Oscillatory shear stress stimulates adhesion molecule expression in cultured human endothelium

Secrets of the code: Do vascular endothelial cells use ion channels to decipher complex flow signals?

The ability of vascular endothelial cells (ECs) to respond to changes in blood flow is essential for both vasoregulation and arterial wall remodelling, while abnormalities in endothelial responsiveness to flow play an important role in the development of atherosclerosis. Endothelial flow responses also have important implications for the field of vascular tissue engineering. In response to changes in fluid dynamic shear stress, ECs exhibit humoral, metabolic, and structural responses. Significantly, ECs respond differently to different types of shear stress. For instance, steady shear stress elicits a profile of responses that differs drastically from oscillatory shear stress. Although our understanding of flow-induced signaling has advanced greatly over the past two decades, how ECs sense shear forces remains to be established. Furthermore, the mechanisms by which ECs discriminate among different flow waveforms are unknown. Activation of flow-sensitive ion channels is one of the most rapid known responses to flow in ECs. In this paper, we argue in favor of an important role for ion channels in shear stress sensing in ECs and propose that these channels may endow ECs with the ability to resolve components of a complex flow signal and hence distinguish among different types of flow.

Biomechanical modulation of endothelial phenotype: implications for health and disease

The functional phenotypic plasticity of the vascular endothelium relies on the ability of individual endothelial cells to integrate and transduce both humoral and biomechanical stimuli from their surrounding environments. Increasing evidence strongly suggests that biomechanical stimulation is a critical determinant of endothelial gene expression and the functional phenotypes displayed by these cells in several pathophysiological conditions. Herein we discuss the types of biomechanical forces that endothelial cells are constantly exposed to within the vasculature, explain how these biomechanical stimuli serve as regulators of endothelial function and discuss the increasing evidence that "atherosclerosis-protective" or "atherosclerosis-prone" haemodynamic environments can be important causative factors for atherogenesis via the differential regulation of endothelial transcriptional programmes.

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.

Computational study of fluid mechanical disturbance induced by endovascular stents

Arterial restenosis following stent deployment may be influenced by the local flow environment within and around the stent. We have used computational fluid dynamics to investigate the flow field in the vicinity of model stents positioned within straight and curved vessels. Our simulations have revealed the presence of flow separation and recirculation immediately downstream of stents. In steady flow within straight vessels, the extent of flow disturbance downstream of the stent increases with both Reynolds number and stent wire thickness but is relatively insensitive to stent interwire spacing. In curved vessels, flow disturbance downstream of the stent occurs along both the inner and outer vessel walls with the extent of disturbance dependent on the angle of vessel curvature. In pulsatile flow, the regions of flow disturbance periodically increase and decrease in size. Non-Newtonian fluid properties lead to a modest reduction in flow disturbance downstream of the stent. In more realistic stent geometries such as stents modeled as spirals or as intertwined rings, the nature of stent-induced flow disturbance is exquisitely sensitive to stent design. These results provide an understanding of the flow physics in the vicinity of stents and suggest strategies for stent design optimization to minimize flow disturbance.

Endothelial protein kinase C isoform identity and differential activity of PKCzeta in an athero-susceptible region of porcine aorta

Endothelial protein kinase C (PKC) signaling was investigated in different regions of normal porcine aorta. The locations map to differential atherosclerotic susceptibility and correlate with sites of disturbed (DF) or undisturbed (UF) local flow profiles. Endothelial lysates were isolated from the inner curvature of the aortic arch (DF; athero-susceptible) and a nearby UF region of the descending thoracic aorta (UF; athero-protected), and in some experiments a distant athero-protected UF site, the common carotid artery. Total endothelial PKC activity in the DF regions was 145% to 240% of that in both UF locations (P<0.05), whereas the UF regions were not significantly different from each other. PKC protein isoforms alpha, beta, epsilon, iota, lambda, and zeta were expressed in similar proportions in both aortic regions, suggesting that differences of kinase activity were not directly attributable to expression levels. Inhibition of members of the "conventional" and "novel" PKC families had no differential effect on regional kinase activity. However, inhibition of PKCzeta, a member of the "atypical" PKC family, reduced the DF lysate kinase activity to that of UF levels (NS P=0.35). Differential phosphorylation of PKCzeta Thr410 and Thr560, along with increased levels of PKCzeta degradation products in UF endothelial lysates, suggested posttranslational modification of PKCzeta as the basis for site-specific differences in vivo. Steady-state regional heterogeneity of an important family of regulatory proteins in intact arterial endothelium in vivo may link localized athero-susceptibility and the associated hemodynamic environment.

Anti-inflammatory strategies in hypertension: focus on COX-1 and COX-2

An increasing body of evidence suggests that elevated levels of blood pressure may induce a proinflammatory response. Indeed, both C-reactive protein and blood pressure are independent determinants of cardiovascular risk, and, in combination, each parameter has additional predictive value. Hence, strategies targeted to lower blood pressure and reduce vascular inflammation may potentially provide clinical benefit. In this review, we discuss the role of chronic low-grade inflammation in the context of cardiovascular disease with a focus on roles of cyclooxygenase-1 and -2 in potential anti-inflammatory treatment strategies.

Differential responses of the Nrf2-Keap1 system to laminar and oscillatory shear stresses in endothelial cells

The Nrf2-Keap1 system coordinately regulates cytoprotective gene expression via the antioxidant responsive element (ARE). The expression of several ARE-regulated genes was found to be up-regulated in endothelial cells by laminar shear stress, suggesting that Nrf2 contributes to the anti-atherosclerosis response via the ARE. To gain further insight into the roles that Nrf2 plays in the development of atherosclerosis, we examined how Nrf2 regulates gene expression in response to anti-atherogenic laminar flow (L-flow) or pro-atherogenic oscillatory flow (O-flow). Exposure of human aortic endothelial cells (HAECs) to L-flow, but not to O-flow, induced the expression of cytoprotective genes, such as NAD(P)H quinone oxidoreductase 1 (NQO1) by 5-fold and heme oxygenase-1 by 8-fold. The critical contribution of Nrf2 to the expression induced by L-flow was ascertained in siRNA-mediated knock-down experiments. Two cyclooxygenase-2 (COX-2) specific inhibitors attenuated Nrf2 nuclear accumulation in the acute phase of L-flow exposure. A downstream product of COX-2, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), activated the Nrf2 regulatory pathway in HAECs through binding to the cysteines of Keap1. These results demonstrate that 15d-PGJ2 is essential for L-flow to activate Nrf2 and induce anti-atherosclerotic gene expression. Whereas both L-flow and O-flow induced the nuclear accumulation of Nrf2 to comparable levels, chromatin immunoprecipitation analysis revealed that Nrf2 binding to the NQO1 ARE was significantly diminished in the case of O-flow compared with that of L-flow. These results suggest that O-flow inhibits Nrf2 activity at the DNA binding step, thereby suppressing athero-protective gene expression and hence predisposing the blood vessels to the formation of atherosclerosis.

