From the
Departments of Pathology and ¶Medicine (Gastroenterology Division), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215
Received for publication, September 21, 2002 , and in revised form, March 12, 2003.
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ABSTRACT |
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INTRODUCTION |
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Most VPF/VEGF biological activities are mediated by its interaction with two high affinity receptor tyrosine kinases, Flt-1 (VEGFR-1) and KDR (VEGFR-2, FLK-1 in mice) (13, 14, 15, 16, 17). A third receptor, neuropilin, which binds to VEGF165 but not VEGF121, has been recognized, but less is known about neuropilin's capacity to initiate endothelial cell signaling (18, 19). A large body of work has demonstrated that KDR, not Flt-1, is responsible for VPF/VEGF-stimulated cell proliferation and migration in cultured EC and for microvascular permeability (20, 21, 22, 23, 24). However, Flt-1 functions to down-regulate KDR-mediated cultured EC proliferation as shown by two different VPF/VEGF receptor chimeric fusion systems in which the N-terminal domains of KDR and Flt-1 were replaced by the N-terminal domain of either epidermal growth factor receptor or colony-stimulating factor-1 receptor (23, 24). Further studies indicated that the signaling pathway required for Flt-1-mediated down-regulation of EC proliferation involves activation of phosphatidylinositol 3-kinase, small GTPase Rac1, CDC42, and, surprisingly, pertussis toxin-sensitive G proteins (24, 25).
The signal transduction pathways mediated by KDR involve KDR phosphorylation (20, 21, 22, 23, 24), phospholipase C (PLC) activation (24, 26, 27, 28, 29, 30), inositol 1,4,5-trisphosphate accumulation (31), intracellular Ca2+ mobilization (24, 32), and protein kinase C and MAPK activation (24, 27, 28, 29, 30). Whereas PLC activation is involved in VPF/VEGF-induced HUVEC proliferation and migration, intracellular Ca2+ mobilization and MAPK are required for VPF/VEGF-induced HUVEC proliferation but not migration (24). Our recent studies have shown that the Gq/11 family of the heterotrimeric GTP-binding proteins and G subunits mediate VPF/VEGF-induced HUVEC migration through the small GTPase RhoA (33). However, it is not clear whether G protein Gq/11 proteins are involved in VPF/VEGF-stimulated proliferation. In this study, we show for the first time that inhibition of Gq/11 protein expression by a Gq/11-specific antisense oligonucleotide blocked VPF/VEGF-stimulated HUVEC proliferation and activation of signaling molecules in VPF/VEGF-stimulated HUVEC that are mediated by KDR, not by Flt-1. Moreover, Gq/11 proteins are not activated by tyrosine phosphorylation but through physical interaction with KDR. Surprisingly, the interaction of Gq/11 proteins with KDR does not require KDR tyrosine phosphorylation, and the Gq/11-specific antisense oligonucleotide blocks the interaction of Gq/11 with KDR and inhibits KDR phosphorylation. Furthermore, the G
11 constitutively active mutant, G
11(G209L), not the G
q constitutively active mutant, G
q(G209L), activates phosphorylation of KDR and MAPK. However, expression of the G
minigene, h
ARK1(495), inhibits VPF/VEGF-stimulated HUVEC proliferation, MAPK phosphorylation, and intracellular Ca2+ mobilization but has no effect on KDR phosphorylation.
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EXPERIMENTAL PROCEDURES |
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Cell CulturePrimary human umbilical vein endothelial cells (HUVEC; obtained from Clonetics) were cultured with or without transduced with retroviruses as described (24). Only cells from passage 3 or 4 that were 80% confluent were used for experiments. Transduction of HUVEC with EGDR- or EGLT-bearing retroviruses was carried out as described previously (24, 33).
