PT-1 selectively activates AMPK-γ1 complexes in mouse skeletal muscle, but activates all three γ subunit complexes in cultured human cells by inhibiting the respiratory chain

The mechanism of UNC-51-like kinase 1 and the applications of small molecule modulators in cancer treatment

Autophagy is a process of self-renewal in cells, which not only provides the necessary nutrients for cells, but also clears necrotic organelles. Autophagy disorders are closely related to diseases such as cancer. UNC-51-like kinase 1 (ULK1) is a serine/threonine protein kinase that plays a crucial role in receiving input from energy and nutrient sensors, activating autophagy to maintain cellular homeostasis under stressful conditions. In recent years, targeting ULK1 has become a highly promising strategy for cancer treatment. This review introduces the regulatory mechanism of ULK1 in autophagy through the AMPK/mTOR/ULK1 pathway and reviews the research progress of ULK1 activators and inhibitors and their applications in cancer treatment. In addition, we analyze the binding modes between ULK1 and modulators through virtual molecular docking, which will provide a reliable basis and theoretical guidance for the design and development of new therapeutic drugs targeting ULK1.

Host cell sensing and restoration of mitochondrial function and metabolism within Helicobacter pylori VacA intoxicated cells

IMPORTANCE

Persistent human gastric infection with Helicobacter pylori is the single most important risk factor for development of gastric malignancy, which is one of the leading causes of cancer-related deaths worldwide. An important virulence factor for Hp colonization and severity of gastric disease is the protein exotoxin VacA, which is secreted by the bacterium and modulates functional properties of gastric cells. VacA acts by damaging mitochondria, which impairs host cell metabolism through impairment of energy production. Here, we demonstrate that intoxicated cells have the capacity to detect VacA-mediated damage, and orchestrate the repair of mitochondrial function, thereby restoring cellular health and vitality. This study provides new insights into cellular recognition and responses to intracellular-acting toxin modulation of host cell function, which could be relevant for the growing list of pathogenic microbes and viruses identified that target mitochondria as part of their virulence strategies.

AMPK: An odyssey of a metabolic regulator, a tumor suppressor, and now a contextual oncogene

Metabolic reprogramming is a unique but complex biochemical adaptation that allows solid tumors to tolerate various stresses that challenge cancer cells for survival. Under conditions of metabolic stress, mammalian cells employ adenosine monophosphate (AMP)-activated protein kinase (AMPK) to regulate energy homeostasis by controlling cellular metabolism. AMPK has been described as a cellular energy sensor that communicates with various metabolic pathways and networks to maintain energy balance. Earlier studies characterized AMPK as a tumor suppressor in the context of cancer. Later, a paradigm shift occurred in support of the oncogenic nature of AMPK, considering it a contextual oncogene. In support of this, various cellular and mouse models of tumorigenesis and clinicopathological studies demonstrated increased AMPK activity in various cancers. This review will describe AMPK's pro-tumorigenic activity in various malignancies and explain the rationale and context for using AMPK inhibitors in combination with anti-metabolite drugs to treat AMPK-driven cancers.

AMPK and Diseases: State of the Art Regulation by AMPK-Targeting Molecules

5′-adenosine monophosphate (AMP)-activated protein kinase (AMPK) is an enzyme that regulates cellular energy homeostasis, glucose, fatty acid uptake, and oxidation at low cellular ATP levels. AMPK plays an important role in several molecular mechanisms and physiological conditions. It has been shown that AMPK can be dysregulated in different chronic diseases, such as inflammation, diabetes, obesity, and cancer. Due to its fundamental role in physiological and pathological cellular processes, AMPK is considered one of the most important targets for treating different diseases. Over decades, different AMPK targeting compounds have been discovered, starting from those that activate AMPK indirectly by altering intracellular AMP:ATP ratio to compounds that activate AMPK directly by binding to its activation sites. However, indirect altering of intracellular AMP:ATP ratio influences different cellular processes and induces side effects. Direct AMPK activators showed more promising results in eliminating side effects as well as the possibility to engineer drugs for specific AMPK isoforms activation. In this review, we discuss AMPK targeting drugs, especially concentrating on those compounds that activate AMPK by mimicking AMP. These compounds are poorly described in the literature and still, a lot of questions remain unanswered about the exact mechanism of AMP regulation. Future investigation of the mechanism of AMP binding will make it possible to develop new compounds that, in combination with others, can activate AMPK in a synergistic manner.

Impairment of Glucose Metabolism and Suppression of Stemness in MCF-7/SC Human Breast Cancer Stem Cells by Nootkatone

Targeting cancer stem cell metabolism has emerged as a promising therapeutic strategy for cancer treatment. Breast cancer stem cells (BCSCs) exert distinct metabolism machinery, which plays a major role in radiation and multidrug resistance. Therefore, exploring the mechanisms involved in energy utilization of BCSCs could improve the effectiveness of therapeutic strategies aimed at their elimination. This study was conducted to clarify the glucose metabolism machinery and the function of nootkatone, a bioactive component of grapefruit, in regulating glucose metabolism and stemness characteristics in human breast carcinoma MCF-7 stem cells (MCF-7SCs). In vivo experiments, transcriptomic analysis, seahorse XF analysis, MTT assay, Western blotting, mammosphere formation, wound healing, invasion assay, flow cytometric analysis, reverse transcription-quantitative polymerase chain reaction, and in silico docking experiments were performed. MCF-7SCs showed a greater tumorigenic capacity and distinct gene profile with enrichment of the genes involved in stemness and glycolysis signaling pathways compared to parental MCF-7 cells, indicating that MCF-7SCs use glycolysis rather than oxidative phosphorylation (OXPHOS) for their energy supply. Nootkatone impaired glucose metabolism through AMPK activation and reduced the stemness characteristics of MCF-7SCs. In silico docking analysis demonstrated that nootkatone efficiently bound to the active site of AMPK. Therefore, this study indicates that regulation of glucose metabolism through AMPK activation could be an attractive target for BCSCs.

Role of the AMPK signalling pathway in the aetiopathogenesis of ocular diseases

BACKGROUND

AMP-activated protein kinase (AMPK) plays a precise role as a master regulator of cellular energy homeostasis. AMPK is activated in response to the signalling cues that exhaust cellular ATP levels such as hypoxia, ischaemia, glucose depletion and heat shock. As a central regulator of both lipid and glucose metabolism, AMPK is considered to be a potential therapeutic target for the treatment of various diseases, including eye disorders.

OBJECTIVE

To review all the shreds of evidence concerning the role of the AMPK signalling pathway in the pathogenesis of ocular diseases.

METHOD

Scientific data search and review of available information evaluating the influence of AMPK signalling on ocular diseases.

RESULTS

Review highlights the significance of AMPK signalling in the aetiopathogenesis of ocular diseases, including cataract, glaucoma, diabetic retinopathy, retinoblastoma, age-related macular degeneration, corneal diseases, etc. The review also provides the information on the AMPK-associated pathways with reference to ocular disease, which includes mitochondrial biogenesis, autophagy and regulation of inflammatory response.

