Regulation of sgk by aldosterone and its effects on the epithelial Na+ channel

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 ald osterone induces the serum- and glucocorticoid-dependent kinase sgk and tha t coexpressing ENaC with this kinase in Xenopus laevis oocytes increases th e amiloride-sensitive Nat current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P Firestone GL, Verrey F, and Pearce D. PI oc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further ch aracterize regulation of sgk by aldosterone in native mammalian epithelia a nd 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 demonstr ated in rat kidney and colon but not in lung. Induction of sgk by aldostero ne was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was de tected as early as 30 min after the hormonal application and was independen t of de novo protein synthesis. The observed aldosterone dose-response rela tionships suggest that the response is mediated, at least in part, by occup ancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopu s 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 t o sgk.