Role of the EGF receptor in PPARγ-mediated sodium and water transport in human proximal tubule cells

Aim/hypothesis This study aimed to determine the interaction between the EGF receptor (EGFR) and peroxisome proliferator-activated receptor. (PPAR gamma) and the role of EGFR in sodium and water transport in the proximal tubule. Methods Primary human proximal tubule cells (PTCs) were exposed to high glucose in the presence and absence of pioglitazone. Total and phospho-EGFR levels and EGFR mRNA expression were determined by western blot and real-time PCR, respectively. Sodium-hydrogen exchanger-3 (NHE3), PPAR gamma and aquaporin 1 (AQP1) levels were determined by western blot. The role of EGFR was elucidated using the EGFR tyrosine kinase inhibitor, PKI166. The role of PPAR gamma in high-glucose conditions was determined using specific PPAR gamma small interfering (si) RNA. P-EGFR, PPAR gamma, AQP1 and NHE3 production in a rat model of diabetes (streptozotocin-induced hypertensive Ren-2 transgenic [mRen2]27 rats) and controls, with or without pioglitazone treatment, was determined by immunohistochemistry. The PPAR gamma and EGFR interaction was determined by chromatin immunoprecipitation assay, and the effect of pioglitazone on EGFR activation by luciferase assay. Results PTCs exposed to both high glucose and pioglitazone increased protein abundance of P-EGFR, NHE3, AQP1 and PPAR gamma. Pioglitazone-induced upregulation of NHE3 and AQP1 was abolished by PKI166. High-glucose-induced increases in P-EGFR, NHE3 and AQP1 were decreased with PPAR gamma siRNA. AQP1 and NHE3 but not PPAR gamma were increased in a diabetic rat model and further increased by pioglitazone treatment. Pioglitazone induced PPAR gamma binding to the EGFR promoter and subsequent downstream activation. Conclusions/interpretation Our data suggest that EGFR activation mediates PPAR gamma-induced sodium and water reabsorption via upregulation of NHE3 and AQP1 channels in the proximal tubule. EGFR inhibition may be a therapeutic strategy in the treatment of diabetic nephropathy and in limiting salt and water retention, which currently restricts the use of PPAR gamma agonists.