Vascular Biology |
From the Institute for General and Experimental Pathology, University of Innsbruck Medical School (Y.H.), and the Institute for Biomedical Aging Research, Austrian Academy of Sciences (Y.Z., Q.X.), Innsbruck, Austria; the Bristol Heart Institute (A.H.B., A.C.N.), Bristol, UK; and the Department of Cardiological Sciences, St Georges Hospital Medical School (Q.X.), London, UK.
Correspondence to Prof Qingbo Xu, Department of Cardiological Sciences, St Georges Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK. E-mail q.xu{at}sghms.ac.uk
| Abstract |
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Key Words: vein grafts gene transfer tissue inhibitor of metalloproteinase-2 matrix metalloproteinases mouse model
| Introduction |
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Matrix metalloproteinases (MMPs) can degrade most of the vascular extracellular matrix components, including elastin and collagen.48 Their expression could be regulated by the interplay of inflammatory cells and cytokines present in the vein grafts.911 Increased proteolysis by MMPs has been associated with neointimal formation in response to endothelial cell injury.12,13 Tissue inhibitors of MMPs (TIMPs) are a family of 4 members, TIMP-1, -2, -3, and -4.14 All TIMPs share the ability to inhibit MMPs by binding to the active site in 1:1 stoichiometry.14
Recently, the role of MMPs in vascular pathologies has been substantiated by TIMP overexpression studies. TIMP-1 or -2 gene transfer results in the reduction of neointimal lesions of rat carotid arteries after balloon injury.1517 TIMP-3 overexpression inhibits vein graft neointimal formation in a porcine model.18 However, no data are available concerning the effects of TIMP gene transfer on vascular remodeling, ie, vessel diameter changes. Using the mouse model of vein bypass grafts, we studied the role of local TIMP-2 overexpression in vein graft remodeling and demonstrated that TIMP-2 gene transfer significantly reduces the diameters of vein grafts in vivo.
| Methods |
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Local Gene Transfer and ß-Galactosidase Staining
The replication-defective recombinant adenoviruses engineered to overexpress human TIMP-2 from the cytomegalovirus immediate-early promoter (RAdTIMP-2) and ß-galactosidase (RAdLacZ) have been described previously.20 RAdTIMP-2 or RAdLacZ was added locally on the adventitia surface of the grafted veins at 1x1010 plaque-forming units/mL in a total volume of 50 µL and incubated at room temperature for 20 minutes. The remaining liquid RAdTIMP-2 or RAdLacZ was gently removed, and the wound was closed. After 1, 2, 4, and 8 weeks, the mice were euthanized, and vein grafts were harvested and fixed for 60 minutes in 2% formaldehyde plus 0.2% glutaraldehyde in PBS. The procedure for determining ß-galactosidase activity in tissues and sections was similar to that described by Sanes et al.21 Briefly, tissues or sections were incubated at 30°C or 37°C for 18 hours in PBS supplemented with 1 mg/mL 5-bromo-4-chloro-3-indolyl ß-D-galactopyranoside (X-Gal, Sigma Chemical Co), 5 mmol/L potassium ferricyanide, 5 mmol/L potassium ferrocyanide, and 2 mmol/L MgCl2. Tissues were rinsed with 3% dimethyl sulfoxide in PBS and stored in PBS at 4°C.
Histology and Lesion Quantification
For histological analysis, in vivo perfusion with 4% phosphate-buffered formaldehyde was performed, as described previously.19 Vein grafts were harvested by cutting the implanted segments from the native vessels at the cuff end. The grafts were dehydrated in graded ethanol baths, cleared in xylol, embedded in paraffin, and sectioned. Because only 1 or 2 layers of cells are in the media of mouse veins and because there is no morphological border between the neointima and media, neointimal lesions were defined as the region between the lumen and adventitia, which contains capillary blood vessels. For lesional area measurement, sections were reviewed by using a BX60 microscope (Olympus Optical Co Ltd) equipped with a Sony 3CCD camera and television monitor.22 The lesion was defined as the region between the lumen and the adventitia. By use of a transmission scanning microscope (model LSM 510, Zeiss), which was equipped with a 488-nm argon ion laser and Plan Neofluar x10/0.3 oculars and was connected by the program Start LSM 510, images were first scanned and saved and then overlaid by different linings to trace the lumen and adventitia. The lesion area was determined by subtracting the area of the lumen from the area enclosed by the line inside the adventitia. Six to 8 cross sections were obtained by selecting the first of every 3 sections from each graft. Six animals per group were used. Areas were measured and recorded in square micrometers. In the statistical analyses, the individual values for the area from each animal at each time point (4 and 8 weeks) were averaged. The outer diameter of vessels was calculated against the lining length between the adventitia and media/neointima.
