Adhesion-activating phorbol ester increases the mobility of leukocyte integrin LFA-1 in cultured lymphocytes

Integrin-mediated drug resistance in multiple myeloma

Drug resistance remains a major obstacle to the treatment of many hematopoietic malignancies such as multiple myeloma. Although much research has been focused on acquired resistance phenotypes, we believe that de novo drug resistance mechanisms may be an important component in protecting cells from initial drug exposure. It is now realized that many of the biological processes associated with this disease, including cell survival, may come as a result of the direct interactions of malignant plasma cells with the bone marrow microenvironment. This review examines the role of cell adhesion to one bone marrow component, fibronectin (FN), and the impact it may have on response to cytotoxic drugs. We discuss the influence of the integrin VLA-4 (alpha4beta1) on cell adhesion mediated drug resistance (CAM-DR) as well as the effects of chronic drug exposure on integrin function. Data presented here demonstrates that drug selection can make a non-adherent cell line adherent to FN through inside-out integrin activation and consequently cause a decrease in sensitivity to drug. We also speculate on the possible mediators of this intrinsic mechanism of drug resistance which may, along with the integrins themselves, become promising therapeutic targets in cancer treatment.

Dynamic regulation of activated leukocyte cell adhesion molecule-mediated homotypic cell adhesion through the actin cytoskeleton

Restricted expression of activated leukocyte cell adhesion molecule (ALCAM) by hematopoietic cells suggests an important role in the immune system and hematopoiesis. To get insight into the mechanisms that control ALCAM-mediated adhesion we have investigated homotypic ALCAM-ALCAM interactions. Here, we demonstrate that the cytoskeleton regulates ALCAM-mediated cell adhesion because inhibition of actin polymerization by cytochalasin D (CytD) strongly induces homotypic ALCAM-ALCAM interactions. This induction of cell adhesion is likely due to clustering of ALCAM at the cell surface, which is observed after CytD treatment. Single-particle tracking demonstrated that the lateral mobility of ALCAM in the cell membrane is increased 30-fold after CytD treatment. In contrast, both surface distribution and adhesion of a glycosylphosphatidylinositol (GPI)-anchored ALCAM mutant are insensitive to CytD, despite the increase in lateral mobility of GPI-ALCAM upon CytD treatment. This demonstrates that clustering of ALCAM is essential for cell adhesion, whereas enhanced diffusion of ALCAM alone is not sufficient for cluster formation. In addition, upon ligand binding, both free diffusion and the freely dragged distance of wild-type ALCAM, but not of GPI-ALCAM, are reduced over time, suggesting strengthening of the cytoskeleton linkage. From these findings we conclude that activation of ALCAM-mediated adhesion is dynamically regulated through actin cytoskeleton-dependent clustering.

Expression of MacMARCKS restores cell adhesion to ICAM-1-coated surface

To evaluate the role of MacMARCKS, a major substrate of protein kinase C, in cell adhesion, we selected a macrophage cell line, Wehi 274.1.7. Although surface expression of beta2-integrins can be detected on these cells, they lack the phorbol ester- or chemokine-induced adhesion to ICAM-1-coated surface, an event mediated by beta2-integrins. Concomitantly, these cells lack expression of both MacMARCKS and its homologue, MARCKS. When wild type MacMARCKS was expressed in these cells, the phorbol ester-induced adhesion to ICAM-1-coated surface increased approximately 5-fold compared to vector transfected control cells. To further investigate the potential physiological role of MacMARCKS in this adhesion event, we also tested the effect of monocyte chemotactic protein-1, and a 3-fold increase in the adhesion to ICAM-1-coated surface was observed with MacMARCKS-transfected cells. Therefore, these data suggest that MacMARCKS is an essential component in regulating cell adhesion.

The parting of the endothelium: miracle, or simply a junctional affair?

Leukocyte extravasation from the blood across the endothelium is vital for the functioning of the immune system. Our understanding of the early steps of this process has developed rapidly. However, it is still unclear how leukocytes undergo the final step, migrating through the junctions that mediate adhesion between adjacent endothelial cells, while preserving the barrier function of the endothelium. The first stage of transmigration - tethering and rolling - is mediated by interactions between selectins on the surface of leukocytes and glycosylated proteins such as GlyCAM-1 on the surface of endothelial cells. Stimulation of the leukocyte by chemokines then induces tight adhesion, which involves binding of activated leukocyte integrins to endothelial ICAM-1/VCAM-1 molecules. Passage of the leukocyte across the endothelium appears to require delocalization of certain endothelial cell molecules and proteolytic degradation of junctional complexes.

R-Ras contains a proline-rich site that binds to SH3 domains and is required for integrin activation by R-Ras

R-Ras contains a proline-rich motif that resembles SH3 domain-binding sites but that has escaped notice previously. We show here that this site in R-Ras is capable of binding SH3 domains and that the SH3 domain binding may be important for R-Ras function. A fusion protein containing the SH3 domains of the adaptor protein Nck interacted strongly with the R-Ras proline-rich sequence and with the intact protein. The binding was independent of whether R-Ras was in its GDP or GTP form. The Nck binding, which was mediated by the second of the three SH3 domains of Nck, was obliterated by mutations in the proline-rich sequence of R-Ras. The interaction of Nck with R-Ras could also be shown in yeast two-hybrid assays and by co-immunoprecipitation in human cells transfected with Nck and R-Ras. Previous results have shown that the expression of a constitutively active R-Ras mutant, R-Ras(38V), converts mouse 32D monocytic cells into highly adherent cells. Introducing the proline mutations into R-Ras(38V) suppressed the effect of R-Ras on 32D cell adhesion while not affecting GTP binding. These results reveal an unexpected regulatory pathway that controls R-Ras through an SH3 domain interaction. This pathway appears to be important for the ability of R-Ras to control cell adhesion.

Bi-directional signal transduction by integrin receptors

The integrin family of cell surface glycoproteins functions primarily as receptors for extracellular matrix ligands. There are now many well characterized integrin-ligand interactions which are known to influence many aspects of cell behaviour including cell morphology, cell adhesion, cell migration as well as cellular proliferation and differentiation. However, in fulfilling these functions, integrins are not simple adhesion receptors that physically mediate connections across the plasma membrane. Rather, integrin function itself is highly regulated, largely through the formation of specific associations with both structural and regulatory components within cells. It is these intracellular interactions which allow integrin function to effect many biochemical signalling pathways and therefore to impinge upon complex cellular activities. Recently, much research has focused on elucidating the molecular mechanisms which control integrin function and the molecular processes which transduce integrin-mediated signalling events. In this review, we discuss progress in the field of integrin signal transduction including, where applicable, potential therapeutic applications arising from the research.

Sequential binding of CD11a/CD18 and CD11b/CD18 defines neutrophil capture and stable adhesion to intercellular adhesion molecule–1

The relative contributions of CD11a/CD18 and CD11b/CD18 to the dynamics and strength of neutrophil adhesion to intercellular adhesion molecule (ICAM)-1–transfected cells were examined over the time course of chemotactic stimulation. Suspensions of neutrophils and transfectants were sheared in a cone-plate viscometer, and formation of heterotypic aggregates was measured by 2-color flow cytometry. The 2-body collision theory was used to compute adhesion efficiency, defined as the proportion of collisions between neutrophils and target cells that resulted in capture. ICAM-1 surface density and shear rate both regulated adhesion efficiency. Target cells expressing approximately 1000 ICAM-1 sites/μm2 (Ilow) were captured with an efficiency of 0.15 at 100 s−1, which decreased to zero at 300 s−1. At 8-fold higher ICAM-1 expression (Ihigh) corresponding to levels measured on interleukin-1–stimulated endothelium, efficiency was 0.3 at 100 s−1 and remained above background to 900 s−1. Shear alone was sufficient for CD11a/CD18-mediated adhesion to ICAM-1, and stimulation with formyl-methionyl-leucyl-phenylalanine boosted capture efficiency through CD11a/CD18 by 4-fold. In comparison, CD11b/CD18 supported one third of this efficiency, but was necessary for aggregate stability over several minutes of shear and at shear stresses exceeding 5 dyne/cm2. Hydrodynamics influenced capture efficiency predominantly through the collisional contact duration, predicted to be approximately 9 milliseconds for successful capture of Ilow and 4 milliseconds for Ihigh. The implication is that an increase in ICAM-1 from resting levels to those on inflamed endothelium effectively increases the permissible shear in which capture through β2-integrins may occur. Neutrophil adhesion to ICAM-1 appears to be a cooperative and sequential process of CD11a-dependent capture followed by CD11b-mediated stabilization.

