title: Serum and glucocorticoid-inducible kinase1 increases plasma membrane wt-CFTR in human airway epithelial cells by inhibiting its endocytic retrieval.
abstract: BACKGROUND
  Chloride (Cl) secretion by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) located in the apical membrane of respiratory epithelial cells plays a critical role in maintenance of the airway surface liquid and mucociliary clearance of pathogens. Previously, we and others have shown that the serum and glucocorticoid-inducible kinase-1 (SGK1) increases wild type CFTR (wt-CFTR) mediated Cl transport in Xenopus oocytes by increasing the amount of wt-CFTR protein in the plasma membrane. However, the effect of SGK1 on the membrane abundance of wt-CFTR in airway epithelial cells has not been examined, and the mechanism whereby SGK1 increases membrane wt-CFTR has also not been examined. Thus, the goal of this study was to elucidate the mechanism whereby SGK1 regulates the membrane abundance of wt-CFTR in human airway epithelial cells.
  
  
  RESULTS
  We report that elevated levels of SGK1, induced by dexamethasone, increase plasma membrane abundance of wt-CFTR. Reduction of SGK1 expression by siRNA (siSGK1) and inhibition of SGK1 activity by the SGK inhibitor GSK 650394 abrogated the ability of dexamethasone to increase plasma membrane wt-CFTR. Overexpression of a constitutively active SGK1 (SGK1-S422D) increased plasma membrane abundance of wt-CFTR. To understand the mechanism whereby SGK1 increased plasma membrane wt-CFTR, we examined the effects of siSGK1 and SGK1-S442D on the endocytic retrieval of wt-CFTR. While siSGK1 increased wt-CFTR endocytosis, SGK1-S442D inhibited CFTR endocytosis. Neither siSGK1 nor SGK1-S442D altered the recycling of endocytosed wt-CFTR back to the plasma membrane. By contrast, SGK1 increased the endocytosis of the epidermal growth factor receptor (EGFR).
  
  
  CONCLUSIONS
  This study demonstrates for the first time that SGK1 selectively increases wt-CFTR in the plasma membrane of human airway epithelia cells by inhibiting its endocytic retrieval from the membrane.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, United States of America.
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Bomberger
authors.affiliation: Department of Microbiology and Immunology and of Physiology, The Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, United States of America.
authors.forename: Bonita A
authors.initials: BA
authors.lastname: Coutermarsh
authors.affiliation: Department of Microbiology and Immunology and of Physiology, The Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, United States of America.
authors.forename: Roxanna L
authors.initials: RL
authors.lastname: Barnaby
authors.affiliation: Mt. Desert Island Biological Laboratory, Salisbury Cove, Maine, United States of America.
authors.forename: J Denry
authors.initials: JD
authors.lastname: Sato
authors.affiliation: Mt. Desert Island Biological Laboratory, Salisbury Cove, Maine, United States of America.
authors.forename: M Christine
authors.initials: MC
authors.lastname: Chapline
authors.affiliation: Department of Microbiology and Immunology and of Physiology, The Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, United States of America ; Mt. Desert Island Biological Laboratory, Salisbury Cove, Maine, United States of America.
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Stanton
mesh_terms: D001565:Benzoates / Q000494:pharmacology; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism*; D016764:Cell Polarity; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism; D004790:Enzyme Induction; D004847:Epithelial Cells / Q000201:enzymology*; D066246:ErbB Receptors / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects; D005938:Glucocorticoids / Q000494:pharmacology; D006801:Humans; D017874:Immediate-Early Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D021381:Protein Transport; D020545:Respiratory Mucosa / Q000378:metabolism; D000099184:Serum-Glucocorticoid Regulated Kinases
chemical_list: D001565:Benzoates; D019086:Bridged Bicyclo Compounds, Heterocyclic; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D003907:Dexamethasone; D066246:ErbB Receptors; D005938:Glucocorticoids; D017874:Immediate-Early Proteins; D017346:Protein Serine-Threonine Kinases; D000099184:Serum-Glucocorticoid Regulated Kinases; C505032:CFTR protein, human; C532254:2-cyclopentyl-4-(5-phenyl-1H-pyrrolo(2,3-b)pyridin-3-yl)-benzoic acid; C512478:EGFR protein, human