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Phospholipase D (PLD) has been implicated in the signal transduction pathways initiated by several mitogenic protein tyrosine kinases. We demonstrate for the first time that most notably PLD2 and to a lesser extent the PLD1 isoform are tyrosine phosphorylated by c-Src tyrosine kinase via direct association. Moreover, epidermal growth factor induced tyrosine phosphorylation of PLD2 and its interaction with c-Src in A431 cells. Interaction between these proteins is via the pleckstrin homology domain of PLD2 and the catalytic domain of c-Src. Coexpression of PLD1 or PLD2 with c-Src synergistically enhances cellular proliferation compared with expression of either molecule. While PLD activity as a lipid-hydrolyzing enzyme is not affected by c-Src, wild-type PLDs but not catalytically inactive PLD mutants significantly increase c-Src kinase activity, up-regulating c-Src-mediated paxillin phosphorylation and extracellular signal-regulated kinase activity. These results demonstrate the critical role of PLD catalytic activity in the stimulation of Src signaling. In conclusion, we provide the first evidence that c-Src acts as a kinase of PLD and PLD acts as an activator of c-Src. This transmodulation between c-Src and PLD may contribute to the promotion of cellular proliferation via amplification of mitogenic signaling pathways.
Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine, the major membrane phospholipid, to form phosphatidic acid (PA) and choline. PA is generally recognized as the signaling product of PLD and functions as an effector in multiple physiological processes. The PLD pathway is thought to play a critical role in regulating cell responses that contribute to mitotic signaling and transformation (14, 17, 23, 28, 36). To date, two PLD isoforms have been cloned and characterized. PLD1 has a low basal activity and is up-regulated by small G proteins (ARF, Rho, and Ral), protein kinase C (PKC), and phosphatidylinositol 4, 5-bisphosphate (PIP2) in vitro. In contrast, PLD2 has a high basal activity, requires PIP2, and is up-regulated by ADP-ribosylation factor and protein kinase C (6, 16). Although many studies focusing on the regulation of PLD have been reported, the cellular role of PLD remains still unclear.
Protein tyrosine phosphorylation is important in regulating signaling pathways through its effects on protein-protein interactions. Tyrosine kinase receptors such as epidermal growth factor (EGF) and platelet-derived growth factor receptors stimulate PLD activity in some cell types, suggesting that PLD activity is modulated by tyrosine phosphorylation (2, 13, 18). Recently, Min et al. (33) and Marcil et al. (31) reported that pervanadate induced tyrosine phosphorylation on PLD1 in Swiss 3T3 fibroblasts and HL60 cells, respectively. Expression of PLD1 or PLD2 in HEK 293 cells revealed that PLD2, but not PLD1, is constitutively associated with the EGF receptor and becomes phosphorylated on tyrosine-11 upon stimulation with EGF (49). Mutation of tyrosine-11 to phenylalanine did not alter the magnitude of EGF stimulation. Thus, it was suggested that tyrosine phosphorylation of PLD2 is important for interaction with SH2-containing proteins but not for its intrinsic activity as a lipid-hydrolyzing enzyme. Not only the identity of the kinase responsible for phosphorylation of PLD but also the role of phosphorylated PLD in cellular responses needs to be clarified. Furthermore, in neutrophils stimulated by fMLP (N-formyl-methionyl-leucyl-phenylalanine) and monosodium urate crystals, PLD1 was activated but not tyrosine phosphorylated (32). Therefore, the physiological relevance of tyrosine phosphorylation as an activation mechanism of PLD remains to be established.
PLD has been reported to be overexpressed and hyperactivated in some human cancers (40, 54, 55, 59). Consistent with these findings, PLD can facilitate mitogenesis and oncogenic transformation (17, 28, 36), and protein tyrosine kinase (PTK) activity occurs widely in mitogenic signaling (12). Jiang et al. (20-22) demonstrated that v-Src (pp60v-src), the viral counterpart of the nonreceptor tyrosine kinase c-Src (pp60c-src), stimulated PLD activity in fibroblasts and that small G proteins Ras and GTPase Ral were involved in PLD activation. The src oncogene encodes a membrane-localized tyrosine-specific protein kinase whose enzymatic activity is necessary to induce oncogenic transformation (7, 24, 42). Because the oncogenicity of c-Src correlates with its kinase activity, the Src oncoprotein probably induces transformation by phosphorylating critical cellular proteins that control cell proliferation. Src kinase activity and sometimes Src protein levels were found to be elevated in a wide variety of human cancers, with a frequent correlation between Src kinase activity and the degree of malignancy and/or invasiveness (3, 19). Recently, it was reported that elevated expression of PLD1 or PLD2 resulted in the transformation of rat fibroblasts overexpressing c-Src (23). However, little is known about how PLD cooperates with c-Src to transform cells.
