Regulation of genetic expression in shear stress-stimulated endothelial cells

Non-motile primary cilia as fluid shear stress mechanosensors

Primary cilia are sensory organelles that transmit extracellular signals into intracellular biochemical responses. Structural and functional defects in primary cilia are associated with a group of human diseases, known as ciliopathies, with phenotypes ranging from cystic kidney and obesity to blindness and mental retardation. Primary cilia mediate mechano- and chemosensation in many cell types. The mechanosensory function of the primary cilia requires the atypical G-protein-coupled receptor polycystin-1 and the calcium-permeable nonselective cation channel polycystin-2. Mechanical stimulations such as fluid-shear stress of the primary cilia initiate intracellular calcium rise, nitric oxide release, and protein modifications. In this review, we describe a set of protocols for cell culture to promote ciliation, mechanical stimulations of the primary cilia, and measurements of calcium rise and nitric oxide release induced by fluid shear stress.

Fluid Shear Stress Effects on Endothelial Cell Cytosolic pH

Fluid flow can modulate endothelial cell intracellular pH (pH(i)). Venous and arterial shear stresses of 1.4 and 14 dyn/cm2, respectively, induced intracellular acidification. The kinetics of the process and magnitude of acidification were dependent on the level of shear stress. Endothelial cells exposed to a venous shear stress were able to recover from the acidification, whereas cells exposed to an arterial shear stress remained acidic. Addition of SITS (1 mM), a HCO(3) (-)/CI(-) exchange inhibitor, greatly reduced the shear stress induced acidification, suggesting that the HCO(3) (-)/C1(-) exchanger is activated by shear stress. Shear stress may activate the exchanger by lowering the [HCO(3) (-)] at the cell surface via convective mass transfer. Altering the HCO(3) (-) gradient across the cell membrane activates the exchanger and, as a consequence, results in intracellular acidification. Perfusion with media containing ATP (10 microM) altered the kinetics of flow-induced acidification observed at both shear stress levels. ATP modulation of pH(i) may be coupled to the rise in [Ca(2+)](j) known to occur with ATP stimulation. To summarize, media perfusion induces intracellular acidification in endothelial cells, and there is evidence to suggest that pH(i) may serve as a second messenger to modulate flow associated changes in endothelial cell metabolism.

Mechanical force induces type I collagen expression in human periodontal ligament fibroblasts through activation of ERK/JNK and AP-1

Type I collagen (COL I) is the predominant collagen in the extracellular matrix of periodontal ligament (PDL), and its expression in PDL fibroblasts (PLF) is sensitive to mechanical force. However, the mechanism by which PLF induces COL I to respond to mechanical force is unclear. This study examined the nature of human PLF in mediating COL I expression in response to centrifugal force. Signal transduction pathways in the early stages of mechanotransduction involved in the force-driven regulation of COL I expression were also investigated. Centrifugal force up-regulated COL I without cytotoxicity and activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 kinase. ERK and JNK inhibitor blocked the expression of COL I but p38 kinase inhibitor had no effect. Centrifugal force activated activator protein-1 (AP-1) through dimerization between c-Fos and c-Jun transcription factors. ERK and JNK inhibitors also inhibited AP-1-DNA binding, c-Fos nuclear translocation, and c-Jun phosphorylation that were increased in the force-exposed PLF. Further, transfecting the cells with c-Jun antisense oligonucleotides almost completely abolished the force-induced increase of c-Jun phosphorylation and COL I induction. Our findings suggest that mechanical signals are transmitted into the nucleus by ERK/JNK signaling pathways and then stimulate COL I expression through AP-1 activation in force-exposed human PLF.

Stress transmission in the lung: pathways from organ to molecule

Gas exchange, the primary function of the lung, can come about only with the application of physical forces on the macroscale and their transmission to the scale of small airway, small blood vessel, and alveolus, where they serve to distend and stabilize structures that would otherwise collapse. The pathway for force transmission then continues down to the level of cell, nucleus, and molecule; moreover, to lesser or greater degrees most cell types that are resident in the lung have the ability to generate contractile forces. At these smallest scales, physical forces serve to distend the cytoskeleton, drive cytoskeletal remodeling, expose cryptic binding domains, and ultimately modulate reaction rates and gene expression. Importantly, evidence has now accumulated suggesting that multiscale phenomena span these scales and govern integrative lung behavior.

