Signalling properties of an HIV-encoded angiogenic peptide mimicking vascular endothelial growth factor activity

HIV-1 expresses a multifunctional protein called TAT (transacting transcrip tional activator), the function of which in vivo is tightly correlated with the incidence of Kaposi's sarcoma in AIDS patients. TAT is angiogenic and apparently binds to receptors specific for vascular endothelial growth fact or (VEGF). Amino acids 46-60 of HIV-TAT, known as the basic peptide, have b een shown to be responsible for its functional interaction with VEGF recept ors, To characterize further the binding properties of this peptide, its co ding sequence was fused to the reading frame of bacterial thioredoxin, allo wing the production of large amounts of chimaeric polypeptides in bacteria in a biologically active form. Binding of chimaeric proteins to VEGF recept ors was studied in vitro in endothelial cell cultures expressing either of the two receptors, Chimaeric thioredoxin proteins carrying the basic domain of TAT bound to both VEGF receptors with affinities similar to those of HI V-TAT or VEGF. Interestingly, these polypeptides competed only partially wi th VEGF for receptor binding. implying different binding sites for the TAT peptide and VEGF, This suggests that TAT binds VEGF receptors at new sites that might be useful targets for pharmacological intervention during pathol ogical angiogenesis. The thioredoxin/basic-peptide chimaeras are functional agonists that mediate VEGF receptor signalling: (1) they stimulate the gro wth of endothelial cells; (2) together with basic fibroblast growth factor they cause tube formation of endothelial cells in collagen gels; (3) they i nduce blood vessel formation on the chicken chorioallantoic membrane; and ( 4) they activate VEGF receptor kinase and mitogen-activated protein kinase activity.