A Novel Role for Glucocorticoid-induced Leucine Zipper Protein in Epithelial Sodium Channel-mediated Sodium Transport

Alain Vandewalle,Ting Ting Zhang,Jian Wang,David Pearce,Rama Soundararajan

doi:10.1074/jbc.m508658200

Alain Vandewalle, Ting Ting Zhang + Show 3 more

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https://doi.org/10.1074/jbc.m508658200

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Abstract

The steroid hormone aldosterone stimulates sodium (Na+) transport in tight epithelia by altering the expression of target genes that regulate the activity and trafficking of the epithelial sodium channel (ENaC). We performed microarray analysis to identify aldosterone-regulated transcripts in mammalian kidney epithelial cells (mpkC-CD(c14)). One target, glucocorticoid-induced leucine zipper protein (GILZ), was previously identified by serial analysis of gene expression (SAGE); however, its function in epithelial ion transport was unknown. Here we show that GILZ expression is rapidly stimulated by aldosterone in mpkCCD(c14) and that GILZ, in turn, strongly stimulates ENaC-mediated Na+ transport by inhibiting extracellular signal-regulated kinase (ERK) signaling. In Xenopus oocytes with activated ERK, heterologous GILZ expression consistently inhibited phospho-ERK expression and markedly stimulated ENaC-mediated Na+ current, in a manner similar to that of U0126 (a pharmacologic inhibitor of ERK signaling). In mpkCCD(c14) cells, GILZ transfection similarly consistently inhibited phospho-ERK expression and stimulated transepithelial Na+ transport. Furthermore, aldosterone treatment of mpkCCD(c14) cells suppressed phospho-ERK levels with a time course that paralleled their increase of Na+ transport. Finally, GILZ expression markedly increased cell surface ENaC expression in epidermal growth factor-treated mammalian kidney epithelial cells, HEK 293. These observations suggest a novel link between GILZ and regulation of epithelial sodium transport through modulation of ERK signaling and could represent an important pathway for mediating aldosterone actions in health and disease.

Highlights

Hormone-regulated sodium (Naϩ) transport in tight epithelia is essential for the control of circulatory volume, blood pressure, and extracellular fluid composition in vertebrates
It is notable that inhibitors of phosphatidylinositol 3Ј-kinase do not completely abolish aldosterone-induced Naϩ transport [16], further supporting the idea that aldosterone may act through multiple pathways to stimulate Naϩ reabsorption, some of which are SGK1- and phosphatidylinositol 3Ј-kinaseduced leucine zipper protein; mGILZ, mouse glucocorticoid-induced leucine zipper protein (GILZ); CCD, cortical collecting duct; EGF, epidermal growth factor; ERK, extracellular signal-regulated kinase (1/2); FKBP5, FK506-binding protein 5; MEK, mitogen-activated protein kinase/ERK kinase; SGK1, serum- and glucocorticoid-induced kinase-1; HEK cells, human kidney epithelial cells; RT, reverse transcription; IRES, internal ribosomal entry site; EGFP, enhanced green fluorescent protein
Taken together with earlier in vivo data [20], our present data suggest that GILZ is a complementary mediator of the early phase of aldosterone action that participates in an analogous way to inhibit ERK signaling

Summary

Introduction

Hormone-regulated sodium (Naϩ) transport in tight epithelia is essential for the control of circulatory volume, blood pressure, and extracellular fluid composition in vertebrates. The broad aim of the present study, was to identify and characterize key genes (other than SGK1) that stimulate ENaC-mediated Naϩ transport and mediate the early response to aldosterone Toward this end, we performed microarray analysis in the aldosteroneresponsive cortical collecting duct (CCD) cell line, mpkCCDc14, which exhibit typical electrophysiologic features of a tight epithelium, express functional ENaC, and respond to aldosterone (1 ␮M) with increased amiloride-sensitive Naϩ currents [19]. We performed microarray analysis in the aldosteroneresponsive cortical collecting duct (CCD) cell line, mpkCCDc14, which exhibit typical electrophysiologic features of a tight epithelium, express functional ENaC, and respond to aldosterone (1 ␮M) with increased amiloride-sensitive Naϩ currents [19] This manuscript and associated additional data deposited in the NCBI Gene Expression Omnibus (GEO) data base (www.ncbi.nlm.nih.gov/geo) report the results of this screen and characterization of one up-regulated transcript, glucocorticoid-induced leucine zipper protein (GILZ). In contrast to other nephron segments, ERK1/2 appears to be constitutively active in distal nephron cells (24, 26 –28), consistent with tonic inhibition of Naϩ transport by the ERK pathway

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