Overexpression of Proteins in HUVECRetrovirus preparation and HUVEC infection with retrovirus were carried out as described (24, 33). Briefly, 293T cells were seeded at a density of 6 x 106 cells/100-mm plate 24 h before transfection. DNA transfection was carried out with the EffecteneTM transfection reagent (Qiagen, Valencia, CA). Two µg of each target gene (pMMP-EGDR, pMMP-LacZ, etc.), 1.5 µg of pMD.MLV gag.pol, and 0.5 µg of pMD.G DNA, encoding the cDNAs of the proteins that are required for virus packaging (kindly provided by Dr. Mulligan), were mixed in 300 µl of EC buffer. 32 µl of enhancer was added to the DNA mixture. After incubation at room temperature for 2 min, 30 µl of Effectene was added to the DNA mixture and incubated at room temperature for 5 min. The DNA mixture was added dropwise to 293T cells. The medium was changed after 16 h. The retrovirus was isolated 48 h after transfection and used immediately for infection or frozen at -70 °C.
24 h before infection, HUVECs were seeded at a density of 0.3 x 106 cells/100-mm plate. 1 ml of retrovirus solution (2 x 107 plaque-forming units/ml) and 5 ml of fresh medium were added to cells with 10 µg/ml polybrene. The medium was changed after 16 h, and cells were ready for experiments 48 h after infection.
The expression plasmids containing Gq, G
q(Q209L), G
11, and G
11(209L) cDNAs were obtained from Guthrie cDNA Resource Center, Guthrie Research Institute (Sayre, PA). Transfection of plasmids to HUVEC was carried out as described (34).
Synthesis and Transfection of Antisense Oligonucleotides3'-End FITC-labeled phosphorothioated Gq/11 antisense oligonucleotide (ODN-Gq/11), 5'-CCATGCGGTTCTCATTGTCTG-3', and a 3'-end fluorescein isothiocyanate-labeled phosphorothioated random oligonucleotide (ODN-RD), 5'-CCCTTATTTACTACTTTCGC-3' (35), were synthesized by Genemed Synthesis (Genemed Synthesis, South San Francisco, CA).
Proliferation AssaysAssays were carried out as described (24, 33). Briefly, HUVEC (with or without oligonucleotide transfection) were serum-starved (0.1% serum) for 24 h and then stimulated with 10 ng/ml VEGF for 20 h. 1 µCi/ml [3H]thymidine was added to each well, and 4 h later the cells were washed, fixed, and lysed. Data were expressed as -fold activation with the stimulated cells compared with control treated cells. The data shown represent the means with S.D. of triplicate determinations per experimental condition, and the experiments were repeated at least three times.
Intracellular Ca2+ ReleaseSerum-starved HUVEC with or without transfection were loaded with Fura-2 AM and stimulated with 10 ng/ml VPF/VEGF. The assay was carried out as described (24, 33). All experiments were repeated at least three times.
CDC42 Activation AssaysThe CDC42 activity assay was carried out as described (25). Briefly, 24-h serum-starved HUVEC with or without oligonucleotide transfection were stimulated with 10 ng/ml VPF/VEGF for 1 min. Cells were washed and lysed. Cell lysates were centrifuged at 14,000 rpm for 3 min. The supernatant was incubated with 50 µl of freshly prepared GST-Pak-CRIB beads at 4 °C for 45 min. The proteins bound to beads were washed and analyzed by SDS-PAGE with antibodies against CDC42. All experiments were repeated at least three times.
Immunoprecipitation and ImmunoblottingHUVEC, with or without virus infection or oligonucleotide transfection, were serum-starved for 24 h and stimulated with 10 ng/ml VPF/VEGF or EGF as indicated for various time periods. Stimulation was halted by the addition of ice-cold phosphate-buffered saline, and cells were washed three times with ice-cold phosphate-buffered saline and lysed with cold radioimmune precipitation buffer (20 mM Tris-HCl, pH 7.5, 0.15 M NaCl, 1% Triton X-100, 1 µM phenylmethylsulfonyl fluoride, 1 mM Na3VO4, 1 mM EGTA, 1 µg/ml leupeptin, 0.5% aprotinin, 2 µg/ml pepstatin A). Cell lysates were collected after centrifugation at 14,000 x g for 15 min at 4 °C. One mg of lysate protein was incubated with antibodies as indicated for 1 h, and with 50 µl of protein A-conjugated agarose beads at 4 °C for another 1 h. For immunoprecipitation with phosphorylated tyrosine, cellular extracts were incubated with agarose-conjugated antiphosphorylated tyrosine for 2 h. Beads were washed with radioimmune precipitation buffer three times, and immunoprecipitates were resuspended in 2x SDS sample buffer for Western blot analysis. For the experiments with pertussis toxin, this was added 16 h before stimulation. All experiments were repeated at least three times.
