Activation of endothelial cell mitogen activated protein kinase ERK1/2 by extracellular HIV-1 Tat protein

Extracellular Tat protein, the transactivating factor of the human immunode ficiency virus type 1 (HIV-1), modulates gene expression, growth, and angio genic activity in endothelial cells by interacting with the vascular endoth elial growth factor (VEGF) receptor-2 (Flk-1/KDR). Recombinant Tat protein, produced as glutathione-S-transferase chimera (GST -Tat), activates mitogen-activated protein kinase (MAPK) ERK1/2 in human, m urine, and bovine endothelial cells whereas GST is ineffective. In bovine a ortic endothelial cells, GST-Tat and the 165 amino acid VEGF isoform (VEGF( 165)) induce transient ERK1/2 phosphorylation with similar potency and kine tics. The synthetic peptide Tat(41-60). but not peptides Tat(1-21) and Tat( 71-86), causes ERK1/2 phosphorylation. thus implicating Tat/KDR interaction in the activation of this signalling pathway. Accordingly, GST-Tat induces ERK1/2 phosphorylation in KDR-transfected porcine aortic endothelial cells but not in parental cells. MAPK kinase inhibitors PD098059 and U0126 preve nt ERK1/2 phosphorylation by Tat. However, they do not affect the angiogeni c activity exerted by Tat in the murine Matrigel plug and chick embryo chor ioallantoic membrane assays. Blocking of MAPK kinase activity impairs inste ad the angiogenic response to VEGF(165) and to fibroblast growth factor-2 ( FGF-2). Our data demonstrate that ERK1/2 activation following the interaction of HI V-1 Tar protein with endothelial cell Flk-1/KDR receptor does not represent an absolute requirement for a full angiogenic response to this growth fact or that appears to utilize mechanism(s) at least in part distinct from thos e triggered by other prototypic angiogenic growth factors.