Vascular endothelial growth factor-induced endothelial cell proliferation is regulated by interaction between VEGFR-2, SH-PTP1 and eNOS

VEGF receptor-2 plays a critical role in endothelial cell proliferation during angiogenesis. However, regulation of receptor activity remains incompletely explained. Here, we demonstrate that VEGF stimulates microvascular endothelial cell proliferation in a dose-dependent manner with VEGF-induced proliferation being greatest at 5 and 100 ng/ml and significantly reduced at intermediate concentrations (>50% at 20 ng/ml). Neutralization studies confirmed that signaling occurs via VEGFR-2. In a similar fashion, ERK/MAPK is strongly activated in response to VEGF stimulation as demonstrated by its phosphorylation, but with a decrease in phosphoryation at 20 ng/ml VEGF. Immunoblotting analysis revealed that VEGF did not cause a dose-dependent change in expression of VEGFR-2 but instead resulted in reduced phosphorylation of VEGFR-2 when cells were exposed to 10 and 20 ng/ml of VEGF. VEGFR-2 dephosphorylation was associated with an increase in the protein tyrosine phosphatase, SH-PTP1, and endothelial nitric oxide synthase (eNOS). Immunoprecipitation and selective immunoblotting confirmed the association between VEGFR-2 dephosphorylation and the upregulation of SH-PTP1 and eNOS. Transfection of endothelial cells with antisense oligonucleotide against VEGFR-2 completely abolished VEGF-induced proliferation, whereas anti SH-PTP1 dramatically increased VEGF-induced proliferation by 1 and 5-fold at 10 and 200 ng/ml VEGF, respectively. Suppression of eNOS expression only abolished endothelial cell proliferation at VEGF concentrations above 20 ng/ml. Taken together, these results indicate that activation of VEGFR-2 by VEGF enhances SH-PTP1 activity and eNOS expression, which in turn lead to two diverse events: one is that SH-PTP1 dephosphorylates VEGFR-2 and ERK/MAPK, which weaken VEGF mitogenic activity, and the other is that eNOS increases nitric oxide production which in turn lowers SH-PTP1 activity via S-nitrosylation.     

Caveolin-1 expression is critical for vascular endothelial growth factor-induced ischemic hindlimb collateralization and nitric oxide-mediated angiogenesis

Nitric oxide (NO) is a powerful angiogenic mediator acting downstream of vascular endothelial growth factor (VEGF). Both the endothelial NO synthase (eNOS) and the VEGFR-2 receptor colocalize in caveolae. Because the structural protein of these signaling platforms, caveolin, also represses eNOS activity, changes in its abundance are likely to influence the angiogenic process in various ways. In this study, we used mice deficient for the caveolin-1 gene (Cav-/-) to examine the impact of caveolae suppression in a model of adaptive angiogenesis obtained after femoral artery resection. Evaluation of the ischemic tissue perfusion and histochemical analyses revealed that contrary to Cav+/+ mice, Cav-/- mice failed to recover a functional vasculature and actually lost part of the ligated limbs, thereby recapitulating the effects of the NOS inhibitor L-NAME administered to operated Cav+/+ mice. We also isolated endothelial cells (ECs) from Cav-/- aorta and showed that on VEGF stimulation, NO production and endothelial tube formation were dramatically abrogated when compared with Cav+/+ ECs. The Ser1177 eNOS phosphorylation and Thr495 dephosphorylation but also the ERK phosphorylation were similarly altered in VEGF-treated Cav-/- ECs. Interestingly, caveolin transfection in Cav-/- ECs redirected the VEGFR-2 in caveolar membranes and restored the VEGF-induced ERK and eNOS activation. However, when high levels of recombinant caveolin were reached, VEGF exposure failed to activate ERK and eNOS. These results emphasize the critical role of caveolae in ensuring the coupling between VEGFR-2 stimulation and downstream mediators of angiogenesis. This study also provides new insights to understand the paradoxical roles of caveolin (eg, repressing basal enzyme activity but facilitating activation on agonist stimulation) in cardiovascular pathophysiology.     

