Different effects of high and low shear stress on platelet-derived growth factor isoform release by endothelial cells: consequences for smooth muscle cell migration

Disturbed flow regulates protein disulfide isomerase A1 expression via microRNA-204

Redox processes can modulate vascular pathophysiology. The endoplasmic reticulum redox chaperone protein disulfide isomerase A1 (PDIA1) is overexpressed during vascular proliferative diseases, regulating thrombus formation, endoplasmic reticulum stress adaptation, and structural remodeling. However, both protective and deleterious vascular effects have been reported for PDIA1, depending on the cell type and underlying vascular condition. Further understanding of this question is hampered by the poorly studied mechanisms underlying PDIA1 expression regulation. Here, we showed that PDIA1 mRNA and protein levels were upregulated (average 5-fold) in the intima and media/adventitia following partial carotid ligation (PCL). Our search identified that miR-204-5p and miR-211-5p (miR-204/211), two broadly conserved miRNAs, share PDIA1 as a potential target. MiR-204/211 was downregulated in vascular layers following PCL. In isolated endothelial cells, gain-of-function experiments of miR-204 with miR mimic decreased PDIA1 mRNA while having negligible effects on markers of endothelial activation/stress response. Similar effects were observed in vascular smooth muscle cells (VSMCs). Furthermore, PDIA1 downregulation by miR-204 decreased levels of the VSMC contractile differentiation markers. In addition, PDIA1 overexpression prevented VSMC dedifferentiation by miR-204. Collectively, we report a new mechanism for PDIA1 regulation through miR-204 and identify its relevance in a model of vascular disease playing a role in VSMC differentiation. This mechanism may be regulated in distinct stages of atherosclerosis and provide a potential therapeutic target.

Biomechanical and Mechanobiological Drivers of the Transition From PostCapillary Pulmonary Hypertension to Combined Pre-/PostCapillary Pulmonary Hypertension

Combined pre-/postcapillary pulmonary hypertension (Cpc-PH), a complication of left heart failure, is associated with higher mortality rates than isolated postcapillary pulmonary hypertension alone. Currently, knowledge gaps persist on the mechanisms responsible for the progression of isolated postcapillary pulmonary hypertension (Ipc-PH) to Cpc-PH. Here, we review the biomechanical and mechanobiological impact of left heart failure on pulmonary circulation, including mechanotransduction of these pathological forces, which lead to altered biological signaling and detrimental remodeling, driving the progression to Cpc-PH. We focus on pathologically increased cyclic stretch and decreased wall shear stress; mechanotransduction by endothelial cells, smooth muscle cells, and pulmonary arterial fibroblasts; and signaling-stimulated remodeling of the pulmonary veins, capillaries, and arteries that propel the transition from Ipc-PH to Cpc-PH. Identifying biomechanical and mechanobiological mechanisms of Cpc-PH progression may highlight potential pharmacologic avenues to prevent right heart failure and subsequent mortality.

KCa3.1 Inhibition Decreases Size and Alters Composition of Atherosclerotic Lesions Induced by Low, Oscillatory Flow

Low, oscillatory flow/shear patterns are associated with atherosclerotic lesion development. Increased expression of K_Ca3.1 has been found in Vascular Smooth Muscle (VSM), macrophages and T-cells in lesions from humans and mice. Increased expression of K_Ca3.1, is also required for VSM cell proliferation and migration. Previously, we showed that the specific K_Ca3.1 inhibitor, TRAM-34, could inhibit coronary neointimal development following balloon injury in swine. Atherosclerosis develops in regions with a low, oscillatory (i.e. atheroprone) flow pattern. Therefore, we used the Partial Carotid Ligation (PCL) model in high-fat fed, Apoe^−/− mice to determine the role of K_Ca3.1 in atherosclerotic lesion composition and development. PCL was performed on 8–10 week old male Apoe^−/− mice and subsequently placed on a Western diet (TD.88137, Teklad) for 4 weeks. Mice received daily s.c. injections of TRAM-34 (120 mg/kg) or equal volumes of vehicle (peanut oil, PO). 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) treatment reduced lesion size ~50% (p < 0.05). In addition, lesions from TRAM-34 treated mice contained less collagen (6% ± 1% vs. 15% ± 2%; p < 0.05), fibronectin (14% ± 3% vs. 32% ± 3%; p < 0.05) and smooth muscle content (19% ± 2% vs. 29% ± 3%; p < 0.05). Conversely, TRAM-34 had no effect on total cholesterol (1455 vs. 1334 mg/dl, PO and TRAM, resp.) or body weight (29.1 vs. 28.8 g, PO and TRAM, resp.). Medial smooth muscle of atherosclerotic carotids showed diminished RE1-Silencing Transcription Factor (REST)/Neural Restrictive Silencing Factor (NRSF) expression, while REST overexpression in vitro inhibited smooth muscle migration. Together, these data support a downregulation of REST/NRSF and upregulation of K_Ca3.1 in determining smooth muscle and matrix content of atherosclerotic lesions.

High fluid shear stress prevents atherosclerotic plaque formation by promoting endothelium denudation and synthetic phenotype of vascular smooth muscle cells

Low blood fluid shear stress (SS) promotes vascular remodeling and atherosclerosis; however, the effects of high (H)SS on vascular remodeling and atherogenesis is not fully clarified. The major goal of this study was to investigate the role of HSS in atherosclerotic plaque formation. A perivascular SS modifier was implanted in the right carotid artery of apolipoprotein E (ApoE)^−/− mice to induce HSS, whereas the left carotid artery represented undisturbed (U)SS as a control in vivo. In vitro modeling used human umbilical vein endothelial cells and vascular smooth muscle cells exposed to HSS (2.5 Pa) using a parallel-plate flow system. The results demonstrated that there were no plaque formations or endothelial cells in the HSS regions of the carotid artery in ApoE^−/− mice. The number of umbilical vein endothelial cells was markedly decreased in a time-dependent manner in HSS. HSS significantly decreased α-smooth muscle actin and increased osteopontin protein expression levels compared with USS in vascular smooth muscle cells (P<0.05). In addition, HSS significantly increased the protein expression levels of collagen α1(XVIII) chain/endostatin and matrix metalloproteinase-8 in vascular smooth muscle cells. These data indicated that HSS may prevent atherosclerotic plaque formation through endothelium denudation and contractile-to-synthetic phenotypic conversion of smooth muscle cells.

High Coronary Wall Shear Stress Worsens Plaque Vulnerability: A Systematic Review and Meta-Analysis

Aim:

The aim of this meta-analysis is to assess the impact of wall shear stress (WSS) severity on arterial plaque vulnerability.

