Induction of nitric oxide synthase mRNA by shear stress requires intracellular calcium and G-protein signals and is modulated by PI 3 kinase

New Dawn for Atherosclerosis: Vascular Endothelial Cell Senescence and Death

Endothelial cells (ECs) form the inner linings of blood vessels, and are directly exposed to endogenous hazard signals and metabolites in the circulatory system. The senescence and death of ECs are not only adverse outcomes, but also causal contributors to endothelial dysfunction, an early risk marker of atherosclerosis. The pathophysiological process of EC senescence involves both structural and functional changes and has been linked to various factors, including oxidative stress, dysregulated cell cycle, hyperuricemia, vascular inflammation, and aberrant metabolite sensing and signaling. Multiple forms of EC death have been documented in atherosclerosis, including autophagic cell death, apoptosis, pyroptosis, NETosis, necroptosis, and ferroptosis. Despite this, the molecular mechanisms underlying EC senescence or death in atherogenesis are not fully understood. To provide a comprehensive update on the subject, this review examines the historic and latest findings on the molecular mechanisms and functional alterations associated with EC senescence and death in different stages of atherosclerosis.

Hilaire C, Schulz E, Kröller-Schön S, Keaney JF. PGC1α Regulates the Endothelial Response to Fluid Shear Stress via Telomerase Reverse Transcriptase Control of Heme Oxygenase-1

OBJECTIVE

Fluid shear stress (FSS) is known to mediate multiple phenotypic changes in the endothelium. Laminar FSS (undisturbed flow) is known to promote endothelial alignment to flow, which is key to stabilizing the endothelium and rendering it resistant to atherosclerosis and thrombosis. The molecular pathways responsible for endothelial responses to FSS are only partially understood. In this study, we determine the role of PGC1α (peroxisome proliferator gamma coactivator-1α)-TERT (telomerase reverse transcriptase)-HMOX1 (heme oxygenase-1) during shear stress in vitro and in vivo. Approach and Results: Here, we have identified PGC1α as a flow-responsive gene required for endothelial flow alignment in vitro and in vivo. Compared with oscillatory FSS (disturbed flow) or static conditions, laminar FSS (undisturbed flow) showed increased PGC1α expression and its transcriptional coactivation. PGC1α was required for laminar FSS-induced expression of TERT in vitro and in vivo via its association with ERRα(estrogen-related receptor alpha) and KLF (Kruppel-like factor)-4 on the TERT promoter. We found that TERT inhibition attenuated endothelial flow alignment, elongation, and nuclear polarization in response to laminar FSS in vitro and in vivo. Among the flow-responsive genes sensitive to TERT status, HMOX1 was required for endothelial alignment to laminar FSS.

CONCLUSIONS

These data suggest an important role for a PGC1α-TERT-HMOX1 axis in the endothelial stabilization response to laminar FSS.

Exercise-mediated adaptations in vascular function and structure: Beneficial effects in coronary artery disease

Exercise exerts direct effects on the vasculature via the impact of hemodynamic forces on the endothelium, thereby leading to functional and structural adaptations that lower cardiovascular risk. The patterns of blood flow and endothelial shear stress during exercise lead to atheroprotective hemodynamic stimuli on the endothelium and contribute to adaptations in vascular function and structure. The structural adaptations observed in arterial lumen dimensions after prolonged exercise supplant the need for acute functional vasodilatation in case of an increase in endothelial shear stress due to repeated exercise bouts. In contrast, wall thickness is affected by rather systemic factors, such as transmural pressure modulated during exercise by generalized changes in blood pressure. Several mechanisms have been proposed to explain the exercise-induced benefits in patients with coronary artery disease (CAD). They include decreased progression of coronary plaques in CAD, recruitment of collaterals, enhanced blood rheological properties, improvement of vascular smooth muscle cell and endothelial function, and coronary blood flow. This review describes how exercise via alterations in hemodynamic factors influences vascular function and structure which contributes to cardiovascular risk reduction, and highlights which mechanisms are involved in the positive effects of exercise on CAD.

Endothelial function in patients with atrial fibrillation

Atrial fibrillation (AF) is the most common heart arrhythmia and is associated with poor outcomes. The adverse effects of AF are mediated through multiple pathways, including endothelial dysfunction, as measured by flow-mediated dilatation. Flow-mediated dilatation has demonstrated endothelial dysfunction in several conditions and is associated with poor outcomes including mortality, yet can be improved with medical therapy. It is thus a useful tool in assessing endothelial function in patients. Endothelial dysfunction is present in patients with AF and is associated with poor outcomes. These patients are generally older and have other co-morbidities such as hypertension, hypercholesterolaemia and diabetes. The precise process by which AF is affiliated with endothelial damage/dysfunction remains elusive. This review explores the endothelial structure, its physiology and how it is affected in patients with AF. It also assesses the utility of flow mediated dilatation as a technique to assess endothelial function in patients with AF. Key MessagesEndothelial function is affected in patients with atrial fibrillation as with other cardiovascular conditions.Endothelial dysfunction is associated with poor outcomes such as stroke, myocardial infarction and death, yet is a reversible condition.Flow-mediated dilatation is a reliable tool to assess endothelial function in patients with atrial fibrillation.Patients with atrial fibrillation should be considered for endothelial function assessment and attempts made to reverse this condition.

Shear Stress Attenuates Inward Remodeling in Cultured Mouse Thoracodorsal Arteries in an eNOS-Dependent, but Not Hemodynamic Manner, and Increases Cx37 Expression

BACKGROUND

Arteries chronically constricted in culture remodel to smaller diameters. Conversely, elevated luminal shear stress (SS) promotes outward remodeling of arteries in vivo and prevents inward remodeling in culture in a nitric oxide synthase (NOS)-dependent manner.

OBJECTIVES

To determine whether SS-induced prevention of inward remodeling in cultured arteries is specifically eNOS-dependent and requires dilation, and whether SS alters the expression of eNOS and other genes potentially involved in remodeling.

METHODS

Female mouse thoracodorsal arteries were cannulated, pressurized to 80 mm Hg, and cultured for 2 days with low SS (<7 dyn/cm2), high SS (≥15 dyn/cm2), high SS + L-NAME (NOS inhibitor, 10-4 M), or high SS in arteries from eNOS-/- mice. In separate arteries cultured 1 day with low or high SS, eNOS and connexin (Cx) 37, Cx40, and Cx43 mRNA were assessed with real-time PCR.

RESULTS

High SS caused little change in passive diameters after culture (-4.7 ± 2.0%), which was less than low SS (-18.9 ± 1.4%; p < 0.0001), high SS eNOS-/- (-18.0 ± 1.5; p < 0.001), or high SS + L-NAME (-12.0 ± 0.6%; nonsignificant) despite similar constriction during culture. Cx37 mRNA expression was increased (p < 0.05) with high SS, but other gene levels were not different.

CONCLUSIONS

eNOS is involved in SS-induced prevention of inward remodeling in cultured small arteries. This effect does not require NO-mediated dilation. SS increased Cx37.

Endothelial dysfunction in cerebral aneurysms

Endothelial cell (EC) dysfunction is known to contribute to cerebral aneurysm (CA) pathogenesis. Evidence shows that damage or injury to the EC layer is the first event in CA formation. The mechanisms behind EC dysfunction in CA disease are interrelated and include hemodynamic stress, hazardous nitric oxide synthase (NOS) activity, oxidative stress, estrogen imbalance, and endothelial cell-to-cell junction compromise. Abnormal variations in hemodynamic stress incite pathological EC transformation and inflammatory zone formation, ultimately leading to destruction of the vascular wall and aneurysm dilation. Hemodynamic stress activates key molecular pathways that result in the upregulation of chemotactic cytokines and adhesion molecules, leading to inflammatory cell recruitment and infiltration. Concurrently, oxidative stress damages EC-to-EC junction proteins, resulting in interendothelial gap formation. This further promotes leukocyte traffic into the vessel wall and the release of matrix metalloproteinases, which propagates vascular remodeling and breakdown. Abnormal hemodynamic stress and inflammation also trigger adverse changes in NOS activity, altering proper EC mediation of vascular tone and the local inflammatory environment. Additionally, the vasoprotective hormone estrogen modulates gene expression that often suppresses these harmful processes. Crosstalk between these sophisticated pathways contributes to CA initiation, progression, and rupture. This review aims to outline the complex mechanisms of EC dysfunction in CA pathogenesis.

