Abstract
The human signaling molecules Tie1 and Tie2 receptor tyrosine kinases (RTKs) play important pathophysiological roles in many diseases, including different cancers. The activity of Tie1 is mediated mainly through the downstream angiopoietin-1 (Ang1)-dependent activation of Tie2, rendering both Tie 1 and the Tie1/Tie2/Ang1 axis attractive putative targets for therapeutic intervention. However, the development of inhibitors that target Tie1 and an understanding of their effect on Tie2 and on the Tie1/Tie2/Ang1 axis remain unfulfilled tasks, due, largely, to the facts that Tie1 is an orphan receptor and is difficult to produce and use in the quantities required for immune antibody library screens. In a search for a selective inhibitor of this orphan receptor, we sought to exploit the advantages (e.g., small size that allows binding to hidden epitopes) of non-immune nanobodies and to simultaneously overcome their limitations (i.e., low expression and stability). We thus performed expression, stability, and affinity screens of yeast-surface-displayed naïve and predesigned synthetic (non-immune) nanobody libraries against the Tie1 extracellular domain. The screens yielded a nanobody with high expression and good affinity and specificity for Tie1, thereby yielding preferential binding for Tie1 over Tie2. The stability, selectivity, potency, and therapeutic potential of this synthetic nanobody were profiled using in vitro and cell-based assays. The nanobody triggered Tie1-dependent inhibition of RTK (Tie2, Akt, and Fak) phosphorylation and angiogenesis in endothelial cells, as well as suppression of human glioblastoma cell viability and migration. This study opens the way to developing nanobodies as therapeutics for different cancers associated with Tie1 activation.