Graft-extrinsic cells predominate in vein graft arterialization

Phylogenetic and Physiological Diversity of Cultivable Actinomycetes Isolated From Alpine Habitats on the Qinghai-Tibetan Plateau

Actinomycetes in extreme alpine habitat have attracted much attention due to their unique physiological activities and functions. However, little is known about their ecological distribution and diversity. Here, we explored the phylogenetic relationship and physiological heterogeneity of cultivable actinomycetes from near-root soils of different plant communities in the Laohu Ditch (2200 - 4200 m) and Gaize County area (5018 - 5130 m) on the Qinghai-Tibetan Plateau. A total of 128 actinomycete isolates were obtained, 16S rDNA-sequenced and examined for antimicrobial activities and organic acid, H₂S, diffusible pigments, various extracellular enzymes production. Seventy three isolates of the total seventy eight isolates from the Laohu Ditch, frequently isolated from 2200 to 4200 m, were closely related to Streptomyces spp. according to the 16S rDNA sequencing, while four isolates within the genus Nocardia spp. were found at 2200, 2800, and 3800 m. In addition, one potential novel isolate with 92% sequence similarity to its nearest match Micromonospora saelicesensis from the GenBank database, was obtained at 2200 m. From the Gaize County area, fifty Streptomyces isolates varied in diversity at different sites from 5018 to 5130 m. The investigation of phenotypic properties of 128 isolates showed that 94.5, 78.9, 68, 64.8, 53, 51.6, 50, 36.7, 31.2, and 22.7% of the total isolates produced catalase, lipase 2, urease, protease, H₂S, lipase 3, amylase, lipase 1, diffusible pigment and organic acid, respectively. The antimicrobial assays of the total isolates revealed that 5, 28, 19, and 2 isolates from Streptomyces spp. exhibited antimicrobial activity against Escherichia coli, Staphylococcus aureus, Candida albicans, and Pseudomonas aeruginosa, respectively. This study intends to bring helpful insights in the exploitation and utilization of alpine actinomycetes for novel bioactive compounds discovery.

Mature Vascular Smooth Muscle Cells, but Not Endothelial Cells, Serve as the Major Cellular Source of Intimal Hyperplasia in Vein Grafts

OBJECTIVE

Neointima formation is a primary cause of intermediate to late vein graft (VG) failure. However, the precise source of neointima cells in VGs remains unclear. Approach and Results: Herein we clarify the relative contributions of mature vascular smooth muscle cells (SMCs) and endothelial cells (ECs) to neointima formation in a mouse model of VG remodeling via the genetic-inducible fate mapping approaches. Regardless of the magnitude of neointima formation, the recipient arterial and the donor venous SMCs contributed ≈55% of the neointima cells at the anastomotic regions, whereas only donor venous SMCs donated ≈68% of the neointima cells at the middle bodies. A small portion of the SMC-derived cells became non-SMC cells, most likely vascular stem cells, and constituted 2% to 11% of the cells in each major layer of VGs. In addition, the recipient arterial ECs were the major cellular source of re-endothelialization but did not contribute to neointima formation. The donor venous ECs donated ≈17% neointima cells in the VGs with mild neointima formation and conditional media from ECs after endothelial-to-mesenchymal transition suppressed vascular SMC dedifferentiation.

CONCLUSIONS

The recipient arterial and donor venous mature SMCs dominate but contribute distinctly to intimal hyperplasia at the anastomosis and the middle body regions of VGs. The recipient arterial ECs are the major cellular source of re-endothelialization but do not donate neointima formation in VGs. Only the donor venous ECs undergo endothelial-to-mesenchymal transition. Endothelial-to-mesenchymal transition is marginal for generating neointima cells but is likely required for controlling the quality of VG remodeling.

Role of smooth muscle cells in coronary artery bypass grafting failure

Atherosclerosis is the underlying pathology of many cardiovascular diseases. The formation and rupture of atherosclerotic plaques in the coronary arteries results in angina and myocardial infarction. Venous coronary artery bypass grafts are designed to reduce the consequences of atherosclerosis in the coronary arteries by diverting blood flow around the atherosclerotic plaques. However, vein grafts suffer a high failure rate due to intimal thickening that occurs as a result of vascular cell injury and activation and can act as 'a soil' for subsequent atherosclerotic plaque formation. A clinically-proven method for the reduction of vein graft intimal thickening and subsequent major adverse clinical events is currently not available. Consequently, a greater understanding of the underlying mechanisms of intimal thickening may be beneficial for the design of future therapies for vein graft failure. Vein grafting induces inflammation and endothelial cell damage and dysfunction, that promotes vascular smooth muscle cell (VSMC) migration, and proliferation. Injury to the wall of the vein as a result of grafting leads to the production of chemoattractants, remodelling of the extracellular matrix and cell-cell contacts; which all contribute to the induction of VSMC migration and proliferation. This review focuses on the role of altered behaviour of VSMCs in the vein graft and some of the factors which critically lead to intimal thickening that pre-disposes the vein graft to further atherosclerosis and re-occurrence of symptoms in the patient.

Perfusion Tissue Culture Initiates Differential Remodeling of Internal Thoracic Arteries, Radial Arteries, and Saphenous Veins

Adaptive remodeling processes are essential to the maintenance and viability of coronary artery bypass grafts where clinical outcomes depend strongly on the tissue source. In this investigation, we utilized an ex vivo perfusion bioreactor to culture porcine analogs of common human bypass grafts: the internal thoracic artery (ITA), the radial artery (RA), and the great saphenous vein (GSV), and then evaluated samples acutely (6 h) and chronically (7 days) under in situ or coronary-like perfusion conditions. Although morphologically similar, primary cells harvested from the ITA illustrated lower intimal and medial, but not adventitial, cell proliferation rates than those from the RA or GSV. Basal gene expression levels were similar in all vessels, with only COL3A1, SERPINE1, FN1, and TGFB1 being differentially expressed prior to culture; however, over half of all genes were affected nominally by the culturing process. When exposed to coronary-like conditions, RAs and GSVs experienced pathological remodeling not present in ITAs or when vessels were studied in situ. Many of the remodeling genes perturbed at 6 h were restored after 7 days (COL3A1, FN1, MMP2, and TIMP1) while others (SERPINE1, TGFB1, and VCAM1) were not. The findings elucidate the potential mechanisms of graft failure and highlight strategies to encourage healthy ex vivo pregraft conditioning.

Trimetazidine Attenuates Cardiac Dysfunction in Endotoxemia and Sepsis by Promoting Neutrophil Migration

Aims: Cardiac dysfunction can be a fatal complication during severe sepsis. The migration of neutrophils is significantly impaired during severe sepsis. We sought to determine the role of trimetazidine (TMZ) in regulation of neutrophil migration to the heart in a mouse model of sepsis and endotoxemia, and to identify the mechanism whereby TMZ confers a survival advantage.

Methods and Results: C57/BL6 mice were (1) injected with LPS followed by 24-h TMZ administration, or (2) treated with TMZ (20 mg/kg/day) for 1 week post cecal ligation and puncture (CLP) operation. Echocardiography and Millar system detection showed that TMZ alleviated cardiac dysfunction and histological staining showed the failure of neutrophils migration to heart in both LPS- and CLP-induced mice. Bone marrow transplantation revealed that TMZ-pretreated bone marrow cells improved LPS- and CLP-induced myocardial dysfunction and enhanced neutrophil recruitment in heart. In CXCL2-mediated chemotaxis assays, TMZ increased neutrophils migration via AMPK/Nrf2-dependent up-regulation of CXCR2 and inhibition of GRK2. Furthermore, using luciferase reporter gene and chromatin immunoprecipitation assays, we found that TMZ promoted the binding of the Nrf2 and CXCR2 promoter regions directly. Application of CXCR2 inhibitor completely reversed the protective effects of TMZ in vivo. Co-culture of neutrophils and cardiomyocytes further validated that TMZ decreased LPS-induced cardiomyocyte pyroptosis by targeting neutrophils.