CONCLUSION

The study concludes the role of AMPK in ocular diseases. There is growing interest in the therapeutic utilization of the AMPK pathway for ocular disease treatment. Furthermore, inhibition of AMPK signalling might represent more pertinent strategy than AMPK activation for ocular disease treatment. Such information will guide the development of more effective AMPK modulators for ocular diseases.[Formula: see text].

CARS senses cysteine deprivation to activate AMPK for cell survival

Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) is an important cellular metabolite-sensing enzyme that can directly sense changes not only in ATP but also in metabolites associated with carbohydrates and fatty acids. However, less is known about whether and how AMPK senses variations in cellular amino acids. Here, we show that cysteine deficiency significantly triggers calcium/calmodulin-dependent protein kinase kinase 2 (CaMKK2)-mediated activation of AMPK. In addition, we found that CaMKK2 directly associates with cysteinyl-tRNA synthetase (CARS), which then binds to AMPKγ2 under cysteine deficiency to activate AMPK. Interestingly, we discovered that cysteine inhibits the binding of CARS to AMPKγ2, and thus, under cysteine deficiency conditions wherein the inhibitory effect of cysteine is abrogated, CARS mediates the binding of AMPK to CaMKK2, resulting in the phosphorylation and activation of AMPK by CaMKK2. Importantly, we demonstrate that blocking AMPK activation leads to cell death under cysteine-deficient conditions. In summary, our study is the first to show that CARS senses the absence of cysteine and activates AMPK through the cysteine-CARS-CaMKK2-AMPKγ2 axis, a novel adaptation strategy for cell survival under nutrient deprivation conditions.

Mitochondria-localized AMPK responds to local energetics and contributes to exercise and energetic stress-induced mitophagy

Mitochondria form a complex, interconnected reticulum that is maintained through coordination among biogenesis, dynamic fission, and fusion and mitophagy, which are initiated in response to various cues to maintain energetic homeostasis. These cellular events, which make up mitochondrial quality control, act with remarkable spatial precision, but what governs such spatial specificity is poorly understood. Herein, we demonstrate that specific isoforms of the cellular bioenergetic sensor, 5' AMP-activated protein kinase (AMPKα1/α2/β2/γ1), are localized on the outer mitochondrial membrane, referred to as mitoAMPK, in various tissues in mice and humans. Activation of mitoAMPK varies across the reticulum in response to energetic stress, and inhibition of mitoAMPK activity attenuates exercise-induced mitophagy in skeletal muscle in vivo. Discovery of a mitochondrial pool of AMPK and its local importance for mitochondrial quality control underscores the complexity of sensing cellular energetics in vivo that has implications for targeting mitochondrial energetics for disease treatment.

A conserved role for AMP-activated protein kinase in NGLY1 deficiency

Mutations in human N-glycanase 1 (NGLY1) cause the first known congenital disorder of deglycosylation (CDDG). Patients with this rare disease, which is also known as NGLY1 deficiency, exhibit global developmental delay and other phenotypes including neuropathy, movement disorder, and constipation. NGLY1 is known to regulate proteasomal and mitophagy gene expression through activation of a transcription factor called "nuclear factor erythroid 2-like 1" (NFE2L1). Loss of NGLY1 has also been shown to impair energy metabolism, but the molecular basis for this phenotype and its in vivo consequences are not well understood. Using a combination of genetic studies, imaging, and biochemical assays, here we report that loss of NGLY1 in the visceral muscle of the Drosophila larval intestine results in a severe reduction in the level of AMP-activated protein kinase α (AMPKα), leading to energy metabolism defects, impaired gut peristalsis, failure to empty the gut, and animal lethality. Ngly1-/- mouse embryonic fibroblasts and NGLY1 deficiency patient fibroblasts also show reduced AMPKα levels. Moreover, pharmacological activation of AMPK signaling significantly suppressed the energy metabolism defects in these cells. Importantly, the reduced AMPKα level and impaired energy metabolism observed in NGLY1 deficiency models are not caused by the loss of NFE2L1 activity. Taken together, these observations identify reduced AMPK signaling as a conserved mediator of energy metabolism defects in NGLY1 deficiency and suggest AMPK signaling as a therapeutic target in this disease.

AMPK allostery: A therapeutic target for the management/treatment of diabetic nephropathy

Diabetic nephropathy (DN) is a chronic complication of diabetes mellitus (DM) with approximately 30-40% of patients with DM developing nephropathy, and it is the leading cause of end-stage renal diseases and diabetic morbidity. The pathogenesis of DN is primarily associated with irregularities in the metabolism of glucose and lipid leading to hyperglycemia-induced oxidative stress, which has been a major target together with blood pressure regulation in the control of DN progression. However, the regulation of 5' adenosine monophosphate-activated protein kinase (AMPK), a highly conserved protein kinase for maintaining energy balance and cellular growth and repair has been implicated in the development of DM and its complications. Therefore, targeting AMPK pathway has been explored as a therapeutic strategy for the treatment of diabetes and its complication, although most of the mechanisms have not been fully elucidated. In this review, we discuss the structure of AMPK relevant to understanding its allosteric regulation and its role in the pathogenesis and progression of DN. We also identify therapeutic agents that modulate AMPK and its downstream targets with their specific mechanisms of action in the treatment of DN.

The influence of hypoxia and energy depletion on the response of endothelial cells to the vascular disrupting agent combretastatin A-4-phosphate

Combretastatin A-4 phosphate (CA4P) is a microtubule-disrupting tumour-selective vascular disrupting agent (VDA). CA4P activates the actin-regulating RhoA-GTPase/ ROCK pathway, which is required for full vascular disruption. While hypoxia renders tumours resistant to many conventional therapies, little is known about its influence on VDA activity. Here, we found that active RhoA and ROCK effector phospho-myosin light chain (pMLC) were downregulated in endothelial cells by severe hypoxia. CA4P failed to activate RhoA/ROCK/pMLC but its activity was restored upon reoxygenation. Hypoxia also inhibited CA4P-mediated actinomyosin contractility, VE-cadherin junction disruption and permeability rise. Glucose withdrawal downregulated pMLC, and coupled with hypoxia, reduced pMLC faster and more profoundly than hypoxia alone. Concurrent inhibition of glycolysis (2-deoxy-D-glucose, 2DG) and mitochondrial respiration (rotenone) caused profound actin filament loss, blocked RhoA/ROCK signalling and rendered microtubules  CA4P-resistant. Withdrawal of the metabolism inhibitors restored the cytoskeleton and CA4P activity. The AMP-activated kinase AMPK was investigated as a potential mediator of pMLC downregulation. Pharmacological AMPK activators that generate AMP, unlike allosteric activators, downregulated pMLC but only when combined with 2DG and/or rotenone. Altogether, our results suggest that Rho/ROCK and actinomyosin contractility are regulated by AMP/ATP levels independently of AMPK, and point to hypoxia/energy depletion as potential modifiers of CA4P response.

