Role of platelets in smooth muscle cell proliferation and migration after vascular injury in rat carotid artery

Roles of vasodilatory prostaglandins in mitogenesis of vascular smooth muscle cells

Vasodilatory prostaglandins (PGI2, PGE1) and synthetic prostacyclin mimetics inhibit smooth muscle cell proliferation in vitro after stimulation by growth factors. Similar results are obtained in vivo after endothelial injury, suggesting that vasodilatory prostaglandins might also control smooth muscle cell proliferation in vivo. However, available data from clinical trials are conflicting and currently do not support the concept that these compounds might be successfully used to suppress excessive smooth muscle cell growth in response to tissue injury, specifically restenosis after PTCA. One possible explanation for these different results is an agonist-induced down-regulation of prostacyclin receptors in vascular smooth muscle cells. It is possible that enhanced endogenous prostacyclin biosynthesis, subsequent to induction of COX-2 and/or in relation to the formation of a neointima from media smooth muscle cells, might have a similar effect. There is still uncertainty regarding the cellular signal transduction pathways and their possibly complex interaction, although cAMP-dependent reactions are probably involved. In addition, vasodilatory prostaglandins might also interfere with the generation and action of other growth modulating factors, including PDGF, hepatocyte growth factor and nitric oxide. In conclusion, vasodilatory prostaglandins might be considered as growth modulating endogenous mediators in vascular smooth muscle cells.

Ultrastructural localisation of nitric oxide synthase, endothelin and binding sites of lectin (from Bandeirea simplicifolia) in the rat carotid artery after balloon catheter injury

An immunocytochemical and cytochemical study has been made on the ultrastructural localisation of type III (endothelial) nitric oxide synthase, endothelin-1 and the binding sites of lectin from Bandeirea simplicifolia to the endothelium surface-associated glycoproteins in the rat left common carotid artery at 1 and 28 d after Fogarty embolectomy balloon catheter-induced injury. Controls were carotid arteries from sham operated rats. In the controls, the immunoreactivity to nitric oxide synthase-III and endothelin-1 was localised in different proportions in vascular endothelial cells (36.9% +/- 4.3 and 7.6% +/- 2.7, respectively); immunoreactivity was confined to the cytoplasm and the membranes of intracellular organelles and structures. In contrast, staining with lectin was localised on the luminal surface of all endothelial cells. 1 d after injury, platelets were adherent to the endothelium-denuded intima. Some of the platelets displayed, immunoreactivity to nitric oxide synthase-III and endothelin-1 and were stained with lectin. 28 d after injury, a neointimal thickening of substantial size was present. Subpopulations of the regrown endothelial cells covering the luminal surface of the neointima showed positive immunoreactivity to nitric oxide synthase-III and endothelin-1 but there was a significant decrease in the proportion of nitric oxide synthase-III-containing endothelial cells (17.2% +/- 1.9; P < 0.001) and a significant increase in the proportion of endothelin-1-containing endothelial cells (36.9% +/- 4.7; P < 0.001) compared with the controls. Staining with lectin was associated with the cell membrane of all endothelial cells and in addition with cells located 'deeper' in the neointima which showed lectin-positive plasmalemma, Golgi complex and multivesicular bodies/lysosomes. In conclusion, regenerated endothelial cells of the neointima showed reduced population (2-fold) of nitric oxide synthase-III-and increased population (5-fold) endothelin-1-positive cells. The subendothelial location of some lectin-stained cells after balloon catheter injury indicates the heterogeneity of the neointima and suggests that some of these cells are involved in early angiogenesis. 24 h and 28 d after injury some platelets showed positive immunoreactivity for nitric oxide synthase-III and endothelin-1.

Nitric oxide synthase: role in the genesis of vascular disease

The product of nitric oxide (NO) synthase is the most potent endogenous vasodilator known. No not only is a potent vasodilator, it also inhibits platelet adherence and aggregation, reduces adherence of leukocytes to the endothelium, and suppresses proliferation of vascular smooth muscle cells. A number of disorders are associated with reduced synthesis and/or increased degradation of vascular NO. These include hypercholesterolemia, diabetes mellitus, hypertension, and tobacco use. The endothelial dysfunction caused by these disorders contributes to the alterations in vascular function and structure observed in these conditions. A reduction in the activity of vascular NO likely plays a significant role in the development of atherosclerosis. Insights into the mechanisms by which NO production or activity is altered in these states will lead to new therapeutic strategies in the treatment of a number of vascular disorders, including hypertension, atherosclerosis, restenosis, and thrombosis.

Role of platelets in restenosis after percutaneous coronary revascularization

The role of platelets in the process of restenosis after percutaneous coronary intervention is not fully understood. After vascular injury there is extensive platelet activation, adhesion, aggregation and secretion. Through the liberation of growth factors, such as platelet-derived growth factor, and surface expression of cell adhesion molecules, such as the glycoprotein IIb/IIIa integrin, platelets appear to be a pivotal mediator of the vascular injury response. Experimental models have demonstrated that profound, prolonged thrombocytopenia, or blockade of the IIb/IIIa receptor, may reduce neointimal hyperplasia after arterial balloon injury. However, multiple clinical trials testing conventional or new platelet agents have not yielded any salutary effects. The recent finding that abciximab, a monoclonal antibody fragment directed against IIb/IIIa, reduced clinical restenosis after coronary angioplasty by 26% in patients raises questions about the mechanism of benefit. The alpha v beta 3 vitronectin receptor is responsible for binding endothelial cells to platelets, and it also has a key role in modulating smooth muscle cell migration. It is possible that the antibody fragment exerts its effect on restenosis by means of alpha v beta 3, because abciximab fully cross-reacts to this integrin owing to the shared beta 3 subunit. To date, the other platelet glycoprotein IIb/IIIa inhibitors, including Integrelin, Tirofiban, Lamifiban and Xemilofiban, are specific in binding to this particular integrin. Considerable further study is necessary to unravel the effects of platelets on the restenosis process.

Proliferative response of human and minipig smooth muscle cells after coronary angioplasty to growth factors and platelets

OBJECTIVES

Platelets aggregating at the site of angioplasty, shown to be a potent proliferative stimulus for cultured smooth muscle cells (SMC), could contribute to proliferation after angioplasty.

METHODS

SMC were cultivated from human aorta and restenosed coronary lesions as well as from minipig aorta and from normal and post angioplasty coronary artery segments (n = 6 per source). 3H-thymidine incorporation was used as a measure of proliferation.

RESULTS

3H-thymidine incorporation varied greatly after passage 7 in all cell lines, but was significantly higher in SMC from human coronary restenosed lesions compared to those from human aorta and minipig coronary post angioplasty segments in passage 2 (44 +/- 6.4 x 10(3) cpm/5000 SMC vs 20 +/- 3.9 and 12.1 +/- 2.1). However, all SMC exhibited a dramatic increase of 3H-incorporation after passage 7. Growth factors stimulated 3H-thymidine incorporation either dose dependently (PDGF-BB and bFGF) or only very modestly (PDGF-AA, EGF, IGF-1). The most potent stimulation was seen with PDGF-BB, 50 ng/ml, and was 17 +/- 6% (human restenosed) and 16 +/- 8% (minipig post angioplasty) of the values observed after stimulation with 10% fetal calf serum. The most effective combination of growth factors, PDGF-BB (50 ng/ml) + bFGF(20 ng/ml) + IGF-1 (50 ng/ml), produced a 3H-thymidine incorporation of 44 +/- 10% (human restenosed) and 42 +/- 11% (minipig post angioplasty) of FCS values. Stimulation by isolated platelets was dose dependent and significantly higher: 75 +/- 19% and 70 +/- 15% of FCS values for those SMC.

CONCLUSIONS

1) SMC from all sources studied exhibit significant changes of proliferation with increasing passages, excluding the comparability of data obtained with cells in different passages. 2) Data obtained with SMC from any source might not apply for SMC from human coronary restenosed lesions. 3) Currently tested growth factors do not fully account for the proliferative effect of platelets on cultured SMC.

