Published online before print August 17, 2007, doi: 10.1161/CIRCRESAHA.107.151415
© 2007 American Heart Association, Inc.
Molecular Medicine |
Loss of the 
7 Integrin Promotes Extracellular Signal-Regulated Kinase Activation and Altered Vascular Remodeling
From the Department of Pharmacology (J.V.W., N.L., C.A.S., M.E., P.S., W.T.G., D.J.B.), Nevada Transgenic Center (P.S., D.J.B.), and Department of Physiology (K.D.K.), University of Nevada School of Medicine, Reno.
Correspondence to Dean J. Burkin, PhD, Department of Pharmacology, University of Nevada School of Medicine, Reno, NV 89557. E-mail dburkin{at}medicine.nevada.edu
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Abstract |
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Vascular smooth muscle cell (VSMC) proliferation and migration are underlying factors in the development and progression of cardiovascular disease. Studies have shown that altered expression of vascular integrins and extracellular matrix proteins may contribute to the vascular remodeling observed after arterial injury and during disease. We have recently shown that loss of the
7ß1 integrin results in VSMC hyperplasia. To investigate the cellular mechanisms underlying this phenotype, we have examined changes in cell signaling pathways associated with VSMC proliferation. Several studies have demonstrated the mitogen-activated protein kinase signaling pathway is activated in response to vascular injury and disease. In this study, we show that loss of the 7 integrin in VSMCs results in activation of the extracellular signal-regulated kinase and translocation of the activated kinase to the nucleus. Forced expression of the 7 integrin or use of the mitogen-activated protein kinase kinase 1 inhibitor U0126 in 7 integrin–deficient VSMCs suppressed extracellular signal-regulated kinase activation and restored the differentiated phenotype to 7 integrin–null cells in a manner dependent on Ras signaling. 7 Integrin–null mice displayed profound vascular remodeling in response to injury with pronounced neointimal formation and reduced vascular compliance. These findings demonstrate that the 7ß1 integrin negatively regulates extracellular signal-regulated kinase activation and suggests an important role for this integrin as part of a signaling complex regulating VSMC phenotype switching.
Key Words: 7ß1 integrin • ERK MAP kinase • VSMC proliferation • vascular remodeling • integrin knockout mice
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Introduction |
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Vascular smooth muscle cell (VSMC) proliferation and migration are major underlying factors in the development and progression of various forms of cardiovascular disease, including atherosclerosis, postangioplasty restenosis, transplant arteriopathy, and pulmonary hypertension.1 Vascular remodeling during disease or injury involves altered expression of extracellular matrix proteins and cell surface integrins.2–4 After arterial injury, laminin expression is reduced and fibronectin accumulates around VSMCs.5,6 These changes coincide with a phenotypic switch in which contractile VSMCs adopt a proliferative phenotype, possibly as part of a developmental program associated with wound repair.2,4
Integrins are transmembrane mechanosensors that relay signals from the extracellular matrix to the cell cytoskeleton and/or cell signaling pathways to modulate cell shape, adhesion, differentiation, proliferation, and contraction.7 Various integrins modulate cell proliferation, usually by crosstalk with proliferative cell signaling pathways or in cooperation with growth factor receptors.8
The 7ß1 integrin is a major laminin-binding receptor in VSMCs, and expression of this integrin increases after differentiation.9 Previous studies using blocking antibodies and peptides have demonstrated that the 7ß1 integrin mediates adhesion of VSMCs to laminin in vitro.9,10 Expression of this integrin has also been shown to be modulated by chemically induced injury and platelet-derived growth factor in cultured rat VSMCs.10,11 We have previously demonstrated that embryonic loss of the 7 integrin results in vascular defects and partial embryonic lethality, whereas in adult mice, loss of the 7 integrin results in VSMC hyperplasia.12 Loss of the 7ß1 integrin in VSMCs leads to altered expression of other integrin chains, which may contribute to the vascular phenotype observed in 7 integrin–null mice.12 These observations have led to the hypothesis that the 7ß1 integrin promotes the contractile phenotype of VSMCs, but the mechanism of this regulation is unclear.
Activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase signaling pathway is critical to the progression of VSMC proliferation in response to vascular injury or disease.13,14 Mitogen activation of ERK has been shown to depend on cell adhesion; however, ERK activation can also occur independently of mitogens.15,16 Interestingly, laminin has been demonstrated to maintain the differentiated state of VSMCs through activation of the p38 MAP kinase signaling pathway.5 In contrast, fibronectin promotes the synthetic phenotype through activation of ERK MAP kinase pathway.5 Although there is mounting evidence that changes in extracellular matrix–dependent cell adhesion can regulate MAP kinase activity,17 the role that the 7ß1 integrin plays in regulating the activity or localization of ERK within VSMCs remains unknown.
Because our previous work demonstrated that 7 integrin–null mice exhibit VSMC hyperplasia, and activation of the ERK MAP kinase signaling is important for VSMC proliferation, we hypothesized that loss of this integrin may lead to increased ERK activation. To test this hypothesis, VSMC proliferation, differentiation, and ERK phosphorylation were examined in 7 integrin–deficient mice and isolated VSMCs. Our results show for the first time that loss of the 7 integrin in VSMCs in vivo results in ERK activation. Forced expression of the 7 integrin or the use of Ras and MAP kinase kinase 1 (MEK1) inhibitors suppressed ERK signaling in 7 integrin–null cells and promoted expression of differentiation markers. Finally, 7 integrin–null mice displayed extreme vascular remodeling in response to injury accompanied by pronounced neointimal formation. These results demonstrate the 7ß1 integrin promotes the differentiated phenotype of VSMCs through suppression of ERK activation and reveal an important role for this integrin as part of an inhibitory signaling complex that suppresses VSMC proliferation.
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Materials and Methods |
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7 Integrin–Null Mice and Isolation of Mouse VSMCsWild-type (C57BL6 strain) and
7 integrin–null (C57BL6-7ßgal strain) mice were euthanized in accordance with a protocol approved by the University of Nevada, Reno Animal Care and Use Committee. Aortic mouse VSMCs were isolated from 10-week-old mice as described in the online data supplement, available at http://circres.ahajournals.org.
Production of an Ad-itga7 Adenovirus and VSMC Transduction Protocol
The rat 7 integrin cDNA (IMAGE Clone 7313230) in pExpress-1 was obtained from American Type Tissue Collection. Recombinant adenovirus was generated using the AdEasy Adenoviral Vector System (Stratagene, La Jolla, CA) to produce the Ad-itga7. For details regarding virus production, see the online data supplement.
Western Blot Analysis
Protein from vascular tissue or VSMCs was extracted as described in the online data supplement. To detect the 7 integrin, smooth muscle -actin (SMA), tropomyosin, and smooth muscle myosin heavy chain (SMMHC), 15 to 20 µg of protein was loaded on polyacrylamide gels (Bio-Rad Laboratories Inc, Hercules, Calif). To detect phosphorylated (p)ERK1/2 and ERK1/2, 50 µg of protein was loaded on a polyacrylamide gel.
