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Controlled induction of focal adhesion disassembly and migration in primary fibroblasts.

Fibroblast migration is an integral component of biological processes such as wound healing and embryogenesis. Previous experiments examining fibroblast locomotion from tissue explants have shown that migrating fibroblasts lack, or contain only transient, focal adhesions (focal contacts). Focal adhesions are specialized regions of tight cell-matrix interaction, assembled by a complex process of transmembrane signalling. Although the explant model has been used for studying several aspects of fibroblast locomotion, it is limited by the lack of control over migration, and only a small percentage of the cells actually locomoting. Therefore, we have developed an in vitro model for cultured fibroblast strains where the presence or absence of focal adhesions can be manipulated, and in the latter case 70% of these cells become locomotory. The stimulus used to decrease the percentage of cells containing focal adhesions, and hence enhance locomotion, was newborn rat heart-conditioned medium (HCM). Addition of HCM to rat embryo fibroblasts induced both chemokinesis and chemotaxis. Cells disassembled focal adhesions on a variety of extracellular matrix substrates after approximately 6 h of stimulation with HCM; conversely, removal of HCM promoted reformation of focal adhesions within 12-24 h. HCM-stimulated fibroblasts which lacked focal adhesions concomitantly lacked F-actin stress fibers and focal concentrations of vinculin and talin. Therefore, fibroblast migration can be readily controlled in an on-off manner through conditioned medium, which influences the absence or presence of focal adhesions.

3T3 Cells↗

Cross talk between MyD88 and focal adhesion kinase pathways.

Focal adhesion kinase (FAK) is a nonreceptor protein tyrosine kinase involved in signaling downstream of integrins, linking bacterial detection, cell entry, and initiation of proinflammatory response through MAPKs and NF-kappaB activation. In this study, using protein I/II from Streptococcus mutans as a model activator of FAK, we investigated the potential link between FAK and TLR pathways. Using macrophages from TLR- or MyD88-deficient mice, we report that MyD88 plays a major role in FAK-dependent protein I/II-induced cytokine release. However, response to protein I/II stimulation was independent of TLR4, TLR2, and TLR6. The data suggest that there is a cross talk between FAK and MyD88 signaling pathways. Moreover, MyD88-dependent, LPS-induced IL-6 secretion by human and murine fibroblasts required the presence of FAK, confirming that MyD88 and FAK pathways are interlinked.

Adaptor Proteins, Signal Transducing↗

Insulin and insulin-like growth factor-1 stimulate dephosphorylation of paxillin in parallel with focal adhesion kinase.

Paxillin and focal adhesion kinase (pp125FAK) co-localize in focal adhesions; recently insulin has been shown to stimulate the dephosphorylation of pp125FAK; however, its effect on paxillin is unknown. We show that insulin and IGF-1 can stimulate the dephosphorylation of paxillin in CHOT (overexpress human insulin receptors) and CHO delta CT69 (overexpress insulin receptors lacking C-terminal 69 amino acids) cells. Furthermore, the insulin-receptor C-terminus is not needed for either insulin or IGF-1 to stimulate paxillin or pp125FAK dephosphorylation in the CHO (Chinese-hamster ovary) cell lines used.

Animals↗

Focal adhesion regulation of cell behavior.

Focal adhesions lie at the convergence of integrin adhesion, signaling and the actin cytoskeleton. Cells modify focal adhesions in response to changes in the molecular composition, two-dimensional (2D) vs. three-dimensional (3D) structure, and physical forces present in their extracellular matrix environment. We consider here how cells use focal adhesions to regulate signaling complexes and integrin function. Furthermore, we examine how this regulation controls complex cellular behaviors in response to matrices of diverse physical and biochemical properties. One event regulated by the physical structure of the ECM is phosphorylation of focal adhesion kinase (FAK) at Y397, which couples FAK to several signaling pathways that regulate cell proliferation, survival, migration, and invasion.

Adaptor Proteins, Signal Transducing↗

Low density lipoprotein receptor-related protein is a calreticulin coreceptor that signals focal adhesion disassembly.