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RESULTS |
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Gq/11 Proteins Are Required for the KDR-mediated, Not Flt-1-mediated, Signaling Pathway in VPF/VEGF-stimulated HUVECNext, we examined the effect of ODN-Gq/11 on the activation of signaling molecules mediated by KDR and Flt-1. As shown in Fig. 2a, ODN-Gq/11, not the control ODN-RD, completely inhibits intracellular Ca2+ mobilization that is mediated by KDR but not Flt-1 (24). We have previously shown that Flt-1, but not KDR, can activate CDC42, a member of the Rho family of small GTPases (25). Therefore, we examined the effect of ODN-Gq/11 on CDC42 activation. To do this, serumstarved HUVEC that were transfected with ODN-Gq/11 or nontransfected were stimulated with VPF/VEGF for 1 min, and cellular extracts were subjected to a CDC42 activation assay as described under "Experimental Procedures." The data indicate that ODN-Gq/11 has no effect on CDC42 activation (Fig. 2b).
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It is known that MAPK phosphorylation is required for VPF/VEGF-stimulated HUVEC proliferation (24, 27, 28, 29, 30). Therefore, we tested whether Gq/11 proteins are required for VPF/VEGF-stimulated MAPK phosphorylation in HUVEC. Serum-starved HUVEC, which were transfected with either ODN-Gq/11 or ODN-RD or nontransfected, were stimulated with VPF/VEGF for 10 min. Cellular extracts were subjected to Western blot analysis using an antibody specifically against phosphorylated MAPK. Surprisingly, ODN-Gq/11 partially inhibits VPF/VEGF-stimulated MAPK phosphorylation in HUVEC, whereas ODN-RD has no effect (Fig. 3a). Similar results were also obtained when cells were treated with VPF/VEGF for 15 and 30 min (data not shown). Because both KDR and Flt-1 are present in HUVEC, in order to further characterize the partial inhibition of MAPK phosphorylation by the Gq/11 antisense oligonucleotide, we used the recently developed chimeric receptors, EGDR and EGLT, in which the extracellular domains of KDR and Flt-1 were replaced with that of epidermal growth factor receptor. Serum-starved HUVEC with or without transduction of LacZ as a control, EGDR, or EGLT was stimulated with VPF/VEGF or EGF for the indicated time. Cellular extracts were used to determine the levels of phosphorylated MAPK. As shown in Fig. 3b, both EGDR and EGLT can mediate MAPK phosphorylation; however, HUVEC transduced with LacZ do not show any detectable MAPK phosphorylation. Furthermore, we used this receptor chimera system to further confirm whether ODN-Gq/11 inhibits only KDR-mediated and not Flt-1-mediated MAPK phosphorylation. EGDR/HUVEC and EGLT/HUVEC, transfected with either ODN-Gq/11 or pertussis toxin, were stimulated with EGF for 10 min. Cellular extracts were used to determine MAPK phosphorylation. The data show that ODN-Gq/11 completely inhibits EGDR-mediated MAPK phosphorylation but has no effect on MAPK phosphorylation mediated by EGLT (Fig. 3c). Moreover, our data also demonstrate that pertussis toxin completely inhibits Flt-1-mediated MAPK phosphorylation but has no effect on KDR-mediated MAPK phosphorylation (Fig. 3c). These results indicate that Gq/11 proteins are required for KDR-mediated signaling pathways and that pertussis toxin-sensitive Gi/o proteins are required for Flt-1 signaling.
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VPF/VEGF Stimulates Interaction between KDR and Gq/11 but Not Tyrosine Phosphorylation of Gq/11Recently, it was reported that platelet-derived growth factor (PDGF) receptor stimulation leads to tyrosine phosphorylation of Gi protein (37). Therefore, we tested whether VPF/VEGF could stimulate Gq/11 phosphorylation. Serum-starved HUVEC were stimulated with VPF/VEGF for 20 s, 40 s, 1 min, and 2 min as indicated. Cellular extracts were immunoprecipitated with an antibody against phosphorylated tyrosine, and the immunoprecipitates were then subjected to immunoblot analysis using antibodies against KDR and Gq/11. The results show that Gq/11 is not phosphorylated (Fig. 4, bottom panel), whereas KDR is phosphorylated at 1 min (Fig. 4, top panel). These results indicate that Gq/11 proteins are not activated by tyrosine phosphorylation.
