The phosphorylation of endogenous Nedd4-2 In Na+—absorbing human airway epithelial cells

Abstract

Neural precursor cell expressed, developmentally down-regulated protein 4-2 (Nedd4-2) mediates the internalisation / degradation of epithelial Na+ channel subunits (α-, β- and γ-ENaC). Serum / glucocorticoid inducible kinase 1 (SGK1) and protein kinase A (PKA) both appear to inhibit this process by phosphorylating Nedd4-2-Ser221, -Ser327 and -Thr246. This Nedd4-2 inactivation process is thought to be central to the hormonal control of Na+ absorption. The present study of H441 human airway epithelial cells therefore explores the effects of SGK1 and / or PKA upon the phosphorylation / abundance of endogenous Nedd4-2; the surface expression of ENaC subunits, and electrogenic Na+ transport. Effects on Nedd4-2 phosphorylation/abundance and the surface expression of ENaC were monitored by western analysis, whilst Na+ absorption was quantified electrometrically. Acutely (20 min) activating PKA in glucocorticoid-deprived (24 h) cells increased the abundance of Ser221-phosphorylated, Ser327-phosphorylated and total Nedd4-2 without altering the abundance of Thr246-phosphorylated Nedd4-2. Activating PKA under these conditions did not cause a co-ordinated increase in the surface abundance of α-, β- and γ-ENaC and had only a very small effect upon electrogenic Na+ absorption. Activating PKA (20 min) in glucocorticoid-treated (0.2 µM dexamethasone, 24 h) cells, on the other hand, increased the abundance of Ser221-, Ser327- and Thr246-phosphorylated and total Nedd4-2; increased the surface abundance of α-, β- and γ-ENaC and evoked a clear stimulation of Na+ transport. Chronic glucocorticoid stimulation therefore appears to allow cAMP-dependent control of Na+ absorption by facilitating the effects of PKA upon the Nedd4-2 and ENaC subunits.