Disruption of cytoskeletal structures mediates shear stress-induced endothelin-1 gene expression in cultured porcine aortic endothelial cells

Hemodynamic force dictates endothelial angiogenesis through MIEN1-ERK/MAPK-signaling axis

It is well-recognized that blood flow at branches and bends of arteries generates disturbed shear stress, which plays a crucial in driving atherosclerosis. Flow-generated fluid shear stress (FSS), as one of the key hemodynamic factors, is appreciated for its critical involvement in regulating angiogenesis to facilitate wound healing and tissue repair. Endothelial cells can directly sense FSS but the mechanobiological mechanism by which they decode different patterns of FSS to trigger angiogenesis remains unclear. In the current study, laminar shear stress (LSS, 15 dyn/cm2) was employed to mimic physiological blood flow, while disturbed shear stress (DSS, ranging from 0.5 ± 4 dyn/cm2) was applied to simulate pathological conditions. The aim was to investigate how these distinct types of blood flow regulated endothelial angiogenesis. Initially, we observed that DSS impaired angiogenesis and downregulated endogenous vascular endothelial growth factor B (VEGFB) expression compared to LSS. We further found that the changes in membrane protein, migration and invasion enhancer 1 (MIEN1) play a role in regulating ERK/MAPK signaling, thereby contributing to endothelial angiogenesis in response to FSS. We also showed the involvement of MIEN1-directed cytoskeleton organization. These findings suggest the significance of shear stress in endothelial angiogenesis, thereby enhancing our understanding of the alterations in angiogenesis that occur during the transition from physiological to pathological blood flow.

Vascular mechanotransduction

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

CFTR limits F-actin formation and promotes morphological alignment with flow in human lung microvascular endothelial cells

Micro- and macrovascular endothelial dysfunction in response to shear stress has been observed in cystic fibrosis (CF), and has been associated with inflammation and oxidative stress. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) regulates endothelial actin cytoskeleton dynamics and cellular alignment in response to flow. Human lung microvascular endothelial cells (HLMVEC) were cultured with either the CFTR inhibitor GlyH-101 (20 µM) or CFTRinh-172 (20 µM), tumor necrosis factor (TNF)-α (10 ng/ml) or a vehicle control (0.1% dimethyl sulfoxide) during 24 and 48 h of exposure to shear stress (11.1 dynes/cm2 ) or under static control conditions. Cellular morphology and filamentous actin (F-actin) were assessed using immunocytochemistry. [Nitrite] and endothelin-1 ([ET-1]) were determined in cell culture supernatant by ozone-based chemiluminescence and ELISA, respectively. Treatment of HLMVECs with both CFTR inhibitors prevented alignment of HLMVEC in the direction of flow after 24 and 48 h of shear stress, compared to vehicle control (both p < 0.05). Treatment with TNF-α significantly increased total F-actin after 24 h versus control (p < 0.05), an effect that was independent of shear stress. GlyH-101 significantly increased F-actin after 24 h of shear stress versus control (p < 0.05), with a significant (p < 0.05) reduction in cortical F-actin under both static and flow conditions. Shear stress decreased [ET-1] after 24 h (p < 0.05) and increased [nitrite] after 48 h (p < 0.05), but neither [nitrite] nor [ET-1] was affected by GlyH-101 (p > 0.05). CFTR appears to limit cytosolic actin polymerization, while maintaining a cortical rim actin distribution that is important for maintaining barrier integrity and promoting alignment with flow, without effects on endothelial nitrite or ET-1 production.

Editorial: Understanding molecular interactions that underpin vascular mechanobiology

Cells are exposed to a variety of mechanical forces in their daily lives, especially endothelial cells that are stretched from vessel distention and are exposed to hemodynamic shear stress from a blood flow. Exposure to excessive forces can induce a disease, but the molecular details on how these cells perceive forces, transduce them into biochemical signals and genetic events, i.e., mechanotransduction, and integrate them into physiological or pathological changes remain unclear. However, seminal studies in endothelial cells over the past several decades have begun to elucidate some of these signals. These studies have been highlighted in APL Bioengineering and elsewhere, describing a complex temporal pattern where forces are sensed immediately by ion channels and force-dependent conformational changes in surface proteins, followed by biochemical cascades, cytoskeletal contraction, and nuclear remodeling that can affect long-term changes in endothelial morphology and fate. Key examples from the endothelial literature that have established these pathways include showing that integrins and Flk-1 or VE-cadherin act as shear stress transducers, activating downstream proteins such as Cbl and Nckβ or Src, respectively. In this Editorial, we summarize a recent literature highlighting these accomplishments, noting the engineering tools and analysis methods used in these discoveries while also highlighting unanswered questions.

Enhancement of non-invasive trans-membrane drug delivery using ultrasound and microbubbles during physiologically relevant flow

Sonoporation has been associated with drug delivery across cell membranes and into target cells, yet several limitations have prohibited further advancement of this technology. Higher delivery rates were associated with increased cellular death, thus implying a safety-efficacy trade-off. Meanwhile, there has been no reported study of safe in vitro sonoporation in a physiologically relevant flow environment. The objective of our study was not only to evaluate sonoporation under physiologically relevant flow conditions, such as fluid velocity, shear stress and temperature, but also to design ultrasound parameters that exploit the presence of flow to maximize sonoporation efficacy while minimizing or avoiding cellular damage. Human umbilical vein endothelial cells (EA.hy926) were seeded in flow chambers as a monolayer to mimic the endothelium. A peristaltic pump maintained a constant fluid velocity of 12.5 cm/s. A focused 0.5 MHz transducer was used to sonicate the cells, while an inserted focused 7.5 MHz passive cavitation detector monitored microbubble-seeded cavitation emissions. Under these conditions, propidium iodide, which is normally impermeable to the cell membrane, was traced to determine whether it could enter cells after sonication. Meanwhile, calcein-AM was used as a cell viability marker. A range of focused ultrasound parameters was explored, with several unique bioeffects observed: cell detachment, preservation of cell viability with no membrane penetration, cell death and preservation of cell viability with sonoporation. The parameters were then modified further to produce safe sonoporation with minimal cell death. To increase the number of favourable cavitation events, we lowered the ultrasound exposure pressure to 40 kPapk-neg and increased the number of cavitation nuclei by 50 times to produce a trans-membrane delivery rate of 62.6% ± 4.3% with a cell viability of 95% ± 4.2%. Furthermore, acoustic cavitation analysis showed that the low pressure sonication produced stable and non-inertial cavitation throughout the pulse sequence. To our knowledge, this is the first study to demonstrate a high drug delivery rate coupled with high cell viability in a physiologically relevant in vitro flow system.

Specification of the mouse cardiac conduction system in the absence of Endothelin signaling

Coordinated contraction of the heart is essential for survival and is regulated by the cardiac conduction system. Contraction of ventricular myocytes is controlled by the terminal part of the conduction system known as the Purkinje fiber network. Lineage analyses in chickens and mice have established that the Purkinje fibers of the peripheral ventricular conduction system arise from working myocytes during cardiac development. It has been proposed, based primarily on gain-of-function studies, that Endothelin signaling is responsible for myocyte-to-Purkinje fiber transdifferentiation during avian heart development. However, the role of Endothelin signaling in mammalian conduction system development is less clear, and the development of the cardiac conduction system in mice lacking Endothelin signaling has not been previously addressed. Here, we assessed the specification of the cardiac conduction system in mouse embryos lacking all Endothelin signaling. We found that mouse embryos that were homozygous null for both ednra and ednrb, the genes encoding the two Endothelin receptors in mice, were born at predicted Mendelian frequency and had normal specification of the cardiac conduction system and apparently normal electrocardiograms with normal QRS intervals. In addition, we found that ednra expression within the heart was restricted to the myocardium while ednrb expression in the heart was restricted to the endocardium and coronary endothelium. By establishing that ednra and ednrb are expressed in distinct compartments within the developing mammalian heart and that Endothelin signaling is dispensable for specification and function of the cardiac conduction system, this work has important implications for our understanding of mammalian cardiac development.

