1 binding partner. Further immunoprecipitation and immunocytochemistry analyses demonstrated that RacGAP1 is recruited to IQGAP1 and active β1 integrin, and that suppression of RacGAP1 expression triggered elevated Rac1 activity during spreading on fibronectin. Consistent with these findings, reduced expression of filamin-A, IQGAP1 or RacGAP1 triggered unconstrained membrane protrusion and disrupted directional cell migration on fibrillar extracellular matrices. These findings suggest a model whereby integrin engagement, followed by filamin-A, IQGAP1 and RacGAP1 recruitment, deactivates Rac1 to constrain its activity spatially and thereby coordinate directional cell migration.

p38 signaling and receptor recycling events in a microfluidic endothelial cell adhesion assay

Adhesion-based microfluidic cell separation has proven to be very useful in applications ranging from cancer diagnostics to tissue engineering. This process involves functionalizing microchannel surfaces with a capture molecule. High specificity and purity capture can be achieved using this method. Despite these advances, little is known about the mechanisms that govern cell capture within these devices and their relationships to basic process parameters such as fluid shear stress and the presence of soluble factors. This work examines how the adhesion of human endothelial cells (ECs) is influenced by a soluble tetrapeptide, Arg-Glu-Asp-Val (REDV) and fluidic shear stress. The ability of these ECs to bind within microchannels coated with REDV is shown to be governed by shear- and soluble-factor mediated changes in p38 mitogen-activated protein kinase expression together with recycling of adhesion receptors from the endosome.

Survivin promotion of melanoma metastasis requires upregulation of α5 integrin

Survivin is an apoptotic and mitotic regulator that is overexpressed in melanoma and a poor prognostic marker in patients with metastatic disease. We recently showed that Survivin enhances melanoma cell motility through Akt-dependent upregulation of α5 integrin. However, the functional role of Survivin in melanoma metastasis is not clearly understood. We found that overexpression of Survivin in LOX and YUSAC2 human melanoma cells increased colony formation in soft agar, and this effect was abrogated by knockdown of α5 integrin by RNA interference. We employed melanoma cell xenografts to determine the in vivo effect of Survivin overexpression on melanoma metastasis. Although Survivin overexpression did not affect primary tumor growth of YUSAC2 or LOX subcutaneous tumors, or indices of proliferation or apoptosis, it significantly increased expression of α5 integrin in the primary tumors and formation of metastatic colonies in the lungs. Additionally, Survivin overexpression resulted in enhanced lung colony formation following intravenous (i.v.) injection of tumor cells in vivo and increased adherence to fibronectin-coated plastic in vitro. Importantly, in vivo inhibition of α5 integrin via intraperitoneal injection of an α5β1 integrin-blocking antibody significantly slowed tumor growth and reduced Survivin-enhanced pulmonary metastasis. Knockdown of α5 integrin in cells prior to i.v. injection also blocked Survivin-enhanced lung colony formation. These findings support a direct role for Survivin in melanoma metastasis, which requires α5 integrin and suggest that inhibitors of α5 integrin may be useful in combating this process.

β1 integrin regulates Arg to promote invadopodial maturation and matrix degradation

β1 integrin is a major regulator of invadopodium maturation. Studies reveal that β1 integrin–mediated adhesion is a key upstream switch that induces Arg-dependent cortactin phosphorylation, actin polymerization, and MMP recruitment to invadopodia for extracellular matrix degradation.

β1 integrin has been shown to promote metastasis in a number of tumor models, including breast, ovarian, pancreatic, and skin cancer; however, the mechanism by which it does so is poorly understood. Invasive membrane protrusions called invadopodia are believed to facilitate extracellular matrix degradation and intravasation during metastasis. Previous work showed that β1 integrin localizes to invadopodia, but its role in regulating invadopodial function has not been well characterized. We find that β1 integrin is required for the formation of mature, degradation-competent invadopodia in both two- and three-dimensional matrices but is dispensable for invadopodium precursor formation in metastatic human breast cancer cells. β1 integrin is activated during invadopodium precursor maturation, and forced β1 integrin activation enhances the rate of invadopodial matrix proteolysis. Furthermore, β1 integrin interacts with the tyrosine kinase Arg and stimulates Arg-dependent phosphorylation of cortactin on tyrosine 421. Silencing β1 integrin with small interfering RNA completely abrogates Arg-dependent cortactin phosphorylation and cofilin-dependent barbed-end formation at invadopodia, leading to a significant decrease in the number and stability of mature invadopodia. These results describe a fundamental role for β1 integrin in controlling actin polymerization–dependent invadopodial maturation and matrix degradation in metastatic tumor cells.

