Oxidized alkyl phospholipids stimulate sodium transport in proximal tubules via PPAR-dependent pathway
The pleiotropic effects of oxidized phospholipids (oxPLs) have been identified. 1-O-hexadecyl-2-azelaoyl-sn-glycero-3-phosphocholine (azPC), an oxPL formed from alkyl phosphatidylcholines, is a potent peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}) agonist. Although it has been reported that thiazolidinediones can induce volume expansion by enhancing renal sodium and water retention, the role of azPC, an endogenous PPAR{gamma} agonist, in renal transport functions is unknown. In the present study, we investigated the effect of azPC on renal proximal tubule (PT) transport using isolated PTs and kidney cortex tissues. We showed that azPC rapidly stimulated Na+/HCO3- cotransporter 1 activity and luminal Na+/H+ exchanger (NHE) activities in a dose-dependent manner, at submicromolar concentrations, in isolated PTs from rats and humans. Additionally, the stimulatory effects were completely blocked by a specific PPAR{gamma} antagonist, 2-chloro-5-nitro-N-phenylbenzamide (GW9662), and a mitogen-activated protein/extracellular signal-regulated kinase (MEK) inhibitor, PD98059. Treatment with an siRNA against PPAR{gamma} significantly suppressed the expression of PPAR{gamma} mRNA, and it completely blocked the stimulation of both Na+/HCO3- cotransporter 1 and NHE activities by azPC. Moreover, azPC induced extracellular signal-regulated kinase (ERK) phosphorylation in rat and human kidney cortex tissues, and the induced ERK phosphorylation by azPC was completely suppressed by GW9662 and PD98059. These results suggest that azPC stimulates renal PT sodium-coupled bicarbonate transport via the PPAR{gamma}/MEK/ERK pathway. The stimulatory effects of azPC on PT transport may be partially involved in the development of volume expansion.