Regulation of PDGF-B in endothelial cells exposed to cyclic strain

High pulse pressure impairs cerebral artery endothelial function in young, but not old, mice

One of the hallmarks of vascular aging is increased pulse pressure. This elevated pulse pressure is associated with deleterious effects on cerebral vascular function; however, it is unknown if age modulates the susceptibility to high pulse pressure. To examine the effects of age on the cerebral artery response to pulse pressure, we studied isolated cerebral arteries collected from young (6.1 ± 0.2 mo) and old (26.7 ± 0.5 mo) male C57BL/6 mice. Isolated cerebral arteries were exposed ex vivo to static pressure, low pulse pressure (25 mmHg), and high pulse pressure (50 mmHg). In cerebral arteries from young mice, endothelium-dependent dilation was similar between the static and low pulse pressure conditions. Exposure to high pulse pressure impaired endothelium-dependent dilation in cerebral arteries from young mice, mediated by less nitric oxide bioavailability and greater oxidative stress. Cerebral arteries from old mice had impaired cerebral artery endothelium-dependent dilation at static pressure compared with young cerebral arteries. However, exposure to low or high pulse pressure did not cause any further impairments to endothelium-dependent dilation in old cerebral arteries compared with static pressure. The old cerebral arteries had less distension during exposure to high pulse pressure and greater stiffness compared with young cerebral arteries. These results indicate that acute exposure to high pulse pressure impairs endothelium-dependent dilation in young, but not old, cerebral arteries. The greater stiffness of cerebral arteries from old mice potentially protects against the negative consequences of high pulse pressure.

Endothelial cell signaling and ventilator-induced lung injury: molecular mechanisms, genomic analyses, and therapeutic targets

Mechanical ventilation is a life-saving intervention in critically ill patients with respiratory failure due to acute respiratory distress syndrome (ARDS). Paradoxically, mechanical ventilation also creates excessive mechanical stress that directly augments lung injury, a syndrome known as ventilator-induced lung injury (VILI). The pathobiology of VILI and ARDS shares many inflammatory features including increases in lung vascular permeability due to loss of endothelial cell barrier integrity resulting in alveolar flooding. While there have been advances in the understanding of certain elements of VILI and ARDS pathobiology, such as defining the importance of lung inflammatory leukocyte infiltration and highly induced cytokine expression, a deep understanding of the initiating and regulatory pathways involved in these inflammatory responses remains poorly understood. Prevailing evidence indicates that loss of endothelial barrier function plays a primary role in the development of VILI and ARDS. Thus this review will focus on the latest knowledge related to 1) the key role of the endothelium in the pathogenesis of VILI; 2) the transcription factors that relay the effects of excessive mechanical stress in the endothelium; 3) the mechanical stress-induced posttranslational modifications that influence key signaling pathways involved in VILI responses in the endothelium; 4) the genetic and epigenetic regulation of key target genes in the endothelium that are involved in VILI responses; and 5) the need for novel therapeutic strategies for VILI that can preserve endothelial barrier function.

The Assembly and Application of 'Shear Rings': A Novel Endothelial Model for Orbital, Unidirectional and Periodic Fluid Flow and Shear Stress

Deviations from normal levels and patterns of vascular fluid shear play important roles in vascular physiology and pathophysiology by inducing adaptive as well as pathological changes in endothelial phenotype and gene expression. In particular, maladaptive effects of periodic, unidirectional flow induced shear stress can trigger a variety of effects on several vascular cell types, particularly endothelial cells. While by now endothelial cells from diverse anatomic origins have been cultured, in-depth analyses of their responses to fluid shear have been hampered by the relative complexity of shear models (e.g., parallel plate flow chamber, cone and plate flow model). While these all represent excellent approaches, such models are technically complicated and suffer from drawbacks including relatively lengthy and complex setup time, low surface areas, requirements for pumps and pressurization often requiring sealants and gaskets, creating challenges to both maintenance of sterility and an inability to run multiple experiments. However, if higher throughput models of flow and shear were available, greater progress on vascular endothelial shear responses, particularly periodic shear research at the molecular level, might be more rapidly advanced. Here, we describe the construction and use of shear rings: a novel, simple-to-assemble, and inexpensive tissue culture model with a relatively large surface area that easily allows for a high number of experimental replicates in unidirectional, periodic shear stress studies on endothelial cells.

Strain-induced mechanotransduction through primary cilia, extracellular ATP, purinergic calcium signaling, and ERK1/2 transactivates CITED2 and downregulates MMP-1 and MMP-13 gene expression in chondrocytes

OBJECTIVE

To determine the strain-induced signaling pathways involved in regulating the transactivation of the transcription regulator Cbp/p300 Interacting Transactivator with ED-rich tail 2 (CITED2) and downstream targets in chondrocytes.

METHODS

Primary human chondrocytes or C28/I2 chondrocytic cells were subjected to various strain regimes. C57BL/6 mice were subjected to treadmill running. Loss-of-function was carried out using siRNA or inhibitors specific for targeted molecules. mRNA levels were assayed by RT-qPCR, and proteins by western blotting, immunofluorescence, and/or immunohistochemical staining. CITED2 promoter activity was assayed in chondrocytes using wild-type or mutant constructs.

RESULTS

Cyclic strain at 5%, 1 Hz induced CITED2 expression and suppressed expression of matrix metalloproteinase (MMP)-1 and -13 at the messenger RNA (mRNA) and protein levels in human chondrocytes. Abolishing primary cilia through knockdown of intraflagellar transport protein (IFT88) attenuated CITED2 gene expression and decreased protein levels. Similar effects were observed with inhibitors of extracellular adenosine triphosphate (ATP) or P2 purinergic receptors, or antagonists of Ca(2+) signaling. Knockdown of IFT88 in articular chondrocytes in vivo diminished treadmill induced-CITED2 expression and upregulated MMPs. Knockdown of hypoxia-inducible factor (HIF)1α, specificity protein 1 (Sp1), or deletion of the shear stress response element (SSRE) in the CITED2 promoter limited cyclic strain-induced transactivation of CITED2. However, the strain induced-transactivation of CITED2 was abolished only on knockdown of HIF1α, Sp1, and SSRE or by loss-of-function of IFT88 or extracellular-signal-regulated kinases (ERK)1/2.

