Vascular insufficiency and
retinal ischemia precede many proliferative retinopathies and stimulate secretion of various vasoactive
growth factors, including
vascular endothelial growth factor (
VEGF) and
placenta growth factor (PlGF). It is unclear, however, how PlGF, which is elevated in proliferative
diabetic retinopathy and is a
VEGF homolog that binds only to
VEGF receptor (VEGFR)-1, promotes
pathological angiogenesis. When primary microvascular endothelial cells were grown on
collagen gels, PlGF-containing
ligands upregulated Bcl-2 expression and stimulated the formation of capillary-like tube networks that were retained for up to 14 days in culture. The inhibition of
VEGFR-1 results in a dramatic decrease in the number of capillary connections, indicating that
VEGFR-1 ligands promote branching angiogenesis. In contrast,
VEGF-induced tube formations and Bcl-2 expression were significantly decreased at the end of this period. Flow cytometry analysis of
annexin-V/
propidium iodide-stained cells revealed that PlGF and PlGF/
VEGF heterodimer inhibited apoptosis in serum-deprived endothelial cells. These two
growth factors stimulated a survival signaling pathway
phosphatidylinositol 3-kinase (PI3K), as identified by increased Akt phosphorylation and because blocking PI3K signalling by adenovirus-mediated overexpression of wild-type
phosphatase and
tensin homolog on chromosome 10 (PTEN) disrupted angiogenesis and decreased Bcl-2 expression by PlGF and PlGF/
VEGF heterodimer, whereas a dominant-negative PTEN mutant enhanced endothelial sprout formation and Bcl-2 expression. Together, these findings indicate that PlGF-containing
ligands contribute to
pathological angiogenesis by prolonging cell survival signals and maintaining vascular networks.