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

Stress relaxation of fibroblasts activates a cyclic AMP signaling pathway

Mechanical force regulates gene expression and cell proliferation in a variety of cell types, but the mechanotransducers and signaling mechanisms involved are highly speculative. We studied the fibroblast signaling mechanism that is activated when cells are switched from mechanically stressed to mechanically relaxed conditions, i.e., stress relaxation. Within 10 min after initiation of stress relaxation, we observed a transient 10-20-fold increase in cytoplasmic cyclic AMP (cAMP) and a threefold increase in protein kinase A activity. The increase in cAMP depended on stimulation of adenylyl cyclase rather than inhibition of phosphodiesterase. Generation of cAMP was inhibited by indomethacin, and release of arachidonic acid was found to be an upstream step of the pathway. Activation of signaling also depended on influx of extracellular Ca2+ because addition of EGTA to the incubations at concentrations just sufficient to exceed Ca2+ in the medium inhibited the stress relaxation-dependent increase in free arachidonic acid and cAMP. This inhibition was overcome by adding CaCl2 to the medium. On the other hand, treating fibroblasts in mechanically stressed cultures with the calcium ionophore A23187-stimulated arachidonic acid and cAMP production even without stress relaxation. In summary, our results show that fibroblast stress relaxation results in activation of a Ca(2+)-dependent, adenylyl cyclase signaling pathway. Overall, the effect of stress relaxation on cAMP and PKA levels was equivalent to that observed after treatment of cells with forskolin.

New perspectives in hypertension research. Potentials of vascular biology

The vessel wall was once considered to be a passive conduit responding to the circulating endocrine system. However, the emergence of molecular and vascular biology in hypertension research has redefined our understanding of the role of the vasculature as a vital organ in the pathogenesis of hypertension. It is now recognized that the vasculature can regulate its own tone by a variety of previously unknown autocrine and/or paracrine vasoactive systems. Recent evidence indicates that the process of vascular remodeling in hypertension appears to be mediated by locally generated factors within the vessel wall. This review examines the implications of this new paradigm in hypertension, focusing on five topics that have developed through the emergence of molecular vascular biology: the discovery and characterization of novel biologically active molecules synthesized by the vessel wall, the molecular mechanisms and consequences of vascular remodeling, the developmental biology of the blood vessel and the relation to pathobiology, the use of in vivo gene transfer to test hypotheses in vivo, and novel treatment strategies based on gene therapy of the vessel wall.

Fluid shear stress induces a biphasic response of human monocyte chemotactic protein 1 gene expression in vascular endothelium

The focal distribution of atherosclerotic lesions in the arterial tree is related to the local shear stress generated by blood flow, but the molecular basis of the atherogenic response of endothelial cells in these lesion-prone areas is still unclear. We report that shear stress mediates a biphasic response of monocyte chemotactic protein 1 (MCP-1) gene expression in vascular endothelial cells (EC). Northern blot analysis indicated that the level of MCP-1 mRNA in human umbilical vein EC (HUVEC) subjected to a shear stress of 16 dynes/cm2 (1 dyne = 10 microN) for 1.5 hr increased by 2- to 3-fold when compared with static cells. The MCP-1 gene expression decreased to the basal level at 4 hr and then declined further to become completely quiescent at 5 hr after the onset of shear. Once the gene expression was fully suppressed, it remained quiescent even after static incubation for 1.5 hr and would not respond to reshearing after this static incubation. However, if the postshearing incubation extended from 1.5 to 24 hr, the MCP-1 mRNA returned to the basal level and was then able to increase after the reapplication of shear stress. Nuclear run-on experiments showed that the shear-induced increased MCP-1 mRNA in HUVEC was regulated at the transcriptional level. By using cycloheximide, it was shown that de novo protein synthesis was not necessary for the induction of MCP-1 by shear stress. The biphasic response of MCP-1 gene expression was found in experiments in which the applied shear stress was 6, 16, or 32 dynes/cm2, and it was observed not only in HUVEC but also in HeLa cells, glioma cell lines, and skin fibroblasts. This in vitro study demonstrates that the response of MCP-1 gene to shear stress represents an immediate early gene activation and suggests that this gene is probably suppressed in EC that have been exposed to a constant shear stress.

Quantitative studies of endothelial cell adhesion. Directional remodeling of focal adhesion sites in response to flow forces

Focal adhesion sites were observed in cultured endothelial cells by tandem scanning confocal microscopy and digitized image analysis, techniques that provide real-time images of adhesion site area and topography in living cells. Image subtraction demonstrated that in the presence of unidirectional steady laminar flow (shear stress [tau] = 10 dyn/cm2) a substantial fraction of focal adhesion sites remodeled in the direction of flow. In contrast, focal adhesions of control (no flow) cells remodeled without preferred direction. In confluent monolayers subjected to shear stresses of 10 dyn/cm2, cells began to realign in the direction of flow after 7-9 h. This was accompanied by redistribution of intracellular stress fibers, alignment of individual focal adhesion sites, and the coalescence of smaller sites resulting in fewer, but larger, focal adhesions per cell. Cell adhesion, repeatedly calculated in the same cells as a function of the areas of focal contact and the separation distances between membrane and substratum, varied by < 10% during both short (30 min), or prolonged (< or = 24 h), periods of exposure to flow. Consistent with these measurements, the gains and losses of focal adhesion area as each site remodeled were approximately equivalent. When the glass substratum was coated with gelatin, rates of remodeling were inhibited by 47% during flow (tau = 10 dyn/cm2). These studies: (a) reveal the dynamic nature of focal adhesion; (b) demonstrate that these sites at the ablumenal endothelial membrane are both acutely and chronically responsive to frictional shear stress forces applied to the opposite (lumenal) cell surface; and (c) suggest that components of the focal adhesion complex may be mechanically responsive elements coupled to the cytoskeleton.

