Fibronectin-stimulated signaling from a focal adhesion kinase-c-Src complex: involvement of the Grb2, p130cas, and Nck adaptor proteins

Src family kinases are required for integrin but not PDGFR signal transduction

Src family kinases (SFKs) have been implicated as important regulators of ligand-induced cellular responses including proliferation, survival, adhesion and migration. Analysis of SFK function has been impeded by extensive redundancy between family members. We have generated mouse embryos harboring functional null mutations of the ubiquitously expressed SFKs Src, Yes and Fyn. This triple mutation leads to severe developmental defects and lethality by E9.5. To elucidate the molecular mechanisms underlying this phenotype, SYF cells (deficient for Src, Yes and Fyn) were derived and tested for their ability to respond to growth factors or plating on extracellular matrix. Our studies reveal that while Src, Yes and Fyn are largely dispensable for platelet-derived growth factor (PDGF)-induced signaling, they are absolutely required to mediate specific functions regulated by extracellular matrix proteins. Fibronectin-induced tyrosine phosphorylation of focal adhesion proteins, including the focal adhesion kinase FAK, was nearly eliminated in the absence of Src, Yes and Fyn. Furthermore, consistent with previous reports demonstrating the importance of FAK for cell migration, SYF cells displayed reduced motility in vitro. These results demonstrate that SFK activity is essential during embryogenesis and suggest that defects observed in SYF triple mutant embryos may be linked to deficiencies in signaling by extracellular matrix-coupled receptors.

Macrophage stimulating protein-induced epithelial cell adhesion is mediated by a PI3-K-dependent, but FAK-independent mechanism

Macrophage stimulating protein (MSP) is a growth and motility factor that mediates its activity via the RON/STK receptor tyrosine kinase. MSP promotes integrin-dependent epithelial cell migration, which suggests that MSP may regulate integrin receptor functions. Integrins are cell surface receptors for extracellular matrix. Epithelial cell adhesion and motility are mediated by integrins. We studied the enhancement by MSP of cell adhesion and the molecular mechanisms mediating this effect. MSP decreased the time required for adhesion of 293 and RE7 epithelial cells to substrates coated with collagen or fibronectin. Prevention of adhesion by an RGD-containing peptide showed that the cell-substrate interaction was mediated by integrins. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K), blocked MSP-dependent adhesion, which shows that PI3-K is in the MSP-induced adhesion pathway. MSP also affected focal adhesion kinase (FAK) which is important for some types of cell adhesion and motility. Although MSP caused PI3-K-independent tyrosine phosphorylation and activation of FAK, experiments with dominant-negative FAK constructs showed that FAK does not mediate the effects of MSP on cell adhesion or motility. Thus PI3-K, but not FAK, mediates MSP-induced integrin-dependent adhesion of epithelial cells. Also, we found ligand-independent association between RON and beta1 integrin, which is additional evidence for a relationship between these two receptor systems.

Signaling via fibroblast growth factor receptor-1 is dependent on extracellular matrix in capillary endothelial cell differentiation

Differentiation of endothelial cells, i.e., formation of a vessel lumen, is a prerequisite for angiogenesis. The underlying molecular mechanisms are ill defined. We have studied a brain capillary endothelial cell line (IBEC) established from H-2Kb-tsA58 transgenic mice. These cells form hollow tubes in three-dimensional type I collagen gels in response to fibroblast growth factor-2 (FGF-2). Culture of IBEC on collagen gels in the presence of FGF-2 protected cells from apoptosis and allowed tube formation (i.e., differentiation) but not growth of the cells. FGF-induced differentiation, but not cell survival, was inhibited by treatment of the cells with an anti-beta1-integrin IgG. Changes in integrin expression in the collagen-gel cultures could not be detected. Rather, cell-matrix interactions critical for endothelial cell differentiation were created during the culture, as indicated by the gradual increase in tyrosine phosphorylation of focal adhesion kinase in the collagen-gel cultures. Inclusion of laminin in the collagen gels led to FGF-2-independent formation of tube structures, but cells were not protected from apoptosis. These data indicate that FGF receptor-1 signal transduction in this cell model results in cell survival. Through mechanisms dependent on cell-matrix interactions, possibly involving the alpha3beta1-integrin and laminin produced by the collagen-cultured IBE cells, FGF stimulation also leads to differentiation of the cells.

Protein kinase C regulates integrin-induced activation of the extracellular regulated kinase pathway upstream of Shc

Adhesion of fibroblasts to extracellular matrices via integrin receptors is accompanied by extensive cytoskeletal rearrangements and intracellular signaling events. The protein kinase C (PKC) family of serine/threonine kinases has been implicated in several integrin-mediated events including focal adhesion formation, cell spreading, cell migration, and cytoskeletal rearrangements. However, the mechanism by which PKC regulates integrin function is not known. To characterize the role of PKC family kinases in mediating integrin-induced signaling, we monitored the effects of PKC inhibition on fibronectin-induced signaling events in Cos7 cells using pharmacological and genetic approaches. We found that inhibition of classical and novel isoforms of PKC by down-regulation with 12-0-tetradeconoyl-phorbol-13-acetate or overexpression of dominant-negative mutants of PKC significantly reduced extracellular regulated kinase 2 (Erk2) activation by fibronectin receptors in Cos7 cells. Furthermore, overexpression of constitutively active PKCalpha, PKCdelta, or PKCepsilon was sufficient to rescue 12-0-tetradeconoyl-phorbol-13-acetate-mediated down-regulation of Erk2 activation, and all three of these PKC isoforms were activated following adhesion. PKC was required for maximal activation of mitogen-activated kinase kinase 1, Raf-1, and Ras, tyrosine phosphorylation of Shc, and Shc association with Grb2. PKC inhibition does not appear to have a generalized effect on integrin signaling, because it does not block integrin-induced focal adhesion kinase or paxillin tyrosine phosphorylation. These results indicate that PKC activity enhances Erk2 activation in response to fibronectin by stimulating the Erk/mitogen-activated protein kinase pathway at an early step upstream of Shc.

Signaling transduction pathway of angiotensin II in human mesangial cells: mediation of focal adhesion and GTPase activating proteins

Human mesangial cells (HMCs) respond to angiotensin II stimulation, which modulates their physiological activities, i.e., contraction and proliferation. It has been revealed that focal adhesion kinase (FAK) and paxillin participate in the angiotensin II-mediated signaling and cytoskeletal rearrangements at focal adhesion. We investigated the influences of cell adhesion upon angiotensin II effects in HMCs. In adherent cells, both FAK and paxillin were tyrosine phosphorylated by angiotensin II, while the cell detachment completely inhibited the tyrosine phosphorylation of paxillin. Activation of p44/42 mitogen-activated protein (MAP) kinase by angiotensin II was accentuated in suspended cells. Moreover, p190, a member of Rho GTPase activating protein (GAP), and RasGAP were coprecipitated with paxillin in adherent cells and angiotensin II stimulation reduced the formation of paxillin-p190 and paxillin-RasGAP complexes. These results suggest that the formation of focal adhesion complexes accelerated by accumulation of mesangial matrices may inhibit the proliferation of HMCs by modulating MAP kinase activity and be related to mesangial cell depletion.

The Drosophila SH2-SH3 adapter protein Dock is expressed in embryonic axons and facilitates synapse formation by the RP3 motoneuron

The Dock SH2-SH3 domain adapter protein, a homolog of the mammalian Nck oncoprotein, is required for axon guidance and target recognition by photoreceptor axons in Drosophila larvae. Here we show that Dock is widely expressed in neurons and at muscle attachment sites in the embryo, and that this expression pattern has both maternal and zygotic components. In motoneurons, Dock is concentrated in growth cones. Loss of zygotic dock function causes a selective delay in synapse formation by the RP3 motoneuron at the cleft between muscles 7 and 6. These muscles often completely lack innervation in late stage 16 dock mutant embryos. RP3 does form a synapse later in development, however, because muscles 7 and 6 are normally innervated in third-instar mutant larvae. The absence of zygotically expressed Dock also results in subtle defects in a longitudinal axon pathway in the embryonic central nervous system. Concomitant loss of both maternally and zygotically derived Dock dramatically enhances these central nervous system defects, but does not increase the delay in RP3 synaptogenesis. These results indicate that Dock facilitates synapse formation by the RP3 motoneuron and is also required for guidance of some interneuronal axons The involvement of Dock in the conversion of the RP3 growth cone into a presynaptic terminal may reflect a role for Dock-mediated signaling in remodeling of the growth cone's cytoskeleton.

