Endothelial transcriptome profiles in vivo in complex arterial flow fields

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Vitexin inhibits APEX1 to counteract the flow-induced endothelial inflammation

Vascular endothelial cells are exposed to shear stresses with disturbed vs. laminar flow patterns, which lead to proinflammatory vs. antiinflammatory phenotypes, respectively. Effective treatment against endothelial inflammation and the consequent atherogenesis requires the identification of new therapeutic molecules and the development of drugs targeting these molecules. Using Connectivity Map, we have identified vitexin, a natural flavonoid, as a compound that evokes the gene-expression changes caused by pulsatile shear, which mimics laminar flow with a clear direction, vs. oscillatory shear (OS), which mimics disturbed flow without a clear direction. Treatment with vitexin suppressed the endothelial inflammation induced by OS or tumor necrosis factor-α. Administration of vitexin to mice subjected to carotid partial ligation blocked the disturbed flow-induced endothelial inflammation and neointimal formation. In hyperlipidemic mice, treatment with vitexin ameliorated atherosclerosis. Using SuperPred, we predicted that apurinic/apyrimidinic endonuclease1 (APEX1) may directly interact with vitexin, and we experimentally verified their physical interactions. OS induced APEX1 nuclear translocation, which was inhibited by vitexin. OS promoted the binding of acetyltransferase p300 to APEX1, leading to its acetylation and nuclear translocation. Functionally, knocking down APEX1 with siRNA reversed the OS-induced proinflammatory phenotype, suggesting that APEX1 promotes inflammation by orchestrating the NF-κB pathway. Animal experiments with the partial ligation model indicated that overexpression of APEX1 negated the action of vitexin against endothelial inflammation, and that endothelial-specific deletion of APEX1 ameliorated atherogenesis. We thus propose targeting APEX1 with vitexin as a potential therapeutic strategy to alleviate atherosclerosis.

SWI/SNF (BAF) complexes: From framework to a functional role in endothelial mechanotransduction

Endothelial cells (ECs) are constantly subjected to an array of mechanical cues, especially shear stress, due to their luminal placement in the blood vessels. Blood flow can regulate various aspects of endothelial biology and pathophysiology by regulating the endothelial processes at the transcriptomic, proteomic, miRNomic, metabolomics, and epigenomic levels. ECs sense, respond, and adapt to altered blood flow patterns and shear profiles by specialized mechanisms of mechanosensing and mechanotransduction, resulting in qualitative and quantitative differences in their gene expression. Chromatin-regulatory proteins can regulate transcriptional activation by modifying the organization of nucleosomes at promoters, enhancers, silencers, insulators, and locus control regions. Recent research efforts have illustrated that SWI/SNF (SWItch/Sucrose Non-Fermentable) or BRG1/BRM-associated factor (BAF) complex regulates DNA accessibility and chromatin structure. Since the discovery, the gene-regulatory mechanisms of the BAF complex associated with chromatin remodeling have been intensively studied to investigate its role in diverse disease phenotypes. Thus far, it is evident that (1) the SWI/SNF complex broadly regulates the activity of transcriptional enhancers to control lineage-specific differentiation and (2) mutations in the BAF complex proteins lead to developmental disorders and cancers. It is unclear if blood flow can modulate the activity of SWI/SNF complex to regulate EC differentiation and reprogramming. This review emphasizes the integrative role of SWI/SNF complex from a structural and functional standpoint with a special reference to cardiovascular diseases (CVDs). The review also highlights how regulation of this complex by blood flow can lead to the discovery of new therapeutic interventions for the treatment of endothelial dysfunction in vascular diseases.

KLF4 Upregulation in Atherosclerotic Thoracic Aortas: Exploring the Protective Effect of Colchicine-based Regimens in a Hyperlipidemic Rabbit Model

BACKGROUND

Inflammatory dysregulation of KLF4 is related to atheromatosis. In the present study, we explored the impact of colchicine-based regimens on the development of thoracic aortic atheromatosis and KLF4 expression.

METHODS

Twenty-eight New Zealand White rabbits were divided to 4 groups. The control group (n = 6) was fed standard chow, group A (n = 6) was fed chow enriched with 1% w/w cholesterol, group B (n = 8) was fed the same cholesterol-enriched diet plus 2 mg/kg body weight/day colchicine and 250 mg/kg body weight/day fenofibrate, while group C (n = 8) was also fed the same diet plus 2 mg/kg body weight/day colchicine and 15 mg/kg body weight/day N-acetylcysteine. After 7 weeks, all animals were euthanized, and their thoracic aortas were isolated. Atherosclerotic plaque area was estimated with morphometric analysis. KLF4 expression was quantified with quantitative RT-PCR.

