Re-evaluating the general(ized) roles of AMPK in cellular metabolism

Molecular profiling of high-level athlete skeletal muscle after acute endurance or resistance exercise - A systems biology approach

OBJECTIVE

Long-term high-level exercise training leads to improvements in physical performance and multi-tissue adaptation following changes in molecular pathways. While skeletal muscle baseline differences between exercise-trained and untrained individuals have been previously investigated, it remains unclear how training history influences human multi-omics responses to acute exercise.

METHODS

We recruited and extensively characterized 24 individuals categorized as endurance athletes with >15 years of training history, strength athletes or control subjects. Timeseries skeletal muscle biopsies were taken from M. vastus lateralis at three time-points after endurance or resistance exercise was performed and multi-omics molecular analysis performed.

RESULTS

Our analyses revealed distinct activation differences of molecular processes such as fatty- and amino acid metabolism and transcription factors such as HIF1A and the MYF-family. We show that endurance athletes have an increased abundance of carnitine-derivates while strength athletes increase specific phospholipid metabolites compared to control subjects. Additionally, for the first time, we show the metabolite sorbitol to be substantially increased with acute exercise. On transcriptional level, we show that acute resistance exercise stimulates more gene expression than acute endurance exercise. This follows a specific pattern, with endurance athletes uniquely down-regulating pathways related to mitochondria, translation and ribosomes. Finally, both forms of exercise training specialize in diverging transcriptional directions, differentiating themselves from the transcriptome of the untrained control group.

CONCLUSIONS

We identify a "transcriptional specialization effect" by transcriptional narrowing and intensification, and molecular specialization effects on metabolomic level Additionally, we performed multi-omics network and cluster analysis, providing a novel resource of skeletal muscle transcriptomic and metabolomic profiling in highly trained and untrained individuals.

High-fat diet and palmitate inhibits FNDC5 expression via AMPK-Zfp57 pathway in mouse muscle cells

Irisin, a muscle-secreted cytokine involved in maintaining glucose homeostasis and improving insulin resistance, is generated from the precursor fibronectin type Ⅲ domain-containing protein 5 (FNDC5) by specific proteases. Zinc-finger protein Zfp57, a transcription factor that maintains the methylation during early embryonic development, is also reported to be associated with diabetes mellitus. However, the association between Zfp57 and FNDC5 is still unclear. In our study, we explored the detailed regulatory effect of Zfp57 on FNDC5 expression. In this study, we found that high-fat diet or saturated fatty acid palmitate increased the Zfp57 expression and decreased FNDC5 expression in muscle tissue or C2C12 myotubes. RNA sequencing analysis disclosed effects of the high-fat diet on genes associated with insulin resistance and the AMP-activated protein kinase (AMPK) signaling pathway in muscle tissue of mice. Chromatin immunoprecipitation experiments revealed that Zfp57 binds the FNDC5 gene promoter at positions -308 to -188. Moreover, Zfp57 overexpression inhibited FNDC5 expression, and Zfp57 knockdown alleviated the inhibitory effect of palmitate on FNDC5 expression in C2C12 myotubes. In addition, in vivo and in vitro studies demonstrated that activation of the AMPK pathway by 5-Aminoimidazole-4-carboxamide riboside (AICAR) or metformin mitigated the inhibitory effect of Zfp57 on FNDC5 expression and improved insulin resistance. These findings collectively suggest that high-fat diet and palmitate inhibit the AMPK pathway to increase Zfp57 expression, which in turn induces FNDC5 inhibition, to further aggravate insulin resistance.

Astilbin lowers the effective caffeine dose for decreasing lipid accumulation via activating AMPK in high-fat diet-induced obese mice

BACKGROUND

Caffeine has an anti-obesity effect, although chronic excessive caffeine consumption also causes caffeinism, which is marked by increased anxiety or depression, amongst other symptoms. The present study aimed to investigate whether the addition of flavonoids such as astilbin can reduce the caffeine dose needed to inhibit obesity.

RESULTS

ICR mice (n = 80) were fed with normal diet, high-fat diet (HFD), HFD supplemented with astilbin, caffeine, or astilbin + caffeine for 12 weeks. When diets supplemented with astilbin, 0.3 g kg^-1 diet caffeine had the same effect as 0.6 g kg^-1 diet caffeine alone, and 0.6 g kg^-1 diet caffeine combined with astilbin most effectively inhibited HFD-induced obesity. Astilbin improved the anti-obesity effects of caffeine on lipid accumulation via the activation of AMP-activated protein kinase α (AMPKα). (i) Activated AMPKα decreased lipid biosynthesis by suppressing the activity or mRNA expression of 3-hydroxy-3-methylglutaryl-CoA reductase, sterol regulatory element binding protein 1c and its target gene fatty acid synthase. (ii) Activated AMPKα also up-regulated lipolysis by enhancing the expression of adipose triglyceride lipase and increasing the phosphorylation of hormone-sensitive lipase. (iii) Finally, activated AMPKα increased carnitine acyltransferase and acyl-CoA oxidase activities, which further promoted fatty acid β-oxidation.

CONCLUSION

The results obtained in the present study indicate that astilbin may decrease the effective dose of caffeine needed for an anti-obesity effect and also suggest that it suppresses fat accumulation via the activation of AMPK. © 2020 Society of Chemical Industry.

Muscle-Specific Lipid Hydrolysis Prolongs Lifespan through Global Lipidomic Remodeling

A growing body of evidence suggests that changes in fat metabolism may have a significant effect on lifespan. Accumulation of lipid deposits in non-adipose tissue appears to be critical for age-related pathologies and may also contribute to the aging process itself. We established a model of lipid storage in muscle cells of C. elegans to reveal a mechanism that promotes longevity non-cell-autonomously. Here, we describe how muscle-specific activation of adipose triglyceride lipase (ATGL) and the phospholipase A₂ (PLA₂) ortholog IPLA-7 collectively affect inter-tissular communication and systemic adaptation that requires the activity of AMP-dependent protein kinase (AMPK) and a highly conserved nuclear receptor outside of the muscle. Our data suggest that muscle-specific bioactive lipid signals, or "lipokines," are generated following triglyceride breakdown and that these signals impinge on a complex network of genes that modify the global lipidome, consequently extending the lifespan.

