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Circulation Research. 2003;93:1066-1073
Published online before print October 23, 2003, doi: 10.1161/01.RES.0000102867.54523.7F
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(Circulation Research. 2003;93:1066.)
© 2003 American Heart Association, Inc.

Molecular Medicine

Mechanisms of Hepatocyte Growth Factor–Mediated Vascular Smooth Muscle Cell Migration

Harry Ma, Tina M. Calderon, Tamar Kessel, Anthony W. Ashton, Joan W. Berman

From the Departments of Pathology (H.M., T.M.C., T.K., J.W.B.), Microbiology and Immunology (J.W.B.), and Medicine, Division of Molecular Cardiology (A.W.S.), Albert Einstein College of Medicine, Bronx, NY.

Correspondence to Dr Joan W. Berman, Albert Einstein College of Medicine, Department of Pathology, Forchheimer 727, 1300 Morris Park Ave, Bronx, NY 10461. E-mail berman{at}aecom.yu.edu


*    Abstract
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 *Abstract
down arrowIntroduction
 down arrowMaterials and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
The migration of vascular smooth muscle cells (SMCs) from the media into the neointima and their subsequent proliferation is important in the pathogenesis of atherosclerosis. This process is regulated by multiple factors, including growth factors, and involves changes in the interaction of SMCs with the extracellular matrix and in intracellular signaling cascades that regulate cell movement. We demonstrated previously that hepatocyte growth factor (HGF) is expressed in human atherosclerotic plaques. Although HGF has been shown to promote SMC migration, the mechanisms involved in this process have not been characterized fully. In this study, inhibitory antibodies were used to determine which integrins mediated HGF-induced SMC migration. Inhibition of ß1 or ß3 integrin resulted in a significant decrease in migration. Subsequent experiments were performed to characterize additional biochemical mechanisms involved in HGF-mediated migration. HGF induced the redistribution of focal adhesions, the activation of focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) and their increased association with ß1 and ß3 integrins, and the production of pro-matrix metalloproteinase-2. Migration levels were significantly reduced by cotreatment of SMCs with the extracellular signal–regulated kinase 1/2 (ERK1/2) inhibitor, UO126, the p38 inhibitor, SB203580, or the phosphatidylinositol-3 kinase inhibitor, LY294002. In HGF-treated SMCs, focal adhesion redistribution and FAK and Pyk2 activation were decreased by ERK1/2 inhibition. Neither SB203580 nor LY294002 inhibited HGF-induced ERK1/2 activation. Thus, ERK1/2 signaling may play an important role in HGF-mediated SMC migration by contributing to focal adhesion redistribution and FAK and Pyk2 activation.


Key Words: atherosclerosis • smooth muscle cells • migration • integrins • cell signaling


*    Introduction
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 up arrowAbstract
 *Introduction
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 down arrowResults
 down arrowDiscussion
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Vascular smooth muscle cell (SMC) migration, regulated by proatherogenic factors located within the vessel wall, is important in the pathogenesis of atherosclerosis and is the main cause of restenosis after balloon angioplasty. This process involves changes in the interaction of SMCs with the extracellular matrix (ECM) and in intracellular signaling cascades that regulate cell movement. Integrins are cell-surface heterodimeric molecules responsible for cell adhesion to the ECM. Multiple integrins mediate binding to specific ECM proteins, such as collagen ({alpha}1ß1, {alpha}2ß1, and {alpha}vß3).1,2 In addition to their adhesive function, integrins also initiate signaling pathways through their ability to activate secondary intracellular signaling molecules, including tyrosine and serine/threonine kinases.3 Integrins and growth factor receptors can induce signaling events that act synergistically to modulate cell function.4 Thus, cross talk between growth factor receptor and integrin signaling pathways can regulate the cellular machinery necessary for directed cell migration.

