A novel role for glucocorticoid-induced leucine zipper protein in epithelial sodium channel-mediated sodium transport.

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.