The integrin beta1 subunit transmembrane domain regulates phosphatidylinositol 3-kinase-dependent tyrosine phosphorylation of Crk-associated substrate

Our previous studies on the transmembrane domain of human integrin subunits have shown that a conserved basic amino acid in both subunits of integrin heterodimers is positioned in the plasma membrane in the absence of interacting proteins. To investigate the possible functional role of the lipid-embedded lysine in the mouse integrin beta1 subunit, this amino acid was replaced with leucine, and the mutated beta1 subunit (beta1A(K756L)) was stably expressed in beta1-deficient GD25 cells. The extracellular domain of beta1A(K756L) integrins possesses a competent conformation for ligand binding as determined by the ability to mediate cell adhesion, and by the presence of the monoclonal antibody 9EG7 epitope. However, the spreading of GD25-beta1A(K756L) cells on fibronectin and laminin-1 was impaired, and the rate of migration of GD25-beta1A(K756L) cells on fibronectin was reduced compared with GD25-beta1A cells. Phosphorylation of tyrosines in focal adhesion kinase (FAK) and the Y416 in c-Src in response to beta1A(K756L)-mediated adhesion was similar to that induced by wild-type beta1. The tyrosine phosphorylation level of paxillin, a downstream target of FAK/Src, was unaffected by the beta1 mutation, whereas tyrosine phosphorylation of CAS was strongly reduced. The results demonstrate that CAS is a target for phosphorylation both by FAK-dependent and -independent pathways after integrin ligation. The latter pathway was inhibited by wortmannin and LY294002, implicating that it required an active phosphatidylinositol 3-kinase. Furthermore, the K756L mutation in the beta1 subunit was found to interfere with beta1-induced activation of Akt. The results from this study identify phosphatidylinositol 3-kinase as an early component of a FAK-independent integrin signaling pathway triggered by the membrane proximal part of the beta1 subunit.     

Tyrosine phosphorylation of the integrin beta 3 subunit regulates beta 3 cleavage by calpain

Outside-in signaling of beta(3) integrins induces and requires phosphorylation at tyrosine 747 (Tyr(747)) and tyrosine 759 (Tyr(759)) of the beta(3) subunit, but the mechanism for this requirement is unclear. On the other hand, a key consequence of integrin signaling, cell spreading, is inhibited by calpain cleavage of beta(3) cytoplasmic domain. Here we show that beta(3) tyrosine phosphorylation inhibits calpain cleavage. Mutating both tyrosines to phenylalanine sensitizes beta(3) to calpain cleavage. Furthermore, phosphorylation at Tyr(747) and Tyr(759) of beta(3) in the focal adhesion sites and the leading edge of spreading platelets was differentially regulated. Selective dephosphorylation of Tyr(759) is associated with calpain cleavage at Tyr(759). Thus, one mechanism by which tyrosine phosphorylation promotes integrin signaling and cell spreading is its inhibition of calpain cleavage of the beta(3) cytoplasmic domain.     

Adhesive ligand binding to integrin alpha IIb beta 3 stimulates tyrosine phosphorylation of novel protein substrates before phosphorylation of pp125FAK

