title: Nedd4-2 phosphorylation induces serum and glucocorticoid-regulated kinase (SGK) ubiquitination and degradation.
abstract: Serum and glucocorticoid-regulated kinase (SGK) plays a key role in the regulation of epithelial Na+ transport. SGK phosphorylates Nedd4-2, an E3 ubiquitin-protein ligase that targets the epithelial Na+ channel (ENaC) for degradation. Phosphorylation increases Na+ transport by reducing Nedd4-2 binding to ENaC, which increases ENaC expression at the cell surface. Thus, SGK expression must be tightly controlled to maintain Na+ homeostasis. This occurs in part by regulation of SGK transcription; a variety of signals including steroid hormones (aldosterone and glucocorticoids) increase SGK levels by inducing transcription of SGK. However, SGK has a short half-life, suggesting that SGK levels might also be controlled by regulation of SGK degradation. Here we found that SGK degradation is mediated in part by Nedd4-2. Consistent with this model, overexpression of Nedd4-2 decreased steady-state levels of SGK in a dose-dependent manner by increasing SGK ubiquitination and degradation in the 26S proteasome. Conversely, silencing of Nedd4-2 by RNA interference stabilized SGK. Nedd4-2 phosphorylation potentiates SGK degradation; degradation was reduced by Nedd4-2 and SGK mutations that disrupt phosphorylation or by inhibition of SGK kinase activity. Together with previous work, the data support a model in which SGK and Nedd4-2 regulate one another in a reciprocal manner. SGK phosphorylates Nedd4-2, which reduces Nedd4-2 binding and inhibition of ENaC. Conversely, phosphorylation increases Nedd4-2-mediated degradation of SGK. Thus, by phosphorylating Nedd4-2, SGK induces its own degradation. This feedback inhibition may fine-tune the regulation of epithelial Na+ absorption.
authors.affiliation: Department of Internal Medicine and Physiology, University of Iowa College of Medicine, Iowa City, Iowa 52242, USA.
authors.forename: Ruifeng
authors.initials: R
authors.lastname: Zhou
authors.forename: Peter M
authors.initials: PM
authors.lastname: Snyder
mesh_terms: D000818:Animals; D015153:Blotting, Western; D019556:COS Cells; D002462:Cell Membrane / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D056827:Endosomal Sorting Complexes Required for Transport; D004848:Epithelium / Q000378:metabolism; D005786:Gene Expression Regulation; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D017874:Immediate-Early Proteins; D008954:Models, Biological; D008957:Models, Genetic; D009154:Mutation; D000075702:Nedd4 Ubiquitin Protein Ligases; D009687:Nuclear Proteins / Q000378:metabolism*; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000378:metabolism; D015398:Signal Transduction; D012964:Sodium / Q000737:chemistry / Q000378:metabolism; D013997:Time Factors; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism*; D000099184:Serum-Glucocorticoid Regulated Kinases
chemical_list: D018076:DNA, Complementary; D056827:Endosomal Sorting Complexes Required for Transport; D049452:Green Fluorescent Proteins; D017874:Immediate-Early Proteins; D000075702:Nedd4 Ubiquitin Protein Ligases; D009687:Nuclear Proteins; D046988:Proteasome Endopeptidase Complex; D017346:Protein Serine-Threonine Kinases; D034741:RNA, Small Interfering; D012964:Sodium; D025801:Ubiquitin; D044767:Ubiquitin-Protein Ligases; D000099184:Serum-Glucocorticoid Regulated Kinases; C000618483:Nedd4 protein, human; C000618485:Nedd4L protein, human; C061553:ATP dependent 26S protease