The role of shear stress in the pathogenesis of atherosclerosis

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

Flow-conditioned HUVECs support clustered leukocyte adhesion by coexpressing ICAM-1 and E-selectin

Endothelial sequestration of circulating monocytes is a key event in early atherosclerosis. Hemodynamics is proposed to regulate monocyte-endothelial cell interactions by direct cell activation and establishment of flow environments that are conducive or prohibitive to cell-cell interaction. We investigated fluid shear regulation of monocyte-endothelial cell adhesion in vitro using a disturbed laminar shear system that models in vivo hemodynamics characteristic of lesion-prone vascular regions. Human endothelial cell monolayers were flow conditioned for 6 h before evaluation of monocyte adhesion under static and dynamic flow conditions. Results revealed a distinctive clustered cell pattern of monocyte adhesion that strongly resembles in vivo leukocyte adhesion in early- and late-stage atherosclerosis. Clustered monocyte cell adhesion correlated with endothelial cells coexpressing intercellular adhesion molecule-1 (ICAM-1) and E-selectin as result of a flow-induced, selective upregulation of E-selectin expression in a subset of ICAM-1-expressing cells. Clustered monocyte cell adhesion assayed under static conditions exhibited a spatial variation in size and frequency of occurrence, which demonstrates differential regulation of endothelial cell adhesiveness by the local flow environment. Dynamic adhesion studies conducted with circulating monocytes resulted in clustered cell adhesion only within the disturbed flow region, where the monocyte rate of motion is sufficiently low for cell-cell interaction. These studies provide evidence and reveal mechanisms of local hemodynamic regulation of endothelial adhesiveness and endothelial monocyte interaction that lead to localized monocyte adhesion and potentially contribute to the focal origin of arterial diseases such as atherosclerosis.

C-reactive protein and arterial stiffness in older adults: the Rotterdam Study

Arterial stiffness is one of the characteristics of vascular aging. Increases in pulse pressure, which reflect an increase in the stiffness of the large arteries, are associated with elevated C-reactive protein (CRP) levels. This may suggest a role of inflammation in the development of arterial stiffness. We investigated the relation between measures of arterial stiffness and CRP within the framework of the Rotterdam Study, a population-based cohort study including subjects aged 55 years and older. The carotid-femoral pulse wave velocity and the distensibility coefficient of the carotid artery were used as measures of arterial stiffness. Data on both arterial stiffness and CRP were available for 866 participants. In adjusted models, levels of CRP were linearly associated with pulse wave velocity (regression coefficient 0.088, 95% CI 0.006-0.170). Adjusted mean values of pulse wave velocity were significantly different across tertiles of CRP, being higher in the highest tertile of CRP. However, no significant association between CRP and carotid distensibility was observed.

Endothelial cell superoxide generation: regulation and relevance for cardiovascular pathophysiology

The endothelial generation of reactive oxygen species (ROS) is important both physiologically and in the pathogenesis of many cardiovascular disorders. ROS generated by endothelial cells include superoxide (O2-*), hydrogen peroxide (H2O2), peroxynitrite (ONOO-*), nitric oxide (NO), and hydroxyl (*OH) radicals. The O2-* radical, the focus of the current review, may have several effects either directly or through the generation of other radicals, e.g., H2O2 and ONOO-*. These effects include 1) rapid inactivation of the potent signaling molecule and endothelium-derived relaxing factor NO, leading to endothelial dysfunction; 2) the mediation of signal transduction leading to altered gene transcription and protein and enzyme activities ("redox signaling"); and 3) oxidative damage. Multiple enzymes can generate O2-*, notably xanthine oxidase, uncoupled NO synthase, and mitochondria. Recent studies indicate that a major source of endothelial O2-* involved in redox signaling is a multicomponent phagocyte-type NADPH oxidase that is subject to specific regulation by stimuli such as oscillatory shear stress, hypoxia, angiotensin II, growth factors, cytokines, and hyperlipidemia. Depending on the level of oxidants generated and the relative balance between pro- and antioxidant pathways, ROS may be involved in cell growth, hypertrophy, apoptosis, endothelial activation, and adhesivity, for example, in diabetes, hypertension, atherosclerosis, heart failure, and ischemia-reperfusion. This article reviews our current knowledge regarding the sources of endothelial ROS generation, their regulation, their involvement in redox signaling, and the relevance of enhanced ROS generation and redox signaling to the pathophysiology of cardiovascular disorders where endothelial activation and dysfunction are implicated.

Poor coronary collateral circulation is associated with higher concentrations of soluble adhesion molecules in patients with single-vessel disease

OBJECTIVE

As the endothelium and inflammatory cells play a crucial role in the development of collaterals after a sudden or slowly progressing stenosis of coronary arteries, the levels of soluble endothelial adhesion molecules (CAMs) including vascular cell adhesion molecule (VCAM-1) intercellular adhesion molecule-1 (ICAM-1) and E-selectin were compared between patients with poor coronary collaterals and patients with well-developed collaterals.

METHODS

In the study, 97 non-diabetic subjects with single-vessel disease were included. Collateral supply to the stenotic coronary artery was determined by angiographic grading system of 0-3 (Rentrop et al. J Am Coll Cardiol 1985; 5:587-592). Serum levels of adhesion molecules were measured by enzyme-linked immunosorbent assay.

RESULTS

Patients were divided into two groups according to the collateral degree (group A: 50 patients with grade 0 and 1; group B: 47 patients with grade 2 and 3 collaterals). The groups were well matched with respect to baseline clinical and angiographic characteristics. Levels of soluble VCAM-1 (mean+/-SEM; 875+/-26.6 versus 742.7+/-35.1 ng/ml; P=0.004), ICAM-1 (322.4+/-12.4 versus 269.4+/-13.3 ng/ml; P=0.005), and E-selectin (43.6+/-2.6 versus 33+/-2.4 ng/ml; P=0.004) were found to be significantly higher in group A in comparison with group B. In addition, when patients were divided into four groups according to the collateral degree, patients with grade 0 collaterals had the highest values and those with grade 3 collaterals had the lowest values for all these molecules.

CONCLUSIONS

We concluded that poor collateral circulation is associated with increased levels of soluble CAMs in patients with obstructive coronary artery disease. However, further studies are needed to elucidate the exact role of these inflammatory markers in the setting of poor collateral circulation.

Nitric oxide is significantly reduced in ex vivo porcine arteries during reverse flow because of increased superoxide production

Oscillatory and negative flows occur normally in the cardiovascular system, which predispose those regions to atherosclerosis. Nitric oxide (NO) production increases in proportion to the magnitude of flow and is known to be athero-protective. What is not known, however, is the effect of flow reversal on NO concentration ([NO]). The hypothesis of the present study is that [NO] is reduced in reverse flow. An additional hypothesis is that the reduction in [NO] is mediated through an increase in superoxide production during flow reversal. These hypotheses were tested in an ex vivo preparation of porcine elastic and muscular arteries. The flow of a physiological solution through the vessels was regulated in the forward and reverse direction and the effluent was assayed for nitrite levels using a combination of a diazo coupling method and high performance liquid chromatography. Our results show that [NO] is significantly reduced during reverse flow. Furthermore, addition of tempol (superoxide dismutase-mimetic) which is a superoxide scavenger returns the [NO] during reverse flow to mirror those of forward flow. These results have important implications since the action of superoxide is implicated in many cardiovascular diseases, and the present finding suggests that flow reversal should be added to the list.

Distinct endothelial phenotypes evoked by arterial waveforms derived from atherosclerosis-susceptible and -resistant regions of human vasculature

Atherosclerotic lesion localization to regions of disturbed flow within certain arterial geometries, in humans and experimental animals, suggests an important role for local hemodynamic forces in atherogenesis. To explore how endothelial cells (EC) acquire functional/dysfunctional phenotypes in response to vascular region-specific flow patterns, we have used an in vitro dynamic flow system to accurately reproduce arterial shear stress waveforms on cultured human EC and have examined the effects on EC gene expression by using a high-throughput transcriptional profiling approach. The flow patterns in the carotid artery bifurcations of several normal human subjects were characterized by using 3D flow analysis based on actual vascular geometries and blood flow profiles. Two prototypic arterial waveforms, "athero-prone" and "athero-protective," were defined as representative of the wall shear stresses in two distinct regions of the carotid artery (carotid sinus and distal internal carotid artery) that are typically "susceptible" or "resistant," respectively, to atherosclerotic lesion development. These two waveforms were applied to cultured EC, and cDNA microarrays were used to analyze the differential patterns of EC gene expression. In addition, the differential effects of athero-prone vs. athero-protective waveforms were further characterized on several parameters of EC structure and function, including actin cytoskeletal organization, expression and localization of junctional proteins, activation of the NF-kappaB transcriptional pathway, and expression of proinflammatory cytokines and adhesion molecules. These global gene expression patterns and functional data reveal a distinct phenotypic modulation in response to the wall shear stresses present in atherosclerosis-susceptible vs. atherosclerosis-resistant human arterial geometries.

