FUNCTIONAL UP-REGULATION OF THE VASCULAR ENDOTHELIAL GROWTH-FACTOR RECEPTOR KDR BY HYPOXIA

Background Vascular endothelial growth factor (VEGF) is a specific end othelial mitogen and chemoattractant that has been shown to be useful for inducing therapeutic angiogenesis in ischemic myocardium and found to stimulate mitogenicity and chemotaxis of endothelial cells through the receptor tyrosine kinase KDR. Although VEGF expression is upregul ated by hypoxic stimuli, regulation of KDR remained unknown under thes e conditions. Methods and results With the use of human umbilical vein endothelial cells and transfected porcine aortic endothelial cells, K DR protein was found to be upregulated under hypoxic conditions (2% O- 2) in both cell types. This process of KDR upregulation was found to b e reversible, was maximal after 24 hours of hypoxia, and was regulated on a posttranscriptional level. Furthermore, the susceptibility for V EGF-induced mitogenicity was enhanced under hypoxic conditions as show n by [H-3]-thymidine incorporation assay. The activated state of incre ased VEGF function in hypoxic endothelial cells was associated with el evated tyrosine phosphorylation of KDR as demonstrated by anti-phospho tyrosine blot. Conclusions These data indicate that hypoxia stimulates VEGF-dependent signaling not only by upregulation of VEGF ligand but also by functional upregulation of a specific signaling receptor. Ther efore, these data provide evidence that the endothelium plays an activ e role in hypoxia-induced angiogenesis.