Venous endothelial changes in therapeutic arteriovenous fistulae

Identification of the Crucial Gene in Overflow Arteriovenous Fistula by Bioinformatics Analysis

Objective: The aim was to study the preliminary screening of the crucial genes in intimal hyperplasia in the venous segment of arteriovenous (AV) fistula and the underlying potential molecular mechanisms of intimal hyperplasia with bioinformatics analysis. Methods: The gene expression profile data (GSE39488) was analyzed to identify differentially expressed genes (DEGs). We performed Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of DEGs. Gene set enrichment analysis (GSEA) was used to understand the potential activated signaling pathway. The protein-protein interaction (PPI) network was constructed with the STRING database and Cytoscape software. The Venn diagram between 10 hub genes and gene sets of 4 crucial signaling pathways was used to obtain core genes and relevant potential pathways. Furthermore, GSEAs were performed to understand their biological functions. Results: A total of 185 DEGs were screened in this study. The main biological function of the 111 upregulated genes in AV fistula primarily concentrated on cell proliferation and vascular remodeling, and the 74 downregulated genes in AV fistula were enriched in the biological function mainly relevant to inflammation. GSEA found four signaling pathways crucial for intimal hyperplasia, namely, MAPK, NOD-like, Cell Cycle, and TGF-beta signaling pathway. A total of 10 hub genes were identified, namely, EGR1, EGR2, EGR3, NR4A1, NR4A2, DUSP1, CXCR4, ATF3, CCL4, and CYR61. Particularly, DUSP1 and NR4A1 were identified as core genes that potentially participate in the MAPK signaling pathway. In AV fistula, the biological processes and pathways were primarily involved with MAPK signaling pathway and MAPK-mediated pathway with the high expression of DUSP1 and were highly relevant to cell proliferation and inflammation with the low expression of DUSP1. Besides, the biological processes and pathways in AV fistula with the high expression of NR4A1 similarly included the MAPK signaling pathway and the pathway mediated by MAPK signaling, and it was mainly involved with inflammation in AV fistula with the low expression of NR4A1. Conclusion: We screened four potential signaling pathways relevant to intimal hyperplasia and identified 10 hub genes, including two core genes (i.e., DUSP1 and NR4A1). Two core genes potentially participate in the MAPK signaling pathway and might serve as the therapeutic targets of intimal hyperplasia to prevent stenosis after AV fistula creation.

Phlebosclerosis in lower extremities veins - a systematic review

Phlebosclerosis is a venous wall degenerative disease which has gained little popularity in the literature due to its uncertain clinical significance. The objective of this review is to evaluate the epidemiology, etiology and clinical significance of phlebosclerosis in lower extremities veins, particularly the effect of preexisting phlebosclerosis of the great saphenous vein on vein graft patency. Medline was searched from inception until November 1, 2019. Reference lists of included studies were scanned. Only articles published after 1949 were included. Two reviewers independently screened titles/abstracts and full-text papers for any study design in relation to phlebosclerosis in lower extremities veins and abstracted data. A total of 16 Cohort studies and one case-control study (3708 participants, mean age 61.8 years, 59.3 % men, and 40.7 % women) were included after screening 317 titles and abstracts, and 80 full-text articles. The incidence of phlebosclerosis ranged from 1.5-9.7 % depending on the radiological features. On the contrary, the incidence of the phlebosclerotic great saphenous vein prior to its use as a vein graft was 26.9-91 % on histological examination. The small saphenous vein was the most common location of phlebosclerosis followed by the great saphenous vein. There is a link between phlebosclerosis and age, venous insufficiency and haemodialysis. As for the vein graft patency seven studies demonstrated a correlation between preexisting phlebosclerosis and vein graft stenosis, whereas three studies failed to prove any association. In conclusion, the radiological incidence of phlebosclerosis depended on the ultrasound findings. Its presence in the great saphenous vein prior to its use as a vein graft is established on histological examination. The small saphenous vein is mainly affected. Risk factors included age, haemodialysis, and venous insufficiency. Preexisting wall thickness of the great saphenous vein graft seemed to affect negatively its patency in bypass surgery.

