Platelet-derived growth factor promotes smooth muscle migration and intimal thickening in a rat model of balloon angioplasty

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.

Differential effects of platelet-derived growth factor isotypes on human smooth muscle cell proliferation and migration are mediated by distinct signaling pathways

BACKGROUND

Platelet-derived growth factor (PDGF) is a potent mitogen and chemoattractant for vascular smooth muscle cells (SMCs). Three isotypes of PDGF (BB, AB, and AA) have been identified; each of these isotypes may have differing effects on the behaviour of vascular SMCs. In this study we evaluated the influence of PDGF isotypes on proliferation and migration of human venous SMCs and explored the signaling pathways through which these effects are mediated.

METHODS

Proliferation was measured by a 72-hour assay of cell number, and migration was evaluated by a 4-hour microchemotaxis assay. The effects of PDGF isotypes on the activities of the signaling proteins mitogen-activated protein kinase (MAP-K), p 125 focal adhesion kinase (p125FAK), and tensin were measured by immunoprecipitation of these proteins and subsequent phosphorylation on myelin basic protein (in MAP-K) and Western blotting with antiphosphotyrosine (in tensin and p125FAK).

RESULTS

All three isotypes stimulated SMC proliferation (PDGF-BB > AB > AA). PDGF-BB and -AB, but not -AA, stimulated chemotaxis. All three isotypes activated MAP-K with an intensity that corresponded to their proliferative effects. PDGF-BB and -AB tyrosine phosphorylated tensin and p125FAK, whereas PDGF-AA had no effect on either of these proteins.

CONCLUSIONS

For human vascular SMCs the physiologic effects and the signaling pathways that mediate these effects are specific for each of the three PDGF isotypes. These data also suggest an association between MAP-K and SMC proliferation and between the proteins, p125FAK and tensin, and migration.

Atherosclerosis and the vascular biology of aging

With advancing age, a series of structural, architectural and compositional modifications take place in the vasculature. The diameter of the vessels tends to increase, and thickening of intimal and medial layers is often observed. In the subendothelial space, blood-derived leukocytes and an increased amount of "activated" smooth muscle cells are present. Extracellular matrix accumulates and becomes particularly rich in glycosaminoglycans. Collagen content increases, while elastin fibers appear progressively disorganized, thinner, and frequently fragmented. These changes in the normal architecture of the vessel wall, that could be referred to as "the vasculopathy of aging", are likely to be the consequence of adaptive mechanisms to maintain normal conditions of flow, mechanical stress and/or wall tension. Although many of these features are similar to the histological findings of the atherosclerotic vessels, atherosclerosis and age-related "vasculopathy" are two distinct phenomena. Nonetheless, several experimental observations in animal models suggest a special link between "the vasculopathy of aging" and atherosclerotic disease, and suggest a particular predisposition of the old vessel to develop the atherosclerotic lesion. Compared to vessels from young animals, older ones show a greater reactivity to mechanical injury and to chronic insults. This may reflect changes in the biology of the vessels that are "intrinsic" to the aging process. Indeed, aging affects the function and responsiveness of the endothelium and vascular smooth muscle cells. Endothelial permeability is increased with age, while ability to produce vasoactive substances declines. Smooth muscle cells from old individuals show a growth advantage over the young ones, and display an increased ability to migrate toward chemoattractants. Moreover, the accumulation of advanced glycation end products (AGEs) occurring with aging can trigger a series of cellular events, such as cellular oxidative stress, expression of leukocyte adhesion molecules, endothelial transmigration of monocytes, and smooth muscle cell chemotaxis, all considered important prelesional events in the atherogenesis process. Taken together, the changes occurring with aging, while unproven to initiate lesion formation per se, are likely to accelerate the development of the atherosclerotic plaque and contribute to increased severity of this disease in the elderly.

Expression of platelet-derived growth factor B-chain in neointimal smooth muscle cells of balloon injured rabbit femoral arteries

In order to obtain information about the developmental mechanisms of restenosis after angioplasty, we investigated an association between the expression of platelet-derived growth factors (PDGFs) and neointimal cell accumulation in rabbit femoral arteries subjected to balloon angioplasty. Northern analysis demonstrated that mRNA expression of PDGF B-chain (PDGF-B) increased markedly in the injured arteries, peaking at day 7 (sevenfold), and the transcripts remained augmented until day 21. Also transcripts of PDGF beta-receptor (PDGFR-beta) and alpha-receptor increased by 3- and 2.5-fold, respectively, but those of PDGF A-chain showed only a slight increase (1.5-fold). In situ hybridization and immunohistochemistry demonstrated the concordant expression of mRNA and protein for PDGF-B in the smooth muscle cells (SMCs) of injured vessels throughout the experiment. PDGF-B expression peaked in neointimal SMCs at day 7. In accordance with PDGF-B expression, cellular proliferation in neointima peaked at day 7, being followed by a dramatic increase of neointimal areas thereafter. Further, we demonstrated PDGFR-beta immunoreactivity in these neointimal cells with PDGF-B expression. Our data provide evidence that PDGF-B may stimulate vascular SMC proliferation and contribute to neointimal formation after angioplasty.

Identification of a potential role for the adventitia in vascular lesion formation after balloon overstretch injury of porcine coronary arteries

BACKGROUND

In the present series of experiments, we examined the onset of cell proliferation and growth factor expression after balloon overstretch injury to porcine coronary arteries.

METHODS AND RESULTS

Domestic juvenile swine underwent balloon overstretch injury to the left anterior descending and circumflex coronary arteries with standard percutaneous transluminal coronary angioplasty balloon catheters. To identify proliferating cells, 5-bromo-2-deoxyuridine (BrDU) was administered over a period of 24 hours before the animals were killed at either 1, 3, 7, or 14 days after injury. Immunohistochemistry was performed with monoclonal antibodies to BrDU and smooth muscle cell markers. Three days after injury, a large number of proliferating cells were located in the adventitia, with significantly fewer positive cells found in the media and lumen. Seven days after injury, proliferating cells were found primarily in the neointima, extending along the luminal surface. In situ hybridization for PDGF A-chain and beta-receptor mRNAs revealed that the expression of these two genes was closely correlated with the sites of proliferation at each time point. Studies in which BrDU was injected between days 2 and 3 and the animals were killed on day 14 suggested that the proliferating adventitial cells may migrate into the neointima.

CONCLUSIONS

These data suggest that adventitial myofibroblasts contribute to the process of vascular lesion formation by proliferating, synthesizing growth factors, and possibly migrating into the neointima. Increased synthesis of alpha-smooth muscle actin observed in the adventitial cells after arterial injury may constrict the injured vessel and contribute to the process of arterial remodeling and late lumen loss after angioplasty.