Oxidized LDL inhibits vascular endothelial growth factor-induced endothelial cell migration by an inhibitory effect on the Akt/endothelial nitric oxide synthase pathway

BACKGROUND: Oxidized LDL (oxLDL) inhibits endothelial cell (EC) migration. Stimulating ECs with vascular endothelial growth factor (VEGF) leads to the activation of Akt/protein kinase B, which in turn activates endothelial nitric oxide synthase (eNOS) by phosphorylation on serine 1177. VEGF-induced cell migration is dependent on the generation of nitric oxide (NO). Therefore, we investigated whether oxLDL affects EC migration by an inhibitory effect on the Akt/eNOS pathway. METHODS AND RESULTS: During an in vitro "scratched wound assay," oxLDL dose-dependently inhibited the VEGF-induced migration of human umbilical vein endothelial cells. Western blot analysis revealed that oxLDL dose- and time-dependently led to dephosphorylation and thus deactivation of Akt. Moreover, oxLDL inhibited the VEGF-induced generation of NO, as detected and quantified using a fluorescent NO indicator, 4,5-diaminofluorescein diacetate. Overexpression of a constitutively active Akt construct (Akt T308D/S473D) or a phosphomimetic eNOS construct (eNOS S1177D) almost completely reversed the inhibitory effect of oxLDL on VEGF-induced EC migration and NO generation. CONCLUSIONS: Our data indicate that oxLDL-induced dephosphorylation of Akt, followed by impaired eNOS activation, reduces the intracellular level of NO and thereby inhibits VEGF-induced EC migration.     

Thromboxane A2 receptor signaling inhibits vascular endothelial growth factor-induced endothelial cell differentiation and migration

Vascular endothelial growth factor (VEGF) is an important patho-physiological mediator of angiogenesis. VEGF-induced endothelial cell (EC) migration and angiogenesis often occur in complicated environments containing multiple agents capable of modifying the response. Thromboxane (TX) A2 is released from multiple cell types and is a prime mediator of pathogenesis of many vascular diseases. Human EC express both TXA2 receptor (TP) isoforms; however, the effects of individual TP isoforms on VEGF-induced EC migration and angogenesis are unknown. We report here that the TXA2 mimetic [1S-(1alpha, 2beta(5Z), 3alpha(1E, 3R), 4alpha]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxab icyclo-[2.2.1]heptan-2yl]-5'-heptenoic acid (IBOP) (100 nmol/L) is a potent antagonist (IC50 30 nmol/L) of VEGF-induced EC migration and differentiation. TPbeta, but not TPalpha, expression is required for the inhibition of VEGF-induced migration and angiogenesis. IBOP costimulation suppressed nitric oxide (NO) release from VEGF-treated EC through decreased activation of Akt, eNOS, and PDK1. TPbeta costimulation also ablated the increase in focal adhesion formation in response to VEGF. This mechanism was characterized by decreased recruitment of focal adhesion kinase (FAK) and vinculin to the alpha(v)beta3 integrin and reduced FAK and Src activation in response to VEGF. Addition of NO donors together with transfection of a constitutively active Src construct could circumvent the blockade of VEGF-induced migration by TP; however, neither intervention alone was sufficient. Thus, TP stimulation appears to limit angiogenesis, at least in part, by inhibiting the pro-angiogenic cytokine VEGF. These data further support a role for antagonism of TP activation in enhancing the angiogenic response in tissues exposed to elevated TXA2 levels in which revascularization is important.     

Vascular endothelial growth factor triggers signaling cascades mediating multiple myeloma cell growth and migration.

Multiple myeloma (MM) remains incurable, with a median survival of 3 to 4 years. This study shows direct effects of vascular endothelial growth factor (VEGF) upon MM and plasma cell leukemia (PCL) cells. The results indicate that VEGF triggers tumor cell proliferation via a protein kinase C (PKC)-independent Raf-1-MEK-extracellular signal-regulated protein kinase pathway, and migration via a PKC-dependent pathway. These observations provide the framework for novel therapeutic strategies targeting VEGF signaling cascades in MM.     

Role of endothelial nitric oxide synthase in endothelial cell migration.