CONCLUSIONS

CITED2 transactivation is a critical event in signaling generated by strain and transduced by primary cilia, extracellular ATP, P2 purinergic receptors, and Ca(2+) signaling. Strain-induced CITED2 transactivation requires HIF1α, Sp1, and an intact SSRE and leads to the downregulation of MMPs such as MMP-1 and MMP-13.

Use of nanoscale mechanical stimulation for control and manipulation of cell behaviour

The ability to control cell behaviour, cell fate and simulate reliable tissue models in vitro remains a significant challenge yet is crucial for various applications of high throughput screening e.g. drug discovery. Mechanotransduction (the ability of cells to convert mechanical forces in their environment to biochemical signalling) represents an alternative mechanism to attain this control with such studies developing techniques to reproducibly control the mechanical environment in techniques which have potential to be scaled. In this review, the use of techniques such as finite element modelling and precision interferometric measurement are examined to provide context for a novel technique based on nanoscale vibration, also known as "nanokicking". Studies have shown this stimulus to alter cellular responses in both endothelial and mesenchymal stem cells (MSCs), particularly in increased proliferation rate and induced osteogenesis respectively. Endothelial cell lines were exposed to nanoscale vibration amplitudes across a frequency range of 1-100 Hz, and MSCs primarily at 1 kHz. This technique provides significant potential benefits over existing technologies, as cellular responses can be initiated without the use of expensive engineering techniques and/or chemical induction factors. Due to the reproducible and scalable nature of the apparatus it is conceivable that nanokicking could be used for controlling cell behaviour within a wide array of high throughput procedures in the research environment, within drug discovery, and for clinical/therapeutic applications.

STATEMENT OF SIGNIFICANCE

The results discussed within this article summarise the potential benefits of using nanoscale vibration protocols for controlling cell behaviour. There is a significant need for reliable tissue models within the clinical and pharma industries, and the control of cell behaviour and stem cell differentiation would be highly beneficial. The full potential of this method of controlling cell behaviour has not yet been realised.

Lifelong Cyclic Mechanical Strain Promotes Large Elastic Artery Stiffening: Increased Pulse Pressure and Old Age-Related Organ Failure

The arterial wall is under a huge mechanical constraint imposed by the cardiac cycle that is bound to generate damage with time. Each heartbeat indeed imposes a pulsatile pressure that generates a vascular stretch. Lifetime accumulation of pulsatile stretches will eventually induce fatigue of the elastic large arterial walls, such as aortic and carotid artery walls, promoting their stiffening that will gradually perturb the normal blood flow and local pressure within the organs, and lead to organ failure. The augmented pulse pressure induced by arterial stiffening favours left ventricular hypertrophy because of the repeated extra work against stiff high-pressure arteries, and tissue damage as a result of excessive pulsatile pressure transmitted into the microcirculation, especially in low resistance/high-flow organs such as the brain and kidneys. Vascular aging is therefore characterized by the stiffening of large elastic arteries leading to a gradual increase in pulse pressure with age. In this review we focus on the effect of age-related stiffening of large elastic arteries. We report the clinical evidence linking arterial stiffness and organ failure and discuss the molecular pathways that are activated by the increase of mechanical stress in the wall. We also discuss the possible interventions that could limit arterial stiffening with age, such as regular aerobic exercise training, and some pharmacological approaches.

Mechanotransduction in the endothelium: role of membrane proteins and reactive oxygen species in sensing, transduction, and transmission of the signal with altered blood flow

SIGNIFICANCE

Changes in shear stress associated with alterations in blood flow initiate a signaling cascade that modulates the vascular phenotype. Shear stress is "sensed" by the endothelium via a mechanosensitive complex on the endothelial cell (EC) membrane that has been characterized as a "mechanosome" consisting of caveolae, platelet endothelial cell adhesion molecule (PECAM), vascular endothelial growth factor receptor 2 (VEGFR2), vascular endothelial (VE)-cadherin, and possibly other elements. This shear signal is transduced by cell membrane ion channels and various kinases and results in the activation of NADPH oxidase (type 2) with the production of reactive oxygen species (ROS).

RECENT ADVANCES

The signaling cascade associated with stop of shear, as would occur in vivo with various obstructive pathologies, leads to cell proliferation and eventual revascularization.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Although several elements of mechanosensing such as the sensing event, the transduction, transmission, and reception of the mechanosignal are now reasonably well understood, the links among these discrete steps in the pathway are not clear. Thus, identifying the mechanisms for the interaction of the K(ATP) channel, the kinases, and ROS to drive long-term adaptive responses in ECs is necessary. A critical re-examination of the signaling events associated with complex flow patterns (turbulent, oscillatory) under physiological conditions is also essential for the progress in the field. Since these complex shear patterns may be associated with an atherosclerosis susceptible phenotype, a specific challenge will be the pharmacological modulation of the responses to altered signaling events that occur at specific sites of disturbed or obstructed flow.

Microvascular guidance: a challenge to support the development of vascularised tissue engineering construct

The guidance of endothelial cell organization into a capillary network has been a long-standing challenge in tissue engineering. Some research efforts have been made to develop methods to promote capillary networks inside engineered tissue constructs. Capillary and vascular networks that would mimic blood microvessel function can be used to subsequently facilitate oxygen and nutrient transfer as well as waste removal. Vascularization of engineering tissue construct is one of the most favorable strategies to overpass nutrient and oxygen supply limitation, which is often the major hurdle in developing thick and complex tissue and artificial organ. This paper addresses recent advances and future challenges in developing three-dimensional culture systems to promote tissue construct vascularization allowing mimicking blood microvessel development and function encountered in vivo. Bioreactors systems that have been used to create fully vascularized functional tissue constructs will also be outlined.

