Characterisation of angiotensin II receptor mediated responses and inhibition of intimal hyperplasia in experimental vein grafts by the specific angiotensin II receptor inhibitor, L158,809

Role of the renin-angiotensin system in the pathogenesis of intimal hyperplasia: therapeutic potential for prevention of vein graft failure?

The saphenous vein remains the most widely used conduit for peripheral and coronary revascularization despite a high rate of vein graft failure. The most common cause of vein graft failure is intimal hyperplasia. No agents have been proven to be successful for the prevention of intimal hyperplasia in human subjects. The renin-angiotensin system is essential in the regulation of vascular tone and blood pressure in physiologic conditions. However, this system mediates cardiovascular remodeling in pathophysiologic states. Angiotensin II is becoming increasingly recognized as a potential mediator of intimal hyperplasia. Drugs modulating the renin-angiotensin system include angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. These drugs are powerful inhibitors of atherosclerosis and cardiovascular remodeling, and they are first-line agents for management of several medical conditions based on class I evidence that they delay progression of cardiovascular disease and improve survival. Several experimental models have demonstrated that these agents are capable of inhibiting intimal hyperplasia. However, there are no data supporting their role in prevention of intimal hyperplasia in patients with vein grafts. This review summarizes the physiology of the renin-angiotensin system, the role of angiotensin II in the pathogenesis of cardiovascular remodeling, the medical indications for these agents, and the experimental data supporting an important role of the renin-angiotensin system in the pathogenesis of intimal hyperplasia.

Mineralocorticoid receptor expression in human venous smooth muscle cells: a potential role for aldosterone signaling in vein graft arterialization

Experimental studies have suggested a role for the local renin-angiotensin-aldosterone system in the response to vascular injury. Clinical data support that aldosterone, via activation of the mineralocorticoid receptor (MR), is an important mediator of vascular damage in humans with cardiovascular disease. In mineralocorticoid-sensitive target tissue, aldosterone specificity for MR is conferred enzymatically by the cortisol-inactivating enzyme 11β-hydroxysteroid-dehydrogenase-2 (11βHSD2). However, the role of MR/aldosterone signaling in the venous system has not been explored. We hypothesized that MR expression and signaling in venous smooth muscle cells contributes to the arterialization of venous conduits and the injury response in vein bypass grafts. MR immunostaining was observed in all samples of excised human peripheral vein graft lesions and in explanted experimental rabbit carotid interposition vein grafts, with minimal staining in control greater saphenous vein. We also found upregulated transcriptional expression of both MR and 11βHSD2 in human vein graft and rabbit vein graft, whereas control greater saphenous vein expressed minimal MR and no detectable 11βHSD2. The expression of MR and 11βHSD2 was confirmed in cultured human saphenous venous smooth muscle cells (hSVSMCs). Using an adenovirus containing a MR response element-driven reporter gene, we demonstrate that MR in hSVSMCs is capable of mediating aldosterone-induced gene activation. The functional significance for MR signaling in hSVSMCs is supported by the aldosterone-induced increase of angiotensin II type-1 receptor gene expression that was inhibited by the MR antagonist spironolactone. The upregulation of MR and 11βHSD2 suggests that aldosterone-mediated tissue injury plays a role in vein graft arterialization.

New therapeutic possibilities for vein graft disease in the post-edifoligide era

Vein graft neointimal hyperplasia involves proliferation and migration of vascular smooth muscle cells into the vessel intima, and ultimately engenders accelerated atherosclerosis and vein graft failure. Since a myriad of stimuli provoke smooth muscle cell proliferation, molecular therapies for vein graft disease have targeted mechanisms fundamental to all cell proliferation - the 'cell-cycle' machinery. Preclinically, the most successful of these therapies has been edifoligide (E2F decoy), a double-stranded oligodeoxynucleotide that binds to the transcription factor known as E2F. Recently, PRoject of Ex vivo vein GRaft Engineering via Transfection (PREVENT) III and IV demonstrated that edifoligide failed to benefit human vein grafts employed to treat lower-extremity ischemia and coronary heart disease, respectively. The clinical failure of edifoligide calls into question previous models of vein graft disease and lends credence to recent animal studies demonstrating that vein graft arterialization substantially involves the immigration into the vein graft of a variety of vascular progenitor cells. Future vein graft disease therapies will likely target not only proliferation of graft-intrinsic cells, but also immigration of graft-extrinsic cells.