Shear stress contributes to t-PA mRNA expression in human endothelial progenitor cells and nonthrombogenic potential of small diameter artificial vessels

Seeding endothelial progenitor cells (EPCs) onto the surface of vascular grafts has been proved to be a promising strategy to improve nonthrombogenic potentials of small diameter artificial vessels. Here, we investigated whether in vitro shear stress modulates the tissue-type plasminogen activator (t-PA) secretion and mRNA expression in human EPCs and improves patency of the EPC-seeded polyurethane small diameter vascular grafts implanted in the canine carotid artery in vivo. In vitro shear stress, in a dose-dependent manner, increased t-PA secretion and mRNA expression of human EPCs. The in vivo implantation of EPC-seeded vascular grafts remained highly patent in shear stress pretreatment compared with stationary condition. The present findings demonstrate for the first time that in vitro shear stress can enhance t-PA secretion and gene expression in human EPCs, which contributes to improvement in nonthrombogenic potentials of EPC-seeded small diameter artificial vessels with maintenance of in vivo highly patency rate.

Solving the advection-diffusion equations in biological contexts using the cellular Potts model

The cellular Potts model (CPM) is a robust, cell-level methodology for simulation of biological tissues and morphogenesis. Both tissue physiology and morphogenesis depend on diffusion of chemical morphogens in the extra-cellular fluid or matrix (ECM). Standard diffusion solvers applied to the cellular potts model use finite difference methods on the underlying CPM lattice. However, these methods produce a diffusing field tied to the underlying lattice, which is inaccurate in many biological situations in which cell or ECM movement causes advection rapid compared to diffusion. Finite difference schemes suffer numerical instabilities solving the resulting advection-diffusion equations. To circumvent these problems we simulate advection diffusion within the framework of the CPM using off-lattice finite-difference methods. We define a set of generalized fluid particles which detach advection and diffusion from the lattice. Diffusion occurs between neighboring fluid particles by local averaging rules which approximate the Laplacian. Directed spin flips in the CPM handle the advective movement of the fluid particles. A constraint on relative velocities in the fluid explicitly accounts for fluid viscosity. We use the CPM to solve various diffusion examples including multiple instantaneous sources, continuous sources, moving sources, and different boundary geometries and conditions to validate our approximation against analytical and established numerical solutions. We also verify the CPM results for Poiseuille flow and Taylor-Aris dispersion.

Microfluidic sorting of mammalian cells by optical force switching

Microfluidic-based devices have allowed miniaturization and increased parallelism of many common functions in biological assays; however, development of a practical technology for microfluidic-based fluorescence-activated cell sorting has proved challenging. Although a variety of different physical on-chip switch mechanisms have been proposed, none has satisfied simultaneously the requirements of high throughput, purity, and recovery of live, unstressed mammalian cells. Here we show that optical forces can be used for the rapid (2-4 ms), active control of cell routing on a microfluidic chip. Optical switch controls reduce the complexity of the chip and simplify connectivity. Using all-optical switching, we have implemented a fluorescence-activated microfluidic cell sorter and evaluated its performance on live, stably transfected HeLa cells expressing a fused histone-green fluorescent protein. Recovered populations were verified to be both viable and unstressed by evaluation of the transcriptional expression of two genes, HSPA6 and FOS, known indicators of cellular stress.

Hydrodynamic forces applied on intercellular bonds, soluble molecules, and cell-surface receptors

Cells and biomolecules exposed to blood circulation experience hydrodynamic forces that affect their function. We present a methodology to estimate fluid forces and force loading rates applied on cellular aggregates, cell-surface proteins, and soluble molecules. Low Reynolds-number hydrodynamic theory is employed. Selected results are presented for biological cases involving platelets, neutrophils, tumor cells, GpIb-like cell-surface receptors, and plasma von Willebrand factor (vWF)-like soluble proteins. Calculations reveal the following: 1), upon application of constant linear shear, cell aggregates and biomolecules experience time-varying forces due to their tumbling motion. 2), In comparison to neutrophil homotypic aggregates, the maximum force applied on neutrophil-platelet aggregates is approximately threefold lower. Thus, alterations in cell size may dramatically alter adhesion molecule requirement for efficient cell binding. Whereas peak forces on homotypic cell doublets are tensile, shear forces dominate in heterotypic doublets with radius ratio <0.3. 3), The peak forces on platelet GpIb and von Willebrand factor are of comparable magnitude. However, they are orders-of-magnitude lower than those applied on intercellular bonds. Charts are provided to rapidly evaluate the magnitude of hydrodynamic force and rotation time-period occurring in any given experiment. The calculation scheme may find application in studies of vascular biology and receptor biophysics.