Thrombospondin (TSP) signals focal adhesion disassembly (the intermediate adhesive state) through interactions with cell surface calreticulin (CRT). TSP or a peptide (hep I) of the active site induces focal adhesion disassembly through binding to CRT, which activates phosphoinositide 3-kinase (PI3K) and extracellular signal-related kinase (ERK) through Galphai2 proteins. Because CRT is not a transmembrane protein, it is likely that CRT signals as part of a coreceptor complex. We now show that low density lipoprotein receptor-related protein (LRP) mediates focal adhesion disassembly initiated by TSP binding to CRT. LRP antagonists (antibodies, receptor-associated protein) block hep I/TSP-induced focal adhesion disassembly. LRP is necessary for TSP/hep I signaling because TSP/hep I is unable to stimulate focal adhesion disassembly or ERK or PI3K signaling in fibroblasts deficient in LRP. LRP is important in TSP-CRT signaling, as shown by the ability of hep I to stimulate association of Galphai2 with LRP. The isolated proteins LRP and CRT interact, and LRP and CRT are associated with hep I in molecular complexes extracted from cells. These data establish a mechanism of cell surface CRT signaling through its coreceptor, LRP, and suggest a novel function for LRP in regulating cell adhesion.

Animals↗

Live-cell monitoring of tyrosine phosphorylation in focal adhesions following microtubule disruption.

Tyrosine phosphorylation of focal adhesion components is involved in the regulation of focal adhesion formation and turnover, yet the underlying molecular mechanisms are still poorly defined. In the present study, we have used quantitative fluorescence microscopy to investigate the dynamic relationships between the incorporation of new components into growing focal adhesions and tyrosine phosphorylation of these sites. For this purpose, a new approach for monitoring phosphotyrosine levels in live cells was developed, based on a 'phosphotyrosine reporter' consisting of yellow fluorescent protein fused to two consecutive phosphotyrosine-binding Src-homology 2 (SH2)-domains derived from pp60(c-Src). This YFP-dSH2 localized to cell-matrix adhesions and its intensity was linearly correlated with that of an anti-phosphotyrosine antibody labeling. The differential increase in vinculin and phosphotyrosine levels was examined in live cells by two-color time-lapse movies of CFP-vinculin and YFP-dSH2. In this study, focal adhesion growth was triggered by microtubule disruption, which was previously shown to stimulate focal adhesion development by inducing cellular contraction. We show here that, 2 minutes after addition of the microtubule-disrupting drug nocodazole, the local densities of the focal adhesion-associated proteins vinculin, paxillin and focal adhesion kinase (FAK) are significantly elevated and the focal adhesion area is increased, whereas elevation in tyrosine phosphorylation inside the growing adhesions occurs only a few minutes later. Phosphotyrosine and FAK density reach their maximum levels after 10 minutes of treatment, whereas vinculin and paxillin levels as well as focal adhesion size continue to grow, reaching a plateau at about 30 minutes. Our findings suggest that protein recruitment and growth of focal adhesions are an immediate and direct result of increased contractility induced by microtubule disruption, whereas tyrosine phosphorylation is activated later.

Amino Acid Sequence↗

Focal adhesion kinase and its potential involvement in tumor invasion and metastasis.

BACKGROUND: Integrins are cell surface receptors which, in part, mediate the adhesion of cells to the extracellular matrix. In addition to providing a molecular "glue" essential for tissue organization and survival, integrins are dynamic signaling molecules. Integrins allow normal, nontransformed cells to sense that they are adhered to the extracellular matrix, thus providing a cell survival signal. This signal allows cells to proliferate in the presence of growth factors and in some instances prevents apoptosis. Integrins also mediate cell migration as it occurs in normal processes such as angiogenesis, wound healing, immune system function, and development. Aberrances in the expression and function of integrins contribute to many disease states including cancer. RESULTS: Focal adhesion kinase (FAK) becomes phosphorylated and activated during integrin-mediated cell adhesion. Focal adhesion kinase is a signal transducer of integrins (and certain soluble growth factors). Cells derived from FAK -/- mouse embryos exhibit reduced migration relative to wild-type cells. Cells which overexpress FAK show increased migration relative to wild-type cells. Focal adhesion kinase promotes cell survival under certain in vitro conditions. Focal adhesion kinase is overexpressed in invasive and metastatic colon, breast, thyroid, and prostate cancers. Enhanced FAK immunostaining is detected in small populations of preinvasive (carcinoma in situ) oral cancers and in large populations of cells in invasive oral cancers. CONCLUSIONS: Focal adhesion kinase is probably not a classical oncogene but may be involved in the progression of cancer to invasion and metastasis. It is hypothesized that overexpression of FAK in subpopulations of tumor cells leads to populations of cells with a high propensity toward invasion and metastasis. Focal adhesion kinase would have a dual role in this regard: Overexpression of FAK leads to (1) increased cell migration and (2) increased cell survival under anchorage-independent conditions. Further work is needed to test this model and to determine whether FAK represents a viable target for anticancer therapy.