Tyrosine phosphorylation of multiple platelet proteins is stimulated by thrombin and other agonists that cause platelet aggregation and secretion. The phosphorylation of a subset of these proteins, including a protein tyrosine kinase, pp125FAK, is dependent on the platelet aggregation that follows fibrinogen binding to integrin alpha IIb beta 3. In this report, we examined whether fibrinogen binding, per se, triggers a process of tyrosine phosphorylation in the absence of exogenous agonists. Binding of soluble fibrinogen was induced with Fab fragments of an anti-beta 3 antibody (anti-LIBS6) that directly exposes the fibrinogen binding site in alpha IIb beta3. Proteins of 50-68 KD and 140 kD became phosphorylated on tyrosine residues in a fibrinogen- dependent manner. This response did not require prostaglandin synthesis, an increase in cytosolic free calcium, platelet aggregation or granule secretion, nor was it associated with tyrosine phosphorylation of pp125FAK. Tyrosine phosphorylation of the 50-68-kD and 140-kD proteins was also observed when (a) fibrinogen binding was stimulated by agonists such as epinephrine, ADP, or thrombin instead of by anti-LIBS6; (b) fragment X, a dimeric plasmin-derived fragment of fibrinogen was used instead of fibrinogen; or (c) alpha IIb beta 3 complexes were cross-linked by antibodies, even in the absence of fibrinogen. In contrast, no tyrosine phosphorylation was observed when the ligand consisted of monomeric cell recognition peptides derived from fibrinogen (RGDS or gamma 400-411). Fibrinogen-dependent tyrosine phosphorylation was inhibited by cytochalasin D. These studies demonstrate that fibrinogen binding to alpha IIb beta 3 initiates a process of tyrosine phosphorylation that precedes platelet aggregation and the phosphorylation of pp125FAK. This reaction may depend on the oligomerization of integrin receptors and on the state of actin polymerization, organizational processes that may juxtapose tyrosine kinases with their substrates.     

Site-selective regulation of platelet-derived growth factor beta receptor tyrosine phosphorylation by T-cell protein tyrosine phosphatase

The platelet-derived growth factor (PDGF) beta receptor mediates mitogenic and chemotactic signals. Like other tyrosine kinase receptors, the PDGF beta receptor is negatively regulated by protein tyrosine phosphatases (PTPs). To explore whether T-cell PTP (TC-PTP) negatively regulates the PDGF beta receptor, we compared PDGF beta receptor tyrosine phosphorylation in wild-type and TC-PTP knockout (ko) mouse embryos. PDGF beta receptors were hyperphosphorylated in TC-PTP ko embryos. Fivefold-higher ligand-induced receptor phosphorylation was observed in TC-PTP ko mouse embryo fibroblasts (MEFs) as well. Reexpression of TC-PTP partly abolished this difference. As determined with site-specific phosphotyrosine antibodies, the extent of hyperphosphorylation varied among different autophosphorylation sites. The phospholipase Cgamma1 binding site Y1021, previously implicated in chemotaxis, displayed the largest increase in phosphorylation. The increase in Y1021 phosphorylation was accompanied by increased phospholipase Cgamma1 activity and migratory hyperresponsiveness to PDGF. PDGF beta receptor tyrosine phosphorylation in PTP-1B ko MEFs but not in PTPepsilon ko MEFs was also higher than that in control cells. This increase occurred with a site distribution different from that seen after TC-PTP depletion. PDGF-induced migration was not increased in PTP-1B ko cells. In summary, our findings identify TC-PTP as a previously unrecognized negative regulator of PDGF beta receptor signaling and support the general notion that PTPs display site selectivity in their action on tyrosine kinase receptors.     

An integrin phosphorylation switch: the effect of beta3 integrin tail phosphorylation on Dok1 and talin binding

Integrins play a fundamental role in cell migration and adhesion; knowledge of how they are regulated and controlled is vital for understanding these processes. Recent work showed that Dok1 negatively regulates integrin activation, presumably by competition with talin. To understand how this occurs, we used NMR spectroscopy and x-ray crystallography to investigate the molecular details of interactions with integrins. The binding affinities of beta3 integrin tails for the Dok1 and talin phosphotyrosine binding domains were quantified using 15N-1H hetero-nuclear single quantum correlation titrations, revealing that the unphosphorylated integrin tail binds more strongly to talin than Dok1. Chemical shift mapping showed that unlike talin, Dok1 exclusively interacts with the canonical NPXY motif of the beta3 integrin tail. Upon phosphorylation of Tyr 747 in the beta3 integrin tail, however, Dok1 then binds much more strongly than talin. Thus, we show that phosphorylation of Tyr 747 provides a switch for integrin ligand binding. This switch may represent an in vivo mechanism for control of integrin receptor activation. These results have implications for the control of integrin signaling by proteins containing phosphotyrosine binding domains.     