Conclusion: Our findings suggested TMZ as a potential therapeutic agent for septic or endotoxemia associated cardiac dysfunction in mice.

STUDY HIGHLIGHTS 

What is the current knowledge on the topic?

Migration of neutrophils is significantly impaired during severe sepsis, but the underlying mechanisms remain unknown.

What question did this study address?

The effects of TMZ on cardiac dysfunction via neutrophils migration.

What this study adds to our knowledge

TMZ attenuated LPS-induced cardiomyocyte pyroptosis and cardiac dysfunction by promoting neutrophils recruitment to the heart tissues via CXCR2.

How this might change clinical pharmacology or translational science

Our findings suggested TMZ as a potential therapeutic agent for septic cardiac dysfunction.

Inflammation in Vein Graft Disease

Bypass surgery is one of the most frequently used strategies to revascularize tissues downstream occlusive atherosclerotic lesions. For venous bypass surgery the great saphenous vein is the most commonly used vessel. Unfortunately, graft efficacy is low due to the development of vascular inflammation, intimal hyperplasia and accelerated atherosclerosis. Moreover, failure of grafts leads to significant adverse outcomes and even mortality. The last couple of decades not much has changed in the treatment of vein graft disease (VGD). However, insight is the cellular and molecular mechanisms of VGD has increased. In this review, we discuss the latest insights on VGD and the role of inflammation in this. We discuss vein graft pathophysiology including hemodynamic changes, the role of vessel wall constitutions and vascular remodeling. We show that profound systemic and local inflammatory responses, including inflammation of the perivascular fat, involve both the innate and adaptive immune system.

Linking gene dynamics to vascular hyperplasia - Toward a predictive model of vein graft adaptation

Reductionist approaches, where individual pieces of a process are examined in isolation, have been the mainstay of biomedical research. While these methods are effective in highly compartmentalized systems, they fail to account for the inherent plasticity and non-linearity within the signaling structure. In the current manuscript, we present the computational architecture for tracking an acute perturbation in a biologic system through a multiscale model that links gene dynamics to cell kinetics, with the overall goal of predicting tissue adaptation. Given the complexity of the genome, the problem is made tractable by clustering temporal changes in gene expression into unique patterns. These cluster elements form the core of an integrated network that serves as the driving force for the response of the biologic system. This modeling approach is illustrated using the clinical scenario of vein bypass graft adaptation. Vein segments placed in the arterial circulation for treatment of advanced occlusive disease can develop an aggressive hyperplastic response that narrows the lumen, reduces blood flow, and induces in situ thrombosis. Reducing this hyperplastic response has been a long-standing but unrealized goal of biologic researchers in the field. With repeated failures of single target therapies, the redundant response pathways are thought to be a fundamental issue preventing progress towards a solution. Using the current framework, we demonstrate how theoretical genomic manipulations can be introduced into the system to shift the adaptation to a more beneficial phenotype, where the hyperplastic response is mitigated and the risk of thrombosis reduced. Utilizing our previously published rabbit vein graft genomic data, where grafts were harvested at time points ranging from 2 hours to 28 days and under differential flow conditions, and a customized clustering algorithm, five gene clusters that differentiated the low flow (i.e., pro-hyperplastic) from high flow (i.e., anti-hyperplastic) response were identified. The current analysis advances these general associations to create a model that identifies those genes sets most likely to be of therapeutic benefit. Using this approach, we examine the range of potential opportunities for intervention via gene cluster over-expression or inhibition, delivered in isolation or combination, at the time of vein graft implantation.

Vascular Adaptation: Pattern Formation and Cross Validation between an Agent Based Model and a Dynamical System

Myocardial infarction is the global leading cause of mortality (Go et al., 2014). Coronary artery occlusion is its main etiology and it is commonly treated by Coronary Artery Bypass Graft (CABG) surgery (Wilson et al, 2007). The long-term outcome remains unsatisfactory (Benedetto, 2016) as the graft faces the phenomenon of restenosis during the post-surgery, which consists of re-occlusion of the lumen and usually requires secondary intervention even within one year after the initial surgery (Harskamp, 2013). In this work, we propose an extensive study of the restenosis phenomenon by implementing two mathematical models previously developed by our group: a heuristic Dynamical System (DS) (Garbey and Berceli, 2013), and a stochastic Agent Based Model (ABM) (Garbey et al., 2015). With an extensive use of the ABM, we retrieved the pattern formations of the cellular events that mainly lead the restenosis, especially focusing on mitosis in intima, caused by alteration in shear stress, and mitosis in media, fostered by alteration in wall tension. A deep understanding of the elements at the base of the restenosis is indeed crucial in order to improve the final outcome of vein graft bypass. We also turned the ABM closer to the physiological reality by abating its original assumption of circumferential symmetry. This allowed us to finely replicate the trigger event of the restenosis, i.e. the loss of the endothelium in the early stage of the post-surgical follow up (Roubos et al., 1995) and to simulate the encroachment of the lumen in a fashion aligned with histological evidences (Owens et al., 2015). Finally, we cross-validated the two models by creating an accurate matching procedure. In this way we added the degree of accuracy given by the ABM to a simplified model (DS) that can serve as powerful predictive tool for the clinic.

A single nucleotide polymorphism of cyclin-dependent kinase inhibitor 1B (p27Kip1) associated with human vein graft failure affects growth of human venous adventitial cells but not smooth muscle cells

BACKGROUND

Cyclin-dependent kinase inhibitor 1B (p27^Kip1) is a cell-cycle inhibitor whose -838C>A single nucleotide polymorphism (rs36228499; hereafter called p27 SNP) has been associated with the clinical failure of peripheral vein grafts, but the functional effects of this SNP have not been demonstrated.

METHODS

Human saphenous vein adventitial cells and intimal/medial smooth muscle cells (SMCs) were derived from explants obtained at the time of lower extremity bypass operations. We determined the following in adventitial cells and SMCs as a function of the p27 SNP genotype: (1) p27 promoter activity, (2) p27 messenger (m)RNA and protein levels, and (3) growth and collagen gel contraction. Deoxyribonuclease I footprinting was also performed in adventitial cells and SMCs.

RESULTS

p27 promoter activity, deoxyribonuclease I footprinting, p27 mRNA levels, and p27 protein levels demonstrated that the p27 SNP is functional in adventitial cells and SMCs. Both cell types with the graft failure protective AA genotype had more p27 mRNA and protein. As predicted because of higher levels of p27 protein, adventitial cells with the AA genotype grew slower than those of the CC genotype. Unexpectedly, SMCs did not show this genotype-dependent growth response.

CONCLUSIONS

These results support the functionality of the p27 SNP in venous SMCs and adventitial cells, but an effect of the SNP on cell proliferation is limited to only adventitial cells. These data point to a potential role for adventitial cells in human vein graft failure and also suggest that SMCs express factors that interfere with the activity of p27.

Vein graft failure: from pathophysiology to clinical outcomes

Occlusive arterial disease is a leading cause of morbidity and mortality worldwide. Aside from balloon angioplasty, bypass graft surgery is the most commonly performed revascularization technique for occlusive arterial disease. Coronary artery bypass graft surgery is performed in patients with left main coronary artery disease and three-vessel coronary disease, whereas peripheral artery bypass graft surgery is used to treat patients with late-stage peripheral artery occlusive disease. The great saphenous veins are commonly used conduits for surgical revascularization; however, they are associated with a high failure rate. Therefore, preservation of vein graft patency is essential for long-term surgical success. With the exception of 'no-touch' techniques and lipid-lowering and antiplatelet (aspirin) therapy, no intervention has hitherto unequivocally proven to be clinically effective in preventing vein graft failure. In this Review, we describe both preclinical and clinical studies evaluating the pathophysiology underlying vein graft failure, and the latest therapeutic options to improve patency for both coronary and peripheral grafts.