Evidence of a limb- and shear stress stimulus profile-dependent impact of high-intensity cycling training on flow-mediated dilation

Lower limb endurance training can improve conduit artery flow-mediated dilation (FMD) in response to transient increases in shear stress (reactive hyperemia; RH-FMD) in both the upper and lower limbs. Sustained increases in shear stress recruit a partially distinct transduction pathway and elicit a physiologically relevant FMD response (SS-FMD) that provides distinct information regarding endothelial function. However, the impact of training on SS-FMD is not well understood. The purpose of this study was to determine the impact of cycling training on handgrip exercise-induced brachial artery (BA) FMD (BA SS-FMD) and calf plantar-flexion-induced superficial femoral artery (SFA) FMD (SFA SS-FMD). RH-FMD was also assessed in both arteries. Twenty-eight young males were randomized to control (n = 12) or training (n = 16) groups. The training group cycled 30 min/day, 3 days/week for 4 weeks at 80% heart rate reserve. FMD was assessed in the BA and SFA before and after the intervention via Duplex ultrasound. Results are means ± SD. Training did not impact SS-FMD in either artery, and SFA RH-FMD was also unchanged (p > 0.05). When controlling for the shear rate stimulus via covariate analysis, BA RH-FMD improved in the training group (p = 0.05) (control - pre-intervention: 5.7% ± 2.4%, post-intervention: 5.3% ± 2.4%; training - pre-intervention: 5.4% ± 2.5%, post-intervention: 7.2% ± 2.4%). Thus, endurance training resulted in nonuniform adaptations to endothelial function, with an isolated impact on the BA's ability to transduce a transient increase in shear stress. Novelty Training did not alter SS-FMD in the arm or leg. RH-FMD was augmented in the arm only. Thus training adaptations were limb- and shear stress profile-specific.

Low WSS and High OSI Measured by 3D Cine PC MRI Reflect High Pulmonary Artery Pressures in Suspected Secondary Pulmonary Arterial Hypertension

Purpose:

Institutional Review Board (IRB)-approved prospective study was conducted to test whether objective and quantitative hemodynamic markers wall shear stress (WSS) and oscillatory shear index (OSI) measured by three-dimensional (3D) cine phase-contrast (PC) can reflect pulmonary arterial hypertension (PAH).

Patients and Methods:

Seventeen consecutive patients of suspected secondary PAH were examined for pulmonary artery pressures (PAPs) with right heart catheterization (RHC) and three-dimensional (3D) cine PC MR. Based on the RHC data, patients were subdivided into two groups of 12 non-PAH (median age of 74.5 years) and 5 PAH (median age of 77 years) patients. Based on 3D cine PC magnetic resonance (MR), hemodynamic parameters including spatially averaged systolic WSS (sWSS), diastolic WSS (dWSS), mean WSS (mWSS), OSI and blood vessel section area (BVSA) at the pulmonary arterial trunk were calculated. Streamline images in the pulmonary arteries were also assessed. All the parameters were compared between non-PAH and PAH groups.

Results:

sWSS (N/m²) and mWSS (N/m²) of PAH group was lower than that of non-PAH group (0.594 ± 0.067 vs. 0.961 ± 0.590, P = 0.001), (0.365 ± 0.035 vs. 0.489 ± 0.132, P = 0.027). OSI of PAH group was higher than that of non-PAH (0.214 ± 0.026 vs. 0.130 ± 0.046, P = 0.001). sWSS, mWSS, and dWSS were inversely correlated and OSI was positively correlated to mean PAP or systolic PAP with r values of –0.638 (P = 0.005), –0.643 (P = 0.005), –0.485 (P = 0.049), and 0.625 (P = 0.007); or –0.622 (P = 0.008), –0.629 (P = 0.007), –0.484 (P = 0.049), and 0.594 (P = 0.012), respectively. sWSS was also inversely correlated to BVSA with r value of –0.488 (P = 0.049), and OSI was correlated to BVSA with r value of 0.574 (P = 0.016). Vortex or helical flows were observed more frequently in PAH patients.

Conclusions:

The low sWSS and mWSS as well as high OSI measured with 3D cine PC MR could be potential hemodynamic markers for the increased PAP in suspected secondary PAH patients.

Mechanisms of Amplified Arteriogenesis in Collateral Artery Segments Exposed to Reversed Flow Direction

OBJECTIVE

Collateral arteriogenesis, the growth of existing arterial vessels to a larger diameter, is a fundamental adaptive response that is often critical for the perfusion and survival of tissues downstream of chronic arterial occlusion(s). Shear stress regulates arteriogenesis; however, the arteriogenic significance of reversed flow direction, occurring in numerous collateral artery segments after femoral artery ligation, is unknown. Our objective was to determine if reversed flow direction in collateral artery segments differentially regulates endothelial cell signaling and arteriogenesis.

APPROACH AND RESULTS

Collateral segments experiencing reversed flow direction after femoral artery ligation in C57BL/6 mice exhibit increased pericollateral macrophage recruitment, amplified arteriogenesis (30% diameter and 2.8-fold conductance increases), and remarkably permanent (12 weeks post femoral artery ligation) remodeling. Genome-wide transcriptional analyses on human umbilical vein endothelial cells exposed to reversed flow conditions mimicking those occurring in vivo yielded 10-fold more significantly regulated transcripts, as well as enhanced activation of upstream regulators (nuclear factor κB [NFκB], vascular endothelial growth factor, fibroblast growth factor-2, and transforming growth factor-β) and arteriogenic canonical pathways (protein kinase A, phosphodiesterase, and mitogen-activated protein kinase). Augmented expression of key proarteriogenic molecules (Kruppel-like factor 2 [KLF2], intercellular adhesion molecule 1, and endothelial nitric oxide synthase) was also verified by quantitative real-time polymerase chain reaction, leading us to test whether intercellular adhesion molecule 1 or endothelial nitric oxide synthase regulate amplified arteriogenesis in flow-reversed collateral segments in vivo. Interestingly, enhanced pericollateral macrophage recruitment and amplified arteriogenesis was attenuated in flow-reversed collateral segments after femoral artery ligation in intercellular adhesion molecule 1(-/-) mice; however, endothelial nitric oxide synthase(-/-) mice showed no such differences.

CONCLUSIONS

Reversed flow leads to a broad amplification of proarteriogenic endothelial signaling and a sustained intercellular adhesion molecule 1-dependent augmentation of arteriogenesis. Further investigation of the endothelial mechanotransduction pathways activated by reversed flow may lead to more effective and durable therapeutic options for arterial occlusive diseases.

Mechanisms regulating endothelial permeability

The endothelial monolayer partitioning underlying tissue from blood components in the vessel wall maintains tissue fluid balance and host defense through dynamically opening intercellular junctions. Edemagenic agonists disrupt endothelial barrier function by signaling the opening of the intercellular junctions leading to the formation of protein-rich edema in the interstitial tissue, a hallmark of tissue inflammation that, if left untreated, causes fatal diseases, such as acute respiratory distress syndrome. In this review, we discuss how intercellular junctions are maintained under normal conditions and after stimulation of endothelium with edemagenic agonists. We have focused on reviewing the new concepts dealing with the alteration of adherens junctions after inflammatory stimulus.

Endothelial shear stress and blood viscosity in peripheral arterial disease

This review examines the emerging role of endothelial shear stress (ESS) and blood viscosity on the initiation and progression of atherosclerosis in peripheral arterial disease. Among the variables determining ESS, blood viscosity has to date been the most overlooked by clinical researchers. Blood viscosity is a laboratory assessment that is minimally invasive and modifiable using pharmacologic therapy as well as by hemodilution. Monitoring and controlling blood viscosity not only modulates ESS, but also reduces peripheral vascular resistance and increases blood flow to the lower extremities.

Histone deacetylase 1 reduces NO production in endothelial cells via lysine deacetylation of NO synthase 3

The lysine acetylation state of nonhistone proteins may be regulated through histone deacetylases (HDACs). Evidence suggests that nitric oxide (NO) synthase 3 (NOS3; endothelial NOS) is posttranslationally lysine acetylated, leading to increased NO production in the endothelium. We tested the hypothesis that NOS3 is lysine acetylated and that upregulated HDAC1-mediated deacetylation leads to reduced NO production in endothelial cells. We determined that NOS3 is basally lysine acetylated in cultured bovine aortic endothelial cells (BAECs). In BAECs, HDAC1 is expressed in the nucleus and cytosol and forms a novel protein-protein interaction with NOS3. Overexpression of HDAC1 in BAECs resulted in a significant reduction in NOS3 lysine acetylation (control = 1.0 ± 0.1 and HDAC1 = 0.59 ± 0.08 arbitrary units, P < 0.01) and significantly blunted basal nitrite production (control 287.7 ± 29.1 and HDAC1 172.4 ± 31.7 pmol·mg(-1)·h(-1), P < 0.05) as well as attenuating endothelin-1-stimulated nitrite production (control = 481.8 ± 50.3 and HDAC1 243.1 ± 48.2 pmol·mg(-1)·h(-1), P < 0.05). While HDAC1 knockdown with small-interfering RNA resulted in no change in NOS3 acetylation level, yet increased basal nitrite production (730.6 ± 99.1 pmol·mg(-1)·h(-1)) and further exaggerated increases in endothelin-1 stimulated nitrite production (1276.9 ± 288.2 pmol·mg(-1)·h(-1)) was observed. Moreover, overexpression or knockdown of HDAC1 resulted in no significant effect on NOS3 protein expression or NOS3 phosphorylation sites T497, S635, or S1179. Thus these data indicate that upregulated HDAC1 decreases NOS3 activity, most likely through direct lysine deacetylation of NOS3. We propose that HDAC1-mediated deacetylation of NOS3 may represent a novel target for endothelial dysfunction.