Stimulation of beta 1 integrin induces tyrosine phosphorylation of vascular endothelial growth factor receptor-3 and modulates cell migration.

Interactions between integrins and tyrosine kinase receptors can modulate a variety of cell functions. We observed a cooperative interaction between the beta(1) integrin and vascular endothelial growth factor receptor-3 (VEGFR-3 or Flt4) that appeared to be required for cell migration. By using VEGFR-3-transfected 293 cells (293/VEGFR-3) or primary dermal microvascular endothelial cells (DMEC), we found that stimulation with either soluble or immobilized extracellular matrix (ECM) proteins, collagen or fibronectin (FN), resulted in the increased tyrosine phosphorylation of VEGFR-3 in the absence of a cognate ligand. This increased tyrosine phosphorylation of VEGFR-3 was diminished by pretreatment with a blocking antibody against the beta(1) integrin. Cross-linking with anti-beta(1) integrin antibody induced a similar degree of tyrosine phosphorylation of VEGFR-3. Stimulation with collagen or FN induced an association between beta(1) integrin and VEGFR-3 in both 293/VEGFR-3 and primary DMEC cells. Collagen or FN-induced tyrosine phosphorylation of VEGFR-3 was inhibited by treatment with cytochalasin D, an inhibitor of actin polymerization. Collagen or FN was able to induce the migration of 293/VEGFR-3 or DMEC cells to a limited extent. However, migration was dramatically enhanced when a gradient of the cognate ligand, VEGF-D, was added. VEGF-D failed to induce cell migration in the absence of ECM proteins. Introducing a mutation at the kinase domain of VEGFR-3 or treatment with blocking antibody against either VEGFR-3 or beta(1) integrin inhibited cell migration induced by ECM and VEGF-D, indicating that signals from both beta(1) integrin and VEGFR-3 are required for this cell function.     

Tyrosine phosphorylation of beta3 integrin provides a binding site for Pyk2

Integrins expressed on leukocytes possess the ability to maintain themselves in a non-adhesive state, thus preventing unwarranted adhesion and uncontrolled inflammation. Leukocyte adhesion is regulated through the modulation of integrin receptors such as alpha(V)beta(3). Firm adhesion to the extracellular matrix and directed cellular motility requires the reorganization of the actin cytoskeleton. The ability of beta(3) to recruit signaling and scaffolding molecules to propagate alpha(V)beta(3) -mediated signals is regulated in part by the phosphorylation of the beta(3) cytoplasmic tail. The identities of integrin-associated signaling molecules within alpha(V)beta(3) podosomes and in particular the proximal binding partners of the beta(3) cytoplasmic tail are not completely known. Here we show that alpha(V)beta(3) ligation induces Pyk2-Tyr-402 phosphorylation and its association with the beta(3) cytoplasmic tail in a beta(3)-Tyr-747 phosphorylation-dependent manner. Pyk2 binding to the beta(3) cytoplasmic tail is direct and dependent upon Pyk2-Tyr-402 and beta(3) -Tyr-747 phosphorylations. These data identify Pyk2 as a phosphorylated beta(3) binding partner, providing a potential structural and signaling platform to achieve alpha(V)beta(3) -mediated remodeling of the actin cytoskeleton.     

Threonine phosphorylation sites in the beta 2 and beta 7 leukocyte integrin polypeptides