Targeting constitutively activated β1 integrins inhibits prostate cancer metastasis

Disseminated prostate cancer cells must survive in circulation for metastasis to occur. Mechanisms by which these cells survive are not well understood. By immunohistochemistry of human tissues, we found that levels of β1 integrins and integrin-induced autophosphorylation of FAK (pFAK-Y397) are increased in prostate cancer cells in primary prostate cancer and lymph node metastases, suggesting that β1 integrin activation occurs in metastatic progression of prostate cancer. A conformation-sensitive antibody, 9EG7, was used to examine β1 integrin activation. We found that β1 integrins are constitutively activated in highly metastatic PC3 and PC3-mm2 cells, with less activation in low metastatic LNCaP and C4-2B4 cells. Increased β1 integrin activation as well as the anoikis resistance in prostate cancer cells correlated with metastatic potential in vivo. Knockdown of β1 integrin abrogated anoikis resistance in PC3-mm2 cells. In agreement with β1 integrin activation, PC3-mm2 cells strongly adhered to type I collagen and fibronectin, a process inhibited by the β1 integrin-neutralizing antibody mAb 33B6. mAb 33B6 also inhibited the phosphorylation of β1 integrin downstream effectors, focal adhesion kinase (FAK) and AKT, leading to a 3-fold increase in PC3-mm2 apoptosis. Systemic delivery of mAb 33B6 suppressed spontaneous metastasis of PC3-mm2 from the prostate to distant lymph nodes following intraprostatic injection and suppressed metastasis of PC3-mm2 to multiple organs following intracardiac injection. Thus, constitutively activated β1 integrins play a role in survival of PC3-mm2 cells in circulation and represent a potential target for metastasis prevention.

Integrin activation States and eosinophil recruitment in asthma

Eosinophil arrest and recruitment to the airway in asthma are mediated, at least in part, by integrins. Eosinophils express α4β1, α6β1, αLβ2, αMβ2, αXβ2, αDβ2, and α4β7 integrins, which interact with counter-receptors on other cells or ligands in the extracellular matrix. Whether a given integrin-ligand pair mediates cell adhesion and migration depends on the activation state of the integrin. Integrins exist in an inactive bent, an intermediate-activity extended closed, and a high-activity extended open conformation. Integrin activation states can be monitored by conformation-specific monoclonal antibodies (mAbs). Studies in mice indicate that both β1 and β2 integrins mediate eosinophil recruitment to the lung. In vitro studies indicate that α4β1 and αMβ2 are the principal integrins mediating eosinophil adhesion, including to vascular cell adhesion molecule-1 and the novel αMβ2 ligand periostin. In vivo, blood eosinophils have intermediate-activity β1 integrins, as judged by mAb N29, apparently resulting from eosinophil binding of P-selectin on the surface of activated platelets, and have a proportion of their β2 integrins in the intermediate conformation, as judged by mAb KIM-127, apparently due to exposure to low concentrations of interleukin-5 (IL-5). Airway eosinophils recovered by bronchoalveolar lavage (BAL) after segmental antigen challenge have high-activity β1 integrins and high-activity αMβ2 that does not require IL-5. Here we review information on how the activation states of eosinophil β1 and β2 integrins correlate with measurements of eosinophil recruitment and pulmonary function in asthma. Blood eosinophil N29 reactivity is associated with decreased lung function under various circumstances in non-severe asthma and KIM-127 with BAL eosinophil numbers, indicating that intermediate-activity α4β1 and αMβ2 of blood eosinophils are important for eosinophil arrest and consequently for recruitment and aspects of asthma.

