Src-dependent,neutrophil-mediated vascular hyperpermeability and beta-catenin modification

The hyperpermeability response of microvessels in inflammation involves complex signaling reactions and structural modifications in the endothelium. Our goal was to determine the role of Src-family kinases (Src) in neutrophil-mediated venular hyperpermeability and possible interactions between Src and endothelial barrier components. We found that inhibition of Src abolished the increases in albumin permeability caused by C5a-activated neutrophils in intact, perfused coronary venules, as well as in cultured endothelial monolayers. Activated neutrophils increased Src phosphorylation at Tyr416, which is located in the catalytic domain, and decreased phosphorylation at Tyr527 near the carboxyl terminus, events consistent with reports that phosphorylating and transforming activities of Src are upregulated by Tyr416 phosphorylation and negatively regulated by Tyr527 phosphorylation. Furthermore, neutrophil stimulation resulted in association of Src with the endothelial junction protein beta-catenin and beta-catenin tyrosine phosphorylation. These phenomena were abolished by blockage of Src activity. Taken together, our studies link for the first time neutrophil-induced hyperpermeability to a pathway involving Src kinase activation, Src/beta-catenin association, and beta-catenin tyrosine phosphorylation in the microvascular endothelium.     

Src kinase regulation by phosphorylation and dephosphorylation

Src and Src-family protein-tyrosine kinases are regulatory proteins that play key roles in cell differentiation, motility, proliferation, and survival. The initially described phosphorylation sites of Src include an activating phosphotyrosine 416 that results from autophosphorylation, and an inhibiting phosphotyrosine 527 that results from phosphorylation by C-terminal Src kinase (Csk) and Csk homologous kinase. Dephosphorylation of phosphotyrosine 527 increases Src kinase activity. Candidate phosphotyrosine 527 phosphatases include cytoplasmic PTP1B, Shp1 and Shp2, and transmembrane enzymes include CD45, PTPalpha, PTPepsilon, and PTPlambda. Dephosphorylation of phosphotyrosine 416 decreases Src kinase activity. Thus far PTP-BL, the mouse homologue of human PTP-BAS, has been shown to dephosphorylate phosphotyrosine 416 in a regulatory fashion. The platelet-derived growth factor receptor protein-tyrosine kinase mediates the phosphorylation of Src Tyr138; this phosphorylation has no direct effect on Src kinase activity. The platelet-derived growth factor receptor and the ErbB2/HER2 growth factor receptor protein-tyrosine kinases mediate the phosphorylation of Src Tyr213 and activation of Src kinase activity. Src kinase is also a substrate for protein-serine/threonine kinases including protein kinase C (Ser12), protein kinase A (Ser17), and CDK1/cdc2 (Thr34, Thr46, and Ser72). Of the three protein-serine/threonine kinases, only phosphorylation by CDK1/cdc2 has been demonstrated to increase Src kinase activity. Although considerable information on the phosphoprotein phosphatases that catalyze the hydrolysis of Src phosphotyrosine 527 is at hand, the nature of the phosphatases that mediate the hydrolysis of phosphotyrosine 138 and 213, and phosphoserine and phosphothreonine residues has not been determined.     

Secretin regulates paracellular permeability in canine gastric monolayers by a Src kinase-dependent pathway

Previous studies found that epidermal growth factor (EGF) decreased paracellular permeability in gastric mucosa, but the other physiological regulators and the molecular mechanisms mediating these responses remain undefined. We investigated the role of secretin and Src in regulating paracellular permeability because secretin regulates gastric chief cell function and Src mediates events involving the cytoskeletal-membrane interface, respectively. Confluent monolayers were formed from canine gastric epithelial cells in short-term culture on Transwell filter inserts. Resistance was monitored in the presence of secretin with or without specific kinase inhibitors. Tyrosine phosphorylation of Src at Tyr(416) was measured with a site-specific phosphotyrosine antibody. Basolateral, but not apical, secretin at concentrations from 1 to 100 nM dose dependently increased resistance; this response was rapid and sustained over hours. PP2 (10 microM), a selective Src tyrosine kinase inhibitor, but not the inactive isomer PP3, abolished the increase in resistance by secretin but only modestly attenuated apical EGF effects. AG-1478 (100 nM), a specific EGF receptor tyrosine kinase inhibitor, attenuated the resistance increase to EGF but not secretin. Secretin, but not EGF, induced tyrosine phosphorylation of Src at Tyr(416) in a dose-dependent fashion, with the maximal response observed at 1 min. PP2, but not PP3, dramatically inhibited this tyrosine phosphorylation. Secretin increases paracellular resistance in gastric mucosa through a Src-mediated pathway, while the effect of EGF is Src independent. Src appears to mediate the physiological effects of this G(s)-coupled receptor in primary epithelial cells.     

