title: FXYD5 modulates Na+ absorption and is increased in cystic fibrosis airway epithelia.
abstract: FXYD5, also known as dysadherin, belongs to a family of tissue-specific regulators of the Na(+)-K(+)-ATPase. We determined the kinetic effects of FXYD5 on Na(+)-K(+)-ATPase pump activity in stably transfected Madin-Darby canine kidney cells. FXYD5 significantly increased the apparent affinity for Na(+) twofold and decreased the apparent affinity for K(+) by 60% with a twofold increase in V(max) of K(+), a pattern that would increase activity and Na(+) removal from the cell. To test the effect of increased Na(+) uptake on FXYD5 expression, we analyzed Madin-Darby canine kidney cells stably transfected with an inducible vector expressing all three subunits of the epithelial Na(+) channel (ENaC). Na(+)-K(+)-ATPase activity increased sixfold after 48-h ENaC induction, but FXYD5 expression decreased 75%. FXYD5 expression was also decreased in lung epithelia from mice that overexpress ENaC, suggesting that chronic Na(+) absorption by itself downregulates epithelial FXYD5 expression. Patients with cystic fibrosis (CF) display ENaC-mediated hyperabsorption of Na(+) in the airways, accompanied by increased Na(+)-K(+)-ATPase activity. However, FXYD5 was significantly increased in the lungs and nasal epithelium of CF mice as assessed by RT-PCR, immunohistochemistry, and immunoblot analysis (P < 0.001). FXYD5 was also upregulated in nasal scrapings from human CF patients compared with controls (P < 0.02). Treatment of human tracheal epithelial cells with a CFTR inhibitor (I-172) confirmed that loss of CFTR function correlated with increased FXYD5 expression (P < 0.001), which was abrogated by an inhibitor of NF-kappaB. Thus FXYD5 is upregulated in CF epithelia, and this change may exacerbate the Na(+) hyperabsorption and surface liquid dehydration observed in CF airway epithelia.
authors.affiliation: Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA. txm20@case.edu
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Miller
authors.forename: Pamela B
authors.initials: PB
authors.lastname: Davis
mesh_terms: D000042:Absorption; D000293:Adolescent; D000328:Adult; D000818:Animals; D002460:Cell Line; D003550:Cystic Fibrosis / Q000473:pathology / Q000503:physiopathology*; D004285:Dogs; D005260:Female; D006801:Humans; D007473:Ion Channels; D007668:Kidney; D008168:Lung / Q000187:drug effects / Q000502:physiology / Q000503:physiopathology; D008297:Male; D008562:Membrane Glycoproteins / Q000502:physiology*; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D051379:Mice; D008840:Microfilament Proteins; D009363:Neoplasm Proteins / Q000502:physiology*; D012016:Reference Values; D020545:Respiratory Mucosa / Q000187:drug effects / Q000502:physiology* / Q000503:physiopathology; D012964:Sodium / Q000378:metabolism*
chemical_list: C451410:FXYD5 protein, human; C506911:FXYD5 protein, mouse; D007473:Ion Channels; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D008840:Microfilament Proteins; D009363:Neoplasm Proteins; D012964:Sodium