Release of platelet-derived growth factor activity from pig venous arterial grafts

Vein graft failure

After the creation of an autogenous lower extremity bypass graft, the vein must undergo a series of dynamic structural changes to stabilize the arterial hemodynamic forces. These changes, which are commonly referred to as remodeling, include an inflammatory response, the development of a neointima, matrix turnover, and cellular proliferation and apoptosis. The sum total of these processes results in dramatic alterations in the physical and biomechanical attributes of the arterialized vein. The most clinically obvious and easily measured of these is lumen remodeling of the graft. However, although somewhat less precise, wall thickness, matrix composition, and endothelial changes can be measured in vivo within the healing vein graft. Recent translational work has demonstrated the clinical relevance of remodeling as it relates to vein graft patency and the systemic factors influencing it. By correlating histologic and molecular changes in the vein, insights into potential therapeutic strategies to prevent bypass failure and areas for future investigation are explored.

Local application of rapamycin inhibits vein graft restenosis in rabbits

OBJECTIVE

The aim of this study was to investigate whether local application of rapamycin reduced neointimal formation in a rabbit model of venous disease.

METHODS

Each rabbit (n = 30; 2.5-3.5 kg) received a treated and a control graft. For the treated graft, 0.3 rapamycin mg was applied locally in Pluronic gel. The control graft received only the Pluronic gel. Grafts were harvested at 28 days for morphometric, immunohistochemical, and flow cytometry analysis.

RESULTS

In the control group, the intimal thickness was 63.72 ± 14.0 μm; in treated group, it was 77.76 ± 14.9 μm (P < .05). Immunohistochemically, proliferation cell nuclear antigen-positive cells were present in the control group and in the treatment group but not in normal external jugular veins. The control group showed much stronger expression than the treatment group (P < .05). Flow cytometry showed, among the control group, decreased G(0)G(1)-stage cells and increased S/G(2)M-stage cells. Among the treatment group, S/G(2)M stage cells were decreased compared with the control. The progression indexes of the control and treatment groups were 29.3 ± 7.15 and 20.1 ± 9.48, respectively, a remarkable decrease (P < .05). Proliferating cells in the control group were apparently inhibited by rapamycin. The treatment group showed positive staining for P27(kip1), but neither the control group nor the normal external jugular veins showed positive results (P < .05). The degree of reduction in intimal thickness and inhibition of proliferating cells in the treatment group correlated with the expression of P27(kip1).

CONCLUSIONS

We observed that perivascular application of rapamycin inhibited neointimal hyperplasia of vein grafts in a rabbit model, an effect that appeared to result from increased P27(kip1) expression.

In vivo modulation of Nogo-B attenuates neointima formation

Nogo-B was recently identified as a novel vascular marker; the normally high vascular expression of Nogo-B is rapidly lost following vascular injury. Here we assess the potential therapeutic effects of Ad-Nogo-B delivery to injured vessels in vivo. Nogo-B overexpression following Ad-Ng-B infection of vascular smooth muscle cells (VSMCs) was shown to block proliferation and migration in a dose-dependent manner in vitro. We next assessed the effects of Ad-Ng-B treatment on neointima formation in two in vivo models of acute vascular injury. Adventitial delivery of Ad-Ng-B to wire-injured murine femoral arteries led to a significant decrease in the intimal area [0.014 mm(2) versus 0.030 mm(2) (P = 0.049)] and the intima:media ratio [0.78 versus 1.67 (P = 0.038)] as compared to the effects of Ad-beta-Gal control virus at 21 days after injury. Similarly, lumenal delivery of Ad-Ng-B to porcine saphenous veins prior to carotid artery grafting significantly reduced the intimal area [2.87 mm(2) versus 7.44 mm(2) (P = 0.0007)] and the intima:media ratio [0.32 versus 0.55 (P = 0.0044)] as compared to the effects following the delivery of Ad- beta-Gal, at 28 days after grafting. Intimal VSMC proliferation was significantly reduced in both the murine and porcine disease models. Gene delivery of Nogo-B exerts a positive effect on vascular injury-induced remodeling and reduces neointimal development in two arterial and venous models of vascular injury.

Accelerated intimal hyperplasia in aortocoronary internal mammary vein grafts in minipigs

Background

More than 50% of aortocoronary saphenous vein grafts are occluded 10 years after surgery. Intimal hyperplasia is the initial critical step in the progression toward occlusion. Internal mammary veins, which are physiologically prone to less hydrostatic pressure, may undergo an accelerated progression to intimal hyperplasia and thus be suitable for investigation of the mechanisms of aortocoronary vein graft disease.

Methods

Six minipigs underwent aortocoronary bypass grafting using standard cardiopulmonary bypass and cardioplegic arrest. Mammary vein were grafted in a reversed manner from ascending aorta to left anterior descending coronary artery (LAD). The proximal LAD was ligated, rendering the anterior left ventricle vein graft-dependent. Minipigs were killed after 4 weeks, and vein grafts were harvested. Histological and immunohistological investigation were performed with respect to morphometric analysis, endothelial damage/dysfunction (v-Willebrand-factor (vWF)), smooth muscle cells (α-smooth actin) and proliferation rate (proliferation marker Ki 67).