Activation of beta 1 but not beta 3 integrin increases cell traction forces

Cell-generated traction forces induce integrin activation, leading to focal adhesion growth and cell spreading. It remains unknown, however, whether integrin activation feeds back to impact the generation of cytoskeletal tension. Here, we used elastomeric micropost arrays to measure cellular traction forces in wildtype and integrin-null cells. We report that activation of β1 but not β3 integrin, by either increasing density of immobilized fibronectin or treating with manganese, elicited fibroblast spreading and cytoskeletal tension. Furthermore, this force generation required Rho kinase and myosin activity. These findings suggest that integrin activation and cell traction forces comprise a bi-directional signaling unit of cell adhesion.

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.

Gain of glycosylation in integrin α3 causes lung disease and nephrotic syndrome

Integrins are transmembrane αβ glycoproteins that connect the extracellular matrix to the cytoskeleton. The laminin-binding integrin α3β1 is expressed at high levels in lung epithelium and in kidney podocytes. In podocytes, α3β1 associates with the tetraspanin CD151 to maintain a functional filtration barrier. Here, we report on a patient homozygous for a novel missense mutation in the human ITGA3 gene, causing fatal interstitial lung disease and congenital nephrotic syndrome. The mutation caused an alanine-to-serine substitution in the integrin α3 subunit, thereby introducing an N-glycosylation motif at amino acid position 349. We expressed this mutant form of ITGA3 in murine podocytes and found that hyperglycosylation of the α3 precursor prevented its heterodimerization with β1, whereas CD151 association with the α3 subunit occurred normally. Consequently, the β1 precursor accumulated in the ER, and the mutant α3 precursor was degraded by the ubiquitin-proteasome system. Thus, these findings uncover a gain-of-glycosylation mutation in ITGA3 that prevents the biosynthesis of functional α3β1, causing a fatal multiorgan disorder.

Regulation of adhesion site dynamics by integrin traffic

The dynamic control of integrin-mediated cell adhesion to extracellular matrix proteins is crucial for several physiological and pathological phenomena as diverse as embryonic morphogenesis, muscle contraction, tissue repair, and cancer cell dissemination. On one hand, the intrinsic conformational plasticity of integrins, which can be bidirectionally modulated by their ligands and cytosolic adaptors in combination with physical forces, is a key regulatory parameter. On the other hand, endo-exocytic integrin traffic logistics represent an additional important mode of control. Herein, we highlight how these two apparently parallel and independent strategies for tuning integrin function appear instead to be indissolubly intermingled, as eukaryotic cells have evolved distinct molecular strategies and endosomal pathways to traffic ligand-bound and ligand-free integrins.

Distinct roles of AKT isoforms in regulating β1-integrin activity, migration, and invasion in prostate cancer

On the basis of an RNA interference screen, we identify AKT1 and AKT2 as inhibitors of β1-integrin activity and invasion in prostate cancer. AKT1 siRNA induces β-integrin activity and up-regulation of RTKs known to function in cooperation with integrins. In contrast, AKT2 siRNA up-regulates microRNA-200, which increases integrin activity.

AKT1 and AKT2 kinases have been shown to play opposite roles in breast cancer migration and invasion. In this study, an RNA interference screen for integrin activity inhibitors identified AKT1 as an inhibitor of β1-integrin activity in prostate cancer. Validation experiments investigating all three AKT isoforms demonstrated that, unlike in breast cancer, both AKT1 and AKT2 function as negative regulators of cell migration and invasion in PC3 prostate cancer cells. Down-regulation of AKT1 and AKT2, but not AKT3, induced activation of cell surface β1-integrins and enhanced adhesion, migration, and invasion. Silencing of AKT1 and AKT2 also resulted in increased focal adhesion size. Importantly, the mechanisms involved in integrin activity regulation were distinct for the two AKT isoforms. Silencing of AKT1 relieved feedback suppression of the expression and activity of several receptor tyrosine kinases, including EGFR and MET, with established cross-talk with β1-integrins. Silencing of AKT2, on the other hand, induced up-regulation of the microRNA-200 (miR-200) family, and overexpression of miR-200 was sufficient to induce integrin activity and cell migration in PC3 cells. Taken together, these data define an inhibitory role for both AKT1 and AKT2 in prostate cancer migration and invasion and highlight the cell type–specific actions of AKT kinases in the regulation of cell motility.