The ULK1/2 and AMPK Inhibitor SBI-0206965 Blocks AICAR and Insulin-Stimulated Glucose Transport

The small molecule kinase inhibitor SBI-0206965 was originally described as a specific inhibitor of ULK1/2. More recently, it was reported to effectively inhibit AMPK and several studies now report its use as an AMPK inhibitor. Currently, we investigated the specificity of SBI-0206965 in incubated mouse skeletal muscle, measuring the effect on analog 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)-stimulated AMPK-dependent glucose transport and insulin-stimulated AMPK-independent glucose uptake. Pre-treatment with 10 µM SBI-0206965 for 50 min potently suppressed AICAR-stimulated glucose transport in both the extensor digitorum longus (EDL) and soleus muscle. This was despite only a modest lowering of AICAR-stimulated AMPK activation measured as ACC2 Ser212, while ULK1/2 Ser555 phosphorylation was prevented. Insulin-stimulated glucose transport was also potently inhibited by SBI-0206965 in soleus. No major changes were observed on insulin-stimulated cell signaling. No general effect of SBI-0206965 on intracellular membrane morphology was observed by transmission electron microscopy. As insulin is known to neither activate AMPK nor require AMPK to stimulate glucose transport, and insulin inhibits ULK1/2 activity, these data strongly suggest that SBI-0206965 has a non-specific off-target inhibitory effect on muscle glucose transport. Thus, SBI-0206965 is not a specific inhibitor of the AMPK/ULK-signaling axis in skeletal muscle, and data generated with this inhibitor must be interpreted with caution.

TLR9 and beclin 1 crosstalk regulates muscle AMPK activation in exercise

The activation of adenosine monophosphate-activated protein kinase (AMPK) in skeletal muscle coordinates systemic metabolic responses to exercise¹. Autophagy-a lysosomal degradation pathway that maintains cellular homeostasis²-is upregulated during exercise, and a core autophagy protein, beclin 1, is required for AMPK activation in skeletal muscle³. Here we describe a role for the innate immune-sensing molecule Toll-like receptor 9 (TLR9)⁴, and its interaction with beclin 1, in exercise-induced activation of AMPK in skeletal muscle. Mice that lack TLR9 are deficient in both exercise-induced activation of AMPK and plasma membrane localization of the GLUT4 glucose transporter in skeletal muscle, but are not deficient in autophagy. TLR9 binds beclin 1, and this interaction is increased by energy stress (glucose starvation and endurance exercise) and decreased by a BCL2 mutation^3,5 that blocks the disruption of BCL2-beclin 1 binding. TLR9 regulates the assembly of the endolysosomal phosphatidylinositol 3-kinase complex (PI3KC3-C2)-which contains beclin 1 and UVRAG-in skeletal muscle during exercise, and knockout of beclin 1 or UVRAG inhibits the cellular AMPK activation induced by glucose starvation. Moreover, TLR9 functions in a muscle-autonomous fashion in ex vivo contraction-induced AMPK activation, glucose uptake and beclin 1-UVRAG complex assembly. These findings reveal a heretofore undescribed role for a Toll-like receptor in skeletal-muscle AMPK activation and glucose metabolism during exercise, as well as unexpected crosstalk between this innate immune sensor and autophagy proteins.

AMP-activated protein kinase: the current landscape for drug development

Since the discovery of AMP-activated protein kinase (AMPK) as a central regulator of energy homeostasis, many exciting insights into its structure, regulation and physiological roles have been revealed. While exercise, caloric restriction, metformin and many natural products increase AMPK activity and exert a multitude of health benefits, developing direct activators of AMPK to elicit beneficial effects has been challenging. However, in recent years, direct AMPK activators have been identified and tested in preclinical models, and a small number have entered clinical trials. Despite these advances, which disease(s) represent the best indications for therapeutic AMPK activation and the long-term safety of such approaches remain to be established.

Polyphenols targeting diabetes via the AMP-activated protein kinase pathway; future approach to drug discovery

Regarding the widespread progression of diabetes, its related complications and detrimental effects on human health, investigations on this subject seems compulsory. AMP-activated protein kinase (AMPK) is a serine/threonine kinase and a key player in energy metabolism regulation. AMPK is also considered as a prime target for pharmaceutical and therapeutic studies on disorders such as diabetes, metabolic syndrome and obesity, where the body energy homeostasis is imbalanced. Following the activation of AMPK (physiological or pharmacological), a cascade of metabolic events that improve metabolic health is triggered. While there are several publications on this subject, this is the first report that has focused solely on polyphenols targeting diabetes via AMPK pathway. The multiple characteristics of polyphenolic compounds and their favorable influence on diabetes pathogenesis, as well as their intersections with the AMPK signaling pathway, indicate that these compounds have a beneficial effect on the regulation of glucose homeostasis. PPs could potentially occupy a significant position in the future anti-diabetic drug market.

Electroporated GLUT4-7myc-GFP detects in vivo glucose transporter 4 translocation in skeletal muscle without discernible changes in GFP patterns

NEW FINDINGS

What is the central question of this study? Resolving the mechanism(s) leading to glucose transporter 4 (GLUT4) translocation to the muscle surface membrane has great therapeutic potential. However, the measurement of GLUT4 translocation is technically challenging. Here, we asked whether electroporation of GLUT4-7myc-GFP into skeletal muscle could be used as a tool to study GLUT4 translocation in vivo. What is the main finding and its importance? By acutely inducing GLUT4-7myc-GFP expression in skeletal muscle, we verified that in vivo exercise and AICAR stimulation increased the GLUT4 presence in the sarcolemma measured as myc signal. Importantly, the increased myc signal in the sarcolemma was not accompanied by major visual changes in the distribution of the GFP signal.

ABSTRACT

Insulin and exercise lead to translocation of the glucose transporter 4 (GLUT4) to the surface membrane of skeletal muscle fibres. This process is pivotal for facilitating glucose uptake into skeletal muscle. To study this, a robust assay is needed to measure the translocation of GLUT4 in adult skeletal muscle directly. Here, we aimed to validate a simple GLUT4 translocation assay using a genetically encoded biosensor in mouse skeletal muscle. We transfected GLUT4-7myc-GFP into mouse muscle to study live GLUT4 movement and to evaluate GLUT4 insertion in the muscle surface membrane after in vivo running exercise and pharmacological activation of AMP-activated protein kinase (AMPK). Transfection led to expression of GLUT4-7myc-GFP that was dynamic in live flexor digitorum brevis fibres and which, upon insulin stimulation, exposed the myc epitope extracellularly. Running exercise, in addition to AMPK activation by 5-aminoimidazole-4-carboxamide ribonucleotide, induced ∼125 and ∼100% increase, respectively, in extracellularly exposure of GLUT4 in the surface membrane of tibialis anterior muscle. Interestingly, the clear increase in surface-exposed GLUT4 content induced by insulin, exercise or AMPK activation was not accompanied by any discernible reorganization of the GLUT4-GFP signal. In conclusion, we provide a detailed description of an easy-to-use translocation assay to study GLUT4 accumulation at the surface membrane induced by exercise and exercise-mimicking stimuli. Notably, our analyses revealed that increased GLUT4 surface membrane accumulation was not accompanied by a discernible change in the GLUT4 localization pattern.