Results

Mean intimal area of vein grafts was increased compared to ungrafted mammary veins. Intimal hyperplasia in vein grafts was characterized by massive accumulation of smooth muscle cells with a high proliferation rate and endothelial perturbation. Significant (p = 0.001) intimal hyperplasia of the grafted mammary vein compared to the ungrafted mammary vein was found. These changes were absent in ungrafted mammary veins.

Conclusion

The present study demonstrates a pig model of aortocoronary vein graft intimal hyperplasia which is characterized by an accelerated progression within internal mammary veins. The model is suitable to investigate the pathophysiology of aortocoronary vein graft intimal hyperplasia as well as therapeutic approaches.

Gene expression changes evoked in a venous segment exposed to arterial flow

OBJECTIVE

This study was conducted to characterize the coordinated molecular changes evoked in the structure and composition of the wall of a venous segment when exposed to fistula flow.

METHODS

An arteriovenous shunt was created in adult C57BL/6J mice. Remodeled veins and contralateral control jugular veins were isolated 7 days after surgery. Total RNA was isolated, linearly amplified, and the transcriptional profiles of this early adaptive response were obtained by microarray analysis. Histologic and immunohistochemical analyses were performed on remodeled veins and control veins isolated on days 1, 3, 5, and 7 after surgery to further examine distinct spatial and temporal aspects of this early process.

RESULTS

There were 131 significantly upregulated and 165 downregulated genes in the remodeled vein compared with the control jugular vein. Genes involved in extracellular matrix reorganization were highly upregulated. Movat's pentachrome staining revealed ground substance on day 3 that was not observed on day 5. The appearance of elastin fibers was first observed on day 7. Morphometric analysis demonstrated maximum wall thickness on day 3. Immunohistochemical analysis revealed the presence of tenascin-C, thrombospondin, lysyl oxidase, and osteopontin in different cell types at different time points throughout the first week after surgery.

CONCLUSION

Major changes in the organization of the extracellular matrix occur during the early response of venous remodeling. Elastin, tenascin-C, thrombospondin, lysyl oxidase, and osteopontin are expressed within the wall of the remodeling vein resulting in the de novo formation of an extracellular matrix scaffold that may be part of a critical adaptation program being evoked to allow the vessel to cope with its new biomechanical environment.

CLINICAL RELEVANCE

The Kidney Dialysis Outcomes Quality Initiative has proposed the construction of arteriovenous fistulas as the primary vascular access for hemodialysis. As the vein is exposed to arterial flow, the vein wall dilates and a vascular remodeling process is triggered. With continued exposure, intimal hyperplasia occurs at the anastomosis that in many cases leads to failure. However, the molecular mechanisms by which the outflow vein remodels into a mature fistula remain incompletely understood. By investigating venous remodeling in a fistula model, candidate genes important for the remodeling process are discovered and their functional significance examined. Thus, the identification of relevant genes involved in this process should provide insight into arteriovenous fistula maturation and may suggest novel approaches for achieving higher patency rates.

Effect of vein graft harvesting on endothelial nitric oxide synthase and nitric oxide production

BACKGROUND

Although the saphenous vein is the most commonly used conduit for coronary artery bypass surgery occlusion rates are high, with more than 50% grafts failing within 10 years. Nitric oxide, a potent vasodilator, also inhibits platelet aggregation, thrombus formation and vascular smooth muscle cell proliferation, is implicated in various vascular pathologies, including graft failure.

METHODS

Saphenous veins were obtained from patients undergoing bypass surgery harvested by conventional methods and with minimal handling, using a "no-touch" technique. Tissue distribution and protein expression of endothelial nitric oxide synthase was compared using immunohistochemistry and Western blot analysis. Nitric oxide generation was assessed using the citrulline assay.

RESULTS

There was injury to conventional compared with no-touch vein segments, in particular to the lumenal endothelium and tunica adventitia. This injury was accompanied by an absence of endothelial nitric oxide synthase immunostaining at regions of endothelial denudation and damaged adventitial layer of conventional veins and a significant reduction (p < 0.05) in endothelial nitric oxide synthase protein expression. Furthermore, nitric oxide release from conventional tissue extracts was significantly (p < 0.05) lower than no-touch vein segments.

CONCLUSIONS

Our results show that there is a reduction in endothelial nitric oxide synthase and nitric oxide release in saphenous veins harvested by conventional surgical methods compared with those prepared atraumatically. These observations may influence graft performance.

Pharmacologic inhibition of vein graft neointimal hyperplasia

Although arterial conduits are widely used and have improved the long-term results of coronary artery bypass grafting, vein grafts remain important additional conduits in coronary surgery. Newer studies show a saphenous vein graft patency of 60% or more at 10 years postoperatively. The pathology of vein graft disease consists of thrombosis, neointimal hyperplasia, and vein graft atherosclerosis, which limit graft longevity. Therapeutic strategies to prevent vein graft disease include external stenting, pharmacotherapy, and gene therapy. The potential benefits of a pharmacologic approach are as follows: (1) Drugs with a broad clinical experience can be used; (2) side effects of systemic application can be minimized by local therapy; and (3) no vascular injury, such as pressurizing the vein for a viral transfection approach, is necessary. The different sites for pharmacotherapy in vein graft disease are reviewed in this article.

