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

Abstract

This study aimed to determine the interaction between the EGF receptor (EGFR) and peroxisome proliferator-activated receptor γ (PPARγ) and the role of EGFR in sodium and water transport in the proximal tubule. 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), PPARγ 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 PPARγ in high-glucose conditions was determined using specific PPARγ small interfering (si)RNA. P-EGFR, PPARγ, 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 PPARγ and EGFR interaction was determined by chromatin immunoprecipitation assay, and the effect of pioglitazone on EGFR activation by luciferase assay. PTCs exposed to both high glucose and pioglitazone increased protein abundance of P-EGFR, NHE3, AQP1 and PPARγ. Pioglitazone-induced upregulation of NHE3 and AQP1 was abolished by PKI166. High-glucose-induced increases in P-EGFR, NHE3 and AQP1 were decreased with PPARγ siRNA. AQP1 and NHE3 but not PPARγ were increased in a diabetic rat model and further increased by pioglitazone treatment. Pioglitazone induced PPARγ binding to the EGFR promoter and subsequent downstream activation. Our data suggest that EGFR activation mediates PPARγ-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 PPARγ agonists.