Antiproliferative and c-myc mRNA suppressive effect of tranilast on newborn human vascular smooth muscle cells in culture

1. Newborn human vascular smooth muscle cells (VSMCs) proliferated faster and were more sensitive to platelet-derived growth factor-BB (PDGF-BB) than those from adults. In this study, we investigated mechanism of the inhibitory effect of tranilast on PDGF-BB-induced proliferation of VSMCs from newborns. 2. Tranilast (30-300 microM) concentration-dependently inhibited the VSMC proliferation in randomly growing cultures stimulated with PDGF-BB. 3. Tranilast (30-300 microM) concentration-dependently inhibited the [3H]-thymidine incorporation into DNA in VSMCs that had been synchronized by 48 h serum depletion and then stimulated by addition of PDGF-BB. However, tranilast had little influence on unscheduled DNA synthesis in quiescent cells or on RNA and protein synthesis, unlike aphidicolin, actimomycin D, and cycloheximide. 4. In synchronized VSMC cultures, tranilast still inhibited the PDGF-BB-induced DNA synthesis even when added 18 h after stimulation of the quiescent cells. The mode of the antiproliferative action of tranilast was different from that of NiCl2, genistein, or staurosporin. In addition, flow cytometry of synchronized VSMCs treated with tranilast revealed a blockade of PDGF-inducible cell-cycle progression at the G1/S checkpoint. 5. Northern blotting showed that tranilast (30-300 microM) concentration-dependently suppressed constitutive c-myc mRNA expression even when added 18 h after PDGF-BB-stimulation of quiescent VSMCs. Tranilast still had an inhibitory effect on the induction of c-myc mRNA when de novo protein synthesis was inhibited by cycloheximide and did not shorten the degradation of c-myc mRNA at the post-transcriptional level, demonstrating that tranilast directly inhibited c-myc mRNA expression at the transcriptional level. 6. These results suggest that the inhibitory effect of tranilast on PDGF-BB-induced proliferation is due to S-phase blockade and may be, at least in part, involved in the direct suppression of c-myc gene expression. Tranilast did not cause cell toxicity and may therefore hold promising potential for the prevention of vascular proliferative diseases.

Expression of platelet derived growth factor B chain and beta receptor in human coronary arteries after percutaneous transluminal coronary angioplasty: an immunohistochemical study

OBJECTIVE

To evaluate whether expression of platelet derived growth factor B (PDGF-B) protein is associated with expression of its receptor protein in human coronary arteries after angioplasty and to identify cells involved.

BACKGROUND

PDGF is considered an important growth factor in the repair process of the vessel wall after angioplasty. In situ hybridisation has revealed expression of PDGF-A and -B chain messenger ribonucleic acid (mRNA) in human coronary arteries at sites of postangioplasty injury.

METHODS

Target and non-target sites of eight coronary arteries were studied immunohistochemically for PDGF-B and PDGF-beta receptor proteins in relation to macrophages, T lymphocytes, smooth muscle cells, and HLA-DR positive cells.

RESULTS

The PDGF-B and PDGF-beta receptor proteins were expressed in areas with distinct repair, containing alpha actin negative spindle cells, macrophages and, at later stages, alpha actin positive smooth muscle cells as well. When the neointima was composed mainly of alpha actin smooth muscle cells, PDGF-B expression was rare and PDGF-beta receptor expression was negative.

CONCLUSIONS

There is expression of PDGF-B and PDGF-beta receptor proteins at sites of postangioplasty repair in human coronary arteries. The associated cells are mainly macrophages and alpha actin negative spindle cells; the latter may be dedifferentiated smooth muscle cells. A link between PDGF expression and the postangioplasty time interval suggests a relation with cell differentiation as part of the maturation of the repair tissue. Mutual expression of both the growth factor and its receptor protein strongly suggests that in humans a PDGF mediated repair process occurs, with involvement of smooth muscle cells and macrophages.

Effects of platelet glycoprotein IIb/IIIa receptor blockade by a chimeric monoclonal antibody (abciximab) on acute and six-month outcomes after percutaneous transluminal coronary angioplasty for acute myocardial infarction. EPIC investigators

Percutaneous transluminal coronary angioplasty (PTCA) for acute myocardial infarction is an attractive alternative to thrombolysis, but is still limited by recurrent ischemia and restenosis. We determined whether adjunctive platelet glycoprotein IIb/IIIa receptor blockade improved outcomes in patients undergoing direct and rescue PTCA in the Evaluation of c7E3 for Prevention of Ischemic Complications (EPIC) trial. Of the 2,099 patients undergoing percutaneous intervention who randomly received chimeric 7E3 Fab (c7E3) as a bolus, a bolus and 12-hour infusion, or placebo, 42 underwent direct PTCA for acute myocardial infarction and 22 patients had rescue PTCA after failed thrombolysis. The primary composite end point comprised death, reinfarction, repeat intervention, or bypass surgery. Outcomes were assessed at 30 days and 6 months. Baseline characteristics were similar in direct and rescue PTCA patients. Pooling the 2 groups, c7E3 bolus and infusion reduced the primary composite end point by 83% (26.1% placebo vs 4.5% c7E3 bolus and infusion, p = 0.06). No reinfarctions or repeat urgent interventions occurred in c7E3 bolus and infusion patients at 30 days, although there was a trend toward more deaths in c7E3-treated patients. Major bleeding was increased with c7E3 (24% vs 13%, p = 0.28). At 6 months, ischemic events were reduced from 47.8% with placebo to 4.5% with c7E3 bolus and infusion (p = 0.002), particularly reinfarction (p = 0.05) and repeat revascularization (p = 0.002). We conclude that adjunctive c7E3 therapy during direct and rescue PTCA decreased acute ischemic events and clinical restenosis in the EPIC trial. These data provide initial evidence of benefit for glycoprotein IIb/IIIa receptor blockade during PTCA for acute myocardial infarction.

Progression and regression of myointimal hyperplasia in experimental vein grafts depends on platelet-derived growth factor and basic fibroblastic growth factor production

PURPOSE

The factors that lead to myointimal hyperplasia (MH) in arterial vein grafts (AVGs) are unknown. Platelet-derived growth factor (PDGF) and basic fibroblastic growth factor (bFGF) are two powerful mitogens for smooth muscle cells that have been implicated in the genesis of MH. The aim of this study was to analyze the correlation between progression and regression of MH and production of PDGF and bFGF in experimental vein grafts.

