Mechanisms of cell shape change: the cytomechanics of cellular response to chemical environment and mechanical loading

Monitoring apoptosis in intact cells by high-resolution magic angle spinning 1 H NMR spectroscopy

Apoptosis maintains an equilibrium between cell proliferation and cell death. Many diseases, including cancer, develop because of defects in apoptosis. A known metabolic marker of apoptosis is a notable increase in ¹ H NMR-observable resonances associated with lipids stored in lipid droplets. However, standard one-dimensional NMR experiments allow the quantification of lipid concentration only, without providing information about physical characteristics such as the size of lipid droplets, viscosity of the cytosol, or cytoskeletal rigidity. This additional information can improve monitoring of apoptosis-based cancer treatments in intact cells and provide us with mechanistic insight into why these changes occur. In this paper, we use high-resolution magic angle spinning (HRMAS) ¹ H NMR spectroscopy to monitor lipid concentrations and apparent diffusion coefficients of mobile lipid in intact cells treated with the apoptotic agents cisplatin or etoposide. We also use solution-state NMR spectroscopy to study changes in lipid profiles of organic solvent cell extracts. Both NMR techniques show an increase in the concentration of lipids but the relative changes are 10 times larger by HRMAS ¹ H NMR spectroscopy. Moreover, the apparent diffusion rates of lipids in apoptotic cells measured by HRMAS ¹ H NMR spectroscopy decrease significantly as compared with control cells. Slower diffusion rates of mobile lipids in apoptotic cells correlate well with the formation of larger lipid droplets as observed by microscopy. We also compared the mean lipid droplet displacement values calculated from the two methods. Both methods showed shorter displacements of lipid droplets in apoptotic cells. Our results demonstrate that the NMR-based diffusion experiments on intact cells discriminate between control and apoptotic cells. Apparent diffusion measurements in conjunction with ¹ H NMR spectroscopy-derived lipid signals provide a novel means of following apoptosis in intact cells. This method could have potential application in enhancing drug discovery by monitoring drug treatments in vitro, particularly for agents that cause portioning of lipids such as apoptosis.

Vasoactive agonists exert dynamic and coordinated effects on vascular smooth muscle cell elasticity, cytoskeletal remodelling and adhesion

In this study, we examined the ability of vasoactive agonists to induce dynamic changes in vascular smooth muscle cell (VSMC) elasticity and adhesion, and tested the hypothesis that these events are coordinated with rapid remodelling of the cortical cytoskeleton. Real-time measurement of cell elasticity was performed with atomic force microscopy (AFM) and adhesion was assessed with AFM probes coated with fibronectin (FN). Temporal data were analysed using an Eigen-decomposition method. Elasticity in VSMCs displayed temporal oscillations with three components at approximately 0.001, 0.004 and 0.07 Hz, respectively. Similarly, adhesion displayed a similar oscillatory pattern. Angiotensin II (ANG II, 10(-6) M) increased (+100%) the amplitude of the oscillations, whereas the vasodilator adenosine (ADO, 10(-4) M) reduced oscillation amplitude (-30%). To test whether the oscillatory changes were related to the architectural alterations in cortical cytoskeleton, the topography of the submembranous actin cytoskeleton (100-300 nm depth) was acquired with AFM. These data were analysed to compare cortical actin fibre distribution and orientation before and after treatment with vasoactive agonists. The results showed that ANG II increased the density of stress fibres by 23%, while ADO decreased the density of the stress fibres by 45%. AFM data were supported by Western blot and confocal microscopy. Collectively, these observations indicate that VSMC cytoskeletal structure and adhesion to the extracellular matrix are dynamically altered in response to agonist stimulation. Thus, vasoactive agonists probably invoke unique mechanisms that dynamically alter the behaviour and structure of both the VSMC cytoskeleton and focal adhesions to efficiently support the normal contractile behaviour of VSMCs.

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

The mechanobiological aetiopathogenesis of tendinopathy: is it the over-stimulation or the under-stimulation of tendon cells?

