VEGF-dependent signaling in retinal microvascular endothelial cells

We examined the effect of vascular endothelial growth factor (VEGF) on intracellular signal transduction pathways using isolated bovine microvascular endothelial cells (BREC). When cell growth was determined by [3H]thymidine incorporation, it was significantly stimulated by VEGF stimulation. In situ hybridization results also demonstrated that c-fos expression was enhanced by the stimulation. Although BREC expressed Flt-1 and Flk-1 as VEGF receptors at similar levels, VEGF stimulation preferentially enhanced the activity of Flt-1 tyrosine kinase. This stimulation initiated an increase in the level of GTP-form Ras and the activation of mitogen activated protein kinase (MAPK). On the other hand, BREC expressed the Janus kinase (Jak) family members Jak1, Jak2, and Tyk2, and the signal transducers and activators of transcription (Stat) family members Stat1, Stat3, and Stat6. These molecules were tyrosine phosphorylated under culture conditions used, and the phosphorylation of Tyk2 and Stat6 was specifically enhanced by VEGF stimulation. These results demonstrate that, in addition to Ras/MAPK pathways, the Flt-1/Tyk2/Stat6 pathway is important in VEGF signaling in BREC. These signal transduction systems may regulate the growth of retinal endothelial cells.