Src family kinase involvement in muscarinic receptor-induced tyrosine phosphorylation in differentiated SH-SY5Y cells

Muscarinic receptor-mediated changes in protein tyrosine phosphorylation were examined in differentiated human neuroblastoma SH-SY5Y cells. Treatment of differentiated cells with 1 mM carbachol caused rapid increases in the tyrosine phosphorylation of focal adhesion kinase (FAK), Cas, and paxillin. The src family kinase-selective inhibitor PP1 reduced carbachol-stimulated tyrosine phosphorylation of FAK, Cas, and paxillin by 50 to 75%. In contrast, carbachol-stimulated activation of ERK1/2 was unaffected by PP1. Src family kinase activation by carbachol was further demonstrated by increased carbachol-induced tyrosine phosphorylation of the src-substrate, p120, and tyrosine phosphorylation of the src family kinase activation-associated autophosphorylation site. Site-specific FAK phosphotyrosine antibodies were used to determine that the carbachol-stimulated increase in the autophosphorylation of FAK was unaffected by pretreatment with PP1, whereas the carbachol-stimulated increase in the src family kinase-mediated phosphotyrosine of FAK was completely blocked by pretreatment with PP1. In SH-SY5Y cell lines stably overexpressing Fyn, the phosphotyrosine immunoreactivity of FAK was 625% that of control cells. Thus, muscarinic receptors activate protein tyrosine phosphorylation in differentiated cells, and the tyrosine phosphorylation of FAK, Cas, and paxillin, but not ERK1/2, is mediated by a src family tyrosine kinase activated in response to stimulation of muscarinic receptors.     

Crk-associated substrate tyrosine phosphorylation sites are critical for invasion and metastasis of SRC-transformed cells

Crk-associated substrate (CAS, p130Cas) is a major tyrosine phosphorylated protein in cells transformed by v-crk and v-src oncogenes. We recently reported that reexpression of CAS in CAS-deficient mouse embryo fibroblasts transformed by oncogenic Src promoted an invasive phenotype associated with enhanced cell migration through Matrigel, organization of actin into large podosome ring and belt structures, activation of matrix metalloproteinase-2, and elevated tyrosine phosphorylation of the focal adhesion proteins FAK and paxillin. We have now extended these studies to examine the mechanism by which CAS achieves these changes and to evaluate the potential role for CAS in promoting in vivo tumor growth and metastasis. Whereas the presence or absence of CAS did not alter the primary growth of subcutaneous-injected Src-transformed mouse embryo fibroblasts, CAS expression was required to promote lung metastasis following removal of the primary tumor. The substrate domain YxxP tyrosines, the major sites of CAS phosphorylation by Src that mediate interactions with Crk, were found to be critical for promoting both invasive and metastatic properties of the cells. The ability of CAS to promote Matrigel invasion, formation of large podosome structures, and tyrosine phosphorylation of Src substrates, including FAK, paxillin, and cortactin, was also strictly dependent on the YxxP tyrosines. In contrast, matrix metalloproteinase-2 activation was most dependent on the CAS SH3 domain, whereas the substrate domain YxxP sites also contributed to this property. Thus multiple CAS-mediated signaling events are implicated in promoting invasive and metastatic properties of Src-transformed cells.     

The role of focal adhesion kinase binding in the regulation of tyrosine phosphorylation of paxillin

Focal adhesion kinase (FAK) and paxillin are focal adhesion-associated, phosphotyrosine-containing proteins that physically interact. A previous study has demonstrated that paxillin contains two binding sites for FAK. We have further characterized these two binding sites and have demonstrated that the binding affinity of the carboxyl-terminal domain of FAK is the same for each of the two binding sites. The presence of both binding sites increases the affinity for FAK by 5-10-fold. A conserved paxillin sequence called the LD motif has been implicated in FAK binding. We show that mutations in the LD motifs in both FAK-binding sites are required to dramatically impair FAK binding in vitro. A paxillin mutant containing point mutations in both FAK-binding sites was characterized. The mutant exhibited reduced levels of phosphotyrosine relative to wild type paxillin in subconfluent cells growing in culture, following cell adhesion to fibronectin and in src-transformed fibroblasts. These results suggest that paxillin must bind FAK for maximal phosphorylation in response to cell adhesion and that FAK may function to direct tyrosine phosphorylation of paxillin in the process of transformation by the src oncogene.     