In the present study, we demonstrate that PLD associates with c-Src. For the first time, we provide the evidence that PLD is an effector for c-Src activation as well as a physiologically relevant substrate by c-Src kinase. Therefore, we hypothesize that this intimate cross talk between PLD and c-Src contributes to cell proliferation by amplifying Src signaling pathways.
Dulbecco's modified Eagle's medium (DMEM), fetal bovine serum, and Lipofectamine were purchased from Invitrogen. Protein A-Sepharose and glutathione-Sepharose 4B were from Amersham Bioscience Biotech. Hydrogen peroxide and phorbol myristate acetate were from Sigma. EGF and anti-phosphotyrosine antibody (P-Tyr) (4G10) were from Upstate Biotechnology, and PP2 was from Calbiochem. Antipaxillin antibody was from Transduction Laboratories (Lexington, Ky.). The antibody to c-Src was from Santa Cruz. Phospho-Tyr416 c-Src, the activated form of the kinase (8, 10), phospho-paxillin, phospho-ERK, and ERK were assessed by using an antibody from Cell Signaling. A polyclonal antibody that recognizes both PLD1 and PLD2 was generated as previously described (34). Phosphatidylbutanol (PtdBut) standard was from Avanti Polar Lipid. [9, 10-3H]myristate was purchased from Perkin-Elmer Life Sciences. Silica gel 60 A thin-layer chromatography plates were from Whatman. Horseradish peroxidase-conjugated anti-mouse immunoglobulin G (IgG) and anti-rabbit IgG were from Kirkegaard & Perry Lab (Gaithersburg, Md.). The ECL Western blotting detection kit was from Amersham Bioscience.
Human A431 epidermoid carcinoma cells were purchased from the American Type Culture Collection. The establishment of mouse fibroblast cells overexpressing wild-type PLD2 was described previously (36). FaO rat hepatoma cell lines overexpressing wild-type, dominant negative mutant, and dominant active mutant of c-Src were obtained by transfection by using Lipofectamine according to the manufacturer's instructions. Transfected cells were selected with G418 (400 μg/ml) for 21 days at 37°C. At that time antibiotic-resistant colonies were pooled and expanded for further analysis under selective conditions. COS-7 cells were transfected with human PLD1, PLD2, catalytically inactive mutants PLD1-K898R, PLD2-K758R, N-terminally deleted mutants of PLD2 Δ183(184-933) and PLD2 Δ313(314-933), Lck, Fyn, and wild-type and mutant c-Src (K295M) expression vectors by using Lipofectamine. All cells were maintained in DMEM supplemented with 10% fetal bovine serum.
The c-Src truncation mutants (SH3, SH2, or kinase domains) with coding regions corresponding to amino acids 88 to 137, 144 to 245, and 265 to 523, respectively, were created by PCR amplification. The forward primers used to amplify the truncation mutants of c-Src with coding region starting at amino acid 88, 144, or 265 were 5′-CCGGAATTCATGGCTCTCTACGACTACGAGTCC-3′, 5′-CGGAATTCATGCAGGCTGAAGAGTGGTAC-3′, and 5′-CCGGAATTCATGGAGTCGCTGCGGCTGGAGGTGAA-3′, respectively. The reverse primers that were used to amplify the truncation mutants of c-Src constructs that end at amino acid 137, 245, or 523 were 5′-CCGCTCGAGCTAGACATAGTTACTGGGGATGTA-3′ , 5′-CGCTCGAGTCAGCAGACGTTGGTCAGGCG-3′, and 5′-CCGCTCGAGCTATGTCGAGGTGAAGTAGTCCTC-3′, respectively. The PCR fragments were then subcloned in frame with the N-terminal fusion glutathione S-transferase (GST) at the EcoRI/XhoI site in the pGEX4T-1 vector (Amersham Bioscience Biotech). All the constructs were sequenced to verify the coding regions of c-Src. The full-length PLD2 and its two deletion mutants with coding regions corresponding to amino acids 184 to 933 or 314 to 933 were created by PCR. A cDNA fragment encoding PLD2 Δ183(184-933) was amplified from pcDNA3.1-PLD2 by PCR using sense (5′-GAATTCATGACAGAGTTCCTGGAAGTCAGTGC-3′) and antisense (5′-GCGGCCGCTATGTCCACACTTCTAGGGGGATC-3′) oligonucleotides. It was then digested with EcoRI and NotI and ligated together with the EcoRI/NotI fragment encoding PLD2 Δ183(184-933) into pCMV-Flag5b (Stratagene) vector. A cDNA fragment encoding hPLD2 Δ313(314-933) was amplified from pcDNA3.1-PLD2 by PCR using sense (5′-GAATTCATGGGCAGAGACTTCCTACAGCTGCAC-3′) and antisense (5′-CTCGAGTGTCCACACTTCTAGGGGGATC-3′) oligonucleotides. The PCR fragments were digested by EcoRI and XhoI and ligated together with the same digestion fragment into pCMV-Flag5b expression vector. Each deletion construct was confirmed by DNA sequencing.