Role of Hypoxia and Metabolism in the Development of Neointimal Hyperplasia in Arteriovenous Fistulas

For patients with end-stage renal disease requiring hemodialysis, their vascular access is both their lifeline and their Achilles heel. Despite being recommended as primary vascular access, the arteriovenous fistula (AVF) shows sub-optimal results, with about 50% of patients needing a revision during the year following creation. After the AVF is created, the venous wall must adapt to new environment. While hemodynamic changes are responsible for the adaptation of the extracellular matrix and activation of the endothelium, surgical dissection and mobilization of the vein disrupt the vasa vasorum, causing wall ischemia and oxidative stress. As a consequence, migration and proliferation of vascular cells participate in venous wall thickening by a mechanism of neointimal hyperplasia (NH). When aggressive, NH causes stenosis and AVF dysfunction. In this review we show how hypoxia, metabolism, and flow parameters are intricate mechanisms responsible for the development of NH and stenosis during AVF maturation.

Increased Oxidative Stress and Hypoxia Inducible Factor-1 Expression during Arteriovenous Fistula Maturation

BACKGROUND

The poor clinical results that are frequently reported for arteriovenous fistulae (AVF) for hemodialysis are typically due to failure of AVF maturation. We hypothesized that early AVF maturation is associated with generation of reactive oxygen species and activation of the hypoxia-inducible factor-1 (HIF-1) pathway, potentially promoting neointimal hyperplasia. We tested this hypothesis using a previously reported mouse AVF model that recapitulates human AVF maturation.

METHODS

Aortocaval fistulae were created in C57Bl/6 mice and compared with sham-operated mice. AVFs or inferior vena cavas were analyzed using a microarray, Amplex Red for extracellular H2O2, quantitative polymerase chain reaction, immunohistochemistry, and immunoblotting for HIF-1α and immunofluorescence for NOX-2, nitrotyrosine, heme oxygenase-1 (HO-1), and vascular endothelial growth factor (VEGF)-A.

RESULTS

Oxidative stress was higher in AVF than that in control veins, with more H2O2 (P = 0.007) and enhanced nitrotyrosine immunostaining (P = 0.005). Immunohistochemistry and immunoblot showed increased HIF-1α immunoreactivity in the AVF endothelium; HIF-1 targets NOX-2, HO-1 and VEGF-A were overexpressed in the AVF (P < 0.01). AVF expressed increased numbers of HIF-1α (P < 0.0001) and HO-1 (P < 0.0001) messenger RNA transcripts.

CONCLUSIONS

Oxidative stress increases in mouse AVF during early maturation, with increased expression of HIF-1α and its target genes NOX-2, HO-1, and VEGF-A. These results suggest that clinical strategies to improve AVF maturation could target the HIF-1 pathway.

Future research directions to improve fistula maturation and reduce access failure

With the increasing prevalence of end-stage renal disease, there is a growing need for hemodialysis. Arteriovenous fistulae (AVF) are the preferred type of vascular access for hemodialysis, but maturation and failure continue to present significant barriers to successful fistula use. AVF maturation integrates outward remodeling with vessel wall thickening in response to drastic hemodynamic changes in the setting of uremia, systemic inflammation, oxidative stress, and pre-existent vascular pathology. AVF can fail due to both failure to mature adequately to support hemodialysis and development of neointimal hyperplasia that narrows the AVF lumen, typically near the fistula anastomosis. Failure due to neointimal hyperplasia involves vascular cell activation and migration and extracellular matrix remodeling with complex interactions of growth factors, adhesion molecules, inflammatory mediators, and chemokines, all of which result in maladaptive remodeling. Different strategies have been proposed to prevent and treat AVF failure based on current understanding of the modes and pathology of access failure; these approaches range from appropriate patient selection and use of alternative surgical strategies for fistula creation, to the use of novel interventional techniques or drugs to treat failing fistulae. Effective treatments to prevent or treat AVF failure require a multidisciplinary approach involving nephrologists, vascular surgeons, and interventional radiologists, careful patient selection, and the use of tailored systemic or localized interventions to improve patient-specific outcomes. This review provides contemporary information on the underlying mechanisms of AVF maturation and failure and discusses the broad spectrum of options that can be tailored for specific therapy.

Assessment of micro-mechanical variations in experimental arteriovenous fistulae using atomic force microscopy

PURPOSE

This study presents a method to quantify micro-stiffness variations in experimental arteriovenous fistulae (AVF).

