Vascular Endothelial Growth Factor Increases Permeability of the Blood-Tumor Barrier via Caveolae-Mediated Transcellular Pathway

The first goal of this study was to determine the effect of vascular endothelial growth factor (VEGF) on permeability of the blood-tumor barrier (BTB). The second goal was to determine possible cellular mechanisms by which VEGF increases permeability of the BTB. In the rat C6 glioma model, the permeability of the BTB was significantly increased after VEGF injection at dose of 0.05 ng/g and reached its peak at 45 min. Meanwhile, we observed that the density of pinocytotic vesicles of brain microvascular endothelial cells (BMECs) in the BTB increased dramatically by transmission electron microscopy. The immunohistochemistry and western blot analysis revealed that the expression level of caveolae structure proteins caveolin-1 and caveolin-2 in BMECs was increased after VEGF injection, peaked at 45 min, and then decreased to the untreated level. The time peak of expression level of caveolin-1 and caveolin-2 was identical with the peak time of permeability of the BTB and the density of pinocytotic vesicles. All of these results strongly indicated that VEGF increased permeability of the BTB caused by enhancement of the density of pinocytotic vesicles, and the molecular mechanism might be associated with upregulated expression of caveolin-1 and caveolin-2.