Upregulation of Na+/H+ exchanger by the AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is activated upon energy depletion and serves to restore energy balance by stimulating energy production and limiting energy utilization. Specifically, it enhances cellular glucose uptake by stimulating GLUT and SGLT1 and glucose utilization by stimulating glycolysis. During O2 deficiency glycolytic degradation of glucose leads to formation of lactate and H+, thus imposing an acid load to the energy-deficient cell. Cellular acidification inhibits glycolysis and thus impedes glucose utilization. Maintenance of glycolysis thus requires cellular H+ export. The present study explored whether AMPK influences Na+/H+ exchanger (NHE) activity and/or Na+-independent acid extrusion. NHE1 expression was determined by RT-PCR and Western blotting. Cytosolic pH (pHi) was estimated utilizing BCECF fluorescence and Na+/H+ exchanger activity from the Na+-dependent re-alkalinization (ΔpHi) after an ammonium pulse. As a result, human embryonic kidney (HEK) cells express NHE1. The pHi and ΔpHi in those cells were significantly increased by treatment with AMPK stimulator AICAR (1 mM) and significantly decreased by AMPK inhibitor compound C (10 μM). The effect of AICAR on pHi and ΔpHi was blunted in the presence of the Na+/H+ exchanger inhibitor cariporide (10 μM), but not by the H+ ATPase inhibitor bafilomycin (10 nM). AICAR significantly enhanced lactate formation, an effect significantly blunted in the presence of cariporide. These observations disclose a novel function of AMPK, i.e. regulation of cytosolic pH.

Research highlights
► AMP-activated protein kinase (AMPK) activates the Na+/H+ exchanger (NHE1) thus increasing cytosolic pH in human embryonic kidney (HEK) cells.
► AMPK thus stimulates the highly pH-sensitive glycolytic flux.
► Inhibition of the the Na+/H+ exchanger by cariporide abrogates the stimulation of NHE1 by AMPK and thus decreases pH and glycolytic flux.
► The stimulation of NHE1 is thus a prerequisite for AMPK-dependent stimulation of energy generation from glycolysis.