In the hormonal control of sodium transport in the lung via glucocorticoids there is substantial evidence that supports a central role for serum- and glucocorticoid-kinase(SGK1). The activity of this kinase is dependent upon phosphorylation of two distinct residues by the target of rapamycin complex 2 (TORC2) and phosphoinositidedependent Kinase 1 (PDK1), both of which are dependent upon Phosphoinositide-3-Kinase (PI3K). However, SGK1 knockout mice do not display any pulmonary abnormalities. Thus the role that SGK1 plays is not fully understood. In this thesis I have explored the role of this kinase in dexamethasone-induced ENaC activity in the H441 human bronchiolar cell model. Dexamethasone-deprived cells do not display ENaC activity as there is no amiloride sensitive current in these cells. Groups of dexamethasone-treated H441 cells do display ENaC activity; however single cells do not display ENaC activity despite displaying an increase in current. Thus cell-cell contact is vital to the development of amiloride sensitivity. SGK1 activity does not mimic the electrophysiological effects of dexamethasone-stimulation as there is no amiloride sensitivity seen after ~3 hours despite a clear increase in SGK1 activity. Furthermore after ~24 hours stimulation, there is a clear amiloride sensitive current although SGK1 activity is comparable to that of dexamethasone-deprived cells. These findings further question whether SGK1 is required for dexamethasone-evoked ENaC activity. Inhibition of PI3K, TORC2 and SGK1 abolished ENaC activity. However inhibition of TORC1 had no effect upon dexamethasone induced ENaC activity. Thus demonstrates that the maintenance of glucocorticoid induced ENaC activity, in pulmonary epithelium, is dependent upon the PI3K-TORC2-SGK1 signalling pathway. Furthermore PI3K plays a permissive, but critical role as its activity is required for SGK1 activity. However without SGK1 activity dexamethasone induced ENaC activity cannot be maintained.
|Date of Award||2011|
|Sponsors||George John and Sheilah Livanos Charitable Trust|
|Supervisor||Stephen Land (Supervisor) & Stuart Wilson (Supervisor)|