The cytoplasmic tail of most integrin molecules lacks endogenous enzymatic activity, and therefore the recruitment of intracellular adaptor proteins is necessary to connect activated integrins to the cytoskeleton and to cytoplasmic kinases, leading to the formation of focal adhesion complexes.5–7 Cytoskeletal proteins, including paxillin and vinculin, and the signaling molecules focal adhesion kinase (FAK), proline-rich tyrosine kinase 2 (Pyk2), and phosphatidylinositol-3 kinase (PI3-K) are recruited to focal adhesion complexes during cell migration.8 Nonmigrating cells establish areas of focal contact throughout the cell to maintain stable adhesion to the ECM. When a migratory stimulus is introduced, there is turnover of focal contacts with disassembly of areas of cell-matrix interaction and reestablishment of focal adhesion complexes at the leading edge of cells to allow for cell movement and resistance to contractile forces during migration.9

FAK, a nonreceptor tyrosine kinase, is recruited to focal adhesion complexes and is postulated to integrate growth factor and integrin signals involved in cell migration.7 Pyk2, a member of the FAK family, also localizes to focal adhesion complexes and is activated by multiple stimuli, including growth factors, integrin ligation, and G-protein–coupled receptors.10,11 Both FAK and Pyk2 have been shown to be involved in actin cytoskeleton reorganization and intracellular signaling during cell migration.12–14

We recently demonstrated that hepatocyte growth factor (HGF), a mesenchymal-derived protein that regulates the growth and motility of various cell types, is expressed in human atherosclerotic plaques, colocalizing with SMCs, microvascular endothelial cells, and monocytes/macrophages.15 The only known receptor for HGF, c-met, was also expressed by plaque SMCs (unpublished data, 2002), suggesting that these cells can respond to HGF in vivo. In endothelial and epithelial cells, HGF induces cell migration, proliferation, and tubule formation.16,17 Recently it was shown that HGF induces SMC migration in vitro, although the mechanisms involved in the establishment of the migratory phenotype have not been characterized fully.18 In this study, we demonstrated that HGF-mediated SMC migration is dependent on ß1 and ß3 integrins and is characterized by a redistribution of focal adhesions to the leading edges of migrating cells. HGF also induced tyrosine phosphorylation of FAK and Pyk2, as well as their association with ß1 and ß3 integrins, and increased the expression of pro-matrix metalloproteinase-2 (pro-MMP-2). Inhibition of extracellular signal–regulated kinase 1/2 (ERK1/2) significantly reduced SMC migration to HGF and also inhibited the redistribution of focal adhesions and the activation of FAK and Pyk2 in HGF-treated SMCs.


*    Materials and Methods
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 *Materials and Methods
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Cells and Reagents
Murine aortic SMCs, a gift from Dr Alison Schecter (Mount Sinai School of Medicine, NY),19 were cultured in DMEM with 10% FCS (passages 12 through 26). Recombinant HGF was obtained from Sigma and Genentech. Platelet-derived growth factor (PDGF) and genestein were purchased from Sigma. Integrin antibodies (ß integrin screening kit [ECM440] and {alpha} integrin and IHC kit [ECM430]) were purchased from Chemicon. U0126, SB203580, and LY294002 were purchased from Calbiochem.

Migration Assay
Migration assays were performed using 0.1% gelatin-coated Transwell (Corning Costar Corp) 12-well tissue culture inserts with 8-µm pores. SMCs were made quiescent in DMEM plus 0.3% FCS for 48 hours before use. Cells (5x104) in 0.1 mL DMEM were added to the upper chamber, and DMEM, with or without HGF or PDGF, was added to the lower chamber. Inserts were incubated for 6 hours at 37°C with 5% CO2. SMCs that did not migrate were scraped off the membrane, and cells that migrated were fixed with methanol and visualized by staining with autohematoxylin (Invitrogen). Migrated cells were counted in five randomly chosen fields at x200 magnification. Each treatment was performed in duplicate.

To inhibit integrin-matrix interactions, SMCs in suspension were incubated with integrin antibodies or negative control IgGs (10 µg/mL) for 30 minutes before addition to the migration chamber. SMCs were also pretreated (30 minutes) with the inhibitors genistein, UO126, SB203580, or LY294002. Cell viability, as determined by trypan blue staining, was not altered by these pretreatments.

