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Signaling mechanism of extracellular RNA in endothelial cells

Silvia Fischer^*, Miwako Nishio^*, Saskia C. Peters, Marlene Tschernatsch, Maureen Walberer, Susanne Weidemann^||, Regina Heidenreich^||, Pierre O. Couraud^¶, Babette B. Weksler^#, Ignacio A. Romero^**, Tibo Gerriets and Klaus T. Preissner^*,1

^* Department of Biochemistry, Medical School,

Department of Animal Welfare and Ethology, and

Department of Experimental Neurology, Medical School, Justus-Liebig-University, Giessen, Germany;

Department of Neurology, University of Cologne, Cologne, Germany;

^|| Department of Dermatology, Eberhard-Karls-University, Tübingen, Germany;

^¶ Institut Cochin, Université Paris Descartes, Paris, France;

^# Weill Cornell Medical College, New York, New York, USA; and

^** Open University, Milton Keynes, UK

¹ Correspondence: Department of Biochemistry, Medical School Justus-Liebig-Universität, Friedrichstrasse 24, D-35392 Giessen, Germany. E-mail: klaus.t.preissner{at}biochemie.med.uni-giessen.de

Extracellular RNA has been shown to induce vascular endothelial growth factor (VEGF)-dependent hyperpermeability in vivo as well as in vitro. Studies were performed to investigate the mechanism of these effects. For permeability studies primary cultures of porcine brain-derived microvascular endothelial cells (BMECs) and for all other analytical studies the human brain endothelial cell line HCMEC/D3 or human umbilical vein endothelial cells (HUVECs) were used. RNA, but not DNA, initiated signaling events by binding of VEGF to neuropilin-1, followed by VEGF-R2 phosphorylation, activation of phospholipase C (PLC), and intracellular release of Ca²⁺. Activation of these pathways by RNA also resulted in the release of von Willebrand Factor from Weibel-Palade bodies. Pretreatment of cells with heparinase totally abrogated the RNA-induced permeability changes, whereas RNA together with VEGF completely restored VEGF-R2-mediated hyperpermeability. Although poly:IC increased the interleukin-6 release via activation of toll-like receptor-3 (TLR-3), permeability changes mediated by poly:IC or RNA remained unchanged after blocking TLR-3 or NF-kB activation. These results indicate that extracellular RNA serves an important cofactor function to engage VEGF for VEGF-R2-dependent signal transduction, reminiscent of the coreceptor mechanism mediated by proteoglycans, which might be of relevance for the mobilization and cellular activities of RNA-binding cytokines in general.—Fischer, S., Nishio, M., Peters, S. C., Tschernatsch, M., Walberer, M., Weidemann, S., Heidenreich, R., Couraud, P. O., Weksler, B. B., Romero, I. A., Gerriets, T., Preissner, K. T. Signaling mechanism of extracellular RNA in endothelial cells.

Key Words: VEGF-receptor • neuropilin-1