MATERIALS

In 64 inbred Lewis rats, a 1-cm segment of inferior vena cava was inserted at the level of the abdominal aorta. The segments of inferior vena cava were obtained from syngenic rats. In 48 rats, the AVG was explanted 3 days (n = 8), 7 days (n = 8), 4 weeks (n = 24), and 12 weeks (n = 8) after surgery. In 16 rats the vein graft was explanted after being in the arterial system for 4 weeks and was reimplanted as a venous-venous bypass in syngenic Lewis rats. Reimplanted vein grafts (RVGs) were explanted 2 weeks (n = 8) and 8 weeks (n = 8) later. Grafts were analyzed by light and electron microscopy, morphometry, and histochemistry, and were put in organ culture to assess PDGF and bFGF production and mitogenic activity.

RESULTS

We observed MH formation in AVGs and MH regression in RVGs (p < 0.001).PDGF and bFGF production correlated with the degree of MH (p < 0.01). Histochemistry showed PDGF and bFGF in the area of MH in AVG, which disappeared in RVG. Conditioned media from AVG had greater mitogenic activity than RVG or control veins.

CONCLUSION

MH formation and regression in experimental vein grafts correlate with PDGF and bFGF production.

Platelet-derived growth factor-A mRNA expression in fetal, normal adult, and atherosclerotic human aortas. Analysis by competitive polymerase chain reaction

BACKGROUND

To understand which growth factors are important for growth of atherosclerotic plaques, it is necessary to know the factor's relative abundance and how its gene is regulated in relation to cell proliferation. We tested whether platelet-derived growth factor-A (PDGF-A) mRNA levels correlated with cell proliferation in developing aorta, normal adult aorta, and atherosclerotic plaques.

METHODS AND RESULTS

We developed a competitive reverse transcription-polymerase chain reaction (RT-PCR) assay to measure human PDGF-A mRNA levels in small tissue samples. A mutated PDGF-A synthetic RNA was used as an internal standard to compete with endogenous PDGF-A mRNA for amplification. The assay is highly sensitive and much more precise than routine RT-PCR. Correction for heteroduplex pairing between the endogenous and mutant PCR products correlates precisely with synthetic RNA standards and quantitative Northern blotting. Immunostaining with the proliferation marker (proliferating cell nuclear antigen) showed the following rank order of proliferation: fetal aorta >> atherosclerotic plaque > normal aortic media. PDGF-A mRNA levels, however, did not correlate with proliferation. Normal adult aorta contained the most PDGF-A mRNa (34.0+/-7.6 amol/microgram total RNA). Fetal aortas were intermediate (10.2+/-1.6 amol/microgram total RNA); advanced atherosclerotic plaques contained the least PDGF-A mRNA (0.3+/-0.1 amol/microgram total RNA). PDGF-A protein was readily detectable in normal media by immunostaining. Advanced plaques generally had less cell-associated PDGF-A protein, although A-chain was also detected in plaque matrix.

CONCLUSIONS

PDGF-A mRNA and protein do not correlate with proliferation among these three groups. The significance of high levels of PDGF-A mRNA in the "quiescent" aortic media is unknown but it clearly does not promote cell replication.

Effect of thrombin inhibition with desulfatohirudin on early kinetics of cellular proliferation after balloon angioplasty in atherosclerotic rabbits

BACKGROUND

Thrombin may have a pivotal role in restenosis after angioplasty. Hirudin, a potent thrombin inhibitor, reduces luminal narrowing by plaque after angioplasty in a rabbit model of atherosclerosis. Because cellular proliferation is believed to be an important mechanism for restenosis and thrombin has been shown to be a potent smooth muscle cell mitogen in vitro, we hypothesized that the mechanism of the effect of hirudin on limiting luminal narrowing by plaque occurs via inhibition of cellular proliferation.

METHODS AND RESULTS

Femoral atherosclerosis was induced in 108 rabbits, and balloon angioplasty was performed. At angioplasty, group 1 rabbits (n=38) were treated with a 2-hour infusion of hirudin, and group 2 rabbits (n=41) were treated with heparin. Group 3 rabbits (n=29) were treated with hirudin (n=15) or heparin (n=14) and killed at 7 or 28 days to determine cross-sectional area narrowing by plaque and cellular proliferation with the use of bromodeoxyuridine labeling. At 29, 71, or 167 hours after angioplasty, group 1 and 2 rabbits were injected with 3H-thymidine and killed 1 hour later, and labeling indexes were determined. A significant increase in the index of 3H-thymidine-labeled nuclei was observed in the intima of "ballooned" arteries compared with "nonballooned" atherosclerotic arteries at both 30 hours (0.06+/-0.05 versus 0.01+/-0.01, P<.01) and 72 hours (0.10+/-0.06 versus 0.004+/-0.004, P<.01). By 7 days, the index of labeled cells was similar to baseline (0.04+/-0.03 versus 0.01+/-0.01, P=.12). Hirudin had no effect on the 3H-thymidine labeling indexes at any of the time points studied despite the fact that hirudin treatment in group 3 rabbits resulted in less cross-sectional area narrowing by plaque at both 7 and 28 days after angioplasty (41+/-16 versus 24+/-12 at 7 days and 60+/-21 versus 44+/-17 at 28 days, heparin versus hirudin; P<.03).

CONCLUSIONS

Balloon angioplasty resulted in a marked increase in cellular proliferation that peaked at 72 hours. A 2-hour infusion of hirudin failed to reduce early 3H-thymidine labeling, suggesting that inhibition of cell proliferation within the first 7 days after angioplasty is not the predominant mechanism by which hirudin exerts its effect on limiting luminal narrowing by plaque 28 days after balloon angioplasty in this animal model.

Egr-1-induced endothelial gene expression: a common theme in vascular injury

A number of pathophysiologically relevant genes, including platelet-derived growth factor B-chain (PDGF-B), are induced in the vasculature after acute mechanical injury. In rat aorta, the activated expression of these genes was preceded by a marked increase in the amount of the early-growth-response gene product Egr-1 at the endothelial wound edge. Egr-1 interacts with a novel element in the proximal PDGF-B promoter, as well as with consensus elements in the promoters of other genes induced by endothelial injury. This interaction is crucial for injury-induced PDGF-B promoter-dependent expression. Sp1, whose binding site in the PDGF-B promoter overlaps that of Egr-1, occupies this element in unstimulated cells and is displaced by increasing amounts of Egr-1. These findings implicate Egr-1 in the up-regulated expression of PDGF-B and other potent mediators in mechanically injured arterial endothelial cells.

Growth factor production after polytetrafluoroethylene and vein arterial grafting: an experimental study

PURPOSE

Occlusion caused by myointimal hyperplasia appears to be the main reason of late failure of polytetrafluoroethylene (PTFE) arterial bypass grafts. Evidence exists that growth factors are involved in the genesis of myointimal hyperplasia. The aim of this study was to assess the release of platelet-derived growth factor (PDGF) and basic fibroblastic growth factor (bFGF) by PTFE arterial grafts.