Comparison of the effects of the thrombin inhibitor r-hirudin in four animal models of neointima formation after arterial injury

Thrombin has been implicated as a contributing factor to restenosis after vessel reopening procedures. We compared the ability of the direct thrombin inhibitor recombinant (r-) hirudin to reduce neointimal growth in different animal models of arterial injury. Carotid arteries of rats, rabbits, and hypercholesterolemic minipigs were injured by withdrawal of an inflated balloon catheter. In addition, we used a double-lesion model in rabbits, which involved balloon angioplasty of a preexisting lesion induced by carotid denudation 4 weeks earlier. r-Hirudin was given in all four animal models as a short-term application (bolus of 1 mg/kg i.v. immediately before injury, followed by infusion of 1 mg.kg-1.h-1 for 2 hours, and an injection of 6 mg/kg SC). Additionally, we investigated the effects of prolonged treatment (intravenous infusion for 3 and 14 days) in rats. Inhibition of thrombin was monitored by determination of activated partial thromboplastin time, and histomorphometric analysis of the arteries was performed after 2 (rats) or 4 (rabbits and minipigs) weeks. In rabbits, short-term r-hirudin treatment reduced neointimal area by 59% (single-injury model, P = .05) and 44% (double-injury model, P = .02). In rats and minipigs no inhibition of neointimal growth was observed after short-term r-hirudin application. A 3- or 14-day infusion of r-hirudin in rats, however, resulted in 25% (P = .007) and 27% (P = .003) reductions in neointimal area, respectively. In conclusion, there is considerable interspecies variation in the time frame of susceptibility for reduction of neointimal growth by inhibition of thrombin after arterial injury. These results demonstrate the importance of testing potential antirestenotic treatments in an array of different animal models.

Differential expression of alpha 1 type VIII collagen in injured platelet-derived growth factor-BB--stimulated rat carotid arteries

Migration of smooth muscle cells from media to intima is critical for the development of neointimal thickening after balloon catheter injury of the rat carotid artery. The present experiments were designed to identify molecules expressed by smooth muscle cells migrating in vivo in the injured artery. Cell migration was maximized by infusing recombinant platelet-derived growth factor-BB (PDGF-BB) after a minimal filament denudation of the rat carotid artery, whereas cell proliferation was minimized by injecting an antibody against basic fibroblast growth factor (bFGF). This treatment caused an eightfold increase in smooth muscle cell migration into the intima but only a twofold increase in intimal smooth muscle cell replication rates. Differential display screening was used to isolate cDNAs that were overexpressed in the injured PDGF-BB-treated versus unmanipulated rat carotids. One of the clones isolated hybridized to a 4.2-kb mRNA species and shared 90% sequence homology to mouse alpha 1 type VIII collagen. Northern and Western blots confirmed overexpression of type VIII collagen in the injured PDGF-BB-treated vessels. In a separate series of experiments, we performed filament denudation injury and administered antibodies to inhibit the actions of endogenous bFGF and PDGF-BB, thereby decreasing smooth muscle cell migration, and found that type VIII collagen mRNA expression varied with migration. Using a different arterial injury model (balloon catheter injury), we showed that expression of type VIII collagen was maximal 2 to 4 days after injury, in coincidence with cell migration from the media to the intima. This molecule constitutes an important component of smooth muscle cell response to vessel injury and may play an important functional role in mediating migration.

Expression of inducible nitric oxide synthase inhibits platelet adhesion and restores blood flow in the injured artery

NO generated by endothelial cells is vasoprotective by antagonizing platelet adhesion and smooth muscle contraction. Since vascular smooth muscle cells (VSMCs) produce NO in response to cytokine stimulation and after arterial injury, we speculated that NO produced by VSMCs could compensate for the loss of endothelium. Using balloon catheter denudation of the rat carotid artery as a model for arterial injury and restenosis, we have evaluated the time course of expression of inducible NO synthase (iNOS) by in situ hybridization and immunohistochemistry and studied the effect of iNOS on platelet adhesion and blood flow of the injured vessel. iNOS mRNA and protein were expressed in the innermost layer of the media from day 1 and were subsequently detected in the neointima, whereas no expression was detectable in the uninjured artery. Systemic administration of N omega-nitro-L-arginine methyl ester (L-NAME) resulted in a twofold to threefold increase in the adhesion of 111In-labeled platelets to the injured vessel wall. Platelet adhesion was also enhanced threefold by local delivery of L-NAME from a gel surrounding the injured vessel, whereas the stereoisomer, D-NAME, had no effect. Finally, inhibition of NO synthase led to a 24% reduction of the blood flow in the injured carotid artery. These results demonstrate that arterial injury triggers the expression of iNOS in the lesion and that NO produced by iNOS inhibits platelet adhesion and restores blood flow. This could explain the disappearance of platelet thrombi from deendothelialized arterial surfaces within a few days after injury and indicates the importance of NO generated by iNOS for the maintenance of vascular tone. Thus, expression of iNOS in lesions may represent a protective mechanism that compensates for the loss of endothelium.

Swine model of coronary restenosis: effect of a second injury

Looking for a coronary artery restenosis model closer to human pathology, a protocol of balloon injury/reinjury (plaque of dilatation) in swine coronary artery was designed. Pig coronary arteries (n = 24) were dilated for this study: 12, group 1, once (sacrifice at 10.0 +/- 2.2 weeks); 6, group 2, twice at 2-wk intervals (sacrifice at 5.2 +/- 0.2 wk); 6, group 3, twice at 4-wk intervals (sacrifice at 9.3 +/- 1.9 wk). A single overdilatation resulted in an eccentric neointimal hyperplasia representing half of the wall area (group 1, 45.6 +/- 5.1%). In animals (groups 2 and 3) subjected to redilatation, fracture length, ratio of fracture length to internal elastic lamina (IEL) circumference, and neointimal hyperplasia response were similar to those observed in group 1. In group 3, the shape of the lesion appeared more concentric and the fracture of the IEL more fragmented than in group 1. Although this model of injury/reinjury did not lead to more severe intimal hyperplasia, performing a second angioplasty at the same site did lead to a more concentric intimal response, related to multiple fractures of the IEL.

Restenosis--an open file

The main procedural drawback to percutaneous coronary angioplasty is restenosis of the treated site within 6 months. Despite advances in equipment, technique, and adjunctive therapies, restenosis has occurred in approximately one-third to one-half of all patients. The biology of restenosis can be divided into plaque persistence and recoil, thrombus formation and transformation, and cellular proliferation and vascular remodeling. Animal models of restenosis have helped to elucidate these mechanisms of restenosis and provide a means to test pharmacologic and mechanical strategies to reduce stenosis recurrence. While numerous agents have been tested in animal models, until recently none has translated into benefit in large-scale clinical trials. Two therapeutic "hopefuls" which have recently emerged in clinical practice are the potent platelet inhibitors, glycoprotein IIb/IIIa receptor antagonists, and intracoronary metallic stents. The IIb/IIIa receptor antagonists target thrombus formation at the angioplasty site, thereby minimizing abrupt vessel closure acutely and neointimal growth chronically, while intracoronary stents safely produce a large coronary arterial lumen acutely and prevent vessel recoil. Separately, these therapeutic strategies have been shown to reduce clinical restenosis 20-30% at 6-month follow-up. With these encouraging results, the future will certainly provide more pharmacologic and mechanical therapies targeting restenosis. With increased understanding of the restenotic process and continued refinement of effective treatments, it may be possible one day to prevent stenosis recurrence.

Estradiol attenuates directed migration of vascular smooth muscle cells in vitro

Although the cardiovascular benefits of the hormone estrogen are at least, in part, mediated by its antiproliferative effect on vascular smooth muscle, its action on the migration of these cells is unknown. To explore this relationship, female rat aortic smooth muscle cells were grown in hormone-free medium, and the effect of various concentrations of beta-estradiol on directed cellular migration was measured in vitro using a microwell Boyden chamber apparatus. Migration of smooth muscle cells to the known chemoattractants platelet-derived growth factor, insulin-like growth factor-1, and fibronectin (all at peak doses for migratory activity) was attenuated by beta-estradiol (0.5 to 10 ng/ml) in a concentration-dependent manner relative to control cells treated with vehicle (0.01% ethanol). This response was insensitive to pretreatment with indomethacin and was stereospecific (17 alpha-estradiol lacked response). Like beta-estradiol, the synthetic estrogen diethylstilbestrol attenuated directed smooth muscle cell migration whereas the male hormone testosterone was ineffective. Additional studies showed that beta-estradiol-mediated suppression of migration was inhibited by the anti-estrogen ICI 164,384 and the gene transcription inhibitor actinomycin D. These are the first results demonstrating a reduction in directed smooth muscle cell migration by beta-estradiol. The mechanism of this estrogen-mediated response appears to involve conventional estrogen receptors.