While there is a significant amount of information available on the clinical presentation(s) and pathological changes associated with tendinopathy, the precise aetiopathogenesis of this condition remains a topic of debate. Classically, the aetiology of tendinopathy has been linked to the performance of repetitive activities (so-called overuse injuries). This has led many investigators to suggest that it is the mechanobiologic over-stimulation of tendon cells that is the initial stimulus for the degradative processes which have been shown to accompany tendinopathy. Although several studies have been able to demonstrate that the in vitro over-stimulation of tendon cells in monolayer can result in a pattern(s) of gene expression seen in clinical cases of tendinopathy, the strain magnitudes and durations used in these in vitro studies, as well as the model systems, may not be clinically relevant. Using a rat tail tendon model, we have studied the in vitro mechanobiologic response of tendon cells in situ to various tensile loading regimes. These studies have led to the hypothesis that the aetiopathogenic stimulus for the degenerative cascade which precedes the overt pathologic development of tendinopathy is the catabolic response of tendon cells to mechanobiologic under-stimulation as a result of microscopic damage to the collagen fibres of the tendon. In this review, we examine the rationale for this hypothesis and provide evidence in support of this theory.

Mathematical Models of Cell Motility

Cell motility is an essential biological action in the creation, operation and maintenance of our bodies. Developing mathematical models elucidating cell motility will greatly advance our understanding of this fundamental biological process. With accurate models it is possible to explore many permutations of the same event and concisely investigate their outcome. While great advancements have been made in experimental studies of cell motility, it now has somewhat fallen on mathematical models to taking a leading role in future developments. The obvious reason for this is the complexity of cell motility. Employing the processing power of today's computers will give researches the ability to run complex biophysical and biochemical scenarios, without the inherent difficulty and time associated with in vitro investigations. Before any great advancement can be made, the basics of cell motility will have to be well-defined. Without this, complicated mathematical models will be hindered by their inherent conjecture. This review will look at current mathematical investigations of cell motility, explore the reasoning behind such work and conclude with how best to advance this interesting and challenging research area.

Isostaticity and controlled force transmission in the cytoskeleton: A model awaiting experimental evidence

A new model is proposed for force transmission through the cytoskeleton (CSK). A general discussion is first presented on the physical principles that underlie the modeling of this phenomenon. Some fundamental problems of conventional models--continuous and discrete--are examined. It is argued that mediation of focused forces is essential for good control over intracellular mechanical signals. The difficulties of conventional continuous models in describing such mediation are traced to a fundamental assumption rather than to their being continuous. Relevant advantages and disadvantages of continuous and discrete modeling are discussed. It is concluded that favoring discrete models is based on two misconceptions, which are clarified. The model proposed here is based on the idea that focused propagation of mechanical stimuli in frameworks over large distances (compared to the mesh size) can only occur when considerable regions of the CSK are isostatic. The concept of isostaticity is explained and a recently developed continuous isostaticity theory is briefly reviewed. The model enjoys several advantages: it leads to good control over force mediation; it explains nonuniform stresses and action at a distance; it is continuous, making it possible to model force propagation over long distances; and it enables prediction of individual force paths. To be isostatic, or nearly so, CSK networks must possess specific structural characteristics, and these are quantified. Finally, several experimental observations are interpreted using the new model and implications are discussed. It is also suggested that this approach may give insight into the dynamics of reorganization of the CSK. Many of the results are amenable to experimental measurements, providing a testing ground for the proposed picture, and generic experiments are suggested.

Mechanics of crawling cells

Crawling of keratocytes derived from aquatic vertebrates represents a very useful model system for the investigation of cell locomotion because of its ease of handling and the clear structural separation of a thin cytoplasmic layer, the lamella, from the cell body containing the nucleus and other organelles. Spreading of spherical keratocytes results in fried egg shaped cells, which on withdrawing their lamella at one side become polarized and start moving. Hydrostatic pressure, tension at the cortex, traction forces exerted on the adhesion sites and inside the cells along filamentous structures are required to gain a certain shape. Traction forces have been made visible using scanning acoustic microscopy. This method also allowed for the demonstration of cytoplasmic fluxes inside a moving keratocyte and changes of forces while a migrating cell is changing its direction of locomotion. The pros and cons for actin polymerization at the leading front providing the driving force for crawling are discussed on the basis of structural and experimental results: do they stringently identify polymerization of actin as the only driving machinery. Such a mechanism not only should explain the advancement of the leading edge but also the movement of the whole cell, i.e. the material flux taking place from the cell body to the periphery. Even if the lamella periphery itself may be motile by actin turnover this scheme may represent an oversimplification if applied to the whole cell. Considering the complexity of a whole cell simplifying model systems may not lead to adequate descriptions of the mechanisms as they occur within cells with a highly complex structure, although the model might be consistent and sufficient to describe, i.e. crawling in general.