RESULTS

Group A developed significantly more atherosclerosis compared to group B (MD: 13.67, 95% CI: 7.49-19.84) and C (MD: 20.29, 95% CI: 14.12-26.47). Colchicine with N-acetylcysteine resulted in more pronounced reduction in the extent of atherosclerotic plaques compared to colchicine/fibrate (MD: 6.62, 95% CI: 0.90-12.34). Group A exhibited significantly greater KLF4 expression compared to group B (MD: 4.94, 95% CI: 1.11-8.77) and C (MD: 9.94, 95% CI: 6.11-13.77). Combining colchicine with N-acetylcysteine instead of fenofibrate (MD: 5.00, 95% CI: 1.45-8.54) led to a more robust reduction in KLF4 expression.

CONCLUSIONS

In the present hyperlipidemic animal model, colchicine-based regimens curtailed de novo atherogenesis and KLF4 overexpression in thoracic aortas.

Endothelial Cell Mechano-Metabolomic Coupling to Disease States in the Lung Microvasculature

Lungs are the most vascular part of humans, accepting the totality of cardiac output in a volume much smaller than the body itself. Due to this cardiac output, the lung microvasculature is subject to mechanical forces including shear stress and cyclic stretch that vary with the cardiac and breathing cycle. Vessels are surrounded by extracellular matrix which dictates the stiffness which endothelial cells also sense and respond to. Shear stress, stiffness, and cyclic stretch are known to influence endothelial cell state. At high shear stress, endothelial cells exhibit cell quiescence marked by low inflammatory markers and high nitric oxide synthesis, whereas at low shear stress, endothelial cells are thought to "activate" into a pro-inflammatory state and have low nitric oxide. Shear stress' profound effect on vascular phenotype is most apparent in the arterial vasculature and in the pathophysiology of vascular inflammation. To conduct the flow of blood from the right heart, the lung microvasculature must be rigid yet compliant. It turns out that excessive substrate rigidity or stiffness is important in the development of pulmonary hypertension and chronic fibrosing lung diseases via excessive cell proliferation or the endothelial-mesenchymal transition. Recently, a new body of literature has evolved that couples mechanical sensing to endothelial phenotypic changes through metabolic signaling in clinically relevant contexts such as pulmonary hypertension, lung injury syndromes, as well as fibrosis, which is the focus of this review. Stretch, like flow, has profound effect on endothelial phenotype; metabolism studies due to stretch are in their infancy.

The biophysics and mechanics of blood from a materials perspective

Cells actively interact with their microenvironment, constantly sensing and modulating biochemical and biophysical signals. Blood comprises a variety of non-adherent cells that interact with each other and with endothelial and vascular smooth muscle cells of the blood vessel walls. Blood cells are further experiencing a range of external forces by the hemodynamic environment and they also exert forces to remodel their local environment. Therefore, the biophysics and material properties of blood cells and blood play an important role in determining blood behaviour in health and disease. In this Review, we discuss blood cells and tissues from a materials perspective, considering the mechanical properties and biophysics of individual blood cells and endothelial cells as well as blood cell collectives. We highlight how blood vessels provide a mechanosensitive barrier between blood and tissues and how changes in vessel stiffness and flow shear stress can be correlated to plaque formation and exploited for the design of vascular grafts. We discuss the effect of the properties of fibrin on blood clotting, and investigate how forces exerted by platelets are correlated to disease. Finally, we hypothesize that blood and vascular cells are constantly establishing a mechanical homeostasis, which, when imbalanced, can lead to hematologic and vascular diseases.

Role of flow-sensitive microRNAs and long noncoding RNAs in vascular dysfunction and atherosclerosis

Atherosclerosis is the primary underlying cause of myocardial infarction, ischemic stroke, and peripheral artery disease. The disease preferentially occurs in arterial regions exposed to disturbed blood flow, in part, by altering expression of flow-sensitive coding- and non-coding genes. In this review, we summarize the role of noncoding RNAs, [microRNAs (miRNAs) and long noncoding RNAs(lncRNAs)], as regulators of gene expression and outline their relationship to the pathogenesis of atherosclerosis. While miRNAs are small noncoding genes that post-transcriptionally regulate gene expression by targeting mRNA transcripts, the lncRNAs regulate gene expression by diverse mechanisms, which are still emerging and incompletely understood. We focused on multiple flow-sensitive miRNAs such as, miR-10a, -19a, -23b, -17~92, -21, -663, -92a, -143/145, -101, -126, -712, -205, and -155 that play a critical role in endothelial function and atherosclerosis by targeting inflammation, cell cycle, proliferation, migration, apoptosis, and nitric oxide signaling. Flow-dependent regulation of lncRNAs is just emerging, and their role in vascular dysfunction and atherosclerosis is unknown. Here, we discuss the flow-sensitive lncRNA STEEL along with other lncRNAs studied in the context of vascular pathophysiology and atherosclerosis such as MALAT1, MIAT1, ANRIL, MYOSLID, MEG3, SENCR, SMILR, LISPR1, and H19. Also discussed is the use of these noncoding RNAs as potential biomarkers and therapeutics to reduce and regress atherosclerosis.