PLD activity was assessed by measuring the formation of [3H]PtdBut, the product of PLD-mediated transphosphatidylation, in the presence of 1-butanol. After 20 h of transfection with Lipofectamine, COS-7 cells in six-well plates were serum starved in the presence of 1 μCi of [3H]myristic acid/ml. After overnight starvation, the cells were washed three times with 5 ml of phosphate-buffered saline (PBS) and preequilibrated in serum-free DMEM for 1 h. For the final 10 min of preincubation, 0.3% butan-1-ol was included. At the end of the preincubation, cells were treated with agonists for the indicated times. The extraction and characterization of lipids by thin-layer chromatography were performed as previously described (34).
Cells or tissues were washed twice with ice-cold PBS and then lysed in the extraction buffer (20 mM HEPES [pH 7.2], 1% Triton X-100, 1% sodium deoxycholate, 0.2% sodium dodecyl sulfate [SDS], 200 mM NaCl, 1 mM Na3VO4, 1 mM NaF, 10% glycerol, 10 μg of leupeptin/ml, 10 μg of aprotinin/ml, 1 mM phenylmethylsulfonyl fluoride). The resulting cell lysates were spun at 15,000 × g in an Eppendorf microcentrifuge for 10 min at 4°C to pellet the unbroken cells. The supernatant was then precleared for 30 min with preimmune IgG and protein A Sepharose at 4°C with rocking. Protein concentrations were determined by using the Bradford method with bovine serum albumin as a standard (4). Equal protein aliquots of precleared cell lysates (1 mg) were incubated with the indicated antibodies and 40 μl of a 1:1 slurry of protein A-Sepharose beads for 4 h at 4°C. The immune complexes were collected by centrifugation, washed five times with a buffer (20 mM Tris [pH 7.5], 1 mM EDTA, 1 mM EGTA, 150 mM NaCl, 2 mM Na3VO4, 10% glycerol, and 1% Nonidet P-40), and resuspended in sample buffer. Immune complexes were boiled in SDS sample buffer.
Protein samples were analyzed by SDS-polyacrylamide gel electrophoresis (PAGE) on 8% gels and were transferred to a nitrocellulose membrane. The blots were then blocked with 5% nonfat milk and incubated with the appropriate primary antibodies followed by incubation with horseradish peroxidase-conjugated secondary antibody. Immunoreactive bands were detected by enhanced chemiluminescence.
Wild-type human PLD2, N-terminal 184-amino-acid-deleted mutant PLD2 Δ183(184-933), and the N-terminal 313-amino-acid-deleted mutant PLD2 Δ313(314-933) were transfected in COS-7 cells as described previously (1). Transfected cells were lysed in lysis buffer. After centrifugation, aliquots of the soluble extract were coimmunoprecipitated and subjected to immunoblot analysis by using an antibody against PLD or c-Src as described above. After cotransfecting COS-7 cells with PLD2 and c-Src, a GST pull-down assay was performed by using GST fusion proteins attached to glutathione-Sepharose beads. Clarified lysates (1 mg) were incubated with 2 μg of GST fusion proteins immobilized on glutathione-Sepharose beads in a final volume of 500 μl of the extraction buffer for 1 h at 4°C. Protein complexes were collected by centrifugation and washed four times with washing buffer (1% Triton X-100, 150 mM NaCl, 20 mM Tris-HCl [pH 8.0], 20 mM NaF, 200 μM sodium orthovanadate, 1 mM phenylmethylsulfonyl fluoride, 10 μg of leupeptin/ml, 10 μg of aprotinin/ml). Associated protein complexes were resolved by SDS-PAGE and transferred to a nitrocellulose membrane, followed by immunoblot analysis with anti-PLD antibody.
