The AMPK protein is a heterotrimer composed of a catalytic α subunit and two regulatory β and γ subunits, in a ratio of 1α:1β:1γ, all of which are required for the formation of a stable and fully functional AMPK complex [
8,
9]. While some minor differences could be shown, depending on the fibre types, for the γ regulatory subunit [
33], it is well established that the α1 and α2 catalytic subunits are both expressed in human and rodent skeletal muscle, and that the α2β2γ1 complex constitutes the majority of AMPK heterotrimers in this metabolic tissue, showing a high degree of consistency between species [
33‐
37]. In the present study, the mRNA expression pattern and protein levels of the AMPK subunits found in rodent livers is in agreement with those previously reported [
35,
38], leading to many possible combinations of heterotrimers containing either α1 or α2 catalytic subunits, and β1, γ1 and γ2 regulatory subunits. As far as we know, our finding showing that the abundance of hepatic AMPK subunits differs in humans compared with rodents is unprecedented. Thus, it might be speculated that the resulting differences in heterotrimeric composition of AMPK could affect the regulation of kinase activity, notably its sensitivity toward AMP. Indeed, it is striking that the activation of AMPK by metformin seemed more potent, although the increase of the AMP:ATP ratio in humans compared with rat primary hepatocytes was similar (Fig.
2), suggesting that the human AMPK α1β2γ1 or α1β2γ2 truncated complexes could be more sensitive to subtle changes in cellular energy status. However, this is in contradiction with previous in vitro results showing that, in rat liver extract, AMPK complexes containing the α2/β2 isoforms had a greater dependence on AMP [
35,
39]. Interestingly, it has been very recently shown that ADP, like AMP, could also bind to the γ subunit, leading to modulation of the AMP-triggered phosphorylation of AMPK on Thr172 [
10]. It seems therefore possible that subtle species-specific differences in the metformin-induced increase of the ADP:ATP ratio could also be involved. Unfortunately, the determination of intracellular ADP levels by HPLC was technically not possible in our conditions, so further investigations are required to clarify this point. It is worth mentioning that AMPK has also been reported to have a differential and tissue-specific localisation pattern in mammalian cells, with the AMPKα1 subunit being mainly localised in the cytosol, and the AMPKα2 and β2 subunits being localised to the nucleus and cytosolic fractions [
39‐
42]. While not as yet clarified, the subcellular localisation of the kinase might have an important functional role, such as regulation of gene expression by phosphorylation of nuclear targets of AMPK. This would therefore suggest that differences in the composition of AMPK complexes between rodents and human hepatocytes could result in different physiological outcomes, including putative nuclear regulatory functions.
Taken together, the species-specific differences shown in AMPKαβγ complexes in the liver imply that pharmaceutical activation of hepatic AMPK could have different effects in rodents and humans.