VEGFR-1 and VEGFR-2: two non-identical twins with a unique physiognomy

Angiogenesis involves endothelial cell differentiation, proliferation, migration and cord formation, which lead to tubulogenesis to form vessels. One group of growth factor receptors implicated in angiogenesis is the VEGFR family of receptor tyrosine kinases. VEGFR-1 and VEGFR-2 are closely related receptor tyrosine kinases and have both common and specific ligands. VEGFR-1 is a kinase-impaired RTK whereas VEGFR-2 is a highly active kinase. Despite their differential kinase activation potentials, both VEGFR-1 and VEGFR-2 are required for normal development and angiogenesis. VEGFR-1 regulates angiogenesis by mechanisms that involve ligand-trapping, receptor homo- and heterodimerization. VEGFR-2 stimulates variety of signaling pathways and broad biological responses in vitro. The mechanisms that govern VEGFR-2 activation, its ability to recruit signaling proteins and to undergo downregulation are highly regulated by its carboxyl terminus. This review highlights recent insights into the mechanism of activation of VEGFR-1 and VEGFR-2, and focuses on the signaling pathways employed by VEGFR-1 and VEGFR-2 that regulate angiogenesis.