Methods:

We systematically searched electronic databases and selected studies which assessed the relationship between WSS measured by intravascular ultrasound and coronary artery plaque features. In 7 studies, a total of 615 patients with 28 276 arterial segments (median follow-up: 7.71 months) were identified. At follow-up, the pooled analysis showed high WSS to be associated with regression of plaque fibrous area, weighted mean difference (WMD) −0.11 (95% CI: −0.20 to −0.02, P = .02) and fibrofatty area, WMD −0.09 (95% CI: −0.17 to −0.01, P = .02), reduction in plaque total area, WMD −0.09 (95% CI: −0.14 to −0.04, P = .007) and increased necrotic core area, and WMD 0.04 (95% CI: 0.01-0.09, P = .03) compared with low WSS. Dense calcium deposits remained unchanged in high and low WSS (0.01 vs 0.02 mm²; P > .05). High WSS resulted in profound remodeling (40% vs 18%, P < .05) and with more constructive remodeling than low WSS (78% vs 40%, P < .01).

Conclusions:

High WSS in coronary arteries is associated with worsening plaque vulnerability and more profound arterial wall remodeling compared with low WSS.

Vascular autophagy in health and disease

Homeostasis is maintained within organisms through the physiological recycling process of autophagy, a catabolic process that is intricately involved in the mobilization of nutrients during starvation, recycling of cellular cargo, as well as initiation of cellular death pathways. Specific to the cardiovascular system, autophagy responds to both chemical (e.g. free radicals) and mechanical stressors (e.g. shear stress). It is imperative to note that autophagy is not a static process, and measurement of autophagic flux provides a more comprehensive investigation into the role of autophagy. The overarching themes emerging from decades of autophagy research are that basal levels of autophagic flux are critical, physiological stressors may increase or decrease autophagic flux, and more importantly, aberrant deviations from basal autophagy may elicit detrimental effects. Autophagy has predominantly been examined within cardiac or vascular smooth muscle tissue within the context of disease development and progression. Autophagic flux within the endothelium holds an important role in maintaining vascular function, demonstrated by the necessary role for intact autophagic flux for shear-induced release of nitric oxide however the underlying mechanisms have yet to be elucidated. Within this review, we theorize that autophagy itself does not solely control vascular homeostasis, rather, it works in concert with mitochondria, telomerase, and lipids to maintain physiological function. The primary emphasis of this review is on the role of autophagy within the human vasculature, and the integrative effects with physiological processes and diseases as they relate to the vascular structure and function.

Endothelial/Epithelial Mesenchymal Transition in Ascending Aortas of Patients With Bicuspid Aortic Valve

Thoracic aortic aneurysm (TAA) is the progressive enlargement of the aorta due to destructive changes in the connective tissue of the aortic wall. Aneurysm development is silent and often first manifested by the drastic events of aortic dissection or rupture. As yet, therapeutic agents that halt or reverse the process of aortic wall deterioration are absent, and the only available therapeutic recommendation is elective prophylactic surgical intervention. Being born with a bicuspid instead of the normal tricuspid aortic valve (TAV) is a major risk factor for developing aneurysm in the ascending aorta later in life. Although the pathophysiology of the increased aneurysm susceptibility is not known, recent studies are suggestive of a transformation of aortic endothelium into a more mesenchymal state i.e., an endothelial-to-mesenchymal transition in these individuals. This process involves the loss of endothelial cell features, resulting in junction instability and enhanced vascular permeability of the ascending aorta that may lay the ground for increased aneurysm susceptibility. This finding differentiates and further emphasizes the specific characteristics of aneurysm development in individuals with a bicuspid aortic valve (BAV). This review discusses the possibility of a developmental fate shared between the aortic endothelium and aortic valves. It further speculates about the impact of aortic endothelium phenotypic shift on aneurysm development in individuals with a BAV and revisits previous studies in the light of the new findings.

Hemodynamics in Cardiac Development

The beating heart is subject to intrinsic mechanical factors, exerted by contraction of the myocardium (stretch and strain) and fluid forces of the enclosed blood (wall shear stress). The earliest contractions of the heart occur already in the 10-somite stage in the tubular as yet unsegmented heart. With development, the looping heart becomes asymmetric providing varying diameters and curvatures resulting in unequal flow profiles. These flow profiles exert various wall shear stresses and as a consequence different expression patterns of shear responsive genes. In this paper we investigate the morphological alterations of the heart after changing the blood flow by ligation of the right vitelline vein in a model chicken embryo and analyze the extended expression in the endocardial cushions of the shear responsive gene Tgfbeta receptor III. A major phenomenon is the diminished endocardial-mesenchymal transition resulting in hypoplastic (even absence of) atrioventricular and outflow tract endocardial cushions, which might be lethal in early phases. The surviving embryos exhibit several cardiac malformations including ventricular septal defects and malformed semilunar valves related to abnormal development of the aortopulmonary septal complex and the enclosed neural crest cells. We discuss the results in the light of the interactions between several shear stress responsive signaling pathways including an extended review of the involved Vegf, Notch, Pdgf, Klf2, eNos, Endothelin and Tgfβ/Bmp/Smad networks.

Numerical Simulation of Hemodynamic Changes in Central Veins after Tunneled Cuffed Central Venous Catheter Placement in Patients under Hemodialysis

The tunneled central venous catheter (CVC) plays an important role for hemodialysis patients, but CVC-related thrombosis in the central veins remain problematic. This study is the first try to numerically find out what hemodynamic parameters are predisposed to the initiation and formation of thrombus after CVC insertion. And the potential relationship between hemodynamic parameters and the incidence rates of thrombosis occurrence was explored. The results revealed that the CVC insertion led to a significant increase of hydraulic resistance, wide-ranging abnormally high wall shear stress (WSS), and a great loss of flow rotation in the vein. Moreover, the clinical data showed that thrombosis mainly occurred at sections where most blood flow lost spiral rotation after the CVC insertion, but no corresponding match was observed between the occurrence of thrombosis and the flow velocity or WSS. We speculate that the destruction of the flow rotation in the central vein is a precursor to the thrombus formation around CVC, and an introduction of spiral flow with the CVC insertion may possibly help to protect the central vein from thrombosis. Further animal and clinical experiments should be carried out to test and verify this speculation.

Notch Signaling in Endothelial Cells: Is It the Therapeutic Target for Vascular Neointimal Hyperplasia?

Blood vessels respond to injury through a healing process that includes neointimal hyperplasia. The vascular endothelium is a monolayer of cells that separates the outer vascular wall from the inner circulating blood. The disruption and exposure of endothelial cells (ECs) to subintimal components initiate the neointimal formation. ECs not only act as a highly selective barrier to prevent early pathological changes of neointimal hyperplasia, but also synthesize and release molecules to maintain vascular homeostasis. After vascular injury, ECs exhibit varied responses, including proliferation, regeneration, apoptosis, phenotypic switching, interacting with other cells by direct contact or secreted molecules and the change of barrier function. This brief review presents the functional role of the evolutionarily-conserved Notch pathway in neointimal hyperplasia, notably by regulating endothelial cell functions (proliferation, regeneration, apoptosis, differentiation, cell-cell interaction). Understanding endothelial cell biology should help us define methods to prompt cell proliferation, prevent cell apoptosis and dysfunction, block neointimal hyperplasia and vessel narrowing.