Animals↗

Focal adhesion kinase is redistributed to focal complexes and mediates cell spreading in macrophages in response to M-CSF.

The macrophage colony-stimulating factor (M-CSF, CSF-1) regulates survival, proliferation and differentiation of mononuclear phagocytes, as well as macrophage motility and morphology. The latter features are usually regulated by ECM-mediated activation of integrins and subsequent tyrosine phosphorylation of cellular proteins, including focal adhesion kinase (FAK). FAK is phosphorylated by downstream receptor tyrosine kinases as well. We addressed the question whether M-CSF regulates FAK tyrosine phosphorylation in macrophages, and found that M-CSF induces FAK phosphorylation at all known tyrosine residues. This phosphorylation was dependent on Src. Extracellularly-regulated kinase (ERK), Jun N-terminal kinase (JNK) and phosphatidylinositol-3-kinase (PI3K) were found to be negatively involved in M-CSF-induced FAK phosphorylation, as their inhibition resulted in FAK hyper-phosphorylation. Following M-CSF treatment, FAK and the active forms of M-CSFR and Src were redistributed to the cytoskeleton, where active ERK, JNK and PI3K were detectable. Immunofluorescence showed the presence of FAK and its active form in focal complexes following M-CSF treatment. Moreover, cell spreading and adhesion were impaired when FAK tyrosine phosphorylation was abrogated by either transfection with FRNK, a dominant negative form of FAK, or treatment with a number of inhibitors of upstream FAK-activating signals. These results point to a relevant role for FAK in the regulation of cell spreading and adhesion in macrophages.

Animals↗

Role of focal adhesion proteins in signal transduction and oncogenesis.

The focal adhesion is a structure that is formed when a cell comes into contact with the extracellular matrix. Originally, the focal adhesion was thought to only provide structural support for the actin-based cytoskeleton of the cell. However, the last decade has yielded considerable information linking various protein components of the focal adhesion to signal transduction pathways. Examples of focal adhesion proteins include the catalytically active p125FAK, SH2-containing tensin, and the multifunctional LIM domain-containing paxillin. The interactions of focal adhesion proteins may be altered after cellular transformation. This review details how certain focal adhesion proteins are associated in cellular signaling as well as transformation. The importance of various GTP-binding proteins in interacting with and forming the focal adhesion, and the influence they have on neoplastic transformation, are discussed. A key feature of this review is how oncogenes and their respective oncoproteins affect the focal adhesion. Classically, v-src transformation of adherent cells has been studied to characterize focal adhesions, namely, because of the distinct morphological changes that occur in the focal adhesion concomitantly with src transformation. Similarly, the BCR/ABL oncogene, which causes chronic myelogenous leukemia, leads to adhesion defects and can alter the properties of focal adhesion proteins. Thus, we describe some of the relevant interactions between the focal adhesion and the Src and BCR/ABL oncoproteins, respectively.

Animals↗

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.

Animals↗

Molecular recognition of paxillin LD motifs by the focal adhesion targeting domain.