Acquired Resilience: An Evolved System of Tissue Protection in Mammals

This review brings together observations on the stress-induced regulation of resilience mechanisms in body tissues. It is argued that the stresses that induce tissue resilience in mammals arise from everyday sources: sunlight, food, lack of food, hypoxia and physical stresses. At low levels, these stresses induce an organised protective response in probably all tissues; and, at some higher level, cause tissue destruction. This pattern of response to stress is well known to toxicologists, who have termed it hormesis. The phenotypes of resilience are diverse and reports of stress-induced resilience are to be found in journals of neuroscience, sports medicine, cancer, healthy ageing, dementia, parkinsonism, ophthalmology and more. This diversity makes the proposing of a general concept of induced resilience a significant task, which this review attempts. We suggest that a system of stress-induced tissue resilience has evolved to enhance the survival of animals. By analogy with acquired immunity, we term this system 'acquired resilience'. Evidence is reviewed that acquired resilience, like acquired immunity, fades with age. This fading is, we suggest, a major component of ageing. Understanding of acquired resilience may, we argue, open pathways for the maintenance of good health in the later decades of human life.

Endothelial Mechanotransduction, Redox Signaling and the Regulation of Vascular Inflammatory Pathways

The endothelium that lines the interior of blood vessels is directly exposed to blood flow. The shear stress arising from blood flow is “sensed” by the endothelium and is “transduced” into biochemical signals that eventually control vascular tone and homeostasis. Sensing and transduction of physical forces occur via signaling processes whereby the forces associated with blood flow are “sensed” by a mechanotransduction machinery comprising of several endothelial cell elements. Endothelial “sensing” involves converting the physical cues into cellular signaling events such as altered membrane potential and activation of kinases, which are “transmission” signals that cause oxidant production. Oxidants produced are the “transducers” of the mechanical signals? What is the function of these oxidants/redox signals? Extensive data from various studies indicate that redox signals initiate inflammation signaling pathways which in turn can compromise vascular health. Thus, inflammation, a major response to infection or endotoxins, can also be initiated by the endothelium in response to various flow patterns ranging from aberrant flow to alteration of flow such as cessation or sudden increase in blood flow. Indeed, our work has shown that endothelial mechanotransduction signaling pathways participate in generation of redox signals that affect the oxidant and inflammation status of cells. Our goal in this review article is to summarize the endothelial mechanotransduction pathways that are activated with stop of blood flow and with aberrant flow patterns; in doing so we focus on the complex link between mechanical forces and inflammation on the endothelium. Since this “inflammation susceptible” phenotype is emerging as a trigger for pathologies ranging from atherosclerosis to rejection post-organ transplant, an understanding of the endothelial machinery that triggers these processes is very crucial and timely.

Endothelial Cell Dysfunction and the Pathobiology of Atherosclerosis

Dysfunction of the endothelial lining of lesion-prone areas of the arterial vasculature is an important contributor to the pathobiology of atherosclerotic cardiovascular disease. Endothelial cell dysfunction, in its broadest sense, encompasses a constellation of various nonadaptive alterations in functional phenotype, which have important implications for the regulation of hemostasis and thrombosis, local vascular tone and redox balance, and the orchestration of acute and chronic inflammatory reactions within the arterial wall. In this review, we trace the evolution of the concept of endothelial cell dysfunction, focusing on recent insights into the cellular and molecular mechanisms that underlie its pivotal roles in atherosclerotic lesion initiation and progression; explore its relationship to classic, as well as more recently defined, clinical risk factors for atherosclerotic cardiovascular disease; consider current approaches to the clinical assessment of endothelial cell dysfunction; and outline some promising new directions for its early detection and treatment.

