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Arteriosclerosis, Thrombosis, and Vascular Biology. 2008;28:2042-2048
Published online before print September 11, 2008, doi: 10.1161/ATVBAHA.108.175109
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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2008;28:2042.)
© 2008 American Heart Association, Inc.


Cell Biology/Signaling

Hepatocyte Growth Factor Inhibits VEGF-Forkhead–Dependent Gene Expression in Endothelial Cells

Md. Ruhul Abid; Robert J. Nadeau; Katherine C. Spokes; Takashi Minami; Dan Li; Shou-Ching Shih; William C. Aird

From the Center for Vascular Biology Research (M.R.A., R.J.N., K.C.S., W.C.A.), Department of Medicine and Division of Molecular and Vascular Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass; the Research Center for Advanced Science and Technology (T.M.), University of Tokyo, Japan; the Department of Pathology (D.L., S.-C.S.), Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass.

Correspondence to Ruhul Abid, MD, PhD, Molecular and Vascular Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, E/RW-663, 330 Brookline Avenue, Boston, MA 02215. E-mail rabid{at}bidmc.harvard.edu


*    Abstract
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Objective— Recently, we reported that the forkhead transcription factor, FKHR/FOXO1, is required for vascular endothelial growth factor (VEGF)–mediated upregulation of a number of genes in endothelial cells. Here, we tested the hypothesis that hepatocyte growth factor (HGF), a potent activator of PI3K-Akt in endothelial cells, is capable of depleting the nucleus of FKHR/FOXO1 and thus inhibiting VEGF induction of this class of genes.

Methods and Results— Incubation of human coronary artery endothelial cells with HGF induced prolonged PI3K/Akt-dependent phosphorylation and nuclear exclusion of FKHR/FOXO1. HGF-mediated inhibition of FKHR/FOXO1 activity resulted in secondary attenuation of VEGF-induced expression of FKHR/FOXO1-dependent genes including vascular cell adhesion molecule-1, manganese superoxide dismutase, endothelial specific molecule-1, CBP/p300 interacting transactivator with ED-rich tail-2, bone morphogenetic protein-2, matrix metalloproteinase (MMP)-10, and MGC5618. At a functional level, preincubation of HGF resulted in inhibition of VEGF-induced vascular cell adhesion molecule (VCAM)-1–mediated monocyte adhesion to endothelial cells. HGF-mediated inhibition of VEGF-inducible VCAM-1 expression and monocyte adhesion was reversed by overexpression of constitutively active phosphorylation-resistant triple mutant (TM)-FKHR.

Conclusion— These findings suggest that physiological agonists of PI3K-Akt signaling pathway may modulate VEGF-FKHR/FOXO1–dependent gene expression in endothelial cells. The data underscore the importance of the "set point" of the endothelial cell when considering mechanisms of signal transduction.


Key Words: HGF • VEGF • forkhead • endothelial cells • gene expression


*    Introduction
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Vascular endothelial growth factor (VEGF) plays a critical role in endothelial cell survival, proliferation, migration, and is involved in wound repair, angiogenesis of ischemic tissue, tumor growth, microvascular permeability, vascular protection, and hemostasis.1–8 VEGF has been shown to activate a number of different intracellular signaling pathways, including PKC, PI3K and Akt, MEK1/2, p38 MAPK, and phospholipase C{gamma}.9–13 VEGF-mediated activation of signaling intermediates, in turn, results in altered activity of transcription factors, including NF-{kappa}B, Egr-1, NFAT-1, Ets-1, Stat-3/5, and forkhead transcription factors.14–20

Hepatocyte growth factor (HGF) (also known as scatter factor) is a high–molecular weight multifunctional polypeptide growth factor. Although HGF originally was described for its ability to stimulate proliferation of liver cells,21 it is now recognized to interact with its receptor, c-MET, in other cell types. For example, in endothelial cells HGF has been shown to promote migration, proliferation, survival, and barrier function.22–25 Under in vivo conditions, HGF has been implicated in angiogenesis.23 In cultured endothelial cells, HGF stimulates several signaling pathways and transcription factors including Rac, ERK1/2, p38 MAPK, PI3K, Src, AKT, FKHR/FOXO1, CREB, and ATF.25–28 However, in contrast to VEGF, HGF has not been shown to activate NF-{kappa}B.26,29

