Hypoxia-Induced Hyperpermeability in Brain Microvessel Endothelial Cells Involves VEGF-Mediated Changes in the Expression of Zonula Occludens-1

Abstract

In vivo, hypoxia is known to damage the blood–brain barrier (BBB) leading to the development of vasogenic brain edema. Primary cultures of porcine brain derived microvascular endothelial cells were used as an in vitro BBB model to evaluate the mechanisms by which hypoxia regulates paracellular permeability. Paracellular passage across endothelial cell monolayers is regulated by specialized intercellular structures like the tight junctions (TJ). Zonula occludens-1 (ZO-1), a protein of the TJ, lines the cytoplasmic face of intact TJ. The continuity of the ZO-1 expression was disrupted during 24 h of hypoxia which correlated with a decrease of the protein level to 32 ± 8% and with a twofold increase in the phosphorylation of ZO-1 in comparison to values determined at the start of the experiment. The localization and expression level of ZO-1 were maintained during hypoxia in the presence of a polyclonal antibody to vascular endothelial growth factor (VEGF) demonstrating that hypoxia-induced changes of the ZO-1 expression are mediated by VEGF. The effect of hypoxia on the ZO-1 distribution probably is not tissue- or cell-specific because similar changes of ZO-1 distribution were observed when the rat brain endothelial cell line RBE4 or the murine epithelial cell line CSG was used. Furthermore, ZO-1 changes correlated with small changes in actin distribution. These results suggest that hypoxia increases the paracellular flux across the cell monolayer via the release of VEGF, which in turn leads to the dislocalization, decreased expression, and enhanced phosphorylation of ZO-1.