Fluid Mechanical Forces and Endothelial Mitochondria: A Bioengineering Perspective

Endothelial cell dysfunction is the hallmark of every cardiovascular disease/condition, including atherosclerosis and ischemia/reperfusion injury. Fluid shear stress acting on the vascular endothelium is known to regulate cell homeostasis. Altered hemodynamics is thought to play a causative role in endothelial dysfunction. The dysfunction is associated with/preceded by mitochondrial oxidative stress. Studies by our group and others have shown that the form and/or function of the mitochondrial network are affected when endothelial cells are exposed to shear stress in the absence or presence of additional physicochemical stimuli. The present review will summarize the current knowledge on the interconnections among intracellular Ca²⁺ - nitric oxide - mitochondrial reactive oxygen species, mitochondrial fusion/fission, autophagy/mitophagy, and cell apoptosis vs. survival. More specifically, it will list the evidence on potential regulation of the above intracellular species and processes by the fluid shear stress acting on the endothelium under either physiological flow conditions or during reperfusion (following a period of ischemia). Understanding how the local hemodynamics affects mitochondrial physiology and the cell redox state may lead to development of novel therapeutic strategies for prevention or treatment of the endothelial dysfunction and, hence, of cardiovascular disease.

The gene expression of human endothelial cells is modulated by subendothelial extracellular matrix proteins: short-term response to laminar shear stress

Vascular surgery for atherosclerosis is confronted by the lack of a suitable bypass material. Tissue engineering strives to produce bio-artificial conduits to provide resistance to thrombosis. The objectives of our study were to culture endothelial cells (EC) on composite assemblies of extracellular matrix proteins, and to evaluate the cellular phenotype under flow. Cell-adhesive assemblies were fabricated on glass slides as combinations of collagen (Co), laminin (LM), and fibronectin (FN), resulting in three samples: Co, Co/LM, and Co/FN. Surface topography, roughness, and wettability were determined. Human saphenous vein EC were harvested from cardiac patients, cultured on the assemblies and submitted to laminar shear stress (SS) of 12 dyn/cm(2) for 40, 80, and 120 min. Cell retention was assessed and qRT-PCR of adhesion genes (VE-cadherin, vinculin, KDR, CD-31 or PECAM-1, β1-integrins) and metabolic genes (t-PA, NF-κB, eNOS and MMP-1) was performed. Quantitative immunofluorescence of VE cadherin, vinculin, KDR, and vonWillebrand factor was performed after 2 and 6 h of flow. Static samples were excluded from shearing. The cells reached confluence with similar growth curves. The cells on Co/LM and Co/FN were resistant to flow up to 120 min but minor desquamation occurred on Co corresponding with temporary downregulation of VE cadherin and vinculin-mRNA and decreased fluorescence of vinculin. The cells seeded on Co/LM initially more upregulated vinculin-mRNA and also the inflammatory factor NF-κB, and the cells plated on Co/FN changed the expression profile minimally in comparison with the static control. Fluorescence of VE cadherin and vonWillebrand factor was enhanced on Co/FN. The cells cultured on Co/LM and Co/FN increased the vinculin fluorescence and expressed more VE cadherin and KDR-mRNA than the cells on Co. The cells plated on Co/FN upregulated the mRNA of VE cadherin, CD-31, and MMP 1 to a greater extent than the cells on Co/LM and they enhanced the fluorescence of VE cadherin, KDR, and vonWillebrand factor. Some of these changes sustained up to 6 h of flow, as confirmed by immunofluorescence. Combined matrices Co/LM and Co/FN seem to be more suitable for EC seeding and retention under flow. Moreover, Co/FN matrix promoted slightly more favorable cellular phenotype than Co/LM under SS of 2-6 h.

Hemodynamic disturbed flow induces differential DNA methylation of endothelial Kruppel-Like Factor 4 promoter in vitro and in vivo

RATIONALE

Hemodynamic disturbed flow (DF) is associated with susceptibility to atherosclerosis. Endothelial Kruppel-Like Factor 4 (KLF4) is an important anti-inflammatory atheroprotective transcription factor that is suppressed in regions of DF.

OBJECTIVE

The plasticity of epigenomic KLF4 transcriptional regulation by flow-mediated DNA methylation was investigated in vitro and in arterial tissue.

