The role of phosphatidylinositol 3-kinase in vascular endothelial growth factor signaling

Visible light exposure induces VEGF gene expression through activation of retinoic acid receptor-alpha in retinoblastoma Y79 cells

Neovascularization of the retina and choroids is the pathological hallmark of many retinopathies, but its molecular mechanisms remain unclear. Vascular endothelial growth factor (VEGF), which is induced by hypoxia or cytokines, plays a critical role in the abnormal growth of blood vessels. In this study, we report that visible light exposure induces VEGF gene expression in retinoblastoma Y79 cells. Fluorescent light exposure (700 lux, wavelength 400 approximately 740 nm) caused a significant increase in VEGF transcripts and protein levels. Such an induction seemed to be specific to certain cells, including photoreceptor cells, because light-induced VEGF expression was not observed in either nontransformed cells, such as retinal pigment epithelium cells, and bovine aortic endothelial cells or transformed cells, such as CV-1 and HepG2 cells. Pertussis toxin and guanosine 5'-[beta-thio]diphosphate, specific inhibitors for rhodopsin-associated G protein, blunted this induction. Progressive deletion and site-specific mutation analyses indicate that light stimulation increases VEGF promoter activity through G+C-rich sequence, which is proven by Sp1 binding sites by supershift assays. Electrophoretic mobility shift assays show that light stimulation increases Sp1 binding. Synthetic retinoic acid receptor-alpha (RARalpha) antagonist completely abrogated light-mediated increase in VEGF expression. Transfection of Y79 cells with dominant negative mutant of RARalpha significantly attenuated the light-mediated induction of VEGF promoter activity. In conclusion, our data indicate that light exposure increases VEGF expression through the mechanisms involving activation of Sp1 and RARalpha signaling in Y79 cells. This study provides new insight into the role of visible light in the transcription and induction of VEGF gene expression.

Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis

New blood vessel formation (angiogenesis) is a fundamental event in the process of tumor growth and metastatic dissemination. Hence, the molecular basis of tumor angiogenesis has been of keen interest in the field of cancer research. The vascular endothelial growth factor (VEGF) pathway is well established as one of the key regulators of this process. The VEGF/VEGF-receptor axis is composed of multiple ligands and receptors with overlapping and distinct ligand-receptor binding specificities, cell-type expression, and function. Activation of the VEGF-receptor pathway triggers a network of signaling processes that promote endothelial cell growth, migration, and survival from pre-existing vasculature. In addition, VEGF mediates vessel permeability, and has been associated with malignant effusions. More recently, an important role for VEGF has emerged in mobilization of endothelial progenitor cells from the bone marrow to distant sites of neovascularization. The well-established role of VEGF in promoting tumor angiogenesis and the pathogenesis of human cancers has led to the rational design and development of agents that selectively target this pathway. Studies with various anti-VEGF/VEGF-receptor therapies have shown that these agents can potently inhibit angiogenesis and tumor growth in preclinical models. Recently, an anti-VEGF antibody (bevacizumab), when used in combination with chemotherapy, was shown to significantly improve survival and response rates in patients with metastatic colorectal cancer and thus, validate VEGF pathway inhibitors as an important new treatment modality in cancer therapy.

Luteolin inhibits vascular endothelial growth factor-induced angiogenesis; inhibition of endothelial cell survival and proliferation by targeting phosphatidylinositol 3'-kinase activity

In an attempt to identify phytochemicals contributing to the well-documented preventive effect of plant-based diets on cancer incidence and mortality, we have previously shown that certain flavonoids inhibit in vitro angiogenesis. Here, we show that the flavonoid luteolin inhibited tumor growth and angiogenesis in a murine xenograft model. Furthermore, luteolin inhibited vascular endothelial growth factor (VEGF)-induced in vivo angiogenesis in the rabbit corneal assay. In agreement, luteolin inhibited both VEGF-induced survival and proliferation of human umbilical vein endothelial cells (HUVECs) with an IC(50) of about 5 mumol/L. Luteolin inhibited VEGF-induced phosphatidylinositol 3'-kinase (PI3K) activity in HUVECs, and this inhibition was critical for both the antisurvival and antimitotic affects of the compound. Indeed, luteolin abolished VEGF-induced activation of Akt, a downstream target of PI3K conveying both survival and mitotic downstream signals. Because overexpression of a constitutively active form of Akt rescued HUVECs only from the antisurvival effects of luteolin, the result indicated that luteolin targeted mainly the survival signals of the PI3K/Akt pathway. With regard to its antimitotic activity, luteolin inhibited VEGF-induced phosphorylation of p70 S6 kinase (S6K), a downstream effector of PI3K responsible for G(1) progression. Indeed, VEGF-induced proliferation of HUVECs was sensitive to rapamycin, an inhibitor of p70 S6K activation. Surprisingly, luteolin did not affect VEGF-induced phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases, a pathway that is considered important for the mitotic effects of VEGF. Thus, blockade of PI3K by luteolin was responsible for the inhibitory effects of the compound on VEGF-induced survival and proliferation of HUVECs. The antisurvival effects of luteolin were mediated via blockage of PI3K/Akt-dependent pathways, whereas inhibition of the PI3K/p70 S6K pathway mediated the antimitotic effects of the compound.

Human microvascular endothelial cells immortalized with human telomerase catalytic protein: a model for the study of in vitro angiogenesis

Human microvascular endothelial cell-1 (HMEC-1) generated by transfection with SV40 large T antigen has been the prevailing model for in vitro studies on endothelium. However, the transduction of SV40 may lead to unwanted cell behaviors which are absent in primary cells. Thus, establishing a new microvascular endothelial cell line, which is capable of maintaining inherent features of primary endothelial cells, appears to be extremely important. Here, we immortalized primary human microvascular endothelial cells (pHMECs) by engineering the human telomerase catalytic protein (hTERT) into the cells. Endothelial cell-specific markers were examined and the angiogenic responses were characterized in these cells (termed as HMVECs, for human microvascular endothelial cells). We found that VEGF receptor 2 (Flk-1/KDR), tie1, and tie2 expression is preserved in HMVEC, whereas Flk-1/KDR is absent in HMEC-1. In addition, HMVEC showed similar angiogenic responses to VEGF as HMEC-1. Furthermore, the HMVEC line was found to generate a prominent angiogenic response to periostin, a potent angiogenic factor identified recently. The data indicate that HMVEC may serve as a suitable in vitro endothelium model.

The pathophysiologic role of VEGF in hematologic malignancies: therapeutic implications

Besides its role as an essential regulator of physiologic and pathologic angiogenesis, vascular endothelial growth factor (VEGF) triggers growth, survival, and migration of leukemia and multiple myeloma cells; plays a pivotal role in hematopoiesis; inhibits maturation of dendritic cells; and increases osteoclastic bone-resorbing activity as well as osteoclast chemotaxis. Dysregulation of VEGF expression and signaling pathways therefore plays an important role in the pathogenesis and clinical features of hematologic malignancies, in particular multiple myeloma. Direct and indirect targeting of VEGF and its receptors therefore may provide a potent novel therapeutic approach to overcome resistance to therapies and thereby improve patient outcome.

The vascular endothelial growth factor family and its receptors

This article focuses on describing the biology of vascular endothelial growth factor (VEGF) and its receptors as well as the regulation of their expression. A thorough understanding of the VEGF system is paramount in optimizing antiangiogenic therapies as a component of antineoplastic regimens.

Platelet factor 4 disrupts the intracellular signalling cascade induced by vascular endothelial growth factor by both KDR dependent and independent mechanisms

The mechanism by which the CXC chemokine platelet factor 4 (PF-4) inhibits endothelial cell proliferation is unclear. The heparin-binding domains of PF-4 have been reported to prevent vascular endothelial growth factor 165 (VEGF(165)) and fibroblast growth factor 2 (FGF2) from interacting with their receptors. However, other studies have suggested that PF-4 acts via heparin-binding independent interactions. Here, we compared the effects of PF-4 on the signalling events involved in the proliferation induced by VEGF(165), which binds heparin, and by VEGF(121), which does not. Activation of the VEGF receptor, KDR, and phospholipase Cgamma (PLCgamma) was unaffected in conditions in which PF-4 inhibited VEGF(121)-induced DNA synthesis. In contrast, VEGF(165)-induced phosphorylation of KDR and PLCgamma was partially inhibited by PF-4. These observations are consistent with PF-4 affecting the binding of VEGF(165), but not that of VEGF(121), to KDR. PF-4 also strongly inhibited the VEGF(165)- and VEGF(121)-induced mitogen-activated protein (MAP) kinase signalling pathways comprising Raf1, MEK1/2 and ERK1/2: for VEGF(165) it interacts directly or upstream from Raf1; for VEGF(121), it acts downstream from PLCgamma. Finally, the mechanism by which PF-4 may inhibit the endothelial cell proliferation induced by both VEGF(121) and VEGF(165), involving disruption of the MAP kinase signalling pathway downstream from KDR did not seem to involve CXCR3B activation.

