Dual effects of heparin on VEGF binding to VEGF receptor-1 and transduction of biological responses

Vascular endothelial growth factor (VEGF) regulates blood vessel formation by binding to the receptor tyrosine kinases VEGF receptor-1 (Flt-1) or VEGF receptor-2 (KDR) and to the structurally unrelated neuropilins. As exon 7-containing isoforms of VEGF bind to heparin, angiogenesis may be modulated by heparin/heparan sulfate. We analyzed the effect of heparin on VEGF(165)-binding and activation of VEGF receptor-1 in porcine aortic endothelial cells, which lack expression of VEGF receptor-2 and neuropilins. Heparin decreased binding of (125)I-VEGF to 50% at 5 microg/ml and cross-linking of (125)I-VEGF to VEGF receptor-1 on intact cells was similarly decreased. Schatchard analyses showed that the affinity for binding of (125)I-VEGF to VEGF receptor-1 was decreased in the presence of heparin. In contrast, VEGF receptor-1 kinase activity was elevated when cells were treated simultaneously with VEGF and heparin. In accordance, VEGF-induced tyrosine phosphorylation of phospholipase Cgamma (PLCgamma) and DNA synthesis were augmented by heparin. However, basal PLCgamma tyrosine phosphorylation and DNA synthesis levels were to some extent increased by incubation of cells with heparin alone. We conclude that although heparin decreases binding of VEGF to VEGF receptor-1, the remaining binding results in more efficient kinase activation. Taken together, there is no loss of VEGF/VEGF receptor-1 function in the presence of heparin.