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.

Trimetazidine prevents macrophage-mediated septic myocardial dysfunction via activation of the histone deacetylase sirtuin 1

BACKGROUND AND PURPOSE

Sepsis is a systemic inflammatory response accompanied by excessive production of inflammatory cytokines and cardiovascular dysfunction. Importantly, macrophage-derived pro-inflammatory agents play a key role in cardiovascular impairment in sepsis. Here we have investigated the effects of trimetazidine (TMZ) on pro-inflammatory responses of macrophages in endotoxin-induced myocardial dysfunction.

EXPERIMENTAL APPROACH

Mice pretreated with TMZ were injected i.p. with LPS and cardiac function evaluated. Levels of macrophage infiltration, macrophage inflammatory response and cardiomyocyte apoptosis were measured using immunohistochemical staining, elisa, real-time RT-PCR, Western blot, TUNEL and flow cytometry assays.

KEY RESULTS

Pretreatment with TMZ prevented LPS-induced myocardial dysfunction and apoptosis. TMZ also lowered levels of pro-inflammatory cytokines in serum and cardiac tissue and myocardial macrophage infiltration. Bone marrow transplantation indicated that TMZ alleviated LPS-induced myocardial dysfunction via decreasing macrophage infiltration. TMZ reduced expression of pro-inflammatory cytokines in LPS-stimulated cardiac and peritoneal macrophages. Co-culture of TMZ-pretreated macrophages with cardiomyocytes and conditioned media from TMZ-pretreated macrophages both decreased LPS-induced cardiomyocyte apoptosis. The anti-apoptosis effects of TMZ resulted from decrease of pro-inflammatory cytokines, partly due to normalizing the sirtuin 1 (Sirt1)/AMP-activated protein kinase (AMPK)/Nrf2/haem oxygenase-1 and Sirt1/PPARα pathways in macrophages. Cytokine secretion was also regulated by ROS, which were attenuated by TMZ via activation of Sirt1, AMPK and PPARα.

CONCLUSIONS AND IMPLICATIONS

TMZ protected against LPS-induced myocardial dysfunction and apoptosis, accompanied by inhibition of macrophage pro-inflammatory responses. Our studies suggest that TMZ might represent a novel therapeutic agent to prevent and treat sepsis-induced myocardial dysfunction.

MK2 inhibitory peptide delivered in nanopolyplexes prevents vascular graft intimal hyperplasia

Autologous vein grafts are commonly used for coronary and peripheral artery bypass but have a high incidence of intimal hyperplasia (IH) and failure. We present a nanopolyplex (NP) approach that efficiently delivers a mitogen-activated protein kinase (MAPK)-activated protein (MAPKAP) kinase 2 inhibitory peptide (MK2i) to graft tissue to improve long-term patency by inhibiting pathways that initiate IH. In vitro testing in human vascular smooth muscle cells revealed that formulation into MK2i-NPs increased cell internalization, endosomal escape, and intracellular half-life of MK2i. This efficient delivery mechanism enabled MK2i-NPs to sustain potent inhibition of inflammatory cytokine production and migration in vascular cells. In intact human saphenous vein, MK2i-NPs blocked inflammatory and migratory signaling, as confirmed by reduced phosphorylation of the posttranscriptional gene regulator heterogeneous nuclear ribonucleoprotein A0, the transcription factor cAMP (adenosine 3',5'-monophosphate) element-binding protein, and the chaperone heat shock protein 27. The molecular effects of MK2i-NPs caused functional inhibition of IH in human saphenous vein cultured ex vivo. In a rabbit vein transplant model, a 30-min intraoperative graft treatment with MK2i-NPs significantly reduced in vivo IH 28 days posttransplant compared with untreated or free MK2i-treated grafts. The decrease in IH in MK2i-NP-treated grafts in the rabbit model also corresponded with decreased cellular proliferation and maintenance of the vascular wall smooth muscle cells in a more contractile phenotype. These data indicate that nanoformulated MK2 inhibitors are a promising strategy for preventing graft failure.

Surgical marking pen dye inhibits saphenous vein cell proliferation and migration in saphenous vein graft tissue

OBJECTIVE

Markers containing dyes such as crystal violet (CAS 548-62-9) are routinely used on the adventitia of vein bypass grafts to avoid twisting during placement. Because little is known about how these dyes affect vein graft healing and function, we determined the effect of crystal violet on cell migration and proliferation, which are responses to injury after grafting.

METHODS

Fresh human saphenous veins were obtained as residual specimens from leg bypass surgeries. Portions of the vein that had been surgically marked with crystal violet were analyzed separately from those that had no dye marking. In the laboratory, they were split into easily dissected inner and outer layers after removal of endothelium. This cleavage plane was within the circular muscle layer of the media. Cell migration from explants was measured daily as either (1) percentage of migration-positive explants, which exclusively measures migration, or (2) number of cells on the plastic surrounding each explant, which measures migration plus proliferation. Cell proliferation and apoptosis (Ki67 and TUNEL staining, respectively) were determined in dye-marked and unmarked areas of cultured vein rings. The dose-dependent effects of crystal violet were measured for cell migration from explants as well as for proliferation, migration, and death of cultured outer layer cells. Dye was extracted from explants with ethanol and quantified by spectrophotometry.

RESULTS

There was significantly less cell migration from visibly blue compared with unstained outer layer explants by both methods. There was no significant difference in migration from inner layer explants adjacent to blue-stained or unstained sections of vein because dye did not penetrate to the inner layer. Ki67 staining of vein in organ culture, which is a measure of proliferation, progressively increased up to 6 days in nonblue outer layer and was abolished in the blue outer layer. Evidence of apoptosis (TUNEL staining) was present throughout the wall and not different in blue-stained and unstained vein wall segments. Blue outer layer explants had 65.9 ± 8.0 ng dye/explant compared with 2.1 ± 1.3 for nonblue outer layer explants. Dye applied in vitro to either outer or inner layer explants dose dependently inhibited migration (IC50∼10 ng/explant). The IC50s of crystal violet for outer layer cell proliferation and migration were 0.1 and 1.2 μg/mL, whereas the EC50 for death was between 1 and 10 μg/mL.

CONCLUSIONS

Crystal violet inhibits venous cell migration and proliferation, indicating that alternative methods should be considered for marking vein grafts.

TGF-β signaling mediates endothelial-to-mesenchymal transition (EndMT) during vein graft remodeling

Veins grafted into an arterial environment undergo a complex vascular remodeling process. Pathologic vascular remodeling often results in stenosed or occluded conduit grafts. Understanding this complex process is important for improving the outcome of patients with coronary and peripheral artery disease undergoing surgical revascularization. Using in vivo murine cell lineage-tracing models, we show that endothelial-derived cells contribute to neointimal formation through endothelial-to-mesenchymal transition (EndMT), which is dependent on early activation of the Smad2/3-Slug signaling pathway. Antagonism of transforming growth factor-β (TGF-β) signaling by TGF-β neutralizing antibody, short hairpin RNA-mediated Smad3 or Smad2 knockdown, Smad3 haploinsufficiency, or endothelial cell-specific Smad2 deletion resulted in decreased EndMT and less neointimal formation compared to controls. Histological examination of postmortem human vein graft tissue corroborated the changes observed in our mouse vein graft model, suggesting that EndMT is operative during human vein graft remodeling. These data establish that EndMT is an important mechanism underlying neointimal formation in interpositional vein grafts, and identifies the TGF-β-Smad2/3-Slug signaling pathway as a potential therapeutic target to prevent clinical vein graft stenosis.

