An endothelial TLR4-VEGFR2 pathway mediates lung protection against oxidant-induced injury

Abstract

TLR4 deficiency causes hypersusceptibility to oxidant-induced injury. We investigated the role of TLR4 in lung protection, using used bone marrow chimeras; cellspecific transgenic modeling; and lentiviral delivery in vivo to knock down or express TLR4 in various lung compartments; and lung-specific VEGF transgenic mice to investigate the effect of TLR4 on VEGF-mediated protection. C57/BL6 mice were exposed to 100% oxygen in anenclosed chamber and assessed for survival and lung injury. Primary endothelial cells were stimulated with recombinant VEGF and exposed to hyperoxia or hydrogen peroxide. Endotheliumspecific expression of human TLR4 (as opposed to its expression in epithelium or immune cells) increased the survival of TLR4-deficent mice in hyperoxia by 24 h and decreased LDH release and lung cell apoptosis after 72 h of exposure by 30%. TLR4 expression was necessary and sufficient for the protective effect of VEGF in the lungs and in primary endothelial cells in culture. TLR4 knockdown in hibited VEGF signaling throughVEGF receptor 2 (VEGFR2), Akt, and ERK pathways in lungs and primary endothelial cells and decreased the availability of VEGFR2 at the cell surface. These findings demonstrate a novel mechanism through which TLR4, an innate pattern receptor, interacts with an endothelial survival pathway.