Endothelial mechanosensors: the gatekeepers of vascular homeostasis and adaptation under mechanical stress

Endothelial cells (ECs) not only serve as a barrier between blood and extravascular space to modulate the exchange of fluid, macromolecules and cells, but also play a critical role in regulation of vascular homeostasis and adaptation under mechanical stimulus via intrinsic mechanotransduction. Recently, with the dissection of microdomains responsible for cellular responsiveness to mechanical stimulus, a lot of mechanosensing molecules (mechanosensors) and pathways have been identified in ECs. In addition, there is growing evidence that endothelial mechanosensors not only serve as key vascular gatekeepers, but also contribute to the pathogenesis of various vascular disorders. This review focuses on recent findings in endothelial mechanosensors in subcellular microdomains and their roles in regulation of physiological and pathological functions under mechanical stress.

Single-cell imaging of mechanotransduction in endothelial cells

Endothelial cells (ECs) are constantly exposed to chemical and mechanical microenvironment in vivo. In mechanotransduction, cells can sense and translate the extracellular mechanical cues into intracellular biochemical signals, to regulate cellular processes. This regulation is crucial for many physiological functions, such as cell adhesion, migration, proliferation, and survival, as well as the progression of disease such as atherosclerosis. Here, we overview the current molecular understanding of mechanotransduction in ECs associated with atherosclerosis, especially those in response to physiological shear stress. The enabling technology of live-cell imaging has allowed the study of spatiotemporal molecular events and unprecedented understanding of intracellular signaling responses in mechanotransduction. Hence, we also introduce recent studies on mechanotransduction using single-cell imaging technologies.

Flow detection and calcium signalling in vascular endothelial cells

Blood vessels alter their morphology and function in response to changes in blood flow, and their responses are based on blood flow detection by the vascular endothelium. Endothelial cells (ECs) covering the inner surface of blood vessels sense shear stress generated by flowing blood and transmit the signal into the interior of the cell, which evokes a cellular response. The EC response to shear stress is closely linked to the regulation of vascular tone, blood coagulation and fibrinolysis, angiogenesis, and vascular remodelling, and it plays an important role in maintaining the homoeostasis of the circulatory system. Impairment of the EC response to shear stress leads to the development of vascular diseases such as hypertension, thrombosis, aneurysms, and atherosclerosis. Rapid progress has been made in elucidating shear stress mechanotransduction by using in vitro methods that apply controlled levels of shear stress to cultured ECs in fluid-dynamically designed flow-loading devices. The results have revealed that shear stress is converted into intracellular biochemical signals that are mediated by a variety of membrane molecules and microdomains, including ion channels, receptors, G-proteins, adhesion molecules, the cytoskeleton, caveolae, the glycocalyx, and primary cilia, and that multiple downstream signalling pathways become activated almost simultaneously. Nevertheless, neither the shear-stress-sensing mechanisms nor the sensor molecules that initially sense shear stress are yet known. Their identification would contribute to a better understanding of the pathophysiology of the vascular diseases that occur in a blood flow-dependent manner and to the development of new treatments for them.

Effects of shear stress cultivation on cell membrane disruption and intracellular calcium concentration in sonoporation of endothelial cells

Microbubble facilitated ultrasound (US) application can enhance intracellular delivery of drugs and genes in endothelial cells cultured in static condition by transiently disrupting the cell membrane, or sonoporation. However, endothelial cells in vivo that are constantly exposed to blood flow may exhibit different sonoporation characteristics. This study investigates the effects of shear stress cultivation on sonoporation of endothelial cells in terms of membrane disruption and changes in the intracellular calcium concentration ([Ca(2+)](i)). Sonoporation experiments were conducted using murine brain microvascular endothelial (bEnd.3) cells and human umbilical vein endothelial cells (HUVECs) cultured under static or shear stress (5 dyne/cm(2) for 5 days) condition in a microchannel environment. The cells were exposed to a short US tone burst (1.25 MHz, 8 μs duration, 0.24 MPa) in the presence of Definity™ microbubbles to facilitate sonoporation. Membrane disruption was assessed by propidium iodide (PI) and changes in [Ca(2+)](i) measured by fura-2AM. Results from this study show that shear stress cultivation significantly reduced the impact of ultrasound-driven microbubbles activities on endothelial cells. Cells cultured under shear stress condition exhibited much lower percentage with membrane disruption and changes in [Ca(2+)](i) compared to statically cultured cells. The maximum increases of PI uptake and [Ca(2+)](i) were also significantly lower in the shear stress cultured cells. In addition, the extent of [Ca(2+)](i) waves in shear cultured HUVECs was reduced compared to the statically cultured cells.

The role of cellular adaptation to mechanical forces in atherosclerosis

Atherosclerosis is a chronic inflammatory disease that originates at regions of arteries exposed to disturbances in fluid flow and results in progressive plaque formation in those areas. Recent work on cellular responses to flow has identified potential mechanosensors and pathways that may influence disease progression. These results led us to hypothesize that the same mechanisms that mediate adaptive responses in the vasculature become maladaptive at sites of disturbed flow. Subsequent changes in gene expression and matrix remodeling help to entrain these inflammatory pathways. These events synergize with systemic risk factors such as hyperlipidemia, smoking, and diabetes, leading to disease progression.

Endothelin receptor B-mediated induction of c-jun and AP-1 in response to shear stress in human endothelial cellsThis article is one of a selection of papers published in the special issue (part 2 of 2) on Forefronts in Endothelin

In vivo, endothelial cells are constantly exposed to shear stress by flowing blood. Short-term exposure of endothelial cells to shear stress has been shown to induce endothelin-1 release. It is currently unknown, however, whether this shear stress-dependent endothelin-1 release affects the expression and activity of transcription factors. In this study, primary cultures of human endothelial cells from the umbilical vein were exposed to laminar shear stress in a cone-and-plate viscometer. Laminar shear stress for 30 min induced a 2-fold increase in mRNA expression of c-jun , but not c-fos, in human endothelial cells. Blockade of endothelin receptor subtype B (ETB) with BQ788 prevented this shear stress-dependent induction of c-jun expression. The induction of c-jun by shear stress involved protein kinase C and endothelial NO synthase. In addition, exposure of endothelial cells to arterial laminar shear stress for 1 h increased the binding of transcription factor AP-1 to its consensus sequence by 1.7-fold in electrophoretic mobility shift assays. This induction was also mediated by an ETB-dependent pathway. Supershift analysis supports an AP-1 complex containing c-jun, but not c-fos, in human endothelial cells. In conclusion, our data suggest endothelin-1-mediated induction of c-jun expression and activation of AP-1 (possibly as a c-jun homodimer) by laminar shear stress in human endothelial cells.

