Ectodomain shedding and regulated intracellular proteolysis can determine the fate or function of cell surface proteins. Fms-related tyrosine kinase (FLT) or VEGF receptor 1 is a high-affinity cell surface VEGF-A receptor tyrosine kinase that is constitutively cleaved to release an NH2-terminal VEGF-A binding ectodomain that, once shed, can antagonize the effects of VEGF-A in the extracellular milieu. We evaluated the effect of VEGF-A on FLT1 cleavage in native cells and in transient and stable expression systems. We demonstrate that VEGF-A inhibits FLT1 ectodomain cleavage in a time- and dose-dependent manner, whereas VEGF-A knockdown in HEK293 cells increases ectodomain shedding. Although kinase insert domain receptor (KDR) or VEGF receptor 2, analogous to FLT1, is also subject to extracellular and intracellular cleavage, VEGF-A does not inhibit KDR cleavage. VEGF-A inhibition of FLT1 cleavage is not dependent on FLT1 tyrosine kinase activity or the intracellular FLT1 residues. N-acetylleucylleucylnorleucinal (ALLN), a proteasomal inhibitor; bafilomycin A, an inhibitor of endosomal acidification; and dynasore, a dynamin inhibitor, all increase the abundance of FLT1 and the shed ectodomain, indicating that FLT1 is subject to dynamin-mediated endocytosis and susceptible to proteasomal and lysosomal degradation. VEGF-A inhibition of cleavage is not reversed by ALLN, bafilomycin A, or dynasore. However, a 30 AA deletion in the extracellular immunoglobulin 7 domain leads to enhanced cleavage of Flt1 with a significant reduction of the VEGF inhibitory effect. Our results indicate that the inhibition of FLT1 ectodomain cleavage by VEGF-A is dependent neither on receptor activation nor on internalization nor a consequence of receptor degradation and likely represents a direct inhibitory effect on receptor cleavage.
Keywords: ligand-mediated endocytosis; proteolytic cleavage; receptor tyrosine kinase.