METHODS

In 15 inbred Lewis rats a 1 cm long segment of PTFE was interposed at the level of the abdominal aorta. In a control of another 15 Lewis rats in a vein graft was implanted at the level of the abdominal aorta. Animals were killed four weeks after implantation and the tissue was studied in organ culture for release of PDGF AA, PDGF BB, and bFGF.

RESULTS

PTFE grafts released a greater quantity of PDGF AA than did control vein grafts (28 +/- 4 ng/cm2/72 hr vs 7 +/- 2 ng/cm2/72 hr). Similarly, PTFE grafts released a greater quantity of bFGF than did arterial vein grafts (308 +/- 22 ng/cm(2)/72hr vs 204 +/- 20 ng/cm2/72 hr).

CONCLUSIONS

We conclude that PTFE arterial grafts released a high quantity of growth factor, which could explain, in part, the occurrence of distal anastomotic myointimal hyperplasia.

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 nitric oxide in the effect of blood flow on neointima formation

PURPOSE

Neointima formation after arterial injury is inhibited by increased blood flow. The object of this study was to determine whether nitric oxide mediates the effect of increased blood flow on neointima formation.

METHOD

Balloon catheter-denuded rat carotid arteries were exposed to increased blood flow or control blood flow by ligation of the contralateral carotid artery. Beginning 2 days before balloon denudation, rats were given either saline vehicle alone or the nitric oxide synthase inhibitor N-nitro-L-arginine-methyl ester (L-NAME) at a dose of 10 mg/kg/day or 2 mg/kg/day intraperitoneally. The normalized neointima area was measured 14 days after denudation.

RESULTS

Blood flow was significantly increased by ligation of the contralateral carotid artery for all drug treatments (p<0.008). In rats given saline vehicle only, normalized neointima area was significantly reduced after increased blood flow compared with control blood flow (0.33+/-0.04 compared with 0.48+/-0.03; p=0.006). Systolic blood pressure was significantly elevated by treatment with high-dose L-NAME (p=0.002 compared with vehicle), but was not altered by low-dose L-NAME (p=NS compared with vehicle). Normalized neointima area was not significantly reduced after increased carotid blood flow for rats treated with either dose of L-NAME (p=NS).

CONCLUSION

The inhibition of neointima formation by increased blood flow was abolished with hypertensive and nonhypertensive doses of the nitric oxide synthase inhibitor L-NAME, which suggests that the L-NAME effects are independent of systemic hemodynamic alterations. It is concluded that flow-induced inhibition of neointima formation is mediated in part by nitric oxide.

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

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

A proliferation analysis of arterial neointimal hyperplasia: lessons for antiproliferative restenosis therapies

Medial smooth muscle cell proliferation is frequently implicated as the major cause of coronary restenosis. Although antiproliferative agents have shown efficacy in animal studies, they are ineffective in human trials. To better understand these discrepancies, we performed a mathematical kinetic analysis of cellular proliferation in the neointimal hyperplasia of rats, pigs, and patients. A model was derived using a differential expression for proliferation, proportional to the number of cells present. Additional terms were included for inhibition of proliferation proportional to neointimal mass and time. The resulting equation was solved in closed form for the number of cells and proliferation rate. These equations were validated in the rat carotid artery injury model from published data. The model was then applied to the porcine coronary injury model, and then to clinical data obtained from angiographic human studies. Peak cellular proliferative activity in patients occurs at 16 days and continues at lower levels for much longer periods of time. Less than 10 generations of cells are sufficient to develop clinically significant restenosis. Conversely, proliferation rates in the two animal models (rats and pigs) are maximal at roughly 2 and 6 days, respectively, also continuing at low levels for extended time periods. Cell proliferation in restenosis is a highly controlled process, with comparatively few cell generations causing enough neointima for arterial obstruction to occur. Substantial cell kinetic differences occur across species. The rat exhibits high proliferation rates and rapid doubling times compared to patients and pigs, and is thus a highly 'proliferative' model. Such differences may be responsible for discrepant animal model and clinical trial results. These data may help determine the timing and strategy of therapy against clinical restenosis.

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.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Elaboration of type-1 plasminogen activator inhibitor from adipocytes. A potential pathogenetic link between obesity and cardiovascular disease

BACKGROUND

Obesity is known to predispose to attenuated fibrinolysis attributable to increased concentrations in plasma of type-1 plasminogen activator inhibitor (PAI-1), the primary physiological inhibitor of endogenous fibrinolysis. PAI-1 is present in neointimal vascular smooth muscle cells and lipid-laden macrophages.

METHODS AND RESULTS

The present study was designed to determine whether PAI-1 expression occurs in adipose tissue as well, thereby potentially contributing to increased cardiovascular risk associated with obesity. 3T3-L1 preadipocytes were differentiated into adipocytes by exposing them to isobutylxanthine (0.5 mmol/L) and dexamethasone (0.25 mumol/L) over 7 days and incubated for 24 hours with transforming growth factor-beta (TGF-beta), known to augment PAI-1 synthesis in several cell types and to be released from platelets when they are activated. TGF-beta increased PAI-1 activity in the conditioned media of the 3T3-L1-derived cells in a concentration-dependent fashion without significantly affecting cell proliferation. Western blotting and immunoprecipitation of 35S-labeled PAI-1 showed that the increased PAI-1 activity paralleled increased PAI-1 protein. Northern blotting showed that increased PAI-1 mRNA preceded increased accumulation of PAI-1 activity and protein in the conditioned media. Furthermore, TGF-beta (10 ng/g body wt) administered in vivo increased PAI-1 activity in mouse plasma and PAI-1 mRNA expression in mouse adipose tissue.

CONCLUSIONS

Increased plasma PAI-1 activity in obese human subjects may result from PAI-1 release from an increased mass of adipose tissue, particularly in association with thrombosis and elaboration of TGF-beta from platelet alpha-granules into the circulation. The increased PAI-1 may exacerbate vascular disease by shifting the balance between thrombosis and thrombolysis toward thrombosis and consequently exposing luminal surfaces of vessels to mitogens associated with microthrombi over protracted intervals.