The mammalian members of the winged helix, or forkhead, transcription factors include FKHR (FOXO1), FKHRL1 (FOXO3a), and AFX (FOXO4).19 Previous studies have demonstrated the presence of FKHR/FOXO1 and AFX in endothelial cells.19 Exposure of endothelial cells to several agonists, including VEGF, angiopoietin, or angiotensin II resulted in PI3K-Akt–mediated phosphorylation and nuclear exclusion of FKHR/FOXO1, and subsequent downregulation of proapoptotic and antiproliferative FKHR/FOXO1 target genes, such as GADD45A and p27kip1.19,30,31 More recently, we demonstrated that VEGF requires FKHR/FOXO1 activity for the induction of certain genes, including VCAM-1, manganese superoxide dismutase (MnSOD), endothelial specific molecule (ESM)-1, CBP/p300 interacting transactivator with ED-rich tail (CITED)-2, bone morphogenetic protein (BMP)-2, MMP-10, and MGC5618.20 These VEGF-inducible forkhead-responsive genes are referred to as Class II genes, to distinguish them from the classical VEGF-repressible forkhead-responsive transcripts (Class I).20 Together with the observation that FKHR/FOXO1 is essential for embryonic vascular development,32,33 these data suggest that FKHR/FOXO1 is an essential transcription factor that is involved in the coordinated regulation of a distinct set of genes in endothelial cells involved in cell maintenance and health.

The history of signal input, or the "set point" of the endothelial cell, is an important determinant of subsequent signaling. For example, the term "ischemic preconditioning" describes the cytoprotective effect of ischemia-reperfusion or hypoxia on tissues or endothelial cells.34 Repeated challenge of endothelial cells with lipopolysaccharide (LPS) leads to endotoxin tolerance characterized by temporary insensitivity to subsequent LPS challenge. Other preconditioning regimens that have been demonstrated to alter endothelial cell signaling include hypoxia,35 heat shock,36 and insulin-like growth factor (IGF)-1.37 As a final example, we recently demonstrated that preincubation of endothelial cells with tumor necrosis factor (TNF)-{alpha} results in altered thrombin-mediated gene expression by modulating nuclear-cytoplasmic trafficking of p65 NF-{kappa}B.38 In the present study, we tested the hypothesis that preconditioning of endothelial cells with the PI3K-AKT agonist, HGF would affect the ability of VEGF to activate Class II genes by limiting the availability of FKHR/FOXO1 in the nucleus (the functional equivalent of FKHR/FOXO1 knockdown). We show that HGF does indeed attenuate VEGF induction of FKHR-dependent genes. These findings add a new level of complexity to forkhead signaling in endothelial cells.


*    Methods
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Cell Culture and Reagents
Human coronary artery endothelial cells (HCAECs) were grown in endothelial growth medium-2-MV (EGM-2-MV) BulletKit (Clonetics) at 37°C and 5% CO2. Endothelial cells from passage 3 to 6 were used for all experiments. Cells were serum-starved in 0.5% FBS before treatment with 50 ng/mL human VEGF-A165 or human 20 ng/mL HGF (PeproTech Inc). Where indicated, cells were preincubated for 30 minutes with 20 ng/mL HGF and then washed with PBS before VEGF treatment. Monoclonal anti–hVCAM-1 neutralizing antibody was purchased from Chemicon (Millipore).

Adenoviruses
HCAECs were transduced with replication-deficient adenoviruses encoding the cDNAs of β-galactosidase (Adv) and triple mutant (TM)-FKHR/FOXO1 as previously described.20,26 The triple mutant version of FKHR/FOXO1 contains T24A, S256A, and S319A and is resistant to agonist-induced phosphorylation and nuclear export. Adenovirus expressing DN-Akt was previously described.19 Briefly, HCAECs were plated at a density of 1x106 cells per 10-cm plate and adenoviruses were added to the cells at five multiplicity of infection (5 MOI) after 5 hour of plating. Cells were then allowed to grow for 36 hours followed by overnight serum starvation. Expression of adenovirus-based FKHR/FOXO1 and Akt was confirmed using RT-polymerase chain reaction (PCR) analyses (supplemental Figure I, available online at http://atvb.ahajournals.org, shows FKHR/FOXO1) and also by Western blots as described previously.19,20

siRNA-Mediated Inhibition of Endogenous FKHR/FOXO1
HCAECs were grown to 70% to 80% confluence in 6-cm plates and transfected with siRNA against the following FKHR/FOXO1 target sequences: siFKHR2 CAG CGC CGA CTT CAT GAG CAA (Qiagen) in Opti-MEM containing Lipofectin (10 µg/mL) for 4 hours as previously described.20 The cells were then incubated in EGM-2 medium for 24 hours and serum-starved in 0.5% serum for 12 to 16 hours before VEGF treatment for the times indicated.

