Flt-1 but not KDR/Flk-1 tyrosine kinase is a receptor for placenta growth factor, which is related to vascular endothelial growth factor

Placenta Growth Factor (PIGF) is a new member of vascular endothelial growth factor (VEGF) family. Although VEGF binds FIt family Fit-I and KDR/FIk-I tyrosine kinases at high affinity for signal transduction, biological activities and the receptors of PIGF have not been extensively studied. Reverse transcription-PCR showed that P1GF-2, a subtype of PIGF-I that bears a basic amino acid-rich domain, is more abundant than P1GF-I and thus is the major subtype in human placenta. Using antibodies specific to PIGF-I or -2 as markers, we obtained large amounts of PIGFs in the baculovirus expression system. PIGF-2 had growthstimulatory activity on human umbilical vein endothelial cells and vascular permeability activity in the Miles assay at levels about 10-fold lower than those of VEGF. All PIGF-I activities were lower than those of P1GF-2.