Nitzan Resnick

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.

Hemodynamic forces are complex regulators of endothelial gene expression.

Vascular endothelial cells, by virtue of their unique anatomical position, are constantly exposed to the fluid mechanical forces generated by flowing blood. In vitro studies with model flow systems have demonstrated that wall shear stresses can modulate various aspects of endothelial structure and function. Certain of these effects appear to result from the regulation of expression of endothelial genes at the transcriptional level. Recent molecular biological studies have defined a “shear stress response element” (SSRE) in the promoter of the human platelet‐ derived growth factor (PDGF)‐B chain gene that interacts with DNA binding proteins in the nuclei of shears stressed endothelial cells to up‐regulate transcriptional activity. Insertion of this element into reporter genes also renders them shear‐inducible. Further characterization of this and other positive (and negative) shear‐responsive genetic regulatory elements, as well as their transactivating factors, should enhance our understanding of vascular endothelium as an integrator of humoral and biomechanical stimuli in health and disease.—Resnick, N., Gimbrone, M. A., Jr., Hemodynamic forces are complex regulators of endothelial gene expression. FASEBJ., 9, 874‐882 (1995)

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.

Vascular Endothelial Cells Respond to Spatial Gradients in Fluid Shear Stress by Enhanced Activation of Transcription Factors

The vascular endothelium is exposed to a spectrum of fluid mechanical forces generated by blood flow; some of these, such as fluid shear stress, can directly modulate endothelial gene expression. Previous work by others and in our laboratory, using an in vitro uniform laminar shear stress model, has identified various shear stress response elements (SSREs) within the promoters of certain endothelial genes that regulate their expression by interacting with various transcription factors, including nuclear factor-kappaB (NF-kappaB), early growth response-1 (Egr-1), and activator protein-1 (AP-1, composed of c-Jun/c-Jun and c-Jun/c-Fos protein dimers). In the current study, we have examined the topographical patterns of NF-kappaB, Egr-1, c-Jun, and c-Fos activation in a specially designed in vitro disturbed laminar shear stress model, which incorporates regions of significant spatial shear stress gradients similar to those found in atherosclerosis-prone arterial geometries in vivo (eg, arterial bifurcations, curvatures, ostial openings). Using newly developed quantitative image analysis techniques, we demonstrate that endothelial cells subjected to disturbed laminar shear stress exhibit increased levels of nuclear localized NF-kappaB, Egr-1, c-Jun, and c-Fos, compared with cells exposed to uniform laminar shear stress or maintained under static conditions. In addition, individual cells display a heterogeneity in responsiveness to disturbed flow, as measured by the amount of NF-kappaB, Egr-1, c-Jun, and c-Fos in their nuclei. This differential regulation of transcription factor expression by disturbed versus uniform laminar shear stress indicates that regional differences in blood flow patterns in vivo-in particular, the occurrence of spatial shear stress gradients-may represent important local modulators of endothelial gene expression at anatomic sites predisposed for atherosclerotic development.

VEGF receptor 2 and the adherens junction as a mechanical transducer in vascular endothelial cells

Blood-flow interactions with the vascular endothelium represents a specialized example of mechanical regulation of cell function that has important physiological and pathophysiological cardiovascular consequences. Yet, the mechanisms of mechanostransduction are not understood fully. This study shows that shear stress induces a rapid induction as well as nuclear translocation of the vascular endothelial growth factor (VEGF) receptor 2 and promotes the binding of the VEGF receptor 2 and the adherens junction molecules, VE-cadherin and β-catenin, to the endothelial cytoskeleton. These changes are accompanied by the formation of a complex containing the VEGF receptor 2–VE-cadherin–β-catenin. In endothelial cells lacking VE-cadherin, shear stress did not augment nuclear translocation of the VEGF receptor 2 and phosphorylation of Akt1 and P38 as well as transcriptional induction of a reporter gene regulated by a shear stress-responsive promoter. These results suggest that VEGF receptor 2 and the adherens junction act as shear-stress cotransducers, mediating the transduction of shear-stress signals into vascular endothelial cells.

