Paxillin, a tyrosine phosphorylated focal adhesion-associated protein binds to the carboxyl terminal domain of focal adhesion kinase

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.

Activation of human endothelial cells via S-endo-1 antigen (CD146) stimulates the tyrosine phosphorylation of focal adhesion kinase p125(FAK)

S-Endo-1 antigen (CD146), a transmembrane receptor also known as MUC18/MCAM, is a member of the immunoglobulin superfamily and belongs to a group of cell adhesion molecules. CD146 is highly expressed on the whole vascular tree. We demonstrate here that engagement of CD146 on human endothelial cells isolated from cord blood results in tyrosine phosphorylation of a large panel of cellular proteins, although no tyrosine phosphorylation of CD146 was detected. In particular, CD146 cross-linking induces the tyrosine phosphorylation of the protein tyrosine kinase p125(FAK) as well as p125(FAK) association with paxillin, both events being inhibited by cytochalasin D. No direct association of CD146 with p125(FAK) was observed. Consistent with these data, CD146 associates with p59(fyn), a Src family kinase known to phosphorylate p125(FAK). The identification of a signaling pathway initiated by CD146 engagement and which includes p59(fyn), p125(FAK), and paxillin indicates that CD146 participates in outside-in signaling in endothelial cells.

Shear stress and the endothelium

Shear stress and the endothelium. Vascular endothelial cells (ECs) in vivo are influenced by two distinct hemodynamic forces: cyclical strain due to vessel wall distention by transmural pressure, and shear stress, the frictional force generated by blood flow. Shear stress acts at the apical cell surface to deform cells in the direction of blood flow; wall distention tends to deform cells in all directions. The shear stress response differs, at least partly, from the cyclical strain response, suggesting that cytoskeletal strain alone cannot explain it. Acute shear stress in vitro elicits rapid cytoskeletal remodeling and activates signaling cascades in ECs, with the consequent acute release of nitric oxide and prostacyclin; activation of transcription factors nuclear factor (NF)kappaB, c-fos, c-jun and SP-1; and transcriptional activation of genes, including ICAM-1, MCP-1, tissue factor, platelet-derived growth factor-B (PDGF-B), transforming growth factor (TGF)-beta1, cyclooxygenase-II, and endothelial nitric oxide synthase (eNOS). This response thus shares similarities with EC responses to inflammatory cytokines. In contrast, ECs adapt to chronic shear stress by structural remodeling and flattening to minimize shear stress. Such cells become very adherent to their substratum and show evidence of differentiation. Increased adhesion following chronic shear stress has been exploited to generate vascular grafts with confluent EC monolayers, retained after implantation in vivo, thus overcoming a major obstacle to endothelialization of vascular prostheses.

Signal transduction of mechanical stresses in the vascular wall

The vascular wall is constantly subjected to a variety of mechanical forces in the form of stretch (tensile stress), due to blood pressure, and shear stress, due to blood flow. Alterations in either of these stresses are known to result in vascular remodeling, an adaptation characterized by modified morphology and function of the blood vessels, allowing the vessels to cope with physiological or pathological conditions. The processes involved in vascular remodeling include cellular hypertrophy and hyperplasia, as well as enhanced protein synthesis or extracellular matrix protein reorganization. In vitro studies using vascular cells have attempted to identify the mechanisms behind structural alterations. Possible pathways include ion channels, integrin interaction between cells and the extracellular matrix, activation of various tyrosine kinases (such as c-Src, focal adhesion kinase, and mitogen-activated protein kinases), and autocrine production and release of growth factors. These pathways lie upstream of de novo synthesis of immediate response genes and total protein synthesis, both of which are likely to be involved in the process of vascular remodeling.

Integrin-mediated signaling events in human endothelial cells

Vascular endothelial cells are important in a variety of physiological and pathophysiological processes. The growth and functions of vascular endothelial cells are regulated both by soluble mitogenic and differentiation factors and by interactions with the extracellular matrix; however, relatively little is known about the role of the matrix. In the present study, we investigate whether integrin-mediated anchorage to a substratum coated with the extracellular matrix protein fibronectin regulates growth factor signaling events in human endothelial cells. We show that cell adhesion to fibronectin and growth factor stimulation trigger distinct initial tyrosine phosphorylation events in endothelial cells. Thus, integrin-dependent adhesion of endothelial cells leads to tyrosine phosphorylation of both focal adhesion kinase and paxillin, but not of several growth factor receptors. Conversely, EGF stimulation causes receptor autophosphorylation, with no effect on focal adhesion kinase or paxillin tyrosine phosphorylation. Adhesion to fibronectin, in the absence of growth factors, leads to activation of MAPK. In addition, adhesion to fibronectin also potentiates growth factor signaling to MAPK. Thus, polypeptide growth factor activation of MAPK in anchored cells is far more effective than in cells maintained in suspension. Other agonists known to activate MAPK were also examined for their ability to activate MAPK in an anchorage-dependent manner. The neuropeptide bombesin, the bioactive lipid lysophosphatidic acid (LPA), and the cytokine tumor necrosis factor alpha, which signal through diverse mechanisms, were all able to activate MAPK to a much greater degree in fibronectin-adherent cells than in suspended cells. In addition, tumor necrosis factor alpha activation of c-Jun kinase (JNK) was also much more robust in anchored cells. Together, these data suggest a cooperation between integrins and soluble mitogens in efficient propagation of signals to downstream kinases. This cooperation may contribute to anchorage dependence of mitogenic cell cycle progression.

Expression and characterization of splice variants of PYK2, a focal adhesion kinase-related protein

Focal adhesion kinase and the recently identified proline-rich tyrosine kinase 2 (PYK2), also known as cell adhesion kinase β, related adhesion focal tyrosine kinase or calcium-dependent protein tyrosine kinase, define a new family of non-receptor protein tyrosine kinases. Activation of PYK2 has been implicated in multiple signaling events, including modulation of ion channels, T- and B-cell receptor signaling and cell death. Mechanisms underlying the functional diversity of PYK2 are unclear. Here, we provide evidence for two novel alternatively expressed isoforms of PYK2. One isoform, designated PYK2s (PYK2 splice form), appears to be a splice variant of PYK2 lacking 42 amino acids within the C-terminal domain. A second isoform, referred to as PRNK (PYK2-related non-kinase), appears to be specified by mRNAs that encode only part of the C-terminal domain of PYK2. Northern blot analysis indicates that the unspliced PYK2 is expressed at high levels in the brain and poorly expressed in the spleen, whereas PYK2s and PRNK are expressed in the spleen. In situ hybridization studies of rat brain demonstrate that the unspliced PYK2 is selectively expressed at high levels in hippocampus, cerebral cortex and olfactory bulb, whereas PYK2s and PRNK are expressed at low levels in all regions of rat brain examined. Immunofluorescence analysis of ectopically expressed PRNK protein shows that PRNK, in contrast to full-length PYK2, is localized to focal adhesions by sequences within the focal adhesion targeting domain. In addition, PYK2, but not PRNK, interacts with p130(cas)and Graf. These results imply that PRNK may selectively regulate PYK2 function in certain cells by binding to some but not all PYK2 binding partners, and the functional diversity mediated by PYK2 may be due in part to complex alternative splicing.

