Stimulation of ENaC Activity by Rosiglitazone is PPARγ-Dependent and Correlates with SGK1 Expression Increase

Abstract

Thiazolidinediones (TZDs) are peroxisome proliferator-activated receptor gamma (PPARγ) agonists used to treat type 2 diabetes. TZD treatment induces side effects such as peripheral fluid retention, often leading to discontinuation of therapy. Previous studies have shown that PPARγ activation by TZD enhances the expression or function of the epithelial sodium channel (ENaC) through different mechanisms. However, the effect of TZDs on ENaC activity is not clearly understood. Here, we show that treating Xenopus laevis oocytes expressing ENaC and PPARγ with the TZD rosiglitazone (RGZ) produced a twofold increase of amiloride-sensitive sodium current (Iam), as measured by two-electrode voltage clamp. RGZ-induced ENaC activation was PPARγ-dependent since the PPARγ antagonist GW9662 blocked the activation. The RGZ-induced Iam increase was not mediated through direct serum- and glucocorticoid-regulated kinase (SGK1)-dependent phosphorylation of serine residue 594 on the human ENaC α-subunit but by the diminution of ENaC ubiquitination through the SGK1/Nedd4-2 pathway. In accordance, RGZ increased the activity of ENaC by enhancing its cell surface expression, most probably indirectly mediated through the increase of SGK1 expression.

PPARγ-Induced Stimulation of Amiloride-Sensitive Sodium Current in Renal Collecting Duct Principal Cells is Serum and Insulin Dependent

“…However, the precise mechanisms underlying these effects remain controversial. We have recently shown that RGZ increases ENaC activity in Xenopus oocytes [26]. With the present data, we demonstrate that RGZ-induced stimulation of the amiloride-sensitive sodium current in mpkCCD c14 cells is serum and insulin dependent.…”

“…Furthermore, RXR expression has been shown in the nephron [13] in which activation of this receptor is provided by 9-cis retinoic acid, a component of FBS [30]. Therefore, it is most likely that stimulation of ENaC activity by PPARγ agonists obtained in our studies and described by other investigators [12,22,26] may in part be due to the activation of RXR by retinoic acid.…”

“…In contrast, RGZ had no effect on sodium transport in these cells [33]. Our recent findings have shown that RGZ treatment of PPARγ/ENaC expressing Xenopus oocytes increases ENaC activity [26]. This activation was abolished by the PPARγ antagonist GW9662.…”

“…Tiwari and colleagues demonstrated an increase of γENaC in kidney plasma membrane-enriched fractions in rats treated with PPARγ agonists [25]. Recently, we have shown that RGZ treatment of PPARγ/ENaC-expressing Xenopus oocytes increases ENaC activity, this stimulation being correlated with an increase in SGK expression [26]. In addition, fluid retention induced by GI262570 treatment, another TZD, has been reported to be correlated with the stimulation of ENaC-Na/K ATPase system in rat [27].…”

Fluid Retention Caused by Rosiglitazone Is Related to Increases in AQP2 andαENaC Membrane Expression

“…Na + reabsorption is mediated via the amiloride-sensitive epithelial sodium channel (ENaC), which exhibits high selectivity for sodium [ 15 ] and is a central requirement for Na + reabsorption across renal epithelia. ENaC expression and translocation to the plasma membrane are tightly regulated by a diverse array of hormonal [ 16 , 17 ] and physical factors [ 18 ]. Our experiments show that ROS (10 μ M) remarkably raised the membrane level of α ENaC, and thus more and more Na + flowed into the lumen, which accelerated water reabsorption and caused more serious fluid retention.…”