Focal Adhesion Staining
SMCs were plated on 0.1% gelatin-coated coverslips for 24 hours and then incubated in serum-free media for another 24 hours before HGF treatment (25 ng/mL). Cells were fixed and permeabilized with 3.7% formaldehyde/0.2% Triton X-100, blocked with 2% BSA/PBS, and incubated with anti-vinculin antibody (1:200, Sigma) or an isotype-matched, irrelevant IgG (1:100) followed by Cy3-conjugated donkey anti-mouse IgG (1:2000, Jackson Immunoresearch Laboratories, West Grove, Pa). Coverslips were mounted with aqueous mounting media and visualized by confocal microscopy (Bio Rad Radiance 2000 Laser Scanning Confocal microscopy).

Immunoprecipitation and Western Blotting
SMCs were lysed with 1% Nonidet P-40, 50 mmol/L Tris-HCl, pH 7.4, 150 mmol/L NaCl, 2 mmol/L EGTA, 0.1 mmol/L Na3VO4, 10 mmol/L NaF, 1 mmol/L PMSF, 50 µL/mL protease inhibitor cocktail for mammalian cell extracts (Sigma), 60 nmol/L okadaic acid, and 40 µmol/L phenylarsine oxide. For immunoprecipitations, antibodies (4 µg) were coupled to protein G-agarose beads and incubated with 400 µg of protein (as determined by Bradford assay) overnight. Beads were pelleted, washed, boiled in sample buffer, and subjected to SDS-polyacrylamide gel electrophoresis. Protein lysates (10 or 20 µg) were analyzed by Western blotting with monoclonal antibodies to phosphotyrosine (1:2000, NeoMarkers), FAK, Pyk2 (1:1000, Santa Cruz Biotechnology), ERK1/2, phospho-ERK1/2 (Thr202/Tyr204), p38, phospho-p38 (Thr180/Tyr182), AKT, or phospho-AKT (Ser473) (1:1000, Cell Signaling) followed by chemiluminescence detection (Perkin Elmer).

Integrin Antibody Array Analysis
Antibodies (0.5 µg) against integrin subunits {alpha}1 through {alpha}6, {alpha}v, ß1, and activated ß1 (ß1A) (Chemicon) were immobilized on PVDF membranes. Membranes were blocked, incubated with protein lysates (0.25 mg/mL) from untreated or HGF-treated (25 ng/mL) SMCs (15 minutes), and immunoblotted with HRP-labeled FAK antibody (Santa Cruz Biotechnology). FAK-integrin association was detected by chemiluminescence.

Gelatin Zymography
SMCs, cultured in serum-free media for 24 hours, were treated with different concentrations of HGF (diluted in 0.1% BSA/PBS) for 24 hours, and control cultures were treated with equivalent amounts of 0.1% BSA/PBS alone. MMP activity in conditioned media was determined by zymography.20 Briefly, conditioned media was collected and supernatant proteins were precipitated with an equal volume of cold acetone, pelleted, and resuspended in PBS. Samples (1 µg) were loaded onto 7.5% SDS-polyacrylamide gels containing 0.1% gelatin. After electrophoresis, gels were washed in 2.5% Triton X-100 and incubated at 37°C in development buffer (50 mmol/L Tris, 200 mmol/L NaCl, 5 mmol/L CaCl2, and 0.02% Brij) overnight. Gels were stained with 0.2% Coomassie blue and destained (30% methanol and 10% acetic acid). Gel Images were captured digitally using the Fluorchem Imaging System (Alpha Innotech Corporation).

Statistical Analysis
Data are presented as mean±SEM. Statistical analysis was performed using a paired one-tailed Student’s t test. Values of P<=0.05 were considered significant.