The cytoplasmic domains of integrins play a key role in a variety of integrin-mediated events including adhesion, migration, and signaling. The molecular mechanisms that enhance integrin function are still incompletely understood. Because protein kinases are known to be involved in the signaling and the activation of integrins, the role of phosphorylation has been studied by several groups. The beta(2) leukocyte integrin subunit has previously been shown to become phosphorylated in leukocytes on cytoplasmic serine and functionally important threonine residues. We have now mapped the phosphorylated threonine residues in activated T cells. After phorbol ester stimulation, all three threonine residues (758-760) of the threonine triplet became phosphorylated but only two at a time. CD3 stimulation leads to a strong threonine phosphorylation of the beta(2) integrin, but differed from phorbol ester activation in that phosphorylation occurred only on threonine 758. The other leukocyte-specific integrin, beta(7), has also been shown to need the cytoplasmic domain and leukocyte-specific signal transduction elements for integrin activation. Cell activation with phorbol ester, and interestingly, through the TCR-CD3 complex, caused beta(7) integrin binding to VCAM-1. Additionally, cell activation led to increased phosphorylation of the beta(7) subunit, and phosphoamino acid analysis revealed that threonine residues became phosphorylated after cell activation. Sequence analysis by manual radiosequencing by Edman degradation established that threonine phosphorylation occurred in the same threonine triplet as in beta(2) phosphorylation.     

Role of band 3 tyrosine phosphorylation in the regulation of erythrocyte glycolysis.

Previous studies demonstrated that the in vitro tyrosine phosphorylation of the human erythrocyte anion transporter, band 3, prevented the binding of various glycolytic enzymes to the N terminus of the cytoplasmic tail. Since these enzymes are inhibited in their bound state, the functional consequences of band 3 tyrosine phosphorylation in the red cell should be to activate the enzymes and elevate glycolysis. We searched for various enhancers of band 3 tyrosine phosphorylation using a novel assay designed to measure the phosphotyrosine levels at the band 3 tyrosine phosphorylation/glycolytic enzyme-binding site. This assay measures the extent of phosphorylation of a synthetic band 3 peptide entrapped within resealed red cells. Using this assay, three distinct compounds, all mild oxidants, were found to stimulate the tyrosine phosphorylation of band 3. All three compounds were also found to elevate glycolytic rates in intact erythrocytes. Moreover, the antitumor drug adriamycin was found to coordinately prevent these agents from stimulating both band 3 tyrosine phosphorylation and erythrocyte glycolysis. These results suggest a possible function for a protein tyrosine kinase in human erythrocytes, to regulate glycolysis through the tyrosine phosphorylation of band 3.     

Evidence against a direct role of the integrin alpha2beta1 in collagen-induced tyrosine phosphorylation in human platelets.

In the present study we have investigated whether the collagen receptor alpha2beta1 (GPIa-IIa; GP, glycoprotein) regulates protein tyrosine phosphorylation in platelets directly through activation of tyrosine kinases or indirectly through modification of the response to GPVI. The interaction of collagen with alpha2beta1 was inhibited in two distinct ways, using the metalloprotease jararhagin, which cleaves the beta1 subunit, or the antibody P1E6 which competes with binding of collagen to the integrin. The two inhibitors caused a shift to the right in the collagen concentration response curves for protein tyrosine phosphorylation and platelet activation consistent with a causal relationship between the two events. There was no change in the overall pattern of tyrosine phosphorylation in response to high concentrations of collagen in the presence of alpha2beta1 blockade demonstrating that the integrin is not required for this event. In contrast, jararhagin and P1E6 had a small, almost negligible inhibitory effect against responses to the GPVI-selective agonist collagen-related peptide (CRP) and the G protein-coupled receptor agonist thrombin. Crosslinking of alpha2beta1 in solution or by adhesion to a monolayer using a variety of antibodies to either subunit of the integrin did not induce detectable protein tyrosine phosphorylation in whole cell lysates. The snake venom toxin trimucytin-stimulated a similar pattern of tyrosine phosphorylation to that induced by crosslinking of GPVI which was maintained in the presence of jararhagin. Trimucytin may therefore induce activation via GPVI rather than alpha2beta1 as previously thought. These observations show that the integrin alpha2beta1 is not required for regulation of tyrosine phosphorylation by collagen.     

