No abstract

While vascular endothelial cells are normally exposed to shear stress, there is no flow and consequently reduced shear stress in capillary sprouts. To explore, if this lack in mechano‐stimulation alters the endothelial phenotype, we analysed the influence of shear stress on the activity of transcription factor FOXO1 and the expression of its target gene angiopoietin‐2 (ang2), an antagonist of the endothelial tyrosine kinase receptor tie2.Endothelial cells were exposed to shear stress using a cone‐and‐plate system with or without inhibition of PI3K (using LY294,002) or FOXO1 (using siRNA) and analysed by real time RT‐PCR, Western blots, immunoprecipitation, and fluorescence microscopy.Shear stress (6°dyn/cm2) increased phosphorylation of Akt and FOXO1 in a PI3K‐dependent manner and FOXO1 was translocated out of the nucleus. In addition, expression of FOXO1 on the mRNA‐ and protein level was strongly reduced by shear stress. Suppression of FOXO1 by shear stress was accompanied by a decrease of ang2‐mRNA and ‐protein, which was similarly be achieved by siRNA against FOXO1. Following flow stop, ang2‐mRNA expression increased within two hours to steady state values under static conditions.So, in capillary sprouts, lacking shear stress activates endothelial FOXO1 by which ang2 is rapidly induced. Thus, the tie2‐system will be inactivated during capillary sprouting, which may turn the vessel wall more susceptible to additional angiogenic factors. By contrast, the onset of flow will activate the tie2‐system and thereby contribute to stabilize the vessel wall.