Participation of Syk in the VEGFR-2 signaling pathway. (A-B) Syk-dependent migration of HUVECs and HDMECs in response to VEGF-A. HUVECs were infected with either control (PINCO) or full-length CD36 vector (supplemental Figure 4B-C). Cells were serum-starved, and some were treated with 4.7μM Syk inhibitor (BAY 61-3606) while others were left untreated, as described in the endothelial kinase assay section in “Methods.” Results are expressed as a percentage of migrated cells (mean ± SE, n = 8) in 2 independent experiments. P values were calculated with an unpaired Student t test against HUVECs or HDMECs with Syk inhibitor or without inhibitor and VEGF-A. In these experiments, we resuspended cells in PBS with BSA to avoid any interference from the media or serum. (C) A proposed model for the TSP-1 regulation of the VEGF-A–induced VEGFR-2 signaling pathway. (a) Binding of TSP-1 to CD36 and/or VEGFR-2, and their associated integrins and tetraspanins (not shown) brings the 2 receptors in close proximity. (b) VEGF-A initiates angiogenesis by binding to VEGFR-2 and inducing its autophosphorylation, which, ultimately promotes Syk phosphorylation directly or through other adaptor proteins. (c) Syk activation further phosphorylates VEGFR-2 at Y1175 and mediates endothelial cell migration. (d) In the absence of VEGF-A, the TSP-1-CD36 complex only promotes Fyn activation, which inhibits endothelial cell migration and returns the angiogenic switch to the “off” position. The dashed lines suggest the possible involvement of other signaling and adaptor proteins.