Expression and function of a recombinant PDGF B gene in porcine arteries

Platelet-derived growth factor (PDGF) B is a mitogen and chemoattractant for smooth muscle cells in vitro, and expression of a recombinant PDGF B gene in porcine arteries stimulates intimal thickening. To define the mechanisms by which PDGF B gene expression induces intimal thickening in vivo, we examined its effects on smooth muscle cell proliferation and migration, extracellular matrix synthesis, and inflammatory cell infiltration in intimal lesions of pig arteries after direct gene transfer of a recombinant PDGF B gene. PDGF B gene expression was associated with rapid formation of an intima, including 3- to 10-fold increases in intimal thickness and intima-to-media area ratio 4 to 21 days after gene transfer compared with control transfected arteries. Intimal smooth muscle cell proliferation was detected at 2 days, peaked at 7 days (P < .01), and declined by 14 days, although the total number of intimal nuclei progressively increased to 21 days (P < .01). Calculations of expected-to-observed ratios of intimal cells, based on BrdC proliferation indexes, demonstrated that the increases in intimal cell number on days 2 through 7 could not be accounted for by proliferation alone, suggesting that recombinant PDGF BB acts to stimulate cell proliferation and migration of smooth muscle cells into the intima. Extracellular matrix deposition and procollagen synthesis were observed after 7 days (P < .01) and were associated with a decline in cell density in the intima, suggesting that extracellular matrix synthesis may contribute to progressive intimal thickening in response to PDGF B gene expression. There was minimal accumulation of inflammatory cells, including macrophages and CD3(+) lymphocytes, in transfected arteries. These data suggest that PDGF B gene expression promotes intimal expansion by both proliferation and migration of smooth muscle cells followed by synthesis of extracellular matrix and therefore acts through several mechanisms to play a role in the pathogenesis of intimal lesions in vivo.

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.

Immunological mechanisms in atherosclerosis

Immunological mechanisms seem to be potent modulators of the atherosclerotic process. The presence of substantial numbers of T-lymphocytes in the lesion and local and circulating autoantibodies to plaque components suggests that a specific immune response is operating. Focal expression of adhesion molecules and local secretion of chemoattractants could mediate the recruitment of inflammatory cells to the lesion. Local cytokine and growth factor networks may operate later, controlling cell migration and proliferation. Although it is still important to realize the complexity of these mechanisms, the ongoing characterization of the molecular mechanisms in atherogenesis may lead to new strategies for intervention with the disease process.

Mononuclear leukocytes invade rabbit arterial intima during thickening formation via CD18-and VLA-4-dependent mechanisms and stimulate smooth muscle migration

The role of mononuclear leukocytes for the migration of smooth muscle cells (SMCs) during intimal thickening was investigated in the rabbit model of electrically stimulated carotid artery. The approach was to inhibit leukocyte entry into the arterial intima with antibodies against the adhesion molecules very late activation antigen-4 (VLA-4) and CD11/CD18. In electrically stimulated control rabbits treated either with saline or a nonspecific antibody, all types of granulocytes, monocytes, and lymphocytes migrated across an intact endothelium into the acellular subendothelial space, followed by the movement of SMCs from the media into the intima within 36 hours of applying electrical current. Treatment of the rabbits with monoclonal antibody (mAb) HP1/2 directed toward the alpha 4 subunit (CD49d) of VLA-4 inhibited mononuclear leukocyte invasion (consisting of monocytes and lymphocytes) by approximately 70% compared with the IgG-treated control rabbits and completely abolished the minimal influx of basophils and eosinophils after 36 hours. Neutrophil infiltration, however, remained unaffected by anti-VLA-alpha 4 treatment. Under these conditions, SMC migration across the internal elastic lamina was reduced by 50%. The use of mAb HP1/2 together with mAb 60.3 (directed to the beta 2 chain of CD11/CD18) completely abolished the influx of monocytes, lymphocytes, and all types of granulocytes into the arterial intima. This complete blockade of leukocyte infiltration resulted in a 70% reduction of intimal SMC accumulation. Together with our previous findings excluding neutrophils as stimulators of SMC migration, the present results indicate that mononuclear leukocyte promote lesion development by stimulating SMC migration.

Inhibition of PDGF- and TGF-beta 1-induced collagen synthesis, migration and proliferation by tranilast in vascular smooth muscle cells from spontaneously hypertensive rats

Vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) proliferate faster and are more sensitive to transforming growth factor-beta 1 (TGF-beta 1) than those of normotensive Wistar-Kyoto rats. We studied the in vitro effects of tranilast, an anti-allergic drug, on the proliferation, migration and extracellular matrix synthesis in the SHR-VSMC. There were many inhibitory effects of tranilast (30-300 microM) on SHR-VSMC. One is the effect on the proliferation stimulated with fetal bovine serum (FBS), TGF-beta 1 and platelet-derived growth factor-BB (PDGF-BB). Another is the effect on the PDGF-BB-induced migration. Lastly, tranilast exhibited inhibitory effects on spontaneous collagen synthesis and TGF-beta 1-induced collagen and glycosaminoglycan synthesis. On the other hand, collagen induced the VSMC migration concentration-dependently. These results suggest that tranilast may prevent restenosis after percutaneous transluminal coronary angioplasty.

In vivo effect of TGF- beta 1. Enhanced intimal thickening by administration of TGF- beta 1 in rabbit arteries injured with a balloon catheter

The in vivo effect of transforming growth factor-beta 1 (TGF-beta 1) was studied in a model system in which arterial intimal thickening was induced by injury of rabbit arteries with a balloon catheter (BCI). Intimal area and its ratio to medial area in carotid arteries after BCI were significantly higher in rabbits treated with 10 micrograms/kg TGF-beta 1 and 10 mg/kg aspirin i.v. QD (TGF-beta 1 group) than in those treated with 10 mg/kg aspirin i.v. QD only (control group). Intimal cell numbers in the TGF-beta 1 and control groups were not significantly different from each other, but matrix volume in the intimal layer was significantly higher in the TGF-beta 1 group. By immunohistochemical and Northern blot analyses, the fibronectin content in carotid intimal and medial layers was greater in the TGF-beta 1 group compared with that in the control group. Thus, in intimal thickenings induced by BCI. TGF-beta 1 mainly enhanced the formation of matrix containing fibronectin. Moreover, the mRNAs of TGF-beta 1 and type II receptors were detected in carotid arteries 7 and 14 days after, but not before, BCI. Thus, TGF-beta 1 influences the process of intimal thickening induced by BCI through a receptor-mediated mechanism in vivo. The significance of this fact is discussed in relation to the development of atherosclerosis.