Repeated ethanol exposure influences key enzymes in cholesterol and lipid homeostasis via the AMPK pathway in the rat prefrontal cortex

Cholesterol homeostasis has been proposed to be implicated in the development of addiction. However, the effects of ethanol on cholesterol homeostasis within the brain are not well understood. One of the most important regulators of cholesterol homeostasis is HMG-CoA reductase (HMG-CoAR), the rate-limiting enzyme of cholesterol biosynthesis. We examined the phosphorylation of HMG-CoAR and the other key regulator of lipid synthesis, acetyl-CoA carboxylase (ACC), following acute or chronic treatment with ethanol (0.5, 1, or 2 g/kg) in the rat prefrontal cortex. The phosphorylation of AMP-activated protein kinase (AMPK), which regulates the HMG-CoAR activity, and its well-known upstream regulators, was also studied. The phosphorylation of HMG-CoAR and ACC were transiently increased by ethanol treatment only in animals previously treated chronically with ethanol. Acute administration to naïve animals did not induce the phosphorylation, regardless of dosage. Similarly, the phosphorylation of AMPK and the upstream regulators, LKB1 and CaMK4, were transiently increased only in chronically ethanol-treated animals. In naïve animals, a high dose (2 g/kg) of ethanol decreased phosphorylation. The phosphorylation of TAK1, another upstream kinase of AMPK, was increased only from 30 min to 24 h after the chronic treatment with ethanol. Together, these results indicate that repeated exposure is required for the activating effect of ethanol on HMG-CoAR and ACC. This effect seems to be mediated by the AMPK system, and may contribute to the long-lasting neuroadaptation involved in the development of alcohol dependence.

The Impact of Moderate-Intensity Continuous or High-Intensity Interval Training on Adipogenesis and Browning of Subcutaneous Adipose Tissue in Obese Male Rats

This study compares the effect of two types of exercise training, i.e., moderate-intensity continuous training (MICT) or high-intensity interval training (HIIT) on the browning of subcutaneous white adipose tissue (scWAT) in obese male rats. Effects on fat composition, metabolites, and molecular markers of differentiation and energy expenditure were examined. Forty male Wistar rats were assigned to lean (n = 8) or obese (n = 32) groups and fed either a standard chow or high-fat obesogenic diet for 10 weeks. Eight lean and obese rats were then blood and tissue sampled, and the remaining obese animals were randomly allocated into sedentary, MICT, or HIIT (running on a treadmill 5 days/week) groups that were maintained for 12 weeks. Obesity increased plasma glucose and insulin and decreased irisin and FGF-21. In scWAT, this was accompanied with raised protein abundance of markers of adipocyte differentiation, i.e., C/EBP-α, C/EBP-β, and PPAR-γ, whereas brown fat-related genes, i.e., PRDM-16, AMPK/SIRT1/PGC-1α, were reduced as was UCP1 and markers of fatty acid transport, i.e., CD36 and CPT1. Exercise training increased protein expression of brown fat-related markers, i.e., PRDM-16, AMPK/SIRT1/PGC-1α, and UCP1, together with gene expression of fatty acid transport, i.e., CD36 and CPT1, but decreased markers of adipocyte differentiation, i.e., C/EBP-α, C/EBP-β, and plasma glucose. The majority of these adaptations were greater with HIIT compared to MICT. Our findings indicate that prolonged exercise training promotes the browning of white adipocytes, possibly through suppression of adipogenesis together with white to beige trans-differentiation and is dependent on the intensity of exercise.

Metformin regulates TRPM6, a potential explanation for magnesium imbalance in type 2 diabetes patients

Metformin therapy is associated with lower serum magnesium (Mg²⁺) levels in type 2 diabetes patients. The TRPM6 channel determines the fine-tuning of Mg²⁺ (re)absorption in intestine and kidney. Therefore, we aimed to investigate the short- and long-term effects of metformin on TRPM6. Patch clamp recordings and biotinylation assays were performed upon 1 h of incubation with metformin in TRPM6-transfected HEK293 cells. Additionally, 24 h of treatment of mDCT15 kidney and hCaco-2 colon cells with metformin was applied to measure the effects on endogenous TRPM6 expression by quantitative real-time PCR. To assess Mg²⁺ absorption, ²⁵Mg²⁺ uptake measurements were performed using inductively coupled plasma mass spectrometry. Short-term effects of metformin significantly increased TRPM6 activity and its cell surface trafficking. In contrast, long-term effects significantly decreased TRPM6 mRNA expression and ²⁵Mg²⁺ uptake. Metformin lowered TRPM6 mRNA levels independently of insulin- and AMPK-mediated pathways. Moreover, in type 2 diabetes patients, metformin therapy was associated with lower plasma Mg²⁺ concentrations and fractional excretion of Mg²⁺. Thereby, short-term metformin treatment increases TRPM6 activity explained by enhanced cell surface expression. Conversely, long-term metformin treatment results in downregulation of TRPM6 gene expression in intestine and kidney cells. This long-term effect translated in an inverse correlation between metformin and plasma Mg²⁺ concentration in type 2 diabetes patients.

Thyroxine Alleviates Energy Failure, Prevents Myocardial Cell Apoptosis, and Protects against Doxorubicin-Induced Cardiac Injury and Cardiac Dysfunction via the LKB1/AMPK/mTOR Axis in Mice

Background

Previous studies have demonstrated that energy failure is closely associated with cardiac injury. Doxorubicin (DOX) is a commonly used clinical chemotherapy drug that can mediate cardiac injury through a variety of mechanisms. Thyroxine is well known to play a critical role in energy generation; thus, this study is aimed at investigating whether thyroxine can attenuate DOX-induced cardiac injury by regulating energy generation.