Identification of osteopontin as a novel ligand for the integrin alpha8 beta1 and potential roles for this integrin-ligand interaction in kidney morphogenesis

Epithelio-mesenchymal interactions during kidney organogenesis are disrupted in integrin alpha8 beta1-deficient mice. However, the known ligands for integrin alpha8 beta1-fibronectin, vitronectin, and tenascin-C-are not appropriately localized to mediate all alpha8 beta1 functions in the kidney. Using a method of general utility for determining the distribution of unknown integrin ligands in situ and biochemical characterization of these ligands, we identified osteopontin (OPN) as a ligand for alpha8 beta1. We have coexpressed the extracellular domains of the mouse alpha8 and beta1 integrin subunits as a soluble heterodimer with one subunit fused to alkaline phosphatase (AP) and have used the alpha8 beta1-AP chimera as a histochemical reagent on sections of mouse embryos. Ligand localization with alpha8 beta1-AP in developing bone and kidney was observed to be overlapping with the distribution of OPN. In "far Western" blots of mouse embryonic protein extracts, bands were detected with sizes corresponding to fibronectin, vitronectin, and unknown proteins, one of which was identical to the size of OPN. In a solid-phase binding assay we demonstrated that purified OPN binds specifically to alpha8 beta1-AP. Cell adhesion assays using K562 cells expressing alpha8 beta1 were used to confirm this result. Together with a recent report that anti-OPN antibodies disrupt kidney morphogenesis, our results suggest that interactions between OPN and integrin alpha8 beta1 may help regulate kidney development and other morphogenetic processes.

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.

Activation of protein tyrosine kinase PYK2 by the m1 muscarinic acetylcholine receptor

Several G protein-coupled receptors are known to direct the tyrosine phosphorylation, and in some cases the activation, of diverse tyrosine kinases. Using a stable cell line approach, we characterize the activation of PYK2, a tyrosine kinase structurally related to focal adhesion kinase, by the G protein-coupled m1 muscarinic acetylcholine receptor. We find that PYK2 tyrosine kinase activity is critical for the m1 receptor-stimulated tyrosine phosphorylation of PYK2. Furthermore, we identify two tyrosine residues that are subject to phosphorylation in response to muscarinic signaling and show that this phosphorylation induces two cytosolic proteins, c-Src and Grb2, to bind to PYK2. This is the first demonstration of the significance played by distinct PYK2 tyrosine residues in G protein-coupled signaling to this kinase. By comparison, though m1 receptors induce the tyrosine phosphorylation of the cytoskeletal protein paxillin, the association of paxillin with PYK2 is unaffected by muscarinic signaling. We also provide evidence that PYK2 specifically phosphorylates the carboxyl-terminal cytosolic portion of the potassium channel Kv1.2 in a manner regulated by the m1 receptor. These results delineate molecular events attending the m1 muscarinic receptor stimulation of this tyrosine kinase and establish PYK2 as an effector of the m1 muscarinic receptor in the regulation of multiple cell functions.

Growth hormone stimulates the tyrosine phosphorylation and association of p125 focal adhesion kinase (FAK) with JAK2. Fak is not required for stat-mediated transcription

We have demonstrated that growth hormone (GH) activates focal adhesion kinase (FAK), and this activation results in the tyrosine phosphorylation of two FAK substrates, paxillin and tensin. The activation of FAK is time-dependent (maximal activation at 5-15 min) and dose-dependent (maximal activation at 0.05 nM). FAK and paxillin are constitutively associated in the unstimulated state, remain associated during the stimulation phase, and recruit tyrosine-phosphorylated tensin to the complex after GH stimulation. Half of the carboxyl-terminal region of the GH receptor is dispensable for FAK activation, but FAK activation does require the proline-rich box 1 region of the GH receptor, indicative that FAK is downstream of JAK2. FAK associates with JAK2 but not JAK1 after GH stimulation of cells. Using FAK-replete and FAK-deficient cells, we also show that FAK is not required for STAT-mediated transcriptional activation by GH. The use of FAK in the signal transduction pathway utilized by GH may be central to many of the pleiotropic effects of GH, including cytoskeletal reorganization, cell migration, chemotaxis, mitogenesis, and/or prevention of apoptosis and gene transcription.

Activation of distinct alpha5beta1-mediated signaling pathways by fibronectin's cell adhesion and matrix assembly domains

The interaction of cells with fibronectin generates a series of complex signaling events that serve to regulate several aspects of cell behavior, including growth, differentiation, adhesion, and motility. The formation of a fibronectin matrix is a dynamic, cell-mediated process that involves both ligation of the alpha5beta1 integrin with the Arg-Gly-Asp (RGD) sequence in fibronectin and binding of the amino terminus of fibronectin to cell surface receptors, termed "matrix assembly sites," which mediate the assembly of soluble fibronectin into insoluble fibrils. Our data demonstrate that the amino-terminal type I repeats of fibronectin bind to the alpha5beta1 integrin and support cell adhesion. Furthermore, the amino terminus of fibronectin modulates actin assembly, focal contact formation, tyrosine kinase activity, and cell migration. Amino-terminal fibronectin fragments and RGD peptides were able to cross-compete for binding to the alpha5beta1 integrin, suggesting that these two domains of fibronectin cannot bind to the alpha5beta1 integrin simultaneously. Cell adhesion to the amino-terminal domain of fibronectin was enhanced by cytochalasin D, suggesting that the ligand specificity of the alpha5beta1 integrin is regulated by the cytoskeleton. These data suggest a new paradigm for integrin-mediated signaling, where distinct regions within one ligand can modulate outside-in signaling through the same integrin.

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.

Paxillin phosphorylation and association with Lck and Pyk2 in anti-CD3- or anti-CD45-stimulated T cells

Antibodies to either CD3 or CD45 have been shown to induce dramatic changes in cell morphology, increased tyrosine phosphorylation of cellular proteins, and the association of a subset of these proteins with the tyrosine kinase Lck. The current study was initiated to determine the identity of the tyrosine-phosphorylated 70-80 kDa protein that becomes Lck-associated after stimulation with anti-CD45 or anti-CD3. We demonstrate that the cytoskeletal protein paxillin becomes tyrosine-phosphorylated when cells are plated on immobilized antibodies specific for CD45 or CD3. Only tyrosine-phosphorylated paxillin is associated with Lck, suggesting that the association is through the SH2 domain of Lck. Consistent with this we demonstrate that the SH2 domain of Lck binds tyrosine-phosphorylated paxillin. In contrast, the association of paxillin with the FAK-related kinase Pyk2 was found to be constitutive and not altered by the phosphorylation of either protein. Finally, we establish that the phosphorylation of paxillin is dependent on the expression of Lck. Taken together, these results demonstrate that paxillin is physically associated with kinases from two different families in T cells and suggest that paxillin may function as an adaptor protein linking cellular signals with cytoskeletal changes during T cell activation.