Role of intimal hyperplasia and arterial remodeling after balloon angioplasty: an experimental study in the atherosclerotic rabbit model

The arterial response to injury appears to be an important factor in the development or restenosis. Traditionally, intimal hyperplasia has been thought to be the primary mechanism responsible for restenosis. However, recent studies have found that arterial remodeling is a major determinant of lumen loss after balloon angioplasty. In this study, we evaluated the actual separate contributions of intimal hyperplasia and arterial remodeling to the restenotic process after balloon angioplasty in the atherosclerotic rabbit model. One month after induction of focal atherosclerotic lesions, femoral arteries were randomized to receive treatment with either two or six balloon inflations. One group of rabbits was euthanized immediately after angioplasty to evaluate the initial degree of injury with each dilation strategy ("acute group"), and the rest were euthanized 28 days after angioplasty ("chronic group"). Arteries that had been treated with six inflations had a higher injury score than those treated with two (4.0+/-3.0 versus 1.9+/-1.5, P<.05). In the chronic group, there was a significant increase in intimal area in the six inflation-treated arteries compared with the two-inflation group (0.617+/-0.06 versus 0.432+/-0.05 mm2, P<.004). However, there was no significant difference in lumen cross-sectional area between groups. By multivariate analysis, the most important independent predictor of lumen area was the external elastic lamina (EEL) area, although the degree of intimal thickening was also a significant independent predictor. There was a strong, positive correlation between intimal area and EEL area: the larger the intimal area, the larger the EEL area (r=.703, P<.0001). The intimal area was similar in both restenotic and nonrestenotic lesions. In contrast, EEL area was significantly larger (due to remodeling) in nonrestenotic lesions. This study confirms previous findings that the degree of injury determines the degree of neointimal proliferation and supports recent findings that chronic arterial remodeling plays a major role in the final lumen area. Understanding and controlling the remodeling process rather than concentrating solely on intimal hyperplasia may yield better results after balloon angioplasty in the future.

Chronic inhalation of nitric oxide inhibits neointimal formation after balloon-induced arterial injury

Systemic and local intravascular NO administration inhibits neointimal formation after vascular injury in animal models. NO appears to attenuate smooth muscle proliferation both directly and indirectly by preventing the release of growth factors. Inhalation of low concentrations of NO dilates pulmonary vascular smooth muscle but does not cause systemic vasodilatation. Recently, NO inhalation was found to inhibit platelet function in vivo. We studied the effects of NO inhalation on neointimal formation after balloon-induced injury of the adult rat carotid artery. Beginning 60 minutes before carotid injury, rats breathed either air with 0 or 80 ppm NO for 14 days. Rats were killed, carotid arteries were fixed and paraffin-embedded, and neointimal formation was measured by analyzing the ratio of intimal to medial areas (I/M ratio) in carotid artery cross sections. Intimal hyperplasia was evident in both groups of animals, but I/M ratios were 43% less in animals breathing 80 ppm NO for 2 weeks than in animals breathing air alone (0.78 +/- 0.12 and 1.37 +/- 0.11 [mean +/- SE], respectively; P < .02). Similarly, 1 week after carotid injury, neointimal formation was less in rats breathing 80 ppm NO than in rats breathing air alone (I/M ratio, 0.39 +/- 0.11 versus 0.76 +/- 0.06; P < .02). Breathing 20 ppm NO for 2 weeks or 80 ppm NO for 1 week followed by air alone for 1 week did not attenuate neointimal formation measured at 14 days. In anesthetized rats breathing 80 ppm NO or air alone for 1 hour, neither systemic blood pressure nor bleeding time differed. These observations demonstrate that inhaling 80 ppm NO inhibits neointimal formation after balloon-induced carotid artery injury in rats. NO inhalation may represent a safe and novel method of preventing restenosis after percutaneous angioplasty.

Induction of DNA synthesis by a single transient mechanical stimulus of human vascular smooth muscle cells. Role of fibroblast growth factor-2

BACKGROUND

Although mechanical vascular injury leads to smooth muscle cell proliferation that contributes to restenosis after balloon angioplasty, the role of the single transient mechanical stimulation of smooth muscle cells in this process is unknown.

METHODS AND RESULTS

To test the hypothesis that a single transient mechanical stimulus can increase DNA synthesis, human vascular smooth muscle cells cultured in a three-dimensional collagen gel system were subjected to transient compression. Transient compression (5-minute duration) of smooth muscle cell-collagen gel cultures in defined serum-free conditions led to delayed increases in [3H]thymidine incorporation. At 12 to 24 hours after compression, there was a 3.3 +/- 0.5-fold (P<.001 versus control) and 3.0 +/- 0.6-fold (P<.002 versus control) increase for 60% and 80% strain, respectively; at 24 to 36 hours after compression, there was a 1.8 +/- 0.5-fold (P<.05 versus control) and 4.3 +/- 0.8-fold (P<.001 versus control) increase. Also, serum-free media conditioned by transiently compressed gel cultures induced DNA synthesis in control, unstimulated smooth muscle cell cultures, suggesting the release of growth factors by transient compression. Although neutralizing antibodies against platelet-derived growth factor did not affect the mechanical induction of DNA synthesis, a neutralizing monoclonal antibody against fibroblast growth factor-2 (FGF-2) decreased this induction by 89% and completely blocked the increase in DNA synthesis caused by media conditioned by transiently compressed gels. Media conditioned by transient compression contained elevated levels of FGF-2 (17 +/- 5 versus 2 +/- 2 pg/mL for control, P<.005) with no increase in lactate dehydrogenase activity, suggesting release of FGF-2 with sublethal cellular injury.

CONCLUSIONS

A single transient mechanical stimulus increases DNA synthesis in human vascular smooth muscle cells, in part by autocrine or paracrine FGF-2 release.

Role of cimetidine in the prevention of intimal hyperplasia in rat carotid artery

In vitro studies have revealed that histamine increases smooth muscle cell proliferation and migration, an effect abolished by the H2 antagonist cimetidine. This study examined the effect of cimetidine on intimal hyperplasia in vivo. Thirty male Wistar rats underwent endothelial denudation of the left carotid artery; 15 received cimetidine 350 mg per kg per day for four weeks and 15 received vehicle only. Four weeks after injury the left carotid arteries were perfusion fixed and harvested. Morphometric analysis revealed that there was no significant difference between the intima:media ratio of rats treated with cimetidine (median (interquartile range (i.q.r.)) 1.69 (0.59)) and those treated with vehicle (median (i.q.r.) 1.59 (0.59), P = 0.28). Similarly, the percentage luminal reduction was not significantly different between the groups (median (i.q.r.) 54.7 (19.9) per cent and 45.8 (13.4) per cent respectively, P = 0.30). It is concluded that treatment with cimetidine does not reduce the formation of intimal hyperplasia in rat carotid arteries de-endothelialized with a balloon catheter.

T-cell lymphokines, interleukin-4 and gamma interferon, modulate the induction of vascular smooth muscle cell tissue plasminogen activator and migration by serum and platelet-derived growth factor

Platelet-derived growth factor (PDGF)-induced smooth muscle cell (SMC) fibrinolysis is necessary for SMC migration. In order to determine whether the T-cell lymphokines interleukin-4 (IL-4) and gamma interferon (gamma-IFN) affect SMC fibrinolysis and migration, we examined the effects of human recombinant IL-4 and gamma-IFN on human aortic SMC tissue-type plasminogen activator (TPA), urokinase-type plasminogen activator (UPA), and plasminogen activator inhibitor type-1 (PAI-1) antigen production, as determined by enzyme-linked immunosorbent assays. Although IL-4 had no direct effect on SMC TPA antigen, IL-4 potentiated SMC TPA antigen levels and activity in conditioned media and cellular lysates in media containing 2% fetal bovine serum but did not change UPA or PAI-1 production. gamma-IFN attenuated IL-4 augmentation of SMC TPA antigen production in conditioned media, although gamma-IFN itself had no direct effects on SMC TPA and PAI-1 antigen production. IL-4 augmented PDGF induction of SMC TPA antigen. gamma-IFN inhibited PDGF induction of SMC TPA antigen and IL-4 potentiation of this process. gamma-IFN diminished the promigratory effects of both IL-4 and PDGF on in vitro SMC migration. Tranexamic acid, a plasmin inhibitor, abrogated the stimulation of SMC migration by IL-4. Therefore, IL-4 and gamma-IFN modulate the induction of SMC TPA and SMC migration by 2% fetal bovine serum and PDGF.