Four-dimensional flow assessment of pulmonary artery flow and wall shear stress in adult pulmonary arterial hypertension: results from two institutions

PURPOSE

To compare pulmonary artery flow using Cartesian and radially sampled four-dimensional flow-sensitive (4D flow) MRI at two institutions.

METHODS

Nineteen healthy subjects and 17 pulmonary arterial hypertension (PAH) subjects underwent a Cartesian 4D flow acquisition (institution 1) or a three-dimensional radial acquisition (institution 2). The diameter, peak systolic velocity (Vmax), peak flow (Qmax), stroke volume (SV), and wall shear stress (WSS) were computed in two-dimensional analysis planes at the main, right, and left pulmonary artery. Interobserver variability, interinstitutional differences, flow continuity, and the hemodynamic measurements in healthy and PAH subjects were assessed.

RESULTS

Vmax, Qmax, SV, and WSS at all locations were significantly lower (P < 0.05) in PAH compared with healthy subjects. The limits of agreement were 0.16 m/s, 2.4 L/min, 10 mL, and 0.31 N/m(2) for Vmax, Qmax, SV, and WSS, respectively. Differences between Qmax and SV using Cartesian and radial sequences were not significant. Plane placement and acquisition exhibited isolated, site-based differences between Vmax and WSS.

CONCLUSIONS

4D flow MRI was used to detect differences in pulmonary artery hemodynamics for PAH subjects. Flow and WSS in healthy and PAH subject cohorts were similar between Cartesian- and radial-based 4D flow MRI acquisitions with minimal interobserver variability.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Effect of shear stress and substrate on endothelial DAPK expression, caspase activity, and apoptosis

Background

In the vasculature, misdirected apoptosis in endothelial cells leads to pathological conditions such as inflammation. Along with biochemical and molecular signals, the hemodynamic forces that the cells experience are also important regulators of endothelial functions such as proliferation and apoptosis. Laminar shear stress inhibits apoptosis induced by serum depletion, oxidative stress, and tumor necrosis factor α (TNFα). Death associated protein kinase (DAPK) is a positive regulator of TNFα induced apoptotic pathway. Here we investigate the effect of shear stress on DAPK in endothelial cells on glass or silicone membrane substrate. We have already shown a link between shear stress and DAPK expression and apoptosis in cells on glass. Here we transition our study to endothelial cells on non-glass substrates, such as flexible silicone membrane used for cyclic strain studies.

Results

We modified the classic parallel plate flow chamber to accommodate silicone membrane as substrate for cells, and validated the chamber for cell viability in shear stress experiments. We found that adding shear stress significantly suppressed TNFα induced apoptosis in cells; while shearing cells alone also increased apoptosis on either substrate. We also found that shearing cells at 12 dynes/cm² for 6 hours resulted in increased apoptosis on both substrates. This shear-induced apoptosis correlated with increased caspase 3/7 activities and DAPK expression and activation via dephosphorylation of serine 308.

Conclusion

These data suggest that shear stress induced apoptosis in endothelial cells via increased DAPK expression and activation as well as caspase-3/7 activity. Most in vitro shear stress studies utilize the conventional parallel plate flow chamber where cells are cultured on glass, which is much stiffer than what cells encounter in vivo. Other mechanotransduction studies have utilized the flexible silicone membrane as substrate, for example, in cyclic stretch studies. Thus, this study bridges the gap between shear stress studies on cells plated on glass to studies on different stiffness of substrates or mechanical stimulation such as cyclic strain. We continue to explore the mechanotransduction role of DAPK in endothelial apoptosis, by using substrates of physiological stiffness for shear stress studies, and by using silicone substrate in cyclic stretch devices.

eNOS genotype modifies the effect of leisure-time physical activity on serum triglyceride levels in a Japanese population

Background

Nitric oxide is a key molecule not only in the cardiovascular system, but also in the metabolic-endocrine system. The purpose of this study was to examine possible associations of the NOS3 T-786C polymorphism (rs2070744) with serum lipid levels on the basis of lifestyle factors for tailoring prevention of dyslipidemia.

Methods

For this cross-sectional study, a total of 2226 subjects aged 35 to 69 years (1084 men and 1142 women) were selected from Japanese participants in the Japan Multi-Institutional Collaborative Cohort (J-MICC) Study. They were recruited in eight areas throughout Japan between February 2004 and November 2008.

Results

In a stratified analysis by leisure-time physical activity, the likelihood of hypertriglyceridemia (serum triglyceride levels ≥ 150 mg/dL) among subjects with the C allele was significantly lower than those without it in the active group (OR = 0.43, 95% CI = 0.22-0.84 in the fasting group), but not in the sedentary group. A gene-environment interaction between the T-786C polymorphism and leisure-time physical activity for hypertriglyceridemia was significant (P = 0.007 in the fasting group). Additionally, serum triglyceride levels (mean ± SD) across leisure-time physical activity classes decreased significantly only in the TC + CC genotype group (111 ± 60 mg/dL for sedentary, 95 ± 48 mg/dL for moderately active, 88 ± 44 mg/dL for very active, P for trend = 0.008 in the fasting group), but not in the TT genotype group. Total cholesterol, high-density lipoprotein (HDL) cholesterol, and non-HDL cholesterol levels had no significant association with the polymorphism.

Conclusions

This study suggests that the NOS3 T-786C polymorphism modifies the effect of leisure-time physical activity on serum triglyceride levels.

Hemorheology and microvascular disorders

The present review presents basic concepts of blood rheology related to vascular diseases. Blood flow in large arteries is dominated by inertial forces exhibited at high flow velocities, while viscous forces (i.e., blood rheology) play an almost negligible role. When high flow velocity is compromised by sudden deceleration as at a bifurcation, endothelial cell dysfunction can occur along the outer wall of the bifurcation, initiating inflammatory gene expression and, through mechanotransduction, the cascade of events associated with atherosclerosis. In sharp contrast, the flow of blood in microvessels is dominated by viscous shear forces since the inertial forces are negligible due to low flow velocities. Shear stress is a critical parameter in microvascular flow, and a force-balance approach is proposed for determining microvascular shear stress, accounting for the low Reynolds numbers and the dominance of viscous forces over inertial forces. Accordingly, when the attractive forces between erythrocytes (represented by the yield stress of blood) are greater than the shear force produced by microvascular flow, tissue perfusion itself cannot be sustained, leading to capillary loss. The yield stress parameter is presented as a diagnostic candidate for future clinical research, specifically, as a fluid dynamic biomarker for microvascular disorders. The relation between the yield stress and diastolic blood viscosity (DBV) is described using the Casson model for viscosity, from which one may be able determine thresholds of DBV where the risk of microvascular disorders is high.

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.

Vibration disrupts vascular function in a model of metabolic syndrome

Vibration-induced white finger (VWF) is a disorder seen in workers exposed to hand-transmitted vibration, and is characterized by cold-induced vasospasms and finger blanching. Because overweight people with metabolic syndrome are pre-disposed to developing peripheral vascular disorders, it has been suggested that they also may be at greater risk of developing VWF if exposed to occupational vibration. We used an animal model of metabolic syndrome, the obese Zucker rat, to determine if metabolic syndrome alters vascular responses to vibration. Tails of lean and obese Zucker rats were exposed to vibration (125 Hz, 49 m/s(2) r.m.s.) or control conditions for 4 h/d for 10 d. Ventral tail arteries were collected and assessed for changes in gene expression, levels of reactive oxygen species (ROS) and for responsiveness to vasomodulating factors. Vibration exposure generally reduced the sensitivity of arteries to acetylcholine (ACh)-induced vasodilation. This decrease in sensitivity was most apparent in obese rats. Vibration also induced reductions in vascular nitric oxide concentrations and increases in vascular concentrations of ROS in obese rats. These results indicate that vibration interferes with endothelial-mediated vasodilation, and that metabolic syndrome exacerbates these effects. These findings are consistent with idea that workers with metabolic syndrome have an increased risk of developing VWF.