Complex Formation with Focal Adhesion Kinase: A Mechanism to Regulate Activity and Subcellular Localization of Src Kinases

Tyrosine phosphorylation of focal adhesion kinase (FAK) creates a high-affinity binding site for the src homology 2 domain of the Src family of tyrosine kinases. Assembly of a complex between FAK and Src kinases may serve to regulate the subcellular localization and the enzymatic activity of members of the Src family of kinases. We show that simultaneous overexpression of FAK and pp60(c-src) or p59(fyn) results in the enhancement of the tyrosine phosphorylation of a limited number of cellular substrates, including paxillin. Under these conditions, tyrosine phosphorylation of paxillin is largely cell adhesion dependent. FAK mutants defective for Src binding or focal adhesion targeting fail to cooperate with pp60(c-src) or p59(fyn) to induce paxillin phosphorylation, whereas catalytically defective FAK mutants can direct paxillin phosphorylation. The negative regulatory site of pp60(c-src) is hypophosphorylated when in complex with FAK, and coexpression with FAK leads to a redistribution of pp60(c-src) from a diffuse cellular location to focal adhesions. A FAK mutant defective for Src binding does not effectively induce the translocation of pp60(c-src) to focal adhesions. These results suggest that association with FAK can alter the localization of Src kinases and that FAK functions to direct phosphorylation of cellular substrates by recruitment of Src kinases.     

Focal Adhesion Targeting: The Critical Determinant of FAK Regulation and Substrate Phosphorylation

The focal adhesion kinase (FAK) is discretely localized to focal adhesions via its C-terminal focal adhesion-targeting (FAT) sequence. FAK is regulated by integrin-dependent cell adhesion and can regulate tyrosine phosphorylation of downstream substrates, like paxillin. By the use of a mutational strategy, the regions of FAK that are required for cell adhesion-dependent regulation and for inducing tyrosine phosphorylation of paxillin were determined. The results show that the FAT sequence was the single region of FAK that was required for each function. Furthermore, the FAT sequence of FAK was replaced with a focal adhesion-targeting sequence from vinculin, and the resulting chimera exhibited cell adhesion-dependent tyrosine phosphorylation and could induce paxillin phosphorylation like wild-type FAK. These results suggest that subcellular localization is the major determinant of FAK function.     

Human caco-2 motility redistributes FAK and paxillin and activates p38 MAPK in a matrix-dependent manner

The signals involved in restitution during mucosal healing are poorly understood. We compared focal adhesion kinase (FAK) and paxillin protein and phosphorylation, extracellular signal-regulated kinase (ERK) 1, ERK2, and p38 activation, as well as FAK and paxillin organization in static and migrating human intestinal Caco-2 cells on matrix proteins and anionically derivatized polystyrene dishes (tissue culture plastic). We also studied effects of FAK, ERK, and p38 blockade in a monolayer-wounding model. Compared with static cells, cells migrating across matrix proteins matrix-dependently decreased membrane/cytoskeletal FAK and paxillin and cytosolic FAK. Tyrosine phosphorylated FAK and paxillin changed proportionately to FAK and paxillin protein. Conversely, cells migrating on plastic increased FAK and paxillin protein and phosphorylation. Migration matrix-dependently activated p38 and inactivated ERK1 and ERK2. Total p38, ERK1, and ERK2 did not change. Caco-2 motility was inhibited by transfection of FRNK (the COOH-terminal region of FAK) and PD-98059, a mitogen-activated protein kinase-ERK kinase inhibitor, but not by SB-203580, a p38 inhibitor, suggesting that FAK and ERK modulate Caco-2 migration. In contrast to adhesion-induced phosphorylation, matrix may regulate motile intestinal epithelial cells by altering amounts and distribution of focal adhesion plaque proteins available for phosphorylation as well as by p38 activation and ERK inactivation. Motility across plastic differs from migration across matrix.     

