Introduction AMP activated protein kinase AMPK is
AMP-activated protein kinase (AMPK) is mainly known as a highly conserved and ubiquitously expressed energy sensor that is highly sensitive to changes in cellular energy levels (by sensing increases in AMP:ATP and ADP:ATP ratios) and makes appropriate adjustment to balance the consumption of ATP with its synthesis . In this context, the overall effect of AMPK activation is to increase the rate of catabolic (ATP-generating) processes and decrease the rate of anabolic (ATP-consuming) processes in an attempt to restore cellular energy homeostasis. This is accomplished by the phosphorylation of downstream targets involved in the regulation of numerous metabolic pathways as well as long-term adaptive changes through transcriptional regulation. In addition to its role in maintaining intracellular energy homeostasis, AMPK also coordinates metabolism at the whole-body level . Therefore, AMPK represents a point of conversion of regulatory signals monitoring systemic and cellular energy status, making AMPK signaling an obvious target for the treatment of several metabolic disorders , . The discovery that muscle contraction activates AMPK in skeletal muscle leading to increased glucose uptake and evidence supporting that skeletal muscle AMPK signaling network is not compromised in type 2 diabetic individuals reinforced the interest for targeting AMPK via physical exercise or pharmacological interventions , . Moreover, a growing number of downstream targets that are phosphorylated when AMPK is activated independently of the canonical AMP and ADP inputs have been recently identified, expanding AMPK function beyond its critical role in the regulation of energy homeostasis , , . It was recently demonstrated that AMPK is able to sense glucose availability independently of changes in WAY-262611 receptor nucleotides by a mechanism involving the formation of complexes with axin on the surface of the lysosome . Given recent reports highlighting the critical regulatory role played by AMPK in autophagy, anti-inflammatory response and cell growth , , it is not surprising to see the heightened interest in the development of AMPK-targeted therapeutics for the potential to offer significant human health benefits.
In this review, we explore the promise and potential challenges of exploiting the versatility of AMPK signaling for novel therapeutic targeting strategies. We provide here an update on recently developed direct AMPK activators, notably focusing on their mechanisms of action and therapeutic opportunities. A detailed description of the role of AMPK in the regulation of whole body and cellular functions is out of scope of the current review and the reader is invited to consult other recent reviews dealing with this specific topic , , , .
Structure and regulation of AMPK AMPK is a heterotrimeric protein kinase comprised of an alpha (α) catalytic subunit in combination with scaffolding beta (β) and regulatory gamma (γ) subunits (Fig. 1). These subunits are encoded by seven genes (PRKAA1, PRKAA2, PRKAB1, PRKAB2, PRKAG1, PRKAG2, PRKAG3), enabling the formation of a diverse collection of αβγ heterotrimer combinations. The seven isoforms (α1, α2, β1, β2, γ1, γ2 and γ3) can theoretically combine to form up to 12 possible heterotrimers with eventually different regulation (expression, turnover, subcellular localization), regulatory properties (sensitivity to various inputs), and functions (specific substrate phosphorylation, different outputs) for unique impact on cellular homeostasis. In addition, reported PRKAG2 transcript variants differentially expressed in adult tissues (with a transcript predominantly expressed in the heart) may also add to the diversity in the composition of AMPK heterotrimers , .
Diversity of the AMPK heterotrimers The complexity of AMPK system with multiple isoform combination presents unique challenges for drug discovery because each AMPK heterotrimer can be viewed as a potential and specific drug target. In addition, there has been only limited knowledge on the AMPK heterotrimeric complexes tissue-specific distribution and specific roles. The question of whether the different isoform combinations have distinct functions remains only partially answered. However, insights in human AMPK molecular diversity and specificity are beginning to emerge and will help to better define therapeutic modalities. In human skeletal muscle, although all seven subunit isoforms of AMPK are expressed at mRNA level, a limited number of distinct combinations of AMPK heterotrimeric complexes have been detected . Only 3 of the 12 possible AMPK heterotrimeric complexes are present to a significant extent (α2/β2/γ1 ≫ α2/β2/γ3 ≥ α1/β2/γ1), with a high degree of consistency between species (Fig. 2) , . However, although present at one-fifth of all heterotrimers, α2/β2/γ3 is the primary complex to be activated in response to high-intensity exercise protocols, indicating differential regulation and distinct responsibilities of heterotrimers in the control of cellular response to energy stress . The activation of AMPK in the skeletal muscle has been shown to play a role in the regulation of the glucose uptake by enhancing, similarly to insulin, the translocation of GLUT4 to the surface membrane . Like Akt which is involved in the signaling of insulin, AMPK is able to phosphorylate the Rab GTPase-activating proteins Tre-2/USP6, BUB2, cdc16 domain family, member 1 (TBC1D1) and TBC1D4 . Once phosphorylated, these downstream targets are inhibited, which leads to the translocation of GLUT4 to the membrane and then enables the entry of glucose in muscle cells. Interestingly, there is a differential phosphorylation of these targets according to the nature of AMPK heterotrimeric complex activated in response to physical activity. In exercised human skeletal muscle, activity of AMPK α2β2γ3 heterotrimer has been correlated with the phosphorylation of TBC1D1 at Ser237 and Thr596, whereas activity of AMPK α2β2γ1 heterotrimer correlated to the phosphorylation of TBC1D4 at Ser341 and Ser704 . Similar TBC1D1 phosphorylation signature was reported in exercised skeletal muscle from type 2 diabetic subjects indicating the competence of insulin resistant skeletal muscle for AMPK activation . It is noteworthy that the nature of the different AMPK heterotrimeric complexes also varies between organs (Fig. 2), raising the possibility to target AMPK pharmacologically in a tissue-specific manner. In human hepatocytes, AMPK α1β2γ1 was identified as the predominant complex , . In contrast, in rodent hepatocytes, the expression of AMPK isoforms was clearly different leading to multiple possible combination of heterotrimers containing either α1 or α2 and β1, γ1 and γ2 (Fig. 1) , . This observation of interspecies divergence with respect to the tissue specificity of AMPK heterotrimers is of particular interest for drug discovery purposes as it questions the translation of preclinical data to clinical studies.