p38 MAPK mediates gamma-irradiation-induced endothelial cell apoptosis, and vascular endothelial growth factor protects endothelial cells through the phosphoinositide 3-kinase-Akt-Bcl-2 pathway

Therapeutic radiation is widely used in cancer treatments. The success of radiation therapy depends not only on the radiosensitivity of tumor cells but also on the radiosensitivity of endothelial cells lining the tumor vasculature. Vascular endothelial growth factor (VEGF) plays a critical role in protecting endothelial cells against a number of antitumor agents including ionizing radiation. Strategies designed to overcome the survival advantage afforded to endothelial cells by VEGF might aid in enhancing the efficacy of radiation therapy. In this report we examined the signaling cascade(s) involved in VEGF-mediated protection of endothelial cells against gamma-irradiation. gamma-Irradiation-induced apoptosis of human dermal microvascular endothelial cells (HDMECs) was predominantly mediated through the p38 MAPK pathway as an inhibitor of p38 MAPK (PD169316), and dominant negative mutants of p38 MAPK could significantly enhance HDMEC survival against gamma-irradiation. Inhibition of the PI3K and MAPK pathways markedly up-regulated gamma-irradiation-mediated p38 MAPK activation resulting in enhanced HDMEC apoptosis. In contrast, VEGF-treated HDMECs were protected from gamma-irradiation-induced apoptosis predominantly through the PI3K/Akt pathway. Bcl-2 expression was markedly elevated in VEGF-treated HDMECs, and it was significantly inhibited by the PI3K inhibitor LY294002. HDMECs exposed to irradiation showed a significant decrease in Bcl-2 expression. In contrast, VEGF-stimulated HDMECs, when irradiated, maintained higher levels of Bcl-2 expression. Taken together our results suggest that gamma-irradiation induces endothelial cell apoptosis predominantly via the activation of p38 MAPK, and VEGF protects endothelial cells against gamma-irradiation predominantly via the PI3K-Akt-Bcl-2 signaling pathway.

Decay-accelerating factor induction on vascular endothelium by vascular endothelial growth factor (VEGF) is mediated via a VEGF receptor-2 (VEGF-R2)- and protein kinase C-alpha/epsilon (PKCalpha/epsilon)-dependent cytoprotective signaling pathway and is inhibited by cyclosporin A

Decay-accelerating factor (DAF), a membrane-bound complement regulatory protein, is up-regulated on endothelial cells (ECs) following treatment with vascular endothelial growth factor (VEGF), providing enhanced protection from complement-mediated injury. We explored the signaling pathways involved in this response. Incubation of human umbilical vein ECs with VEGF induced a 3-fold increase in DAF expression. Inhibition by flk-1 kinase inhibitor SU1498 and failure of placental growth factor (PlGF) to up-regulate DAF confirmed the role of VEGF-R2. The response was also blocked by pretreatment with phospholipase C-gamma (PLCgamma) inhibitor U71322 and protein kinase C (PKC) antagonist GF109203X. In contrast, no effect was seen with nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine (l-NMMA). Use of PKC agonists and isozyme-specific pseudosubstrate peptide antagonists suggested a role for PKCalpha and -epsilon in VEGF-mediated DAF up-regulation. This was confirmed by transfection of ECs with PKCalpha and -epsilon dominant-negative constructs, which in combination completely abrogated induction of DAF by VEGF. In contrast, LY290042, a phosphoinositide 3-kinase (PI3K) inhibitor, significantly augmented DAF expression, suggesting a negative regulatory role for phosphoinositide 3-kinase. The widely used immunosuppressive drug cyclosporin A (CsA) inhibited DAF induction by VEGF in a dose-dependent manner. The VEGF-induced DAF expression was functionally effective, significantly reducing complement-mediated EC lysis, and this cytoprotective effect was reversed by CsA. These data provide evidence for a VEGF-R2-, phospholipase C-gamma-, and PKCalpha/epsilon-mediated cytoprotective pathway in ECs. This may represent an important mechanism for the maintenance of vascular integrity during chronic inflammation involving complement activation. Moreover, inhibition of this pathway by CsA may play a role in CsA-mediated vascular injury.

Selective inhibition of vascular endothelial growth factor receptor-2 (VEGFR-2) identifies a central role for VEGFR-2 in human aortic endothelial cell responses to VEGF

Vascular endothelial growth factor receptors (VEGFR) are considered essential for angiogenesis. The VEGFR-family proteins consist of VEGFR-1/Flt-1, VEGFR-2/KDR/Flk-1, and VEGFR-3/Flt-4. Among these, VEGFR-2 is thought to be principally responsible for angiogenesis. However, the precise role of VEGFRs1-3 in endothelial cell biology and angiogenesis remains unclear due in part to the lack of VEGFR-specific inhibitors. We used the newly described, highly selective anilinoquinazoline inhibitor of VEGFR-2 tyrosine kinase, ZM323881 (5-[[7-(benzyloxy) quinazolin-4-yl]amino]-4-fluoro-2-methylphenol), to explore the role of VEGFR-2 in endothelial cell function. Consistent with its reported effects on VEGFR-2 [IC(50) < 2 nM], ZM323881 inhibited activation of VEGFR-2, but not of VEGFR-1, epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), or hepatocyte growth factor (HGF) receptor. We studied the effects of VEGF on human aortic endothelial cells (HAECs), which express VEGFR-1 and VEGFR-2, but not VEGFR-3, in the absence or presence of ZM323881. Inhibition of VEGFR-2 blocked activation of extracellular regulated-kinase, p38, Akt, and endothelial nitric oxide synthetase (eNOS) by VEGF, but did not inhibit p38 activation by the VEGFR-1-specific ligand, placental growth factor (PIGF). Inhibition of VEGFR-2 also perturbed VEGF-induced membrane extension, cell migration, and tube formation by HAECs. Vascular endothelial growth factor receptor-2 inhibition also reversed VEGF-stimulated phosphorylation of CrkII and its Src homology 2 (SH2)-binding protein p130Cas, which are known to play a pivotal role in regulating endothelial cell migration. Inhibition of VEGFR-2 thus blocked all VEGF-induced endothelial cellular responses tested, supporting that the catalytic activity of VEGFR-2 is critical for VEGF signaling and/or that VEGFR-2 may function in a heterodimer with VEGFR-1 in human vascular endothelial cells.

Vascular endothelial growth factor receptor-1 and receptor-2 initiate a phosphatidylinositide 3-kinase–dependent clonogenic response in acute myeloid leukemia cells

OBJECTIVE

Vascular endothelial growth factor (VEGF) interacts with two high-affinity receptor tyrosine kinases (RTK) on vascular endothelium to initiate complementary but disparate biologic responses. We previously reported that acute myeloid leukemia (AML) cells express VEGF and one or both VEGF-A receptors, Flt-1 (VEGFR-1) and KDR (VEGFR-2). To evaluate receptor-selective trophic response to VEGF-A in AML cells, we investigated receptor-specific ligand activation responsible for VEGF-initiated clonogenic response.

MATERIALS AND METHODS

Using KG1 (VEGFR-1+/VEGFR-2+) and HL60 (VEGFR-1+) cells with differential VEGF receptor display, we investigated ligand-induced clonogenic response and receptor-initiated signaling after stimulation with VEGF-A, the VEGFR-1 selective ligand placental growth factor (PlGF), or receptor-specific antibody agonists.

RESULTS

Recombinant human (rhu)-VEGF increased S-phase fraction and stimulated colony formation in both KG1 and HL60 cells. Ligation of VEGFR-1 or VEGFR-2 with receptor-specific antibody agonists triggered equivalent and concentration-dependent stimulation of colony recovery in KG1 cells, whereas clonogenic response in HL60 cells was restricted to VEGFR-1 activation by antibody or PlGF. In serum-deprived KG1 and HL60 cells, rhu-VEGF stimulated rapid and sustained phosphorylation of Akt/PKB that was inhibited by the phosphatidyl inositol 3-kinase (PI3-K) kinase inhibitor wortmannin. Preincubation with wortmannin inhibited VEGF-induced colony formation in a concentration-dependent fashion. rhu-VEGF-induced clonogenic response and Akt phosphorylation was abolished by the VEGF-RTK inhibitor SU-5416 at concentrations greater than 10 microM, whereas MEK inhibition by PD98059 (1 and 10 microM) was ineffective. In vivo suppression of Akt phosphorylation was confirmed in myeloblast lysates from three patients with advanced myeloid malignancies treated with SU5416.

CONCLUSION

These data indicate that VEGF interaction with either VEGFR-1 or VEGFR-2 initiates a clonogenic response in AML cells that is PI3-kinase dependent. RTK inhibitors with broad specificity for angiogenic receptors represent novel therapeutics that merit further clinical investigation in AML.