Paxillin enables attachment-independent tyrosine phosphorylation of focal adhesion kinase and transformation by RAS

Paxillin and HIC5 are closely related adapter proteins that regulate cell migration and are tyrosine-phosphorylated by focal adhesion kinase (FAK). Paxillin, HIC5, and FAK tyrosine phosphorylation increase upon cell attachment and decrease upon detachment from extracellular matrix. Unexpectedly, we found that although FAK tyrosine phosphorylation in attached cells did not require paxillin, in detached fibroblasts there was remaining FAK tyrosine phosphorylation that required expression of paxillin and was not supported by HIC5. The support of attachment-independent FAK tyrosine phosphorylation required the paxillin LIM domains and suggested that paxillin might facilitate oncogenic transformation. Paxillin but not HIC5 augmented anchorage-independent cell proliferation induced by RAS. Both anchorage-independent FAK tyrosine phosphorylation and RAS-induced colony formation required multiple docking sites on paxillin, including LD4 (docking sites for FAK-Src and GIT1/2-PIX-NCK-PAK complex), LD5, and all four carboxyl-terminal LIM domains (that bind tubulin and PTP-PEST). Analysis using paxillin mutants dissociated domains of paxillin that are required for regulation of cell migration from domains that are required for anchorage-independent cell proliferation and demonstrated essential functions of the paxillin LIM domains that are not found in HIC5 LIM domains. These results highlight the role of paxillin in facilitating attachment-independent signal transduction implicated in cancer.     

Protein kinase C-mediated tyrosine phosphorylation of paxillin and focal adhesion kinase requires cytoskeletal integrity and is uncoupled to mitogen-activated protein kinase activation in human hepatoma cells

Treatment of cultured human hepatoma HepG2 cells with the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), results in an increase in tyrosine phosphorylation of several proteins, including the focal adhesion kinase (FAK) and paxillin using anti-phosphotyrosine Western blotting and immunoprecipitation. However, when cells are in suspension or in the presence of cytochalasin D which disrupts the intracellular network of actin microfilaments, TPA loses its ability to stimulate tyrosine phosphorylation of FAK and paxillin but it still activates mitogen-activated protein kinase (MAPK) and induces PKC translocation from cytosol to the membrane in HepG2 cells. On the other hand, PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, blocks TPA-induced MAPK activation but has no effect on TPA-induced tyrosine phosphorylation. Our findings suggest that TPA-induced tyrosine phosphorylation of FAK and paxillin in human hepatoma cells is PKC dependent and requires the integrity of the cell cytoskeleton but is uncoupled to the signal transduction pathway of PKC leading to the translocation of PKC and MAPK activation.     

FAK regulates tyrosine phosphorylation of CAS, paxillin, and PYK2 in cells expressing v-Src, but is not a critical determinant of v-Src transformation.

FAK (focal adhesion kinase) is a nonreceptor protein-tyrosine kinase activated by tyrosine phosphorylation following integrin-mediated cell adhesion. Oncogenic Src promotes enhanced and deregulated FAK tyrosine phosphorylation which has been proposed to contribute to altered cell growth and/or morphological properties associated with transformation. In this study, an inducible FAK expression system was used to study the potential role of FAK in v-Src transformation. Our results portray FAK as a major v-Src substrate that also plays a role in recruiting v-Src to phosphorylate substrates CAS (Crk-associated substrate) and paxillin. The FAK Tyr-397 autophosphorylation site was necessary for this scaffolding function, but was not required for v-Src to stably interact with and phosphorylate FAK. FAK was also shown to negatively regulate v-Src mediated phosphorylation of the FAK-related kinase PYK2. Despite these effects, FAK does not play an essential role in targeting v-Src to major cellular substrates including CAS and paxillin. Nor is FAK strictly required to achieve the altered morphological and growth characteristics of v-Src transformed cells.     

