Plasma levels of endothelial microparticles (EMPs) are markers of cardiovascular diseases and contribute to the pathogenesis of atherosclerosis. Laminar shear stress (SS) protects against plaque formation contrarily to oscillatory and low SS. We thus investigated whether different flow patterns (laminar, low or static) could affect EMP release.

HUVECs were subjected to high and low SS (15, 1.5 dynes /cm²) or no SS (static) for 24hours. EMPs were isolated from the culture medium and characterized by annexin V labeling using flow cytometry analysis. HUVECs exposed to high SS for 24hours emitted 4 fold less AnnexinV+ EMPs compared to static (p<0.001) and 3 fold less compared to low SS (p<0.01) conditions. This flowdependent EMP shedding was associated with the presence of 1.7 %±0.05, 3.75 %±0.1, 0.4 %±0.06 of TUNEL positive HUVECs in static, low and high conditions, respectively. Treatment of HUVECs with L-NAME (10-4M) significantly increased EMPs in all conditions when compared to untreated cells. Similar EMPs increase was obtained when cells were treated with KT5720, a PKA inhibitor (10-6M) under low and high flow. We further characterized EMPs by determining their surface expression of ICAM-1. ICAM-1 expression on EMPs was significantly increased in low SS and inhibited by high SS when compared to static conditions. Adhesion assays with EMPs-stimulated HUVECs (24hours) increased U937 cells capacity to attach in low SS-EMPs exposed to HUVECs compared to static and high SS EMPs stimulations. This effect was abolished with LFA-pretreated U937 before adhesion suggesting the pro-adhesive properties of low SS-EMPs bearing ICAM-1 at their surface.

Altogether, these findings indicate that high SS decreased the thrombotic and adhesive properties of EMPs, which might in part explain their anti-atherogenic effects, whereas low SS induced the shedding of prothrombotic, ICAM-1 positive EMPs, suggesting a novel way by which low SS might affect atherosclerosis.