Focal adhesions (FAs) are large submembrane signaling complexes formed at sites of cellular attachment to the extracellular matrix. The interaction of LD motifs with their targets plays an important role in the assembly of FAs. We have determined the molecular basis for the recognition of two paxillin LD motifs by the FA targeting (FAT) domain of FA kinase using a combination of X-ray crystallography, solution NMR, and homology modeling. The four-helix FAT domain displays two LD binding sites on opposite sites of the molecule that bind LD peptides in a helical conformation. Threading studies suggest that the LD-interacting domain of p95PKL shares a common four-helical core with the FAT domain and the tail of vinculin, defining a structural family of LD motif binding modules.

Amino Acid Motifs↗

Mechanisms of focal adhesion kinase regulation.

Focal adhesion kinase (FAK) is a tyrosine kinase whose phosphorylation state and activity is tightly linked to cell adhesion to the extracellular matrix through integrin receptors. FAK's regulation by adhesion places it in a key position to be able to influence cellular events that are either dependent on cell adhesion like cell proliferation and survival, or that require modulation of cell adhesion like cell migration. FAK's involvement in cellular pathways that regulate cell growth and cell movement suggests that it may contribute to the development of cancer or other diseases. FAK's possible involvement in these pathways makes it a potential drug target. In this review we will focus on the developing view how FAK's activity and phosphorylation are regulated within the cell. Specifically, we will address the contribution of integrins and growth factor dependent pathways to FAK's activation. The role of the tyrosine kinase Src in FAK's regulation will be discussed. The contribution of various negative regulators of FAK's phosphorylation on its regulation including phosphatases and proteases will be discussed. Lastly, the emerging role of FAK's amino terminal FERM like domain in FAK's regulation will be explored. FAK's function within a cell are tightly linked to its phosphorylation state, thus understanding its normal regulation in the cell will provide important insight into drug development by highlighting novel regulatory mechanisms within FAK that potentially may be exploited.

Amino Acid Sequence↗

Direct interaction of focal adhesion kinase with p190RhoGEF.

Focal adhesion kinase (FAK) is a protein-tyrosine kinase that associates with multiple cell surface receptors and signaling proteins through which it can modulate the activity of several intracellular signaling pathways. FAK activity can influence the formation of distinct actin cytoskeletal structures such as lamellipodia and stress fibers in part through effects on small Rho GTPases, although the molecular interconnections of these events are not well defined. Here, we report that FAK interacts with p190RhoGEF, a RhoA-specific GDP/GTP exchange factor, in neuronal cells and in brain tissue extracts by co-immunoprecipitation and co-localization analyses. Using a two-hybrid assay and deletion mutagenesis, the binding site of the FAK C-terminal focal adhesion targeting (FAT) domain was identified within the C-terminal coiled-coil domain of p190RhoGEF. Binding was independent of a LD-like binding motif within p190RhoGEF, yet FAK association was disrupted by a mutation (Leu-1034 to Ser) that weakens the helical bundle structure of the FAK FAT domain. Neuro-2a cell binding to laminin increased endogenous FAK and p190RhoGEF tyrosine phosphorylation, and co-transfection of a dominant-negative inhibitor of FAK activity, termed FRNK, inhibited lamininstimulated p190RhoGEF tyrosine phosphorylation and p21 RhoA GTP binding. Overexpression of FAK in Neuro-2a cells increased both endogenous p190RhoGEF tyrosine phosphorylation and RhoA activity, whereas these events were inhibited by FRNK co-expression. Because insulin-like growth factor 1 treatment of Neuro-2a cells increased FAK tyrosine phosphorylation and enhanced p190RhoGEF-mediated activation of RhoA, our results support the conclusion that FAK association with p190RhoGEF functions as a signaling pathway downstream of integrins and growth factor receptors to stimulate Rho activity.

Animals↗

Stromal cell-derived factor-1alpha stimulates tyrosine phosphorylation of multiple focal adhesion proteins and induces migration of hematopoietic progenitor cells: roles of phosphoinositide-3 kinase and protein kinase C.