The 7 integrin was identified with a rabbit polyclonal antibody anti-7B (B2 347) (Dr Stephen Kaufman, University of Illinois, Urbana) at a dilution of 1:2000. pERK1/2 and ERK1/2 were detected with a rabbit polyclonal anti-pMAPK44/42 antibody (Cell Signaling Technology Inc, Danvers, Mass) and a goat polyclonal anti-ERK (Santa Cruz Biotechnology Inc, Santa Cruz, Calif) at a dilution of 1:1000, respectively. SMA was identified with a mouse polyclonal anti–-SMA antibody (Sigma Aldrich, St Louis, Mo) at a dilution of 1:2000, and SMMHC was detected with a rabbit polyclonal anti-SMMHC antibody (Biomedical Technologies Inc, Stoughton, Mass) at a dilution of 1:1000. The smooth muscle isoform of tropomyosin was detected with a mouse monoclonal anti-tropomyosin antibody (T 2780) (Sigma Aldrich). Primary antibodies were detected as described previously.18 Blots were normalized for protein loading as published previously.18
Immunofluorescence
Primary aortic VSMCs were seeded (5x104) on coverslips and fixed in either cold methanol or 4% paraformaldehyde for 2 minutes, washed in PBS, then washed 6 times with a 0.05% Triton solution (0.05% Triton X-100, 15 mmol/L sodium citrate, 150 mmol/L NaCl), and then blocked in PBS containing 5% BSA for 30 minutes. pERK1/2 was detected using pMAPK44/42 (Cell Signaling Technologies Inc) at a 1:100 dilution followed by a 1:1000 dilution of fluorescein isothiocyanate–conjugated anti-rabbit secondary antibody (Jackson ImmunoResearch Laboratories, West Grove, Pa). A Cy3-labeled anti-SMA monoclonal antibody (Sigma Aldrich) was used at a dilution of 1:500. Coverslips were mounted in Vectashield+DAPI (Vector Laboratories, Burlingame, Calif). A Zeiss Axioskop 2 Plus fluorescence microscope was used to visualize sections, and images were captured with a Zeiss Axiocam HRc digital camera and Axiovision 4.1 software.
ERK Localization in VSMCs
Cultured VSMCs were subjected to immunofluorescence using anti-pERK1/2. Nuclei positive for pERK1/2 were counted in wild-type and 7 integrin–null mice and expressed as a percentage of the total number of nuclei in the field. The percentage of pERK1/2-positive nuclei was determined over 10 random fields at x400 magnification. The average percentage of cells with nuclear located ERK1/2 in each genotype were calculated, and statistical significance (P<0.05) was determined using a Student’s t test.
Inhibition of MEK1/2
Approximately 5x104 wild-type and 7 integrin–null aortic VSMCs were seeded on 35-mm plates containing coverslips. Cells were treated with either U0126 (1,4-diamin-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene) (10 µmol/L) (Calbiochem, San Diego, Calif) to inhibit MEK1/2 or U0124 (10 µmol/L, a nonactive form of U0126). Cells were incubated in DMEM lacking serum for 8 days, and then inhibitors were added for 48 hours. Coverslips were removed from each cell culture plate for immunofluorescence to detect pERK1/2 (Cell Signaling Technology Inc) and SMA (Sigma Aldrich). The number of cells positive for SMA and nuclear localization of pERK1/2 was counted in 10 random fields. Data were reported as the percentage of SMA-positive cells with nuclear localization of pERK1/2.
Inhibition of Ras
Approximately 1x106 wild-type and 7 integrin–null VSMCs were seeded on 100-mm plates for Western blot analysis, and 5x104 wild-type and 7 integrin–null VSMCs were seeded on 35-mm plates containing coverslips for immunofluorescence. Cells were growth arrested by adding DMEM lacking serum for 8 days. To inhibit Ras, cells were treated after 7 days of differentiation for 12 hours with the Ras inhibitor manumycin A (50 ng/mL) (Calbiochem). Cells were harvested for Western blot analysis, and coverslips were subjected to immunofluorescence to detect pERK1/2 and SMA. The number of cells positive for SMA, and nuclear localization of pERK1/2 in 10 random fields was counted.
Carotid Artery Ligation Model and Morphometric Analysis
Carotid artery ligation was performed as previously described19 (see the online data supplement). Morphometric analysis was performed on sequential sections from ligated and nonligated arteries (wild type, n=6; 7 integrin null, n=6) using AxioVision measurement software module (Carl Zeiss Imaging, Jena, Germany).
