Contribution of the endothelium to intimal thickening in normocholesterolemic and hypercholesterolemic rabbits

In vivo endothelialization and neointimal hyperplasia assessment after rabbit carotid endarterectomy with bovine pericardium

Background

Previous studies have reported that the use of a patch in carotid endarterectomy (CEA) surgery can reduce the rate of restenosis and perioperative complications. The goal of this study was to compare the short- and medium-term outcomes of endothelialization and neointimal hyperplasia of patch closure (PC) angioplasty in CEA with direct closure (DC) in a rabbit model. A bovine pericardial patch (BPP) was used in the PC procedures.

Methods

Two carotid arteries were dried by air flow to simulate endarterectomy and selected for PC and DC in each rabbit. Different animals were sacrificed at 1, 2, 3, 4, and 8 weeks after the procedure. The endarterectomized segments were extracted and examined microscopically with histopathological and immunohistochemical analysis, and electron-microscopy measurements.

Results

In all, 19 rabbits were included in this study; 3 rabbits were placed in a 2-week postoperative group and 4 rabbits were placed in the 1-, 3-, 4-, and 8-week postoperative group respectively. Hematoxylin-eosin (HE) staining showed neointima on the PC side at an early stage (1-week postoperatively), and intimal hyperplasia could be seen on both sides. Immunohistochemical analysis showed that Ki-67 was higher on the PC side than on the DC side at an early stage (1,661.5±1,122.9 cells/mm², P=0.060). In the 2-week postoperative group, von Willebrand factor (vWF) was higher on the DC side (−377.0±155.6 cells/mm², P=0.052). Alpha-smooth muscle actin (α-SMA) values were comparable on both sides (P>0.05). Electron microscopy measurements showed that functional endothelial cells exhibited a cobblestone-like morphology and were nicely elongated in the direction of blood flow.

Conclusions

The use of BPP in PC angioplasty during CEA can maintain stability and also provide rapid endothelialization. PC with BPP has comparable ability of efficient endothelialization with DC, but is more likely to have early endothelialization.

Hypercholesterolemia and oxidative stress inhibit endothelial cell healing after arterial injury

OBJECTIVE

Endothelial cell (EC) migration is essential for arterial healing after angioplasty. Oxidized low-density lipoproteins and oxidative stress decrease EC migration in vitro. The objective of this study was to determine the effect of hypercholesterolemia and oxidative stress on EC healing after an arterial injury.

METHODS

C57BL/6 wild-type mice were placed in one of eight groups: chow diet (n = 11), high-cholesterol (HC) diet (n = 11), chow diet plus paraquat (n = 11), HC diet plus paraquat (n = 11), chow diet plus N-acetylcysteine (NAC) (n = 11), HC diet plus NAC (n = 11), chow diet plus paraquat and NAC (n = 11), and HC diet plus paraquat and NAC (n = 11). After 2 weeks on the assigned diet with or without NAC, the carotid artery was injured using electrocautery. Animals in the paraquat groups were given 1 mg/kg intraperitoneally to increase oxidative stress. After 120 hours, Evans Blue dye was infused intravenously to stain the area of the artery that remained deendothelialized. This was used to calculate the percentage of re-endothelialization. Plasma and tissue samples were analyzed for measures of oxidative stress.

RESULTS

The HC diet increased oxidative stress and reduced EC healing compared with a chow diet, with EC covering 26.8% ± 2.8% and 48.1% ± 5.2% (P < .001) of the injured area, respectively. Administration of paraquat decreased healing in both chow and HC animals to 18.1% ± 3.5% (P < .001) and 9.8% ± 4.6% (P < .001), respectively. Pretreatment with NAC (120 mmol/L in drinking water) for 2 weeks prior to injury, to decrease oxidative stress, improved EC healing to 39.9% ± 5.7% (P < .001) in hypercholesterolemic mice and to 30.7% ± 3.6% (P < .001) in the paraquat group. NAC treatment improved healing to 24.6% ± 3.4% (P < .001) in hypercholesterolemic mice treated with paraquat.

