title: Down-regulation of the Na+-coupled phosphate transporter NaPi-IIa by AMP-activated protein kinase.
abstract: OBJECTIVE
  The Na(+)-coupled phosphate transporter NaPi-IIa is the main carrier accomplishing renal tubular phosphate reabsorption. It is driven by the electrochemical Na(+) gradient across the apical cell membrane, which is maintained by Na(+) extrusion across the basolateral cell membrane through the Na(+)/K(+) ATPase. The operation of NaPi-IIa thus requires energy in order to avoid cellular Na(+) accumulation and K(+) loss with eventual decrease of cell membrane potential, Cl(-) entry and cell swelling. Upon energy depletion, early inhibition of Na(+)-coupled transport processes may delay cell swelling and thus foster cell survival. Energy depletion is sensed by the AMP-activated protein kinase (AMPK), a serine/threonine kinase stimulating several cellular mechanisms increasing energy production and limiting energy utilization. The present study explored whether AMPK influences the activity of NAPi-IIa.
  
  
  METHODS
  cRNA encoding NAPi-IIa was injected into Xenopus oocytes with or without additional expression of wild-type AMPK (AMPK(α1)-HA+AMPK(β1)-Flag+AMPK(γ1)-HA), of inactive AMPK(αK45R) (AMPK(α1K45R)+AMPK(β1)-Flag+AMPK(γ1)-HA) or of constitutively active AMPK(γR70Q) (AMPK(α1)-HA+AMPK(β1)-Flag+AMPKγ1(R70Q)). NaPi-IIa activity was estimated from phosphate-induced current in dual electrode voltage clamp experiments.
  
  
  RESULTS
  In NaPi-IIa-expressing, but not in water-injected Xenopus oocytes, the addition of phosphate (1 mM) to the extracellular bath solution generated a current (Ip), which was significantly decreased by coexpression of wild-type AMPK and of AMPK(γR70Q) but not of AMPK(αK45R). The phosphate-induced current in NaPi-IIa- and AMPK-expressing Xenopus ooocytes was significantly increased by AMPK inhibitor Compound C (20 µM). Kinetic analysis revealed that AMPK significantly decreased the maximal transport rate.
  
  
  CONCLUSIONS
  The AMP-activated protein kinase AMPK is a powerful regulator of NaPi-IIa and thus of renal tubular phosphate transport. © 2013 S. Karger AG, Basel.
authors.affiliation: University of Tübingen, Department of Physiology, Tübingen, Germany.
authors.forename: Miribane
authors.initials: M
authors.lastname: Dërmaku-Sopjani
authors.forename: Ahmad
authors.initials: A
authors.lastname: Almilaji
authors.forename: Tatsiana
authors.initials: T
authors.lastname: Pakladok
authors.forename: Carlos
authors.initials: C
authors.lastname: Munoz
authors.forename: Zohreh
authors.initials: Z
authors.lastname: Hosseinzadeh
authors.forename: María
authors.initials: M
authors.lastname: Blecua
authors.forename: Mentor
authors.initials: M
authors.lastname: Sopjani
authors.forename: Florian
authors.initials: F
authors.lastname: Lang
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000235:genetics / Q000502:physiology*; D000818:Animals; D020134:Catalytic Domain / Q000235:genetics; D015536:Down-Regulation / Q000235:genetics*; D005260:Female; D049449:Luminescence; D009154:Mutation; D009865:Oocytes / Q000201:enzymology / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D014982:Xenopus laevis
chemical_list: D047428:Protein Kinase Inhibitors; C493554:SLC34A1 protein, human; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa; D055372:AMP-Activated Protein Kinases