Endothelium-derived nitric oxide (NO) and its precursor L-arginine have been implied to promote angiogenesis, but little is known about the precise mechanism. The inhibition of endogenous NO formation by Nomega-nitro-L-arginine methyl ester (L-NAME) (1 mmol/L) but not its inactive enantiomer D-NAME (1 mmol/L) inhibited endothelial cell sprouting from the scratched edge of the cultured bovine aortic endothelial cell monolayer. Inhibition of endogenous NO release by L-NAME was confirmed by amperometric measurement using an NO-specific electrode. In the modified Boyden chamber, L-NAME (1 mmol/L) significantly inhibited endothelial cell migration, whereas L-NAME did not affect endothelial DNA synthesis as assessed by analysis of [3H]thymidine incorporation. We then examined alteration of endothelial cell adhesion molecule expression after the inhibition of NO by L-NAME in cultured human umbilical vein endothelial cells. In both normoxic and hypoxic conditions, L-NAME (1 mmol/L) inhibited surface expression of integrin alphavbeta3, which is an important integrin facilitating endothelial cell survival and angiogenesis. However, L-NAME did not affect the expression of platelet endothelial cell adhesion molecule-1, intercellular adhesion molecule-1, vascular endothelial adhesion molecule-1, gap junction protein connexin 43, and VE-cadherin, which have been reported to potentially affect angiogenesis. In summary, inhibition of endothelial NO synthase by L-NAME attenuated endothelial cell migration but not proliferation in vitro. Furthermore, endogenous endothelium-derived NO maintains the functional expression of integrin alphavbeta3, a mediator for endothelial migration, survival, and angiogenesis. Endothelium-derived NO, thus, may play an important role in mediating angiogenesis by supporting endothelial cell migration, at least partly, via an integrin-dependent mechanism.     

Vascular endothelial growth factor binds to fibrinogen and fibrin and stimulates endothelial cell proliferation

Vascular development and response to injury are regulated by several cytokines and growth factors including the members of the fibroblast growth factor and vascular endothelial cell growth factor (VEGF) families. Fibrinogen and fibrin are also important in these processes and affect many endothelial cell properties. Possible specific interactions between VEGF and fibrinogen that could play a role in coordinating vascular responses to injury are investigated. Binding studies using the 165 amino acid form of VEGF immobilized on Sepharose beads and soluble iodine 125 ((125)I)-labeled fibrinogen demonstrated saturable and specific binding. Scatchard analysis indicated 2 classes of binding sites with dissociation constants (K(d)s) of 5.9 and 462 nmol/L. The maximum molar binding ratio of VEGF:fibrinogen was 3.8:1. Further studies characterized binding to fibrin using (125)I-labeled VEGF- and Sepharose-immobilized fibrin monomer. These also demonstrated specific and saturable binding with 2 classes of sites having K(d)s of 0.13 and 97 nmol/L and a molar binding ratio of 3.6:1. Binding to polymerized fibrin demonstrated one binding site with a K(d) of 9.3 nmol/L. Binding of VEGF to fibrin(ogen) was independent of FGF-2, indicating that there are distinct binding sites for each angiogenic peptide. VEGF bound to soluble fibrinogen in medium and to surface immobilized fibrinogen or fibrin retained its capacity to support endothelial cell proliferation. VEGF binds specifically and saturably to fibrinogen and fibrin with high affinity, and this may affect the localization and activity of VEGF at sites of tissue injury. (Blood. 2000;96:3772-3778)     

Phosphoinositide-dependent kinase 1 and p21-activated protein kinase mediate reactive oxygen species-dependent regulation of platelet-derived growth factor-induced smooth muscle cell migration