Methods

First, the effect of DOX on adenosine diphosphate (ADP) and adenosine triphosphate (ATP) ratios in mice was assessed. In addition, thyroxine was given to mice before they were treated with DOX to investigate the effects of thyroxine on DOX-induced cardiac injury. Furthermore, to determine whether the liver kinase b1 (LKB1)/adenosine 5′-monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) axis mediated the effect of thyroxine on DOX-induced cardiac injury, thyroxine was given to DOX-treated LKB1 knockout (KO) mice.

Results

DOX treatment time- and dose-dependently increased the ADP/ATP ratio. Thyroxine treatment also reduced lactate dehydrogenase (LDH) and creatine kinase myocardial band (CK-MB) levels in both serum and heart tissue and alleviated cardiac dysfunction in DOX-treated mice. Furthermore, thyroxine reversed DOX-induced reductions in LKB1 and AMPK phosphorylation; mitochondrial complex I, III, and IV activity; and mitochondrial swelling and reversed DOX-induced increases in mTOR pathway phosphorylation and myocardial cell apoptosis. These effects of thyroxine on DOX-treated mice were significantly attenuated by LKB1 KO.

Conclusions

Thyroxine alleviates energy failure, reduces myocardial cell apoptosis, and protects against DOX-induced cardiac injury via the LKB1/AMPK/mTOR axis in mice. Thyroxine may be a new agent for the clinical prevention of cardiac injury in tumor patients undergoing chemotherapy with DOX.

Hypoxia preconditioning promotes endurance exercise capacity of mice by activating skeletal muscle Nrf2

Elite endurance athletes are used to train under hypoxic/high-altitude conditions, which can elicit certain stress responses in skeletal muscle and helps to improve their physical performance. Nuclear factor erythroid 2-related factor 2 (Nrf2) regulates cellular redox homeostasis and metabolism in skeletal muscle, playing important roles in adaptation to various stresses. In this study, Nrf2 knockout (KO) and wild-type (WT) mice were preconditioned to 48 h of hypoxia exposure (11.2% oxygen), and the effects of hypoxia preconditioning (HP) on exercise capacity and exercise-induced changes of antioxidant status, energetic metabolism, and mitochondrial adaptation in skeletal muscle were evaluated. Nrf2 knockout (KO) and wild-type (WT) mice were exposed to normoxia or hypoxia for 48 h before taking incremental treadmill exercise to exhaustion under hypoxia. The skeletal muscles were collected immediately after the incremental treadmill exercise to evaluate the impacts of HP and Nrf2 on the exercise-induced changes. The results indicate the absence of Nrf2 did not affect exercise capacity, although the mRNA expression of certain muscular genes involved in antioxidant, glycogen and fatty acid catabolism was decreased in Nrf2 KO mice. However, 48-h HP enhanced exercise capacity in WT mice but not in Nrf2 KO mice, and the exercise capacity of WT mice was significantly higher than that of Nrf2 KO mice. These findings suggest HP promotes exercise capacity of mice with the participation of the Nrf2 signal in skeletal muscle.

NEW & NOTEWORTHY Hypoxia preconditioning (HP) activated the nuclear factor erythroid 2-related factor 2 (Nrf2) signal, which was involved in HP-elicited adaptation responses to hypoxia, oxidative, and metabolic stresses in skeletal muscle. On the other hand, Nrf2 deficiency abolished the enhanced exercise capacity after the 48-h HP. Our results indicate that Nrf2 plays an essential role in the exercise capacity-enhancing effect of HP, possibly by modulating muscular antioxidative responses, the mRNA expression of muscular genes involved in glycogen and fatty acid metabolism, as well as mitochondrial biogenesis, and through the cross talk with AMPK and hypoxia-inducible factor-1α signaling.

Dietary supplement of Smilax china L. ethanol extract alleviates the lipid accumulation by activating AMPK pathways in high-fat diet fed mice

Background

Obesity has become a public health concern worldwide because it is linked to numerous metabolic disorders, such as hyperlipidemia, hypertension and cardiovascular disease. Therefore, there is an urgent need to develop new therapeutic strategies that are efficacious and have minimal side effects in obesity treatment. This study examined the effect of dietary supplement of Smilax china L. ethanol extract (SCLE) on high-fat diet (HFD) induced obesity.

Methods

Fifty ICR mice were fed a normal diet, high-fat diet (HFD) or HFD supplemented with 0.25, 0.5% or 1% SCLE for 8 weeks. Body weight, intraperitioneal adipose tissue (IPAT) weight, serum biochemical parameters, and liver lipids were measured. Activity, mRNA and protein expressions of lipid metabolism-related enzymes were analyzed.

Results

Over 0.5% SCLE had reduced cholesterol biosynthesis by the activation of AMP-activated protein kinase (AMPK), which subsequently suppressed the mRNA expression of both sterol regulatory element binding protein-2 and 3-hydroxy-3-methyl-glutaryl-CoA reductase. Thus, the plasma and liver cholesterol concentrations in the HFD-fed mice were decreased. AMPK activation caused by SCLE also significantly upregulated lipolysis by enhancing adipose triglyceride lipase and hormone-sensitive lipase activities. This accelerated triglyceride hydrolysis and fatty acid release. Finally, SCLE increased carnitine palmitoyltransferase 1 and acyl-CoA oxidase activities, which further promoted fatty acid β-oxidation.

Conclusion

SCLE could lead to a decrease in body weight gain and fat mass by inhibiting the lipid synthesis and promoting lipolysis and β-oxidation in HFD fed mice. The underlying mechanism is probably associated with regulating AMPK pathway.

