Gene expression profiling of human aortic endothelial cells exposed to disturbed flow and steady laminar flow

Effects of disturbed flow on vascular endothelium: pathophysiological basis and clinical perspectives

Vascular endothelial cells (ECs) are exposed to hemodynamic forces, which modulate EC functions and vascular biology/pathobiology in health and disease. The flow patterns and hemodynamic forces are not uniform in the vascular system. In straight parts of the arterial tree, blood flow is generally laminar and wall shear stress is high and directed; in branches and curvatures, blood flow is disturbed with nonuniform and irregular distribution of low wall shear stress. Sustained laminar flow with high shear stress upregulates expressions of EC genes and proteins that are protective against atherosclerosis, whereas disturbed flow with associated reciprocating, low shear stress generally upregulates the EC genes and proteins that promote atherogenesis. These findings have led to the concept that the disturbed flow pattern in branch points and curvatures causes the preferential localization of atherosclerotic lesions. Disturbed flow also results in postsurgical neointimal hyperplasia and contributes to pathophysiology of clinical conditions such as in-stent restenosis, vein bypass graft failure, and transplant vasculopathy, as well as aortic valve calcification. In the venous system, disturbed flow resulting from reflux, outflow obstruction, and/or stasis leads to venous inflammation and thrombosis, and hence the development of chronic venous diseases. Understanding of the effects of disturbed flow on ECs can provide mechanistic insights into the role of complex flow patterns in pathogenesis of vascular diseases and can help to elucidate the phenotypic and functional differences between quiescent (nonatherogenic/nonthrombogenic) and activated (atherogenic/thrombogenic) ECs. This review summarizes the current knowledge on the role of disturbed flow in EC physiology and pathophysiology, as well as its clinical implications. Such information can contribute to our understanding of the etiology of lesion development in vascular niches with disturbed flow and help to generate new approaches for therapeutic interventions.

Endothelialization of PVA/gelatin cryogels for vascular tissue engineering: effect of disturbed shear stress conditions

Mechanically, poly(vinyl alcohol) (PVA)-based cryogels are extremely well suited for vascular tissue engineering applications. However, their surface properties lead to a slow rate of endothelialization, and the mode of cell attachment leaves the endothelium susceptible to removal under physiological shear stress conditions. In this study, abrupt and ramped disturbed shear stress conditions created by a turbulent orbital flow were used to examine endothelialization on PVA/gelatin cryogels. Cell proliferation rate and apoptosis were evaluated by fluorescent activated cell sorter (FACS) analysis, and the expression of cell-adhesion molecules was used to evaluate the response of cells on cryogels to static and shear conditions by real-time polymerase chain reaction (RT-PCR). Application of a ramped shear stress had a profound effect on endothelial cell proliferation (22.30 +/- 0.20-fold increase), necrosis (eliminated), apoptosis (1.04 +/- 0.18 increase), and overall facilitation of endothelialization while concomitantly increasing nitric oxide (NO) synthesis levels. Ramped shear stress was also effective in helping the retention of the endothelial cells on the cryogel surface, whereas abrupt application caused widespread removal. Under static conditions, Selectin-P expression decreased, whereas both inter-cellular adhesion molecule (ICAM) and platelet endothelial cell adhesion molecule (PECAM)-I expression increased on cryogels over a 10-day culture period. Under both shear stress conditions, Selectin-P expression was decreased both on cryogels and tissue culture polystyrene (TCPS). Controlled application of disturbed shear stress shortens endothelialization times on cryogel surfaces, in contrast to the established antiproliferative effect of shear stress caused by laminar flow, without compromising their functionality. This demonstrates how such mechanical stimuli can be exploited to alter cellular behavior and facilitate the required outcomes for tissue engineering applications.

A defect in Δ6 and Δ5 desaturases may be a factor in the initiation and progression of insulin resistance, the metabolic syndrome and ischemic heart disease in South Asians

The high incidence of insulin resistance and the metabolic syndrome in South Asians remains unexplained. I propose that a defect in the activity of Δ⁶ and Δ⁵ desaturases and consequent low plasma and tissue concentrations of polyunsaturated fatty acids such as γ-linolenic acid (GLA), dihomo-γ-linolenic acid (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and formation of their anti-inflammatory products prostaglandin E₁ (PGE₁), prostacyclin (PGI₂), PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could be responsible for the high incidence of insulin resistance, the metabolic syndrome and ischemic heart disease (IHD) in South Asians. This proposal is supported by the observation that South Asian Indians have lower plasma and tissue concentrations of GLA, DGLA, AA, EPA and DHA, the precursors of PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, and nitrolipids, the endogenous molecules that prevent platelet aggregation, vasoconstriction, thrombus formation, leukocyte activation and possess anti-inflammatory action and thus, are capable of preventing the development of insulin resistance, atherosclerosis, hypertension, type 2 diabetes mellitus and premature ischemic heart disease. Genetic predisposition, high carbohydrate intake, lack of exercise, tobacco use and low birth weight due to maternal malnutrition suppress the activity of Δ⁶ and Δ⁵ desaturases that leads to low plasma and tissue concentrations of polyunsaturated fatty acids and their products. This implies that adequate provision of polyunsaturated fatty acids and co-factors needed for their metabolism, and efforts to enhance the formation of their beneficial metabolites PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could form a novel approach in the prevention and management of these diseases in this high-risk population.