Length–Tension Relationships
Carotid arteries from 10-week-old wild-type and 7 integrin–null mice were carefully dissected as previously published20 (see the online data supplement). Passive tension was measured at each stretch point and used to develop a passive length–tension relationship. Active tension was measured after treatment with KCl and used to develop an active length–tension relationship. The passive and active length–tension relationships were developed using Graph Pad-Prizm 3.0 nonlinear regression analysis (Graph Pad Software, San Diego, Calif). The data are shown as the means±SEM. Data were considered significant at P<0.05 using a 1-way ANOVA.
Statistical Analysis
All averaged data are reported as the means±SD unless otherwise stated. Student’s t tests were preformed using SigmaStat 1.0 software (Jandel Corporation, San Rafael, Calif). To compare multiple groups, 1-way ANOVA was performed using SigmaStat 1.0 software and ad hoc testing (Jandel Corporation). P<0.05 was considered statistically significant.
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Results |
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Loss of
7 Integrin Results in Continued Proliferation of Cultured VSMCsTo determine whether loss of the
7 integrin caused continued cell proliferation in vitro, the average number of VSMCs was counted. As expected, wild-type cells underwent growth arrest between days 2 to 4, so that after 8 days in differentiation medium, the cell population had stabilized (Figure 1A). In contrast, 7 integrin–null cells continued to proliferate (Figure 1A). Therefore, hyperplasia observed in 7 integrin–null mice was maintained in cultured primary VSMCs.
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Reduced Differentiation in Cultured 7 Integrin–Null VSMCs
To determine whether loss of the 7 integrin resulted in decreased differentiation in vitro, wild-type and 7 integrin–null VSMCs were differentiated for 8 days, and the percentage of cells expressing high levels of SMA was determined. Wild-type cells showed a progressive increase in expression of SMA, so that by 8 days, approximately 90% of cells expressed this early differentiation marker (Figure 1B). In contrast, only 30% of 7 integrin–null VSMCs were SMA positive after 8 days of differentiation, despite greater cell numbers (Figure 1B). These results indicate that loss of the 7 integrin results in either a reduced capacity for differentiation or instability in differentiation.
Loss of the 7 Integrin Results in Reduced Expression of VSMC Contractile Proteins
To confirm whether loss of the 7 integrin promotes cell proliferation, Western blot analysis of SMA (early marker for differentiation), tropomyosin (intermediate marker for differentiation), and SMMHC (late marker for differentiation) was performed.21 After 8 days of differentiation, wild-type and 7 integrin–null VSMCs were grown in serum-free media. Wild-type cells had an 
2-fold increase in SMA, tropomyosin, and SMMHC from day 0 to day 8 (Figure 2A through 2C). In contrast, expression of SMA, tropomyosin, and SMMHC did not significantly increase in 7 integrin–null cells (Figure 2A through 2C). The percentage of 7 integrin–null cells expressing SMA in Figure 1 correlates with the reduced expression of VSMC contractile proteins in Figure 2. Together, these results indicate that VSMCs that lack the 7 integrin exhibit a reduced capacity to express contractile proteins after serum withdrawal.
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7 Integrin Regulates the MAP Kinase Pathway in VSMCs
To investigate whether the VSMC proliferation observed in 7 integrin–null mice and cells was attributable to the activation of the MAP kinase signaling pathway, protein extracted from wild-type and 7 integrin–null aortae and cultured aortic VSMCs was subjected to Western blot analysis. Blots were probed sequentially with antibodies against pERK1/2 and total ERK1/2. ERK1/2 phosphorylation in the aortae from 7 integrin–null mice was 2-fold higher compared with wild type (Figure 3A). ERK1/2 phosphorylation in cultured VSMCs isolated from 7 integrin–null mice was 1.5-fold higher than cells from wild-type animals (Figure 3B). These data provide strong evidence for activation of the MAP kinase signaling pathway in the vascular smooth muscle of 7 integrin–null mice and in primary cultured VSMCs isolated from these mice.
