Regulation of the endothelial apelin/APJ system by hemodynamic fluid flow

Although the apelin/APJ system is abundantly expressed in vascular endothelial cells (EC), it has not yet been considered to be regulated by fluid flow. The aim of this study was to explore the influence of shear stress on the expression of apelin/APJ in human EC. Therefore, gene and protein expression were assessed after flow exposure; cell supernatants were collected for measurements of NO and apelin; APJ or apelin knockdown were performed using siRNA. Our data show that gene and protein expression of apelin and APJ are modulated by fluid flow depending on the magnitude of shear stress. Moreover, apelin-12 activated NO production via PI3K/Akt signaling in human EC. In contrast, apelin-13 additionally activated Erk1/2 phosphorylation and enhanced EC proliferation. Knockdown of APJ inhibited phosphorylation of PI3K and impaired flow-induced eNOS and PECAM-1 expression. Knockdown of apelin had no influence on flow-induced APJ and PECAM-1 expression, but derogated eNOS expression under static and flow conditions. The present study reveals a flow-mediated adjustment of the apelin/APJ system in human EC in which APJ expression is induced by shear stress independently of its ligand. Furthermore, apelin-12 signaling is an essential regulatory element in endothelial NO synthesis.