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Vascular Endothelial Growth Factor (VEGF) Pathway

  • Monique Nilsson
    Affiliations
    Department of Cancer Biology, University of Texas, M. D. Anderson Cancer Center, Houston, TX
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  • John V. Heymach
    Correspondence
    Address for correspondence: John V. Heymach, MD, PhD, Dept. of Thoracic/Head and Neck Medical Oncology, M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Unit 432, Houston, TX 77030-4009
    Affiliations
    Department of Cancer Biology, University of Texas, M. D. Anderson Cancer Center, Houston, TX

    Department of Thoracic/Head and Neck Medical Oncology, University of Texas, M. D. Anderson Cancer Center, Houston, TX
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      Angiogenesis, the growth of new capillary blood vessels, is critical for the growth and metastatic spread of tumors.
      • Folkman J
      Tumor angiogenesis: therapeutic implications.
      The search for tumor-derived factors that stimulate angiogenesis led to the identification of vascular endothelial growth factor (VEGF, also known as VEGF-A) as an endothelial mitogen.
      • Leung DW
      • Cachianes G
      • Kuang WJ
      • et al.
      Vascular endothelial growth factor is a secreted angiogenic mitogen.
      This protein had previously been identified as vascular permeability-inducing factor secreted by tumor cells (VPF, for vascular permeability factor).
      • Senger DR
      • Galli SJ
      • Dvorak AM
      • et al.
      Tumor cells secrete a vascular permeability factor that promotes accumulation of ascites fluid.
      The VEGF gene undergoes alternative splicing to yield at least five different isoforms, ranging in size from 121 to 206 amino acids, of which VEGF165 is the predominant form.

      THE VEGF FAMILY

      VEGF is now known to be the prototypic member of a family of structurally related dimeric proteins including VEGF-B, VEGF-C, VEGF-D, and VEGF-E, as well as placental-growth factor (PlGF) -1 and -2.
      • Hicklin DJ
      • Ellis LM
      Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis.
      • Folkman J
      • Heymach JV
      • et al.
      VEGF is essential for development because homozygous or heterozygous deletion of the VEGF gene is embryonically lethal.
      • Carmeliet P
      • Ferreira V
      • Breier G
      • et al.
      Abnormal blood vessel development and lethality in embryos lacking a single VEGF allele.
      Indeed, VEGF family members are important in physiological angiogenic processes in the adult including wound healing, ovulation, and pregnancy, as well as pathological conditions such as cancer.
      • Hicklin DJ
      • Ellis LM
      Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis.
      VEGF ligands activate angiogenic programs through binding of several receptors. VEGFR-1 (Flt-1) binds VEGF, VEGF-B, and PlGF -1,2 and promotes recruitment of endothelial progenitors and monocyte migration. VEGFR-2 (Flk-1/KDR) is expressed on nearly all endothelial cells and binds VEGF, VEGF-C, VEGF-D, and VEGF-E. Signal transduction through VEGFR2 has been shown to regulate endothelial cell proliferation, migration, and survival.
      • Hicklin DJ
      • Ellis LM
      Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis.
      In healthy adults, expression of VEGFR-3 is limited to lymphatic endothelium,
      • Kaipainen A
      • Korhonen J
      • Mustonen T
      • et al.
      Expression of the fms-like tyrosine kinase 4 gene becomes restricted to lymphatic endothelium during development.
      although VEGFR-3 may also be expressed on tumor-associated blood vessels. Through binding to VEGF-C and VEGF-D, VEGFR-3 is thought to facilitate the outgrowth of lymphatic vessels. Neuropilin (NRP)-1,2 have been demonstrated to be coreceptors for VEGF. NRP-1 binds VEGF165 and PlGF, and NRP-2 binds VEGF165 and VEGF-C.
      • Hicklin DJ
      • Ellis LM
      Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis.
      Unlike other VEGFRs, NRP-1,2 lack intracellular signaling domains. Although the specific role of NRP-1,2 in angiogenesis is not fully known, NRP-1,2 bind VEGF ligands and enhance their affinity to other VEGFRs.
      • Soker S
      • Takashima S
      • Miao HQ
      • et al.
      Neuropilin-1 is expressed by endothelial and tumor cells as an isoform-specific receptor for vascular endothelial growth factor.

