Involvement of VEGFR-2 (kdr/flk-1) but not VEGFR-1 (flt-1) in VEGF-A and VEGF-C-induced tube formation by human microvascular endothelial cells in fibrin matrices in vitro.

Different forms of vascular endothelial growth factor (VEGF) and their cellular receptors (VEGFR) are associated with angiogenesis, as demonstrated by the lethality of VEGF-A, VEGFR-1 or VEGFR-2 knockout mice. Here we have used an in vitro angiogenesis model, consisting of human microvascular endothelial cells (hMVEC) cultured on three-dimensional (3D) fibrin matrices to investigate the roles of VEGFR-1 and VEGFR-2 in the process of VEGF-A and VEGF-C-induced tube formation. Soluble VEGFR-1 completely inhibited the tube formation induced by the combination of VEGF-A and TNFα (VEGF-A/TNFα). This inhibition was not observed when tube formation was induced by VEGF-C/TNFα or bFGF/TNFα. Blocking monoclonal antibodies specific for VEGFR-2, but not antibodies specifically blocking VEGFR-1, were able to inhibit the VEGF-A/TNFα-induced as well as the VEGF-C/TNFα-induced tube formation in vitro. PlGF-2, which interacts only with VEGFR-1, neither induced tube formation in combination with TNFα, nor inhibited or stimulated by itself the VEGF-A/TNFα-induced tube formation in vitro. These data indicate that VEGF-A or VEGF-C activation of the VEGFR-2, and not of VEGFR-1, is involved in the formation of capillary-like tubular structures of hMVEC in 3D fibrin matrices used as a model of repair-associated or pathological angiogenesis in vitro.