Endothelial AMP-Activated Kinase α1 Phosphorylates eNOS on Thr495 and Decreases Endothelial NO Formation

AMP-activated protein kinase (AMPK) is frequently reported to phosphorylate Ser1177 of the endothelial nitric-oxide synthase (eNOS), and therefore, is linked with a relaxing effect. However, previous studies failed to consistently demonstrate a major role for AMPK on eNOS-dependent relaxation. As AMPK also phosphorylates eNOS on the inhibitory Thr495 site, this study aimed to determine the role of AMPKα1 and α2 subunits in the regulation of NO-mediated vascular relaxation. Vascular reactivity to phenylephrine and acetylcholine was assessed in aortic and carotid artery segments from mice with global (AMPKα^-/-) or endothelial-specific deletion (AMPKα^ΔEC) of the AMPKα subunits. In control and AMPKα1-depleted human umbilical vein endothelial cells, eNOS phosphorylation on Ser1177 and Thr495 was assessed after AMPK activation with thiopental or ionomycin. Global deletion of the AMPKα1 or α2 subunit in mice did not affect vascular reactivity. The endothelial-specific deletion of the AMPKα1 subunit attenuated phenylephrine-mediated contraction in an eNOS- and endothelium-dependent manner. In in vitro studies, activation of AMPK did not alter the phosphorylation of eNOS on Ser1177, but increased its phosphorylation on Thr495. Depletion of AMPKα1 in cultured human endothelial cells decreased Thr495 phosphorylation without affecting Ser1177 phosphorylation. The results of this study indicate that AMPKα1 targets the inhibitory phosphorylation Thr495 site in the calmodulin-binding domain of eNOS to attenuate basal NO production and phenylephrine-induced vasoconstriction.

Lactate administration activates the ERK1/2, mTORC1, and AMPK pathways differentially according to skeletal muscle type in mouse

Skeletal muscle is described as an endocrine organ, constitutively or intermittently secreting bioactive molecules. The signaling pathways by which these molecules mediate changes in skeletal muscle and regulate interorgan crosstalk are only partly understood. Lactate is widely described as a signaling molecule in different cells, but the role of lactate as a signaling molecule in mature skeletal muscle has not been fully unveiled. The aim of this study was to determine the role of lactate on activation of signaling pathways in adult mouse skeletal muscle. Male mice were injected intraperitoneally with lactate or saline, and tissues were dissected after 40 min. Phosphorylation levels of relevant proteins in muscle were assessed by Western blotting. After lactate administration, we found an increase in p-ERK1/2Thr202/Tyr204 (3.5-fold; P = 0.004) and p-p70S6KThr389 (1.9-fold; P = 0.01) in quadriceps; and an increase in p-rpS6Ser235/236 in both quadriceps (6.3-fold; P = 0.01) and EDL (2.3-fold; P = 0.01), without changes in soleus. There was a tendency toward an increase in p-AMPKThr172 (1.7-fold; P = 0.08), with a significant increase in p-ACCSer79 (1.5-fold; P = 0.04) in soleus, without changes in quadriceps and EDL. These results support the hypothesis that lactate plays a role in the molecular signaling related to hypertrophy and to oxidative metabolism on adult skeletal muscle and suggest that this activation depends on the skeletal muscle type. The mechanisms that underlie the effect of lactate in mature skeletal muscles remain to be established.

A Method for Rapid Screening of Anilide-Containing AMPK Modulators Based on Computational Docking and Biological Validation

Adenosine 5'-monophsphate-activated protein kinase (AMPK) is a crucial energy sensor for maintaining cellular homeostasis. Targeting AMPK may provide an alternative approach in treatment of various diseases like cancer, diabetes, and neurodegenerations. Accordingly, novel AMPK activators are frequently identified from natural products in recent years. However, most of such AMPK activators are interacting with AMPK in an indirect manner, which may cause off-target effects. Therefore, the search of novel direct AMPK modulators is inevitable and effective screening methods are needed. In this report, a rapid and straightforward method combining the use of in silico and in vitro techniques was established for selecting and categorizing huge amount of compounds from chemical library for targeting AMPK modulators. A new class of direct AMPK modulator have been discovered which are anilides or anilide-like compounds. In total 1,360,000 compounds were virtually screened and 17 compounds were selected after biological assays. Lipinski's rule of five assessment suggested that, 13 out of the 17 compounds are demonstrating optimal bioavailability. Proton acceptors constituting the structure of these compounds and hydrogen bonds with AMPK in the binding site appeared to be the important factors determining the efficacy of these compounds.

Promise and challenges for direct small molecule AMPK activators

AMP-activated protein kinase (AMPK) is an evolutionary conserved and ubiquitously expressed serine/threonine kinase playing a central role in the coordination of energy homeostasis. Based on the beneficial outcomes of its activation on metabolism, AMPK has emerged as an attractive target for the treatment of metabolic diseases. Identification of novel downstream targets of AMPK beyond the regulation of energy metabolism has renewed considerable attention in exploiting AMPK signaling for novel therapeutic targeting strategies including treatment of cancer and inflammatory diseases. The complexity of AMPK system with tissue- and species-specific expression of multiple isoform combination regulated by various inputs, post-traductional modifications and subcellular locations presents unique challenges for drug discovery. Here, we review the most recent advances in the understanding of the mechanism(s) of action of direct small molecule AMPK activators and the potential therapeutic opportunities.

AMPK in skeletal muscle function and metabolism

Skeletal muscle possesses a remarkable ability to adapt to various physiologic conditions. AMPK is a sensor of intracellular energy status that maintains energy stores by fine-tuning anabolic and catabolic pathways. AMPK’s role as an energy sensor is particularly critical in tissues displaying highly changeable energy turnover. Due to the drastic changes in energy demand that occur between the resting and exercising state, skeletal muscle is one such tissue. Here, we review the complex regulation of AMPK in skeletal muscle and its consequences on metabolism (e.g., substrate uptake, oxidation, and storage as well as mitochondrial function of skeletal muscle fibers). We focus on the role of AMPK in skeletal muscle during exercise and in exercise recovery. We also address adaptations to exercise training, including skeletal muscle plasticity, highlighting novel concepts and future perspectives that need to be investigated. Furthermore, we discuss the possible role of AMPK as a therapeutic target as well as different AMPK activators and their potential for future drug development.—Kjøbsted, R., Hingst, J. R., Fentz, J., Foretz, M., Sanz, M.-N., Pehmøller, C., Shum, M., Marette, A., Mounier, R., Treebak, J. T., Wojtaszewski, J. F. P., Viollet, B., Lantier, L. AMPK in skeletal muscle function and metabolism.