Defining the short-term effects of pharmacological 5'-AMP activated kinase modulators on mitochondrial polarization, morphology and heterogeneity

Background

Under aerobic growth conditions, mitochondria are the major producers of cellular ATP and crucial for the proper performance of organs and tissues. This applies especially to cells with high energy demand, such as the renal proximal tubule epithelium. Mitochondrial dysfunction contributes to the pathology of human health conditions, including various kidney diseases. The improvement of mitochondrial function ameliorates some of these pathologies. This can potentially be achieved with pharmacological compounds. For example, long-term treatment with activators of 5'-AMP activated kinase (AMPK) enhances mitochondrial biogenesis. However, pharmacological damage control during acute cell injury requires that the short-term effects of these compounds and the impact on healthy cells are also understood. It was our objective to define the changes elicited by established modulators of AMPK activity in healthy renal proximal tubule cells.

Methods

Our work combines confocal microscopy with quantitative image analysis, 3D image reconstruction and Western blotting to provide novel insights into the biology of mitochondria. Specifically, we evaluated the effects of pharmacological AMPK modulators (compound C, AICAR, phenformin, resveratrol) on mitochondrial polarization, morphology and heterogeneity. Microscopic studies generated information at the single cell and subcellular levels. Our research focused on LLC-PK1 cells that are derived from the renal proximal tubule. Mitochondrial heterogeneity was also examined in MCF7 breast cancer cells.

Results

Pharmacological agents that affect AMPK activity in renal proximal tubule cells can alter mitochondrial organization and the electrochemical potential across the inner mitochondrial membrane. These changes were compound-specific. Short-term incubation with the AMPK inhibitor compound C caused mitochondrial hyperpolarization. This was accompanied by mitochondrial fragmentation. By contrast, AMPK activators AICAR, phenformin and resveratrol had little impact. We further show that the biological properties of mitochondria are determined by their subcellular location. Mitochondria at the cell periphery displayed higher MitoTracker/Tom70 values as compared to organelles located in the vicinity of the nucleus. This was not limited to renal proximal tubule cells, but also observed in MCF7 cells. Pharmacological AMPK modulators altered these location-dependent properties in a compound-specific fashion. While the region-dependent differences were enhanced with phenformin, they were ameliorated by resveratrol.

Discussion

We evaluated the rapid changes in mitochondrial characteristics that are induced by pharmacological AMPK modulators. Our research supports the concept that pharmacological agents that target AMPK can rearrange mitochondrial networks at the single cell level. Collectively, these insights are relevant to the development of proper strategies for the short-term adjustment of mitochondrial performance.

A comparison of LKB1/AMPK/mTOR metabolic axis response to global ischaemia in brain, heart, liver and kidney in a rat model of cardiac arrest

BACKGROUND

Cellular energy failure in high metabolic rate organs is one of the underlying causes for many disorders such as neurodegenerative diseases, cardiomyopathies, liver and renal failures. In the past decade, numerous studies have discovered the cellular axis of LKB1/AMPK/mTOR as an essential modulator of cell homeostasis in response to energy stress. Through regulating adaptive mechanisms, this axis adjusts the energy availability to its demand by a systematized control on metabolism. Energy stress, however, could be sensed at different levels in various tissues, leading to applying different strategies in response to hypoxic insults.

METHODS

Here the immediate strategies of high metabolic rate organs to time-dependent short episodes of ischaemia were studied by using a rat model (n = 6/group) of cardiac arrest (CA) (15 and 30 s, 1, 2, 4 and 8 min CA). Using western blot analysis, we examined the responses of LKB1/AMPK/mTOR pathway in brain, heart, liver and kidney from 15 s up to 8 min of global ischaemia. The ratio of ADP/ATP was assessed in all ischemic and control groups, using ApoSENSOR bioluminescent assay kit.

RESULTS

Brain, followed by kidney showed the early dephosphorylation response in AMPK (Thr172) and LKB1 (Ser431); in the absence of ATP decline (ADP/ATP elevation). Dephosphorylation of AMPK was followed by rephosphorylation and hyperphosphorylation, which was associated with a significant ATP decline. While heart's activity of AMPK and LKB1 remained at the same level during short episodes of ischaemia, liver's LKB1 was dephosphorylated after 2 min. AMPK response to ischaemia in liver was mainly based on an early alternative and a late constant hyperphosphorylation. No significant changes was observed in mTOR activity in all groups.

CONCLUSION

Together our results suggest that early AMPK dephosphorylation followed by late hyperphosphorylation is the strategy of brain and kidney in response to ischaemia. While the liver seemed to get benefit of its AMPK system in early ischameia, possibly to stabilize ATP, the level of LKB1/AMPK activity in heart remained unchanged in short ischaemic episodes up to 8 min. Further researches must be conducted to elucidate the molecular mechanism underlying LKB1/AMPK response to oxygen supply.

Mechanisms linking mechanotransduction and cell metabolism

Throughout their lifetimes, all cells experience force. These forces are sensed by cell surface adhesion receptors, such as the cadherins and integrins. Much attention has focused on identifying how these adhesion receptors transmit force. In contrast, less is known regarding how these force-activated pathways are integrated with other cellular processes. In this review, we describe how cadherins and integrins transmit force, and discuss how these adhesion receptors are linked to cell metabolism. We focus on understanding this connection by highlighting how the cadherins and integrins interact with a master regulator of energy homeostasis, AMP-activated protein kinase (AMPK) and its upstream activator, Liver Kinase B1 (LKB1). We consider why there is a need for force transmission to be coupled to metabolism and highlight the major unanswered questions in the field.