Discovery of novel mechanosensitive genes in vivo using mouse carotid artery endothelium exposed to disturbed flow

Recently, we showed that disturbed flow caused by a partial ligation of mouse carotid artery rapidly induces atherosclerosis. Here, we identified mechanosensitive genes in vivo through a genome-wide microarray study using mouse endothelial RNAs isolated from the flow-disturbed left and the undisturbed right common carotid artery. We found 62 and 523 genes that changed significantly by 12 hours and 48 hours after ligation, respectively. The results were validated by quantitative polymerase chain reaction for 44 of 46 tested genes. This array study discovered numerous novel mechanosensitive genes, including Lmo4, klk10, and dhh, while confirming well-known ones, such as Klf2, eNOS, and BMP4. Four genes were further validated for protein, including LMO4, which showed higher expression in mouse aortic arch and in human coronary endothelium in an asymmetric pattern. Comparison of in vivo, ex vivo, and in vitro endothelial gene expression profiles indicates that numerous in vivo mechanosensitive genes appear to be lost or dysregulated during culture. Gene ontology analyses show that disturbed flow regulates genes involved in cell proliferation and morphology by 12 hours, followed by inflammatory and immune responses by 48 hours. Determining the functional importance of these novel mechanosensitive genes may provide important insights into understanding vascular biology and atherosclerosis.

Genomics of premature atherosclerotic vascular disease

Atherosclerotic vascular disease is a systemic process with a common pathophysiology but with different disease manifestations depending on the vascular site. Over the past two decades, significant efforts have gone toward determining the genomic factors contributing to atherosclerotic vascular disease. Substantial information has been generated regarding the genomics of atherosclerotic coronary heart disease, and recently, several genomic analyses have looked at the cerebrovascular and peripheral vascular beds. This article reviews genomic investigations of atherosclerotic vascular disease in the coronary, cerebrovascular, and peripheral arteries. In this review, we have tried to restrict the discussion to studies of premature atherosclerosis, particularly those using non-biased genomic techniques.

The response of human aortic endothelial cells in a stenotic hemodynamic environment: effect of duration, magnitude, and spatial gradients in wall shear stress

Inflammation plays a key role in the development and stability of coronary plaques. Endothelial cells alter their expression in response to wall shear stress (WSS). Straight/tubular and asymmetric stenosis models were designed to study the localized expression of atheroprone molecules and inflammatory markers due to the presence of the spatial wall shear stress gradients created by an eccentric plaque. The effects of steady wall shear stress duration (0-24 h) and magnitude (4.5-18 dynes/cm(2)) were analyzed in human abdominal aortic endothelial cells through quantitative real-time polymerase chain reaction (PCR) and immunofluorescence analysis in straight/tubular models. Regional expression was assessed by immunofluorescence and confocal microscopy in stenosis models. Under steady fully developed flow, endothelial cells exhibited a sustained increase in levels of atheroprotective genes with WSS duration and magnitude. The local response in the stenosis model showed that expression of endothelial nitric oxide synthase and Kruppel-like factor 2 is magnitude rather than gradient dependent. A WSS magnitude dependent transient increase in translocation of transcription factor nuclear factor kappaB was observed. Intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and E-selectin exhibited a sustained increase in protein expression with time. The mRNA levels of these molecules were transiently upregulated and this was followed by a decrease in expression to levels lower than static controls. Regionally, increased inflammatory marker expression was observed in regions of WSS gradients both proximal and distal to the stenosis when compared with the uniform flow regions, whereas the atheroprotective markers were expressed to a greater extent in regions of elevated WSS magnitudes. The results from the straight/tubular model cannot explain the regional variation seen in the stenosis models. This may help explain the localization of inflammatory cells at the shoulders of plaques in vivo.

Endothelial shear stress: a critical determinant of arterial remodeling and arterial stiffness in humans--a carotid 3.0-T MRI study

BACKGROUND

Low endothelial shear stress (ESS) elicits endothelial dysfunction. However, the relationship between ESS and arterial remodeling and arterial stiffness is unknown in humans. We developed a 3.0-T MRI protocol to evaluate the contribution of ESS to arterial remodeling and stiffness.

METHODS AND RESULTS

Fifteen young (aged 26 ± 3 years) and 15 older (aged 57 ± 3 years) healthy volunteers as well as 15 patients with cardiovascular disease (aged 63 ± 10 years) were enrolled. Phase-contrast MRI of the common carotid arteries was used to derive ESS data from the spatial velocity gradients close to the arterial wall. ESS measurements were performed on 3 occasions and showed excellent reproducibility (intraclass correlation coefficient, 0.79). Multiple linear regression analysis accounting for age and blood pressure revealed that ESS was an independent predictor of the following response variables: carotid wall thickness (regression coefficient [b], -0.19 mm(2) per N/m(2); P=0.02), lumen area (b, -15.5 mm(2) per N/m(2); P<0.001), and vessel size (b, -24.0 mm(2) per N/m(2); P<0.001). Segments of the artery wall exposed to lower ESS were significantly thicker than segments exposed to higher ESS within the same artery (P=0.009). Furthermore, ESS was associated with arterial compliance, accounting for age, blood pressure, and wall thickness (b, -0.003 mm(2)/mm Hg per N/m(2); P=0.04).

CONCLUSIONS

Our carotid MRI data show that ESS is an important determinant of arterial remodeling and arterial stiffness in humans. The data warrant further studies to evaluate use of carotid ESS as a noninvasive tool to improve the understanding of individual cardiovascular disease risk and to assess novel drug therapies in cardiovascular disease prevention.

The critical role of hemodynamics in the development of cerebral vascular disease

Atherosclerosis and intracranial saccular aneurysms predictably localize in areas with complex arterial geometries such as bifurcations and curvatures. These sites are characterized by unique hemodynamic conditions that possibly influence the risk for these disorders. One hemodynamic parameter in particular has emerged as a key regulator of vascular biology—wall shear stress (WSS). Variations in geometry can change the distribution and magnitude of WSS, thus influencing the risk for vascular disorders. Computer simulations conducted using patient-specific data have suggested that departures from normal levels of WSS lead to aneurysm formation and progression. In addition, multiple studies indicate that disturbed flow and low WSS predispose patients to extracranial atherosclerosis, and particularly to carotid artery disease. Conversely, in the case of intracranial atherosclerosis, more studies are needed to provide a firm link between hemodynamics and atherogenesis. The recognition of WSS as an important factor in cerebral vascular disease may help to identify individuals at risk and guide treatment options.

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.

