Inhibition of Rac1 GTPase downregulates vascular endothelial growth factor receptor-2 expression by suppressing Sp1-dependent DNA binding in human endothelial cells.

RhoA, Rac1 and CDC42 are small GTP-binding proteins of the Rho family that play a crucial role in regulation of the actin-based cytoskeleton. In addition to cell growth regulation, they are implicated in transcriptional activation, oncogenic transformation and angiogenesis. The small Rho-GTPases have been linked to vascular endothelial growth factor (VEGF)-induced signalling pathways, but their role has not yet been elucidated. As signalling via the VEGF receptor-2 (VEGFR2) pathway is critical for angiogenic responses in cancer, wound repair and ischaemic and inflammatory diseases, we investigated whether the small Rho-GTPase Rac1 influences VEGFR2 expression in human endothelial cells. In this study, we show that a dominant negative Rac1 expression vector led to a pronounced decrease in VEGFR2 mRNA and protein expression. To identify minimal promoter requirements and potential applications of the small Rho-GTPases, we used VEGFR2 promoter-reporter gene constructs containing various deletions. The inhibitory effects of dominant negative Rac1 on the transcriptional activity of the VEGFR2 promoter localized to an element between -77 and -60 that contains an Sp1 transcription factor binding site. Electrophoretic mobility shift assays demonstrated that constitutive Sp1-dependent DNA binding decreased with Rac1 inhibition. Hence, repression of the small Rho GTPase Rac1 seems to be an additional critical molecular mechanism in the regulation of VEGFR2 expression.