ABSTRACT: Loss of ultrafiltration capacity in peritoneal dialysis (PD) patients is often associated with peritoneal vascular changes, manifest as diabetes-like vasculopathy and angiogenesis. The endothelial monolayer lining the vessel lumen controls vessel barrier function and thereby influences peritoneal substrate transport and ultrafiltration. The dipeptide alanyl-glutamine (AlaGln) has recently shown cytoprotective effects when added to PD fluid, including preservation of mesothelial cells in vitro (Kratochwill et al., Nephrol Dial Transplant, 2012, doi:10.1093/ndt/gfr459) and attenuation of the exuberant angiogenesis seen in long-term rodent models of PD (Ferrantelli et al., Kidney international, 2016, doi:10.1016/j.kint.2015.12.005). These effects have been reflected in early phase clinical trials as restoration of effluent cell stress responses and improved biomarkers of peritoneal health (Kratochwill et al., PLoS One, 2016, doi:10.1371/journal.pone.0165045; Vychytil et al., Kidney international, 2018, doi:10.1016/j.kint.2018.08.031). In a randomized clinical phase II trial, 8 weeks treatment with AlaGln in PD fluid decreased peritoneal protein loss. This clinically important effect might be explained by preserved peritoneal membrane and vessel integrity, but the role of AlaGln in preservation of peritoneal endothelial cell function remains unclear. In this study, we investigated to what extent in vivo observed biological processes and pathways are replicated by in vitro exposure of human umbilical vein endothelial cells (HUVEC) to conventional PD fluid. HUVECs were exposed to experimental solutions for up to 24 h. All test fluids were sterile-filtered before usage. Each experiment consisted of three independent samples in biological replicates on separate culture plates. For PD fluid incubation, cells were first exposed for 1 h to pure glucose-based PD fluid (Dianeal PD4 3.86% glucose, Baxter, Castlebar, Ireland), or to the same PD fluid supplemented with AlaGln dipeptide (8 mM, Dipeptiven; Fresenius Kabi, Bad Homburg, Germany), or to normal medium without growth factors as control. Cells were subsequently exposed for 24 h to the above solutions diluted 1:1 with culture medium and brought to 2% FCS. Cells were then washed three times (250 mM sucrose, 10 mM Tris/HCl, pH 7) and lysed in 125 µL per well of lysis buffer (30 mM Tris, pH 8.5, 7 M urea, 2 M thiourea, 4% CHAPS, 1 mM EDTA, one tablet of Complete Protease Inhibitor (Roche, Basel; Switzerland) per 100 ml, and one tablet of PhosStop Protease Inhibitor (Roche) per 100 mL). Total protein concentration was determined with the 2D-Quant kit (GE Healthcare, Uppsala, Sweden) per manufacturer’s manual. Lysates were stored at -80°C until further processing. We investigated the molecular mechanisms of endothelial cell pathology during in vivo and in vitro PD fluid exposure by proteomic and bioinformatic analysis of the endothelial cell response to hyperglycemic stress, integrating current understanding of PD-related cellular injury and stress responses (Bender et al., Nephrol Dial Transplant, 2011, doi:10.1093/ndt/gfq484; Kratochwill et al., BioMed research international, 2015, doi:10.1155/2015/628158; Kratochwill et al., J Proteome Res, 2009, doi:10.1021/pr800916s; Lechner et al., J Proteome Res, 2010, doi:10.1021/pr9011574). We then evaluated the effect of AlaGln addition to conventional PD fluid on these parameters in endothelial cells. This strategy allowed characterization of endothelial cell injury and stress responses during exposure to conventional hyperglycemic PD fluid, and their modulation by added AlaGln.
Klaus Kratochwill 