For quantitative real-time PCR, Western and Northern blot analyses, immunolocalization studies, nuclear and cytoplasmic cellular fractionation, and monocyte adhesion assay, please see supplemental materials.

Statistical Analyses
All values are presented as mean±SD where appropriate. Statistical significance between 2 groups was determined by use of a paired t test, and values of P<0.05 were considered significant.


*    Results
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HGF Induces Early and Prolonged Phosphorylation of Akt and FKHR/FOXO1 in Primary Human Endothelial Cells
HCAECs were incubated in the absence or presence of VEGF or HGF for varying times and assayed for phosphorylation of Akt and FKHR/FOXO1 by Western blot analysis. In response to VEGF, Akt was phosphorylated at 5 minutes, returning to basal levels by 30 minutes (Figure 1A, upper panel). VEGF-mediated phosphorylation of FKHR/FOXO1 occurred at 15 minutes and returned to basal levels by 60 minutes (Figure 1B). In contrast, HGF-induced phosphorylation of Akt and FKHR/FOXO1 occurred at 5 minutes and did not return to their basal levels until after 2 hours (Figure 1A and 1B and data not shown). VEGF- and HGF-mediated phosphorylation of FKHR/FOXO1 was inhibited by the PI3K-inhibitor LY294002 (data not shown) and dominant-negative (DN) Akt (Figure 1C and 1D). Together, these data indicate that whereas both VEGF- and HGF-mediated phosphorylation of FKHR/FOXO1 occurs through a PI3K-Akt signaling pathway, the effect of HGF on FKHR/FOXO1 phosphorylation in HCAEC occurs earlier and is more sustained.


Figure 1
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Figure 1. HGF induces greater and more sustained phosphorylation of endogenous FKHR/FOXO1 in HCAECs compared to VEGF. Serum-starved HCAECs were incubated with either VEGF (50 ng/mL) or HGF (20 ng/mL) for the times indicated. HCAECs were harvested for total protein, and Western blots were carried out as described in Methods. A, Membranes were probed for Ser473-Akt (p-Akt) and total Akt. B through D, Membranes were probed for Ser256-FKHR/FOXO1 (p-FKHR/FOXO1) and total FKHR/FOXO1. B, Bar graph in the bottom panel shows the corresponding quantitation of FKHR/FOXO1 phosphorylation relative to total FKHR/FOXO1 in 3 independent experiments. C and D, HCAECs were transduced with control adenovirus (Adv) or adenovirus expressing dominant-negative Akt (DN-Akt) 2 days before growth factor treatment. Membranes were probed for Ser256-FKHR/FOXO1 (p-FKHR/FOXO1) and total FKHR/FOXO1. Western blots shown are representative of 3 independent experiments. *P<0.05.

HGF-Mediated Nuclear Exclusion of FKHR/FOXO1 in Primary Human Endothelial Cells Is Prolonged Compared to VEGF
Phosphorylation of FKHR/FOXO1 has been shown to promote translocation of the transcription factor from the nucleus to cytoplasm.19,39 In immunofluorescence assays, incubation of HCAECs with VEGF resulted in cytoplasmic translocation of FKHR/FOXO1 by 15–30 minutes, an effect that was reversed by 60 minutes (supplemental Figure II). HGF-mediated nuclear exclusion of FKHR/FOXO1 occurred by 15 minutes. However, in contrast to VEGF, the effect of HGF on cytoplasmic translocation persisted at 60 minutes, returning to basal levels only after 2 hours (Figure 2). HCAECs were also processed for cytoplasmic and nuclear fractions and assayed for FKHR/FOXO1 by Western blot. In these assays, nuclear-to-cytoplasmic translocation of FKHR/FOXO1 by VEGF was maximal at 30 minutes and returned to the basal levels within 60 minutes, whereas HGF-induced cytoplasmic translocation was maximal from 30 to 60 minutes but did not return to the basal levels until after 2 hours (supplemental Figure III). In accordance with the phosphorylation data (please see Figure 1C and 1D), LY294002, and DN-Akt inhibited VEGF- and HGF-mediated cytoplasmic translocation of FKHR/FOXO1 (supplemental Figure IV shows the effect of DN-Akt), suggesting that PI3K-Akt-induced phosphorylation is critical for subcellular localization of FKHR/FOXO1 in HCAECs. Consistent with the early and sustained effect of HGF phosphorylation and subcellular localization of FKHR/FOXO1, preincubation of HCAECs with HGF for 30 minutes preempted the effect of VEGF (supplemental Figure V). Together, these findings suggest that HGF induces sustained nuclear exclusion of FKHR/FOXO1 and that preincubation with HGF limits the availability of nuclear FKHR/FOXO1 in VEGF-treated cells.