In situ cell nucleus deformation in tendons under tensile load; a morphological analysis using confocal laser microscopy

Cell and cell nucleus deformations have been implicated in the mechanotransduction of mechanical loads acting on tissues. While in situ cell nucleus deformation in response to increasing tissue strains has been examined in articular cartilage this phenomenon has not been investigated in tendons. To examine in situ cell nuclei deformation in tendons undergoing tensile strain rat tail tendons were harvested from adult Sprague-Dawley rats and stained with acridine orange to highlight the cell nuclei. The tendons were mounted on a custom-designed, low-load, tensile testing device affixed to the mechanical stage of a confocal laser microscope. Cells within the tendons were isolated for analysis. Images of individual cells were captured at 0% strain as well as sequentially at 2%, 4% and 6% grip-to-grip tendon strain. Digital images of the cell nuclei were then measured in the x (length) and y (height) axis and deformation expressed as a percentage of cell nuclei strain. In addition, centroid-to-centroid distances of adjacent cell nuclei within each image were measured and used to calculate local tissue strain. There was a weak (r2 = 0.34) but significant (p < 0.01) correlation between local tissue strain and cell nucleus strain in the x axis. The results of this study support the hypothesis that in situ cell nucleus deformation does occur during tensile loading of tendons. This deformation may play a significant role in the mechanical signal transduction pathway of this tissue.

Dynamic study of cell mechanical and structural responses to rapid changes of calcium level

Cell shape control is complex since it may involve multiple cytoskeletal components and metabolic pathways. Here we present a kinetic study of the mechanical and structural responses of cells from the monocytic THP-1 line to a rapid increase of cytosolic calcium level. Cells were exposed to ionomycin in a medium of varying calcium concentration and they were probed at regular intervals for (1) cortical rigidity as determined with micropipette aspiration, and (2) content and distribution of polymerized actin, myosin or ABP-280, as determined with flow cytometry and/or confocal microscopy. An increase of free intracellular calcium level induced: (1) a biphasic deformability change with marked stiffening within a second, and significant softening a minute later; (2) a biphasic change of actin polymerization with initial decrease (within less than a second) and rapid recovery (within a few seconds); (3) a topographical redistribution of microfilaments with an oscillatory behavior of the cortical fraction, while no substantial redistribution of myosin or ABP-280 was detected. It is suggested that a regulation of cell rigidity might be achieved without any structural change by suitable modulation of the lifetime of bridges formed between microfilaments by actin binding proteins.

Measurements of mechanical properties of the blastula wall reveal which hypothesized mechanisms of primary invagination are physically plausible in the sea urchin Strongylocentrotus purpuratus

Computer simulations showed that the elastic modulus of the cell layer relative to the elastic modulus of the extracellular layers predicted the effectiveness of different force-generating mechanisms for sea urchin primary invagination [L. A. Davidson, M. A. R. Koehl, R. Keller, and G. F. Oster (1995) Development 121, 2005-2018]. Here, we measured the composite elastic modulus of the cellular and extracellular matrix layers in the blastula wall of Strongylocentrotus purpuratus embryos at the mesenchyme blastula stage. Combined, these two layers exhibit a viscoelastic response with an initial stiffness ranging from 600 to 2300 Pa. To identify the cellular structures responsible for this stiffness we disrupted these structures and correlated the resulting lesions to changes in the elastic modulus. We treated embryos with cytochalasin D to disrupt the actin-based cytoskeleton, nocodazole to disrupt the microtubule-based cytoskeleton, and a gentle glycine extraction to disrupt the apical extracellular matrix (ECM). Embryos treated less than 60 min in cytochalasin D showed no change in their time-dependent elastic modulus even though F-actin was severely disrupted. Similarly, nocodazole had no effect on the elastic modulus even as the microtubules were severely disrupted. However, glycine extraction resulted in a 40 to 50% decrease in the elastic modulus along with a dramatic reduction in the hyalin protein at the apical ECM, thus implicating the apical ECM as a major mechanical component of the blastula wall. This finding bears on the mechanical plausibility of several models for primary invagination.