Determination of Blood TIMP-2
Blood was collected from mice when vena cava (donor) or vein grafts were harvested. Heparin-plasma TIMP-2 was measured by using TIMP-2 kits (Amercon) according to the manufacturers instructions.
Immunofluorescence Double Staining
For double staining, frozen sections were incubated with the rat monoclonal antibody against MAC-1 and visualized with swine anti-rat immunoglobulin conjugated with TRITC (Dakopatts). Sections were rinsed, stained with rabbit anti-human TIMP-2 antibody (Chemicon International, Inc), and developed with swine anti-rabbit immunoglobulin-FITC (Dakopatts). For double staining of smooth muscle cells, sections were incubated with
-actin-Cy3 (Sigma) and then labeled for TIMP-2. Sections were examined in a confocal microscope equipped with appropriate filter combinations for the 2-wavelength method (Bio-Rad).
In Situ Zymography
MMP inhibitory activity was determined by in situ zymography as described previously.20 Briefly, 8-µm frozen sections on glass slides were coated with LM-1 photographic emulsion (Kodak) diluted 1:2 with incubation medium (50 mmol/L Tris, 50 mmol/L NaCl, 10 mmol/L CaCl2, and 0.05% Brij 35, pH 7.6) and incubated overnight at 37°C. Sections were developed in the light with Kodak D-19 developer and fixed. Gelatinolytic activity was identified as white holes of lysis on a black background. Frozen sections were analyzed at days 14 and 28 after infections.
Statistical Analysis
ANOVA was performed when >2 groups were compared. A paired Student t test was used to assess differences between 2 groups after ANOVA. A value of P<0.05 was considered significant.
| Results |
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To statistically analyze vein graft remodeling, Figure 2 summarizes data of the neointima area and the vessel diameter measured microscopically. Neointimal areas of the adenovirus vector-treated and RAdTIMP-2-treated groups were increased at 4 weeks (Figure 2a). A significant increase in the neointimal areas of the vessel wall in adenovirus vector-treated (8 weeks, n=6) groups was found. RAdTIMP-2 gene transfer led to a decrease of neointimal areas compared with adenovirus vector treatment alone (Figure 2a). In other words, the neointimal lesions enhanced by the adenovirus vector were completely blocked by TIMP-2 gene transfer. Interestingly, overexpression of TIMP-2 in vein grafts resulted in a significant reduction in the vessel diameter (819±96 versus 624±67 µm, P<0.05; Figure 2b), which is similar to carotid arteries in size (598±73 µm in diameter).
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The neointimal lesion has an inflammatory nature characterized by mononuclear cell infiltration in the early stage of vein bypass grafts.19 Local application of adenovirus vector resulted in enhanced inflammatory response (Figure 3a). Occasionally, focal necrosis in the basal region of the neointima and lymphoid tissue in the adventitia were found (Figure 3b and 3c). The cell density of 4-week grafts treated with gene transfer was significantly higher than that in untreated controls, but such a difference disappeared at 8 weeks (Figure 3d). To study the effects of ß-galactosidase on neointimal formation and inflammatory response, we compared the difference in vein grafts treated with either adenovirus carrying the LacZ gene or adenovirus empty vector. There were no significant differences in neointima lesions and cell densities between the 2 groups (data not shown). It seems that adenovirus is the main source inducing inflammatory responses. The dose of adenovirus used in the present study was chosen on the basis of the results of our preliminary experiments, which indicated that it was the lowest dose that resulted in higher TIMP-2 expression in vein grafts.
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Adenovirus-Mediated Gene Transfer
Ex vivo adenovirus-mediated gene transfer to pig vein grafts has been demonstrated to be highly efficient.18 Because of the thinner vessel walls of mouse vein grafts, adenovirus-mediated gene transfer was carried out in vivo after vein grafting. When the vein grafts were harvested and developed with X-Gal, weak staining for ß-galactosidase activity was observed at 7 days (Figure 4a), strong staining was observed at 2 weeks (Figure 4b), and almost no staining was observed at 4 and 8 weeks (Figure 4c and 4d). However, when the sections from 4-week grafts were developed,
5% of the cells in the vessel wall were blue (Figure 4d), but blue staining was not apparent in the 8-week sections (Figure 4f).