Adenovirus-mediated over-expression of the cyclin/cyclin-dependent kinase inhibitor, p21 inhibits vascular smooth muscle cell proliferation and neointima formation in the rat carotid artery model of balloon angioplasty

Vascular smooth muscle cell (VSMC) proliferation after arterial injury is important in the pathogenesis of a number of vascular proliferative disorders, including atherosclerosis and restenosis after balloon angioplasty. Thus, a better understanding of the molecular mechanisms underlying VSMC proliferation in response to arterial injury would have important therapeutic implications for patients with atherosclerotic vascular disease. The p21 protein is a negative regulator of mammalian cell cycle progression that functions both by inhibiting cyclin dependent kinases (CDKs) required for the initiation of S phase, and by binding to and inhibiting the DNA polymerase delta co-factor, proliferating cell nuclear antigen (PCNA). In this report, we show that adenovirus-mediated over-expression of human p21 inhibits growth factor-stimulated VSMC proliferation in vitro by efficiently arresting VSMCs in the G1 phase of the cell cycle. This p21-associated cell cycle arrest is associated both with significant inhibition of the phosphorylation of the retinoblastoma gene product (Rb) and with the formation of complexes between p21 and PCNA in VSMCs. In addition, we demonstrate that localized arterial infection with a p21-encoding adenovirus at the time of balloon angioplasty significantly reduced neointimal hyperplasia in the rat carotid artery model of restenosis. Taken together, these studies demonstrate the important role of p21 in regulating Rb phosphorylation and cell cycle progression in VSMC, and suggest a novel cytostatic gene therapy approach for restenosis and related vascular proliferative disorders.

Relation of coronary angioscopic findings at coronary angioplasty to angiographic restenosis

BACKGROUND

Discordant results have been reported regarding morphological predictors of restenosis after percutaneous transluminal coronary angioplasty (PTCA). These discrepancies may be related to the limitations of angiography in the study of plaque morphology.

METHODS AND RESULTS

We studied 117 consecutive patients who underwent successful PTCA and who underwent coronary angioscopy before and immediately after the procedure. Angiographic follow-up was performed in 99 (85%) patients. We analyzed the relationship between angioscopic variables at the time of PTCA and the occurrence of restenosis assessed by quantitative coronary angiography. Plaque shape and color had no effect on late loss in luminal diameter (late loss: smooth lesions, 0.55 +/- 0.68 mm; complex lesions, 0.76 +/- 0.60 mm; white plaques, 0.51 +/- 0.56 mm; yellow plaques, 0.65 +/- 0.72 mm; P = NS). An angioscopic protruding thrombus at the PTCA site was associated with significantly greater loss in luminal diameter (late loss: no thrombus, 0.47 +/- 0.54 mm; lining thrombus, 0.59 +/- 0.67 mm; protruding thrombus, 1.07 +/- 0.77 mm; P < .05). Dissection assessed by angioscopy immediately after PTCA had no effect on late loss in luminal diameter (late loss: no dissection, 0.60 +/- 0.60 mm; simple dissection, 0.82 +/- 0.75 mm; complex dissection, 0.57 +/- 0.80 mm; P = NS).

CONCLUSIONS

These results show that coronary angioscopy may be helpful in predicting the risk of restenosis after PTCA. The high rate of angiographic recurrence observed when PTCA is performed at thrombus-containing lesions supports a role for thrombus in the process of luminal renarrowing after PTCA.

Migration behavior of human smooth muscle cells cultivated from restenotic and primary lesions

Subintimal smooth muscle cell (SMC) migration is considered an essential determinant of arteriosclerosis and neointimal formation. In this study, a cell culture model was established to characterize migration activity of SMCs originating from restenotic and primary lesions. Plaques from symptomatic stenoses of 32 patients (19 men, 13 women; 4 carotid, 17 peripheral, 11 coronary lesions) were removed by percutaneous atherectomy or direct operative approach. Ten patients suffered from recurrent stenosis. Cell cultures were established by explantation of tissue samples. By indirect immunofluorescence microscopy, SMCs were shown to be the predominant cell type of all advanced lesions irrespective of their origin. The spontaneous cellular motility of SMCs was analyzed in vitro by means of a computer-assisted observation system. Cells of all groups exhibited random motility. SMC migratory velocity was found to be significantly (P < 0.001) greater in cells from restenotic lesions than in those from primary plaques. In conclusion, migration behavior of human SMCs originating from arteriosclerotic lesions may be quantified in vitro as a functional determinant characterizing restenotic versus primary lesions.

Intracellular signaling pathways required for rat vascular smooth muscle cell migration. Interactions between basic fibroblast growth factor and platelet-derived growth factor

Intracellular signaling pathways activated by both PDGF and basic fibroblast growth factor (bFGF) have been implicated in the migration of vascular smooth muscle cells (VSMC), a key step in the pathogenesis of many vascular diseases. We demonstrate here that, while bFGF is a weak chemoattractant for VSMCs, it is required for the PDGF-directed migration of VSMCs and the activation of calcium/calmodulin-dependent protein kinase II (CamKinase II), an intracellular event that we have previously shown to be important in the regulation of VSMC migration. Neutralizing antibodies to bFGF caused a dramatic reduction in the size of the intracellular calcium transient normally seen after PDGF stimulation and inhibited both PDGF-directed VSMC migration and CamKinase II activation. Partially restoring the calcium transient with ionomycin restored migration and CamKinase II activation as did the forced expression of a mutant CamKinase II that had been "locked" in the active state by site-directed mutagenesis. These results suggest that bFGF links PDGF receptor stimulation to changes in intracellular calcium and CamKinase II activation, reinforcing the central role played by CamKinase II in regulating VSMC migration.

Phenotypic modulation of smooth muscle cells during the formation of neointimal thickenings in the rat carotid artery after balloon injury: an electron-microscopic and stereological study

The formation of neointimal thickenings in the rat carotid artery after balloon injury was studied by a combination of electron-microscopic and stereological methods. All smooth muscle cells in the normal media had a contractile phenotype, the cytoplasm being dominated by myofilaments. Seven days after endothelial denudation, the smooth muscle cells in the innermost part of the media had assumed a synthetic phenotype by loss of myofilaments and formation of a large endoplasmic reticulum and Golgi complex. These cells moved through fine openings in the internal elastic lamina and gave rise to a growing neointima by proliferation and secretion of extracellular matrix components. Fourteen days after the operation, the neointima had almost reached its final size, and mitoses were no longer noted. Nevertheless, the cells maintained a synthetic phenotype with prominent secretory organelles, although myofilaments had started to become more abundant again. They were surrounded by an extracellular matrix made up of collagen fibrils and coalescing patches of elastin. Thirty-five days after the operation, an endothelial cell layer had reformed and covered most of the luminal vessel surface. In parallel, the smooth muscle cells in the neointima had returned to a contractile phenotype with a cytoplasm dominated by myofilaments. These findings provide a morphological basis for further analysis of the cellular and molecular interactions involved in the formation of neointimal thickenings after endothelial injury, and for the search for agents interfering with this process.

VCL, an antagonist of the platelet GP1b receptor, markedly inhibits platelet adhesion and intimal thickening after balloon injury in the rat

BACKGROUND

Arterial injury is immediately followed by platelet adhesion at the site of injury, a process that requires the interaction of subendothelial von Willebrand factor with the platelet GP1b receptor. VCL, a recombinant von Willebrand factor GP1b binding domain, inhibits platelet binding to von Willebrand factor. The aim of this study was to determine whether VCL inhibits platelet adhesion at the site of arterial injury and affects neointimal thickening after injury in rats.

