Analysis of Phosphate Transporters in Peritoneal Cells and Tissues and Their Transport Kinetics In Vitro

Peritoneal dialysis (PD) is limited by insufficient phosphate removal, leading to adverse cardiovascular outcomes in patients with chronic kidney disease. To advance the understanding of the molecular mechanisms of peritoneal phosphate transport, RNAseq data of phosphate transporters in four PD-relevant cell lines were analyzed. The expression and localization of the respective proteins were validated by immunostaining in these cells. The transcriptomics of omental arterioles from children on PD were analyzed. In vitro Transwell models of an immortalized mesothelial cell line (MeT-5A) and human umbilical vein endothelial cells (HUVECs) and respective co-cultures were established, enabling quantification of phosphate transport across mesothelial and endothelial monolayers. Sodium phosphonoformate tribasic hexahydrate (PFA) and Tenapanor were used to inhibit transcellular and paracellular transport pathways. Cell viability and integrity markers were measured over the experimental periods. SLC20A1 and SLC20A2 were expressed across all studied cell types, while SLC34A2 and SLC34A3 were mesothelial cell-specific. Omental arterioles of children on low-glucose-degradation-product (GDP) PD showed higher SLC20A1 expression vs. stage 5 chronic kidney disease (CKD5) and healthy controls. Permeability for phosphate was lower across MeT-5A compared with HUVEC monolayers and was not further reduced in co-culture. Inhibitors reduced both transcellular and paracellular transport to 75% in MeT-5A and 65% in co-cultures, while no effects were observed in HUVEC alone, suggesting the mesothelial cell layer as a significant barrier for phosphate transport. Our studies provide first analyses combining findings on molecular phosphate transporters in peritoneal cells and arterioles and introducing a Transwell model for quantitative studies of phosphate kinetics.