Caspase-mediated cleavage of focal adhesion kinase pp125FAK and disassembly of focal adhesions in human endothelial cell apoptosis

Normal endothelial and epithelial cells undergo apoptosis when cell adhesion and spreading are prevented, implying a requirement for antiapoptotic signals from the extracellular matrix for cell survival. We investigated some of the molecular changes occurring in focal adhesions during growth factor deprivation-induced apoptosis in confluent monolayers of human umbilical vein endothelial cells. Among the first morphologic changes after initiation of the apoptotic process are membrane blebbing, loss of focal adhesion sites, and retraction from the matrix followed by detachment. We observe a specific proteolytic cleavage of focal adhesion kinase (pp125FAK), an important component of the focal adhesion complex, and identify pp125FAK as a novel substrate for caspase-3 and caspase-3-like apoptotic caspases. The initial cleavage precedes detachment, and coincides with loss of pp125FAK and paxillin from focal adhesion sites and their redistribution into the characteristic membrane blebs of apoptotically dying cells. Cleavage of pp125FAK differentially affects its association with signaling and cytoskeletal components of the focal adhesion complex; binding of paxillin, but not pp130(Cas) (Cas, Crk-associated substrate) and vinculin, to the COOH terminally truncated pp125FAK is abolished. Therefore, caspase-mediated cleavage of pp125FAK may be participating in the disassembly of the focal adhesion complex and actively interrupting survival signals from the extracellular matrix, thus propagating the cell death program.

Cell density modulates protein-tyrosine phosphorylation

The growth of normal cells is arrested at saturating cell density in a process termed contact inhibition. An understanding of how cells communicate their contact with one another is critical for determining how cancers develop and spread. Because the molecular details of how fibroblasts communicate density changes are unclear, we examined cell density itself as a source of signaling events rather than examine specific receptors. A technique was developed to measure tyrosine phosphorylation acutely as a function of cell density. The tyrosine phosphorylation of a number of proteins was found to be modified in response to cell density. Three of these proteins were identified as Src, paxillin, and focal adhesion kinase (FAK), all of which show an increase in their tyrosine phosphate levels with increasing density. All of these proteins are found in focal adhesions, and both FAK and paxillin are believed to be localized exclusively in focal adhesions. Thus, changing cell density alters tyrosine phosphorylation of focal adhesion components.

Cellular phenotypic transformation during early embryogenesis: a role for focal adhesion kinase?

We have used the gastrulating chick embryo as a model for studying the potential role of focal adhesion kinase (FAK) in phenotypic transformation. In the gastrulating embryo, there is a well-defined epithelial to mesenchymal transformation as the upper epithelial epiblast layer of cells ingresses at the primitive streak to form the invasive mesenchymal mesoderm layer and the epithelioid endoderm layer. Immunolocalization showed that FAK was expressed primarily in the apical cytoplasm of the epiblast layer, together with some regions of the mesoderm and endoderm. Hensen's node and the primitive streak, where the transformation occurs, showed very low immunoreactivity. Levels of FAK in these individual tissues were quantified by densitometric analysis of Western blots, and FAK activation was quantified by stripping these blots and reprobing for phosphotyrosine. Immunoprecipitation indicated that the phosphotyrosine bands corresponded with the FAK bands on the blots. Although the blots confirmed that FAK was highly expressed in the epiblast, the level of FAK activation was highest in the endoderm, despite relatively low expression of the protein. Similar quantitative blotting was carried out using cells from each of the three layers cultured on different substrata. The results indicated that cells cultured on fibronectin, laminin, and Matrigel expressed differing levels of FAK, with differing levels of tyrosine phosphorylation, depending on the cell type and the substratum. We conclude that FAK is developmentally regulated during gastrulation, and that this regulation could be influenced by the changing substratum encountered by the differentiating cells during this process. However, the apical localization of FAK in much of the epiblast appears to preclude a consistent focal contact-like association of this molecule with integrins in vivo, and we therefore suggest that in the embryo, FAK may be involved in integrin-mediated signalling pathways without physical association with cell-substratum contacts.Key words: chick, embryo, gastrulation, phenotypic transformation, FAK.

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.

A suppressive role of p125FAK protein tyrosine kinase in hydrogen peroxide-induced apoptosis of T98G cells

Protein tyrosine phosphorylation was examined on a human glioblastoma cell line, T98G, after exposure to oxidative stress in vitro. Hydrogen peroxide (1 mM) markedly induced tyrosine phosphorylation of a 125 kDa protein at 30 min after stimulation. The 125-kDa molecule phosphorylated was revealed to be a focal adhesion kinase (FAK). Tyrosine phosphorylation of p125FAK continued at least up to 5 h, and decreased after 8 h concomitant with apoptosis. Tyrosine phosphorylation of p125FAK was blocked by herbimycin A, a potent inhibitor of protein tyrosine kinases, while apoptosis was accelerated. When T98G cells were incubated with FAK antisense oligonucleotide, apoptosis was also accelerated. These results suggest that tyrosine phosphorylation of p125FAK plays a suppressive role in hydrogen peroxide-induced apoptosis.

Rapid impact of beta-amyloid on paxillin in a neural cell line

Beta-amyloid1-42 (Abeta) is a naturally occuring peptide whose accumulation in the brain is putatively coupled to Alzheimer's disease pathogenesis. Deleterious effects of Abeta on neurons have been linked to the inappropriate activation of signaling pathways within the cell (reviewed in Yankner, 1996), including tyrosine phosphorylation of focal adhesion kinase (FAK) (Zhang et al., 1994, 1996a,b). Here we have investigated the effects of Abeta on paxillin in a neural cell line. Paxillin, a substrate for FAK, is thought to act as a signal "integrator," functioning to link other proteins into multi-molecular signaling complexes (reviewed in Turner, 1994). Treatment of the rat central nervous system B103 cell line with aggregates of Abeta was found to induce the tyrosine phosphorylation of paxillin within 30 min, nearly 24 hr prior to significant cell death. Particularly striking was a subsequent "mobilization" of paxillin to the cytoskeleton in Abeta-treated cells. The amount of paxillin associated with the cytoskeleton in Abeta-treated cells was increased 10-fold over controls. The Abeta-induced paxillin accumulation could be visualized immunocytochemically, with an increase in number and size of paxillin-labeled focal contacts upon treatment with Abeta. This effect was specific, in that vinculin, another focal contact protein, was unaffected by Abeta. Disruption of f-actin, which inhibits both Abeta-induced neurotoxicity (Furukawa and Mattson, 1995) and focal contact signaling in B103 cells (Zhang et al., 1996b) was found to block the cytoskeletal paxillin accumulation. The rapid tyrosine phosphorylation and cytoskeletal mobilization of paxillin links Abeta to the activation of focal contact signaling events that may influence neuronal cytoskeletal architecture, gene expression, synaptic plasticity and cell viability.