AMPK signaling in skeletal muscle during exercise: Role of reactive oxygen and nitrogen species

Reactive oxygen and nitrogen species (RONS) are generated during exercise depending on intensity, duration and training status. A greater amount of RONS is released during repeated high-intensity sprint exercise and when the exercise is performed in hypoxia. By activating adenosine monophosphate-activated kinase (AMPK), RONS play a critical role in the regulation of muscle metabolism but also in the adaptive responses to exercise training. RONS may activate AMPK by direct an indirect mechanisms. Directly, RONS may activate or deactivate AMPK by modifying RONS-sensitive residues of the AMPK-α subunit. Indirectly, RONS may activate AMPK by reducing mitochondrial ATP synthesis, leading to an increased AMP:ATP ratio and subsequent Thr(172)-AMPK phosphorylation by the two main AMPK kinases: LKB1 and CaMKKβ. In presence of RONS the rate of Thr(172)-AMPK dephosphorylation is reduced. RONS may activate LKB1 through Sestrin2 and SIRT1 (NAD(+)/NADH.H(+)-dependent deacetylase). RONS may also activate CaMKKβ by direct modification of RONS sensitive motifs and, indirectly, by activating the ryanodine receptor (Ryr) to release Ca(2+). Both too high (hypoxia) and too low (ingestion of antioxidants) RONS levels may lead to Ser(485)-AMPKα1/Ser(491)-AMPKα2 phosphorylation causing inhibition of Thr(172)-AMPKα phosphorylation. Exercise training increases muscle antioxidant capacity. When the same high-intensity training is applied to arm and leg muscles, arm muscles show signs of increased oxidative stress and reduced mitochondrial biogenesis, which may be explained by differences in RONS-sensing mechanisms and basal antioxidant capacities between arm and leg muscles. Efficient adaptation to exercise training requires optimal exposure to pulses of RONS. Inappropriate training stimulus may lead to excessive RONS formation, oxidative inactivation of AMPK and reduced adaptation or even maladaptation. Theoretically, exercise programs should be designed taking into account the intrinsic properties of different skeletal muscles, the specific RONS induction and the subsequent signaling responses.

Resistin's, obesity and insulin resistance: the continuing disconnect between rodents and humans

PURPOSE

This review aimed to discuss the conflicting findings from resistin research in rodents and humans as well as recent advances in our understanding of resistin's role in obesity and insulin resistance.

METHODS

A comprehensive review and synthesis of resistin's role in obesity and insulin resistance as well as conflicting findings from resistin research in rodents and humans.

RESULTS

In rodents, resistin is increased in high-fat/high-carbohydrate-fed, obese states characterized by impaired glucose uptake and insulin sensitivity. Resistin plays a causative role in the development of insulin resistance in rodents via 5' AMP-activated protein kinase (AMPK)-dependent and AMPK-independent suppressor of cytokine signaling-3 (SOCS-3) signaling. In contrast to rodents, human resistin is primarily secreted by peripheral-blood mononuclear cells (PBMCs) as opposed to white adipocytes. Circulating resistin levels have been positively associated with central/visceral obesity (but not BMI) as well as insulin resistance, while other studies show no such association. Human resistin has a role in pro-inflammatory processes that have been conclusively associated with obesity and insulin resistance. PBMCs, as well as vascular cells, have been identified as the primary targets of resistin's pro-inflammatory activity via nuclear factor-κB (NF-κB, p50/p65) and other signaling pathways.

CONCLUSION

Mounting evidence reveals a continuing disconnect between resistin's role in rodents and humans due to significant differences between these two species with respect to resistin's gene and protein structure, differential gene regulation, tissue-specific distribution, and insulin resistance induction as well as a paucity of evidence regarding the resistin receptor and downstream signaling mechanisms of action.

The Kinome of Pacific Oyster Crassostrea gigas, Its Expression during Development and in Response to Environmental Factors

Oysters play an important role in estuarine and coastal marine habitats, where the majority of humans live. In these ecosystems, environmental degradation is substantial, and oysters must cope with highly dynamic and stressful environmental constraints during their lives in the intertidal zone. The availability of the genome sequence of the Pacific oyster Crassostrea gigas represents a unique opportunity for a comprehensive assessment of the signal transduction pathways that the species has developed to deal with this unique habitat. We performed an in silico analysis to identify, annotate and classify protein kinases in C. gigas, according to their kinase domain taxonomy classification, and compared with kinome already described in other animal species. The C. gigas kinome consists of 371 protein kinases, making it closely related to the sea urchin kinome, which has 353 protein kinases. The absence of gene redundancy in some groups of the C. gigas kinome may simplify functional studies of protein kinases. Through data mining of transcriptomes in C. gigas, we identified part of the kinome which may be central during development and may play a role in response to various environmental factors. Overall, this work contributes to a better understanding of key sensing pathways that may be central for adaptation to a highly dynamic marine environment.

Effects of pharmacological AMP deaminase inhibition and Ampd1 deletion on nucleotide levels and AMPK activation in contracting skeletal muscle

AMP-activated protein kinase (AMPK) plays a central role in regulating metabolism and energy homeostasis. It achieves its function by sensing fluctuations in the AMP:ATP ratio. AMP deaminase (AMPD) converts AMP into IMP, and the AMPD1 isoenzyme is expressed in skeletal muscles. Here, effects of pharmacological inhibition and genetic deletion of AMPD were examined in contracting skeletal muscles. Pharmacological AMPD inhibition potentiated rises in AMP, AMP:ATP ratio, AMPK Thr172, and acetyl-CoA carboxylase (ACC) Ser218 phosphorylation induced by electrical stimulation, without affecting glucose transport. In incubated extensor digitorum longus and soleus muscles from Ampd1 knockout mice, increases in AMP levels and AMP:ATP ratio by electrical stimulation were potentiated considerably compared with muscles from wild-type mice, whereas enhanced AMPK activation was moderate and only observed in soleus, suggesting control by factors other than changes in adenine nucleotides. AMPD inhibitors could be useful tools for enhancing AMPK activation in cells and tissues during ATP-depletion.

Is 5´-AMP-Activated Protein Kinase Both Jekyll and Hyde in Bladder Cancer?