Low-affinity LFA-1/ICAM-3 interactions augment LFA-1/ICAM-1-mediated T cell adhesion and signaling by redistribution of LFA-1

Although ICAM-3 is implicated in both adhesion and signal transduction events of leukocytes, its low affinity for LFA-1 compared to other ligands of LFA-1 has puzzled many investigators. Here we investigated the role of ICAM-3 in supporting LFA-1-mediated ICAM-1 binding and subsequently cell signaling. We observed that although ICAM-3 binds poorly to LFA-1 expressed on resting T cells, it specifically facilitates and increases LFA-1-mediated adhesion to the high affinity ligand of LFA-1, ICAM-1. We demonstrate that low-affinity binding of LFA-1 to ICAM-3 together with ICAM-1 alters the cell surface distribution of LFA-1 dramatically, inducing large clusters of LFA-1 that facilitate ICAM-1 binding after LFA-1 activation. We found that LFA-1-mediated ICAM-1 cell-cell interactions such as T cell proliferation greatly depend on low affinity LFA-1/ICAM-3 interactions that enhance stable LFA-1/ICAM-1 cell-cell contact. Taken together, these data demonstrate that low affinity LFA-1 binding to ICAM-3 regulates strong LFA-1/ICAM-1-mediated adhesion by driving LFA-1 into clusters to facilitate cell-cell interactions that take place in the immune system.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

Alpha 4 beta 1 integrin/VCAM-1 interaction activates alpha L beta 2 integrin-mediated adhesion to ICAM-1 in human T cells

Modulation of integrin affinity and/or avidity provides a regulatory mechanism by which leukocyte adhesion to endothelium is strengthened or weakened at different stages of emigration. In this study, we demonstrate that binding of high-affinity alpha 4 beta 1 integrins to VCAM-1 strengthens alpha L beta 2 integrin-mediated adhesion. The strength of adhesion of Jurkat cells, a human leukemia T cell line, or MnCl2-treated peripheral blood T cells to immobilized chimeric human VCAM-1/Fc, ICAM-1/Fc, or both was quantified using parallel plate flow chamber leukocyte detachment assays in which shear stress was increased incrementally (0.5-30 dynes/cm2). The strength of adhesion to VCAM-1 plus ICAM-1, or to a 40-kDa fragment of fibronectin containing the CS-1 exon plus ICAM-1, was greater than the sum of adhesion to each molecule alone. Treatment of Jurkat or blood T cells with soluble cross-linked VCAM-1/Fc or HP2/1, a mAb to alpha 4, significantly increased adhesion to ICAM-1. These treatments induced clustering of alpha L beta 2 integrins, but not the high-affinity beta 2 integrin epitope recognized by mAb 24. Up-regulated adhesion to ICAM-1 was abolished by cytochalasin D, an inhibitor of cytoskeletal rearrangement. Taken together, our data suggest that the binding of VCAM-1 or fibronectin to alpha 4 beta 1 integrins initiates a signaling pathway that increases beta 2 integrin avidity but not affinity. A role for the cytoskeleton is implicated in this process.

Soluble VCAM-1 binding to α4 integrins is cell-type specific and activation dependent and is disrupted during apoptosis in T cells

Soluble vascular cell adhesion molecule-1 (sVCAM-1) is generated during inflammation and can alter lymphocyte functions. The authors report that the binding of sVCAM-1 to 4 integrin-bearing cells is a dynamically regulated, active cellular process. Binding of recombinant sVCAM-1 to 4 integrins on peripheral blood mononuclear cells was cell-type specific. Circulating CD16+ NK cells constitutively bound sVCAM-1 with high affinity, whereas a subpopulation of T-lymphocytes, primarily CD45RO+ (memory), bound sVCAM-1 only after phorbol ester stimulation. sVCAM-1 binding to homogenous stable cell lines was also cell-type specific, and required active cellular processes because it was blocked by the inhibition of ATP synthesis and by Fas-induced apoptosis. Indeed, the loss of high-affinity VCAM-1 binding was an early event in apoptosis. Furthermore, an H-Ras/Raf-initiated signaling pathway also suppressed sVCAM-1 binding to 4β1 integrins. Collectively, these results showed that the capacity of 4 integrins to bind VCAM-1 is actively regulated and that this regulation may control 4 integrin-dependent cellular functions.

Primary megakaryocytes reveal a role for transcription factor NF-E2 in integrin alpha IIb beta 3 signaling

Platelet integrin alphaIIbbeta3 responds to intracellular signals by binding fibrinogen and triggering cytoskeletal reorganization, but the mechanisms of alphaIIbbeta3 signaling remain poorly understood. To better understand this process, we established conditions to study alphaIIbbeta3 signaling in primary murine megakaryocytes. Unlike platelets, these platelet precursors are amenable to genetic manipulation. Cytokine-stimulated bone marrow cultures produced three arbitrary populations of alphaIIbbeta3-expressing cells with increasing size and DNA ploidy: small progenitors, intermediate-size young megakaryocytes, and large mature megakaryocytes. A majority of the large megakaryocytes bound fibrinogen in response to agonists, while almost none of the smaller cells did. Fibrinogen binding to large megakaryocytes was inhibited by Sindbis virus-mediated expression of isolated beta3 integrin cytoplasmic tails. Strikingly, large megakaryocytes from mice deficient in the transcription factor NF-E2 failed to bind fibrinogen in response to agonists, despite normal surface expression of alphaIIbbeta3. Furthermore, while megakaryocytes from wild-type mice spread on immobilized fibrinogen and exhibited filopodia, lamellipodia and Rho-dependent focal adhesions and stress fibers, NF-E2-deficient megakaryocytes adhered poorly. These studies establish that agonist-induced activation of alphaIIbbeta3 is controlled by NF-E2-regulated signaling pathways that mature late in megakaryocyte development and converge at the beta3 cytoplasmic tail. Megakaryocytes provide a physiologically relevant and tractable system for analysis of bidirectional alphaIIbbeta3 signaling.

Function of the cytoskeleton in human neutrophils and methods for evaluation

The cytoskeleton plays a critical role in the determination of cell shape and serves as a scaffold for critical cellular enzymes and adhesion molecules. It provides structural integrity for the cell and regulates the function of many biochemical events that are critical to cellular function. The microfilamentous cytoskeleton participates in force generation necessary for shape change and motion. In neutrophils and other motile cells, polymerization of actin likely drives extension of the lamellae and participates in force generation through interaction with myosin, by polymerization alone and by osmotic mechanisms. Here, we will focus on the microfilamentous cytoskeleton in the neutrophil and briefly review its function as well as some direct and indirect methods that have been used to asses its role in neutrophil function. The discussion will address general approaches and leaves the details of the methods to the references.

