AMP-activated protein kinase-independent inhibition of hepatic mitochondrial oxidative phosphorylation by AICA riboside

AICA riboside (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside) has been extensively used in cells to activate the AMPK (AMP-activated protein kinase), a metabolic sensor involved in cell energy homoeostasis. In the present study, we investigated the effects of AICA riboside on mitochondrial oxidative; phosphorylation. AICA riboside was found to dose-dependently inhibit the oligomycin-sensitive JO(2) (oxygen consumption rate) of isolated rat hepatocytes. A decrease in P-i (inorganic phosphate), ATP, AMP and total adenine nuclectide contents was also observed with AICA riboside concentrations > 0.1 mM. Interestingly, in hepatocytes from mice lacking both alpha 1 and alpha 2 AMPK catalytic subunits, basal JO(2) and expression of several mitochondrial proteins were significantly reduced compared with wild-type mice, suggesting that mitochondrial biogenesis was perturbed. However, inhibition of JO(2) by AICA riboside was still present in the mutant mice and thus was clearly not mediated by AMPK. In permeabilized hepatocytes, this inhibition was no longer evident, suggesting that it could be due to intracellular accumulation of Z nuclectides and/or loss of adenine nucleotides and P-i. ZMP did indeed inhibit respiration in isolated rat mitochondria through a direct effect on the respiratory-chain complex I. In addition, inhibition of JO(2) by AICA riboside was also potentiated in cells incubated with fructose to deplete adenine nucleotides and P-i. We conclude that AICA riboside inhibits cellular respiration by an AMPK-independent mechanism that likely results from the combined intracellular P-i depletion and ZMP accumulation. Our data also demonstrate that the cellular effects of AICA riboside are not necessarily caused by AMPK activation and that their interpretation should be taken with caution.