Effects of Pulsatile Shear Stress on Signaling Mechanisms Controlling Nitric Oxide Production, Endothelial Nitric Oxide Synthase Phosphorylation, and Expression in Ovine Fetoplacental Artery Endothelial Cells

During gestation, placental blood flow, endothelial nitric oxide (NO) production, and endothelial cell nitric oxide synthase (eNOS) expression are elevated dramatically. Shear stress can induce flow-mediated vasodilation, endothelial NO production, and eNOS expression. Both the activity and expression of eNOS are closely regulated because it is the rate-limiting enzyme essential for NO synthesis. The authors adapted CELLMAX artificial capillary modules to study the effects of pulsatile flow/shear stress on ovine fetoplacental artery endothelial (OFPAE) cell NO production, eNOS expression, and eNOS phosphorylation. This model allows for the adaptation of endothelial cells to low physiological flow environments and thus prolonged shear stresses. The cells were grown to confluence at 3 dynes/cm2, then were exposed to 10, 15, or 25 dynes/cm2 for up to 24 h and NO production, eNOS mRNA, and eNOS protein expression were elevated by shear stress in a graded fashion (p < .05). Production of NO by OFPAE cells exposed to pulsatile shear stress was de novo; i.e., inhibited by L-NMMA (N(G)-monomethyl-L-arginine) and reversed by excess NOS substrate L-arginine. Rises in NO production at 25 dynes/cm2 (8-fold) exceeded (p < .05) that seen for eNOS protein (3.6-fold) or eNOS mRNA (1.5-fold). Acute rises in NO production with shear stress occurred by eNOS activation, whereas prolonged NO rises were via elevations in both eNOS expression and enzyme activation. The authors therefore used Western analysis to investigate the signaling mechanisms underlying pulsatile shear stress-induced increases in eNOS phosphorylation and protein expression by "flow-adapted" OFPAE cells. Increasing shear stress from 3 to 15 dynes/cm2 very rapidly increased eNOS Ser1177, ERK1/2 (extracellular signal-regulated kinase 1 and 2) and Akt, but not p38 MAPK (p38 mitogen-activated protein kinase) phosphorylation by Western analysis. Phosphorylation of eNOS Ser1177 under shear stress was elevated by 20 min, a response that was blocked by PI-3K (phosphatidylinositol 3-kinase) inhibitors wortmannin and LY294002, but not the MEK (MAPK kinase) inhibitor UO126. Basic fibroblast growth factor (bFGF) enhanced eNOS protein levels in static culture via a MEK-mediated mechanism, but it could not further augment the elevated eNOS protein levels induced by 15 dynes/cm2 shear stress. Blocking of either signaling pathways or p38 MAPK did not change the shear stress-induced increase in eNOS protein levels. Therefore, shear stress induced rapid eNOS phosphorylation on Ser1177 in OFPAE cells through a PI-3K-dependent pathway. The bFGF-induced rise in eNOS protein levels in static culture was much less than those observed under flow and was blocked by inhibiting MEK. Prolonged shear stress-stimulated increases in eNOS protein levels were not affected by inhibition of MEK- or PI-3K-mediated pathways. In conclusion, pulsatile shear stress greatly induces NO production by OFPAE cells through the mechanisms of both PI-3K-mediated eNOS activation and elevations in eNOS protein levels; bFGF does not further stimulate eNOS expression under flow condition.

Small GTPases in mechanosensitive regulation of endothelial barrier

Alterations in vascular permeability are defining feature of diverse processes including atherosclerosis, inflammation, ischemia/reperfusion injury, and ventilator-induced lung injury. Clinical observations and experimental studies support an essential role of mechanical forces in pathophysiologic regulation of lung barrier. Accumulating data demonstrate that decreased levels of blood flow and increased cyclic stretch of lung tissues associated with lung mechanical ventilation at high tidal volumes increase vascular permeability, activate inflammatory cytokine production, alveolar flooding, leukocyte infiltration, and hypoxemia, and increase morbidity and mortality. Potential synergism between pathologic mechanical stimulation and inflammatory molecules resulting in vascular leak and lung injury becomes increasingly recognized. This review will discuss a role of Rho family of small GTPases in the mechanochemical regulation of pulmonary endothelial permeability associated with ventilator induced lung injury.

Nitric oxide and endothelial cellular senescence

Cellular senescence is characterized by permanent exit from the cell cycle and the appearance of distinct morphological and functional changes associated with an impairment of cellular homeostasis. Many studies support the occurrence of vascular endothelial cell senescence in vivo, and the senescent phenotype of endothelial cells can be transformed from anti-atherosclerotic to pro-atherosclerotic. Thus, endothelial cell senescence promotes endothelial dysfunction and may contribute to the pathogenesis of age-associated vascular disorders. Emerging evidence suggests that increasing nitric oxide (NO) bioavailability or endothelial NO synthase (eNOS) activity activates telomerase and delays endothelial cell senescence. In this review, we discuss the potential mechanisms underlying the ability of NO to prevent endothelial cell senescence and describe the possible changes in the NO-mediated anti-senescence effect under pathophysiological conditions, including oxidative stress and hyperglycemia. Further understanding of the mechanisms underlying the anti-senescence effect of NO in endothelial cells will provide insights into the potential of eNOS-based anti-senescence therapy for age-associated vascular disorders.

Long-term cyclic stretch controls pulmonary endothelial permeability at translational and post-translational levels

We have previously described differential effects of physiologic (5%) and pathologic (18%) cyclic stretch (CS) on agonist-induced pulmonary endothelial permeability. This study examined acute and chronic effects of CS on agonist-induced intracellular signaling and cell morphology in the human lung macro- and microvascular endothelial cell (EC) monolayers. Endothelial permeability was assessed by analysis of morphological changes, parameters of cell contraction and measurements of transendothelial electrical resistance. Exposure of both microvascular and macrovascular EC to 18% CS for 2-96 h increased thrombin-induced permeability and monolayer disruption. Interestingly, the ability to promote thrombin responses was present in EC cultures exposed to 48-96 h of CS even after replating onto non-elastic substrates. In turn, physiologic CS preconditioning (72 h) attenuated thrombin-induced paracellular gap formation and MLC phosphorylation in replated EC cultures. Long-term preconditioning at 18% CS (72 h) increased the content of signaling and contractile proteins including Rho GTPase, MLC, MLC kinase, ZIP kinase, PAR1, caldesmon and HSP27 in the pulmonary microvascular and macrovascular cells. We conclude that short term CS regulates EC permeability via modulation of agonist-induced signaling, whereas long-term CS controls endothelial barrier at both post-translational level and via magnitude-dependent regulation of pulmonary EC phenotype, signaling and contractile protein expression.

Activation of endothelial nitric oxide synthase by proanthocyanidin-rich fraction from Croton celtidifolius (Euphorbiaceae): involvement of extracellular calcium influx in rat thoracic aorta

The present study investigates the mechanisms related to the endogenous nitric oxide synthase (eNOS) activation in the relaxant effects of a proanthocyanidin-rich fraction (PRF), obtained from Croton celtidifolius Baill barks, in rat thoracic aorta rings with endothelium. In vessels pre-contracted with phenylephrine (Phe), PRF (0.1 - 100 microg/mL) induced a concentration-dependent relaxation. This effect was significantly reduced by endothelium denudation, by N(omega)-nitro-L-arginine, and by 1H[1,2,3]oxadiazolo[4,3-alpha]quinoxalin. However, the vasorelaxant effect was not altered by indomethacin, atropine, tetraethylammonium, and charybdotoxin plus apamin. In thoracic aorta rings pre-contracted with phorbol-12,13-dibuyrate, PRF also induced a concentration-dependent relaxation. The PRF-induced relaxation disappeared in the absence of extracellular calcium in the medium and decreased significantly in the presence of lanthanum. A sulfhydryl alkylating agent, N-ethylmaleimide, and a phospholipase C (PLC) blocker, neomycin, significantly decreased PRF-induced vasorelaxation. In vessels pre-contracted with Phe, the PRF-induced vasorelaxant effect was not altered by quinacrine and ONO-RS-082, genistein and thyrphostin A-23, GF109203, and pertussis toxin and cholera toxin. The results suggest that the PRF-induced vasorelaxant effect is endothelium-dependent and involves the NO/cGMP pathway. We hypothesize that the activation of eNOS is due to an increase of intracellular calcium derived from PLC activation and an N-ethylmaleimide sensitive pathway.

Magnitude-dependent effects of cyclic stretch on HGF- and VEGF-induced pulmonary endothelial remodeling and barrier regulation

Mechanical ventilation at high tidal volumes compromises the blood-gas barrier and increases lung vascular permeability, which may lead to ventilator-induced lung injury and pulmonary edema. Using pulmonary endothelial cell (ECs) exposed to physiologically [5% cyclic stretch (CS)] and pathologically (18% CS) relevant magnitudes of CS, we evaluated the potential protective effects of hepatocyte growth factor (HGF) on EC barrier dysfunction induced by CS and vascular endothelial growth factor (VEGF). In static culture, HGF enhanced EC barrier function in a Rac-dependent manner and attenuated VEGF-induced EC permeability and paracellular gap formation. The protective effects of HGF were associated with the suppression of Rho-dependent signaling triggered by VEGF. Five percent CS promoted HGF-induced enhancement of the cortical F-actin rim and activation of Rac-dependent signaling, suggesting synergistic barrier-protective effects of physiological CS and HGF. In contrast, 18% CS further enhanced VEGF-induced EC permeability, activation of Rho signaling, and formation of actin stress fibers and paracellular gaps. These effects were attenuated by HGF pretreatment. EC preconditioning at 5% CS before HGF and VEGF further promoted EC barrier maintenance. Our data suggest synergistic effects of HGF and physiological CS in the Rac-mediated mechanisms of EC barrier protection. In turn, HGF reduced the barrier-disruptive effects of VEGF and pathological CS via downregulation of the Rho pathway. These results support the importance of HGF-VEGF balance in control of acute lung injury/acute respiratory distress syndrome severity via small GTPase-dependent regulation of lung endothelial permeability.