Regulation of integrin function by T cell activation: points of convergence and divergence

Lymphocyte adhesiveness is dynamically regulated in response to conditions in the extracellular environment. One mechanism of regulation of integrin adhesion receptors involves a rapid, but transient, increase in integrin function upon T lymphocyte activation. These integrin activating signals can be initiated either via ligation of Ig superfamily members that are coupled to tyrosine kinase cascades, such as the CD3/T cell receptor, CD2, and CD28, or by G protein-coupled receptors for chemokines. Analysis of integrin activation induced by CD3/TCR, CD2 and CD28 suggests a critical role for phosphoinositide 3-OH kinase (PI 3-K). This review summarizes recent insights into PI 3-K-dependent regulation of integrin function in leukocytes, including the mechanisms by which these receptors are coupled to PI 3-K, and potential downstream effectors of PI 3-K that regulate integrin-mediated adhesion in leukocytes.

Enhancement of endothelial cell migration and in vitro tube formation by TAP20, a novel beta 5 integrin-modulating, PKC theta-dependent protein

Migration, proliferation, and tube formation of endothelial cells are regulated by a protein kinase C isoenzyme PKCtheta. A full-length cDNA encoding a novel 20-kD protein, whose expression was PKCtheta-dependent, was identified in endothelial cells, cloned, characterized, and designated as theta-associated protein (TAP) 20. Overexpression of TAP20 decreased cell adhesion and enhanced migration on vitronectin and tube formation in three-dimensional culture. An antiintegrin alphavbeta5 antibody prevented these TAP20 effects. Overexpression of TAP20 also decreased focal adhesion formation in alphavbeta3-deficient cells. The interaction between TAP20 and beta5 integrin cytoplasmic domain was demonstrated by protein coprecipitation and immunoblotting. Thus, the discovery of TAP20, which interacts with integrin beta5 and modulates cell adhesion, migration, and tube formation, further defines a possible pathway to angiogenesis dependent on PKCtheta.

The Talin head domain binds to integrin beta subunit cytoplasmic tails and regulates integrin activation

The beta subunit cytoplasmic domains of integrin adhesion receptors are necessary for the connection of these receptors to the actin cytoskeleton. The cytoplasmic protein, talin, binds to beta integrin cytoplasmic tails and actin filaments, hence forming an integrin-cytoskeletal linkage. We used recombinant structural mimics of beta(1)A, beta(1)D and beta(3) integrin cytoplasmic tails to characterize integrin-binding sites within talin. Here we report that an integrin-binding site is localized within the N-terminal talin head domain. The binding of the talin head domain to integrin beta tails is specific in that it is abrogated by a single point mutation that disrupts integrin localization to talin-rich focal adhesions. Integrin-cytoskeletal interactions regulate integrin affinity for ligands (activation). Overexpression of a fragment of talin containing the head domain led to activation of integrin alpha(IIb)beta(3); activation was dependent on the presence of both the talin head domain and the integrin beta(3) cytoplasmic tail. The head domain of talin thus binds to integrins to form a link to the actin cytoskeleton and can thus regulate integrin function.

Phosphorylated form of MacMARCKS is essential to LFA-1-dependent cell-cell adhesion of U937 monocytic cells

MacMARCKS (MRP, F52), a protein kinase C (PKC) substrate, is involved in the activation of beta2-integrin. To determine the role of the PKC-mediated phosphorylation of MacMARCKS in this process, human U937 monocytic cells were transfected with cDNAs encoding wild type or mutant MacMARCKS. We observed that the expression of the exogenous wild type MacMARCKS greatly enhanced LFA-1-mediated cell-cell adhesion in U937 cells treated with phorbol 12-myristate 13-acetate (PMA). This MacMARCKS-stimulated adhesion depended on the phosphorylation status of MacMARCKS: whereas phosphorylated MacMARCKS enhanced adhesion, unphosphorylated MacMARCKS inhibited it. However, phosphorylated MacMARCKS alone could not induce LFA-1-mediated cell-cell adhesion unless phorbol esters were added, suggesting that the phosphorylation of other proteins might also be involved. Okadaic acid, a phosphatase inhibitor, induced LFA-1-mediated cell-cell adhesion only in the cells expressing wild type or phosphorylated MacMARCKS and not in the cells expressing unphosphorylated MacMARCKS. Therefore, we conclude that the phosphorylated form of MacMARCKS is essential to LFA-1-mediated cell-cell adhesion.

Role of the integrin-associated protein CD9 in binding between sperm ADAM 2 and the egg integrin alpha6beta1: implications for murine fertilization

CD9 is a tetraspan protein that associates with several beta1 integrins, including alpha6beta1. Because alpha6beta1 is present on murine eggs and interacts with the sperm-surface glycoprotein ADAM 2 (fertilin beta), we first asked whether CD9 is present on murine eggs and whether it functions in sperm-egg binding and fusion. CD9 is present on the plasma membrane of oocytes in the ovary as well as on eggs isolated from the oviduct. The anti-CD9 mAb, JF9, potently inhibits sperm-egg binding and fusion in vitro in a dose-dependent manner. JF9 also disrupts binding of fluorescent beads coated with native fertilin or a recombinant fertilin beta disintegrin domain. (Both ligands bind to the egg via alpha6beta1.) Immunohistochemistry showed that CD9 is undetectable in the uterine epithelium, appears basolaterally and as prominent apical patches on the epithelium in the region between the uterus and the oviduct, and then persists apically in the oviduct. The integrin alpha6A subunit is found in similar apical patches in the region between the uterus and oviduct, but is confined to the basal aspect of the epithelium in the uterus and oviduct. Hence, alpha6A and CD9 both are expressed on the apical epithelial surface at the uterine-oviduct junction. These findings correlate with the observation that fertilin beta "knockout" sperm traverse the uterus but do not progress into the oviduct, contributing to the infertility of fertilin beta(-/-) male mice. Our results suggest that high-avidity binding between fertilin beta (ADAM 2) and alpha6beta1 requires cooperation between alpha6beta1 and CD9. Such cooperation may assist sperm passage into the oviduct as well as sperm-egg interactions.

The actin cytoskeleton regulates LFA-1 ligand binding through avidity rather than affinity changes

To elucidate the role of the cytoskeleton regulating avidity or affinity changes in the leukocyte adhesion receptor lymphocyte function-associated antigen-1 (LFA-1) (alpha(L)beta(2)), we generated mutant cytoplasmic LFA-1 receptors and expressed these into the erythroleukemic cell line K562. We determined whether intercellular adhesion molecule-1 (ICAM-1)-mediated adhesion of LFA-1, lacking parts of its cytoplasmic tails, is regulated through receptor diffusion/clustering and/or by altered ligand binding affinity. All cytoplasmic deletion mutants that lack the complete beta(2) cytoplasmic tail and/or the conserved KVGFFKR sequence in the alpha(L) cytoplasmic tail were constitutively active and expressed high levels of the activation epitopes NKI-L16 and M24. Surprisingly, whereas these mutants showed a clustered cell surface distribution of LFA-1, the ligand-binding affinity as measured by titration of soluble ligand ICAM-1 remained unaltered. The notion that redistribution of LFA-1 does not alter ligand-binding affinity is further supported by the finding that disruption of the cytoskeleton by cytochalasin D did not alter the binding affinity nor adhesion to ICAM-1 of these mutants. Most cytoplasmic deletion mutants that spontaneously bound ICAM-1 were not capable to spread on ICAM-1, demonstrating that on these mutants LFA-1 is not coupled to the actin cytoskeleton. From these data we conclude that LFA-1-mediated cell adhesion to ICAM-1 is predominantly regulated by receptor clustering and that affinity alterations do not necessarily coincide with strong ICAM-1 binding.