Effect of protein kinase inhibitors on the stretch-elicited c-Fos and c-Jun up-regulation in human PDL osteoblast-like cells

Osteoblastic cells transduce signals of mechanical loading that plays a key role in maintaining bone formation. In an attempt to elucidate the biochemical events associated with the conversion of mechanical stress to biological outcome, we examined cultured human periodontal ligament (hPDL) osteoblastic cells exposed to continuous stretch, in terms of cellular parameters correlating known signaling cascades to the initial phase of osteoblast-specific transcriptional control. Time-course experiments revealed that mechanical stretch-loaded hPDL cells exhibit a very rapid and relatively sustained increase in the abundance of the immediate-early gene products, c-Fos and c-Jun, components of the activator protein-1 (AP-1) transcription factor. Moreover, this increase in protein levels was accompanied by hyperphosphorylation and thereby potentiation of c-Jun, the principal modulator of AP-1 activity. Importantly, these inductive effects were partly or completely abolished by pre-incubating the cells with SB 203580, PD 098059, and the novel compound Y-27632, inhibitors of p38 mitogen-activated protein kinase (MAPK), MAPK kinase (MEK), and Rho-associated protein kinase (RhoK), respectively. These results consolidate AP-1 as the pivotal downstream effector in the early response of hPDL cells to continuous mechanical stretching, via the coordinate stimulation of de novo synthesis and post-translational regulation of AP-1 proteins. This "integrating" function of AP-1 is mediated through a mechanotransduction circuit that incorporates elements of well-defined upstream signaling protein kinase systems.

Hemodynamic factors of thrombus formation in coronary aneurysms associated with Kawasaki disease

BACKGROUND

We studied the mechanism of thrombus formation in coronary aneurysms based on rheological findings.

METHODS

We studied 43 coronary aneurysms in 33 patients with Kawasaki disease (mean (+/- SD) age 6.1 +/- 4.3 years). These lesions were divided into three groups on the basis of maximum diameter: (i) small (group S); (ii) medium sized (group M); and (iii) large (group L) aneurysms. Using a Doppler flow guidewire and a pressure-monitoring guidewire, we measured coronary flow velocity and perfusion pressure inside aneurysms and in adjacent normal-looking vessels. We calculated the average peak velocity (APV) index, the mean coronary perfusion pressure (P) index and shear index.

RESULTS

The APV index and shear index decreased significantly (p < 0.005) as the aneurysm size increased (APV index in groups S, M and L was 0.893 +/- 0.149, 0.573 +/- 0.242 and 0.128 +/- 0.131, respectively; shear index in groups S, M and L was 0.750 +/- 0.149, 0.328 +/- 0.153 and 0.020 +/- 0.028, respectively). However, coronary perfusion pressure showed no relationship to aneurysm size and was not significantly different from that in adjacent normal-looking vessels.

CONCLUSIONS

The results of the present study suggest that the stagnation of flow and the reduction of shear stress in coronary aneurysms could initiate thrombus formation.

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.

Fluid shear stress as a regulator of gene expression in vascular cells: possible correlations with diabetic abnormalities

Diabetes mellitus is associated with increased frequency, severity and more rapid progression of cardiovascular diseases. Metabolic perturbations from hyperglycemia result in disturbed endothelium-dependent relaxation, activation of coagulation pathways, depressed fibrinolysis, and other abnormalities in vascular homeostasis. Atherosclerosis is localized mainly at areas of geometric irregularity at which blood vessels branch, curve and change diameter, and where blood is subjected to sudden changes in velocity and/or direction of flow. Shear stress resulting from blood flow is a well known modulator of vascular cell function. This paper presents what is currently known regarding the molecular mechanisms responsible for signal transduction and gene regulation in vascular cells exposed to shear stress. Considering the importance of the hemodynamic environment of vascular cells might be vital to increasing our understanding of diabetes.

