¹Cancer Research Center, The Burnham Institute, 10901 N Torrey Pines Road, La Jolla, CA 92037, USA

Correspondence: K Vuori, E-mail: kvuori@burnham.org

Received 27 January 2003; Revised 4 April 2003; Accepted 9 April 2003.

Abstract

Organization of the actin cytoskeleton in eucaryotic cells is controlled by small GTPases of the Rho family. Rac becomes activated by growth factor stimulation and integrin-mediated cell adhesion to extracellular matrix and is known to have a crucial role in lamellipodia formation, cell spreading and migration. At present, the intracellular pathways that connect cell surface receptors to Rac activation are poorly characterized. It has been reported previously that integrin-mediated cell attachment induces activation of the EGF receptor (EGFR) in the absence of EGF. We demonstrate here that this activation is instrumental for integrin-dependent Rac activation. Thus, we found that cells in which EGFR activity had been inhibited failed to spread and form lamellipodia on fibronectin. Failure to spread coincided with inhibition of adhesion-induced GTP loading of Rac and also with inhibition of the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Subsequent studies demonstrated that an activated form of PI 3-kinase restored Rac GTP loading in the presence of EGFR inhibition, while a dominant-negative form of PI 3-kinase blocked Rac GTP loading in fibronectin-adherent cells. Our further functional studies identified Vav2, a known exchange factor for Rac, as a crucial downstream component in EGFR- and PI 3-kinase-dependent Rac activation upon integrin-mediated cell adhesion. Our results provide a mechanistic insight into integrin-dependent Rac activation, and identify a novel role for EGFR, PI 3-kinase and Vav2 in this pathway.

Most types of normal cells respond to interaction with the extracellular matrix (ECM) by spreading out to acquire a flattened morphology. These morphological changes are regulated by the Rho-family of small GTPases, which become activated upon ligation of the integrin family of cell-surface receptors to ECM. When cells first adhere to ECM, they spread rapidly by extending filopodia-like projections and lamellipodia. It has been demonstrated that these events are mediated by the Rho-family members Cdc42 and Rac, respectively (Price et al., 1998). At a later time point of adhesion, Rho-dependent assembly of large focal adhesions and actin stress fibers takes place (Barry et al., 1997; Clark et al., 1998). This coordinated activation of Rho GTPases has been shown to be instrumental for the regulation of several biological effects downstream of integrins, including cell motility and proliferation (Evers et al., 2000; Danen and Yamada, 2001).

It has been demonstrated that integrin-mediated adherence to ECM leads to activation of RTKs in the absence of a soluble growth factor ligand. Thus, adhesion to fibronectin and collagen type I leads to phosphorylation of the PDGF -receptor in fibroblasts (Sundberg and Rubin, 1996). Most notably, the EGF receptor (EGFR) has been reported to become activated when fibroblasts or endothelial cells adhere to fibronectin in the absence of EGF (Moro et al., 1998). Further studies have revealed that adhesion-induced EGFR phosphorylation is dependent upon Src activity and that the tyrosine residues phosphorylated in these conditions are different from the ones that become phosphorylated upon EGF stimulation (Moro et al., 2002). Integrin-mediated cell adhesion induces Shc phosphorylation and Shc/Grb2/Sos complex formation, which has been suggested to be a mechanism by which integrins activate the Ras–MAP kinase pathway (Wary et al., 1996). In support of this, inhibition of adhesion-induced EGFR activation using the pharmacological inhibitor AG1478 has been shown to inhibit Shc phosphorylation and MAPK activation (Moro et al., 1998). This indicates that EGFR activity mediates activation of various intracellular signaling pathways downstream of integrins.

In the study highlighted above, Moro et al. used NIH 3T3 cells overexpressing EGFR to demonstrate that integrin ligation results in EGFR activation in an EGF-independent manner. Our studies in COS-7 and 293 T cells are in accordance with these findings. Thus, we found that attachment of serum-starved COS-7 and 293 T cells on fibronectin induced EGFR phosphorylation (Figure 1 and not shown). We further found that treatment of the cells with the EGFR inhibitor AG1478 inhibited integrin-mediated tyrosine phosphorylation of EGFR and Shc (Figure 1). Recent studies have suggested that EGFR also mediates integrin-dependent phosphatidylinositol 3-kinase (PI 3-kinase) activation (Tiganis et al., 1999). In support of this observation, we found that adhesion-induced activation of the serine/threonine kinase Akt, which is a target of PI 3-kinase (King et al., 1997), is dependent on EGFR activity (Figure 1). Importantly, our studies indicated that not all adhesion-induced signaling events are dependent on EGFR activity. Thus, we observed that adhesion-dependent tyrosine phosphorylation of the focal adhesion kinase FAK was not affected by pretreatment of COS-7 cells with AG1478 (Figure 1). Similar results were obtained in 293 T and NIH 3T3 + EGFR cells (not shown).