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In G protein-coupling receptor signaling pathways, G proteins are activated by its interaction with the G protein-coupling receptor. Therefore, we tested whether Gq/11 proteins interact with KDR in VPF/VEGF-stimulated HUVEC. HUVEC were stimulated with 10 ng/ml VPF/VEGF for 5, 10, and 20 s. Cellular extracts were immunoprecipitated with an antibody against Gq/11, and the immunoprecipitates were then immunoblotted with an antibody against KDR. The data show that VPF/VEGF stimulates interaction between KDR and G
q/11 as early as 5 s after treatment and that this interaction is transient (Fig. 5a). Furthermore, when cellular extracts from serum-starved HUVEC transfected with ODN-Gq/11 or ODN-RD were immunoprecipitated with an antibody against Gq/11 and then immunoblotted with an antibody against KDR, ODN-Gq/11 completely inhibited the association of KDR with Gq/11 proteins, but ODN-RD had no effect (Fig. 5b). The data confirm that the interaction of KDR and Gq/11 is specific.
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Interaction of KDR and Gq/11 Does Not Require KDR PhosphorylationNext, we tested whether KDR phosphorylation is required for KDR and Gq/11 interaction. We utilized the chimeric receptor, EGDR. In HUVEC transduced with LacZ, EGF did not stimulate the interaction of KDR with Gq/11 (Fig. 6a). However, when serum-starved HUVEC transduced with EGDR were stimulated with EGF for various times, the interaction of EGDR and Gq/11 was found to be in a similar time course to that of VPF/VEGF-stimulated HUVEC (Fig. 6a). Previously, we showed that EGDR(Y915F), a mutated receptor with partial phosphorylation activity, cannot induce HUVEC migration but maintains its ability to induce cell proliferation. However, EGDR(Y1059F), a mutated receptor that could not be tyrosine-phosphorylated, cannot stimulate HUVEC proliferation but still induces HUVEC migration (33). Therefore, we overexpressed these two EGDR mutants in HUVEC to test their effect on KDR and Gq/11 protein interaction. The results show that like wild type EDGR, both EGDR(Y951F) and EGDR(Y1059F) interact with Gq/11 (Fig. 6b). Our data clearly demonstrate that tyrosine phosphorylation of KDR is not required for interaction of KDR with Gq/11 protein.
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G11 Protein Activation Is Required for KDR PhosphorylationSo far, our data indicate that, after VPF/VEGF stimulation in HUVEC, 1) Gq/11 are not tyrosine-phosphorylated in response to VPF/VEGF; 2) Gq/11 proteins physically interact with KDR; and 3) Gq/11 and KDR interaction does not require KDR phosphorylation. Therefore, we further characterized the correlation between Gq/11 activation and KDR phosphorylation. First we examined whether Gq/11 proteins are required for KDR phosphorylation. To do this, serum-starved HUVEC transfected with ODN-Gq/11 and ODN-RD were stimulated with VPF/VEGF for 1 min, and KDR phosphorylation was determined. To our surprise, KDR phosphorylation was completely blocked by ODN-Gq/11 but not by ODN-RD (Fig. 6a), indicating that Gq/11 proteins are required for KDR phosphorylation. Then we examined whether ectopic expression of wild type Gq/11 proteins could potentiate VPF/VEGF-induced phosphorylation of KDR and MAPK and whether expression of constitutively active Gq/11 mutants alone could stimulate phosphorylation of KDR and MAPK. For this purpose, serum-starved HUVEC that were nontransfected or transfected with wild type G
q, G
11, and their constitutively active mutants, G
q(Q209L) and G
11(Q209L) were stimulated with 10 ng/ml VPF/VEGF or vehicle for 1 min for KDR phosphorylation or for 10 min for MAPK phosphorylation. The similar levels of the Gq/11 protein expression were first confirmed by immunoblot analysis using an antibody that recognized both G
q and G
11 (data not shown). The results show that in nonstimulated cells, only G
11(Q209L) caused phosphorylation of KDR (Fig. 7b) and MAPK (Fig. 7c). However, in VPF/VEGF-stimulated HUVEC, transfection of G
q, G
11, or G
q(Q209L) did not change the levels of phosphorylation of KDR (Fig. 7b) and MAPK (Fig. 7c). These data clearly demonstrate that G
11, not G
q, is required, and activated G
11 is sufficient for VPF/VEGF-stimulated KDR phosphorylation and the downstream signaling pathway.