CONCLUSION

Re-endothelialization of arterial injuries is reduced in hypercholesterolemic mice and is inversely correlated with oxidative stress. An oral antioxidant decreases oxidative stress and improves EC healing.

CLINICAL RELEVANCE

Vascular injury following cardiovascular intervention, including cardiac and peripheral arterial angioplasty and stenting, is associated with inflammation and oxidative stress. Hypercholesterolemia is also associated with increased oxidative stress. Oxidative stress, regardless of the source, induces cellular dysfunction in endothelial and smooth muscle cells that reduce healing after arterial injury. Decreasing oxidative stress with an exogenously administered antioxidant can improve endothelial cell healing, and this is important to control intimal hyperplasia and reduce the thrombogenicity of the vessel.

Different inflammatory response and oxidative stress in neointimal hyperplasia after balloon angioplasty and stent implantation in cholesterol-fed rabbits

Inflammatory responses appear to play an important role in the occurrence of restenosis following coronary intervention. However, the contribution of C-reactive protein (CRP) and oxidative stress to restenosis after balloon angioplasty and stent implantation remains unclear. The aim of this study was to examine this issue using hyperlipidemic rabbits. Rabbits were divided into two groups; they were fed with a 0.5% cholesterol diet and with a mixed 0.5% cholesterol and 0.5% probucol diet. Each group of rabbits underwent balloon injury and stent implantation in right and left iliac arteries, respectively. Eight weeks after the intervention, we examined luminal stenosis, neointimal hyperplasia, immunoreactivity for macrophage, CRP and oxidized phosphatidylcholine (oxPC), and also the expression of CRP mRNA. The degrees of neointimal hyperplasia and immunopositive areas (%) for macrophage, CRP, and oxPC in the neointima were significantly higher after stent implantation than after balloon injury, but CRP mRNA was undetectable in either artery. Anti-oxidant probucol reduced angiographic stenosis, neointimal hyperplasia, and macrophage- and oxPC-positive areas much more significantly after stenting. The results demonstrate that the inflammatory response to the development of neointimal hyperplasia differs after balloon injury and stent implantation and that CRP deposition and oxidative stress might be involved more significantly in neointimal development after stent implantation.

Mechanisms of monocyte recruitment in vascular repair after injury

The inflammatory response to acute vessel wall injury has been increasingly recognized to play a decisive role in neointima formation. In particular, the exuberant infiltration with monocytes aggravates neointimal growth and can thereby promote restenosis. The adhesion of circulating monocytes to the site of mechanical injury represents the key event in monocyte recruitment, and this review highlights recent insights into the molecular mechanisms of monocyte adhesion throughout the course of neointimal growth. An acute and a chronic phase of monocyte recruitment after vascular injury can be discerned. The adhesion of platelets to the denuded subendothelial matrix is the hallmark of the acute phase providing an adhesive substrate for monocytes, whereas chronic monocyte recruitment is regulated by the interaction with neointimal smooth muscle cells and recovering endothelial cells. Clearly, the mechanisms of monocyte rolling and adhesion differ considerably between these diverse substrates. This review is particularly focused on the contribution of chemokines and adhesion molecules to monocyte recruitment to injured vessels according to the different stages of neointimal growth, and on closely related functions of the chemokine-like molecule macrophage migration inhibitory factor. Understanding the complex molecular interactions of the injured vessel wall with circulating monocytes may enable therapeutic targeting to prevent the development of restenosis.