The Tyrosine Kinase Csk Dimerizes through Its SH3 Domain

The Src family kinases possess two sites of tyrosine phosphorylation that are critical to the regulation of kinase activity. Autophosphorylation on an activation loop tyrosine residue (Tyr 416 in commonly used chicken c-Src numbering) increases catalytic activity, while phosphorylation of a C-terminal tyrosine (Tyr 527 in c-Src) inhibits activity. The latter modification is achieved by the tyrosine kinase Csk (C-terminal Src Kinase), but the complete inactivation of the Src family kinases also requires the dephosphorylation of the activation loop tyrosine. The SH3 domain of Csk recruits the tyrosine phosphatase PEP, allowing for the coordinated inhibition of Src family kinase activity. We have discovered that Csk forms homodimers through interactions mediated by the SH3 domain in a manner that buries the recognition surface for SH3 ligands. The formation of this dimer would therefore block the recruitment of tyrosine phosphatases and may have important implications for the regulation of Src kinase activity.     

Angiotensin II induces focal adhesion kinase/paxillin phosphorylation and cell migration in human umbilical vein endothelial cells

In the present study, we demonstrated that Ang II provokes a transitory enhancement of focal adhesion kinase (FAK) and paxillin phosphorylation in human umbilical endothelial cells (HUVEC). Moreover, Ang II induces a time- and dose-dependent augmentation in cell migration, but does not affect HUVEC proliferation. The effect of Ang II on FAK and paxillin phosphorylation was markedly attenuated in cells pretreated with wortmannin and LY294002, indicating that phosphoinositide 3-kinase (PI3K) plays an important role in regulating FAK activation. Similar results were observed when HUVEC were pretreated with genistein, a non-selective tyrosine kinases inhibitor, or with the specific inhibitor PP2 for Src family kinases, demonstrating the involvement of protein tyrosine kinases, and particularly Src family of tyrosine kinases, in the downstream signalling pathway of Ang II receptors. Furthermore, FAK and paxillin phosphorylation was markedly blocked after treatment of HUVEC with AG1478, a selective inhibitor of epidermal growth factor receptor (EGFR) phosphorylation. Pretreatment of cells with inhibitors of PI3K, Src family tyrosine kinases, and EGFR also decreased HUVEC migration. In conclusion, these results suggest that Ang II mediates an increase in FAK and paxillin phosphorylation and induces HUVEC migration through signal transduction pathways dependent on PI3K and Src tyrosine kinase activation and EGFR transactivation.     

Src protein-tyrosine kinase structure and regulation

Src and Src-family protein kinases are proto-oncogenes that play key roles in cell morphology, motility, proliferation, and survival. v-Src (a viral protein) is encoded by the chicken oncogene of Rous sarcoma virus, and Src (the cellular homologue) is encoded by a physiological gene, the first of the proto-oncogenes. From the N- to C-terminus, Src contains an N-terminal 14-carbon myristoyl group, a unique segment, an SH3 domain, an SH2 domain, a protein-tyrosine kinase domain, and a C-terminal regulatory tail. The chief phosphorylation sites of Src include tyrosine 416 that results in activation from autophosphorylation and tyrosine 527 that results in inhibition from phosphorylation by C-terminal Src kinase. In the restrained state, the SH2 domain forms a salt bridge with phosphotyrosine 527, and the SH3 domain binds to the kinase domain via a polyproline type II left-handed helix. The SH2 and SH3 domains occur on the backside of the kinase domain away from the active site where they stabilize a dormant enzyme conformation. Protein-tyrosine phosphatases such as PTPalpha displace phosphotyrosine 527 from the Src SH2 domain and mediate its dephosphorylation leading to Src kinase activation. C-terminal Src kinase consists of an SH3, SH2, and kinase domain; it lacks an N-terminal myristoyl group and a C-terminal regulatory tail. Its X-ray structure has been determined, and the SH2 lobe occupies a position that is entirely different from that of Src. Unlike Src, the C-terminal Src kinase SH2 and SH3 domains stabilize an active enzyme conformation. Amino acid residues in the alphaD helix near the catalytic loop in the large lobe of C-terminal Src kinase serve as a docking site for the physiological substrate (Src) but not for an artificial substrate (polyGlu(4)Tyr).     