Low shear stress-induced interleukin-8 mRNA expression in endothelial cells is mechanotransduced by integrins and the cytoskeleton

Shear stress is thought to be important in maintaining vascular homeostasis and regulating vascular remodeling. The authors have previously shown that low shear stress increases interleukin (IL)-8 gene expression in endothelial cells. However, the detailed events that contribute to the regulation of this expression remain to be identified. In this study, the authors examined whether the shear stress-induced IL-8 mRNA expression in endothelial cells is mechanotransduced by integrins and the cytoskeleton. Exposure of endothelial cells to low shear stress (4.2 dyne/cm(2)) rapidly increased the expression of IL-8 mRNA. The induced IL-8 mRNA expression was inhibited by GRGDNP, which blocked the binding of integrins to the extracellular matrix. Additionally, this increase was attenuated by both anti-alpha(v)beta(3) integrin and anti-beta(1) integrin antibodies. Treatment of endothelial cells with either low shear stress or cytochalasin D resulted in the disruption of the cytoskeleton. Following the disruption of filamentous F-actin, the IL-8 mRNA expression in endothelial cells also increased, indicating that the shear stress-induced IL-8 mRNA expression may be mediated by the disruption of actin fibers. Taken together, these data suggest that integrins and the actin cytoskeleton play important roles in regulating the shear stress-induced IL-8 gene expression. Supplementary materials are available for the Materials and Methods section of this article. Go to the publisher's online of edition of Endothelium for this free supplemental resource.

Arterial adaptations to chronic changes in haemodynamic function: coupling vasomotor tone to structural remodelling

Healthy mature arteries are usually extremely quiescent tissues with cell proliferation rates much below 1%/day and with extracellular matrix constituents exhibiting half-lives of years to decades. However, chronic physiological or pathological changes in haemodynamic function elicit arterial remodelling processes that may involve substantial tissue synthesis, degradation or turnover. Although these remodelling processes accommodate changing demands placed upon the cardiovascular system by physiological adaptations, they can compromise further perfusion in the context of arterial occlusive disease and they entrench hypertension and may exacerbate its progression. Recent findings indicate that some of the most important such remodelling responses involve the integrated effects of persistently altered vascular tone that feed into restructuring responses, with common signalling pathways frequently interacting in the control of both phases of the response. Current efforts to define these signals and their targets may provide new directions for therapeutic interventions to treat important vascular disorders.

Proteomic analysis of vascular endothelial cells in response to laminar shear stress

Isotope-coded affinity tags (cICAT) coupled with mass spectrometric analysis is one of the leading technologies for quantitative proteomic profiling and protein quantification. We performed proteomic analysis of bovine aortic endothelial cells (BAEC) in response to laminar shear stress using cICAT labeling coupled with LC-MS/MS. Protein expressions in BAEC under 15 dynes/cm2 of shear stress for 10 min, 3 h, and 6 h were compared with matched stationary controls. Analysis of each sample produced 1800-2400 proteins at >or=75% confidence level. We found 142, 213, and 186 candidate proteins that were up- or down-regulated by at least two-fold after 10 min, 3 h, and 6 h of shear stress, respectively. Some of these proteins have known cellular functions and they encompass many signaling pathways. The signaling pathways that respond to shear stress include those of integrins, G-protein-coupled receptors, glutamate receptors, PI3K/AKT, apoptosis, Notch and cAMP-mediated signaling pathways. The validity of the mass spectrometric analysis was also confirmed by Western blot and confocal immunofluorescence microscopy. The present quantitative proteomic analysis suggests novel potential regulatory mechanisms in vascular endothelial cells in response to shear stress. These results provide preliminary footprints for further studies on the signaling mechanisms induced by shear stress.

Acute increases of renal medullary osmolality stimulate endothelin release from the kidney

Experiments conducted in vitro suggest that high osmolality stimulates endothelin production and release by renal tubular epithelial cells. Whether hyperosmotic solutions exert similar effects in vivo is unknown. Therefore, we tested the hypothesis that increasing renal medullary osmolality enhances urinary excretion of endothelin in anesthetized rats. Isosmotic NaCl (284 mosmol/kgH2O) was infused either intravenously (1.5 ml/h) or into the renal medullary interstitium (0.5 ml/h) during a 1-h equilibration period and 30-min baseline urine collection period, followed by either isosmotic or hyperosmotic NaCl (921 or 1,664 mosmol/kgH2O iv; 1,714 mosmol/kgH2O into renal medulla) for two further 30-min periods. Compared with isosmotic NaCl, infusion of hyperosmotic NaCl into the renal medulla significantly increased the endothelin excretion rate ( P < 0.05; from 0.30 ± 0.02 to 0.49 ± 0.03 fmol/min). Intravenous infusion of hyperosmotic NaCl also significantly increased endothelin excretion rate in a concentration-dependent manner (from 0.79 ± 0.07 to 1.77 ± 0.16 fmol/min and 0.59 ± 0.04 to 1.11 ± 0.08 fmol/min for 1,664 and 921 mosmol/kgH2O, respectively). To differentiate between effects of osmolality and NaCl, similar experiments were performed using mannitol solutions. Compared with isosmotic mannitol, medullary interstitial infusion of hyperosmotic mannitol (1,820 mosmol/kgH2O) significantly increased endothelin excretion rate ( P < 0.05; from 0.54 ± 0.03 to 0.94 ± 0.12 fmol/min). Thus exposing the renal medulla to hyperosmotic concentrations of either NaCl or mannitol stimulates endothelin release in vivo, consistent with medullary osmolality being an important regulator of renal endothelin synthesis.

Assessment of vascular wall shear stress and implications for atherosclerotic disease

Mechanical characteristics of circulation and vasculature, such as blood flow in combination with geometrical features, participate in the generation of several pathophysiological processes, such as development of blood vessel structure, chronic vessel tone regulation, atherosclerosis and others. We aim to briefly describe the basic mechanical and haemodynamic phenomena related to the forces applied to arterial walls, especially shear stresses and the shear-induced physiologic and pathophysiologic processes focusing on those related to atherosclerotic disease.

Mechanosensation and endothelin in astrocytes--hypothetical roles in CNS pathophysiology

Endothelin (ET) is a potent autocrine mitogen produced by reactive and neoplastic astrocytes. ET has been implicated in the induction of astrocyte proliferation and other transformations engendered by brain pathology, and in promoting the malignant behavior of astrocytomas. Reactive astrocytes containing ET are found in the periphery/penumbra of a wide array of CNS pathologies. Virtually all brain pathology deforms the surrounding parenchyma, either by direct mass effect or edema. Mechanical stress is a well established stimulus for ET production and release by other cell types, but has not been well studied in the brain. However, numerous studies have illustrated that astrocytes can sense mechanical stress and translate it into chemical messages. Furthermore, the ubiquitous reticular meshwork formed by interconnected astrocytes provides an ideal morphology for sensing and responding to mechanical disturbances. We have recently demonstrated stretch-induced ET production by astrocytes in vitro. Inspired by this finding, the purpose of this article is to review the literature on (1) astrocyte mechanosensation, and (2) the endothelin system in astrocytes, and to consider the hypothesis that mechanical induction of the ET system may influence astrocyte functioning in CNS pathophysiology. We conclude by discussing evidence supporting future investigations to determine whether specific inhibition of stretch-activated ion channels may represent a novel strategy for treating or preventing CNS disturbances, as well as the relevance to astrocyte-derived tumors.