Restenosis of the Coronary Arteries: Past, Present, Future Directions

Restenosis is a pathologic response to vascular injury, characterized by neointimal hyperplasia and progressive narrowing of a stented vessel segment. Although advances in stent design have led to a dramatic reduction in the incidence of restenosis, it continues to represent the most common cause of target lesion failure following percutaneous coronary intervention. Efforts to maximize restenosis prevention, through careful consideration of modifiable risk factors and an individualized approach, are critical, as restenosis, once established, can be particularly difficult to treat. Novel approaches are on the horizon that have the potential to alter the natural history of this stubborn disease.

Patient-Specific, Multi-Scale Modeling of Neointimal Hyperplasia in Vein Grafts

Neointimal hyperplasia is amongst the major causes of failure of bypass grafts. The disease progression varies from patient to patient due to a range of different factors. In this paper, a mathematical model will be used to understand neointimal hyperplasia in individual patients, combining information from biological experiments and patient-specific data to analyze some aspects of the disease, particularly with regard to mechanical stimuli due to shear stresses on the vessel wall. By combining a biochemical model of cell growth and a patient-specific computational fluid dynamics analysis of blood flow in the lumen, remodeling of the blood vessel is studied by means of a novel computational framework. The framework was used to analyze two vein graft bypasses from one patient: a femoro-popliteal and a femoro-distal bypass. The remodeling of the vessel wall and analysis of the flow for each case was then compared to clinical data and discussed as a potential tool for a better understanding of the disease. Simulation results from this first computational approach showed an overall agreement on the locations of hyperplasia in these patients and demonstrated the potential of using new integrative modeling tools to understand disease progression.

Shear Stress in Autophagy and Its Possible Mechanisms in the Process of Atherosclerosis

Autophagy can eliminate harmful components and maintain cellular homeostasis in response to a series of extracellular insults in eukaryotes. More and more studies show that autophagy plays vital roles in the development of atherosclerosis. Atherosclerosis is a multifactorial disease and shear stress acts as a key role in its process. Understanding the role of shear stress in autophagy may offer insight into atherosclerosis therapies, especially emerging targeted therapy. In this article, we retrospect related studies to summarize the present comprehension of the association between autophagy and atherosclerosis onset and progression.

Engineering of the human vessel wall with hair follicle stem cells in vitro

Hair follicle stem cells (HFSCs) are increasingly used as a stem cell paradigm in vascular tissue engineering due to the fact that they are a rich source of easily accessible multipotent adult stem cells. Promising results have been demonstrated with small diameter (less than 6 mm) tissue engineered blood vessels under low blood pressure, however engineering large vessels (>6 mm in diameter) remains a challenge due to the fact it demands a higher number of seed cells and higher quality biomechanical properties. The aim of the current study was to engineer a large vessel (6 mm in diameter) with differentiated smooth muscle cells (SMCs) induced from human (h)HFSCs using transforming growth factor‑β1 and platelet‑derived growth factor BB in combination with low‑serum culture medium. The cells were seeded onto polyglycolic acid and then wrapped around a silicone tube and further cultured in vitro. A round vessel wall was formed subsequent to 8 weeks of culture. Histological examination indicated that layers of smooth muscle‑like cells and collagenous fibres were oriented in the induced group. In contrast, disorganised cells and collagenous fibres were apparent in the undifferentiated group. The approach developed in the current study demonstrated potential for constructing large muscular vessels with differentiated SMCs induced from hHFSCs.

High shear stress induces atherosclerotic vulnerable plaque formation through angiogenesis

Rupture of atherosclerotic plaques causing thrombosis is the main cause of acute coronary syndrome and ischemic strokes. Inhibition of thrombosis is one of the important tasks developing biomedical materials such as intravascular stents and vascular grafts. Shear stress (SS) influences the formation and development of atherosclerosis. The current review focuses on the vulnerable plaques observed in the high shear stress (HSS) regions, which localizes at the proximal region of the plaque intruding into the lumen. The vascular outward remodelling occurs in the HSS region for vascular compensation and that angiogenesis is a critical factor for HSS which induces atherosclerotic vulnerable plaque formation. These results greatly challenge the established belief that low shear stress is important for expansive remodelling, which provides a new perspective for preventing the transition of stable plaques to high-risk atherosclerotic lesions.

Device-Based In Vitro Techniques for Mechanical Stimulation of Vascular Cells: A Review

The most common cause of death in the developed world is cardiovascular disease. For decades, this has provided a powerful motivation to study the effects of mechanical forces on vascular cells in a controlled setting, since these cells have been implicated in the development of disease. Early efforts in the 1970 s included the first use of a parallel-plate flow system to apply shear stress to endothelial cells (ECs) and the development of uniaxial substrate stretching techniques (Krueger et al., 1971, “An in Vitro Study of Flow Response by Cells,” J. Biomech., 4(1), pp. 31–36 and Meikle et al., 1979, “Rabbit Cranial Sutures in Vitro: A New Experimental Model for Studying the Response of Fibrous Joints to Mechanical Stress,” Calcif. Tissue Int., 28(2), pp. 13–144). Since then, a multitude of in vitro devices have been designed and developed for mechanical stimulation of vascular cells and tissues in an effort to better understand their response to in vivo physiologic mechanical conditions. This article reviews the functional attributes of mechanical bioreactors developed in the 21st century, including their major advantages and disadvantages. Each of these systems has been categorized in terms of their primary loading modality: fluid shear stress (FSS), substrate distention, combined distention and fluid shear, or other applied forces. The goal of this article is to provide researchers with a survey of useful methodologies that can be adapted to studies in this area, and to clarify future possibilities for improved research methods.

A PARAMETRIC NUMERICAL INVESTIGATION OF LOCAL HAEMODYNAMICS IN THE END-TO-SIDE ANASTOMOSIS OF CERVICAL-TO-PETROUS BYPASS BASED ON REAL GEOMETRY OF INTERNAL CAROTID ARTERY

Bypass reconstructed from the cervical segment of internal carotid artery (ICA) to its petrous segment allows high-volume bypass flow without any risk of intracranial dissection. The purpose of this study was to investigate the geometric effect on the end-to-side anastomosis of cervical-to-petrous bypass, for its close relationship to local haemodynamic and the long-term performance of bypass. We focused on two controllable geometric parameters: diameter ratio (Φ) and angle (α) between the graft and host arteries. Different models covering a range of Φ (0.75, 1 and 1.25) and α (30°, 45°, 60° and 90°) were constructed based on real geometry of human ICA. Numerical simulations of blood flow were performed in physiological flow condition. The flow patterns, flow distributions, time-average wall shear stress (TAWSS) and oscillatory shear index (OSI) in different models were compared. Our results showed geometric factors have influence on both the local haemodynamic parameters and the flow velocity through downstream branches. Of models with different geometric parameters, the model with Φ ≥ 1 or α = 45° were the most optimized considering haemodynamic performance.

