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Transfection with APJ siRNA lead to a loss of shear stress dependent eNOS up-regulation and adhesion.Ĭonclusion: Our results show for the first time a shear stress regulated adjustment of the Apelin/APJ system in human endothelial cells depending on arterial or venous flow conditions. In order to demonstrate shear stress dependency of the apelin/APJ system, we transfected HUVEC with APJ siRNA and exposed them to our standard flow conditions. Incubation with apelin-12 for up to 15 min lead to a significant increase in PI3 kinase and Akt phosphorylation under static and dynamic conditions (p=0.02), whereas incubation with apelin-13 had no significant influence on PI3k/Akt signalling and lead to phosphorylation of Erk 1/2 instead. Western Blot Analysis underlined apelin isoform dependent cellular signaling. Incubation of HUVEC with apelin-12 and -13 revealed a dose-dependent increase of NO production only after incubation with apelin-12 (P<0.01). Furthermore, in ECs APJ gene and protein expression increased after exposure to shear stress (p<0.05), which could be confirmed by IF. Interestingly, HCAEC showed an opposite behaviour (p<0.001). In HUVEC, expression of apelin was increased by 40% under low shear stress, whereas under high shear stress conditions apelin expression decreased by 50% (p<0.01). Results: After application of shear stress, apelin gene and protein expression were regulated in ECs only. Additionally, HUVEC were transfected with APJ siRNA. The supernatant was collected for nitrite measurements. After shear stress exposure cells were harvested and prepared for real-time qPCR and Western Blot analysis or fixed and stained for anti-APJ (IF). Static controls were kept under similar conditions. Cells were stimulated with apelin-12 and apelin-13. Methods: We applied hemodynamic shear stress (1.5 dyne/cm 2 for venous, 20 dyne/cm 2 for arterial conditions) on HUVEC, HCAEC, and HCASMC using a parallel plate flow chamber. We suppose that the apelin/APJ system is involved in the transduction of mechanical forces into intracellular signaling. Application of fluid shear stress to endothelial cells elicits the formation of NO via PI3K/AKT signaling and phosphorylation of the endothelial NO synthase (eNOS). Namely, the isoforms apelin-12 and apelin-13 seem to be most active in the cardio vasculature. Introduction: The apelin/APJ system is being considered to play a crucial role in cardiovascular function.