Calmodulin-dependent protein kinase kinase-β activates AMPK without forming a stable complex – synergistic effects of Ca2+ and AMP
Résumé
Activation of AMPK by phosphorylation at Thr-172 is catalyzed by at least two distinct upstream kinases, i.e. the tumour suppressor LKB1, and calmodulin-dependent protein kinase kinase-β (CaMKKβ). The sequence around Thr-172 is highly conserved between the two catalytic subunit isoforms of AMPK and the twelve AMPK-related kinases, and LKB1 has been shown to act upstream of all of them. We now report that none of the AMPK-related kinases tested could be phosphorylated or activated in intact cells or cell-free assays by CaMKKβ, although we did observe a slow phosphorylation and activation of BRSK1 by CaMKKα. Despite recent reports, we could not find any evidence that the α and/or β subunits of AMPK formed a stable complex with CaMKKβ. We also showed that increasing AMP concentrations in HeLa cells (which lack LKB1) had no effect on basal AMPK phosphorylation, but enhanced the ability of agents that increase intracellular Ca2+ to activate AMPK. This is consistent with the effect of AMP on phosphorylation of Thr-172 being due to inhibition of dephosphorylation, and confirms that the effect of AMP is independent of the upstream kinase utilized.
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