The platelet cytoskeleton regulates the affinity of the integrin alpha(IIb)beta(3) for fibrinogen

Agonist-generated inside-out signals enable the platelet integrin alpha(IIb)beta(3) to bind soluble ligands such as fibrinogen. We found that inhibiting actin polymerization in unstimulated platelets with cytochalasin D or latrunculin A mimics the effects of platelet agonists by inducing fibrinogen binding to alpha(IIb)beta(3). By contrast, stabilizing actin filaments with jasplakinolide prevented cytochalasin D-, latrunculin A-, and ADP-induced fibrinogen binding. Cytochalasin D- and latrunculin A-induced fibrinogen was inhibited by ADP scavengers, suggesting that subthreshold concentrations of ADP provided the stimulus for the actin filament turnover required to see cytochalasin D and latrunculin A effects. Gelsolin, which severs actin filaments, is activated by calcium, whereas the actin disassembly factor cofilin is inhibited by serine phosphorylation. Consistent with a role for these factors in regulating alpha(IIb)beta(3) function, cytochalasin D- and latrunculin A-induced fibrinogen binding was inhibited by the intracellular calcium chelators 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid acetoxymethyl ester and EGTA acetoxymethyl ester and the Ser/Thr phosphatase inhibitors okadaic acid and calyculin A. Our results suggest that the actin cytoskeleton in unstimulated platelets constrains alpha(IIb)beta(3) in a low affinity state. We propose that agonist-stimulated increases in platelet cytosolic calcium initiate actin filament turnover. Increased actin filament turnover then relieves cytoskeletal constraints on alpha(IIb)beta(3), allowing it to assume the high affinity conformation required for soluble ligand binding.

Role of abnormal integrin-cytoskeletal interactions in impaired beta1 integrin function in chronic myelogenous leukemia hematopoietic progenitors

Abnormal circulation and unregulated proliferation of chronic myelogenous leukemia (CML) progenitors is related, at least in part, to BCR/ABL induced abnormalities in beta1 integrin-mediated adhesion and signaling. The BCR/ABL oncogene has several potential interactions with cytoskeletal elements that are important for normal integrin signaling. In the present study, we evaluated whether abnormalities in beta1 integrin-cytoskeletal interactions were present in primary CML progenitors and contributed to defective integrin function. beta1 integrin-cytoskeletal interactions were studied in CML and normal CD34+ primary hematopoietic progenitors as well as BCR/ABL-transfected or mock-transfected M07e cells. In normal CD34+ progenitors, antibody-mediated cross-linking of beta1 integrins resulted in their redistribution into caps via a process requiring receptor-cytoskeletal interactions. CML CD34+ cells demonstrated significantly impaired beta1 integrin capping. This defect was related to the presence of the BCR/ABL gene, because capping also was impaired in BCR/ABL-transfected M07e cells. Defective receptor capping was not seen for non-integrin receptors. In addition, CML CD34- and M07eBCR/ABL cells also demonstrated increased actin polymerization and altered actin cytoskeletal organization. Further studies suggested that impaired beta1 integrin capping and defective integrin-mediated adhesion and proliferation inhibition in CML cells were related to abnormally enhanced integrin-cytoskeletal association and restricted receptor mobility. Finally, interferon alpha, which restores integrin-mediated adhesion and signaling in CML progenitors, also enhanced integrin capping in CD34+ cells. These studies suggest that p210BCR/ABL induces abnormal association of integrin receptors with the cytoskeleton and restricted receptor mobility and provide new insights into mechanisms underlying abnormal integrin function in CML progenitors.

Immune complex-induced integrin activation and L-plastin phosphorylation require protein kinase A

Integrins in resting leukocytes are poorly adhesive, and cell activation is required to induce integrin-mediated adhesion. We recently demonstrated a close correlation between phosphorylation of Ser(5) in L-plastin (LPL), a leukocyte-specific 67-kDa actin bundling protein, and activation of alpha(M)beta(2)-mediated adhesion in polymorphonuclear neutrophils (PMN) (Jones, S. L., Wang, J., Turck, C. W., and Brown, E. J. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 9331-9336). However, the kinase that phosphorylates LPL Ser(5) has not been identified. We found that cAMP-dependent protein kinase (PKA), but not a variety of other serine kinases, can specifically phosphorylate LPL and LPL-derived peptides on Ser(5) in vitro. The cell-permeable cAMP analog 8-bromo-cAMP and the adenylate cyclase activator forskolin both induce LPL phosphorylation in cells. Two PKA inhibitors, H89 and KT5720, inhibited immune complex (IC)-stimulated LPL phosphorylation as well as IC-induced activation of alpha(M)beta(2)-mediated adhesion in PMN. The dose response of H89 inhibition of PMN adhesion correlated with its inhibition of LPL phosphorylation in response to IC. IC stimulation also transiently increased intracellular cAMP concentration in PMN. Thus, PKA functions in an integrin activation pathway initiated by IC binding to Fcgamma receptors in addition to its better known role as a negative regulator of cell activation by G protein-coupled receptors. In contrast, LPL Ser(5) phosphorylation and PMN adhesion induced by formylmethionyl-leucylphenylalanine or phorbol myristate acetate were not affected by PKA inhibitors, suggesting that a different kinase(s) is responsible for LPL phosphorylation in response to these agonists. Phosphoinositidyl 3-kinase also is required for FcgammaR but not formylmethionyl-leucylphenylalanine- or phorbol myristate acetate-induced LPL phosphorylation and activation of alpha(M)beta(2). Two phosphoinositidyl 3-kinase inhibitors blocked FcgammaR-induced cAMP accumulation, demonstrating that this kinase acts upstream of PKA. These data demonstrate a necessary role for PKA in IC-induced integrin activation and LPL phosphorylation.

Mechanisms and consequences of affinity modulation of integrin alpha(V)beta(3) detected with a novel patch-engineered monovalent ligand

Integrin alpha(V)beta(3) mediates diverse responses in vascular cells, ranging from cell adhesion, migration, and proliferation to uptake of adenoviruses. However, the extent to which alpha(V)beta(3) is regulated by changes in receptor conformation (affinity), receptor diffusion/clustering (avidity), or post-receptor events is unknown. Affinity regulation of the related integrin, alpha(IIb)beta(3), has been established using a monovalent ligand-mimetic antibody, PAC1 Fab. To determine the role of affinity modulation of alpha(V)beta(3), a novel monovalent ligand-mimetic antibody (WOW-1) was created by replacing the heavy chain hypervariable region 3 of PAC1 Fab with a single alpha(V) integrin-binding domain from multivalent adenovirus penton base. Both WOW-1 Fab and penton base bound selectively to activated alpha(V)beta(3), but not to alpha(IIb)beta(3), in receptor and cell binding assays. alpha(V)beta(3) affinity varied with the cell type. Unstimulated B-lymphoblastoid cells bound WOW-1 Fab poorly (apparent K(d) = 2.4 microM), but acute stimulation with phorbol 12-myristate 13-acetate increased receptor affinity >30-fold (K(d) = 80 nM), with no change in receptor number. In contrast, alpha(V)beta(3) in melanoma cells was constitutively active, but ligand binding could be suppressed by overexpression of beta(3) cytoplasmic tails. Up-regulation of alpha(V)beta(3) affinity had functional consequences in that it increased cell adhesion and spreading and promoted adenovirus-mediated gene transfer. These studies establish that alpha(V)beta(3) is subject to rapid regulated changes in affinity that influence the biological functions of this integrin.