Combretastatin A4 phosphate has tumor antivascular activity in rat and man as demonstrated by dynamic magnetic resonance imaging

PURPOSE

Combretastatin A4 phosphate (CA4P) is a novel vascular targeting agent. Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) studies were performed to examine changes in parameters related to blood flow and vascular permeability in tumor and normal tissue after CA4P treatment.

MATERIALS AND METHODS

Changes in kinetic DCE-MRI parameters (transfer constant [Ktrans] and area under contrast medium-time curve [AUC]) over 24 hours after treatment with CA4P were measured in 18 patients in a phase I trial and compared with those obtained in the rat P22 carcinosarcoma model, using the same imaging technique. Rats were treated with 30 mg/kg of CA4P; patients received escalating doses from 5 to 114 mg/m2.

RESULTS

A similar pattern and time course of change in tumor and normal tissue parameters was seen in rats and humans. Rat tumor Ktrans was reduced by 64% 6 hours after treatment with CA4P (30 mg/kg). No significant reductions in kidney or muscle parameters were seen. Significant reductions were seen in tumor Ktrans in six of 16 patients treated at >or= 52 mg/m2, with a significant group mean reduction of 37% and 29% at 4 and 24 hours, respectively, after treatment. The mean reduction in tumor initial area under the gadolinium-diethylenetriamine pentaacetic acid concentration-time curve (AUC) was 33% and 18%, respectively, at these times. No reduction was seen in muscle Ktrans or in kidney AUC in group analysis of the clinical data.

CONCLUSION

CA4P acutely reduces Ktrans in human as well as rat tumors at well-tolerated doses, with no significant changes in kidney or muscle, providing proof of principle that this drug has tumor antivascular activity in rats and humans.

Scholarly review of geometry and compliance: biomechanical perspectives on vascular injury and healing

Mechanical stress and strain upon cardiovascular tissue are important factors that influence the ultimate configuration of clinically observed disease entities. Although mechanical forces can stimulate cellular changes and response, structural or geometric alterations introduced by disease processes can, in turn, influence local hemodynamic conditions. Dynamic interactions of structural parameters, such as arterial compliance and geometry, can further contribute to the final determination of the mechanical conditions and outcome of the vessel. Manipulation of vascular compliance and geometry may, therefore, have desirable effects. In this article, fundamental vascular biomechanical forces are defined and their association with cellular response and clinical disease processes are introduced. The interplay between vascular geometry and compliance is emphasized, and the potential for mechanical solutions to vascular diseases are explored.

Hydrodynamic damage to animal cells

Animal cells are affected by hydrodynamic forces that occur in culture vessel, transfer piping, and recovery operations such as microfiltration. Depending on the type, intensity, and duration of the force, and the specifics of the cell, the force may induce various kinds of responses in the subject cells. Both biochemical and physiological responses are observed, including apoptosis and purely mechanical destruction of the cell. This review examines the kinds of hydrodynamic forces encountered in bioprocessing equipment and the impact of those forces on cells. Methods are given for quantifying the magnitude of the specific forces, and the response thresholds are noted for the common types of cells cultured in free suspension, supported on microcarriers, and anchored to stationary surfaces.

Shear-induced changes in endothelin-1 secretion of microvascular endothelial cells

Human glomerular microvascular endothelial cell (HGMEC) culture monolayers were maintained in static culture as controls or subjected to steady laminar shear stress of 0.5, 1.0, or 1.5 N/m2. Over 25 h of shear, the cumulative secretion of ET-1 was 705.4 pg/cm2 in the control, 820.7 pg/cm2 at 0.5 N/m2, 1063.2 pg/cm2 at 1.0 N/m2, and 644.7 pg/cm2 at 1.5 N/m2. The average ET-1 secretion rate for the HGMEC monolayers exposed to 0.5, 1.0, or 1.5 N/m2 of shear stress was 32.83 +/- 2.01 pg/cm2 x h, 42.53 +/- 3.74 pg/cm2 x h, and 25.79 +/- 1.29 pg/cm2 x h, respectively. The average ET-1 secretion rate of the static controls was 28.22 +/- 3.11 pg/cm2 x h. The results showed that low shear stress (0.5 N/m2) elevated and high shear stress (1.5 N/m2) suppressed secretion of ET-1, while an intermediate level of shear stress (1.0 N/m2) led to the maximum secretion of ET-1, and furthermore, ET-1 secretion varied with the duration of shear in a nonlinear fashion, and the logistic equations may be used to describe relationship between the duration of shear and the ET-1 secretion. The major secretion period of ET-1 occurred between 5.3 and 22.3 h, with the peak secretion rate occurring at approximately 10.7-15.2 h. Our findings showed also that the major secretion period and peak secretion rate of HGMECs varied with the level of shear stress. Thus, the response of cultured human microvascular endothelial cells to shear stress differed from that of large-vessel endothelial cell cultures in terms of ET-1 secretion. In addition to the level of shear stress, the duration of shear is an important determinant in ET-1 secretion. Consequently, the heterogeneity of vascular endothelial cells and the duration of shear should both be considered in future research on the secretion of vascular endothelial cell cultures.

Stimulated release of endothelin-converting enzyme is simultaneous with tissue-type plasminogen activator and decrease in coronary heart disease

OBJECTIVE

During the process of atherosclerosis the endothelium changes both structurally and functionally. We examined whether shedding of endothelin-converting enzyme (ECE), a metalloprotease responsible for endothelin production, is concomitant with tissue-type plasminogen activator (t-PA), and how atherosclerosis affects ECE release.

DESIGN

Fourteen healthy volunteers and 24 patients with angiographically verified coronary heart disease (CHD) were investigated. ECE and t-PA releases were measured by a provocation test (20-min venous occlusion).

RESULTS

Serum ECE activities were comparable in both groups before the venous occlusion test (in CHD patients 205 +/- 24 vs in healthy controls 204 +/- 40 pmol/ml/h, p = NS). However, delta-ECE (= the difference between, after, and before the venous occlusion test) was significantly lower in CHD patients than in controls (203 +/- 36 vs 338 +/- 43 pmol/ml/h, p = 0.02, respectively). Delta-t-PA was similar in both groups (22.3 +/- 4.4 vs 14.5 +/- 4.6 ng/ml, p = NS, respectively). Furthermore, t-PA and ECE values correlated in the CHD group in all pre-, post-venous occlusion test, and delta-venous occlusion test values (r = 0.56, p = 0.009; r = 0.62, p = 0.003; r = 0.54, p = 0.01, respectively).

CONCLUSION

Vascular ECE release can be stimulated, and it is concomitant with t-PA release. A common location in endothelium may explain this simultaneous shedding. However, ECE levels do not rise in patients with CHD as markedly as in healthy patients. Atherosclerosis may explain reduced shedding of ECE.