Figure 2
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Figure 2. HGF-mediated nuclear exclusion of FKHR/FOXO1 in HCEACs is more sustained compared to VEGF. HGF-mediated subcellular localization of FKHR/FOXO1 was assayed as described in Methods. The photomicrographs show FKHR/FOXO1 (red, left), nuclear DAPI (blue, middle), and merged FKHR/FOXO1 and DAPI (right). The bar graph shows the corresponding quantitative analyses of the subcellular localization of FKHR/FOXO1 and was generated by counting nuclear and cytoplasmic localization of FKHR/FOXO1 in 200 HCAECs cells per time point using NIH ImageJ and McMaster Biophotonics as described in Methods; N indicates nuclear; C, cytoplasmic. The data presented are from 3 independent experiments. *P<0.05.

Temporal Modulation of FKHR/FOXO1 by HGF Attenuates VEGF-Mediated Induction of FKHR/FOXO1-Dependent Genes in HCAECs
Based on the above findings, we reasoned that HGF-mediated depletion of FKHR/FOXO1 from the nucleus would impair the ability of VEGF to induce those genes that depend on FKHR/FOXO1 as a positive transcription factor (ie, Class II genes). To test this hypothesis, HCAECs were serum-starved, preincubated with HGF, treated in the absence or presence of VEGF, and harvested for total RNA. In real-time PCR analyses, pretreatment of HCAECs with HGF for 30 minutes blocked VEGF-mediated induction of FKHR/FOXO1-dependent genes including VCAM-1, MnSOD, BMP2, ESM-1, MMP10, CITED2, and MGC5618 (Table). Representative Northern blot analyses for MnSOD and VCAM-1 are shown in Figure 3A. To determine whether this inhibitory effect of HGF was attributabl to unavailability of nonphosphorylated FKHR/FOXO1 in the nucleus, we transduced HCAEC with adenoviruses expressing either β-gal (Adv) or phosphorylation-defective constitutively active triple mutant (TM)-FKHR/FOXO1. As shown in the Table and Figure 3B, the ability of HGF to attenuate VEGF-mediated induction of VCAM-1, MnSOD, BMP2, ESM-1, MMP10, CITED2, and MGC5618 expression was reversed by nuclear-localized TM-FKHR/FOXO1. To rule out an effect of HGF preconditioning on VEGFR2 phosphorylation, we carried out immunoprecipitation of VEGFR2 followed by Western blotting with an antiphospho-tyrosine antibody. Preincubation of HCAECs with HGF did not significantly alter VEGF-induced phosphorylation or mRNA expression of VEGFR2 (supplemental Figure VI). A previous study demonstrated that HGF inhibits VEGF activation of NF-{kappa}B.40 However, consistent with our previous results,20 we failed to demonstrate an effect of HGF on VEGF-induced NF-{kappa}B signaling. Specifically, HGF had no effect on VEGF-mediated phosphorylation of I{kappa}B or NF-{kappa}B p65 DNA binding (supplemental Figure VII). Together, these data suggest that HGF limits the availability of nuclear-localized FKHR/FOXO1, which in turn impairs VEGF-inducible/FKHR/FOXO1-dependent gene expression.