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VSMCs From 7 Integrin–Null Mice Exhibit Nuclear Localization of ERK1/2
Phosphorylation of ERK1/2 results in translocation of the kinase into the nucleus, where it regulates transcription factors to promote cell growth and proliferation. Wild-type and 7 integrin–null VSMCs were differentiated in DMEM containing 0% serum for 8 days. Only 2% of differentiated wild-type cells showed nuclear localization of pERK1/2 (Figure 4A and 4B). In contrast, 44.8% of 7 integrin–null cells had pERK1/2-positive nuclei (Figure 4A and 4B). These results provide evidence that loss of the 7 integrin in differentiated VSMCs results in nuclear localization of activated ERK, which may act to promote continuous cellular proliferation and corresponds with increased ERK phosphorylation shown in Figure 3.
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MEK1/2 Inhibition Restores Expression of Differentiation Markers in 7 Integrin–Null VSMCs
To determine whether inhibition of the MAP kinase cell signaling pathway reduces proliferation in 7 integrin–null cells, VSMCs were differentiated for 8 days in serum-free medium and treated for 48 hours with either the MEK1/2 inhibitor U0126 or the nonactive control compound U0124. Inhibition of ERK1/2 phosphorylation was confirmed by Western blot analysis (Figure 5A). Treatment of cells with U0124 did not affect the nuclear localization of pERK in either wild-type or 7 integrin–null VSMCs (Figure 5B). Only 2% of differentiated wild-type cells showed nuclear localization of pERK. In contrast, we observed pERK nuclear localization in 45% of 7 integrin–null cells. Treatment with U0124 had no effect on ERK localization. In contrast, treatment with U0126 significantly reduced the percentage of 7 integrin–null VSMCs with nuclear pERK to almost wild-type levels (Figure 5B).
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We next analyzed the percentage of 7 integrin–null VSMCs that expressed SMA after 8 days in serum-free medium (Figure 5C). In untreated or U0124 treated cells, >90% of wild-type cells expressed SMA, whereas only 40% of 7 integrin–null cells expressed SMA. In contrast, treatment with U0126 resulted in 85% of 7 integrin–null cells expressing SMA (Figure 5C). Together, these data show that pharmacological inhibition of the MAP kinase signaling pathway in 7 integrin–null cells blocks the nuclear localization of pERK1/2 and restores expression of an early marker for VSMC differentiation close to wild-type levels.
Ras Inhibition Reduces ERK1/2 Activation in 7 Integrin–Null Cells
To determine whether ERK1/2 activation observed in 7 integrin–null cells was Ras dependent, wild-type and 7 integrin–null VSMCs were growth arrested for 8 days and Ras activity inhibited by treatment with manumycin A for 12 hours. Manumycin A is a potent inhibitor of Ras farnesyltransferase and has been shown to block Ras and phosphorylation of ERK1/2.22 Inhibition of activated ERK1/2 was confirmed by Western blot analysis (Figure 6A). The number of cells with nuclear localized ERK1/2 was determined by immunofluorescence (Figure 6B). Only 2% of wild-type cells were positive for nuclear localization of pERK1/2, whereas 45% of 7 integrin–null cells showed nuclei with ERK1/2 localization. Use of the inhibitor manumycin A reduced nuclear localization of pERK1/2 in 7 integrin–null cells to 13% (Figure 6B).
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Next, we determined whether the Ras inhibitor restored differentiation to 7 integrin–null VSMCs. Whereas 94% of wild-type cells were positive for SMA, only 52% of 7 integrin–null cells expressed SMA (Figure 6C). Treatment with manumycin A resulted in 79% of 7 integrin–null cells expressing SMA after serum deprivation (Figure 6C). Together, these results suggest that inhibition of the Ras signaling pathway blocks activated ERK1/2 and increases expression of SMA in 7 integrin–null VSMCs.