Three-dimensional 10% cyclic strain reduces bovine aortic endothelial cell angiogenic sprout length and augments tubulogenesis in tubular fibrin hydrogels

The development of a functional microvasculature is critical to the long-term survival of implanted tissue-engineered constructs. Dynamic culture conditions have been shown to significantly modulate phenotypic characteristics and stimulate proliferation of cells within hydrogel-based tissue engineered blood vessels. Although prior work has described the effects uniaxial or equibiaxial mechanical stimulation has on endothelial cells, no work has outlined effects of three-dimensional mechanical stimulation on endothelial cells within tubular vessel analogues. We demonstrate here that 7 days of 10% cyclic volumetric distension has a deleterious effect on the average length and density of angiogenic sprouts derived from pellets of bovine aortic endothelial cells. Although both groups demonstrated lumen formation, the sprouts grown under dynamic culture conditions typically had wider, less-branching sprout patterns. These results suggest that prolonged mechanical stimulation could represent a cue for angiogenic sprouts to preferentially develop larger lumens over cellular migration and subsequent sprout length.

c-Jun regulates shear- and injury-inducible Egr-1 expression, vein graft stenosis after autologous end-to-side transplantation in rabbits, and intimal hyperplasia in human saphenous veins

Coronary artery bypass graft failure represents an unsolved problem in interventional cardiology and heart surgery. Late occlusion of autologous saphenous vein bypass grafts is a consequence of neointima formation underpinned by smooth muscle cell (SMC) migration and proliferation. Poor long term patency and the lack of pharmacologic agents that prevent graft failure necessitate effective alternative therapies. Our objective here was to evaluate the effect of targeted inhibition of the bZIP transcription factor c-Jun on intimal hyperplasia in human saphenous veins and vein graft stenosis after autologous end-to-side transplantation. DNAzymes targeting c-Jun attenuated intimal hyperplasia in human saphenous vein explants. Adenovirus-forced c-Jun expression stimulated SMC proliferation, proliferating cell nuclear antigen, and MMP-2 expression. c-Jun DNAzymes abrogated Adeno-c-Jun-inducible SMC growth and wound repair and reduced intimal thickening in jugular veins of New Zealand white rabbits 4 weeks after autologous end-to-side transplantation to carotid arteries. Conversely, in a DNAzyme-free setting, Adeno-c-Jun potentiated neointima formation in the veins compared with Adeno-LacZ. Inducible c-Jun expression is ERK1/2- and JNK-dependent but p38-independent. Injury- and shear-inducible c-Jun controls early growth response-1. These data demonstrate that strategies targeting c-Jun may be useful for the prevention of vein graft stenosis. Control of one important shear-responsive transcription factor by another indicates the existence of transcriptional amplification mechanisms that magnify the vascular response to cell injury or stress through inducible transcriptional networks.

Histone deacetylase-1 is enriched at the platelet-derived growth factor-D promoter in response to interleukin-1beta and forms a cytokine-inducible gene-silencing complex with NF-kappab p65 and interferon regulatory factor-1

Understanding the mechanisms governing cytokine control of growth factor expression in smooth muscle cells would provide invaluable insight into the molecular regulation of vascular phenotypes and create future opportunities for therapeutic intervention. Here, we report that the proinflammatory cytokine interleukin (IL)-1beta suppresses platelet-derived growth factor (PDGF)-D promoter activity and mRNA and protein expression in smooth muscle cells. NF-kappaB p65, induced by IL-1beta, interacts with a novel element in the PDGF-D promoter and inhibits PDGF-D transcription. Interferon regulatory factor-1 (IRF-1) is also induced by IL-1beta and binds to a different element upstream in the promoter. Immunoprecipitation and chromatin immunoprecipitation experiments showed that IL-1beta stimulates p65 interaction with IRF-1 and the accumulation of both factors at the PDGF-D promoter. Mutation of the IRF-1 and p65 DNA-binding elements relieved the promoter from IL-1beta-mediated repression. PDGF-D repression by IL-1beta involves histone deacetylation and interaction of HDAC-1 with IRF-1 and p65. HDAC-1 small interfering RNA ablates complex formation with IRF-1 and p65 and abrogates IRF-1 and p65 occupancy of the PDGF-D promoter. Thus, HDAC-1 is enriched at the PDGF-D promoter in cells exposed to IL-1beta and forms a cytokine-inducible gene-silencing complex with p65 and IRF-1.

Injury-induced platelet-derived growth factor receptor-alpha expression mediated by interleukin-1beta (IL-1beta) release and cooperative transactivation by NF-kappaB and ATF-4: IL-1beta facilitates HDAC-1/2 dissociation from promoter

Platelet-derived growth factors are a family of potent mitogens and chemoattractants for fibroblasts and other cells of mesenchymal origin. Platelet-derived growth factor (PDGF) dimeric ligands (composed of A-, B-, C-, and D-chains) exert their biological activity through high affinity interactions with cell surface receptor subunits (alpha and beta). PDGF-receptor-alpha is widely implicated in the pathogenesis of hyperplastic fibrotic disease, yet the molecular mechanisms controlling its expression in response to injury are poorly understood. Here we show that PDGF-R alpha expression is induced in fibroblasts by mechanical injury and interleukin (IL)-1beta, which was abolished by neutralizing IL-1beta antibodies in the culture supernatant or inhibitors of NF-kappaB. Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed the existence of a new NF-kappaB binding site at -531/-521 bp in the PDGF-R alpha promoter. We have recently shown that ATF-4 is also induced by injury (Malabanan, K. P., Kanellakis, P., Bobik, A., and Khachigian, L. M. (2008) Circ. Res. 103, 378-387), and we demonstrate here that ATF-4 binds a novel element -259/-254 and stimulates PDGF-R alpha transcription. ATF-4 and NF-kappaB interact, occupy the PDGF-R alpha promoter, and induce PDGF-R alpha transcription in a cooperative manner. IL-1beta facilitates the dissociation of histone deacetylase (HDAC)-1/2 from the PDGF-R alpha promoter, whereas the HDAC inhibitors suberoylanilide hydroxamic acid and trichostatin A potentiate IL-1beta induction of PDGF-R alpha transcription. These findings, taken together, demonstrate that injury stimulates IL-1beta secretion by fibroblasts, which activates NF-kappaB and ATF-4 and stimulates interaction with the PDGF-R alpha promoter, triggering PDGF-R alpha transcription. Physical and functional interactions between NF-kappaB and ATF-4 have not been reported in any gene. This is also the first report of HDAC regulation of PDGF-R alpha transcription.

