REGULATION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR RECEPTOR-2 (FLK-1) EXPRESSION IN VASCULAR ENDOTHELIAL-CELLS

We have previously reported the existence of a synergistic interaction between vascular endothelial growth factor (VEGF) and basic fibroblas t growth factor (bFGF) in the induction of angiogenesis in vitro. Here we demonstrate that bFGF increases VEGF receptor-a (VEGFR-2/Flk-1) ex pression: mRNA levels were increased by 4.5- to 8.0-fold and total pro tein by 2.0- to 3.5-fold, in bovine microvascular endothelial (BME), a ortic endothelial (BAE), and transformed fetal aortic (GM7373) endothe lial cells. VEGF itself did not affect VEGFR-2 expression, and neither bFGF nor VEGF altered expression of FG;F receptor-1. We also show tha t synergism occurs at the level of proliferation when this is measured in a three-dimensional but not in a conventional two-dimensional assa y. Differences in the level of VEGFR-2 expression were also observed w hen cells were grown on or within collagen gels under different condit ions: mRNA levels were lowest under sparse conditions, increased 20- t o 26-fold at confluence, and increased even further (57-fold) when cel ls were cultured in suspension in three-dimensional collagen gels. Fin ally, a synergistic increase was seen in the level of expression of ur okinase and urokinase receptor mRNAs when cells were exposed to bFGF a nd VEGF for 4 days. These findings demonstrate that the level of VEGFR -2 expression can be modulated by environmental factors including cyto kines and the geometry of the culture conditions and provide some insi ght into the mechanisms of synergism between bFGF and VEGF in the indu ction of angiogenesis in vitro. (C) 1998 Academic Press.