Smooth muscle cell migration in response to platelet-derived growth factor (PDGF) is a key event in several vascular pathologies, including atherosclerosis and restenosis. PDGF increases intracellular levels of reactive oxygen species (ROS) in vascular smooth muscle cells (VSMCs), but the ROS sensitivity of migration and of the signaling pathways leading to migration are largely unknown. In VSMCs, PDGF dose-dependently increased migration compared with nonstimulated cells, with a maximum increase at 10 ng/mL. Pretreatment with the antioxidant N-acetyl-cysteine, the flavin-containing enzyme inhibitor diphenylene iodonium, or the glutathione peroxidase mimetic ebselen significantly attenuated migration (PDGF alone, 5.0+/-1.1-fold; NAC, 1.8+/-0.2-fold; diphenylene iodonium, 1.4+/-0.3-fold migration; and ebselen, 2.0+/-0.5-fold migration), as did overexpression of catalase. Pretreatment of VSMCs with the Src inhibitor PP1 or dominant-negative Rac adenovirus significantly inhibited migration, but only Src activation was attenuated by ROS inhibitors. Phosphorylation of the Src- and Rac-effector p21-activated protein kinase (PAK) 1 on Thr423 (the phosphoinositide-dependent kinase-1 [PDK1] site) was attenuated by ROS inhibition, and infection of VSMCs with dominant-negative PAK1 adenovirus attenuated migration. Moreover, kinase-inactive K111N-PDK1 inhibited PAK1 phosphorylation on Thr423, and both K111N-PDK1 and Y9F-PDK1 significantly inhibited VSMC migration. PDK1 tyrosine phosphorylation was also ROS dependent. These data indicate that PDGF-induced VSMC migration is ROS dependent and identify the Src/PDK1/PAK1 signaling pathway as an important ROS-sensitive mediator of migration. Such information is critical to understanding the role of ROS in vascular diseases in which migration of VSMCs is an important component.     

Mediation of basic fibroblast growth factor-induced lactotropic cell proliferation by Src-Ras-mitogen-activated protein kinase p44/42 signaling

Basic fibroblast growth factor (bFGF), which is secreted from folliculostellate cells in the anterior pituitary, is known to be involved in the communication between folliculostellate cells and lactotropes during estradiol-induced lactotropic cell proliferation. We studied the role of MAPK p44/42 in bFGF-regulated cell proliferation using enriched lactotropes and the lactotrope-derived PR1 cell line. In cell cultures, bFGF increased cell proliferation of PR1 cells and enriched lactotropes. In both of these cell populations, bFGF also increased phosphorylation of MAPK p44/42. U0126, an inhibitor of MAPK p44/42, blocked the bFGF-induced activation of MAPK p44/42 as well as the bFGF-induced cell proliferation of enriched lactotropes and PR1 cells. Treatment of PR1 cells with bFGF increased the activity of Ras p21, whereas overexpression of a dominant negative mutant of Ras p21 abrogated the bFGF-induced activation of MAPK p44/42 in these cells. Furthermore, the Src kinase inhibitor PP1 suppressed bFGF-induced activation of MAPK p44/42 in both enriched lactotropes and PR1 cells. The Src kinase inhibitor PP1 also reduced bFGF activation of Ras p21 and cell proliferation in PR1 cells. On the other hand, the bFGF-induced activation of MAPK p44/42 in enriched lactotropes and PR1 cells was not affected by protein kinase C inhibitors. These data suggest that bFGF induction of lactotropic cell proliferation is possibly mediated by activation of Src kinase, Ras p21, and MAPK p44/42.     

Influence of extracellular magnesium on capillary endothelial cell proliferation and migration.

We investigated the role of extracellular magnesium on capillary endothelial cell migration and proliferation, components of endothelial cell function that play an important role in angiogenesis and wound healing. Cell migration and proliferation were tested in six different MgSO4 concentrations and in various culture conditions. The Boyden chamber procedure was used to evaluate migration of bovine adrenal cortex capillary endothelial cells. We found that low magnesium concentration inhibited cell migration, but a dose-dependent increase in migration was observed when magnesium level was increased beyond the normal serum concentration (up to 2.4 mM magnesium; p less than 0.0001). Cell proliferation was also inhibited by very low magnesium concentration, an effect observed under all conditions studied. When cell proliferation was stimulated by acidic or basic fibroblast growth factors, it appeared that a ceiling was reached, an increasing magnesium concentration had no additional stimulatory effect. However, a dose-dependent increase in proliferation (p less than 0.005) was observed when magnesium concentration was increased above the normal serum level (0.8 mM) in culture conditions that did not cause marked cell proliferation. Thus, magnesium has an important role in endothelial cell migration and proliferation: very low extracellular magnesium concentrations inhibit and supranormal levels enhance both migration and proliferation. These results suggest that magnesium deficiency might adversely influence the healing and reendothelialization of vascular injuries and the healing of myocardial infarction and might also result in delayed or inadequate angiogenesis, effects potentially leading to infarct expansion and inadequate collateral development.     