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G Is Not Required for KDR PhosphorylationIn G protein-coupling receptor systems, heterotrimeric G proteins, after interaction with activated receptors, dissociate into
and
subunits that trigger different downstream pathways. Recently, we demonstrated that G
subunits are required for VPF/VEGF-stimulated KDR-mediated migration and Flt-1-mediated antiproliferation in HUVEC by utilizing a G
minigene, h
ARK1(495) (25, 36). Here, we further tested whether G
is required for the signaling pathway that regulates VPF/VEGF-stimulated HUVEC proliferation. Serum-starved HU-VEC transduced with LacZ and h
ARK1(495) were stimulated with 10 ng/ml VPF/VEGF or bFGF and subjected to a DNA synthesis assay. As shown in Fig. 8a, overexpression of h
ARK1(495) completely inhibited DNA synthesis in HUVEC stimulated with VPF/VEGF but had no effect on bFGF-stimulated DNA synthesis. We also examined the effect of h
ARK1(495) on VPF/VEGF-induced MAPK phosphorylation. Serum-starved HUVEC infected with viruses expressing LacZ or h
ARK1(495) were stimulated with 10 ng/ml VPF/VEGF for 10 min. Equal amounts of cellular protein were subjected to immunoblot analysis with an antibody against phosphorylated MAPK (Fig. 8b, top panel) as well as an antibody against nonphosphorylated MAPK to confirm equal protein loading (Fig. 8b, bottom panel). Overexpression of h
ARK1(495) completely inhibited VPF/VEGF-stimulated MAPK phosphorylation (Fig. 8b, top panel). Since VPF/VEGF stimulates intracellular Ca2+ mobilization in HUVEC, we tested whether h
ARK1(495) affects the response of VPF/VEGF. The results show that h
ARK1(495) completely inhibited VPF/VEGF-stimulated intracellular Ca2+ mobilization (Fig. 8c). In certain G protein-coupling receptor systems, released G
subunits following receptor stimulation transactivate receptor tyrosine kinases, in particular the EGF receptor, which in turn induces MAPK activation (38, 39). Therefore, we examined whether G
is required for KDR phosphorylation. Serum-starved HU-VEC with or without transduction of LacZ or h
ARK1(495) were stimulated with 10 ng/ml VPF/VEGF for 1 min. Cellular extracts were subjected to immunoprecipitate with an antibody against KDR and then immunoblotted with an antibody against phosphorylated tyrosine. Surprisingly, h
ARK1(495) had no effect on VPF/VEGF-stimulated KDR phosphorylation (Fig. 8d). Our data indicated that G
subunits are not required for VPF/VEGF-stimulated KDR phosphorylation but mediate the downstream signaling pathway after KDR phosphorylation.