HUVEC ICAM-1 and VCAM-1 synthesis in response to potentially athero-prone and athero-protective mechanical and nicotine chemical stimuli

Previous mechano-transduction studies have investigated the endothelial cell (EC) morphological response to mechanical stimuli; generally consisting of a wall shear stress (WSS) and a cyclic tensile hoop strain (THS). More recent studies have investigated the EC biochemical response (intercellular adhesion molecule, ICAM-1, and vascular cellular adhesion molecule, VCAM-1, expression) to idealized mechanical stimuli. However, current literature is lacking in the area of EC biochemical response to combinations of physiological WSS and THS mechanical stimuli. The objective of this study is to investigate the EC response to physiological WSS and THS stimuli and to compare this response to that of ECs exposed to idealized steady WSS and cyclic THS of the same magnitudes. This study also investigated the EC response to a nicotine chemical stimulus combined with a suspected athero-prone physiological mechanical stimulus. A bioreactor was designed to apply a range of combinations of physiological WSS and THS waveforms. The bioreactor was calibrated and validated using computational fluid dynamics and video extensometry techniques. The bioreactor was used to investigated the biochemical response exhibited by human umbilical vein endothelial cells (HUVECs) exposed to physiological athero-protective (first bioreactor test case, pulsatile WSS combined with pulsatile THS) and athero-prone (second bioreactor test case, oscillating WSS combined with pulsatile THS) mechanical environments. The final testing environment (third bioreactor test case) combined a nicotine chemical stimulus with the mechanical stimuli of the second bioreactor test case. In first and second bioreactor test cases, the addition of a pulsatile THS to the WSS resulted in opposite trends of ICAM-1 down-regulation and up-regulation, respectively. This outcome suggests that the effect of the additional pulsatile THS depends on the state of the applied WSS waveform. Similarly, in first and second bioreactor test cases, the addition of a pulsatile THS to the WSS resulted in a VCAM-1 up-regulation. However, it has been previously shown that the addition of a cyclic THS to a high- or low-steady WSS resulted in a VCAM-1 down-regulation, indicating that the EC response to idealized mechanical stimuli (steady WSS and cyclic THS) is not comparable to physiological mechanical stimuli (unsteady WSS and pulsatile THS), even though in both situations the average magnitude of WSS and THS applied were similar. In third bioreactor test case, a nicotine chemical stimulus induced a substantial VCAM-1 up-regulation and a moderate ICAM-1 up-regulation. The addition of the mechanical stimuli of the second bioreactors test case resulted in a greater VCAM-1 up-regulation than what was expected, considering the observations of the previous second bioreactor test case alone. This study found that the EC biochemical response to physiological mechanical stimuli is not comparable to the previously observed EC response to idealized mechanical stimuli, even though in both environments the mechanical stimuli were of a similar magnitude. Also, the level of VCAM-1 expressed by the nicotine stimulated ECs showed an elevated level of sensitivity to the athero-prone mechanical stimuli.

Characterization of umbilical cord blood-derived late outgrowth endothelial progenitor cells exposed to laminar shear stress

Endothelial progenitor cells isolated from umbilical cord blood (CB-EPCs) represent a promising source of endothelial cells for synthetic vascular grafts and tissue-engineered blood vessels since they are readily attainable, can be easily isolated, and possess a high proliferation potential. The objective of this study was to compare the functional behavior of late outgrowth CB-EPCs with human aortic endothelial cells (HAECs). CB-EPCs and HAECs were cultured on either smooth muscle cells in a coculture model of a tissue-engineered blood vessels or fibronectin adsorbed to Teflon-AF-coated glass slides. Late outgrowth CB-EPCs expressed endothelial cell-specific markers and were negative for the monocytic marker CD14. CB-EPCs have higher proliferation rates than HAECs, but are slightly smaller in size. CB-EPCs remained adherent under supraphysiological shear stresses, oriented and elongated in the direction of flow, and expressed similar numbers of alpha(5)beta(1) and alpha(v)beta(3) integrins and antithrombotic genes compared to HAECs. There were some differences in mRNA levels of E-selectin and vascular cell adhesion molecule 1 between CB-EPCs and HAECs; however, protein levels were similar on the two cell types, and CB-EPCs did not support adhesion of monocytes in the absence of tumor necrosis factor-alpha stimulation. Although CB-EPCs expressed significantly less endothelial nitric oxide synthase protein after exposure to flow than HAECs, nitric oxide levels induced by flow were not significantly different. These results suggest that late outgrowth CB-EPCs are functionally similar to HAECs under flow conditions and are a promising cell source for cardiovascular therapies.

Regional gene expression of LOX-1, VCAM-1, and ICAM-1 in aorta of HIV-1 transgenic rats

Background

Increased prevalence of atherosclerotic cardiovascular disease in HIV-infected patients has been observed. The cause of this accelerated atherosclerosis is a matter of controversy. As clinical studies are complicated by a multiplicity of risk-factors and a low incidence of hard endpoints, studies in animal models could be attractive alternatives.

Methodology/Principal Findings

We evaluated gene expression of lectin-like oxidized-low-density-lipoprotein receptor-1 (LOX-1), vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1) in HIV-1 transgenic (HIV-1Tg) rats; these genes are all thought to play important roles in early atherogenesis. Furthermore, the plasma level of sICAM-1 was measured. We found that gene expressions of LOX-1 and VCAM-1 were higher in the aortic arch of HIV-1Tg rats compared to controls. Also, the level of sICAM-1 was elevated in the HIV-1Tg rats compared to controls, but the ICAM-1 gene expression profile did not show any differences between the groups.

Conclusions/Significance

HIV-1Tg rats have gene expression patterns indicating endothelial dysfunction and accelerated atherosclerosis in aorta, suggesting that HIV-infection per se may cause atherosclerosis. This transgenic rat model may be a very promising model for further studies of the pathophysiology behind HIV-associated cardiovascular disease.