The 5´-AMP-activated protein kinase (AMPK) is a key regulator of cellular metabolism and energy homeostasis in mammalian tissues. Metabolic adaptation is a critical step in ensuring cell survival during metabolic stress. Because of its critical role in the regulation of glucose homeostasis and carbohydrate, lipid, and protein metabolism, AMPK is involved in many human diseases, including cancers. Although AMPK signaling was originally characterized as a tumor-suppressive signaling pathway, several lines of evidence suggest that AMPK plays a much broader role and cannot simply be defined as either an oncogenic regulator or tumor suppressor. Notably, several recent studies demonstrated that the antitumorigenic effects of many indirect AMPK activators, such as metformin, do not depend on AMPK. Conversely, activation of AMPK induces the progression of cancers, emphasizing its oncogenic effect. Bladder cancer can be divided into two groups: non–muscle-invasive bladder cancer (NMIBC) and muscle-invasive bladder cancer (MIBC). The molecular mechanisms underlying these two types of cancer are distinct: NMIBC is associated with activation of the Ras pathway, whereas MIBC is characterized by loss of major tumor suppressors. Importantly, both pathways are connected to the mammalian target of rapamycin (mTOR) pathway. In addition, our recent metabolomic findings suggest that β-oxidation of fatty acids is an important factor in the development of bladder cancer. Both mTOR and β-oxidation are tightly associated with the AMPK pathway. Here, I summarize and discuss the recent findings on the two distinct roles of AMPK in cancer, as well as the relationship between bladder cancer and AMPK.

Metformin attenuates palmitic acid-induced insulin resistance in L6 cells through the AMP-activated protein kinase/sterol regulatory element-binding protein-1c pathway

AMP-activated protein kinase (AMPK) is an important effector of metformin action on glucose uptake in skeletal muscle cells. We recently reported that metformin improved insulin receptor substrate-1 (IRS-1)-associated insulin signaling by downregulating sterol regulatory element-binding protein-1c (SREBP-1c) expression. In this study, we investigated whether AMPK activation and SREBP-1c inhibition contribute to the beneficial effects of metformin on IRS-1-associated insulin signaling in L6 myotubes. L6 muscle cells were incubated with palmitic acid (PA) to induce insulin resistance and then treated with metformin and/or the AMPK inhibitor, compound C. AMPK, SREBP-1c, IRS-1 and Akt protein expression levels were determined by western blot analysis. The effects of metformin on SREBP-1c gene transcription were determined by a luciferase reporter assay. Glucose uptake was evaluated using the 2-NBDG method. In the PA-treated L6 cells, metformin treatment enhanced AMPK phosphorylation, reduced SREBP-1c expression and increased IRS-1 and Akt protein expression, whereas treatment with compound C blocked the effects of metformin on SREBP-1c expression and the IRS-1 and Akt levels. Moreover, metformin suppressed SREBP-1c promoter activity and promoted glucose uptake through AMPK. The results from this study demonstrate that metformin ameliorates PA-induced insulin resistance through the activation of AMPK and the suppression of SREBP-1c in skeletal muscle cells.

Activation of AMP-activated protein kinase and phosphorylation of glycogen synthase kinase3 β mediate ursolic acid induced apoptosis in HepG2 liver cancer cells

Despite the antitumour effect of ursolic acid observed in several cancers, the underlying mechanism remains unclear. Thus, in the present study, the roles of AMP-activated protein kinase (AMPK) and glycogen synthase kinase 3 beta (GSK3β) were examined in ursolic acid induced apoptosis in HepG2 hepatocellular carcinoma cells. Ursolic acid significantly exerted cytotoxicity, increased the sub-G1 population and the number of ethidium homodimer and terminal deoxynucleotidyl transferase(TdT) mediated dUTP nick end labeling positive cells in HepG2 cells. Also, ursolic acid enhanced the cleavages of poly-ADP-ribose polymerase (PARP) and caspase3, attenuated the expression of astrocyte elevated gene (AEG1) and survivin in HepG2 cells. Interestingly, ursolic acid increased the phosphorylation of AMPK and coenzyme A carboxylase and also enhanced phosphorylation of GSK3β at inactive form serine 9, whereas ursolic acid attenuated the phosphorylation of AKT and mTOR in HepG2 cells. Conversely, AMPK inhibitor compound C or GSK3β inhibitor SB216763 blocked the cleavages of PARP and caspase 3 induced by ursolic acid in HepG2 cells. Furthermore, proteosomal inhibitor MG132 suppressed AMPK activation, GSK3β phosphorylation, cleaved PARP and deceased AEG-1 induced by ursolic acid in HepG2 cells. Overall, our findings suggest that ursolic acid induced apoptosis in HepG2 cells via AMPK activation and GSK3β phosphorylation as a potent chemopreventive agent.

AMP-activated protein kinase as regulator of P2Y(6) receptor-induced insulin secretion in mouse pancreatic β-cells

5'-AMP-activated protein kinase (AMPK) and its pharmacological modulators have been targeted for treating type 2 diabetes. Extracellular uridine 5'-diphosphate (UDP) activates P2Y6 receptors (P2Y6Rs) in pancreatic β-cells to release insulin and reduce apoptosis, which would benefit diabetes. Here, we studied the role of P2Y6R in activation of AMPK in MIN6 mouse pancreatic β-cells and insulin secretion. Treatment with a potent P2Y6R dinucleotide agonist MRS2957 (500nM) activated AMPK, which was blocked by P2Y6R-selective antagonist MRS2578. Also, MRS2957 induced phosphorylation of acetyl-coenzyme A carboxylase (ACC), a marker of AMPK activity. Calcium chelator BAPTA-AM, calmodulin-dependent protein kinase kinase (CaMKK) inhibitor STO-069 and IP3 receptor antagonist 2-APB attenuated P2Y6R-mediated AMPK phosphorylation revealing involvement of intracellular Ca(2+) pathways. P2Y6R agonist induced insulin secretion at high glucose, which was reduced by AMPK siRNA. Thus, P2Y6R has a crucial role in β-cell function, suggesting its potential as a therapeutic target in diabetes.