METHODS

AVF created by anastomosing the superficial epigastric vein to the femoral artery in Sprague-Dawley rats were allowed to remodel for 21 days before being harvested and preserved in culture medium. A custom atomic force microscope was used to measure microvascular stiffness (Young's modulus) in three areas of the AVF: the inflow artery, the juxta-anastomotic area, and the outflow vein. Morphometric measurements and collagen and elastin contents were also determined.

RESULTS

Atomic force microscopy indentation revealed an increased stiffness in the juxta-anastomotic area of the AVF compared to the outflow vein and inflow artery. The juxta-anastomotic area was also significantly stiffer than the contralateral vein. The lack of elasticity (higher Young's modulus) of the juxta-anastomotic region was associated with a thicker vascular wall that was rich in collagen but poor in elastin.

CONCLUSIONS

This study demonstrates for the first time the feasibility of using atomic force microscopy to measure local stiffness variations in experimental AVF. This technique could be instrumental in advancing our understanding of how micro-spatial organization of the AVF wall determines the overall biomechanical performance of this type of vascular access.

Hemodynamic Shear Stress and Endothelial Dysfunction in Hemodialysis Access

Surgically-created blood conduits used for chronic hemodialysis, including native arteriovenous fistulas (AVFs) and synthetic AV grafts (AVGs), are the lifeline for kidney failure patients. Unfortunately, each has its own limitations: AVFs often fail to mature to become useful for dialysis and AVGs often fail due to stenosis as a result of neointimal hyperplasia, which preferentially forms at the graft-venous anastomosis. No clinical therapies are currently available to significantly promote AVF maturation or prevent neointimal hyperplasia in AVGs. Central to devising strategies to solve these problems is a complete mechanistic understanding of the pathophysiological processes. The pathology of arteriovenous access problems is likely multi-factorial. This review focuses on the roles of fluid-wall shear stress (WSS) and endothelial cells (ECs). In arteriovenous access, shunting of arterial blood flow directly into the vein drastically alters the hemodynamics in the vein. These hemodynamic changes are likely major contributors to non-maturation of an AVF vein and/or formation of neointimal hyperplasia at the venous anastomosis of an AVG. ECs separate blood from other vascular wall cells and also influence the phenotype of these other cells. In arteriovenous access, the responses of ECs to aberrant WSS may subsequently lead to AVF non-maturation and/or AVG stenosis. This review provides an overview of the methods for characterizing blood flow and calculating WSS in arteriovenous access and discusses EC responses to arteriovenous hemodynamics. This review also discusses the role of WSS in the pathology of arteriovenous access, as well as confounding factors that modulate the impact of WSS.

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.

Phlebosclerosis in lower extremities veins

Objectives: To determine the prevalence, distribution and extent of phlebosclerosis in an outpatient population referred to vascular clinic.

Design: Prospective cohort study.

Patients and Methods: Seven hundred and forty consecutive patients, 302 (41%) men with a cohort median age of 49 (range, 18–84), were referred to the vascular clinic for detection of lower extremity acute or chronic venous disease. A total of 1000 limbs were assessed using duplex ultrasonography. Deep veins were not assessed. Characteristics of venous wall and lumen were studied. Phlebosclerosis was diagnosed based on the increased echogenicity and thickness or calcification of the venous wall. Patients with known trauma or any surgery in the lower extremities were excluded. Two control groups were used for comparison, one of consecutive patients undergoing vein mapping ( n = 100) and the other of younger volunteers free of any vein disease ( n = 25).

Results: Of the 1000 limbs (right: 458, left: 542) studied by duplex ultrasound, 21 limbs had intense brightness of the venous wall. Fifteen of them had also calcifications and 61 wall thickening. The prevalence of phlebosclerosis was significantly higher in the patient group when compared with control groups 1 (P = 0.019) and 2 (P = 0.011). The mean age of patients with phlebosclerosis was higher compared with the whole patient group (57 versus 49 years, P < 0.0001). Phlebosclerosis affected all superficial veins with greater prevalence in the small saphenous vein (SSV). The mean length of the phlebosclerotic lesion was 2 cm (range 1–7 cm).

Conclusion: The prevalence of phlebosclerosis in the lower extremities appears to be low with no significant sex differences. Age and chronic venous disease are important factors for its development. It may be present in the absence of thrombosis or reflux. A greater prevalence of phlebosclerotic lesions was found in the SSV.

Inhibitory effect of brachytherapy on intimal hyperplasia in arteriovenous fistula1

PURPOSE

To determine whether endovascular radiation can inhibit intimal hyperplasia in a swine model of hemodialysis access.