Assessment of subacute inflammatory and proliferative response to coronary stenting in a porcine model by local gene expression studies and histomorphometry

The aim of the study was to analyse inflammatory and proliferative response early after coronary stenting by angiography, histomorphometry and local gene expression analysis using quantitative rt-PCR. Therefore, eight German domestic pigs underwent stenting of the left coronary artery. Selective coronary angiography was performed after 14 days. Explanted coronary arteries were examined histomorphometrically after methacrylate-embedding. Snap-frozen samples were examined for local gene expression of TGF-beta, TNF-alpha, GM-CSF, VEGF, PDGF and Fas Ligand (FasL) by real-time quantitative rt-PCR normalized to the housekeeping gene GAPDH and compared to unstented coronary arteries. All stented coronaries were patent with only little neointima formation. The median vessel diameter was 2.55 mm (range 2.43-2.68 mm). Histopathology revealed little inflammatory response limited to the tissue surrounding the stent struts; luminal area ranged from 84% to 91%. Compared to unstented control arteries, no significant differences in local gene expression were detected for VEGF, PDGF, TGF-beta, TNF-alpha and GM-CSF. Expression of FasL was upregulated as little as 1.7-fold (p=0.01). We conclude that, in native coronary arteries, no significant upregulation of investigated genes regulating vascular remodelling, inflammation or fibrogenesis was demonstrated 14 days after stenting. Whether upregulation of FasL as a marker gene of apoptosis is transient and biological significant requires further investigation.

Mechanical Stretch Induces Phosphorylation of p38-MAPK and Apoptosis in Human Saphenous Vein

Objective— Failure of saphenous vein grafts remains a major limitation of coronary bypass surgery. The aims of the present study were to determine whether pressure distension of human saphenous vein induces the activation of p38-MAPK and to determine its role in apoptosis.

Methods and Results— Phosphorylated p38 was detected at basal levels in human saphenous vein obtained immediately after harvesting. Distended saphenous vein showed significantly higher levels of phosphorylated p38 compared with control vein ( P <0.01) and nondistended saphenous vein maintained for 3 and 6 hours after harvesting (both P <0.01). Apoptosis in distended and nondistended vein was significantly higher at 24 hours compared with control vein, with distended vein showing increased apoptosis compared with nondistended saphenous vein at all time points investigated ( P <0.001). Immunolocalization showed co-localization of phosphorylated p38 and apoptosis. Inhibition of p38 activity reduced the apoptotic index of cultured vascular smooth muscle cells by 72.1%±1.2% and cultured distended saphenous vein segments by 72.7%±0.9%.

Conclusions— Pressure distension of intact human saphenous vein induces activation of p38, and this is associated with apoptosis. Inhibition of p38 kinase activity in saphenous vein smooth muscle cells and intact vein reduces apoptosis. These findings contribute to our understanding of the mechanisms of saphenous vein graft failure.

Tumor necrosis factor-alpha -308 gene polymorphism is associated with synthetic hemodialysis graft failure

BACKGROUND

Progressive venous stenosis mediated, in part, by inflammatory cytokines is a major cause of synthetic hemodialysis graft failure. A tumor necrosis factor-alpha (TNF-alpha) gene polymorphism (G to A, position -308) has been shown to increase plasma cytokine levels and severity of diseases with an underlying inflammatory component.

METHODS

We genotyped 67 patients with synthetic polytetrafluoroethylene (PTFE) grafts and examined the association of the high-(AA or GA) and low- (GG) production TNF-alpha-08 genotypes with the rate of graft failures/thrombosis and graft survival.

RESULTS

Hemodialysis patients with the high-production TNF-alpha genotypes had a significantly increased rate of PTFE graft failure at 90 days (37.2% versus 14%) and 1 year (62.8% versus 34.4%) after graft placement compared with patients with the low-production genotype (respectively). Hemodialysis patients with the high-production TNF-alpha genotypes had significantly lower cumulative PTFE graft survival at 1 year (29.4% +/- 11.1% versus 71.2 +/- 6.8%) and 2 years (22.1% +/- 10.5% versus 48.2 +/- 8.1%) compared with patients with the low-production genotype (respectively). Patients with the A allele had approximately twice the mean thrombosis rate compared with those who had the low-production TNF-alpha genotype (3.3 +/- 0.8 versus 1.7 +/- 0.4 thromboses/patient/year, respectively; mean +/- SEM, p < .05).

CONCLUSIONS

These data suggest that the TNF-alpha -308 A allele is associated with increased PTFE graft thrombosis and failure in hemodialysis patients.