Interactions between oxygen homeostasis, food availability, and hydrogen sulfide signaling

The ability to sense and respond to stressful conditions is essential to maintain organismal homeostasis. It has long been recognized that stress response factors that improve survival in changing conditions can also influence longevity. In this review, we discuss different strategies used by animals in response to decreased O(2) (hypoxia) to maintain O(2) homeostasis, and consider interactions between hypoxia responses, nutritional status, and H(2)S signaling. O(2) is an essential environmental nutrient for almost all metazoans as it plays a fundamental role in development and cellular metabolism. However, the physiological response(s) to hypoxia depend greatly on the amount of O(2) available. Animals must sense declining O(2) availability to coordinate fundamental metabolic and signaling pathways. It is not surprising that factors involved in the response to hypoxia are also involved in responding to other key environmental signals, particularly food availability. Recent studies in mammals have also shown that the small gaseous signaling molecule hydrogen sulfide (H(2)S) protects against cellular damage and death in hypoxia. These results suggest that H(2)S signaling also integrates with hypoxia response(s). Many of the signaling pathways that mediate the effects of hypoxia, food deprivation, and H(2)S signaling have also been implicated in the control of lifespan. Understanding how these pathways are coordinated therefore has the potential to reveal new cellular and organismal homeostatic mechanisms that contribute to longevity assurance in animals.

TG accumulation inhibitory effects of Jinqi formula by AMPK signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Jinqi formula is a traditional Chinese anti-diabetic formula containing three ingredients (Coptidis rhizoma, Astragali rhadix and Lonicerae japonicae Flos).

MATERIALS AND METHODS

The active fractions of Jinqi formula were purified and HPLC analyses were used for quality control. The anti-adipogenic effects of Jinqi formula were analyzed in vitro using 3T3-L1 cells and in vivo with KK-A(y) mice. RT-PCR and Western blot were used to confirm genes and proteins of interest, respectively.

RESULTS

In vitro study showed that Jinqi formula suppressed the accumulation of triglyceride (TG) and free fatty acids (FFA) in mature 3T3-L1 cells by increasing the expression and tyrosine phosphorylation of 5'-AMP-activated protein kinase (AMPK), as well as decreasing the expression of Acetyl CoA Carboxylase (ACC), Fatty Acid Synthase (FAS) and Hormone Sensitive Lipase (HSL). In vivo study demonstrated that Jinqi formula reduced body weight without changing food intake in KK-A(y) mice, and decreased the levels of serum glucose, TG, FFA. In addition, consistent with the in vitro study results, Jinqi formula increased the expression and tyrosine phosphorylation of AMPK in the liver and muscular tissues of the KK-A(y) mice. Furthermore, Jingqi formula suppressed the expression of ACC and HSL and upregulated the expression of IRS-1 in the liver. Whereas in the skeletal muscles, Jingqi formula decreased the expression of ACC and increased the expression of GLUT-4 and IRS-2.

CONCLUSIONS

Jingqi formula inhibits TG accumulation at least in part via the stimulation of AMPK activity in a multi-target manner.

The filamentous growth MAPK Pathway Responds to Glucose Starvation Through the Mig1/2 transcriptional repressors in Saccharomyces cerevisiae

In the budding yeast S. cerevisiae, nutrient limitation induces a MAPK pathway that regulates filamentous growth and biofilm/mat formation. How nutrient levels feed into the regulation of the filamentous growth pathway is not entirely clear. We characterized a newly identified MAPK regulatory protein of the filamentous growth pathway, Opy2. A two-hybrid screen with the cytosolic domain of Opy2 uncovered new interacting partners including a transcriptional repressor that functions in the AMPK pathway, Mig1, and its close functional homolog, Mig2. Mig1 and Mig2 coregulated the filamentous growth pathway in response to glucose limitation, as did the AMP kinase Snf1. In addition to associating with Opy2, Mig1 and Mig2 interacted with other regulators of the filamentous growth pathway including the cytosolic domain of the signaling mucin Msb2, the MAP kinase kinase Ste7, and the MAP kinase Kss1. As for Opy2, Mig1 overproduction dampened the pheromone response pathway, which implicates Mig1 and Opy2 as potential regulators of pathway specificity. Taken together, our findings provide the first regulatory link in yeast between components of the AMPK pathway and a MAPK pathway that controls cellular differentiation.

AMP-activated protein kinase (AMPK) controls the aging process via an integrated signaling network

Efficient control of energy metabolic homeostasis, enhanced stress resistance, and qualified cellular housekeeping are the hallmarks of improved healthspan and extended lifespan. AMPK signaling is involved in the regulation of all these characteristics via an integrated signaling network. Many studies with lower organisms have revealed that increased AMPK activity can extend the lifespan. Experiments in mammals have demonstrated that AMPK controls autophagy through mTOR and ULK1 signaling which augment the quality of cellular housekeeping. Moreover, AMPK-induced stimulation of FoxO/DAF-16, Nrf2/SKN-1, and SIRT1 signaling pathways improves cellular stress resistance. Furthermore, inhibition of NF-κB signaling by AMPK suppresses inflammatory responses. Emerging studies indicate that the responsiveness of AMPK signaling clearly declines with aging. The loss of sensitivity of AMPK activation to cellular stress impairs metabolic regulation, increases oxidative stress and reduces autophagic clearance. These age-related changes activate innate immunity defence, triggering a low-grade inflammation and metabolic disorders. We will review in detail the signaling pathways of this integrated network through which AMPK controls energy metabolism, autophagic degradation and stress resistance and ultimately the aging process.