Gene polymorphism association studies in dialysis: vascular access

Complications of the vascular access site for hemodialysis are a major cause of morbidity, suboptimal dialysis, and hospitalization. Vascular access for dialysis that is achieved by central venous catheters is associated with complications such as infection and thrombosis. Arteriovenous fistulas and grafts are also at risk for infectious complications. Further, proliferation of the venous wall with secondary thrombosis is a common pathophysiological process that leads to vascular access dysfunction. Genetic polymorphisms that contribute to vascular access failure are found among factors of the coagulation cascade, and host mediators that induce endothelial dysfunction as well as vessel wall proliferation. The two most common mutations of coagulation factors seem to influence the risk of central venous catheter and fistula thrombosis. Indeed, both the single nucleotide polymorphism of the factor V gene at amino acid position 506 (factor V Leiden mutation) and the prothrombin 20210 polymorphism have been associated with thrombotic complications of the vascular access. Among the endothelium-directed factors, a polymorphism of the methylene tetrahydrofolate reductase gene coding for an enzyme that degrades the endothelium toxic product homocysteine, has been associated with fistula failure. While the angiotensin converting enzyme polymorphism does not seem to be associated with vascular access complications, polymorphisms of the profibrogenic cytokine transforming growth factor-beta1 are associated with the prognosis of native arteriovenous fistulae. The role of pro- and anti-inflammatory cytokine gene polymorphisms as prognostic factors for vascular access is yet to be clearly defined.

Role of Gαq in smooth muscle cell proliferation

BACKGROUND

G protein-linked receptors are involved in the processes that lead to intimal hyperplasia. This study examined the role of Galphaq signaling pathways in vascular smooth muscle cell (SMC) proliferation in vitro.

METHODS

Rat pulmonary artery SMCs were cultured in vitro. Standard assays of cellular DNA synthesis, proliferation, phospholipase C-beta (PLCbeta) activation, and extracellular signal-regulated kinase (ERK1/2) phosphorylation were used to study the response to angiotensin II (a specific Galphaq agonist; 0.1-100 micromol/L) in the presence and absence of GP-2A (a competitive Galphaq inhibitor; 10 micromol/L) and the PLCbeta inhibitor U73122 (10micromol/L).

RESULTS

Angiotensin II induced SMC DNA synthesis and cell proliferation. DNA synthesis was inhibited by both Galphaq inhibitor, GP-2A, and PLCbeta inhibitor U73122, in a dose-dependent manner (66% +/- 7% of angiotensin II alone at 10 micromol/L for GP-2A [P <.05] and 63% +/- 6% for U73122). GP-2A completely inhibited angiotensin II-induced Galphaq-mediated PLCbeta phosphorylation. Activation of ERK1/2 by angiotensin II was significantly reduced by GP-2A (P <.05) and by PLCbeta inhibition (P <.05).