The Aqueous Outflow System as a Mechanical Pump

PURPOSE

To describe a new aqueous outflow model involving a mechanical pump.

MATERIALS AND METHODS

Laboratory materials include human and monkey eyes; methods include the dissecting microscope, light microscopy, scanning electron microscopy, transmission electron microscopy, and tracer studies. Clinical methods involve human subject slit lamp, gonioscopy, and operating microscope examination.

RESULTS

Laboratory evidence demonstrates that aqueous outflow tissues are responsive to intraocular pressure induced deformation. Deformation occurs in response to small pressure gradients. Laboratory evidence also demonstrates the presence of valves discharging aqueous to Schlemm's canal. The laboratory model predicts pulsatile aqueous discharge in vivo. Clinical in vivo evidence demonstrates pulsatile aqueous flow from the anterior chamber into Schlemm's canal, from Schlemm's canal into collector channels, and from Schlemm's canal into aqueous and episcleral veins, all synchronous with the ocular pulse.

CONCLUSIONS

Aqueous outflow tissue deformation caused by normal intraocular pressure transients induces pulsatile one-way discharge of aqueous to the vascular system. The model identifies biomechanical coupling of intraocular pressure with aqueous outflow tissue deformation and also sites of high flow capable of inducing shear stress. These mechanotransduction mechanisms, well characterized as a means of controlling pressure and flow in the vascular system, also provide a means of regulatory feedback to control intraocular pressure and aqueous flow.

Sphingosine kinase-1 mediates TNF-α-induced MCP-1 gene expression in endothelial cells: upregulation by oscillatory flow

Atherosclerosis is a focal inflammatory disease and preferentially occurs in areas of low fluid shear stress and oscillatory flow, whereas the risk of atherosclerosis is decreased in regions of high fluid shear stress and steady laminar flow. Sphingosine kinase-1 (SphK1) catalyzes the conversion of sphingosine to sphingosine-1 phosphate (S1P), a sphingolipid metabolite that plays important roles in angiogenesis, inflammation, and cell growth. In the present study, we demonstrated that exposure of human aortic endothelial cells to oscillatory flow (shear stress, ±5 dyn/cm2for 48 h) resulted in a marked increase in SphK1 mRNA levels compared with endothelial cells kept in static culture. In contrast, laminar flow (shear stress, 20 dyn/cm2for 48 h) decreased SphK1 mRNA levels. We further investigated the role of SphK1 in TNF-α-induced expression of inflammatory genes, such as monocyte chemoattractant protein-1 (MCP-1) and VCAM-1 by using small interfering RNA (siRNA) specifically for SphK1. Treatment of endothelial cells with SphK1 siRNA suppressed TNF-α-induced increase in MCP-1 mRNA levels, MCP-1 protein secretion, and activation of p38 MAPK. SphK1 siRNA also inhibited TNF-α-induced cell surface expression of VCAM-1, but not ICAM-1, protein. Exposure of endothelial cells to S1P led to an increase in MCP-1 protein secretion and MCP-1 mRNA levels and activation of NF-κB-mediated transcriptional activity. Treatment of endothelial cells with the p38 MAPK inhibitor SB-203580 suppressed S1P-induced MCP-1 protein secretion. These data suggest that SphK1 mediates TNF-α-induced MCP-1 gene expression through a p38 MAPK-dependent pathway and may participate in oscillatory flow-mediated proinflammatory signaling pathway in the vasculature.

Relationship between C reactive protein and pulse pressure is not mediated by atherosclerosis or aortic stiffness

BACKGROUND

Pulse pressure (PP), C reactive protein (CRP) and soluble intercellular adhesion molecule-1 (sICAM-1) levels have been associated with cardiovascular prognosis. Interestingly, previous reports have shown that PP was associated both with CRP and sICAM-1. The mechanisms underlying these associations remain unknown. On the one hand, it has been shown that PP influences, via endothelial function, the expression of various molecules, which in turn may generate inflammation. On the other hand, inflammation-induced changes in the arterial wall, modifying the PP, may be the confounding factor of these relationships. The aim of the present study was to investigate the role played by the arterial structure and the aortic stiffness on these relationships.

METHODS

In a cross-sectional population sample of 891 healthy subjects, carotid-femoral pulse wave velocity and blood pressure were measured in the supine position. The common carotid intima-media thickness and the presence of plaques were assessed by ultrasonography. CRP and sICAM-1 levels were measured by an immunonephelemetric method and an immunoenzymatic method, respectively.

RESULTS

A positive relationship was found between PP and CRP (P < 0.001). This relationship remained after adjustment for classical cardiovascular risk factors, and successively for mean blood pressure, intima-media thickness, presence of plaques and pulse wave velocity (P < 0.05). No significant association was observed between PP and sICAM-1.

CONCLUSIONS

The results of this study demonstrate that changes in arterial structure and in arterial stiffness are not confounding factors in the relationship between PP and CRP.

Sterol-responsive element-binding protein (SREBP) 2 down-regulates ATP-binding cassette transporter A1 in vascular endothelial cells: a novel role of SREBP in regulating cholesterol metabolism

ATP-binding cassette transporter A1 (ABCA1) is a pivotal regulator of cholesterol efflux from cells to apolipoproteins, whereas sterol-responsive element-binding protein 2 (SREBP2) is the key protein regulating cholesterol synthesis and uptake. We investigated the regulation of ABCA1 by SREBP2 in vascular endothelial cells (ECs). Our results showed that sterol depletion activated SREBP2 and increased its target, low density lipoprotein receptor mRNA, with a concurrent decrease in the ABCA1 mRNA. Transient transfection analysis revealed that sterol depletion decreased the ABCA1 promoter activity by 50%, but low density lipoprotein receptor promoter- and the sterol-responsive element-driven luciferase activities were increased. Overexpression of the N terminus of SREBP2 (SREBP2(N)), an active form of SREBP2, also inhibited the ABCA1 promoter activity. Functionally adenovirus-mediated SREBP2(N) expression increased cholesterol accumulation and decreased apoA-I-mediated cholesterol efflux. The conserved E-box motif was responsible for the SREBP2(N)-mediated inhibition since mutation of the E-box increased the basal activity of the ABCA1 promoter and abolished the inhibitory effect of SREBP2(N). Furthermore sterol depletion and SREBP2(N) overexpression induced the binding of SREBP2(N) to both consensus and ABCA1-specific E-box. Chromatin immunoprecipitation assay demonstrated that serum starvation enhanced the association of SREBP2 and the ABCA1 promoter in ECs. To correlate this mechanism pathophysiologically, we found that oscillatory flow caused the activation of SREBP2 and therefore attenuated ABCA1 promoter activity in ECs. Thus, this SREBP-regulated mechanism may control the efflux of cholesterol, which is a newly defined function of SREBP2 in ECs in addition to its role in cholesterol uptake and biosynthesis.