A functional genetic screen reveals new regulators of β1-integrin activity

β1 integrins constitute a large group of widely distributed adhesion receptors, which regulate the ability of cells to interact with their surroundings. This regulation of the expression and activity of integrins is crucial for tissue homeostasis and development and contributes to inflammation and cancer. We report an RNA interference screen to uncover genes involved in the regulation of β1-integrin activity using cell spot microarray technology in cancer cell lines. Altogether, ten cancer and two normal cell lines were used to identify regulators of β1 integrin activity. Cell biological analysis of the identified β1-integrin regulatory genes revealed that modulation of integrin activity can influence cell invasion in a three-dimensional matrix. We demonstrate with loss-of-function and rescue experiments that CD9 activates and MMP8 inactivates β1 integrins and that both proteins associate with β1 integrins in cells. Furthermore, CD9 and MMP8 regulate cancer cell extravasation in vivo. Our discovery of new regulators of β1-integrin activity highlight the complexity of integrin activity regulation and provide a set of new genes involved in regulation of integrin function.

Distinct recycling of active and inactive β1 integrins

Integrin trafficking plays an important role in cellular motility and cytokinesis. Integrins undergo constant endo/exocytic shuttling to facilitate the dynamic regulation of cell adhesion. Integrin activity toward the components of the extracellular matrix is regulated by the ability of these receptors to switch between active and inactive conformations. Several cellular signalling pathways have been described in the regulation of integrin traffic under different conditions. However, the interrelationship between integrin activity conformations and their endocytic fate have remained incompletely understood. Here, we have investigated the endocytic trafficking of active and inactive β1 integrins in cancer cells. Both conformers are endocytosed in a clathrin- and dynamin-dependent manner. The net endocytosis rate of the active β1 integrins is higher, whereas endocytosis of the inactive β1 integrin is counteracted by rapid recycling back to the plasma membrane via an ARF 6- and early endosome antigen 1-positive compartment in an Rab 4a- and actin-dependent manner. Owing to these distinct trafficking routes, the two receptor pools display divergent subcellular localization. At steady state, the inactive β1 integrin is mainly on the plasma membrane, whereas the active receptor is predominantly intracellular. These data provide new insights into the endocytic traffic of integrins and imply the possibility of a previously unappreciated crosstalk between pathways regulating integrin activity and traffic.

NOX2-derived reactive oxygen species are crucial for CD29-induced pro-survival signalling in cardiomyocytes

AIMS

The highly expressed cell adhesion receptor CD29 (β(1)-integrin) is essential for cardiomyocyte growth and survival, and its loss of function causes severe heart disease. However, CD29-induced signalling in cardiomyocytes is ill defined and may involve reactive oxygen species (ROS). A decisive source of cardiac ROS is the abundant NADPH oxidase (NOX) isoform NOX2. Because understanding of NOX-derived ROS in the heart is still poor, we sought to test the role of ROS and NOX in CD29-induced survival signalling in cardiomyocytes.

METHODS AND RESULTS

In neonatal rat ventricular myocytes, CD29 activation induced intracellular ROS formation (oxidative burst) as assessed by flow cytometry using the redox-sensitive fluorescent dye dichlorodihydrofluorescein diacetate. This burst was inhibited by apocynin and diphenylene iodonium. Further, activation of CD29 enhanced NOX activity (lucigenin-enhanced chemiluminescence) and activated the MEK/ERK and PI3K/Akt survival pathways. CD29 also induced phosphorylation of the inhibitory Ser9 on the pro-apoptotic kinase glycogen synthase kinase-3β in a PI3K/Akt- and MEK-dependent manner, and improved cardiomyocyte viability under conditions of oxidative stress. The ROS scavenger MnTMPyP or adenoviral co-overexpression of the antioxidant enzymes superoxide dismutase and catalase inhibited CD29-induced pro-survival signalling. Further, CD29-induced protective pathways were lost in mouse cardiomyocytes deficient for NOX2 or functional p47(phox), a regulatory subunit of NOX.