Nitric oxide-dependent stimulation of endothelial cell proliferation by sustained high flow

Little is understood about endothelial cell (EC) responses to high flow, which mediate adaptive outward remodeling as well as cerebral aneurysm development. Opposite EC behaviors have been reported in vivo including cell loss during aneurysm initiation and cell proliferation during adaptive outward remodeling. This study aims at elucidating the EC growth response to elevated wall shear stress (WSS) and determining if nitric oxide (NO) is involved. A confluent EC monolayer was subjected to steady-state, laminar flow with WSS ranging from 15 to 100 dyn/cm(2) for 24 and 48 h. Cells oriented to the direction of the flow with a time course that varied with WSS. At 48 h, all cells were aligned with the flow. EC proliferation was examined using bromodeoxyuridine (BrdU) incorporation. The percentage of proliferating ECs rose linearly from 15 to 50 dyn/cm(2) to more than sixfold at 50-100 dyn/cm(2) compared with the accepted physiological baseline of 15-20 dyn/cm(2). In addition, terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling (TUNEL) staining revealed that apoptosis decreased with increasing WSS. These results demonstrate that high WSS stimulates EC proliferation and suppresses apoptosis. Furthermore, immunostaining revealed increased endothelial nitric oxide synthase (eNOS) production with increasing WSS. NOS inhibition with N(omega)-nitro-l-arginine methyl ester (l-NAME) drastically reduced the WSS-stimulated proliferation, indicating a critical role of NO production in the stimulation of EC proliferation by high WSS.

Relative reduction of endothelial nitric-oxide synthase expression and transcription in atherosclerosis-prone regions of the mouse aorta and in an in vitro model of disturbed flow

Atherosclerosis develops in distinct regions of the arterial tree. Defining patterns and mechanisms of endothelial cell gene expression in different regions of normal arteries is key to understanding the initial molecular events in atherogenesis. In this study, we demonstrated that the expression of endothelial nitric-oxide synthase (eNOS), an atheroprotective gene, and its phosphorylation on Ser(1177), a marker of activity, were lower in regions of the normal mouse aorta that are predisposed to atherosclerosis. The same expression pattern was observed in mouse strains that are both susceptible and resistant to atherosclerosis, and the topography of eNOS expression was inverse to p65, the main nuclear factor-kappaB subunit. Modeling of disturbed and uniform laminar flow in vitro reproduced the expression patterns of eNOS and p65 that were found in vivo. Heterogeneous nuclear RNA expression and RNA polymerase II chromosome immunoprecipitation studies demonstrated that regulation of transcription contributed to increased eNOS expression in response to shear stress. In vivo, the transcription of eNOS was reduced in regions of the mouse aorta predisposed to atherosclerosis, as defined by reporter gene expression in eNOS promoter-beta-galactosidase reporter transgenic mice. These data suggest that disturbed hemodynamic patterns found at arterial branches and curvatures uniquely modulate endothelial cell gene expression by regulating transcription, potentially explaining why these regions preferentially develop atherosclerosis when risk factors such as hypercholesterolemia are introduced.

Importance of hemodynamic forces as signals for exercise-induced changes in endothelial cell phenotype

Current evidence indicates that the ability of physical activity to sustain a normal phenotype of arterial endothelial cells (ECs) plays a central role in the beneficial effects of exercise (Ex) on atherosclerotic disease. Here we evaluate the strength of evidence that shear stress (SS) and/or circumferential wall stress (stretch) are the primary signals, produced by bouts of Ex, that signal altered gene expression in arterial ECs, thereby resulting in a less atherogenic EC phenotype. Current literature indicates that SS is a signal for expression of antiatherogenic genes in cultured ECs, in ECs of isolated arteries, and in ECs of arteries in intact animals. Furthermore, SS levels in the arteries of humans during Ex are in the range that produces beneficial changes. In contrast, complex flow profiles within recirculation zones and/or oscillatory flow patterns can cause proatherogenic gene expression in ECs. In vivo evidence indicates that Ex decreases oscillatory flow/SS in some portions of the arterial tree but may increase oscillatory flow in other areas of the arterial tree. Circumferential wall stress can increase expression of some beneficial EC genes as well, but circumferential wall stress also increases production of reactive oxygen species and increases the expression of adhesion factors and other proatherogenic genes. Interactions of arterial pressure and fluid SS play an important role in arterial vascular health and likely contribute to how Ex bouts signal changes in EC gene expression. It is also clear that other local and circulating factors interact with these hemodynamic signals during Ex to produce the healthy arterial EC phenotype. We conclude that available evidence suggests that exercise signals formation of beneficial endothelial cell phenotype at least in part through changes in SS and wall stretch in the arteries.

Reduced store-operated Ca2+ entry in pulmonary endothelial cells from chronically hypoxic rats

Chronic hypoxia (CH)-induced pulmonary hypertension may influence basal endothelial cell (EC) intracellular Ca(2+) concentration ([Ca(2+)](i)). We hypothesized that CH decreases EC [Ca(2+)](i) associated with membrane depolarization and reduced Ca(2+) entry. To test this hypothesis, we assessed 1) basal endothelial Ca(2+) in pressurized pulmonary arteries and freshly isolated ECs, 2) EC membrane potential (E(m)), 3) store-operated Ca(2+) current (I(SOC)), and 4) store-operated Ca(2+) (SOC) entry in arteries from control and CH rats. We found that basal EC Ca(2+) was significantly lower in pressurized pulmonary arteries and freshly isolated ECs from CH rats compared with controls. Similarly, ECs in intact arteries from CH rats were depolarized compared with controls, although no differences were observed between groups in isolated cells. I(SOC) activation by 1 muM thapsigargin displayed diminished inward current and a reversal potential closer to 0 mV in cells from CH rats compared with controls. In addition, SOC entry determined by fura 2 fluorescence and Mn(2+) quenching revealed a parallel reduction in Ca(2+) entry following CH. We conclude that differences in the magnitude of SOC entry exist between freshly dispersed ECs from CH and control rats and correlates with the decrease in basal EC [Ca(2+)](i). In contrast, basal EC Ca(2+) influx is unaffected and membrane depolarization is limited to intact arteries, suggesting that E(m) may not play a major role in determining basal EC [Ca(2+)](i) following CH.

Effects of interleukin-1 on calcium signaling and the increase of filamentous actin in isolated and in situ articular chondrocytes

OBJECTIVE

To determine whether interleukin-1 (IL-1) initiates transient changes in the intracellular concentration of [Ca2+]i and the organization of filamentous actin (F-actin) in articular chondrocytes.

METHODS

Articular chondrocytes within cartilage explants and enzymatically isolated chondrocytes were loaded with Ca(2+)-sensitive fluorescence indicators, and [Ca2+]i was measured using confocal fluorescence ratio imaging during exposure to 10 ng/ml IL-1alpha. Inhibitors of Ca2+ mobilization (Ca(2+)-free medium, thapsigargin [inhibitor of Ca-ATPases], U73122 [inhibitor of phospholipase C], and pertussis toxin [inhibitor of G proteins]) were used to determine the mechanisms of increased [Ca2+]i. Cellular F-actin was quantified using fluorescently labeled phalloidin. Toxin B was used to determine the role of the Rho family of small GTPases in F-actin reorganization.

RESULTS

In isolated cells on glass and in in situ chondrocytes within explants, exposure to IL-1 induced a transient peak in [Ca2+]i that was generally followed by a series of decaying oscillations. Thapsigargin, U73122, and pertussis toxin inhibited the percentage of cells responding to IL-1. IL-1 increased F-actin content in chondrocytes in a manner that was inhibited by toxin B.

CONCLUSION

Both isolated and in situ chondrocytes respond to IL-1 with transient increases in [Ca2+]i via intracellular Ca2+ release mediated by the phospholipase C and inositol trisphosphate pathways. The influx of Ca2+ from the extracellular space and the activation of G protein-coupled receptors also appear to contribute to these mechanisms. These findings suggest that Ca2+ mobilization may be one of the first signaling events in the response of chondrocytes to IL-1.