PAR1 activation initiates integrin engagement and outside-in signalling in megakaryoblastic CHRF-288 cells

To better understand the means by which cells such as human platelets regulate the binding of the integrin alphaIIbbeta3 to fibrinogen, we have examined agonist-initiated inside-out and outside-in signalling in CHRF-288 cells, a megakaryoblastic cell line that expresses alphaIIbbeta3 and the human thrombin receptor, PAR1. The results show several notable similarities and differences. (1) Activation of PAR1 caused CHRF-288 cells to adhere and spread on immobilized fibrinogen in an alphaIIbbeta3-dependent manner, but did not support the binding of soluble fibrinogen or PAC-1, an antibody specific for activated alphaIIbbeta3. (2) Direct activation of protein kinase C with PMA or disruption of the actin cytoskeleton with low concentrations of cytochalasin D also caused CHRF-288 cells to adhere to fibrinogen. (3) Despite the failure to bind soluble fibrinogen, activation of PAR1 in CHRF-288 cells caused phosphoinositide hydrolysis, arachidonate mobilization and the phosphorylation of p42MAPK, phospholipase A2 and the Rac exchange protein, Vav, all of which occur in platelets. PAR1 activation also caused an increase in cytosolic Ca2+, which, when prevented, blocked adhesion to fibrinogen. (4) Finally, as in platelets, adhesion of CHRF-288 cells to fibrinogen was followed by a burst of integrin-dependent ('outside-in') signalling, marked by FAK phosphorylation and a more prolonged phosphorylation of p42MAPK. However, in contrast to platelets, adhesion to fibrinogen had no effect on Vav phosphorylation. Collectively, these observations show that signalling initiated through PAR1 in CHRF-288 cells can support alphaIIbbeta3 binding to immobilized ligand, but not the full integrin activation needed to bind soluble ligand. This would suggest that there has been an increase in integrin avidity without an accompanying increase in affinity. Such increases in avidity are thought to be due to integrin clustering, which would also explain the results obtained with cytochalasin D. The failure of alphaIIbbeta3 to achieve the high affinity state in CHRF-288 cells was not due to the failure of PAR1 activation to initiate a number of signalling events that normally accompany platelet activation nor did it prevent at least some forms of outside-in signalling. However, at least one marker of outside-in signalling, the augmentation of Vav phosphorylation seen during platelet aggregation, did not occur in CHRF-288 cells.

The cytoskeletal association of CD11/CD18 leukocyte integrins in phorbol ester-activated cells correlates with CD18 phosphorylation

Leukocyte adhesion is a regulated process, which involves CD11/CD18 leukocyte integrins. CD11/CD18 acidity may be regulated intracellularly, and the CD18 polypeptide has previously been shown to become phosphorylated on serine and threonine after phorbol ester activation of T cells. Increased adhesiveness is believed to be mediated by regulating the overall avidity of cellular contact. CD11/CD18 integrins have earlier been reported to interact with several cytoskeletal proteins. We have now studied the involvement of the CD18 phosphorylation in cytoskeletal associations. We have investigated the distribution of phosphorylated CD18 between soluble, cytoskeletal and nuclear fractions of T cell detergent lysates. A significant amount of phosphorylated CD18 polypeptides was observed to fraction along with the cytoskeleton, while the majority of the cell surface CD18 molecules remained in the soluble fraction. Putative candidates for this altered cytoskeletal binding of CD11/CD18 were shown to be talin and filamin, which were observed to bind to CD18 cytoplasmic peptides and co-precipitate with CD18. The importance of the CD18 cytoplasmic domain in the regulation of the leukocyte adhesion was further strengthened by inhibition of phorbol ester-induced T cell adhesion with a phosphorylated lipopeptide corresponding to the cytoplasmic portion of the CD18. These results indicate that the induced CD18 phosphorylation and the altered cytoskeletal binding of the phosphorylated integrin complex may contribute to the increased avidity of CD11/CD18-mediated leukocyte adhesion.

Regulation of Integrin-Mediated T Cell Adhesion by the Transmembrane Protein Tyrosine Phosphatase CD45

The transmembrane protein tyrosine phosphatase CD45 is required for Ag receptor signal transduction in lymphocytes. Recently, a role for CD45 in the regulation of macrophage adhesion has been demonstrated as well. To investigate further the role of CD45 in the regulation of adhesion, we examined integrin-mediated adhesion to fibronectin of two T cell lines and their CD45-deficient variants. The absence of CD45 correlated with enhanced adhesion to fibronectin via integrin α5β1 (VLA-5), but not α4β1 (VLA-4) in both cell lines. Adhesion returned to normal levels upon transfection of wild-type CD45 into the CD45-deficient lines. Transfection of chimeric or mutant molecules expressing some, but not all, CD45 domains and activities demonstrated that both the transmembrane domain and the tyrosine phosphatase activity of CD45 were required for regulation of integrin-dependent adhesion, but the highly glycosylated extracellular domain was dispensable. In contrast, only a catalytically active CD45 cytoplasmic domain was required for TCR signaling. Transfectants that restored normal levels of adhesion to fibronectin coimmunoprecipitated with the transmembrane protein known as CD45-associated protein. These studies demonstrate a novel role for CD45 in adhesion regulation and suggest a possible function for its association with CD45-associated protein.

ICAM-2 and a Peptide from Its Binding Domain Are Efficient Activators of Leukocyte Adhesion and Integrin Affinity

Cell adhesion mediated by the CD11/CD18 integrins and their ligands, the ICAMs, is required for many leukocyte functions. In resting cells the integrins are nonadhesive, but when activated they become adhesive for their ligands. Previous findings have shown that a peptide derived from the first Ig domain of ICAM-2 (P1) binds to LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) and activates leukocyte aggregation. Because its mechanism of action has remained poorly understood, we have now studied the peptide-induced ligand binding in detail. Here we show that P1 was able to induce CD11/CD18-dependent adhesion of human T lymphocytes to immobilized, purified ICAM-1, -2, and -3. The optimal peptide concentration was 150 μg/ml, whereas concentrations higher than 400 μg/ml did not have any stimulatory effect. The increase in adhesion was detectable within 10 min of treatment with the peptide; it was dependent on energy, divalent cations, temperature, and an intact cytoskeleton but was unaffected by protein kinase C and protein tyrosine kinase inhibitors. Peptide treatment resulted in strong stimulation of the binding of soluble, recombinant ICAMs to T lymphocytes, showing that the integrin affinity toward its ligands was increased. Importantly, soluble ICAM-2Fc was also able to induce T lymphocyte adhesion to purified ICAM-1, -2, and -3, and it was a more potent stimulatory molecule than ICAM-1Fc or ICAM-3Fc.

Disruption of filamentous actin inhibits human macrophage fusion

The foreign body reaction to implanted biomaterials, characterized by the presence of macrophages and foreign body giant cells (FBGC), can result in structural and functional failure of the implant. Recently, we have shown that interleukin-4 and interleukin-13 can independently induce human macrophage fusion to form FBGC via a macrophage mannose receptor (MR) -mediated pathway. The MR is believed to mediate both endocytosis of glycoproteins and phagocytosis of microorganisms, which bear terminal mannose, fucose, N-acetylglucosamine, or glucose residues. Polarization of microfilaments to closely apposed macrophage membranes as observed with fluorescence confocal microscopy led us to ask whether MR-mediated fusion occurred via a filamentous actin-dependent pathway. Cytochalasins B and D and latrunculin-A, agents that disrupt microfilaments, inhibited macrophage fusion in a concentration-dependent manner. The concentrations of cytochalasins D and B that inhibited fusion did not significantly decrease macrophage adhesion, spreading, or motility but did inhibit internalization of Candida albicans during interleukin-13-enhanced, MR-mediated phagocytosis. Very low concentrations of cytochalasin B (< 2 microM) induced a slight enhancement of macrophage fusion. Taken together, the results of this study suggest that cytokine-induced, MR-mediated macrophage fusion requires an intact F-actin cytoskeleton and that the mechanism of fusion is similar to phagocytosis.--DeFife, K. M., Jenney, C. R., Colton, E., Anderson, J. M. Disruption of filamentous actin inhibits human macrophage fusion.