Decreased neointimal formation in Nox2-deficient mice reveals a direct role for NADPH oxidase in the response to arterial injury

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced, in part, from NADPH oxidase in response to host invasion and tissue injury. Defects in NADPH oxidase impair host defense; however, the role of ROS and RNS in the response to tissue injury is not known. We addressed this issue by subjecting leukocyte oxidase (Nox2)-deficient (Nox2-/-) mice to arterial injury. Femoral artery injury was associated with increased Nox2 expression, ROS/RNS production, and oxidative protein and lipid modification in wild-type mice. In Nox2-/- mice, RNS-mediated protein oxidation, as monitored by protein nitrotyrosine content, was significantly diminished. This was accompanied by reduced neointimal proliferation, as monitored by intimal thickness and intimal/medial ratio, in Nox2-/- compared to wild-type mice. In addition, Nox2 deficiency led to reduced cellular proliferation and leukocyte accumulation. These data indicate that Nox2-mediated oxidant production has a requisite role in the response to tissue injury.

Effect of vitamin E on aortic lipid oxidation and intimal proliferation after arterial injury in cholesterol-fed rabbits

Oxidized low-density lipoproteins (LDL) are implicated in atherosclerosis. However, large-scale intervention studies designed to test whether antioxidants, such as vitamin E, can ameliorate cardiovascular disease have generated ambivalent results. This may relate to the fact that the mechanism whereby lipid oxidation is initiated in vivo is unknown and the lack of direct evidence for a deficiency of antioxidants in atherosclerotic lesions. Further, there is little evidence to suggest that vitamin E acts as an antioxidant for lipid peroxidation in vivo. Here we tested the antioxidant effect of dietary vitamin E (alpha-tocopherol) supplementation on intimal proliferation and lipid oxidation in balloon-injured, hypercholesterolemic rabbits. alpha-Tocopherol supplementation increased vascular content of alpha-tocopherol over 30-fold compared to nonsupplemented and alpha-tocopherol-deficient chows. Balloon injury resulted in oxidized lipid deposition in the aorta. Maximum levels of primary lipid oxidation products, measured as hydroperoxides of esterified lipid (LOOH) and oxidized linoleate (HODE), were 0.22 and 1.10 nmol/mg, representing 0.21 and 0.39% of the precursor molecule, respectively. Secondary lipid oxidation products, measured as oxysterols, were maximal at 5.60 nmol/mg or 1.48% of the precursor compound. Vascular HODE and oxysterols were significantly reduced by vitamin E supplementation. However, the intima/media ratio of aortic vessels increased with vitamin E supplementation, suggesting that the antioxidant promoted intimal proliferation. Thus, the study demonstrates a dissociation of aortic lipid oxidation and lesion development, and suggests that vitamin E does not prevent lesion development in this animal model.

Atherosclerotic plaques composed of a large core of foam cells covered with thin fibrous caps in twice-injured carotid arterial specimens obtained from high cholesterol diet-fed rabbits

We attempted to find atherosclerotic plaques including a large lipid core and thin fibrous cap in twice-injured arterial specimens obtained from high cholesterol diet (HCD) fed rabbits. Rabbits fed a HCD were subjected to carotid artery injury using a balloon catheter. After 2 or 4 weeks of cholesterol feeding, a second mild injury was induced in the same region as the first injury. The rabbits were given a standard diet for 2 weeks after the second injury. Typical atherosclerotic plaques with a fibrous cap formed by smooth muscle cells and extracellular matrix overlying a core formed by macrophage foam cells were observed in the lesion. Gelatin proteolytic activities were found in homogenates containing either media or intima from the injured artery, and activated matrix metalloproteinase-2 (MMP2) was detected. With prolongation of the HCD feeding period (interval between injuries) from 2 weeks to 4 weeks, typical plaque was observed more frequently. Furthermore, the neointimal area and the macrophage foam cells area increased, as did gelatin-proteolytic activity. Since the typical atherosclerotic plaques observed in the present study have some histopathological and pathogenic characteristics in common with unstable atherosclerotic plaque, we expect that the typical atherosclerotic plaque found in the present study will be useful for basic studies of plaque stabilization and prevention of acute coronary syndromes.