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Table. HGF-Mediated Inhibition of VEGF-Induced Gene Expression (HGF+VEGF) Is Reversed by TMFKHR (TM-FKHR+HGF+BEGF) in HCAECs as Determined by Fold Induction or Reduction Using RT-PCR Analyses


Figure 3
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Figure 3. HGF-mediated inhibition of VEGF-induced class II gene expression is reversed by constitutively active TM-FKHR/FOXO1. A, HCAECs were serum-starved overnight and then preincubated in the absence (control) or presence of HGF for 30 minutes, followed by incubation without (–) or with VEGF (+) for 4 hours. Total RNA was extracted and Northern blots analyses were performed using the MnSOD probe; the same membrane was stripped and reprobed for VCAM-1 transcript. Ethidium bromide-stained 28S RNA is shown as a loading control. MnSOD expression is quantitated in the bottom panel (C indicates control; V, VEGF; H, HGF; HV, pretreatment with HGF followed by VEGF). The data are expressed as mean±SD of 3 independent experiments. B, HCAECs were transduced with adenoviruses expressing β-galactosidase (Adv) or triple-mutant (TM)-FKHR/FOXO1. Cells were treated in the absence (control, C) or presence of VEGF (V) or HGF (H) for 4 hours. Alternatively, cells were preincubated with HGF for 30 minutes and then treated with VEGF for 4 hour (HV). *P<0.05 control vs VEGF-treated; {dagger}P<0.05 control adenovirus (Adv)-transduced plus VEGF-treated vs TM-FKHR/FOXO1 adenovirus-transduced plus VEGF, or HGF-pretreated followed by VEGF treatment, as indicated.

FKHR/FOXO1 Is Required for VEGF-Mediated Monocyte Adhesion to HCAECs
We next wished to determine whether the inhibitory effect of HGF on VEGF signaling was functionally relevant. To that end, we asked whether HGF-mediated attenuation of VCAM-1 expression in VEGF-treated endothelial cells influenced monocyte adhesion. As shown in Figure 4, VEGF resulted in more than 3-fold induction of U937 adhesion to HCAECs. Preincubation of HCAECs with HGF resulted in inhibition of VEGF-induced monocyte adhesion (Figure 4, upper panel). This effect was reversed by overexpression of a constitutively active TM-FKHR/FOXO1 (Figure 4, lower panel). In contrast, siRNA-mediated knockdown of siFKHR/FOXO1 in HCAECs resulted in a decrease in the VEGF-induced monocyte adhesion, irrespective of the presence of HGF (supplemental Figure VIIIA). Finally, neutralizing anti–VCAM-1 antibody inhibited VEGF-inducible monocyte adhesion to HCAECs (supplemental Figure VIIIB). These data suggest that HGF-mediated phosphorylation and nuclear exclusion of FKHR/FOXO1 and subsequent attenuation of VEGF-inducible VCAM-1 expression results in reduced monocyte adhesion to endothelial cells.


Figure 4
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Figure 4. HGF-mediated inhibition of VEGF-induced monocyte adhesion to HCAECs is reversed by TM-FKHR/FOXO1. HCAECs were transduced with either Adv or TM-FKHR/FOXO1 as described in Methods. The cells were serum-starved for 12 to 16 hours before HGF or VEGF stimulation. Where combined HGF and VEGF treatments are indicated, HCAEC monolayers were treated with HGF for 30 minutes before the addition of VEGF. U937 monocyte adhesion on HCAEC monolayer was determined as described in Methods. Bar graph results shown are mean±SD (standard deviation). *P<0.05 control vs VEGF-treated; {dagger}P<0.05 VEGF-treated vs HGF-pretreated plus VEGF-treated. The results are obtained from 3 independent experiments. Quantitative bar graphs with probability value (<0.05) are shown.


*    Discussion
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*Discussion
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VEGF activation of PI3K-Akt results in phosphorylation and nuclear exclusion of the FKHR/FOXO1. Nuclear exclusion, in turn, leads to downregulation of genes that are primarily regulated by forkhead (eg, p27kip1, BTG-1). However, there is a distinct set of forkhead-responsive genes that are actually upregulated by VEGF.20 VEGF-mediated induction of these genes requires FKHR/FOXO1 and at least one other positive acting factor, such as NF-{kappa}B or NFAT.20 The existence of the latter gene class has two implications. First, for VEGF to induce the expression of forkhead-responsive genes there must either be residual FKHR/FOXO1 in the nucleus, or timely reentry of the transcription factor into the nucleus. Second, preincubation of endothelial cells with an agonist that selectively alters PI3K-Akt-FKHR/FOXO1 signaling should function as a rheostat for VEGF effect on this gene class. Consistent with this hypothesis, we have demonstrated that HGF, a physiological agonist that activates PI3K-Akt, but not NF-{kappa}B,26 blocks the stimulatory effect of VEGF on Class II genes including VCAM-1 and secondary monocyte adhesion.

