Intimal Hyperplasia in Mouse Vein Grafts Is Regulated by Flow

Distinct Temporal Pattern of the Prediction of Lumen Remodeling of Lower Extremity Vein Bypass Grafts by Initial Local Hemodynamics

We predicted human lower extremity vein bypass graft remodeling by hemodynamics. Computed tomography and duplex ultrasound scans of 55 patients were performed at 1 week and 1, 6, and 12 months post-implantation to obtain wall shear stress (WSS) and oscillatory shear index (OSI) at 1-mm intervals via computational fluid dynamics simulations. Graft remodeling was quantified by computed tomography-measured lumen diameter changes in the early (1 week-1 month), intermediate (1-6 months), and late (6-12 months) periods. Linear mixed-effect models were constructed to examine the overall relationship between remodeling and initial hemodynamics using the average data of all cross sections within the same graft. A significant association of graft remodeling with WSS (p < 0.001) and time (p = 0.001) was found; however, the effect size decreased with time (every 2.7 dyne/cm² increase of WSS was associated with a 0.39, 0.35, 0.002 mm diameter increase in the three periods, respectively). The association of remodeling with OSI was significant only in the intermediate period (every 0.1 increase of OSI was associated with a 0.25 mm lumen diameter decrease, p = 0.004). Therefore, the association of graft lumen remodeling with local hemodynamics has a distinct temporal pattern; WSS and OSI are predictive of remodeling only in certain postoperative periods.

Ephrin type-B receptor 4 activation reduces neointimal hyperplasia in human saphenous vein in vitro

BACKGROUND

Vein bypass is an essential therapy for patients with advanced peripheral and coronary artery disease despite development of neointimal hyperplasia. We have shown that stimulation of the receptor tyrosine kinase ephrin type-B receptor 4 (Eph-B4) with its ligand ephrin-B2 prevents neointimal hyperplasia in murine vein grafts. This study determines whether Eph-B4 in adult human veins is capable of phosphorylation and activation of downstream signaling pathways, as well as functional to release nitric oxide (NO) and prevent neointimal hyperplasia in vitro.

METHODS

Discarded human saphenous veins were taken from the operating room and placed in organ culture without or with ephrin-B2/Fc (2 μg/mL) for 14 days, and the neointima/media ratio was measured in matched veins. Primary human umbilical vein endothelial cells were treated with ephrin-B2/Fc (2 μg/mL) and examined with quantitative polymerase chain reaction, Western blot, immunoassays, and for release of NO. Ephrin-B2/Fc (2 μg/mL) was placed on the adventitia of saphenous veins treated with arterial shear stress for 24 hours in a bioreactor and activated Eph-B4 examined with immunofluorescence.

RESULTS

The baseline intima/media ratio in saphenous vein rings was 0.456 ± 0.097, which increased to 0.726 ± 0.142 in untreated veins after 14 days in organ culture but only to 0.630 ± 0.132 in veins treated with ephrin-B2/Fc (n = 19, P = .017). Ephrin-B2/Fc stimulated Akt, endothelial NO synthase and caveolin-1 phosphorylation, and NO release (P = .007) from human umbilical vein endothelial cells (n = 6). Ephrin-B2/Fc delivered to the adventitia stimulated endothelial Eph-B4 phosphorylation after 24 hours of arterial stress in a bioreactor (n = 3).

CONCLUSIONS

Eph-B4 is present and functional in adult human saphenous veins, with intact downstream signaling pathways capable of NO release and prevention of neointimal hyperplasia in vitro. Adventitial delivery of ephrin-B2/Fc activates endothelial Eph-B4 in saphenous veins treated with arterial shear stress in vitro. These results suggest that stimulation of Eph-B4 function may be a candidate strategy for translation to human clinical trials designed to inhibit venous neointimal hyperplasia.

Mouse vein graft hemodynamic manipulations to enhance experimental utility

Mouse models serve as a tool to study vein graft failure. However, in wild-type mice, there is limited intimal hyperplasia, hampering efforts to identify anti-intimal hyperplasia therapies. Furthermore, vein graft wall remodeling has not been well quantified in mice. We hypothesized that simple hemodynamic manipulations can reproducibly augment intimal hyperplasia and remodeling end points in mouse vein grafts, thereby enhancing their experimental utility. Mouse inferior vena cava-to-carotid interposition isografts were completed using an anastomotic cuff technique. Three flow restriction manipulations were executed by ligating outflow carotid branches, creating an outflow common carotid stenosis, and constructing a midgraft stenosis. Flowmetry and ultrasonography were used perioperatively and at day 28. All ligation strategies decreased the graft flow rate and wall shear stress. Morphometry showed that intimal thickness increased by 26% via carotid branch ligation and by 80% via common carotid stenosis. Despite similar mean flow rates and shear stresses among the three manipulations, the flow waveform amplitudes were lowest with common carotid stenosis. The disordered flow of the midgraft stenosis yielded poststenotic dilatation. The creation of an outflow common carotid stenosis generates clinically relevant (poor runoff) vein graft low wall shear stress and offers a technically flexible method for enhancing the intimal hyperplasia response. Midgraft stenosis exhibits poststenotic positive wall remodeling. These reproducible approaches offer novel strategies for increasing the utility of mouse vein graft models.

Rationale and practical techniques for mouse models of early vein graft adaptations

Mouse models serve as relatively new yet powerful research tools to study intimal hyperplasia and wall remodeling of vein bypass graft failure. Several model variations have been reported in the past decade. However, the approach demands thoughtful preparation, selected sophisticated equipment, microsurgical technical expertise, advanced tissue processing, and data acquisition. This review compares several described models and aims (building on our personal experiences) to practically aid the investigators who want to utilize mouse models of vein graft failure.

