Substrate-Immobilized HIV-1 Tat Drives VEGFR2/α _v β ₃ –Integrin Complex Formation and Polarization in Endothelial Cells

Objective— The HIV-1 transactivating factor (Tat) possesses features typical of both cell-adhesive and angiogenic growth factor (AGF) proteins, inducing endothelial cell (EC) adhesion and proangiogenic activation. Tat was exploited to investigate the events triggered by EC adhesion to substrate-bound AGF that lead to proangiogenic activation. Methods and Results— Immobilized Tat induces actin cytoskeleton organization, formation of α v β 3 integrin + focal adhesion plaques, and recruitment of vascular endothelial growth factor receptor-2 (VEGFR2) in the ventral plasma membrane of adherent ECs. Also, acceptor photobleaching fluorescence resonance energy transfer demonstrated that VEGFR2/α v β 3 coupling occurs at the basal aspect of Tat-adherent ECs. Cell membrane fractionation showed that a limited fraction of α v β 3 integrin and VEGFR2 does colocalize in lipid rafts at the basal aspect of Tat-adherent ECs. VEGFR2 undergoes phosphorylation and triggers pp60src/ERK 1/2 activation. The use of lipid raft disrupting agents and second messenger inhibitors demonstrated that intact lipid rafts and the VEGFR2/pp60src/ERK 1/2 pathway are both required for cytoskeleton organization and proangiogenic activation of Tat-adherent ECs. Conclusion— Substrate-immobilized Tat causes VEGFR2/α v β 3 complex formation and polarization at the basal aspect of adherent ECs, VEGFR2/pp60src/ERK 1/2 phosphorylation, cytoskeleton organization, and proangiogenic activation. These results provide novel insights in the AGF/tyrosine kinase receptor/integrin cross-talk.