CONCLUSION

Inhibition of Galphaq decreases PLCbeta and ERK1/2 phosphorylation, leading to decreased SMC proliferation in vitro. Understanding specific signal transduction pathways will be an integral component of anti-restenosis therapy.Clinical Relevance The universal response of a blood vessel to injury is chronic wound healing, which includes the development of intimal hyperplasia and subsequent remodeling of the vessel wall. This can lead to luminal narrowing in as many as 30% of patients undergoing angioplasty. Neointimal formation is the principal cause of in-stent recurrent stenosis. Intimal hyperplasia is in part produced by smooth muscle cell (SMC) proliferation. Understanding the keys to the proliferation of SMCs will enable therapies to be developed that may inhibit the initial development of intimal hyperplasia. Whereas in the past many studies focused on the multiple mechanical, humoral, and cellular elements that induce SMC proliferation, molecular therapeutics focuses on key choke points within the cell that can be used to inhibit proliferation. One of these key choke points is signal transduction. Galphaq is one of the ubiquitous signal transduction proteins on the membrane of SMCs. Inhibiting G proteins, such as Galphaq, would enable interference with a significant amount of the mechanical, humeral, and cellular elements that produce SMC proliferation, and thus decrease the development of intimal hyperplasia. The present study identifies and begins to map out the role of Galphaq in SMC proliferation and investigates the possible use of a small peptide in its inhibition. Other data suggest that inhibition of other G proteins will also decrease intimal hyperplasia. This is therefore a fertile area for the development of therapeutics to inhibit intimal hyperplasia. The direct relevance to the clinician is that this study identifies a transduction pathway that may be inhibited, and points in the direction of a possible molecular therapeutic target that would be beneficial as an adjunct to angioplasty or as part of a drug-eluding stent regimen.

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

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

Local inhibition of tyrosine kinase activity markedly attenuates the development of intimal hyperplasia in experimental vein grafts

BACKGROUND

Intimal hyperplasia is due to the migration and proliferation of vascular smooth muscle cells after bypass surgery. Tyrosine kinases are involved in many signal transduction pathways including cell proliferation. This study examines the effects of local treatment with the tyrosine kinase inhibitor, tyrphostin AG-51, on the formation of intimal hyperplasia in vein grafts.

MATERIALS AND METHODS

Thirty-nine New Zealand White rabbits underwent interposition bypass grafting of the carotid artery using the jugular vein. In the first group (TKI), tyrphostin AG-51 (5 mg), dissolved in 600 microliter of dimethyl sulfoxide and Ringer's lactate (2:1, v:v), was used to incubate the veins ex vivo prior to grafting and delivered locally in 2.5 ml of 30% pluronic gel after grafting. The second group (DMSO) received the same treatment but without tyrphostin. In the third group (control), tyrphostin and DMSO were omitted from the incubation and gel delivery solutions. Postoperatively, vein grafts were harvested on Day 3 for Western analysis using an antiphosphotyrosine antibody (PY-20) to assess for tyrosine kinase activity, and on Day 28 for either morphologic or contractile function studies.

RESULTS

Local application of the TKI to vein grafts resulted in a 49% reduction in intimal hyperplasia compared to DMSO-treated vein grafts (31 +/- 4 micrometer vs. 61 +/- 5 micrometer, P < 0.01). Treatment with DMSO alone reduced intimal hyperplasia by 28% compared to control (85 +/- 4 micrometer, P < 0.05). The contractile responses in the DMSO and TKI-treated vein grafts were equivalent. Western analysis showed a 39-fold decrease in tyrosine phosphorylation with TKI treatment compared to control.

CONCLUSION

This study demonstrates that local short-term treatment with TKI produces a 49% reduction in intimal hyperplasia and suggests that phosphorylation of tyrosine residues is involved in the signaling pathways leading to the development of intimal hyperplasia in vein grafts.

Local effects of nitric oxide supplementation and suppression in the development of intimal hyperplasia in experimental vein grafts

OBJECTIVES

The universal response of vein grafts after insertion into the arterial circulation is the development of intimal hyperplasia; smooth muscle cell proliferation and connective tissue deposition, which may be modulated in part by dysfunctional endothelial nitric oxide (NO) metabolism. This study examines the effects of single dose, local application by pluronic gel of a NO donor, S-nitroso-N-acetylpenicillamine (SNAP) and an NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) on the formation of intimal hyperplasia.