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DISCUSSION |
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It is well known that G protein-coupled receptors can transduce their signaling through heterotrimeric G protein-mediated EGF receptor transactivation (38, 39). Recently, a number of studies have indicated that receptor tyrosine kinases can also transduce their signaling through heterotrimeric G proteins. For example, platelet-derived growth factor receptor stimulates MAPK activation via direct tyrosine phosphorylation of pertussis toxin-sensitive Gi protein (37). Overexpression of G
q(Q209L), a constitutively active form of G
q that cannot promote DNA synthesis by itself, stimulates DNA synthesis in platelet-derived growth factor-mediated NIH-3T3 cells (44). Another receptor tyrosine kinase, insulin-like growth factor-1 receptor, stimulates MAPK and cell proliferation also via physical interaction with Gi and G
subunits (41, 42). Interestingly, insulin-stimulated glucose transport is through Gq/11-dependent PI-3 kinase rather than G protein Gi and G
subunit pathways (40). In this study, we present evidence that VPF/VEGF stimulates KDR-mediated HUVEC proliferation and MAPK phosphorylation via induction of rapid but transient interaction between Gq/11 proteins and KDR. In order to determine whether the Gq/11 and KDR interaction require the tyrosine kinase activity of KDR, we used two EGDR mutants, EGDR(Y951F) and EGDR(Y1059F), in which each of the autophosphorylation sites of KDR was mutated to phenylalanine (33). As compared with wild type EGDR, EGDR(Y951F) shows a partial defect in its kinase activity, whereas EGDR(Y1059F) completely loses its kinase activity (33). In terms of ability to stimulate HUVEC proliferation, EGDR(Y951F) is similar to wild type EGDR, whereas EGDR(Y1059F) completely loses this activity when HUVEC transduced with the respective mutants are treated with EGF (33). With these mutant receptors, our current study indicates that these kinase-defective EGDR(Y951F) and kinase-deficient EGDR(Y1059F) receptors still interact with Gq/11 proteins to a similar extent as wild type EGDR. The results suggest that the tyrosine kinase activity of KDR is not essential for Gq/11 and KDR interaction. Surprisingly, our results show that the Gq/11 antisense oligonucleotide blocks KDR phosphorylation and a constitutively active form of G
11, G
11(Q209L), activates KDR phosphorylation in the absence of VPF/VEGF stimulation. Thus, our conclusion is that interaction of KDR and G11 occurs upstream of KDR phosphorylation in VPF/VEGF-stimulated HUVEC, which is supported by our data indicating that Gq/11 cannot be tyrosine-phosphorylated by VPF/VEGF stimulation. Our finding is in contrast with two recent studies by Alderton et al. (37) and De Vivo and Iyengar (44) showing that PDGF receptor stimulates MAPK activation via direct tyrosine phosphorylation of pertussis toxin-sensitive Gi protein, or activated Gq potentiates PDGF-stimulated DNA synthesis.
Recently, Thuringer et al. (45) have reported that stimulation of Gq/11-coupled bradykinin B2 receptor induces tyrosine phosphorylation of KDR, resulting in increased endothelial nitric-oxide synthase activity. These results are consistent with our current observations that upon stimulation, KDR rapidly binds and activates heterotrimeric G proteins G11, which in turn increases the intrinsic kinase activity and autophosphorylation of KDR. Interestingly, Tanimoto et al. (46) showed that sphingosine 1-phosphate, via binding to its Gi-coupled receptor endothelial differentiation 1, stimulates tyrosine phosphorylation of KDR and that inhibition of KDR expression or intrinsic kinase activity decreases sphingosine 1-phosphate-induced activation of Akt and eNOS. The results of Tanimoto et al. (46) indicate that sphingosine 1-phosphate-induced KDR transactivation is independent of metalloprotenase and VPF/VEGF release but requires intracellular calcium release and Src tyrosine kinase. Although it is not clear how the B2 receptors and sphingosine 1-phosphate stimulate KDR transactivation, their molecular mechanisms seem to be different. Therefore, the questions remain to be solved regarding how KDR binds and activates G11 and how activated G
11 interacts and stimulates the tyrosine activity of KDR. The answer to the latter point may provide a common mechanism whereby Gq/11-coupled receptors transactivate KDR.