Activation of m3 muscarinic receptors induces rapid tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin in rat pancreatic acini

Tyrosine phosphorylation plays a key role in transmembrane and cytoplasmic signal transduction mechanisms stimulated by oncogenes, integrins, growth factors, neuropeptides, and bioactive lipids. Moreover, recent studies show that stimulation of odd-numbered muscarinic receptors increases the tyrosine phosphorylation of several proteins in different cellular types. The present study was aimed at examining whether activation of m3 muscarinic receptors in rat pancreatic acini evokes tyrosine phosphorylation of p125(FAK), and its substrates, p130(cas) and paxillin. Results show that stimulation of pancreatic acini with carbachol resulted in a rapid and transient increase in tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin. Tyrosine phosphorylation of these proteins occurred in a time- and concentration-dependent manner. Simultaneous blockage of both PKC activation and increases in [Ca(2+)](i) partially decreased p125(FAK), p130(cas), and paxillin tyrosine phosphorylation stimulated by carbachol. Pretreatment of pancreatic acini with Clostridium botulinum C3 transferase, which specifically inactivates p21(rho), partially inhibited carbachol-induced p125(FAK), p130(cas), and paxillin tyrosine phosphorylation. In contrast, this treatment had no effect on amylase release stimulated by carbachol. Cytochalasin D, which disrupts actin microfilaments network, completely inhibited carbachol stimulated tyrosine phosphorylation of these proteins without having significant effects in carbachol-stimulated amylase secretion. These results dissociate tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin from amylase secretion after m3 muscarinic receptors occupation in rat pancreatic acini. Taken together, these data suggest that (a) activation of m3 muscarinic receptors in rat pancreatic acini increases tyrosine phosphorylation of p125(FAK) and its substrates, p130(cas) and paxillin by diacylglycerol-activated PKC- and calcium- dependent, and independent pathways, (b) these responses require activation of p21(rho) and an intact actin cytoskeleton, and (c) p125(FAK), p130(cas), and paxillin are unlikely related to secretion in rat pancreatic acinar cells.     

Beta 2 integrin-dependent tyrosine phosphorylation of paxillin in human neutrophils treated with tumor necrosis factor

The focal adhesion protein paxillin undergoes tyrosine phosphorylation in response to signals mediated by integrins, neuropeptides and oncogene products, possibly via activation of the focal adhesion- associated kinase, p125FAK. In the present work, tumor necrosis factor- alpha (TNF) stimulated tyrosine phosphorylation of paxillin in human neutrophils. Cell adhesion and participation of the beta 2 integrin CD18 were necessary, but not sufficient, for the response. Adherent neutrophils also tyrosine phosphorylated paxillin in response to phorbol ester, formylmethionyl-leucyl-phenylalanine and opsonized bacteria. In contrast, p125FAK was constitutively tyrosine phosphorylated in a manner unaffected by adherence and/or TNF. Thus, cytokines and microbial products are among the stimuli that can induce the tyrosine phosphorylation of paxillin, and kinases other than p125FAK may be responsible. This is the first identification of paxillin and p125FAK in human cells and neutrophils, and one of the few identifications of a specific protein that undergoes tyrosine phosphorylation in response to any agonist in neutrophils or in response to TNF in any cell.     

Focal Adhesion and Stress Fiber Formation Is Regulated by Tyrosine Phosphatase Activity

Tyrosine phosphorylation of cytoskeletal proteins plays an important role in the regulation of focal adhesions and stress fiber organization. In the present study we examined the role of tyrosine phosphatases in this process using p125FAK and paxillin as substrates. We show that tyrosine phosphatase activity in Swiss 3T3 cells was markedly increased when actin stress fibers were disassembled by cell detachment from the substratum, by serum starvation, or by cytochalasin D treatment. This activity was blocked by phenylarsine oxide, an inhibitor of a specific class of tyrosine phosphatases characterized by two vicinal thiol groups in the active site. Phenylarsine oxide treatment of serum-starved cells induced increased tyrosine phosphorylation of p125FAK and paxillin in a dose-dependent manner and induced assembly of focal adhesions and actin stress fibers, showing that inhibition of one or more phenylarsine oxide-sensitive tyrosine phosphatases is a sufficient stimulus for triggering focal adhesion and actin stress fiber formation in adherent cells.     