Structure, strength, failure, and remodeling of the pulmonary blood-gas barrier

The pulmonary blood-gas barrier needs to satisfy two conflicting requirements. It must be extremely thin for efficient gas exchange, but also immensely strong to withstand the extremely high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the blood-gas barrier on the thin side is attributable to the type IV collagen in the basement membranes. However, when the wall stresses rise to very high levels, ultrastructural changes in the barrier occur, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include intense exercise in elite human athletes. Some animals, such as Thoroughbred racehorses, consistently break their alveolar capillaries during galloping, causing hemorrhage. Pathophysiological conditions causing stress failure include neurogenic pulmonary edema, high-altitude pulmonary edema, left heart failure, and overinflation of the lung. Remodeling of the capillary wall occurs in response to increased wall stress, a good example being the thickening of the capillary basement membrane in diseases such as mitral stenosis. The blood-gas barrier is able to maintain its extreme thinness with sufficient strength only through continual regulation of its wall structure. Recent experimental work suggests that rapid changes in gene expression for extracellular matrix proteins and growth factors occur in response to increases in capillary wall stress. How the blood-gas barrier is regulated to be extremely thin but sufficiently strong is a central issue in lung biology.

Novel optical methodologies in studying mechanical signal transduction in mammalian cells

For the last 3 decades evidence has been accumulating that some types of mammalian cells respond to their mechanically active environment by altering their morphology, growth rate, and metabolism. The study of such responses is very important in understanding, physiological and pathological conditions ranging from bone formation to atherosclerosis. Obtaining this knowledge has been the goal for an active research area in bioengineering termed cell mechanotransduction. The advancement of optical methodologies used in cell biology research has given the tools to elucidate cellular mechanisms that would otherwise be impossible to visualize. Combined with molecular biology techniques, they give engineers invaluable tools in understanding the chemical pathways involved in mechanotransduction. Herein we briefly review the current knowledge on mechanical signal transduction in mammalian cells, focusing on the application of novel optical techniques in the ongoing research.

Mechanical factors affecting hemostasis and thrombosis

Both physical and chemical factors can influence the activity of platelets and coagulation factors responsible for the formation of thrombotic and hemostatic masses in the vicinity of an injured vessel wall. Studies performed in controlled shear devices (viscometers) have indicated that physical factors alone can induce platelet aggregation, even in the absence of exogenous chemical factors. The physical considerations which appear to be important for the local activation of hemostatic/thrombotic mechanisms appear to be related to the magnitude of the shear rate/stress, the duration of the applied physical force and the local geometry. Blood flow alone has multiple influences on platelet and coagulative mechanisms. It has been well established that at physiologically encountered shear conditions, increases in the local shear rate enhance the attachment of platelets to the vessel wall and the growth of platelet aggregates on adherent platelets. In contrast, increases in local shear conditions inhibit the production of fibrin formation on surfaces where tissue factor (TF) is exposed. At levels of shear rate/stress high as compared to normal physiological conditions, but comparable to those observed at the apex of severely stenosed vessels, platelet aggregate formation is dependent on the duration of the exposure time. Considerable advances in our understanding of flow-related mechanisms have evolved from the use of well-defined perfusion chambers employing parallel flow streamlines. However, processes leading to hemostasis and thrombosis generally occur in more complicated flow situations where flow streamlines are not parallel and in which abnormally high, as well as abnormally low, shear rates and shear stress levels may be encountered in close proximity to each other.

Shear stress as an inhibitor of vascular smooth muscle cell proliferation. Role of transforming growth factor-beta 1 and tissue-type plasminogen activator

We examined whether shear stress can inhibit vascular smooth muscle cell (VSMC) proliferation in vitro directly. Human VSMCs were exposed to fluid flow for 24 hours using a cone-plate apparatus, and their proliferation was inhibited significantly by shear stresses of 1.4 and 2.8 Pa (14 and 28 dyne/cm2), according to the magnitude. Next, we investigated whether transforming growth factor-beta 1 (TGF beta 1), which is known to be an important cytokine that suppresses VSMC proliferation, is the predominant mediator of shear-induced inhibition of VSMC growth. After exposure of VSMCs to shear stress (2.8 Pa) for 24 hours, gene expression of TGF beta 1 and, interestingly, tissue-type plasminogen activator, which converts plasminogen to plasmin, an activator of TGF beta 1, increased twofold and fivefold, respectively. The levels of both latent and active forms of TGF beta 1 in conditioned media of VSMCs exposed to fluid flow increased significantly. An anti-TGF beta 1 antibody reversed shear-induced inhibition of VSMC growth significantly. We concluded that shear stress inhibited VSMC proliferation in vitro and this inhibition was mediated predominantly by TGF beta 1 in an autocrine manner. These data suggest that shear stress plays an important role as an inhibitor of atherogenesis in endothelium-desquamated lesions.