Cytoskeletal structure regulates endothelial cell immunogenicity independent of fluid shear stress

The cardiovascular disease atherosclerosis is directly linked to the functions of endothelial cells (ECs), which are affected by fluid shear stress (FSS). High, unidirectional FSS causes EC elongation with aligned cytoskeletal components and nonimmunogenic EC functions that protect against atherosclerosis. In contrast, low, oscillatory FSS is associated with cobblestone-shaped ECs with randomly oriented cytoskeletons and proinflammatory EC functions that promote atherosclerosis. Whether EC shape plays a role in EC immunogenic functions, independent of FSS, has not been previously determined. The goal of this study was to determine the effect of EC elongation and cytoskeletal alignment on the expression of inflammatory genes and functions. With the use of micropatterned lanes, EC elongation and cytoskeletal alignment were achieved in the absence of FSS. EC gene expression of key inflammation markers determined that the elongation and cytoskeletal alignment of micropattern-elongated ECs (MPECs) alone significantly downregulated VCAM-1 while having no effect on E-selectin and ICAM-1. The positive control of FSS-elongated ECs promoted E-selectin and VCAM-1 downregulation and upregulation of ICAM-1. Functionally, monocytic U937 cells formed weaker interactions on the surface of MPECs compared with cobblestone ECs. Interestingly, MPEC expression of the known FSS-dependent transcription factor krüppel-like factor 2 (KLF2), which promotes a nonimmunogenic EC phenotype, was significantly upregulated in MPECs compared with cobblestone ECs. Cytoskeletal regulation of KLF2 expression was shown to be dependent on microtubules. Therefore, the cellular elongation and cytoskeletal alignment of MPECs regulated immunogenic gene expression and functions and may act synergistically with FSS to create an EC surface with reduced inflammatory capability.

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.

Systems analysis of the role of bone morphogenic protein 4 in endothelial inflammation

Shear stress is an important factor in the onset and progression of atherosclerosis. High and unidirectional laminar stress is seen as protective, while low and oscillatory shear stress is considered pro-inflammatory and pro-atherogenic. The mechanosensitive response of endothelial cells is governed by a complex system of genes, proteins, and signals that operate at distinctly different time scales. We propose a dynamic mathematical model that quantitatively describes this mechanosensing system and permits novel insights into its functioning. The model, the first of its kind, is constructed within the guidelines of Biochemical Systems Theory and accounts for different time scales by means of approximated delays. Parameter values are obtained directly from biochemical observations in an ad hoc fashion. The model reflects most documented observations well and leads to a number of predictions and novel hypotheses. In particular, it demonstrates the crucial role of Bone Morphogenic Protein 4 and p47(phox)-dependent NADPH oxidases in endothelial inflammation.

Increased synthetic phenotype behavior of smooth muscle cells in response to in vitro balloon angioplasty injury model

Restenosis remains a common problem following balloon angioplasty, and it has been speculated that changes in the mechanical environment due to endovascular interventions are correlated with shifts in smooth muscle cell (SMC) phenotype. In order to study SMC response to forces similar to those exerted during balloon angioplasty, an in vitro concurrent shear and tensile forces simulator has been developed. After 24 hr of exposure to cyclic tension (5%) and shear (0.1-0.5 dynes/cm(2)) following simulated angioplasty injury (12% stretch), rat aortic SMCs exhibited significant synthetic behavior. These responses included increased cell proliferation, apoptosis, and cell hypertrophy compared to cells exposed to strain alone. While all SMCs exposed to dynamic stimuli (strain, strain+balloon injury, strain+balloon injury+shear) demonstrated a decrease in contractile protein expression, the injury group also exhibited significantly greater expression of the synthetic marker vimentin. These in vitro findings agree with in vivo events following balloon angioplasty and present a refined dynamic model to be implemented for better understanding of SMC activation and prevention of responses through pharmacological treatment.

Shear stress inhibits homocysteine-induced stromal cell-derived factor-1 expression in endothelial cells

RATIONALE

Hyperhomocysteinemia contributes to vascular dysfunction and risks of cardiovascular diseases. Stromal cell-derived factor (SDF)-1, a chemokine expressed by endothelial cells (ECs), is highly expressed in advanced atherosclerotic lesions. The interplays among homocysteine, chemokines, and shear stress in regulating vascular endothelial function are not clearly understood.

OBJECTIVE

To investigate the mechanisms for modulations of EC SDF-1 expression by homocysteine and shear stress.

METHODS AND RESULTS

Homocysteine stimulation induced dose- and time-dependent SDF-1 expression and phosphorylation of mitogen-activated protein kinases extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. By using specific inhibitors, small interfering (si)RNA, and dominant negative mutants, we demonstrated that activation of JNK pathway is critical for the homocysteine-induced SDF-1 expression. Transcription factor ELISA and chromatin immunoprecipitation assays showed that homocysteine increased Sp1- and AP-1-DNA binding activities in ECs. Inhibition of Sp1 and AP-1 activations by specific siRNA blocked the homocysteine-induced SDF-1 promoter activity and expression. Preshearing of ECs for 1 to 4 hours at 20 dyn/cm2 inhibited the homocysteine-induced JNK phosphorylation, Sp1 and AP-1 activation, and SDF-1 expression. The homocysteine-induced SDF-1 expression was suppressed by NO donor. Inhibitor or siRNA for endothelial NO synthase abolished the shear inhibition of SDF-1 expression.

CONCLUSIONS

Our findings serve to elucidate the molecular mechanisms underlying the homocysteine induction of SDF-1 expression in ECs and the shear stress protection against this induction.

Upregulation of glutathione peroxidase offsets stretch-induced proatherogenic gene expression in human endothelial cells

OBJECTIVE

Localization of atherosclerotic plaques typically correlates with areas of biomechanical strain where shear stress is decreased while stretch, thought to promote atherogenesis through enhanced oxidative stress, is increased.