Negative regulation of the E3 ubiquitin ligase itch via Fyn-mediated tyrosine phosphorylation

Conjugation of ubiquitin (Ub) to a protein substrate targets the substrate for degradation or functional modification, which is tightly controlled by diverse mechanisms including phosphorylation of the substrate. An emerging mechanism involves regulation of the E3 Ub ligase, for example, the JNK-dependent phosphorylation and activation of Itch E3 ligase, which controls the turnover of Jun proteins and T cell differentiation. Here we show that Itch is also modulated by an Src kinase Fyn via tyrosine phosphorylation at the Tyr371 residue. Fyn associates with Itch, and loss of Fyn results in reduced Itch phosphorylation. Importantly, tyrosine phosphorylation of Itch appears to reduce its interaction with its substrate JunB. The turnover of JunB is accelerated in Fyn-deficient T cells, which is further reconstituted by Itch Tyr371 mutation. Thus, in contrast to the activation pathway mediated by serine/threonine phosphorylation, tyrosine phosphorylation of Itch plays a negative role in modulating Itch-promoted ubiquitination.     

Modulation of Thr phosphorylation of integrin beta1 during muscle differentiation

By using transient elevations of cytosolic free calcium levels triggered by integrin antibody or laminin (Kwon, M. S., Park, C. S., Choi, K., Park, C.-S., Ahnn, J., Kim, J. I., Eom, S. H., Kaufman, S. J., and Song, W. K. (2000) Mol. Biol. Cell 11, 1433-1443), we have demonstrated that protein phosphatase 2A (PP2A) is implicated in the regulation of reversible phosphorylation of integrin. In E63 skeletal myoblasts, the treatment of PP2A inhibitors such as okadaic acid and endothall induces an increase of phosphorylation of integrin beta1A and thereby inhibits integrin-induced elevation of cytosolic calcium level and formation of focal adhesions. None of these effects were in differentiated myotubes expressing the alternate beta1D isoform. In the presence of okadaic acid, PP2A in association with integrin beta1A was reduced on myoblasts, whereas beta1D on myotubes remained bound with PP2A. Both co-immunoprecipitation and in vitro phosphatase assays revealed that dephosphorylation of residues Thr788-Thr789 in the integrin beta1A cytoplasmic domain is dependent upon PP2A activity. Mutational analysis of the cytoplasmic domain and confocal microscopy experiments indicated that substitution of Thr788-Thr789 with Asn788-Asn789 is of critical importance for regulating the function of integrin beta1. These results suggest that PP2A may be a primary regulator of threonine phosphorylation of integrin beta1A and subsequent activation of downstream signaling molecules. Taken together, we propose that dephosphorylation of residues Thr788-Thr789 in the cytoplasmic domain of integrin beta1A may contribute to the linkage of integrins to focal adhesion sites and induce the association with cytoskeleton proteins. The switch of integrin beta1A to beta1D isoform in myotubes therefore may be a mechanism to escape from phospho-regulation by PP2A and promotes a more stable association of the cytoskeleton with the extracellular matrix.     

Threonine phosphorylation of integrin beta3 in calyculin A-treated platelets is selectively sensitive to 5'-iodotubercidin

Exposure of platelets to toxins (calyculin A or okadaic acid) that inhibit protein serine/threonine phosphatases types 1 and 2A, at concentrations that block aggregatory and secretory responses, results in the phosphorylation of several platelet proteins including integrin beta(3). Since protein phosphorylation represents a balance between kinase and phosphatase activities, this increase in phosphorylation reflects either the removal of phosphatases that oppose constitutively active kinases known to reside in the platelet (e.g., casein kinase 2) or the activation of endogenous kinases. In this study, we demonstrate that the addition of calyculin A promotes the activation of several endogenous platelet protein kinases, including p42/44(mapk), p38(mapk), Akt/PKB, and LKB1. Using a pharmacologic approach, we assessed whether inhibition of these and other enzymes block phosphorylation of beta(3). Inhibitors of p38(mapk), casein kinase, AMP kinase, protein kinase C, and calcium-calmodulin-dependent kinases did not block phosphorylation of beta(3) on thr(753). In contrast, 5'-iodotubercidin, at 50 muM, blocks beta(3) phosphorylation without affecting the efficacy of calyculin A to inhibit platelet aggregation and spreading. These data dissociate threonine phosphorylation of beta(3) molecules and inhibition of platelet responses by protein phosphatase inhibitors.     