Figure 1.

Effect of the EGFR inhibitor AG1478 on adhesion-induced phosphorylation of the EGFR, Shc, FAK and Akt. Serum-starved COS-7 cells were trypsinized, washed in 0.5 mg/ml soybean trypsin inhibitor and resuspended in DMEM/0.5% BSA at 10⁵ cells/ml. Cells were kept in suspension at 37°C for 30 min. DMSO (carrier) or 1 M AG1478 (Calbiochem, San Diego, CA, USA) was added and cells were kept in suspension for an additional 30 min. Cells were lysed immediately or seeded on dishes coated with 20 g/ml fibronectin and incubated at 37°C for 30 min prior to lysis. Cells were lysed in 1% NP–40, 150 mM NaCl, 50 mM Tris-Cl pH 8.0, 5 mM EDTA, 10 mM NaF, 10 mM Na₄P₂O₇, 0.4 mM Na₃VO₄ and protease inhibitors. The EGFR, Shc and FAK were immunoprecipitated from the lysate with the following antibodies: anti-EGFR (1005) and anti-FAK (A-17) (Santa Cruz Biotechnology, Santa Cruz, CA, USA) and anti-Shc (Signal Transduction, San Diego, CA, USA). Tyrosine phosphorylation levels (pTyr) of the immunoprecipitated proteins were evaluated by immunoblotting with a PY-20 antibody (Signal Transduction). To study Akt phosphorylation, 50 g total cell lysate (TCL) was probed with an antiphospho-Akt (Ser 473) (Cell Signaling, Beverly, MA, USA) specific antibody. Blots were stripped and reprobed with the indicated antibodies (same as above for EGFR, Shc and FAK, and anti-Akt1 (C-20) (Santa Cruz Biotechnology)) to show equal loading

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In order to gain further insight into the role of EGFR in integrin signaling pathways, we examined adhesion-induced reorganization of actin cytoskeleton while inhibiting EGFR activity. As shown in Figure 2 and as published previously (Dolfi et al., 1998), phalloidin staining demonstrated extensive membrane ruffling and lamellipodia formation in COS-7 cells that had been allowed to spread on fibronectin for 30 min. When the cells had been pretreated for 30 min with AG1478, there was an obvious defect in spreading and ruffling on fibronectin (Figure 2). This effect was not due to toxicity by AG1478 as confirmed by trypan blue staining (not shown). Similar results were obtained when EGFR signaling was inhibited using genetic means by expressing a dominant-negative form of EGFR lacking the C-terminus (EGFRC). Thus, phalloidin staining revealed that these cells do not spread well or form prominent lamellipodia on fibronectin (Figure 2).

Figure 2.

Treatment with AG1478 inhibits cell spreading on fibronectin. COS-7 cells were mock-transfected or transfected with Myc-tagged dominant-negative EGFR, EGFRC (Moro et al., 1998), Myc-tagged RacV12 or Myc-RacN17 (Ridley et al., 1992) by using Lipofectamine with PLUS reagent (Invitrogen, Carlsbad, CA, USA). Serum-starved cells were prepared as described in Figure 1. Cells were seeded on coverslips coated with 20 g/ml fibronectin and incubated at 37°C for 30 min. Adherent cells were fixed with a 4% paraformaldehyde solution and the cell membranes were permeabilized by 0.5% Triton X-100 in PBS. The actin cytoskeleton was visualized by staining with FITC-phalloidin (Molecular Probes, Eugene, OR, USA). Myc-tagged EGFRC-, RacN17- or RacV12-transfected cells were visualized by incubating the cells with anti-Myc (9E10) (Santa Cruz Biotechnology) antibody followed by anti-mouse Alexa 546 staining (Molecular Probes)

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Activation of the small GTPase Rac is necessary for the formation of lamellipodia and spreading on fibronectin (Price et al., 1998). The morphology induced by AG1478 and the dominant-negative form of the EGFR therefore suggested a possible defect in adhesion-dependent activation of Rac signaling pathways. Indeed, the morphology induced by AG1478 especially was reminiscent of the morphology of cells expressing RacN17, a dominant-negative form of Rac (Figure 2). In order to study this further, a constitutively active form of Rac, RacV12, was transiently expressed in COS-7 cells. As shown in Figure 2, RacV12 induced formation of prominent lamellipodia in untreated serum-starved cells, and significantly rescued the morphological defects in AG1478-treated cells. These results suggest that EGFR activity is required for cell spreading on fibronectin, and that inhibition of the tyrosine kinase activity of the EGFR may lead to the inhibition of adhesion-dependent Rac activation.