Src-mediated aryl hydrocarbon and epidermal growth factor receptor cross talk stimulates colon cancer cell proliferation

The aryl hydrocarbon receptor (AhR) mediates many toxic effects of environmental pollutants. AhR also interacts with multiple growth factor-driven signaling pathways. In the course of examining effects of growth factors on proliferation of human colon cancer cells, we identified cross talk between AhR and the epidermal growth factor receptor (EGFR). In the present work, we explored underlying signal transduction mechanisms and functional consequences of this interaction. With the use of two human colon cancer cell lines, H508 and SNU-C4, we examined the effects of AhR ligands including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on cell proliferation and activation of EGFR, ERK1/2, and Src kinases. In colon cancer cells, 5-day incubation with TCDD stimulated a twofold dose-dependent increase in cell proliferation that was detectable with 1 nM and maximal with 30 nM TCDD. TCDD induced dose- and time-dependent phosphorylation of EGFR (Tyr845) and ERK1/2; maximal phosphorylation was observed 5 to 10 min after addition of 30 nM TCDD. Both TCDD-induced ERK1/2 phosphorylation and cell proliferation were abolished by AhR small interfering RNA, AhR-specific inhibitor CH223191, Src kinase inhibitor PP2, neutralizing antibodies against matrix metalloproteinase 7, heparin-binding-EGF-like growth factor and EGFR, EGFR inhibitors (AG1478 and PD168393), and MEK1 inhibitor PD98059. Coimmunoprecipitation experiments revealed that AhR forms a protein complex with Src and regulates Src activity by phosphorylating Src (Tyr416) and dephosphorylating Src (Tyr527). These data support novel observations that, in human colon cancer cells, Src-mediated cross talk between aryl hydrocarbon and EGFR results in ERK1/2 activation, thereby stimulating cell proliferation.     

Lithium-induced inhibition of Src tyrosine kinase in rat cerebral cortical neurons: a role in neuroprotection against N-methyl-D-aspartate receptor-mediated excitotoxicity

The neuroprotective effects of lithium, a mood stabilizer, against glutamate-induced excitotoxicity in rat cortical neurons were associated with a decrease in Tyr1472 phosphorylation of the N-methyl-D-aspartate (NMDA) receptor NR2B subunit and a loss of receptor activity. Since this receptor tyrosine phosphorylation is mediated by the Src-family tyrosine kinases, we investigated the effects of lithium on the Src kinase activity. Levels of phosphorylated Src kinase at Tyr416, an index of Src activation, were reduced after treatment with LiCl (1 mM) for more than 3 days. Protein levels of Src-family kinases such as Src, Fyn, and Yes were unchanged by lithium treatment. The activities of cytosolic protein tyrosine kinase and protein phosphatase were also unchanged by lithium treatment, indicating the selectivity and the modulation. Moreover, the levels of postsynaptic densities (PSD) and SynGAP, the scaffolding proteins of the NMDA receptor complex, were unaltered by lithium. A Src kinase inhibitor, SU6656, and an NR2B antagonist, ifenprodil, partially blocked glutamate excitotoxicity. Our results suggest that lithium-induced inactivation of Src kinase contributes to this drug-induced NMDA receptor inhibition and neuroprotection against excitotoxicity.     

Csk inhibition of c-Src activity requires both the SH2 and SH3 domains of Src.

The protein tyrosine kinase c-Src is negatively regulated by phosphorylation of Tyr527 in its carboxy-terminal tail. A kinase that phosphorylates Tyr527, called Csk, has recently been identified. We expressed c-Src in yeast to test the role of the SH2 and SH3 domains of Src in the negative regulation exerted by Tyr527 phosphorylation. Inducible expression of c-Src in Schizosaccharomyces pombe caused cell death. Co-expression of Csk counteracted this effect. Src proteins mutated in either the SH2 or SH3 domain were as lethal as wild type c-Src, but were insensitive to Csk, even though they were substrates for Csk in vivo. Peptide binding experiments revealed that Src proteins with mutant SH3 domains adopted a conformation in which the SH2 domain was not interacting with the tail. These data support the model of an SH2 domain-phosphorylated tail interaction repressing c-Src activity, but expand it to include a role for the SH3 domain. We propose that the SH3 domain contributes to the maintenance of the folded, inactive configuration of the Src molecule by stabilizing the SH2 domain-phosphorylated tail interaction. Moreover, the system we describe here allows for further study of the regulation of tyrosine kinases in a neutral background and in an organism amenable to genetic analysis.     