Specific activation of leukocyte beta2 integrins lymphocyte function-associated antigen-1 and Mac-1 by chemokines mediated by distinct pathways via the alpha subunit cytoplasmic domains

We show that CC chemokines induced a sustained increase in monocyte adhesion to intercellular adhesion molecule-1 that was mediated by Mac-1 (alphaMbeta2) but not lymphocyte function-associated antigen-1 (LFA-1; alphaLbeta2). In contrast, staining for an activation epitope revealed a rapid and transient up-regulation of LFA-1 activity by monocyte chemotactic protein-1 (MCP-1) in monocytes and Jurkat CCR2 chemokine receptor transfectants or by stromal-derived factor-1alpha in Jurkat cells. Differential kinetics for activation of Mac-1 (sustained) and LFA-1 (transient) avidity in response to stromal-derived factor-1alpha were confirmed by expression of alphaM or alphaL in alphaL-deficient Jurkat cells. Moreover, expression of chimeras containing alphaL and alphaM cytoplasmic domain exchanges indicated that alpha cytoplasmic tails conferred the specific mode of regulation. Coexpressing alphaM or chimeras in mutant Jurkat cells with a "gain of function" phenotype that results in constitutively active LFA-1 demonstrated that Mac-1 was not constitutively active, whereas constitutive activity was mediated via the alphaL cytoplasmic tail, implying the presence of distinct signaling pathways for LFA-1 and Mac-1. Transendothelial chemotaxis of monocytes in response to MCP-1 was dependent on LFA-1; however, Mac-1 was involved at MCP-1 concentrations stimulating its avidity, showing differential contributions of beta2 integrins. Our data suggest that a specific regulation of beta2 integrin avidity by chemokines may be important in leukocyte extravasation and may be triggered by distinct activation pathways transduced via the alpha subunit cytoplasmic domains.

Affinity Modulation of Very Late Antigen-5 Through Phosphatidylinositol 3-Kinase in Mast Cells

Adhesiveness of integrins is up-regulated rapidly by a number of molecules, including growth factors, cytokines, chemokines, and other cell surface receptors, through a mechanism termed inside-out signaling. The inside-out signaling pathways are thought to alter integrin affinity for ligand, or cell surface distribution of integrin by diffusion/clustering. However, it remains to be clarified whether any physiologically relevant agonists induce a rapid change in the affinity of β1 integrins and how ligand-binding affinity is modulated upon stimulation. In this study, we reported that affinity of β1 integrin very late Ag-5 (VLA-5) for fibronectin was rapidly increased in bone marrow-derived mast cells by Ag cross-linking of FcεRI. Ligand-binding affinity of VLA-5 was also augmented by receptor tyrosine kinases when the phospholipase Cγ-1/protein kinase C pathway was inhibited. Wortmannin suppressed induction of the high affinity state VLA-5 in either case. Conversely, introduction of a constitutively active p110 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) increased the binding affinity for fibronectin. Failure of a constitutively active Akt to stimulate adhesion suggested that the affinity modulation mechanisms mediated by PI 3-kinase are distinct from the mechanisms to control growth and apoptosis by PI 3-kinase. Taken together, our findings demonstrated that the increase of affinity of VLA-5 was induced by physiologically relevant stimuli and PI 3-kinase was a critical affinity modulator of VLA-5.

Evidence that distinct states of the integrin alpha6beta1 interact with laminin and an ADAM

Integrins can exist in different functional states with low or high binding capacity for particular ligands. We previously provided evidence that the integrin alpha6beta1, on mouse eggs and on alpha6-transfected cells, interacted with the disintegrin domain of the sperm surface protein ADAM 2 (fertilin beta). In the present study we tested the hypothesis that different states of alpha6beta1 interact with fertilin and laminin, an extracellular matrix ligand for alpha6beta1. Using alpha6-transfected cells we found that treatments (e.g., with phorbol myristate acetate or MnCl2) that increased adhesion to laminin inhibited sperm binding. Conversely, treatments that inhibited laminin adhesion increased sperm binding. Next, we compared the ability of fluorescent beads coated with either fertilin beta or with the laminin E8 fragment to bind to eggs. In Ca2+-containing media, fertilin beta beads bound to eggs via an interaction mediated by the disintegrin loop of fertilin beta and by the alpha6 integrin subunit. In Ca2+-containing media, laminin E8 beads did not bind to eggs. Treatment of eggs with phorbol myristate acetate or with the actin disrupting agent, latrunculin A, inhibited fertilin bead binding, but did not induce laminin E8 bead binding. Treatment of eggs with Mn2+ dramatically increased laminin E8 bead binding, and inhibited fertilin bead binding. Our results provide the first evidence that different states of an integrin (alpha6beta1) can interact with an extracellular matrix ligand (laminin) or a membrane-anchored cell surface ligand (ADAM 2).

TCR, LFA-1, and CD28 Play Unique and Complementary Roles in Signaling T Cell Cytoskeletal Reorganization

T cells interacting with APCs undergo rearrangement of surface receptors and cytoskeletal elements to face the zone of contact with the APC. This polarization process is thought to affect T cell signaling by organizing a specialized domain on the T cell surface and to direct T cell effector function toward the appropriate APC. We have investigated the contribution of TCR, CD28, and LFA-1 signaling to T cell cytoskeletal polarization by assaying the response of an Ag-specific Th1 clone toward a panel of transfected APCs expressing MHC class II alone or in combination with ICAM-1 or B7-1. We show that polarization of talin, an actin-binding protein, occurs in response to integrin engagement. In contrast, reorientation of the T cell microtubule-organizing center (MTOC) is dependent on and directed toward the site of TCR signaling, regardless of whether integrins or costimulatory molecules are engaged. MTOC reorientation in response to peptide-MHC complexes is sensitive to the phosphatidylinositol 3-kinase inhibitor wortmannin. CD28 coengagement overcomes this sensitivity, as does activation via Ab cross-linking of the TCR or via covalent peptide-MHC complexes, suggesting that phosphatidylinositol 3-kinase is not required per se but rather plays a role in signal amplification. Engagement of TCR in trans with LFA-1 results in separation of MTOC reorientation and cortical cytoskeletal polarization events, indicating that the two processes are not directly mechanistically linked. These studies show that T cells mobilize individual cytoskeletal components in response to distinct and specific cell surface interactions.

High Affinity Very Late Antigen-4 Subsets Expressed on T Cells Are Mandatory for Spontaneous Adhesion Strengthening But Not for Rolling on VCAM-1 in Shear Flow

The very late Ag-4 (VLA-4) integrin supports both rolling and firm adhesion of leukocytes on VCAM-1 under shear flow. The molecular basis for the unique ability of a single adhesion molecule to mediate these versatile adhesive processes was investigated. VLA-4 occurs in multiple activation states, with different affinities to ligand. In this study we tested how these states regulate VLA-4 adhesiveness under shear flow in Jurkat T cells and PBL. VLA-4 on nonstimulated Jurkat cells supported rolling and spontaneous arrest on VCAM-1, whereas a Jurkat activation mutant with reduced VLA-4 affinity failed to spontaneously arrest after tethering to or during rolling on VCAM-1. The contribution of VLA-4 affinity for ligand to rolling and spontaneous arrests on immobilized VCAM-1 was dissected using soluble VLA-4 ligands, which selectively block high affinity states. VLA-4 saturation with ligand completely blocked spontaneous adhesion strengthening post-tethering to VCAM-1, but did not impair rolling on the endothelial ligand. High affinity VLA-4 was found to comprise a small subset of VLA-4 on resting Jurkat cells and PBL. This subset is essential for firm adhesion but not for tethering or rolling adhesions on VCAM-1. Interestingly, low and high affinity VLA-4 states were found to mediate similar initial tethering to ligand. High affinity VLA-4, constitutively expressed on circulating T cells, may control their early adhesion strengthening on VCAM-1-expressing endothelium before exposure to vascular chemokines and activation of additional integrins.