Residual stresses in oscillating thoracic arteries reduce circumferential stresses and stress gradients

The purpose of this paper is to examine the effects of residual stresses and strains in the oscillating arteries on the stress distribution in the vascular wall. We employ a static theory of large elastic deformations for orthotropic material (Chuong and Fung, 1986, J. biomech. Engng 108, 189-192) with the acceleration term added to make the theory dynamic. We use the static elastic parameters of residual stresses in our analysis because the dynamic parameters are not available in the literature. Our analysis reveals that the effect of considering the residual stresses is to decrease the very large circumferential stresses at the inner wall by 62% and reduces the stress gradient through the arterial wall by 94% compared to the case when residual stresses are ignored. Thus, because the arteries do contain residual stresses, the consequent lower stresses at the inner wall and the reduced stress gradient may reduce the progression of atheroma. Our computations show that the stress gradients do not depend on the heart rate.

Leucocyte adhesion under flow conditions: principles important in tissue engineering

An understanding of inflammatory responses is important in a wide variety of tissue engineering applications. This review describes the current understanding of a central aspect of inflammatory responses, the adhesion of leucocytes to blood vessel walls prior to their emigration into tissues. These highly specific adhesive interactions are mediated by three main families of receptors: the selectins, integrins, and members of the immunoglobulin superfamily. Under flow conditions, the various receptors make distinct contributions to a multistep process of adhesion in which leucocytes roll, adhere firmly, and eventually transmigrate. Two examples in which these principles are important in tissue engineering research, lymphocyte adherence in transplant rejection and monocyte adherence in atherosclerosis, are discussed in the last part of the paper.

Nuclear factor-kappa B interacts functionally with the platelet-derived growth factor B-chain shear-stress response element in vascular endothelial cells exposed to fluid shear stress

Hemodynamic forces, such as fluid shear stress, that act on the endothelial lining of the cardiovascular system can modulate the expression of an expanding number of genes crucial for homeostasis and the pathogenesis of vascular disease. A 6-bp core element (5'-GAGACC-3'), defined previously as a shear-stress response element is present in the promoters of many genes, including the PDGF B-chain, whose expression is modulated by shear stress. The identity of the nuclear protein(s) binding to this element has not yet been elucidated. Using electrophoretic mobility shift assays and in vitro DNase I footprinting, we demonstrate that nuclear factor-kappa B p50-p65 heterodimers, which accumulate in the nuclei of cultured vascular endothelial cells exposed to fluid shear stress, bind to the PDGF-B shear-stress response element in a specific manner. Mutation of this binding motif abrogated its interaction with p50-p65 and abolished the ability of the promoter to mediate increased gene expression in endothelial cells exposed to shear stress. Transient cotransfection studies indicate that p50-p65 is able to activate PDGF-B shear-stress response element-dependent reporter gene expression in these cells. These findings thus implicate nuclear factor-kappa B in the transactivation of an endothelial gene responding to a defined fluid mechanical force.

Shear stress-mediated changes in the expression of leukocyte adhesion receptors on human umbilical vein endothelial cells in vitro