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Recombinant TIMP-2 Expressed in Macrophages
To determine cell types locally expressing recombinant TIMP-2, double immunofluorescence staining on sections of vein grafts was performed. TIMP-2 was not detected in vein grafts infected by adenovirus expressing ß-galactosidase at 2 weeks (Figure 5a), although low levels of TIMP-2 were found in the neointima 4 weeks after infection (Figure 5d). In contrast, RAdTIMP-2-infected vein grafts at 2 and 4 weeks showed high levels of immunostaining for TIMP-2 (Figure 5b, 5c, 5e, and 5f). Interestingly, most TIMP-2-positive cells were identified to be MAC-1+ cells, ie, monocytes/macrophages (Figure 5b and 5c), but not smooth muscle cells (Figure 5e and 5f). Abundant TIMP-2 and MAC-1 double positive cells were observed at 2 weeks (Figure 5b), and these positive cells were still found 4 weeks after gene transfer (Figure 5c). The possible mechanism of macrophages expressing high levels of TIMP-2 may involve secondary infections of RAdTIMP-2 that adhered to the tissues of adventitia and/or was released from necrotic cells infected by the adenovirus. Furthermore, increased immunostaining related to the extracellular matrix of vein grafts infected by RAdTIMP-2 was also observed (Figure 5b, 5c, 5e, and 5f), indicating that TIMP-2 released from the infected cells may also be associated with the extracellular matrix proteins. In addition, serum TIMP-2 was not detectable in all animals (data not shown).
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TIMP-2 Overexpression Inhibits MMP Activity
Elevated levels of pro-MMPs and activated gelatinases (MMP-2 and MMP-9) are associated with the formation of vein graft lesions. Moderate levels of MMP activities were detected in control uninfected vein grafts at 2 weeks (Figure 6a), and elevated MMP activities were found in vein grafts infected with adenovirus expressing LacZ genes (Figure 6b). Overexpression of TIMP-2 significantly reduced gelatinase activities in the intima/media and adventitia (Figure 6c). In 4-week vein grafts, MMP activities were at similar levels in untreated and vector-infected groups (Figure 6d and 6e), whereas lower levels of MMP activities in TIMP-2-overexpressing vein grafts were observed (Figure 6f). Thus, TIMP-2 overexpression effectively inhibits MMP activities in the vein grafts of mouse models.
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| Discussion |
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Previous reports have documented that TIMP-2 gene transfer blocks vascular smooth muscle cell migration and reduces neointimal thickness in carotid arteries after balloon injury and in cultured human vein segments.1517,20 The mechanisms of the reduction of neointimal lesions by TIMP-2 gene overexpression appears to involve the inhibition of MMP activities that are essential for smooth muscle cells to disassociate from surrounding matrix cages.26 In the present study, we have demonstrated that TIMP-2 gene transfer blocks neointimal hyperplasia of vein grafts induced by adenovirus infection. There is evidence that adenovirus as a gene transfer vector evokes inflammatory responses.27,28 Similarly, we have also found that adenovirus vector treatment significantly enhances neointimal lesions in vein grafts and that such enhancement is completely overcome by TIMP-2 overexpression (Figure 2a). If a new vector that does not result in the enhancement of neointimal lesions could be available, TIMP-2 gene transfer might prevent or retard neointimal hyperplasia of vein grafts. The detailed mechanism by which TIMP-2 exerts its role in influencing vein graft remodeling remains to be elucidated. Given the fact that maturation of the neointimal lesions is associated with smooth muscle cell migration and a greater preponderance of extracellular matrix, matrix turnover should be 1 of the main factors influencing neointimal formation and/or vein graft remodeling. Therefore, the matrix, rather than being merely a system of scaffolding for the surrounding cells, is a dynamic structure that is central to the control of vascular remodeling.
Evidence indicates that TIMP-2 proteins can be intracellularly produced and released into the extracellular space.29 In this mouse model, we measured serum TIMP-2 concentrations in all groups that were at a similar level (data not shown), suggesting that locally transferred recombinant human TIMP-2 does not lead to significantly elevated peripheral TIMP-2 levels. Furthermore, anti-human TIMP-2 antibody titers in mouse serum were lower (
320 measured by ELISA). Possibly, a single local application of TIMP-2 is not enough to evoke a strong immune response. However, repeated use of human TIMP-2 in the mouse model may result in side effects that are due to immune response.
In the present study, we demonstrated, for the first time, that local TIMP-2 gene overexpression significantly reduces vein graft diameter, ie, remodeling to an artery-like vessel via inhibition of matrix metalloproteinase activity. In the mouse model, decreased MMP activities inhibited by TIMP-2 in vein grafts could result in matrix accumulation constituting the vascular skeleton, which limits the vessel to a diameter similar to that of the carotid artery, because the diameter of the vena cava is larger than that of the carotid artery. Similarly, the diameter of human saphenous vein used for bypass grafts is larger than that of the coronary artery. We speculate that such enlarged or ballooned lumen-induced local alteration in hemodynamic force could be important for the development of later atherosclerosis. TIMP-2 gene overexpression reduces the distinction between grafted veins and anastomosed arteries. Such graft remodeling toward anastomosed arteries might be beneficial for maintaining normal blood flow through the vein grafts. Thus, the remodeling of vein grafts to artery-like vessels resulting from local TIMP-2 gene transfer may lead to prolonged patency.
| Acknowledgments |
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Received October 24, 2000; accepted April 20, 2001.
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