Activation of alphaVbeta3 on vascular cells controls recognition of prothrombin

Regulation of vascular homeostasis depends upon collaboration between cells of the vessel wall and blood coagulation system. A direct interaction between integrin alphaVbeta3 on endothelial cells and smooth muscle cells and prothrombin, the pivotal proenzyme of the blood coagulation system, is demonstrated and activation of the integrin is required for receptor engagement. Evidence that prothrombin is a ligand for alphaVbeta3 on these cells include: (a) prothrombin binds to purified alphaVbeta3 via a RGD recognition specificity; (b) prothrombin supports alphaVbeta3-mediated adhesion of stimulated endothelial cells and smooth muscle cells; and (c) endothelial cells, either in suspension and in a monolayer, recognize soluble prothrombin via alphaVbeta3. alphaVbeta3-mediated cell adhesion to prothrombin, but not to fibrinogen, required activation of the receptor. Thus, the functionality of the alphaVbeta3 receptor is ligand defined, and prothrombin and fibrinogen represent activation- dependent and activation-independent ligands. Activation of alphaVbeta3 could be induced not only by model agonists, PMA and Mn2+, but also by a physiologically relevant agonist, ADP. Inhibition of protein kinase C and calpain prevented activation of alphaVbeta3 on vascular cells, suggesting that these molecules are involved in the inside-out signaling events that activate the integrin. The capacity of alphaVbeta3 to interact with prothrombin may play a significant role in the maintenance of hemostasis; and, at a general level, ligand selection by alphaVbeta3 may be controlled by the activation state of this integrin.

Cytoskeletal interactions with the leukocyte integrin beta2 cytoplasmic tail. Activation-dependent regulation of associations with talin and alpha-actinin

Circulating leukocytes are nonadherent but bind tightly to endothelial cells following activation. The increased avidity of leukocyte integrins for endothelial ligands following activation is regulated, in part, by interaction of the beta2 subunit cytoplasmic tail with the actin cytoskeleton. We propose a mechanism to explain how tethering of the actin cytoskeleton to leukocyte integrins is regulated. In resting leukocytes, beta2 integrins are constitutively linked to the actin cytoskeleton via the protein talin. Activation of cells induces proteolysis of talin and dissociation from the beta2 tail. This phase is transient, however, and is followed by reattachment of actin filaments to integrins that is mediated by the protein alpha-actinin. The association of alpha-actinin with integrins may stabilize the cytoskeleton and promote firm adhesion to and migration across the endothelium. Glutathione S-transferase-beta2 tail fusion protein/mutagenesis experiments suggest that the affinity of alpha-actinin binding to the beta2 tail is regulated by a change in the conformation of the tail that unmasks a cryptic alpha-actinin binding domain. Positive and inhibitory domains within the beta2 tail regulate alpha-actinin binding: a single 11-amino acid region (residues 736-746) is necessary and sufficient for alpha-actinin binding, and a regulatory domain between residues 748-762 inhibits constitutive association of the beta2 tail with alpha-actinin.

Making a little affinity go a long way: a topological view of LFA-1 regulation

Lymphocytes utilize adhesion to navigate in the body and to transiently interact with a variety of potential antigen presenting cells. Interactions of adhesion molecules are governed by the law of mass action and the less understood rules of apposed biological membranes. Biochemical parameters such as adhesion molecule affinity only tell part of the story. Factors such as lateral mobility, membrane alignment and cytoskeletal interactions are equally important in determining the final outcome. Therefore it is important to determine mechanisms by which the properties of cell membranes and the cytoskeleton reinforce or hinder adhesion molecule interactions. Work from my lab has shown that one mechanism by which lymphocyte adhesion molecules cooperate is to align adhering membranes with nanometer precision. Here, I discuss a model for LFA-1 regulation that is dependent on three independent processes: LFA-1 lateral mobility, ligand induced generation of a small amount of high affinity LFA-1 and local membrane alignment. I propose that coordination of these processes allows rapid interconversion between stable adhesion and detachment.

Adhesion and signaling mediated by the cytoplasmic tails of leucocyte integrins

Integrins not only mediate cell adhesion but also contribute to a variety of other cellular processes including proliferation, cytokine production, cytotoxicity and apoptosis. They act as bi-directional signal transducers, mediating signaling from inside-to-outside the cell and from outside-to-inside the cell. Evidence is emerging that signaling through leukocyte integrins (beta 2 and beta 7) is distinct from signaling by the more widely distributed beta 1 integrins. Here we discuss the role of the cytoplasmic domains of leukocyte integrins and that of cytosolic proteins that bind integrins in mediating signal transduction. Distinct sites in the alpha as well as the common beta chain contribute to this process. We also show that beta 2 integrin distribution on the cell surface is of particular relevance for leukocytes to rapidly alter their adhesive state and display their highly dynamic adhesive behavior. From these and recently published findings the picture is arising that particular cell functions may be supported by integrin-specific signaling pathways.

Integrin associated proteins

Integrin cytoplasmic domains may interact directly with serveral different cytoskeletal proteins and intracellular signaling molecules. Also, integrins interact directly with other transmembrane structures, including transmembrane-4 superfamily (TM4SF) proteins. New evidence suggests that TM4SF proteins may act as linkers between extracellular integrin alpha chain domains and intracellular signaling molecules, such as phosphatidylinositol 4-kinase and protein kinase C.

Regulation of integrin-mediated adhesion by muscarinic acetylcholine receptors and protein kinase C in small cell lung carcinoma

STUDY OBJECTIVES

Improved understanding of the phenotypic characteristics of small cell lung cancer (SCLC) cells may facilitate the development of new therapies for this bronchogenic malignancy with early metastases. Herein we investigate whether activation of the M3 subtype of muscarinic acetylcholine receptor (mAChR) expressed on SCLC cells affects beta1-integrin-mediated adhesion of these cells.

DESIGN

Adhesion of the SCLC cell lines SCC-9 and NCI-H345 to extracellular matrix (ECM) proteins was investigated. Cell adhesion was quantified by labeling the cells with either toluidine blue dye and measuring optical density or 3H-thymidine and measuring beta-activity. Fluorescence-activated cell sorting was used to quantify the SCLC cell surface expression of beta1-integrins.

SETTING

Experiments were conducted in the Molecular Pharmacology Laboratory, Guthrie Research Institute.

MEASUREMENTS AND RESULTS

Activation of mAChR with the agonist carbachol (10 microM, 1.5 h) significantly increases adhesion of the SCC-9 SCLC cell line to the ECM proteins laminin and collagen types I and IV. In contrast, mAChR activation does not alter the adhesion of SCC-9 cells to vitronectin, fibronectin, poly-L-lysine, or bovine serum albumin. Carbachol also does not alter the adhesion of NCI-H345 SCLC cells that lack functional mAChR. Preincubation of SCC-9 cells with the AIIB2 blocking antibody to beta1-integrin inhibits mAChR-induced adhesion to ECM proteins. Immunofluorescence analysis indicates that mAChR activation does not alter the surface expression of beta1-integrins by SCC-9 cells. Direct stimulation of protein kinase C (PKC) by treatment with phorbol 12-myristate 13-acetate (PMA) (10 nM, 1.5 h) increases the adhesion of both the SCC-9 and NCI-H345 cell lines to ECM proteins. These results indicate that direct activation of PKC or stimulation of M3 mAChR (which results in increased PKC activity) increases the binding activity of beta1-integrins, resulting in increased adhesion of SCLC cells to ECM proteins.

CONCLUSIONS

The ability of mAChR to regulate SCLC proliferation and adhesion suggests that activation of these receptors may be used to alter SCLC tumorigenesis and metastasis.

Integrin affinity modulation

Integrin cell-adhesion receptors mediate interactions between cells and the extracellular matrix. Dynamic regulation of integrin adhesive function is termed 'activation' or 'inside-out' signalling. Activation is key to integrin function in processes as diverse as cell migration, the organization of the extracellular matrix and platelet aggregation. Consequently, there has been an intense effort to elucidate the molecular mechanism of integrin activation. This has resulted in the recent identification of novel cytoplasmic partners for integrins and the emerging characterization of the signal-transduction pathways that regulate integrin 'inside-out' signalling. Here, the authors review the recent developments that have provided us with an increased understanding of the basis of integrin activation.