Timing of nitric oxide donor supplementation determines endothelin-1 regulation and quality of lung preservation for transplantation

Nitroglycerin (NTG) given to donor lungs improves lung preservation for transplantation, but the mechanism(s) underlying this therapeutic benefit remain incompletely understood. Furthermore, it is not known whether the therapeutic window of opportunity for NTG administration is temporally-restricted. Because endothelin-1 (ET-1), a potent vasoconstrictor, and nitric oxide (NO) are reciprocally regulated in vitro, we hypothesized that early administration of the NO donor NTG may suppress ET-1 and thereby improve lung preservation. Using an isogeneic rat left lung transplantation model, four groups were studied (n = 12 transplant/group): (1) NTG given during flush/ preservation (Early NTG); (2) NTG given in the ex vivo flush (Late NTG); (3) No NTG; and (4) a nonselective ET-receptor antagonist (PD156252) given during flush/preservation. Early NTG decreased vascular tone in lung grafts measured ex vivo as well as in vivo following lung transplantation, and resulted in improved survival (100%) and gas exchange (pO2 209 +/- 19 mm Hg) compared with Late (17%, 62 +/- 16 mm Hg) or No NTG (25%, 59 +/- 9 mm Hg) (P < 0.05 for Early NTG versus all other groups for both survival and pO2). PD156252 was associated with an intermediate level of survival (50%) and function (104 +/- 23 mm Hg). Transplanted lung graft ET-1 mRNA, measured by Northern blotting and in situ hybridization, and protein, measured by Western blotting and immunohistochemistry, were suppressed only with Early NTG (P < 0.05 versus all other groups). Post-transplantation benefits of NTG are restricted to lung grafts which received NTG during the early harvest and immersion periods, and are coincident with suppression of graft ET-1 expression. When viewed in the context of improved graft survival and function with ET-1 receptor blockade, these data suggest that early administration of NTG to donor lungs improves primary graft function, in part, by suppressing graft ET-1 expression.

Crosstalk between endothelin and nitric oxide in the control of vascular tone

Several lines of evidence indicate that nitric oxide (NO) impairs endothelin (ET) production/action in vitro. Acute pressor responses caused by the blockade of NO formation with arginine analogues in vivo are blunted by selective ET(A) or dual ET(A)/ET(B) receptor blockade whereas blockade of NO formation magnifies ET-induced constriction of various vascular territories. Given that ET receptor blockade has normally limited effects on mean arterial pressure, the reversal of pressor responses caused by the blockade of NO formation with ET receptor blockade most likely reflects a significant crosstalk between NO and ET. Suppression of NO formation also leads to significant increases in ET production caused by agents targeting the endothelium, such as acetylcholine and thrombin. In addition, the inhibitory effect of shear stress on endothelial cells ET production also involves NO as an intermediate.Paradoxically, chronic exposure to organic nitrates which causes nitrate tolerance leads to an augmented vascular ET content. An increased angiotensin II (AII) production is apparently pivotal in this process. This article reviews observations pointing to the importance of NO/ET interactions as a fundamental and common regulatory mechanism shared across species. As a consequence of this crosstalk between NO and ET, experimental strategies designed to assess endothelial NO-dependent activity by the blockade of NO formation may be mitigated by magnified ET-dependent influences.

Patterns of living beta-actin movement in wounded human coronary artery endothelial cells exposed to shear stress

We previously demonstrated that physiologic levels of shear stress enhance endothelial repair. Cell spreading and migration, but not proliferation, were the major mechanisms accounting for the increases in wound closure rate (Albuquerque et al., 2000, Am. J. Physiol. Heart Circ. Physiol. 279, H293-H302). However, the patterns and movements of beta-actin filaments responsible for cell motility and translocation in human coronary artery endothelial cells (HCAECs) have not been previously investigated under physiologic flow. HCAECs transfected with beta-actin-GFP were cultured on type I collagen-coated coverslips. Confluent cell monolayers were subjected to laminar shear stress of 12 dynes/cm(2) for 18 h in a parallel-plate flow chamber to attain cellular alignment and then wounded by scraping with a metal spatula and subsequently exposed to a laminar shear stress of 20 dynes/cm(2) (S-W-sH) or static (S-W-sT) conditions. Time-lapse imaging and deconvolution microscopy was performed during the first 3 h after imposition of S-W-sH or S-W-sT conditions. The spatial and temporal dynamics of beta-actin-GFP motility and translocation during wound closure in HCAEC monolayers were analyzed under both conditions. Compared with HCAEC under S-W-sT conditions, our data show that HCAEC under S-W-sH conditions demonstrated greater beta-actin-GFP motility, filament and clumping patterns, and filament arcs used during cellular attachment and detachment. These findings demonstrate intriguing patterns of beta-actin organization and movement during wound closure in HCAEC exposed to physiological flow.

Increased contribution of L-arginine-nitric oxide pathway in aorta of mice lacking the gene for vimentin

Experiments were designed to investigate endothelial function in the aorta of mice lacking the gene for the cytoskeleton protein vimentin (vim -/- ). Rings with and without endothelium from wild-type (vim +/+ ), heterozygous (vim +/- ), and homozygous (vim -/- ) mice were suspended in organ chambers to record of changes in isometric tension. During phenylephrine contraction, acetylcholine evoked comparable endothelium-dependent relaxations in the three groups. In the presence of Nomega-nitro-L-arginine, acetylcholine caused endothelium-dependent contractions, which were greater in vim -/- than in vim +/+ and vim +/- aortas. Indomethacin did not affect relaxation to acetylcholine in vim +/+ or in vim +/-, but it significantly increased the maximal response in vim -/- (67 +/- 7 vs. 102 +/- 4%). Response to acetylcholine in vim -/- aortas was not affected by cyclooxygenase type 2 inhibitor NS-398, the thromboxane receptor antagonist SQ-29,548, or superoxide dismutase. Relaxations to sodium nitroprusside were not different between vim +/+ and vim -/- mice and were not affected by cyclooxygenase inhibition. Cyclic guanosine monophosphate levels, which were increased to a comparable level by acetylcholine in vim +/+ and vim -/-, were augmented by indomethacin in vim -/- aortas but not in vim +/+ aortas. Expression of endothelial nitric oxide synthase was not different between vim +/+ and vim -/- preparations. These results suggest that despite comparable endothelium-dependent responses to acetylcholine, endothelial cells from vim -/- mice release a cyclooxygenase product that compensates the augmented contribution of nitric oxide.

Twisting integrin receptors increases endothelin-1 gene expression in endothelial cells

A magnetic twisting stimulator was developed based on the previously published technique of magnetic twisting cytometry. Using ligand-coated ferromagnetic microbeads, this device can apply mechanical stresses with varying amplitudes, duration, frequencies, and waveforms to specific cell surface receptors. Biochemical and biological responses of the cells to the mechanical stimulation can be assayed. Twisting integrin receptors with RGD (Arg-Gly-Asp)-containing peptide-coated beads increased endothelin-1 (ET-1) gene expression by >100%. In contrast, twisting scavenger receptors with acetylated low-density lipoprotein-coated beads or twisting HLA antigen with anti-HLA antibody-coated beads did not lead to alterations in ET-1 gene expression. In situ hybridization showed that the increase in ET-1 mRNA was localized in the cells that were stressed with the RGD-coated beads. Blocking stretch-activated ion channels with gadolinium, chelating Ca2+ with EGTA, or inhibiting tyrosine phosphorylation with genistein abolished twist-induced ET-1 mRNA elevation. Abolishing cytoskeletal tension with an inhibitor of the myosin ATPase, with an inhibitor of myosin light chain kinase, or with an actin microfilament disrupter blocked twisted-induced increases in ET-1 expression. Our results are consistent with the hypothesis that the molecular structural linkage of integrin-cytoskeleton is an important pathway for stress-induced ET-1 gene expression.