The fragment of PLD2 (724 to 825 amino acids) was fused to GST. For the production of monoclonal antibody, a female BALB/c mouse (7 to 8 weeks old) was immunized with 50 μg of the purified GST fusion protein in complete Freund's adjuvant. At intervals of 2 weeks, three booster injections were given. Three days before cell fusion, the mouse was further boosted by an injection of 50 μg of the protein in PBS. The spleen cells from the immunized mouse were fused with SP2 myeloma cells. Standard fusion, screening, and cloning procedures were followed. Culture supernatants were screened for the immunoblotting. The cells from the positive wells were cloned twice to ensure monoclonality.
The trypan blue exclusion method was used to quantify cell proliferation. After transfection, cells were harvested by trypsin and washed in PBS. Trypan blue (Sigma) was added to suspended cells at a concentration of 0.4% (wt/vol). Live cell numbers were determined by counting with a hemocytometer.
To detect activated c-Src, clarified whole-cell lysates of transfected cells were resolved by SDS-PAGE and transferred to nitrocellulose membranes. Tyr416-phosphorylated c-Src was detected by using a 1:1,000 dilution of anti-phospho Tyr416 c-Src antibody, with horseradish peroxidase-conjugated goat anti-rabbit IgG as secondary antibody. Identical samples were immunoblotted with c-Src antibody as controls. Immune complexes on nitrocellulose were visualized by enzyme-linked chemiluminescence.
We investigated whether the Src family PTKs were capable of phosphorylating PLD isozymes. Cotransfection experiments were performed with COS-7 cells utilizing Src family PTKs and PLD expression vectors as depicted in Fig. Fig.1A.1A. After transfection, cell lysates were prepared and subjected to immunoprecipitation by using an antibody directed against P-Tyr, and tyrosine phosphorylation of PLD isozymes was monitored by successive immunoblotting for PLD. The anti-PLD antibody generated by us recognized both PLD1 and PLD2. The data presented in Fig. Fig.1A1A show that transfection of vectors containing c-Src, Lck, and Fyn resulted in tyrosine phosphorylation of PLD2, while faint phosphorylation of PLD1 was observed only after a long exposure of the membrane to film (data not shown).
The studies that are described next were focused on c-Src, since c-Src of the tested Src family PTKs induced the strongest tyrosine phosphorylation of PLD. In cells cotransfected with PLD1 or PLD2 plus c-Src, both PLD1 and PLD2 (approximately 125 and 105 kDa, respectively) were tyrosine phosphorylated, whereas no tyrosine-phosphorylated proteins (except for IgG chains) were detected in cells transfected with PLD1 or 2 plus empty vector (Fig. (Fig.1B).1B). Similar results were obtained by immunoblotting analysis with the antibody directed against P-Tyr after immunoprecipitation with anti-PLD antibody. Since equal amounts of PLD1 and PLD2 were immunoprecipitated from all samples (Fig. (Fig.1B),1B), it appears that PLD2 is notably more strongly tyrosine phosphorylated than PLD1. Western blot analysis showed that the 60-kDa tyrosine-phosphorylated protein was c-Src.
The c-Src mutant K295M is kinase inactive due to disrupted phosphotransfer activity (9). Cotransfection of the kinase-inactive mutant c-Src with PLDs did not lead to tyrosine phosphorylation of PLD1 and PLD2, whereas wild-type c-Src did (Fig. (Fig.1C).1C). These results indicate that both PLD1 and PLD2 are tyrosine phosphorylated by the expression of c-Src tyrosine kinase and that PLD2 is its preferred substrate.
Next, we examined whether tyrosine phosphorylation of PLD occurs through its direct interaction with c-Src having kinase activity. Cells were cotransfected with PLD1 or PLD2 plus wild-type or mutant c-Src. Coimmunoprecipitation assays indicated that both PLD1 and PLD2 associated with wild-type c-Src, but not with the kinase-inactive mutant (Fig. (Fig.2).2). PLD2, which was more strongly tyrosine phosphorylated by c-Src than PLD1, revealed a stronger interaction with c-Src. These results demonstrate that the extent of phosphorylation in PLD isozymes by c-Src kinase is closely correlated with the binding affinity between PLD isozymes and c-Src protein.