Suppression of angiogenesis by the plant alkaloid, sanguinarine

Sanguinarine is a benzophenanthridine alkaloid derived from the root of Sanguinaria canadensis. Its principal pharmacologic use is in dental products where it has antibacterial, antifungal, and anti-inflammatory activities that reduce gingival inflammation and supragingival plaque formation. Angiogenesis is indispensable for inflammation, and most angiogenesis is dependent on vascular endothelial growth factor (VEGF). However, the effect of sanguinarine on angiogenesis is not known. In the present study, we examined the effect of sanguinarine on VEGF-induced angiogenesis in vitro and in vivo. Interestingly, sanguinarine markedly suppressed VEGF-induced endothelial cell migration, sprouting, and survival in vitro in a dose-dependent manner at nanomolar concentrations. Furthermore, sanguinarine potently suppressed blood vessel formation in vivo in mouse Matrigel plugs and the chorioallantoic membrane of chick embryos. Our biochemical assays indicated that sanguinarine strongly suppressed basal and VEGF-induced Akt phosphorylation, while it did not produce any changes in VEGF-induced activation of ERK1/2 and PLCgamma1. Therefore, we conclude that sanguinarine is a potent antiangiogenic natural product, and its mode of action could involve the blocking of VEGF-induced Akt activation. Thus, in addition to antibacterial, antifungal, and anti-inflammatory activities, sanguinarine has a novel antiangiogenic role.

Vascular endothelial growth factor stimulates differential signaling pathways in in vivo microcirculation

Vascular endothelial growth factor (VEGF) induces mild vasodilation and strong increases in microvascular permeability. Using intravital microscopy and digital integrated optical intensity image analysis, we tested, in the hamster cheek pouch microcirculation, the hypothesis that differential signaling pathways in arterioles and venules represent an in vivo regulatory mechanism in the control of vascular diameter and permeability. The experimental design involved blocking specific signaling molecules and simultaneously assessing VEGF-induced changes in arteriolar diameter and microvascular transport of FITC-Dextran 150. Inhibition of Akt [indirectly via phosphatidylinositol 3-kinase with LY-294002 or wortmannin] or PKC (with bisindolylmaleimide) reduced VEGF-induced hyperpermeability. However, phosphatidylinositol 3-kinase/Akt inhibition enhanced the early phase and attenuated the late phase of VEGF-induced vasodilation, whereas blocking PKC had no effect. Inhibition of extracellular signal-regulated kinase (ERK)-1/2 (with PD-98059 or AG-126) also reduced VEGF-induced hyperpermeability but did not block VEGF-induced vasodilation. Blockade of endothelial nitric oxide synthase (with N(omega)-monomethyl-l-arginine) inhibited VEGF-induced changes in both permeability and diameter. Furthermore, immunofluorescence studies with human umbilical vein endothelial cells revealed that bisindolylmaleimide, PD-98059, and l-NMMA attenuate VEGF-induced reorganization of vascular endothelial cadherin. Our data demonstrate that 1) endothelial nitric oxide synthase is a common convergence pathway for VEGF-induced changes in arteriolar diameter and microvascular permeability; 2) PKC and ERK-1/2 do not play a major role in VEGF-induced vasodilation in the hamster cheek pouch microcirculation; and 3) Akt, PKC, and ERK-1/2 are elements of the signaling cascade that regulates VEGF-stimulated microvascular hyperpermeability. Our data provide evidence for differential signaling as a regulatory step in VEGF-stimulated microvascular dynamics.

Stem cell factor/c-kit signaling promotes the survival, migration, and capillary tube formation of human umbilical vein endothelial cells

c-kit receptor tyrosine kinase is a marker of progenitor cells, which differentiate into blood and/or vascular endothelial cells, and has an important role in the amplification/mobilization of progenitor cells. c-kit is expressed in mature endothelial cells, but its role there is unclear. Stem cell factor, a c-kit ligand, dose-dependently promoted survival, migration, and capillary tube formation of human umbilical vein endothelial cells. These effects mimicked those of vascular endothelial growth factor, except that stem cell factor did not sufficiently support proliferation of these cells. After exposing cells to this factor, Akt, Erk1/2, and c-kit were immediately (</=5 min) and dose-dependently tyrosinephosphorylated. STI-571, a c-kit inhibitor, dose-dependently attenuated these phosphorylations and inhibited stem cell factor-promoted survival and capillary tube formation over the same dose range. Wortmannin and LY294002, inhibitors of phosphoinositide 3-kinase, and PD98059, an inhibitor of MEK, abrogated survival and capillary tube formation, indicating that Akt and Erk1/2 should promote survival and capillary tube formation of these endothelial cells at a locus downstream to stem cell factor/c-kit signaling. Akt was more strongly phosphorylated, whereas Erk1/2 and p38 were more weakly phosphorylated with stem cell factor than with vascular endothelial growth factor. Phospholipase Cgamma was phosphorylated only with the latter, indicating that stem cell factor/c-kit signaling is somewhat different.

Shear stress and VEGF activate IKK via the Flk-1/Cbl/Akt signaling pathway

Vascular endothelial cells are continuously exposed to mechanical (e.g., shear stress) and chemical (e.g., growth factors) stimuli. It is important to elucidate the mechanisms by which cells perceive and integrate these different stimuli to regulate the downstream signaling pathways. We ( 50 ) have previously reported the shear-induced interplay between two membrane receptors, integrins and Flk-1. In the present study, we investigated the molecular mechanisms regulating the downstream IκB kinase (IKK) pathway in response to shear stress and VEGF. Both shear stress and VEGF induced a transient increase of IKK activity. These effects were inhibited by SU-1498, a specific Flk-1 inhibitor, and by a negative mutant of Casitas B-lineage lymphoma (Cbl) with tyrosine-to-phenylalanine mutations at sites 700, 731, and 774 (Cblnm). Because Flk-1 and Cbl form a complex upon shearing or VEGF applications ( 50 ), these results suggest that shear stress and VEGF activate IKK via the receptor Flk-1 and its recruitment of the adapter protein Cbl. The inhibition of the shear- and VEGF-induced IKK activities by a negative mutant of Akt indicates that Akt acts upstream to IKK in response to shear stress and VEGF. Furthermore, SU-1498 and Cbl-nmabolished the shear- and VEGF-induced Akt activity, indicating that Akt acts at a level downstream to Flk-1 and Cbl. Therefore, our results indicate that the signaling events induced by shear stress and VEGF converge at the membrane receptor Flk-1 and that these stimuli share the Flk-1/Cbl/Akt pathway in activating IKK activation.

Human immunodeficiency virus type 1 tat-mediated cytotoxicity of human brain microvascular endothelial cells

Human immunodeficiency virus (HIV)-1 infection is often complicated with neurologic disorders, but the pathogenesis of HIV-1 encephalopathy is incompletely understood. Tat (HIV-1 transactivator protein) is released from HIV-1-infected cells and has been detected in the sera and cerebrospinal fluid of HIV-1-infected patients. Tat, along with increased inflammatory cytokines such as interferon-gamma (IFN-gamma), have been implicated in the pathogenesis of HIV-1-associated blood-brain barrier dysfunction. The present study examined the effects of Tat and IFN-gamma on human brain microvascular endothelial cells (HBMECs), which constitute the blood-brain barrier. Tat produced cytotoxicity of HBMECs, but required IFN-gamma. IFN-gamma treatment of HBMECs up-regulates vascular endothelial growth factor receptor-2 (VEGFR2/KDR), which is known to be the receptor for Tat. Tat activated KDR in the presence of IFN-gamma, and Tat-mediated cytopathic changes involve its interaction with KDR and phosphatidylinositol 3-kinase (PI3K). Further understanding and characterization of Tat-HBMEC interactions should help us understand HIV-1 neuropathogenesis and develop strategies to prevent HIV-1 encephalopathy.

Activated Ras induces a proangiogenic phenotype in primary endothelial cells

Angiogenic factors alter endothelial cell phenotype to promote the formation of new blood vessels, a process critical for a number of normal and pathological conditions. Ras is required for the induction of the angiogenic phenotype in response to vascular endothelial growth factor (VEGF). However, VEGF generates many signals, several of which are not dependent upon Ras activation. Our current study investigates the sufficiency of Ras activation for driving angiogenic responses. An activated Ras(V12) mutant induces prominent membrane ruffling, branching morphogenesis on three-dimensional collagen, DNA synthesis, and cell migration in primary endothelial cells. An upregulation of PI3K/AKT, Erk, and Jnk signaling pathways accompany these phenotypic changes. The inhibition of Erk blocked cell proliferation, but only partially attenuated migration. Blocking PI3K had no effect on DNA synthesis, but caused a modest reduction in cell migration. Lastly, Jnk played a significant role in both the proliferation and migration response. These effects of Ras(V12) are not the result of increased autocrine secretion of VEGF. These data suggest that the acquisition of activating Ras mutations can lead to a proangiogenic conversion in the phenotype of primary endothelial cells. Furthermore, these data raise the possibility that chronic Ras activation in endothelial cells may be sufficient to promote angiogenesis and the development of vascular anomalies.

