The functioning of the respiratory tract is dependent upon the absorption of Na⁺ from the liquid film covering the lung / airway epithelia and this process is dependent upon the epithelial Na⁺ channel (ENaC). Glucocorticoids play an important role in the induction / maintenance of this phenotype although the mechanisms underlying this are not well understood. H441 airway epithelial cells (e.g. Thomas et al. Am. J. Physiol. Lung Cell. Mol. Physiol. 287: L843-L851, 2004) express a Na⁺ conductance (G_Na) essentially identical to that associated with co-expression of the component subunits of the epithelial Na⁺ channel (ENaC), whilst work from this laboratory showed that G_Na was negligible in hormone-deprived cells and thus established that the activity / expression of this endogenous conductance was strictly dependent upon glucocorticoids stimulation (Clunes et al. J. Physiol. 557: 809–819, 2004). We have therefore explored the signalling pathways that underlie the dexamethasone-induced activation of this endogenous conductance.

Glucocorticoids activate serum and glucocorticoid regulated kinase 1 (SGK1) (see Lang & Cohen. Science STKE 108: RE17, 2001) and this kinase controls the apical abundance of ENaC by regulating channel internalization / degradation (e.g. Snyder. Endocrine Rev. 23: 258–275, 2002). Assays of endogenosu SGK1 activity (see Murray et al., Biochem. J. 385: 1–12, 2005) showed that dexamethasone activates this kinase within ~2 h. This response was abolished by LY294002, an inhibitor of PI3K whilst electrophysiological studies confirmed that dexamethasone also increases G_Na. This response also occurred within 2 h. This electrophysiological response was also abolished by LY294002 and, since PI3K controls SGK1 by phosphorylating SGK1-Thr256 and SGK1-Ser422 (Kobayashi & Cohen. Biochem. J. 339: 319–328, 1999) these data are consistent with the view that glucocorticoids control G_Na via an SGK1-depdendent mechanism. Further support for this came from the fact that transiently expressing constitutively active SGK1 (SGK1-S422D) increased cellular SGK1 activity and induced membrane Na⁺ current. However, despite this, expression of catalytically inactive SGK1 (SGK1-K127A) exerted a dominant negative effect by suppressing the activation of SGK1 but did not prevent the increase in G_Na. Moreover, increasing PI3K activity by transiently expressing a membrane-anchored form of the catalytic PI3K-P110a subunit (CD2-P110a) activated SGK1 and augmented the glucocorticoid-induced G_Na. However, this PI3K-activating construct did not Na⁺ current in hormone-deprived cells. Similarly insulin, a PI3K-activating hormone augmented the dexamethasone-induced G_Na but did not evoke any current in hormone-deprived cells. Whilst the dexamethasone-induced increase in G_Na displays an absolute requirement for PI3K, activating this enzyme does not provide a stimulus sufficient to mimic the electrophysiological effects of glucocorticoid stimulation.