Effect on endothelial cell gene expression of shear stress, oxygen concentration, and low-density lipoprotein as studied by a novel flow cell culture system

A new cell culture system has been developed that reflects the vascular microenvironment. By means of this system the cultured cells are exposed not only to shear stress by the circulating culture medium, but also to an oxygen concentration gradient and certain critical blood components such as low-density lipoprotein (LDL) and monocytes. DNA microarray analysis was performed for human umbilical vein endothelial cells cultured in this system in the absence and presence of laminar flow at a low shear stress, 0.2 dyn/cm(2). In addition to shear stress, either an oxygen concentration gradient, or LDL (1 mg/ml), or both were applied. Many Nrf-2-regulating genes, such as heme oxygenase 1, NAD(P)H quinone oxidoreductase 1, solute carrier family 7 No. 11, and glutamate-cysteine ligase modifier subunit, were induced by laminar flow at very low shear stress regardless of the additional conditions. Certain genes were specifically affected by exposure to the oxygen gradient and/or LDL under shear stress, but the degree was very low. These results suggest that shear stress is the most critical factor affecting gene expression in endothelial cells and that Nrf-2-regulating proteins may contribute to protecting endothelial cells against other vascular stress. This system should provide highly relevant and useful information about both vascular physiology and pathology, in the latter on such urgent matters as the specific steps involved in atherogenesis.

Mitochondrial requirement for endothelial responses to cyclic strain: implications for mechanotransduction

Mechanical strain triggers a variety of cellular responses, but the underlying mechanotransduction process has not been established. Endothelial cells (EC) respond to mechanical strain by upregulating adhesion molecule expression through a signaling process involving reactive oxygen species (ROS), but the site of their generation is unknown. Mitochondria anchor to the cytoskeleton and could function as mechanotransducers by releasing ROS during cytoskeletal strain. In human umbilical vein EC (HUVEC), ROS production increased 221 ± 17% during 6 h of cyclic strain vs. unstrained controls. Mitochondrial inhibitors diphenylene iodonium or rotenone abrogated this response, whereas inhibitors of nitric oxide (NO) synthase (l-nitroarginine), xanthine oxidase (allopurinol), or NAD(P)H oxidase (apocynin) had no effect. The antioxidants ebselen and diethyldithiocarbamate inhibited the increase in ROS, but the NO scavenger Hb had no effect. Thus strain induces ROS release from mitochondria. In other studies, HUVEC were rendered mitochondria deficient (ρ0EC) to determine the requirement for electron transport in the response to strain. Strain-induced 2′7′-dichlorofluorescein fluorescence was attenuated by >80% in ρ0EC compared with HUVEC (43 ± 7 vs. 221 ± 17%). Treatment with cytochalasin D abrogated strain-induced ROS production, indicating a requirement for the actin cytoskeleton. Cyclic strain (6 h) increased VCAM-1 expression in wild-type but not ρ0EC. Increases in NF-κB activation and VCAM-1 mRNA expression during strain were prevented by antioxidants. These findings demonstrate that mitochondria function as mechanotransducers in endothelium by increasing ROS signaling, which is required for strain-induced increase in VCAM-1 expression via NF-κB.

Three-dimensional phase contrast velocity mapping acquisition improves wall shear stress estimation in vivo

The purpose of this study was to evaluate and apply high-resolution three-dimensional phase contrast mapping for estimation of wall shear stress in vivo. A silicon pipe of 8 mm diameter with a 8.3 ml/s steady flow and the entrance of the carotid bifurcation in 10 young healthy volunteers aged 23.6 +/- 3.1 years was studied. Very high resolution three-dimensional and two-dimensional phase contrast mapping sequences with spatial resolutions of 0.31 x 0.31 x 1.5 mm(3) and 0.31 x 0.31 x 3 mm(3), respectively, were compared in vivo and in vitro. Wall shear stress was calculated using multi-sectored, three-dimensional paraboloid fitting. In comparison to the two-dimensional measurements, the three-dimensional method with only half the slice thickness gave higher signal-to-noise ratio and velocity-to-noise ratios both in vivo and in vitro. Wall shear stress derived from the three-dimensional velocity measurements did not differ from the two-dimensional velocity measurements either in vitro or in vivo. Mean wall shear stress was lowest and oscillatory shear index was highest at the outer wall, towards the carotid bifurcation. Three-dimensional velocity mapping increases resolution and image quality and allows estimation of wall shear stress patterns circumferentially and longitudinally in human arteries.

Getting stents to go with the flow

Implantation of expandable stents into stenotic arteries after percutaneous coronary intervention to relieve arterial narrowing has become a standard therapeutic tool. The improvement in vascular interventional technology, and especially stent technology, has, arguably, outstripped understanding of the biologic consequences of opening an obstructed artery. In the case of bifurcation stenoses, new evidence suggests that opening a stenotic subsidiary branch may create unfavorable hemodynamics in the stented main branch that can lead to in-stent restenosis.

The local physicochemical environment conditions the proinflammatory response of endothelial cells and thus modulates leukocyte recruitment

The locations at which vascular endothelial cells recruit leukocytes during physiological or pathological inflammatory responses are influenced by direct effects of local haemodynamics on leukocyte adhesion. However, the expression of genes by endothelial cells, and their ability to respond to inflammatory cytokines also depend on the flow forces to which they are exposed. In addition, cells of the underlying stroma can modify the phenotype and responsiveness of endothelial cells, and hence their ability to recruit leukocytes. Thus, endothelial cells are plastic in their responses, and we hypothesise that the pattern of recruitment of leukocytes to tissues is critically dependent on the variable modulation of the endothelium by the local physicochemical microenvironment.

Differential membrane potential and ion current responses to different types of shear stress in vascular endothelial cells

Vascular endothelial cells (ECs) distinguish among and respond differently to different types of fluid mechanical shear stress. Elucidating the mechanisms governing this differential responsiveness is the key to understanding why early atherosclerotic lesions localize preferentially in arterial regions exposed to low and/or oscillatory flow. An early and very rapid endothelial response to flow is the activation of flow-sensitive K+ and Cl− channels that respectively hyperpolarize and depolarize the cell membrane and regulate several important endothelial responses to flow. We have used whole cell current- and voltage-clamp techniques to demonstrate that flow-sensitive hyperpolarizing and depolarizing currents respond differently to different types of shear stress in cultured bovine aortic ECs. A steady shear stress level of 10 dyn/cm2 activated both currents leading to rapid membrane hyperpolarization that was subsequently reversed to depolarization. In contrast, a steady shear stress of 1 dyn/cm2 only activated the hyperpolarizing current. A purely oscillatory shear stress of 0 ± 10 dyn/cm2 with an oscillation frequency of either 1 or 0.2 Hz activated the hyperpolarizing current but only minimally the depolarizing current, whereas a 5-Hz oscillation activated neither current. These results demonstrate for the first time that flow-activated ion currents exhibit different sensitivities to shear stress magnitude and oscillation frequency. We propose that flow-sensitive ion channels constitute components of an integrated mechanosensing system that, through the aggregate effect of ion channel activation on cell membrane potential, enables ECs to distinguish among different types of flow.

The relationship between high-sensitive C-reactive protein and pulse wave velocity in healthy Japanese men

Although pulse wave velocity (PWV) and high-sensitive C-reactive protein (hsCRP) are known as predictors of future cardiovascular events, their association has not been examined. The present study was conducted to evaluate their association in the general population. In 2668 Japanese men (43 +/- 10 years old), PWV was obtained by volume rendering methods, and hsCRP was determined by the latex aggregation method. PWV showed a significant correlation with logarithm of hsCRP (r = 0.06, P < 0.01). The concentration of hsCRP in the highest quartile of PWV was higher than that in the other three groups (P < 0.01). However, multiple linear regression analyses demonstrated that logarithm of hsCRP was not significantly related to PWV, independent from conventional risk factors. Calculated Framingham risk score (FRS) was higher in the highest quartiles of both hsCRP and PWV than in the other groups (P < 0.05). Thus, while increased hsCRP related to increased PWV, they may be independent predictors of atherosclerotic cardiovascular risk. A prospective study to confirm the independency of their significance in predicting future cardiovascular events and to evaluate the usefulness of the combination of both parameters to screen subjects for cardiovascular risk is necessary.