Extracellular matrix regulates endothelial functions through interaction of VEGFR-3 and integrin ?5?1

Endothelium extracellular matrix (ECM) interactions can provide distinct spatial and molecular signals which control cellular proliferation, migration, and differentiation. Here, we investigated the role of fibronectin (FN), a major ECM protein, on the functions of lymphatic endothelial cells (LEC). We observed that FN, the ligand for integrin alpha5beta1, selectively promoted the growth of LEC as compared with vitronectin (VN) in the presence of the ligand for vascular endothelial growth factor receptor 3 [VEGFR-3 (VEGF-C156S)]. Upon investigating the mechanisms whereby ECM components regulate VEGFR-3 signaling, we found that FN transactivated VEGFR-3 and significantly enhanced the phosphorylation of VEGFR-3 induced by VEGF-C156S as compared to VN. An enhanced association of the integrin subunit alpha5 or beta1 with VEGFR-3, after stimulation with VEGF-C156S, was observed by co-immunoprecipitation. While blockade of integrin alpha5beta1 inhibited the VEGF-C156S-induced phosphorylation of VEGFR-3, no similar effect was obtained by blocking integrin alphavbeta3. FN also protected the endothelial cells from serum deprivation-induced apoptosis. Moreover, while the specific PI3 kinase inhibitor, LY294002, abolished this FN-mediated cell survival, the MAPK kinase inhibitor, PD98059, had no significant effect. Furthermore, a dominant-negative mutant of VEGFR-3 (G857R) reduced VEGF-C156S or FN-mediated cell survival, as well as the activities of PI3 kinase/Akt. Our results indicate that integrin alpha5beta1 participates in the activation of both VEGFR-3 and its downstream PI3 kinase/Akt signaling pathway, which is essential for FN-mediated lymphatic endothelial cell survival and proliferation.

Inactivation of the PTEN gene protein product is associated with the invasiveness and metastasis, but not angiogenesis, of breast cancer

PTEN is a novel tumor-suppressor gene located on chromosomal band 10q23. Loss of PTEN function has been implicated in the progression of several types of cancer, but the correlation between loss of PTEN expression and advanced carcinomas is not well established. The capacity for angiogenesis of a tumor is known to play a very important role in growth and metastasis, and there have been reports that PTEN relates to angiogenesis. In the present study, formalin-fixed and paraffin embedded tissues from 101 patients with breast carcinomas, including 88 cases of invasive ductal carcinomas and 13 cases of ductal carcinoma in situ (DCIS), were evaluated by immunohistochemical methods for the expression of PTEN and vascular endothelial growth factor (VEGF), as well as microvessel density (MVD). The results were compared with the clinicopathologic parameters. There was no loss of PTEN expression in any of the cases of DCIS, but 28 (32%) of the 88 invasive cases did not express PTEN. Loss of PTEN expression was associated with lymph node metastasis (P = 0.03), but did not correlate with tumor size, tumor grade, MVD or recurrence. VEGF expression significantly correlated with lymph node metastasis in invasive ductal carcinoma (P = 0.01). There was no correlation between the expression of PTEN and that of VEGF (P = 0.63). The present study suggests that loss of PTEN expression is common and correlates with tumor progression and lymph node metastasis in breast carcinoma. The relationship between loss of PTEN and progression of breast cancer may not be explained by modulation of angiogenesis.

Activation of receptor-mediated angiogenesis and signaling pathways after VEGF administration in fetal rat CNS explants

The angiogenic role of vascular endothelial growth factor (VEGF) receptors, flk-1 and flt-1, and their downstream signaling pathways, MAPK/ERK and PI-3 kinase, were examined in a fetal rat cortical explant model after exposure to exogenous VEGF. Treatment with VEGF resulted in substantial neovascularization characterized by increased vascular flk-1 receptor expression, whereas flt-1 receptor protein expression was absent. The specific role of flk-1 receptors in the angiogenic process was confirmed by the addition of antisense oligonucleotides (AS-ODNs) to flk-1, which blocked angiogenesis, whereas AS-ODNs to flt-1 had no effect. These results were further supported by the finding that specific chemical inhibition of flk-1 receptors caused disruption of the angiogenic response, whereas inhibition of the flt-1 receptors had no effect. Application of either MAPK/ERK or PI-3 kinase pathway inhibitors disrupted VEGF-induced angiogenesis, thereby indicating that both signaling pathways mediate this process. Thus VEGF binding to the endothelial flk-1 receptor activates the MAPK/ERK and PI-3 kinase pathways, resulting in neoangiogenic events. Of interest is the fact that although VEGF is regarded as a vascular permeability factor, its application to nascent cortical tissue caused an increase in a key physiologic protein of the blood-brain barrier function, glucose transporter-1, suggesting that the cytokine may have a role in blood-brain barrier development.

Vascular endothelial growth factor up-regulates expression of receptor activator of NF-kappa B (RANK) in endothelial cells. Concomitant increase of angiogenic responses to RANK ligand

Vascular endothelial growth factor (VEGF) is known as a key regulator of angiogenesis during endochondral bone formation. Recently, we demonstrated that TNF-related activation-induced cytokine (TRANCE or RANKL), which is essential for bone remodeling, also had an angiogenic activity. Here we report that VEGF up-regulates expression of receptor activator of NF-kappa B (RANK) and increases angiogenic responses of endothelial cells to TRANCE. Treatment of human umbilical vein endothelial cells (HUVECs) with VEGF increased both RANK mRNA and surface protein expression. Although placenta growth factor specific to VEGF receptor-1 had no significant effect on RANK expression, inhibition of downstream signaling molecules of the VEGF receptor-2 (Flk-1/KDR) such as Src, phospholipase C, protein kinase C, and phosphatidylinositol 3'-kinase suppressed VEGF-stimulated RANK expression in HUVECs. Moreover, the MEK inhibitor PD98059 or expression of dominant negative MEK1 inhibited induction of RANK by VEGF but not the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM). VEGF potentiated TRANCE-induced ERK activation and tube formation via RANK up-regulation in HUVECs. Together, these results show that VEGF enhances RANK expression in endothelial cells through Flk-1/KDR-protein kinase C-ERK signaling pathway, suggesting that VEGF plays an important role in modulating the angiogenic action of TRANCE under physiological or pathological conditions.

The role of c-Fes in vascular endothelial growth factor-A-mediated signaling by endothelial cells

c-Fes plays pivotal roles in angiogenic cellular responses of endothelial cells. Here we examined the role of c-Fes in vascular endothelial growth factor-A (VEGF-A)-mediated signaling pathways in endothelial cells. We introduced either wild-type or kinase-inactive c-Fes in porcine aortic endothelial (PAE) cell lines, which endogenously express VEGF receptor (VEGFR)-1, and PAE cells ectopically expressing VEGFR-2 (denoted KDR/PAE cells) and generated stable cell lines. VEGF-A induced autophosphorylation of c-Fes only in KDR/PAE cells, suggesting that VEGFR-2 was required for its activation. Expression of kinase-inactive c-Fes failed to demonstrate dominant negative effect on VEGF-A-induced chemotaxis and capillary morphogenesis. Phosphoinositide 3-kinase (PI3-kinase) was activated in KDR/PAE cells and c-Fes contributed to this process in a kinase activity-dependent manner. However, VEGFR-2, insulin receptor substrate-1, and c-Src were also involved in VEGF-A-induced activation of PI3-kinase, resulting in the compensation in cells expressing kinase-inactive c-Fes. Interestingly, overexpression of wild-type c-Fes in PAE cells induced VEGF-A-independent capillary morphogenesis. Considered collectively, VEGF-A activated PI3-kinase partly through c-Fes and increase in c-Fes kinase activity enhanced capillary morphogenesis by yet unknown signaling pathways.

Analysis of VEGF-responsive genes involved in the activation of endothelial cells

BACKGROUND

Identification of the genes and pathways associated with the activation of endothelial cells (ECs) could help uncover the role of ECs in wound healing, vascular permeability, blood brain barrier function, angiogenesis, diabetic retinopathy, atherosclerosis, psoriasis, and growth of solid tumors.

DESIGN

Herein, we embedded ECs in 3D type I collagen gel, left unstimulated or stimulated with VEGF165, and subjected to suppression subtractive hybridization followed by differential display (SSHDD). Gene fragments obtained from SSHDD were subjected to DNA sequence analysis. Database search with nucleotide sequence were performed using the BLAST algorithm and expression of candidate genes determined by northern blot analysis.

RESULTS

A total of approximately 32 cDNA fragments, including known regulators of angiogenesis, and a set of genes that were not reported to be associated with activation of ECs and angiogenesis previously were identified. We confirmed the mRNA expression of KDR, alpha2 integrin, Stanniocalcin, including a set of 11 candidate genes. Western immunoblotting results indicated that KDR, alpha2 integrin, MMP-1, MMP-2, and VE-cadherin genes were indeed active genes.