Inhibition of cell spreading by expression of the C-terminal domain of focal adhesion kinase (FAK) is rescued by coexpression of Src or catalytically inactive FAK: a role for paxillin tyrosine phosphorylation

pp125FAK is a tyrosine kinase that appears to regulate the assembly of focal adhesions and thereby promotes cell spreading on the extracellular matrix. In some cells, the C terminus of pp125FAK is expressed as a separate protein, pp41/43FRNK. We have previously shown that overexpression of pp41/43FRNK inhibits tyrosine phosphorylation of pp125FAK and paxillin and, in addition, delays cell spreading and focal adhesion assembly. Thus, pp41/43FRNK functions as a negative inhibitor of adhesion signaling and provides a tool to dissect the mechanism by which pp125FAK promotes cell spreading. We report here that the inhibitory effects of pp41/43FRNK expression can be rescued by the co-overexpression of wild-type pp125FAK and partially rescued by catalytically inactive variants of pp125FAK. However, coexpression of an autophosphorylation site mutant of pp125FAK, which fails to bind the SH2 domain of pp60c-Src, or a mutant that fails to bind paxillin did not promote cell spreading. In contrast, expression of pp41/43FRNK and pp60c-Src reconstituted cell spreading and tyrosine phosphorylation of paxillin but did so without inducing tyrosine phosphorylation of pp125FAK. These data provide additional support for a model whereby pp125FAK acts as a "switchable adaptor" that recruits pp60c-Src to phosphorylate paxillin, promoting cell spreading. In addition, these data point to tyrosine phosphorylation of paxillin as being a critical step in focal adhesion assembly.

Protein tyrosine phosphorylation in pancreatic acini: differential effects of VIP and CCK

Cholecystokinin (CCK) and vasoactive intestinal peptide (VIP) stimulate enzyme secretion from pancreatic acini by binding to heptahelical receptors without intrinsic tyrosine kinase activity. Signal transduction by the CCK receptor involves activation of phospholipase C by Gq proteins and activation of tyrosine kinases, whereas occupation of VIP receptors stimulates adenylyl cyclase through binding to Gs proteins. Here, we use electrophoretic separation of cellular proteins and antiphosphotyrosine immunoblotting to demonstrate a VIP-stimulated rapid and dose-dependent increase in tyrosine phosphorylation of proteins migrating at 130, 115, and 93 kDa in freshly isolated rat pancreatic acini. Phosphorylation of these proteins was increased after direct stimulation of adenylyl cyclase or the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase with forskolin or dibutyryl cAMP and was inhibited by the tyrosine kinase inhibitors genistein or tyrphostin 23. Compared with VIP, CCK stimulated tyrosine phosphorylation of additional proteins migrating at 60, 66, and 72/78 kDa. Using two-dimensional electrophoretic separation or immunoprecipitation, the 72/78-kDa phosphoprotein was identified as paxillin. We propose that paxillin might be involved in CCK-but not in VIP-induced exocytosis.

Induction of apoptosis after expression of PYK2, a tyrosine kinase structurally related to focal adhesion kinase

Many cells (e.g., epithelial cells) require attachment to the extracellular matrix (ECM) to survive, a phenomenon known as anchorage-dependent cell survival. Disruption of the cell-ECM interactions mediated by the integrin receptors results in apoptosis. Focal adhesion kinase (FAK), a 125-kD protein tyrosine kinase activated by integrin engagement, appears to be involved in mediating cell attachment and survival. Proline-rich tyrosine kinase 2 (PYK2), also known as cellular adhesion kinase beta (CAKbeta) and related adhesion focal tyrosine kinase, is a second member of the FAK subfamily and is activated by an increase in intracellular calcium levels, or treatment with TNFalpha and UV light. However, the function of PYK2 remains largely unknown. In this study, we show that over-expression of PYK2, but not FAK, in rat and mouse fibroblasts leads to apoptotic cell death. Using a series of deletion mutants and chimeric fusion proteins of PYK2/FAK, we determined that the NH2-terminal domain and tyrosine kinase activity of PYK2 were required for the efficient induction of apoptosis. Furthermore, the apoptosis mediated by PYK2 could be suppressed by over-expressing catalytically active v-Src, c-Src, phosphatidylinositol-3-kinase, or Akt/protein kinase B. In addition, it could also be suppressed by overexpressing an ICE or ICE-like proteinase inhibitor, crmA, but not Bcl2. Collectively, our results suggest that PYK2 and FAK, albeit highly homologous in primary structure, appear to have different functions; FAK is required for cell survival, whereas PYK2 induces apoptosis in fibroblasts.

Cleavage of focal adhesion kinase by caspases during apoptosis

Apoptotic cells undergo characteristic morphological changes that include detachment of cell attachment from the substratum and loss of cell-cell interactions. Attachment of cells to the extracellular matrix and to other cells is mediated by integrins. The interactions of integrins with the extracellular matrix activates focal adhesion kinase (FAK) and suppresses apoptosis in diverse cell types. Members of the tumor necrosis family such as Fas and Apo-2L, also known as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), induce apoptosis in both suspension and adherent cells through the activation of caspases. These caspases, when activated, cleave substrates that are important for the maintenance of nuclear and membrane integrity. In this study, we show that FAK is sequentially cleaved into two different fragments early in Apo-2L-induced apoptosis. We also demonstrate that FAK cleavage is mediated by caspases and that FAK shows unique sensitivity to different caspases. Our results suggest that disruption of FAK may contribute to the morphological changes observed in apoptotic suspension and adherent cells.

Differential signaling by the focal adhesion kinase and cell adhesion kinase beta

pp125(FAK) and CAKbeta/Pyk2/CadTK/RAFTK are related protein-tyrosine kinases. It is therefore of interest whether CAKbeta shares some of the properties of pp125(FAK). Using recombinant glutathione S-transferase fusion proteins, we show that the C-terminal domains of both proteins bind paxillin in vitro. The C-terminal domain of CAKbeta was engineered to be autonomously expressed in chicken embryo cells and, like pp125(FAK) and p41/43(FRNK) (the C-terminal noncatalytic domain of pp125(FAK)), was found to localize to cellular focal adhesions. In contrast, full-length CAKbeta was generally found diffusely distributed throughout the cell, although a fraction of the cells exhibited focal adhesion localization. Vanadate treatment of pp125(FAK)- and CAKbeta-overexpressing CE cells induced a dramatic increase in the phosphotyrosine content of a common set of proteins including tensin, paxillin, and p130(Cas), but some of these substrates, particularly p130(Cas), appeared to be differentially phosphorylated by pp125(FAK) and CAKbeta. Levels of tyrosine phosphorylation were higher in CAKbeta-overexpressing cells, and additional phosphotyrosine-containing species were specifically immunoprecipitated. In addition, vanadate treatment of CE cells overexpressing CAKbeta, but not pp125(FAK) overexpressors, induced a profound morphological change, which could be a consequence of the observed differences in substrate phosphorylation.

Integrin signaling: specificity and control of cell survival and cell cycle progression

Integrin-mediated adhesion to the extracellular matrix plays an important role in regulating cell survival and proliferation. There is now increasing evidence that integrins activate shared as well as subgroup-specific signaling pathways. The signals from these adhesion receptors are integrated with those originating from growth factor and cytokine receptors in order to organize the cytoskeleton, stimulate mitogen-activated protein kinase cascades, and regulate immediate early gene expression. The repertoire of integrins and composition of the extracellular matrix appear to dictate whether a cell will survive, proliferate or exit the cell cycle and differentiate in response to soluble factors.

