On the mechanism of thrombin-induced angiogenesis. Potentiation of vascular endothelial growth factor activity on endothelial cells by up-regulation of its receptors.

Many of the cellular actions of thrombin may contribute to the angiogenesis-promoting effect of thrombin reported previously. In this study, we investigated the interaction between thrombin and vascular endothelial growth factor (VEGF), the specific endothelial cell mitogen and key angiogenic factor. Exposure of human umbilical vein endothelial cells to thrombin sensitizes these cells to the mitogenic activity of VEGF. This thrombin-mediated effect is specific, dose-dependent and requires the activated thrombin receptor. Quantitative reverse transcription- polymerase chain reaction analysis reveals a time- and dose-dependent up-regulation of mRNA for VEGF receptors (KDR and flt-1). Optimal thrombin concentration for maximal expression of mRNA for KDR is 1.5 IU/ml (170% over controls) and appears 8-12 h after thrombin stimulation. Nuclear run-on experiments demonstrate that the up-regulation of KDR mRNA by thrombin occurred at the transcriptional level. In addition, functional protein of KDR receptor is increased to about 200% over control after 12 h of thrombin treatment. The up-regulation of KDR and flt-1 mRNA is also mimicked by the thrombin receptor activating peptide. These findings could explain at least in part the potent angiogenic action of thrombin.