Transcription factor decoys for the prevention of vein bypass graft failure

Failure of vein bypass grafts, performed for either coronary or lower extremity arterial occlusions, is a common clinical problem that incurs significant morbidity and mortality. Vein grafts provide a unique opportunity for genetic interventions, since the target tissue is available for manipulation ex vivo prior to implantation, and prior to the onset of the pathophysiologic events that lead to graft disease. Smooth muscle cell proliferation is a hallmark of neointimal hyperplasia in vein grafts and arteries, and is an attractive target for molecular therapy. Gene blockade strategies can be accomplished by delivery of small oligodeoxynucleotides (ODN) that target specific mRNAs ('antisense') or that competitively inhibit transcription factors ('decoys'). Transcription factors are attractive targets for molecular therapy since they influence the expression of a large number of genes involved in a coordinated cellular program. An ODN decoy approach has been developed targeting the transcription factor E2F, which is critically involved in cell cycle progression. Brief (10 minute) incubation of the ODN in solution, using non-distending pressure, results in efficient delivery of the ODN to >80% of cells in the vein wall. Preclinical studies in a rabbit model of vein grafting and hypercholesterolemia demonstrated a marked reduction in neointima formation, as well as prolonged resistance to graft atherosclerosis. Phase I/II studies conducted in lower extremity and coronary bypass patients have demonstrated safety and feasibility, and have also suggested possible efficacy. Large, randomized multicenter, phase III trials are currently under way to evaluate the efficacy of E2F decoy treatment on preventing vein bypass failure in both peripheral and coronary grafting procedures. These studies herald the arrival of a new class of molecular agents into the armamentarium of cardiovascular therapies.

Macro-porosity is necessary for the reduction of neointimal and medial thickening by external stenting of porcine saphenous vein bypass grafts

BACKGROUND

placing external non-restrictive macro-porous stents around porcine vein grafts prevents neointima formation and medial thickening in both the short and long term. Whether the porosity of the stent material influences this effect, however, has not been determined. Therefore, the effect on neointimal and medial thickening of external macro-porous (polyester) and micro-porous (polytetrafluorethylene) stents of equal diameter were compared. The effect on expression of platelet-derived growth factor (PDGF), a potent mediator of vascular smooth muscle cell migration and proliferation and its receptors was also investigated.

METHODS AND RESULTS

saphenous vein-carotid artery interposition grafting was performed in Landrace pigs with external placement of 8 mm diameter macro- and micro-porous stents contralaterally. One month after surgery, graft wall dimensions, PDGF and PDGF receptor expression and cell proliferation using proliferating cell nuclear antigen (PCNA) were measured on histological sections. Macro-porous stents significantly reduced neointimal and medial thickening compared with micro-porous stents (0.1+/-0.02 vs. 0.25+/-0.03 mm, P<0.002, and 0.10+/-0.02 vs. 0.17+/-0.02 mm, P<0.014, respectively). Macro-porous stents significantly reduced the percentage of cells expressing PDGF and PCNA, compared with micro-porous stents (36+/-9 vs. 80+/-7, P < 0.002, and 11+/-3 vs. 21+/-2, P < 0.02, respectively). The percentage of cells expressing PDGF receptors was similar with both the stent types. Adventitial microvessel formation occurred across macro-porous stents but was markedly suppressed by micro-porous stents.

CONCLUSIONS

porosity is crucial to the efficacy of external stents in reducing neointima formation in porcine vein grafts. Decreases in PDGF expression and cell proliferation accompany the reduction in neointima formation. In addition, macro-porous stents allow adventitial microvessels to connect with the vasculature outside the stent, thereby potentially improving oxygenation. Although external stenting is highly effective in reducing neointima formation after vein grafting, the properties of the stent material necessary for this effect have not been defined. This study establishes that macro-porosity is one essential feature required to reduce PDGF expression cell proliferation and neointima formation.

Prevention of mechanical stretch-induced endothelial and smooth muscle cell injury in experimental vein grafts

Vein grafts are subject to increased tensile stress due to exposure to arterial blood pressure, which has been hypothesized to induce endothelial cell (EC) and smooth muscle cell (SMC) injury. This study was designed to verify this hypothesis and to develop a tissue engineering approach that can be used to prevent these pathological events. Two experimental models were created in rats to achieve these goals: (1) a nonengineered vein graft with increased tensile stress, which was created by grafting a jugular vein into the abdominal aorta using a conventional end-to-end anastomotic technique; and (2) an engineered vein graft with reduced tensile stress, which was created by restricting a vein graft into a cylindrical sheath constructed using a polytetrafluoroethylene membrane. The integrity of ECs in these models was examined by using a silver nitrate staining method, and the integrity of SMCs was assessed by using a fluorescein phalloidin-labeling technique. It was found that nonengineered vein grafts were associated with early EC denudation with a change in EC coverage from 100 percent in normal jugular veins to 36 +/- 10, 28 +/- 12, 18 +/- 9, 44 +/- 15, 80 +/- 13, and 97 +/- 6 percent at 1 and 6 hours and 1, 5, 10, and 30 days, respectively. Similarly, rapid SMC actin filament degradation was found during the early period with a change in SMC coverage from approximately 94 percent in normal jugular veins to 80 +/- 10, 41 +/- 17, 25 +/- 9, 51 +/- 15, 79 +/- 15, 98 +/- 2 percent at 1 and 6 hours and 1, 5, 10, and 30 days, respectively, in nonengineered vein grafts. In engineered vein grafts with reduced tensile stress, EC denudation and SMC actin filament degradation were prevented significantly. These results suggested that mechanical stretch due to increased tensile stress contributed to EC and SMC injury in experimental vein grafts, and these pathological events could be partially prevented when tensile stress was reduced by using a biomechanical engineering approach.