Endothelial microtubule disruption blocks flow-dependent dilation of arterioles

The cytoskeleton is believed to have an important role in the structural and functional integrity of endothelial cells. The role of the endothelial cytoskeleton, specifically microtubules, in the mediation of flow-induced dilation of arterioles has not yet been studied. Thus the aim of our study was to investigate the role of microtubules in the endothelial mechanotransduction of flow-induced dilation of isolated gracilis arterioles of the rat. The active diameter of arterioles at a constant perfusion pressure (80 mmHg) was approximately 63 microm, whereas their passive diameter (Ca(2+)-free solution) was approximately 119 microm. At a constant pressure, increases in flow of the perfusate solution (from 0 to 10 and from 10 to 20 microl/min) elicited increases in diameter up to approximately 95 microm (approximately a 53% increase). Intraluminal administration of nocodazole at concentrations of 5 x 10(-9) and 5 x 10(-8) M had no discernible effects on the structure of endothelial microtubules or on flow-induced dilation, whereas it disassembled microtubules and eliminated flow-induced dilation at a concentration of 5 x 10(-7) M. At this higher concentration, however, the basal diameter and dilations to acetylcholine (10(-8) M), sodium nitroprusside (10(-7) M), arachidonic acid (5 x 10(-6) M), and prostaglandin E2 (10(-8) M) were unaffected. Colchicine (5 x 10(-7) M) also disassembled microtubules and eliminated flow-induced dilation. We concluded that, in isolated arterioles, the integrity of the endothelial cytoskeleton is essential for the transduction of the shear stress signal that results in the release of endothelial factors evoking dilation.

Endothelial gene regulation by laminar shear stress

Endothelial cells, because of their unique localization, are constantly exposed to fluid mechanical forces derived by the flowing blood. These forces, and more specifically shear stresses; affect endothelial structure and function, both in vivo and in vitro, and are implicated as contributing factors in the development of cardiovascular diseases. We have demonstrated earlier that the shear stress selectively induces the transcription of several endothelial genes, and have defined a shear stress response element (SSRE) in the promoter of platelet-derived-growth-factor B (PDGF-B), that is shared by additional endothelial shear stress responsive genes. Here we further characterize this SSRE and the nuclear factors that bind to it, and imply the possible role of the endothelium cytoskeleton in transducing shear stress, leading to the expression of PDGF-B/SSRE constructs in transfected endothelial cells exposed to shear stress. We also present, yet a new shear stress response element in the Platelet Derived Growth Factor A promoter, that contains a binding site to the transcription factors egr1/sp1. These results further demonstrate the complexity of gene regulation by hemodynamic forces, and support the important part that these forces have in the physiology and pathophysiology of the vessel wall.

Flow (shear stress)-induced endothelium-dependent dilation is altered in mice lacking the gene encoding for dystrophin

BACKGROUND

Dystrophin has a key role in striated muscle mechanotransduction of physical forces. Although cytoskeletal elements play a major role in the mechanotransduction of pressure and flow in vascular cells, the role of dystrophin in vascular function has not yet been investigated. Thus, we studied endothelial and muscular responses of arteries isolated from mice lacking dystrophin (mdx mice).

METHODS AND RESULTS

Carotid and mesenteric resistance arteries 120 micrometer in diameter were isolated and mounted in vitro in an arteriograph to control intraluminal pressure and flow. Blood pressure was not affected by the absence of dystrophin. Pressure-induced (myogenic), phenylephrine-induced, and KCl-induced forms of tone were unchanged. Flow (shear stress)-induced dilation in arteries isolated from mdx mice was decreased by 50% to 60%, whereas dilation to acetylcholine or sodium nitroprusside was unaffected. NG-nitro-L-arginine methyl ester-sensitive flow dilation was also decreased in arteries from mdx mice. Thus, the absence of dystrophin was associated with a defect in signal transduction of shear stress. Dystrophin was present in vascular endothelial and smooth muscle cells, as shown by immunolocalization, and localized at the level of the plasma membrane, as seen by confocal microscopy of perfused isolated arteries.

CONCLUSIONS

-This is the first functional study of arteries lacking the gene for dystrophin. Vascular reactivity was normal, with the exception of flow-induced dilation. Thus, dystrophin could play a specific role in shear-stress mechanotransduction in arterial endothelial cells. Organ damage in such diseases as Duchenne dystrophy might be aggravated by such a defective arterial response to flow.

Regulation of the endothelin system by shear stress in human endothelial cells

In this study, the effect of shear stress on the expression of genes of the human endothelin-1 system was examined. Primary cultures of human umbilical vein endothelial cells (HUVEC) were exposed to laminar shear stress of 1, 15 or 30 dyn cm-2 (i.e. 0.1, 1.5 or 3 N m-2) (venous and two different arterial levels of shear stress) in a cone-and-plate viscometer. Laminar shear stress transiently upregulates preproendothelin-1 (ppET-1) mRNA, reaching its maximum after 30 min (approx 1.7-fold increase). In contrast, long-term application of shear stress (24 h) causes downregulation of ppET-1 mRNA in a dose-dependent manner. Arterial levels of shear stress result in downregulation of endothelin-converting enzyme-1 isoform ECE-1a (predominating in HUVEC) to 36.2 +/- 8.5 %, and isoform ECE-1b mRNA to 72.3 +/- 1.9 % of static control level. The endothelin-1 (ET-1) release is downregulated by laminar shear stress in a dose-dependent manner. This downregulation of ppET-1 mRNA and ET-1 release is not affected by inhibition of protein kinase C (PKC), or tyrosine kinase. Inhibition of endothelial NO synthase (L-NAME, 500 microm) prevents downregulation of ppET-1 mRNA by shear stress. In contrast, increasing degrees of long-term shear stress upregulate endothelin receptor type B (ETB) mRNA by a NO- and PKC-, but not tyrosine kinase-dependent mechanism. In conclusion, our data suggest the downregulation of human endothelin synthesis, and an upregulation of the ETB receptor by long-term arterial laminar shear stress. These effects might contribute to the vasoprotective and anti-arteriosclerotic potential of arterial laminar shear stress.

Tissue angiotensin II and endothelin-1 modulate differently the response to flow in mesenteric resistance arteries of normotensive and spontaneously hypertensive rats

In resistance arteries pressure-induced (myogenic) tone (MT) and flow (shear stress)-induced dilation (FD) are potent determinant of vascular resistance. We investigated the role of angiotensin II and endothelin-1 in FD and MT in resistance arteries and their potential change in hypertension. Flow - diameter - pressure relationship was established in situ, under anaesthesia, in two daughter branches of a mesenteric resistance artery (180 microM, n=7 per group) from spontaneously hypertensive (SHR) or normotensive (WKY) rats. One artery was ligated distally, so that it was submitted to pressure only, while the other was submitted to pressure and flow. Drugs were added to the preparation and external diameter, pressure and flow measured continuously. External diameter (with flow) ranged from 150+/-3 to 191+/-7 microM in WKY (n=28) rats and from 168+/-6 to 186+/-6 microM in SHR (n=28). Flow induced a dilation of the non-ligated arteries which was lower in SHR (13+/-5 - 31+/-4 microM vs WKY: 5+/-5 - 44+/-4 microM). In the ligated artery, the diameter did not significantly change, due to MT. In the vessels submitted to flow angiotensin converting enzyme inhibition (perindopril, 10 micromol L(-1)) increased the diameter in SHR (+11+/-2 microM) significantly more than in WKY (+2+/-1 microM). Angiotensin type 1 receptor (AT(1)R) blockade (losartan, 10 micromol L(-1)) increased the diameter in the vessels with flow in SHR only (+6+/-1 microM). Angiotensin type 2 receptor (AT(2)R) blockade (PD 123319, 1 micromol L(-1)) decreased arterial diameter in WKY only (9+/-2). Endothelin-1 type A receptor (ET(A)R) blockade (LU135252, 0.1 micromol L(-1)) increased the diameter only in SHR in the artery submitted to flow (by 6+/-1 microM). Thus FD was counteracted by a flow-dependent AT(1) and ET(A) receptors-activation in SHR whereas in WKY FD AT(2)-dependent dilation is involved.