HGF shares certain properties with VEGF. For example, both growth factors promote endothelial cell proliferation and migration in vitro and induce angiogenesis in vivo.41 However, HGF and VEGF also exert distinct functions in endothelial cells. For example, VEGF increases endothelial cell permeability, whereas HGF promotes barrier function.28 HGF and VEGF induce largely nonoverlapping patterns of gene expression in endothelial cells, suggesting different signal transduction pathways.42 In combination, HGF and VEGF act additively or synergistically to promote endothelial cell survival, proliferation, angiogenesis and downstream gene expression.41 The results of the present study provide evidence for an additional level of cross-talk between HGF and VEGF.

Our data support the hypothesis that HGF inhibits VEGF-mediated gene expression by interfering with the FKHR/FOXO1 arm of the VEGF signaling pathway. Compared with VEGF, HGF induces early, intense, and prolonged phosphorylation and nuclear exclusion of FKHR/FOXO1. By depleting the nucleus of a critical threshold of FKHR/FOXO1 or delaying nuclear reentry of the transcription factor, HGF may deprive the VEGF signal of a necessary transacting protein required for activation of Class II genes (Figure 5). In further support of our hypothesis is the observation that the effect of HGF on VEGF was reversed by constitutively active TM-FKHR/FOXO1.


Figure 5
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Figure 5. Model for HGF inhibition of VEGF-responsive genes that require FKHR/FOXO1 for expression. Left panel, VEGF normally induces expression of Class II genes (ON) via a pathway that requires FKHR/FOXO1 and one or more forkhead-independent transcription factor(s) (indicated by blue oval) for optimal expression. Right panel, HGF or other factors that stimulate PI3K-Akt may promote prolonged phosphorylation and nuclear exclusion of FKHR/FOXO1 and thus attenuate subsequent VEGF induction of Class II genes.

Two alternative explanations for an inhibitory effect of HGF on VEGF-mediated gene expression must be considered. First, it is formally possible that HGF-c-Met signaling inhibits VEGFR2 activation. However, our finding that HGF pretreatment failed to alter VEGF-mediated VEGFR2 phosphorylation argues against this mechanism. Second, HGF may inhibit NF-{kappa}B, which functions with FKHR/FOXO1 to activate many of the Class II genes. Indeed, a previous study showed that when added together, HGF inhibited VEGF-mediated induction of I{kappa}B phosphorylation, p65 nuclear translocation and DNA binding, and VCAM-1/intercellular adhesion molecule-1 (ICAM-1) expression.40 However, we have been unable to demonstrate an effect of HGF on NF-{kappa}B activation in the absence or presence of VEGF. The reason for the discrepancy between the two studies is unclear, but may relate to the use of different cell types (HUVECs in the published report, HCAECs in the current study) or the timing of HGF addition (concomitant with VEGF in the published report, before VEGF in the current study). Regardless of a potential contribution of NF-{kappa}B, our findings point to an important role of the FKHR/FOXO1 pathway in modulating the cross-talk between HGF and VEGF. The functional relevance of this effect is evidenced by the observation that HGF priming reduced VEGF-mediated VCAM-1–dependent adhesion of monocytes. Based on a survey of the HGF-inhibitable Class II genes (Table), we predict that HGF signaling may also attenuate VEGF-inducible superoxide dismutase activity, ESM-1 secretion and MMP10 synthesis.

In summary, we have demonstrated that cross-talk between agonists may influence forkhead-dependent signaling in endothelial cells. Stated another way, the capacity of VEGF to promote expression of these Class II genes is dependent on the "set point" of the cell. These findings raise interesting questions about (patho)physiology. HGF is expressed by many cell types, including endothelial cells and vascular smooth muscle cells. HGF levels are increased in certain diseases, such as cancer and atherosclerosis.43,44 Moreover, recent studies support the therapeutic use of HGF as a means of augmenting angiogenesis. It will be important to consider the potential effects of endogenous or exogenous HGF on the VEGF-FKHR/FOXO1 signaling axis in the endothelium.


*    Acknowledgments
 
Sources of Funding

This work was supported by National Institutes of Health Grant HL077348 (to W.C.A. and M.R.A) and American Heart Association Grant SDG 0453284N (to M.R.A.). T.M. was supported by NIBIO, NEDO, the Science and Technology from Ministry of Education, Culture, Sports, Sciences and Technology, and the Takeda Science foundation, Japan.

Disclosures

None.


*    Footnotes
 
Original received February 27, 2008; final version accepted August 29, 2008.


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*References
 
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