Extensive monocyte recruitment is an early phenomenon associated with the development of atherosclerotic lesions, suggesting an active role for the involvement of adhesion receptors expressed by endothelial cells. In this study we describe the contribution of hemodynamic shear forces in regulating the expression of a few of the monocyte adhesion receptors, including intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and E-selectin on endothelial cells. A parallel plate flow chamber and recirculating flow loop device was used to expose human umbilical vein endothelial cells (HUVECs) to different levels of shear (2-25 dyn/cm2). Subsequently the cells were analyzed either for shear induced changes in the mRNA levels of adhesion receptors by Northern blot analyses or for changes in the surface expression of ICAM-1 using flow cytometry. Results from the fluorescence analysis showed a transient increase in the surface expression of ICAM-1, 12 hr after exposure to 25 dyn/cm2 shear, returning to basal levels within 24 hr. This was quite different from the time dependent response of ICAM-1 to lipopolysaccharide (LPS), where ICAM-1 expression was maximally induced 18-24 hr post-stimulus. ICAM-1 mRNA level appeared slightly elevated after exposure to shear for 1 hr, compared to basal values, but dropped below basal levels within 6 hr. This biphasic response was seen irrespective of the magnitude of applied shear stress. VCAM-1 mRNA expression, in contrast, decreased below the baseline expression within an hour after onset of flow, and appeared to be considerably down-regulated within 6 hr. After exposure to shear for 24 hr, no increase in mRNA levels could be detected for either molecule, at any shear magnitude. E-selectin mRNA was less responsive to shear stress, especially at the lower magnitudes of shear. After an hour of exposure to flow E-selectin mRNA level appeared slightly reduced compared with control levels, but it remained at this level even after 6 hr of flow. These results indicate that the expression of adhesion receptors is sensitive to local shear stresses in a manner that is molecule specific in the short term even though prolonged exposure to flow results in similar down-regulation for both ICAM-1 and VCAM-1.

Mechanical strain increases endothelin-1 gene expression via protein kinase C pathway in human endothelial cells

Vascular endothelial cells (ECs) are constantly subjected to mechanical strain due to relaxation and contraction of vessel walls. The effects of cyclical strain on endothelin-1 (Et-1) secretion and Et-1 mRNA levels in human umbilical vein ECs were examined. Cultured ECs grown on a flexible membrane base were deformed by negative pressure (16 kPa at 60 cycles/min). Cells subjected to strain showed increased Et-1 secretion (0.54 ng/hr/10(6) cells) compared with unstrained control cells (0.22 ng/hr/10(6) cells). Northern blot analysis of cells strained for 2 hours or longer demonstrated a sustained elevated Et-1 mRNA level at more than double the level in unstrained controls. This strain-induced ET-1 mRNA level returned to its basal level 2 hours after the release of strain. Cells treated with actinomycin D before or during strain treatment showed no strain-induced gene expression. Pretreatment of ECs with a protein kinase C (PKC) inhibitor, Calphostin C, strongly inhibited the strain-induced Et-1 gene expression. Pretreatment of ECs with cAMP- or cGMP-dependent protein kinase inhibitors (KT5720 or KT5823) only partially inhibited the increased Et-1 mRNA levels in strain-treated cells. EGTA strongly inhibited the Et-1 gene expression. The intracellular calcium chelator BAPTA/AM also showed an inhibitory effect on Et-1 mRNA levels. We conclude that mechanical strain can stimulate the secretion of Et-1 from ECs by increasing Et-1 mRNA levels via transcription, and that this gene induction is mediated predominantly via the PKC pathway and requires extracellular Ca2+. This strain-induced Et-1 gene expression in ECs may contribute to the regulation of vascular tone and structure in normal and pathological states of the cardiovascular system.

Shear stress selectively upregulates intercellular adhesion molecule-1 expression in cultured human vascular endothelial cells

Hemodynamic forces induce various functional changes in vascular endothelium, many of which reflect alterations in gene expression. We have recently identified a cis-acting transcriptional regulatory element, the shear stress response element (SSRE), present in the promoters of several genes, that may represent a common pathway by which biomechanical forces influence gene expression. In this study, we have examined the effect of shear stress on endothelial expression of three adhesion molecules: intercellular adhesion molecule-1 (ICAM-1), which contains the SSRE in its promoter, and E-selectin (ELAM-1) and vascular cell adhesion molecule-1 (VCAM-1), both of which lack the SSRE. Cultured human umbilical vein endothelial cells, subjected to a physiologically relevant range of laminar shear stresses (2.5-46 dyn/cm2) in a cone and plate apparatus for up to 48 h, showed time-dependent but force-independent increases in surface immunoreactive ICAM-1. Upregulated ICAM-1 expression was correlated with increased adhesion of the JY lymphocytic cell line. Northern blot analysis revealed increased ICAM-1 transcript as early as 2 h after the onset of shear stress. In contrast, E-selectin and vascular cell adhesion molecule-1 transcript and cell-surface protein were not upregulated at any time point examined. This selective regulation of adhesion molecule expression in vascular endothelium suggests that biomechanical forces, in addition to humoral stimuli, may contribute to differential endothelial gene expression and thus represent pathophysiologically relevant stimuli in inflammation and atherosclerosis.