METHODS AND RESULTS

Sprague-Dawley rats were randomized to receive VCL, 4 mg/kg bolus followed by a continuous infusion of 2 mg.kg-1.h-1 for 72 hours, or an identical volume of saline. Balloon injury of the femoral artery was performed 15 minutes after the initial bolus injection of VCL. Scanning electron microscopy performed 1 and 3 days after injury indicated that VCL-treated rats had > 80% reduction in the number of platelets adherent to the vessel wall at the site of injury compared with controls (P < .003). Histological examination at day 14 showed that, compared with controls, VCL-treated rats had a 60% reduction in the intima-media ratio (0.21 +/- 0.03 versus 0.53 +/- 0.06, P = .001) and a reduced luminal area stenosis (12 +/- 3% versus 38 +/- 10%, P = .04). At 28 days after injury, there was no rebound of neointimal thickening in VCL-treated rats (intima-media ratio, 0.19 +/- 0.04; luminal stenosis, 17 +/- 5%). The difference between VCL-treated rats and control rats persisted but was attenuated (intima-media ratio, 0.19 +/- 0.04 versus 0.28 +/- 0.1, P = .162; luminal stenosis, 17 +/- 5% versus 31 +/- 5%, P = .058) as neointimal thickening regressed in untreated rats. With the use of proliferating cell nuclear antigen immunohistochemistry on day 3, VCL had no effect on smooth muscle cell (SMC) proliferation.

CONCLUSIONS

Antagonism of the platelet GP1b receptor by VCL profoundly decreased platelet deposition at the site of balloon injury in the rat femoral artery. This effect was associated with a persistent reduction in neointimal thickening. The lack of effect of VCL on SMC proliferation suggests that the decrease in neointimal thickening may have been mediated through inhibition of SMC migration and/or modulation of the extracellular matrix.

Stenosis enhances role of platelets in growth of regional thrombus and intimal wall thickening in rat carotid arteries

The authors present the results of a study in which stenosis was induced, resulting in either thrombus or intimal wall thickening, in rat carotid arteries. At > or = 75% stenosis in mildly denuded arteries, an acute and occlusive thrombus formation was induced, but the thrombus was significantly reduced in thrombocytopenia. Thrombus formation near the site of stenosis decreased with decreasing degree of stenosis, whereas the percent formation in the distal region (percent total thrombus) increased. Numerous mural platelet microthrombi were noted at the distal region of the stenosed arteries. After chronic 50% stenosis of the carotid artery for 2 weeks, significant intimal thickening was observed, without any occlusive thrombus formation. The combination with mild denudation was critical in eliciting the effect of stenosis. The magnitude of intimal growth in the stenosed artery was marked by day 6 and plateaued thereafter, whereas it was slight in nonstenosed arteries. The 5-bromodeoxyuridine index of the cells of the medial layer at day 3 was significantly increased by the stenosis, and the effect was reversed in thrombocytopenia. Complete reendothelialization of the intimal surface was observed by 7 to 10 days after surgery in the stenosed arteries. These findings suggest that the introduction of stenosis in these arteries enhances the interaction of platelets with the damaged arterial walls under abnormal fluid shear and that this enhancement leads to acute and occlusive thrombus formation associated with more marked stenosis as well as to sustained increase of intimal wall thickness in less marked stenosis.

Antiplatelet drugs. A comparative review

Antiplatelet therapy has become a useful means of preventing acute thromboembolic artery occlusions in cardiovascular diseases. The rationale for this is an enhanced activity of circulating platelets and release of platelet-derived vasoactive mediators, probably due to endothelial dysfunction. This review discusses the current status of 4 major classes of antiplatelet compounds: (i) aspirin and related drugs active via cyclo-oxygenase product formation; (ii) thienopyridines (ticlopidine and clopidogrel); (iii) direct thrombin inhibitors (e.g. hirudin); and (iv) GPIIb/IIIa receptor antagonists [e.g. abciximab (c7E3 Fab)]. It is concluded that aspirin is the drug of choice for long term oral treatment, specifically for secondary prevention of myocardial infarction, and is also a suitable basic but not maximally efficient drug in percutaneous transluminal coronary angioplasty (PTCA) and platelet activation during clot lysis. Ticlopidine has a similar indication and may be superior to aspirin in prevention of ischaemic stroke and peripheral arterial occlusion. Direct thrombin inhibitors and glycoprotein GPIIb/IIIa receptor antagonists need further investigation in clinical trials. To date, these compounds have a higher bleeding risk and currently they are available only for short term parenteral application. They are superior to aspirin in acute platelet-dependent ischaemic syndromes, such as unstable angina, and in connection with therapeutic PTCA because of their high potency in preventing platelet-dependent reocclusion. Future developments include more selective thromboxane inhibitors, i.e. combined-mode agents; nonpeptide clot-specific thrombin inhibitors with longer lasting action and nonpeptide fibrinogen receptor antagonists.

Identification of a 38-kDa heparin-binding glycoprotein (gp38k) in differentiating vascular smooth muscle cells as a member of a group of proteins associated with tissue remodeling

Cultured aortic smooth muscle cells (SMC) exhibit morphological and phenotypic modulation characterized by a change from a substrate attached monolayer culture to a multilayered nodular cell culture in which SMC are imbedded into the extracellular matrix. Associated with nodule formation is a change in the pattern of SMC gene expression including increased expression of a well characterized marker of smooth muscle cell differentiation, SM alpha-actin, and a 38-kDa glycoprotein (gp38k). gp38k has sequence homology with proteins reported to be correlated with tissue remodeling. To characterize the gp38k mRNA we designed degenerate oligonucleotides based on partial polypeptide sequencing to select a cDNA encoding the full-length gp38k. Southern analysis indicates that porcine gp38k is present as a single copy gene. Northern analysis indicates that the increase in gp38k is correlated with an increase in the steady state level of gp38k mRNA; and is present in cultures that have initiated the formation of multilayered foci and nodules. The correlation between SMC differentiation and gp38k expression is further established by using culture conditions that facilitate SMC differentiation. Cultures seeded onto reconstituted extracellular matrix show rapid formation of nodules and increased expression of gp38k mRNA. Comparison of the gp38k and cDNA sequences with nucleotide and protein sequences available through GenBank and SwissProt data banks revealed that molecules homologous to gp38k were present in human, mouse, bovine, and Drosophila tissues, suggesting that the gp38k may be a member of a gene family. Although a function for gp38k has not been identified, this report represents the first report of its correlation with a specific process important in phenotypic and morphological modulation of vascular SMC.

Restenosis after experimental angioplasty. Intimal, medial, and adventitial changes associated with constrictive remodeling

Predicting and preventing arterial restenosis after angioplasty has failed despite considerable research into mechanisms and techniques. We examined the roles of chronic constriction, neointimal-medial growth, and adventitial changes in restenosis in atherosclerotic rabbits. Angioplasty was performed on femoral artery lesions 4 weeks after lesion induction by air drying and cholesterol-supplemented diet. Angiographic and histological evaluation was conducted 3 to 4 weeks after angioplasty. The angiographic minimum luminal diameter (MLD) increased from 1.31 +/- 0.21 to 1.73 +/- 0.41 mm after angioplasty. Loss in MLD by 3 to 4 weeks was 0.95 +/- 0.64 mm. Initial gain and late loss correlated (P = .008). Late residual stenosis, defined histologically as the difference between the luminal areas of a proximal reference site and lesion site normalized by the luminal area of the reference site, was 52 +/- 32%. Histological indices of chronic constriction, neointimal-medial growth, and adventitial growth were defined on the basis of the areas of these arterial wall layers at the lesion site relative to the reference site. Another parameter defined as the ratio of adventitial area to the area of intima+media at the lesion site allowed evaluation of the relative importance of these layers. Surprisingly, late residual stenosis correlated with chronic constriction (P = .0003) but not with neointimal-medial growth or adventitial growth. The ratio of adventitial area to the area of intima+media at the lesion site also correlated with chronic constriction (P = .01). These findings suggest that factors related to arterial remodeling rather than neointimal-medial growth may dominate the response to angioplasty.