Next, we investigated whether the interaction between Src and PLD2 could be induced by extracellular stimuli without the enforced overexpression of these proteins. We used A431 cells which express PLD2 isozyme and EGF receptors as a model system (35). For this study we generated mouse monoclonal antibody specific for PLD2 but not PLD1, and this PLD2 antibody was used for immunoblotting due to its inability to immunoprecipitate PLD protein (Fig. (Fig.3A).3A). Serum-starved cultures of the cells were exposed to EGF in the presence or absence of a selective inhibitor of Src family tyrosine kinase, PP2, and cell lysates were prepared and subjected to immunoprecipitation with antibody against either P-Tyr or c-Src. Tyrosine phosphorylation of PLD2 isozyme and its association with c-Src were monitored by successive immunoblotting for mouse monoclonal anti-PLD2 antibody. As shown in Fig. Fig.3B,3B, EGF induced tyrosine phosphorylation of PLD2 and pretreatment of c-Src kinase inhibitor reduced EGF-induced tyrosine phosphorylation of PLD2. PLD2 was associated with c-Src in response to EGF (Fig. (Fig.3B),3B), suggesting that interaction of PLD2 and c-Src is regulated by a physiological ligand such as EGF.
PLD2 contains a number of functional domains, including conserved catalytic regions (I, II, III, and IV), a PIP2-binding site, a pleckstrin homology (PH) domain, and an N-terminal phox (PX) domain (Fig. (Fig.4A)4A) that has been proposed to mediate a wide variety of protein-protein interactions as well as binding of phospholipids (15, 18, 56). To determine the domain(s) of PLD2 required for c-Src interaction, cells were cotransfected with an N-terminally deleted mutant construct of PLD2, namely Δ183(184-933) or Δ313(314-933) plus c-Src. Using an antibody to PLD that recognizes the C-terminal region of PLD, we found that truncation of the N-terminal 313 amino acids from PLD2 (which removes both the PX domain and the PH domain) resulted in the loss of c-Src association, while truncation of the N-terminal 183 amino acids (which removes the PX domain) did not affect its association (Fig. (Fig.4B).4B). These data indicate that the PH domain of PLD2 is critical for the interaction with c-Src. Moreover, using a GST pull-down assay, we confirmed that the GST PH domain interacted with c-Src (Fig. (Fig.4C4C).
Having identified the PLD2 region required for association with c-Src, we next sought to identify the c-Src region involved in binding to PLD2. GST fusion proteins that contained the specific domains of c-Src, namely, GST-SH3, GST-SH2, and GST-kinase, were prepared (Fig. (Fig.5A).5A). c-Src and PLD2 were coexpressed in COS-7 cells, cell lysates were prepared, incubated with fusion proteins, and subjected to a GST pull-down assay, and proteins were analyzed by immunoblotting with an anti-PLD antibody. We found that the catalytic domain, but not SH3 or SH2, was required for binding to PLD2 (Fig. (Fig.5B5B).
Next, we investigated the roles of PLD and c-Src in the cellular growth. Mouse fibroblasts overexpressing PLD1 or PLD2 (36) were plated at a density of 5 × 104 cells in 12-well plates and transfected with control vector or wild-type or a kinase-inactive mutant c-Src. After incubation for 40 h, the live cells were counted by using the trypan blue exclusion assay. As we reported recently (36), overexpression of PLD in the fibroblast increased growth rates compared to those of vector-transfected cells. Cells transfected with wild-type c-Src displayed increased cell proliferation compared to cells transfected with the control vector or the kinase-inactive mutant (Fig. (Fig.6).6). Coexpression of c-Src and PLD1 or 2 synergistically increased the cellular proliferation over that observed with expression of either molecule. Cells proliferated even at high density, suggesting that these cells overcame density arrest. These results based on the number of cells were similar to those obtained from the proliferation assay based on metabolic conversion of a tetrazolium compound, MTS (3-[4,5-dimethylthiazol-2-yl]-5-[3-carboxylmethoxylphenyl]-2-[4-sulfophenyl]-2H-tetrazolium), to a colored product by living cells (data not shown). These results demonstrate that modulation between PLD2 and c-Src might contribute to enhanced cellular proliferation.