*    Results
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 up arrowIntroduction
 up arrowMaterials and Methods
 *Results
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HGF Induces SMC Migration
To characterize signaling pathways involved in the induction of SMC migration by HGF, an in vitro transwell migration assay was established using inserts coated with gelatin (denatured collagen) to mimic the collagen-rich matrix of atherosclerotic plaques.21 SMCs were added to the top compartment of transwells, HGF or PDGF was added to the lower compartment, and the number of transmigrated cells was quantified after 6 hours. HGF (25 ng/mL) and PDGF (20 ng/mL) induced a significant increase in SMC migration compared with transwells containing DMEM media alone (Figure 1). There was no significant difference between HGF- and PDGF-induced SMC migration. HGF at 50 ng/mL did not induce greater SMC migration than did 25 ng/mL (data not shown). The incubation time required to produce maximal SMC migration was determined in prior experiments (data not shown).



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  		Figure 1. HGF induces SMC migration. SMCs were added to the upper side of gelatin-coated tissue culture inserts and allowed to migrate for 6 hours in response to DMEM with or without HGF (25 ng/mL) or PDGF (20 ng/mL) in the lower chamber. After microscopic evaluation of inserts, migrated cells were expressed as number of cells per high power field (# Cells/HPF). HGF and PDGF induced significant SMC migration compared with migration in the absence of growth factor (*P<0.001, n=4).

Integrin-Mediated Migration
SMCs express the ß1, ß3, ß4, and ß5 as well as the {alpha}1 through {alpha}6 and {alpha}V integrin subunits.22 HGF-induced SMC migration occurred only on matrix-coated inserts and not on uncoated inserts (data not shown); therefore, we performed an initial characterization of the specific integrins involved in the migratory response of SMCs to HGF. SMCs were pretreated with integrin subunit antibodies (ß1 through ß5, {alpha}1 through {alpha}6, {alpha}V) (10 µg/mL) or with negative control IgGs before their addition to gelatin-coated inserts. Inhibition of ß1 or ß3 significantly inhibited HGF-induced SMC migration (Figure 2). Antibodies to ß2, ß4, or ß5 did not inhibit HGF-induced migration. Integrins containing ß1 and ß3 chains, such as {alpha}1ß1, {alpha}2ß1, {alpha}Vß3, {alpha}4ß1, and {alpha}5ß1, are largely responsible for cellular adhesion to different types of collagen.1,2 Anti-{alpha}1-6 and anti-{alpha}V antibodies and negative control IgGs had no effect in the transmigration assays (data not shown). Additional attempts to inhibit integrin-matrix interactions by increasing antibody concentrations or by treating with both ß1 and ß3 antibodies resulted in nonspecific inhibition of migration with the negative control antibodies (data not shown).



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  		Figure 2. HGF-induced SMC migration is mediated by integrins. Before their addition to culture inserts, SMCs were pretreated with antibodies (10 µg/mL) to different ß integrin subunits. Inhibition of ß1 or ß3 integrin significantly reduced HGF-induced SMC migration compared with untreated SMCs (*P<0.04, n=4).

HGF Induces Redistribution of Focal Adhesions
To determine whether HGF induces the redistribution of focal adhesions to the leading edges of migrating cells, HGF-treated SMCs were examined by confocal microscopy for focal adhesion localization, as identified by vinculin immunofluorescence. Untreated SMCs had a homogeneous distribution of focal adhesions (Figure 3A). After the addition of HGF for 15 minutes, there was an overall decrease in the number of focal adhesions as well as redistribution to the leading edges of cells (arrows) (Figure 3B). This effect was transient, because focal adhesions were more evenly distributed throughout the cell by 30 minutes (data not shown), and by 3 hours the distribution of focal adhesions in HGF-treated SMCs (Figure 3D) appeared similar to that of control cells (Figure 3C).



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  		Figure 3. HGF induces redistribution of focal adhesions. A, Control SMCs have an even distribution of focal adhesions, as detected by confocal analysis of vinculin immunofluorescence. B, Treatment with 25 ng/mL of HGF for 15 minutes leads to the redistribution and concentration of focal adhesions to the edges of the cells (arrows). By 3 hours, focal adhesions in control (C) and HGF-treated (D) SMCs were similar (n=3).