Antisense therapy for angioplasty restenosis. Some critical considerations

The high affinity of even relatively short sequences of DNA for their target mRNA suggests that antisense agents represent an ideal method of suppressing specific gene products both in vitro and in vivo. In experiments performed thus far, an effect on the target mRNA in cultured vascular cells and in the vessel wall can be documented. The in vitro activity, toxicity, and pharmacokinetic data of antisense oligonucleotides are encouraging, and the in vivo animal experiments demonstrating suppression of neointimal formation are very promising. If animals trials presently under way show continued suppression not only of intimal formation but also of loss of lumen caliber after a single application, then effective delivery of antisense oligonucleotides is a realistic possibility. Nevertheless, some words of caution regarding the use of antisense oligonucleotides are warranted. Potential nonspecific effects of antisense oligonucleotides should be carefully considered in studies in which antisense agents are used to define biological functions of specific genes. In particular, demonstration that the target mRNA has been suppressed does not prove that other sequences within the mRNA pool have not also been suppressed. Critical control measures include adding back the target mRNA or protein and demonstrating similar biological effects with antisense sequences, which also suppress target gene expression directed at different regions of the target mRNA. At the clinical level, the systemic effects of antisense oligonucleotides, the dosage required, the timing of administration compared with mechanical intervention, and the toxicity of breakdown products all need to be established. In addition, the most appropriate targets for antisense use in restenosis remain largely obscure. Indiscriminate suppression of cell-cycle genes or proto-oncogenes may be as acutely toxic as current anticancer chemotherapy if the site delivery is not completely localized. Furthermore, much of the clinical evidence suggests that restenosis is a chronic process, continuing to develop weeks to months after the procedure. If this is the case, then the current approaches that rely on a transient, local application of an antisense agent may fail. If, however, a target gene is identified that is specific to vascular tissue, then repeated administration of an antisense agent may be tolerated via a systemic route. This approach has proved successful in targeting mutated genes with little suppression of closely related genes and with minimal systemic toxicity. An alternative approach is to transfect the target tissue with a gene that makes it susceptible to systemic delivery of a drug that is not normally toxic to mammalian cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Platelet-derived growth factor. Distinct signal transduction pathways associated with migration versus proliferation

Figure 2 summarizes our current interpretation of data concerning signals from the activated PDGF receptor involved in directed migration and proliferation of human arterial SMC. Binding of PDGF (PDGF-BB or PDGF-AA) causes PDGF-receptor dimerization, tyrosine autophosphorylation, and subsequent binding of several molecules containing SH2 domains to the activated receptor. Binding and activation of PLC gamma by the PDGF receptor leads to PIP2 hydrolysis, resulting in generation of diacylglycerol (DAG) and IP3. Subsequently, intracellular levels of calcium are elevated as a result of IP3-mediated calcium release from intracellular compartments. The decreased levels of PIP2 and increased levels of calcium both favor actin-filament disassembly by inducing capping of actin-filament barbed ends and actin-monomer sequestration. A localized, and transient, actin-filament disassembly enables the cell to extend filopodia towards PDGF, thereby enabling chemotaxis to take place. At a later time and/or in a different compartment, actin-filament assembly is promoted by PDGF by a mechanism that is not completely understood, but that may involve small GTP-binding proteins, such as Rho, and formation of DAG. Migration on collagen requires functional alpha 2 beta 1 integrins, which may either constitute a permissive state required for a cell to migrate, or which may be actively involved in intracellular signals leading to migration. PDGF-induced DNA synthesis and proliferation involves activation of Ras, MAP kinase kinase, and MAP kinase. Cross-talk between PKA signaling and tyrosine-kinase receptor signaling results in PKA inhibition of the MAP kinase cascade, probably at the level of Raf. Activation of PI 3-kinase, or a PI 3-kinase-like enzyme, is also likely to contribute to the mitogenic effects of PDGF in these cells (Bornfeldt, unpublished observation). What determines if a SMC will migrate and/or proliferate in response to PDGF? Results are starting to emerge that show regulation of expression of molecules involved in intracellular signaling with different phenotypic states of SMC. For example, expression of PLC gamma is very low in intact vascular wall (where SMC show a "contractile phenotype"), and induced when SMC are converted to a "synthetic phenotype" in culture. Proliferation and expression of MAP kinase, but not calcium signaling, appear to be regulated by the extracellular matrix, and the profile of integrin expression is different in SMC in culture compared to SMC in the vascular wall. Thus, the relation between expression of signaling molecules involved in migration and signaling molecules involved in proliferation, as well as cross-talk between different signal-transduction pathways, may determine the net effect of PDGF.

Smooth muscle cell proliferation is proportional to the degree of balloon injury in a rat model of angioplasty

BACKGROUND

A variable degree of smooth muscle cell (SMC) proliferation after balloon injury has been reported in previous rat studies. It is unknown whether balloon injury induces c-fos expression and whether it is related to the degree of vascular injury in vivo. Therefore, we tested the hypothesis that proportional increases in neointimal formation and c-fos expression might be present after different degrees of balloon dilation.

METHODS AND RESULTS

Angioplasty of the carotid artery was performed with a balloon catheter. Vascular injury was evaluated at 0, 0.5, 1.0, 1.5, and 2 atm (n = 6 for all). In 40 additional rats, total RNA dot blots were performed to assess the effect of various degrees of balloon injury on c-fos expression. SMC proliferation proportional to the increases of inflation pressure was found between 0 and 2 atm with neointimal areas of 0.002 +/- 0.002, 0.069 +/- 0.014, 0.128 +/- 0.043, 0.190 +/- 0.010, and 0.255 +/- 0.041 mm2, respectively. When the degree of SMC proliferation (neointima and neointima/media ratio) was plotted against balloon inflation pressure, a linear relation was observed (r = .733, P < .001 and r = .755, P < .001, respectively). An increase in c-fos expression proportional to the degree of injury was found 30 minutes after injury.

CONCLUSIONS

Neointimal proliferation produced by balloon injury is related to balloon inflation pressure, supporting the concept of an SMC proliferative response proportional to the degree of injury. The increase in SMC proliferation is associated with a proportional increase in the early expression of the c-fos nuclear proto-oncogene.

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.

FK409, a new nitric-oxide donor, suppresses smooth muscle proliferation in the rat model of balloon angioplasty

The effect of FK409, a new nitric-oxide (NO) donor, on neointimal formation of rat carotid arteries following balloon injury was studied. The intimal thickening at 14 days was strongly suppressed by twice daily administration of FK409 at 10 mg/kg from 2 days before to 13 days after injury. The neointima area and neointima/media ratio were decreased by 48.0% (P < 0.01) and 38.5% (P < 0.01), respectively, compared with control. On the other hand, isosorbide dinitrate (ISDN), a classical nitro-vasodilator, did not suppress intimal thickening even at 100 mg/kg twice a day. An in vivo 5-bromo-2'-dedoxyuridine (BrdU) uptake study revealed that FK409 inhibited the proliferative response of smooth muscle cells (SMC) in media at early stage of injury. In fact, the neointimal formation at 14 days was inhibited by the short term administration of FK409 only from the day of injury to 4 days after at 10 mg/kg twice a day. In cultured rat SMC, FK409 (1-10 mumol/l) markedly enhanced intracellular c-GMP and inhibited the proliferation in 10% FBS-containing medium. These results suggest that FK409 suppresses intimal thickening following balloon injury of the rat carotid artery by inhibition of SMC proliferation.