Cross Talk Between Vascular Smooth Muscle Cells and Monocytes Through Interleukin-1β/Interleukin-18 Signaling Promotes Vein Graft Thickening

Objective—

Interleukin (IL)-1β and IL-18 are key proinflammatory cytokines that play important roles in the pathophysiology of vein graft remodeling. However, the mechanism of IL-1β/IL-18 production and its role in the development of graft remodeling remain unclear.

Approach and Results—

IL-1β/IL-18 were rapidly expressed in venous interposition grafts. Vascular smooth muscle cell (VSMC) death and monocytic inflammasome activation occurred in grafted veins. Necrotic VSMCs induced the expression of IL-1β, IL-18, and other inflammasome-associated proteins in monocytes, which was partially inhibited by their antagonist, recombinant IL-1ra-Fc-IL-18bp. Activated monocytes stimulated proliferation of VSMCs by activating cell growth–related signaling molecules (AKT, STAT3, ERK1/2, and mTOR [AKT/protein kinase B, signal transducer and activator of transcription 3, extracellular signal-regulated kinase 1/2, mammalian target of rapamycin]) and increasing production of platelet-derived growth factor-bb; these effects were suppressed by IL-1ra-Fc-IL-18bp. Activated monocytes also promoted migration of VSMCs, which was independent of IL-1β/IL-18 signaling. Importantly, administration of IL-1ra-Fc-IL-18bp inhibited activation of cell growth–related signaling molecules, VSMC proliferation, and vein graft thickening in vivo.

Conclusions—

Our work identified an interaction among necrotic VSMCs, monocytes, and viable VSMCs through IL-1β/IL-18 signaling, which might be exploited as a therapeutic target in vein graft remodeling.

Smooth muscle cells from the anastomosed artery are the major precursors for neointima formation in both artery and vein grafts

Accumulation of smooth muscle cells (SMC) results in neointima formation in injured vessels. Two graft models consisting of vein and artery grafts were created by anastomosing common carotid arteries to donor vessels. To identify the origin of the neointima cells from anastomosed arteries, we use Wnt1-Cre/reporter mice to label and track SMCs in the common carotid artery. The contribution of SMCs in the neighboring arteries to neointima formation was studied. On evaluating the artery grafts after 1 month, >90 % of the labeled neointima cells were found to have originated from the anastomosing host arteries. Most of the neointima cells were also smooth muscle α-actin positive (SMA-α(+)) and expressed the smooth muscle myosin heavy chain (SMMHC), the SMC terminal differentiation marker. In vein grafts, about 60 % SMA-α-positive cells were from anastomosing arteries. Bone marrow cells did not contribute to neointima SMCs in vein grafts, but did co-stain with markers of inflammatory cells. Wnt1 expression was not detected in the neointima cells in the vein or artery grafts, or the injured femoral arteries. Neointima SMCs showed the synthetic phenotype and were positively labeled with BrdU in vitro and in vivo. Treatment with the IGF-1 receptor inhibitor suppressed SMC proliferation and neointima formation in vein grafts. Our results indicate that SMCs from the neighboring artery are predominantly present in the neointima formed in both vein and artery grafts and that Wnt1-Cre mice can be used to explore the role of SMCs originating from neighboring vessels in vascular remodeling.

Vein graft adaptation and fistula maturation in the arterial environment

Veins are exposed to the arterial environment during two common surgical procedures, creation of vein grafts and arteriovenous fistulae (AVF). In both cases, veins adapt to the arterial environment that is characterized by different hemodynamic conditions and increased oxygen tension compared with the venous environment. Successful venous adaptation to the arterial environment is critical for long-term success of the vein graft or AVF and, in both cases, is generally characterized by venous dilation and wall thickening. However, AVF are exposed to a high flow, high shear stress, low-pressure arterial environment and adapt mainly via outward dilation with less intimal thickening. Vein grafts are exposed to a moderate flow, moderate shear stress, high-pressure arterial environment and adapt mainly via increased wall thickening with less outward dilation. We review the data that describe these differences, as well as the underlying molecular mechanisms that mediate these processes. Despite extensive research, there are few differences in the molecular pathways that regulate cell proliferation and migration or matrix synthesis, secretion, or degradation currently identified between vein graft adaptation and AVF maturation that account for the different types of venous adaptation to arterial environments.

Regulation of smooth muscle cells in development and vascular disease: current therapeutic strategies

Vascular smooth muscle cells (SMCs) exhibit extensive phenotypic diversity and rapid growth during embryonic development, but maintain a quiescent, differentiated state in adult. The pathogenesis of vascular proliferative diseases involves the proliferation and migration of medial vascular SMCs into the vessel intima, possibly reinstating their embryonic gene expression programs. Multiple mitogenic stimuli induce vascular SMC proliferation through cell cycle progression. Therapeutic strategies targeting cell cycle progression and mitogenic stimuli have been developed and evaluated in animal models of atherosclerosis and vascular injury, and several clinical studies. Recent discoveries on the recruitment of vascular progenitor cells to the sites of vascular injury suggest new therapeutic potentials of progenitor cell-based therapies to accelerate re-endothelialization and prevent engraftment of SMC-lineage progenitor cells. Owing to the complex and multifactorial nature of SMC regulation, combinatorial antiproliferative approaches are likely to be used in the future in order to achieve maximal efficacy and reduce toxicity.

Association of vascular access type with inflammatory marker levels in maintenance hemodialysis patients

Aggressive NIH is a common histopathological lesion found at the sites of venous stenosis in arteriovenous fistula (AVF) and arteriovenous grafts (AVG). Inflammatory mediators have been proposed to play a pathogenic role in NIH, but there is paucity of data evaluating this hypothesis in clinical studies or in animal models. Serum levels of inflammatory mediators can potentially identify patients at high risk of AVF and AVG dysfunction. In a cross-sectional cohort study of 754 HD patients who were part of the NIED study cohort, we examined the associations between inflammatory markers including serum interleukin (IL) 1β, IL-6, C-reactive protein (CRP), and tumor necrosis factor-α (TNF-α) and type of vascular access. Unadjusted and multivariate-adjusted linear regression models were used. In addition, time-dependent regression model was used to assess the association between inflammatory markers and mortality. We observed that in the multivariate-adjusted model, inflammatory mediators interleukin-6 (IL-6), interleukin-1L-ß (IL-1ß), and C-reactive protein (CRP), the predicted value in hemodialysis patients, are lowest in patients with AVF and highest in central venous catheter (CVC) and AVG even in case-mix and malnutrition-inflammation complex syndrome (MICS)-adjusted models. IL-6 and CRP levels fall consistently in the same patients when AVG or CVC is changed to AVF and increase if the same patient changes access from AVF to AVG or CVC. Obesity is a risk factor for fistula failure and fistulas are associated with the lowest mortality compared with CVC and AVG. We did not find any statistically significant association between tumor necrosis factor-α (TNF- α) and vascular access outcomes. Higher levels of inflammatory mediators seen in CVC and AVG compared with AVF could potentially explain the higher mortality seen in patients with CVC and AVG compared with AVF.