Direct observations of the mechanical behaviors of the cytoskeleton in living fibroblasts

Cytoskeletal proteins tagged with green fluorescent protein were used to directly visualize the mechanical role of the cytoskeleton in determining cell shape. Rat embryo (REF 52) fibroblasts were deformed using glass needles either uncoated for purely physical manipulations, or coated with laminin to induce attachment to the cell surface. Cells responded to uncoated probes in accordance with a three-layer model in which a highly elastic nucleus is surrounded by cytoplasmic microtubules that behave as a jelly-like viscoelastic fluid. The third, outermost cortical layer is an elastic shell under sustained tension. Adhesive, laminin-coated needles caused focal recruitment of actin filaments to the contacted surface region and increased the cortical layer stiffness. This direct visualization of actin recruitment confirms a widely postulated model for mechanical connections between extracellular matrix proteins and the actin cytoskeleton. Cells tethered to laminin-treated needles strongly resisted elongation by actively contracting. Whether using uncoated probes to apply simple deformations or laminin-coated probes to induce surface-to-cytoskeleton interaction we observed that experimentally applied forces produced exclusively local responses by both the actin and microtubule cytoskeleton. This local accomodation and dissipation of force is inconsistent with the proposal that cellular tensegrity determines cell shape.

Regulation of endothelin-1 gene expression by cell shape and the microfilament network in vascular endothelium

Endothelial synthesis and release of endothelin-1 (ET-1) are exquisitely regulated by external shear and strain. We tested the hypothesis that manipulation of endothelial cell shape can regulate ET-1 gene expression. Treatment of bovine aortic endothelial cell (BAEC) monolayers with cytochalasin D disrupted F-actin and induced cell retraction and rounding, in parallel with time- and dose-dependent specific decreases in ET-1 mRNA levels. Treatments with forskolin, phorbol 12-myristate 13-acetate, staurosporine, and genistein also induced cell shape change and decreased F-actin staining and ET-1 mRNA levels. BAEC plated onto nonadhesive petri dishes coated with decreasing concentrations of synthetic RGD polymer showed RGD dose-dependent decreases in cell spreading and in F-actin microfilament elaboration. These changes were specifically accompanied by decreases in ET-1 peptide secretion (60%) and, via posttranscriptional mechanisms, ET-1 mRNA (94%) and were not due to decreased cell-cell contact. We conclude that the shape and microfilament network of endothelial cells are potent posttranscriptional regulators of ET-1 gene expression.

Effect of mechanical deformation on structure and function of polymorphonuclear leukocytes

The present studies were designed to test the hypothesis that mechanical deformation of polymorphonuclear leukocytes (PMN) leads to functional changes that might influence their transit in the pulmonary capillaries. Human leukocytes were passed through 5- or 3-micron-pore polycarbonate filters under controlled conditions. Morphometric analysis showed that the majority of PMN were deformed and that this deformation persisted longer after filtration through 3-micron filters than through 5-micron filters (P < 0.05) but did not result in the cytoskeletal polarization characteristic of migrating cells. Flow cytometric studies of the filtered PMN showed that there was a transient increase in the cytosolic free Ca2+ concentration after both 3- and 5-micron filtration (P < 0.01) with an increase in F-actin content after 3-micron filtration (P < 0.05). Although L-selectin expression on PMN was not changed by either 5- or 3-micron filtration, CD18 and CD11b were increased by 3-micron filtration (P < 0.05). Priming of the PMN with N-formyl-methionyl-leucyl-phenylalanine (0.5 nM) before filtration resulted in an increase of CD11b by both 5 (P < 0.05)- and 3-micron (P < 0.01) filtration. Neither 5- nor 3-micron filtration induced hydrogen peroxide production. We conclude that mechanical deformation of PMN, similar to what occurs in the pulmonary microvessels, induces both structural and functional changes in the cells, which might influence their passage through the pulmonary capillary bed.