Integrin-dependent tyrosine phosphorylation and growth regulation by Vav

The proto-oncogene product p95Vav (Vav) undergoes rapid phosphorylation on tyrosine following stimulation of the T or B cell antigen receptor, and in response to a variety of other cell surface stimuli. Vav contains, among other, a guanine nucleotide exchange factor domain with homology to the Rho/Rac/CDC42 exchange protein Db1. It has been recently shown that Vav is functionally linked to small GTPases of the Rho family, suggesting that it is an activator of Rho GTPases and may participate in regulation of cytoskeletal organization. The present study shows that cell adhesion to fibronectin triggers rapid phosphorylation of Vav on tyrosine in Vav-transfected CHO cells and in Jurkat T cells. Vav phosphorylation is strongly dependent on adhesion and is mediated by beta 1 integrins. Furthermore, Vav overexpression enhances the adhesion-dependent increase in the rate and extent of phosphorylation on focal adhesion kinase and paxillin, and the formation of stress fibers and lamellipodia. In addition, there is a marked increase in the amount of Vav localized to the triton-insoluble fraction following 1 h of incubation on FN. Finally, Vav increases the growth rate of the cells in an adhesion-dependent manner. Our results strongly implicate Vav as a mediator of integrin signal transduction.     

Interleukin 2-induced tyrosine phosphorylation. Interleukin 2 receptor beta is tyrosine phosphorylated

Interaction of interleukin 2 (IL2) with its high affinity membrane receptor complex (IL2R) is sufficient to induce proliferation of T lymphocytes. However, the biochemical mechanisms by which IL2 induces this process remain unresolved. The IL2R complex consists of at least two distinct polypeptides that bind IL2, a 75-kDa intermediate affinity subunit (IL2R beta) and a 55-kDa low affinity subunit (IL2R alpha). As indicated by Western blotting with anti-phosphotyrosine-specific antibodies and confirmed by phosphoamino acid analysis, we now demonstrate that interaction of the T cell growth factor interleukin 2 (IL2) with its high affinity receptor on IL2-sensitive human peripheral blood lymphoblasts induces tyrosine phosphorylation of proteins of 92, 80, 78, 70-75, and 57 kDa. IL2 induced tyrosine phosphorylation in YT 2C2 cells which express only the 75-kDa intermediate affinity IL2 binding molecule (IL2R beta) but not in cells which either express only the 55-kDa low affinity IL2 receptor molecule (IL2R alpha) or no IL2-binding sites. Therefore, IL2R beta, in the absence of IL2R alpha, appears sufficient to transduce the transmembrane signal leading to tyrosine phosphorylation. Two different antibodies reactive with phosphotyrosine specifically immunoprecipitated IL2R beta cross-linked to radiolabeled IL2. These findings suggest that IL2R beta is a substrate for the tyrosine kinase which is activated by IL2 binding to its receptor. Thus, like several other growth factor receptors, activation of the IL2R results in an increase in tyrosine phosphorylation with the receptor itself serving as one substrate.     

Functions of alpha3beta1 integrin

alpha3beta1 integrin is a laminin receptor with apparently diverse functions. In epithelial cells it acts as a receptor for the basement membrane, whereas in neuronal and possibly tumor cells it mediates migration. Interactions of alpha3beta1 integrin with tetraspanin proteins may provide clues to how it transduces signals that affect cell behavior.     

