Temporal Gradient in Shear But Not Steady Shear Stress Induces PDGF-A and MCP-1 Expression in Endothelial Cells

Abstract

Abstract-Three well-defined laminar flow profiles were created to distinguish the influence of a gradient in shear and steady shear on platelet-derived growth factor A (PDGF-A) and monocyte chemoattractant protein-1 (MCP-1) expression in human endothelial cells. The flow profiles (16 dyne/cm 2 maximum shear stress) were ramp flow (shear stress smoothly transited at flow onset), step flow (shear stress abruptly applied at flow onset), and impulse flow (shear stress abruptly applied for 3 s only). Ramp flow induced only minor expression of PDGF-A and did not increase MCP-1 expression.Step flow increased PDGF-A and MCP-1 mRNA levels 3-and 2-fold at 1.5 hours, respectively, relative to ramp flow. In contrast, impulse flow increased PDGF-A and MCP-1 expression 6-and 7-fold at 1.5 hours, and these high levels were sustained for at least 4 hours. These results indicate that a temporal gradient in shear (impulse flow and the onset of step flow) and steady shear (ramp flow and the steady component of step flow) stimulates and diminishes the expression of PDGF-A and MCP-1, respectively. NO synthase inhibitor N G -amino-L-arginine (L-NAA) was found to markedly enhance MCP-1 and PDGF-A expression induced by step flow, but decrease their expression induced by impulse flow, in a dose-dependent manner. NO donor spermine-NONOate (SPR/NO) dose-dependently reduced the MCP-1 and PDGF-A expression induced by impulse flow. Moreover, impulse flow was found to stimulate sustained (4 hours) IB-␣ degradation and egr-1 mRNA induction. L-NAA prevented IB-␣ degradation, whereas SPR/NO increased IB-␣ resynthesis 2 hours after impulse flow. The exact nature and influence of local shear involved in endothelial dysfunction leading to susceptibility for atherogenesis, however, remains unclear. Endothelial cells (EC) throughout the vasculature experience a variety of flow environments both with spatial variances and with temporal gradients in wall shear stress. In the venous system wall shear stress is lower and minimal gradients in shear stress exist because of the nonpulsatile nature of the blood flow. In the arterial system the flow conditions are generally assumed to be laminar and to present the endothelium with a high mean wall shear stress in addition to large temporal gradients in shear stress. At arterial bifurcations and curvatures, locations known to be highly prone to atherogenesis, disturbed flow patterns may develop that result in low mean wall shear stress, but still present the EC with large temporal gradients in shear stress. 3 These observations, combined with other in vitro evidence, 4 -6 suggest that gradients in shear and steady shear represent different biomechanical stimuli that differentially regulate local endothelial function by distinct signaling pathway, and thus contribute to the characteristic distribution pattern of atherosclerosis.A host of endothelial genes exhibit differential responses to shear stress stimuli that may be involved in the focal localization of atherogenic plaques. 7 The application of step shear...

Endothelial KLF2 Links Local Arterial Shear Stress Levels to the Expression of Vascular Tone-Regulating Genes

“…2 These changes in gene expression result from modulation of either the production or the activity of transcription factors. [32][33][34] By stably overexpressing LKLF in cultured endothelial cells we now show that a distinct group of known shear-stress regulated genes involved in controlling vascular tone, ACE, EDN, ADM, and NOS3, can be transcriptionally regulated by LKLF. Furthermore, knockdown of LKLF using RNA interference revealed that the response of these genes to flow is primarily dependent on LKLF, with the exception of ACE.…”

Use of Ultrasound for Non-Invasive Assessment of Flow-Mediated Dilation

“…While human in vivo studies typically describe shear stress as a mean construct, numerous secondary phenomena associated with flow, including pulsatile flow, retrograde flow, and flow turbulence, can influence the regulation of endothelial cells [67][68][69][70][71][72] .…”

“…In vitro studies suggest that these two distinct fluid stimuli (velocity acceleration vs. steady shear) regulate short-and longterm endothelial function via independent biomechanical pathways 70,71,[73][74][75] . Studies have shown that the rate of velocity acceleration can affect the progression of atherosclerosis 70,73,76) , endothelial cell function 71 , 74) , mechanotransduction 77,78) , calcium kinetics 79,80) , and vascular tone 81,82) . Conditions which affect velocity acceleration include ventricular ischemia 83) , acute myocardial infarction 84) , stenosis 85) , hypertension 86) , and hyperthyroidism 87) .…”

Assessment of Endothelial Function Using Ultrasound

“…6). In vitro studies suggest that these two distinct fluid stimuli (velocity acceleration vs. steady shear) regulate short-and long-term endothelial function via independent biomechanical pathways (Ojha 1994;Bao & Lu et al 1999;White & Haidekker et al 2001;Hsiai & Cho et al 2002DePaola, 1992 (Ojha 1994;Bao & Lu et al 1999;Hsiai & Cho et al 2001), endothelial cell function (White & Haidekker et al 2001 ;Hsiai & Cho et al 2002), mechanotransduction (Hsieh & Li et al 1993;Bao & Clark et al 2000), calcium kinetics (Helmlinger & Berk et al 1995;Blackman & Barbee et al 2000), and vascular tone (Frangos & Eskin et al 1985;Noris & Morigi et al 1995 ). Conditions which affect velocity acceleration include ventricular ischemia (Sabbah & Przybylski et al 1987), acute myocardial infarction (Kezdi & Stanley et al 1969), stenosis (Bassini & Gatti et al 1982), hypertension (Sainz & Cabau et al 1995), and hyperthyroidism (Chemla & Levenson et al 1990).…”