Structure-activity studies of phosphorylated peptide inhibitors of the association of phosphatidylinositol 3-kinase with PDGF-beta receptor

Phosphorylated pentapeptides derived from Tyr751 of the PDGF-beta receptor (pTyr751-Val-Pro-Met-Leu, pTyr = phosphotyrosine) were prepared to examine their ability to inhibit the association of the C-terminal SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) with the PDGF-beta receptor. Peptidic analogs were prepared to examine the importance of the amine and carboxy terminus and specific amino acids via alanine/D-amino acid scans and site specific modifications. Several of these peptides had submicromolar activity. In particular, it was shown that neutralization of the amine and carboxy terminus led to analogs with enhanced activity. In addition, it was determined that only minimal modifications were allowed for pTyr and Met, while the other positions were quite tolerant of modification.

Cigarette smoking accelerates carotid artery intimal hyperplasia in a dose-dependent manner

BACKGROUND AND PURPOSE

Intimal hyperplasia is the single most important cause of early restenosis after carotid endarterectomy. Cigarette smoking is an independent risk factor associated with peripheral vascular disease and cerebrovascular accidents. We undertook a dose-response experiment to determine the effect of cigarette smoke on development of intimal hyperplasia in a rat carotid artery intimal injury model.

METHODS

Seventy-two rats were divided into six equal groups and underwent standardized balloon injury to the carotid artery. Each group received 0 (controls), 1, 2, 3, 6, or 8 cigarettes per day for 4 weeks. Resultant intimal hyperplasia was expressed as a percentage of original lumen replaced by intimal hyperplasia.

RESULTS

Percent intimal hyperplasia development (+/- SD) was as follows: controls (0 cigarettes per day), 17.7 +/- 13.2; 1 cigarette per day, 22.8 +/- 15.0; 2 cigarettes per day, 20.0 +/- 14.7; 3 cigarettes per day, 19.2 +/- 12.1; 6 cigarettes per day, 43.5 +/- 15.5; and 8 cigarettes per day, 36.7 +/- 9.8. Six and 8 cigarettes per day significantly increased the development of intimal hyperplasia after intimal injury (P < .01).

CONCLUSIONS

High-dose cigarette smoke accelerates development of intimal hyperplasia and may pose a significant risk factor in developing carotid restenosis.

Neurotrophin and neurotrophin receptors in vascular smooth muscle cells. Regulation of expression in response to injury

The neurotrophins, a family of related polypeptide growth factors including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-3 and NT-4/5 promote the survival and differentiation of distinctive sets of embryonic neurons. Here we define a new functional role for neurotrophins, as autocrine or local paracrine mediators of vascular smooth muscle cell migration. We have identified neurotrophins, and their cognate receptors, the trk tyrosine kinases, in human and rat vascular smooth muscle cells in vivo. In vitro, cultured human smooth muscle cells express BDNF; NT-3; and trk A, B, and C. Similarly, rat smooth muscle cells expressed all three trk receptors as well as all four neurotrophins. Moreover, NGF induces cultured human smooth muscle cell migration at subnanomolar concentrations. In the rat aortic balloon deendothelialization model of vascular injury, the expression of NGF, BNDF, and their receptors trk A and trk B increased dramatically in the area of injury within 3 days and persisted during the formation of the neointima. In human coronary atherosclerotic lesions, BDNF, NT-3, and NT-4/5, and the trk B and trk C receptors could be demonstrated in smooth muscle cells. These findings suggest that neurotrophins play an important role in regulating the response of vascular smooth muscle cells to injury.

Regulation of insulin-like growth factor (IGF) binding protein-2 synthesis and degradation by platelet-derived growth factor and the IGFs is enhanced by serum deprivation in vascular smooth muscle cells

Growth factors such as platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) stimulated proliferation and migration of vascular smooth muscle cells (SMC). IGF-I bioactivity is modulated by high-affinity binding proteins (IGFBP) which are important regulators of these processes. Porcine vascular SMC synthesize IGFBP-2 and IGFBP-4 in vitro. In the present study, levels of IGFBP-2 in conditioned media (CM) were increased approximately 1.6 to 2.2-fold when cells were exposed to PDGF (20 ng/ml) or insulin (5 micrograms/ml) for 24 hr following a 24 hr incubation in serum-free media, or following a 72 hr exposure to either growth factor. Similar increases in IGFBP-2 mRNA levels were observed. Exposure of cells to PDGF for 24 hr without prior serum deprivation resulted in smaller (47 +/- 11%) increases in IGFBP-2 protein levels but failed to alter mRNA levels. IGF-I, FGF, TGF-beta and EGF failed to increase IGFBP-2 using either experimental paradigm. In contrast, IGFBP-2 protein levels were consistently decreased (75 +/- 14%) after 72 hr of exposure to IGF-II without corresponding decreases in IGFBP-2 mRNA levels. Immunoprecipitation of [35S]methionine-labeled IGFBP-2 indicated that this decrease was not due to a decrease in synthesis of IGFBP-2. Immunoblot analysis of CM from cells treated with IGF-II indicated that the decrease in intact protein corresponded with an increase in two non-IGF binding IGFBP-2 fragments of 22 and 14 kD. Increased abundance of these fragments was also observed following IGF-I exposure, although corresponding decreases in intact IGFBP-2 were not usually observed. The relative abundance of these fragments did not appear to be affected by treatment with PDGF or insulin. In contrast to IGFBP-2, regulation of the levels of IGFBP-4 in CM did not appear to be altered by serum deprivation. Insulin consistently increased IGFBP-4 mRNA and protein levels under all situations. PDGF tended to increase IGFBP-4 protein levels, although this effect was less consistent and not as great as the increase observed with insulin. Treatment with IGF-I or -II consistently decreased IGFBP-4 levels in CM but tended to increase their mRNA levels under all situations. These data indicate that insulin, PDGF, and the IGFs regulate both IGFBP-2 and IGFBP-4. While PDGF and insulin stimulate IGFBP-2 and 4 synthesis, the IGFs appear to activate protease(s) which regulate IGFBP-2 and -4 levels post-translationally.(ABSTRACT TRUNCATED AT 400 WORDS)

Sphingosine-1-phosphate inhibits PDGF-induced chemotaxis of human arterial smooth muscle cells: spatial and temporal modulation of PDGF chemotactic signal transduction