      VEGF SIGNAL TRANSDUCTION

      Although VEGF-A binds VEGFR1 with a higher affinity than VEGFR-2, the biological effects of VEGF-A are thought to be mediated through VEGFR-2. On ligand binding, VEGFR-2 dimerizes, resulting in kinase activation and autophosphorylation of tyrosine residues including Tyr951, Tyr996, Tyr1054, Tyr1175, and Tyr1214.
      • Matsumoto T
      • Claesson-Welsh L
      • et al.
      VEGF receptor signal transduction.
      Phosphorylation of these residues leads to the activation of signal-transduction molecules phospholipase C-γ (PLC-γ), PI3K, Akt, Ras, Src, and MAPK (Figure 1). Phosphorylation of Tyr1175 results in the binding and phosphorylation of PLC-γ, which subsequently stimulates the release of Ca2+ from internal stores and activation of protein kinase C (PKC). Activation of PKC stimulates the Raf/MEK/ERK pathway, which promotes cell proliferation. Ca2+ mobilization and PKC activation are thought to be key signaling events in VEGF-A–induced vascular permeability via activation of endothelial nitric oxide synthase activity.
      Figure thumbnail gr1
      FIGURE 1Signal transduction via the VEGF family. VEGF family members, VEGF, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and PlGF bind three VEGFR tyrosine kinases, resulting in dimerization, receptor autophosphorylation, and activation of downstream signal-transduction pathways. Signal transduction through VEGFR2 is shown. Ligand binding to VEGFR2 activates signal-transduction molecules phospholipase C-γ(PLC-γ), PI3K, Akt, Ras, Src, and MAPK and regulates cell proliferation, migration, survival, and vascular permeability.
      PI3K is a heterodimer composed of a p85 regulatory subunit and a p110 catalytic subunit and is critical in the regulation of cell proliferation, migration, and survival. VEGF-A has been shown to stimulate phosphorylation of p85 and enhance PI3K enzymatic activity. The mechanism by which VEGF-A results in activation of PI3K remains unclear, although studies have implicated a role for Src kinases, β-catenin, and VE-cadherin.
      • Carmeliet P
      • Lampugnani MG
      • Moons L
      • et al.
      Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis.
      • Schlessinger J
      New roles for Src kinases in control of cell survival and angiogenesis.
      VEGFR-2–induced activation of PI3K results in accumulation of phosphatidylinositol-3,4,5-triphosphate, which induces phosphorylation of Akt/PKB. Once activated, Akt/PKB phosphorylates and thus inhibits proapoptotic proteins, BAD, and caspase-9.
      Members of the Src family kinases, Src, Fyn, and Yes, are expressed in endothelial cells. After VEGFR-2 autophosphorylation, T-cell–specific adapter binds Tyr951 and then associates with Src. Src kinases regulate actin stress fiber organization and may mediate VEGF-A–induced PI3K activation. Ligand binding to VEGFR-2 also triggers activation of the Ras pathway, initiating signaling through the Raf-1–MEK–ERK signal cascade
      • Matsumoto T
      • Claesson-Welsh L
      • et al.
      VEGF receptor signal transduction.
      known to be important in growth factor–induced cell proliferation. This activation may occur through multiple routes. The adaptor protein Grb2 is thought to bind pTyr1214 on VEGFR-2, leading to stimulation of the guanine-nucleotide–exchange protein Sos and activation of Ras.
      • Matsumoto T
      • Claesson-Welsh L
      • et al.
      VEGF receptor signal transduction.
      Alternatively, VEGFR2 activation may promote Shc phosphorylation and binding to Grb2, which may induce Ras activation.
      • Kroll J
      • Waltenberger J
      The vascular endothelial growth factor receptor KDR activates multiple signal transduction pathways in porcine aortic endothelial cells.