Angiotensin-converting enzyme and vascular remodeling

Vascular remodeling is the result of a close interplay of changes in vascular tone and structure. In this review, the role of angiotension-converting enzyme (ACE) and the impact of ACE inhibition on vascular remodeling processes during vascular injury and restenosis, hypertension, atherosclerosis, and aneurysm formation are discussed. The role of ACE and angiotensin II (Ang II) in neointimal thickening has been firmly established by animal studies and is mediated by Ang II type 1 (AT(1)) receptor signaling events via monocyte chemoattractant protein-1 and NAD(P)H oxidase. ACE and Ang II are involved in the remodeling of large and resistance arteries during hypertension; here, cell proliferation and matrix remodeling are also regulated by signaling events downstream of the AT(1) receptor. In atherosclerosis, Ang II is involved in the inflammatory and tissue response, mediated by various signaling pathways downstream of the AT(1) receptor. Although ACE inhibition has been shown to inhibit atherosclerotic processes in experimental animal models, results of large clinical trials with ACE inhibitors were not conclusive. Remodeling of vessel dimensions and structure during aneurysm formation is counteracted by ACE inhibition. Here, a direct effect of ACE inhibitors on matrix metalloproteinase activity has to be considered as part of the working mechanism. The role of ACE2 in vascular remodeling has yet to be established; however, ACE2 has been shown to be associated with vascular changes in hypertension and atherosclerosis.

Negative regulation of vascular smooth muscle cell migration by blood shear stress

Vortex blood flow with reduced blood shear stress in a vein graft has been hypothesized to promote smooth muscle cell (SMC) migration and intimal hyperplasia, pathological events leading to vein graft restenosis. To demonstrate that blood shear stress regulates these processes, we developed a modified vein graft model where the SMC response to reduced vortex blood flow was compared with that of control vein grafts. Vortex blood flow induced SMC migration and neointimal hyperplasia in control vein grafts, whereas reduction of vortex blood flow in the modified vein graft strongly suppressed these effects. A venous polymer implant with known fluid shear stress was employed to clarify the molecular mechanism of shear-dependent SMC migration in vivo. In the polymer implant, the phosphorylation of extracellular signal-regulated kinase (ERK1/2) and myosin light chain kinase (MLCK), found primarily in SMCs, increased from day 3 to day 5 and returned toward the control level from day 5 to day 10, with the peak phosphorylation associated with the maximal speed of SMC migration. Treatment with PD-98059 (an inhibitor specific to the ERK1/2 activator MEK1/2) significantly suppressed the phosphorylation of MLCK, suggesting a role for ERK1/2 in regulating the activity of MLCK. Treatment with PD-98059 or ML-7 (an inhibitor specific to MLCK) reduced shear stress-dependent SMC migration, resulting in an SMC distribution independent of fluid shear stress. These results suggest that fluid shear stress regulates SMC migration via the mediation of ERK1/2 and MLCK.

The evaluation of a small-diameter polysaccharide-based arterial graft in rats

Cardiovascular diseases may require surgery such as arterial bypasses that are usually performed with synthetic PTFE and Dacron grafts with diameter above 6mm. For smaller diameter replacement, healthy vascular tissue from the patient is not always available to carry out this type of graft. The purpose of this study was to evaluate the effectiveness of a small-diameter polysaccharide-based scaffold as an alternative arterial replacement. Tubular polysaccharide-based grafts of 2 mm internal diameter were prepared and moulded by a cross-linking technique. Fifteen Wistar adult rats underwent infrarenal aortic bypass with these grafts using microsurgical techniques. Grafts withstood aortic blood pressure and exhibited physiological blood flow, as evaluated with ultrasound techniques and angiographies at 4 and 8 weeks post-surgery. Harvested grafts were morphologically evaluated by light microscopy and immunohistochemistry. Neointima formation at 8 weeks was evidenced through collagen deposition and smooth muscle-like cells circumferential growth on the luminal surface without intimal hyperplasia or aneurysm formation. In conclusion, we described a 2mm polysaccharide-based arterial material being investigated in vivo and demonstrating patency for up to 8 weeks post-surgery with neointima formation and absence of intimal hyperplasia.

Homocysteine Levels, Haemostatic Risk Factors and Patency Rates After Endovascular Treatment of the Common Iliac Arteries

OBJECTIVES

To investigate the impact of clinical risk factors, plasma homocysteine and haemostatic variables on the results after endovascular treatment of symptomatic atherosclerosis of the common iliac artery.

DESIGN

Prospective observational study.

SETTING

University hospital.

PATIENTS AND METHOD

The study included 139 technically successful interventions in 103 patients. Technical success was defined as < or = 30% residual stenosis as seen on the post treatment angiogram. Blood samples for analyses of fasting plasma values of homocysteine, fibrinogen, D-dimer, activated protein C resistance were drawn upon admission. Median follow-up for all procedures was 22 months (range 0-55 months). Patency was defined as freedom from > or = 50% restenosis or reocclusion.

RESULTS

The technical success rate for all procedures was 93%. The 1-year cumulative primary patency rate based on intention to treat was 85%. Multivariate analysis revealed a significant independent association between patency rates and levels of fibrinogen and homocysteine and the nature of the lesion treated (stenosis vs. occlusion).

CONCLUSION

The aetiology of restenoses and reocclusions is probably multifactorial. Procoagulant activity, the nature of the lesion treated and homocysteine levels within and above the upper range of normal limits are important risk factors for failure after endovascular treatment of the common iliac arteries.