Gene therapy for the prevention of vein graft disease

Ischemic cardiovascular disease remains the leading cause of death worldwide. Despite advances in the medical management of atherosclerosis over the past several decades, many patients require arterial revascularization to reduce mortality and alleviate ischemic symptoms. Technological advancements have led to dramatic increases in the use of percutaneous and endovascular approaches, yet surgical revascularization (bypass surgery) with autologous vein grafts remains a mainstay of therapy for both coronary and peripheral artery disease. Although bypass surgery is highly efficacious in the short term, long-term outcomes are limited by relatively high failure rates as a result of intimal hyperplasia, which is a common feature of vein graft disease. The supply of native veins is limited, and many individuals require multiple grafts and repeat procedures. The need to prevent vein graft failure has led to great interest in gene therapy approaches to this problem. Bypass grafting presents an ideal opportunity for gene therapy, as surgically harvested vein grafts can be treated with gene delivery vectors ex vivo, thereby maximizing gene delivery while minimizing the potential for systemic toxicity and targeting the pathogenesis of vein graft disease at its onset. Here we will review the pathogenesis of vein graft disease and discuss vector delivery strategies and potential molecular targets for its prevention. We will summarize the preclinical and clinical literature on gene therapy in vein grafting and discuss additional considerations for future therapies to prevent vein graft disease.

Role of hemodynamic forces in the ex vivo arterialization of human saphenous veins

BACKGROUND

Human saphenous vein grafts are one of the salvage bypass conduits when endovascular procedures are not feasible or fail. Understanding the remodeling process that venous grafts undergo during exposure to arterial conditions is crucial to improve their patency, which is often compromised by intimal hyperplasia. The precise role of hemodynamic forces such as shear stress and arterial pressure in this remodeling is not fully characterized. The aim of this study was to determine the involvement of arterial shear stress and pressure on vein wall remodeling and to unravel the underlying molecular mechanisms.

METHODS

An ex vivo vein support system was modified for chronic (up to 1 week), pulsatile perfusion of human saphenous veins under controlled conditions that permitted the separate control of arterial shear stress and different arterial pressure (7 mm Hg or 70 mm Hg).

RESULTS

Veins perfused for 7 days under high pressure (70 mm Hg) underwent significant development of a neointima compared with veins exposed to low pressure (7 mm Hg). These structural changes were associated with altered expression of several molecular markers. Exposure to an arterial shear stress under low pressure increased the expression of matrix metalloproteinase (MMP)-2 and MMP-9 and tissue inhibitor of metalloproteinase (TIMP)-1 at the transcript, protein, and activity levels. This increase was enhanced by high pressure, which also increased TIMP-2 protein expression despite decreased levels of the cognate transcript. In contrast, the expression of plasminogen activator inhibitor-1 increased with shear stress but was not modified by pressure. Levels of the venous marker Eph-B4 were decreased under arterial shear stress, and levels of the arterial marker Ephrin-B2 were downregulated under high-pressure conditions.

CONCLUSIONS

This model is a valuable tool to identify the role of hemodynamic forces and to decipher the molecular mechanisms leading to failure of human saphenous vein grafts. Under ex vivo conditions, arterial perfusion is sufficient to activate the remodeling of human veins, a change that is associated with the loss of specific vein markers. Elevation of pressure generates intimal hyperplasia, even though veins do not acquire arterial markers.

CLINICAL RELEVANCE

The pathological remodeling of the venous wall, which leads to stenosis and ultimately graft failure, is the main limiting factor of human saphenous vein graft bypass. This remodeling is due to the hemodynamic adaptation of the vein to the arterial environment and cannot be prevented by conventional therapy. To develop a more targeted therapy, a better understanding of the molecular mechanisms involved in intimal hyperplasia is essential, which requires the development of ex vivo models of chronic perfusion of human veins.

Characteristics of the actions by which 5-hydroxytryptamine affects electrical and mechanical activities in rabbit jugular vein graft

BACKGROUND AND PURPOSE

The vasomodulating actions of 5-HT in vein grafts, and the underlying mechanisms, remain to be fully clarified. Here, we characterized the actions by which 5-HT affects electrical and mechanical activities in rabbit autologous jugular vein grafts.

EXPERIMENTAL APPROACH

Smooth muscle cell (SMC) membrane potential and isometric tension were measured in vein grafts 4 weeks after implantation into carotid arteries. Changes in the expression of 5-HT receptor subtypes and in myosin heavy chain isoforms (SM1, SM2 and SMemb) were examined by immunohistochemistry and Western blot analysis.

KEY RESULTS

The walls of grafted veins displayed massive increases in the number of SM1- and SM2-positive SMCs. 5-HT induced a large depolarization and contraction that were each reduced by both 5-HT(2A) - and 5-HT(1B/1D) -receptor antagonists. The 5-HT-induced contraction was not modified by a 5-HT₇ -receptor antagonist. The 5-HT₇ -receptor-selective agonist AS 19 did not induce relaxation during the contraction to prostaglandin F(2α) . Immunohistochemical and Western blot analyses revealed that immunoreactive responses against 5-HT(2A) and 5-HT(1B/1D) receptors were increased in the vein graft.

CONCLUSIONS AND IMPLICATIONS

5-HT is able to induce a large contraction in rabbit autologous jugular vein grafts through (i) an increased number of differentiated contractile SMCs; (ii) an increased number of SMCs expressing contractile 5-HT(2A) - and 5-HT(1B/1D) receptors; and (iii) a down-regulation of the function of the relaxant SMC 5-HT₇ receptors. These changes in the vein graft may help it to resist the higher pressure present on the arterial side of the circulation.

Effect of cellular senescence on the albumin permeability of blood-derived endothelial cells

In this study, we tested the hypotheses that endothelial cells (ECs) derived from human umbilical cord blood (hCB-ECs) exhibit low permeability, which increases as hCB-ECs age and undergo senescence, and that the change in the permeability of hCB-ECs is due to changes in tight junction protein localization and the activity of exchange protein activated by cAMP (Epac)1. Albumin permeability across low-passage hCB-EC monolayers on Transwell membranes was 10 times lower than for human aortic ECs (HAECs) (P < 0.01) but similar to in vivo values in arteries. Expression of the tight junction protein occludin and tyrosine phosphorylation of occludin were less in hCB-ECs than in HAECs (P < 0.05). More hCB-ECs than HAECs underwent mitosis (P < 0.01). hCB-ECs that underwent >44 population doublings since isolation had a significantly higher permeability than hCB-ECs that underwent <31 population doublings (P < 0.05). This age-related increase in hCB-EC permeability was associated with an increase in tyrosine phosphorylation of occludin (P < 0.01); permeability and occludin phosphorylation were reduced by treatment with 2 μM resveratrol. Tyrosine phosphorylation of occludin and cell age influence the permeability of hCB-ECs, whereas levels of EC proliferation and expression of tight junction proteins did not explain the differences between hCB-EC and HAEC permeability. The elevated permeability in late passage hCB-ECs was reduced by 25-40% by elevation of membrane-associated cAMP and activation of the Epac1 pathway. Given the similarity to in vivo permeability to albumin and the high proliferation potential, hCB-ECs may be a suitable in vitro model to study transport-related pathologies and cell aging.

Influence of ischemia before vein grafting on early hyperplasia of the graft and the dynamic changes of the intima after grafting

Background

To investigate both the influence of ischemia before grafting on early hyperplasia of the vein grafts, and the dynamic changes of the intima after grafting in a rabbit model of vein graft disease.

Methods

We performed paired vein graft experiments under different ischemic conditions (15 vs. 60 min; 15 vs. 90 min) in the neck of the rabbits and compared the differences between the grafts. Clopidogrel, an anti-platelet agent, was administered before and after surgery. Twenty-eight days after the grafting procedure, the veins were evaluated microscopically. The dynamic changes of the intima after grafting were evaluated by scanning electron microscopy over time.

Results

The vein grafts subjected to 60- or 90-min ischemia exhibited no differences compared to those subjected to 15-min ischemia in terms of the mean thickness of the intimal, medial, and adventitial layers of the graft. Similarly, there was no difference in the Ki-67 labeling index (proliferation marker) between the vein grafts. Vein grafts with 15-min ischemia lost endothelial cells (ECs) but healed by 3 days post graft, whereas vein grafts with 90-min ischemia suffered serious EC loss, which was restored with new ECs during days 2 to 14 post graft.