Methods for exposing multiple cultures of endothelial cells to different fluid shear stresses and to cytokines, for subsequent analysis of inflammatory function

Endothelial cells are conditioned by physicochemical environmental factors, including shear stress applied by flowing blood. However, the effects of shear conditioning on the functional responses of endothelial cells, such as ability to recruit leukocytes, remain uncertain. Here we describe a system for culturing multiple samples of endothelial cells under flow for prolonged periods, either at different shear stresses, or exposed concurrently to different concentrations of cytokines, for instance, tumour necrosis factor-alpha (TNF). The endothelial cells were cultured in glass capillaries (microslides) that could be conveniently transferred to a flow-based adhesion assay, to test the ability of the cultures to support adhesion and migration of flowing leukocytes. Paired control, 'static' samples were exposed to the identical medium and culture geometry. We found that the type of tubing used in the culture flow circuit and its maintenance at 37 degrees C were critical design factors, which could influence the response to TNF of the static controls which were exposed to recirculated medium. Endothelial cells conditioned by culture under flow showed a reduction in response to TNF, as judged by ability to induce the capture and migration of neutrophils. We found that the higher the shear stress, the weaker the ability to recruit neutrophils. This sensitivity to shear stress was greater if the cells were allowed to stabilise under static conditions for 24 h, compared to cells exposed to flow immediately after seeding. The inhibition of neutrophil recruitment was similar for cultures exposed to steady flow or flow with a pulsatile element (flow oscillation approximately 20% about the mean). Thus, we have developed a versatile culture system which allows investigations of functional modifications of endothelial cells and demonstrates the potential sensitivity of inflammatory responses to the local fluid environment.

Analysis of the effect of disturbed flow on monocytic adhesion to endothelial cells

The preferential adhesion of monocytes to vascular endothelial cells (ECs) at regions near branches and curvatures of the arterial tree, where flow is disturbed, suggests that hemodynamic conditions play significant roles in monocyte adhesion. The present study aims to elucidate the effects of disturbed flow on monocyte adhesion to ECs and the adhesive properties of ECs. We applied, for the first time, the micron-resolution particle image velocimetry (microPIV) technique to analyze the characteristics of the disturbed flow produced in our vertical-step flow (VSF) chamber. The results demonstrated the existence of a higher near-wall concentration and a longer residence time of the monocytic analog THP-1 cells near the step and the reattachment point. THP-1 cells showed prominent adhesion to ECs pretreated with TNFalpha in the regions near the step and the reattachment point, but they showed virtually no adhesion to un-stimulated ECs. Pre-incubation of the TNFalpha-treated ECs with antibodies against intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and E-selectin inhibited the THP-1 adhesion; the maximal inhibition was observed with a combination of these antibodies. Pre-exposure of ECs to disturbed flow in VSF for 24 h led to significant increases in their surface expressions of ICAM-1 and E-selectin, but not VCAM-1, and in the adhesion of THP-1 cells. Our findings demonstrate the importance of complex flow environment in modulating the adhesive properties of vascular endothelium and consequently monocyte adhesion in regions of prevalence of atherosclerotic lesions.

Occlusion of Arterio-Arterial Anastomosis Manifesting as Acute Twin–Twin Transfusion Syndrome

In vivo, ex vivo and modelling studies suggest that arterio-arterial anastomoses (AAAs) protect against haemodynamic imbalance in monochorionic twins and thus the development of TTTS. We report the acute onset of severe TTTS at 34 weeks' gestation in a patient with an antenatally visualized AAA which was shown at injection studies to have been obliterated, presumably by thrombosis. Computer modelling with the relevant clinical data confirmed that occlusion of the AAA alone was sufficient to reproduce the clinical manifestations. A study of the vascular configuration of AAA in the fixed placenta suggested that its small diameter and turbulent flow may have contributed to its occlusion. This case report shows that the unmasking of unbalanced AVA configurations by occlusion of a protective AAA can manifest as TTTS.

Circulating blood cells modulate the atherosclerotic process in apolipoprotein E-deficient mice

The interaction of blood with the arterial tree may play an important role in the development of atherosclerotic lesions. The aims of this study were (1) to determine how anemia or increased hematocrit affect the development of atherosclerosis and (2) to find relationships between hematologic and hemorrheologic variables in apolipoprotein (apo) E-deficient mice. Forty-two mice were randomly divided into 3 groups of 14 mice each. There was no further manipulation in the control group. To induce anemia, the mice from one of the groups were repeatedly bled, drawing approximately 250 microL blood from each mouse twice a week. To increase the hematocrit levels in another group of mice, we injected 20 U recombinant human erythropoietin every other day. The development of lesions and the main variables involved in atherogenesis were compared among groups. Our results show that atherosclerosis was attenuated in the mice that were bled, and this was not accounted for by changes in plasma lipid levels, the distribution of lipoprotein particles, the body iron distribution, or oxidation parameters. Moreover, atherosclerosis was enhanced in the mice treated with the continuous administration of erythropoietin. To ascertain the relationship between hematocrit and whole blood viscosity, we measured both variables in pooled blood from 24 additional mice, which were manipulated to ensure a wide range of values. We found a direct and significant correlation between hematocrit and blood viscosity and between hematocrit and lesion size. Our data support in vivo the idea that hemorrheology has an important role in atherogenesis in this particular animal model.

Oscillatory shear stress stimulates endothelial production of O2- from p47phox-dependent NAD(P)H oxidases, leading to monocyte adhesion

Arterial regions exposed to oscillatory shear (OS) in branched arteries are lesion-prone sites of atherosclerosis, whereas those of laminar shear (LS) are relatively well protected. Here, we examined the hypothesis that OS and LS differentially regulate production of O2- from the endothelial NAD(P)H oxidase, which, in turn, is responsible for their opposite effects on a critical atherogenic event, monocyte adhesion. We used aortic endothelial cells obtained from C57BL/6 (MAE-C57) and p47phox-/- (MAE-p47-/-) mice, which lack a component of NAD(P)H oxidase. O2- production was determined by dihydroethidium staining and an electron spin resonance using an electron spin trap methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine. Chronic exposure (18 h) to an arterial level of OS (+/- 5 dynes/cm2) increased O2- (2-fold) and monocyte adhesion (3-fold) in MAE-C57 cells, whereas chronic LS (15 dynes/cm2, 18 h) significantly decreased both monocyte adhesion and O2- compared with static conditions. In contrast, neither LS nor OS were able to induce O2- production and monocyte adhesion to MAE-p47-/-. Treating MAE-C57 with a cell-permeable superoxide dismutase compound, polyethylene glycol-superoxide dismutase, also inhibited OS-induced monocyte adhesion. In addition, over-expressing p47phox in MAE-p47-/- restored OS-induced O2- production and monocyte adhesion. These results suggest that chronic exposure of endothelial cells to OS stimulates O2- and/or its derivatives produced from p47phox-dependent NAD(P)H oxidase, which, in turn, leads to monocyte adhesion, an early and critical atherogenic event.

Prediction of sites of coronary atherosclerosis progression:In vivo profiling of endothelial shear stress, lumen, and outer vessel wall characteristics to predict vascular behavior

PURPOSE OF REVIEW

Native atherosclerosis and in-stent restenosis are focal and evolve independently. The endothelium regulates arterial behavior by responding to its local environment of hemodynamic stresses, in particular, shear stress. Identification of endothelial shear stress and arterial wall characteristics may allow for the prediction of the progression of atherosclerosis. Accurate identification of arterial segments at high risk for progression may permit preemptive intervention strategies to avoid adverse coronary events.