Reduced cell adhesion during mitosis by threonine phosphorylation of beta1 integrin

Cell shape and adhesion of cultured mammalian cells change dramatically during mitosis, however, how cell cycle-dependent alterations in cell adhesion are regulated remain to be elucidated. We show here that normal human mammary epithelial (HME) cells which became less adhesive and adopted the rounded morphology during the G(2)/M phase of the cell cycle significantly reduced their dependence on beta1 integrin-mediated adhesion to laminin, by using function blocking antibody to beta1 integrin. In G(2)/M cells, both total and cell surface expressions of beta1 integrin were comparable with those in G(1) cells but it was phosphorylated at threonines 788-789 within its cytoplasmic domain and coimmunoprecipitated Ca(2+)/calmodulin-dependent protein kinase (CaMK) II. The threonine phosphorylated beta1 integrin significantly reduced its intracellular linkage with actin, with no significant reduction in the actin expression. In contrast, beta1 integrin in G(1) cells was not threonine phosphorylated but formed a link with actin and coimmunoprecipitated the core enzyme of the serine/threonine protein phosphatase (PP) 2A. The results suggest that reduced beta1 integrin-mediated cell adhesion of HME cells to the substratum during mitosis may be induced by beta1 integrin phosphorylation at threonines 788-789 and its reduced ability to link with the actin cytoskeleton.     

Regulation of the pp72syk protein tyrosine kinase by platelet integrin alpha IIb beta 3.

pp72syk is essential for development and function of several hematopoietic cells, and it becomes activated through tandem SH2 interaction with ITAM motifs in immune response receptors. Since Syk is also activated through integrins, which do not contain ITAMs, a CHO cell model system was used to study Syk activation by the platelet integrin, alpha IIb beta 3. As in platelets, Syk underwent tyrosine phosphorylation and activation during CHO cell adhesion to alpha IIb beta 3 ligands, including fibrinogen. This involved Syk autophosphorylation and the tyrosine kinase activity of Src, and it exhibited two novel features. Firstly, unlike alpha IIb beta 3-mediated activation of pp125FAK, Syk activation could be triggered by the binding of soluble fibrinogen and abolished by truncation of the alpha IIb or beta 3 cytoplasmic tail, and it was resistant to inhibition by cytochalasin D. Secondly, it did not require phosphorylated ITAMs since it was unaffected by disruption of an ITAM-interaction motif in the SH2(C) domain of Syk or by simultaneous overexpression of the tandem SH2 domains. These studies demonstrate that Syk is a proximal component in alpha IIb beta 3 signaling and is regulated as a consequence of intimate functional relationships with the alpha IIb beta 3 cytoplasmic tails and with Src or a closely related kinase. Furthermore, there are fundamental differences in the activation of Syk by alpha IIb beta 3 and immune response receptors, suggesting a unique role for integrins in Syk function.     

Urokinase receptors promote beta1 integrin function through interactions with integrin alpha3beta1

The urokinase receptor (uPAR) is linked to cellular migration through its capacity to promote pericellular proteolysis, regulate integrin function, and mediate cell signaling in response to urokinase (uPA) binding. The mechanisms for these activities remain incompletely defined, although uPAR was recently identified as a cis-acting ligand for the beta2 integrin CD11b/CD18 (Mac-1). Here we show that a major beta1 integrin partner for uPAR/uPA signaling is alpha3. In uPAR-transfected 293 cells uPAR complexed (>90%) with alpha3beta1 and antibodies to alpha3 blocked uPAR-dependent vitronectin (Vn) adhesion. Soluble uPAR bound to recombinant alpha3beta1 in a uPA-dependent manner (K(d) < 20 nM) and binding was blocked by a 17-mer alpha3beta1 integrin peptide (alpha325) homologous to the CD11b uPAR-binding site. uPAR colocalized with alpha3beta1 in MDA-MB-231 cells and uPA (1 nM) enhanced spreading and focal adhesion kinase phosphorylation on fibronectin (Fn) or collagen type I (Col) in a pertussis toxin- and alpha325-sensitive manner. A critical role of alpha3beta1 in uPA signaling was verified by studies of epithelial cells from alpha3-deficient mice. Thus, uPAR preferentially complexes with alpha3beta1, promoting direct (Vn) and indirect (Fn, Col) pathways of cell adhesion, the latter a heterotrimeric G protein-dependent mechanism of signaling between alpha3beta1 and other beta1 integrins.