The stromal cell-derived factor-1 (SDF-1) is an alpha chemokine that binds to the CXCR4 receptor. Knock-out studies in mice demonstrate that this ligand-receptor pair is essential in hematopoiesis. One function of SDF-1 appears to be the regulation of migration of hematopoietic progenitor cells. We previously characterized signal transduction pathways induced by SDF-1alpha in human hematopoietic progenitors and found tyrosine phosphorylation of focal adhesion components, including the related adhesion focal tyrosine kinase (RAFTK), the adaptor molecule p130 Cas, and the cytoskeletal protein paxillin. To better understand the functional role of signaling molecules connecting the CXCR4 receptor to the process of hematopoietic migration, we studied SDF-1alpha-mediated pathways in a model hematopoietic progenitor cell line (CTS), as well as in primary human bone marrow CD34(+) cells. We observed that several other focal adhesion components, including focal adhesion kinase (FAK) and the adaptor molecules Crk and Crk-L, are phosphorylated on SDF-1alpha stimulation. Using a series of specific small molecule inhibitors, both protein kinase C (PKC) and phosphoinositide-3 kinase (PI-3K) appeared to be required for SDF-1alpha-mediated phosphorylation of focal adhesion proteins and the migration of both CTS and primary marrow CD34(+) cells, whereas the mitogen-activated protein kinases ERK-1 and -2 were not. These studies further delineate the molecular pathways mediating hematopoietic progenitor migration and response to an essential chemokine, SDF-1alpha. (Blood. 2000;95:2505-2513)

Animals↗

Stimulation of beta1 integrin down-regulates ICAM-1 expression and ICAM-1-dependent adhesion of lung cancer cells through focal adhesion kinase.

Adhesion molecules are involved in intracellular signaling in various physiological and pathological processes including metastasis and growth of tumor cells. Tumor cells interact with various host cells as well as with extracellular matrices through certain adhesion molecules such as integrins. We here propose that stimulation of beta1 integrin reduces intercellular adhesion molecule (ICAM)-1-mediated interaction of lung cancer cells with CTLs. This concept is based on the following findings: (a) engagement of beta1 integrins on certain lung cancer cells by a specific antibody or by ligand matrices such as fibronectin and collagen markedly reduced ICAM-1 expression on the cell surface and induced sICAM-1; (b) down-regulation of ICAM-1 by stimulation of beta1 integrins was abrogated by tyrosine kinase inhibitors or by transfection of dominant negative truncations of focal adhesion kinase (FAK); (c) engagement of beta1 integrins also reduced ICAM-1-dependent adhesion of lung cancer cells to T cells, a process completely inhibited by tyrosine kinase inhibitors and by transfection of dominant negative forms of FAK; and (d) stimulation of beta1 integrins prevented killing of lung cancer cells by autologous CTLs. In malignant tumors, cancer cells, including lung cancer cells, are surrounded by extracellular matrix proteins such as fibronectin and collagen. This suggests that the engagement of beta1 integrins by matrix proteins potentially occurs in cancer cells in vivo and that continuous stimulation via beta1 integrins reduces ICAM-1-expression, ICAM-1-mediated adhesion of cancer cells to CTLs and their killing by CTLs. Our results suggest that such processes can lead to the escape of lung cancer cells in vivo from immunological surveillance.

Antibodies, Monoclonal↗

Negative regulation of PYK2/related adhesion focal tyrosine kinase signal transduction by hematopoietic tyrosine phosphatase SHPTP1.

Related adhesion focal tyrosine kinase (RAFTK) (also known as PYK2) is a cytoplasmic tyrosine kinase related to the focal adhesion kinase (FAK) p125(FAK). RAFTK is rapidly phosphorylated on tyrosine residues in response to various stimuli, such as tumor necrosis factor-alpha, changes in osmolarity, elevation in intracellular calcium concentration, lysophosphatidic acid, and bradykinin. Overexpression of RAFTK induces activation of c-Jun amino-terminal kinase (also known as stress-activated protein kinase), mitogen-activated protein kinase (MAPK), and p38 MAPK. The present studies demonstrate that RAFTK binds constitutively to the protein tyrosine phosphatase SHPTP1. In contrast to PTP1B, overexpression of wild-type SHPTP1 blocks tyrosine phosphorylation of RAFTK. The results further demonstrate that RAFTK is a direct substrate of SHPTP1 in vitro. Moreover, treatment of PC12 cells with bradykinin is associated with inhibition in tyrosine phosphorylation of RAFTK in the presence of SHPTP1. Furthermore, in contrast to the phosphatase-dead SHPTP1 C453S mutant, overexpression of wild-type SHPTP1 blocks interaction of RAFTK with the SH2-domain of c-Src and inhibits RAFTK-mediated MAPK activation. Significantly, cotransfection of RAFTK with SHPTP1 did not inhibit RAFTK-mediated c-Jun amino-terminal kinase activation. Taken together, these findings suggest that SHPTP1 plays a negative role in PYK2/RAFTK signaling by dephosphorylating RAFTK.