Conclusions

Ninety-minute ischemia before vein grafting can cause serious EC loss, but does not increase early intimal hyperplasia when clopidogrel is administered. Protecting the vein from ischemia and reperfusion injury preserves ECs.

Vein graft neointimal hyperplasia is exacerbated by CXCR4 signaling in vein graft-extrinsic cells

OBJECTIVE

Because vein graft neointimal hyperplasia engenders vein graft failure, and because most vein graft neointimal cells derive from outside the vein graft, we sought to determine whether vein graft neointimal hyperplasia is affected by activity of the CXC chemokine receptor-4 (CXCR4), which is important for bone marrow-derived cell migration.

METHODS

In congenic Cxcr4(-/+) and wild-type (WT) recipient mice, we performed interposition grafting of the common carotid artery with the inferior vena cava (IVC) of either Cxcr4(-/+) or WT mice to create four surgically chimeric groups of mice (n ≥ 5 each), characterized by vein graft donor/recipient: WT/WT; Cxcr4(-/+)/WT; WT/Cxcr4(-/+); and Cxcr4(-/+)/Cxcr4(-/+); vein grafts were harvested 6 weeks postoperatively.

RESULTS

The agonist for CXCR4 is expressed by cells in the arterializing vein graft. Vein graft neointimal hyperplasia was reduced by reducing CXCR4 activity in vein graft-extrinsic cells, but not in vein graft-intrinsic cells: the rank order of neointimal hyperplasia was WT/WT ≈ Cxcr4(-/+)/WT > WT/Cxcr4(-/+) ≈ Cxcr4(-/+)/Cxcr4(-/+); CXCR4 deficiency in graft-extrinsic cells reduced neointimal hyperplasia by 39% to 47% (P < .05). Vein graft medial area was equivalent in all grafts except Cxcr4(-/+)/Cxcr4(-/+), in which the medial area was 60% ± 20% greater (P < .05). Vein graft re-endothelialization was indistinguishable among all three vein graft groups. However, the prevalence of medial leukocytes was 40% ± 10% lower in Cxcr4(-/+)/Cxcr4(-/+) than in WT/WT vein grafts (P < .05), and the prevalence of smooth muscle actin-positive cells was 45% ± 20% higher (P < .05).

CONCLUSIONS

We conclude that CXCR4 contributes to vein graft neointimal hyperplasia through mechanisms that alter homing to the vein graft of graft-extrinsic cells, particularly leukocytes.

CLINICAL RELEVANCE

The utility of autologous vein grafts is severely reduced by neointimal hyperplasia, which accelerates subsequent graft atherosclerosis. Our study demonstrates that vein graft neointimal hyperplasia is aggravated by activity of the cell-surface “CXC” chemokine receptor-4 (CXCR4), which is critical for recruitment of bone marrow-derived cells to sites of inflammation. Our model for CXCR4 deficiency used mice with heterozygous deficiency of Cxcr4. Consequently, our results suggest the possibility that a CXCR4 antagonist--like plerixafor, currently in clinical use--could be applied to vein grafts periadventitially, and perhaps achieve beneficial effects on vein graft neointimal hyperplasia.

Animal models for studying vein graft failure and therapeutic interventions

Vein grafts have been extensively used to bypass blockages in arteries, but are themselves subject to early closure by thrombosis or later obstruction by vein graft disease (neointimal hyperplasia and remodelling). Animal models are a crucial means of testing potential therapeutic and surgical interventions to prevent graft stenosis and occlusion. This review outlines many of the animal models of vein grafting. Recent studies include targeted gene therapy to prevent acute vein graft thrombosis and the use of folic acid to limit graft failure in diabetic pigs.

Myocardial injury after ischemia-reperfusion in mice deficient in Akt2 is associated with increased cardiac macrophage density

Akt2 protein kinase has been shown to promote cell migration and actin polymerization in several cell types, including macrophages. Because migrating macrophages constitute an important inflammatory response after myocardial ischemia, we determined cardiac macrophage expression after ischemia-reperfusion (I/R) injury and cryo-injury in mice lacking Akt2 (Akt2-KO). At 7 days post-I/R, Akt2-KO cardiac tissues showed an increase in immunohistochemical staining for macrophage markers (Galectin 3 and F4/80) compared with wild-type (WT) mice, indicating macrophage density was increased in the injured Akt2-KO myocardium. This change was time dependent because macrophage density was similar between WT and Akt2-KO myocardium at 3 days post-I/R, but by 7 and 14 days post-I/R, macrophage density was significantly increased in Akt2-KO myocardium. Concomitantly, infarct size was larger and cardiac function was reduced in Akt2-KO mice subjected to I/R. However, when cryo-infarction produced similar infarct sizes in the anterior wall in both WT and Akt2-KO mice, macrophage density remained higher in Akt2-KO mouse myocardium, suggesting Akt2 regulates myocardial macrophage density independent of infarct size. Consistently, bone marrow from Akt2-KO mice enhanced myocardial macrophage density in both C57/B6 WT and Akt2-KO recipient mice. Finally, reciprocal ex-vivo coculturing of macrophages and cardiac myocytes showed that activated Akt2-KO peritoneal macrophages had reduced mobility and adhesion when compared with WT littermate controls. Thus, although Akt-2 KO mice did not affect the initial inflammation response after injury and Akt2 deficiency has been shown to impair cell migration or motility in macrophages, our data suggested a novel mechanism in which increasing retention of Akt2-KO macrophages resulted in increasing cardiac Akt2-KO macrophage density in the myocardial space.

Human umbilical cord blood-derived endothelial cells reendothelialize vein grafts and prevent thrombosis

OBJECTIVE

To accelerate vein graft reendothelialization and reduce vein graft thrombosis by infusing human umbilical cord blood-derived endothelial cells (hCB-ECs) because loss of endothelium contributes to vein graft thrombosis and neointimal hyperplasia.

METHODS AND RESULTS

Under steady flow conditions in vitro, hCB-ECs adhered to smooth muscle cells 2.5 to 13 times more than ECs derived from peripheral blood or human aorta (P<0.05). Compared with peripheral blood and human aorta ECs, hCB-ECs had 1.4-fold more cell surface α(5)β(1) integrin heterodimers per cell (P<0.05) and proliferated on fibronectin 4- to 10-fold more rapidly (P<0.05). Therefore, we used hCB-ECs to enhance reendothelialization of carotid interposition vein grafts implanted in NOD.CB17-Prkdc(scid)/J mice. Two weeks postoperatively, vein grafts from hCB-EC-treated mice demonstrated approximately 55% reendothelialization and no luminal thrombosis. In contrast, vein grafts from sham-treated mice demonstrated luminal thrombosis in 75% of specimens (P<0.05) and only approximately 14% reendothelialization. In vein grafts from hCB-EC-treated mice, 33±10% of the endothelium was of human origin, as judged by human major histocompatibility class I expression.

CONCLUSIONS

The hCB-ECs adhere to smooth muscle cells under flow conditions in vitro, accelerate vein graft reendothelialization in vivo, and prevent vein graft thrombosis. Thus, hCB-ECs offer novel therapeutic possibilities for vein graft disease.