RECENT FINDINGS

In vitro studies indicate that low endothelial shear stress upregulates the genetic and molecular responses leading to the initiation and progression of atherosclerosis, and promotes inflammation and formation of other features characteristic of vulnerable plaque. Physiologic endothelial shear stress is vasculoprotective and fosters quiescence of the endothelium and vascular wall. High endothelial shear stress promotes platelet aggregation. Recent studies have now provided evidence that endothelial shear stress and vascular wall morphology along the course of human coronary arteries can be characterized in vivo, and, in serial studies, may actually predict the focal areas in which atherosclerosis progression occurs.

SUMMARY

Rapidly evolving methodologies are able to characterize the arterial wall and the local hemodynamic environmental factors likely responsible for progression of coronary disease in humans. These new diagnostic modalities allow for identification of plaque progression. Future studies need to identify the factors responsible for vulnerable plaque formation. The current availability of drug-eluting stents with a low risk of restenosis allows for consideration of preemptive intervention strategies for these high-risk vascular sites such that future adverse coronary events can be averted.

Shear flow-induced motility of Dictyostelium discoideum cells on solid substrate

Application of a mild hydrodynamic shear stress to Dicytostelium discoideum cells, unable to detach cells passively from the substrate, triggers a cellular response consisting of steady membrane peeling at the rear edge of the cell and periodic cell contact extensions at its front edge. Both processes require an active actin cytoskeleton. The cell movement induced by the hydrodynamic forces is very similar to amoeboid cell motion during chemotaxis, as for its kinematic parameters and for the involvement of phosphatidylinositol(3,4,5)-trisphosphate internal gradient to maintain cell polarity. Inhibition of phosphoinositide 3-kinases by LY294002 randomizes the orientation of cell movement with respect to the flow without modifying cell speed. Two independent signaling pathways are, therefore, induced in D. discoideum in response to external forces. The first increases the frequency of pseudopodium extension, whereas the second redirects the actin cytoskeleton polymerization machinery to the edge opposite to the stressed side of the cell.

Airway distension promotes leukocyte recruitment in rat tracheal circulation

Mechanical distortion of blood vessels is known to activate endothelial cells. Whether airway distension likewise activates the vascular endothelium within the airway wall is unknown. Using intravital microscopy in the rat trachea, we investigated if airway distention with the application of positive end-expiratory pressure (PEEP) caused leukocyte recruitment to the airway. Tracheal postcapillary venules were visualized and leukocyte kinetics monitored in anesthetized, mechanically ventilated rats (80 breaths/minute, 6 ml/kg VT, 1 cm H(2)O PEEP). Leukocyte rolling velocity (Vwbc) and the number of adherent cells were not altered with normal ventilation over the course of 2 hours. Ventilation with sustained PEEP (8 cm H(2)O for 1 hour reduced Vwbc and increased adhesion, reaching a maximum at 1 hour of PEEP. Intermittent (2x and 5x) 8 cm H(2)O PEEP also induced a similar reduction in Vwbc, accompanied by an increase in adhesion. However, leukocyte recruitment after airway distension is localized to the airways because increased PEEP did not induce leukocyte recruitment in the mesenteric microcirculation or when PEEP was applied to the lung distal to the site of measurement. Pretreatment with endothelin receptor and selectin inhibitors blocked the effects of distension on leukocyte recruitment, suggesting their involvement in the proinflammatory response.

Exposure to fluid shear stress modulates the ability of endothelial cells to recruit neutrophils in response to tumor necrosis factor-alpha: a basis for local variations in vascular sensitivity to inflammation

Vascular endothelial cells are able to sense changes in the forces acting on them and respond, for instance, by modifying expression of a range of genes. However, there is little information on how such responses are integrated to modify homeostatic functions. We hypothesized that different shear stresses experienced in different regions of the circulation might influence endothelial sensitivity to inflammatory stimuli. We cultured human endothelial cells in tubes and exposed them for varying periods to shear stresses ranging from those typically found in postcapillary venules to those in arteries. When tumor necrosis factor-alpha was included in the flow cultures, we found startling differential effects of shear stress on the ability of endothelial cells to induce adhesion and migration of flowing neutrophils. Compared with static cultures, endothelial cells cultured at low shear stress (0.3 Pa) captured similar numbers of neutrophils but failed to induce their transendothelial migration. After exposure of endothelial cells to high shear stress (1.0 or 2.0 Pa), capture of neutrophils was largely ablated. The modification in response was detectable after 4 hours of exposure to flow but was much greater after 24 hours. From analysis of gene expression, loss of capture or migration was attributable to reduction in tumor necrosis factor-induced expression of selectins or CXC-chemokines, respectively. Thus, conditioning of endothelial cells by different flow environments may underlie variations in susceptibility to inflammation between different tissues or parts of the vascular tree.

Increased C-reactive protein concentrations in never-treated hypertension

OBJECTIVE

To test whether the plasma concentration of C-reactive protein (CRP), a sensitive marker of systemic inflammation, is increased in patients with newly diagnosed, never-treated hypertension and whether blood pressure and its pulsatile component, pulse pressure, are correlated with plasma CRP concentration independently of a consistent number of cardiovascular risk factors.

DESIGN

Cross-sectional study in a hospital outpatient hypertension clinic.

METHODS

A total of 135 newly diagnosed, never-treated patients with hypertension and 40 healthy matched non-hypertensive controls underwent office and 24-h blood pressure measurement and blood sampling for determination of plasma CRP and serum lipid concentrations.

RESULTS

Plasma CRP concentration was greater in hypertensive individuals (1.85 mg/l, interquartile range 0.74-3.64) than in control individuals (1.01 mg/l, interquartile range 0.67-1.88; P = 0.02). In the entire population, CRP had a significant direct association with office systolic blood pressure and pulse pressure, but not with diastolic blood pressure. Among hypertensive patients, plasma CRP was related to 24-h systolic blood pressure (r = 0.28, P < 0.01) and pulse pressure (r = 0.32, P < 0.01), but not to diastolic blood pressure (r = 0.12, P > 0.2). CRP was also directly associated with body mass index (r = 0.25, P < 0.01), serum low-density lipoprotein cholesterol (r = 0.21, P = 0.03) and serum triglycerides (r = 0.21, P = 0.03). In the multivariate analysis, systolic blood pressure and pulse pressure, but not diastolic blood pressure, were significant predictors of plasma CRP concentration when a consistent number of cardiovascular risk factors was controlled for simultaneously.

CONCLUSIONS

Systolic blood pressure and pulse pressure, but not diastolic blood pressure, are predictors of plasma C-reactive protein concentrations in patients with newly diagnosed, never-treated hypertension, irrespective of the potential proinflammatory action of traditional cardiovascular risk factors.

Monocyte recruitment to endothelial cells in response to oscillatory shear stress

Leukocyte recruitment to endothelial cells is a critical event in inflammatory responses. The spatial, temporal gradients of shear stress, topology, and outcome of cellular interactions that underlie these responses have so far been inferred from static imaging of tissue sections or studies of statically cultured cells. In this report, we developed micro-electromechanical systems (MEMS) sensors, comparable to a single endothelial cell (EC) in size, to link real-time shear stress with monocyte/EC binding kinetics in a complex flow environment, simulating the moving and unsteady separation point at the arterial bifurcation with high spatial and temporal resolution. In response to oscillatory shear stress (tau) at +/- 2.6 dyn/cm2 at a time-averaged shear stress (tau(ave))=0 and 0.5 Hz, individual monocytes displayed unique to-and-fro trajectories undergoing rolling, binding, and dissociation with other monocyte, followed by solid adhesion on EC. Our study quantified individual monocyte/EC binding kinetics in terms of displacement and velocity profiles. Oscillatory flow induces up-regulation of adhesion molecules and cytokines to mediate monocyte/EC interactions over a dynamic range of shear stress +/- 2.6 dyn/cm2 (P=0.50, n=10).