Regulation of early events in integrin signaling by protein tyrosine phosphatase SHP-2

The nontransmembrane protein tyrosine phosphatase SHP-2 plays a critical role in growth factor and cytokine signaling pathways. Previous studies revealed that a fraction of SHP-2 moves to focal contacts upon integrin engagement and that SHP-2 binds to SHP substrate 1 (SHPS-1)/SIRP-1alpha, a transmembrane glycoprotein with adhesion molecule characteristics (Y. Fujioka et al., Mol. Cell. Biol. 16:6887-6899, 1996; M. Tsuda et al., J. Biol. Chem. 273:13223-13229). Therefore, we asked whether SHP2-SHPS-1 complexes participate in integrin signaling. SHPS-1 tyrosyl phosphorylation increased upon plating of murine fibroblasts onto specific extracellular matrices. Both in vitro and in vivo studies indicate that SHPS-1 tyrosyl phosphorylation is catalyzed by Src family protein tyrosine kinases (PTKs). Overexpression of SHPS-1 in 293 cells potentiated integrin-induced mitogen-activated protein kinase (MAPK) activation, and potentiation required functional SHP-2. To further explore the role of SHP-2 in integrin signaling, we analyzed the responses of SHP-2 exon 3(-/-) and wild-type cell lines to being plated on fibronectin. Integrin-induced activation of Src family PTKs, tyrosyl phosphorylation of several focal adhesion proteins, MAPK activation, and the ability to spread on fibronectin were defective in SHP-2 mutant fibroblasts but were restored upon SHP-2 expression. Our data suggest a positive-feedback model in which, upon integrin engagement, basal levels of c-Src activity catalyze the tyrosyl phosphorylation of SHPS-1, thereby recruiting SHP-2 to the plasma membrane, where, perhaps by further activating Src PTKs, SHP-2 transduces positive signals for downstream events such as MAPK activation and cell shape changes.

Protein tyrosine phosphatase-PEST regulates focal adhesion disassembly, migration, and cytokinesis in fibroblasts

In this article, we show that, in transfected COS-1 cells, protein tyrosine phosphatase (PTP)-PEST translocates to the membrane periphery following stimulation by the extracellular matrix protein fibronectin. When plated on fibronectin, PTP-PEST (-/-) fibroblasts display a strong defect in motility. 3 h after plating on fibronectin, the number and size of vinculin containing focal adhesions were greatly increased in the homozygous PTP-PEST mutant cells as compared with heterozygous cells. This phenomenon appears to be due in part to a constitutive increase in tyrosine phosphorylation of p130(CAS), a known PTP-PEST substrate, paxillin, which associates with PTP-PEST in vitro, and focal adhesion kinase (FAK). Another effect of this constitutive hyperphosphorylation, consistent with the focal adhesion regulation defect, is that (-/-) cells spread faster than the control cell line when plated on fibronectin. In the PTP-PEST (-/-) cells, an increase in affinity for the SH2 domains of Src and Crk towards p130(CAS) was also observed. In (-/-) cells, we found a significant increase in the level of tyrosine phosphorylation of PSTPIP, a cleavage furrow-associated protein that interacts physically with all PEST family members. An effect of PSTPIP hyperphosphorylation appears to be that some cells remain attached at the site of the cleavage furrow for an extended period of time. In conclusion, our data suggest PTP-PEST plays a dual role in cell cytoskeleton organization, by promoting the turnover of focal adhesions required for cell migration, and by directly or indirectly regulating the proline, serine, threonine phosphatase interacting protein (PSTPIP) tyrosine phosphorylation level which may be involved in regulating cleavage furrow formation or disassembly during normal cell division.

Expression of β1 integrins in glomerular tissue of Streptozotocin-induced diabetic rats

Based upon the importance of integrins as receptors for extracellular matrix components as well as transducers of extracellular signals, and since major alterations take place in the renal extracellular matrix during diabetes, it is important to study the role played by integrins in the development of the diabetic glomerulosclerosis. Expression of the β1 subunit by renal glomerular cells was evaluated by biochemical and morphological means in short- and long-term diabetic rats. Western blots of isolated rat renal glomeruli demonstrated that the expression of β1 increases along with age as well as with the hyperglycaemic state. These changes were significant as early as 6 weeks of hyperglycaemia. This was further demonstrated by immunocytochemistry, which revealed the presence of the β1 subunit at the level of the plasma membranes of endothelial, epithelial, and mesangial cells. Quantitation of the immunolabelings confirmed the increased expression of β1 under diabetic conditions. Further to this, expression of the focal adhesion kinase (FAK) was evaluated by immunoblotting showing little increase in diabetic conditions. On the other hand, testing the tyrosine phosphorylation of FAK, revealed significant increases in diabetes. To recover the fraction of FAK associated with the β1 subunit, immunoprecipitation of isolated glomeruli homogenates was carried out with the anti- β1 antibody. This demonstrated that the amounts of FAK co-precipitated with β1, as well as its tyrosine-phosphorylation, are in fact reduced in diabetic conditions. Since the changes reported were observed at time points prior to any morphological alteration of the renal extracellular matrix, it appears that modifications in integrins and in their intracellular relays constitute early events that precede the onset of the diabetic nephropathy and must then be associated with the hyperglycaemic condition.Key words: integrins, focal adhesion kinase, tyrosine phosphorylation, renal tissue, diabetes.

Modulation of cell proliferation and differentiation through substrate-dependent changes in fibronectin conformation

Integrin-mediated cell adhesion to extracellular matrices provides signals essential for cell cycle progression and differentiation. We demonstrate that substrate-dependent changes in the conformation of adsorbed fibronectin (Fn) modulated integrin binding and controlled switching between proliferation and differentiation. Adsorption of Fn onto bacterial polystyrene (B), tissue culture polystyrene (T), and collagen (C) resulted in differences in Fn conformation as indicated by antibody binding. Using a biochemical method to quantify bound integrins in cultured cells, we found that differences in Fn conformation altered the quantity of bound alpha5 and beta1 integrin subunits but not alphav or beta3. C2C12 myoblasts grown on these Fn-coated substrates proliferated to different levels (B > T > C). Immunostaining for muscle-specific myosin revealed minimal differentiation on B, significant levels on T, and extensive differentiation on C. Differentiation required binding to the RGD cell binding site in Fn and was blocked by antibodies specific for this site. Switching between proliferation and differentiation was controlled by the levels of alpha5beta1 integrin bound to Fn, and differentiation was inhibited by anti-alpha5, but not anti-alphav, antibodies, suggesting distinct integrin-mediated signaling pathways. Control of cell proliferation and differentiation through conformational changes in extracellular matrix proteins represents a versatile mechanism to elicit specific cellular responses for biological and biotechnological applications.

