Rescue of ΔF508-CFTR by the SGK1/Nedd4-2 Signaling Pathway

The most common mutation in cystic fibrosis (CF) is Delta F508, which is associated with failure of the mutant cystic fibrosis transmembrane conductance regulator (CFTR) to traffic to the plasma membrane. By a still unknown mechanism, the loss of correctly trafficked Delta F508-CFTR results in an excess of the epithelial sodium channel (ENaC) on the apical plasma membrane. ENaC trafficking is known to be regulated by a signaling pathway involving the glucocorticoid receptor, the serum-and glucocorticoid-regulated kinase SGK1, and the ubiquitin E3 ligase Nedd4-2. We show here that dexamethasone rescues functional expression of Delta F508-CFTR. The half-life of Delta F508-CFTR is also dramatically enhanced. Dexamethasone-activated Delta F508-CFTR rescue is blocked either by the glucocorticoid receptor antagonist RU38486 or by the phosphatidylinositol 3-kinase inhibitor LY294002. Co-immunoprecipitation studies indicate that Nedd4-2 binds to both wild-type-and Delta F508-CFTR. These complexes are inhibited by dexamethasone treatment, and CFTR ubiquitination is concomitantly decreased. We further show that knockdown of Nedd4-2 by small interfering RNA also corrects Delta F508-CFTR trafficking. Conversely, knockdown of SGK1 by small interfering RNA completely blocks dexamethasone-activated Delta F508-CFTR rescue. These data suggest that the SGK1/Nedd4-2 signaling pathway regulates both CFTR and ENaC trafficking in CF epithelial cells.