Cyclic strain induces mouse embryonic stem cell differentiation into vascular smooth muscle cells by activating PDGF receptor beta

Embryonic stem (ES) cells are exposed to fluid-mechanical forces, such as cyclic strain and shear stress, during the process of embryonic development but much remains to be elucidated concerning the role of fluid-mechanical forces in ES cell differentiation. Here, we show that cyclic strain induces vascular smooth muscle cell (VSMC) differentiation in murine ES cells. Flk-1-positive (Flk-1+) ES cells seeded on flexible silicone membranes were subjected to controlled levels of cyclic strain and examined for changes in cell proliferation and expression of various cell lineage markers. When exposed to cyclic strain (4-12% strain, 1 Hz, 24 h), the Flk-1+ ES cells significantly increased in cell number and became oriented perpendicular to the direction of strain. There were dose-dependent increases in the VSMC markers smooth muscle alpha-actin and smooth muscle-myosin heavy chain at both the protein and gene expression level in response to cyclic strain, whereas expression of the vascular endothelial cell marker Flk-1 decreased, and there were no changes in the other endothelial cell markers (Flt-1, VE-cadherin, and platelet endothelial cell adhesion molecule 1), the blood cell marker CD3, or the epithelial marker keratin. The PDGF receptor beta (PDGFR beta) kinase inhibitor AG-1296 completely blocked the cyclic strain-induced increase in cell number and VSMC marker expression. Cyclic strain immediately caused phosphorylation of PDGFR beta in a dose-dependent manner, but neutralizing antibody against PDGF-BB did not block the PDGFR beta phosphorylation. These results suggest that cyclic strain activates PDGFR beta in a ligand-independent manner and that the activation plays a critical role in VSMC differentiation from Flk-1+ ES cells.

Fibroblast growth factor-2 expression is altered in lambs with increased pulmonary blood flow and pulmonary hypertension

A lamb model of pulmonary hypertension, developed by inserting an aortopulmonary vascular graft (shunt), displays vascular remodeling and increased pulmonary blood flow characteristic of children with congenital heart disease. The purpose of this study was to determine whether expression of fibroblast growth factor-2 (FGF-2), a smooth muscle cell mitogen, is altered in shunt lambs. FGF-2 mRNA and protein levels were increased in lung tissue extracts from shunt lambs at 4 wk of age relative to age-matched controls (p < 0.05). FGF-2 protein levels were also increased in the pulmonary arteries and serum of shunt lambs (p < 0.05). Pulmonary arterial smooth muscle cells (PASMC) and endothelial cells (PAEC) were isolated from 4 wk-old lambs and subjected to cyclic stretch and laminar shear stress to mimic increased pulmonary blood flow. Stretch and shear increased FGF-2 promoter activity, and intracellular and extracellular FGF-2 protein levels in both cell types (p < 0.05). Exogenous FGF-2 stimulated PASMC proliferation at levels detected in the extracellular medium of sheared cells (p < 0.05). Elevated FGF-2 signaling by PASMC and PAEC exposed to increased pulmonary blood flow may play a role in the pulmonary vascular remodeling associated with the shunt model of pulmonary hypertension secondary to congenital heart disease.

Phosphatase PTEN is inactivated in bovine aortic endothelial cells exposed to cyclic strain

Hemodynamic forces, including cyclic strain (CS) and shear stress (SS), have been recognized as important modulators of vascular cell morphology and function. PTEN (also known as MMAC1/TEP1) is a lipid phosphatase that leads to a decrease in intracellular phosphatidylinositol 3,4,5 trisphosphate (PIP3) and therefore can modulate the stimulating effect of phosphatidylinositol 3-kinase (PI3K). In this study, we focused on the upstream regulators of the PI3K-Akt pathway by assessing Akt, PTEN, casein kinase 2 (CK2) (a kinase that catalyzes phosphorylation of PTEN), and PI3K activity in endothelial cells (EC) exposed to CS. The activity of phospho-PTEN (n = 4) and phospho-CK2 (n = 4) increased in a time-dependent fashion, reaching maximal activity by 10 min of CS stimulation. The peak of phospho-Akt activity (n = 4) occurred later, at 60 min. Akt activity was altered by transfection of EC with dominant negative PTEN plasmids. Furthermore, CS increased PIP3 immunoreactivity in a time-dependent manner, reaching maximal activity after 60 min of CS stimulation, and these effects were affected by transfection of EC with dominant negative PTEN plasmids. Inhibition of PTEN activity had no effect on CS-mediated cell proliferation but inhibited CS-mediated suppression of apoptosis.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.

Shear stress-stimulated endothelial cells induce smooth muscle cell chemotaxis via platelet-derived growth factor-BB and interleukin-1alpha

OBJECTIVE

Vascular smooth muscle cell (SMC) migration is critical to the development of atherosclerosis and neointimal hyperplasia. Hemodynamic forces such as shear stress and cyclic strain stimulate endothelial cell signal-transduction pathways, resulting in the secretion of several factors, including SMC chemoattractants such as platelet-derived growth factor (PDGF). We hypothesized that mechanical forces stimulate endothelial cells to secrete SMC chemoattractants to induce migration via the mitogen-activated protein kinase (MAPK) pathway.

METHODS

Bovine aortic endothelial cells were exposed to shear stress, cyclic strain, or static conditions for 16 hours. The resulting conditioned medium was used as a SMC chemoattractant in a Boyden chamber. Activation of SMC extracellular signal-regulated protein kinase 1/2 (ERK1/2) was assessed by Western blot analysis. Pathways were inhibited with anti-PDGF-BB or anti-interleukin-1alpha (IL-1alpha) antibodies, or the ERK1/2 upstream pathway inhibitor PD98059.

RESULTS

Conditioned medium from endothelial cells exposed to shear stress corresponding to arterial levels of shear stress stimulated SMC migration but lower levels of shear stress or cyclic strain did not. Both PDGF-BB and IL-1alpha were secreted into the conditioned medium by endothelial cells stimulated with shear stress. Both PDGF-BB and IL-1alpha stimulated SMC chemotaxis but were not synergistic, and both stimulated SMC ERK1/2 phosphorylation. Inhibition of PDGF-BB or IL-1alpha inhibited SMC chemotaxis and ERK1/2 phosphorylation.