Molecular mechanisms in intimal hyperplasia

Intimal hyperplasia is the process by which the cell population increases within the innermost layer of the arterial wall, such as occurs physiologically during closure of the ductus arteriosus and during involution of the uterus. It also occurs pathologically in pulmonary hypertension, atherosclerosis, after angioplasty, in transplanted organs, and in vein grafts. The underlying causes of intimal hyperplasia are migration and proliferation of vascular smooth muscle cells provoked by injury, inflammation, and stretch. This review discusses, at a molecular level, both the final common pathways leading to smooth muscle migration and proliferation and their (patho)-physiological triggers. It emphasizes the key roles played by growth factors and extracellular matrix-degrading metalloproteinases, which act in concert to remodel the extracellular matrix and permit cell migration and proliferation.

Gene therapy for vein grafts

Bypass vein graft failure represents the greatest limitation to the current surgical therapy of myocardial and lower extremity ischemia. Elucidation of the molecular and cellular biology of neointimal hyperplasia and subsequent vein graft atherosclerosis has formed a basis for the design and implementation of gene-based therapies to prevent vein graft disease. Manipulation of the genetic regulation of vascular cell cycle progression has been shown to effectively redirect vein graft biology away from neointimal disease and toward medial hypertrophy as a more adaptive form of remodeling in response to stresses of the arterial circulation, and has prevented experimental graft atherosclerosis. Early clinical experience suggests that this approach may provide an early avenue for translation of such a gene-based therapy in humans. Other experimental gene transfer strategies have also been explored in animal models of vein grafts, which may be particularly well suited to the application of genetic manipulation given the direct access to the tissue at the time of disease initiation.

Focal expression of angiotensin II type 1 receptor and smooth muscle cell proliferation in the neointima of experimental vein grafts: relation to eddy blood flow

Eddy flow has been shown to promote focal smooth muscle cell (SMC) proliferation and neointimal formation in experimental vein grafts. This study focuses on whether the angiotensin II type 1 (AT(1)) receptor mediates these events. Experimental vein grafts with and without eddy flow were created in the rat. Losartan was used to assess the influence of the AT(1) receptor on SMC proliferation. In vein grafts with eddy flow, apparent focal expression of AT(1) mRNA and protein was found in the leading region of the proximal focal neointima, where eddy flow occurred, but not in the trailing region, where eddy flow diminished, at days 5, 10, 20, and 30. The rate of SMC proliferation in the leading region (10.9+/-1.4%, 19.5+/-2.2%, 12.2+/-2.0%, and 6.9+/-1.3% at these times, respectively) was significantly higher than that in the trailing region (9.5+/-1.8%, 15.3+/-2.0%, 8.2+/-1.9%, and 3.2+/-0.7%) in these vein grafts. When eddy flow was prevented in engineered vein grafts, no apparent location difference was found in the distribution of AT(1) receptor mRNA and protein in the neointima, and the rate of SMC proliferation (5.3+/-1.0%, 5.8+/-0.9%, 3.4+/-1.0%, and 3.7+/-0.9% at days 5, 10, 20, and 30, respectively) was reduced significantly. In vein grafts with losartan, the rate of SMC proliferation in the leading region of the neointima (9.4+/-1.8%, 10.1+/-1.3%, 8.3+/-0.9%, and 4.2+/-0. 5% at days 5, 10, 20, and 30, respectively) was significantly lower than that in vein grafts without losartan. These results suggested that eddy flow upregulated the AT(1) receptor, which in turn mediated focal SMC proliferation in the neointima of experimental vein grafts.

Partial prevention of monocyte and granulocyte activation in experimental vein grafts by using a biomechanical engineering approach

Leukocytes interact with endothelial cells and contribute to the development of vascular diseases such as thrombosis and atherosclerosis. These processes are possibly influenced by mechanical factors. This study focused on the role of mechanical stretch in the activation of monocytes and granulocytes in experimental vein grafts. Two models were created by using rats: a nonengineered vein graft with increased tensile stress, which was created by grafting a jugular vein into the abdominal aorta, and an engineered vein graft with reduced tensile stress, which was created by restricting the vein graft into a cylindrical sheath constructed by using fixative-treated intestinal tissue. The density of activated monocytes and granulocytes, which attached to the endothelium, and the distribution of the intercellular adhesion molecule (ICAM)-1 in endothelial cells were examined using immunohistological assays. It was found that, in nonengineered vein grafts, the density of activated monocytes and granulocytes increased significantly compared to that in normal jugular veins at day 1, 5, 10 and 20. At each observation time, the cell density in the proximal region of the nonengineered vein grafts was significantly higher than that in the middle and distal regions, and the cell density in the distal region was significantly higher than that in the middle region. These changes were associated with ICAM-1 clustering at day 1 and 5 and focal ICAM-1 un-regulation at day 10 and 20. In engineered vein grafts, the density of activated monocytes and granulocytes decreased significantly compared to that in nonengineered vein grafts at all observation times, although it was significantly higher than that in normal jugular veins. At each observation time, the cell density in the proximal and distal regions was significantly higher than that in the middle region, but no significant difference was found between the proximal and distal regions. ICAM-1 clustering along endothelial cell borders was found at day 1 and 5, but no apparent focal ICAM-1 up-regulation was found at day 10 and 20. These results suggested that mechanical stretch due to exposure to increased tensile stress contributed to the activation of monocytes and granulocytes in experimental vein grafts, and this event could be partially prevented by reducing tensile stress using a biomechanical engineering approach.