CONCLUSION

We have identified a set of 11 VEGF-responsive endothelial cell candidate genes. Their expression in endothelial cell is confirmed by northern blot analyses. This preliminary report forms as a foundation for functional studies to be performed to reveal their roles in EC activation and pathophysiological events associated with the vasculature including tumor growth.

p38 MAPK inhibition is critically involved in VEGFR-2-mediated endothelial cell survival

Vascular endothelial growth factor (VEGF) promotes vasculogenesis, arteriogenesis, and angiogenesis by stimulating proliferation, migration, and cell survival of endothelial cells. VEGF mediates its actions through activation of two receptor tyrosine kinases, VEGFR-1 and VEGFR-2. Serum starvation led to apoptosis of human umbilical vein endothelial cells (HUVEC), which was accompanied by activation of p38 MAPK and caspase-3. Stimulation of both VEGF-receptors resulted in a considerable decrease of apoptosis, which was associated with the inhibition of p38 MAPK and caspase-3 activity. Selective stimulation of VEGFR-2 showed similar results, whereas the isolated activation of VEGFR-1 was without effect. Incubation of HUVEC with SB203580, a p38 MAPK inhibitor, resulted in similar effects as VEGF-stimulation: p38 MAPK and caspase-3 enzyme activity were reduced and apoptosis was prevented. These data indicate that activation of VEGFR-2 prevents endothelial cell apoptosis by inhibiting p38 MAPK phosphorylation and thus, reducing caspase-3 activity.

Vascular endothelial growth factor receptor-2 induces survival of hematopoietic progenitor cells

Vascular endothelial growth factor (VEGF) and its receptors play an essential role in the formation and maintenance of the hematopoietic and vascular compartments. The VEGF receptor-2 (VEGFR-2) is expressed on a population of hematopoietic cells, although its role in hematopoiesis is still unclear. In this report, we have utilized a strategy to selectively activate VEGFR-2 and study its effects in primary bone marrow cells. We found that VEGFR-2 can maintain the hematopoietic progenitor population in mouse bone marrow cultured in the absence of exogenous cytokines. Maintenance of the hematopoietic progenitor population is due to increased cell survival with minimal effect on proliferation. Progenitor survival is mainly mediated by activation of the phosphatidylinositol 3'-kinase/Akt pathway. Although VEGFR-2 also activated Erk1/2 mitogen-activated protein kinase, it did not induce cell proliferation, and blockade of this pathway only partially decreased VEGFR-2-mediated survival of hematopoietic progenitors. Thus, the role of VEGFR-2 in hematopoiesis is likely to maintain survival of hematopoietic progenitors through the activation of antiapoptotic pathways.

Antiangiogenic effect of KR-31372 by apoptosis via mediation of mitochondrial KATP channel opening and the phosphatase and tensin homolog deleted from chromosome 10 phosphorylation

Antiangiogenic action of (2R,3R,4S)-N"-cyano-N-(6-nitro-3,4-dihydro-hydroxy-2-methyl-2-dimethoxymethyl-2H-1-benzopyran-4yl)-N'-benzyl guanidine (KR-31372) was examined with its proapoptotic action in human umbilical vein endothelial cells (HUVECs) compared with diazoxide. KR-31372 as well as diazoxide significantly suppressed the neovascularization in mice induced by the Matrigel-containing recombinant human vascular endothelial growth factor (VEGF)165 in vivo and the basal tube formation of HUVECs in vitro with suppression of proliferation of HUVECs stimulated by VEGF165. KR-31372 and diazoxide enhanced DNA fragmentation associated with increase in phosphatase and tensin homolog deleted from chromosome 10 (PTEN) and decrease in serine/threonine kinase phosphorylation, which were accompanied by augmented Bax and cytochrome c release, and suppressed Bcl-2 in HUVECs. In the U87-MG cells, when transfected with expression vectors for sense PTEN, KR-31372 enhanced DNA fragmentation, but not in naive U87-MG cells. The suppression by KR-31372 and diazoxide of these variables was significantly antagonized by 5-hydroxydecanoic acid, a mitochondrial KATP channel blocker. Taken together, KR-31372 strongly inhibited angiogenesis in HUVECs by proapoptotic mechanism via mediation of 5-hydroxydecanoic acid-inhibitable mitochondrial KATP channel opening and PTEN phosphorylation.

Src and phosphatidylinositol 3-kinase mediate soluble E-selectin-induced angiogenesis

Angiogenesis plays an important role in a variety of pathophysiologic processes, including tumor growth and rheumatoid arthritis. We have previously shown that soluble E-selectin (sE-selectin) is an important angiogenic mediator. However, the mechanism by which sE-selectin mediates angiogenesis is still unknown. In this study, we show that sE-selectin is a potent mediator of human dermal microvascular endothelial cell (HMVEC) chemotaxis, which is predominantly mediated through the Src and the phosphatidylinositiol 3-kinase (PI3K) pathways. Further, sE-selectin induced a 2.2-fold increase in HMVEC tube formation in the Matrigel in vitro assay. HMVECs pretreated with the Src inhibitor (PP2) and the PI3K inhibitor (LY294002) or transfected with Src antisense oligonucleotides or Akt dominant-negative mutants significantly inhibited sE-selectin-mediated HMVEC tube formation. In contrast, HMVECs transfected with an extracellular signal-related kinase 1/2 (ERK1/2) mutant or pretreated with the mitogen-activated protein kinase (MAPK) inhibitor PD98059 failed to show sE-selectin-mediated HMVEC tube formation. Similarly, in the Matrigel-plug in vivo assay, sE-selectin induced a 2.2-fold increase in blood vessel formation, which was significantly inhibited by PP2 and LY294002 but not by PD98059. sE-selectin induced a marked increase in Src, ERK1/2, and PI3K phosphorylation. PI3K and ERK1/2 phosphorylation was significantly inhibited by PP2, thereby suggesting that both of these pathways may be activated via Src kinase. Even though the ERK1/2 pathway was activated by sE-selectin in HMVECs, it seems not to be essential for sE-selectin-mediated angiogenesis. Taken together, our data clearly show that sE-selectin-induced angiogenesis is predominantly mediated through the Src-PI3K pathway.

Hydrogen peroxide formation and actin filament reorganization by Cdc42 are essential for ethanol-induced in vitro angiogenesis

This report focuses on the identification of the molecular mechanisms of ethanol-induced in vitro angiogenesis. The manipulation of angiogenesis is an important therapeutic approach for the treatment of cancer, cardiovascular diseases, and chronic inflammation. Our results showed that ethanol stimulation altered the integrity of actin filaments and increased the formation of lamellipodia and filopodia in SVEC4-10 cells. Further experiments demonstrated that ethanol stimulation increased cell migration and invasion and induced in vitro angiogenesis in SVEC4-10 cells. Mechanistically, ethanol stimulation activated Cdc42 and produced H(2)O(2) a reactive oxygen species intermediate in SVEC4-10 cells. Measuring the time course of Cdc42 activation and H(2)O(2) production upon ethanol stimulation revealed that the Cdc42 activation and the increase of H(2)O(2) lasted more than 3 h, which indicates the mechanisms of the long duration effects of ethanol on the cells. Furthermore, either overexpression of a constitutive dominant negative Cdc42 or inhibition of H(2)O(2) production abrogated the effects of ethanol on SVEC4-10 cells, indicating that both the activation of Cdc42 and the production of H(2)O(2) are essential for the actions of ethanol. Interestingly, we also found that overexpression of a constitutive dominant positive Cdc42 itself was sufficient to produce H(2)O(2) and to induce in vitro angiogenesis. Taken together, our results suggest that ethanol stimulation can induce H(2)O(2) production through the activation of Cdc42, which results in reorganizing actin filaments and increasing cell motility and in vitro angiogenesis.

Regulation of cell-cell interactions by phosphatidic acid phosphatase 2b/VCIP

We identified vascular endothelial growth factor and type I collagen inducible protein (VCIP), also known as phosphatidic acid phosphatase 2b (PAP2b), in a functional assay of angiogenesis. VCIP/PAP2b exhibits an Arg-Gly-Asp (RGD) cell adhesion sequence. Immunoprecipitation and fluorescence-activated cell sorting analyses demonstrated that VCIP-RGD is exposed to the outside of the cell surface. Retroviral transduction of VCIP induced cell aggregation/cell- cell interactions, modestly increased p120 catenin expression and promoted activation of the Fak, Akt and GSK3beta protein kinases. Furthermore, expression of recombinant VCIP promoted adhesion, spreading and tyrosine phosphorylation of Fak, Shc, Cas and paxillin in endothelial cells. GST-VCIP-RGD, but not GST-VCIP-RGE, specifically interacted with a subset of integrins, and these interactions were effectively blocked by anti-alpha(v)beta(3) and anti-alpha(5)beta(1) integrin antibodies, and by PAP2b/VCIP-derived peptides. Interestingly, PAP2b/VCIP is expressed in close proximity to vascular endothelial growth factor, von Willebrand factor and alpha(v)beta(3) integrin in tumor vasculatures. These findings demonstrate an unexpected function of PAP2b/VCIP, and represent an important step towards understanding the molecular mechanisms by which PAP2b/VCIP-induced cell-cell interactions regulate specific intracellular signaling pathways.