Biomechanical regulation of vascular smooth muscle cell functions: from in vitro to in vivo understanding

Vascular smooth muscle cells (VSMCs) have critical functions in vascular diseases. Haemodynamic factors are important regulators of VSMC functions in vascular pathophysiology. VSMCs are physiologically active in the three-dimensional matrix and interact with the shear stress sensor of endothelial cells (ECs). The purpose of this review is to illustrate how haemodynamic factors regulate VSMC functions under two-dimensional conditions in vitro or three-dimensional co-culture conditions in vivo. Recent advances show that high shear stress induces VSMC apoptosis through endothelial-released nitric oxide and low shear stress upregulates VSMC proliferation and migration through platelet-derived growth factor released by ECs. This differential regulation emphasizes the need to construct more actual environments for future research on vascular diseases (such as atherosclerosis and hypertension) and cardiovascular tissue engineering.

Development of vascular smooth muscle contractility by endothelium-derived transforming growth factor β proteins

It is well established that the release of vasodilators and vasoconstrictors from vascular endothelium regulates vascular smooth muscle contraction. In this report, we investigate the role of the endothelium in the development and maintenance of constitutive vascular contractility. For that purpose, contractile activity of cultured bovine aortic smooth muscle cells (BASMCs) embedded in collagen gels was monitored by changes in gel diameter. After culturing for 5 days, ATP- and high KCl solution-induced contractions were significantly enhanced in the gels that were overlaid with bovine aortic endothelial cells (BAECs) or were cultured with conditioned medium of cultured BAECs. ATP-induced Ca(2+) transients, recorded in BASMCs cultured with conditioned medium of BAECs, were markedly augmented, but high KCl-induced Ca(2+) transients were not affected. BASMCs in control gels were spindle shaped, and those in endothelium-treated gels were more elongated and interconnected. The endothelial conditioned medium also strongly affected the intracellular distribution of actin fibers. Conditioned medium of BAECs contained TGFβ1 and TGFβ2. The TGFβ receptor antagonist SB431542 as well as simultaneous treatment with TGFβ1 and TGFβ2 neutralizing antibodies completely reversed the above effects of endothelial conditioned medium on BASMCs. BAECs medium induced phosphorylation of Smad2 and increased ATP-induced phosphorylation of myosin light chain in BASMCs. The present results indicate that the release of TGFβ1 and TGFβ2 from vascular endothelium affects the contractility of vascular smooth muscle cells by altering their morphology and agonist-induced Ca(2+) mobilization.

Exercise training and peripheral arterial disease

Peripheral arterial disease (PAD) is a common vascular disease that reduces blood flow capacity to the legs of patients. PAD leads to exercise intolerance that can progress in severity to greatly limit mobility, and in advanced cases leads to frank ischemia with pain at rest. It is estimated that 12 to 15 million people in the United States are diagnosed with PAD, with a much larger population that is undiagnosed. The presence of PAD predicts a 50% to 1500% increase in morbidity and mortality, depending on severity. Treatment of patients with PAD is limited to modification of cardiovascular disease risk factors, pharmacological intervention, surgery, and exercise therapy. Extended exercise programs that involve walking approximately five times per week, at a significant intensity that requires frequent rest periods, are most significant. Preclinical studies and virtually all clinical trials demonstrate the benefits of exercise therapy, including improved walking tolerance, modified inflammatory/hemostatic markers, enhanced vasoresponsiveness, adaptations within the limb (angiogenesis, arteriogenesis, and mitochondrial synthesis) that enhance oxygen delivery and metabolic responses, potentially delayed progression of the disease, enhanced quality of life indices, and extended longevity. A synthesis is provided as to how these adaptations can develop in the context of our current state of knowledge and events known to be orchestrated by exercise. The benefits are so compelling that exercise prescription should be an essential option presented to patients with PAD in the absence of contraindications. Obviously, selecting for a lifestyle pattern that includes enhanced physical activity prior to the advance of PAD limitations is the most desirable and beneficial.

Role of endothelial shear stress in stent restenosis and thrombosis: pathophysiologic mechanisms and implications for clinical translation

Restenosis and thrombosis are potentially fatal complications of coronary stenting with a recognized multifactorial etiology. The effect of documented risk factors, however, cannot explain the preponderance of certain lesion types, stent designs, and implantation configurations for the development of these complications. Local hemodynamic factors, low endothelial shear stress (ESS) in particular, are long known to critically affect the natural history of atherosclerosis. Increasing evidence now suggests that ESS may also contribute to the development of restenosis and thrombosis upon stenting of atherosclerotic plaques, in conjunction with well-appreciated risk factors. In this review, we present in vivo and mechanistic evidence associating ESS with the localization and progression of neointimal hyperplasia and in-stent clotting. Clinical studies have associated stent design features with the risk of restenosis. Importantly, computational simulations extend these observations by directly linking specific stent geometry and positioning characteristics with the post-stenting hemodynamic milieu and with the stent's thrombogenicity and pro-restenotic potential, thereby indicating ways to clinical translation. An enhanced understanding of the pathophysiologic role of ESS in restenosis and thrombosis might dictate hemodynamically favorable stent designs and deployment configurations to reduce the potential for late lumen loss and thrombotic obstruction. Recent methodologies for in vivo ESS profiling at a clinical level might allow for early identification of patients at high risk for the development of restenosis or thrombosis and might thereby guide individualized, risk-tailored treatment strategies to prevent devastating complications of endovascular interventions.

Small calibre polyhedral oligomeric silsesquioxane nanocomposite cardiovascular grafts: influence of porosity on the structure, haemocompatibility and mechanical properties

There is a significant worldwide demand for a small calibre vascular graft for use as a bypass or replacement conduit. An important feature in determining the success of a graft is the wall structure, which includes porosity, pore size and pore interconnectivity, as these play a crucial role in determining the long-term patency of a bypass graft. In this study we fabricate a small diameter (<5mm) vascular graft from polyhedral oligomeric silsesquioxane-poly(carbonate urea)urethane (POSS-PCU) via an extrusion, phase inversion method using an automated, custom built machine. Through the dispersion of a porogen, sodium bicarbonate (NaHCO(3)), in controlled concentrations (0-55%) we were able to produce grafts with well-defined pore morphologies. The impact of NaHCO(3) concentration on the structure of the graft wall and its influence on the mechanical and haemocompatibility properties are evaluated here. Scanning electron microscopy and mercury porosimetry were used to characterise graft structure. Atomic force microscopy elucidated any changes in surface morphology. The addition of NaHCO(3) improved the pore interconnectivity and increasing the concentration of NaHCO(3) led to grafts with rougher surfaces and larger pore sizes. The ultimate tensile strength and suture retention decreased with increasing concentrations of NaHCO(3), while graft compliance increased. To evaluate haemocompatibility platelets and peripheral blood mononuclear cells (PBMC) were incubated on a range of different graft samples. Platelet adhesion, PBMC surface receptor expression (CD14, CD86, CD69 and HLA-DR) and cytokine release (PF4, IL-1β, IL-6, IL-10, TNFα) were all measured. Increasing numbers of platelets adhered to grafts produced with no NaHCO(3), which exhibited a smooth surface morphology, and PBMC adherent on these grafts expressed higher levels of CD14 and CD86. Whilst the different graft samples induced varying levels of cytokine secretion in vitro, no distinct pattern suggesting a non-trivial relationship was observed.

