Vascular endothelial growth factor receptors in angiogenesis
Abstract: Angiogenesis is a process in which new capillaries sprout from preexisting blood vessels. Vascular endothelial growth factor (VEGF) promotes endothelial cell proliferation and migration and thereby stimulates angiogenesis. VEGF receptors (VEGFRs) have been implicated as key regulatory components in endothelial cells. This thesis presents studies of the molecular mechanisms in angiogenesis by use of different endothelial cell lines and chick embryos. The signal transduction properties of VEGFR-1 and VEGFR-2 were investigated, and the inhibitory effects of a VEGFR-targeted peptide were evaluated In the chick chorioallantoic membrane, angiogenesis was potentiated by administration of exogenous VEGF or fibroblast growth factor, another pro-angiogenic factor. The angiogenic effect was inhibited by phosphoinositide 3-kinase (Pl3K) inhibitors wortmannin and LY294002. The data indicate that P13K activity is required for migration, mitogenicity and survival but not for differentiation of endothelial cells during angiogenesis. To examine the potential heterodimer formation by VEGFR-1 and -2, endothelial cells expressing both receptors were established. Heterodimers were indeed formed and allowed efficient signal transduction. VEGF-induced intracellular Ca2+ fluxes were efficiently transduced by hetero- and homodimer configurations; moreover, each dimer configuration mediated Ca2+ fluxes with a distinct pattern. The function of VEGFR-1 in angiogenesis has been debated. We show that porcine aortic endothelial cells over expressing VEGFR-1 responded to VEGF treatment with increased DNA synthesis and proliferation. Mutants for VEGFR-1 lacking certain potential phosphorylation sites were unable to mediate the mitogenic response. In search for a potent inhibitor of angiogenesis, J63, a peptide corresponding to the substrate-binding motif in the tyrosine kinase domain of VEGFR was evaluated. Vessel development in the chorioallantoic membrane was suppressed when exposed to J63 together with VEGF. Advances in the development of inhibitors for VEGF receptor function may provide a useful approach for the treatment of angiogenesis-dependent diseases.
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