PKC- and MAPK-independent upregulation of VEGF receptor expressions in human umbilical venous endothelial cells following VEGF stimulation.

Regulatory mechanisms of expressions of receptors for vascular endothelial growth factor (VEGF) are an important issue to clarify angiogenesis in the liver. Effects of exogenous VEGF on VEGF receptor expressions and intracellular signal transduction system were examined using human umbilical venous endothelial cells (HUVECs). HUVECs were cultured in BEM-2 solution containing VEGF for 5 days, and subjected to experiments 24 h after replacement of the medium to EBM-2 solution without VEGF. Addition of VEGF at concentrations up to 100 ng/ml increased the incorporation of 3H-thymidine into the DNA in the cells in a dose-related manner. Similar increases were found in mRNA expressions of VEGF receptors, VEGFR-1 and VEGFR-2, assessed by Northern blotting and in phosphorylation of mitogen activated protein kinase (MAPK) by Western blotting. Such VEGF-induced upregulation of DNA synthesis and phosporylation of MAPK in the cells were attenuated by the addition of GFX, a specific inhibitor of protein kinase C (PKC), but mRNA expression of VEGFR-2 evaluated by quantitative competitive RT-PCR method was not changed. It is suggested that exogenous VEGF upregulated mRNA expressions of VEGF receptors through intracellular signal transduction independent of the PKC-to-MAPK pathway in HUVECs.