Abstract
The role of SGK1 (serum- and glucocorticoid-induced protein kinase 1) in the glucocorticoid induction of alpha-ENaC (epithelial Na+ channel alpha subunit) gene transcription was explored by monitoring the transcriptional activity of a luciferase-linked, alpha-ENaC reporter gene construct (pGL3-KR1) expressed in H441 airway epithelial cells. Dexamethasone evoked a concentration-dependent (EC50 approximately 4 microM) increase in transcriptional activity dependent upon a glucocorticoid response element in the alpha-ENaC sequence. Although dexamethasone also activated endogenous SGK1, artificially increasing cellular SGK1 activity by expressing a constitutively active SGK1 mutant (SGK1-S422D) in hormone-deprived cells did not activate pGL3-KR1. Moreover, expression of catalytically inactive SGK1 (SGK1-K127A) suppressed the activation of endogenous SGK1 without affecting the transcriptional response to dexamethasone. Increasing cellular PI3K (phosphoinositide 3-kinase) activity by expressing a membrane-anchored form of the catalytic PI3K-P110alpha subunit [CD2 (cluster of differentiation 2)-P110alpha] also activated endogenous SGK1 without affecting pGL3-KR1activity. A catalytically inactive form of CD2-P110alpha (R1130P), on the other hand, prevented the dexamethasone-induced activation of SGK1, but did not inhibit the activation of pGL3-KR1. However, expression of SGK1-S422D or CD2-P110alpha enhanced the transcriptional responses to maximally effective concentrations of dexamethasone and this effect occurred with no change in EC50. Dexamethasone-induced (0.3-300 nM) activation of pGL3-KR1 was unaffected by inhibitors of PI3K (PI-103 and wortmanin) and by rapamycin, a selective inhibitor of the TORC1 (target of rapamycin complex 1) signalling complex. Dexamethasone-induced activation of the alpha-ENaC gene promoter can thus occur independently of SGK1/PI3K, although this pathway does provide a mechanism that allows this transcriptional response to dexamethasone to be enhanced.