Association of smooth muscle cell phenotypic modulation with extracellular matrix alterations during neointima formation in rabbit vein grafts

PURPOSE

To clarify the mechanisms of structural changes underlying vein graft stenosis that limits efficacy of bypass grafting operation, we examined the accumulation and distribution of various extracellular matrix (ECM) components during neointima formation in rabbit vein grafts and analyzed their correlation with proliferation and phenotypic modulation of smooth muscle cells (SMCs).

METHODS AND RESULTS

An autologous external jugular vein graft was transplanted into the carotid artery in 25 rabbits. After the restoration of blood flow, the graft was markedly dilated. Medial SMCs in the graft appeared to be injured, and they began to proliferate at day 4 and subsequently migrated and formed the neointima at day 7. The neointima observed at days 7 and 14 contained ECM components, including type I collagen, heparan sulfate, and chondroitin sulfate, and the intimal SMCs were phenotypically modulated from the differentiated-type (SM2-positive and SM embryonic-negative) to the dedifferentiated-type (SM2-negative and SM embryonic-positive) as determined with immunostainings for myosin heavy chain isoforms. The intimal SMC proliferation was maximal at 2 weeks and then decreased rapidly. However, the neointima continued to thicken thereafter throughout the 6-month period of the experiment, and ECM accumulation, such as type I collagen and decorin, a small dermatan sulfate proteoglycan, was a prominent feature observed in the hypocellular region of the deep intima from 2 months after the transplantation. The phenotype of the intimal SMCs gradually returned to the differentiated-type from the deep intima after 2 months, but a small number of the intimal SMCs remained in the dedifferentiated phenotype even at 6 months after the operation.

CONCLUSION

The neointima in the vein graft was formed initially by means of migration and proliferation of the phenotypically modulated, dedifferentiated-type SMCs and continued to thicken by means of sustained ECM accumulation, including type I collagen and decorin, in association with the prolonged presence of the dedifferentiated-type SMCs. These chronologic features in cell kinetics and ECM accumulation may contribute to the frequent occurrence of graft wall thickening that occurs in the vein grafts.

Increased secretion of basement membrane-degrading metalloproteinases in pig saphenous vein into carotid artery interposition grafts

Late saphenous vein bypass graft failure in humans involves medial and neointimal thickening as the result of migration and proliferation of vascular smooth muscle cells (SMCs). Recent work on angioplasty indicates that basement membrane-degrading metalloproteinases (MMPs) cooperate with growth factors to mediate SMC migration and proliferation. We sought evidence here for a similar role in experimental vein grafts in pigs. Tissue levels and secretion of MMP-2 and MMP-9 were compared by quantitative zymography in veins and grafts removed 2 to 168 days after implantation. Pro and active forms of MMP-2 were present in veins, but levels were increased in vein grafts after 7 days (4- and 6-fold, respectively) and 28 days (3-fold for both), returning to values in veins after 168 days. MMP-9 was not detected in veins, was increased in grafts after 2 days, further increased after 7 days (6-fold) and 28 days (15-fold), and declined to undetectable levels by 168 days. Immunocytochemistry detected increased expression of MMP-2 and MMP-9 with the same time course. MMP-2 was widely distributed, whereas MMP-9 was concentrated in highly proliferative SMCs at the superficial layers of the thickened neointima. We conclude that increased production of the basement membrane-degrading MMP-2 and MMP-9 spanned the period of neointima formation and SMC proliferation in experimental vein grafts. MMPs therefore constitute new therapeutic targets for reducing late vein graft failure.

Gene transfer to vein graft wall by HVJ-liposome method: time course and localization of gene expression

BACKGROUND

A novel gene transfer method using liposomes with a viral envelope of hemagglutinating virus of Japan (HVJ) has been reported to be very effective for gene transfection into somatic cells and might be applicable to improve the patency of vein grafts. The present study examined the time course and localization of gene expression to assess the feasibility of ex vivo gene transfer into the vein graft by the HVJ-liposome method.

METHODS

The HVJ-liposome complex containing either beta-galactosidase plasmid DNA (deoxyribonucleic acid) or no genes (controls) (experiment 1) or fluorescein isothiocyanate-labeled oligonucleotides either with or without HVJ-liposomes (experiment 2) was infused into rabbit vein grafts and allowed to incubate before autologous transplantation to carotid arteries.

RESULTS

In experiment 1, all grafts incubated with beta-galactosidase plasmid with HVJ-liposomes showed the blue staining of X-gal 7 days after operation, whereas the controls did not. The blue granules were present in the medial and adventitial tissue and were still present after 14 days. In experiment 2, many fluorescein isothiocyanate-labeled nuclei were observed in the graft wall 2 and 4 days after operation and remained present mainly in the media of HVJ-liposome-treated grafts after 7 and 14 days, when no fluorescein isothiocyanate activity was observed without HVJ-liposome treatment.