Alternatively spliced EDA segment regulates fibronectin-dependent cell cycle progression and mitogenic signal transduction

Fibronectin (FN) is comprised of multiple isoforms arising from alternative splicing of a single gene transcript. One of the alternatively spliced segments, EDA, is expressed prominently in embryonic development, malignant transformation, and wound healing. We showed previously that EDA+ FN was more potent than EDA- FN in promoting cell spreading and cell migration because of its enhanced binding affinity to integrin alpha5beta1 (Manabe, R., Oh-e, N., Maeda, T., Fukuda, T., and Sekiguchi, K. (1997) J. Cell Biol. 139, 295-307). In this study, we compared the cell cycle progression and its associated signal transduction events induced by FN isoforms with or without the EDA segment to examine whether the EDA segment modulates the cell proliferative potential of FN. We found that EDA+ FN was more potent than EDA- FN in inducing G1-S phase transition. Inclusion of the EDA segment potentiated the ability of FN to induce expression of cyclin D1, hyperphosphorylation of pRb, and activation of mitogen-activated protein kinase extracellular signal regulated kinase 2 (ERK2). EDA+ FN was also more potent than EDA- FN in promoting FN-mediated tyrosine phosphorylation of p130(Cas), but not focal adhesion kinase, which occurred in parallel with the activation of ERK2, suggesting that p130(Cas) may be involved in activation of ERK2. These results indicated that alternative splicing at the EDA region is a novel mechanism that promotes FN-induced cell cycle progression through up-regulation of integrin-mediated mitogenic signal transduction.

Cell adhesion regulates the interaction between the docking protein p130(Cas) and the 14-3-3 proteins

Integrin ligand binding induces a signaling complex formation via the direct association of the docking protein p130(Cas) (Cas) with diverse molecules. We report here that the 14-3-3zeta protein interacts with Cas in the yeast two-hybrid assay. We also found that the two proteins associate in mammalian cells and that this interaction takes place in a phosphoserine-dependent manner, because treatment of Cas with a serine phosphatase greatly reduced its ability to bind 14-3-3zeta. Furthermore, the Cas-14-3-3zeta interaction was found to be regulated by integrin-mediated cell adhesion. Thus, when cells are detached from the extracellular matrix, the binding of Cas to 14-3-3zeta is greatly diminished, whereas replating the cells onto fibronectin rapidly induces the association. Consistent with these results, we found that the subcellular localization of Cas and 14-3-3 is also regulated by integrin ligand binding and that the two proteins display a significant co-localization during cell attachment to the extracellular matrix. In conclusion, our results demonstrate that 14-3-3 proteins participate in integrin-activated signaling pathways through their interaction with Cas, which, in turn, may contribute to important biological responses regulated by cell adhesion to the extracellular matrix.

Identification of Grb4/Nckbeta, a src homology 2 and 3 domain-containing adapter protein having similar binding and biological properties to Nck

Adapter proteins made up of Src homology (SH) domains mediate multiple cellular signaling events initiated by receptor protein tyrosine kinases. Here we report that Grb4 is an adapter protein closely related to but distinct from Nck that is made up of three SH3 domains and one SH2 domain. Northern analysis indicated that both genes are expressed in multiple tissues. Both Nck and Grb4 proteins could associate with receptor tyrosine kinases and the SH3-binding proteins PAK, Sos1, and PRK2, and they synergized with v-Abl and Sos to induce gene expression via the transcription factor Elk-1. Although neither protein was transforming on its own, both Nck and Grb4 cooperated with v-Abl to transform NIH 3T3 cells and influenced the morphology and anchorage-dependent growth of wild type Ras-transformed cells. Nck and Grb4 therefore appear to be functionally redundant.

Stable association of PYK2 and p130(Cas) in osteoclasts and their co-localization in the sealing zone

Bone resorption is initiated by osteoclast attachment to the mineralized matrix, cytoskeletal reorganization, cellular polarization, and the formation of the sealing zone. The present study examines the interaction between PYK2 and p130(Cas) (Crk-associated substrate), suggested to be part of the signaling pathway initiated by osteoclast adhesion. Using murine osteoclast-like cells (OCLs) and their mononuclear precursors (pOCs), generated in a co-culture of bone marrow and osteoblastic MB1.8 cells, we show that: 1) p130(Cas) is tyrosine-phosphorylated upon adhesion of pOCs to vitronectin or ligation of beta3 integrins; 2) p130(Cas) colocalizes with PYK2 and the cytoskeletal proteins F-actin, vinculin, and paxillin in the podosomal-rich ring-like structures of OCLs plated on glass and in the sealing zone in actively resorbing OCLs on bone; 3) p130(Cas) and PYK2 form a stable complex in pOCs, independent of tyrosine phosphorylation of either molecule, and this complex is present in Src (-/-) OCLs, in which neither protein is phosphorylated or associated with the osteoclast adhesion structure; 4) the association of p130(Cas) and PYK2 is mediated by the SH3 domain of p130(Cas) and the C-terminal domain of PYK2. These findings suggest that p130(Cas) and its association with PYK2 may play an important role in the adhesion-dependent signaling that leads to cytoskeletal reorganization and formation of the sealing zone during osteoclast activation.

Regulation of fibroblast motility by the protein tyrosine phosphatase PTP-PEST

The protein tyrosine phosphatase PTP-PEST is a cytosolic enzyme that displays a remarkable degree of selectivity for tyrosine-phosphorylated p130(Cas) as a substrate, both in vitro and in intact cells. We have investigated the physiological role of PTP-PEST using Rat1 fibroblast-derived stable cell lines that we have engineered to overexpress PTP-PEST. These cell lines exhibit normal levels of tyrosine phosphorylation of the majority of proteins but have significantly lower levels of tyrosine phosphorylation of p130(Cas) than control cells. Initial cellular events occurring following integrin-mediated attachment to fibronectin (cell attachment and spreading) are essentially unchanged in cells overexpressing PTP-PEST; similarly, the extent and time course of mitogen-activated protein kinase activation in response to integrin engagement is unchanged. In contrast, the reduced phosphorylation state of p130(Cas) is associated with a considerably reduced rate of cell migration and a failure of cells overexpressing PTP-PEST to accomplish the normally observed redistribution of p130(Cas) to the leading edge of migrating cells. Furthermore, cells overexpressing PTP-PEST demonstrate significantly reduced levels of association of p130(Cas) with the Crk adaptor protein. Our results suggest that one physiological role of PTP-PEST is to dephosphorylate p130(Cas), thereby controlling tyrosine phosphorylation-dependent signaling events downstream of p130(Cas) and regulating cell migration.

De novo expression of pp125FAK in human macrophages regulates CSK distribution and MAP kinase activation but does not affect focal contact structure

The protein tyrosine kinase pp125FAK (focal adhesion kinase, or FAK) is expressed by a variety of cell types and has been implicated in integrin-mediated signaling events. We explored the potential functions of FAK by expressing it de novo in a cell type lacking FAK. We showed previously that cultured human macrophages lack FAK yet still have well-formed focal contacts. Adenovirus-mediated expression of FAK results in the appearance of FAK protein, which localizes to focal contacts and becomes tyrosine-phosphorylated without perturbing overall cell morphology or focal contacts. FAK associates with CSK 48 h after infection and recruits it to focal contacts. Tyrosine phosphorylation of p130cas but not of paxillin is stimulated after FAK expression. The phosphorylation of p130cas is lost at 48 h in parallel with CSK accumulation in focal contacts. The ERK2 form of MAP kinase is similarly activated at 12-24 h, but it also returns to low levels at 48 h. These findings demonstrate that FAK can be reconstituted to focal contacts in cells that lack it without affecting cell morphology or focal contact structure. FAK can regulate the distribution and activities of elements of the MAP kinase signaling pathway.

Dissociation of FAK/p130(CAS)/c-Src complex during mitosis: role of mitosis-specific serine phosphorylation of FAK

At mitosis, focal adhesions disassemble and the signal transduction from focal adhesions is inactivated. We have found that components of focal adhesions including focal adhesion kinase (FAK), paxillin, and p130(CAS) (CAS) are serine/threonine phosphorylated during mitosis when all three proteins are tyrosine dephosphorylated. Mitosis-specific phosphorylation continues past cytokinesis and is reversed during post-mitotic cell spreading. We have found two significant alterations in FAK-mediated signal transduction during mitosis. First, the association of FAK with CAS or c-Src is greatly inhibited, with levels decreasing to 16 and 13% of the interphase levels, respectively. Second, mitotic FAK shows decreased binding to a peptide mimicking the cytoplasmic domain of beta-integrin when compared with FAK of interphase cells. Mitosis-specific phosphorylation is responsible for the disruption of FAK/CAS binding because dephosphorylation of mitotic FAK in vitro by protein serine/threonine phosphatase 1 restores the ability of FAK to associate with CAS, though not with c-Src. These results suggest that mitosis-specific modification of FAK uncouples signal transduction pathways involving integrin, CAS, and c-Src, and may maintain FAK in an inactive state until post-mitotic spreading.