In order to measure directly the level of activation of Rac, pull-down experiments were performed using the p21-binding domain (PBD) of PAK to selectively isolate the GTP-bound form of Rac (Glaven et al., 1999; del Pozo et al., 2000). As published before (del Pozo et al., 2000), cells that had been kept in suspension for 1 h demonstrated low levels of GTP-Rac (Figure 3a). Adhesion to fibronectin in the absence of serum induced activation of Rac by 10 min. The levels of activated Rac further increased by 20 min and were sustained by 30 min. Expression of the dominant-negative form of the EGF receptor (EGFRC) completely blocked GTP loading of Rac induced by adhesion to fibronectin at all time points studied (Figure 3a). These results correlated with the defect in cell spreading observed by immunofluorescence in the same conditions.

Figure 3.

Adhesion-induced Rac, PAK and JNK activations are inhibited when the tyrosine kinase activity of EGFR is inhibited. (a,b) 293 T cells were mock-transfected or transfected with EGFRC. Serum-starved cells were trypsinized, washed once in 0.5 mg/ml soybean trypsin inhibitor in PBS and resuspended in DMEM/0.5% BSA. Samples were kept in suspension for 1 h at 37°C before lysis or incubated on fibronectin-coated dishes for the indicated times. Cells were lysed in 25 mM HEPES pH 7.5, 150 mM NaCl, 1% NP-40, 10 mM MgCl₂, 1 mM EDTA, 10% glycerol and protease inhibitors. A sample was kept for total Rac, Cdc42 and EGFRC level analysis. The remaining lysate was used for the pull-down assay to isolate the GTP-loaded forms of Rac or Cdc42 using the p21-binding domain of PAK1 fused to glutathione S-transferase (GST-PBD) (Glaven et al., 1999). GST-PBD fusion protein was prepared as previously described (Abassi and Vuori, 2002). Blots from the PBD pull-downs or total cell lysates (TCL) were probed with an (a) anti-Rac (Upstate Biotechnology, Lake Placid, NY, USA) or (b) anti-Cdc42 (Signal Transduction) antibody. Expression of the truncated form of the EGFR (EGFRC) was detected in the TCL samples with an anti-Myc antibody. Blots were scanned and the intensity of the bands was measured using the NIH Image program. Ratios of GTP-Rac/total Rac and GTP-Cdc42/total Cdc42 are shown in lower panels in a and b, respectively. (c) Serum-starved cells 293 T were prepared as described in Figure 1. Cells in suspension or adhering to fibronectin were lysed in 50 mM HEPES pH 7.6, 250 mM NaCl, 3 mM EDTA, 3 mM EGTA, 0.5% NP-40, 100 M NA₃VO₄ and protease inhibitors. Endogenous PAK1 was immunoprecipitated with anti-PAK1 (N-20) antibody (Santa Cruz Biotechnology). Each sample was divided in two and 3/4 of the beads were resuspended in 2 loading buffer and used for immunoblotting using the anti-PAK1 antibody. The kinase assay was performed by incubating the other 1/4 of the immunoprecipitated PAK1 in reaction buffer (50 mM HEPES pH 7.6, 10 mM MgCl₂, 10 M cold ATP, 10 g MBP and 5 Ci ³²P-ATP) at 30°C for 10 min. The reaction was stopped by adding an equal volume of 2 loading buffer and heating at 100°C for 5 min. The samples were separated by SDS–PAGE, the gel was stained with Coomassie blue stain to confirm equal amounts of MBP and subjected to autoradiography. Blots were scanned and the intensity of the bands was measured using the NIH Image program. MBP phosphorylation relative to the amount of PAK1 protein is shown in the lower panel. (d) Stably adherent, serum-starved cells were treated with either DMSO or 1 M AG1478 for 1 h prior to trypsinization. Detached cells were washed once with 0.5 mg/ml soybean trypsin inhibitor in PBS. The suspended sample was lysed immediately with 1 lysis buffer from the Cell Signaling nonradioactive JNK kinase assay kit. The other samples were resuspended in the initial media containing DMSO or AG1478, seeded on fibronectin-coated dishes and incubated at 37°C for the indicated times. Equal amounts of proteins of cell lysates were used for the kinase assays, which were performed as described in the manufacturer's instructions. Samples were analysed by immunoblotting using an antiphospho-c-Jun antibody (Ser 63) provided in the kit