Next, we attempted to examine the underlying mechanisms of the modulation between PLD and c-Src, possibly leading to enhancement of cellular proliferation. We first investigated whether PLD phosphorylation by c-Src has any effect on its phospholipase activity. Constitutively active c-Src mutant was coexpressed with PLD1 or 2 in COS-7 cells, and in vivo PLD activity was measured following EGF stimulation. The results of these experiments showed that EGF-induced PLD activity in cells cotransfected with PLD1 or 2 plus constitutively active c-Src was similar to that in cells cotransfected with PLDs plus control vector (Fig. (Fig.7A).7A). Transient expression of the dominant negative mutant c-Src also did not affect PLD activity (data not shown). Similar data were obtained by using FaO hepatoma cells stably overexpressing the constitutively active mutant c-Src, in which EGF-induced PLD activity was not different from that in cells expressing empty vector or inactive c-Src (Fig. (Fig.7B).7B). These results suggest that tyrosine phosphorylation of PLD by c-Src does not increase its phospholipase activity. Therefore, other regulatory mechanism(s) contributing to the promotion of cellular proliferation by interaction between PLD and c-Src may exist.
Next, we investigated whether PLD might lead to an increase in Src kinase activity. It has become clear that autophosphorylation of Tyr416 in the activation loop is critical for full activation of Src-family tyrosine kinases (26, 53, 57). First, COS-7 cells were transiently cotransfected with c-Src and various PLD2 constructs, and the lysates were assayed for c-Src activity by immunoblotting with an antibody specific for c-Src autophosphorylated at Tyr416 (anti-phospho-Y416 c-Src ). The data presented in Fig. Fig.9A9A show that cells coexpressing wild-type PLD2 and c-Src contained high levels of autophosphorylated kinase. Truncation of the N-terminal region containing the PX and PH domains of PLD2 (which eliminates 85% of the basal activity) resulted in decreased c-Src activation, whereas truncation of the N-terminal 183 amino acids from PLD2 (which removes the PX domains but still yields a catalytically active enzymes) induced an activation of c-Src similar to that in cells transfected with wild-type PLD2 and c-Src (Fig. (Fig.8A).8A). Since the N-terminal 313-amino-acid-truncated PLD2 mutant that does not bind to c-Src (PLD2 Δ313(314-933) failed to activate c-Src (Fig. (Fig.4B),4B), we further examined whether PLD catalytic activity or the binding of PLD isozyme with c-Src is more critical for c-Src activation by PLD isozyme. Vector or PLD alone did not induce autophosphorylation of Tyr416 in transfected cells. Interestingly, not only wild-type PLD2 but also wild-type PLD1 significantly stimulated c-Src kinase activity compared to cells transfected with c-Src alone. In contrast, both catalytically inactive mutants of PLD1 and PLD2 reduced c-Src kinase activity (Fig. (Fig.8B).8B). As shown in Fig. Fig.2,2, PLD1 has a much lower binding affinity with c-Src than does PLD2. Furthermore, a catalytically inactive PLD2 mutant also interacted with c-Src to the same extent as wild-type PLD2 (Fig. (Fig.8C).8C). These results demonstrate that the catalytic activity of PLD isozyme rather than its binding with c-Src is required for c-Src activation by PLD.
c-Src has been reported to phosphorylate paxillin in integrin signaling or various mitogenic signaling pathways (58). Therefore, we investigated whether PLD affects c-Src-induced phosphorylation of paxillin. Transient cotransfection of c-Src and PLD into COS-7 cells showed that overexpression of wild-type PLDs, but not catalytically inactive mutant PLDs, increased tyrosine phosphorylation of paxillin compared to cells expressing c-Src and empty vector (Fig. (Fig.9).9). Tyrosine phosphorylation of paxillin was not detected in cells transfected with vector or PLD only. This result suggests that PLD with a catalytic activity might be involved in the potentiation of tyrosine phosphorylation of paxillin, possibly through activation of c-Src. In addition, it has been reported that overexpression or activation of c-Src is involved in the activation of the Ras/ERK pathway in COS-7 cells (29). Using phospho-specific antibodies that bind only activated forms of ERK, we examined whether PLD enhanced c-Src-stimulated ERK activity. Transient expression of wild-type PLD1 or PLD2 plus c-Src resulted in increased ERK activity compared to that observed with cells transfected with c-Src and empty vector or catalytically inactive PLD mutant plus c-Src (Fig. (Fig.9).9). Activation of ERK was not detected in cells transfected with vector or PLD only. Taken together, these results suggest that activation of the Src signaling cascade is also dependent upon PLD activity.