Lateral zone of cell-cell adhesion as the major fluid shear stress-related signal transduction site

It has been proposed previously that actin filaments and cell adhesion sites are involved in mechanosignal transduction. In this study, we present certain morphological evidence that supports this hypothesis. The 3D disposition of actin filaments and phosphotyrosine-containing proteins in endothelial cells in situ was analyzed by using confocal microscopy and image reconstruction techniques. Surgical coarctations were made in guinea pig aortas, and the same 3D studies were conducted on such areas 1 week later. Stress fibers (SFs) were present at both basal and apical regions of endothelial cells regardless of coarctation, and several phosphotyrosine-containing proteins were associated with SF ends. Apical SFs had one end attached to the apical cell membrane and the other attached to either the basal membrane or the lateral cell border. Within the coarctation area, the actin filament-containing and vinculin-containing structures became prominent, especially at the apical and the lateral regions. Substantially higher levels of anti-phosphotyrosine and anti-Src staining were detected in the constricted area, particularly at the cell-cell apposition, whereas the anti-focal adhesion kinase, anti-CT10-related kinase, anti-platelet endothelial cell adhesion molecule-l, anti-vinculin, and phalloidin staining intensities increased only slightly after coarctation. We propose that apical SFs directly transmit the mechanical force of flow from the cell apex to the lateral and/or basal SF anchoring sites and that the SF ends associated with signaling molecules are sites of signal transduction. Our results support the idea that the cell apposition area is the major fluid shear stress-dependent mechanosignal transduction site in endothelial cells.

Visualization of human umbilical vein endothelial cells by acoustic microscopy

The morphology and acoustic properties of the human umbilical vein endothelial cells (HUVECs) were evaluated using a scanning acoustic microscope system. HUVECs were cultured for 4 days and exposed to the endotoxin for 4 h. The frequency of the scanning acoustic microscope was variable between 100 and 210 MHz. By changing the measuring frequency, ultrasonic amplitude and phase were measured and the quantitative value of attenuation was calculated. Before and after endotoxin stimuli, HUVECs were observed by scanning acoustic microscopy and the attenuation was measured. The acoustic images were successfully obtained to identify the outer shape of the HUVEC and the location of the nucleus in the cell. The attenuation of the nucleus is higher than that of the cytoplasm. The attenuation of the cytoplasm was increased and became inhomogeneous after endotoxin exposure. This finding would be related to the change of F-actin filaments, which is the main component of the cytoskeleton. Scanning acoustic microscopy is useful for assessing the cellular viscoelastic properties since it can detect both the morphological and acoustic changes without contacting the cellular surface.

Sustained JNK activation induces endothelial apoptosis: studies with colchicine and shear stress

The disruption of microtubules by treating bovine aortic endothelial cells with 10(-7)-10(-5) M colchicine caused apoptosis, as evidenced by DNA laddering and TdT-mediated dUTP nick end labeling fluorescence staining. Colchicine treatment also induced a sustained activation of c-Jun NH(2)-terminal kinase (JNK) that lasted for >/=12 h. The blockade of JNK activity by using the negative interfering mutant JNK(K-R) markedly decreased the apoptosis induced by colchicine. Exposure of bovine aortic endothelial cells to laminar shear stress (12 dyn/cm(2)) caused a transient (<2 h) activation of JNK, and there was no induction of apoptosis. The sustained activation of JNK may play a significant role in the apoptosis induced by colchicine.

Mechanotransduction of rat aortic vascular smooth muscle cells requires RhoA and intact actin filaments

The growth-promoting effect of mechanical stress on vascular smooth muscle cells (VSMCs) has been implicated in the progress of vascular disease in hypertension. Extracellular signal-regulated kinases (ERKs) have been implicated in cellular responses, such as vascular remodeling, induced by mechanical stretch. However, it remains to be determined how mechanical stretch activates ERKs. The cytoskeleton seems the most likely candidate for force transmission into the interior of the cell. Therefore, we examined (1) whether the cytoskeleton involves mechanical stretch-induced signaling, (2) whether Rho is activated by stretch, and (3) whether Rho mediates the stretch-induced signaling in rat cultured VSMCs. Mechanical stretch activated ERKs, with a peak response observed at 20 minutes, followed by a significant increase in DNA synthesis. Treatment with the ERK kinase-1 inhibitor, PD98059, inhibited the stretch-induced increase in DNA synthesis. Cytochalasin D, which selectively disrupts the network of actin filaments, markedly inhibited stretch-induced ERK activation. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to mechanical stretch. Botulinum C3 exoenzyme, which inactivates Rho p21 (known to participate in the reorganization of the actin cytoskeleton), attenuated stretch-induced ERK activation. Inhibition of Rho kinase (p160ROCK) also suppressed stretch-induced ERK activation dose dependently. Our results suggest that mechanotransduction in VSMCs is dependent on intact actin filaments, that Rho is activated by stretch, and that Rho/p160ROCK mediates stretch-induced ERK activation and vascular hyperplasia.

Adhesion-induced intracellular signalling in endothelial cells depends on the nature of the matrix

The adhesion of a human microvascular endothelial cell line to its own matrix was studied in comparison with adhesion of the same cells to fibronectin or thrombospondin-1. These endothelial cells adhered preferentially to their matrix whereas an equal cell number was attached to fibronectin or thrombospondin-1. The adhesion of cells to thrombospondin-1 was mediated by the N-terminal heparin binding domain of thrombospondin-1 as shown by the use of a recombinant fragment, N18. Cells adhering to their matrix displayed a morphology and a cytoskeleton organization very similar to that observed in vivo with an apical immunostaining for actin stress fibers and a fine basal labeling for vinculin. Cells on fibronectin were extensively spread and rapidly assembled stress fibers and focal contacts. Cells adherent to thrombospondin-1 presented large lamellae rich in actin but devoid of vinculin and only few actin fibers were observed. Depending on the substratum used, adhering endothelial cells displayed also different tyrosine phosphorylation patterns on electrophoresis. Our observations indicate that endothelial cells adhering to their matrix present an activation state intermediate between that induced by a "hyperadhesive" protein like fibronectin and that generated by a moderate, indeed anti-adhesive, protein like thrombospondin-1.

Differential roles of endothelin-1 in the development of secondary pulmonary hypertension in patients with left heart failure with or without acute exacerbation

BACKGROUND

The aim of this study was to investigate the contribution of endothelin-1 (ET-1) to the development of secondary pulmonary hypertension (PH) in patients with left heart failure (HF).

METHODS AND RESULTS

The subjects were 40 patients with left HF with (group 1; n = 20) and without (group 2; n = 20) acute exacerbation. Before treatment, the ET-1 level in the pulmonary capillary wedge region was three times greater in patients of group 2 than group 1, although there was no significant difference in mean pulmonary artery pressure (mPAP) or pulmonary vascular resistance index (PVRI) between the two groups. Also, the ET-1 level significantly correlated with mPAP and PVRI for both groups, but with different slopes of the regression lines. After treatment of group 1, the extent of reduction in the ET-1 level significantly correlated with that in mPAP and in PVRI, whereas the ET-1 level itself correlated with mPAP, with the regression lines approximating those of group 2.