CONCLUSION

p47(phox)-dependent, NOX2-derived ROS are mandatory for CD29-induced pro-survival signalling in cardiomyocytes. These findings go in line with a growing body of evidence suggesting that ROS can be beneficial to the cell and support a crucial role for NOX2-derived ROS in cell survival in the heart.

β2 integrin adhesion complexes maintain the integrity of HIV-1 assembly compartments in primary macrophages

In human monocyte-derived macrophages (MDM), human immunodeficiency virus type 1 (HIV-1) assembly takes place primarily on complex intracellular plasma membrane domains connected to the cell surface by closely apposed membrane sheets or narrow channels. Some of the membranes associated with these compartments are decorated by thick (≈30 nm), electron-dense, cytoplasmic coats. Here we show by immunolabelling of ultrathin cryosections that the β2 integrin CD18, together with the αM and αX integrins (CD11b and CD11c), is clustered at these coated domains, and that the coats themselves contain the cytoskeletal linker proteins talin, vinculin and paxillin that connect the integrin complexes to the actin cytoskeleton. Intracellular plasma membrane-connected compartments (IPMC) with CD18-containing focal adhesion-like coats are also present in uninfected MDM. These compartments become more prominent as the cells mature in tissue culture and their appearance correlates with increased expression of CD18, CD11b/c and paxillin. Depletion of CD18 by RNA interference leads to parallel down-regulation of CD11b and CD11c, as well as of paxillin, and the disappearance of the adhesion-like coats. In addition, CD18 knockdown alters the appearance of virus-containing IPMC in HIV-infected MDM, indicating that the β2 integrin/focal adhesion-like coat structures are involved in the organization of these compartments.

Bartonella henselae engages inside-out and outside-in signaling by integrin β1 and talin1 during invasome-mediated bacterial uptake

The VirB/D4 type IV secretion system (T4SS) of the bacterial pathogen Bartonella henselae (Bhe) translocates seven effector proteins (BepA-BepG) into human cells that subvert host cellular functions. Two redundant pathways dependent on BepG or the combination of BepC and BepF trigger the formation of a bacterial uptake structure termed the invasome. Invasome formation is a multi-step process consisting of bacterial adherence, effector translocation, aggregation of bacteria on the cell surface and engulfment, and eventually, complete internalization of the bacterial aggregate occurs in an F-actin-dependent manner. In the present study, we show that Bhe-triggered invasome formation depends on integrin-β1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin β1 in a fibronectin- and VirB/D4 T4SS-independent manner and that activated integrin β1 is essential for both effector translocation and the actin rearrangements leading to invasome formation. Furthermore, we show that talin1, but not talin2, is required for inside-out activation of integrin β1 during invasome formation. Finally, integrin-β1-mediated outside-in signaling by FAK, Src, paxillin and vinculin is necessary for invasome formation. This is the first example of a bacterial entry process that fully exploits the bi-directional signaling capacity of integrin receptors in a talin1-specific manner.

A syndecan-4 hair trigger initiates wound healing through caveolin- and RhoG-regulated integrin endocytosis

Cell migration during wound healing requires adhesion receptor turnover to enable the formation and disassembly of cell-extracellular matrix contacts. Although recent advances have improved our understanding of integrin trafficking pathways, it is not known how extracellular ligand engagement controls receptor dynamics. Using atomic force microscopy, we have measured cell avidity for fibronectin and defined a mechanism for the outside-in regulation of α(5)β(1)-integrin. Surprisingly, adhesive strength was attenuated by the syndecan-4-binding domain of fibronectin due to a rapid triggering of α(5)β(1)-integrin endocytosis. Association of syndecan-4 with PKCα was found to trigger RhoG activation and subsequent dynamin- and caveolin-dependent integrin uptake. Like disruption of syndecan-4 or caveolin, gene disruption of RhoG in mice was found to retard closure of dermal wounds due to a migration defect of the fibroblasts and keratinocytes of RhoG null mice. Thus, this syndecan-4-regulated integrin endocytic pathway appears to play a key role in tissue repair.