Chronic exercise in dogs increases coronary vascular nitric oxide production and endothelial cell nitric oxide synthase gene expression

Recently, we have shown that chronic exercise increases endothelium-derived relaxing factor (EDRF)/nitric oxide (NO)-mediated epicardial coronary artery dilation in response to brief occlusion and acetylcholine. This finding suggests that exercise can provide a stimulus for the enhanced production of EDRF/NO, thus possibly contributing to the beneficial effects of exercise on the cardiovascular system. Therefore, the purpose of the present study was to examine whether chronic exercise could influence the production of NO (measured as the stable degradation product, nitrite) and endothelial cell NO synthase (ECNOS) gene expression in vessels from dogs after chronic exercise. To this end, dogs were exercised by running on a treadmill (9.5 km/h for 1 hour, twice daily) for 10 days, and nitrite production in large coronary vessels and microvessels and ECNOS gene expression in aortic endothelial extracts were assessed. Acetylcholine (10(-7) to 10(-5) mol/L) dose-dependently increased the release of nitrite (inhibited by nitro-L-arginine) from coronary arteries and microvessels in control and exercised dogs. Moreover, acetylcholine-stimulated nitrite production was markedly enhanced in large coronary arteries and microvessels prepared from hearts of dogs after chronic exercise compared with hearts from control dogs. One potential mechanism that may contribute to the enhanced production of nitrite in vessels from exercised dogs may be the induction of the calcium-dependent ECNOS gene. Steady-state mRNA levels for ECNOS were significantly higher than mRNA levels for von Willebrand's factor (vWF, a specific endothelial cell marker) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH, a constitutively expressed gene) in exercised dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Lysophosphatidylcholine transcriptionally induces growth factor gene expression in cultured human endothelial cells

Lysophosphatidylcholine (lyso-PC) is a major phospholipid component of atherogenic lipoproteins (e.g., oxidized LDL and beta-VLDL) and also can be generated through the action of leukocyte-secreted phospholipase A2 at sites of inflammation. We have previously reported that lyso-PC can activate cultured endothelia, resulting in the selective upregulation of adhesion molecules, such as vascular cell adhesion molecule-1 and intercellular adhesion molecule-1. In this study, we have found that lyso-PC increased steady state mRNA levels for two smooth muscle/fibroblast-directed growth factors, the A and B chains of PDGF and heparin-binding EGF-like protein (HB-EGF), in cultured human endothelial cells. Lyso-PC did not upregulate the expression of certain other inducible endothelial genes, including E-selectin, IL-8, or monocyte chemoattractant protein-1 in the same cells, in contrast to the coordinate pattern of activation typically observed with other stimuli, such as TNF alpha, bacterial endotoxin, or PMA. Nuclear runoff assays documented an increased transcriptional rate for the HB-EGF gene in lyso-PC-treated cells. Northern blot analyses, after actinomycin D treatment, further indicated that the increased amounts of mRNA for HB-EGF, PDGF A and B chains, and intercellular adhesion molecule-1 were not dependent upon message stabilization. We conclude that lyso-PC can induce growth factor gene expression in cultured endothelial cells and thus may contribute to the migration and proliferation of smooth muscle cells and fibroblasts in various response-to-injury settings in vivo.

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.

Shear stress-induced reorganization of the surface topography of living endothelial cells imaged by atomic force microscopy

We report the first topographical data of the surface of living endothelial cells at sub-light-microscopic resolution, measurements essential for a detailed understanding of force distribution in the endothelium subjected to flow. Atomic force microscopy was used to observe the surface topography of living endothelial cells in confluent monolayers maintained in static culture or subjected to unidirectional shear stress in laminar flow (12 dyne/cm2 for 24 hours). The surface of polygonal unsheared cells was smooth, with mean excursion of surface undulation between peak height (over the nucleus) and minima (at intercellular junctions) of 3.4 +/- 0.7 microns (mean +/- SD); the mean height to length ratio was 0.11 +/- 0.02. In cells that were aligned in the direction of flow after a 24-hour exposure to laminar shear stress, height differentials were significantly reduced (mean, 1.8 +/- 0.5 micron), and the mean height to length ratio was 0.045 +/- 0.009. Calculation of maximum shear stress and maximum gradient of shear stress (delta tau/delta x, where tau is shear stress at the cell surface) at constant flow velocity revealed substantial streamling of aligned cells that reduced delta tau/delta x by more than 50% at a nominal shear stress of 10 dyne/cm2. Aligned cells exhibited ridges extending in the direction of flow that represented imprints of submembranous F-actin stress-fiber bundles mechanically coupled to the cell membrane. The surface ridges, approximately 50 nm in height and 200 to 1000 nm in width, were particularly prominent in the periphery of the aligned cells.(ABSTRACT TRUNCATED AT 250 WORDS)