The FGF-2-derived peptide FREG inhibits melanoma growth in vitro and in vivo

Previous data report that fibroblast growth factor-2 (FGF-2)-derived peptide FREG potently inhibits FGF-2-dependent angiogenesis in vitro and in vivo. Here, we show that FREG inhibits up to 70% in vitro growth and invasion/migration of smooth muscle and melanoma cells. Such inhibition is mediated by platelet-derived growth factor-receptor-α (PDGF-Rα); in fact, proliferation and migration were restored upon PDGF-Rα neutralization. Further experiments demonstrated that FREG interacts with PDGF-Rα both in vitro and in vivo and stimulates its phosphorylation. We have previously shown that overexpressing PDGF-Rα strongly inhibits melanoma growth in vivo; we, therefore, hypothesized that PDGF-Rα agonists may represent a novel tool to inhibit melanoma growth in vivo. To support this hypothesis, FREG was inoculated intravenously (i.v.) in a mouse melanoma model and markedly inhibited pulmonary metastases formation. Immunohistochemical analyses showed less proliferation, less angiogenesis, and more apoptosis in metastasized lungs upon FREG treatment, as compared to untreated controls. Finally, in preliminary acute toxicity studies, FREG showed no toxicity signs in healthy animals, and neither microscopic nor macroscopic toxicity at the liver, kidney, and lungs level. Altogether, these data indicate that FREG systemic treatment strongly inhibits melanoma metastases development and indicate for the first time that agonists of PDGF-Rα may control melanoma both in vitro and in vivo.

Expression of platelet-derived growth factor ligand and receptor in cerebral arteriovenous and cavernous malformations

The aim of this study was to investigate the expression of platelet-derived growth factor (PDGF) ligands A and B and receptors α and β in cerebral arteriovenous and cavernous malformations. Fifteen arteriovenous malformation (AVM) and 15 cerebral cavernous malformation (CCM) tissue samples were immunostained for PDGF ligands A and B, PDGF receptors (PDGFR) α and β, and vascular endothelial growth factor. Tissues were compared in terms of expression levels within various vascular layers, and the results were confirmed using western blotting. AVM had higher levels of PDGF-A expression than CCM (p = 0.004, 0.009, 0.001, and 0.027, for endothelium, media, adventitia, and perilesional tissue, respectively) and western blotting showed that there was higher expression of PDGFR-α in AVM tissues. In contrast, CCM endothelium, media, and adventitia had higher PDGF-B expression compared with AVM (p = 0.007, 0.001, and 0.039, respectively). PDGFR-β expression was also significantly higher in the endothelium of CCM tissue (p = 0.007). Overexpression of PDGF ligands and receptors in AVM and CCM may mean that therapeutic strategies targeting the PDGF pathway could be useful in the treatment of these two malformations.

Shear stress and plaque development

Although traditional cardiovascular risk factors 'prime the soil' for atherogenesis systemically, atherosclerosis primarily occurs in a site-specific manner with a predilection towards the inner wall of curvatures and outer wall of bifurcations with sparing of flow-dividers. Wall shear stress is a frictional force exerted parallel to the vessel wall that leads to alteration of the endothelial phenotype, endothelial cell signaling, gene and protein expression leading to a proinflammatory phenotype, reduced nitric oxide availability and disruption of the extracellular matrix, which in turn leads to plaque development. Clinical and experimental data are emerging that suggest the pathobiology associated with abnormal wall shear stress results in atherosclerotic plaque development and progression.

The role of low endothelial shear stress in the conversion of atherosclerotic lesions from stable to unstable plaque

PURPOSE OF REVIEW

Local hemodynamic factors are major determinants of the natural history of individual atherosclerotic plaque progression in coronary arteries. The purpose of this review is to summarize the role of low endothelial shear stress (ESS) in the transition of early, stable plaques to high-risk atherosclerotic lesions.

RECENT FINDINGS

Low ESS regulates multiple pathways within the atherosclerotic lesion, resulting in intense vascular inflammation, progressive lipid accumulation, and formation and expansion of a necrotic core. Upregulation of matrix-degrading proteases promotes thinning of the fibrous cap, severe internal elastic lamina fragmentation, and extracellular matrix remodeling. In the setting of plaque-induced changes of the local ESS, coronary regions persistently exposed to very low ESS develop excessive expansive remodeling, which further exacerbates the proinflammatory low ESS stimulus. Recent studies suggest that the effect of recognized cardioprotective medications may be mediated by attenuation of the proinflammatory effect of the low ESS environment in which a plaque develops.

SUMMARY

Low ESS determines the severity of vascular inflammation, the status of the extracellular matrix, and the nature of wall remodeling, all of which synergistically promote the transition of stable lesions to thin cap fibroatheromata that may rupture with subsequent formation of an occlusive thrombus and result in an acute coronary syndrome.

Platelet-derived growth factor-receptor alpha strongly inhibits melanoma growth in vitro and in vivo

Cutaneous melanoma is the most aggressive skin cancer; it is highly metastatic and responds poorly to current therapies. The expression of platelet-derived growth factor receptors (PDGF-Rs) is reported to be reduced in metastatic melanoma compared with benign nevi or normal skin; we then hypothesized that PDGF-Ralpha may control growth of melanoma cells. We show here that melanoma cells overexpressing PDGF-Ralpha respond to serum with a significantly lower proliferation compared with that of controls. Apoptosis, cell cycle arrest, pRb dephosphorylation, and DNA synthesis inhibition were also observed in cells overexpressing PDGF-Ralpha. Proliferation was rescued by PDGF-Ralpha inhibitors, allowing to exclude nonspecific toxic effects and indicating that PDGF-Ralpha mediates autocrine antiproliferation signals in melanoma cells. Accordingly, PDGF-Ralpha was found to mediate staurosporine cytotoxicity. A protein array-based analysis of the mitogen-activated protein kinase pathway revealed that melanoma cells overexpressing PDGF-Ralpha show a strong reduction of c-Jun phosphorylated in serine 63 and of protein phosphatase 2A/Balpha and a marked increase of p38gamma, mitogen-activated protein kinase kinase 3, and signal regulatory protein alpha1 protein expression. In a mouse model of primary melanoma growth, infection with the Ad-vector overexpressing PDGF-Ralpha reached a significant 70% inhibition of primary melanoma growth (P < .001) and a similar inhibition of tumor angiogenesis. All together, these data demonstrate that PDGF-Ralpha strongly impairs melanoma growth likely through autocrine mechanisms and indicate a novel endogenous mechanism involved in melanoma control.