Expression of matrix metalloproteinase-9 in endothelial cells is differentially regulated by shear stress. Role of c-Myc

Atherosclerotic plaques preferentially localize to areas of the vasculature with complex laminar or oscillatory blood flow. Prior data implicate matrix metalloproteinases (MMPs) in the initiation and progression of atherosclerotic lesions. In cultured endothelial cells, oscillatory but not unidirectional shear significantly increases MMP-9 mRNA as well as secretion of the MMP-9 protein (p < 0.05). In contrast, cell-associated protein levels of Tissue Inhibitor of MMP 1 (TIMP-1), an inhibitor of MMP-9, are insensitive to the shear regimen. To investigate transcriptional regulation of MMP-9 gene expression, we utilized retroviral-based reporter constructs containing different lengths of the human MMP-9 promoter. The activity of the full MMP-9 promoter is 3-fold higher (p < 0.05) in unidirectional shear compared with static conditions, and the activity is further increased approximately 10-fold by oscillatory shear (p < 0.01) over unidirectional flow. Our data identify a shear-sensitive binding site at -152 in the MMP-9 promoter. We show that the c-Myc transcription factor binds specifically to this site and that reporter constructs in which the c-Myc binding site was abolished lacked the shear responsiveness of native MMP-9 reporter constructs. Our results suggest that endothelial MMP-9 expression is flow-sensitive and is up-regulated by oscillatory flow via activation of c-Myc. This effect may contribute to the development and progression of atherosclerotic lesions in areas of vasculature that are subject to disturbed flow.

Bone morphogenic protein 4 produced in endothelial cells by oscillatory shear stress stimulates an inflammatory response

Atherosclerosis is now viewed as an inflammatory disease occurring preferentially in arterial regions exposed to disturbed flow conditions, including oscillatory shear stress (OS), in branched arteries. In contrast, the arterial regions exposed to laminar shear (LS) are relatively lesion-free. The mechanisms underlying the opposite effects of OS and LS on the inflammatory and atherogenic processes are not clearly understood. Here, through DNA microarrays, protein expression, and functional studies, we identify bone morphogenic protein 4 (BMP4) as a mechanosensitive and pro-inflammatory gene product. Exposing endothelial cells to OS increased BMP4 protein expression, whereas LS decreased it. In addition, we found BMP4 expression only in the selective patches of endothelial cells overlying foam cell lesions in human coronary arteries. The same endothelial patches also expressed higher levels of intercellular cell adhesion molecule-1 (ICAM-1) protein compared with those of non-diseased areas. Functionally, we show that OS and BMP4 induced ICAM-1 expression and monocyte adhesion by a NFkappaB-dependent mechanism. We suggest that BMP4 is a mechanosensitive, inflammatory factor playing a critical role in early steps of atherogenesis in the lesion-prone areas.

The response of endothelial cells to fluid shear stress using a co-culture model of the arterial wall

An endothelial cell (EC) smooth muscle cell (SMC) co-culture model of the arterial wall was used to study the effect of fluid shear stress on EC behavior. This model, in addition to being a more realistic tissue analogue, is a valuable research tool for studying the effects of mechanical stimulation upon the behavior of both SMCs and ECs. In the present study, a 10% cyclic strain was used to alter the characteristics of an SMC-seeded collagen gel. This form of strain preconditioning resulted in a rearrangement of the vessel wall that yielded circumferentially oriented cells and collagen fibrils. The preconditioned collagen gel was subsequently seeded with ECs and exposed to fluid-induced shear stress (10 dynes/cm2) for 48 hr. In the absence of flow, ECs seeded on slab constructs were oriented with the underlying collagen fibrils. Sheared constructs exhibited ECs oriented in the flow direction. Shear stress also affected EC proliferation, reducing the total number of dividing ECs by as much as 48 percent compared to unsheared constructs. The shear-induced reduction in proliferation was further enhanced when constructs were first strain-preconditioned (64% reduction). Moreover, conditioned media from shear stress experiments inhibited proliferation of ECs seeded on tissue culture plastic. These results suggest that EC response to fluid shear stress in a collagen co-culture model is influenced by the underlying substrate, and one that in this study is modified by strain preconditioning.

Pressure distension stimulates the expression of endothelial adhesion molecules in the human saphenous vein graft

BACKGROUND

Mechanical trauma occurring during saphenous vein graft harvesting plays a major role in graft failure after coronary bypass surgery. There is increasing evidence that neutrophil-endothelial interaction is involved in the pathogenesis of early graft occlusion. This study evaluates the effect of pressure distension on the expression of endothelial adhesion molecules in human saphenous vein.

METHODS

Segments of saphenous vein graft (SVG) were collected from 20 patients undergoing coronary bypass surgery. We evaluated the expression of intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and P-selectin on SVG endothelium under basal conditions and after pressure distension at 300 mm Hg. In the same experimental setting we also evaluated adhesion of both unstimulated and activated neutrophils to the endothelium of SVG.

RESULTS

Control endothelial cells exhibited only a weak staining for intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and P-selectin, whereas the levels of adhesion molecules increased significantly in the distended veins. Similarly, significantly greater adhesion of both unstimulated and activated neutrophils was observed in distended veins compared with control veins.

CONCLUSIONS

Pressure distension of SVG before coronary bypass surgery induces upregulation of endothelial adhesion molecules, with subsequent increase in neutrophil adhesion to the endothelium. Neutrophil adhesion to endothelial cells may contribute to early failure of SVG.

Cap G, a gelsolin family protein modulating protective effects of unidirectional shear stress

Atherosclerosis is a progressive and complex pathophysiological process occurring in large arteries. Although it is of multifactorial origin, the disease develops at preferential sites along the vasculature in regions experiencing specific hemodynamic conditions that are predisposed to endothelial dysfunction. The exact mechanisms allowing endothelial cells to discriminate between plaque-free and plaque-prone flows remain to be explored. To investigate such mechanisms, we performed a proteomic analysis on endothelial cells exposed in vitro to these two-flow patterns. A few spots on the two-dimensional gel had an intensity that was differentially regulated by plaque-free versus plaque-prone flows. One of them was further investigated and identified as macrophage-capping protein (Cap G), a member of the gelsolin protein superfamily. A 2-fold increase of Cap G protein and a 5-fold increase of Cap G mRNA were observed in cells exposed to a plaque-free flow as compared with static cultures. This increase was not observed in cells exposed to plaque-prone flow. Plaque-free flow induced a corresponding increase in nuclear and cytoskeletal-associated Cap G. Finally, overexpression of Cap G in transfection assays increased the motility potential of endothelial cells. These observations together with the known functions of Cap G suggest that Cap G may contribute to the protective effect exerted by plaque-free flow on endothelial cells. On the contrary, in cells exposed to a plaque-prone flow, no induction of Cap G expression could be observed.

Cellular adhesion molecules and cardiovascular disease. Part I. Their expression and role in atherogenesis

Atherosclerosis is now recognized to be an inflammatory disease, and several inflammatory markers have been shown to be associated with both the presence and future risk of symptomatic cardiovascular disease. Cellular adhesion molecules, particularly members of the selectin family and immunoglobulin superfamily, are intimately involved in the recruitment of leucocytes to sites of inflammation, including developing atherosclerotic lesions. Their role in the pathogenesis of atherosclerosis has been clearly demonstrated using knockout mice models. Plasma levels of adhesion molecules, which have been shed from the cell surface, have been associated with the presence of clinical atherosclerotic disease, although published studies differ in their findings. This limited consensus in the literature may be explained either by unrecognized confounding factors, or perhaps by the unpredictable relationship between cell surface expression and activity of cellular adhesion molecules and their shedding into the plasma. While cell surface activity of adhesion molecules appears critical in the development of atherosclerotic lesions, the measurement of plasma levels of soluble adhesion molecules may offer little additional benefit for individual patients in the prediction of the extent of atherosclerotic disease above the assessment of conventional cardiovascular risk factors.