Vascular smooth muscle cell migration mediated by thrombin and urokinase receptor

To determine whether thrombin directly modifies mobility of vascular smooth muscle cells (SMC), in Transwell systems (modified Boyden chambers), we exposed SMC to alpha-thrombin. In concentrations as low as 1 NIH U/ml, thrombin induced migration as well as proliferation of SMC. Inhibition of protein synthesis by cycloheximide (2 micrograms/ml) obviated thrombin's chemotactic effect. Neither gamma-thrombin nor D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (PPACK)-inactivated alpha-thrombin (both used as controls) exerted a chemotactic effect. Concomitant hirudin or antithrombin III plus heparin inhibited chemotaxis by thrombin when added up to 2 h after addition of thrombin. alpha-Thrombin increased SMC synthesis of urokinase receptor (uPAR) and its cell surface expression as shown by metabolic labeling and immunoprecipitation as well as by flow cytometry. Thus alpha-thrombin, in concentrations thought to be present in vivo at sites of vascular injury, can stimulate not only proliferation but also migration of vascular SMC though a mechanism(s) possibly involving synthesis of uPAR, which is known to influence migration in diverse types of cells. Accordingly, both proliferation and migration dependent on thrombin may accelerate atherosclerosis, restenosis, or both after interventions such as angioplasty.

Potentiation of the hyporeactivity induced by in vivo endothelial injury in the rat carotid artery by chronic treatment with fish oil

1. The present study investigates whether or not chronic feeding of rats with a diet enriched in fish oil affects the reactivity of balloon-injured carotid arteries. The left carotid arteries were injured in vivo by the repeated passage of a balloon catheter. Both the right (control artery) and the left carotid arteries were excised 24 h after the injury, and suspended in organ chambers for the measurement of changes in isometric tension in the presence of indomethacin. 2. Phenylephrine evoked similar concentration-contraction curves in the right (control) carotid arteries without endothelium from control and fish oil-fed rats. Balloon injury decreased the contractility of carotid arteries to phenylephrine in both types of rats and the pEC50 for phenylephrine was significantly decreased in balloon-injured arteries from control rats compared to those obtained in arteries from fish oil-fed rats (pEC50 7.59 +/- 0.1 and 7.28 +/- 0.06, respectively) while maximal contractions were similar (1.93 +/- 0.15 g and 1.79 +/- 0.12 g, respectively). 3. The treatment of control right carotid arteries without endothelium with either NG-nitro-L-arginine (an inhibitor of nitric oxide synthase) or superoxide dismutase (which protects nitric oxide from degradation) did not affect significantly the contractions to phenylephrine in either group. In these preparations, methylene blue (an inhibitor of soluble guanylate cyclase) decreased slightly but significantly maximal contractions to phenylephrine in both groups. The treatment of balloon-injured carotid arteries with NG-nitro-L-arginine or methylene blue partly restored contractions to phenylephrine in arteries from both types of rat. Superoxide dismutase further depressed the contractility to the alpha l-adrenoceptor agonist in balloon-injured arteries from control diet-fed rats but had no effect in balloon-injured preparations from fish oil-fed rats.4. 3-Morpholino-sydnonimine (SIN-1, a donor of nitric oxide) evoked similar concentration-dependent relaxations in control and balloon-injured carotid arteries from both types of rat.5. Balloon injury caused an increase in the tissue content of cyclic GMP in carotid arteries from control diet-fed rats. This production of cyclic GMP was abolished by N0-nitro-L-arginine. Superoxide dismutase potentiated significantly the production of cyclic GMP caused by balloon injury in control but not in fish oil-fed rats.6 These observations confirm that in vivo balloon injury causes the production of nitric oxide in the injured blood vessel wall. This production of nitric oxide from L-arginine accounts for the decreased contractility to phenylephrine and the accumulation of cyclic GMP in balloon-injured arteries. They further indicate that chronic feeding of rats with fish oil potentiates the L-arginine-nitric oxide pathway in the injured vessel leading to an enhanced hyporeactivity to phenylephrine.

Local lesion-related factors and restenosis after coronary angioplasty. Evidence from a quantitative angiographic study in patients with unstable angina undergoing double-vessel angioplasty

BACKGROUND

Restenosis rates are high when coronary angioplasty is performed in patients with unstable angina. The relative contributions of local and systemic factors to this excess risk of restenosis are unclear. To assess these, we compared changes in minimal lumen diameter and the incidence of restenosis, determined by quantitative coronary angiography, after coronary angioplasty at culprit and nonculprit lesions dilated in the course of a single procedure in patients with unstable angina.

METHODS AND RESULTS

We identified 67 consecutive patients with unstable angina in whom two lesions, in different vessels, were dilated during the same procedure. Lesions were designated as culprit or nonculprit on the basis of the location of ECG changes during chest pain combined with assessment of the angiographic characteristics of the lesions. With these criteria, 43 patients had identifiable culprit lesions. Stenosis severity before and immediately after angioplasty and at follow-up was assessed with quantitative angiography. Angiographic follow-up was performed in 91% (39 patients) of this subgroup. Culprit lesions were more severe (P < .02) than nonculprit lesions. The late loss at culprit lesions (0.87 +/- 0.75 mm) was significantly (P < .01) greater than the equivalent value for nonculprit lesions (0.33 +/- 0.69 mm). With a categorical definition (> 50% stenosis at follow-up), restenosis occurred at 67% of culprit lesions and at 32% of nonculprit lesions (P < .01).

CONCLUSIONS

The greater loss in minimal lumen diameter and the consequent higher rate of restenosis at culprit compared with nonculprit lesions suggest that local "lesion-related" factors are an important determinant of the high rate of restenosis when coronary angioplasty is performed in patients with unstable angina.

Enhanced migratory activity of vascular smooth muscle cells with high expression of platelet-derived growth factor A and B

Proliferation and migration of vascular smooth muscle cells (SMCs) are major events in atherogenesis. It is known that platelet-derived growth factor (PDGF) stimulates both of these processes in a paracrine fashion, whereas autocrine stimulation has been shown only for proliferation. The aim of this study was to investigate the influence of PDGF expression in SMCs on migratory activity of these cells. SMCs were cultivated from the vascular tissue of 23 patients. Cellular motility was analyzed by a computer-assisted motion analysis system; 54 images per sample, obtained during an observation period of eighteen hours, were analyzed. PDGF-A and PDGF-B mRNA levels were determined by quantitative polymerase chain reaction (PCR) following reverse transcription. To quantitate mRNA content of SMCs, the authors coamplified cDNA copies of mRNA from cells and from a synthetic reference RNA in the same reaction vessel. Cells derived from atherosclerotic lesions produced a 1.6-fold increase of PDGF-A (P < 0.05) and a 5-fold increase of PDGF-B mRNA (P < 0.05) as compared with those from normal vessels. The migratory velocity (range 11.1-49.2 microns/hr) was independent of PDGF-A and PDGF-B mRNA expression. A significant correlation between levels of PDGF-A mRNA and PDGF-B mRNA and the degree of directional changes of SMCs on the covered track (klinokinesis) was found (P < 0.05).

CONCLUSION

PDGF-A and PDGF-B mRNA expression is significantly correlated with positive klinokinesis without affecting migratory velocity. This finding reflects enhanced migratory activity of SMCs. Besides its known mitogenic effects, the authors present evidence that PDGF may act as an autocrine motogen* in SMCs.

Cytostatic gene therapy for vascular proliferative disorders with a constitutively active form of the retinoblastoma gene product

Vascular smooth muscle cell (SMC) proliferation in response to injury is an important etiologic factor in vascular proliferative disorders such as atherosclerosis and restenosis after balloon angioplasty. The retinoblastoma gene product (Rb) is present in the unphosphorylated and active form in quiescent primary arterial SMCs, but is rapidly inactivated by phosphorylation in response to growth factor stimulation in vitro. A replication-defective adenovirus encoding a nonphosphorylatable, constitutively active form of Rb was constructed. Infection of cultured primary rat aortic SMCs with this virus inhibited growth factor-stimulated cell proliferation in vitro. Localized arterial infection with the virus at the time of balloon angioplasty significantly reduced SMC proliferation and neointima formation in both the rat carotid and porcine femoral artery models of restenosis. These results demonstrate the role of Rb in regulating vascular SMC proliferation and suggest a gene therapy approach for vascular proliferative disorders associated with arterial injury.