CONCLUSION

Shear stress stimulates endothelial cells to secrete several SMC chemoattractants, including PDGF-BB and IL-1alpha; both PDGF-BB and IL-1alpha stimulate SMC chemotaxis via the ERK1/2 signal-transduction pathway. These results suggest that the response to vascular injury may have a common pathway amenable to pharmacologic manipulation.

CLINICAL RELEVANCE

One difficulty in the pharmacologic treatment of atherosclerosis or neointimal hyperplasia leading to restenosis is the multiplicity of activated pathways and thus potential treatment targets. This study demonstrates that shear stress, a hemodynamic force that may be a biologically relevant stimulus to induce vascular pathology, stimulates endothelial cells to secrete PDGF-BB and IL-1alpha. Both of these mediators stimulate the SMC ERK1/2 pathway to induce migration, a critical event in the pathogenesis of atherosclerosis and neointimal hyperplasia. Therefore, this study suggests a relevant common target pathway in SMC that is amenable to manipulation for clinical treatment.

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

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

Gene expression profiles of livers from thermally injured rats

The liver plays an important role in a severe thermal injury by modulating immune function, inflammatory processes and the acute phase response, which are an orchestrated attempt to restore homeostasis. Using high-density oligonucleotide arrays, we examined the gene expression profile in the livers of rats between 2 and 240 h after a 40% total body surface area (TBSA) burn. Alterations in gene expression unique to a thermal injury were identified. Approximately 39 genes out of 8700 genes on each array across all the time points showed a significant change in expression patterns. Real time reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analyses verified significant changes in early growth response-1 (Egr-1) messenger RNA (mRNA) and protein levels corresponding to the array data. Significant increases in serum levels of alpha-2-macroglobulin that correspond to changes in its mRNA levels were observed at 6 and 24 h after burn, p<0.05. The genomic pattern for liver in the hypermetabolic phase after the burn injury involves transcription factors, stress and inflammatory responses, cytoskeletal and extracellular matrix modifications, and regulation of cell proliferation and differentiation. During the initial phase of thermal injury gene expression profiles in the liver may provide some insight into how cellular protection mechanisms and systemic hypermetabolism are initiated and controlled. The genome wide changes observed may provide a rational therapeutic strategy to improve burn care.

Novel roles of neuropeptide processing enzymes: EC3.4.24.15 in the neurome

Neuropeptide processing metalloenzymes, such as angiotensin converting enzyme, neprilysin, endothelin converting enzyme, neurolysin, and EC3.4.24.15 (EP24.15), are central to the formation and degradation of bioactive peptides. We present EP24.15 as a paradigm for novel functions ascribed to these enzymes in the neurome. Although the neurome typically encompasses proteomes of the brain and central nervous system, exciting new roles of these neuropeptidases have been demonstrated in other organ systems. We discuss the involvement of EP24.15 with clinical sequelae involving the use of gonadotropin-releasing hormone (GnRH; LHRH) analogs that act as enzyme inhibitors, in vascular physiology (blood pressure regulation), and in the hematologic system (immune surveillance). Hemodynamic forces, such as cyclic strain and shear stress, on vascular cells, induce an increase in EP24.15 transcription, suggesting that neuropeptidase-mediated hydrolysis of pressor/depressor peptides is likely regulated by changes in hemodynamic force and blood pressure. Lastly, EP24.15 regulates surface expression of major histocompatibility complex Class I proteins in vivo, suggesting that EP24.15 may play an important role in maintenance of immune privilege in sites of increased endogenous expression. In these extraneural systems, regulation of both neuropeptide and other peptide substrates by neuropeptidases indicates that the influence of these enzymes may be more global than was anticipated previously, and suggests that their attributed role as neuropeptidases underestimates their physiologic actions in the neural system.

Articular cartilage functional histomorphology and mechanobiology: a research perspective

The histomorphogenesis of articular cartilage is regulated during skeletal development by the intermittent forces and motions imposed at diarthrodial joints. A key feature in this development is the formation of the superficial, transitional, radial, and calcified cartilage zones through the cartilage thickness. The histomorphological, biological, and mechanical characteristics of these zones can be correlated with the distributions of pressures, deformations, and pressure-induced fluid flow that are created in vivo. In a mature joint, cyclic loads produce cyclic hydrostatic fluid pressure through the entire cartilage thickness that is comparable in magnitude to the applied joint pressure. Prolonged physical activity can cause the total cartilage thickness to decrease about 5%, although the consolidation strains vary tremendously in the different zones. The superficial zone can experience significant fluid exudation and consolidation (compressive strains) in the range of 60% while the radial zone experiences relatively little fluid flow and consolidation. The topological variation in the histomorphologic appearance of articular cartilage is influenced by the local mechanical loading of chondrocytes in the different zones. Patterns of stress, strain, and fluid flow created in the joint result in spatial and temporal changes in the rates of synthesis and degradation of matrix proteins. When viewed over the course of a lifetime, even subtle difference in these cellular processes can affect the micro- and macro-morphology of articular cartilage. This hypothesis is supported by in vivo and ex vivo experiments where load-induced changes in matrix synthesis and catabolism, gene expression, and signal transduction pathways have been observed.

Unique genes induced by mechanical stress in periodontal ligament cells

OBJECTIVES

The aim of this study is to isolate mechanical stress-induced genes (MSGens) from human periodontal ligament (PDL) cells and to analyze profiles of the mRNA expression of these genes.

BACKGROUND

Differential expression of genes in PDL cells under physiological stress such as occlusal force is thought to be orchestrated not only for the remodeling of PDL itself but also for the repair and regeneration of periodontal tissues. However, little is known about the genes expressed in PDL cells under mechanical stress.

METHODS

The cDNA from mechanical stress-applied human PDL cells was subtracted against the cDNA from static control cells. The subtracted cDNA was amplified by polymerase chain reaction (PCR) and cloned for further analysis.