Role of tyrosine kinases in extracellular matrix-mediated modulation of arterial smooth muscle cell phenotype

Fibronectin (FN) promotes the modulation of freshly isolated arterial smooth muscle cells (SMCs) from a contractile to a synthetic phenotype by interacting with integrins on the cell surface. This process is characterized by a structural and functional transformation of the cells, including a reorganization of the cytoskeleton, the formation of a large secretory apparatus, and the acquisition of proliferative capacity. In this study we have investigated the role of integrin signaling through tyrosine kinases in the structural changes that occur in SMCs during primary culture on FN. A gradual increase in phosphotyrosine staining in focal adhesions and a concomitant increase in tyrosine phosphorylation of proteins including focal adhesion kinase were observed. In contrast, cells seeded on laminin formed few focal adhesions, and tyrosine phosphorylation of proteins was less than in cells cultured on FN. Treatment of cells cultured on FN with the tyrosine kinase inhibitor genistein strongly suppressed focal adhesion formation, cell spreading, and cytoskeletal reorganization. In addition, electron microscopic analysis demonstrated that the phenotypic modulation was slowed down. These results indicate that the ability of extracellular matrix components to promote a change in the phenotypic properties of SMCs depends on the assembly of focal adhesions with associated tyrosine kinase activity.

Integrin-mediated activation of focal adhesion kinase is independent of focal adhesion formation or integrin activation. Studies with activated and inhibitory beta3 cytoplasmic domain mutants

Integrin alphaIIbbeta3 functions as the fibrinogen receptor on platelets and mediates platelet aggregation and clot retraction. Among the events that occur during either "inside-out" or "outside-in" signaling through alphaIIbbeta3 is the phosphorylation of focal adhesion kinase (pp125(FAK)) and the association of pp125(FAK) with cytoskeletal components. To examine the role of pp125(FAK) in these integrin-mediated events, pp125(FAK) phosphorylation and association with the cytoskeleton was determined in cells expressing two mutant forms of alphaIIbbeta3: alphaIIbbeta3(D723A/E726A), a constitutively active integrin in which the putative binding site for pp125(FAK) is altered, and alphaIIbbeta3(F727A/K729E/F730A), in which the putative binding site for alpha-actinin is altered. Both mutants were expressed on the cell surface and were able to bind ligand, either spontaneously or upon activation. Whereas cells expressing alphaIIbbeta3(D723A/E726A) were able to form focal adhesions and stress fibers upon adherence to fibrinogen, cells expressing alphaIIbbeta3(F727A/K729E/F730A) adhere to fibrinogen, but had reduced focal adhesions and stress fibers. pp125(FAK) is recruited to focal adhesions in adherent cells expressing alphaIIbbeta3(D723A/E726A) and is phosphorylated in adherent cells or in cells in suspension in the presence of fibrinogen. In adherent cells expressing alphaIIbbeta3(F727A/K729E/F730A), pp125(FAK) was phosphorylated despite reduced formation of focal adhesions and stress fibers. We conclude that activation of pp125(FAK) can be dissociated from two important events in integrin signaling, the assembly of focal adhesions in adherent cells and integrin activation following ligand occupation.

Paxillin is tyrosine-phosphorylated by and preferentially associates with the calcium-dependent tyrosine kinase in rat liver epithelial cells

We and others have recently cloned a non-receptor, calcium-dependent tyrosine kinase (CADTK; also known as PYK2, CAKbeta, and RAFTK) that shares both overall domain structure and 45% amino acid identity with p125(FAK). We have studied the signaling, activation, and potential function of these related enzymes in GN4 rat liver epithelial cells that express CADTK and p125(FAK) at roughly similar levels. p125(FAK) is nearly fully tyrosine-phosphorylated in resting GN4 cells. In contrast, while CADTK is not tyrosine-autophosphorylated in untreated cells, angiotensin II increases CADTK Tyr(P) by 5-10-fold. With regard to signaling, CADTK activation is correlated with stimulation of c-Jun N-terminal kinase and p70(S6K) pathways but not with the stimulation of mitogen-activated protein kinase or p90(RSK). In this report we assessed the contribution of CADTK and p125(FAK) to tyrosine phosphorylation of focal contact proteins. In adherent GN4 cells, the constitutive activity of p125(FAK) was correlated with basal paxillin, tensin, and p130(CAS) tyrosine phosphorylation. A rapid increase in the tyrosine phosphorylation of each protein was detected after treatment with angiotensin II or other agonists that stimulate CADTK; the prolonged 3-4-fold increase in paxillin tyrosine phosphorylation was the most substantial change. In the WB cell line that expresses 3-fold less CADTK than GN4 cell line agonist-dependent paxillin tyrosine phosphorylation is similarly reduced. Immunoprecipitation of CADTK from GN4 cells revealed CADTK. paxillin complexes that persisted in 500 mM NaCl but not in 0.1% SDS cell lysis buffer. The complexes were largely independent of the tyrosine phosphorylation state of either protein. Surprisingly, we did not detect p125(FAK).paxillin complexes in immunoprecipitates using either of two p125(FAK) antibodies. When CADTK and p125(FAK) were transiently overexpressed in 293(T) cells, both enzymes associated with paxillin, but the avidity of CADTK appeared to be greater. In addition, in transfected 293(T) cells, complexes between CADTK and another potential substrate, p130(CAS), were detected. In summary, in GN4 rat liver epithelial cells stimulation of CADTK was highly correlated with paxillin tyrosine phosphorylation; in addition, CADTK but not p125(FAK) was complexed to paxillin at detectable levels. This suggests that agonist-dependent cytoskeletal changes in epithelial cells might proceed, in part, by CADTK-dependent mechanisms.

Identification of integrin-stimulated sites of serine phosphorylation in FRNK, the separately expressed C-terminal domain of focal adhesion kinase: a potential role for protein kinase A

Focal adhesion kinase (pp125(FAK)) is a protein tyrosine kinase that is localized to focal adhesions in many cell types and which undergoes tyrosine phosphorylation after integrin binding to extracellular matrix. In some cells the C-terminal non-catalytic domain of pp125(FAK) is expressed as a separate protein referred to as FRNK (FAK-related, non-kinase). We have previously shown that overexpression of FRNK inhibits tyrosine phosphorylation of pp125(FAK) and its substrates as well as inhibiting cell spreading on fibronectin. In this report we identify Ser148 and Ser151 as residues in FRNK that are phosphorylated after tyrosine phosphorylation of pp125(FAK) and in response to integrin binding to fibronectin. Tyrosine phosphorylation of pp125(FAK) appears to be an early event after integrin occupancy, and serine phosphorylation of FRNK occurs significantly later. Treatment of fibroblasts with a series of protein kinase A inhibitors delayed serine phosphorylation of FRNK as well as cell spreading on fibronectin and tyrosine phosphorylation of pp125(FAK). However, these PKA inhibitors are unlikely to delay cell spreading simply by preventing serine phosphorylation of FRNK, as overexpression of FRNK containing mutations of Ser148 and Ser151 either singly or jointly to either alanine or glutamate residues did not significantly alter the ability of FRNK to act as an inhibitor of pp125(FAK).