Cytomegalovirus infection enhances mRNA expression of platelet-derived growth factor-BB and transforming growth factor-beta 1 in rat aortic allografts. Possible mechanism for cytomegalovirus-enhanced graft arteriosclerosis

We have recently demonstrated that rat cytomegalovirus (RCMV) infection induces an early inflammatory response in the adventitia (perivasculitis) and in the subendothelial space (endothelialitis) as well as doubles smooth muscle cell (SMC) proliferation and intimal thickening of rat aortic allografts performed from the DA (AG-B4, RT1a) to the WF (AG-B2, RT1v) strain. In this study, the impact of RCMV infection on the structure of inflammation in the allograft adventitia and on the expression of SMC growth factors in the allograft vascular wall was investigated. The recipient rats were inoculated with 10(5) plaque-forming U of RCMV Maastricht strain or left noninfected and used as controls. The allografts were removed at 7 days and 1 and 3 months after transplantation and processed for morphometry and immunohistochemistry. RNA was isolated for reverse transcriptase polymerase chain reaction (RT-PCR). RCMV infection was associated with significantly upregulated presence (P < .05) of T helper (W3/25), T cytotoxic (OX8), and natural killer (3.2.3) cells in the allograft adventitia 7 days after transplantation but not thereafter. More monocyte/macrophages (OX42) were also present in RCMV-infected allografts, but the difference was not significant. Concomitantly, RCMV infection significantly enhanced (P < .05) the expression of major histocompatibility complex class II (OX6) and almost doubled (P = NS) the expression of interleukin-2R (CD25), intercellular adhesion molecule-1 (CD54;1A29), and lymphocyte function-associated antigen-1 alpha-chain (CD11a; WT.1) in the adventitial inflammatory infiltrate. RCMV infection was linked with an early, prominent expression of both PDGF-BB mRNA at 7 days (P < .05) and at 1 month (P < .025) and of transforming growth factor-beta 1 mRNA at 7 days (P < .025) and at 1 month (P < .025) after transplantation. A less-prominent mRNA upregulation of acidic fibroblast growth factor (P < .05) was associated with RCMV infection at 7 days and at 1 month, as well as of epidermal growth factor at 1 month after transplantation, when compared with noninfected allografts, although the mRNA expression in both groups was below the levels of nontransplanted DA aortas. RCMV infection almost doubled basic fibroblast growth factor mRNA expression (P = NS) in the allograft vascular wall at 7 days and at 1 month. RCMV infection had no additional impact on insulin-like growth factor-1 mRNA expression when compared with noninfected allografts.(ABSTRACT TRUNCATED AT 400 WORDS)

Patterns of tyrosine phosphorylation differ in vascular hypertrophy and hyperplasia

Vascular smooth muscle cells (VSMC) undergo hypertrophy when exposed to thromboxane A2 and hyperplasia when exposed to phorbol 12-myristate 13-acetate (PMA) or platelet-derived growth factor (PDGF). Each of these three agonists stimulate rapid tyrosine phosphorylation of numerous VSMC proteins. The current studies were undertaken to identify proteins that are specifically tyrosine phosphorylated in one or the other growth response. All three agonists increased the phosphotyrosine content of multiple proteins. In Western analysis of phosphotyrosine immunoprecipitates, the hyperplastic agents PDGF and PMA increased tyrosine phosphorylation of phospholipase C-gamma 1 (PLC-gamma 1), GTPase-activating protein (GAP), and phosphatidylinositol-3-kinase (PI-3-kinase), while the hypertrophic agonist thromboxane failed to tyrosine-phosphorylate either of these three substrates. Tyrosine kinase inhibition with herbimycin A (5 microM) prevented agonist-stimulated tyrosine phosphorylation of PLC-gamma 1, GAP, and PI-3-kinase. In growth studies, herbimycin A inhibited PMA- and PDGF-induced hyperplasia but not thromboxane-stimulated hypertrophy. These results indicate that tyrosine phosphorylation of PLC-gamma 1, GAP, and PI-3-kinase are specific responses for VSMC hyperplasia but not thromboxane-stimulated hypertrophy.

Characterization of thrombin receptor expression during vascular lesion formation

Blood vessels respond to injury by initiating cell proliferation and migration that result in vascular lesion formation. To determine the roles of thrombin and the thrombin receptor in this process, we characterized thrombin receptor expression in normal and injured arteries, thrombin receptor-mediated smooth muscle cell mitogenesis, and the regulation of thrombin receptor mRNA expression in vitro. Thrombin receptor mRNA was not detected in normal rat or baboon arteries by in situ hybridization. Immunohistochemistry using an antithrombin receptor antibody (TR-R9), directed against the thrombin cleavage site of the rat aortic smooth muscle cell thrombin receptor, revealed low-level staining for thrombin receptor protein in endothelial cells and smooth muscle cells of normal arteries. In contrast, balloon catheter injury increased thrombin mRNA expression in medial smooth muscle cells within 6 hours. This increased thrombin receptor expression continued within the media and in neointimal cells throughout vascular lesion formation, predominantly in areas of active cell proliferation. In vitro, alpha-thrombin stimulates rat aortic smooth muscle cell proliferation in a concentration-dependent manner. That thrombin receptor activation is required for the mitogenic response was confirmed by demonstrating that the polyclonal antibody TR-R9 inhibits thrombin-induced cell proliferation. Thrombin receptor mRNA synthesis was induced by both basic fibroblast growth factor (maximal stimulation of 1.8-fold at 1 hour) and platelet-derived growth factor (maximal stimulation of 2.4-fold at 8 and 24 hours) in quiesced cultured rat aortic smooth muscle cells. In summary, upregulation of smooth muscle cell thrombin receptor expression occurs very early after vascular injury and continues throughout neointimal development.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine kinase inhibition prevents deformation-stimulated vascular smooth muscle growth

The goal of this study was to determine the role of tyrosine phosphorylation in transducing deformation-stimulated vascular smooth muscle growth. Rat aorta-derived vascular smooth muscle cells were cultured on flexible silicone elastomer membranes and subjected to cyclic deformation (15 cycles per minute, deformed 2 seconds, relaxed 2 seconds). Deformation significantly increased proto-oncogene expression, [3H]thymidine incorporation, [3H]leucine incorporation, and cell number. Time course studies showed an 8-hour lag between initiation of cell deformation and onset of [3H]thymidine incorporation, with peak levels achieved after 18 to 24 hours. Western analysis of protein blots from deformed cells (10 minutes) demonstrated increased levels of phosphotyrosine-containing proteins having molecular weights of 110 to 130 and 70 to 80 kD. Deformation-stimulated tyrosine phosphorylation was prevented by the tyrosine kinase inhibitor Herbimycin A. Tyrosine kinase inhibition also prevented deformation-stimulated vascular smooth muscle cell growth as measured by [3H]thymidine incorporation. Cyclic deformation stimulates vascular smooth muscle proliferation through activation of tyrosine kinases. Inhibition of tyrosine phosphorylation is an effective means of preventing deformation-induced vascular smooth muscle growth in vitro.

Modulation of activator protein-1 (AP-1) transcription factor and protein kinase C by hydrogen peroxide and D-alpha-tocopherol in vascular smooth muscle cells

The effects of hydrogen peroxide D-alpha-tocopherol and of D-beta-tocopherol on proliferation, protein kinase C and activator protein-1 (AP-1) activation have been studied in vascular smooth muscle cells. Cell proliferation, when activated by foetal calf serum, was inhibited by D-alpha-tocopherol. Protein kinase C activity was stimulated by hydrogen peroxide in a manner similar to phorbol myristate acetate; in the latter case, but not in the former, D-alpha-tocopherol inhibited the reaction. Hydrogen peroxide prevented phorbol-myristate-acetate-stimulated AP-1 binding to DNA but stimulated it if protein kinase C was down-regulated or inhibited. D-alpha-Tocopherol promoted AP-1 activation in quiescent cells but prevented its activation by phorbol myristate acetate. None of the described effects of D-alpha-tocopherol were shared by D-beta-tocopherol, suggesting a non-antioxidant mechanism as the basis of its action. The data show that hydrogen peroxide and D-alpha-tocopherol affect more than one element in the cell signal-transduction cascade.

