title: Hypotonic stress upregulates β- and γ-ENaC expression through suppression of ERK by inducing MKP-1.
abstract: We investigated a physiological role for ERK, a member of the MAPK family, in the hypotonic stimulation of epithelial Na(+) channel (ENaC)-mediated Na(+) reabsorption in renal epithelial A6 cells. We show that hypotonic stress causes a major dephosphorylation of ERK following a rapid transient phosphorylation. PD98059 (a MEK inhibitor) increases dephosphorylated ERK and enhances the hypotonic-stress-stimulated Na(+) reabsorption. ERK dephosphorylation is mediated by MAPK phosphatase (MKP). Hypotonic stress activates p38, which in turn induces MKP-1 and to a lesser extent MKP-3 mRNA expression. Inhibition of p38 suppresses MKP-1 induction, preventing hypotonic stress from dephosphorylating ERK. Inhibition of MKP-1 and -3 by the inhibitor NSC95397 also suppresses the hypotonicity-induced dephosphorylation of ERK. NSC95397 reduces both β- and γ-ENaC mRNA expression and ENaC-mediated Na(+) reabsorption stimulated by hypotonic stress. In contrast, pretreatment with PD98059 significantly enhances mRNA and protein expression of β- and γ-ENaC even under isotonic conditions. However, PD98059 only stimulates Na(+) reabsorption in response to hypotonic stress, suggesting that ERK inactivation by itself (i.e., under isotonic conditions) is not sufficient to stimulate Na(+) reabsorption, even though ERK inactivation enhances β- and γ-ENaC expression. Based on these results, we conclude that hypotonic stress stimulates Na(+) reabsorption through at least two signaling pathways: 1) induction of MKP-1 that suppresses ERK activity and induces β- and γ-ENaC expression, and 2) promotion of translocation of the newly synthesized ENaC to the apical membrane.
authors.affiliation: Department of Molecular Cell Physiology, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Naomi
authors.initials: N
authors.lastname: Niisato
authors.forename: Mariko
authors.initials: M
authors.lastname: Ohta
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Eaton
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Marunaka
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D054638:Dual Specificity Phosphatase 1 / Q000378:metabolism*; D054642:Dual Specificity Phosphatase 6 / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000187:drug effects* / Q000378:metabolism; D053503:Epithelial Sodium Channels / Q000378:metabolism*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D005419:Flavonoids / Q000494:pharmacology; D007038:Hypotonic Solutions / Q000494:pharmacology*; D007668:Kidney / Q000166:cytology / Q000378:metabolism*; D048670:MAP Kinase Kinase 4 / Q000378:metabolism; D023421:Models, Animal; D009285:Naphthoquinones / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D015854:Up-Regulation / Q000187:drug effects*; D014982:Xenopus laevis; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
chemical_list: C451261:2,3-bis(2-hydroxyethylsulfanyl)-(1,4)naphthoquinone; D004791:Enzyme Inhibitors; D053503:Epithelial Sodium Channels; D005419:Flavonoids; D007038:Hypotonic Solutions; D009285:Naphthoquinones; D048049:Extracellular Signal-Regulated MAP Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D054638:Dual Specificity Phosphatase 1; D054642:Dual Specificity Phosphatase 6; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one