Loss of matrix-dependent cytoskeletal tyrosine kinase signals may regulate intestinal epithelial differentiation during mucosal healing

Intestinal epithelial restitution and the migratory phenotype appear regulated by the extracellular matrix. Since integrin-associated adhesion to matrix triggers tyrosine kinase activity, we hypothesized that matrix-specific tyrosine kinase signals might modulate the intestinal epithelial migratory phenotype, particularly via focal adhesion kinase. Caco-2 cells were seeded at two densities on collagen I, laminin, fibronectin, and tissue culture plastic. Four days later the first cells were confluent, whereas the second cells were not contact inhibited and expressed migratory lamellipodia. Cells were fractionated into membrane/cytoskeletal and cytosolic fractions. Cytoskeletal tyrosine kinase activity in static cells was matrix dependent and, unlike cytoscolic tyrosine kinase, correlated with adhesion, highest on collagen and lowest on plastic. Migrating cells exhibited matrix-dependent increases in cystosolic tyrosine kinase activity. Cytosolic changes in tyrosine kinase activity in motile cells exceeded membrane/cytoskeletal changes. However, matrix-dependent variations in increase in cytosolic tyrosine kinase activity correlated inversely with changes in cytoskeletal tyrosine kinase activity, suggesting cytoskeletal tyrosine kinase translocation to the cytosol during motility. Indeed cytoskeletal focal adhesion kinase activity decreased during migration on collagen. Tyrosine kinase inhibition by genistein both inhibited migration and stimulated expression of brush-border enzymes downregulated during motility. Although enterocyte-matrix interactions alter both cytosolic and cytoskeletal tyrosine kinase activity, matrix-dependent cytoskeletal events are likely to regulate adhesion and differentiation in static cells. Loss of matrix-dependent cytoskeletal tyrosine kinase signals such as focal adhesion kinase during restitution may trigger a phenotypic switch to the "dedifferentiated" migrating intestinal epithelial phenotype.

Regulation of the cell cycle by focal adhesion kinase

In this report, we have analyzed the potential role and mechanisms of integrin signaling through FAK in cell cycle regulation by using tetracycline-regulated expression of exogenous FAK and mutants. We have found that overexpression of wild-type FAK accelerated G1 to S phase transition. Conversely, overexpression of a dominant-negative FAK mutant DeltaC14 inhibited cell cycle progression at G1 phase and this inhibition required the Y397 in DeltaC14. Biochemical analyses indicated that FAK mutant DeltaC14 was mislocalized and functioned as a dominant-negative mutant by competing with endogenous FAK in focal contacts for binding signaling molecules such as Src and Fyn, resulting in a decreases of Erk activation in cell adhesion. Consistent with this, we also observed inhibition of BrdU incorporation and Erk activation by FAK Y397F mutant and FRNK, but not FRNKDeltaC14, in transient transfection assays using primary human foreskin fibroblasts. Finally, we also found that DeltaC14 blocked cyclin D1 upregulation and induced p21 expression, while wild-type FAK increased cyclin D1 expression and decreased p21 expression. Taken together, these results have identified FAK and its associated signaling pathways as a mediator of the cell cycle regulation by integrins.

Characterization of Cbl-Nck and Nck-Pak1 interactions in myeloid FcgammaRII signaling

Fc receptors modulate inflammatory processes, including phagocytosis, serotonin and histamine release, superoxide production, and secretion of cytokines. Aggregation of FcgammaRIIa, the low-affinity receptor for monomeric IgG, activates nonreceptor protein tyrosine kinases such as Lyn, Hck, and Syk, potentially driving the phosphorylation of the downstream adaptor proteins, including Cbl and/or Nck. Previous work from our laboratory using interferon-gamma-differentiated U937 (U937IF) myeloid cells investigated mechanisms which regulate Fcgamma receptor-induced assembly of adaptor complexes. Herein we report that FcgammaRII receptor signaling in U937IF and HEL cells involves Cbl and Nck, suggesting that Cbl-Nck interactions may link FcgammaRII to downstream activation of Pak kinase. FcgammaRII crosslinking induced the phosphorylation of Cbl and Nck on tyrosine. The alphaCbl immunoprecipitations revealed constitutive binding of Nck and Grb2 to Cbl and FcgammaRII-inducible binding of CrkL to Cbl. The interactions of Cbl with Nck and CrkL were phosphorylation dependent since dephosphorylation of cellular proteins with potato acid phosphatase abrogated binding. GST-Nck fusion protein pulldown experiments show that Cbl and Pak1 bind to the second SH3 domain of Nck. A specific Src inhibitor, PP1, was shown to completely abrogate the FcgammaR-induced superoxide response, correlating with a decrease in Cbl and Nck tyrosine phosphorylation. Our results provide the first evidence that Src is required for FcgammaR activation of the respiratory burst in myeloid cells and suggest that Cbl-Nck, Cbl-Pak1, and Nck-Pak1 interactions may regulate this response.

Growth hormone stimulates the formation of a multiprotein signaling complex involving p130(Cas) and CrkII. Resultant activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK)

We have demonstrated previously that growth hormone (GH) activates focal adhesion kinase (FAK), and this activation results in the tyrosine phosphorylation of two FAK substrates, namely paxillin and tensin. We now show here in Chinese hamster ovary cells stably transfected with rat GH receptor cDNA that human (h)GH induces the formation of a large multiprotein signaling complex centered around another FAK-associated protein, p130(Cas) and the adaptor protein CrkII. hGH stimulates the tyrosine phosphorylation of both p130(Cas) and CrkII, their association, and the association of multiple other tyrosine-phosphorylated proteins to the complex. Both the c-Src and c-Fyn tyrosine kinases are tyrosine phosphorylated and activated by cellular hGH stimulation and form part of the multiprotein signaling complex as does tensin, paxillin, IRS-1, the p85 subunit of phosphatidylinositol 3-kinase, C3G, SHC, Grb-2, and Sos-1. c-Cbl and Nck are also tyrosine-phosphorylated by cellular stimulation with hGH and associate with the p130(Cas)-CrkII complex. c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) is activated in response to hGH in accordance with the formation of the abovementioned signaling complex, and hGH stimulated JNK/SAPK activity is increased in CrkII overexpressing NIH3T3 cells compared with vector transfected NIH3T3 cells. The formation of such a large multiprotein signaling complex by GH, with the resultant activation of multiple downstream effector molecules, may be central to many of the pleiotropic effects of GH.

Nck-2, a novel Src homology2/3-containing adaptor protein that interacts with the LIM-only protein PINCH and components of growth factor receptor kinase-signaling pathways

Many of the protein-protein interactions that are essential for eukaryotic intracellular signal transduction are mediated by protein binding modules including SH2, SH3, and LIM domains. Nck is a SH3- and SH2-containing adaptor protein implicated in coordinating various signaling pathways, including those of growth factor receptors and cell adhesion receptors. We report here the identification, cloning, and characterization of a widely expressed, Nck-related adaptor protein termed Nck-2. Nck-2 comprises primarily three N-terminal SH3 domains and one C-terminal SH2 domain. We show that Nck-2 interacts with PINCH, a LIM-only protein implicated in integrin-linked kinase signaling. The PINCH-Nck-2 interaction is mediated by the fourth LIM domain of PINCH and the third SH3 domain of Nck-2. Furthermore, we show that Nck-2 is capable of recognizing several key components of growth factor receptor kinase-signaling pathways including EGF receptors, PDGF receptor-beta, and IRS-1. The association of Nck-2 with EGF receptors was regulated by EGF stimulation and involved largely the SH2 domain of Nck-2, although the SH3 domains of Nck-2 also contributed to the complex formation. The association of Nck-2 with PDGF receptor-beta was dependent on PDGF activation and was mediated solely by the SH2 domain of Nck-2. Additionally, we have detected a stable association between Nck-2 and IRS-1 that was mediated primarily via the second and third SH3 domain of Nck-2. Thus, Nck-2 associates with PINCH and components of different growth factor receptor-signaling pathways via distinct mechanisms. Finally, we provide evidence indicating that a fraction of the Nck-2 and/or Nck-1 proteins are associated with the cytoskeleton. These results identify a novel Nck-related SH2- and SH3-domain-containing protein and suggest that it may function as an adaptor protein connecting the growth factor receptor-signaling pathways with the integrin-signaling pathways.