Forced 7 Integrin Expression Restores VSMC Differentiation
The replication defective adenovirus, Ad-itga7, was used to investigate whether forced 7 integrin expression could restore differentiation in cultured 7 integrin–deficient VSMCs. VSMCs were infected with Ad-itga7 or the control virus Ad-GFP and cultured for 6 days in serum-free medium. Ad-GFP control did not alter SMA expression in 7 integrin–null cells (Figure 7A). In contrast, 7 integrin–null cells infected with Ad-itga7 showed the same percentage of cells expressing SMA as wild type (Figure 7A). These results demonstrate that the 7ß1 integrin regulates key molecular pathways in vascular smooth muscle, which maintain VSMC differentiation.
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Restoration of 7 Integrin Expression Inhibits ERK Activation
To determine whether the 7ß1 integrin negatively regulates ERK activation in VSMCs, we used Ad-itga7 to restore expression of the integrin in differentiated 7 integrin–null cells (Figure 7B). In untreated and Ad-GFP (empty vector)-treated 7 integrin–deficient cells incubated for 8 days in serum-free medium, we observed an 30% increase in ERK1/2 activation compared with wild-type controls. In contrast, treatment with Ad-itga7 reduced ERK phosphorylation to below wild-type levels (Figure 7B). These results indicate that restoration of 7 integrin expression suppresses activation of the ERK MAP kinase signaling pathway in 7 integrin–deficient VSMCs.
Vascular Remodeling in 7 Integrin–Null Mice After Ligation Injury
To determine whether vascular damage in 7 integrin–null mice leads to neointima formation, wild-type and 7 integrin–null carotid arteries were ligated for 28 days. Carotid arteries lacking the 7 integrin developed a dramatic neointima as well as a thickening of the medial layer and narrowing of the lumen after ligation injury (Figure 8A through 8C). In contrast, wild-type ligated vessels did not form a measurable neointima after 28 days (Figure 8A and 8C). Morphometric analysis was used to assess medial wall thickness and neointima formation from sections 1000 µm proximal from the ligation site. The medial layer of the undamaged contralateral carotid artery in 7 integrin–null mice was increased 1.3-fold compared with the wild type, reflecting the hyperplasia observed in these mice (Figure 8B). After ligation, a 1.6-fold increase in medial wall thickness was observed in 7 integrin–null arteries compared with the wild type (Figure 8B). These results indicate loss of the 7ß1 integrin contributes to dramatic vascular remodeling and neointimal development.
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Loss of the 7 Integrin Increases the Passive Length–Tension Relationship of Carotid Arteries
To determine whether loss of the 7 integrin affected blood vessel function, the passive and active length–tension relationships for carotid arteries from wild-type and 7 integrin–null mice were measured (Figure 8D and 8E). The contractile response to 90 mmol/L KCl in wild-type and 7 integrin–null vessels showed similar maximum active force development at all lengths, suggesting that the contractility of 7 integrin–null carotid arteries was not grossly altered (Figure 8D). However, a significant upward shift in the passive length–tension curve was observed (P<0.05) (Figure 8E). This result is consistent with changes in the composition and/or deposition of extracellular matrix in the carotid arteries of 7 integrin–null mice resulting in a less compliant blood vessel.
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Discussion |
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In this study, we have demonstrated that loss of the
7 integrin in VSMCs results in activation of the MAP kinase cell signaling pathway, cellular proliferation, and neointimal expansion. Restoring 7 integrin expression by using MAP kinase or Ras inhibitors in 7 integrin–null cells suppressed ERK activation and promoted the expression of differentiation markers. Although integrin-mediated cell adhesion signaling is often a positive regulator that promotes anchorage-dependent growth, this study suggests that integrins may also negatively regulate proliferative pathways to promote VSMC contractile phenotype.
Studies have demonstrated that the MAP kinase signaling pathway is rapidly activated in response to vascular injury.14 In hypertensive rats, ERK and c-Jun NH2-terminal kinases are chronically activated in VSMCs, suggesting that the MAP kinase signaling pathways play a critical role in the progression of pulmonary hypertension.13 We have demonstrated that 7 integrin–null mice exhibit vascular defects both in utero and in the adult.12 Our results show that loss of the 7 integrin in vivo results in chronic ERK activation and suggest a role for the 7ß1 integrin in regulating cell signaling pathways that mediate VSMC phenotype.