METHODS AND RESULTS

In human cultured endothelial cells, nitric oxide synthase expression was exclusively shear stress-dependent whereas expression of glutathione peroxidase-1 (GPx-1), but not that of Cu(2+)/Zn(2+)-superoxide dismutase or Mn(2+)-superoxide dismutase, was upregulated solely in response to cyclic stretch. GPx-1 expression was also enhanced in isolated mouse arteries perfused at high pressure. Combined pharmacological and decoy oligodeoxynucleotide blockade revealed that activation of p38 MAP kinase followed by nuclear translocation of CCAAT/enhancer binding protein plays a pivotal role in stretch-induced GPx-1 expression in human endothelial cells. Antisense oligodeoxynucleotide knockdown of GPx-1 reinforced both their capacity to generate hydrogen peroxide and the transient stretch-induced expression of CD40, monocyte chemoatractant protein-1, and vascular cell adhesion molecule-1. Consequently, THP-1 monocyte adhesion to the GPx-1-depleted cells was augmented.

CONCLUSIONS

Stretch-induced proatherosclerotic gene expression in human endothelial cells seems to be hydrogen peroxide-mediated. The concomitant rise in GPx-1 expression, but not that of other antioxidant enzymes, may comprise an adaptive mechanism through which the cells maintain their antiatherosclerotic properties in spite of a decreased bioavailability of nitric oxide.

Can eccentric arterial plaques alone cause flow stagnation points and favour thrombus incorporation?

We have used an experimental model of aorta stenosis, with a Plexiglas plug, simulating a stable atheromatous plaque that promotes local turbulence and thrombosis. With animal survival of more than 24 h, we followed the partial fibrinolysis of the thrombus as well as its posterior organization and incorporation to the arterial wall as a neointima for up to 30 days. The mushroom plug form permitted the development of recirculation and stasis areas around it, favouring this evolution. Despite noted limitations, this study demonstrates that thrombus incorporation can contribute to plaque extension, as it can promote recirculation and stasis areas.

T helper type 17-related cytokine expression is increased in the bronchial mucosa of stable chronic obstructive pulmonary disease patients

There are increased numbers of activated T lymphocytes in the bronchial mucosa of stable chronic obstructive pulmonary disease (COPD) patients. T helper type 17 (Th17) cells release interleukin (IL)-17 as their effector cytokine under the control of IL-22 and IL-23. Furthermore, Th17 numbers are increased in some chronic inflammatory conditions. To investigate the expression of interleukin (IL)-17A, IL-17F, IL-21, IL-22 and IL-23 and of retinoic orphan receptor RORC2, a marker of Th17 cells, in bronchial biopsies from patients with stable COPD of different severity compared with age-matched control subjects. The expression of IL-17A, IL-17F, IL-21, IL-22, IL-23 and RORC2 was measured in the bronchial mucosa using immunohistochemistry and/or quantitative polymerase chain reaction. The number of IL-22(+) and IL-23(+) immunoreactive cells is increased in the bronchial epithelium of stable COPD compared with control groups. In addition, the number of IL-17A(+) and IL-22(+) immunoreactive cells is increased in the bronchial submucosa of stable COPD compared with control non-smokers. In all smokers, with and without disease, and in patients with COPD alone, the number of IL-22(+) cells correlated significantly with the number of both CD4(+) and CD8(+) cells in the bronchial mucosa. RORC2 mRNA expression in the bronchial mucosa was not significantly different between smokers with normal lung function and COPD. Further, we report that endothelial cells express high levels of IL-17A and IL-22. Increased expression of the Th17-related cytokines IL-17A, IL-22 and IL-23 in COPD patients may reflect their involvement, and that of specific IL-17-producing cells, in driving the chronic inflammation seen in COPD.

Flow-Mediated Signaling Modulates Endothelial Cell Phenotype

The focal development of atherosclerosis in the vascular tree may be explained in part by the local nature of blood flow. Bifurcations and branching points, prone to early atherogenesis, experience disturbed and oscillatory flow, whereas straight vascular regions, resistant to atherosclerosis, are exposed to steady laminar flow. A large number of studies suggest that the antiatherosclerotic effects of laminar flow are in part due to the ability of flow to modulate endothelial cell phenotype. Under steady laminar flow, endothelial cells generate molecules that promote a vasoactive, anticoagulant, anti-inflammatory, and growth-inhibitory surface. In contrast, disturbed flow induces a proliferative, prothrombotic, and adhesive phenotype. Endothelial cells are able to sense the variations of flow via mechanosensitive cell surface proteins and to transduce these signals via intracellular pathways to transcription factors in the nucleus leading to phenotypic changes. This review summarizes the "outside-in" signaling events initiated by flow that modulate endothelial cell phenotype.

Hemodynamic forces, vascular oxidative stress, and regulation of BMP-2/4 expression

Changes in the hemodynamic environment (e.g., hypertension, disturbed-flow conditions) are known to promote atherogenesis by inducing proinflammatory phenotypic alterations in endothelial and smooth muscle cells; however, the mechanisms underlying mechanosensitive induction of inflammatory gene expression are not completely understood. Bone morphogenetic protein-2 and -4 (BMP-2/4) are TGF-beta superfamily cytokines that are expressed by both endothelial and smooth muscle cells and regulate a number of cellular processes involved in atherogenesis, including vascular calcification and endothelial activation. This review considers how hemodynamic forces regulate BMP-2/4 expression and explores the role of mechanosensitive generation of reactive oxygen species by NAD(P)H oxidases in the control of BMP signaling.

Capability of human umbilical cord blood progenitor-derived endothelial cells to form an efficient lining on a polyester vascular graft in vitro

One of the goals of vascular tissue engineering is to create functional conduits for small-diameter bypass grafting. The present biocompatibility study was undertaken to check the ability of cord blood progenitor-derived endothelial cells (PDECs) to take the place of endothelial cells in vascular tissue engineering. After isolation, culture and characterization of endothelial progenitor cells, the following parameters were explored, with a commercial knitted polyester prosthesis (Polymaille C, Laboratoires Pérouse, France) impregnated with collagen: cell adhesion and proliferation, colonization, cell retention on exposure to flow, and the ability of PDECs to be regulated by arterial shear stress via mRNA levels. PDECs were able to adhere to commercial collagen-coated vascular grafts in serum-free conditions, and were maintained but did not proliferate when seeded at 2.0 x 10(5) cm(-2). Cellularized conduits were analyzed by histology and histochemical staining, demonstrating collagen impregnation and the endothelial characteristics of the colonizing cells. Thirty-six hours after cell seeding the grafts were maintained for 6 h of either static conditions (controls) or application of pulsatile laminar shear stress, which restored the integrity of the monolayer. Finally, quantitative real-time RT-PCR analysis performed at 4 and 8 h from cells lining grafts showed that MMP1 mRNA only was increased at 4h whereas vWF, VE-cadherin and KDR were not significantly modified at 4 and 8 h. Our results show that human cord blood PDECs are capable of forming an efficient lining and to withstand shear stress.

