Functions of the VEGF receptor-1 (FLT-1) in the vasculature.

Vascular endothelial growth factor (VEGF) is a major inducer of angiogenesis and vasculogenesis. Two distinct receptors for VEGF, the tyrosine kinase receptors VEGFR-1 (Flt-1) and VEGFR-2 (Flk-1/KDR), have been identified. Transfection studies could demonstrate biological activities for the Flk-1/KDR-, but not for the Flt-1-receptor, which led to the hypothesis that Flt-1 is a decoy receptor for VEGF. However, Flt-1 is biologically active in non-endothelial cells, namely monocytes, which exclusively express this receptor. In addition, the Flt-1 ligand placenta growth factor (PlGF), which is unable to bind and activate Flk-1/KDR, elicits activities in both monocytes and endothelial cells. The implications of Flt-1 mediated monocyte transmigration through endothelial monolayers and induction of the procoagulant tissue factor on monocytes for the field of vascular medicine are discussed.