Regulation ofsgkby aldosterone and its effects on the epithelial Na⁺channel

Abstract

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

A Novel Pathway of Epithelial Sodium Channel Activation Involves a Serum- and Glucocorticoid-inducible Kinase Consensus Motif in the C Terminus of the Channel's α-Subunit

“…In primary cultures of rabbit renal collecting duct cells (16) and in rat distal nephron, aldosterone was shown to stimulate the expression of SGK1 mRNA (17). This effect correlates well with the stimulatory effect of aldosterone on sodium transport and appears to be mediated by the mineralocorticoid receptor (18). Immunohistochemical studies have shown that in the renal collecting duct SGK1 is not expressed in intercalated cells but in principal cells consistent with a specific co-expression of SGK1 and ENaC (19).…”

“…Immunohistochemical studies have shown that in the renal collecting duct SGK1 is not expressed in intercalated cells but in principal cells consistent with a specific co-expression of SGK1 and ENaC (19). SGK1 mRNA and protein levels are also increased by aldosterone in distal colon (17,18,20). These findings suggest that SGK1 is a mediator of aldosterone action in classic mineralocorticoid target tissues.…”

“…Moreover, in the same study it was demonstrated that the rate of channel endocytosis was similar in oocytes co-expressing ENaC and SGK1 compared with control oocytes expressing ENaC alone. Recently, another group has reported that a point mutation (Y618A) in the ␤-subunit of ENaC, which is known to impair regulation of ENaC by Nedd4, had no significant effect on the stimulatory effect of SGK on ENaC currents (18). Taken together, these findings argue against an exclusive effect of SGK1 on the rate of Nedd4-2-mediated channel retrieval.…”

“…This was initially demonstrated by co-expression experiments in Xenopus laevis oocyte (16,18,21,22) and more recently in cultured renal epithelial cells (23,24). SGK1 is thought to be an important molecular target that integrates multiple endocrine inputs regulating epithelial sodium transport (24, 25).…”

Transcriptome of a mouse kidney cortical collecting duct cell line: Effects of aldosterone and vasopressin

“…2 A). Aldosterone further stimulated GILZ mRNA expression by 11-fold increase after 4 h, and by 7-fold after 24 h. To date, only a similar rapid and high induction of gene expression caused by aldosterone in mammalian epithelia has been reported for the recently identified serum and glucocorticoid-regulated kinase (sgk-kinase) shown to significantly regulate the ENaC activity (11,33,34). GILZ belongs to the TSC-22͞DIP family of proteins that were initially proposed to function as transcription factors (28).…”