Hepatocyte growth factor induces ERK-dependent paxillin phosphorylation and regulates paxillin-focal adhesion kinase association.

Hepatocyte growth factor (HGF) modulates cell adhesion, migration, and branching morphogenesis in cultured epithelial cells, events that require regulation of cell-matrix interactions. Using mIMCD-3 epithelial cells, we studied the effect of HGF on the focal adhesion proteins, focal adhesion kinase (FAK) and paxillin and their association. HGF was found to increase the tyrosine phosphorylation of paxillin and to a lesser degree FAK. In addition, HGF induced association of paxillin and activated ERK, correlating with a gel retardation of paxillin that was prevented with the ERK inhibitor U0126. The ability of activated ERK to phosphorylate and induce gel retardation of paxillin was confirmed in vitro in both full-length and amino-terminal paxillin. Several potential ERK phosphorylation sites in paxillin flank the paxillin-FAK association domains, so the ability of HGF to regulate paxillin-FAK association was examined. HGF induced an increase in paxillin-FAK association that was inhibited by pretreatment with U0126 and reproduced by in vitro phosphorylation of paxillin with ERK. The prevention of the FAK-paxillin association with U0126 correlated with an inhibition of the HGF-mediated FAK tyrosine phosphorylation and inhibition of HGF-dependent cell spreading and adhesion. An examination of cellular localization of FAK and paxillin demonstrated that HGF caused a condensation of focal adhesion complexes at the leading edges of cell processes and FAK-paxillin co-localization in these large complexes. Thus, these data suggest that HGF can induce serine/threonine phosphorylation of paxillin most probably mediated directly by ERK, resulting in the recruitment and activation of FAK and subsequent enhancement of cell spreading and adhesion.     

GIT1 utilizes a focal adhesion targeting-homology domain to bind paxillin

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins for the ADP-ribosylation factor family of small GTP-binding proteins, but also serve as adaptors to link signaling proteins to distinct cellular locations. One role for GIT proteins is to link the PIX family of Rho guanine nucleotide exchange factors and their binding partners, the p21-activated protein kinases, to remodeling focal adhesions by interacting with the focal adhesion adaptor protein paxillin. We here identified the C-terminal domain of GIT1 responsible for paxillin binding. Combining structural and mutational analyses, we show that this region folds into an anti-parallel four-helix domain highly reminiscent to the focal adhesion targeting (FAT) domain of focal adhesion kinase (FAK). Our results suggest that the GIT1 FAT-homology (FAH) domain and FAT bind the paxillin LD4 motif quite similarly. Since only a small fraction of GIT1 is bound to paxillin under normal conditions, regulation of paxillin binding was explored. Although paxillin binding to the FAT domain of FAK is regulated by tyrosine phosphorylation within this domain, we find that tyrosine phosphorylation of the FAH domain GIT1 is not involved in regulating binding to paxillin. Instead, we find that mutations within the FAH domain may alter binding to paxillin that has been phosphorylated within the LD4 motif. Thus, despite apparent structural similarity in their FAT domains, GIT1 and FAK binding to paxillin is differentially regulated.     

Echistatin inhibits pp125FAK autophosphorylation, paxillin phosphorylation and pp125FAK-paxillin interaction in fibronectin-adherent melanoma cells.