Intact Cell Assays to Monitor AMPK and Determine the Contribution of the AMP-Binding or ADaM Sites to Activation

AMP-activated protein kinase (AMPK) is extremely sensitive to cellular stress, so that nonphysiological activation of the kinase can readily occur during harvesting of cells or tissues. In this chapter we describe methods to harvest cells and tissues, and for kinase assays, that preserve the physiological activation status of AMPK as far as possible. Note that similar care with methods of cell or tissue harvesting is required when AMPK function is monitored by Western blotting, rather than by kinase assays. We also describe methods to determine whether compounds that activate AMPK in intact cells do so indirectly by interfering with cellular ATP synthesis or directly by binding to AMPK and, if the latter, whether this occurs by binding at the AMP-binding sites on the γ subunit or at the ADaM site located between the α and β subunits.

Novel direct AMPK activator suppresses non-small cell lung cancer through inhibition of lipid metabolism

Drug resistance is becoming an obstacle in anti-cancer therapies. For target-based therapy of lung cancer, gefitinib, as the first generation of tyrosine kinase inhibitors (TKIs), demonstrated good initial response to the non-small cell lung cancer (NSCLC) patients whom harbors epidermal growth factor receptor (EGFR) mutation. However, within one year, additional EGFR mutation occurred, leading to eventual gefitinib-resistance. Therefore, it is urgently to discover novel effective small molecule inhibitors for those patients. Abnormal energy metabolism is accepted as new cancer hallmark. Recently, a metabolism rate-limiting enzyme 5’-adenosine menophosphate-activated protein kinase (AMPK) has become a promising anti-cancer target. In this study, we have identified a novel direct AMPK agonist, D561-0775 from a compound library by using molecular docking screening technique. We demonstrated that D561-0775 exhibited significant inhibitory effect on gefitinib-resistant NSCLC cell lines but less cytotoxicity on normal cells. Furthermore, D561-0775 demonstrated a remarkable in vitro AMPK enzyme activation effect. Taken together, D561-0775 showed potential anti-cancer activity via inducing apoptosis, cell cycle arrest, suppressing glycolysis and cholesterol synthesis after activation of AMPK in gefitinib-resistant H1975 cells. D561-0775 has provided a new chemical structure that could be developed as cancer drug for gefitinib-resistant NSCLC patients through inhibition lipid metabolism by directly targeting at AMPK directly.

AMPK: guardian of metabolism and mitochondrial homeostasis

Cells constantly adapt their metabolism to meet their energy needs and respond to nutrient availability. Eukaryotes have evolved a very sophisticated system to sense low cellular ATP levels via the serine/threonine kinase AMP-activated protein kinase (AMPK) complex. Under conditions of low energy, AMPK phosphorylates specific enzymes and growth control nodes to increase ATP generation and decrease ATP consumption. In the past decade, the discovery of numerous new AMPK substrates has led to a more complete understanding of the minimal number of steps required to reprogramme cellular metabolism from anabolism to catabolism. This energy switch controls cell growth and several other cellular processes, including lipid and glucose metabolism and autophagy. Recent studies have revealed that one ancestral function of AMPK is to promote mitochondrial health, and multiple newly discovered targets of AMPK are involved in various aspects of mitochondrial homeostasis, including mitophagy. This Review discusses how AMPK functions as a central mediator of the cellular response to energetic stress and mitochondrial insults and coordinates multiple features of autophagy and mitochondrial biology.

Mechanisms of Paradoxical Activation of AMPK by the Kinase Inhibitors SU6656 and Sorafenib

SU6656, a Src kinase inhibitor, was reported to increase fat oxidation and reduce body weight in mice, with proposed mechanisms involving AMP-activated protein kinase (AMPK) activation via inhibition of phosphorylation of either LKB1 or AMPK by the Src kinase, Fyn. However, we report that AMPK activation by SU6656 is independent of Src kinases or tyrosine phosphorylation of LKB1 or AMPK and is not due to decreased cellular energy status or binding at the ADaM site on AMPK. SU6656 is a potent AMPK inhibitor, yet binding at the catalytic site paradoxically promotes phosphorylation of Thr172 by LKB1. This would enhance phosphorylation of downstream targets provided the lifetime of Thr172 phosphorylation was sufficient to allow dissociation of the inhibitor and subsequent catalysis prior to its dephosphorylation. By contrast, sorafenib, a kinase inhibitor in clinical use, activates AMPK indirectly by inhibiting mitochondrial metabolism and increasing cellular AMP:ADP and/or ADP:ATP ratios.

Heterotypic endosomal fusion as an initial trigger for insulin-induced glucose transporter 4 (GLUT4) translocation in skeletal muscle

KEY POINTS

Comprehensive imaging analyses of glucose transporter 4 (GLUT4) behaviour in mouse skeletal muscle was conducted. Quantum dot-based single molecule nanometry revealed that GLUT4 molecules in skeletal myofibres are governed by regulatory systems involving 'static retention' and 'stimulus-dependent liberation'. Vital imaging analyses and super-resolution microscopy-based morphometry demonstrated that insulin liberates the GLUT4 molecule from its static state by triggering acute heterotypic endomembrane fusion arising from the very small GLUT4-containing vesicles in skeletal myofibres. Prior exposure to exercise-mimetic stimuli potentiated this insulin-responsive endomembrane fusion event involving GLUT4-containing vesicles, suggesting that this endomembranous regulation process is a potential site related to the effects of exercise.

ABSTRACT

Skeletal muscle is the major systemic glucose disposal site. Both insulin and exercise facilitate translocation of the glucose transporter glucose transporter 4 (GLUT4) via distinct signalling pathways and exercise also enhances insulin sensitivity. However, the trafficking mechanisms controlling GLUT4 mobilization in skeletal muscle remain poorly understood as a resuly of technical limitations. In the present study, which employs various imaging techniques on isolated skeletal myofibres, we show that one of the initial triggers of insulin-induced GLUT4 translocation is heterotypic endomembrane fusion arising from very small static GLUT4-containing vesicles with a subset of transferrin receptor-containing endosomes. Importantly, pretreatment with exercise-mimetic stimuli potentiated the susceptibility to insulin responsiveness, as indicated by these acute endomembranous activities. We also found that AS160 exhibited stripe-like localization close to sarcomeric α-actinin and that insulin induced a reduction of the stripe-like localization accompanying changes in its detergent solubility. The results of the present study thus provide a conceptual framework indicating that GLUT4 protein trafficking via heterotypic fusion is a critical feature of GLUT4 translocation in skeletal muscles and also suggest that the efficacy of the endomembranous fusion process in response to insulin is involved in the benefits of exercise.