Serum level of soluble intercellular adhesion molecule-1 correlates with pulmonary arterial pressure in children with congenital heart disease

BACKGROUND

Endothelial activation and vascular inflammation are thought to be the mechanisms of pulmonary hypertension. Increased expression of the intercellular adhesion molecule (ICAM-1) and raised serum level of its soluble form (sICAM-1) are found in various conditions associated with endothelial activation.

METHODS

Serum samples from 31 children (14 boys and 17 girls; age, 4.9 +/- 4.6 years) with congenital heart disease (CHD) collected at the time of cardiac catheterization were analyzed for sICAM-1 level. Uni- and multivariable stepwise linear regression analyses were performed for the following variables against the sICAM-1 level: age, hemoglobin, serum creatinine, systemic arterial pressure (SAP), pulmonary arterial pressure (PAP), pulmonary blood flow (Qp) and resistance (Rp), systemic blood flow (Qs) and resistance (Rs), Qp/Qs, Rp/Rs, and pulmonary and systemic oxygen saturation.

RESULTS

The sICAM-1 levels in children who had CHD with and without pulmonary hypertension were 411 +/- 110 and 344 +/- 81 ng/ml, respectively (p = 0.11). In the univariable models, age, serum creatinine, systolic PAP, mean PAP, diastolic PAP, Rp, and Rp/Rs were significantly correlated with sICAM-1 level. In the multiple stepwise regression model, only mean PAP remained as an independent predictor of sICAM-1 level (r = 0.55; p = 0.002).

CONCLUSION

Children with CHD and pulmonary hypertension had a trend toward elevated sICAM-1 compared with CHD children who had no pulmonary hypertension. A linear correlation was found between mean pulmonary arterial pressure and sICAM-1 level.

The subendothelial extracellular matrix modulates JNK activation by flow

Atherosclerosis begins as local inflammation of artery walls at sites of disturbed flow. JNK (c-Jun NH(2)-terminal kinase) is thought to be among the major regulators of flow-dependent inflammatory gene expression in endothelial cells in atherosclerosis. We now show that JNK activation by both onset of laminar flow and long-term oscillatory flow is matrix-specific, with enhanced activation on fibronectin compared to basement membrane protein or collagen. Flow-induced JNK activation on fibronectin requires new integrin ligation and requires both the mitogen-activated protein kinase kinase MKK4 and p21-activated kinase. In vivo, JNK activation at sites of early atherogenesis correlates with the deposition of fibronectin. Inhibiting p21-activated kinase reduces JNK activation in atheroprone regions of the vasculature in vivo. These results identify JNK as a matrix-specific, flow-activated inflammatory event. Together with other studies, these data elucidate a network of matrix-specific pathways that determine inflammatory events in response to fluid shear stress.

Regulation of shear-induced nuclear translocation of the Nrf2 transcription factor in endothelial cells

Background

Vascular endothelial cells (ECs) constantly experience fluid shear stresses generated by blood flow. Laminar flow is known to produce atheroprotective effects on ECs. Nrf2 is a transcription factor that is essential for the antioxidant response element (ARE)-mediated induction of genes such as heme-oxygenase 1 (HO-1). We previously showed that fluid shear stress increases intracellular reactive oxygen species (ROS) in ECs. Moreover, oxidants are known to stimulate Nrf2. We thus examined the regulation of Nrf2 in cultured human ECs by shear stress.

Results

Exposure of human umbilical vein endothelial cells (HUVECs) to laminar shear stress (12 dyne/cm²) induced Nrf2 nuclear translocation, which was inhibited by a phosphatidylinositol 3-kinase (PI3K) inhibitor, a protein kinase C (PKC) inhibitor, and an antioxidant agent N-acetyl cysteine (NAC), but not by other protein kinase inhibitors. Therefore, PI3K, PKC, and ROS are involved in the signaling pathway that leads to the shear-induced nuclear translocation of Nrf2. We also found that shear stress increased the ARE-binding activity of Nrf2 and the downstream expression of HO-1.

Conclusion

Our data suggest that the atheroprotective effect of laminar flow is partially attributed to Nrf2 activation which results in ARE-mediated gene transcriptions, such as HO-1 expression, that are beneficial to the cardiovascular system.

Vascular endothelial responses to altered shear stress: pathologic implications for atherosclerosis

Atherosclerosis preferentially develops at branches and curvatures of the arterial tree, where blood flow is disturbed from a laminar pattern, and wall shear stress is non-uniform and has an irregular distribution. Vascular endothelial cells (ECs), which form an interface between the flowing blood and the vessel wall, are exposed to blood flow-induced shear stress. There is increasing evidence suggesting that laminar blood flow and sustained high shear stress modulate the expression of EC genes and proteins that function to protect against atherosclerosis; in contrast, disturbed blood flow and the associated low and reciprocating shear stress upregulate proatherosclerotic genes and proteins that promote development of atherosclerosis. Understanding of the effects of shear stress on ECs will provide mechanistic insights into its role in the pathogenesis of atherosclerosis. The aim of this review article is to summarize current findings on the effects of shear stress on ECs, in terms of their signal transduction, gene expression, structure, and function. These endothelial cellular responses have important relevance to understanding the pathophysiological effects of altered shear stress associated with atherosclerosis and thrombosis and their complications.