MATERIALS AND METHODS

Polytetrafluoroethylene arteriovenous grafts (6 mm in diameter) were placed between the common carotid artery and the external jugular vein bilaterally in 8 pigs. Two days after the surgery, fistulography was performed. Gamma radiation (12 Gy) was delivered endovascularly to one of the grafts at the venous anastomosis by using an iridium(192) source. Thus, the other graft in each pig served as an untreated control. Fistulas were evaluated with fistulography or venography 6 week after radiation. All grafts were then harvested for histological and immunohistochemical examination.

RESULTS

Seven grafts on the treated side and 5 grafts on the control side remained patent for at least 6 weeks. Angiography demonstrated that the percentage stenosis at the venous anastomosis was significantly lower for the treated group (15.9 +/- 14.1 versus 32.6 +/- 16.7%, P = 0.045). Histopathologic analyses revealed that the mean intimal area and maximal intimal thickness were significantly lower with reduced smooth muscle cell proliferation at the venous anastomosis on the treated side compared to the control side (0.68 +/- 0.30 versus 1.06 +/- 0.29 mm(2), P = 0.017, and 0.18 +/- 0.08 versus 0.26 +/- 0.07 mm, P = 0.004, respectively). The residual lumen was significantly greater for the treated group (1.59 +/- 0.42 versus 1.06 +/- 0.37 mm(2), P = 0.031). No significant differences were found in the area, nor maximal thickness in the vein either proximal or distal to the anastomosis between the two groups.

CONCLUSIONS

In an animal model of hemodialysis access, brachytherapy with iridium(192) delivered 2 days after graft implantation reduces intimal hyperplasia and stenosis at the venous anastomosis. The reduced smooth-muscle cells found in the radiated veins suggests that brachytherapy may exert its effect on neointimal formation by inhibition of smooth muscle cell proliferation.

Lipid metabolism and molecular changes in normal and atherosclerotic vessels

OBJECTIVES

a positive correlation between cholesterol esterification, acyl-CoA:cholesterol acyltransferase (ACAT), multidrug resistance (MDR1) gene expression and atherosclerotic lesions has been shown in human arteries. The objective of this study was to map the expression of MDR1, ACAT genes and the cholesteryl ester content in normal, atherosclerotic and varicose human vessels.

MATERIALS

vascular segments were obtained from seven cadaveric donors, 27 patients undergoing vascular surgery for severe atherosclerotic disease and 11 patients with saphenous vein varicosities.

METHODS

lipid analysis and RT-PCR of MDR1 and ACAT mRNAs were performed.

RESULTS

an increase in cholesteryl ester content and in ACAT and MDR1 expression was demonstrated in relation to the age in the arteries prone to atherosclerosis; this expression was maximal in arteries from symptomatic patients. In resistant arteries and in veins cholesteryl ester accumulation was rare and light, while ACAT and MDR1 expression was not related to the age of the subjects.

CONCLUSIONS

the results showed that an increase in MDR1 and ACAT expression may be responsible for the accumulation of cholesteryl esters as well as for cell growth rate acceleration in vessel sites prone to atherosclerosis.

Venous morphology predicts class of chronic venous insufficiency

OBJECTIVES

this study aimed to determine specific morphological differences in long saphenous veins from patients with various grades of chronic venous insufficiency.

DESIGN

comparable veins from a control group were compared with patients with either primary varicose veins or those with associated skin changes including venous ulcers.

MATERIALS

below-knee segments of saphenous vein were examined from a total of 64 patients.

METHODS

veins were examined for elastic-tissue disruption and endothelial-cell changes and comparisons made between clinical groups.

RESULTS

elastic-tissue disruption, as measured by fragmentation of the elastic lamina and the percentage of the intimal-medial boundary containing elastin, increased with increasing severity of venous disease. Moreover, endothelial cells became more densely packed, as measured by endothelial cell and endothelial-cell nuclei density, with increasing severity of disease. Other measures such as the density of multinucleated "giant" endothelial cells and the number of nuclei per "giant" cell did not correlate with venous disease, however.

CONCLUSIONS

this study demonstrates that several morphological characteristics of superficial saphenous veins correlate with severity of venous disease. In particular, the alterations to the structure of elastic tissue within these veins appears indicative of the progressive nature of chronic venous insufficiency.