SHARPIN is an endogenous inhibitor of β1-integrin activation

Regulated activation of integrins is critical for cell adhesion, motility and tissue homeostasis. Talin and kindlins activate β1-integrins, but the counteracting inhibiting mechanisms are poorly defined. We identified SHARPIN as an important inactivator of β1-integrins in an RNAi screen. SHARPIN inhibited β1-integrin functions in human cancer cells and primary leukocytes. Fibroblasts, leukocytes and keratinocytes from SHARPIN-deficient mice exhibited increased β1-integrin activity, which was fully rescued by re-expression of SHARPIN. We found that SHARPIN directly binds to a conserved cytoplasmic region of integrin α-subunits and inhibits recruitment of talin and kindlin to the integrin. Therefore, SHARPIN inhibits the critical switching of β1-integrins from inactive to active conformations.

TRAF6 ubiquitinates TGFβ type I receptor to promote its cleavage and nuclear translocation in cancer

Transforming growth factor β (TGFβ) is a pluripotent cytokine promoting epithelial cell plasticity during morphogenesis and tumour progression. TGFβ binding to type II and type I serine/threonine kinase receptors (TβRII and TβRI) causes activation of different intracellular signaling pathways. TβRI is associated with the ubiquitin ligase tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6). Here we show that TGFβ, via TRAF6, causes Lys63-linked polyubiquitination of TβRI, promoting cleavage of TβRI by TNF-alpha converting enzyme (TACE), in a PKCζ-dependent manner. The liberated intracellular domain (ICD) of TβRI associates with the transcriptional regulator p300 to activate genes involved in tumour cell invasiveness, such as Snail and MMP2. Moreover, TGFβ-induced invasion of cancer cells is TACE- and PKCζ- dependent and the TβRI ICD is localized in the nuclei of different kinds of tumour cells in tissue sections. Thus, our data reveal a specific role for TβRI in TGFβ mediated tumour invasion.

TGFβ can function as both a tumour suppressor and tumour promoter under different cellular contexts. Here, the cleavage product of the TGFβ type I receptor is shown to be generated in a TGFβ-dependent manner, and can induce the expression of genes involved in tumour cell invasion.

Reconstruction of integrin activation

Integrins are integral membrane proteins that mediate cell-matrix and cell-cell adhesion. They are important for vascular development and hematopoiesis, immune and inflammatory responses, and hemostasis. Integrins are also signaling receptors that can transmit information bidirectionally across plasma membranes. Research in the past 2 decades has made progress in unraveling the mechanisms of integrin signaling and brings the field to the moment of attempting synthetic reconstruction of the signaling pathways in vitro. Reconstruction of biologic processes provides stringent tests of our understanding of the process, as evidenced by studies of other biologic machines, such as ATP synthase, lactose permease, and G-protein-coupled receptors. Here, we review recent progress in reconstructing integrin signaling and the insights that we have gained through these experiments.

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.

The integrin-binding motif RGDS induces protein tyrosine phosphorylation without activation in Bufo arenarum (Amphibia) oocytes

Integrins are cell adhesion molecules that are thought to be involved in sperm–oocyte interaction. Nevertheless, their function in mammalian fertilization is still controversial, as different species behave differently. In amphibians, their role is mainly supported byXenopus laevisstudies, where RGDS peptide induces oocyte activation. We recently provided evidence suggesting the presence and involvement of integrins in the interaction of the oocyte plasma membrane (PM) with sperm in the amphibianBufo arenarum. In order to understand the role of integrin homologs in oocytes and their possible contribution to egg activation mechanisms, we examined the presence of integrin subunits and the effect of RGDS peptide on oocytes and during fertilization. Western blot studies detected integrin subunits α5, αV and β1 in oocytes. In sperm, we could detect only the αV integrin subunit. We found that RGDS peptide was unable to elicit egg activation or MAPK dephosphorylation, but can induce reversible inhibition of fertilization. A similar partial inhibition was produced by an anti-β1 integrin antibody. Using an anti-phosphotyrosine antibody we found major changes in phosphotyrosine-containing proteins in egg extracts minutes after fertilization. Cytosol and PMs isolated from oocytes and fertilized eggs showed additional fertilization-induced phosphorylated proteins. Some of these were also present in cytosol and PMs from RGDS-treated oocytes (partially mimicking fertilization). These findings suggest thatB. arenarumfertilization involves integrins (e.g. β1 subunit) as adhesion proteins. Our data support the view that RGDS-binding receptors may function as signaling receptors inB. arenarumoocytes, but integrin engagement by RGDS is not sufficient for oocyte activation.