Upstream regulation of the MAP kinase pathway is complex, involving extracellular stimuli that include mechanical stretch, neurotransmitters, growth factors, and cytokines, all exerting their effects through cell surface receptors and the cell cytoskeleton. Increased activation of ERK in 7 integrin–deficient vascular smooth muscle suggests the 7ß1 integrin may directly or indirectly suppress this proliferative signaling pathway in a Ras-dependant manner in normal, differentiated VSMCs.
Restoration of 7 integrin expression in 7 integrin–deficient VSMCs suppressed ERK activation and restored VSMC differentiation to wild-type levels. These data provide evidence for a clear relationship between 7 integrin expression and activation of the MAP kinase pathway. The Grb2 (growth factor receptor–bound protein 2) can associate with the cytoplasmic domain of integrins.23,24 Grb2 mediates recruitment to the cell periphery of Ras guanine nucleotide exchange factor Sos (Son-of-sevenless), leading to Ras activation. Activated Ras then stimulates ERK phosphorylation, which translocates into the nucleus to promote cellular growth and proliferation.25,26
An indirect mode of regulation by the 7ß1 integrin may involve altered adhesion of VSMCs with the extracellular matrix. VSMCs in the artery wall are normally surrounded by a basement membrane rich in extracellular matrix proteins that include collagen IV, laminin, and elastin, which promote the contractile phenotype and regulate vascular tone.1,5,6,27 In uninjured blood vessels, VSMCs are unresponsive to mitogens, suggesting that the extracellular matrix may act as a barrier to proliferative signals.28 Loss of 7ß1 integrin may result in reduced continuity between VSMCs and laminin in the surrounding basal lamina, increasing the availability of cell surface receptors on VSMCs to interact with and respond to growth factors.
We demonstrate that 7 integrin–null mice exhibit a severe vascular remodeling phenotype. Injury to the blood vessel resulted in profound neointimal expansion and the development of a thicker medial layer. Loss of the suppressive effects of the 7ß1 integrin on the ERK MAP kinase signaling or chronic ERK activation may promote the severe vascular remodeling phenotype observed in these animals. Together, these results suggest the 7ß1 integrin may be a major genetic modifier in the progression of cardiovascular disease.
Recent studies demonstrate that changes in the vascular microenvironment can have significant effects on vascular remodeling. Loss of the ß3 integrin in VSMCs inhibits ligation-induced neointimal development.29 The absence of fibulin-5, an elastin-binding protein, promotes an exaggerated vascular remodeling response after carotid ligation-induced injury.30 The 7ß1 integrin has been shown to promote laminin deposition in skeletal muscle myoblasts.31 If the 7ß1 integrin regulates laminin deposition in VSMCs, then loss of this integrin could lead to increased access of mitogenic stimuli, reduced VSMC adhesion, and increased VSMC migration.
This study shows that loss of the 7 integrin translates into a less compliant blood vessel. Reduced compliance was also observed in diaphragm muscle from 1-year-old 7 integrin–null mice.32 Changes in the deposition and/or composition of extracellular matrix or expression of the repertoire of integrins in vascular smooth muscle may contribute to this phenotype.
The phenotype of VSMCs is critical to the progression of vasculoproliferative diseases.2 Results from the present study provide evidence that integrins are critical to regulating signaling pathways that mediate VSMC phenotype. Understanding how changes in the local vascular environment translates to altered integrin activation and MAP kinase cell signaling may lead to novel targets for therapies for cardiovascular disease.
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Acknowledgments |
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Sources of Funding
This study was supported by National Center for Research Resources/NIH grants P20 RR018751-01 and P20 RR15581-04 (to D.J.B.) and P20 RR018751-07 (to W.T.G.).
Disclosures
None.
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Footnotes |
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Original received February 28, 2007; revision received July 10, 2007; accepted August 3, 2007.
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