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Adhesion to collagen type I has also been shown to induce EGFR phosphorylation in the absence of its growth factor ligand (Moro et al., 1998). We therefore examined Rac activation induced by adhesion to collagen type I in 293 T cells and observed a similar time course of activation as induced by adhesion to fibronectin. Similar to what was observed on fibronectin, Rac activation in cells plated on collagen I was inhibited by expression of EGFRC (not shown). These results demonstrate that the requirement of EGFR for adhesion-induced Rac activation is not limited to fibronectin; it remains to be determined whether this role can be extended to other substrates of the extracellular matrix.

Pull-down assays using the PBD of PAK were also performed to measure Cdc42 activation on fibronectin with or without expression of EGFRC. Just as for Rac, Cdc42 was activated by 10 min incubation of cells on fibronectin-coated dishes and stayed active until 30 min (Figure 3b). Importantly, expression of EGFRC had no effect on Cdc42 GTP loading at any time point examined. In addition, adhesion-induced activation of RhoA, as measured by affinity precipitation with the Rho binding domain of rhotekin, was not affected upon inhibition of EGFR activity (data not shown). These results indicate that activation of the EGF receptor by adhesion to fibronectin is necessary for the activation of Rac, but not of Cdc42 or RhoA.

In order to further examine the effects of EGFR activity on adhesion-dependent Rac signaling pathways, we studied the effects of EGFR inhibition on PAK and JNK activity. Adhesion-induced activation of these two kinases is known to occur in a Rac-dependent manner (Dolfi et al., 1998; Price et al., 1998). The kinase activity of PAK was measured by an in vitro kinase assay using MBP as a substrate. As shown in Figure 3c, and as published before (Price et al., 1998), PAK activation was found to be transient, the activity being maximal at 10 min of incubation on fibronectin. The kinase activity then returned to baseline levels by 20 min incubation on fibronectin. Pretreatment of the cells with AG1478 resulted in a significant inhibition of adhesion-induced PAK activation at all time points tested. These results are in accordance with the decreased levels of Rac-GTP in the same conditions.

The kinase activity of JNK was measured with a nonradioactive in vitro kinase assay by using GST-c-Jun as a substrate. In COS-7 cells, JNK activation was clearly detectable by 15 min incubation on fibronectin and further increased by 30 min (Figure 3d). Again, treatment with AG1478 prior to seeding on fibronectin-coated dishes significantly inhibited adhesion-dependent activation of JNK. Similar results were obtained using NIH 3T3+EGFR cells (results not shown).

We next examined which signaling pathways upstream of Rac might be affected by inhibition of the tyrosine kinase activity of the EGFR. Like all GTPases, Rac is activated by switching from a GDP to a GTP-loaded form, a process dependent on guanine nucleotide exchange factors (GEFs). Products of PI 3-kinase have been shown to activate some GEFs, namely Vav and SOS, by binding to the pleckstrin homology (PH) domain of the exchange factor (Han et al., 1998; Nimnual et al., 1998). As shown in Figure 1 and previously by Tiganis et al., 1999, adhesion-dependent activation of Akt, which is a downstream target of PI 3-kinase, is inhibited by treatment of cells with AG1478. For these reasons, PBD pull-down experiments were performed with cells cotransfected with EGFRC and an active form of the catalytic subunit of PI3-kinase, p110* (Hu et al., 1995). As shown in Figure 4a, left panel, expression of p110* reversed the effect of EGFRC on adhesion-induced Rac activation. As an additional line of investigation, we examined the effect of a dominant-negative form of PI3-kinase, p85iSH2 (Klippel et al., 1993; Dhand et al., 1994) on adhesion-induced Rac activation. PBD pull-down assays demonstrated that expression of p85iSH2 inhibited adhesion-induced Rac GTP loading in a concentration-dependent manner (Figure 4a, left panel). We also performed a time course of Rac activation on fibronectin when cells were pretreated with 25M LY294002, a pharmacological inhibitor of PI 3-kinase activity (Figure 4a, right panel), and again observed inhibition of Rac activation.

Figure 4.