Endothelial progenitor cells in acute ischemic kidney injury: strategies for increasing the cells' renoprotective competence

Acute ischemic kidney injury is the most frequent cause of acute renal failure in daily clinical practice. It has become increasingly recognized that microvascular endothelial cell dysfunction (ED) in peritubular capillaries inhibits the process of postischemic renal reperfusion. ED can serve as therapeutic target in the management of acute ischemic kidney injury. Postischemic reflow can be restored by systemic administration of either mature endothelial cells or of endothelial progenitor cells. Endothelial progenitor cells EPCs can be cultured from the peripheral circulation of humans and different animals. The cells act vasoprotectively by direct and indirect mechanisms. The protective effects of EPCs in acute ischemic kidney injury can be stimulated by preincubating the cells with different agonistic mediators. This paper summarizes the currently available data on strategies to improve the renoprotective activity of EPCs in acute ischemic kidney injury.

Divalent cations regulate the folding and activation status of integrins during their intracellular trafficking

Integrins are divalent cation-dependent, αβ heterodimeric adhesion receptors that control many fundamental aspects of cell behaviour by bi-directional signalling between the extracellular matrix and intracellular cytoskeleton. The activation state of cell surface integrins is tightly regulated by divalent cation occupancy of the ligand-binding pocket and by interaction with cytoplasmic adaptor proteins, such as talin. These agents elicit gross conformational changes across the entire molecule, which specify the activation state. Much less is known about the activation state of newly synthesised integrins or the role of cations during the early folding and trafficking of integrins. Here we use a number of well-characterised, conformation-specific antibodies to demonstrate that β1-integrins adopt the bent, inactive conformation after assembly with α-integrins in the endoplasmic reticulum. Folding and assembly are totally dependent on the binding of Ca(2+) ions. In addition, Ca(2+) binding prevents integrin activation before its arrival at the cell surface. Activation at the cell surface occurs only following displacement of Ca(2+) with Mg(2+) or Mn(2+). These results demonstrate the essential roles played by divalent cations to facilitate folding of the β-integrin subunit, to prevent inappropriate intracellular integrin signalling, and to activate ligand binding and signalling at the cell surface.

Blood vessel tubulogenesis requires Rasip1 regulation of GTPase signaling

Cardiovascular function depends on patent blood vessel formation by endothelial cells (ECs). However, the mechanisms underlying vascular "tubulogenesis" are only beginning to be unraveled. We show that endothelial tubulogenesis requires the Ras interacting protein 1, Rasip1, and its binding partner, the RhoGAP Arhgap29. Mice lacking Rasip1 fail to form patent lumens in all blood vessels, including the early endocardial tube. Rasipl null angioblasts fail to properly localize the polarity determinant Par3 and display defective cell polarity, resulting in mislocalized junctional complexes and loss of adhesion to extracellular matrix (ECM). Similarly, depletion of either Rasip1 or Arhgap29 in cultured ECs blocks in vitro lumen formation, fundamentally alters the cytoskeleton, and reduces integrin-dependent adhesion to ECM. These defects result from increased RhoA/ROCK/myosin II activity and blockade of Cdc42 and Rac1 signaling. This study identifies Rasip1 as a unique, endothelial-specific regulator of Rho GTPase signaling, which is essential for blood vessel morphogenesis.