MATERIALS

Forty New Zealand white rabbits underwent jugular vein interposition grafting of the common carotid artery.

DESIGN

Ten animals were controls, 10 animals had the outer surface of the vein graft coated with 30% pluronic gel (2.5 ml), and 10 each were immersed for 15 min prior to insertion in Ringer lactate containing 10(-3) M of SNAP or L-NAME and then had their vein grafts coated with 2.5 ml of gel containing either SNAP (10(-3) M) or L-NAME (10(-3) M), which allows for sustained delivery for up to 6 h. On the 28th post operative day, the animals were sacrificed and vein grafts were harvested for morphology by electron microscopy (SEM and TEM) and dimensional analysis by videomorphometry.

RESULTS

All vein grafts developed intimal hyperplasia. On SEM the vein grafts had a confluent layer of endothelial cells with multiple layers of smooth muscle cells representing intimal hyperplasia in TEM. There were no demonstrable morphological differences between the four groups. Local treatment with SNAP produced a significant 36% decrease in mean intimal thickness (72 +/- 4 microns vs. 45 +/- 4 microns; mean +/- S.E.M.; p < 0.01) without a change in medial thickness compared to gel-only treated groups (58 +/- 6 microns vs. 61 +/- 7 microns; p = ns). Inhibition of NO synthase by L-NAME had no effect on the development of intimal hyperplasia (72 +/- 4 microns vs. 79 +/- 10 microns; p = ns); medial thickness was also unchanged.

CONCLUSION

These data confirm the protective effect of NO in vascular injury and suggest that NO synthase activity is either absent or reduced to such a level that further inhibition in this short time course is not relevant to the pathophysiology of vein graft intimal hyperplasia.

Oral monoterpene therapy (perillyl alcohol) reduces vein graft intimal hyperplasia

The development of intimal hyperplasia is recognized as a major impediment to graft patency. D-Limonenes are monoterpenes with a recognized cytostatic effect on cell proliferation by inhibiting posttranslational isoprenylation of p21ras and other small G-proteins. This study examines the effect of perillyl alcohol, an oral hydroxylated D-limonene, on the development of intimal hyperplasia and its associated smooth muscle cell physiological responses in an experimental model of vein bypass grafting. Twenty New Zealand white rabbits had a right carotid interposition bypass graft using the ipsilateral external jugular vein. Ten animals received chronic oral therapy with a perillyl alcohol (200 mg/kg/day; begun 5 days before surgery and continued until harvest) and 10 control animals received vehicle only. All animals were sacrificed on the 28th postoperative day. Vein grafts were harvested either for morphology/videomorphometry (n = 6 per group) or for in vitro isometric tension studies (n = 4; four 5-mm rings per graft). The cell proliferation and incorporation of [3H]thymidine into the cellular DNA of serum-stimulated rabbit aortic smooth muscle cells was also assessed in the presence of increasing concentrations of perillyl alcohol (10(-9)-10(-4) M). Perillyl alcohol treated vein grafts showed a 22% reduction in overall mean intimal thickness (54 +/- 4 microns vs 69 +/- 3 microns; P = 0.006) but a 25% increase in overall mean medial thickness (86 +/- 4 microns vs 61 +/- 3 microns). The intimal ratio of the perillyl alcohol treated vein grafts decreased by 27% compared to controls. Perillyl alcohol induced norepinephrine and serotonin hypersensitivity in vein grafts compared to controls. The IC50 for perillyl alcohol was 176 nM with maximal inhibition at 5 microM. Incubation of smooth muscle cell cultures with increasing concentrations of perillyl alcohol showed a dose-dependent decrease in in vitro cellular proliferation, maximal at 1 microM. Therapy with perillyl alcohol alters the early development of intimal hyperplasia reducing the intimal response but increasing the medial response without significant changes in the physiological responses of the smooth muscle cells. Modulating G-proteins will affect the intimal hyperplastic response in vein grafts.