A unique signal transduction from FLT tyrosine kinase, a receptor for vascular endothelial growth factor VEGF

Flt-1 (fms-like tyrosine kinase-1), a receptor-type tyrosine kinase of sharing similar features with two other flt-family encoded proteins VEGFR2/KDR/Flk-1/VEGFR2/KDR/Flk-1 and VEGFR3/Flt-4, has been recently identified as a receptor for Vascular Endothelial Growth Factor (VEGF) known to induce the proliferation of vascular endothelial cells. In this study, we demonstrate that Flt-1 encodes for a 180 kDa glycoprotein, binds VEGF with high affinity, undergoes autophosphorylation but does not generate any mitogenic response in transfected NIH3T3 fibroblasts. Interestingly, the immediate early gene c-Myc was not induced, whereas the c-fos was induced very weakly in Flt-1 expressing NIH3T3 cells. A comparative analysis of the Flt-1 signal cascade in the environment of endothelial cells with that of Flt-1 expressing NIH3T3 cells showed that VEGF induced phosphorylation of PLC gamma and GAP complex on tyrosine in both type of cells. However, a strong activation of MAP kinases was observed only in endothelial cells. Further, different from many other Receptor Tyrosine Kinases, tyrosine phosphorylation of Shc protein, an important adaptor for signal transduction from many receptor kinases, was very weak in both Flt-1-NIH3T3 cells and endothelial cells. These results suggest that Flt-1 kinase utilizes a unique signal transduction system in endothelial cells, and the activation of the Flt-1 kinase is insufficient to trigger a mitogenic response in NIH3T3 fibroblasts.