Overexpression of AMP-metabolizing enzymes controls adenine nucleotide levels and AMPK activation in HEK293T cells

We investigated whether overexpression of AMP-metabolizing enzymes in intact cells would modulate oligomycin-induced AMPK activation. Human embryonic kidney (HEK) 293T cells were transiently transfected with increasing amounts of plasmid vectors to obtain a graded increase in overexpression of AMP-deaminase (AMPD) 1, AMPD2, and soluble 5'-nucleotidase IA (cN-IA) for measurements of AMPK activation and total intracellular adenine nucleotide levels induced by oligomycin treatment. Overexpression of AMPD1 and AMPD2 slightly decreased AMP levels and oligomycin-induced AMPK activation. Increased overexpression of cN-IA led to reductions in the oligomycin-induced increases in AMP and ADP concentrations by ∼70 and 50%, respectively, concomitant with a 50% decrease in AMPK activation. The results support the view that a rise in ADP as well as AMP is important for activation of AMPK, which can thus be regulated by the adenylate energy charge. The control coefficient of cN-IA on AMP was 0.3-0.7, whereas the values for AMPD1 and AMPD2 were <0.1, suggesting that in this model cN-IA exerts a large proportion of control over intracellular AMP. Therefore, small molecule inhibition of cN-IA could be a strategy for AMPK activation.

AMP-activated protein kinase pathway and bone metabolism

There is increasing evidence that osteoporosis, similarly to obesity and diabetes, could be another disorder of energy metabolism. AMP-activated protein kinase (AMPK) has emerged over the last decade as a key sensing mechanism in the regulation of cellular energy homeostasis and is an essential mediator of the central and peripheral effects of many hormones on the metabolism of appetite, fat and glucose. Novel work demonstrates that the AMPK signaling pathway also plays a role in bone physiology. Activation of AMPK promotes bone formation in vitro and the deletion of α or β subunit of AMPK decreases bone mass in mice. Furthermore, AMPK activity in bone cells is regulated by the same hormones that regulate food intake and energy expenditure through AMPK activation in the brain and peripheral tissues. AMPK is also activated by antidiabetic drugs such as metformin and thiazolidinediones (TZDs), which also impact on skeletal metabolism. Interestingly, TZDs have detrimental skeletal side effects, causing bone loss and increasing the risk of fractures, although the role of AMPK mediation is still unclear. These data are presented in this review that also discusses the potential roles of AMPK in bone as well as the possibility for AMPK to be a future therapeutic target for intervention in osteoporosis.

Human kinome profiling identifies a requirement for AMP-activated protein kinase during human cytomegalovirus infection

Human cytomegalovirus (HCMV) modulates numerous cellular signaling pathways. Alterations in signaling are evident from the broad changes in cellular phosphorylation that occur during HCMV infection and from the altered activity of multiple kinases. Here we report a comprehensive RNAi screen, which predicts that 106 cellular kinases influence growth of the virus, most of which were not previously linked to HCMV replication. Multiple elements of the AMP-activated protein kinase (AMPK) pathway scored in the screen. As a regulator of carbon and nucleotide metabolism, AMPK is poised to activate many of the metabolic pathways induced by HCMV infection. An AMPK inhibitor, compound C, blocked a substantial portion of HCMV-induced metabolic changes, inhibited the accumulation of all HCMV proteins tested, and markedly reduced the production of infectious progeny. We propose that HCMV requires AMPK or related activity for viral replication and reprogramming of cellular metabolism.

The effect of cyclosporine A on the phosphorylation of the AMPK pathway in the rat hippocampus

Cyclosporine A (CsA), an immunosuppressant and calcineurin inhibitor, induces hyperlipidemia in humans and animals. AMP-activated protein kinase (AMPK) is involved in metabolic homeostasis and lipid metabolism through modulating downstream molecules acetyl CoA carboxylase (ACC) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoAR). AMPK activity is regulated by the phosphorylation at the Thr-172 residue by its upstream liver kinase B 1 (LKB1), Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) or transforming growth-factor-β-activated kinase 1 (TAK1). AMPK can be deactivated through dephosphorylation by protein phosphatase 2Cα (PP2Cα). In this study, we demonstrated that phosphorylation at Thr-172-AMPK increased with a concurrent increase in the phosphorylation of Ser-431-LKB1 and Thr-184/187-TAK1 in the rat hippocampus at 5 h after an intraperitoneal CsA (50 mg/kg) injection. CsA did not affect the phosphorylation of Thr-196-Ca(2+)/calmodulin-dependent protein kinase 4 (CaMK4) and the amount of PP2Cα. An increased phosphorylation of Ser-79-ACC and Ser-872-HMG-CoAR was also observed. In conclusion, our data indicate that CsA activates the AMPK pathway in the rat hippocampus, which suggests that CsA affects the regulatory signaling pathway of lipid metabolism in the rat brain.

AMP-activated protein kinase: a potential player in Alzheimer's disease

AMP-activated protein kinase (AMPK) stimulates energy production via glucose and lipid metabolism, whereas it inhibits energy consuming functions, such as protein and cholesterol synthesis. Increased cytoplasmic AMP and Ca(2+) levels are the major activators of neuronal AMPK signaling. Interestingly, Alzheimer's disease (AD) is associated with several abnormalities in neuronal energy metabolism, for example, decline in glucose uptake, mitochondrial dysfunctions and defects in cholesterol metabolism, and in addition, with problems in maintaining Ca(2+) homeostasis. Epidemiological studies have also revealed that many metabolic and cardiovascular diseases are risk factors for cognitive impairment and sporadic AD. Emerging studies indicate that AMPK signaling can regulate tau protein phosphorylation and amyloidogenesis, the major hallmarks of AD. AMPK is also a potent activator of autophagic degradation which seems to be suppressed in AD. All these observations imply that AMPK is involved in the pathogenesis of AD. However, the responses of AMPK activation are dependent on stimulation and the extent of activating stress. Evidently, AMPK signaling can repress and delay the appearance of AD pathology but later on, with increasing neuronal stress, it can trigger detrimental effects that augment AD pathogenesis. We will outline the potential role of AMPK function in respect to various aspects affecting AD pathogenesis.