Quantifying cardiovascular flow dynamics during early development

The relationship between developing biologic tissues and their dynamic fluid environments is intimate and complex. Increasing evidence supports the notion that these embryonic flow-structure interactions influence whether development will proceed normally or become pathogenic. Genetic, pharmacological, or surgical manipulations that alter the flow environment can thus profoundly influence morphologic and functional cardiovascular phenotypes. Functionally deficient phenotypes are particularly poorly described as there are few imaging tools with sufficient spatial and temporal resolution to quantify most intra-vital flows. The ability to visualize biofluids flow in vivo would be of great utility in functionally phenotyping model animal systems and for the elucidation of the mechanisms that underlie flow-related mechano-sensation and transduction in living organisms. This review summarizes the major methodological advances that have evolved for the quantitative characterization of intra-vital fluid dynamics with an emphasis on assessing cardiovascular flows in vertebrate model organisms.

Transcriptional and posttranscriptional regulation of endothelial nitric oxide synthase expression

The ability of the endothelium to produce nitric oxide is essential to maintenance of vascular homeostasis; disturbance of this ability is a major contributor to the pathogenesis of vascular disease. In vivo studies have demonstrated that expression of endothelial nitric oxide synthase (eNOS) is vital to endothelial function and have led to the understanding that eNOS expression is subject to modest but significant degrees of regulation. Subsequently, numerous physiological and pathophysiological stimuli have been identified that modulate eNOS expression via mechanisms that alter steady-state eNOS mRNA levels. These mechanisms involve changes in the rate of eNOS gene transcription (transcriptional regulation) and alteration of eNOS mRNA processing and stability (posttranscriptional regulation). In cultured endothelial cells, shear stress, transforming growth factor-beta1, lysophosphatidylcholine, cell growth, oxidized linoleic acid, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, and hydrogen peroxide have been shown to increase eNOS expression. In contrast, tumor necrosis factor-alpha, hypoxia, lipopolysaccaride, thrombin, and oxidized LDL can decrease eNOS mRNA levels. For many of these stimuli, both transcriptional and posttranscriptional mechanisms contribute to regulation of eNOS expression. Recent studies have begun to further define signaling pathways responsible for changes in eNOS expression and have characterized cis- and trans-acting regulatory elements. In addition, a role has been identified for epigenetic control of eNOS mRNA levels. This review will discuss transcriptional and posttranscriptional regulation of eNOS with emphasis on the molecular mechanisms that have been identified for these processes.

Induction of KLF2 by fluid shear stress requires a novel promoter element activated by a phosphatidylinositol 3-kinase-dependent chromatin-remodeling pathway

Fluid shear stress maintains vascular homeostasis by influencing endothelial gene expression. One mechanism by which shear stress achieves this is through the induction of transcription factors including Krüppel-like factor 2(KLF2). We have previously reported that a 62-bp region of the KLF2 promoter is responsible for its shear stress-induced expression via the binding of nuclear factors. In this study, we find that the 62-bp shear stress response region contains a 30-bp tripartite palindrome motif. Electrophoretic mobility supershift and chromatin immunoprecipitation assays demonstrate that PCAF (P-300/cAMP-response element-binding protein-binding protein-associated factor)) and heterogeneous nuclear ribonucleoprotein D bind this region as components of the shear stress regulatory complex. We have also characterized a PI3K-dependent/Akt-independent pathway responsible for shear stress-induced KLF2 nuclear binding, promoter activation, and mRNA expression. Furthermore, the shear stress response region of the KLF2 promoter was specifically immunoprecipitated by antibodies against acetylated histones H3 and H4 in shear-stressed but not static hemangioendothelioma cells. The acetylation of these histones was blocked by PI3K inhibition. Finally, we have found that KLF2 increases endothelial nitric-oxide synthase expression in murine endothelial cultures, an effect that is also blocked by PI3K inhibition. These results define the DNA regulatory element, signal transduction pathway, and molecular mechanism activating the flow-dependent expression of a vital endothelial transcription factor.

Unique Morphology and Focal Adhesion Development of Valvular Endothelial Cells in Static and Fluid Flow Environments

BACKGROUND

The influence of mechanical forces on cell function has been well documented for many different cell types. Endothelial cells native to the aortic valve may play an important role in mediating tissue responses to the complex fluid environment, and may therefore respond to fluid flow in a different manner than more characterized vascular endothelial cells.

METHODS AND RESULTS

Porcine endothelial cells of aortic and aortic valvular origin were subjected to 20 dynes/cm2 steady laminar shear stress for up to 48 hours, with static cultures serving as controls. The aortic valve endothelial cells were observed to align perpendicular to flow, in direct contrast to the aortic endothelial cells, which aligned parallel to flow. Focal adhesion complexes reorganized prominently at the ends of the long axis of aligned cells. Valvular endothelial cell alignment was dependent on Rho-kinase signaling, whereas vascular endothelial cell alignment was dependent on both Rho-kinase and phosphatidylinositol 3-kinase signal pathways.

CONCLUSIONS

These differences in response to mechanical forces suggest a unique phenotype of valvular endothelial cells not mimicked by vascular endothelial cells, and could have implications for cardiovascular cell biology and cell-source considerations for tissue-engineered valvular substitutes.

Src kinase mediates angiotensin II-dependent increase in pulmonary endothelial nitric oxide synthase

We have previously demonstrated that angiotensin II (Ang II) stimulates nitric oxide (NO) production in bovine pulmonary artery endothelial cells (BPAECs) by increasing NO synthase (NOS) expression via the type 2 receptor. The purpose of this study was to identify the Ang II-dependent signaling pathway that mediates this increase in endothelial NOS (eNOS). The Ang II-dependent increase in eNOS expression is prevented when BPAECs are pretreated with the tyrosine kinase inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine, which also blocked Ang II-dependent mitogen-activated protein kinase (MAPK) kinase/extracellular-regulated protein kinase (MEK)-1 and MAPK phosphorylation, suggesting that Src is upstream of MAPK in this pathway. Transfection of BPAECs with an Src dominant negative mutant cDNA prevented the Ang II-dependent Src activation and increase in eNOS protein expression. PD98059, a MEK-1 inhibitor, prevented the Ang II-dependent phosphorylation of extracellular-regulated protein kinases 1 and 2 and increase in eNOS expression. Neither AG1478, an epidermal growth factor receptor kinase inhibitor, nor AG1295, a platelet derived growth factor receptor kinase inhibitor, had any effect on Ang II-stimulated Src activity, MAPK activation, or eNOS expression. Pertussis toxin prevented the Ang II-dependent increase in Src activity, MAPK activation, and eNOS expression. These data suggest that Ang II stimulates Src tyrosine kinase via a pertussis toxin-sensitive pathway, which in turn activates the MAPK pathway, resulting in increased eNOS protein expression in BPAECs.

Hypoxia increases Hsp90 binding to eNOS via PI3K-Akt in porcine coronary artery endothelium

This study examines the molecular mechanisms by which hypoxia regulates phosphorylated endothelial nitric oxide synthase (eNOS) activity and NO production in porcine coronary artery endothelial cells (PCAEC). Exposure to hypoxia (pO(2)=10 mmHg) for periods up to 3 h resulted in a time-dependent increase in eNOS protein expression and an early (15 min) and sustained increase in eNOS phosphorylation at Ser-1177. Exposure to hypoxia for 30 min led to a doubling in eNOS activity (control=6.2+/-4.4 vs hypoxia=14.1+/-5.0 fmol cGMP/microg protein, P<0.05) and NO release (control=5.9+/-0.8 vs hypoxia=11.8+/-1.2 nM/microg protein, P<0.05). Hypoxia also led to a significant increase in Akt phosphorylation and upregulation of Hsp90 binding to eNOS. Pretreatment of cells with either 1 microg/ml geldanamycin (a specific inhibitor of Hsp90) or 500 nM wortmannin (a specific PI3 kinase inhibitor) suppressed hypoxia-stimulated Akt and eNOS phosphorylation and significantly attenuated hypoxia-stimulated Hsp90 binding to eNOS. Both eNOS activity and NO production were inhibited by geldanamycin and wortmannin. Although hypoxia led to early activation of p42/44 mitogen-activated protein kinases (MAPK), inhibition of their pathway by PD98059 did not suppress hypoxia-stimulated eNOS phosphorylation and eNOS activity. These data demonstrate that hypoxia leads to increased eNOS phosphorylation via stimulated Hsp90 binding to eNOS and activation of the PI3-Akt pathway. We conclude that a coordinated interaction between Hsp90 and PI3-Akt may be an important mechanism by which eNOS activity and NO production is upregulated in hypoxic heart.