It has recently become apparent that PLD activity plays a significant role in transduction of intracellular signals (11, 27, 51). Since tyrosine kinases have been shown to form networks with a diverse array of cellular signal transduction pathways, a PLD-tyrosine kinase link provides novel mechanisms for PLD signaling to modulate a broad range of physiological responses. Therefore, it is crucial to understand how these two signaling components communicate with each other.
Several studies indicate that PTKs participate in the PLD activation via an unidentified mechanism (38). However, the biological significance of phosphorylation as one of the activation mechanisms of PLD still remains to be determined, since PLD activity has not been correlated with its phosphorylation level in many studies (18, 32). In addition, the nature of the tyrosine kinase has not yet been defined. Recent studies demonstrated that PLD1 and PLD2 were phosphorylated in cell-free preparations by p38MAP kinase (39) but such phosphorylation did not enhance PLD activity. It was suggested that an ERK-dependent tyrosine kinase might mediate the effect of norepinephrine on tyrosine phosphorylation of PLD2 (41). Although PLD is phosphorylated on serine and threonine residues in intact cells in response to phorbol myristate acetate (25) or on tyrosine residues by pervanadate (31, 33), it is still unclear whether this phosphorylation of PLD is of physiological relevance to enhanced hydrolysis of phosphatidylcholine or other phospholipid substrates.
Although Src tyrosine kinase has been shown to play a role in PLD-mediated signaling (23, 43), the link between c-Src and PLD was not clear. In this study, we demonstrate for the first time that Src family protein kinases induced tyrosine phosphorylation of PLD isozymes, most notably PLD2, by transient transfection. Moreover, EGF induced tyrosine phosphorylation of PLD2 in A431 cells. However, EGF-induced PLD activity was not altered by coexpression of c-Src in COS-7 cells. In addition, EGF-induced PLD activity in hepatoma cells overexpressing constitutive active mutant c-Src was not different from the activity shown in cells transfected with empty vector or dominant inactive c-Src. These results suggest that tyrosine phosphorylation of PLD by c-Src does not increase PLD phospholipase activity. This correlates with a previous report that overexpression of active c-Src or kinase-negative mutant c-Src did not affect PLD activity in rat mesangial cells (44) but is in contrast to observations with fibroblasts transformed by the viral counterpart and oncogenic PTK v-Src (20). A comparison of the effects of pp60c-src and pp60v-src would suggest that only the oncogenic tyrosine kinase is able to interfere with the Ras pathway. The significance of PLD activation by v-src tyrosine kinase is unknown. Moreover, the molecular basis by which PLD activity is differentially regulated by v-Src and c-Src remains to be clarified. In some pathways, the enzymatic activity of PLD as a lipid hydrolytic enzyme might not be strictly required for PLD function, pointing to the multifaceted nature of the PLD protein. Consistent with this hypothesis, it was reported that the mitogenic activity of phospholipase C-γ1, which is known to play an important role in cell proliferation, does not exclusively result from the enzymatic activity of the lipase and that another activity inherent to the phospholipase C-γ1 molecule can also induce DNA synthesis in quiescent cells (50). Although it is possible that nonphospholipase activities of PLD through tyrosine phosphorylation play a role in PLD-mediated cellular signaling, at present it is still not clear how tyrosine phosphorylation of PLD contributes to PLD-mediated signal transduction in cells. Tyrosine phosphorylation of PLD may not be critical for proliferation because PLD1 and PLD2 show enhancement in a similar potency proliferation by coexpression of c-Src synergistically, even though the extent of tyrosine phosphorylation of PLD1 is very different from that of PLD2. We cannot exclude the possibility that tyrosine-phosphorylated residues in PLD2 by c-Src may provide the docking sites to recruit the proteins with the SH2 domain, leading to amplification of Src signaling cascades. The biological significance of PLD tyrosine phosphorylation remains to be determined in detail.