HGF Induces FAK and Pyk2 Phosphorylation
FAK is an important component of focal adhesions. PDGF, a potent inducer of SMC migration, stimulates the phosphorylation of FAK and paxillin, and these phosphorylation events correlate with the migratory response of several cell types to chemotactic factors.23 Pyk2 is also expressed by SMC and phosphorylates paxillin.24 To determine whether FAK or Pyk2 is activated in HGF-treated SMCs, FAK and Pyk2 were immunoprecipitated from HGF-treated SMCs and analyzed by Western blotting with a phosphotyrosine antibody. HGF treatment for 5 minutes resulted in increased levels of phosphorylated FAK, which remained elevated to a lesser extent after 15 minutes of treatment (Figure 4) and returned to baseline levels by 30 minutes (data not shown). The time course of HGF-induced FAK phosphorylation correlated with the redistribution of focal adhesions in HGF-treated SMCs. HGF-induced Pyk2 phosphorylation was similar to FAK (Figure 4).



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  		Figure 4. HGF induces tyrosine phosphorylation of FAK and Pyk2. Immunoprecipitation (IP) of FAK and Pyk2 from SMCs, followed by Western blotting (WB) for phosphotyrosine, demonstrated HGF (25 ng/mL)-induced phosphorylation of both FAK and Pyk2. Blots were stripped and incubated with FAK and Pyk2 antibodies to demonstrate equal protein loading (n=2).

HGF Increases FAK and Pyk2 Association With ß1 and ß3 Integrins
During migration, focal adhesion formation involves many proteins, including integrins, that bind to the ECM as well as to intracellular signaling molecules such as FAK and Pyk2.25–27 We demonstrated that HGF-induced migration is dependent on either ß1 or ß3 integrin interactions with gelatin and that HGF induces the phosphorylation of FAK and Pyk2. Therefore, we examined whether HGF treatment of SMCs resulted in the association of FAK or Pyk2 with these integrins. Immunoprecipitation of ß1 or ß3 integrins indicates that there is an increased association of FAK and Pyk2 with both integrins after HGF treatment for 5 minutes (Figure 5A). In addition, an immunoprecipitation assay using an antibody array consisting of {alpha} and ß1 integrin antibodies bound to PVDF membranes was performed to additionally characterize integrin-FAK interactions in HGF-treated cells. Arrays were incubated with protein lysates from untreated or HGF-treated (15 minutes) SMCs followed by immunoblotting with a FAK antibody (Figure 5B). Increased levels of FAK were associated with activated ß1 (ß1A) and {alpha}1, {alpha}6, and {alpha}v integrins in HGF-treated cells compared with untreated cells, as quantified by densitometry (Figure 5C). These data suggest that FAK and Pyk2 recruitment to ß1, ß3, {alpha}1, {alpha}6, or {alpha}v integrins may contribute to HGF-induced SMC migration.



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  		Figure 5. HGF induces FAK and Pyk2 association with ß and {alpha} integrins. A, Immunoprecipitation of ß1 and ß3 integrins followed by Western blot analysis for FAK and Pyk2 showed that treatment with HGF (25 ng/mL) for 5 minutes resulted in increased association of ß1 and ß3 integrins with both FAK and Pyk2 (n=2). B, Antibody array of membrane-bound antibodies to {alpha} and ß1 integrins was incubated with protein lysates from untreated or HGF (25 ng/mL)-treated (15 minutes) SMCs followed by immunoblotting for FAK. The negative control was membrane-bound IgG antibody. C, HGF induced increased FAK association with activated ß1 (ß1A), {alpha}1, {alpha}6, and {alpha}v integrins, as quantified by densitometry (n=1).

