Vascular endothelial growth factor up-regulates ICAM-1 expression via the phosphatidylinositol 3 OH-kinase/AKT/nitric oxide pathway and modulates migration of brain microvascular endothelial cells

Endothelium of the cerebral blood microvessels, which constitutes the major component of the blood-brain barrier, controls leukocyte and metastatic ca ncer cell adhesion and trafficking into the brain parenchyma. In this study , using rat primary brain microvascular endothelial cells (BMEC), we demons trate that the vascular endothelial growth factor (VEGF), a potent promoter of angiogenesis, up-regulates the expression of the intracellular adhesion molecule-1 (ICAM-1) through a novel pathway that includes phosphatidylinos itol 3 OH-kinase (PI3K), AKT, and nitric oxide (NO), resulting in the migra tion of BMEC. Upon VEGF treatment, AKT is phosphorylated in a PI3K-dependen t manner. AKT activation leads to NO production and release and activation- deficient AKT attenuates NO production stimulated by VEGF. Transfection of the constitutive myr-AKT construct significantly increased basal NO release in BMEC. In these cells, VEGF and the endothelium-derived NO synergistical ly up-regulated the expression of ICAM-1, which was mediated by the PI3K pa thway. This activity was blocked by the PI3K-specific inhibitor, wortmannin , Furthermore, VEGF and NO significantly increased BmEC migration, which wa s mediated by the up-regulation of ICAM-1 expression and was dependent on t he integrity of the PI3K/AKT/NO pathway. This effect was abolished by wortm annin, by the specific ICAM-1 antibody, by the specific inhibitor of NO syn thase, N-G-L-monomethyl-arginine (L-NMMA) or by a combination of wortmannin , ICAM-1 antibody, and L-NMMA. These findings demonstrate that the angiogen ic factor VEGF up-regulates ICAM-1 expression and signals to ICAM-1 as an e ffector molecule through the PI3K/AKT/NO pathway, which leads to brain micr ovessel endothelial cell, migration. These observations may contribute to a better understanding of BMEC angiogenesis and the physiological as well as pathophysiological function of the blood-brain barrier, whose integrity is crucial for normal brain function.