Insulin receptor substrate-1 as a signaling molecule for focal adhesion kinase pp125(FAK) and pp60(src)

Insulin receptor substrate-1 (IRS-1) is a major substrate of insulin and insulin-like growth factor-I receptors, which upon phosphorylation on tyrosine docks several signaling molecules. Recently, IRS-1 was found to interact with alphav beta3 integrins upon insulin stimulation. Integrins are transmembrane proteins that play an important role in adhesion between cells and between cells and extracellular matrix. One of the major proteins implicated in integrin signaling is pp125(FAK), a cytosolic tyrosine kinase, which upon integrin engagement becomes tyrosine-phosphorylated and subsequently binds to c-Src. Here, we established a mammalian two-hybrid system to show that pp125(FAK) binds to IRS-1. This association depends largely on the C terminus of pp125(FAK) but not on pp125(FAK) tyrosine kinase activity. Furthermore, we observed co-immunoprecipitation of pp125(FAK) with IRS-1 in 293 cells, suggesting a possible biological function of this association. When IRS-1 was expressed in 293 cells together with pp125(FAK) or Src, we found extensive IRS-1 tyrosine phosphorylation. In pp125(FAK)-expressing cells, this was concomitant with increased association of IRS-1 with Src homology 2-containing proteins such as growth factor receptor-bound protein 2, phosphatidylinositol (PI) 3-kinase p85alpha subunit, and Src homology 2-containing protein-tyrosine phosphatase-2. In addition, pp125(FAK)-induced association of IRS-1 with PI 3-kinase resulted in increased PI 3-kinase activity. In contrast, no change in mitogen-activated protein kinase activity was observed, indicating that pp125(FAK)-induced association between IRS-1 and growth factor receptor-bound protein 2 does not affect the mitogen-activated protein kinase pathway. Moreover, we found that engagement of integrins induced IRS-1 tyrosine phosphorylation. Considering our results together, we suggest that integrins and insulin/insulin-like growth factor-I receptor signaling pathways converge at an early point in the signaling cascade, which is the IRS-1 protein.

Involvement of focal adhesion kinase in invasin-mediated uptake

High-efficiency entry of the enteropathogenic bacterium Yersinia pseudotuberculosis into nonphagocytic cells is mediated by the bacterial outer membrane protein invasin. Invasin-mediated uptake requires high affinity binding of invasin to multiple beta1 chain integrin receptors on the host eukaryotic cell. Previous studies using inhibitors have indicated that high-efficiency uptake requires tyrosine kinase activity. In this paper we demonstrate a requirement for focal adhesion kinase (FAK) for invasin-mediated uptake. Overexpression of a dominant interfering form of FAK reduced the amount of bacterial entry. Specifically, the autophosphorylation site of FAK, which is a reported site of c-Src kinase binding, is required for bacterial internalization, as overexpression of a derivative lacking the autophosphorylation site had a dominant interfering effect as well. Cultured cells expressing interfering variants of Src kinase also showed reduced bacterial uptake, demonstrating the involvement of a Src-family kinase in invasin-promoted uptake.

Pyk2 and Src-family protein-tyrosine kinases compensate for the loss of FAK in fibronectin-stimulated signaling events but Pyk2 does not fully function to enhance FAK- cell migration

The focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) links transmembrane integrin receptors to intracellular signaling pathways. We show that expression of the FAK-related PTK, Pyk2, is elevated in fibroblasts isolated from murine fak-/- embryos (FAK-) compared with cells from fak+/+ embryos (FAK+). Pyk2 was localized to perinuclear regions in both FAK+ and FAK- cells. Pyk2 tyrosine phosphorylation was enhanced by fibronectin (FN) stimulation of FAK- but not FAK+ cells. Increased Pyk2 tyrosine phosphorylation paralleled the time-course of Grb2 binding to Shc and activation of ERK2 in FAK- cells. Pyk2 in vitro autophosphorylation activity was not enhanced by FN plating of FAK- cells. However, Pyk2 associated with active Src-family PTKs after FN but not poly-L-lysine replating of the FAK- cells. Overexpression of both wild-type (WT) and kinase-inactive (Ala457), but not the autophosphorylation site mutant (Phe402) Pyk2, enhanced endogenous FN-stimulated c-Src in vitro kinase activity in FAK- cells, but only WT Pyk2 overexpression enhanced FN-stimulated activation of co-transfected ERK2. Interestingly, Pyk2 overexpression only weakly augmented FAK- cell migration to FN whereas transient FAK expression promoted FAK- cell migration to FN efficiently compared with FAK+ cells. Significantly, repression of endogenous Src-family PTK activity by p50(csk) overexpression inhibited FN-stimulated cell spreading, Pyk2 tyrosine phosphorylation, Grb2 binding to Shc, and ERK2 activation in the FAK- but not in FAK+ cells. These studies show that Pyk2 and Src-family PTKs combine to promote FN-stimulated signaling events to ERK2 in the absence of FAK, but that these signaling events are not sufficient to overcome the FAK- cell migration defects.

Direct binding of p130(Cas) to the guanine nucleotide exchange factor C3G

p130(Cas) (Cas; crk-associated substrate) belongs to a new family of docking molecules. It contains one Src homology (SH) 3 domain in its amino-terminal region followed by a region containing binding motifs for SH2 and SH3 domains. To gain further insight into Cas signaling we used the SH3 domain of Cas in a two-hybrid screen to search a human placenta library for binding partners. The screen confirmed a previous finding of its binding to the focal adhesion kinase (FAK) but also identified C3G, a guanine nucleotide exchange factor. We found direct interaction between Cas and C3G in vitro and in vivo. A series of analysis with C3G deletion mutants revealed a proline-rich Cas-binding site (Ala0-Pro1-Pro2-Lys3-Pro4-Pro5-Leu6-Pro7) located NH2-terminal to the previously characterized Crk binding motifs in C3G. Mutagenesis studies showed that Pro1, Lys3, and Pro4 within the ligand-binding site are critical for high affinity interaction. These results, combined with sequence alignments of proline-rich binding elements from proteins known for Cas binding, define the consensus sequence XXPXKPX which is recognized by the CasSH3 domain. Cas shows structural characteristics of a docking molecule and may serve to bring C3G to specific compartments within the cell.

Identification of Nck family genes, chromosomal localization, expression, and signaling specificity

Already a dozen molecules share binding to the Src homology (SH) 3 domains of human Nck, an SH3-SH3-SH3-SH2 adapter protein. We reason that there may be multiple gene members of Nck to accommodate the large binding repertoires. Here we report identification of novel human and mouse Nck genes and rename them as the Nckalpha and Nckbeta genes (including the human Nckalpha, human Nckbeta, mouse Nckalpha, and mouse Nckbeta genes). Nckalpha and Nckbeta share 68% amino acid identity, whereas the two Nckalpha and two Nckbeta across the species show 96% identity to each other. The human Nckbeta gene is mapped to 2q12, whereas the human Nckalpha gene has previously been mapped at 3q21. Antibodies specifically against Nckalpha and Nckbeta detect Nckalpha and Nckbeta with an identical molecular mass in the same cells of various origins. Ectopically expressed Nckbeta, but not its SH2 domain mutant, strongly inhibits epidermal growth factor- and platelet-derived growth factor-stimulated DNA synthesis. Consistently, epidermal growth factor receptor and platelet-derived growth factor receptor preferentially interact with Nckbeta over Nckalpha in vitro. This study indicates that Nck is a multiple gene family and that each gene may have its own signaling specificity. Because previous anti-Nck (human Nckalpha) antibodies cross-react with Nckbeta, reassessment of those studies with specific Nck genes would be necessary.