Complexes of gamma-tubulin with nonreceptor protein tyrosine kinases Src and Fyn in differentiating P19 embryonal carcinoma cells

Nonreceptor protein tyrosine kinases of the Src family have been shown to play an important role in signal transduction as well as in regulation of microtubule protein interactions. Here we show that gamma-tubulin (gamma-Tb) in P19 embryonal carcinoma cells undergoing neuronal differentiation is phosphorylated and forms complexes with protein tyrosine kinases of the Src family, Src and Fyn. Elevated expression of both kinases during differentiation corresponded with increased level of proteins phosphorylated on tyrosine. Immunoprecipitation experiments with antibodies against Src, Fyn, gamma-tubulin, and with anti-phosphotyrosine antibody revealed that gamma-tubulin appeared in complexes with these kinases. In vitro kinase assays showed tyrosine phosphorylation of proteins in gamma-tubulin complexes isolated from differentiated cells. Pretreatment of cells with Src family selective tyrosine kinase inhibitor PP2 reduced the amount of phosphorylated gamma-tubulin in the complexes. Binding experiments with recombinant SH2 and SH3 domains of Src and Fyn kinases revealed that protein complexes containing gamma-tubulin bound to SH2 domains and that these interactions were of SH2-phosphotyrosine type. The combined data suggest that Src family kinases might have an important role in the regulation of gamma-tubulin interaction with tubulin dimers or other proteins during neurogenesis.     

Tyrosine phosphorylation regulates the biochemical and biological properties of pp60c-src

To investigate the importance of tyrosine phosphorylation in the regulation of pp60c-src, we have substituted phenylalanine for tyrosine at positions 416, 519, and 527. Cells expressing the 527 or the 519/527 mutant but not the 416 or the 519 mutant were morphologically transformed, grew in soft agar, and formed foci. In addition, the 527 and 519/527 mutants had elevated kinase activities in vitro. Modifying Tyr 416 to phenylalanine in the 527 or the 519/527 mutants only partially inhibited their kinase activities yet abolished their ability to induce focus formation and promote growth in soft agar. These results suggest that two events must occur to activate the full transforming potential of pp60c-src: hypophosphorylation at Tyr 527 and hyperphosphorylation at Tyr 416.     

Activation and suppression of pp60c-src transforming ability by mutation of its primary sites of tyrosine phosphorylation

pp60c-src is phosphorylated in vivo at tyrosine 527, a residue not present in pp60v-src (its transforming homolog), and not at tyrosine 416, its site of in vitro autophosphorylation. To test the hypothesis that tyrosine phosphorylation regulates pp60c-src biological activity, we constructed and studied pp60c-src mutants in which Tyr 527 and Tyr 416 were separately or coordinately altered to phenylalanine. Tyr----Phe 527 mutation strongly activated pp60c-src transforming and kinase activities, whereas the additional introduction of a Tyr----Phe 416 mutation suppressed these activities. Tyr----Phe 416 mutation of normal pp60c-src eliminated its partial transforming activity, which suggests that transient or otherwise restricted phosphorylation of Tyr 416 is important for pp60c-src function even though stable phosphorylation is not observed in vivo.     

Coincident regulation of PKCdelta in human platelets by phosphorylation of Tyr311 and Tyr565 and phospholipase C signalling

PKC (protein kinase C)d plays a complex role in platelets, having effects on both positive and negative signalling functions. It is phosphorylated on tyrosine residues in response to thrombin and collagen, and it has recently been shown that Tyr311 is phosphorylated in response to PAR (protease-activated receptor) 1 and PAR4 receptor activation. In the present study, we show that Tyr311 and Tyr565 are phosphorylated in response to thrombin, and have examined the interplay between phosphorylation and the classical lipid-mediated activation of PKCd. Phosphorylation of both Tyr311 and Tyr565 is dependent on Src kinase and PLC (phospholipase C) activity in response to thrombin. Importantly, direct allosteric activation of PKCd with PMA also induced phosphorylation of Tyr311 and Tyr565, and this was dependent on the activity of Src kinases, but not PLC. Membrane recruitment of PKCd is essential for phosphorylation of this tyrosine residue, but tyrosine phosphorylation is not required for membrane recruitment of PKCd. Both thrombin and PMA induce recruitment of PKCd to the membrane, and for thrombin, this recruitment is a PLC-dependent process. In order to address the functional role of tyrosine residue phosphorylation of PKCd, we demonstrate that phosphorylation can potentiate the activity of the kinase, although phosphorylation does not play a role in membrane recruitment of the kinase. PKCd is therefore regulated in a coincident fashion, PLC-dependent signals recruiting it to the plasma membrane and by phosphorylation on tyrosine residues, potentiating its activity.     

