Sitagliptin does not interfere with VEGF-A signaling in retinal endothelial cells in vitro

Sitagliptin, an inhibitor of dipeptidyl peptidase-4 widely used in type 2 diabetes therapy, impairs the barrier formed by retinal endothelial cells (REC) during prolonged exposure. Because diabetic macular edema is treated with inhibitors of vascular endothelial growth factor (VEGF)-A signaling, we now studied (1) if sitagliptin interfered with this therapeutic approach and (2) if VEGF-A is involved in sitagliptin-induced barrier dysfunction.Confluent immortalized bovine REC (iBREC) were exposed for two days to sitagliptin and/or tivozanib, an inhibitor of the VEGF receptor 2, with or without VEGF-A. The cell index as an indicator of permeability was continuously measured, and cells and supernatants were harvested. Expressions of regulators of para- and transcellular flow, i.e., claudin-1, claudin-5, and plasmalemma vesicle-associated protein (PLVAP), were determined by subsequent Western blot analyses, and potential secretion of VEGF-A was measured by ELISA.Sitagliptin-exposed iBREC did not secrete VEGF-A, and, accordingly, tivozanib did not prevent the sitagliptin-induced decline of the cell index and the increased expression of tight junction (TJ) protein claudin 1. More TJ-protein claudin-5 was isolated together with proteins from membranes or organelles from sitagliptin-treated iBREC, which did not express PLVAP. The VEGF-A₁₆₅-induced declines of the cell index and TJ-protein claudin-1 were prevented by tivozanib, and this process was not modulated by sitagliptin.The underlying mechanisms of barrier dysfunctions caused by sitagliptin or VEGF-A are different and independent. Most importantly, the DPP-4 inhibitor does not interfere with blocking VEGF signaling to correct VEGF-A-dependent barrier dysfunction.