METHODS AND RESULTS

To recapitulate dominant flow characteristics of atheroprotected and atherosusceptible arteries, human aortic endothelial cells were subjected to pulsatile undisturbed flow or oscillatory DF containing a flow-reversing phase. Differential CpG site methylation was measured by methylation-specific polymerase chain reaction, bisulfite pyrosequencing, and restriction enzyme-polymerase chain reaction. The methylation profiles of endothelium from disturbed and undisturbed flow sites of adult swine aortas were also investigated. In vitro, DF increased DNA methylation of CpG islands within the KLF4 promoter that significantly contributed to suppression of KLF4 transcription; the effects were mitigated by DNA methyltransferase (DNMT) inhibitors and knockdown of DNMT3A. Contributory mechanisms included DF-induced increase of DNMT3A protein (1.7-fold), DNMT3A enrichment (11-fold) on the KLF4 promoter, and competitive blocking of a myocyte enhancer factor-2 binding site in the KLF4 promoter near the transcription start site. DF also induced DNMT-sensitive propathological expression of downstream KLF4 transcription targets nitric oxide synthase 3, thrombomodulin, and monocyte chemoattractant protein-1. In support of the in vitro findings, swine aortic endothelium isolated from DF regions expressed significantly lower KLF4 and nitric oxide synthase 3, and bisulfite sequencing of KLF4 promoter identified a hypermethylated myocyte enhancer factor-2 binding site.

CONCLUSIONS

Hemodynamics influence endothelial KLF4 expression through DNMT enrichment/myocyte enhancer factor-2 inhibition mechanisms of KLF4 promoter CpG methylation with regional consequences for atherosusceptibility.

Flow-dependent cellular mechanotransduction in atherosclerosis

Atherosclerosis depends on risk factors such as hyperlipidemia, smoking, hypertension and diabetes. Although these risk factors are relatively constant throughout the arterial circulation, atherosclerotic plaques occur at specific sites where flow patterns are disturbed, with lower overall magnitude and complex changes in speed and direction. Research over the past few decades has provided new insights into the cellular mechanisms of force transduction and how mechanical effects act in concert with conventional risk factors to mediate plaque formation and progression. This Commentary summarizes our current understanding of how mechanotransduction pathways synergize with conventional risk factors in atherosclerosis. We attempt to integrate cellular studies with animal and clinical data, and highlight major questions that need to be answered to develop more effective therapies.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Transcriptional regulation of endothelial dysfunction in atherosclerosis: an epigenetic perspective

Atherosclerosis is a progressive human pathology that encompasses several stages of development. Endothelial dysfunction represents an early sign of lesion within the vasculature. A number of risk factors for atherosclerosis, including hyperlipidemia, diabetes, and hypertension, target the vascular endothelium by re-programming its transcriptome. These profound alterations taking place on the chromatin rely on the interplay between sequence specific transcription factors and the epigenetic machinery. The epigenetic machinery, in turn, tailor individual transcription events key to atherogenesis to intrinsic and extrinsic insults dictating the development of atherosclerotic lesions. This review summarizes our current understanding of the involvement of the epigenetic machinery in endothelial injury during atherogenesis.

Altered resting hemodynamics in lower-extremity arteries of individuals with spinal cord injury

OBJECTIVE

To investigate lower-extremity arterial hemodynamics in individuals with spinal cord injury (SCI). We hypothesized that oscillatory shear index would be altered and resting mean shear would be higher in the lower-extremity arteries of SCI.

RESEARCH

Cross-sectional study of men and women with SCIs compared to able-bodied controls.

SUBJECTS

Subjects included 105 ages 18-72 years with American Spinal Injury Association (ASIA) Impairment Scale grades A, B, or C and injury duration at least 5 years. Subjects were matched for age and cardiovascular disease risk factors with 156 able-bodied controls.

METHODS

Diameter and blood velocity were determined with subject at rest via ultrasound in superficial femoral, popliteal, brachial, and carotid arteries. Mean shear, antegrade shear, retrograde shear, and oscillatory shear index were calculated.

RESULTS

Oscillatory shear index was lower in SCI compared to controls for superficial femoral (0.16 ± 0.10 vs. 0.26 ± 0.06, P < 0.01) and popliteal arteries (0.20 ± 0.11 vs. 0.26 ± 0.05, P < 0.01). Mean shear rate was higher in SCI compared to controls for superficial femoral (43.54 ± 28.0 vs. 20.48 ± 13.1/second, P < 0.01) and popliteal arteries (30.43 ± 28.1 vs. 11.68 ± 9.5/second, P < 0.01).

CONCLUSIONS

The altered resting hemodynamics in SCI are consistent with an atheroprotective hemodynamic environment.