Src tyrosine kinase regulates insulin-induced activation of protein kinase C (PKC) delta in skeletal muscle

Insulin stimulation of skeletal muscle results in rapid activation of protein kinase Cdelta (PKCdelta), which is associated with its tyrosine phosphorylation and physical association with insulin receptor (IR). The mechanisms underlying tyrosine phosphorylation of PKCdelta have not been determined. In this study, we investigated the possibility that the Src family of nonreceptor tyrosine kinases may be involved upstream insulin signaling. Studies were done on differentiated rat skeletal myotubes in primary culture. Insulin caused an immediate stimulation of Src and induced its physical association with both IR and PKCdelta. Inhibition of Src by treatment with the Src family inhibitor PP2 reduced insulin-stimulated Src-PKCdelta association, PKCdelta tyrosine phosphorylation and PKCdelta activation. PP2 inhibition of Src also decreased insulin-induced IR tyrosine phosphorylation, IR-PKCdelta association and association of Src with both PKCdelta and IR. Finally, inhibition of Src decreased insulin-induced glucose uptake. We conclude that insulin activates Src tyrosine kinase, which regulates PKCdelta activity. Thus, Src tyrosine kinase may play an important role in insulin-induced tyrosine phosphorylation of both IR and PKCdelta. Moreover, both Src and PKCdelta appear to be involved in IR activation and subsequent downstream signaling.     

p130cas but not paxillin is essential for Caco-2 intestinal epithelial cell spreading and migration on collagen IV

We have previously observed that collagen IV regulates Caco-2 intestinal epithelial cell spreading and migration via Src kinase and stimulates Src-dependent tyrosine phosphorylation of p130cas. We observed that collagen IV also stimulated Src-dependent phosphorylation of both paxillin Tyr31 and paxillin Tyr118. Caco-2 transfection with paxillin or p130cas siRNAs inhibited expression of these proteins by more than 90% for at least 5 days after transfection. Although p130cas siRNA inhibited cell spreading on collagen IV by 33%, three different paxillin siRNAs did not inhibit cell spreading. p130cas siRNA did not affect Src Tyr416 or Src Tyr527 phosphorylation, FAK Tyr397 phosphorylation, or Src-dependent phosphorylation of FAK Tyr925, suggesting that p130cas did not inhibit cell spreading by altering FAK or Src activity. Rat p130cas expression after siRNA knock-out of endogenous human p130cas in Caco-2 cells reduced cell spreading inhibition by 71%. In contrast, expression of rat p130cas from which the Src-phosphorylated substrate domain was deleted did not rescue siRNA inhibition of cell spreading. Combined treatment with siRNAs to Crk and CrkL, which bind to the p130cas substrate domain, inhibited cell spreading by 54%. Both p130cas siRNA and the combined Crk/CrkL siRNAs strongly inhibited (52 and 46% inhibition, respectively) Caco-2 sheet migration on collagen IV and noticeably inhibited lamellipodial extension, whereas paxillin siRNA only inhibited migration by 18% and did not noticeably affect lamellipodial extension. These results suggest that Src may regulate Caco-2 migration on collagen IV via both p130cas and paxillin but that Src phosphorylation of p130cas is more important for this process.     

Src tyrosine kinase inhibitor PP2 suppresses ERK1/2 activation and epidermal growth factor receptor transactivation by X-irradiation

Exposure of MDA-MB-468 cells to ionizing radiation (IR) caused biphasic activation of ERK as indicated by its phosphorylation at Thr202/Tyr204. Specific epidermal growth factor receptor (EGFR) inhibitor AG1478 and specific Src inhibitor PP2 inhibited IR-induced ERK1/2 activation but phosphatidylinositol-3 kinase inhibitor wortmannin did not. IR caused EGFR tyrosine phosphorylation, whereas it did not induce EGFR autophosphorylation at Tyr992, Tyr1045, and Tyr1068 or Src-dependent EGFR phosphorylation at Tyr845. SHP-2, which positively regulates EGFR/Ras/ERK signaling cascade, became activated by IR as indicated by its phosphorylation at Tyr542. This activation was inhibited by PP2 not by AG1478, which suggests Src-dependent activation of SHP-2. Src and PTPalpha, which positively regulates Src, became activated as indicated by phosphorylation at Tyr416 and Tyr789, respectively. These data suggest that IR-induced ERK1/2 activation involves EGFR through a Src-dependent pathway that is distinct from EGFR ligand activation.     