In the G protein-coupled receptor system, a ligand interacts with the receptor to stimulate G protein interaction with the ligand-bound receptors. The release of G subunits will activate Src kinase proteins, which, in turn, transactivate protein kinase receptors, such as epidermal growth factor receptor (38, 39). However, this mechanism does not apply to VPF/VEGF signaling pathways. Our data indicate that VPF/VEGF binds to KDR and induces the G protein to interact with KDR, which, in turn, stimulates KDR phosphorylation. However, the G
minigene, h
ARK1(495), does not inhibit KDR phosphorylation; rather, it inhibits intracellular Ca2+ mobilization and MAPK phosphorylation, which are required for VPF/VEGF-stimulated HUVEC proliferation (24, 33). Overexpression of h
ARK1(495) also inhibits RhoA activation, which is required for VPF/VEGF-stimulated HUVEC migration (36). Therefore, there are two possibilities: 1) KDR phosphorylation is required for G
dissociation from G
subunits, or 2) both G
and KDR phosphorylation are required for activation of a downstream signaling molecule. The first assumption seems unlikely, because kinase-deficient receptor, EGDR(Y1059F), is still able to interact with Gq/11 proteins and mediate VPF/VEGF-stimulated HUVEC migration (33), and free G
subunits are required for HUVEC migration (36). These data indicate that G
subunits function without KDR phosphorylation to regulate HUVEC migration. It is known that, unlike bFGF, which directly binds and activates the Grb2-Sos-Ras-Raf-1-MAPK cascade, VPF/VEGF utilizes a pathway of PLC activation, intracellular Ca2+ mobilization, and MAPK phosphorylation (24, 33, 47). However, the mitogenic activity of VEGF is much weaker than bFGF (Fig. 1b), and the intracellular Ca2+ mobilization is much weaker responding to VPF/VEGF than to the ligands of the G protein-coupled receptor. Therefore, it is possible that neither released G
nor KDR phosphorylation is enough to regulate the downstream signaling pathways, whereas both G
and KDR phosphorylation are required to activate different PLC members to regulate intracellular Ca2+ mobilization. This hypothesis can be supported by the facts that 1) KDR phosphorylation is required for PLC
phosphorylation (48); 2) a recent study show that leukotriene D4, a G protein-coupled receptor ligand, can stimulate association between G
subunits, c-Src, and PLC
; and 3) G
-activated c-Src may phosphorylate and activate PLC
(49). Therefore, our assumption is that maximal activation of PLC
by VPF/VEGF and subsequent intracellular calcium mobilization in endothelial cells may require two components: KDR tyrosine phosphorylation and released G
subunits. The future study will need to be carried out to further characterize how the G
11 subunit, KDR, G
subunits, and PLC
interact with one another and characterize the role of each component in KDR signaling to cell proliferation and migration. Also, the Gq/11-binding sites within the 1.8-kDa intracellular portion of KDR and the KDR-interacting sites within the G
q/11 subunits will need to be identified.
In summary, our studies have identified a novel signaling pathway in which, upon VPF/VEGF binding, KDR interacts with and activates a G11 family protein, resulting in KDR phosphorylation, MAPK activation, and cell proliferation. Released G
subunits are not required for KDR phosphorylation but may function together with KDR phosphorylation to trigger downstream signaling pathways. Together with our previous reports that Gq/11 proteins are also required for KDR-mediated RhoA activation, which leads to HUVEC migration (36), and that pertussis toxin-sensitive G proteins are involved in Flt-1-mediated down-regulation of HUVEC proliferation (25), we conclude that KDR and Flt-1 employ heterotrimeric G proteins G11
and Gi/o in their signaling pathways, respectively. This study not only significantly contributes to our understanding of the molecular mechanism of VPF/VEGF-induced vasculogenesis and angiogenesis but also provides a novel tool for cancer therapy by targeting the specific heterotrimeric G protein, G11
, to inhibit VPF/VEGF-induced angiogenesis.
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FOOTNOTES |
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A National Research Service Award fellow.
|| To whom correspondence should be addressed: Dept. of Pathology, Beth Israel Deaconess Medical Center and Harvard Medical School, 330 Brookline Ave., RN270H, Boston, MA 02215. Tel.: 617-667-7853; Fax: 617-667-3591; E-mail: dmukhopa{at}caregroup.harvard.edu.
1 The abbreviations used are: VPF, vascular permeability factor; VEGF, vascular endothelial growth factor; HUVEC, human umbilical vein endothelial cell(s); EC, endothelial cell(s); MAPK, mitogen-activated protein kinase; PLC, phospholipase C; bFGF, basic fibroblast growth factor; PDGF, platelet-derived growth factor; EGF, epidermal growth factor; EGDR, the fusion receptor of EGFR and KDR; EGLT, the fusion receptor of EGFR and Flt-1.
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ACKNOWLEDGMENTS |
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REFERENCES |
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