Granulocyte-macrophage colony-stimulating factor signals for increased glucose transport via phosphatidylinositol 3-kinase- and hydrogen peroxide-dependent mechanisms

Granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulates cellular glucose uptake by decreasing the apparent K(m) for substrate transport through facilitative glucose transporters on the plasma membrane. Little is known about this signal transduction pathway and the role of the alpha subunit of the GM-CSF receptor (alpha GMR) in modulating transporter activity. We examined the function of phosphatidylinositol 3-kinase (PI 3-kinase) in GM-CSF-stimulated glucose uptake and found that PI 3-kinase inhibitors, wortmannin and LY294002, completely blocked the GM-CSF-dependent increase of glucose uptake in Xenopus oocytes expressing the low affinity alpha GMR and in human cells expressing the high affinity alpha beta GMR complex. We identified a Src homology 3 domain-binding motif in alpha GMR at residues 358-361 as a potential interaction site for the PI 3-kinase regulatory subunit, p85. Physical evidence for p85 binding to alpha GMR was obtained by co-immunoprecipitation with antibodies to alpha GMR and p85, and an alpha GMR mutant with alteration of the Src homology 3 binding domain lost the ability to bind p85. Experiments with a construct eliminating most of the intracellular portion of alpha GMR showed a 50% reduction in GM-CSF-stimulated glucose uptake with residual activity blocked by wortmannin. Searching for a proximally generated diffusible factor capable of activating PI 3-kinase, we identified hydrogen peroxide (H(2)O(2)), generated by ligand or antibody binding to alpha GMR, as the initiating factor. Catalase treatment abrogated GM-CSF- or anti-alpha GMR antibody-stimulated glucose uptake in alpha GMR-expressing oocytes, and H(2)O(2) activated PI 3-kinase and led to some stimulation of glucose uptake in uninjected oocytes. Human myeloid cell lines and primary explant human lymphocytes expressing high affinity GM-CSF receptors responded to alpha GMR antibody with increased glucose uptake. These results identify the early events in the stimulation of glucose uptake by GM-CSF as involving local H(2)O(2) generation and requiring PI 3-kinase activation. Our findings also provide a mechanistic explanation for signaling through the isolated alpha subunit of the GM-CSF receptor.

Angiogenic effects of interleukin 8 (CXCL8) in human intestinal microvascular endothelial cells are mediated by CXCR2

Angiogenesis plays a critical role in metastasis and tumor growth. Human tumors, including colorectal adenocarcinoma, secrete angiogenic factors, inducing proliferation and chemotaxis of microvascular endothelial cells, eventually leading to tumor neovascularization. The chemokine interleukin 8 (IL-8; CXCL8) exerts potent angiogenic properties on endothelial cells through interaction with its cognate receptors CXCR1 and CXCR2. As CXCR1 and CXCR2 expression is differentially regulated in tissue-specific endothelial cells and effects of IL-8 on intestinal endothelial cells are not defined, we characterized the potential IL-8-induced angiogenic mechanisms in primary cultures of human intestinal microvascular endothelial cells (HIMEC) and IL-8 receptor expression in human intestinal microvessels. CXCR1 and CXCR2 expression on HIMEC were defined using reverse transcriptase-PCR, immunohistochemistry, flow cytometry, and Western blot analysis. IL-8-induced downstream signaling events were assessed using immunoblot analysis and immunofluorescence. The angiogenic effects of IL-8 on HIMEC were determined using proliferation and chemotaxis assays. HIMEC responded to IL-8 with rapid stress fiber assembly, chemotaxis, enhanced proliferation, and phosphorylation of extracellular signal-regulated protein kinase 1/2 (ERK 1/2). HIMEC express CXCR2, but not CXCR1. Neutralizing antibodies to CXCR2 diminished IL-8-induced chemotaxis and stress fiber assembly. Specific inhibitors of ERK 1/2 and phosphoinositide 3-kinase abrogated endothelial tube formation and IL-8-induced chemotaxis in HIMEC. IL-8 elicits angiogenic responses in microvascular endothelial cells isolated from human intestine by engaging CXCR2. We confirmed tissue expression of CXCR2 in human intestinal microvessels. Supported by the notion that malignant colonic epithelial cells overexpress IL-8, CXCR2 blockade may be a novel target for anti-angiogenic therapy in colorectal adenocarcinoma.

The angiogenic regulator CD13/APN is a transcriptional target of Ras signaling pathways in endothelial morphogenesis

Angiogenesis, the formation of new blood vessels, is a critical step for tumor growth and metastasis and an integral component of the pathologic inflammatory response in arthritis and the proliferative retinopathies. The CD13/aminopeptidase N (CD13/APN) metalloprotease is an important regulator of angiogenesis where its expression on activated blood vessels is induced by angiogenic signals. Here, we show that cytokine induction of CD13/APN in endothelial cells is regulated by distinct Ras effector pathways involving Ras/mitogen-activated protein kinase (MAPK) or PI-3K. Signals transduced by activated Ras, Raf, and mitogen-induced extracellular kinase (MEK) stimulate transcription from the CD13/APN proximal promoter. Inhibition of these pathways and extracellular signal-regulated serine/threonine kinase (ERK-2) and PI-3K by expression of dominant-negative proteins or chemical inhibitors prevented induction of CD13/APN transcription in response to basic fibroblast growth factor (bFGF). We show that Ras-induced signal transduction is required for growth factor-induced angiogenesis, because inhibition of downstream mediators of Ras signaling (MEK or PI-3K) abrogated endothelial cell migration, invasion, and morphogenesis in vitro. Reintroduction of CD13/APN, a shared downstream target of these pathways, overrode the suppressive effect of these inhibitors and restored the function of endothelial cells in migration/invasion and capillary morphogenesis assays. Similarly, inhibition of MEK abrogated cell invasion and the formation of endothelial-lined capillaries in vivo, which was effectively rescued by addition of exogenous CD13/APN protein. These studies provide strong evidence that CD13/APN is an important target of Ras signaling in angiogenesis and is a limiting factor in angiogenic progression.

Src family kinases and nitric oxide production are required for hepatocyte growth factor-stimulated endothelial cell growth

Hepatocyte growth factor (HGF) is a potent mitogen for vascular endothelial cells (EC); however, signal transduction pathways for HGF-stimulated EC growth remain unclear. In the present study we investigated the role of Src family kinases and nitric oxide (NO) in HGF-stimulated EC growth. Human umbilical vein endothelial cells (HUVEC) were stimulated with HGF and NO was measured by an NOx analyzing HPLC system. Activation of ERK1/2 and p38 MAPK was assessed by Western blot. NO production in HUVEC increased 1.8-fold by HGF. A Src family kinases inhibitor PP1 inhibited HGF-stimulated NO production by 71%. HUVEC growth increased 1.9-fold in cell number by HGF. PP1 and Nitro-L-arginine methylester (L-NAME) inhibited HGF-stimulated HUVEC growth by 51 and by 71%. ERK1/2 and p38 MAPK were phosphorylated by HGF and a MEK inhibitor PD98059 and a p38 MAPK inhibitor SB203580 inhibited HGF-stimulated HUVEC growth by 66% and by 58%; however, HGF-induced phosphorylation of ERK1/2 and p38 MAPK was not inhibited by L-NAME, indicating that NO is not an upstream activator of ERK1/2 and p38 MAPK. These findings demonstrated that Src family kinases regulate HGF-stimulated NO production in HUVEC and that HGF stimulates HUVEC growth through NO-dependent and NO-independent pathways.

Vascular endothelial growth factor promotes proliferation of cortical neuron precursors by regulating E2F expression

Neurogenesis, or the production of new neurons, is regulated by physiological and pathological processes including aging, stress, and brain injury. Many mitogenic and trophic factors that regulate proliferation of nonneuronal cells are also involved in neurogenesis. These include vascular endothelial cell growth factor (VEGF), which stimulates the incorporation of bromodeoxyuridine (BrdU) into neuronal precursor cells in vitro and in the adult rat brain in vivo. Using BrdU labeling as an index of cell proliferation, we found that the in vitro neuroproliferative effect of VEGF was associated with up-regulation of E2F family transcription factors, cyclin D1, cyclin E, and cdc25. VEGF also increased nuclear expression of E2F1, E2F2, and E2F3, consistent with regulation of the G1/S phase transition of the cell cycle. The proliferative effect of VEGF was inhibited by the extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059, the phospholipase C (PLC) inhibitor U73122, the protein kinase C (PKC) inhibitor GF102390X, and the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin, indicating involvement of multiple signaling pathways. These findings help to provide a molecular basis for some of the recently identified neuronal effects of VEGF.

Regulation of Notch1 and Dll4 by vascular endothelial growth factor in arterial endothelial cells: implications for modulating arteriogenesis and angiogenesis

Notch and its ligands play critical roles in cell fate determination. Expression of Notch and ligand in vascular endothelium and defects in vascular phenotypes of targeted mutants in the Notch pathway have suggested a critical role for Notch signaling in vasculogenesis and angiogenesis. However, the angiogenic signaling that controls Notch and ligand gene expression is unknown. We show here that vascular endothelial growth factor (VEGF) but not basic fibroblast growth factor can induce gene expression of Notch1 and its ligand, Delta-like 4 (Dll4), in human arterial endothelial cells. The VEGF-induced specific signaling is mediated through VEGF receptors 1 and 2 and is transmitted via the phosphatidylinositol 3-kinase/Akt pathway but is independent of mitogen-activated protein kinase and Src tyrosine kinase. Constitutive activation of Notch signaling stabilizes network formation of endothelial cells on Matrigel and enhances formation of vessel-like structures in a three-dimensional angiogenesis model, whereas blocking Notch signaling can partially inhibit network formation. This study provides the first evidence for regulation of Notch/Delta gene expression by an angiogenic growth factor and insight into the critical role of Notch signaling in arteriogenesis and angiogenesis.