MEK kinase 1 is critically required for c-Jun N-terminal kinase activation by proinflammatory stimuli and growth factor-induced cell migration

Exposure of eukaryotic cells to extracellular stimuli results in activation of mitogen-activated protein kinase (MAPK) cascades composed of MAPKs, MAPK kinases (MAP2Ks), and MAPK kinase kinases (MAP3Ks). Mammals possess a large number of MAP3Ks, many of which can activate the c-Jun N-terminal kinase (JNK) MAPK cascade when overexpressed, but whose biological function is poorly understood. We examined the function of the MAP3K MEK kinase 1 (MEKK1) in proinflammatory signaling. Using MEKK1-deficient embryonic stem cells prepared by gene targeting, we find that, in addition to its function in JNK activation by growth factors, MEKK1 is required for JNK activation by diverse proinflammatory stimuli, including tumor necrosis factor alpha, IL-1, double-stranded RNA, and lipopolysaccharide. MEKK1 is also essential for induction of embryonic stem cell migration by serum factors, but is not required for activation of other MAPKs or the IkappaB kinase signaling cascade.     

Differential regulation of cell migration and proliferation through proline-rich tyrosine kinase 2 in endothelial cells

Proline-rich tyrosine kinase 2 (Pyk2), a member of the focal adhesion kinase family, is thought to act as a key component in vasculogenesis and angiogenesis. Therefore, we studied the effect of mutant Pyk2 expression on the migration and proliferation in endothelial cells (ECs). Two types of mutant Pyk2 were examined by adenovirus vectors AxCA-Pyk2K457A, expressing a kinase inactive mutant, and AxCA-Pyk2Y402F, expressing a tyrosine autophosphorylation site mutant, in addition to AxCA-Pyk2, expressing wild-type Pyk2. Migration of ECs infected with AxCA-Pyk2Y402F increased to a level similar to that of ECs infected with AxCA-Pyk2. The size of effect was dependent on the amount of applied adenoviruses within the range of 3-30 multiplicity of infection. In contrast, AxCA-Pyk2K457A infection did not show any significant effect on cell migration. Western blotting showed that both phosphorylation of Pyk2 Y(881) and association of p130(Cas) with Pyk2 were enhanced in ECs infected with AxCA-Pyk2Y402F as well as with AxCA-Pyk2, but not in ECs infected with AxCA-Pyk2K457A. Therefore, signaling mediated by Pyk2 Y(881) and p130(Cas) may be involved in the migration of ECs infected either with AxCA-Pyk2Y402F or with AxCA-Pyk2. In proliferation assay, AxCA-Pyk2 infection suppressed EC proliferation significantly; however, neither AxCA-Pyk2Y402F nor AxCA-Pyk2K457A showed such an inhibitory effect. Thus, the two Pyk2 mutants revealed that Pyk2 signaling differentially regulates cell migration and proliferation pathways.     

Predominant role of endothelial nitric oxide synthase in vascular endothelial growth factor-induced angiogenesis and vascular permeability

Nitric oxide (NO) plays a critical role in vascular endothelial growth factor (VEGF)-induced angiogenesis and vascular hyperpermeability. However, the relative contribution of different NO synthase (NOS) isoforms to these processes is not known. Here, we evaluated the relative contributions of endothelial and inducible NOS (eNOS and iNOS, respectively) to angiogenesis and permeability of VEGF-induced angiogenic vessels. The contribution of eNOS was assessed by using an eNOS-deficient mouse, and iNOS contribution was assessed by using a selective inhibitor [l-N(6)-(1-iminoethyl) lysine, l-NIL] and an iNOS-deficient mouse. Angiogenesis was induced by VEGF in type I collagen gels placed in the mouse cranial window. Angiogenesis, vessel diameter, blood flow rate, and vascular permeability were proportional to NO levels measured with microelectrodes: Wild-type (WT) > or = WT with l-NIL or iNOS(-/-) > eNOS(-/-) > or = eNOS(-/-) with l-NIL. The role of NOS in VEGF-induced acute vascular permeability increase in quiescent vessels also was determined by using eNOS- and iNOS-deficient mice. VEGF superfusion significantly increased permeability in both WT and iNOS(-/-) mice but not in eNOS(-/-) mice. These findings suggest that eNOS plays a predominant role in VEGF-induced angiogenesis and vascular permeability. Thus, selective modulation of eNOS activity is a promising strategy for altering angiogenesis and vascular permeability in vivo.