Targeting AMPK: From Ancient Drugs to New Small-Molecule Activators

The AMP-activated protein kinase (AMPK) is an evolutionary conserved and ubiquitously expressed serine/threonine kinase mainly acting as a key regulator of cellular energy homeostasis. AMPK is a heterotrimeric protein complex, consisting of a catalytic α subunit and two regulatory β and γ subunits, whose activity is tightly regulated by changes in adenine nucleotides and several posttranslational modifications. Once activated in response to energy deficit, AMPK concomitantly inhibits ATP-consuming anabolic processes and promotes ATP-generating catabolic pathways via direct phosphorylation of multiple downstream effectors, leading to restoration of cellular energy balance. A growing number of energy/nutrient-independent functions of AMPK are also regularly reported, progressively expanding its role to regulation of non-metabolic cellular processes. Historically, AMPK as a therapeutic target has attracted much of interest due to its potential impact on metabolic disorders, such as obesity and type 2 diabetes, but has also recently received considerable renewed attention in the framework of cancer studies, highlighting the persistent need for selective, reversible, potent, and tissue-specific activators. In this chapter, we review the most recent advances in the understanding of the mechanism(s) of action of the current portfolio of AMPK activators, including plant-derived natural compounds and newly discovered small-molecule agonists directly targeting various AMPK subunits.

Role of AMP-Activated Protein Kinase for Regulating Post-exercise Insulin Sensitivity

Skeletal muscle insulin resistance precedes development of type 2 diabetes (T2D). As skeletal muscle is a major sink for glucose disposal, understanding the molecular mechanisms involved in maintaining insulin sensitivity of this tissue could potentially benefit millions of people that are diagnosed with insulin resistance. Regular physical activity in both healthy and insulin-resistant individuals is recognized as the single most effective intervention to increase whole-body insulin sensitivity and thereby positively affect glucose homeostasis. A single bout of exercise has long been known to increase glucose disposal in skeletal muscle in response to physiological insulin concentrations. While this effect is identified to be restricted to the previously exercised muscle, the molecular basis for an apparent convergence between exercise- and insulin-induced signaling pathways is incompletely known. In recent years, we and others have identified the Rab GTPase-activating protein, TBC1 domain family member 4 (TBC1D4) as a target of key protein kinases in the insulin- and exercise-activated signaling pathways. Our working hypothesis is that the AMP-activated protein kinase (AMPK) is important for the ability of exercise to insulin sensitize skeletal muscle through TBC1D4. Here, we aim to provide an overview of the current available evidence linking AMPK to post-exercise insulin sensitivity.

Structure and Regulation of AMPK

AMP-activated protein kinase is a family of heterotrimeric serine/threonine protein kinases that come in twelve different flavors. They serve an essential function in all eukaryotes of conserving cellular energy levels. AMPK complexes are regulated by changes in cellular AMP:ATP or ADP:ATP ratios and by a number of neutraceuticals and some of the widely-used diabetes medications such as metformin and thiazolinonediones. Moreover, biochemical activities of AMPK are tightly regulated by phosphorylation or dephosphorylation by upstream kinases and phosphatases respectively. Efforts are underway in many pharmaceutical companies to discover direct AMPK activators for the treatment of cardiovascular and metabolic diseases such as diabetes, non-alcoholic steatohepatitis (NASH) and diabetic nephropathy. Many advances have been made in the AMPK structural biology arena over the last few years that are beginning to provide detailed molecular insights into the overall topology of these fascinating enzymes and how binding of small molecules elicit subtle conformational changes leading to their activation and protection from dephosphorylation. In the brief review below on AMPK structure and function, we have focused on the recent crystallographic results especially on specific molecular interactions of direct synthetic AMPK activators which lead to biased activation of a sub-family of AMPK isoforms.

Activation of Skeletal Muscle AMPK Promotes Glucose Disposal and Glucose Lowering in Non-human Primates and Mice

The AMP-activated protein kinase (AMPK) is a potential therapeutic target for metabolic diseases based on its reported actions in the liver and skeletal muscle. We evaluated two distinct direct activators of AMPK: a non-selective activator of all AMPK complexes, PF-739, and an activator selective for AMPK β1-containing complexes, PF-249. In cells and animals, both compounds were effective at activating AMPK in hepatocytes, but only PF-739 was capable of activating AMPK in skeletal muscle. In diabetic mice, PF-739, but not PF-249, caused a rapid lowering of plasma glucose levels that was diminished in the absence of skeletal muscle, but not liver, AMPK heterotrimers and was the result of an increase in systemic glucose disposal with no impact on hepatic glucose production. Studies of PF-739 in cynomolgus monkeys confirmed translation of the glucose lowering and established activation of AMPK in skeletal muscle as a potential therapeutic approach to treat diabetic patients.

Benzimidazole derivative small-molecule 991 enhances AMPK activity and glucose uptake induced by AICAR or contraction in skeletal muscle

AMP-activated protein kinase (AMPK) plays diverse roles and coordinates complex metabolic pathways for maintenance of energy homeostasis. This could be explained by the fact that AMPK exists as multiple heterotrimer complexes comprising a catalytic α-subunit (α1 and α2) and regulatory β (β1 and β2)- and γ (γ1, γ2, γ3)-subunits, which are uniquely distributed across different cell types. There has been keen interest in developing specific and isoform-selective AMPK-activating drugs for therapeutic use and also as research tools. Moreover, establishing ways of enhancing cellular AMPK activity would be beneficial for both purposes. Here, we investigated if a recently described potent AMPK activator called 991, in combination with the commonly used activator 5-aminoimidazole-4-carboxamide riboside or contraction, further enhances AMPK activity and glucose transport in mouse skeletal muscle ex vivo. Given that the γ3-subunit is exclusively expressed in skeletal muscle and has been implicated in contraction-induced glucose transport, we measured the activity of AMPKγ3 as well as ubiquitously expressed γ1-containing complexes. We initially validated the specificity of the antibodies for the assessment of isoform-specific AMPK activity using AMPK-deficient mouse models. We observed that a low dose of 991 (5 μM) stimulated a modest or negligible activity of both γ1- and γ3-containing AMPK complexes. Strikingly, dual treatment with 991 and 5-aminoimidazole-4-carboxamide riboside or 991 and contraction profoundly enhanced AMPKγ1/γ3 complex activation and glucose transport compared with any of the single treatments. The study demonstrates the utility of a dual activator approach to achieve a greater activation of AMPK and downstream physiological responses in various cell types, including skeletal muscle.

Recent progress in the identification of adenosine monophosphate-activated protein kinase (AMPK) activators

Adenosine monophosphate-activated protein kinase (AMPK), a serine/threonine heterotrimeric protein kinase, is a critical regulator of cellular and whole body energy homeostasis. There are twelve known AMPK isoforms that are differentially expressed in tissues and species. Dysregulation of AMPK signaling is associated with a multitude of human pathologies. Hence isoform-selective activators of AMPK are actively being sought for the treatment of cardiovascular and metabolic diseases. The present review summarizes the status of direct AMPK activators from the patent and published literature.