VEGF(165) promotes survival of leukemic cells by Hsp90-mediated induction of Bcl-2 expression and apoptosis inhibition

Similar to endothelial cells (ECs), vascular endothelial growth factor (VEGF) induces Bcl-2 expression on VEGF receptor-positive (VEGFR(+)) primary leukemias and cell lines, promoting survival. We investigated the molecular pathways activated by VEGF on such leukemias, by performing a gene expression analysis of VEGF-treated and untreated HL-60 leukemic cells. One gene to increase after VEGF stimulation was heat shock protein 90 (Hsp90). This was subsequently confirmed at the protein level, on primary leukemias and leukemic cell lines. VEGF increased the expression of Hsp90 by interacting with KDR and activating the mitogen-activated protein kinase cascade. In turn, Hsp90 modulated Bcl-2 expression, as shown by a complete blockage of VEGF-induced Bcl-2 expression and binding to Hsp90 by the Hsp90-specific inhibitor geldanamycin (GA). GA also blocked the VEGF-induced Hsp90 binding to APAF-1 on leukemic cells, a mechanism shown to inhibit apoptosis. Notably, VEGF blocked the proapoptotic effects of GA, correlating with its effects at the molecular level. Earlier, we showed that in some leukemias, a VEGF/KDR autocrine loop is essential for cell survival, whereas here we identified the molecular correlates for such an effect. We also demonstrate that the generation of a VEGF/VEGFR autocrine loop on VEGFR(+) cells such as ECs, also protected them from apoptosis. Infection of ECs with adenovirus-expressing VEGF resulted in elevated Hsp90 levels, increased Bcl-2 expression, and resistance to serum-free or GA-induced apoptosis. In summary, we demonstrate that Hsp90 mediates antiapoptotic and survival-promoting effects of VEGF, which may contribute to the survival advantage of VEGFR(+) cells such as subsets of leukemias.

PTEN modulates vascular endothelial growth factor-mediated signaling and angiogenic effects

Phosphatidylinositol 3-kinase is activated by vascular endothelial growth factor (VEGF), and many of the angiogenic cellular responses of VEGF are regulated by the lipid products of phosphatidylinositol 3-kinase. The tumor suppressor PTEN has been shown to down-regulate phosphatidylinositol 3-kinase signaling, yet the effects of PTEN on VEGF-mediated signaling and angiogenesis are unknown. Inhibition of endogenous PTEN in cultured endothelial cells by adenovirus-mediated overexpression of a dominant negative PTEN mutant (PTEN-C/S) enhanced VEGF-mediated Akt phosphorylation, and this effect correlated with decreases in caspase-3 cleavage, caspase-3 activity, and DNA degradation after induction of apoptosis with tumor necrosis factor-alpha. Overexpression of PTEN-C/S also enhanced VEGF-mediated endothelial cell proliferation and migration. In contrast, overexpression of wild-type PTEN inhibited the anti-apoptotic, proliferative, and chemotactic effects of VEGF. Moreover, PTEN-C/S increased the length of vascular sprouts in the rat aortic ring assay and modulated VEGF-mediated tube formation in an in vitro angiogenesis assay, whereas PTEN-wild type inhibited these effects. Taken together, these findings demonstrate that PTEN potently modulates VEGF-mediated signaling and function and that PTEN is a viable target in therapeutic approaches to promote or inhibit angiogenesis.

Identification of functional VEGF receptors on human platelets

Platelets secrete platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) upon stimulation. We have demonstrated that platelets have functionally active PDGF alpha-receptors, a transmembrane tyrosine kinase involved in negative feedback regulation. Here we demonstrate the presence of the related VEGF receptors fms-like tyrosine kinase-1 and kinase-insert domain region on human platelets. VEGF itself did not cause platelet aggregation. However, addition of exogenous VEGF to SFRLLN or thrombin-stimulated platelets potentiated platelet aggregation. Moreover, thrombin-induced phosphoinositide 3-kinase and mitogen-activated protein kinase activity were enhanced in the presence of VEGF.

Reactive oxygen species as downstream mediators of angiogenic signaling by vascular endothelial growth factor receptor-2/KDR

Recent evidence shows the involvement of reactive oxygen species (ROS) in the mitogenic cascade initiated by the tyrosine kinase receptors of several growth factor peptides. We have asked whether also the vascular endothelial growth factor (VEGF) utilizes ROS as messenger intermediates downstream of the VEGF receptor-2 (VEGFR-2)/KDR receptor given that the proliferation of endothelial cells during neoangiogenesis is physiologically regulated by oxygen and likely by its derivative species. In porcine aortic endothelial cells stably expressing human KDR, receptor activation by VEGF is followed by a rapid increase in the intracellular generation of hydrogen peroxide as revealed by the peroxide-sensitive probe dichlorofluorescein diacetate. Genetic and pharmacological studies suggest that such oxidant burst requires as upstream events the activation of phosphatidylinositol 3-kinase and the small GTPase Rac-1 and is likely initiated by lipoxygenases. Interestingly, ROS generation in response to VEGF is not blocked but rather potentiated by endothelial nitric-oxide synthase inhibitors diphenyleneiodonium and N(G)methyl-l-arginine, ruling out the possibility of nitric oxide being the oxidant species here detected in VEGF-stimulated cells. Inhibition of KDR-dependent generation of ROS attenuates early signaling events including receptor autophosphorylation and binding to a phospholipase C-gamma-glutathione S-transferase fusion protein. Moreover, catalase, the lipoxygenase inhibitor nordihydroguaiaretic acid, the synthetic ROS scavenger EUK-134, and phosphatidylinositol 3-kinase inhibitor wortmannin all reduce ERK phosphorylation in response to VEGF, and antioxidants prevent VEGF-dependent mitogenesis. Finally, cell culture and stimulation in a nearly anoxic environment mimic the effect of ROS scavenger on receptor and ERK phosphorylation, reinforcing the idea that ROS are necessary components of the mitogenic signaling cascade initiated by KDR. These data identify ROS as a new class of intracellular angiogenic mediators and may represent a potential premise for new antioxidant-based antiangiogenic therapies.

Rapid transactivation of the vascular endothelial growth factor receptor KDR/Flk-1 by the bradykinin B2 receptor contributes to endothelial nitric-oxide synthase activation in cardiac capillary endothelial cells

Bradykinin (BK) and vascular endothelial growth factor (VEGF)-165 stimulate vasodilatation, microvascular permeability, and angiogenesis via the activation of the B2-type and KDR/Flk-1 receptors. To delineate the signal transduction pathways distal to the receptor activation in microvascular permeability, we compared their effects on two downstream targets, i.e. endothelial nitric-oxide (NO) synthase (eNOS) and F-actin, in primary cultures of cardiac capillary endothelial cells. The two mediators induced a similar cytoskeletal reorganization and both the translocation and activation of eNOS, leading to NO release within the first minutes of cell exposure. At the same time, BK produced the tyrosine phosphorylation and internalization of KDR/Flk-1 as did VEGF itself. This transactivation was blocked by the selective inhibitor of VEGF receptor tyrosine kinase activity but not by inhibitors of epidermal growth factor receptor or protein kinase C activity. The selective inhibitor of VEGF receptor tyrosine kinase activity totally prevented the effects of VEGF but only partially inhibited NO release induced by BK without affecting the concomitant cytoskeletal reorganization. Thus, BK transactivated KDR/Flk-1 through an intrinsic kinase activity of KDR/Flk-1, resulting in a further eNOS activation in endothelial cells. This represents a novel mechanism whereby a G protein-coupled receptor activates a receptor tyrosine kinase to generate biological response.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Selective induction of neuropilin-1 by vascular endothelial growth factor (VEGF): a mechanism contributing to VEGF-induced angiogenesis

Neuropilin (NRP) 1, previously identified as a neuronal receptor that mediates repulsive growth cone guidance, has been shown recently to function also in endothelial cells as an isoform-specific receptor for vascular endothelial growth factor (VEGF)(165) and as a coreceptor in vitro of VEGF receptor 2. However, its potential role in pathologic angiogenesis remains unknown. In the present study, we first show that VEGF selectively up-regulates NRP1 but not NRP2 via the VEGF receptor 2-dependent pathway. By NRP1 binding analysis, we showed that its induction by VEGF accompanies functional receptor expression. Endothelial proliferation stimulated by VEGF(165) was inhibited significantly by antibody perturbation of NRP1. In a murine model of VEGF-dependent angioproliferative retinopathy, intense NRP1 mRNA expression was observed in the newly formed vessels. Furthermore, selective NRP1 inhibition in this model suppressed neovascular formation substantially. These results suggest that VEGF cannot only activate endothelial cells directly but also can contribute to robust angiogenesis in vivo by a mechanism that involves up-regulation of its cognate receptor expression.