Adhesion-induced Rac activation is dependent upon PI 3-kinase and Vav2 activity. (a) Left panel: 293 T cells were transfected with either an empty vector (EV), EGFRC, an active from of PI 3-kinase, p110* (Hu et al., 1995) or 1, 2.5 or 5 g of a dominant form of PI 3-kinase, p85iSH2 (Klippel et al., 1993; Dhand et al., 1994) separately or together as indicated. Cells were prepared as described in Figure 3a and kept in suspension for 1 h or allowed to adhere on fibronectin for 20 min. PBD pull-downs to isolate GTP-Rac were performed as described in Figure 3a. GTP-Rac was detected by immunoblotting after the pull-down by probing with an anti-Rac antibody. Total Rac was detected from total cell lysates (TCL) using the same antibody. Expressions of EGFRC and p85iSH2 were detected from the TCL samples with anti-Myc and anti-p85 (Signal Transduction) antibodies, respectively. Right panel: Serum-starved 293 T cells were prepared as described in Figure 1 except that they were treated with 25 M LY294002 (Calbiochem) or DMSO. Cells were lysed or seeded onto fibronectin-coated dishes and incubated at 37°C for the indicated time. Rac activity assay was carried as described in Figure 3A. (b) 293 T cells were transfected with either an empty vector (EV), 500 ng and 1 g L342R/L343SVav2 (dbl mutant), WT-Vav2 (Marignani and Carpenter, 2001) or EGFRC separately or together as indicated. Cell treatment and Rac activity assays were carried out as in (a) EGFRC levels were detected from TCL samples using an anti-Myc antibody. L342R/L343SVav2 and WTVav2 were detected from cell lysate using a polyclonal anti-Vav2 (obtained from Dr Robert Abraham). (c) COS-7 cells were prepared for immunofluorescence as described in Figure 2. The actin cytoskeleton was visualized by staining with FITC-phalloidin. Myc-tagged p110*-transfected cells were visualized by incubating the cells with anti-Myc (9E10) antibody followed by anti-mouse Alexa 546 staining. WT-Vav2-transfected cells were identified by staining with a polyclonal Vav2antibody, followed by incubation with a TRITC-conjugated anti-rabbit antibody (Sigma, St Louis, MO, USA). (d) 293 T cells were transfected with an empty vector (EV), GFP-tagged SH3–SH2–SH3 Vav2 (Liu and Burridge, 2000), L342R/L343SVav2 or p110* separately or together as indicated. Cell treatment and Rac activity assays were carried out as in (a). L342R/L343SVav2 was detected from the TCL sample with the Vav2-antibody The SH3–SH2–SH3 domains of Vav2 were detected from the TCL sample with a monoclonal GFP antibody (Chemicon International, Temecula, CA, USA)

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We next examined whether p110* rescues Rac activation upon EGFR inhibition by activating one of its known target exchange factors. We first investigated the effects of the ubiquitously expressed isoform of Vav, Vav2, by expressing a T7-tagged version of the wild-type protein (Marignani and Carpenter, 2001). As previously reported (Marignani and Carpenter, 2001), expression of WT-Vav2 significantly increased Rac-GTP levels in adherent cells (Figure 4b). Its expression also efficiently rescued the inhibitory effect of EGFRC. Conversely, expression of a dominant-negative form of Vav2, which lacks an active dbl domain (L342R/L343S) (Marignani and Carpenter, 2001), inhibited adhesion-induced Rac activation in a dose-dependent manner. These studies are on par with the recent findings by Marignani and Carpenter (2001), who reported an inhibition of cell spreading on fibronectin upon expression of the dbl mutant of Vav2. Importantly, an active form of PI 3-kinase was unable to rescue Rac activation in fibronectin-adherent cells when a dominant-negative form of Vav2 was expressed (Figure 4d). Thus, our studies suggest that PI 3-kinase and Vav2 likely function, in this molecular order, downstream of EGFR in mediating adhesion-dependent Rac activation. We also performed immunofluorescence studies to examine whether expression of p110* and WT-Vav2 could rescue the morphological defects induced by AG1478. As shown in Figure 4c, COS-7 cells expressing either of these constructs spread and form prominent lamellipodia on fibronectin both in the presence and absence of AG1478. These results demonstrate that, in the absence of EGFR activity, p110* and WT-Vav2 can rescue fibronectin-induced Rac GTP loading and activation of its downstream effectors leading to reorganization of the actin cytoskeleton.