A comparative study of policosanol vs lovastatin on intimal thickening in rabbit cuffed carotid artery

Policosanol is a cholesterol-lowering drug isolated from sugar cane wax, which acts by inhibiting cholesterol biosynthesis. Previous studies have demonstrated that policosanol inhibited smooth muscle cell (SMC) proliferation in the cuffed carotid artery of the rabbit and in arterial wall damage induced by forceps in the central artery of the ear of rabbits. The present study was undertaken to compare the effects of policosanol and lovastatin on SMC proliferation in the cuffed carotid artery of rabbits. Collars were placed around the left carotid for 7 and 15 days. The contralateral artery was sham operated. We studied eight experimental groups: two controls groups receiving vehicle for 7 and 15 days, respectively, a satellite sham operated control group, four groups treated with policosanol at 5 and 25 mg kg(-1)for 7 and 15 days and a reference group receiving lovastatin at 20 mg kg(-1)for 15 days. Samples of arteries were examined by light and electron microscopy. To evaluate intimal thickening the cross-sectional areas of intima and media were measured. Neointima was significantly reduced in treated animals compared with controls, but the reduction in lovastatin animals was significantly lower than in policosanol-treated groups. The SMC proliferation was studied by the immunohistochemical detection of proliferating cell nuclear antigen and the reduction observed in policosanol-treated rabbits was significantly larger than in lovastatin-treated animals. It is concluded that the protective effect of policosanol against neointima formation in this experimental model was slightly better than that of lovastatin.

Effect of policosanol on intimal thickening in rabbit cuffed carotid artery

We studied the effect of policosanol on smooth muscle cell proliferation in the cuffed carotid artery of the rabbit. Policosanol is a mixture of higher aliphatic primary alcohols isolated from sugar cane wax, with cholesterol lowering effects proved in experimental models and patients with type II hypercholesterolemia. It acts by inhibiting cholesterol biosynthesis. The positioning of a nonocclusive silicone collar around the rabbit carotid artery results in the formation of a neointima. We wished to determine whether policosanol orally administered prevented intimal thickening. Collars were placed around the left carotid for 15 days. The contralateral artery was sham operated. We included three experimental groups: a control received vehicle and two others policosanol at 5 and 25 mg Kg until sacrificed. Samples of arteries were examined by light and electron microscopy. To evaluate intimal thickening the cross-sectional area of intima and media were measured. Neointima was significantly reduced in policosanol-treated animals compared with controls. The smooth muscle cell proliferation was studied by the immunohistochemical detection of proliferating cell nuclear antigen and a significant reduction was observed in policosanol treated rabbits. It is concluded that policosanol has a protective effect on the neointima formation in this experimental model.

Expression of tissue factor in the rabbit aorta after balloon injury

Tissue factor (TF) is a primary initiator of the extrinsic pathway of blood coagulation. Recently TF has been shown to be overexpressed in atherosclerotic lesions and it is thought to contribute to the thrombogenicity of the plaques. We studied TF expression in the media and the neointima of rabbit aortas at various intervals after balloon injury. TF protein was immunohistochemically detected in smooth muscle cells (SMCs) of the inner layer of the media at 2 h after injury and was subsequently detected in SMCs in the neointima, whereas no TF expression was detected in the uninjured aortas except for the adventitia. Immunohistochemical and immunoelectron microscopic studies revealed that TF-positive SMCs were of an immature or synthetic phenotype and TF protein was detected in the rough endoplasmic reticulum in SMCs. TF mRNA in the intima and media increased at 2 h after injury and returned to near baseline levels at 12-24 h, whereas TF activity also increased at 2 h and continued at similar levels over the next 72 h. TF mRNA and activity increased markedly at 2-8 weeks after injury. These data suggest that TF is rapidly induced in the medial SMCs and hereafter is constitutively expressed in the neointima. TF expressed in the neointima may contribute to hypercoagulable properties of injured arteries.

