Glucocorticoids can activate the alpha-EnaC gene promoter independently of SGK1

Niall McTavish, Jennet Getty, Ann Burchell, Stuart M. Wilson

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    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 rnonitoring 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 similar to 4 mu M) 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-P110 alpha subunit [CD2 (cluster of differentiation 2)-P110 alpha] also activated endogenous SGK1 without affecting pGL3-KR1 activity. A catalytically inactive form of CD2-P110 alpha (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-P110 alpha 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.

    Original languageEnglish
    Pages (from-to)189-197
    Number of pages9
    JournalBiochemical Journal
    Volume423
    DOIs
    Publication statusPublished - 15 Oct 2009

    Keywords

    • airway epithelium
    • epithelial Na+ channel (ENaC) dexamethasone
    • phosphoinositide 3-kinase (PI3K)
    • reporter gene
    • serum- and glucocorticoid-induced protein kinase 1 (SGK 1)
    • EPITHELIAL SODIUM-CHANNEL
    • INDUCIBLE PROTEIN-KINASE
    • LUNG LIQUID
    • 5'-FLANKING REGION
    • RESPONSE ELEMENT
    • NA+ TRANSPORT
    • SERUM
    • CELLS
    • PHOSPHORYLATION
    • EXPRESSION

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