Adaptation of endothelial cells to physiologically-modeled, variable shear stress

Endothelial cell (EC) function is mediated by variable hemodynamic shear stress patterns at the vascular wall, where complex shear stress profiles directly correlate with blood flow conditions that vary temporally based on metabolic demand. The interactions of these more complex and variable shear fields with EC have not been represented in hemodynamic flow models. We hypothesized that EC exposed to pulsatile shear stress that changes in magnitude and duration, modeled directly from real-time physiological variations in heart rate, would elicit phenotypic changes as relevant to their critical roles in thrombosis, hemostasis, and inflammation. Here we designed a physiological flow (PF) model based on short-term temporal changes in blood flow observed in vivo and compared it to static culture and steady flow (SF) at a fixed pulse frequency of 1.3 Hz. Results show significant changes in gene regulation as a function of temporally variable flow, indicating a reduced wound phenotype more representative of quiescence. EC cultured under PF exhibited significantly higher endothelial nitric oxide synthase (eNOS) activity (PF: 176.0±11.9 nmol/10⁵ EC; SF: 115.0±12.5 nmol/10⁵ EC, p = 0.002) and lower TNF-a-induced HL-60 leukocyte adhesion (PF: 37±6 HL-60 cells/mm²; SF: 111±18 HL-60/mm², p = 0.003) than cells cultured under SF which is consistent with a more quiescent anti-inflammatory and anti-thrombotic phenotype. In vitro models have become increasingly adept at mimicking natural physiology and in doing so have clarified the importance of both chemical and physical cues that drive cell function. These data illustrate that the variability in metabolic demand and subsequent changes in perfusion resulting in constantly variable shear stress plays a key role in EC function that has not previously been described.

Inflammation in atherosclerosis: a cause or a result of vascular disorders?

Sound data support the concept that in atherosclerosis, inflammation and dyslipidemia intersect each other and that irrespective of the initiator, both participate from the early stages to the ultimate fate of the atheromatous plaque. The two partakers manoeuvre a vicious circle in atheroma formation: dyslipidaemia triggers an inflammatory process and inflammation elicits dyslipidaemia. Independent of the initial cause, the atherosclerotic lesions occur focally, in particular arterial-susceptible sites, by a process that, although continuous, can be arbitrarily divided into a sequence of consecutive stages that lead from fatty streak to the fibro-lipid plaque and ultimately to plaque rupture and thrombosis. In the process, the initial event is a change in endothelial cells (EC) constitutive properties. Then, the molecular alarm signals send by dysfunctional EC are decoded by specific blood immune cells (monocytes, T lymphocytes, neutrophils, mast cells) and by the resident vascular cells, that respond by initiating a robust inflammatory process, in which the cells and the factors they secrete hasten the atheroma development. Direct and indirect crosstalk between the cells housed within the nascent plaque, complemented by the increase in risk factors of atherosclerosis lead to atheroma development and outcome. The initial inflammatory response can be regarded as a defense/protective reaction mechanism, but its further amplification, speeds up atherosclerosis. In this review, we provide an overview on the role of inflammation and dyslipidaemia and their intersection in atherogenesis. The data may add to the foundation of a novel attitude in the diagnosis and treatment of atherosclerosis.

Lessons from the endothelial junctional mechanosensory complex

Mechanotransduction plays a key role in both normal physiology and in diseases such as cancer, atherosclerosis and hypertension. Nowhere is this more evident than in the vascular system, where fluid shear stress from blood flow plays a critical role in shaping the blood vessels and in determining their function and dysfunction. Responses to flow are mediated in part by a complex of proteins comprised of PECAM-1, VE-cadherin and VEGFR2 at endothelial cell-cell junctions; all proteins that clearly have other, non-mechanical functions. We review recent progress toward understanding the functions and mechanisms of mechanotransduction by this complex and suggest some principles that may apply more broadly.

Endothelial expression of human ABCA1 in mice increases plasma HDL cholesterol and reduces diet-induced atherosclerosis

The role of endothelial ABCA1 expression in reverse cholesterol transport (RCT) was examined in transgenic mice, using the endothelial-specific Tie2 promoter. Human ABCA1 (hABCA1) was significantly expressed in endothelial cells (EC) of most tissues except the liver. Increased expression of ABCA1 was not observed in resident peritoneal macrophages. ApoA-I-mediated cholesterol efflux from aortic EC was 2.6-fold higher (P < 0.0001) for cells from transgenic versus control mice. On normal chow diet, Tie2 hABCA1 transgenic mice had a 25% (P < 0.0001) increase in HDL-cholesterol (HDL-C) and more than a 2-fold increase of eNOS mRNA in the aorta (P < 0.04). After 6 months on a high-fat, high-cholesterol (HFHC) diet, transgenic mice compared with controls had a 40% increase in plasma HDL-C (P < 0.003) and close to 40% decrease in aortic lesions (P < 0.02). Aortas from HFHC-fed transgenic mice also showed gene expression changes consistent with decreased inflammation and apoptosis. Beneficial effects of the ABCA1 transgene on HDL-C levels or on atherosclerosis were absent when the transgene was transferred onto ApoE or Abca1 knockout mice. In summary, expression of hABCA1 in EC appears to play a role in decreasing diet-induced atherosclerosis in mice and is associated with increased plasma HDL-C levels and beneficial gene expression changes in EC.

