Both Kdr and Flt1 play a vital role in hypoxia-induced Src-PLD1-PKCγ-cPLA2 activation and retinal neovascularization

To understand the mechanisms of Src-PLD1-PKC gamma-cPLA(2) activation by vascular endothelial growth factor A (VEGFA), we studied the role of Kdr and Flt1. VEGFA, while having no effect on Flt1 phosphorylation, induced Kdr phosphorylation in human retinal microvascular endothelial cells (HRMVECs). Depletion of Kdr attenuated VEGFA-induced Src-PLD1-PKC gamma-cPLA(2) activation. Regardless of its phosphorylation state, downregulation of Flt1 also inhibited VEGFA-induced Src-PLD1-PKC gamma-cPLA(2) activation, but only modestly. In line with these findings, depletion of either Kdr or Flt1 suppressed VEGFA-induced DNA synthesis, migration, and tube formation, albeit more robustly with Kdr downregulation. Hypoxia induced tyrosine phosphorylation of Kdr and Flt1 in mouse retina, and depletion of Kdr or Flt1 blocked hypoxia-induced Src-PLD1-PKC gamma-cPLA(2) activation and retinal neovascularization. VEGFB induced Flt1 tyrosine phosphorylation and Src-PLD1-PKC gamma-cPLA(2) activation in HRMVECs. Hypoxia induced VEGFA and VEGFB expression in retina, and inhibition of their expression blocked hypoxia-induced Kdr and Flt1 activation, respectively. Furthermore, depletion of VEGFA or VEGFB attenuated hypoxia-induced Src-PLD1-PKC gamma-cPLA(2) activation and retinal neovascularization. These findings suggest that although VEGFA, through Kdr and Flt1, appears to be the major modulator of Src-PLD1-PKC gamma-cPLA(2) signaling in HRMVECs, facilitating their angiogenic events in vitro, both VEGFA and VEGFB mediate hypoxia-induced Src-PLD1-PKC gamma-cPLA(2) activation and retinal neovascularization via activation of Kdr and Flt1, respectively.