Activation of cardiac AMP-activated protein kinase by LKB1 expression or chemical hypoxia is blunted by increased Akt activity

AMP-activated protein kinase (AMPK) plays a major role in the regulation of cardiac energy substrate utilization and can be negatively regulated by Akt activation in the heart. It has recently been shown that Akt directly phosphorylates AMPK alpha(1)/alpha(2) on Ser485/491 in vitro and prevents the AMPK kinase ( =AMPKK) LKB1 from phosphorylating AMPK alpha at its primary activation site, Thr172 (S Horman, D Vertommen, R Heath, D Neumann, V Mouton, A Woods, U Schlattner, T Wallimann, D Carling, L Hue, and MH Rider. J Biol Chem 281: 5335-5340, 2006). To determine whether this is also the case in the cardiac myocyte, neonatal rat cardiac myocytes (NRCM) were infected with a recombinant adenovirus expressing a constitutively active mutant of Akt1 (myrAkt1) and then with or without adenoviruses expressing the active LKB1 complex. Expression of myrAkt1 blunted LKB-induced phosphorylation of AMPK alpha at Thr172, which resulted in a dramatic decrease in phosphorylation of AMPK's target, acetyl CoA-carboxylase. This decrease in AMPK activity was associated with prior Akt1-dependent phosphorylation of AMPK alpha 1/alpha 2 at Ser485/491. To investigate whether Akt1 activation was also able to prevent other AMPKKs from phosphorylating AMPK alpha, we subjected NRCM to chemical hypoxia and noted a marked increase in phosphorylation of AMPK alpha at Thr172, despite no change in LKB1 activity. NRCM expressing myrAkt1 demonstrated increased phosphorylation of AMPK alpha 1/alpha 2 at Ser485/491 and a complete inhibition of chemical hypoxia-induced phosphorylation of AMPK alpha at Thr172. Taken together, our data show that activation of Akt1 is able to prevent activation of cardiac AMPK by LKB1 and at least one other AMPKK, likely by prior phosphorylation of AMPK alpha 1/alpha 2 at Ser485/491.