Interactions of the cytoplasmic domain of P-selectin with clathrin-coated pits enhance leukocyte adhesion under flow

Flowing leukocytes tether to and roll on P-selectin, a receptor on endothelial cells that is rapidly internalized in clathrin-coated pits. We asked whether the association of P-selectin with clathrin-coated pits contributes to its adhesive function. Under flow, rolling neutrophils accumulated efficiently on CHO cells expressing wild-type P-selectin or a P-selectin construct with a substitution in the cytoplasmic domain that caused even faster internalization than that of the wild-type protein. By contrast, far fewer rolling neutrophils accumulated on CHO cells expressing P-selectin constructs with a deletion or a substitution in the cytoplasmic domain that impaired internalization. Neutrophils rolled on the internalization-competent constructs with greater adhesive strength, slower velocity, and more uniform motion. Flowing neutrophils tethered equivalently to internalization-competent or internalization-defective P-selectin, but after tethering, they rolled further on internalization-competent P-selectin. Confocal microscopy demonstrated colocalization of alpha-adaptin, a component of clathrin-coated pits, with wild-type P-selectin, but not with P-selectin lacking the cytoplasmic domain. Treatment of CHO cells or endothelial cells with hypertonic medium reversibly impaired the clathrin-mediated internalization of P-selectin and its ability to support neutrophil rolling. Interactions of the cytoplasmic domain of P-selectin with clathrin-coated pits provide a novel mechanism to enhance leukocyte adhesion under flow.

A role for the actin-bundling protein L-plastin in the regulation of leukocyte integrin function

Regulation of leukocyte integrin avidity is a crucial aspect of inflammation and immunity. The actin cytoskeleton has an important role in the regulation of integrin function, but the cytoskeletal proteins involved are largely unknown. Because inflammatory stimuli that activate integrin-mediated adhesion in human polymorphonuclear neutrophils (PMN) and monocytes cause phosphorylation of the actin-bundling protein L-plastin, we tested whether L-plastin phosphorylation was involved in integrin activation. L-plastin-derived peptides that included the phosphorylation site (Ser-5) rapidly induced leukocyte integrin-mediated adhesion when introduced into the cytosol of freshly isolated primary human PMN and monocytes. Substitution of Ala for Ser-5 abolished the ability of the peptide to induce adhesion. Peptide-induced adhesion was sensitive to pharmacologic inhibition of phosphoinositol 3-kinase and protein kinase C, but adhesion induced by a peptide containing a phosphoserine at position 5 was insensitive to inhibition. These data establish a novel role for L-plastin in the regulation of leukocyte adhesion and suggest that many signaling events implicated in integrin regulation act via induction of L-plastin phosphorylation.

Complementary roles for receptor clustering and conformational change in the adhesive and signaling functions of integrin alphaIIb beta3

Integrin alphaIIb beta3 mediates platelet aggregation and "outside-in" signaling. It is regulated by changes in receptor conformation and affinity and/or by lateral diffusion and receptor clustering. To document the relative contributions of conformation and clustering to alphaIIb beta3 function, alphaIIb was fused at its cytoplasmic tail to one or two FKBP12 repeats (FKBP). These modified alphaIIb subunits were expressed with beta3 in CHO cells, and the heterodimers could be clustered into morphologically detectable oligomers upon addition of AP1510, a membrane-permeable, bivalent FKBP ligand. Integrin clustering by AP1510 caused binding of fibrinogen and a multivalent (but not monovalent) fibrinogen-mimetic antibody. However, ligand binding due to clustering was only 25-50% of that observed when alphaIIb beta3 affinity was increased by an activating antibody or an activating mutation. The effects of integrin clustering and affinity modulation were additive, and clustering promoted irreversible ligand binding. Clustering of alphaIIb beta3 also promoted cell adhesion to fibrinogen or von Willebrand factor, but not as effectively as affinity modulation. However, clustering was sufficient to trigger fibrinogen-independent tyrosine phosphorylation of pp72(Syk) and fibrinogen-dependent phosphorylation of pp125(FAK), even in non-adherent cells. Thus, receptor clustering and affinity modulation play complementary roles in alphaIIb beta3 function. Affinity modulation is the predominant regulator of ligand binding and cell adhesion, but clustering increases these responses further and triggers protein tyrosine phosphorylation, even in the absence of affinity modulation. Both affinity modulation and clustering may be needed for optimal function of alphaIIb beta3 in platelets.

Regulation of alphaIIb beta3 function in human B lymphocytes

We studied the function of the platelet integrin alphaIIb beta3 using a B lymphocyte model in which alphaIIb beta3 can be induced to interact with fibrinogen using phorbol myristate acetate (PMA). To determine whether a G protein-coupled receptor could also activate alphaIIb beta3 in lymphocytes, we coexpressed the human formyl peptide receptor (fPR) and alphaIIb beta3, finding that the fPR agonist formyl Met-Leu-Phe (fMLP)-stimulated lymphocyte adherence to immobilized fibrinogen and binding of soluble fibrinogen to the lymphocyte surface. The response to fMLP, but not PMA, was abrogated by pertussis toxin, indicating that the fPR was coupled to the G-protein Galphai, whereas the protein kinase C inhibitor bisindolylmaleimide I inhibited the response to both fMLP and PMA, indicating that signaling from the fPR included protein kinase C. On the other hand, the tyrosine kinase inhibitor genistein, the Syk inhibitor piceatannol, and the RhoA inhibitor C3 exoenzyme had no effect, implying that neither tyrosine phosphorylation nor the GTPase RhoA were involved. Furthermore, whereas micromolar concentrations of cytochalasin D inhibited the PMA-stimulated interaction of alphaIIb beta3 with fibrinogen, nanomolar concentrations actually induced fibrinogen binding to unstimulated cells. Our studies demonstrate that alphaIIb beta3 expressed in B lymphocytes can be activated by a physiologic agonist and outline an activating pathway that includes Galphai, protein kinase C, and the actin cytoskeleton.

RhoA and the Function of Platelet Integrin αIIbβ3

Integrins respond to “inside-out” signals, which enable them to bind adhesive ligands, and ligand binding initiates “outside-in” signals that mediate anchorage-dependent cellular responses. RhoA is a GTPase that regulates certain actin rearrangements and transcriptional events. It has also been implicated in integrin signaling, but the exact relationship is not understood. To examine this further, platelets were incubated with C3 exoenzyme to adenine diphosphate (ADP)-ribosylate and inactivate RhoA, and the function of integrin αIIbβ3 was studied. Despite inactivation of ≥ 90% of RhoA, platelets exhibited normal inside-out signaling, as monitored by agonist-induced binding of a fibrinogen-mimetic anti-αIIbβ3 antibody and normal fibrinogen-dependent aggregation. On the other hand, RhoA inactivation decreased the adhesion of agonist-stimulated platelets to fibrinogen (P &lt; .04) and the formation of vinculin-rich focal adhesions in platelets that did adhere (P &lt; .001). These effects were selective because fibrin clot retraction, a response also dependent on αIIbβ3 and actin contractility, was unaffected by C3, as was the content of F-actin in resting or agonist-stimulated platelets. Similar results were obtained in a Chinese hamster ovary (CHO) cell model system of αIIbβ3: C3 exoenzyme (or overexpression of dominant-negative N19RhoA) failed to influence integrin activation state, but it blocked the formation of focal adhesions in cells spread on fibrinogen. These studies establish that RhoA plays a highly selective role in αIIbβ3 signaling, and they identify a subset of responses to integrin ligation that may be uniquely dependent on the actin rearrangements regulated by this GTPase.