AMPK: Mechanisms of Cellular Energy Sensing and Restoration of Metabolic Balance

Abstract

AMPK is a highly conserved master regulator of metabolism, which restores energy balance during metabolic stress both at the cellular and physiological levels. The identification of numerous AMPK targets has helped explain how AMPK restores energy homeostasis. Recent advancements illustrate novel mechanisms of AMPK regulation, including changes in subcellular localization and phosphorylation by non-canonical upstream kinases. Notably, the therapeutic potential of AMPK is widely recognized and heavily pursued for treatment of metabolic diseases such as diabetes, but also obesity, inflammation, and cancer. Moreover, the recently solved crystal structure of AMPK has shed light both into how nucleotides activate AMPK and, importantly, also into the sites bound by small molecule activators, thus providing a path for improved drugs.

Keywords: AMPK; energy restoration; metabolic adaptation; therapeutic activation; trimeric complex.

Figure 1. Domains and structure of the AMPK complex

A) AMPK exists as a trimeric complex consisting of a catalytic subunit (α) and two regulatory subunits (β and γ). The main protein domains are shown. Abbreviations: AID (autoinhibitory domain), CTD (C-terminus domain), NTD (N-terminus domain), CBM (carbohydrate-binding module), CBS (cystathionine β-synthase repeats), RIM (regulatory-subunit-interacting motif), ST-loop (serine/threonine enriched loop). B) The crystal structure of the AMPK α2β1γ1 trimeric complex is shown. Major structural domains are indicated. The structure shows the activator A769662 bound to a pocket formed by the interface between the kinase domain and the CBM. Also shown are two AMP molecules bound to site 3 and site 4, respectively, and phospho-Thr172. The ST-loop in the α-subunit and the myristoylation site in the β-subunit are not resolved in this structure. Structure sourced and adapted from PDB file 4CFF, using PyMOL software.

Figure 2. Upstream regulation of AMPK and metabolic consequences of AMPK activation

Phosphorylation by upstream kinases is the main AMPK activating event. The regulation of AMPK phosphorylation can be nucleotide-dependent (canonical regulation), resulting from changes in AMP:ATP or ADP:ATP ratios that are induced by a variety of energy stresses. Several important alternative modes of AMPK regulation have been described, which can be classified as nucleotide-independent regulation. In response to energy stress AMPK restores ATP levels by acutely inhibiting ATP-consuming biosynthetic pathways while simultaneously activating catabolic pathways that regenerate ATP through the breakdown of macromolecules. Some of the metabolic processes that are affected by AMPK activation are shown.

Figure 3. Substrates of AMPK regulate multiple metabolic processes in cells

AMPK is phosphorylated and activated by LKB1 and CAMKKβ in response to stimuli that increase AMP/ADP levels (energy stress) or Ca²⁺ flux, respectively. Once active, AMPK induces metabolic changes through the phosphorylation of substrates. Some of the best established metabolic processes regulated by AMPK are shown, together with the relevant substrates.

Figure 4. AMPK activating compounds

Compounds that activate AMPK can be divided into three categories, depending on the mechanism used to activate AMPK. Examples of some of these compounds are shown.