HGF-Mediated Migration Is Dependent on ERK1/2, p38, and PI3-K
We demonstrated that FAK and Pyk2 are activated in HGF-treated SMCs. FAK and Pyk2 are proposed to function as regulators of the ERK1/2 signaling pathway in SMCs.12,28,29 PDGF-induced migration of SMCs has been shown to be dependent on the activation of ERK1/2.28 To determine whether ERK1/2 activation is involved in HGF-induced SMC migration, cells were pretreated with the ERK1/2 inhibitor, U0126, and migration assays were performed. ERK1/2 inhibition significantly reduced HGF-mediated SMC migration (Figure 6A). SMCs were also pretreated with genistein, a general tyrosine kinase inhibitor, to confirm the specificity of HGF signaling through its tyrosine kinase receptor, c-met. Genistein completely inhibited HGF-induced SMC migration (Figure 6A). The level of SMC migration was lower than in previous experiments and is most likely attributable to differences in HGF preparations (HGF from Sigma induced less SMC migration compared with HGF from Genentech). Western blot analysis showed that increased ERK1/2 phosphorylation in SMCs treated with HGF was detected at 1 minute, with maximal increases occurring from 3 to 15 minutes of treatment (Figure 6B). UO126 abrogated HGF-induced (15-minute) ERK1/2 phosphorylation.



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  		Figure 6. HGF-induced SMC migration is dependent on tyrosine kinases, ERK1/2, p38, and PI3-K. A, SMCs were pretreated with genestein (Gen), UO126 (UO), SB203580 (SB), or LY294002 (LY) for 30 minutes before use in the migration assay. HGF significantly induced untreated SMC migration (*P<0.003). SMCs pretreated with each inhibitor exhibited significantly decreased migration to HGF (**P<0.05, n=4). B, SMCs treated with HGF (25 ng/mL) (1, 3, 5, and 15 minutes) or HGF (25 ng/mL) plus UO126 (10 µmol/L) (15 minutes) were analyzed by Western blot for phosphorylated ERK1/2 (Phospho ERK1/2). The blot was stripped and incubated with ERK1/2 antibody to demonstrate protein loading (n=1). C, SMCs treated with HGF (25 ng/mL) and UO126, SB203580, or LY294002 (10 µmol/L) (15 minutes) were analyzed by Western blot for phosphorylated ERK1/2, p38, and AKT. HGF treatment induced ERK1/2, p38, and AKT phosphorylation, and inhibition of p38 or PI3-K did not reduce HGF-induced ERK1/2 phosphorylation. Blots were stripped and incubated with ERK1/2, p38, and AKT antibodies to demonstrate protein loading (n=2).

Additional signaling molecules, including p38 and PI3-K, have been shown to be involved in HGF-induced migration of other cell types. To determine the role of p38 and PI3-K in HGF-mediated SMC migration, assays were performed with cells pretreated with the p38 inhibitor SB203580 or the PI3-K inhibitor LY294002. Inhibition of p38 or PI3-K significantly reduced HGF-induced migration (Figure 6A). Western blot analyses were performed to determine whether p38 or PI3-K activation was involved in the HGF-induced activation of ERK1/2 in SMCs. Figure 6C illustrates that HGF induced the phosphorylation of p38 and AKT, a downstream substrate of PI3-K. Inhibition of either p38 or PI3-K did not block ERK1/2 phosphorylation in HGF-treated SMCs (Figure 6C).

ERK1/2-Dependent Focal Adhesion Redistribution
Studies have suggested that activated ERK1/2 is associated with focal adhesion complexes.11,30,31 We examined whether inhibition of ERK1/2 affected the redistribution of focal adhesion complexes after HGF stimulation. Figure 7B illustrates HGF-induced redistribution of focal adhesions. When SMCs were pretreated with U0126, the redistribution of focal adhesion complexes was inhibited, in particular the disassembly of existing focal adhesions (Figure 7D). Because FAK and Pyk2 are regulators of focal adhesion activity, we examined whether ERK1/2 inhibition affected HGF-induced FAK and Pyk2 activation. Pretreatment of SMCs with U0126 resulted in minimal but reproducible decreases in the activation of FAK and Pyk2 (Figure 7E).