Integrin-mediated activation of MAP kinase is independent of FAK: evidence for dual integrin signaling pathways in fibroblasts

Integrin-mediated cell adhesion causes activation of MAP kinases and increased tyrosine phosphorylation of focal adhesion kinase (FAK). Autophosphorylation of FAK leads to the binding of SH2-domain proteins including Src-family kinases and the Grb2-Sos complex. Since Grb2-Sos is a key regulator of the Ras signal transduction pathway, one plausible hypothesis has been that integrin-mediated tyrosine phosphorylation of FAK leads to activation of the Ras cascade and ultimately to mitogen activated protein (MAP) kinase activation. Thus, in this scenario FAK would serve as an upstream regulator of MAP kinase activity. However, in this report we present several lines of evidence showing that integrin-mediated MAP kinase activity in fibroblasts is independent of FAK. First, a beta1 integrin subunit deletion mutant affecting the putative FAK binding site supports activation of MAP kinase in adhering fibroblasts but not tyrosine phosphorylation of FAK. Second, fibroblast adhesion to bacterially expressed fragments of fibronectin demonstrates that robust activation of MAP kinase can precede tyrosine phosphorylation of FAK. Finally, we have used FRNK, the noncatalytic COOH-terminal domain of FAK, as a dominant negative inhibitor of FAK autophosphorylation and of tyrosine phosphorylation of focal contacts. Using retroviral infection, we demonstrate that levels of FRNK expression sufficient to completely block FAK tyrosine phosphorylation were without effect on integrin-mediated activation of MAP kinase. These results strongly suggest that integrin-mediated activation of MAP kinase is independent of FAK and indicate the probable existence of at least two distinct integrin signaling pathways in fibroblasts.

Monocyte cells and cancer cells express novel paxillin isoforms with different binding properties to focal adhesion proteins

The versatility of integrin functions is mediated by engagement of a number of proteins that assemble with integrins. Among them, paxillin is one of the important molecules interacting with a variety of signaling molecules and cytoskeletal building blocks. We report here that paxillin is not a single molecule with a unique physiological property. We identified two human paxillin isoforms, beta and gamma. These isoforms have distinct amino acid insertions; each consists of a distinct exon, at the same site of previously reported paxillin (paxillin alpha). Several proteins were co-precipitated with paxillin, and we found that beta bound to focal adhesion kinase but weakly to vinculin, and gamma bound to vinculin but only weakly to focal adhesion kinase, although both bound equally to talin. No additional proteins were found to bind to beta and gamma over those binding to alpha. Unlike the alpha isoform, beta and gamma mRNAs were not detected in normal tissues, but several cancer cells expressed both alpha and beta proteins simultaneously. All three isoform proteins were expressed in promonocytic cells with ratios comparable with each other, and the expression patterns were altered during differentiation of floating promonocytic cells into adherent macrophage-like cells. Therefore, each isoform of paxillin exhibits distinct expression and different biochemical as well as physiological properties and thereby appears to act as a distinct module involved in different functions of integrins.

Preferential localization of tyrosine-phosphorylated paxillin in focal adhesions

Focal adhesions are sites for integrin-mediated attachment of cultured cells to the extracellular matrix. Localization studies have shown that focal adhesions can be stained by antiphosphotyrosine antibodies, but the role of tyrosine-phosphorylated proteins in focal adhesions is not known. By using ventral plasma membranes prepared from chicken embryo fibroblasts spread on the substrate, we present evidence for the preferential localization of a minor pool of tyrosine-phosphorylated paxillin in focal adhesions. Ventral plasma membranes showed an enrichment in beta 1-integrins, and in several tyrosine-phosphorylated polypeptides, while focal adhesion proteins like vinculin and paxillin, although localized to focal adhesions in ventral plasma membranes, were not particularly enriched in these preparations compared to whole cell lysates. Biochemical and morphological analysis of ventral plasma membranes showed a dramatic increase in the level of tyrosine-phosphorylation of the pool of paxillin localized to the adhesive sites, when compared to the paxillin present in whole cell lysates. The observed preferential localization of tyrosine-phosphorylated paxillin to focal adhesions may represent a general mechanism to compartmentalize focal adhesion components from large non-phosphorylated, cytosolic pools.

Complexes of focal adhesion kinase (FAK) and Crk-associated substrate (p130(Cas)) are elevated in cytoskeleton-associated fractions following adhesion and Src transformation. Requirements for Src kinase activity and FAK proline-rich motifs

The focal adhesion kinase (FAK) and Crk-associated substrate, p130(Cas) (Cas), have been implicated in diverse signaling pathways including those mediated by integrins, G-protein-coupled receptors, tyrosine kinase receptors, and the v-src and v-crk oncogenes. The recent identification of a direct interaction between FAK and Cas prompted the examination of potential regulation of FAK.Cas complexes by factors that result in concomitant increase in their phosphotyrosine content, namely cell adhesion and transformation by Src. Both conditions resulted in elevated FAK.Cas complex levels in nonionic detergent-insoluble fractions, indicating increased association with the cytoskeleton. For activated Src, this effect requires an active Src catalytic domain but not its Src homology 2 (SH2) or Src homology 3 (SH3) domains. FAK kinase domain tyrosines 576 and 577 are also required, suggesting that direct phosphorylation of these sites by Src may influence the solubility and/or stability of the complex. FAK-Cas association was only observed in the context of Cas binding to at least one of two distinct proline-rich sites on FAK. These findings firmly establish a direct interaction between FAK and Cas and demonstrate that Src can influence the subcellular localization of the complex by a tyrosine phosphorylation-dependent mechanism.

Signaling through focal adhesion kinase

Focal adhesion kinase (FAK) is a nonreceptor protein-tyrosine kinase implicated in controlling cellular responses to the engagement of cell-surface integrins, including cell spreading and migration, survival and proliferation. Aberrant FAK signaling may contribute to the process of cell transformation by certain oncoproteins, including v-Src. Progress toward elucidating the events leading to FAK activation following integrin-mediated cell adhesion, as well as events downstream of FAK, has come through the identification of FAK phosphorylation sites and interacting proteins. A signaling partnership is formed between FAK and Src-family kinases, leading to tyrosine phosphorylation of FAK and associated 'docking' proteins Cas and paxillin. Subsequent recruitment of proteins containing Src homology 2 domains, including Grb2 and c-Crk, to the complex is likely to trigger adhesion-induced cellular responses, including changes to the actin cytoskeleton and activation of the Ras-MAP kinase pathway.

