NF-κB Inhibits Sodium Transport via Down-regulation of SGK1 in Renal Collecting Duct Principal Cells

Sophie De Seigneux,Valérie Leroy,Hafida Ghzili,Martine Rousselot,Søren Nielsen,Bernard C Rossier,Pierre-Yves Martin,Eric Féraille

doi:10.1074/jbc.m803812200

Abstract

Tubulointerstitial inflammation is a common feature of renal diseases. We have investigated the relationship between inflammation and Na(+) transport in the collecting duct (CD) using the mCCD(cl1) and mpkCDD(cl4) principal cell models. Lipopolysaccharide (LPS) decreased basal and aldosterone-stimulated amiloride-sensitive transepithelial current in a time-dependent manner. This effect was associated with a decrease in serum and glucocorticoid-regulated kinase 1 (SGK1) mRNA and protein levels followed by a decrease in epithelial sodium channel (ENaC) alpha-subunit mRNA levels. The LPS-induced decrease in SGK1 expression was confirmed in isolated rat CD. This decreased expression of either SGK1 or the ENaC alpha-subunit was not due to enhanced degradation of mRNA. In contrast, LPS inhibited transcriptional activity of the SGK1 promoter measured by luciferase-reporter gene assay. The effect of LPS was not mediated by inhibition of mineralocorticoid or glucocorticoid receptor, because expression of both receptors was unchanged and blockade of either receptor by spironolactone or RU486, respectively, did not prevent the down-regulation of SGK1. The effect of LPS was mediated by the canonical NF-kappaB pathway, as overexpression of a constitutively active mutant, IKKbeta (inhibitor of nuclear factor kappaB kinase-beta) decreased SGK1 mRNA levels, and knockdown of p65 NF-kappaB subunit by small interfering RNA increased SGK1 mRNA levels. Chromatin immunoprecipitation showed that LPS increased p65 binding to two NF-kappaB sites along the SGK1 promoter. In conclusion, we show that activation of the NF-kappaB pathway down-regulates SGK1 expression, which might lead to decreased ENaC alpha-subunit expression, ultimately resulting in decreased Na(+) transport.

Highlights

Critical regulation of Naϩ and water reabsorption occurring in the collecting duct (CD)2 is handled mainly by systemic hor
The results show that LPS did not LPS-induced Down-regulation of serum and glucocorticoid-regulated kinase 1 (SGK1) Expression Is Not increase the rate of SGK1 or ENaC ␣-subunit mRNA degrada- Mediated by Decreased mineralocorticoid receptor (MR) and glucocorticoid receptors (GR) Activity—To determine tion
The results of the present study reveal that proinflammatory cytokines and LPS antagonize electrogenic Naϩ transport at least partly via NF-␬B-dependent inhibition of SGK1 transcription

Summary

Introduction

Critical regulation of Naϩ and water reabsorption occurring in the collecting duct (CD) is handled mainly by systemic hor-. Protein kinase SGK1 is a key player in regulating Naϩ transport in the CD It increases apical plasma membrane expression and activity of the apical epithelial sodium channel (ENaC) by reducing NEDD4 ubiquitin ligase-dependent internalization [7]. The canonical pathway is the major NF-␬B pathway in most cells and is activated by various stimuli including proinflammatory cytokines and bacterial products such as lipopolysaccharide (LPS) This pathway is dependent on the nuclear factor ␬B kinase-␤ (IKK␤), which phosphorylates I␬B␣ protein and induces its degradation by the proteasome, leading to the release of p65-p50 NF-␬B heterodimers. The alternative NF-␬B pathway is triggered by various factors such as CD40R, LT␤R, and BAFFR and is strictly dependent on protein kinase IKK␣ activation, leading to regulated cleavage of p100 NF-␬B subunit and generation of p52-RelB NF-␬B heterodimers This noncanonical pathway is involved mainly in the maintenance and development of lymphoid organs. In this work we demonstrate that: 1) basal and aldosterone-stimulated amiloride-sensitive electrogenic ion transport and SGK1 expression are significantly decreased by proinflammatory stimuli independently of MR or GR inhibition; and 2) the canonical NF-␬B pathway directly down-regulates SGK1 expression, which may participate in reducing ENaC ␣-subunit expression, leading altogether to a decrease in sodium transport

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