Mechanics and Function of the Pulmonary Vasculature: Implications for Pulmonary Vascular Disease and Right Ventricular Function

The relationship between cardiac function and the afterload against which the heart muscle must work to circulate blood throughout the pulmonary circulation is defined by a complex interaction between many coupled system parameters. These parameters range broadly and incorporate system effects originating primarily from three distinct locations: input power from the heart, hydraulic impedance from the large conduit pulmonary arteries, and hydraulic resistance from the more distal microcirculation. These organ systems are not independent, but rather, form a coupled system in which a change to any individual parameter affects all other system parameters. The result is a highly nonlinear system which requires not only detailed study of each specific component and the effect of disease on their specific function, but also requires study of the interconnected relationship between the microcirculation, the conduit arteries, and the heart in response to age and disease. Here, we investigate systems-level changes associated with pulmonary hypertensive disease progression in an effort to better understand this coupled relationship.

Endothelial metallothionein expression and intracellular free zinc levels are regulated by shear stress

We examined the effects of fluid shear stress on metallothionein (MT) gene and protein expression and intracellular free zinc in mouse aorta and in human umbilical vein endothelial cells (HUVECs). Immunostaining of the endothelial surface of mouse aorta revealed increased expression of MT protein in the lesser curvature of the aorta relative to the descending thoracic aorta. HUVECs were exposed to high steady shear stress (15 dyn/cm(2)), low steady shear stress (1 dyn/cm(2)), or reversing shear stress (mean of 1 dyn/cm(2), 1 Hz) for 24 h. Gene expression of three MT-1 isoforms, MT-2A, and zinc transporter-1 was upregulated by low steady shear stress and reversing shear stress. HUVECs exposed to 15 dyn/cm(2) had increased levels of free zinc compared with cells under other shear stress regimes and static conditions. The increase in free zinc was partially blocked with an inhibitor of nitric oxide synthesis, suggesting a role for shear stress-induced endothelial nitric oxide synthase activity. Cells subjected to reversing shear stress in zinc-supplemented media (50 μM ZnSO(4)) had increased intracellular free zinc, reduced surface intercellular adhesion molecule-1 expression, and reduced monocyte adhesion compared with cells exposed to reversing shear stress in normal media. The sensitivity of intracellular free zinc to differences in shear stress suggests that intracellular zinc levels are important in the regulation of the endothelium and in the progression of vascular disease.

Infection of the endothelium by members of the order Rickettsiales

The vascular endothelium is the main target of a limited number of infectious agents, Rickettsia, Ehrlichia ruminantium, and Orientia tsutsugamushi are among them. These arthropod-transmitted obligately-intracellular bacteria cause serious systemic diseases that are not infrequently lethal. In this review, we discuss the bacterial biology, vector biology, and clinical aspects of these conditions with particular emphasis on the interactions of these bacteria with the vascular endothelium and how it responds to intracellular infection. The study of these bacteria in relevant in vivo models is likely to offer new insights into the physiology of the endothelium that have not been revealed by other models.

Endothelial cell responses to atheroprone flow are driven by two separate flow components: low time-average shear stress and fluid flow reversal

To simulate the effects of shear stress in regions of the vasculature prone to developing atherosclerosis, we subjected human umbilical vein endothelial cells to reversing shear stress to mimic the hemodynamic conditions at the wall of the carotid sinus, a site of complex, reversing blood flow and commonly observed atherosclerosis. We compared the effects of reversing shear stress (time-average: 1 dyn/cm(2), maximum: +11 dyn/cm(2), minimum: -11 dyn/cm(2), 1 Hz), arterial steady shear stress (15 dyn/cm(2)), and low steady shear stress (1 dyn/cm(2)) on gene expression, cell proliferation, and monocyte adhesiveness. Microarray analysis revealed that most differentially expressed genes were similarly regulated by all three shear stress regimens compared with static culture. Comparisons of the three shear stress regimens to each other identified 138 genes regulated by low average shear stress and 22 genes regulated by fluid reversal. Low average shear stress induced increased cell proliferation compared with high shear stress. Only reversing shear stress exposure induced monocyte adhesion. The adhesion of monocytes was partially inhibited by the incubation of endothelial cells with ICAM-1 blocking antibody. Increased heparan sulfate proteoglycan expression was observed on the surface of cells exposed to reversing shear stress. Heparinase III treatment significantly reduced monocyte adhesion. Our results suggest that low steady shear stress is the major impetus for differential gene expression and cell proliferation, whereas reversing flow regulates monocyte adhesion.