Shear-dependence of endothelial functions

Endothelial cells are subjected to shear forces which influence important cell functions. Shear stress induces cell elongation and formation of stress fibers, increases permeability, pinocytosis and lipoprotein internalization, is involved in the formation of atherosclerotic lesions, increases the production of tissue plasminogen activator, and enhances von Willebrand factor release and hence platelet aggregation. It decreases adherence of erythrocytes and leukocytes, and increases the release of prostacyclin, endothelium derived relaxing factor, histamine and other compounds, but decreases erythropoietin secretion. The mechanism of signal transduction to the endothelial cell is not known exactly; shear-sensitive ion channels seem to be involved. It is concluded that a better understanding of shear-dependent endothelial functions will influence pathophysiologic concepts and therapeutic interventions.

1992 ALZA Distinguished Lecture: bioengineering and vascular biology

The vascular system is naturally dynamic; fluid mechanics and mass transfer are closely integrated with blood and vascular cell function. We are beginning to understand how local wall shear stress and strain modulate endothelial cell metabolism at the gene level. This knowledge may help explain the focal nature of many vascular pathologies, including atherosclerosis. Understanding mechanical control of gene regulation at the level of specific promoter elements and transcription factors involved will lead to development of novel constructs for localized delivery of specific gene products in regions of high or low shear stress or strain in the vascular system. In addition, recent research has shown how local fluid mechanics can alter receptor specificity in cell-to-cell and cell-to-matrix protein adhesion and aggregation. Knowledge of the specific molecular sequences involved in cell-to-cell recognition will allow development of targeted therapeutics, with applications in thrombosis, inflammation, cancer metastasis, and sickle-cell anemia. Bioengineers are uniquely qualified to be leaders in this field, because advances require a synthesis of cell and molecular biology with systems analysis, transport phenomena, and quantitative modeling. Rapid progress in tissue engineering applications will require this new kind of biomedical engineer, which represents both a challenge and an opportunity for our profession.

Platelet-derived growth factor B chain promoter contains a cis-acting fluid shear-stress-responsive element

The endothelial lining of blood vessels is constantly exposed to fluid mechanical forces generated by flowing blood. In vitro application of fluid shear stresses to cultured endothelial cells influences the expression of multiple genes, as reflected by changes in their steady-state mRNA levels. We have utilized the B chain of platelet-derived growth factor (PDGF-B) as a model to investigate the mechanisms of shear-stress-induced gene regulation in cultured bovine aortic endothelial cells (BAECs). Northern blot analysis revealed elevated endogenous PDGF-B transcript levels in BAECs, after exposure to a physiological level of laminar shear stress (10 dynes/cm2; 1 dyne = 100 mN) for 4 h. A transfected reporter gene, consisting of a 1.3-kb fragment of the human PDGF-B promoter coupled to chloramphenicol acetyltransferase (CAT), indicated a direct effect on transcriptional activity. Transfection of a series of PDGF-B-CAT deletion mutants led to the characterization of a cis-acting component within the PDGF-B promoter that was necessary for shear-stress responsiveness. In gel-shift assays, overlapping oligonucleotide probes of this region formed several protein-DNA complexes with nuclear extracts prepared from both static and shear-stressed BAECs. A 12-bp component (CTCTCAGAGACC) was identified that formed a distinct pattern of complexes with nuclear proteins extracted from shear-stressed BAECs. This shear-stress-responsive element does not encode binding sites for any known transcription factor but does contain a core binding sequence (GAGACC), as defined by deletion mutation in gel-shift assays. Interestingly, this putative transcription factor binding site is also present in the promoters of certain other endothelial genes, including tissue plasminogen activator, intercellular adhesion molecule 1, and transforming growth factor beta 1, that also are induced by shear stress. Thus, the expression of PDGF-B and other pathophysiologically relevant genes in vascular endothelium appears to be regulated, in part, by shear-stress-induced transcription factors interacting with a common promoter element.