      APPROACHES TO INHIBITING THE VEGF PATHWAY

      Several different types of agents have been developed to target the VEGF pathway. These include monoclonal antibodies against VEGF (i.e., bevacizumab) and proteins that bind VEGF such as VEGF Trap, a molecule generated as a fusion protein of the VEGFR extracellular domain and the Fc portion of immunoglobulin G1. Other strategies to target the VEGF pathway include antibodies that block the receptor (IMC-1121b) as well as small-molecule inhibitors of the receptor tyrosine kinase such as sunitinib, sorafenib, and ZD6474 (Table 1). Receptor tyrosine kinase inhibitors are typically administered orally, and because of the structural similarity of the different receptor tyrosine kinases, they generally inhibit multiple kinases in addition to the VEGF receptors. Representative agents targeting the VEGF pathway that are currently in clinical testing are listed in Table 1. The clinical activity of other therapeutic agents such as COX-2 inhibitors, thalidomide, and EGFR inhibitors may also be attributable, at least in part, to reduced expression of VEGF and other proangiogenic factors.
      TABLE 1Examples of Agents in Clinical Development Targeting the VEGF Pathway
      AgentTarget within VEGF pathwayTypeCompany
      Bevacizumab
      FDA approved. RTKI, receptor tyrosine kinase inhibitor.
      VEGFmABGenentech
      VEGF trapVEGFEC receptor domainRegeneron
      IMC-1121BVEGFR-2mAbImclone
      Sunitinib
      FDA approved. RTKI, receptor tyrosine kinase inhibitor.
      VEGFR-1,2,3RTKIPfizer
      Sorafenib
      FDA approved. RTKI, receptor tyrosine kinase inhibitor.
      VEGFR-2,3RTKIBayer
      ZD6474,VEGFR-2,3RTKIAstraZeneca
      AZD2171VEGFR-1,2,3RTKIAstraZeneca
      VatalinibVEGFR-2RTKINovartis
      AMG706VEGFR-1,2,3RTKIAmgen
      Ag-013736VEGFR-1,2,3RTKIPfizer
      a FDA approved. RTKI, receptor tyrosine kinase inhibitor.

      VEGF PATHWAY INHIBITORS: CLINICAL RESULTS

      The VEGF pathway has now been validated as a therapeutic target by phase III randomized clinical trials for patients with several different tumor types. In patients with previously untreated non-small cell lung cancer, the addition of bevacizumab to carboplatin and paclitaxel prolonged survival by approximately 2 months compared with treatment with carboplatin and paclitaxel alone.
      • Sandler AB
      • Gray R
      • Brahmer J
      • et al.
      Randomized phase II/III Trial of paclitaxel (P) plus carboplatin (C) with or without bevacizumab (NSC # 704865) in patients with advanced non-squamous non-small cell lung cancer (NSCLC): An Eastern Cooperative Oncology Group (ECOG) Trial - E4599 (Abstract).
      Patients with squamous cell histology were excluded from this trial because of a high rate of life-theatening hemoptysis observed in phase II testing of the same regimen. The addition of bevacizumab to chemotherapy has provided benefit in other tumor types as well, including colorectal, breast, and renal-cell cancer.
      • Sandler AB
      • Gray R
      • Brahmer J
      • et al.
      Randomized phase II/III Trial of paclitaxel (P) plus carboplatin (C) with or without bevacizumab (NSC # 704865) in patients with advanced non-squamous non-small cell lung cancer (NSCLC): An Eastern Cooperative Oncology Group (ECOG) Trial - E4599 (Abstract).
      • Hurwitz H
      • Fehrenbacher L
      • Novotny W
      • et al.
      Bevacizumab plus irinotecan, fluorouracil, and leucovorin for metastatic colorectal cancer.
      Sorafenib and sunitinib have also demonstrated significant clinical activity, particularly in renal-cell carcinoma.
      • Escudier B
      • Szczylik C
      • Eisen T
      • et al.
      Randomized Phase III trial of the Raf kinase and VEGFR inhibitor sorafenib (BAY 43-9006) in patients with advanced renal cell carcinoma (RCC) (Abstract).
      • Motzer RJ
      • Michaelson MD
      • Redman BG
      • et al.
      Activity of SU11248, a multitargeted inhibitor of vascular endothelial growth factor receptor and platelet-derived growth factor receptor, in patients with metastatic renal cell carcinoma.
      Currently, there are ongoing trials using VEGF pathway inhibitors for virtually every type of tumor, and it seems likely that they will become a standard component of treatment for a broad range of tumor types. But despite this progress, major challenges remain. These include developing biomarkers to predict which patients are most likely to respond to treatment; improving management of toxicities, particularly hemoptysis and cardiovascular/thromboembolic complications; and understanding mechanisms of therapeutic resistance so that combinations of VEGF inhibitors with other types of therapy can be more rationally designed.