Application of multiphase computational fluid dynamics to analyze monocyte adhesion

Study of the mechanisms of monocyte adhesion initiating atheroslerotic lesions has engaged investigators for decades. Single-phase computational fluid dynamics (CFD) analyses fail to account for particulate migration. Consequently, inconsistencies arise when correlating adhesion with wall shear stress (WSS). The purpose of this paper is to present, to our knowledge, the first computational analysis of in vitro U937 monocyte-like human cell adhesion data using a coupled multiphase CFD-population balance adhesion model. The CFD model incorporates multiphase non-Newtonian hemodynamic models to compute the spatial distributions of freely flowing monocytes and WSSs in control volumes adjacent to the wall. Measurements of monocyte adhesion onto an E-selectin-coated flow model that included an idealized stenosis and an abrupt expansion were available from the literature. In this study, we develop a new monolayer population balance adhesion model, based on the widely accepted mechanism of ligand-receptor binding, coupled to the CFD results. The monolayer population balance model accounts for the interactions of freely flowing, rolling, and adhering monocytes with surfaces via first-order reactions, transport of rolling cells in the monolayer, and the concept of a WSS detachment threshold, clearly evident in the adhesion experiments. The new paradigm of coupling the multiphase hemodynamic CFD model with the proposed adhesion model is illustrated by determining and interpreting the model parameters for experimental datasets having Reynolds numbers of 100 and 140. The coupled multiphase CFD adhesion model is able to simultaneously predict the spatial variations in freely flowing monocytes, their adherent number density, and carrier fluid WSSs adjacent to ligand-coated flow cell surfaces.

Expression of CYP1A1 and CYP1B1 in human endothelial cells: regulation by fluid shear stress

AIMS

CYP1A1 and CYP1B1, members of the cytochrome P450 protein family, are regulated by fluid shear stress. This study describes the effects of duration, magnitude and pattern of shear stress on CYP1A1 and CYP1B1 expressions in human endothelial cells, towards the goal of understanding the role(s) of these genes in pro-atherogenic or anti-atherogenic endothelial cell functions.

METHODS AND RESULTS

We investigated CYP1A1 and CYP1B1 expressions under different durations, levels, and patterns of shear stress. CYP1A1 and CYP1B1 mRNA, protein, and enzymatic activity were maximally up-regulated at > or =24 h of arterial levels of shear stress (15-25 dynes/cm2). Expression of both genes was significantly attenuated by reversing shear stress when compared with 15 dynes/cm2 steady shear stress. Small interfering RNA knockdown of CYP1A1 resulted in significantly reduced CYP1B1 and thrombospondin-1 expression, genes regulated by the aryl hydrocarbon receptor (AhR). Immunostaining of human coronary arteries showed constitutive CYP1A1 and CYP1B1 protein expressions in endothelial cells. Immunostaining of mouse aorta showed nuclear localization of AhR and increased expression of CYP1A1 in the descending thoracic aorta, whereas reduced nuclear localization of AhR and attenuated CYP1A1 expression were observed in the lesser curvature of the aortic arch.

CONCLUSION

CYP1A1 and CYP1B1 gene and protein expressions vary with time, magnitude, and pattern of shear stress. Increased CYP1A1 gene expression modulates AhR-regulated genes. Based on our in vitro reversing flow data and in vivo immunostained mouse aorta, we suggest that increased expression of both genes reflects an anti-atherogenic endothelial cell phenotype.

Lung ischemia: a model for endothelial mechanotransduction

Endothelial cells in vivo are constantly exposed to shear associated with blood flow and altered shear stress elicits cellular responses (mechanotransduction). This review describes the role of shear sensors and signal transducers in these events. The major focus is the response to removal of shear as occurs when blood flow is compromised (i.e., ischemia). Pulmonary ischemia studied with the isolated murine lung or flow adapted pulmonary microvascular endothelial cells in vitro results in endothelial generation of reactive oxygen species (ROS) and NO. The response requires caveolae and is initiated by endothelial cell depolarization via K(ATP) channel closure followed by activation of NADPH oxidase (NOX2) and NO synthase (eNOS), signaling through MAP kinases, and endothelial cell proliferation. These physiological mediators can promote vasodilation and angiogenesis as compensation for decreased tissue perfusion.