Integrin-mediated tyrosine phosphorylation and redistribution of paxillin during neuronal adhesion

Integrins are important receptors for neuronal adhesion to laminin, which is one of the best promoters of neurite outgrowth. The present study was carried out to understand some of the intracellular mechanisms which allow integrin-mediated neurite extension on laminin. In chicken retinal neurons, integrin-mediated adhesion to laminin and antibody-induced integrin clustering caused an increase in tyrosine phosphorylation of paxillin and focal adhesion kinase. The kinetics of phosphorylation and dephosphorylation of these proteins were different in neurons plated on laminin, compared to neurons in which the receptors were clustered with anti-integrin antibodies. Analysis of sucrose velocity gradients could not show any association of paxillin and focal adhesion kinase with the integrin receptors. On the other hand, by using digitonin and milder extraction conditions, we found an enrichment of the tyrosine-phosphorylated polypeptides in the cytoskeletal, digitonin-insoluble fraction. Furthermore, neuronal adhesion induced a dramatic increase in the fraction of tyrosine-phosphorylated paxillin recovered with the digitonin-insoluble fraction, suggesting redistribution of this protein following adhesion of neurons to laminin. Localization studies on the detergent-insoluble fraction showed codistribution of both paxillin and focal adhesion kinase with integrins. We also found that paxillin tyrosine phosphorylation, but not paxillin expression, is developmentally regulated in the retina. Our results show that integrin-mediated neuronal adhesion leads to the accumulation of a pool of highly phosphorylated proteins at adhesion sites. There they may be responsible for the reorganization of the cytoskeleton, which underlies the process of neurite extension.

Paxillin association in vitro with integrin cytoplasmic domain peptides

Short cytoplasmic domains of integrin heterodimers are crucial for transduction of signals generated by adhesion of cells to the extracellular matrix. Here, we describe the use of peptides mimicking the intracellular tails of integrin alpha5beta1 to assay in vitro associations with cytoskeletal proteins. Our results suggest that the focal adhesion protein, paxillin, may interact directly with the intracellular region of the integrin beta1 subunit. Paxillin is known to form stable complexes with several signaling molecules, including focal adhesion kinase. Physical interaction between paxillin and the beta1 cytoplasmic domain suggests a model in which paxillin may function as a key intermediary in integrin-mediated signal transduction.

Reduced cell attachment and phosphorylation of focal adhesion kinase associated with expression of a mutant insulin receptor

Insulin signaling results in rapid changes to the cell cytoskeleton, and it has recently been shown that insulin stimulates the dephosphorylation of the cytoskeletal-associated tyrosine kinase, focal adhesion kinase (pp125(FAK)). We report here that mutation of two tryptic cleavage sites (Lys164 and Lys582 --> Asn; 2N) in the insulin receptor alpha-subunit results in a cell-line (CHO.2N-10) with altered morphology associated with an increase in cell size, a decrease in cell adhesiveness, and a decrease in pp125(FAK) tyrosine phosphorylation in the absence of insulin (45.2 +/- 9.7% compared to nontransfected Chinese hamster ovary (CHO) cells). In contrast to pp125(FAK), paxillin phosphorylation was similar in all cell lines despite lower levels (61.0 +/- 10.4% compared to CHO cells) of paxillin protein in CHO.2N-10 cells. We observed comparable protein levels of pp125(FAK) and the structural focal adhesion protein, vinculin, in all cell lines. Despite underphosphorylation of pp125(FAK) in the basal state, insulin stimulation of CHO.2N-10 cells still resulted in dephosphorylation of pp125(FAK). CHO.2N-10 and CHO.T (overexpress wild-type insulin receptor) cells have similar insulin binding characteristics, insulin-stimulated autokinase and peptide phosphorylation, and insulin-stimulated pp185/IRS-1 phosphorylation. Our results suggest that the insulin receptor may play an important role in cell-matrix interactions, such as modulating cell adhesion and inducing cell architecture changes.

Identification of LIM3 as the principal determinant of paxillin focal adhesion localization and characterization of a novel motif on paxillin directing vinculin and focal adhesion kinase binding

Paxillin is a 68-kD focal adhesion phosphoprotein that interacts with several proteins including members of the src family of tyrosine kinases, the transforming protein v-crk, and the cytoskeletal proteins vinculin and the tyrosine kinase, focal adhesion kinase (FAK). This suggests a function for paxillin as a molecular adaptor, responsible for the recruitment of structural and signaling molecules to focal adhesions. The current study defines the vinculin- and FAK-interaction domains on paxillin and identifies the principal paxillin focal adhesion targeting motif. Using truncation and deletion mutagenesis, we have localized the vinculin-binding site on paxillin to a contiguous stretch of 21 amino acids spanning residues 143-164. In contrast, maximal binding of FAK to paxillin requires, in addition to the region of paxillin spanning amino acids 143-164, a carboxyl-terminal domain encompassing residues 265-313. These data demonstrate the presence of a single binding site for vinculin, and at least two binding sites for FAK that are separated by an intervening stretch of 100 amino acids. Vinculin- and FAK-binding activities within amino acids 143-164 were separable since mutation of amino acid 151 from a negatively charged glutamic acid to the uncharged polar residue glutamine (E151Q) reduced binding of vinculin to paxillin by >90%, with no reduction in the binding capacity for FAK. The requirement for focal adhesion targeting of the vinculin- and FAK-binding regions within paxillin was determined by transfection into CHO.K1 fibroblasts. Significantly and surprisingly, paxillin constructs containing both deletion and point mutations that abrogate binding of FAK and/or vinculin were found to target effectively to focal adhesions. Additionally, expression of the amino-terminal 313 amino acids of paxillin containing intact vinculin- and FAK-binding domains failed to target to focal adhesions. This indicated other regions of paxillin were functioning as focal adhesion localization motifs. The carboxyl-terminal half of paxillin (amino acids 313-559) contains four contiguous double zinc finger LIM domains. Transfection analyses of sequential carboxyl-terminal truncations of the four individual LIM motifs and site-directed mutagenesis of LIM domains 1, 2, and 3, as well as deletion mutagenesis, revealed that the principal mechanism of targeting paxillin to focal adhesions is through LIM3. These data demonstrate that paxillin localizes to focal adhesions independent of interactions with vinculin and/or FAK, and represents the first definitive demonstration of LIM domains functioning as a primary determinant of protein subcellular localization to focal adhesions.

Talin contains three actin-binding sites each of which is adjacent to a vinculin-binding site

We have determined the sequence of chicken talin (2,541 amino acids, M(r) 271,881) which is very similar (89% identity) to that of the mouse protein. Alignments with the Caenorhabditis elegans and Dictyostelium discoideum talin sequences show that the N- and C-terminal regions of the protein are conserved whereas the central part of the molecule is more divergent. By expressing overlapping talin polypeptides as fusion proteins, we have identified at least three regions of the protein which can bind F-actin: residues 102–497, 951–1,327 and 2,269-2,541. The N-terminal binding site contains a region with homology to the ERM family of actin-binding proteins, and the C-terminal site is homologous to the yeast actin-binding protein Sla2p. Each of the actin-binding sites is close to, but distinct from a binding site for vinculin, a protein which also binds actin. The Pro1176 to Thr substitution found in talin from Wistar-Furth rats does not destroy the capacity of this region of the protein to bind actin or vinculin. Microinjection studies showed that a fusion protein containing the N-terminal actin-binding site localised weakly to stress fibres, whereas one containing the C-terminal site initially localised predominantly to focal adhesions. The former was readily solubilised, and the latter was resistant to Triton extraction. The N-terminal talin polypeptide eventually disrupted actin stress fibres whereas the C-terminal polypeptide was without effect. However, a larger C-terminal fusion protein also containing a vinculin-binding site did disrupt stress fibres and focal adhesions. The results suggest that, although both the N- and C-terminal regions of talin bind actin, the properties of these two regions of the protein are distinct.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Evidence for in vivo phosphorylation of the Grb2 SH2-domain binding site on focal adhesion kinase by Src-family protein-tyrosine kinases

