Down-regulation of the Na+-coupled phosphate transporter NaPi-IIa by AMP-activated protein kinase

Abstract

Background/aims: 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.

Conclusion: 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.