CONCLUSIONS

These results demonstrated the feasibility of ex vivo transfection to the medial and adventitial tissue of the vein graft by the HVJ-liposome method and suggest the possibility of its clinical application to prevent vein graft failure.

The effect of radiographic contrast media on human vascular smooth muscle cells

The relation between intravascular radiographic contrast media (RCM) and myointimal hyperplasia after percutaneous transluminal angioplasty is not known. We have investigated the cytotoxic effects of RCM on human vascular smooth muscle cells (VSMCs) and their effect on the growth of these cells. The cytotoxic effects of RCM were studied using human VSMCs. The cells after being grown to confluency were exposed for 60 min to 250 mgI ml-1 of diatrizoate, ioxaglate, iopromide, iotrolan and saturated mannitol solutions. The control group was treated with only 15% fetal calf serum (FCS) containing medium. The viability of the cells was examined using the trypan blue exclusion test. The effect of RCM on growth was assessed by exposing the VSMCs after growth arrest, for either 15 or 60 min to 250 mgI ml-1 of diatrozoate, ioxaglate, iopromide, iotrolan and saturated mannitol solution. There was no significant change in the viability of the VSMCs after 60 min exposure to iopromide, iotrolan, saturated mannitol solution, and after 15 min exposure to diatrizoate or ioxaglate. After exposure to diatrizoate or ioxaglate for 60 min, 16.5 +/- 2.2% or 9.2 +/- 2.6% dead cells were found, respectively (p < 0.05 versus control). In the growth assay of VSMCs, diatrizoate, ioxaglate and saturated mannitol solutions reduced the growth rate (p < 0.05 versus control). No significant change was observed with iopromide and iotrolan. In conclusion, ionic RCM have cytotoxic and cytostatic effects on VSMCs while non-ionic media have no effects. There is no direct stimulatory effect of contrast media on the growth of VSMCs. The cytotoxic and cytostatic effects of contrast media seems to be both osmolality and chemotoxicity dependent. Low osmolar non-ionic RCM are not likely to contribute to the mechanisms responsible for myointimal hyperplasia after angioplasty.

External stenting reduces long-term medial and neointimal thickening and platelet derived growth factor expression in a pig model of arteriovenous bypass grafting

Bypass of stenotic coronary arteries with autologous saphenous vein is an established treatment for ischemic heart disease. However, its long-term clinical success is limited. Late vein graft failure is the result of medial and intimal thickening consequent upon medial vascular smooth muscle cell migration, proliferation and extracellular matrix deposition, followed later by superimposed atherosclerosis. These changes directly compromise graft blood flow and provoke thrombosis. Vein graft wall thickening may represent an adaptation imposed by arterial hemodynamic factors, and these factors have been shown to promote vascular smooth muscle cell migration and proliferation through activation of key mediators including platelet-derived growth factor (PDGF). Many pharmacological interventions aimed at preventing these long-term changes have proven unsuccessful in clinical evaluation. We recently demonstrated in a pig saphenous vein graft model that application of an external polyester stent to the outside of carotid interposition vein grafts reduced intimal hyperplasia and total wall thickness 1 month after implantation. However, it is not known whether the benefits of the stent are maintained in the longer term or what mechanisms underlie its effect. The present study therefore compared morphological changes and PDGF expression in stented grafts and contralateral unstented grafts in the same pigs, 6 months after graft implantation. Reduced medial thickening, neointima formation, and cell proliferation were sustained in externally stented grafts, and these effects were associated with a significant reduction in PDGF expression.

Towards the prevention of vein graft failure

Improved outcome after coronary bypass surgery over the last decade has been attributed largely to the increasing use of arterial conduits and their superior patency rates over that of saphenous vein grafts. In spite of this trend, autologous saphenous vein has remained an important and convenient conduit for a variety of operative scenarios, and is still used for more than 70% of grafts. As a result, vein graft failure continues to represent a significant clinical and economic burden upon the health care service. Between 15 to 30% of saphenous vein grafts occlude within the first year of surgery, increasing to over 50% after 10 years. By this time, more than 10% of patients will require further intervention to alleviate symptoms arising from occluded grafts and progression of native disease. Graft occlusion arises either from early thrombosis or the later onset of 'vein graft disease' and subsequent atherosclerotic changes.

Upregulation of basement membrane-degrading metalloproteinase secretion after balloon injury of pig carotid arteries

Basement membrane-degrading metalloproteinases (gelatinases) appear necessary for vascular smooth muscle cell migration and proliferation in culture and for intimal migration of cells after balloon injury to the rat carotid artery. We investigated in the present study the secretion of gelatinases from pig carotid artery tissue after balloon injury. Segments of injured artery and segments proximal and distal to the area of injury were removed 3, 7, and 21 days after balloon dilatation. Medial explants from these segments were then cultured for 3 days, and the serum-free conditioned media were subjected to gelatin zymography. Production of 72- and 95-kD gelatinases was quantified by densitometry. Balloon-injured segments secreted significantly more 72- and 95-kD gelatinase than did paired distal segments at all time points. Release of both gelatinase activities was increased at 3 and 7 days relative to segments from uninjured arteries but declined again by 21 days after balloon injury. Similar results were found for gelatinase levels in extracts of arterial tissue. Consistent with the protein secretion data, in situ hybridization demonstrated that the mRNAs for both gelatinases were upregulated after balloon injury. Expression was prominent in medial smooth muscle cells, particularly around foci of necrosis, and in neointimal cells 3 and 7 days after balloon injury; 72-kD gelatinase mRNA persisted after 21 days and was prominent in regrown endothelial cells. The upregulation of gelatinase activity paralleled the time course of smooth muscle cell migration and proliferation in this model. We conclude that increased gelatinase production occurs in response to balloon injury and may play a role in permitting migration and proliferation of vascular smooth muscle cells.