A requirement for caveolin-1 and associated kinase Fyn in integrin signaling and anchorage-dependent cell growth

Caveolin-1 functions as a membrane adaptor to link the integrin alpha subunit to the tyrosine kinase Fyn. Upon integrin ligation, Fyn is activated and binds, via its SH3 domain, to Shc. Shc is subsequently phosphorylated at tyrosine 317 and recruits Grb2. This sequence of events is necessary to couple integrins to the Ras-ERK pathway and promote cell cycle progression. These findings reveal an unexpected function of caveolin-1 and Fyn in integrin signaling and anchorage-dependent cell growth.

Protein-tyrosine phosphatase Shp-2 regulates cell spreading, migration, and focal adhesion

Shp-2, a widely expressed cytoplasmic tyrosine phosphatase with two SH2 domains, is believed to participate in signal relay downstream of growth factor receptors. We show here that this phosphatase also plays an important role in the control of cell spreading, migration, and cytoskeletal architecture. Fibroblast cells lacking a functional Shp-2 were impaired in their ability to spread and migrate on fibronectin compared with wild-type cells. Furthermore, Shp-2 mutant cells displayed an increased number of focal adhesions and condensed F-actin aggregation at the cell periphery, properties reminiscent of focal adhesion kinase (FAK)-deficient cells. This is consistent with our previous observations in vivo that mice homozygous for the Shp-2 mutation died at midgestation with similar phenotype to FAK and fibronectin-deficient embryos, having severe defects in mesodermal patterning, particularly the truncation of posterior structures. Biochemical analysis demonstrated that FAK dephosphorylation was significantly reduced in Shp-2 mutant cells in suspension. Furthermore, regulated association of Src SH2 domain with FAK and paxillin during cell attachment and detachment on fibronectin was disrupted in Shp-2 mutant cells. This report defines a unique role of the Shp-2 tyrosine phosphatase in cell motility, which might guide the design of a new strategy for pharmaceutical interference of tumor metastasis.

Bombesin, vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine induce focal adhesion kinase activation in intact Swiss 3T3 cells

Treatment of quiescent Swiss 3T3 cells with bombesin rapidly increased focal adhesion kinase (FAK)-associated tyrosine kinase activity in immune complexes. The effect was rapid (maximum at 2.5 min) and dose dependent (half-maximum response at 0.05 nM). Addition of vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine also elicited a rapid increase in FAK-associated tyrosine kinase activity. Addition of the selective Src inhibitor pyrazolopyrimidine directly to the in vitro kinase assay potently inhibited Src kinase activity induced by bombesin but did not affect the kinase activity of FAK measured by autophosphorylation or by synthetic substrate phosphorylation in paralell assays. In addition, Src activity was not detected in FAK immunoprecipitates using an optimal Src peptide substrate. Thus, agonist-induced tyrosine kinase activity measured in FAK immunoprecipitates is mediated by FAK activation rather than by co-immunoprecipitating Src. Bombesin-induced FAK activation is not dependent either on protein kinase C or Ca2+ mobilization but was completely blocked by treatment with cytochalasin D or by placing the cells in suspension. These findings indicate that FAK activation requires an intact actin cytoskeleton. Our results demonstrate that agonists that act via 7-transmembrane domain receptors stimulate FAK kinase activation.

Human Caco-2 intestinal epithelial motility is associated with tyrosine kinase and cytoskeletal focal adhesion kinase signals

Intestinal epithelial cells assume a specialized phenotype adapted to motility and mucosal healing during mucosal restitution. Since cell-matrix interactions initiate tyrosine kinase (TK) signals, we hypothesized that the regulation of the intestinal epithelial migratory phenotype may also involve TK signals, particularly via focal adhesion kinase (FAK). Caco-2 cells were seeded simultaneously at 26,000 and 6000 cells/cm2. After 4 days, the first cells were confluent, while cells in the second population were not contact-inhibited and expressed migrating lamellipodia. Cells were fractionated into Triton X-100-soluble (membrane/cytoskeletal) and -insoluble (cytosolic) fractions. TK activity in each fraction was assayed by ELISA using a synthetic substrate. FAK protein was assessed by immunoprecipitation with monoclonal anti-FAK and Western blotting. Because active FAK autophosphorylates, we also measured FAK tyrosine phosphorylation, immunoprecipitating with anti-FAK and then Western blotting for phosphotyrosine. TK activity was increased in both cytosolic and membrane/cytoskeletal fractions of migrating cells by 17.6 +/- 3.6 and 28.9 +/- 4.1%, respectively, compared to static cells (n = 11, P < 0.01). FAK protein increased in the cytosolic fraction by 90.4 +/- 20.0% (n = 5, P = 0.01), but did not change in the membrane/cytoskeletal fraction. Tyrosine phosphorylated FAK increased by 62.8 +/- 21.4% in the cytosolic fraction of migrating cells but also by 46.6 +/- 38.4% in the membrane/cytoskeletal fraction (n = 5, P < 0.05). These data suggest that intestinal epithelial cell migration is associated with increases in both cytosolic and cytoskeletal TK activity and upregulation of cytosolic FAK protein. The increase in cytoskeletal FAK phosphorylation without increased FAK protein suggests autophosphorylation and increased cytoskeletal FAK activity. The migrating intestinal epithelial phenotype may therefore be modulated by TK signals including cytoskeletal FAK activity.

Integrin-mediated tyrosine phosphorylation of SHPS-1 and its association with SHP-2. Roles of Fak and Src family kinases

SHPS-1 is a receptor-like glycoprotein that undergoes tyrosine phosphorylation and binds SHP-2, an Src homology 2 domain containing protein tyrosine phosphatase, in response to various mitogens. Cell adhesion to extracellular matrix proteins such as fibronectin and laminin also induced the tyrosine phosphorylation of SHPS-1 and its association with SHP-2. These responses were markedly reduced in cells overexpressing the Csk kinase or in cells that lack focal adhesion kinase or the Src family kinases Src or Fyn. However, unlike Src, focal adhesion kinase did not catalyze phosphorylation of the cytoplasmic domain of SHPS-1 in vitro. Overexpression of a catalytically inactive SHP-2 markedly inhibited activation of mitogen-activated protein (MAP) kinase in response to fibronectin stimulation without affecting the extent of tyrosine phosphorylation of focal adhesion kinase or its interaction with the docking protein Grb2. Overexpression of wild-type SHPS-1 did not enhance fibronectin-induced activation of MAP kinase. These results indicate that the binding of integrins to the extracellular matrix induces tyrosine phosphorylation of SHPS-1 and its association with SHP-2, and that such phosphorylation of SHPS-1 requires both focal adhesion kinase and an Src family kinase. In addition to its role in receptor tyrosine kinase-mediated MAP kinase activation, SHP-2 may play an important role, partly through its interaction with SHPS-1, in the activation of MAP kinase in response to the engagement of integrins by the extracellular matrix.

Propensity for macrophage apoptosis is related to the pattern of expression and function of integrin extracellular matrix receptors

Ligation of integrins to an extracellular matrix activates signal transduction systems which produce multiple responses in different cell types. Adhesion often provides a survival signal to cells; disruption of adhesion frequently results in apoptosis. Our laboratory has utilized apoptosis-sensitive and -resistant cell lines to investigate the role of integrin expression and function in regulation of apoptosis in macrophages. Chronic exposure of murine macrophage-like RAW264.7 cells to apoptosis-inducing agents (bacterial lipopolysaccharide and interferon-gamma) resulted in the generation of a derivative cell line (RES) resistant to apoptosis. Observation of RAW and RES cultures indicated a difference in adhesion between the two cell types. The two cell lines also exhibit significant differences in expression of integrins previously characterized to be important in apoptosis.