Mechanisms of vein graft adaptation to the arterial circulation: insights into the neointimal algorithm and management strategies

For patients with coronary artery disease or limb ischemia, placement of a vein graft as a conduit for a bypass is an important and generally durable strategy among the options for arterial reconstructive surgery. Vein grafts adapt to the arterial environment, and the limited formation of intimal hyperplasia in the vein graft wall is thought to be an important component of successful vein graft adaptation. However, it is also known that abnormal, or uncontrolled, adaptation may lead to abnormal vessel wall remodeling with excessive neointimal hyperplasia, and ultimately vein graft failure and clinical complications. Therefore, understanding the venous-specific pathophysiological and molecular mechanisms of vein graft adaptation are important for clinical vein graft management. Of particular importance, it is currently unknown whether there exist several specific distinct molecular differences in the venous mechanisms of adaptation that are distinct from arterial post-injury responses; in particular, the participation of the venous determinant Eph-B4 and the vascular protective molecule Nogo-B may be involved in mechanisms of vessel remodeling specific to the vein. This review describes (1) venous biology from embryonic development to the mature quiescent state, (2) sequential pathologies of vein graft neointima formation, and (3) novel candidates for strategies of vein graft management. Scientific inquiry into venous-specific adaptation mechanisms will ultimately provide improvements in vein graft clinical outcomes.

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.

Human umbilical cord blood endothelial progenitor cells decrease vein graft neointimal hyperplasia in SCID mice

AIMS

Vein graft endothelial damage is a key step in the development of neointimal hyperplasia, leading to vein graft failure. We sought to determine whether exogenous endothelial progenitor cells could promote vein graft re-endothelialization, and thereby ameliorate neointimal hyperplasia.

METHODS AND RESULTS

Carotid artery interposition grafting was performed with syngeneic inferior vena cavae in mice with severe combined immunodeficiency (SCID). Lineage-negative human umbilical cord blood (hUCB) cells (or medium alone) were injected into vein-grafted mice intra-operatively and 2 weeks post-operatively. In vein grafts from hUCB cell-injected mice, we found human HLA-expressing endothelial cells, as well as increased levels of VEGF and FGF-2. Furthermore, hUCB cells secreted VEGF and FGF-2 in vitro. The markedly enhanced endothelial regeneration, likely resulting from both direct engraftment and paracrine actions of hUCB cells, inhibited inflammatory response, diminished intimal cell proliferation, and reduced neointimal hyperplasia in the vein grafts.

CONCLUSIONS

hUCB cells may accelerate vein graft re-endothelialization via both direct differentiation into endothelial cells and release of paracrine factors to enhance endothelial regeneration and reduce inflammation. These data highlight a potential therapeutic role for cellular therapy in vessel injury.

Hemodynamic influences on abdominal aortic aneurysm disease: Application of biomechanics to aneurysm pathophysiology

"Atherosclerotic" abdominal aortic aneurysms (AAAs) occur with the greatest frequency in the distal aorta. The unique hemodynamic environment of this area predisposes it to site-specific degenerative changes. In this review, we summarize the differential hemodynamic influences present along the length of the abdominal aorta, and demonstrate how alterations in aortic flow and wall shear stress modify AAA progression in experimental models. Improved understanding of aortic hemodynamic risk profiles provides an opportunity to modify patient activity patterns to minimize the risk of aneurysmal degeneration.

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.

Mechano-sensitive transcriptional factor Egr-1 regulates insulin-like growth factor-1 receptor expression and contributes to neointima formation in vein grafts

OBJECTIVE

Vein grafts in a coronary bypass or a hemodialysis access often develop obliterative growth of the neointima. We previously reported that the mechanical stretch-activated insulin-like growth factor-1 receptor (IGF-1/IGF-1R) pathway plays an important role in this remodeling. However, the transcriptional mechanism(s) regulating IGF-1R expression and neointima formation have not been identified.

METHODS AND RESULTS

Deletion and site-specific mutagenesis analysis of IGF-1R promoter identified that the minimal mechano-responsive promoter element (-270--130) contains 2 consensus sequences for binding of early growth reponse-1 (Egr-1) transcriptional factor. Mechanical stretch stimulated both Egr-1 mRNA (4.6-fold) and protein (5.2-fold) in vascular smooth muscle cells. Interposition of a vein into an artery increased Egr-1 mRNA (7.8+/-2.6-fold vs sham). In vascular smooth muscle cells isolated from Egr-1 knockout mice, mechanical stretch could not increase IGF-1R, and vascular smooth muscle cells proliferation was decreased by 47% compared to wild-type cells. Importantly, the neointima area was reduced by at least 50%, and the lumen-to-media ratio increased by 55% in vein grafts of Egr-1 knockout mice compared with results of wild-type mice.

CONCLUSIONS

Egr-1 is a mechano-sensitive transcriptional factor that stimulates IGF-1R transcription, resulting in vascular remodeling of vein grafts.

Adaptive changes in autogenous vein grafts for arterial reconstruction: clinical implications

For patients with the most severe manifestations of lower extremity arterial occlusive disease, bypass surgery using autogenous vein has been the most durable reconstruction. However, the incidence of bypass graft stenosis and graft failure remains substantial and wholesale improvements in patency are lacking. One potential explanation is that stenosis arises not only from over exuberant intimal hyperplasia, but also due to insufficient adaptation or remodeling of the vein to the arterial environment. Although in vivo human studies are difficult to conduct, recent advances in imaging technology have made possible a more comprehensive structural examination of vein bypass maturation. This review summarizes recent translational efforts to understand the structural and functional properties of human vein grafts and places it within the context of the rich existing literature of vein graft failure.

Progenitor cells and vascular disease

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

Long-term engraftment of bone marrow-derived cells in the intimal hyperplasia lesion of autologous vein grafts

Intimal hyperplasia of autologous vein grafts is a critical problem affecting the long-term patency of many types of vascular reconstruction. Within intimal hyperplasia lesions, smooth muscle cells are a major component, playing an essential role in the pathological process. Given that bone marrow-derived cells may differentiate into smooth muscle cells in the neointima of injured arteries, we hypothesized that the bone marrow may serve as a source for some of the smooth muscle cells within intimal hyperplasia lesions of vein grafts. To test this hypothesis, we used an established mouse model for intimal hyperplasia in wild-type mice that had been transplanted with bone marrow from a green fluorescent protein (GFP+/+) transgenic mouse. High-resolution confocal microscopy analysis performed 2 and 8 weeks after grafting demonstrated expression of GFP in 5.4 +/- 0.8% and 11.9 +/- 2.3%, respectively, of smooth muscle cells within intimal hyperplasia lesions. By 16 weeks, GFP expression in smooth muscle cells was not detected by immunohistochemistry; however, real-time PCR revealed that 20.2 +/- 1.7% of the smooth muscle cells captured from the neointima lesion by laser capture microdissection at 16 weeks contained GFP DNA. Our results suggest that bone marrow-derived cells differentiated into smooth muscle cells within the intimal lesion and may provide a novel clinical approach for decreasing intimal hyperplasia in vein grafts.

The source of neointimal cells in vein grafts: does the origin matter?

The focus of this Commentary is the existing plasticity in the origin of neointimal vascular smooth muscle cells and endothelial cells after vein grafting.

Lower extremity vein graft failure: a translational approach

Abstract

Patients with the most severe manifestations of lower extremity arterial occlusive disease often require peripheral bypass surgery for limb salvage and preservation of function. Although good quality saphenous vein offers the most durable conduit for reconstruction, 5-year failure rates are 30–50% and have remained largely unchanged for the past two decades. The majority of these failures occur within the first year of implantation, which is regarded as the most biologically active time during which the vein graft adapts to the arterial environment. Although intimal hyperplasia is generally regarded as the primary culprit of vein graft failure, geometric remodeling of the healing vein graft has recently emerged as a potentially significant contributing factor. While hemodynamic forces, including an increase in shear stress and wall tension, are undoubtedly central to the magnitude and direction of vein graft remodeling, we have determined that these forces alone cannot account for the extent of variability noted in early remodeling patterns. Therefore, we hypothesize that circulating factors, such as mediators of inflammation, may modulate the vein graft response to mechanical forces. This article reviews the definition and diagnosis of vein graft failure and summarizes our current efforts to understand the mechanisms of normal and abnormal vein graft adaptation to the arterial environment.