Shear Stress Regulates Endothelial Nitric-oxide Synthase Promoter Activity through Nuclear Factor κB Binding

We have previously demonstrated that shear stress increases transcription of the endothelial nitric-oxide synthase (eNOS) by a pathway involving activation of the tyrosine kinase c-Src and extracellular signal-related kinase 1/2 (ERK1/2). In the present study sought to determine the events downstream of this pathway. Shear stress activated a human eNOS promoter chloramphenicol acetyl-CoA transferase chimeric construct in a time-dependent fashion, and this could be prevented by inhibition of the c-Src and MEK1/2. Studies using electromobility shift assays, promoter deletions, and promoter mutations revealed that shear activation of the eNOS promoter was due to binding of nuclear factor kappaB subunits p50 and p65 to a GAGACC sequence -990 to -984 base pairs upstream of the eNOS transcription start site. Shear induced nuclear translocation of p50 and p65, and activation of the eNOS promoter by shear could be prevented by co-transfection with a dominant negative I kappa Balpha. Exposure of endothelial cells to shear resulted in Ikappa kinase phosphorylation, and this was blocked by the MEK1/2 inhibitor PD98059 and the cSrc inhibitor PP1, suggesting these signaling molecules are upstream of NFkappaB activation. These experiments indicate that shear stress increases eNOS transcription by NFkappaB activation and p50/p65 binding to a GAGACC sequence present of the human eNOS promoter. While NFkappaB activation is generally viewed as a proinflammatory stimulus, the current data indicate that its transient activation by shear may increase expression of eNOS, which via production of nitric oxide could convey anti-inflammatory and anti-atherosclerotic properties.

eNOS T-786C Genotype, Physical Activity, and Peak Forearm Blood Flow in Females

BACKGROUND

The purpose of the present study was to determine interactive and main effects of the eNOS T-786C gene polymorphism and habitual physical activity level on forearm vascular resistance (FVR) and forearm blood flow (FBF) at rest and during 3 min of reactive hyperemia.

METHODS

We studied healthy, Caucasian (age 25 +/- 1 yr), sedentary (maximal oxygen consumption, [OV0312]O2max: 33.8 +/- 1 mL x kg(-1)x min(-1)), and endurance-trained ([OV0312]O2max: 45.3 +/- 1 mL x kg(-1)x min(-1)) women. FBF was measured using venous occlusion plethysmography before (resting) and after 5 min of arm arterial occlusion at 1 (peak vasodilation), 2, and 3 min of reactive hyperemia. [OV0312]O2max was measured using a standard treadmill protocol, and skinfolds were measured to estimate body composition.

RESULTS

There was a significant interaction between eNOS genotype and physical activity level on resting FVR (P = 0.0003). Sedentary subjects with the TT genotype had the lowest resting FVR, but among the endurance-trained group, the TC+CC genotype group had the lowest resting FVR. This interaction was reflected in the resting FBF values (P = 0.03). After accounting for important covariates, there was a main effect of eNOS genotype on peak FBF (TT, 7.0 +/- 0.3 vs TC+CC, 5.9 +/- 0.4 mL x 100 mL(-1) FAV x min(-1), P = 0.03) and the percent decrease in FVR (TT, -62 +/- 2 vs TC+CC, -51 +/- 4%, P = 0.04) at minute 1.

CONCLUSIONS

These results of the interactive effects suggest that young females possessing a C allele may reduce their resting FVR by improving their cardiovascular fitness level, but TT homozygotes, who may have normal eNOS gene function, may not improve their resting FVR with improvements in cardiovascular fitness. Furthermore, regardless of physical activity level, the TT genotype showed a favorable hemodynamic response during reactive hyperemia compared with the C allele carriers.

Mechanisms of shear stress-induced endothelial nitric-oxide synthase phosphorylation and expression in ovine fetoplacental artery endothelial cells

Placental blood flow, nitric-oxide (NO) levels, and endothelial NO synthase (eNOS) expression increase during human and ovine pregnancy. Shear stress stimulates NO production and eNOS expression in ovine fetoplacental artery endothelial (OFPAE) cells. Because eNOS is the rate-limiting enzyme essential for NO synthesis, its activity and expression are both closely regulated. We investigated signaling mechanisms underlying pulsatile shear stress-induced increases in eNOS phosphorylation and protein expression by OFPAE cells. The OFPAE cells were cultured at 3 dynes/cm2 shear stress, then exposed to 15 dynes/cm2 shear stress. Western blot analysis for phosphorylated ERK1/2, Akt, p38 mitogen activated protein kinase (MAPK), and eNOS showed that shear stress rapidly increased phosphorylation of ERK1/2 and Akt but not of p38 MAPK. Phosphorylation of eNOS Ser1177 under shear stress was elevated by 20 min, a response that was blocked by the phosphatidyl inositol-3-kinase (PI-3K)-inhibitors wortmannin and LY294002 but not by the mitogen activated protein kinase kinase (MEK)-inhibitor UO126. Basic fibroblast growth factor (bFGF) enhanced eNOS protein levels in static culture via a MEK-mediated mechanism, but it could not further augment the elevated eNOS protein levels otherwise induced by the 15 dynes/cm2 shear stress. Blockade of either signaling pathway changed the shear stress-induced increase in eNOS protein levels. In conclusion, shear stress induced rapid eNOS phosphorylation on Ser1177 in OFPAE cells through a PI-3K-dependent pathway. The bFGF-induced rise in eNOS protein levels in static culture was much less than those observed under flow and was blocked by inhibition of MEK. Prolonged shear stress-stimulated increases in eNOS protein were not affected by inhibition of MEK- or PI-3K-mediated pathways.

Physiological mechanisms regulating the expression of endothelial-type NO synthase

Although endothelial nitric oxide synthase (eNOS) is a constitutively expressed enzyme, its expression is regulated by a number of biophysical, biochemical, and hormonal stimuli, both under physiological conditions and in pathology. This review summarizes the recent findings in this field. Shear stress, growth factors (such as transforming growth factor-beta, fibroblast growth factor, vascular endothelial growth factor, and platelet-derived growth factor), hormones (such as estrogens, insulin, angiotensin II, and endothelin 1), and other compounds (such as lysophosphatidylcholine) upregulate eNOS expression. On the other hand, the cytokine tumor necrosis factor-alpha and bacterial lipopolysaccharide downregulate the expression of this enzyme. The growth status of cells, the actin cytoskeleton, and NO itself are also important regulators of eNOS expression. Both transcriptional and posttranscriptional mechanisms are involved in the expressional regulation of eNOS. Different signaling pathways are involved in the regulation of eNOS promoter activity and eNOS mRNA stability. Changes in eNOS expression and activity under pathophysiological conditions and the pharmacological modulation of eNOS expression are subject of a subsequent brief review (part 2) to be published in the next issue of this journal.

Shear stress induces expression of vascular endothelial growth factor receptor Flk-1/KDR through the CT-rich Sp1 binding site

Fluid shear stress is 1 of the major factors that control gene expression in vascular endothelial cells. We investigated the role of shear stress in the regulation of the expression of fetal liver kinase-1/kinase domain region (Flk-1/KDR), a vascular endothelial growth factor receptor, by using human umbilical vein endothelial cells. Laminar shear stress (15 dyne/cm2) elevated Flk-1/KDR mRNA levels by approximately 3-fold for 8 hours, and the expression was upregulated within the range of 5 to 40 dyne/cm2. Deletion analysis of the 5'-flanking region of the Flk-1/KDR gene promoter by use of a luciferase reporter vector revealed that a shear stress-responsive element resided in the sequence between -94 and -31 bp, which contained putative nuclear factor-kappaB, activator protein-2, and GC-rich Sp1 and CT-rich Sp1 binding sites. Electrophoretic mobility shift assay demonstrated that nuclear extract was bound to the GC-rich Sp1 sites and the CT-rich Sp1 site with a similar pattern. However, shear stress enhanced the DNA-protein interactions only on the CT-rich Sp1 site but not on the GC-rich Sp1 sites. A 3-bp mutation in the CT-rich Sp1 site eliminated the response to shear stress in electrophoretic mobility shift assay and luciferase reporter assay. These results suggest that shear stress induces Flk-1/KDR expression through the CT-rich Sp1 binding site.

Endothelial nitric oxide production during in vitro simulation of external limb compression

External pneumatic compression (EPC) is effective in preventing deep vein thrombosis (DVT) and is thought to alter endothelial thromboresistant properties. We investigated the effect of EPC on changes in nitric oxide (NO), a critical mediator in the regulation of vasomotor and platelet function. An in vitro cell culture system was developed to simulate flow and vessel collapse conditions under EPC. Human umbilical vein endothelial cells were cultured and subjected to tube compression (C), pulsatile flow (F), or a combination of the two (FC). NO production and endothelial nitric oxide synthase (eNOS) mRNA expression were measured. The data demonstrate that in the F and FC groups, there is a rapid release of NO followed by a sustained increase. NO production levels in the F and FC groups were almost identical, whereas the C group produced the same low amount of NO as the control group. Conditions F and FC also upregulate eNOS mRNA expression by a factor of 2.08 +/- 0.25 and 2.11 +/- 0.21, respectively, at 6 h. Experiments with different modes of EPC show that NO production and eNOS mRNA expression respond to different time cycles of compression. These results implicate enhanced NO release as a potentially important factor in the prevention of DVT.