Angiostatin induces endothelial cell apoptosis and activation of focal adhesion kinase independently of the integrin-binding motif RGD

Angiostatin, a fragment of plasminogen, has been identified and characterized as an endogenous inhibitor of neovascularization. We show that angiostatin treatment of endothelial cells in the absence of growth factors results in an increased apoptotic index whereas the proliferation index is unchanged. Angiostatin also inhibits migration and tube formation of endothelial cells. Angiostatin treatment has no effect on growth factor-induced signal transduction but leads to an RGD-independent induction of the kinase activity of focal adhesion kinase, suggesting that the biological effects of angiostatin relate to subversion of adhesion plaque formation in endothelial cells.

Tyrosine phosphorylation of p130Cas is involved in actin organization in osteoclasts

Integrin-mediated interaction with the extracellular matrix plays a critical role in the function of osteoclasts, the bone-resorbing cells. This study examines the role of p130Cas (Crk-associated substrate (Cas)) in actin organization in osteoclasts. Multinucleated osteoclast-like cells (OCLs) were obtained in a co-culture of murine bone marrow cells and primary osteoblasts. After plating on culture dishes, OCLs formed a ringlike structure consisting of F-actin dots at cell periphery (actin ring). The percentage of OCLs with actin rings and its diameter increased with time and cell spreading. Tyrosine phosphorylation of a protein (p130) increased with actin ring formation. Treatment with cytochalasin D disrupted actin rings and reduced tyrosine phosphorylation of p130. Using specific antibodies, p130 was identified as Cas. By immunocytochemistry, Cas was localized to the peripheral regions of OCLs and its distribution overlapped that of F-actin. In OCLs derived from Src(-/-) mice, in which osteoclast activity is severely compromised, tyrosine phosphorylation of Cas was markedly reduced. Moreover, Cas was diffusely distributed in the cytoplasm and actin ring formation is not observed. These findings suggest that Src-dependent tyrosine phosphorylation of Cas is involved in the adhesion-induced actin organization associated with osteoclast activation.

Activation of Rho-dependent cell spreading and focal adhesion biogenesis by the v-Crk adaptor protein

The small GTPase RhoA plays a critical role in signaling pathways activated by serum-derived factors, such as lysophosphatidic acid (LPA), including the formation of stress fibers in fibroblasts and neurite retraction and rounding of soma in neuronal cells. Previously, we have shown that ectopic expression of v-Crk, an SH2/SH3 domain-containing adapter proteins, in PC12 cells potentiates nerve growth factor (NGF)-induced neurite outgrowth and promotes the survival of cells when NGF is withdrawn. In the present study we show that, when cultured in 15% serum or lysophosphatidic acid-containing medium, the majority of v-Crk-expressing PC12 cells (v-CrkPC12 cells) display a flattened phenotype with broad lamellipodia and are refractory to NGF-induced neurite outgrowth unless serum is withdrawn. v-Crk-mediated cell flattening is inhibited by treatment of cells with C3 toxin or by mutation in the Crk SH2 or SH3 domain. Transient cotransfection of 293T cells with expression plasmids for p160ROCK (Rho-associated coiled-coil-containing kinase) and v-Crk, but not SH2 or SH3 mutants of v-Crk, results in hyperactivation of p160ROCK. Moreover, the level of phosphatidylinositol-4,5-bisphosphate is increased in v-CrkPC12 cells compared to the levels in mutant v-Crk-expressing cells or wild-type cells, consistent with PI(4)P5 kinase being a downstream target for Rho. Expression of v-Crk in PC12 cells does not result in activation of Rac- or Cdc42-dependent kinases PAK and S6 kinase, demonstrating specificity for Rho. In contrast to native PC12 cells, in which focal adhesions and actin stress fibers are not observed, immunohistochemical analysis of v-CrkPC12 cells reveals focal adhesion complexes which are formed at the periphery of the cell and are connected to actin cables. The formation of focal adhesions correlates with a concomitant upregulation in the expression of focal adhesion proteins FAK, paxillin, alpha3-integrin, and a higher-molecular-weight form of beta1-integrin. Our results indicate that v-Crk activates the Rho-signaling pathway and serves as a scaffolding protein during the assembly of focal adhesions in PC12 cells.

Multiple Grb2-mediated integrin-stimulated signaling pathways to ERK2/mitogen-activated protein kinase: summation of both c-Src- and focal adhesion kinase-initiated tyrosine phosphorylation events

Fibronectin receptor integrin-mediated cell adhesion triggers intracellular signaling events such as the activation of the Ras/mitogen-activated protein (MAP) kinase cascade. In this study, we show that the nonreceptor protein-tyrosine kinases (PTKs) c-Src and focal adhesion kinase (FAK) can be independently activated after fibronectin (FN) stimulation and that their combined activity promotes signaling to extracellular signal-regulated kinase 2 (ERK2)/MAP kinase through multiple pathways upstream of Ras. FN stimulation of NIH 3T3 fibroblasts promotes c-Src and FAK association in the Triton-insoluble cell fraction, and the time course of FN-stimulated ERK2 activation paralleled that of Grb2 binding to FAK at Tyr-925 and Grb2 binding to Shc. Cytochalasin D treatment of fibroblasts inhibited FN-induced FAK in vitro kinase activity and signaling to ERK2, but it only partially inhibited c-Src activation. Treatment of fibroblasts with protein kinase C inhibitors or with the PTK inhibitor herbimycin A or PP1 resulted in reduced Src PTK activity, no Grb2 binding to FAK, and lowered levels of ERK2 activation. FN-stimulated FAK PTK activity was not significantly affected by herbimycin A treatment and, under these conditions, FAK autophosphorylation promoted Shc binding to FAK. In vitro, FAK directly phosphorylated Shc Tyr-317 to promote Grb2 binding, and in vivo Grb2 binding to Shc was observed in herbimycin A-treated fibroblasts after FN stimulation. Interestingly, c-Src in vitro phosphorylation of Shc promoted Grb2 binding to both wild-type and Phe-317 Shc. In vivo, Phe-317 Shc was tyrosine phosphorylated after FN stimulation of human 293T cells and its expression did not inhibit signaling to ERK2. Surprisingly, expression of Phe-925 FAK with Phe-317 Shc also did not block signaling to ERK2, whereas FN-stimulated signaling to ERK2 was inhibited by coexpression of an SH3 domain-inactivated mutant of Grb2. Our studies show that FN receptor integrin signaling upstream of Ras and ERK2 does not follow a linear pathway but that, instead, multiple Grb2-mediated interactions with Shc, FAK, and perhaps other yet-to-be-determined phosphorylated targets represent parallel signaling pathways that cooperate to promote maximal ERK2 activation.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

Integrin signaling and cell growth control

Integrins contribute to cell growth by providing a physical linkage between cytoskeletal structures and the extracellular matrix, and also by participating in various signal transduction processes. The interaction of integrins with matrix ligands can generate signals in and of itself, and can also modulate signals instigated by soluble factors such as peptide mitogens. Cellular events affected by integrin-mediated signaling include motility, cell division, differentiation and programmed cell death. Elucidation of how integrin-mediated cell adhesion controls cell growth is likely to be of fundamental importance in understanding complex biological processes, such as tissue morphogenesis and tumor progression.

Identification of phosphorylated proteins associated with the fibroblast growth factor receptor type I during early Xenopus development

Signaling through the FGF receptor (FGFR) is required for mesoderm induction in Xenopus. Some of the downstream signaling molecules implicated in this developmental process include Ras, Raf and MAP kinase. In a previous report, we demonstrated that PLC gamma 1, Grb-2, SOS and Nck were associated with activated FGFR1s in a signaling complex in Xenopus blastulae. In addition, several unidentified phosphotyrosylproteins were present in the FGFR1 complex. Here we identify three of these proteins as Ras-GAP, the p85 of P13'K and SHP2, while demonstrating that c-Src and She were not associated with the FGFR1. Furthermore, we show that three additional phosphotyrosylproteins from the FGFR1 complex specifically bound to the adaptor molecule Nck.