RESULTS

Among 68 independent clones isolated, 15 contained DNA fragments greater than 250 bp. Reverse Northern analysis revealed a marked induction of MSGen-15 and MSGen-28 mRNA expression in the mechanical stress-applied cells. However, little difference in the magnitude of expression for the other MSGens was detected between the stress-applied cells and the control cells. After nucleotide sequencing and the analysis of homology with known genes, five clones were identified; ribosomal protein S27 (MSGen-9), MRG 15 (MSGen-15), androgen-binding protein (MSGen-18), cathepsin H (MSGen-28), and cytochrome c (MSGen-47). Interestingly, it has been reported that MRG 15 is a novel transcription factor involved in the regulation of cell growth and senescence. The remaining 10 clones, classified into six sequence types, had no significant homology with any known genes.

CONCLUSIONS

These results suggest that many known and unknown genes are expressed in response to mechanical stress in PDL cells, and that a transcription factor, MRG 15, may be responsible for molecular events in PDL cells under mechanical stress.

Role of PP2A in the regulation of p38 MAPK activation in bovine aortic endothelial cells exposed to cyclic strain

We have previously reported that cyclic strain results in rapid phosphorylation of p38 mitogen activated protein kinase (MAPKs). The aim of this study was to examine the role of protein phosphatase type 2A (PP2A) in regulating p38 MAPK activation in bovine aortic endothelial cells exposed to cyclic strain. In this study, we demonstrate that the catalytic subunit of PP2A is tyrosine phosphorylated by cyclic strain, resulting in inhibition of phosphatase activity. Okadaic acid, an inhibitor of PP2A at lower concentrations increased phosphorylation of p-38. Phospho-p38 MAPK physically associated with the catalytic subunit, PP2Ac. Phospho-p38 MAPK was dephosphorylated by purified PP2Ac in cell lysates, but if pretreated with okadaic acid, phospho-p38 MAPK was maintained. Taken together, our result suggests that PP2A plays a regulatory role in p38 MAPK activation in endothelial cells exposed to cyclic strain.

Cyclic strain stimulates early growth response gene product 1-mediated expression of membrane type 1 matrix metalloproteinase in endothelium

SUMMARY

Matrix metalloproteinases (MMPs) are hypothesized to be involved in the processes of endothelial cell (EC) migration and matrix remodeling during angiogenesis. Although hemodynamic forces (such as blood pressure, wall tension, and shear stress) are considered to be strong stimuli for angiogenesis, the role of hemodynamic forces on the regulation of MMPs including membrane type 1 matrix metalloproteinase (MT1-MMP) has not been fully elucidated. To study this, rat microvascular EC were exposed to 60 cycles/minute of 24% maximum strain for up to 24 hours. MT1-MMP mRNA and protein increased in a time-dependent manner through 24 hours of exposure to cyclic strain. Cyclic strain induced early growth response gene product (Egr-1) mRNA and protein within 1 hour. A specific nucleoprotein complex was formed when an oligonucleotide containing binding sites for Sp1 and Egr-1 was incubated with nuclear extracts from EC exposed to 1 hour of cyclic strain. Antibodies to Egr-1 completely supershifted this complex. Increased binding of Egr-1 by cyclic strain to the MT1-MMP promoter correlated with enhanced transcriptional activity. These results suggest that cyclic strain up-regulates the Egr-1-mediated expression of MT1-MMP in rat microvascular EC, emphasizing the importance of hemodynamic forces in the regulation of MT1-MMP in vivo.

Endothelial responses to mechanical stress: where is the mechanosensor?

OBJECTIVE

The endothelium is normally subjected to mechanical deformation resulting from shear stress and from strain associated with stretch of the vessel wall. These stimuli are detected by a mechanosensor that initiates a variety of signaling systems responsible for triggering the functional responses. The identity of the mechanosensor has not been established. This article discusses the different mechanisms of mechanosensing that have been proposed and reviews the literature with respect to signaling systems that are activated in response to stress and strain in endothelium.

DATA SOURCES

Published literature related to mechanotransduction, signal transduction pathways initiated by strain in endothelium, and pathophysiologic effects of abnormal shear forces in diseases.

DATA EXTRACTION AND SYNTHESIS

Proposed mechanisms of mechanosensing include stretch-sensitive ion channels, protein kinases associated with the cytoskeleton, integrin-cytoskeletal interactions, cytoskeletal-nuclear interactions, and oxidase systems capable of generating reactive oxygen species. However, the molecular identity of the mechanosensor is not known, nor is it clear whether multiple sensing mechanisms exist.

CONCLUSIONS

Many responses are initiated in cells subjected to mechanical deformation, including alterations in ion channel conductance, activation of signal transduction pathways, and altered expression of specific genes. Future progress in this field will require a critical distinction between cell systems that become activated during mechanical strain and the identity of the cellular mechanosensor that triggers subsequent responses.

The mechanical behavior of vascular grafts: a review

The development of intimal hyperplasia (IH) near the anastomosis of a vascular graft to artery is directly related to changes in the wall shear rate distribution. Mismatch in compliance and diameter at the end-to-end anastomosis of a compliant artery and rigid graft cause shear rate disturbances that may induce intimal hyperplasia and ultimately graft failure. The principal strategy being developed to prevent IH is based on the design and fabrication of compliant synthetic or innovative tissue-engineered grafts with viscoelastic properties that mirror those of the human artery. The goal of this review is to discuss how mechanical properties including compliance mismatch, diameter mismatch, Young's modulus and impedance phase angle affect graft failure due to intimal hyperplasia.