Epinephrine promotes pulmonary angiitis: evidence for a beta1-adrenoreceptor-mediated mechanism

Epinephrine (Epi) increases lymphocyte traffic to lung. We investigated whether Epi also modulates pulmonary cell-mediated immune responses in vivo. C57BL/6 mice were immunized with hen-egg lysozyme (HEL) on day 0, challenged with HEL intratracheally at day 12, and killed at day 15. Mice received Epi (0.5 mg/kg) subcutaneously during the sensitization phase, days 1-7 (Epi-SP), or the effector phase, days 12-14 (Epi-EP); controls received saline subcutaneously. Epi-SP mice showed increased airway inflammation (P < 0.03) and pulmonary angiitis (P < 0.04) characterized by endothelialitis and subendothelial fibrin deposition. Macrophages and granulocytes were increased in perivascular cuffs in situ (P < 0.001). CD3+ lymphocytes increased in the bronchoalveolar lavage fluid, whereas NK1.1+ and CD4+CD25+ lymphocytes decreased (all P < 0.05). Atenolol, a selective beta1-adrenoreceptor (AR) antagonist, inhibited the increased vascular and airway inflammation and the reduction in CD4+CD25+ lymphocytes (all P < 0.05) yielded by Epi, whereas all alpha/beta-AR blockers inhibited airway inflammation. We conclude that Epi-EP selectively promotes vascular inflammation in vivo via a beta1-receptor-mediated mechanism.

A model for shear stress sensing and transmission in vascular endothelial cells

Arterial endothelial cell (EC) responsiveness to flow is essential for normal vascular function and plays a role in the development of atherosclerosis. EC flow responses may involve sensing of the mechanical stimulus at the cell surface with subsequent transmission via cytoskeleton to intracellular transduction sites. We had previously modeled flow-induced deformation of EC-surface flow sensors represented as viscoelastic materials with standard linear solid behavior (Kelvin bodies). In the present article, we extend the analysis to arbitrary networks of viscoelastic structures connected in series and/or parallel. Application of the model to a system of two Kelvin bodies in parallel reveals that flow induces an instantaneous deformation followed by creeping to the asymptotic response. The force divides equally between the two bodies when they have identical viscoelastic properties. When one body is stiffer than the other, a larger fraction of the applied force is directed to the stiffer body. We have also probed the impact of steady and oscillatory flow on simple sensor-cytoskeleton-nucleus networks. The results demonstrated that, consistent with the experimentally observed temporal chronology of EC flow responses, the flow sensor attains its peak deformation faster than intracellular structures and the nucleus deforms more rapidly than cytoskeletal elements. The results have also revealed that a 1-Hz oscillatory flow induces significantly smaller deformations than steady flow. These results may provide insight into the mechanisms behind the experimental observations that a number of EC responses induced by steady flow are not induced by oscillatory flow.

Norepinephrine enhances adhesion of HIV-1-infected leukocytes to cardiac microvascular endothelial cells

Recent reports have indicated that norepinephrine (NE) enhances HIV replication in infected monocytes and promotes increased expression of select matrix metalloproteinases associated with dilated cardiomyopathy (DCM) in vitro in co-cultures of HIV-infected leukocytes and human cardiac microvascular endothelial cells (HMVEC-C). The influence of NE on HIV infection and leukocyte-endothelial interactions suggests a pathogenic role in AIDS-related cardiovascular disease. This study examined the effects of norepinephrine (NE) and HIV-1 infection on leukocyte adhesion to HMVEC-C. Both flow and static conditions were examined and the expression of selected adhesion molecules and cytokines were monitored in parallel. NE pretreatment resulted in a detectable, dose-dependent increase of leukocyte-endothelial adhesion (LEA) with both HIV-1-infected and -uninfected peripheral blood mononuclear cells (PBMCs) relative to media controls after 48 hr in co-culture with HMVEC-C in vitro. However, the combination of NE plus HIV infection resulted in a significant (P < 0.0001) 18-fold increase in LEA over uninfected media controls. Increased levels in both cell-associated and -soluble ICAM-1 and E-Selectin but not VCAM-1 correlated with increased LEA and with HIV-1 infection or NE pretreatment. Blocking antibodies specific for ICAM-1 or E-Selectin inhibited HIV-NE-induced LEA. These data suggest a model in which NE primes HIV-1-infected leukocytes for enhanced adhesion and localization in HMVEC-C where they can initiate and participate in vascular injury associated with AIDS-related cardiomyopathy.

Shear stress inhibits adhesion molecule expression in vascular endothelial cells induced by coculture with smooth muscle cells

Vascular endothelial cells (ECs), which exist in close proximity to vascular smooth muscle cells (SMCs), are constantly subjected to blood flow-induced shear stress. Although the effect of shear stress on endothelial biology has been extensively studied, the influence of SMCs on endothelial response to shear stress remains largely unexplored. We examined the potential role of SMCs in regulating the shear stress-induced gene expression in ECs, using a parallel-plate coculture flow system in which these 2 types of cells were separated by a porous membrane. In this coculture system, SMCs tended to orient perpendicularly to the flow direction, whereas the ECs were elongated and aligned with the flow direction. Under static conditions, coculture with SMCs induced EC gene expression of intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and E-selectin, while attenuating EC gene expression of endothelial nitric oxide synthase (eNOS). Shear stress significantly inhibited SMC-induced adhesion molecule gene expression. These EC responses under static and shear conditions were not observed in the absence of close communication between ECs and SMCs, and they were also not observed when ECs were cocultured with fibroblasts instead of SMCs. Our findings indicate that under static conditions, coculture with SMCs induces ICAM-1, VCAM-1, and E-selectin gene expression in ECs. These coculture effects are inhibited by shear stress and require specific interaction between ECs and SMCs in close contact.

Modulation of collateral artery growth in a porcine hindlimb ligation model using MCP-1

For an appropriate extrapolation to patients with peripheral arterial obstructive disease, we tested the efficacy of monocyte chemoattractant protein 1 (MCP-1) treatment in a porcine hindlimb ligation model. In 40 minipigs, a femoral artery ligation was performed. Control animals were examined immediately after ligation (n = 4) or after 2 wk of intra-arterial infusion of PBS (n = 11). A second group of animals was evaluated after intra-arterial infusion of 2.0 microg/h of MCP-1 for 48 h (followed by 12 days of PBS; n = 13) or 2 wk continuously (n = 12). In the terminal experiment after 2 wk, resting flow to the leg and peripheral arterial pressures were assessed without vasodilatation. Subsequently, vascular conductance was determined by using a pump-driven extracorporal circulation during maximal vasodilatation. The results showed that resting blood flow to the hindlimb was 53% of the normal after 2 wk of infusion of PBS, compared with 81% in both MCP-1 treatment groups (P < 0.05). Collateral conductance was 645 +/- 346 ml x min(-1) x mmHg(-1) after 2 wk of infusion with PBS, compared with 1,070 +/- 530 and 1,158 +/- 535 ml x min(-1) x mmHg(-1) after 48 h and 2 wk treatment with MCP-1, respectively (P < 0.05). Modulation of the process of arteriogenesis is feasible in this large animal model via intra-arterial infusion of the Cys-Cys-chemokine MCP-1.