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  		Figure 7. Redistribution of focal adhesions and phosphorylation of FAK and Pyk2 is dependent on ERK1/2. A, Untreated SMCs exhibited an even distribution of focal adhesions, as detected by confocal analysis of vinculin immunofluorescence. B, Treatment with 25 ng/mL HGF (15 minutes) resulted in the redistribution and concentration of focal adhesions to the edges of cells (arrows). C, Pretreatment of SMCs with U0126 alone (10 µmol/L) had no affect on focal adhesions in the absence of HGF. D, UO126 (10 µmol/L) inhibited the redistribution of focal adhesions induced by HGF. E, UO126 decreased HGF-induced tyrosine phosphorylation of both FAK and Pyk2, as detected by immunoprecipitation and Western blot analysis (n=2).

HGF Induces Pro-MMP-2 From SMCs
SMC migration is facilitated by MMPs.20,32 Thus, we examined whether HGF could induce the production of the gelatinase, MMP-2. HGF treatment resulted in increased pro-MMP-2 expression, as determined by zymography, compared with control cultures treated with an amount of BSA equivalent to that added to each HGF treatment group (Figure 8).



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  		Figure 8. HGF induces pro-MMP-2 expression. Concentrated supernatants from HGF or BSA-treated SMCs (24 hours) were analyzed by zymography. HGF-treated SMCs exhibited increased pro-MMP-2 expression compared with BSA treatment (n=2).


*    Discussion
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 up arrowMaterials and Methods
 up arrowResults
 *Discussion
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Growth factor stimulation of SMC migration and proliferation is involved in neointimal development during atherosclerotic plaque progression.33 We demonstrated previously that HGF is expressed in atherosclerotic human vessels, colocalizing with neointimal SMCs and monocytes/macrophages.15 We also found c-met expression on SMCs in both the neointima and media (unpublished data, 2002), indicating that these SMCs can respond to HGF. Neointimal expression of HGF and the presence of c-met–positive SMCs within the media and neointima suggest that HGF may be involved in SMC recruitment to the developing plaque. To determine whether SMCs would respond to HGF, migration assays were performed in this study and HGF was found to significantly induce SMC migration. These data suggest that the production of HGF in the plaque may serve as a chemotactic factor for SMC, recruiting cells from the media.

SMC migration in vivo occurs in an ECM composed of proteins that can modulate SMC function through interactions with cell-surface adhesion molecules. Binding of ECM proteins by their respective cell-surface receptors is believed to have more than an anchoring effect. Integrin-mediated binding of matrix proteins results in activation of signal transduction pathways that are important for SMC migration.4 We found in this study that HGF-induced SMC migration occurs on gelatin (denatured collagen)-coated inserts. Increased expression of collagens has been demonstrated in atherosclerotic plaques, and they are often a major component of the fibrous cap.21

SMCs express several integrin subsets that interact with collagen. We examined which subset of integrins that can bind to gelatin/collagen is involved in HGF-induced migration. Our studies demonstrated that inhibition of ß1 or ß3 integrin interactions significantly reduced the migratory effects of HGF on SMCs. Pretreatment of cells with ß1 or ß3 integrin antibodies either directly inhibited migration or inhibited adhesion to the matrix, resulting in an indirect inhibition of migration. Inhibition of individual {alpha} subunits did not affect HGF-induced SMC migration, suggesting that ß1 or ß3 integrins with different {alpha} subunits may interact with gelatin, providing redundancy in the {alpha} subunit contribution to integrin-mediated regulation of SMC migration. Alternatively, the {alpha} subunit of ß1 or ß3 integrins may not contribute to the induction of the migratory phenotype elicited by HGF. Integrin inhibition in vivo results in a decrease in neointimal formation after vascular injury,34 underscoring the importance of integrin-mediated migration in atherosclerosis. Recent studies have shown that HGF can affect integrin avidity to matrix proteins as well as upregulate integrin levels, suggesting that HGF may affect integrin-mediated migration in several ways.35

