How increased VEGF induces glomerular hyperpermeability: a potential signaling pathway of Rac1 activation

Despite growing evidence for a pathogenic role of vascular endothelial growth factor (VEGF) in microvascular complications of diabetes, the underlying mechanism responsible for its detrimental effect remains unknown. In the current study, we hypothesized that some of the detrimental effects of VEGF on microvascular endothelial cells in the diabetic milieu stem from its aberrant signaling, which leads to perturbed tight junction assembly and increased endothelial permeability. Using an integrated in vitro approach, we investigated whether the effect of VEGF on endothelial cell permeability involves Rac1 GTPase activation and tight junction disassembly. Rac1 activity was detected by Western blotting in cell membrane protein as well as pull-down assay. The permeability of glomerular endothelial cells monolayer was detected as transendothelial electronic resistance. Then tyrosine phosphorylated occludin protein was detected by Western blotting after immunoprecipitation. N17Rac1 cells are obtained by transfection of glomerular endothelial cells with a dominant negative mutant of Rac1. The data obtained in this study indicate that activation of Rac1 GTPase contributes to VEGF-induced endothelial cell hyperpermeability. We also observed that Rac1 activation leads to increased endothelial permeability through tyrosine phosphorylation of occludin. Indeed, N17Rac1 cells dramatically attenuated the effect of VEGF on phospho-occludin and endothelial cell permeability. These results, when taken together, provide a framework for understanding the role of VEGF-induced Rac1/phospho-occludin pathway in the integrity of endothelial barrier function in the glomerulus.