Neurovascular unit and the effects of dosage in VEGF toxicity: role for oxidative stress and thrombin.

Bidirectional communication between neurons and vascular cells is important to the maintenance of the central nervous system (CNS) milieu. Vascular endothelial growth factor (VEGF), through its ability to affect both vascular and neuronal cells, is likely a key protein in this process. Despite considerable literature documenting a neuroprotective function for VEGF, overexpression of this protein has also been shown in a wide variety of CNS diseases, including Alzheimer's disease (AD). Increased oxidative stress and elevated thrombin levels have also been documented in AD, specifically in the microvasculature. The aim of the current study is to examine endothelial cells and neurons in vitro to determine the effects of oxidative stress and thrombin on VEGF release as well as the effects of low and high dose VEGF on neuronal viability. The data show that microvessels isolated from AD patients secrete significantly higher levels of VEGF compared to control-derived vessels. Exposure of brain endothelial cells to oxidative stress (sodium nitroprusside, menadione, or hydrogen peroxide) or thrombin significantly increases VEGF expression. Exposure of cultured neurons to oxidative stress increases expression of thrombin. Treating rat cortical neurons with high dose VEGF (≥500 ng/ml) decreases neuronal survival and expression of the anti-apoptotic protein Bcl-2 while increasing proapoptic proteins caspase 3 and phosphorylated p38 MAPK. High dose VEGF also negates the decrease in amyloid-β evoked by low dose VEGF. These results suggest that despite literature supporting neuroprotective effects of this protein, caution is warranted prior to implementation of VEGF as a therapeutic in the brain.