HGF-induced migration is dependent on a redistribution of focal adhesion complexes in epithelial and carcinoma cell lines.36–38 We examined whether HGF induced the formation or redistribution of focal adhesions in SMCs. Focal adhesions are a complex of proteins that link the ECM to the cytoskeleton of the cell through integrin-mediated associations. We demonstrated that HGF induces a redistribution of focal adhesions in SMCs. This process allows migrating cells to attach to the ECM at the leading edge and to detach from the trailing edge. Our data also suggest that HGF-induced activation of ERK1/2 plays a role in the disassembly of existing focal adhesions, possibly through activation of FAK and Pyk2. HGF-induced FAK phosphorylation in epithelial cells was previously shown to be dependent on ERK1/2 activation.37 The time course of ERK1/2 phosphorylation in HGF-treated SMCs suggests that ERK1/2 may contribute to the activation of FAK and Pyk2. ERK1/2 phosphorylation is induced after 1 minute of HGF treatment, and this early activation may contribute to the phosphorylation of FAK and Pyk2, which precedes the redistribution of SMC focal adhesions.

Studies of HGF-mediated migration in tumor cells demonstrated that FAK activation plays a role in cell migration as well as the activation of other signaling proteins, such as PI3-K, ERK1/2, and p38.36–40 We demonstrated that HGF induces FAK activation in SMCs, and an in vivo study documented FAK activation in SMCs during neointimal hyperplasia.41 These findings suggest that FAK, in addition to integrins, may be a mediator of HGF-induced migration. FAK has been shown to link the cytoplasmic domain of ß1 integrins to focal adhesions, implicating FAK as a link between ß1 integrin activation and focal adhesion formation in HGF-treated cells.7,8 Because we have shown that HGF-induced migration of SMCs is dependent on ß1 integrins, the redistribution of focal adhesions that occurs after HGF stimulation may be dependent on FAK activation.

ERK1/2 inhibition significantly reduced SMC migration to HGF. ERK1/2 is required for PDGF-induced SMC migration.28 Once activated, ERK1/2 has been shown to regulate integrin levels in MDCK cells.42 Activation of ERK1/2 by growth factors can lead to the subsequent activation of myosin light chain kinase, which facilitates cell contractility and migration.43 Our data suggest that ERK1/2 activity is also important in regulating the redistribution of focal adhesions during cell migration, and this MAPK may affect the phosphorylation of FAK and Pyk2. However, because ERK1/2 inhibition did not completely abolish HGF-induced SMC migration, it suggests that there are other signal transduction pathways involved in HGF-induced SMC migration. Additional signaling molecules reported to play important roles in cell migration include p3844 and PI3-K.45,46 Inhibition of either of these signaling pathways also significantly inhibited HGF-induced SMC migration. However, ERK1/2 activation seems to be independent of p38 and PI3-K, because SB203580 or LY294002 does not inhibit HGF-induced ERK1/2 phosphorylation. These data suggest that ERK1/2 signaling events involved in HGF-mediated SMC migration may not require p38 or PI3-K.

Migration of cells surrounded by ECM is facilitated by MMPs, and HGF has been shown to induce endothelial cell production of MMPs. However, its effects on MMP production by SMCs were unknown.47 Our data show that HGF can induce the production of pro-MMP-2. Expression of MMP-2 in atherosclerotic plaques has been previously demonstrated.20,48 MMP-2 degrades gelatin/collagen, a major component of the fibrous cap, suggesting that it may participate in plaque instability or rupture. Thus, HGF may contribute to plaque instability by inducing the production of MMPs from SMCs. Conversely, HGF may stabilize the plaque by recruiting more SMCs to the neointima, which then secrete collagen, contributing to fibrous cap formation.


*    Acknowledgments
 
Acknowledgments

This work was supported by National Institutes of Mental Health grant MH52974, National Institutes of Health grant NS11920, and the National Institutes of Health Medical Scientist Training Program.


*    Footnotes
 
Original received December 17, 2002; resubmission received June 13, 2003; revised resubmission received October 9, 2003; accepted October 14, 2003.


*    References
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