The role of Sgk-1 in the upregulation of transport proteins by PPAR-{gamma} agonists in human proximal tubule cells.
BACKGROUND
Cellular sodium and water transport are dysregulated in diabetes mellitus. Synthetic peroxisome proliferator-activated receptor gamma (PPAR-gamma) agonists are currently used in the treatment of type 2 diabetes, but their use is limited by fluid retention. Recent data suggest that PPAR-gamma agonists stimulate distal tubular epithelial Na transport, potentially through the serine glucocorticoid kinase-1 (Sgk-1)-dependent regulation of the epithelial Na channel. We have recently demonstrated that Sgk-1 additionally regulates sodium reabsorption through the proximal tubular sodium hydrogen exchanger-3 (NHE3). However, the effects of PPAR-gamma agonists on Sgk-1, the water channel proteins aquaporins and on sodium transport in human proximal tubule cells (PTCs) have not previously been studied.


METHODS
PTCs were exposed to the PPAR-gamma agonists, pioglitazone and the more selective PPAR-gamma agonist L-805645 with and without the Sgk inhibitor (GSK650394A). PPAR-gamma, Sgk-1, NHE3, AQP 1 and 7 mRNA and protein expression were determined by semi-quantitative PCR and western blot. The Sgk-1-specific effect was determined using Sgk-1 siRNA.


RESULTS
Exposure of PTCs to 10 muM pioglitazone and 8 microM L-805645 increased the mRNA and protein expression of PPAR-gamma (P < 0.005), NHE3 and Sgk-1 (both P < 0.05). The expression of AQPs 1 and 7 was increased by pioglitazone and L-805645 (both P < 0.05). The increases in NHE3 and AQPs 1 and 7 were significantly reduced by pharmacological inhibition of Sgk and when cultures were exposed to Sgk-1-specific siRNA.


CONCLUSIONS
PPAR-gamma agonists enhanced the expression of NHE3, AQP 1 and 7 channels in human proximal tubule cells through Sgk-1-dependent pathways.
Saad
Sonia
S
Renal Research Laboratories, Kolling Institute of Medical Research, University of Sydney, Sydney, Australia.
Agapiou
David J
DJ
Chen
Xin-Ming
XM
Stevens
Veronica
V
Pollock
Carol A
CA
D002352:Carrier Proteins / Q000378:metabolism*; D002478:Cells, Cultured; D006801:Humans; D017874:Immediate-Early Proteins / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000166:cytology* / Q000378:metabolism*; D047495:PPAR gamma / Q000819:agonists* / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D015854:Up-Regulation; D000099184:Serum-Glucocorticoid Regulated Kinases
D002352:Carrier Proteins; D017874:Immediate-Early Proteins; D047495:PPAR gamma; D017346:Protein Serine-Threonine Kinases; D000099184:Serum-Glucocorticoid Regulated Kinases