Role of mitogen-activated protein kinases in pulmonary endothelial cells exposed to cyclic strain

The aim of this study was to examine the role of mitogen-activated protein kinases (MAPKs) activation in bovine pulmonary arterial endothelial cells (EC) exposed to cyclic strain. EC were subjected to 10% average strain at 60 cycles/min. Cyclic strain induced activation of extracellular signal-regulated kinase (ERK; 1.5-fold), c-Jun NH(2)-terminal protein kinase (JNK; 1.9-fold), and p38 (1. 5-fold) with a peak at 30 min. To investigate the functional role of the activated MAPKs, we analyzed cells after treatment with PD-98059, a specific ERK kinase inhibitor, or SB-203580, a catalytic inhibitor for p38, and after transient transfection with JNK(K-R), and MEKK(K-M) the respective catalytically inactive mutants of JNK1 and MAPK kinase kinase-1. Cyclic strain increased activator protein-1 (AP-1) binding activity, which was blocked by PD-98059 and SB-203580. Activity of AP-1-dependent luciferase reporter driven by 12-O-tetradecanoyl-phorbol-13-acetate-responsive element (TRE) was induced by cyclic strain, and this was attenuated by PD-98059, MEKK(K-M), JNK(K-R), and SB-203580. PD-98059 and SB-203850 did not inhibit cell alignment and migration induced by cyclic strain. MEKK(K-M) and JNK(K-R) transfection did not block cyclic strain-induced cell alignment. In conclusion, cyclic strain activates ERK, JNK, and p38, and their activation plays a role in transcriptional activation of AP-1/TRE but not in cell alignment and migration changes in bovine pulmonary arterial EC.

Cyclic strain induces proliferation of cultured embryonic heart cells

Embryonic heart cells undergo cyclic strain as the developing heart circulates blood to the embryo. Cyclic strain may have an important regulatory role in formation of the adult structure. This study examines the feasibility of a computerized cell-stretching device for applying strain to embryonic cardiocytes to allow measurement of the cellular response. A primary coculture of myocytes and a secondary culture of nonmyocytes from stage-31 (7 d) embryonic chick hearts were grown on collagen-coated membranes that were subsequently strained at 2 Hz to 20% maximal radial strain. After 24 h, total cell number increased by 37+/-6% in myocyte cocultures and by 26+/-6% in nonmyocyte cultures over unstrained controls. Lactate dehydrogenase and apoptosis assays showed no significant differences in cell viabilities between strained and unstrained cells. After 2 h strain, bromodeoxyuridine incorporation was 38+/-1.2% versus 19+/-0.2% (P < 0.01) in strained versus unstrained myocyte cocultures, and 35+/-2.1% versus 16+/-0.2% (P = 0.01) in nonmyocyte cultures. MF20 antibody labeling and periodic acid-Schiff (PAS) staining estimated the number of myocytes in strained wells as 50-67% larger than in control wells. Tyrosine phosphorylation may play a role in the cellular response to strain, as Western blot analysis showed an increase in tyrosine phosphorylation of two proteins with approximate molecular weights of 63 and 150 kDa within 2 min of strain. The results of this study indicate that embryonic chick cardiocytes can be cultured in an active mechanical environment without significant detachment and damage and that increased proliferation may be a primary response to strain.

Vasculogenesis and angiogenesis as mechanisms of vascular network formation, growth and remodeling

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or inter-vascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, vascular endothelial growth factor (VEGF) and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

The response of adult human saphenous vein endothelial cells to combined pressurized pulsatile flow and cyclic strain, in vitro

Adult human saphenous vein endothelial cells (HVEC) were cultured in a compliant tubular device and evaluated by Northern hybridization for the effects of combined pressurized pulsatile flow and cyclic strain on the expression of mRNAs for endothelin-1 (ET-1), endothelial cell nitric oxide synthase (ecNOS), tissue plasminogen activator (tPA), and plasminogen activator inhibitor type 1 (PAI-1). The hemodynamic environment was designed to mimic shear stress conditions at the distal anastomosis of a saphenous vein graft, a common site of intimal proliferation. Steady-state mRNA levels in experimental tubes were expressed relative to that in controls. No changes were observed in ET-1 mRNA after 1 and 24 hr, but a 50% decrease in experimental cultures was observed after 48 hr in the vascular simulating device. Similar results were obtained for ecNOS mRNA, although a subgroup (4 of 11) showed a significant decrease (>50%) by 24 hr. For tPA mRNA, no change was observed after 1 hr, but a significant decrease (>60%) was measured after 24 hr and no message was detectable after 48 hr. Steady-state levels for PAI-1 mRNA remained unchanged through 48 hr of treatment. These results show that pressure, pulsatile flow, and cyclic strain, when applied in concert, differentially alter vasoactive and fibrinolytic functions in HVEC. Moreover, the dramatic decrease in steady-state levels of tPA mRNA is consistent with a shift toward an increased thrombotic state.

Effects of increased ambient pressure on colon cancer cell adhesion

Forces such as strain modulate intestinal epithelial biology. Shear and pressure influence other cells. The effects of pressure on human colon cancer cells are poorly understood. Increasing ambient pressure for 30 min by 15 mm Hg over atmospheric stimulated adhesion to matrix proteins of four human colon cancer cell lines and primary cells from three human colon cancers, but not bovine aortic smooth-muscle cells. This effect was energy dependent and cation dependent (blocked by azide and chelation), accompanied by tyrosine phosphorylation of intracellular proteins including focal adhesion kinase, and blocked by tyrosine kinase inhibition (genistein, tyrphostin, and erbstatin) and a functional antibody to the beta1 integrin subunit. Although pressure stimulated adhesion even in a balanced salt solution, baseline and pressure-stimulated adhesion were each substantially diminished in the absence of serum. These data suggest that relatively low levels of increased pressure may stimulate malignant colonocyte adhesion by a cation-dependent beta1-integrin-mediated mechanism, perhaps via focal adhesion kinase-related tyrosine phosphorylation. In addition to elucidating another aspect of physical force regulation of colonocyte biology, these findings may be relevant to the effects of increased pressure engendered by colonic peristalsis, surgical manipulation, or laparoscopic surgery on colon cancer cell adhesion.

Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases

The present study was designed to investigate whether apoptosis occurs in early-stage vein grafts and to determine the mechanisms by which mechanical stress contributes to apoptosis in vascular smooth muscle cells (SMCs). Apoptosis in vessel walls of mouse vein grafts was confirmed by morphological changes and by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL). TUNEL(+) cells in vein grafts 1, 4, and 8 wk postoperatively was 13%, 29%, and 21%, respectively, and apoptosis occurred mainly in veins grafted to arteries, remaining unchanged in vein-to-vein grafts. When mouse, rat, and human arterial SMCs were cultured on a flexible membrane and subjected to cyclic strain stress, apoptosis was observed in a time- and strength-dependent manner. All three types of SMCs showed apoptotic death as confirmed by TUNEL, propidium iodide, and annexin V staining. To further study the signal pathways leading to apoptosis, activities of p38, a subfamily of mitogen-activated protein kinases (MAPKs), were determined. Mechanical stress resulted in p38 MAPK activation, reaching high levels within 8 min. SB 202190, a specific inhibitor for p38 MAPKs, prevented SMC apoptosis in response to mechanical stress. SMC lines stably transfected with a dominant negative rac, an upstream signal transducer, or overexpressing MAPK phosphatase-1, a negative regulator for MAPKs, completely inhibited mechanical stress stimulated p38 activation and abolished mechanical stress-induced apoptosis. Thus, we provide solid evidence that one of the earliest events in venous bypass grafts is apoptosis, in which mechanical stress-induced p38-MAPK activation is responsible for transducing signals leading to apoptosis.-Mayr, M., Li, C., Zou, Y., Huemer, U., Hu, Y., Xu, Q. Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases.

New DNA enzyme targeting Egr-1 mRNA inhibits vascular smooth muscle proliferation and regrowth after injury

Early growth response factor-1 (Egr-1) binds to the promoters of many genes whose products influence cell movement and replication in the artery wall. Here we targeted Egr-1 using a new class of DNA-based enzyme that specifically cleaved Egr-1 mRNA, blocked induction of Egr-1 protein, and inhibited cell proliferation and wound repair in culture. The DNA enzyme also inhibited Egr-1 induction and neointima formation after balloon injury to the rat carotid artery wall. These findings demonstrate the utility of DNA enzymes as biological tools to delineate the specific functions of a given gene, and implicate catalytic nucleic acid molecules composed entirely of DNA as potential therapeutic agents.

Tissue factor activity is increased in human endothelial cells cultured under elevated static pressure

We tested the hypothesis that elevated blood pressure, a known stimulus for vascular remodeling and an independent risk factor for the development of atherosclerotic disease, can modulate basal and cytokine-induced tissue factor (TF; CD 142) expression in cultured human endothelial cells (EC). Using a chromogenic enzymatic assay, we measured basal and tumor necrosis factor-alpha (TNF-alpha; 10 ng/ml, 5 h)-induced TF activities in human aortic EC (HAEC) and vena cava EC (HVCEC) cultured at atmospheric pressure and at 170 mmHg imposed pressure for up to 48 h. Basal TF activities were 22 +/- 10 U/mg protein for HAEC and 14 +/- 9 U/mg protein for HVCEC and were upregulated in both cell types >10-fold by TNF-alpha. Exposure to pressure for 5 h induced additional elevation of basal TF activity by 47 +/- 16% (P < 0.05, n = 6) for HAEC and 17 +/- 5% (P < 0.05, n = 3) for HVCEC. Pressurization also enhanced TF activity in TNF-alpha-treated cells from 240 +/- 28 to 319 +/- 32 U/mg protein in HAEC (P < 0.05, n = 4) and from 148 +/- 25 to 179 +/- 0.8 U/mg protein (P < 0.05, n = 3) in HVCEC. Cytokine stimulation caused an approximately 100-fold increase in steady-state TF mRNA levels in HAEC, whereas pressurization did not alter either TF mRNA or cell surface antigen expression, as determined by quantitative RT-PCR methodology and ELISA. Elevated pressure, however, modulated the EC plasma membrane organization and/or permeability as inferred from the increased cellular uptake of the fluorescent amphipathic dye merocyanine 540 (33 +/- 7%, P < 0.05). Our data suggest that elevated static pressure modulates the hemostatic potential of vascular cells by modifying the molecular organization of the plasma membrane.

PDGF-A expression correlates with blood pressure and remodeling in 1K1C hypertensive rat arteries

We previously demonstrated remodeling of large and small arteries in angiotensin II-treated rats, paralleled by an increased expression of platelet-derived growth factor (PDGF)-A chain mRNA in large arteries. Both remodeling and PDGF-A expression were associated with elevation of blood pressure rather than a direct effect of angiotensin II. To further delineate the role of PDGF-A and elevated blood pressure, we assessed the level of PDGF-A and -B mRNA and protein in the wall of large as well as small arteries in the one-kidney, one-clip (1K1C) hypertensive rat, a non-renin-dependent model of hypertension. Fourteen days after renal artery stenosis, the thoracic aorta and both femoral arteries were collected from 1K1C rats (n = 8) and uninephrectomized controls (n = 8) and immediately processed for morphological measurement, immunohistochemistry, RT-PCR, and Western blotting. Systolic blood pressure was significantly elevated in hypertensive rats (202 +/- 26 mmHg) compared with control rats (122 +/- 7.9 mmHg) and was accompanied by arterial hypertrophy in both aorta and femoral arteries. The mRNA for PDGF-A chain was increased threefold in the thoracic aorta (P < 0.05) of 1K1C rats, whereas the message for PDGF-B was not significantly changed in hypertensive versus control animals. A higher staining of the intima-media was observed by using an anti-PDGF-A chain polyclonal antibody on paraffin-embedded sections. Western blot results indicated an approximately 2-fold increase in PDGF-A protein in aortic and femoral wall of the 1K1C rats. The results showed that both the mRNA and protein for PDGF-A chain are increased and well correlated with the blood pressure and wall area, suggesting a direct effect of elevated pressure on PDGF synthesis, which, in turn, may affect the onset and progression of vascular hypertrophy.

Special communicationthe critical role of mechanical forces in blood vessel development, physiology and pathology

The following extended abstracts were presented at the Research Initiatives in Vascular Disease Conference, Movers and Shakers in the Vascular Tree-Hemodynamic and Biomechanical Factors in Blood Vessel Pathology, sponsored by The Lifeline Foundation and the Cardiovascular & Interventional Radiology Research and Educational Foundation; jointly sponsored by the International Society for Cardiovascular Surgery, North American Chapter, The Society for Vascular Surgery, and The Society of Cardiovascular and Interventional Radiology; in cooperation with the National Institutes of Health-National Heart, Lung &Blood Institute on Mar 11-12, 1999, in Bethesda, Md.