PDGF-DD, a novel mediator of smooth muscle cell phenotypic modulation, is upregulated in endothelial cells exposed to atherosclerosis-prone flow patterns

Platelet-derived growth factor (PDGF)-BB is a well-known smooth muscle (SM) cell (SMC) phenotypic modulator that signals by binding to PDGF alphaalpha-, alphabeta-, and betabeta-membrane receptors. PDGF-DD is a recently identified PDGF family member, and its role in SMC phenotypic modulation is unknown. Here we demonstrate that PDGF-DD inhibited expression of multiple SMC genes, including SM alpha-actin and SM myosin heavy chain, and upregulated expression of the potent SMC differentiation repressor gene Kruppel-like factor-4 at the mRNA and protein levels. On the basis of the results of promoter-reporter assays, changes in SMC gene expression were mediated, at least in part, at the level of transcription. Attenuation of the SMC phenotypic modulatory activity of PDGF-DD by pharmacological inhibitors of ERK phosphorylation and by a small interfering RNA to Kruppel-like factor-4 highlight the role of these two pathways in this process. PDGF-DD failed to repress SM alpha-actin and SM myosin heavy chain in mouse SMCs lacking a functional PDGF beta-receptor. Importantly, PDGF-DD expression was increased in neointimal lesions in the aortic arch region of apolipoprotein C-deficient (ApoE(-/-)) mice. Furthermore, human endothelial cells exposed to an atherosclerosis-prone flow pattern, as in vascular regions susceptible to the development of atherosclerosis, exhibited a significant increase in PDGF-DD expression. These findings demonstrate a novel activity for PDGF-DD in SMC biology and highlight the potential contribution of this molecule to SMC phenotypic modulation in the setting of disturbed blood flow.

Arterial geometry, flow pattern, wall shear and mass transport: potential physiological significance

We have studied numerically steady and unsteady flow in a straight and a helically stented common carotid artery, in order to model porcine experimental results that show reduced intimal hyperplasia (IH) in the helical case. The combination of flow pulsatility and three-dimensionality generates a sweeping motion of the Dean vortices, which overall reduced extremes of both oxygen flux to the vessel wall and wall shear stress (WSS). Since IH and atherosclerosis affect preferentially low WSS regions, these findings imply that vessel three-dimensionality and flow pulsatility can play important protective roles in respect of these diseases. The amplitude and frequency of the velocity waveform are important parameters of the system. Increase in amplitude increases WSS and oxygen flux to the vessel wall. Increase in frequency has a small effect; it increases WSS but has no effect on the oxygen flux to the vessel wall.

Oxygen mass transfer in a model three-dimensional artery

Arterial geometry is commonly non-planar and associated with swirling blood flow. In this study, we examine the effect of arterial three-dimensionality on the distribution of wall shear stress (WSS) and the mass transfer of oxygen from the blood to the vessel wall in a U-bend, by modelling the blood vessels as either cylindrical or helical conduits. The results show that under physiological flow conditions, three-dimensionality can reduce both the range and extent of low WSS regions and substantially increase oxygen flux through the walls. The Sherwood number and WSS distributions between the three-dimensional helical model and a human coronary artery show remarkable qualitative agreement, implying that coronary arteries may potentially be described with a relatively simple idealized three-dimensional model, characterized by a small number of well-defined geometric parameters. The flow pattern downstream of a planar bend results in separation of the Sh number and WSS effects, a finding that implies means of investigating them individually.

Endothelial injury induces vascular smooth muscle cell proliferation in highly localized regions of a direct contact co-culture system

Though previous studies have indicated a relationship between the proliferation of endothelial cells and vascular smooth muscle cells (VSMCs) in co-culture, the results have been contradictory and the signaling mechanism poorly understood. In this transmembrane co-culture study, VSMCs and endothelial cells were grown to confluence on opposite sides of a microporous membrane to mimic the intima/media border of vessels. The endothelial layer was injured, and then cultured for 3 days, resulting in partial re-endothelialization. VSMC proliferation across from the injured/partially recovered endothelial region was significantly higher than across from the de-endothelialized region (a sevenfold increase) and the uninjured region (a threefold increase). ELISA indicated that PDGF, which was undetectable in uninjured co-culture and homotypic controls, increased after injury and the addition of a piperazinyl-quinazoline carboxamide PDGF receptor inhibitor blocked VSMC proliferation across from the injured/partially recovered region. We conclude that co-culture signaling initiated by endothelial cell injury locally stimulates VSMC proliferation and that this signaling could be mediated by PDGF-BB.

Identification of platelet-derived growth factor D in human chronic allograft nephropathy

Chronic allograft nephropathy (CAN), a descriptive term denoting chronic scarring injury of the renal parenchyma and vasculature in allograft kidneys arising from various etiologies including chronic rejection, is the most common cause of late allograft failure, but mediators of this progressive injury largely remain unknown. We hypothesized that platelet-derived growth factor D (PDGF-D) and its specific receptor PDGF-Rbeta may be an important mediator in the pathogenesis of CAN and, hence, sought to identify its expression in this setting. Allograft nephrectomies demonstrating CAN, obtained from patients with irreversible transplant kidney failure (n = 15), were compared with renal tissues without prominent histopathological abnormalities (n = 18) and a series of renal allograft biopsies demonstrating acute vascular rejection (AVR) (n = 12). Antibodies to PDGF-D and PDGF-Rbeta were used for immunohistochemistry. Double and triple immunohistochemistry was used to identify cell types expressing PDGF-D. PDGF-D was widely expressed in most neointimas in arteries exhibiting the chronic arteriopathy of CAN and only weakly expressed in a small proportion of sclerotic arteries in the other 2 groups. Double and triple immunolabeling demonstrated that the neointimal cells expressing PDGF-D were alpha-smooth muscle actin-expressing cells, but not infiltrating macrophages or endothelial cells. PDGF-Rbeta expression evaluated in serial sections was localized to the same sites where neointimal PDGF-D was expressed. PDGF-Rbeta was expressed in interstitial cells more abundantly in the CAN group compared with the normal and AVR groups, without demonstrable colocalization of PDGF-D. PDGF-D is present in the neointima of the arteriopathy of CAN, where it can engage PDGF-Rbeta to promote mesenchymal cell migration, proliferation, and neointima formation. PDGF-D may engage the PDGF-Rbeta to promote interstitial injury in chronic allograft injury, but its sources within the interstitium were unidentified.