Echistatin, a snake-venom RGD-containing protein, was previously shown to disrupt cell-matrix adhesion by a mechanism that involves the reduction of pp125FAK tyrosine phosphorylation levels. The aim of this study was to establish the sequence of events downstream pp125FAK dephosphorylation that could be responsible for echistatin-induced disassembly of actin cytoskeleton and focal adhesions in fibronectin-adherent B16-BL6 melanoma cells. The results obtained show that echistatin induces a decrease of both autophosphorylation and kinase activity of pp125FAK. One hour of cell exposure to echistatin caused a 39% decrease of pp125FAK Tyr397 phosphorylation and a 31% reduction of pp125FAK autophosphorylation activity as measured by immune-complex kinase assay. Furthermore, 1 h of cell treatment by echistatin produced a 63% decrease of paxillin phosphorylation, as well as a reduction in the amount of paxillin bound to pp125FAK. Immunofluorescence analysis of echistatin treated cells showed the concomitant disappearance of both paxillin and pp125FAK from focal adhesions. The reduction of paxillin phosphorylation may represent a critical step in the pathway by which disintegrins exert their biological activity, including the inhibition of experimental metastasis in vivo.     

New Insights into FAK Phosphorylation Based on a FAT Domain-Defective Mutation

Mounting evidence suggests that the FAK N-terminal (FERM) domain controls FAK phosphorylation and function; however, little is known regarding the role of the C terminal (FAT) domain in FAK regulation. We identified a patient-derived FAK mutant, in which a 27-amino acid segment was deleted from the C-terminal FAT domain (named FAK-Del33). When FAK-Del33 was overexpressed in specific tumor cell lines, Y397 phosphorylation increased compared with that observed in cells expressing FAK-WT. Here, we attempt to unveil the mechanism of this increased phosphorylation. Using cell biology experiments, we show that FAK-Del33 is incapable of co-localizing with paxillin, and has constitutively high Y397 phosphorylation. With a kinase-dead mutation, it showed phosphorylation of FAK-Del33 has enhanced through auto-phosphorylation. It was also demonstrated that phosphorylation of FAK-Del33 is not Src dependent or enhanced intermolecular interactions, and that the hyperphosphorylation can be lowered using increasing amounts of transfected FERM domain. This result suggests that Del33 mutation disrupting of FAT''s structural integrity and paxillin binding capacity leads to incapable of targeting Focal adhesions, but has gained the capacity for auto-phosphorylation in cis.     

Protein tyrosine phosphatase-dependent proteolysis of focal adhesion complexes in endothelial cell apoptosis

Adenosine and/or homocysteine causes endothelial cell apoptosis, a mechanism requiring protein tyrosine phosphatase (PTPase) activity. We investigated the role of focal adhesion contact disruption in adenosine-homocysteine endothelial cell apoptosis. Analysis of focal adhesion kinase (FAK), paxillin, and vinculin demonstrated disruption of focal adhesion complexes after 4 h of treatment with adenosine-homocysteine followed by caspase-induced proteolysis of FAK, paxillin, and p130(CAS). No significant changes were noted in tyrosine phosphorylation of FAK or paxillin. Pretreatment with the caspase inhibitor Z-Val-Ala-Asp-fluoromethylketone prevented adenosine-homocysteine-induced DNA fragmentation and FAK, paxillin, and p130(CAS) proteolysis. Asp-Glu-Val-Asp-ase activity was detectable in endothelial cells after 4 h of treatment with adenosine-homocysteine. The PTPase inhibitor sodium orthovanadate did not prevent endothelial cell retraction or FAK, paxillin, or vinculin redistribution. Sodium orthovanadate did block adenosine-homocysteine-induced FAK, paxillin, and p130(CAS) proteolysis and Asp-Glu-Val-Asp-ase activity. Thus disruption of focal adhesion contacts and caspase-induced degradation of focal adhesion contact proteins occurs in adenosine-homocysteine endothelial cell apoptosis. Focal adhesion contact disruption induced by adenosine-homocysteine is independent of PTPase or caspase activation. These studies demonstrate that disruption of focal adhesion contacts is an early, but not an irrevocable, event in endothelial cell apoptosis.     