Tumor necrosis factor receptor-2 signaling attenuates vein graft neointima formation by promoting endothelial recovery

OBJECTIVE

Inflammation appears intricately linked to vein graft arterialization. We have previously shown that tumor necrosis factor (TNF) receptor-1 (TNFR1, p55) signaling augments vein graft neointimal hyperplasia (NH) and remodeling through its effects on vascular smooth muscle cells (SMCs). In this study we examined the role of TNFR2 (p75) signaling in vein graft arterialization.

METHODS AND RESULTS

Inferior vena cava-to-carotid artery interposition grafting was performed between p75-/- and congenic (C57B1/6J) wild-type (WT) mice. Six weeks postoperatively, neointimal and medial dimensions were greater in p75-/- grafts placed into p75-/- recipients (by 42% or 60%, respectively; P<0.05), when compared with WT veins grafted into WT recipients. Relative to WT vein grafts, p75 deficiency augmented early (2-week-old) graft vascular cell adhesion molecule (VCAM)-1 expression (by 2.4-fold, P<0.05), increased endothelial cell apoptosis (2-fold), and delayed graft re-endothelialization. Both cellular proliferation in early, and collagen I content of mature (6-week-old) vein grafts were increased (by 70% and 50%, respectively) in p75-/- grafts. P75 deficiency augmented TNF-induced apoptosis of cultured endothelial cells, but did not affect TNF-stimulated SMC proliferation or migration induced by co-cultured macrophages.

CONCLUSIONS

TNF signaling via p75 reduces vein graft neointimal hyperplasia through mechanisms involving reduction of adhesion molecule expression and endothelial cell apoptosis.

Mouse strain differential neointimal response in vein grafts and wire-injured arteries

BACKGROUND

Neointimal development is seen clinically after both vein grafting and balloon catheterization, but may not represent the same pathology under these 2 conditions. This study compared the degree of neointimal hyperplasia after vein grafting or arterial-injury grafts in 2 strains of mice: C57Bl/6 and FVB.

METHODS AND RESULTS

Jugular vein branches were interpositioned as grafts in the femoral artery of syngenic-matched mice, with graft harvest at 30 days. Wire-injured carotid arteries were grafted to the carotid arteries of syngenic-matched mice, with graft harvest at 14 days. Histomorphometry revealed no strain differences in vein grafts in the extent of position-dependent neointimal thickening or lumen cross-sectional area. Both strains showed significantly thicker neointima and less lumen area at the proximal graft site (vs the mid-graft; p<0.05). In contrast, a significantly greater neointimal thickness was found in the wire-injured carotid grafts of FVB mice vs those of C57Bl/6 mice (p<0.05).

CONCLUSIONS

Neointimal formation shows a vessel-dependent, strain-dependent difference, with greater arterial neointimal thickening in FVB mice. These data suggest that different mechanisms operate for arterial-injury- vs vein-graft-associated neointimal development and that the difference has a genetic basis.

Vascular progenitor cells and atherosclerosis

Vascular regeneration occurs throughout life as a dynamic process. Millions of new endothelial cells are created with essentially the same number of cells undergoing programmed cell death or necrosis every day. As a result, the human vascular tree could be considered to essentially replace its entire endothelial population over a specified number of years. Within this network there is a compartment of vascular progenitor cells that appear to govern this homeostasis throughout life, continuously repopulating cells that die by apoptosis or necrosis. This delicate equilibrium appears to be disrupted in atherosclerotic disease processes as patients with known ischemic heart disease risk factors have been found to have lower numbers of circulating endothelial progenitor cells, which may tip the balance in favor of lesion formation, rather than repair. The aim of this article is to discuss the types of vascular progenitor cells and the mechanisms behind their mobilization, homing and differentiation into mature endothelial cells capable of vascular repair.

Molecular engineering of vein bypass grafts

Surgical bypass of arterial occlusions using autogenous vein provides an effective treatment for many patients with advanced coronary or peripheral atherosclerosis. However, the long-term benefit of bypass surgery is limited by the development of de novo occlusive lesions within the vein graft, which occurs in a significant percentage of patients over time. The pathophysiology of vein graft failure involves a complex interplay between an acute vascular injury response and the hemodynamic adaptation of the vein to arterial forces. Cell proliferation, inflammation, and matrix metabolism are critical components of postimplantation remodeling. Conventional pharmacotherapy has had limited impact on graft failure. Vein grafts present a unique and attractive opportunity for molecular engineering, which is defined for purposes of this review as the local application of genomic (eg, gene transfer or gene inhibition) or proteomic interventions designed to alter the healing response. The critical enabling technologies for these strategies are described, with a perspective on preclinical and clinical development for this indication. The recently completed clinical trials of edifoligide (E2F decoy oligodeoxynucleotide) provide important lessons for future studies. A better understanding of the remodeling response of vein grafts in humans is required to design effective molecular therapies and to define the appropriate target populations and surrogate markers for future clinical trials.

Cytokines and the early vein graft: strategies to enhance durability

This brief review focuses on experimental studies linking the proinflammatory cytokine tumor necrosis factor-alpha to accelerated vein graft failure in the broader historical context of vein graft research. From some perspectives, the field appears ripe for transfer of cytokine knowledge and therapeutic approaches that have evolved in other systems to vascular surgery problems. However, the complexity of vein graft disease suggests that more robust research approaches, such as broadening of the scope beyond focus on single mediators and neointimal hyperplasia, will be necessary to reach translatable strategies to prolong human vein graft durability.

The role of vascular stem cells in atherogenesis and post-angioplasty restenosis

It is well known that atherosclerosis prevails in elderly populations as ageing acts as a recognized risk factor for this disease. Although the pathogenic factors leading to atherosclerosis are highly heterogeneous, traditionally speaking, the causative risk factors include hyperlipidemia, hypertension, diabetes mellitus and smoking, which can damage to endothelial function, and subsequently promote lipid penetration and inflammatory cell infiltration. Damaged endothelial cells (ECs) may be replaced by neighboring cell division, while damaged smooth muscle cells (SMCs) may be replaced by medial SMCs emigrating into the intima during atherogenesis. However, this standpoint is challenged by recent findings that vascular progenitor/stem cells (VPCs) may contribute to atherogenesis and post-angioplasty restenosis. VPCs are a group of primitive cells that have the potential to produce mature, functional cells in the vascular wall. VPCs residing in bone marrow, vascular wall or circulating in the peripheral blood may be stimulated by a variety of pathogenic factors. These stem cells then participate in regeneration, repair and remodeling of the injured arterial wall. This new concept may bring about a great breakthrough in understanding the pathogenesis of atherosclerosis and develop novel therapeutic strategies for coronary heart disease. This article will mainly review the role of VPCs in atherogenesis, thus providing a novel understanding about the pathophysiology of atherosclerosis.

Pharmacologic inhibition of vein graft neointimal hyperplasia

Although arterial conduits are widely used and have improved the long-term results of coronary artery bypass grafting, vein grafts remain important additional conduits in coronary surgery. Newer studies show a saphenous vein graft patency of 60% or more at 10 years postoperatively. The pathology of vein graft disease consists of thrombosis, neointimal hyperplasia, and vein graft atherosclerosis, which limit graft longevity. Therapeutic strategies to prevent vein graft disease include external stenting, pharmacotherapy, and gene therapy. The potential benefits of a pharmacologic approach are as follows: (1) Drugs with a broad clinical experience can be used; (2) side effects of systemic application can be minimized by local therapy; and (3) no vascular injury, such as pressurizing the vein for a viral transfection approach, is necessary. The different sites for pharmacotherapy in vein graft disease are reviewed in this article.