AMPK: regulating energy balance at the cellular and whole body levels

Abstract

AMP-activated protein kinase appears to have evolved in single-celled eukaryotes as an adenine nucleotide sensor that maintains energy homeostasis at the cellular level. However, during evolution of more complex multicellular organisms, the system has adapted to interact with hormones so that it also plays a key role in balancing energy intake and expenditure at the whole body level.

FIGURE 1.

Domain structure of eukaryotic AMPK orthologs The catalytic α-subunits contain conventional serine/threonine kinase domains containing the threonine residue (Thr172 in rat α2) phosphorylated by upstream kinases. The kinase domains are followed (at least in vertebrates) by small domains with a negative effect on kinase activity (auto-inhibitory domains), which are joined to the COOH-terminal domains (α-CTD) by a less well conserved linker. In vertebrates, there is also a flexible serine-/threonine-rich loop (ST loop) within the α-CTD that is phosphorylated by Akt. The β-subunits contain two conserved regions, a carbohydrate-binding module (CBM) that causes the mammalian complex to bind to glycogen particles, and a COOH-terminal domain (β-CTD) that provides the bridge between the α- and γ-subunits. The γ-subunits contain variable NH₂-terminal regions followed by a short sequence involved in binding to the β-subunit, then four tandem repeats of a cystathionine-β-synthase (CBS) motif. These act in pairs to form the binding sites for adenine nucleotides; in mammalian AMPK, there is one site between CBS1 and CBS2 and two between CBS3 and CBS4.

FIGURE 2.

Regulation of mammalian AMPK by adenine nucleotides and Ca²⁺ Cellular energy stress leads to net conversion of ATP to ADP, some of which is converted to AMP by the adenylate kinase reaction (2ADP ↔ ATP + AMP), which maintains AMP at low levels in unstressed cells. AMP then activates AMPK by three mechanisms, all of which are due to binding to one or more sites on the AMPK-γ subunit: 1) promoting Thr172 phosphorylation by LKB1; 2) inhibiting Thr172 dephosphorylation by protein phosphatases; 3) allosteric activation. Only mechanism 2 is mimicked by ADP, and then only at higher concentrations than AMP (20). AMPK can also be activated by increases in cytosolic Ca²⁺, which triggers direct phosphorylation of Thr172 by CaMKKβ.

FIGURE 3.

Regulation of mTORC1 by the insulin/Akt and AMPK signaling pathways The insulin receptor activates phosphoinositide 3-kinase (PI3K), causing production of phosphatidylinositol 3,4,5-trisphosphate (PIP₃), the second messenger that switches on the protein kinase Akt. Akt then has at least three effects: 1) it phosphorylates AMPK, antagonizing its activation by LKB1; 2) it phosphorylates TSC2 at sites that oppose its function as a Rheb-GAP, thus activating mTORC1; 3) it phosphorylates PRAS40, relieving its inhibitory effects on mTORC1. mTORC1 (a complex containing mTOR, Raptor, and other components) then phosphorylates S6K1 and 4E-BP1, promoting cell growth by enhancing translation of specific mRNAs, as well as rRNA synthesis. By contrast, AMPK, which is activated by energy stress or by hormones that increase cytosolic Ca²⁺ such as ghrelin, has two effects: 1) it phosphorylates TSC2 at distinct sites, enhancing its Rheb-GAP activity; 2) it phosphorylates Raptor. These effects act to inhibit mTORC1, protein synthesis, and cell growth.

FIGURE 4.

Model for role of AMPK in controlling feeding behavior in mammals, based on Sternson (81) and Kahn (15) According to this combined model, the key role of AMPK is in presynaptic neurons immediately upstream of NPY/AgRP neurons in the hypothalamus. Ghrelin causes release of inositol 3,4,5-triphosphate (IP₃) in these cells via the G-protein-coupled receptor GHSR1, leading to release of Ca²⁺ via IP₃, thus activating CaMKKβ and hence AMPK (one effect of which is to inhibit mTORC1 via the mechanisms shown in FIGURE 3). The released Ca²⁺ also triggers release of glutamate that activates the downstream NPY/AgRP neurons, initiating feeding via release of AgRP onto second-order neurons. AMPK is also proposed to activate ryanodine receptors (RyR) that release Ca²⁺, setting up a positive feedback loop (PFL) that allows continuous stimulation of the NPY/AgRP neuron even if stimulation by ghrelin ceases. This loop is interrupted and feeding stops when POMC neurons release an opioid onto the presynaptic neuron, inhibiting AMPK via the PI3K-Akt-mTORC1-S6K1 pathway. Insulin also depresses feeding by activating the same pathway in the presynaptic neuron.