Theoretical estimates of mechanical properties of the endothelial cell cytoskeleton

Current modeling of endothelial cell mechanics does not account for the network of F-actin that permeates the cytoplasm. This network, the distributed cytoplasmic structural actin (DCSA), extends from apical to basal membranes, with frequent attachments. Stress fibers are intercalated within the network, with similar frequent attachments. The microscopic structure of the DCSA resembles a foam, so that the mechanical properties can be estimated with analogy to these well-studied systems. The moduli of shear and elastic deformations are estimated to be on the order of 10(5) dynes/cm2. This prediction agrees with experimental measurements of the properties of cytoplasm and endothelial cells reported elsewhere. Stress fibers can potentially increase the modulus by a factor of 2-10, depending on whether they act in series or parallel to the network in transmitting surface forces. The deformations produced by physiological flow fields are of insufficient magnitude to disrupt cell-to-cell or DCSA cross-linkages. The questions raised by this paradox, and the ramifications of implicating the previously unreported DCSA as the primary force transmission element are discussed.

Auxins and Cytokinins as Antipodal Modulators of Elasticity within the Actin Network of Plant Cells

The cytoskeleton of plant and animal cells serves as a transmitter, transducer, and effector of cell signaling mechanisms. In plants, pathways for proliferation, differentiation, intracellular vesicular transport, cell-wall biosynthesis, symbiosis, secretion, and membrane recycling depend on the organization and dynamic properties of actin- and tubulin-based structures that are either associated with the plasma membrane or traverse the cytoplasm. Recently, a new in vivo cytoskeletal assay (cell optical displacement assay) was introduced to measure the tension within subdomains (cortical, transvacuolar, and perinuclear) of the actin network in living plant cells. Cell optical displacement assay measurements within soybean (Glycine max [L.]) root cells previously demonstrated that lipophilic signals, e.g. linoleic acid and arachidonic acid or changes in cytoplasmic pH gradients, could induce significant reductions in the tension within the actin network of transvacuolar strands. In contrast, enhancement of cytoplasmic free Ca2+ resulted in an increase in tension. In the present communication we have used these measurements to show that a similar antipodal pattern of activity exists for auxins and cytokinins (in their ability to modify the tension within the actin network of plant cells). It is suggested that these growth substances exert their effect on the cytoskeleton through the activation of signaling cascades, which result in the production of lipophilic and ionic second messengers, both of which have been demonstrated to directly effect the tension within the actin network of soybean root cells.

Measuring the viscoelastic properties of human platelets with the atomic force microscope

We have measured force curves as a function of the lateral position on top of human platelets with the atomic force microscope. These force curves show the indentation of the cell as the tip loads the sample. By analyzing these force curves we were able to determine the elastic modulus of the platelet with a lateral resolution of approximately 100 nm. The elastic moduli were in a range of 1-50 kPa measured in the frequency range of 1-50 Hz. Loading forces could be controlled with a resolution of 80 pN and indentations of the platelet could be determined with a resolution of 20 nm.

Control of endothelial cell gene expression by flow

The vessel wall is constantly subjected to, and affected by, the stresses resulting from the hemodynamic stimuli of transmural pressure and flow. At the interface between blood and the vessel wall, the endothelial cell plays a crucial role in controlling vessel structure and function in response to changes in hemodynamic conditions. Using bovine aortic endothelium monolayers, we show that fluid shear stress causes simultaneous differential regulation of endothelial-derived products. We also report that the downregulation of endothelin-1 mRNA by flow is a reversible process, and through the use of uncharged dextran supplementation demonstrate it to be shear stress- rather than shear rate-dependent. Recent work on the effect of fluid shear stress on endothelial cell gene expression of a number of potent endothelial products is reviewed, including vasoactive substances, autocrine and paracrine growth factors, thrombosis/fibrinolysis modulators, chemotactic factors, surface receptors and immediate-early genes. The encountered patterns of gene expression responses are classified into three categories: a transient increase with return to baseline (type I), a sustained increase (type II) and a biphasic response consisting of an early transient increase of varying extent followed by a pronounced and sustained decrease (type III). The importance of the dynamic character of the flow stimulus and the magnitude dependence of the response are presented. Potential molecular mechanisms of shear-induced gene regulation, including putative shear stress response elements (SSRE), are discussed. These results suggest exquisite modulation of endothelial cell phenotype by local fluid shear stress and may offer insight into the mechanism of flow-dependent vascular remodeling and the observed propensity of atherosclerosis formation around bifurcations and areas of low shear stress.

