Elevated Vascular Endothelial Growth Factor Receptor-2 Abundance Contributes to Increased Angiogenesis in Vascular Endothelial Growth Factor Receptor-1–Deficient Mice

Background— Vascular endothelial growth factor receptor-1 (VEGFR-1/Flt-1) is a potential therapeutic target for cardiovascular diseases, but its role in angiogenesis remains controversial. Whereas germlineVegfr-1 −/− embryos die of abnormal vascular development in association with excessive endothelial differentiation, mice lacking only the kinase domain appear healthy.Methods and Results— We performed Cre-lox P–mediated knockout to abrogate the expression of all known VEGFR-1 functional domains in neonatal and adult mice and analyzed developmental, pathophysiological, and molecular consequences. VEGFR-1 deficiency promoted tip cell formation and endothelial cell proliferation and facilitated angiogenesis of blood vessels that matured and perfused properly. Vascular permeability was normal at the basal level but elevated in response to high doses of exogenous VEGF-A. In the postinfarct ischemic cardiomyopathy model, VEGFR-1 deficiency supported robust angiogenesis and protected against myocardial infarction. VEGFR-1 knockout led to abundant accumulation of VEGFR-2 at the protein level, increased VEGFR-2 tyrosine phosphorylation transiently, and enhanced serine phosphorylation of Akt and ERK. Interestingly, increased angiogenesis, tip cell formation, vascular permeability, VEGFR-2 accumulation, and Akt phosphorylation could be partially rescued or suppressed by one or more of the following manipulations, including injection of the VEGFR-2 selective inhibitor SU1498, anti-VEGF-A, or introduction ofVegfr-2 +/− heterozygosity intoVegfr-1 somatic knockout mice.Conclusions— Upregulation of VEGFR-2 abundance at the protein level contributes in part to increased angiogenesis in VEGFR-1–deficient mice.