Functional characterization of AMP-activated protein kinase signaling in tumorigenesis

AMP-activated protein kinase (AMPK) is a ubiquitously expressed metabolic sensor among various species. Specifically, cellular AMPK is phosphorylated and activated under certain stressful conditions, such as energy deprivation, in turn to activate diversified downstream substrates to modulate the adaptive changes and maintain metabolic homeostasis. Recently, emerging evidences have implicated the potential roles of AMPK signaling in tumor initiation and progression. Nevertheless, a comprehensive description on such topic is still in scarcity, especially in combination of its biochemical features with mouse modeling results to elucidate the physiological role of AMPK signaling in tumorigenesis. Hence, we performed this thorough review by summarizing the tumorigenic role of each component along the AMPK signaling, comprising of both its upstream and downstream effectors. Moreover, their functional interplay with the AMPK heterotrimer and exclusive efficacies in carcinogenesis were chiefly explained among genetically altered mice models. Importantly, the pharmaceutical investigations of AMPK relevant medications have also been highlighted. In summary, in this review, we not only elucidate the potential functions of AMPK signaling pathway in governing tumorigenesis, but also potentiate the future targeted strategy aiming for better treatment of aberrant metabolism-associated diseases, including cancer.

AMP-activated protein kinase: a cellular energy sensor that comes in 12 flavours

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that is expressed in essentially all eukaryotic cells, suggesting that it arose during early eukaryotic evolution. It occurs universally as heterotrimeric complexes containing catalytic α subunits and regulatory β and γ subunits. Although Drosophila melanogaster contains single genes encoding each subunit, in mammals, each subunit exists as multiple isoforms encoded by distinct genes, giving rise to up to 12 heterotrimeric combinations. The multiple isoforms of each subunit are 2R-ohnologues generated by the two rounds of whole genome duplication that occurred at the evolutionary origin of the vertebrates. Although the differential roles of these isoform combinations remain only partly understood, there are indications that they may have different subcellular locations, different inputs and outputs, and different functions. The multiple isoforms are of particular interest with respect to the roles of AMPK in cancer because the genes encoding some isoforms, such as PRKAA1 and PRKAB2 (encoding α1 and β2), are quite frequently amplified in tumour cells, whereas the genes encoding others, such as PRKAA2 (encoding α2), tend to be mutated, which, in some but not all cases, may result in a loss of function. Thus, although AMPK acts downstream of the tumour suppressor liver kinase B1, and some of its isoform combinations may act as tumour suppressors that restrain the growth and proliferation of tumour cells, other isoform combinations may paradoxically act as oncogenes, perhaps by aiding the survival of tumour cells undergoing environmental stresses such as hypoxia or nutrient deprivation.

α-MSH Stimulates Glucose Uptake in Mouse Muscle and Phosphorylates Rab-GTPase-Activating Protein TBC1D1 Independently of AMPK

The melanocortin system includes five G-protein coupled receptors (family A) defined as MC1R-MC5R, which are stimulated by endogenous agonists derived from proopiomelanocortin (POMC). The melanocortin system has been intensely studied for its central actions in body weight and energy expenditure regulation, which are mainly mediated by MC4R. The pituitary gland is the source of various POMC-derived hormones released to the circulation, which raises the possibility that there may be actions of the melanocortins on peripheral energy homeostasis. In this study, we examined the molecular signaling pathway involved in α-MSH-stimulated glucose uptake in differentiated L6 myotubes and mouse muscle explants. In order to examine the involvement of AMPK, we investigate α-MSH stimulation in both wild type and AMPK deficient mice. We found that α-MSH significantly induces phosphorylation of TBC1 domain (TBC1D) family member 1 (S237 and T596), which is independent of upstream PKA and AMPK. We find no evidence to support that α-MSH-stimulated glucose uptake involves TBC1D4 phosphorylation (T642 and S704) or GLUT4 translocation.

Chronic Activation of γ2 AMPK Induces Obesity and Reduces β Cell Function

Despite significant advances in our understanding of the biology determining systemic energy homeostasis, the treatment of obesity remains a medical challenge. Activation of AMP-activated protein kinase (AMPK) has been proposed as an attractive strategy for the treatment of obesity and its complications. AMPK is a conserved, ubiquitously expressed, heterotrimeric serine/threonine kinase whose short-term activation has multiple beneficial metabolic effects. Whether these translate into long-term benefits for obesity and its complications is unknown. Here, we observe that mice with chronic AMPK activation, resulting from mutation of the AMPK γ2 subunit, exhibit ghrelin signaling-dependent hyperphagia, obesity, and impaired pancreatic islet insulin secretion. Humans bearing the homologous mutation manifest a congruent phenotype. Our studies highlight that long-term AMPK activation throughout all tissues can have adverse metabolic consequences, with implications for pharmacological strategies seeking to chronically activate AMPK systemically to treat metabolic disease.

AMPK activators: mechanisms of action and physiological activities

AMP-activated protein kinase (AMPK) is a central regulator of energy homeostasis, which coordinates metabolic pathways and thus balances nutrient supply with energy demand. Because of the favorable physiological outcomes of AMPK activation on metabolism, AMPK has been considered to be an important therapeutic target for controlling human diseases including metabolic syndrome and cancer. Thus, activators of AMPK may have potential as novel therapeutics for these diseases. In this review, we provide a comprehensive summary of both indirect and direct AMPK activators and their modes of action in relation to the structure of AMPK. We discuss the functional differences among isoform-specific AMPK complexes and their significance regarding the development of novel AMPK activators and the potential for combining different AMPK activators in the treatment of human disease.

Regulation of autophagy in human skeletal muscle: effects of exercise, exercise training and insulin stimulation

KEY POINTS

Regulation of autophagy in human muscle in many aspects differs from the majority of previous reports based on studies in cell systems and rodent muscle. An acute bout of exercise and insulin stimulation reduce human muscle autophagosome content. An acute bout of exercise regulates autophagy by a local contraction-induced mechanism. Exercise training increases the capacity for formation of autophagosomes in human muscle. AMPK activation during exercise seems insufficient to regulate autophagosome content in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin. Studies in rodent muscle suggest that autophagy is regulated by acute exercise, exercise training and insulin stimulation. However, little is known about the regulation of autophagy in human skeletal muscle. Here we investigate the autophagic response to acute one-legged exercise, one-legged exercise training and subsequent insulin stimulation in exercised and non-exercised human muscle. Acute one-legged exercise decreased (P<0.01) lipidation of microtubule-associated protein 1A/1B-light chain 3 (LC3) (∼ 50%) and the LC3-II/LC3-I ratio (∼ 60%) indicating that content of autophagosomes decreases with exercise in human muscle. The decrease in LC3-II/LC3-I ratio did not correlate with activation of 5'AMP activated protein kinase (AMPK) trimer complexes in human muscle. Consistently, pharmacological AMPK activation with 5-aminoimidazole-4-carboxamide riboside (AICAR) in mouse muscle did not affect the LC3-II/LC3-I ratio. Four hours after exercise, insulin further reduced (P<0.01) the LC3-II/LC3-I ratio (∼ 80%) in muscle of the exercised and non-exercised leg in humans. This coincided with increased Ser-757 phosphorylation of Unc51 like kinase 1 (ULK1), which is suggested as a mammalian target of rapamycin complex 1 (mTORC1) target. Accordingly, inhibition of mTOR signalling in mouse muscle prevented the ability of insulin to reduce the LC3-II/LC3-I ratio. In response to 3 weeks of one-legged exercise training, the LC3-II/LC3-I ratio decreased (P<0.05) in both trained and untrained muscle and this change was largely driven by an increase in LC3-I content. Taken together, acute exercise and insulin stimulation reduce muscle autophagosome content, while exercise training may increase the capacity for formation of autophagosomes in muscle. Moreover, AMPK activation during exercise may not be sufficient to regulate autophagy in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin.