Cytostatin, an inhibitor of cell adhesion to extracellular matrix, selectively inhibits protein phosphatase 2A

Cytostatin, which is isolated from a microbial cultured broth as a low molecular weight inhibitor of cell adhesion to extracellular matrix (ECM), has anti-metastatic activity against B16 melanoma cells in vivo. In this study, we examined a target of cytostatin inhibiting cell adhesion to ECM. Cytostatin inhibited tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin upon B16 cell adhesion to fibronectin. While the amount of FAK was not affected by cytostatin, electrophoretically slow-migrating paxillin appeared. Alkaline phosphatase treatment diminished cytostatin-induced slow-migrating paxillin. Furthermore, cytostatin increased intracellular serine/threonine-phosphorylated proteins and was found to be a selective inhibitor of protein phosphatase 2A (PP2A). Cytostatin inhibited PP2A with an IC(50) of 0.09 microgram/ml in a non-competitive manner against a substrate, p-nitrophenyl phosphate, but it had no apparent effect on other protein phosphatases including PP1, PP2B and alkaline phosphatase even at 100 microgram/ml. On the contrary, dephosphocytostatin, a cytostatin analogue, without inhibitory effect on PP2A did not affect B16 cell adhesion including FAK and paxillin. These results indicate that cytostatin inhibits cell adhesion through modification of focal contact proteins such as paxillin by inhibiting a PP2A type protein serine/threonine phosphatase. This is the first report that describes a drug with anti-metastatic ability that inhibits PP2A selectively.     

Dissociation of focal adhesion kinase and paxillin tyrosine phosphorylation induced by bombesin and lysophosphatidic acid from epidermal growth factor receptor transactivation in Swiss 3T3 cells

Tyrosine phosphorylation of the nonreceptor tyrosine kinase p125 focal adhesion kinase (FAK) and the adapter protein paxillin is rapidly increased by multiple agonists, including bombesin (BOM) and lysophosphatidic acid (LPA), through heptahelical G protein-coupled receptors (GPCRs). The pathways involved remain incompletely understood. The experiments presented here were designed to test the role of epidermal growth factor receptor (EGFR) transactivation in the rapid increase of tyrosine phosphorylation of FAK and paxillin induced by GPCR agonists. Our results show that treatment with the selective EGFR tyrosine kinase inhibitor AG 1478, at concentrations that completely blocked the increase in tyrosine phosphorylation of these proteins induced by EGF, did not affect the stimulation of tyrosine phosphorylation of either FAK or paxillin induced by multiple GPCR agonists including LPA, BOM, vasopressin, bradykinin, and endothelin. Similar results were obtained when Swiss 3T3 cells were treated with another highly specific inhibitor of the EGF receptor kinase activity, PD-158780. Collectively, our results clearly dissociate EGFR transactivation from the tyrosine phosphorylation of FAK and paxillin induced by multiple GPCR agonists.     

A role for phosphoinositides in tyrosine phosphorylation of p125 focal adhesion kinase in rat pancreatic acini

Previous studies have shown that different agonists increase tyrosine phosphorylation of the focal adhesion related proteins p125(FAK), p130(Cas), and paxillin in different cell types and that tyrosine phosphorylation depends on the integrity of the actin cytoskeleton. Because phosphoinositides are important for the maintenance of the cytoskeleton, the role of phosphoinositides in the tyrosine phosphorylation of these proteins in response to occupancy of m3 muscarinic and CCK(A) receptors has been investigated in pancreatic acini. Addition of carbachol or CCK-8 to pancreatic acini resulted in rapid increases in the tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin. Pretreatment of pancreatic acini with LY294002 or wortmannin resulted in a concentration-dependent inhibition of tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin stimulated by carbachol or CCK-8. Carbachol- or CCK-8-stimulated tyrosine phosphorylation of these proteins was not inhibited by rapamycin, PD 98059 or SB 203580, and thus it was dissociated from the activation of p70 S6 or MAP kinases. These results indicate that m3 muscarinic and CCK(A) receptor-mediated increase in p125(FAK), p130(Cas), and paxillin tyrosine phosphorylation in pancreatic acini depends on the ability of these cells to synthesise phosphoinositides.