      REFERENCES

        • Folkman J
        Tumor angiogenesis: therapeutic implications.
        N Engl J Med. 1971; 285: 1182-1186
        • Leung DW
        • Cachianes G
        • Kuang WJ
        • et al.
        Vascular endothelial growth factor is a secreted angiogenic mitogen.
        Science. 1989; 246: 1306-1309
        • Senger DR
        • Galli SJ
        • Dvorak AM
        • et al.
        Tumor cells secrete a vascular permeability factor that promotes accumulation of ascites fluid.
        Science. 1983; 219: 983-985
        • Hicklin DJ
        • Ellis LM
        Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis.
        J Clin Oncol. 2006; 23: 1011-1027
        • Folkman J
        • Heymach JV
        • et al.
        Tumor Angiogenesis. 7th Ed. B.C, Decker, Hamilton, Canada2006
        • Carmeliet P
        • Ferreira V
        • Breier G
        • et al.
        Abnormal blood vessel development and lethality in embryos lacking a single VEGF allele.
        Nature. 1996; 380: 435-439
        • Kaipainen A
        • Korhonen J
        • Mustonen T
        • et al.
        Expression of the fms-like tyrosine kinase 4 gene becomes restricted to lymphatic endothelium during development.
        Proc Natl Acad Sci U S A. 1995; 92: 3566-3570
        • Soker S
        • Takashima S
        • Miao HQ
        • et al.
        Neuropilin-1 is expressed by endothelial and tumor cells as an isoform-specific receptor for vascular endothelial growth factor.
        Cell. 1998; 92: 735-745
        • Matsumoto T
        • Claesson-Welsh L
        • et al.
        VEGF receptor signal transduction.
        Sci STKE. 2001; 2001: RE21
        • Carmeliet P
        • Lampugnani MG
        • Moons L
        • et al.
        Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis.
        Cell. 1999; 98: 147-157
        • Schlessinger J
        New roles for Src kinases in control of cell survival and angiogenesis.
        Cell. 2000; 100: 293-296
        • Kroll J
        • Waltenberger J
        The vascular endothelial growth factor receptor KDR activates multiple signal transduction pathways in porcine aortic endothelial cells.
        J Biol Chem. 1997; 272: 32521-32527
        • Sandler AB
        • Gray R
        • Brahmer J
        • et al.
        Randomized phase II/III Trial of paclitaxel (P) plus carboplatin (C) with or without bevacizumab (NSC # 704865) in patients with advanced non-squamous non-small cell lung cancer (NSCLC): An Eastern Cooperative Oncology Group (ECOG) Trial - E4599 (Abstract).
        J Clin Oncol. 2006; 23: LBA4
        • Hurwitz H
        • Fehrenbacher L
        • Novotny W
        • et al.
        Bevacizumab plus irinotecan, fluorouracil, and leucovorin for metastatic colorectal cancer.
        N Engl J Med. 2004; 350: 2335-2342
        • Escudier B
        • Szczylik C
        • Eisen T
        • et al.
        Randomized Phase III trial of the Raf kinase and VEGFR inhibitor sorafenib (BAY 43-9006) in patients with advanced renal cell carcinoma (RCC) (Abstract).
        J Clin Oncol. 2006; 23: LBA4510
        • Motzer RJ
        • Michaelson MD
        • Redman BG
        • et al.
        Activity of SU11248, a multitargeted inhibitor of vascular endothelial growth factor receptor and platelet-derived growth factor receptor, in patients with metastatic renal cell carcinoma.
        J Clin Oncol. 2006; 24: 16-24