Covalent immobilization as a stimulus of cell wall composition changes

Covalent immobilization of yeast cells by an activated diamine spacer is accompanied by increased levels of cell wall proteins, lipids, amino sugars, amino acids and acid phosphatase leakage, and by altered composition of mannoproteins. The observed changes in cell wall composition are attributed to the effect of cell-solid surface contact.

Endothelial expression of thrombomodulin is reversibly regulated by fluid shear stress

The vascular endothelium, by virtue of its position at the interface between blood and the vessel wall, is known to play a critical role in the control of thrombosis and fibrinolysis. Thrombomodulin (TM) is a surface receptor that binds thrombin and is a potent activator of the protein C anticoagulant pathway. Although TM expression is known to be regulated by various cytokines, little is known about its response to ever-present biomechanical stimuli. We have explored the role of fluid shear stress, imparted on the luminal surface of the endothelial cell as a result of blood flow, on the expression of TM mRNA and protein in both bovine aortic endothelial (BAE) and bovine smooth muscle (BSM) cells in an in vitro system. We report in the present study that TM expression is regulated by flow. Subjecting BAE cells to fluid shear stress in the physiological range of magnitude of 15 (moderate shear stress) and 36 (elevated shear stress) dynes/cm2 resulted in a mild transient increase followed by a significant decrease in TM mRNA to 37% and 16% of its resting level, respectively, by 9 hours after the onset of flow. In contrast, shear stress at the low magnitude of 4 dynes/cm2 did not affect TM mRNA levels. The sensitivity of TM mRNA expression by flow was found to be specific to endothelium, since it was not observed in BSM cells exposed to steady laminar shear stress of 15 dynes/cm2. Furthermore, unlike BAE cells, BSM cells did not exhibit altered cell shape nor align in the direction of flow after 24 hours of shear stress at 15 dynes/cm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluid shear stress differentially modulates expression of genes encoding basic fibroblast growth factor and platelet-derived growth factor B chain in vascular endothelium

Fluid shear stress has been shown to be an important regulator of vascular structure and function through its effect on the endothelial cell. We have explored the effect of shear stress on the expression of the heparin-binding growth factors platelet-derived growth factor B chain (PDGF-B) and basic fibroblast growth factor (bFGF) in bovine aortic endothelial cells using a purpose-built cone-plate viscometer. Using morphometric analysis, we have mimicked the endothelial cell shape changes encountered in vivo in response to shear stress and correlated these with changes in gene expression. Steady laminar shear stress of 15 and 36 dyn/cm2 both resulted in endothelial cell shape change, but the higher shear stress induced greater and more uniform alignment in the direction of flow and nuclear protrusion after 24 h. Steady laminar shear stress of both 15 and 36 dyn/cm2 induced a significant 3.9- and 4.2-fold decrease, respectively, in PDGF-B mRNA at 9 h. In contrast, steady laminar shear of 15 dyn/cm2 induced a mild and transient 1.5-fold increase in bFGF mRNA while shear of 36 dyn/cm2 induced a significant 4.8-fold increase at 6 h of shear which remained at 2.9-fold at 9 h. Pulsatile and turbulent shear stress showed the same effect as steady laminar shear stress (all at 15 dyn/cm2 time-average magnitude) on PDGF-B and bFGF mRNA content. Cyclic stretch (20% strain, 20/min) of cells grown on silicone substrate did not significantly affect either PDGF-B or bFGF mRNA levels. These results suggest that expression of each peptide growth factor gene is differentially regulated by fluid shear stress in the vascular endothelial cell. These results may have implications on vascular structure and function in response to hemodynamic forces and present a model for the study of transduction of mechanical stimuli into altered gene expression.