Cell Adhesion Molecules↗

Concentration of pp125 focal adhesion kinase (FAK) at the myotendinous junction.

Focal adhesion kinase is a recently characterized tyrosine kinase that is concentrated at focal contacts in cultured cells. It is thought to play an important role in the regulation of the integrin-based signal transduction mechanism involved in the assembly of this membrane specialization. In this study, we examined the immunocytochemical distribution of focal adhesion kinase in Xenopus skeletal muscle and its role in the formation of two sarcolemmal specializations, the myotendinous junction and the neuromuscular junction, using a monoclonal antibody (2A7) against this protein. Immunoprecipitation of Xenopus embryonic tissues with this antibody demonstrated a single band at a relative molecular mass of 116 kDa. A distinct concentration of immunolabeling for focal adhesion kinase was observed at the myotendinous junction of muscle fibers in vivo. At this site, the labeling for this protein is correlated with an accumulation of phosphotyrosine immunolabeling. Focal adhesion kinase was not concentrated at the neuromuscular junction in muscle cells either in vivo or in vitro. However, it was localized at spontaneously formed acetylcholine receptor clusters in cultured Xenopus myotomal muscle cells, although its distribution was not exactly congruent with that of the receptors. In these cells, the accumulation focal adhesion kinase was induced by polystyrene microbeads. In addition, beads also induce the formation of acetylcholine receptor clusters and myotendinous junction-like specializations. By following the appearance of the focal adhesion kinase relative to the formation of these sarcolemmal specializations at bead-muscle contacts in cultured muscle cells, we conclude that the accumulation of this protein was in pace with the development of the myotendinous junction, but occurred well after the clustering of acetylcholine receptors. These results suggest that focal adhesion kinase may be involved in the development and/or maintenance of the myotendinous junction through an integrin-based signaling system. Although it can accumulate at acetylcholine receptor clusters formed in culture, it does not appear to be involved in the development of the neuromuscular junction.

Animals↗

Ischemia induced focal adhesion kinase cleavage in rat lung.

Focal adhesion kinase (FAK), a 125 kDa protein, is located at the focal adhesion sites that stabilize endothelial cells on the matrix. Ischemic stimuli cause FAK cleavage and FAK depletion from focal adhesion sites. To test this hypothesis under ischemic conditions, we established an isolated rat lung that we ventilated at 40 inflations/min at inspiratory and expiratory pressures of 10 and 3 cm H2O, respectively. After 5 min of blood perfusion at pulmonary arterial and left atrial pressures of 13 and 3 cm H2O respectively, we either stopped (ischemic group, n=10) or continued (perfused group, n=10) perfusion for 2 hr. We then recovered endothelial cells from each lung, using our previously reported magnetic cell separation method. The endothelial lysates were separated into cytosol and membrane fractions, then immunoprecipitated and blotted with anti-FAK, and anti-phospho-FAK antibody. In ischemic as compared to perfused lungs, immunoblotting with anti-FAK antibody revealed a greater prominence of bands at 125 kDa as well as at cleavage products 80, 70 and 44 kDa in the cytosol fraction. Immuno-blotting with anti-phosphotyrosine antibody revealed decreased enhancement of bands at 80, 70 and 44 kDa, also at 125 kDa in the cytosol fraction. We propose that during lung ischemia, FAK is dephosphorylated, cleaved and translocated to the cytosol. This first evidence of the involvement of FAK in lung ischemia suggests that destabilization of focal adhesions may underlie endothelial barrier deterioration, contributing to increased vascular permeability in ischemic lung injury.

Acetylation↗