Angiopoietin-1 negatively regulates expression and activity of tissue factor in endothelial cells

Normally, tissue factor (TF) is not expressed on the surface of endothelial cells, but its expression can be induced by vascular endothelial growth factor (VEGF) and tumor necrosis factor (TNF)-a. However, the signaling pathway(s) affecting this induction is unknown. Using human umbilical vein endothelial cells, we found that inhibitors of guanine-cytosine-rich DNA binding protein and nuclear factor (NF)-kB suppressed VEGF- and TNF-a-induced expression and activity of TF. However, unexpectedly, phosphatidylinositol (PI) 3'-kinase inhibitor enhanced the VEGF- and TNF-a-induced expression and activity of TF. Angiopoietin-1 (Ang1), a strong activator of intracellular PI 3'-kinase/Akt, inhibited the induction of TF by VEGF and TNF-a, whereas Ang1 itself did not produce any significant effect on TF. Selective activation (or inactivation) of PI 3'-kinase/Akt by using adenoviral transfer reduced (or enhanced) TNF-a-induced expression of TF mRNA and protein, regardless of Ang1 treatment. From these results, we conclude that Ang1 inhibits the up-regulation of TF expression, possibly through activation of PI 3'-kinase/Akt in endothelial cells. Ang1 may be useful as an inhibitor of VEGF- and TNF-a-induced coagulation, inflammation, and cancer progression.

Differential mitogenic responses of human macrovascular and microvascular endothelial cells to cytokines underline their phenotypic heterogeneity

A variety of growth factors promote the complex multistep process of angiogenesis. The mitogenic activity of vascular endothelial growth factors (VEGFs) and placental growth factors (PlGFs), known as cytokines acting predominantly on endothelial cells, was tested on human umbilical vein endothelial cells (HUVEC) and microvascular endothelial cells (MIEC) and compared with the potency of the universally acting basic fibroblast growth factor (FGF-2). The cells were seeded at different cell numbers and incubated with various doses of growth factors for a period of 24-72 h in culture medium +/- serum. Proliferation was determined by measuring the optical density after staining the cells with the tetrazolium salt WST-1. VEGF121 and VEGF165 increased the number of HUVEC and MIEC at low and high seeding densities various doses and incubation times. The efficiency of FGF-2 was less pronounced at high seeding densities of the cells under serum-free conditions. PlGF-1 and PlGF-2 stimulated mitogenesis on HUVEC only at low cell numbers and after a short incubation time by 125 +/- 3% and 102 +/- 5% (P < 0.001), respectively. Longer incubation times with the lower seeding density in the absence of FCS did not induce a significant stimulatory effect of the PlGFs. MIEC responded stronger to all growth factors. In particular under serum free conditions, PlGF-1 and PlGF-2 effectively stimulated cell proliferation by 247 +/- 54% (P < 0.01) and 288 +/- 40% (P < 0.05) at low cell numbers, and by 81 +/- 13% (P < 0.05) and 49 +/- 13% (P < 0.01), respectively, at high cell numbers. The addition of fetal calf serum caused a reduced proliferative response of all growth factors on both cell types related to the controls. In conclusion, MIEC and HUVEC differ in their proliferative response to VEGFs, PlGFs and FGF-2.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

Role of PLCgamma and Ca(2+) in VEGF- and FGF-induced choroidal endothelial cell proliferation

Although both vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) receptors have been shown to be important in the regulation of vascular endothelial cell growth, the roles of phospholipase C (PLC)gamma and Ca(2+) in their downstream signaling cascades are still not clear. We have examined the effects of VEGF and FGF on PLCgamma phosphorylation and on changes in intracellular Ca(2+) levels in primary endothelial cells. VEGF stimulation leads to PLCgamma activation and increases in intracellular Ca(2+), which are correlated with mitogen-activated protein (MAP) kinase (MAPK) activation and cell growth. Inhibition of Ca(2+) increases by the Ca(2+) chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM resulted in marked inhibition of MAPK activation, which was shown to be linked to regulation of cell growth in these cells. In contrast, FGF stimulation did not lead to PLCgamma activation or to changes in intracellular Ca(2+) levels, although MAPK phosphorylation and stimulation of cell proliferation were observed. Neither BAPTA-AM nor the PLC inhibitor U-73122 had an effect on these FGF-stimulated responses. These data demonstrate a direct role for PLCgamma and Ca(2+) in VEGF-regulated endothelial cell growth, whereas this signaling pathway is not linked to FGF-mediated effects in primary endothelial cells. Thus endothelial cell-specific factors regulate the ability of VEGF receptors and FGF receptors to couple to this signaling pathway.

An inhibitory role of the phosphatidylinositol 3-kinase-signaling pathway in vascular endothelial growth factor-induced tissue factor expression

Vascular endothelial growth factor (VEGF) is not only essential for vasculogenesis and angiogenesis but is also capable of inducing tissue factor, the prime initiator of coagulation, in endothelial cells. In this study we have analyzed the VEGF-elicited pathways involved in the induction of tissue factor in human umbilical cord vein endothelial cells. Using specific low molecular weight inhibitors we could demonstrate a crucial role of the p38 and Erk-1/2 mitogen-activated protein (MAP) kinases. In contrast, treatment with wortmannin or LY294002, inhibitors of phosphatidylinositol 3 (PI3)-kinase, resulted in a strong enhancement of the VEGF-induced tissue factor production, indicating a negative regulatory role of the PI3-kinase on tissue factor-inducing pathways. Accordingly, transduction with constitutively active Akt led to a reduction of VEGF-induced tissue factor production. Western blot analyses using antibodies specific for phosphorylated p38 showed an enhanced activation of this MAP kinase in human umbilical cord vein endothelial cells when stimulated with VEGF in the presence of wortmannin in comparison to either agent alone. Thus, the negative regulation of the PI3-kinase pathway on endothelial tissue factor activity can be explained at least in part by a suppression of this MAP kinase-signaling pathway. This is the first demonstration of a reciprocal relationship between procoagulant activity and the PI3-kinase-Akt signaling pathway, and it reveals a novel mechanism by which tissue factor expression can be controlled in endothelial cells.

VEGF(165) requires extracellular matrix components to induce mitogenic effects and migratory response in breast cancer cells

The expression of VEGF and the relapse-free survival rate of breast cancer patients are inversely related. While VEGF induces the proliferation and migration of vascular endothelial cells, its function in breast cancer cells is not well studied. We reported previously that fibronectin increased VEGF-dependent migration in breast cancer cells. Since VEGF has an extracellular matrix (ECM)-binding domain and possesses binding affinity for heparin, we sought to determine the effects of VEGF in breast cancer cells and the role of heparin and/or fibronectin in VEGF-induced signaling. Cells grown on plastic were compared to those grown on fibronectin or to those grown on plastic in the presence of heparin, and analysed for intracellular signaling, proliferation and migration in response to VEGF(165). Both heparin and fibronectin enhanced the binding of VEGF to T47D cells. After treatment with VEGF, [(3)H]thymidine incorporation, c-fos induction, and the number of migrating cells were significantly higher ( approximately twofold) in cells grown on fibronectin or in cells grown on plastic in the presence of heparin when compared to those grown on plastic only. Likewise, tyrosine phosphorylation of VEGF receptors, MAPK activity and PI3-kinase activity were all several-fold higher in cells seeded on fibronectin or in the presence of heparin as compared to cells exposed to VEGF alone. VEGF-dependent c-fos induction was found to be regulated through a MAPK-dependent, but PI3-kinase-independent pathway. In contrast, the migration of T47D cells in response to VEGF, in the presence of ECM, was regulated through PI3-kinase. Therefore, VEGF requires ECM components to induce a mitogenic response and cell migration in T47D breast cancer cells.

Isolated lymphatic endothelial cells transduce growth, survival and migratory signals via the VEGF-C/D receptor VEGFR-3

Vascular endothelial growth factor receptor-3 (VEGFR-3/Flt4) binds two known members of the VEGF ligand family, VEGF-C and VEGF-D, and has a critical function in the remodelling of the primary capillary vasculature of midgestation embryos. Later during development, VEGFR-3 regulates the growth and maintenance of the lymphatic vessels. In the present study, we have isolated and cultured stable lineages of blood vascular and lymphatic endothelial cells from human primary microvascular endothelium by using antibodies against the extracellular domain of VEGFR-3. We show that VEGFR-3 stimulation alone protects the lymphatic endothelial cells from serum deprivation-induced apoptosis and induces their growth and migration. At least some of these signals are transduced via a protein kinase C-dependent activation of the p42/p44 MAPK signalling cascade and via a wortmannin-sensitive induction of Akt phosphorylation. These results define the critical role of VEGF-C/VEGFR-3 signalling in the growth and survival of lymphatic endothelial cells. The culture of isolated lymphatic endothelial cells should now allow further studies of the molecular properties of these cells.