PLD associated with c-Src, and it was the catalytic domain, not the SH3 and SH2 domains, of c-Src that was required for binding to PLD2. This is reminiscent of the interaction of Gαs and Gαi with the catalytic domain of c-Src (30). c-Src binds to a region of PLD2 containing PH, which is known to be involved in protein-protein interactions as well as binding of phospholipids (56). Highly tyrosine-phosphorylated PLD2 was strongly associated with c-Src in cotransfection experiments. This was comparable to EGF-induced association of phosphorylated PLD2 with c-Src in A431 cells.
To understand PLD signaling and its biological functions, it is essential to identify the direct targets of PLD. The effectors of PLD might be proteins interacting with it or the signaling molecules that are activated by PA. Therefore, we investigated the possibility that PLD has a role not only as a substrate of c-Src but also as a regulator of c-Src. Coexpression of wild-type PLD1 or PLD2, but not catalytically inactive PLD mutants, along with c-Src, stimulated c-Src kinase activity. There seems to be little difference between PLD1 and PLD2 in c-Src activation, even though the extent of their interaction with c-Src is very different. Although catalytically inactive PLD mutant associated with c-Src, overexpression of the PLD mutant rather decreased c-Src activation. Therefore, c-Src activation via PLD appeared to be dependent upon the catalytic activity of PLD rather than interaction of PLD with c-Src. The present work identifies PLD as a regulator of Src family tyrosine kinases and provides substantial new insights for future investigations in the two fields of PLD function and Src regulators.
Until now, PLDs have not been studied for their ability to increase c-Src kinase activity. In the present study, we demonstrate for the first time that there is an intimate cross-regulation between PLD and c-Src. Critical downstream targets of the Src kinase remain controversial and probably depend on the particular signaling pathway that activated the kinase. Many proposed substrates of c-Src are derived from studies of proteins phosphorylated by the activated viral Src protein, such as phosphatidylinositol 3 kinase, Shc, phospholipase C-γ, paxillin, vinculin, and ERK (5, 29, 48). In the present study, we demonstrated that PLD-induced activation of c-Src increased tyrosine phosphorylation of paxillin and activation of ERK, downstream target molecules of Src. Activation of these molecules was also dependent on the catalytic activity of PLD. Our observations are consistent with the recent report demonstrating that EGF-induced ERK activation is dependent upon PLD activity in A431 cells (46). This may occur by stimulation of c-Src signaling via PLD activity-dependent mechanism. Therefore, our results suggested that PA generated by PLD activity might be involved in the c-Src/ERK pathway. A number of studies have demonstrated that PA induces tyrosine phosphorylation in neutrophils and other cell types (45, 47). While it is tempting to speculate that PA-dependent tyrosine phosphorylation may be involved in activation of c-Src and stimulation of the ERK, additional experiments are required to prove this hypothesis.
Both PLD and c-Src are overexpressed and highly activated in a wide variety of human cancers (19, 40, 54, 55, 59). A connection between activation of Src via PLD and cancer progression would be a significant finding. Consistent with such a connection, Joseph et al. (23) reported that elevated expression of either PLD1 or PLD2 transformed cells overexpressing c-Src. Therefore, it is reasonable to propose that these two proteins have coordinated functions.
The present studies significantly advance our understanding of the cross talk between PLDs and c-Src tyrosine kinases. In summary, our novel findings show that there is an interaction between Src and PLD protein leading to tyrosine phosphorylation of both PLD1 and PLD2 and that this interaction and PLD activity activate c-Src, leading to autophosphorylation, paxillin phosphorylation, and ERK activation. PLD is tyrosine phosphorylated as a novel substrate of c-Src and stimulates Src kinase signaling in a PLD activity-dependent mechanism, inducing cell proliferation (Fig. (Fig.10).10). These characteristics suggest that PLD lies at an interesting nexus of Src-dependent signaling. Further studies aimed at the elucidation of cell growth signal → c-Src → signaling event of PLD phosphorylation and PLD-stimulated c-Src downstream signaling events leading to the gene expression are necessary to gain a better understanding of the functional role of PLD in mediating signals from c-Src tyrosine kinase.
We thank Sung Ho Ryu (POSTECH, Pohang, Korea) for providing cDNA encoding PLD constructs (wild-type and mutant GST-PH) and helpful comments in the preparation of the manuscript and Pamela L. Schwartzberg for providing cDNA encoding wild-type and mutant c-Src (K295M) expression vectors.
This work was supported by a Korea Research Foundation grant (KRF-2002-070-C00074).