Role of endothelial shear stress in the natural history of coronary atherosclerosis and vascular remodeling: molecular, cellular, and vascular behavior

Although the entire coronary tree is exposed to the atherogenic effect of the systemic risk factors, atherosclerotic lesions form at specific arterial regions, where low and oscillatory endothelial shear stress (ESS) occur. Low ESS modulates endothelial gene expression through complex mechanoreception and mechanotransduction processes, inducing an atherogenic endothelial phenotype and formation of an early atherosclerotic plaque. Each early plaque exhibits an individual natural history of progression, regression, or stabilization, which is dependent not only on the formation and progression of atherosclerosis but also on the vascular remodeling response. Although the pathophysiologic mechanisms involved in the remodeling of the atherosclerotic wall are incompletely understood, the dynamic interplay between local hemodynamic milieu, low ESS in particular, and the biology of the wall is likely to be important. In this review, we explore the molecular, cellular, and vascular processes supporting the role of low ESS in the natural history of coronary atherosclerosis and vascular remodeling and indicate likely mechanisms concerning the different natural history trajectories of individual coronary lesions. Atherosclerotic plaques associated with excessive expansive remodeling evolve to high-risk plaques, because low ESS conditions persist, thereby promoting continued local lipid accumulation, inflammation, oxidative stress, matrix breakdown, and eventually further plaque progression and excessive expansive remodeling. An enhanced understanding of the pathobiologic processes responsible for atherosclerosis and vascular remodeling might allow for early identification of a high-risk coronary plaque and thereby provide a rationale for innovative diagnostic and/or therapeutic strategies for the management of coronary patients and prevention of acute coronary syndromes.

Effects of stenting the parent artery on aneurysm filling and gene expression of various potential factors involved in healing of experimental aneurysms

SUMMARY

Intracranial stents are increasingly used in the endovascular treatment of aneurysms, but very little is known regarding their effect on the cellular and molecular evolution of aneurysms. Bilateral venous pouch lateral wall carotid aneurysms were created in 20 dogs. All dogs then underwent angiography and balloon-expandable stenting of one aneurysm four to six weeks later. Fifteen dogs underwent aneurysm harvesting at one day (n=3), four days (n=4), seven days (n=3), and 14 days (n=5) for mRNA expression analysis, using axial sections taken from the aneurysm neck and fundus for RTPCR amplification of four cytokines or growth factors: TNF-a, TGF-b1, MCP-1, and PDGFBB; two adhesion molecules: VCAM-1 and PECAM-1; five matrix modifying agents; MMP- 2, 9, TIMPs 1, 3, 4, and two cellular markers: CD34 and a-SMA. Five other dogs, sacrificed at 12 weeks, were examined for extent of filling of the aneurysm neck with organized tissue and for neointima formation at the aneurysm ostium. Angiography was performed prior to sacrifice in all animals, and compared with initial studies. Eleven out of 20 stented aneurysms showed a favorable angiographic evolution, while none of the 20 nonstented aneurysms improved (p=0.001). Pathology showed partially occluded aneurysms, with neointima formation around the stent struts.Observed trends in mRNA expression, that stenting increased expression of genes involved in organization and neointima formation, agreed with experimental hypotheses, but differences between stented and non-stented aneurysms did not reach statistical significance. Parent vessel stenting was associated with angiographic improvement of aneurysm appearance. Modifications in mRNA expression patterns following stenting deserve further study to better establish potential molecular targets to promote aneurysm healing.

Shear stress protects against endothelial regulation of vascular smooth muscle cell migration in a coculture system

Vascular endothelial cells (ECs) are constantly exposed to blood flow-induced shear stress; these forces strongly influence the behaviors of neighboring vascular smooth muscle cells (VSMCs). VSMC migration is a key event in vascular wall remodeling. In this study, the authors assessed the difference between VSMC migration in VSMC/EC coculture under static and shear stress conditions. Utilizing a parallel-plate coculture flow chamber system and Transwell migration assays, they demonstrated that human ECs cocultured with VSMCs under static conditions induced VSMC migration, whereas laminar shear stress (1.5 Pa, 15 dynes/cm2) applied to the EC side for 12 h significantly inhibited this process. The changes in VSMC migration is mainly dependent on the close interactions between ECs and VSMCs. Western blotting showed that there was a consistent correlation between the level of Akt phosphorylation and the efficacy of shear stress-mediated EC regulation of VSMC migration. Wortmannin and Akti significantly inhibited the EC-induced effect on VSMC Akt phosphorylation and migration. These results indicate that shear stress protects against endothelial regulation of VSMC migration, which may be an atheroprotective function on the vessel wall.

Shear stress-stimulated endothelial cells induce smooth muscle cell chemotaxis via platelet-derived growth factor-BB and interleukin-1alpha

OBJECTIVE

Vascular smooth muscle cell (SMC) migration is critical to the development of atherosclerosis and neointimal hyperplasia. Hemodynamic forces such as shear stress and cyclic strain stimulate endothelial cell signal-transduction pathways, resulting in the secretion of several factors, including SMC chemoattractants such as platelet-derived growth factor (PDGF). We hypothesized that mechanical forces stimulate endothelial cells to secrete SMC chemoattractants to induce migration via the mitogen-activated protein kinase (MAPK) pathway.

METHODS

Bovine aortic endothelial cells were exposed to shear stress, cyclic strain, or static conditions for 16 hours. The resulting conditioned medium was used as a SMC chemoattractant in a Boyden chamber. Activation of SMC extracellular signal-regulated protein kinase 1/2 (ERK1/2) was assessed by Western blot analysis. Pathways were inhibited with anti-PDGF-BB or anti-interleukin-1alpha (IL-1alpha) antibodies, or the ERK1/2 upstream pathway inhibitor PD98059.

RESULTS

Conditioned medium from endothelial cells exposed to shear stress corresponding to arterial levels of shear stress stimulated SMC migration but lower levels of shear stress or cyclic strain did not. Both PDGF-BB and IL-1alpha were secreted into the conditioned medium by endothelial cells stimulated with shear stress. Both PDGF-BB and IL-1alpha stimulated SMC chemotaxis but were not synergistic, and both stimulated SMC ERK1/2 phosphorylation. Inhibition of PDGF-BB or IL-1alpha inhibited SMC chemotaxis and ERK1/2 phosphorylation.

CONCLUSION

Shear stress stimulates endothelial cells to secrete several SMC chemoattractants, including PDGF-BB and IL-1alpha; both PDGF-BB and IL-1alpha stimulate SMC chemotaxis via the ERK1/2 signal-transduction pathway. These results suggest that the response to vascular injury may have a common pathway amenable to pharmacologic manipulation.