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

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.

Pharmacological Targeting of AMP-Activated Protein Kinase and Opportunities for Computer-Aided Drug Design

As a central regulator of metabolism, the AMP-activated protein kinase (AMPK) is an established therapeutic target for metabolic diseases. Beyond the metabolic area, the number of medical fields that involve AMPK grows continuously, expanding the potential applications for AMPK modulators. Even though indirect AMPK activators are used in the clinics for their beneficial metabolic outcome, the few described direct agonists all failed to reach the market to date, which leaves options open for novel targeting methods. As AMPK is not actually a single molecule and has different roles depending on its isoform composition, the opportunity for isoform-specific targeting has notably come forward, but the currently available modulators fall short of expectations. In this review, we argue that with the amount of available structural and ligand data, computer-based drug design offers a number of opportunities to undertake novel and isoform-specific targeting of AMPK.

Differential regulation by AMP and ADP of AMPK complexes containing different γ subunit isoforms

AMPK complexes containing γ1, γ2 or γ3 subunit isoforms were generated by expression in human cells. They displayed differences in all three effects by which adenine nucleotides cause regulation, i.e. in allosteric activation, promotion of phosphorylation and inhibition of dephosphorylation

The γ subunits of heterotrimeric AMPK complexes contain the binding sites for the regulatory adenine nucleotides AMP, ADP and ATP. We addressed whether complexes containing different γ isoforms display different responses to adenine nucleotides by generating cells stably expressing FLAG-tagged versions of the γ1, γ2 or γ3 isoform. When assayed at a physiological ATP concentration (5 mM), γ1- and γ2-containing complexes were allosterically activated almost 10-fold by AMP, with EC₅₀ values one to two orders of magnitude lower than the ATP concentration. By contrast, γ3 complexes were barely activated by AMP under these conditions, although we did observe some activation at lower ATP concentrations. Despite this, all three complexes were activated, due to increased Thr¹⁷² phosphorylation, when cells were incubated with mitochondrial inhibitors that increase cellular AMP. With γ1 complexes, activation and Thr¹⁷² phosphorylation induced by the upstream kinase LKB1 [liver kinase B1; but not calmodulin-dependent kinase kinase (CaMKKβ)] in cell-free assays was markedly promoted by AMP and, to a smaller extent and less potently, by ADP. However, effects of AMP or ADP on activation and phosphorylation of the γ2 and γ3 complexes were small or insignificant. Binding of AMP or ADP protected all three γ subunit complexes against inactivation by Thr¹⁷² dephosphorylation; with γ2 complexes, ADP had similar potency to AMP, but with γ1 and γ3 complexes, ADP was less potent than AMP. Thus, AMPK complexes containing different γ subunit isoforms respond differently to changes in AMP, ADP or ATP. These differences may tune the responses of the isoforms to fit their differing physiological roles.

Skeletal muscle AMP-activated protein kinase γ1(H151R) overexpression enhances whole body energy homeostasis and insulin sensitivity

AMP-activated protein kinase (AMPK) is a major sensor of energy homeostasis and stimulates ATP-generating processes such as lipid oxidation and glycolysis in peripheral tissues. The heterotrimeric enzyme consists of a catalytic α-subunit, a β-subunit that is important for enzyme activity, and a noncatalytic γ-subunit that binds AMP and activates the AMPK complex. We generated a skeletal muscle Cre-inducible transgenic mouse model expressing a mutant γ1-subunit (AMPKγ1(H151R)), resulting in chronic AMPK activation. The expression of the predominant AMPKγ3 isoform in skeletal muscle was reduced in extensor digitorum longus (EDL) muscle (81-83%) of AMPKγ1(H151R) transgenic mice, whereas the abundance and phosphorylation of the AMPK target acetyl-CoA carboxylase was increased in tibialis anterior muscle. Glycogen content was increased 10-fold in gastrocnemius muscle. Whole body carbohydrate oxidation was increased by 11%, and whereas glucose tolerance was unaffected, insulin sensitivity was increased in AMPKγ1(H151R) transgenic mice. Furthermore, perigonadal white adipose tissue mass and serum leptin were reduced in female AMPKγ1(H151R) transgenic mice by 38 and 51% respectively. Conversely, in male AMPKγ1(H151R) transgenic mice, food intake was increased (14%), but body weight and body composition were unaltered, presumably because of increased energy expenditure. In conclusion, transgenic activation of skeletal muscle AMPKγ1 in this model plays an important sex-specific role in skeletal muscle metabolism and whole body energy homeostasis.

The AMPK activator R419 improves exercise capacity and skeletal muscle insulin sensitivity in obese mice

OBJECTIVE

Skeletal muscle AMP-activated protein kinase (AMPK) is important for regulating glucose homeostasis, mitochondrial content and exercise capacity. R419 is a mitochondrial complex-I inhibitor that has recently been shown to acutely activate AMPK in myotubes. Our main objective was to examine whether R419 treatment improves insulin sensitivity and exercise capacity in obese insulin resistant mice and whether skeletal muscle AMPK was important for mediating potential effects.

METHODS

Glucose homeostasis, insulin sensitivity, exercise capacity, and electron transport chain content/activity were examined in wildtype (WT) and AMPK β1β2 muscle-specific null (AMPK-MKO) mice fed a high-fat diet (HFD) with or without R419 supplementation.

RESULTS

There was no change in weight gain, adiposity, glucose tolerance or insulin sensitivity between HFD-fed WT and AMPK-MKO mice. In both HFD-fed WT and AMPK-MKO mice, R419 enhanced insulin tolerance, insulin-stimulated glucose disposal, skeletal muscle 2-deoxyglucose uptake, Akt phosphorylation and glucose transporter 4 (GLUT4) content independently of alterations in body mass. In WT, but not AMPK-MKO mice, R419 improved treadmill running capacity. Treatment with R419 increased muscle electron transport chain content and activity in WT mice; effects which were blunted in AMPK-MKO mice.

CONCLUSIONS

Treatment of obese mice with R419 improved skeletal muscle insulin sensitivity through a mechanism that is independent of skeletal muscle AMPK. R419 also increases exercise capacity and improves mitochondrial function in obese WT mice; effects that are diminished in the absence of skeletal muscle AMPK. These findings suggest that R419 may be a promising therapy for improving whole-body glucose homeostasis and exercise capacity.