Focal adhesion kinase (FAK) is a nonreceptor protein-tyrosine kinase (PTK) that associates with integrin receptors and participates in extracellular matrix-mediated signal transduction events. We showed previously that the c-Src nonreceptor PTK and the Grb2 SH2/SH3 adaptor protein bound directly to FAK after fibronectin stimulation (D. D. Schlaepfer, S.K. Hanks, T. Hunter, and P. van der Geer, Nature [London] 372:786-791, 1994). Here, we present evidence that c-Src association with FAK is required for Grb2 binding to FAK. Using a tryptic phosphopeptide mapping approach, the in vivo phosphorylation of the Grb2 binding site on FAK (Tyr-925) was detected after fibronectin stimulation of NIH 3T3 cells and was constitutively phosphorylated in v-Src-transformed NIH 3T3 cells. In vitro, c-Src phosphorylated FAK Tyr-925 in a glutathione S-transferase-FAK C-terminal domain fusion protein, whereas FAK did not. Using epitope-tagged FAK constructs, transiently expressed in human 293 cells, we determined the effect of site-directed mutations on c-Src and Grb2 binding to FAK. Mutation of FAK Tyr-925 disrupted Grb2 binding, whereas mutation of the c-Src binding site on FAK (Tyr-397) disrupted both c-Src and Grb2 binding to FAK in vivo. These results support a model whereby Src-family PTKs are recruited to FAK and focal adhesions following integrin-induced autophosphorylation and exposure of FAK Tyr-397. Src-family binding and phosphorylation of FAK at Tyr-925 creates a Grb2 SH2-domain binding site and provides a link to the activation of the Ras signal transduction pathway. In Src-transformed cells, this pathway may be constitutively activated as a result of FAK Tyr-925 phosphorylation in the absence of integrin stimulation.

The CRKL adaptor protein transforms fibroblasts and functions in transformation by the BCR-ABL oncogene

The CRKL adaptor protein was recently identified as a substrate for the BCR-ABL tyrosine kinase in patients with chronic myelogenous leukemia, but its function is unknown. Here we report that CRKL is phosphorylated when overexpressed, activates RAS and JUN kinase signaling pathways, and transforms fibroblasts in a RAS-dependent fashion. We examined the potential role of CRKL in BCR-ABL function by deleting the CRKL binding site in BCR-ABL. This mutant BCR-ABL protein shows a 50% reduction in fibroblast transforming activity. The GRB2 adaptor protein has previously been implicated in this pathway, presumably linking BCR-ABL to RAS. To address the relative roles of CRKL and GRB2 in this system, we compared BCR-ABL mutants with defects in binding to one or both adaptors. Whereas each single mutant showed a 2-3-fold loss in transforming activity, the double mutant showed a 15-fold reduction, suggesting that GRB2 and CRKL both contribute to BCR-ABL transformation. These results demonstrate the oncogenic potential of CRKL and provide functional evidence that CRKL plays a role in fibroblast transformation by BCR-ABL in conjunction with other adaptor proteins.

RANTES induces tyrosine kinase activity of stably complexed p125FAK and ZAP-70 in human T cells

The chemokine RANTES is a chemoattractant and activating factor for T lymphocytes. Investigation of the signal transduction mechanisms induced by RANTES in T cells revealed tyrosine phosphorylation of multiple protein species with prominent bands at 70-85 and 120-130 kD. Immunoprecipitation and Western analyses revealed that a protein of 125 kD was identical to the focal adhesion kinase (FAK) pp125FAK. RANTES stimulated phosphorylation of FAK as early as 30 seconds and immunoblots using antiphosphotyrosine monoclonal antibodies revealed that there was consistent phosphorylation of a 68-70 kD species in the pp125FAK immunoprecipitates. Immunoblotting and kinase assays showed this to be two separate proteins, the tyrosine kinase zeta-associated protein (ZAP) 70, and the focal adhesion protein paxillin. These results indicate a potentially important role for RANTES in the generation of T cell focal adhesions and subsequent cell activation via a molecular complex containing FAK, ZAP-70, and paxillin.

Inhibition of focal adhesion kinase (FAK) signaling in focal adhesions decreases cell motility and proliferation

It has been proposed that the focal adhesion kinase (FAK) mediates focal adhesion formation through tyrosine phosphorylation during cell adhesion. We investigated the role of FAK in focal adhesion structure and function. Loading cells with a glutathione-S-transferase fusion protein (GST-Cterm) containing the FAK focal adhesion targeting sequence, but not the kinase domain, decreased the association of endogenous FAK with focal adhesions. This displacement of endogenous FAK in both BALB/c 3T3 cells and human umbilical vein endothelial cells loaded with GST-Cterm decreased focal adhesion phosphotyrosine content. Neither cell type, however, exhibited a reduction in focal adhesions after GST-Cterm loading. These results indicate that FAK mediates adhesion-associated tyrosine phosphorylation, but not the formation of focal adhesions. We then examined the effect of inhibiting FAK function on other adhesion-dependent cell behavior. Cells microinjected with GST-Cterm exhibited decreased migration. In addition, cells injected with GST-Cterm had decreased DNA synthesis compared with control-injected or noninjected cells. These findings suggest that FAK functions in the regulation of cell migration and cell proliferation.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

p130Cas, a substrate associated with v-Src and v-Crk, localizes to focal adhesions and binds to focal adhesion kinase

p130(Cas) (crk associated substrate) has the structural characteristics of an adapter protein, containing multiple consensus SH2 binding sites, an SH3 domain, and a proline-rich domain. The structure of p130(Cas) suggests that it may act to provide a framework for protein-protein interactions; however, as yet, its functional role in cells is unknown. In this report we show that p130(Cas) is localized to focal adhesions. We demonstrate that p130(Cas) associates both in vitro and in vivo with pp125(FAK) (focal adhesion kinase), a kinase implicated in signaling by the integrin family of cell adhesion receptors. p130(Cas) also associates with pp41/43(FRNK) (pp125(FAK)-related, non-kinase), an autonomously expressed form of pp125(FAK) composed of only the C-terminal noncatalytic domain. We show that the association of p130(Cas) with pp125(Fak) and pp41/43(FRNK) is direct, and is mediated by the binding of the SH3 domain of p130(Cas) to a proline-rich sequence present in both the C terminus of pp125(FAK) and in pp41/43(FRNK). In agreement with recent studies we show that p130(Cas) is tyrosine-phosphorylated upon integrin mediated cell adhesion. The association of p130(Cas) with pp125(FAK), a kinase which is activated upon cell adhesion, is likely to be functionally important in integrin mediated signal transduction.