Src tyrosine kinase alters gating of hyperpolarization-activated HCN4 pacemaker channel through Tyr531

We recently discovered that the constitutively active Src tyrosine kinase can enhance hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channel activity by binding to the channel protein. To investigate the mechanism of modulation by Src of HCN channels, we studied the effects of a selective inhibitor of Src tyrosine kinase, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), on HCN4 and its mutant channels expressed in HEK 293 cells by using a whole cell patch-clamp technique. We found that PP2 can inhibit HCN4 currents by negatively shifting the voltage dependence of channel activation, decreasing the whole cell channel conductance, and slowing activation and deactivation kinetics. Screening putative tyrosine residues subject to phosphorylation yielded two candidates: Tyr(531) and Tyr(554). Substituting HCN4-Tyr(531) with phenylalanine largely abolished the effects of PP2 on HCN4 channels. Replacing HCN4-Tyr(554) with phenylalanine did not abolish the effects of PP2 on voltage-dependent activation but did eliminate PP2-induced slowing of channel kinetics. The inhibitory effects of HCN channels associated with reduced Src tyrosine activity is confirmed in HL-1 cardiomyocytes. Finally, we found that PP2 can decrease the heart rate in a mouse model. These results demonstrate that Src tyrosine kinase enhances HCN4 currents by shifting their activation to more positive potentials and increasing the whole cell channel conductance as well as speeding the channel kinetics. The tyrosine residue that mediates most of Src's actions on HCN4 channels is Tyr(531).     

Involvement of Src family tyrosine kinase in apoptosis of human neutrophils induced by protozoan parasite Entamoeba histolytica

Tyrosine kinases are one of the most important regulators for intracellular signal transduction related to inflammatory responses. However, there are no reports describing the effects of tyrosine kinases on neutrophil apoptosis induced by Entamoeba histolytica. In this study, isolated human neutrophils from peripheral blood were incubated with live trophozoites in the presence or absence of tyrosine kinase inhibitors. Entamoeba-induced receptor shedding of CD16 and PS externalization in neutrophils were inhibited by pre-incubation of neutrophils with the broad-spectrum tyrosine kinase inhibitor genistein or the Src family kinase inhibitor PP2. Entamoeba-induced ROS production was also inhibited by genistein or PP2. Moreover, genistein and PP2 blocked the phosphorylation of ERK and p38 MAPK in neutrophils induced by E. histolytica. These results suggest that Src tyrosine kinases may participate in the signaling event for ROS-dependent activation of MAPKs during neutrophil apoptosis induced by E. histolytica.     

The role of protein-tyrosine phosphatase 1B in integrin signaling

Protein-tyrosine phosphatase 1B (PTP1B) is a key negative regulator of insulin and leptin signaling and a novel therapeutic target for the treatment of type 2 diabetes, obesity, and other associated metabolic syndromes. Because PTP1B regulates multiple signal pathways and it can both enhance and antagonize a cellular event, it is important to establish the physiological relevance of PTP1B in these processes. In this study, we utilize potent and selective PTP1B inhibitors to delineate the role of PTP1B in integrin signaling. We show that down-regulation of PTP1B activity with small molecule inhibitors suppresses cell spreading and migration to fibronectin, increases Tyr(527) phosphorylation in Src, and decreases phosphorylation of FAK, p130(Cas), and ERK1/2. In addition, PTP1B "substrate-trapping" mutants bind Tyr(527)-phosphorylated Src and protect it from dephosphorylation by endogenous PTP1B. These results establish that PTP1B promotes integrin-mediated responses in fibroblasts by dephosphorylating the inhibitory pTyr(527) and thereby activating the Src kinase. We also show that PTP1B forms a complex with Src and p130(Cas), and that the proline-rich motif PPRPPK (residues 309-314) in PTP1B is essential for the complex formation. We suggest that the specificity of PTP1B for Src pTyr(527) is mediated by protein-protein interactions involving the docking protein p130(Cas) with both Src and PTP1B in addition to the interactions between the PTP1B active site and the pTyr(527) motif.