FUNCTIONAL CHARACTERIZATION OF SECRETED FORM OF A VEGF RECEPTOR

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A mitogen specific for endothelial cells, termed vascular endothelial growth factor (VEGF), was purified from the conditioned medium of bovine pituitary folliculo stellate cells. VEGF is composed of two presumably identical chains held together by disulfide bonds. The amino acid sequence of VEGF is distantly related (18% overall identity) to the sequence of platelet-derived growth factor (PDGF), which is also a disulfide-linked dimer. The eight cysteines involved in intra- and interchain disulfide bridges in PDGF are identical in VEGF. Recently, expression of the fms-like tyrosine kinase (flt) cDNA in COS cells was shown to confer specific, high-affinity binding to VEGF. Expression of flt in Xenopus laevis oocytes caused the oocytes to release calcium in response to VEGF. These findings show that flt encodes a receptor for VEGF. Flt is a receptor-type tyrosine kinase with seven immunoglobulin- like regions in its extracellular domain. However, a Northern blot analysis using extracellular domain of flt as probe reveals that the majority of flt messages are comprised of mRNAs with size of 2.2 kb and 3.0 kb in human endothelial cells. PCR and sequence analysis indicates that these short forms of flt mRNA code for the first six immunoglobulin- like domains followed shortly by the termination codon. Thus, these are the secreted forms of flt since they do not contain transmembrane domains. The sequence of 3' untranslated region of this secreted form of flt suggests they are synthesized by alternative splicing. Whether this secreted form of flt plays a role in regulating binding of VEGF to endothelial cells remains to be elucidated.