Endothelial Cell Mechanical Responses Are Dependent on Both Fluid Shear Stress and Tensile Strain

Abstract

Vascular endothelial cells are sensitive to their mechanical environment. They are regularly subject to both fluid shear stress as a result of blood flow, as well as cyclic tensile (or compression) strain due to blood vessel dilation (or contraction). The purpose of this study was to characterize physiological responses and morphological changes occurring in vascular endothelial cells when they were exposed to altered mechanical loadings, i.e., fluid shear stress and/or tensile strain. Human coronary artery endothelial cells (HCAEC) were grown to confluence in a 6‐well Flexcell plate coated with fibronectin. Cells were then placed in a programmable shearing and stretching device, where they were exposed to dynamic shear stress and cyclic tensile strain simultaneously at physiological or pathological levels. Three shear stress‐tensile strain conditions were used: 1) shear stress at 1 Pa and cyclic tensile strain at 7%, mimicking normal mechanical loading conditions in a healthy coronary artery; 2) shear stress at 3.7 Pa and tensile strain at 3%, mimicking pathological shear stress/strain conditions occurring at the stenosis (60%) throat in a diseased coronary artery; 3) shear stress at 0.7 Pa and tensile strain at 5%, mimicking pathological shear stress/strain conditions in the recirculation zones downstream of a stenosis. Following 1‐hour shear stress‐strain treatment, cells were fixed and cytoskeletal F‐actin filaments were stained using FITC conjugated phalloidin. Cell morphology was visualized using immunofluorescence microscope; cell area and elongation (ratio of long axis over short axis) were quantified using the Image J software. Under all conditions, cells that were exposed to shear stress and cyclic tensile strain concurrently were much larger compared to untreated cells or cells that were exposed to either shear stress or tensile strain alone. Stenosis conditions caused a significant increase in cell area compared to the other two conditions (normal and recirculation). In contrast, cell elongation decreased significantly under concurrent shear stress and tensile strain, with the lowest values observed under recirculation conditions. PECAM‐1 phosphorylation, an indicator of mechanotransduction pathway activation, was also measured, using a solid phase ELISA approach. The results demonstrated that the simultaneous stimulation from shear stress and tensile strain induced significant changes in endothelial cell surface phosphorylated PECAM‐1 expression, compared to when cells were exposed to only shear stress or tensile strain. These findings indicated the complex interplay between altered flow‐induced shear stress and tensile strain could significantly affect vascular endothelial cell responses. Therefore, to better understand how mechanical conditions affect endothelial cell mechanotransduction and functions, both shear stress and tensile strain need to be considered.