Homeostatic modulation of cell surface KDR and Flt1 expression and expression of the vascular endothelial cell growth factor (VEGF) receptor mRNAs byVEGF

Vascular endothelial cell growth factor (VEGF) is a potent angiogenic facto r expressed during embryonic development, during wound healing, and in path ologies dependent on neovascularization, including cancer. Regulation of th e receptor tyrosine kinases, KDR and Flt-1, to which VEGF binds on endothel ial cells is incompletely understood. Chronic incubation with tumor-conditi oned medium or VEGF diminished I-125-VEGF binding to human umbilical vein e ndothelial cells, incorporation of I-126-VEGF into covalent complexes with KDR and Flt1, and immunoreactive KDR in cell lysates, Receptor down-regulat ion desensitized VEGF activation of mitogen-activated protein kinase (extra cellular signal-regulated kinases 1 and 2) and p38 mitogen-activated protei n kinase, Preincubation with VEGF or tumor-conditioned medium down-regulate d cell surface receptor expression but up-regulated KDR and Flt-1 mRNAs, an effect abrogated by a neutralizing VEGF antibody. Removal of VEGF from the medium led to recovery of I-125-VEGF binding and resensitization of human umbilical vein endothelial cells. Recovery of receptor expression was inhib ited by cycloheximide, indicating that augmented VEGF receptor mRNAs, and n ot receptor recycling from a cytoplasmic pool, restored responsiveness. As the VEGF receptors promote endothelial cell survival, proliferation, and ot her events necessary for angiogenesis, the noncoordinate regulation of VEGF receptor proteins and mRNAs suggests that human umbilical vein endothelial cells are protected against inappropriate or prolonged loss of VEGF recept ors by a homeostatic mechanism important to endothelial cell function.