Vascular permeability induced by VEGF family members in vivo: Role of endogenous PAF and NO synthesis

We previously reported that vascular endothelial growth factor (VEGF) increases vascular permeability through the synthesis of endothelial platelet-activating factor (PAF), while others reported the contribution of nitric oxide (NO). Herein, we addressed the contribution of VEGF receptors and the role played by PAF and NO in VEGF-induced plasma protein extravasation. Using a modified Miles assay, intradermal injection in mice ears of VEGF-A(165), VEGF-A(121), and VEGF-C (1 mu M) which activate VEGFR-2 (Flk-1) receptor increased vascular permeability, whereas a treatment with VEGFR-1 (Flt-1) analogs; PI6F and VEGF-B (1 mu M) had no such effect. Pretreatment of mice with PAF receptor antagonist (LAU8080) or endothelial nitric oxide synthase (eNOS) inhibitor (L-NAME) abrogated protein extravasation mediated by VEGF-A165. As opposed to PAF (0.01-1 mu M), treatment with acetylcholine (ACh; up to 100 mu M; inducer of NO synthesis) or sodium nitroprusside (SNP; up to 1 mu M; NO donor) did not induce protein leakage. Simultaneous pretreatment of mice with eNOS and protein kinase A (PKA) inhibitors restored VEGF-A165 vascular hyperpermeability suggesting that endogenous NO synthesis leads to PKA inhibition, which support maintenance of vascular integrity. Our data demonstrate that VEGF analogs increase vascular permeability through VEGFR-2 activation, and that both endogenous PAF and NO synthesis contribute to VEGF-A(165)-mediated vascular permeability. However, PAF but not NO directly increases vascular permeability per se, thereby, suggesting that PAF is a direct inflammatory mediator, whereas NO serves as a cofactor in VEGF-A(165) proinflammatory activities.