We also attempted to rescue the defect in Rac activation induced by EGFRC by expression of a membrane-targeted, activated form of SOS (Qian et al., 1998). However, this form of SOS was unable to activate Rac when coexpressed with EGFRC in serum-starved cells adherent on fibronectin (results not shown). Likewise, a dominant-negative form of SOS that has a point mutation in the dbl homology domain (Qian et al., 1998) failed to have an effect on adhesion-induced Rac activation (results not shown). Our studies therefore suggest a role for Vav2, but not SOS, as an exchange factor for Rac in integrin signaling pathways downstream of EGFR and PI 3-kinase. Interestingly, Mettouchi et al. (2001)demonstrated recently that SOS and PI 3-kinase mediate mitogen-dependent activation of Rac in adherent cells. This highlights the notion that growth factor receptor pathways activated as a result of cell attachment may differ from those that become activated upon growth factor-mediated stimulation of RTKs.

As a summary, our studies demonstrate the novel finding that activation of Rac and its downstream effectors by adhesion of serum-starved cells to fibronectin are dependent upon activation of the EGF receptor. We further demonstrate that PI 3-kinase and Vav2 activities are necessary for activation of Rac upon cell adhesion. Our additional studies suggest that adhesion to fibronectin activates PI 3-kinase downstream of the EGFR which then activates the exchange activity of Vav2 toward Rac. Recent studies by Marignani and Carpenter (2001) have similarly suggested that Vav2 plays a role in fibronectin-dependent cell spreading. Our studies thus expand these findings by demonstrating that adhesion-dependent Vav2 activation occurs downstream of the EGFR and PI 3-kinase, whose products are known activators of Vav (Han et al., 1998).

A number of studies have suggested that tyrosine phosphorylation of Vav proteins is necessary for their GEF activity in vitro (Crespo et al., 1997; Schuebel et al., 1998; Movilla and Bustelo, 1999), and stimulation of cells with growth factors is known to result in tyrosine phosphorylation of Vav2 (Bustelo, 2000; Liu and Burridge, 2000). Previous studies have reported that adhesion to fibronectin does not induce Vav2 phosphorylation (Liu and Burridge, 2000; Moores et al., 2000; Marignani and Carpenter, 2001). We similarly failed to see an effect of cell attachment on Vav2 phosphorylation (not shown), suggesting that Vav2 becomes engaged in integrin signaling pathways in a tyrosine phosphorylation-independent manner.

Protein–protein interactions are also known to play a role in Vav2 activation. Following stimulation with EGF or PDGF, Vav2 binds to the tyrosine-phosphorylated growth factor receptor via its SH2 domain (Moores et al., 2000; Pandey et al., 2000). This interaction has been shown to be necessary for tyrosine phosphorylation of Vav2 following EGF stimulation (Tamas et al., 2001). It has thus been suggested that the C-terminus of Vav containing the SH2 domain flanked by two SH3 domains serves an autoinhibitory function and its binding to the autophosphorylated receptor alleviates this inhibition. We investigated the possibility that binding of the phosphorylated EGFR to Vav2 could play a role in activating Vav2 in fibronectin-adherent cells. However, our coimmunoprecipitation studies found no evidence of such an interaction (results not shown). In keeping with the model described above, the isolated C-terminal domain of Vav2 was found to act as a dominant-negative for EGF-induced Rac activation (Liu and Burridge (2000); not shown). However, adhesion-induced Rac activation was not affected by expression of the C-terminal domain (SH3–SH2–SH3) of Vav2 (Figure 4d).

Interestingly, products of PI 3-kinase may turn out to be a major mechanism of activation of Vav2. Thus, Tamas et al. (2002) have recently reported that introduction of a point mutation into the Vav2 PH domain or treatment of cells with the PI 3-kinase inhibitor LY294002 prior to EGF stimulation significantly inhibited Vav2 exchange activity. Also, the PH domain has been shown to be required for the transforming and signaling activity of the oncogenic form of Vav2 (Booden et al., 2002). This is in contrast to the other Vav proteins, Vav and Vav3, whose PH domains are dispensable for their activity (Han et al., 1998; Movilla and Bustelo, 1999). Thus, activation of Vav2 by growth factors likely results from the coordination of different cellular events, such as recruitment of Vav2 to the activated receptor, tyrosine phosphorylation by the receptor, activation by products of PI 3-kinase and changes in protein–protein interactions. Although Vav2 is not phosphorylated by adhesion to fibronectin, its activation is also dependent upon PI 3-kinase activity. It remains to be determined what additional mechanisms might be involved in adhesion-dependent regulation of Vav2.