Arterial thrombosis induces early upregulation of intercellular adhesion molecule in the media

PURPOSE

Thrombosed peripheral vessels that are pharmacologically or mechanically recanalized have diminished long-term patency rates compared with vessels that are repaired before occlusion. We hypothesized that thrombosis induces proinflammatory changes in the arterial media that may contribute to postthrombotic vascular remodeling.

METHODS

We studied expression of intercellular adhesion molecule (ICAM), a mediator of leukocyte recruitment, in the arterial wall after thrombosis. Thrombosis was induced in rabbit superficial femoral arteries by embolizing polystyrene beads (Thr-emb) or by ligation (Thr-lig). Control vessels were dissected but not ligated (C-dis) or were subjected to bead embolization and immediate removal (C-emb). Arterial wall ICAM expression was measured by indirect immunohistochemical analysis at 6 hours, 24 hours, and 1 week. Staining intensity was graded from 0 (none) to 4 (intense) by observers who were blinded to the experimental conditions.

RESULTS

No increase in ICAM expression by thrombosed vessels was present at 6 hours. After 24 hours, ICAM expression in the media of thrombosed vessels was increased (Thr-emb, 2.3 +/- 0.5; Thr-lig, 2.0 +/- 0) compared with control vessels (C-dis, 0 +/- 0; C-emb, 0.8 +/- 0.5; p < 0.004). This difference became more marked at 1 week. ICAM staining localized to actin-staining regions of the media.

CONCLUSIONS

Arterial thrombosis, but not surgical injury, induces pronounced early and sustained upregulation of ICAM expression in smooth muscle-containing regions of the arterial media. Upregulation of ICAM is likely to promote recruitment of inflammatory cells or mediate vascular remodeling after luminal thrombosis.

Tin ethyl etiopurpurin significantly inhibits vascular smooth muscle cell proliferation in vivo

Smooth muscle cell proliferation is a major component of restenosis following angioplasty. Hematoporphyrin derivative and other photosensitive compounds inhibit proliferation by causing cellular necrosis upon light activation (photodynamic therapy). Other photosensitive compounds, such as benzoporphyrin derivative, have been suggested as having non-cytotoxic antiproliferative effects without photodynamic therapy, although other studies using benzoporphyrin derivative were negative. Inhibition of smooth muscle cell proliferation was examined in an in vivo rabbit model of vascular injury using a novel synthetic chlorin derivative, tin ethyl etiopurpurin, and benzoporphyrin derivative without photodynamic therapy. Tin ethyl etiopurpurin and benzoporphyrin derivative inhibited smooth muscle cell proliferation by 50-90% of control (p < or = 0.05) without toxic side effects. These results suggest that tin ethyl etiopurpurin and benzoporphyrin derivative without photodynamic therapy may provide a novel and potent antiproliferative therapy that might be useful in the treatment of restenosis.

Effects of inflation pressure of balloon catheter on vascular injuries and subsequent development of intimal hyperplasia in rabbit aorta

Balloon catheter de-endothelialization is the most popular means of arterial injury in experimental animals and has been used as the model system to investigate atherogenesis and restenosis after percutaneous transluminal coronary angioplasty (PTCA). The aim of this study was to examine the relationship between balloon inflation pressure and vascular damage and also subsequent intimal hyperplasia. Retrograde pullback balloon injury of rabbit aortas was made at three different balloon pressures (1.5, 1.75, and 2.0 atm). The medial injuries, such as necrosis of smooth muscle cells and disruption of elastic lamina, were occasionally found in the injured segment of the aorta by balloon catheter at 1.75 atm and more frequently at 2.0 atm. No prominent medial injury was observed in the aortic segment to balloon catheter injury at 1.5 atm; Intimal hyperplasia developed in each animal and increased with time, 2, 4, and 8 weeks after injury. The intimal hyperplasia followed by balloon injury at 1.75 and 2.0 atm was more prominent than that at 1.5 atm, however, the development of the intimal hyperplasia was not parallel to the degree of inflation pressure. On the other hand, decrease of DNA content of the media and reduction of norepinephrine-induced vasoconstriction were observed in a pressure-dependent manner after balloon injury. These findings indicate that intimal hyperplasia is not proportionally correlated to the severity of the vascular injury. The control of inflation pressure is very important in order to examine vascular injuries, subsequent intimal hyperplasia and vasomotor responses in animal models of balloon catheter injury.