Inhibition of platelet activity by S-nitrosoglutathione during coronary angioplasty

Platelet activation is associated with acute vessel occlusion and chronic restenosis after percutaneous transluminal coronary angioplasty (PTCA). Organic nitrates, which act by releasing the vasodilator and anti-platelet agent nitric oxide (NO), have a predominantly vasodilator action and cause hypotension at doses required to inhibit platelet activation. S-nitrosoglutathione (GSNO) is an NO donor with a preferential action on platelets. We investigated platelet activation in patients undergoing PTCA and the effect of GSNO. Blood was sampled from the coronary sinus to measure platelet surface expression of P-selectin and glycoprotein IIb/IIIa as indices of platelet activation. In 7 control patients, PTCA caused a rise in platelet surface expression of P-selectin and glycoprotein IIb/IIIa, which was maximal 5 minutes after PTCA, indicating increased platelet activation despite treatment with aspirin, glyceryl trinitrate, and heparin. 6 patients received an intracoronary infusion of GSNO, starting 10 min before PTCA. GSNO significantly inhibited the PTCA-induced increase in platelet surface expression of P-selectin and glycoprotein IIb/IIIa without altering blood pressure. These findings show that platelets are activated following PTCA and that GSNO can prevent this activation.

Platelet-induced vascular smooth muscle cell proliferation is modulated by the growth amplification factors serotonin and adenosine diphosphate

BACKGROUND

Platelet-mediated mechanisms have been implicated in intimal lesion formation following vascular injury. Although the participation of peptide growth factors has been suspected in this process, little has been known about the possible mitogenic role of other platelet factors that are released at sites of vascular injury.

METHODS AND RESULTS

We tested the hypothesis that platelet products, which are not peptide growth factors, are important modulators of the platelet-induced smooth muscle cell (SMC) proliferative response by acting as growth amplification factors. In these studies, cell proliferation was assessed by [3H]thymidine incorporation, flow cytometry, and direct cell counting. We examined the potential mitogenicity of several platelet products, including serotonin, ADP, norepinephrine, histamine, platelet-activating factor, the thromboxane A2 mimetic U46619, and bradykinin. Of the platelet products tested, serotonin and ADP induced a synergistic response with peptide growth factors. This synergy was greatest at low growth-factor concentrations. Addition of nonaggregated platelets to quiescent SMC cultures strongly stimulated cell proliferation. Since the addition of suramin to platelet-treated cultures markedly inhibited SMC proliferation, peptide growth factors are most likely the primary mitogens mediating this response. However, platelet-induced proliferation was also markedly reduced by the serotonin antagonists ketanserin and LY53857 (44%), and by the ADP antagonist apyrase (35%).

CONCLUSIONS

Therefore, serotonin and ADP contribute significantly, in synergy with peptide growth factors, to the platelet-induced SMC proliferative response. We propose that in vivo serotonin and ADP act as amplification factors for SMC proliferation at sites of vascular injury.

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.

Smooth muscle cell migration and matrix metalloproteinase expression after arterial injury in the rat

We have characterized matrix metalloproteinase expression in the rat carotid artery after two forms of arterial injury, balloon catheter denudation and nylon filament denudation. Gelatinolytic enzymes with molecular masses of 70 and 62 kD were produced constitutively in the rat carotid. Production of an 88-kD gelatinase was induced after balloon catheter injury, and proteinase production continued during the period of migration of smooth muscle cells from the media to the intima, from 6 hours to 6 days after balloon catheter injury. In addition, a marked increase in 62-kD gelatinolytic activity was observed between 4 and 14 days after arterial injury. Gelatinase activities (88 and 62 kD) were also increased after nylon filament denudation but were markedly less after this injury than after balloon catheter injury. These results suggested a correlation between gelatinase activity and smooth muscle cell migration after arterial injury. Administration of a metalloproteinase inhibitor after balloon catheter injury resulted in a 97% reduction in the number of smooth muscle cells migrating into the intima. Therefore, we hypothesize that gelatinase expression directly facilitates smooth muscle cell migration within the media and into the intima. These results suggest that gelatinases are involved in the vascular smooth muscle cell activation and neointimal formation that characterize arterial tissue remodeling after injury.

Indirect angiogenic cytokines upregulate VEGF and bFGF gene expression in vascular smooth muscle cells, whereas hypoxia upregulates VEGF expression only

BACKGROUND

Hypoxia and indirect angiogenic factors may stimulate angiogenesis via induction of endothelial cell mitogen(s). To evaluate this hypothesis, we investigated whether low oxygen tension or cytokines known to promote neovascularization in vivo could modulate the expression of either vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF) in human vascular smooth muscle cells (SMCs).

METHODS AND RESULTS

SMCs were treated with platelet-derived growth factor BB (PDGF-BB) or transforming growth factor-beta 1 (TGF-beta 1) or exposed to low oxygen tension in serum-free medium. Northern analysis detected low basal levels of VEGF and bFGF mRNA in extracts of unstimulated SMCs. However, both VEGF and bFGF transcripts increased after administration of PDGF-BB (10 or 20 ng/mL) or TGF-beta 1 (0.1 to 10 ng/mL). Hypoxia was a potent stimulus for VEGF gene expression but had no apparent effect on bFGF steady-state mRNA levels.

CONCLUSIONS

These results indicate that certain indirect angiogenic cytokines, such as PDGF-BB or TGF-beta 1, may act via induction of bFGF and VEGF gene expression in cells resident near endothelial cells in vivo. Hypoxia constitutes a potent stimulus for VEGF gene expresion but does not regulate bFGF under the same experimental conditions.

Migration of cultured vascular smooth muscle cells through a basement membrane barrier requires type IV collagenase activity and is inhibited by cellular differentiation

The migration of vascular smooth muscle cells (VSMCs) from the tunica media to the neointima is a key event in the development and progression of many vascular diseases and a highly predictable consequence of mechanical injury to the blood vessel. In vivo, VSMCs are surrounded by and embedded in a variety of extracellular matrices (ECMs) that must be traversed during migration. One of the principal barriers to cell movement in the intact vessel is the basement membrane (BM) that surrounds each VSMC and separates the VSMC-containing medial cell layer from the endothelium. We have used a Boyden chamber to monitor the ability of VSMCs to degrade a BM barrier as they migrate toward a chemoattractant and to define the role of extracellular proteases in this process. We show that cultured VSMCs can migrate across a BM barrier and that this ability was dependent on the phenotypic state of the cell. VSMCs maintained in a proliferating or "synthetic" state readily migrated across a BM toward a chemoattractant, whereas the migration of serum-starved/differentiated VSMCs was suppressed by > 80% (P < .001). By use of a number of peptides that inhibit matrix metalloproteinase (MMP) activity, the migration of proliferating VSMCs across the BM barrier was inhibited by > 80% (P < .0001), whereas migration that occurred in the absence of the barrier was unaffected. Northern blotting and zymographic analyses indicated that 72-kD type IV collagenase (MMP2) was the principal MMP expressed and secreted by these cells. Accordingly, antisera capable of selectively neutralizing MMP2 activity also inhibited VSMC migration across the barrier without significantly affecting the migration of VSMCs in the absence of the barrier. Finally, MMP2 activity was also regulated by the phenotypic state of the cells in that MMP2 activity expressed by serum-starved/differentiated VSMCs was < 5% of that measured in proliferating VSMCs. Extrapolating to the in vivo situation in which VSMCs reside in an ECM composed of various BM barriers, these results suggest that VSMC migration in vivo may be dependent on MMP2 activity. That activity, in turn, could be regulated by the phenotypic state of VSMCs and increase as these cells undergo the transition from a quiescent and differentiated state to that of a dedifferentiated, proliferating, and motile phenotype after injury to the vessel.

Colorectal cancer: lessons for genetic counselling and care for families

Cancers of the colon and the rectum are the second leading cause of malignancy in European countries with similar incidence rates for men and women and, therefore, one of the major health concerns. Emphasis is placed on the early detection of a developing neoplasm in order to improve the life expectancy of patients and their quality of life. Colorectal cancer (CRC) is an excellent model for studying the etiology and pathogenesis of a common malignancy and the complex multistage process of carcinogenesis. Abundant clinical and pathological evidence suggests that CRC arises from benign adenomas that proceed through a series of steps to metastatic carcinomas. Following the discovery of oncogenes and, more importantly tumor suppressor genes, Fearon & Vogelstein (1990) proposed a scheme of genetic events which are associated with colorectal tumorigenesis. Genetic linkage studies have recently identified another type of gene for colon cancer susceptibility that seems to act by destabilising the genome.