Glitazone-induced changes in Arg8-vasopressin-stimulated ion transport in MDCK -C7 cells

Glitazones are synthetic ligands for the nuclear-bound peroxisome proliferator-activated receptor gamma (PPARγ). These agents have insulin-sensitizing capabilities and are used to treat Type II diabetes mellitus. However, glitazones can cause fluid retention and weight gain, thereby limiting their usefulness in certain patient populations. The side effect etiology is unknown, but the nature of presentation suggests modulation of salt and water homeostasis processes (ie. ion transport) in cortical collecting duct principal cells. Because PPARγ is expressed in principal cells, receptor ligands could directly modulate tubular mechanisms or secondarily effect salt and water homeostasis via a systemic route. Insulin and Arg8-vasopressin regulate ion transport in the principal cell. We previously showed that glitazones do not alter insulin-stimulated Na+ reabsorption. However, in Madin-Darby Canine Kidney-Clone 7 (MDCKC7) cells, short-circuit current (SCC) electrophysiology revealed that two glitazones—pioglitazone and GI2570—significantly inhibited Arg8-vasopressin-stimulated Cl- secretion and Na+ reabsorption via the cystic fibrosis transmembrane regulator (CFTR) and epithelial Na+ channel (ENaC), respectively. Dose response relationships showed that the IC50 for Cl - secretion closely matched the EC50 for receptor (PPARγ) transactivation by both compounds. The effect on Cl- secretion was also time-dependent. In order to confirm the involvement of PPARγ in glitazoneinduced inhibition of Cl- secretion, two PPARγ antagonists (T0070907 & GW9662) were used. Neither antagonist reversed the inhibitory effects of the glitazones on Cl- secretion, suggesting that PPARγ is not required. No changes in cellular [cAMP], cAMP-dependent protein kinase A (PKA) catalytic subunit α-1 expression or activation, AMP-activated protein kinase (AMPK) expression or activation, or total cellular adenine nucleotide levels were found with glitazone treatment. However, quantitative PCR experiments showed that both glitazones significantly lowered CFTR mRNA expression. This is consistent with a decrease in CFTR-mediated Cl- secretion in the presence of GI2570 or pioglitazone. These results show that two chemically unique glitazones significantly inhibit Arg 8-vasopressin stimulated Cl- secretion by lowering the amount of CFTR message expressed in the cell. These data suggest a heretofore unappreciated role for Cl- transport in renal-driven salt and water homeostatic mechanisms.