Regulation of AMP-activated protein kinase by natural and synthetic activators

Abstract

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that is almost universally expressed in eukaryotic cells. While it appears to have evolved in single-celled eukaryotes to regulate energy balance in a cell-autonomous manner, during the evolution of multicellular animals its role has become adapted so that it also regulates energy balance at the whole body level, by responding to hormones that act primarily on the hypothalamus. AMPK monitors energy balance at the cellular level by sensing the ratios of AMP/ATP and ADP/ATP, and recent structural analyses of the AMPK heterotrimer that have provided insight into the complex mechanisms for these effects will be discussed. Given the central importance of energy balance in diseases that are major causes of morbidity or death in humans, such as type 2 diabetes, cancer and inflammatory disorders, there has been a major drive to develop pharmacological activators of AMPK. Many such activators have been described, and the various mechanisms by which these activate AMPK will be discussed. A particularly large class of AMPK activators are natural products of plants derived from traditional herbal medicines. While the mechanism by which most of these activate AMPK has not yet been addressed, I will argue that many of them may be defensive compounds produced by plants to deter infection by pathogens or grazing by insects or herbivores, and that many of them will turn out to be inhibitors of mitochondrial function.

Keywords: AMP; AMP-activated protein; AMPK activator; Energy balance; Mitochondrial function; Regulatory mechanism; kinase.

Graphical abstract

Figure 1

Structures of AMPK-activating compounds that act via: (A) inhibiting mitochondrial ATP synthesis; (B) pro-drugs converted to active agents inside cells, as shown; and (C) direct activators. (D) shows the mechanism by which antifolate drugs activate AMPK by causing accumulation of ZMP, an intermediate in the synthesis of the purine nucleotides inosine monophosphate (IMP), AMP and guanosine monophosphate (GMP).

Figure 2

Structure of complete α1β2γ1 heterotrimer of AMPK. The model was created with MacPyMol using PDB file 4RER. All molecules are shown in “sphere view”, omitting hydrogen atoms. Domains of the heterotrimer are color coded and labeled as decribed in the text, whereas ancillary ligands (β-cyclodextrin, staurosporine and AMP) are shown with carbon atoms in light gray, oxygen in red and nitrogen in blue. AMP in site 3 is just visible beneath α-RIM2, while AMP in sites 1 and 4 are located around the other side of the γ1 subunit.

Figure 3

Two views of the kinase and auto-inhibitory domains of the α subunit (α-KD:α-AID) in inactive (A) and active (B) conformations. Note the major rotation of the α-AID relative to the α-KD between the two models; in (A), α-AID helix α3 interacts mainly with the α-KD small lobe (and with the hinge between the small and large lobes), but in (B) it interacts mainly with the γ subunit (not shown) instead. Note also that the four side chains of the “regulatory spine” (in white, red, magenta and blue) are out of alignment in (A) but are stacked in alignment in (B), indicating an active conformation. The models were created with MacPyMol using PDB files 4RED (A) and 4RER (B), and are shown in “cartoon” view except for the four residues that form the “regulatory spine” which are in “sphere” view. The view in (A) is of a structure derived from a construct containing only the α-KD and α-AID of human AMPK-α1. The structure crystallized as a dimer, and the α-KD shown is from one molecule while the α-AID shown is from the other molecule within the dimer. Nevertheless, the α-KD:α-AID construct behaved as a monomer in solution, and the structure is very similar to that of an α-KD:α-AID from S. pombe, where the arrangement of the α-AID and α-KD from the same molecule were very similar to that shown here. The view in (B) is of the same structure shown in Fig. 2, but only the α-KD and α-AID are shown.