Role of oxidative stress in adaptive responses in special reference to atherogenesis

Although lipid peroxidation products have been implicated in oxidative stress-related diseases, pretreatment of cells with such compounds at sublethal concentrations shows significant cytoprotective effects against forthcoming oxidative stress. The adaptive response induced by 4-hydroxynonenal (HNE) is critically mediated by gene expression of cytoprotective proteins via NF-E2-related factor 2/Kelch-like-ECH-associated protein 1 (Nrf2/Keap-1) pathway. The physical or mechanical stimuli such as shear stress also impose adaptive responses by inducing gene expression. Laminar shear stress, anti-atherogenic shear stress activates Nrf2/Keap-1 pathway. The transcriptome analysis using DNA microarray reveal high similarity in gene expression profiles of cells treated with HNE and laminar shear stress, providing insight into molecular mechanisms. These findings suggest a general hormetic effect of diverse stimuli in cell cultures and may lead to a reappraisal of the eventual role of reactive oxygen species and lipid peroxidation in organisms.

Cell signaling directing the formation and function of hemogenic endothelium during murine embryogenesis

During developmental hematopoiesis, multilineage hematopoietic progenitors are thought to derive from a subset of vascular endothelium. Herein, we define the phenotype of such hemogenic endothelial cells and demonstrate, on a clonal level, that they exhibit multilineage hematopoietic potential. Furthermore, we have begun to define the molecular signals that regulate their development. We found that the formation of yolk sac hemogenic endothelium and its hematopoietic potential were significantly impaired in the absence of retinoic acid (RA) signaling, and could be restored in RA-deficient (Raldh2(-/-)) embryos by provision of exogenous RA in utero. Thus, we identify a novel, critical role for RA signaling in the development of hemogenic endothelium that contributes to definitive hematopoiesis.

p21-activated kinase signaling regulates oxidant-dependent NF-kappa B activation by flow

Disturbed blood flow induces inflammatory gene expression in endothelial cells, which promotes atherosclerosis. Flow stimulates the proinflammatory transcription factor nuclear factor (NF)-kappaB through integrin- and Rac-dependent production of reactive oxygen species (ROS). Previous work demonstrated that NF-kappaB activation by flow is matrix-specific, occurring in cells on fibronectin but not collagen. Activation of p21-activated kinase (PAK) followed the same matrix-dependent pattern. We now show that inhibiting PAK in cells on fibronectin blocked NF-kappaB activation by both laminar and oscillatory flow in vitro and at sites of disturbed flow in vivo. Constitutively active PAK rescued flow-induced NF-kappaB activation in cells on collagen. Surprisingly, PAK was not required for flow-induced ROS production. Instead, PAK modulated the ability of ROS to activate the NF-kappaB pathway. These data demonstrate that PAK controls NF-kappaB activation by modulating the sensitivity of cells to ROS.

Changes in regional myocardial volume during the cardiac cycle: implications for transmural blood flow and cardiac structure

Although previous studies report a reduction in myocardial volume during systole, myocardial volume changes during the cardiac cycle have not been quantitatively analyzed with high spatiotemporal resolution. We studied the time course of myocardial volume in the anterior mid-left ventricular (LV) wall of normal canine heart in vivo (n = 14) during atrial or LV pacing using transmurally implanted markers and biplane cineradiography (8 ms/frame). During atrial pacing, there was a significant transmural gradient in maximum volume decrease (4.1, 6.8, and 10.3% at subepi, midwall, and subendo layer, respectively, P = 0.002). The rate of myocardial volume increase during diastole was 4.7 +/- 5.8, 6.8 +/- 6.1, and 10.8 +/- 7.7 ml.min(-1).g(-1), respectively, which is substantially larger than the average myocardial blood flow in the literature measured by the microsphere method (0.7-1.3 ml.min(-1).g(-1)). In the early activated region during LV pacing, myocardial volume began to decrease before the LV pressure upstroke. We conclude that the volume change is greater than would be estimated from the known average transmural blood flow. This implies the existence of blood-filled spaces within the myocardium, which could communicate with the ventricular lumen. Our data in the early activated region also suggest that myocardial volume change is caused not by the intramyocardial tissue pressure but by direct impingement of the contracting myocytes on the microvasculature.

Molecular mechanisms in chronic venous insufficiency

Chronic venous disease (CVD) is common. Its manifestations include varicose veins; skin changes such as dermatitis, hyperpigmentation, and lipodermatosclerosis; and chronic leg ulcers. Recent advances in the understanding of its pathophysiology have shown how molecular mechanisms in the inflammatory cascade are involved in these diverse findings. Venous hypertension and associated fluid shear stress alterations on the endothelial surface may initiate this cascade and may lead to adverse changes in the venous wall, venous valves, and skin that can eventually result in varicose veins and in venous ulcers.

Gene response in endothelial cells cultured on engineered surfaces is regulated by shear stress

In vitro endothelialization of small-diameter synthetic vascular prostheses confluently lined with cultured autologous endothelial cells (ECs) before implantation has been shown to increase their patency. Many authors have studied the effects of shear stress on EC gene response seeded on various substrates showing different gene expression profiles according to cell type, flow times, or shear type with different molecular biology techniques, but few studies have reported any EC gene response to shear stress when cells are seeded on vascular grafts. The purpose of this in vitro study was to investigate whether ECs were able to transduce shear stress at the level of the nucleus. Human saphenous vein ECs were seeded on glass slides coated with gelatin or fibrin glue or on 6-mm fibrin-glue-coated grafts. Then cells were exposed to 12 dyn/cm(2) for 4 h and ribonucleic acid (RNA) were extracted. The relative messenger RNA (mRNA) expression was studied using real-time quantitative polymerase chain reaction for the following mRNAs: von Willebrand Factor, tissue-plasminogen activator, CD31, vascular endothelial (VE)-cadherin, beta(1) integrin, and vascular endothelial growth factor receptor type 2. From parallel flow chambers, results have shown similar EC gene response on gelatin and fibrin glue under laminar shear stress with downregulation of prothrombotic genes, as well as upregulation of nonthrombotic genes and upregulation of adhesion molecules such as VE-cadherin, but some discrepancies are noted, with a downregulation of CD31 and kinase insert domain receptor (KDR) for the former, without significant variation for the latter. In comparison, results show upregulation of tissue type plasminogen activator gene and downregulation of KDR, VE-cadherin, and beta(1) integrin genes in ECs lining grafts. To conclude, the major finding of our study is to show that human saphenous vein ECs seeded on fibrin glue (in planar flow chambers or in tubular grafts) can be regulated using shear stress via gene expression changes in a nonthrombotic way.