Direct association of protein-tyrosine phosphatase PTP-PEST with paxillin

Tyrosine phosphorylation of focal adhesion-associated proteins may be involved in the regulation of the cytoskeleton and in the control of signals for growth and survival. The focal adhesion kinase (FAK) functions in regulating tyrosine phosphorylation of several of these proteins, including paxillin, tensin, and p130(cas). Protein- tyrosine phosphatases, the counterparts of protein-tyrosine kinases, also presumably regulate phosphorylation of these proteins. We have tested the hypothesis that FAK intimately associates with a protein-tyrosine phosphatase. Protein-tyrosine phosphatase activity associated with the recombinant C-terminal domain of FAK in vitro and could be coimmunoprecipitated with both FAK and paxillin from lysates of chicken embryo cells. However, the interaction with FAK appeared to be indirect and mediated via paxillin. The protein-tyrosine phosphatase was subsequently identified as protein-tyrosine phosphatase-PEST, a nonreceptor protein-tyrosine phosphatase. The C-terminal noncatalytic domain of protein-tyrosine phosphatase-PEST directly bound to paxillin in vitro. The association of both a protein-tyrosine kinase and a protein-tyrosine phosphatase with paxillin suggests that paxillin may play a critical role in the regulation of the phosphotyrosine content of proteins in focal adhesions.

T cell receptor-mediated tyrosine phosphorylation of Cas-L, a 105-kDa Crk-associated substrate-related protein, and its association of Crk and C3G

Cas-L (pp105), a Crk-associated substrate (p130(Cas))-related protein, was first identified as a 105-kDa protein that is tyrosine-phosphorylated following beta1 integrin cross-linking in T cells. Cas-L contains possible multiple binding sites for the Src homology (SH) 2 domains of various signaling molecules, and appears to be involved in signal transduction through phosphorylated tyrosine-mediated protein-protein interaction. Since Cas-L is preferentially expressed in lymphocytes, it is conceivable that Cas-L plays an important role in lymphocyte-specific signals. Here, we show the involvement of Cas-L in the T cell receptor (TCR)/CD3 signaling pathway. Cas-L is transiently phosphorylated following CD3 cross-linking, and tyrosine-phosphorylated Cas-L binds to Crk and C3G. Furthermore, a Cas-L mutant that lacks the SH3 domain, the binding site for focal adhesion kinase (FAK), is also tyrosine-phosphorylated upon CD3 cross-linking, but not upon beta1 integrin crosslinking, suggesting that FAK is not involved in CD3-dependent Cas-L phosphorylation. Taken together, the present study indicates a novel signaling pathway mediated by tyrosine-phosphorylated Cas-L upon the TCR/CD3 stimulation.

Integrins and GTPases in tumour cell growth, motility and invasion

One of the most exciting aspects of recent research on cell-adhesion receptors is the realization that these molecules can participate in bidirectional signal-transduction processes. Integrins can convey signals from the extracellular matrix to the cell interior, and intracellular events can influence the affinity of integrins for their ligands. Another important emerging concept is that integrins and small GTPases of the Rho family work together to coordinate both cytoskeletal organization and signalling processes. In this review, the authors examine integrin signalling and integrin-GTPase interplay in the context of cancer cell growth and behaviour.

CAS/Crk coupling serves as a "molecular switch" for induction of cell migration

Carcinoma cells selected for their ability to migrate in vitro showed enhanced invasive properties in vivo. Associated with this induction of migration was the anchorage-dependent phosphorylation of p130CAS (Crk-associated substrate), leading to its coupling to the adaptor protein c-CrkII (Crk). In fact, expression of CAS or its adaptor protein partner Crk was sufficient to promote cell migration, and this depended on CAS tyrosine phosphorylation facilitating an SH2-mediated complex with Crk. Cytokine-stimulated cell migration was blocked by CAS lacking the Crk binding site or Crk containing a mutant SH2 domain. This migration response was characterized by CAS/Crk localization to membrane ruffles and blocked by the dominant-negative GTPase, Rac, but not Ras. Thus, CAS/Crk assembly serves as a "molecular switch" for the induction of cell migration and appears to contribute to the invasive property of tumors.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Identification of p130Cas as a mediator of focal adhesion kinase-promoted cell migration

Previously we have demonstrated that focal adhesion kinase (FAK)-promoted migration on fibronectin (FN) by its overexpression in CHO cells is dependent on FAK autophosphorylation at Y397 and subsequent binding of Src to this site. In this report, we have examined the role of FAK association with Grb2 and p130(Cas), two downstream events of the FAK/Src complex that could mediate integrin-stimulated activation of extracellular signal-regulated kinases (Erks). We show that a Y925F FAK mutant was able to promote cell migration as efficiently as FAK and that the transfected FAK demonstrated no detectable association with Grb2 in CHO cells. In contrast, cells expressing a FAK P712/715A mutant demonstrated a level of migration comparable to that of control cells. This mutation did not affect FAK kinase activity, autophosphorylation, or Src association but did significantly reduce p130(Cas) association with FAK. Furthermore, FAK expression in CHO cells increased tyrosine phosphorylation of p130(Cas) and its subsequent binding to several SH2 domains, which depended on both the p130(Cas) binding site and the Src binding site. However, we did not detect increased activation of Erks in cells expressing FAK, and the MEK inhibitor PD98059 did not decrease FAK-promoted cell migration. Finally, we show that coexpression of p130(Cas) further increased cell migration on FN and coexpression of the p130(Cas) SH3 domain alone functioned as a dominant negative mutant and decreased cell migration. Together, these results demonstrate that p130(Cas), but not Grb2, is a mediator of FAK-promoted cell migration and suggest that FAK/ p130(Cas) complex targets downstream pathways other than Erks in mediating FAK-promoted cell migration.

SH2- and SH3-mediated interactions between focal adhesion kinase and Src

Intramolecular SH2 and SH3 interactions mediate enzymatic repression of the Src kinases. One mechanism of activation is disruption of these interactions by the formation of higher affinity SH2 and SH3 interactions with specific ligands. We show that a consensus Src SH3-binding site residing upstream of the Src SH2-binding site in FAK can function as a ligand for the Src SH3 domain. Surface plasmon resonance experiments indicate that a FAK peptide containing both the Src SH2- and SH3-binding sites exhibits increased affinity for Src. Furthermore, the presence of both sites in vitro more potently activates c-Src. A FAK mutant (FAKPro-2) with substitutions destroying the SH3-binding site shows reduced binding to Src in vivo. This mutation also reduces Src-dependent tyrosine phosphorylation on the mutant itself and downstream substrates, such as paxillin. These observations suggest that an SH3-mediated interaction between Src-like kinases and FAK may be important for complex formation and downstream signaling in vivo.

Transformation suppression by protein tyrosine phosphatase 1B requires a functional SH3 ligand

We have recently shown that protein tyrosine phosphatase 1B (PTP1B) associates with the docking protein p130Cas in 3Y1 rat fibroblasts. This interaction is mediated by a proline-rich sequence on PTP1B and the SH3 domain on p130Cas. Expression of wild-type PTP1B (WT-PTP1B), but not a catalytically competent, proline-to-alanine point mutant that cannot bind p130Cas (PA-PTP1B), causes substantial tyrosine dephosphorylation of p130Cas (F. Liu, D. E. Hill, and J. Chernoff, J. Biol. Chem. 271:31290-31295, 1996). Here we demonstrate that WT-, but not PA-PTP1B, inhibits transformation of rat 3Y1 fibroblasts by v-crk, -src, and -ras, but not by v-raf. These effects on transformation correlate with the phosphorylation status of p130Cas and two proteins that are associated with p130Cas, Paxillin and Fak. Expression of WT-PTP1B reduces formation of p130Cas-Crk complexes and inhibits mitogen-activated protein kinase activation by Src and Crk. These data show that transformation suppression by PTP1B requires a functional SH3 ligand and suggest that p130Cas may represent an important physiological target of PTP1B in cells.