CONCLUSIONS

Our findings suggest that ET-1 may have differential roles in the development of secondary PH in patients with left HF with or without acute exacerbation.

Osteoblasts induce osteopontin expression in response to attachment on fibronectin: demonstration of a common role for integrin receptors in the signal transduction processes of cell attachment and mechanical stimulation

Osteopontin is a predominant integrin binding protein of bone and its expression has been shown to be induced by mechanical stimuli within osteoblasts (Toma et al. [1997] J. Bone Miner. Res. 12:1626-1636). The present studies examined if the cell adhesion would mimic the mechano-transduction that stimulated opn mRNA expression and whether integrin receptors were involved in these processes. Osteopontin mRNA expression was induced three- to four-fold, 24 hours after embryonic chicken calvaria osteoblast attachment to fibronectin (FN), however no induction was observed if the cells were plated on tissue culture plastic alone. Osteopontin mRNA induction in response to cell attachment on FN was dependent on new protein synthesis and the activation of a tyrosine protein kinase(s) but unlike mechano-induction was independent of the maintenance of the cell's microfilament structure. Integrin receptor(s) were shown to be involved in mediating the signal transduction processes of both cell attachment and mechanical stimulation since incubation of osteoblasts with the integrin binding peptide RGDS partially blocked the induction of opn expression in response to both stimuli. Interestingly, incubation of the osteoblasts that were adherent on tissue culture plastic alone with the RGDS peptide lead to an induction in opn expression suggesting that integrin occupancy by itself was sufficient to initiate the signal transduction process that induced opn expression. In order to assess the role of integrin occupancy vs. focal adhesion complex formation that accompanies cell attachment, in the signal transduction process that induces opn expression, receptor clustering was stimulated pharmacologically with bombesin or lysophosphatidic acid in osteoblasts attached to tissue culture plastic. Neither compound in the absence of occupancy of the integrin receptors was capable of stimulating opn expression in attached cells, however if the cells were placed in suspension pharmacological mediation of receptor clustering and integrin occupancy were additive in their effect of inducing opn expression. These data demonstrate that induction of opn expression by mechanical stimuli and cell attachment are commonly mediated through integrin receptor(s). However, when cells are attached receptor clustering alone which accompanies focal adhesion formation was incapable of mediating signal transduction suggesting that receptor occupancy was a prerequisite to the signal transduction process that leads to the induction of opn mRNA expression.

Shear stress and the endothelium

Shear stress and the endothelium. Vascular endothelial cells (ECs) in vivo are influenced by two distinct hemodynamic forces: cyclical strain due to vessel wall distention by transmural pressure, and shear stress, the frictional force generated by blood flow. Shear stress acts at the apical cell surface to deform cells in the direction of blood flow; wall distention tends to deform cells in all directions. The shear stress response differs, at least partly, from the cyclical strain response, suggesting that cytoskeletal strain alone cannot explain it. Acute shear stress in vitro elicits rapid cytoskeletal remodeling and activates signaling cascades in ECs, with the consequent acute release of nitric oxide and prostacyclin; activation of transcription factors nuclear factor (NF)kappaB, c-fos, c-jun and SP-1; and transcriptional activation of genes, including ICAM-1, MCP-1, tissue factor, platelet-derived growth factor-B (PDGF-B), transforming growth factor (TGF)-beta1, cyclooxygenase-II, and endothelial nitric oxide synthase (eNOS). This response thus shares similarities with EC responses to inflammatory cytokines. In contrast, ECs adapt to chronic shear stress by structural remodeling and flattening to minimize shear stress. Such cells become very adherent to their substratum and show evidence of differentiation. Increased adhesion following chronic shear stress has been exploited to generate vascular grafts with confluent EC monolayers, retained after implantation in vivo, thus overcoming a major obstacle to endothelialization of vascular prostheses.

The study of the influence of flow on vascular endothelial biology

It is now recognized that the mechanical environment of a cell has an influence on its structure and function. For the vascular endothelial cell that resides at the interface of the flowing blood and the underlying vessel wall, there is mounting evidence of the importance of flow and the associated wall shear stress in the regulation of endothelial biology. Not only is it a sensitive regulator of endothelial structure and function, but different flow environments will influence endothelial cell biology differently. Furthermore, there may be an interaction between the chemical environment of a cell and its mechanical environment. This is illustrated by the inhibition by steady laminar shear stress of the cytokine induction of VCAM-1. Results also are presented in which flow studies have been conducted using a co-culture model of the vessel wall. These experiments provide evidence of a quiescent endothelium; however, much more needs to be done to engineer the cell culture environment to make it more physiologic.

Fluid shear stress transcriptionally induces lectin-like oxidized LDL receptor-1 in vascular endothelial cells

Fluid shear stress has been shown to modulate various endothelial functions, including gene expression. In this study, we examined the effect of fluid shear stress on the expression of lectin-like oxidized LDL receptor-1 (LOX-1), a novel receptor for atherogenic oxidized LDL in cultured bovine aortic endothelial cells (BAECs). Exposure of BAECs to the physiological range of shear stress (1 to 15 dyne/cm2) upregulated LOX-1 protein and mRNA in a time-dependent fashion. LOX-1 mRNA levels peaked at 4 hours, and LOX-1 protein levels peaked at 8 hours. Inhibition of de novo RNA synthesis by actinomycin D totally abolished shear stress-induced LOX-1 mRNA expression. Furthermore, nuclear runoff assay showed that shear stress directly stimulates transcription of the LOX-1 gene. Chelation of intracellular Ca2+ with quin 2-AM completely reduced shear stress-induced LOX-1 mRNA expression; furthermore, the treatment of BAECs with ionomycin upregulated LOX-1 mRNA levels in a dose-dependent manner. Taken together, physiological levels of fluid shear stress can regulate LOX-1 expression by a mechanism dependent on intracellular Ca2+ mobilization. Inducible expression of LOX-1 by fluid mechanics may play a role in localized expression of LOX-1 and atherosclerotic lesion formation in vivo.

Microtubules are involved in early hypertrophic responses of myocardium during pressure overload

Mechanical overloading to cardiac muscle causes fetal contractile protein gene expression and acceleration of protein synthesis. Myocyte microtubules might be involved in these pressure overload-induced hypertrophic responses. We assessed c-fos and fetal contractile protein genes such as beta-myosin heavy chain (MHC) and alpha-skeletal actin using Northern blot analysis and quantified total cardiac protein, DNA, and RNA content in the left ventricular myocardium obtained from four groups of rats: sham-operated rats; sham-operated rats treated with colchicine, which depolymerized microtubules; rats in which acute pressure overload was imposed by abdominal aortic constriction for 3 days (AoC); and AoC rats treated with colchicine (AoC + colchicine). Systolic arterial pressure was elevated to a similar degree in AoC and AoC + colchicine rats. c-fos and beta-MHC mRNA levels were significantly upregulated in AoC rats, which was attenuated by microtubule inhibition. Both RNA content and RNA-to-DNA ratio, the index of the protein synthesis capacity, were increased in AoC rats, which effect was also abolished by colchicine. Furthermore, induction of nonfunctioning microtubules by taxol or deuterium oxide exerted the same inhibitory effects. Thus the hypertrophic responses of the myocardium during pressure overload might depend on the integrity of myocyte microtubules.