Bone Morphogenic Protein Antagonists Are Coexpressed With Bone Morphogenic Protein 4 in Endothelial Cells Exposed to Unstable Flow In Vitro in Mouse Aortas and in Human Coronary Arteries

Background— Exposure to disturbed flow, including oscillatory shear stress, stimulates endothelial cells (ECs) to produce bone morphogenic protein (BMP) 4, which in turn activates inflammation, a critical atherogenic step. BMP activity is regulated by the level of BMP antagonists. Until now it was not known whether shear also regulates the expression of BMP antagonists and whether they play a role in EC pathophysiology.

Methods and Results— BMP antagonists follistatin, noggin, and matrix Gla protein were expressed in cultured bovine and human arterial ECs. Surprisingly, oscillatory shear stress increased expression of the BMP antagonists in ECs, whereas unidirectional laminar shear decreased such expression. Immunohistochemical studies with mouse aortas showed data consistent with in vitro findings: Only ECs in the lesser curvature exposed to disturbed flow, but not those in the greater curvature and straight arterial regions exposed to undisturbed flow, showed coexpression of BMP4 and the BMP antagonists. Similarly, in human coronary arteries, expression of BMP4 and BMP antagonists in ECs positively correlated with the severity of atherosclerosis. Monocyte adhesion induced by oscillatory shear stress was inhibited by knockdown of BMP4 or treatment with recombinant follistatin or noggin, whereas it was increased by knockdown of follistatin and/or noggin.

Conclusions— The present results suggest that ECs coexpress BMP antagonists along with BMP4 in an attempt to minimize the inflammatory response by oscillatory shear stress as part of a negative feedback mechanism. The balance between the agonist, BMP4, and its antagonists may play an important role in the overall control of inflammation and atherosclerosis.

Endothelial cell migration on RGD-peptide-containing PEG hydrogels in the presence of sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a potent chemokinetic agent for endothelial cells that is released by activated platelets. We previously developed Arg-Gly-Asp (RGD)-containing polyethylene glycol biomaterials for the controlled delivery of S1P to promote endothelialization. Here, we studied the effects of cell adhesion strength on S1P-stimulated endothelial cell migration in the presence of arterial levels of fluid shear stress, since an upward shift in optimal cell adhesion strengths may be beneficial for promoting long-term cell adhesion to materials. Two RGD peptides with different integrin-binding specificities were added to the polyethylene glycol hydrogels. A linear RGD bound primarily to beta(3) integrins, whereas a cyclic RGD bound through both beta(1) and beta(3) integrins. We observed increased focal adhesion formation and better long-term adhesion in flow with endothelial cells on linear RGD peptide, versus cyclic RGD, even though initial adhesion strengths were higher for cells on cyclic RGD. Addition of 100 nM S1P increased cell speed and random motility coefficients on both RGD peptides, with the largest increases found on cyclic RGD. For both peptides, much of the increase in cell migration speed was found for smaller cells (<1522 microm(2) projected area), although the large increases on cyclic RGD were also due to medium-sized cells (2288-3519 microm(2)). Overall, a compromise between high cell migration rates and long-term adhesion will be important in the design of materials that endothelialize after implantation.

Vascular adhesion molecules in atherosclerosis

Numerous reports document the role of vascular adhesion molecules in the development and progression of atherosclerosis. Recent novel findings in the field of adhesion molecules require an updated summary of current research. In this review, we highlight the role of vascular adhesion molecules including selectins, vascular cell adhesion molecule (VCAM)-1, intercellular adhesion molecule1 (ICAM-1), PECAM-1, JAMs, and connexins in atherosclerosis. The immune system is important in atherosclerosis, and significant efforts are under way to understand the vascular adhesion molecule-dependent mechanisms of immune cell trafficking into healthy and atherosclerosis-prone arterial walls. This review focuses on the role of vascular adhesion molecules in the regulation of immune cell homing during atherosclerosis and discusses future directions that will lead to better understanding of this disease.

Frequency-dependent response of the vascular endothelium to pulsatile shear stress

As a result of the complex blood flow patterns that occur in the arterial tree, certain regions of the vessel wall experience fluctuations in shear stress that are dominated by harmonic frequencies higher than the heart rate ( 11 ). To assess whether variations in frequency affect endothelial gene expression, the gene expression patterns of cultured porcine aortic endothelium exposed to three sinusoidal waveforms (1, 2, and 3 Hz; amplitude = 15 dyn/cm2) and one physiological waveform were compared with the expression profiles elicited by steady flow. At each frequency, including steady flow, three levels of mean shear stress (0, 7.5, and 15 dyn/cm2) were used. After 24 h shear exposure, RNA was extracted for microarray analysis against 10,665 Sus scrofa oligonucleotides. A two-way ANOVA identified 232 genes of which their transcription was differentially modulated by frequency, while mean shear significantly affected the expression of ∼3,000 genes. One-way ANOVAs showed that the number of frequency-dependent genes increased as the mean shear stress was reduced. At 1 Hz, several inflammatory transcripts were repressed relative to steady flow, including VCAM and IL-8, whereas several atheroprotective transcripts were induced. The anti-inflammatory response at 1 Hz was reversed at 2 Hz. The proinflammatory response evoked by the higher frequency was most pronounced under reversing and oscillatory shear. This study suggests that arterial regions subject to both shear reversal and dominant frequencies that exceed the normal heart rate are at greater risk for atherosclerotic lesion development.

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.