Experimental models of accelerated atherosclerosis syndromes

The processes of angioplasty restenosis, vein graft failure, and transplant atherosclerosis, collectively termed 'syndromes of accelerated atherosclerosis', have been the focus of significant clinical and experimental research. Limitations of clinical studies have forced emphasis onto experimental animal models for the purpose of determining pathophysiology and evaluation of potential therapeutic strategies. However, the apparent failure of many in vivo animal models to predict interventional outcome in humans has raised doubt over their suitability as models of these pathophysiological states. Similar criticism has befallen the use of in vitro techniques for elucidating pathophysiology of the cellular elements. An awareness of the shortcomings of the various experimental models in use would therefore seem prerequisite both to a more critical evaluation of data generated from their use, and to the development of truly effective therapeutic strategies for humans.

Influence of external stent size on early medial and neointimal thickening in a pig model of saphenous vein bypass grafting

BACKGROUND

Late saphenous vein graft failure results from intimal and medial thickening due to migration and proliferation of vascular smooth muscle cells and superimposed atheroma. These changes may represent an adaptation by the vein to its insertion into the arterial system. Using a porcine model of arteriovenous bypass grafting, we recently demonstrated that supporting the graft with a nonrestrictive external Dacron velour stent significantly reduced intimal hyperplasia and total wall thickness. In the present study, we investigated the influence of different external stent sizes on graft wall dimensions and cell proliferation.

METHODS AND RESULTS

Three stent sizes were tested: mildly restrictive, nonrestrictive, and oversized (5, 6, and 8 mm in diameter, respectively). Four weeks after grafting, total wall thickness was decreased 40% by 5-mm stents (P = .02), 66% by 6-mm stents (P = .0004), and 81% by 8-mm stents (P = .02 versus unstented grafts). Neointimal thickness was reduced almost 62% by 6-mm and 72% by 8-mm stents (both P = .01) but not by 5-mm stents. As a result, the encroachment of the intima into the lumen was reduced approximately 70% by 6- or 8-mm stents (P = .02 and P = .01 versus unstented grafts, respectively). Both neointimal and medial cell proliferation were significantly reduced by all three stents compared with unstented grafts.

CONCLUSIONS

External stenting of saphenous vein bypass grafts reduces early intimal and medial hyperplasia. Oversized stents give equally profound suppression of intimal thickening, obviating the need for precise size matching with the graft and greatly simplifying the surgical procedure.

External stenting reduces early medial and neointimal thickening in a pig model of arteriovenous bypass grafting

The long-term clinical success of coronary artery bypass grafting with autologous saphenous vein is limited by progressive medial and neointimal thickening in the graft and superimposed atherosclerosis. We sought to reduce wall thickening by applying an external stent to experimental grafts in pigs. The diameter of the stent was designed to allow unrestricted initial expansion of the vein in response to arterial pressure and the stent material was highly porous so as to minimize adventitial [correction of advential] disruption. Four weeks after graft implantation, stented grafts had a larger lumen (11.2 +/- 6.2 [standard deviation] mm2 versus 7.6 +/- 3.4 mm2, p < 0.05, n = 9) and an almost fourfold thinner media (0.14 +/- 0.08 versus 0.49 +/- 0.22, p < 0.001) and neointima (0.10 +/- 0.07 versus 0.35 +/- 0.24, p < 0.001) than paired unstented grafts in the same animals. Cell proliferation was also greatly reduced by stenting in the neointimal and medial layers. The stenting procedure devised here is readily applicable to clinical coronary bypass grafts.

An essential role for platelet-derived growth factor in neointima formation in human saphenous vein in vitro

The role of platelet-derived growth factor (PDGF), a potent vascular smooth muscle cells (SMC) mitogen and chemoattractant, was investigated during neointima formation in human saphenous vein organ culture. PDGFA and B messenger ribonucleic acid (mRNA) expression was detected by RNase protection assay and in situ hybridisation and PDGF protein by immunocytochemistry. The expression of PDGFA and B mRNA was low in veins before culture while PDGF protein was detected in all cell types. A neointima consisting of densely packed SMC developed after 14 days of culture. The dense packing and high expression of PDGFA and B mRNA in neointimal SMC led to higher PDGF protein concentrations in the neointima, the role of which was examined by culturing with neutralising anti-(human PDGF) antibodies. The anti-PDGF antibodies significantly reduced neointimal thickness by approximately 66% and the number of neointimal cells by approximately 50%, without affecting neointimal or medial proliferation indices or cell viability. These results suggest that PDGF played an essential role in SMC migration into the neointima in human saphenous vein.