Modulus-driven differentiation of marrow stromal cells in 3D scaffolds that is independent of myosin-based cytoskeletal tension

Proliferation and differentiation of cells are known to be influenced by the physical properties of the extracellular environment. Previous studies examining biophysics underlying cell response to matrix stiffness utilized a two-dimensional (2D) culture format, which is not representative of the three-dimensional (3D) tissue environment in vivo. We report on the effect of 3D matrix modulus on human bone marrow stromal cell (hBMSC) differentiation. hBMSCs underwent osteogenic differentiation in poly(ethylene glycol) hydrogels of all modulus (300-fold modulus range, from 0.2 kPa to 59 kPa) in the absence of osteogenic differentiation supplements. This osteogenic differentiation was modulus-dependent and was enhanced in stiffer gels. Osteogenesis in these matrices required integrin-protein ligation since osteogenesis was inhibited by soluble Arginine-Glycine-Aspartate-Serine peptide, which blocks integrin receptors. Immunostained images revealed lack of well-defined actin filaments and microtubules in the encapsulated cells. Disruption of mechanosensing elements downstream of integrin binding that have been identified from 2D culture such as actin filaments, myosin II contraction, and RhoA kinase did not abrogate hBMSC material-driven osteogenic differentiation in 3D. These data show that increased hydrogel modulus enhanced osteogenic differentiation of hBMSCs in 3D scaffolds but that hBMSCs did not use the same mechanosensing pathways that have been identified in 2D culture.

Integrin structure, activation, and interactions

Integrins are large, membrane-spanning, heterodimeric proteins that are essential for a metazoan existence. All members of the integrin family adopt a shape that resembles a large "head" on two "legs," with the head containing the sites for ligand binding and subunit association. Most of the receptor dimer is extracellular, but both subunits traverse the plasma membrane and terminate in short cytoplasmic domains. These domains initiate the assembly of large signaling complexes and thereby bridge the extracellular matrix to the intracellular cytoskeleton. To allow cells to sample and respond to a dynamic pericellular environment, integrins have evolved a highly responsive receptor activation mechanism that is regulated primarily by changes in tertiary and quaternary structure. This review summarizes recent progress in the structural and molecular functional studies of this important class of adhesion receptor.

Requirement of open headpiece conformation for activation of leukocyte integrin alphaXbeta2

Negative stain electron microscopy (EM) and adhesion assays show that alpha(X)beta(2) integrin activation requires headpiece opening as well as extension. An extension-inducing Fab to the beta(2) leg, in combination with representative activating and inhibitory Fabs, were examined for effect on the equilibrium between the open and closed headpiece conformations. The two activating Fabs stabilized the open headpiece conformation. Conversely, two different inhibitory Fabs stabilized the closed headpiece conformation. Adhesion assays revealed that alpha(X)beta(2) in the extended-open headpiece conformation had high affinity for ligand, whereas both the bent conformation and the extended-closed headpiece conformation represented the low affinity state. Intermediate integrin affinity appears to result not from a single conformational state, but from a mixture of equilibrating conformational states.

Preclinical molecular imaging of tumor angiogenesis

Angiogenesis, a course that new blood vessels grow from the existing vasculature, plays important roles both physiologically and pathologically. Angiogenesis can be switched on by growth factors secreted by tumor cells, and in turn supplies more oxygen and nutrition to the tumor. More and more preclinical studies and clinical trials have shown that inhibition of angiogenesis is an effective way to inhibit tumor growth, substantiating the development of anti-angiogenesis therapeutics. Imaging technologies accelerate the translation of preclinical research to the clinic. In oncology, various imaging modalities are widely applied to drug development, tumor early detection and therapy response monitoring. So far, several angiogenesis related imaging agents are promising in cancer diagnosis. However, more effective imaging agents with less side-effect still need to be pursued to visualize angiogenesis process non-invasively. The main purpose of this review is to summarize the recent progresses in preclinical molecular imaging of angiogenesis and to discuss the potential of the current preclinical probes specific to various angiogenesis targets including vascular endothelial growth factor and its receptors (VEGF/VEGFRs), integrin avb3 and matrix metalloproteinases (MMPs). It is predictable that related investigations in the field will benefit cancer research and quicken the anti-angiogenic drug development.