Shear Stress Gradients Remodel Endothelial Monolayers in Vitro via a Cell Proliferation-Migration-Loss Cycle

Wall shear stress has been implicated in the genesis of atherosclerosis because a strong correlation exists between the location of developing arterial lesions and regions where particular gradients in stress occur. Studying the behavior of endothelial cells in such regions may contribute to our understanding of the disease etiology. We report the detailed migratory history of endothelial cells subjected to large shear stress gradients caused by a surface protuberance in an in vitro model system. The history of cell migration, cell division, and cell loss from the surface was continuously monitored in confluent human umbilical vein endothelial cell monolayers for 48 hours after the onset of flow. Individual cells were tracked using time-lapse video microscopy. In contrast to a uniform laminar flow field in which cells were observed to continually rearrange their relative position with no net migration, in a disturbed flow field there was a net migration directed away from the region of high shear gradient. This organized migration pattern under disturbed flow conditions was accompanied by more than a twofold increase in cell motility. In addition, cell division increased in the vicinity of the flow separation (maximum shear stress gradient of 34 dyne/cm2 per mm) whereas cell loss was increased upstream and downstream in the regions where the shear gradient diminishes. These data suggest a steady cell proliferation-migration-loss cycle and indicate that local shear stress gradient may play a key role in the morphological remodeling of the vascular endothelium in vivo.

Hemodynamics, Endothelial Gene Expression, and Atherogenesisa

Vascular endothelium, the continuous lining of the cardiovascular system, forms a multifunctional interface between circulating blood and the various tissues and organs of the body. It constitutes a selectively permeable barrier for macromolecules, as well as non-thrombogenic container that actively maintains the fluidity of blood. It is a metabolically active tissue, serving as the source of multiple factors (peptides, proteins, lipids) that are critical for normal homeostasis. These include growth stimulators and inhibitors [e.g., platelet-derived growth factor, (PDGF), transforming growth factor-beta (TGF-beta), fibroblast growth factor (FGF), and heparin-like glycosaminoglycans]; vasoconstrictors and vasodilators [e.g., endothelin-1 (ET1 ), angiotensin 11, and endothelial-derived relaxing factors, such as nitric oxide]; the various proand anti-thrombotic factors (e.g., tissue factor, thrombomodulin, and von Willebrand factor); fibrinolytic activators and inhibitors (e.g., tissue plasminogen activator (tPA), urokinase, and plasminogen activator inhibitor1 , (PAIl)]; potent arachidonate metabolites (e.g., prostacyclin and PGI,); leukocyte adhesion molecules (e.g., E-Selectin, P-Selectin, ICAM-I, and VCAM-I) and multiple cytokines (e.g., IL-I, IL-6, IL-8, MCP-I, and GM-CSF). This partial list underscores the functional diversity of the endothelial interface in normal physiology and also illustrates its potential contributions to pathophysiological processes in vascular disease. It has been our laboratorys working concept that the vascular endothelium is a dynamically mutable interface, whose structural and functional properties are respon-

Endothelial Gene Regulation by Laminar Shear Stress

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

Transcriptional and post-translation regulation of the Tie1 receptor by fluid shear stress changes in vascular endothelial cells

The interaction between the vascular endothelium and hemodynamic forces (and more specifically, fluid shear stress), induced by the flow of blood, plays a major role in vascular remodeling and in new blood vessels formation via a process termed arteriogenesis. Tie1 is an orphan tyrosine kinase receptor expressed almost exclusively in endothelial cells and is required for normal vascular development and maintenance. The present study demonstrates that Tie1 expression is rapidly down‐regulated in endothelial cells exposed to shear stress, and more so to shear stress changes. This down‐regulation is accompanied by a rapid cleavage of Tie1 and binding of the cleaved Tie1 45 kDa endodomain to Tie2. The rapid cleavage of Tie1 is followed by a transcriptional down‐regulation in response to shear stress. The activity of the Tie1 promoter is suppressed by shear stress and by tumor necrosis factor α. Shear stress‐induced transcriptional suppression of Tie1 is mediated by a negative shear stress response element, localized in a region of 250 bp within the promoter. The rapid down‐regulation of Tie1 by shear stress changes and its rapid binding to Tie2 may be required for destabilization of endothelial cells in order to initiate the process of vascular restructuring.

Signalling pathways in vascular endothelium activated by shear stress: relevance to atherosclerosis.

Major advances in our understanding of how endothelial cells sense and respond to haemodynamic forces and, more specifically, to fluid shear stress have been achieved during the past 3 years. These include definition of potential shear stress receptors and multiple signalling pathways that mediate shear stress regulation of gene expression. A few studies have also pointed to the unique effects of complex shear stress on endothelial activation, thus leading to better understanding of the mechanisms that lead to the development of atherosclerosis.