Activation of the PDGF receptor on human arterial smooth muscle cells (SMC) induces migration and proliferation via separable signal transduction pathways. Sphingosine-1-phosphate (Sph-1-P) can be formed following PDGF receptor activation and therefore may be implicated in PDGF-receptor signal transduction. Here we show that Sph-1-P does not significantly affect PDGF-induced DNA synthesis, proliferation, or activation of mitogenic signal transduction pathways, such as the mitogen-activated protein (MAP) kinase cascade and PI 3-kinase, in human arterial SMC. On the other hand, Sph-1-P strongly mimics PDGF receptor-induced chemotactic signal transduction favoring actin filament disassembly. Although Sph-1-P mimics PDGF, exogenously added Sph-1-P induces more prolonged and quantitatively greater PIP2 hydrolysis compared to PDGF-BB, a markedly stronger calcium mobilization and a subsequent increase in cyclic AMP levels and activation of cAMP-dependent protein kinase. This excessive and prolonged signaling favors actin filament disassembly by Sph-1-P, and results in inhibition of actin nucleation, actin filament assembly and formation of focal adhesion sites. Sph-1-P-induced interference with the dynamics of PDGF-stimulated actin filament disassembly and assembly results in a marked inhibition of cell spreading, of extension of the leading lamellae toward PDGF, and of chemotaxis toward PDGF. The results suggest that spatial and temporal changes in phosphatidylinositol turnover, calcium mobilization and actin filament disassembly may be critical to PDGF-induced chemotaxis and suggest a possible role for endogenous Sph-1-P in the regulation of PDGF receptor chemotactic signal transduction.

Failure of heparin to inhibit intimal hyperplasia in injured baboon arteries. The role of heparin-sensitive and -insensitive pathways in the stimulation of smooth muscle cell migration and proliferation

BACKGROUND

Heparin is a potent inhibitor of smooth muscle cell (SMC) growth and intimal hyperplasia in animal models but has been ineffective in inhibiting restenosis in humans. This difference may relate to flaws in clinical study design or, alternatively, to interspecies differences in SMC response to heparin. To determine whether heparin could inhibit intimal hyperplasia in a species more closely related to humans, we studied the effect of a low-molecular-weight heparin (LMWH) on baboon SMC proliferation and migration in culture and in arteries subjected to experimental angioplasty.

METHODS AND RESULTS

LMWH or saline was infused continuously after experimental angioplasty of baboon peripheral arteries (six animals per group). After 28 days, bromodeoxyuridine (BrdU) was given to label proliferating cells, and balloon-injured arteries were perfusion-fixed in situ and removed for analysis. All arteries had reendothelialized (Evans blue dye exclusion). LMWH increased partial thromboplastin time (LMWH, 81.7 +/- 8.4 seconds versus saline, 34.7 +/- 0.8 seconds [mean +/- SEM]; P = .004) but failed to inhibit intimal thickening or SMC proliferation (intimal area: LMWH, 0.19 +/- 0.03 mm2 versus saline, 0.21 +/- 0.03 mm2; BrdU labeling: LMWH, 2.9 +/- 0.6% versus saline, 2.4 +/- 0.4%; P = NS). In culture, LMWH and standard heparin (100 micrograms/mL) significantly inhibited serum-induced but not platelet-derived growth factor (PDGF-BB)-induced SMC proliferation (% control, serum: LMWH, 60.5 +/- 4.0%, P = .0002; standard heparin, 29.4 +/- 8.2%, P = .0001; % control, PDGF-BB: LMWH, 117.7 +/- 11.3%, P = NS; standard heparin, 90.9 +/- 14.4%, P = NS) and SMC migration (% control, serum: LMWH, 15.3 +/- 1.9%, P = .0198; standard heparin, 26.4 +/- 13.8%, P = .0032; % control, PDGF-BB: LMWH, 98.5 +/- 14.3%, P = NS; standard heparin, 100.0 +/- 13.5%, P = NS).

CONCLUSIONS

LMWH failed to inhibit intimal hyperplasia in a baboon angioplasty model. Furthermore, LMWH blocked serum-induced but not PDGF-BB-induced SMC proliferation and migration in culture. Thus, heparin-sensitive and -insensitive pathways exist for SMC activation. The relative importance of each pathway induced by injury may vary between species and thus account for different responses to heparin.

Inhibition of cellular ras prevents smooth muscle cell proliferation after vascular injury in vivo

Proliferation of smooth muscle cells of the arterial wall in response to local injury is an important aetiologic factor of vascular proliferative disorders such as atherosclerosis and restenosis after angioplasty. Ras proteins are key transducers of mitogenic signals from membrane to nucleus in many cell types. We investigated the role of ras proteins in the vascular response to arterial injury by inactivating cellular ras of rats in which the common carotid artery was subjected to balloon injury. DNA vectors expressing ras transdominant negative mutants, which interfere with ras function, reduced neointimal formation after injury. Our results indicate a key role for ras in smooth muscle cell proliferation and show that the local delivery of transdominant negative mutants of ras in vivo might prevent some of the acute vascular injury caused by balloon injury.

A subpopulation of smooth muscle cells in injured rat arteries expresses platelet-derived growth factor-B chain mRNA

Proliferation of smooth muscle cells (SMCs) and formation of a neointima are characteristics of the response of rat carotid arteries to balloon injury. Rat platelet-derived growth factor (PDGF)-B was cloned, thus allowing us to use species-specific probes to carry out in situ hybridization on the surface of injured arteries. A distinct population of luminal SMCs (7% to 10%) in the developing neointima expressed PDGF-B mRNA, but very few luminal SMCs still expressed PDGF-B (0.5%) when the lesion had stopped growing. Primary SMC cultures revealed expression of PDGF-B mRNA in 1.6% of SMCs derived from normal tunica media and in 11% of SMCs derived from the neointima. These data demonstrate that SMCs in the injured vessel wall are heterogeneous with regard to PDGF-B expression and that subculturing of these cells may give rise to cultures that are either positive or negative for PDGF-B expression. Furthermore, with abundant expression of the PDGF receptor beta-subunit expressed by intimal SMCs, our findings provide evidence that PDGF-B synthesized by these cells may be involved in intimal lesion formation via a paracrine or autocrine mechanism.

Platelet-derived growth factor ligand and receptor expression by large vessel endothelium in vivo

Using an injury model of the rat carotid artery and aorta, we have determined the time course of expression for PDGF ligands and receptors in endothelium by in situ hybridization and immunostaining. Platelet-derived growth factor (PDGF)-A and -B chains-were expressed in endothelial cells at the wound edge, but no expression was detectable in uninjured endothelium. PDGF-alpha receptor was expressed in a similar pattern as PDGF-A chain whereas expression of PDGF-beta receptor was not detected at any time. Expression of PDGF-B chain did not correlate with endothelial cell replication and a neutralizing antibody against PDGF-B had no effect on endothelial regrowth in the denuded aorta. Intimal smooth muscle cells are known to express PDGF-beta receptors and could thus be stimulated to migrate in response to PDGF-B from endothelial cells.