Inhibitory effects of prostacyclin analogue, TFC-132, on aortic neointimal thickening in vivo and smooth muscle cell proliferation in vitro

Effects of a new prostacyclin analogue, TFC-132, on neointimal thickening following intimal mechanical injury and on the proliferation of cultured aortic smooth muscle cells (SMCs) were studied. The intimal injury was induced by indwelling of polyethylene tubing for 24 h in the rabbit aorta. Rabbits were killed 10 days after drawing out the tubing. TFC-132 (0.6 mg/kg or 1.2 mg/kg) was given orally at 8-h intervals through the experiment. The serum concentrations of the analogue rose significantly 1 and 2 h after administration. The mean intimal thickening in the TFC-132 treated groups was significantly thinner than in the control one. Human aortic SMCs were cultured and 3H-thymidine incorporation into DNA (DNA synthesis) was measured at the varying concentrations of TFC-132. The analogue inhibited DNA synthesis of cultured SMCs at 10(-6) and 10(-5) M. These data indicate that a new prostacyclin analogue, TFC-132, has an inhibitory effect on the neointimal thickening after intimal injury and on the aortic SMC proliferation.

Pathobiology of intimal hyperplasia

In the current vascular interventional environment, high restenosis rates have increased awareness of the significance of intimal hyperplasia, a chronic structural lesion that develops after vessel wall injury, and which can lead to luminal stenosis and occlusion. Intimal hyperplasia may be defined as the abnormal migration and proliferation of vascular smooth muscle cells with associated deposition of extracellular connective tissue matrix. The pathology of intimal hyperplasia is reviewed with particular attention to its physiology, pharmacology, cell biology and molecular biology.

Subendothelial layer of pseudointima of polytetrafluoroethylene graft is formed by transformation of fibroblasts migrated from extravascular space

A well organised pseudointima is formed in polytetrafluoroethylene (PTFE) grafts within 4 weeks after implantation into inferior vena cava (IVC) of rabbits. To investigate the process of the subendothelial organisation of pseudointima, an animal experiment was conducted. The outer wall of PTFE graft (30 microns fibril length, 3 mm inner diameter, 3 cm long) was coated with 10 um silicon film in the following ways to prevent cellular ingrowth from the extravascular space: non-coating; full-length coating; half-length coating; and full-length coating excluding 5 mm midportion. These grafts were implanted into rabbit IVC and were harvested 4 weeks later. All the grafts were patent but the lumen of the non-coated area was narrowed by pseudointimal hyperplasia. The degree of the hyperplasia estimated by dried tissue deposit was inversely proportional to the length of the coating. The coverage of the luminal surface with endothelial-like cells was noted at anastomotic areas and also at the surface corresponding to the non-coated area. Light microscopy and immunostaining studies on the non-coated midportion revealed the presence of fibroblasts in the interstices of PTFE and smooth muscle cells and myofibroblasts in the pseudointima. Transmission electron microscopy confirmed the presence of myofibroblasts in the midportion of the non-coated area. No transmural capillary ingrowth was observed in the midportion by histological and immunohistochemical analysis. These observations suggest that the subendothelial layer of pseudointima in PTFE grafts is formed by proliferation and transformation of fibroblasts migrating from the extravascular space and that endothelial-like cells may also be derived from such transformation.