Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa 52242, USA. psynder@blue.weeg.uiowa.edu
The epithelial Na+ channel (ENaC) forms the pathway for Na+ absorption across epithelia, including the kidney collecting duct, where it plays a critical role in Na+ homeostasis and blood pressure control. Na+ absorption is regulated in part by mechanisms that control the expression of ENaC at the apical cell surface. Nedd4 family members (e.g. Nedd4, Nedd4-2) bind to the channel and decrease its surface expression by catalyzing its ubiquitination and degradation. Conversely, serum and glucocorticoid-regulated kinase (SGK), a downstream mediator of aldosterone, increases the expression of ENaC at the cell surface. Here we show that SGK and human Nedd4-2 (hNedd4-2) converge in a common pathway to regulate epithelial Na+ absorption. Consistent with this model, we found that SGK bound to hNedd4-2 and hNedd4. A PY motif in SGK mediated the interaction and was required for SGK to stimulate ENaC. SGK phosphorylated hNedd4-2 (but not hNedd4), altering hNedd4-2 function; phosphorylation reduced the binding of hNedd4-2 to alphaENaC, and hence, the hNedd4-2-mediated inhibition of Na+ absorption. These data suggest that SGK regulates epithelial Na+ absorption in part by modulating the function of hNedd4-2.