CLINICAL RELEVANCE

One difficulty in the pharmacologic treatment of atherosclerosis or neointimal hyperplasia leading to restenosis is the multiplicity of activated pathways and thus potential treatment targets. This study demonstrates that shear stress, a hemodynamic force that may be a biologically relevant stimulus to induce vascular pathology, stimulates endothelial cells to secrete PDGF-BB and IL-1alpha. Both of these mediators stimulate the SMC ERK1/2 pathway to induce migration, a critical event in the pathogenesis of atherosclerosis and neointimal hyperplasia. Therefore, this study suggests a relevant common target pathway in SMC that is amenable to manipulation for clinical treatment.

Revascularization of ischemic tissues by PDGF-CC via effects on endothelial cells and their progenitors

The angiogenic mechanism and therapeutic potential of PDGF-CC, a recently discovered member of the VEGF/PDGF superfamily, remain incompletely characterized. Here we report that PDGF-CC mobilized endothelial progenitor cells in ischemic conditions; induced differentiation of bone marrow cells into ECs; and stimulated migration of ECs. Furthermore, PDGF-CC induced the differentiation of bone marrow cells into smooth muscle cells and stimulated their growth during vessel sprouting. Moreover, delivery of PDGF-CC enhanced postischemic revascularization of the heart and limb. Modulating the activity of PDGF-CC may provide novel opportunities for treating ischemic diseases.

The role of shear stress in the pathogenesis of atherosclerosis

Although the pathobiology of atherosclerosis is a complex multifactorial process, blood flow-induced shear stress has emerged as an essential feature of atherogenesis. This fluid drag force acting on the vessel wall is mechanotransduced into a biochemical signal that results in changes in vascular behavior. Maintenance of a physiologic, laminar shear stress is known to be crucial for normal vascular functioning, which includes the regulation of vascular caliber as well as inhibition of proliferation, thrombosis and inflammation of the vessel wall. Thus, shear stress is atheroprotective. It is also recognized that disturbed or oscillatory flows near arterial bifurcations, branch ostia and curvatures are associated with atheroma formation. Additionally, vascular endothelium has been shown to have different behavioral responses to altered flow patterns both at the molecular and cellular levels and these reactions are proposed to promote atherosclerosis in synergy with other well-defined systemic risk factors. Nonlaminar flow promotes changes to endothelial gene expression, cytoskeletal arrangement, wound repair, leukocyte adhesion as well as to the vasoreactive, oxidative and inflammatory states of the artery wall. Disturbed shear stress also influences the site selectivity of atherosclerotic plaque formation as well as its associated vessel wall remodeling, which can affect plaque vulnerability, stent restenosis and smooth muscle cell intimal hyperplasia in venous bypass grafts. Thus, shear stress is critically important in regulating the atheroprotective, normal physiology as well as the pathobiology and dysfunction of the vessel wall through complex molecular mechanisms that promote atherogenesis.

Cyclic strain-induced endothelial MMP-2: role in vascular smooth muscle cell migration

Matrix metalloproteinases (MMPs) play a vital role in vasculature response to hemodynamic stimuli via the degradation of extracellular matrix substrates. In this study, we investigated the putative role of cyclic strain-induced endothelial MMP-2 (and MMP-9) expression and release in modulating bovine aortic smooth muscle cell (BASMC) migration in vitro. Equibiaxial cyclic strain of bovine aortic endothelial cells (BAECs) leads to elevation in cellular MMP-2 (and MMP-9) expression, activity, and secretion into conditioned media, events which were time- and force-dependent. Subsequent incubation of BASMCs with conditioned media from chronically strained BAECs (5%, 24 h) significantly reduces BASMC migration (38+/-6%), an inhibitory effect which could be completely reversed by targeted siRNA 'knock-down' of MMP-2 (but not MMP-9) expression and activity in BAECs. Moreover, inhibition of strain-mediated MMP-2 expression in BAECs by protein tyrosine kinase (PTK) blockade with genistein (50 microM) was also found to completely reverse this inhibitory effect on BASMC migration. Finally, direct supplementation of recombinant MMP-2 into the BASMC migration assay was found to have no significant effect on migration. However, the effect on BASMC migration of MMP-2 siRNA transfection in BAECs could be reversed by supplementation of recombinant MMP-2 into BAEC media prior to (and for the duration of) strain. These findings reveal a potentially novel role for strain-induced endothelial MMP-2 in regulating vascular SMC migration.

Extracellular HMGB1, a signal of tissue damage, induces mesoangioblast migration and proliferation

High mobility group box 1 (HMGB1) is an abundant chromatin protein that acts as a cytokine when released in the extracellular milieu by necrotic and inflammatory cells. Here, we show that extracellular HMGB1 and its receptor for advanced glycation end products (RAGE) induce both migration and proliferation of vessel-associated stem cells (mesoangioblasts), and thus may play a role in muscle tissue regeneration. In vitro, HMGB1 induces migration and proliferation of both adult and embryonic mesoangioblasts, and disrupts the barrier function of endothelial monolayers. In living mice, mesoangioblasts injected into the femoral artery migrate close to HMGB1-loaded heparin-Sepharose beads implanted in healthy muscle, but are unresponsive to control beads. Interestingly, alpha-sarcoglycan null dystrophic muscle contains elevated levels of HMGB1; however, mesoangioblasts migrate into dystrophic muscle even if their RAGE receptor is disabled. This implies that the HMGB1-RAGE interaction is sufficient, but not necessary, for mesoangioblast homing; a different pathway might coexist. Although the role of endogenous HMGB1 in the reconstruction of dystrophic muscle remains to be clarified, injected HMGB1 may be used to promote tissue regeneration.

Arterial wall shear stress: observations from the bench to the bedside

Shear stress is the tangential force of the flowing blood on the endothelial surface of the blood vessel. Shear is described mathematically or ideal fluids, and in vitro models have enabled researchers to describe the effects of shear on endothelial cells. High shear stress, as found in laminar flow, promotes endothelial cell survival and quiescence, alignment in the direction of flow, and secretion of substances that promote vasodilation and anticoagulation. Low shear stress, or changing shear stress direction as found in turbulent flow, promotes endothelial proliferation and apoptosis, shape change, and secretion of substances that promote vasoconstriction, coagulation, and platelet aggregation. The precise pathways by which endothelial cells sense shear stress to promote their quiescent or activated pathways are currently unknown. Clinical applications include increasing shear stress via creation of an arteriovenous fistula or vein cuff to promote bypass graft flow and patency. Since an abnormal level of shear stress is implicated in the pathogenesis of atherosclerosis, neointimal hyperplasia, and aneurysmal disease, additional research to understand the effects of shear stress on the blood vessel may provide insight to prevent vascular disease.