Endothelial nitric oxide synthase pathophysiology after nonocclusive common carotid artery thrombosis in rats

Although vascular dysregulation has been documented in patients with extracranial vascular disease, transient ischemic attacks, and stroke, the pathomechanisms are poorly understood. To model thromboembolic stroke in rats, photochemically induced nonocclusive common carotid artery thrombosis (CCAT) was used to generate a platelet thrombus in the carotid artery of anesthetized rats. After CCAT, platelet aggregates break off the thrombus, travel to the distal cerebral vasculature, damage blood vessels, and cause small infarctions. The authors hypothesized that deficits in the endothelial nitric oxide synthase (eNOS) pathway may be responsible for vascular dysfunction after embolic stroke. To examine the functional status of the eNOS system, they measured eNOS-dependent dilation after CCAT by applying acetylcholine through a cranial window over the middle cerebral artery. The authors also measured eNOS mRNA and protein in the middle cerebral artery to determine whether functional changes were caused by alterations in expression. eNOS-dependent dilation was reduced at 6 hours, elevated at 24 hours, and returned to baseline 72 hours after CCAT. Endothelial nitric oxide synthase mRNA increased at 2 hours and was followed by a rise in protein 24 hours after CCAT. Changes in the eNOS system may account for some of the observed vascular deficits in patients with cerebrovascular disease.

Estrogen acutely activates prostacyclin synthesis in ovine fetal pulmonary artery endothelium

Prostacyclin (PGI(2)) is a key mediator of pulmonary vasodilation during perinatal cardiopulmonary transition, at a time when fetal plasma estrogen levels are rising. We have previously shown that estradiol-17beta (E(2)) rapidly stimulates nitric oxide production by ovine fetal pulmonary artery endothelial cells (PAEC), and that this occurs through nongenomic mechanisms which are calcium- and tyrosine kinase-mitogen-activated protein (MAP) kinase-dependent. In the present study, we determined if E(2) acutely activates PGI(2) production in PAEC. E(2) (10(-8) M for 15 min) caused a 52% increase in PGI(2), the threshold concentration was 10(-10) M E(2), the effect occurred within 5 min, and it was not related to changes in cyclooxygenase type 1 (COX-1) or COX-2 abundance. Estrogen receptor (ER) alpha and ER beta proteins and mRNAs were found to be constitutively expressed in PAEC, and PGI(2) stimulation with E(2) was fully blocked by both ER antagonism with ICI 182,780, which is not selective for either ER isoform, and the ER beta-specific antagonist RR-tetrahydrochrysene. The rapid response to E(2) was also inhibited by calcium chelation, whereas genistein- or PD98059-induced inhibition of tyrosine kinase and MAP kinase kinase, respectively, had no effect. Thus, E(2) causes rapid stimulation of PGI(2) synthesis in fetal PAEC, this process is mediated by ER beta, and it is calcium-dependent and tyrosine kinase-MAP kinase-independent. These mechanisms may play a role in pulmonary vasodilation in the perinatal period.

Red wine polyphenols increase calcium in bovine aortic endothelial cells: a basis to elucidate signalling pathways leading to nitric oxide production

1. The present study investigates the mechanisms by which polyphenolic compounds from red wine elicit Ca(2+) mobilization in bovine aortic endothelial cells (BAECs). Two polyphenol-containing red wine extracts, red wine polyphenolic compounds (RWPC) and Provinols, and delphinidin, an anthocyanin were used. 2. RWPC stimulated a Ca(2+)-dependent release of nitric oxide (NO) from BAECs accounting for the relaxation of endothelium-denuded rat aortic rings as shown by cascade bioassay. 3. RWPC, Provinols and delphinidin increased cytosolic free calcium ([Ca(2+)](i)), by releasing Ca(2+) from intracellular stores and by increasing Ca(2+) entry. 4. The RWPC-induced increase in [Ca(2+)](i) was decreased by exposure to ryanodine (30 microM), whereas Provinols and delphinidin-induced increases in [Ca(2+)](i) were decreased by bradykinin (0.1 microM) and thapsigargin (1 microM) pre-treatment. 5. RWPC, Provinols and delphinidin-induced increases in [Ca(2+)](i) were sensitive to inhibitors of phospholipase C (neomycin, 3 mM; U73122, 3 microM) and tyrosine kinase (herbimycin A, 1 microM). 6. RWPC, Provinols and delphinidin induced herbimycin A (1 microM)-sensitive tyrosine phosphorylation of several intracellular proteins. 7. Provinols released Ca(2+) via both a cholera (CTX) and pertussis toxins (PTX)-sensitive pathway, whereas delphinidin released Ca(2+) only via a PTX-sensitive mechanism. 8. Our data contribute in defining the mechanisms of endothelial NO production caused by wine polyphenols including the increase in [Ca(2+)](i) and the activation of tyrosine kinases. Furthermore, RWPC, Provinols and delphinidin display differences in the process leading to [Ca(2+)](i) increases in endothelial cells illustrating multiple cellular targets of natural dietary polyphenolic compounds.

Essential role for phosphoinositide 3-kinase in shear-dependent signaling between platelet glycoprotein Ib/V/IX and integrin alpha(IIb)beta(3)

Platelet adhesion and aggregation at sites of vascular injury are critically dependent on the interaction between von Willebrand factor (VWF) and 2 major platelet adhesion receptors, glycoprotein (GP) Ib/V/IX and integrin alpha(IIb)beta(3). GP Ib/V/IX binding to VWF mediates platelet tethering and translocation, whereas activation of integrin alpha(IIb)beta(3) promotes cell arrest. To date, the signaling pathways used by the VWF-GP Ib/V/IX interaction to promote activation of integrin alpha(IIb)beta(3), particularly under shear, have remained poorly defined. In this study, the potential involvement of type 1 phosphoinositide (PI) 3-kinases in this process was investigated. Results show that platelet adhesion and spreading on immobilized VWF results in a specific increase in the PI 3-kinase lipid product, PtdIns(3,4)P(2). Under static conditions, inhibiting PI 3-kinase with LY294002 or wortmannin did not prevent platelet adhesion, integrin alpha(IIb)beta(3) activation, or platelet spreading although it significantly delayed the onset of these events. In contrast, PI 3-kinase inhibition under shear dramatically reduced both platelet adhesion and spreading. Real-time analysis of intracellular calcium demonstrated that under static conditions inhibiting PI 3-kinase delayed the onset of intracellular fluxes in adherent platelets, but did not affect the final magnitude of the calcium response. However, under shear, inhibiting PI 3-kinase dramatically reduced intracellular calcium mobilization and integrin alpha(IIb)beta(3) activation, resulting in impaired thrombus growth. The studies demonstrate a shear-dependent role for PI 3-kinase in promoting platelet adhesion on immobilized VWF. Under static conditions, platelets appear to mobilize intracellular calcium through both PI 3-kinase-dependent and -independent mechanisms, whereas under shear PI 3-kinase is indispensable for VWF-induced calcium release.

Hyper-osmotic stress induces volume change and calcium transients in chondrocytes by transmembrane, phospholipid, and G-protein pathways

Mechanical compression of cartilage is associated with a rise in the interstitial osmotic pressure, which can alter cell volume and activate volume recovery pathways. One of the early events implicated in regulatory volume changes and mechanotransduction is an increase of intracellular calcium ion ([Ca(2+)](i)). In this study, we tested the hypothesis that osmotic stress initiates intracellular Ca(2+) signaling in chondrocytes. Using laser scanning microscopy and digital image processing, [Ca(2+)](i) and cell volume were monitored in chondrocytes exposed to hyper-osmotic solutions. Control experiments showed that exposure to hyper-osmotic solution caused significant decreases in cell volume as well as transient increases in [Ca(2+)](i). The initial peak in [Ca(2+)](i) was generally followed by decaying oscillations. Pretreatment with gadolinium, a non-specific blocker of mechanosensitive ion channels, inhibited this [Ca(2+)](i) increase. Calcium-free media eliminated [Ca(2+)](i) increases in all cases. Pretreatment with U73122, thapsigargin, or heparin (blockers of the inositol phosphate pathway), or pertussis toxin (a blocker of G-proteins) significantly decreased the percentage of cells responding to osmotic stress and nearly abolished all oscillations. Cell volume decreased with hyper-osmotic stress and recovered towards baseline levels throughout the duration of the control experiments. The peak volume change with 550 mOsm osmotic stress, as well as the percent recovery of cell volume, was dependent on [Ca(2+)](i.) These findings indicate that osmotic stress causes significant volume change in chondrocytes and may activate an intracellular second messenger signal by inducing transient increases in [Ca(2+)](i).