AMP-activated protein kinase activates transcription of the UCP3 and HKII genes in rat skeletal muscle

Chronic AMPK inactivation slows SHH medulloblastoma progression by inhibiting mTORC1 signaling and depleting tumor stem cells

We show that inactivating AMPK in a genetic medulloblastoma model depletes tumor stem cells and slows progression. In medulloblastoma, the most common malignant pediatric brain tumor, drug-resistant stem cells co-exist with transit-amplifying cells and terminally differentiated neuronal progeny. Prior studies show that Hk2-dependent glycolysis promotes medulloblastoma progression by suppressing neural differentiation. To determine how the metabolic regulator AMPK affects medulloblastoma growth and differentiation, we inactivated AMPK genetically in medulloblastomas. We bred conditional Prkaa1 and Prkaa2 deletions into medulloblastoma-prone SmoM2 mice and compared SmoM2-driven medulloblastomas with intact or inactivated AMPK. AMPK-inactivation increased event-free survival (EFS) and altered cellular heterogeneity, increasing differentiation and decreasing tumor stem cell populations. Surprisingly, AMPK-inactivation decreased mTORC1 activity and decreased Hk2 expression. Hk2 deletion similarly depleted medulloblastoma stem cells, implicating reduced glycolysis in the AMPK-inactivated phenotype. Our results show that AMPK inactivation disproportionately impairs medulloblastoma stem cell populations typically refractory to conventional therapies.

A blast from the past: To tame time with metformin

The strong evidence of metformin use in subjects affected by type 2 diabetes (T2DM) on health outcomes, together with data from pre-clinical studies, has led the gerontological research to study the therapeutic potential of such a drug as a slow-aging strategy. However, despite clinical use for over fifty years as an anti-diabetic drug, the mechanisms of action beyond glycemic control remain unclear. In this review, we have deeply examined the literature, doing a narrative review from the metformin story, through mechanisms of action to slow down aging potential, from lower organisms to humans. Based on the available evidence, we conclude that metformin, as shown in lower organisms and mice, may be effective in humans' longevity. A complete analysis and follow-up of ongoing clinical trials may provide more definitive answers as to whether metformin should be promoted beyond its use to treat T2DM as a drug that enhances both healthspan and lifespan.

Molecular and biochemical regulation of skeletal muscle metabolism

Skeletal muscle hypertrophy is a culmination of catabolic and anabolic processes that are interwoven into major metabolic pathways, and as such modulation of skeletal muscle metabolism may have implications on animal growth efficiency. Muscle is composed of a heterogeneous population of muscle fibers that can be classified by metabolism (oxidative or glycolytic) and contractile speed (slow or fast). Although slow fibers (type I) rely heavily on oxidative metabolism, presumably to fuel long or continuous bouts of work, fast fibers (type IIa, IIx, and IIb) vary in their metabolic capability and can range from having a high oxidative capacity to a high glycolytic capacity. The plasticity of muscle permits continuous adaptations to changing intrinsic and extrinsic stimuli that can shift the classification of muscle fibers, which has implications on fiber size, nutrient utilization, and protein turnover rate. The purpose of this paper is to summarize the major metabolic pathways in skeletal muscle and the associated regulatory pathways.

Counteracting Health Risks by Modulating Homeostatic Signaling

Homeostasis was initially conceptualized by Bernard and Cannon around a century ago as a steady state of physiological parameters that vary within a certain range, such as blood pH, body temperature, and heart rate^1,2. The underlying mechanisms that maintain homeostasis are explained by negative feedbacks that are executed by the neuronal, endocrine, and immune systems. At the cellular level, homeostasis, such as that of redox and energy steady state, also exists and is regulated by various cell signaling pathways. The induction of homeostatic mechanism is critical for human to adapt to various disruptive insults (stressors); while on the other hand, adaptation occurs at the expense of other physiological processes and thus runs the risk of collateral damages, particularly under conditions of chronic stress. Conceivably, anti-stress protection can be achieved by stressor-mimicking medicinals that elicit adaptive responses prior to an insult and thereby serve as health risk countermeasures; and in situations where maladaptation may occur, downregulating medicinals could be used to suppress the responses and prevent subsequent pathogenesis. Both strategies are preemptive interventions particularly suited for individuals who carry certain lifestyle, environmental, or genetic risk factors. In this article, we will define and characterize a new modality of prophylactic intervention that forestalls diseases via modulating homeostatic signaling. Moreover, we will provide evidence from the literature that support this concept and distinguish it from other homeostasis-related interventions such as adaptogen, hormesis, and xenohormesis.

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.

Advantages of Phosphodiesterase Type 5 Inhibitors in the Management of Glucose Metabolism Disorders: A Clinical and Translational Issue

Among metabolic diseases, carbohydrate metabolism disorders are the most widespread. The most common glucose pathological conditions are acquired and may increase the risk of type 2 diabetes, obesity, heart diseases, stroke, and kidney insufficiency. Phosphodiesterase type 5 inhibitors (PDE5i) have long been used as an effective therapeutic option for the treatment of erectile dysfunction (ED). Different studies have demonstrated that PDE5i, by sensitizing insulin target tissues to insulin, play an important role in controlling the action of insulin and glucose metabolism, highlighting the protective action of these drugs against metabolic diseases. In this review, we report the latest knowledge about the role of PDE5i in the metabolic diseases of insulin resistance and type 2 diabetes, highlighting clinical aspects and potential treatment approaches. Although various encouraging data are available, further in vivo and in vitro studies are required to elucidate the mechanism of action and their clinical application in humans.

Multifaceted roles of TAK1 signaling in cancer

Context-specific signaling is a prevalent theme in cancer biology wherein individual molecules and pathways can have multiple or even opposite effects depending on the tumor type. TAK1 represents a particularly notable example of such signaling diversity in cancer progression. Originally discovered as a TGF-β-activated kinase, over the years it has been shown to respond to numerous other stimuli to phosphorylate a wide range of downstream targets and elicit distinct cellular responses across cell and tissue types. Here we present a comprehensive review of TAK1 signaling and provide important therapeutic perspectives related to its function in different cancers.

Recent Data on Cellular Component Turnover: Focus on Adaptations to Physical Exercise

Significant progress has expanded our knowledge of the signaling pathways coordinating muscle protein turnover during various conditions including exercise. In this manuscript, the multiple mechanisms that govern the turnover of cellular components are reviewed, and their overall roles in adaptations to exercise training are discussed. Recent studies have highlighted the central role of the energy sensor (AMP)-activated protein kinase (AMPK), forkhead box class O subfamily protein (FOXO) transcription factors and the kinase mechanistic (or mammalian) target of rapamycin complex (MTOR) in the regulation of autophagy for organelle maintenance during exercise. A new cellular trafficking involving the lysosome was also revealed for full activation of MTOR and protein synthesis during recovery. Other emerging candidates have been found to be relevant in organelle turnover, especially Parkin and the mitochondrial E3 ubiquitin protein ligase (Mul1) pathways for mitochondrial turnover, and the glycerolipids diacylglycerol (DAG) for protein translation and FOXO regulation. Recent experiments with autophagy and mitophagy flux assessment have also provided important insights concerning mitochondrial turnover during ageing and chronic exercise. However, data in humans are often controversial and further investigations are needed to clarify the involvement of autophagy in exercise performed with additional stresses, such as hypoxia, and to understand the influence of exercise modality. Improving our knowledge of these pathways should help develop therapeutic ways to counteract muscle disorders in pathological conditions.

Age-dependent cardiac function during experimental sepsis: effect of pharmacological activation of AMP-activated protein kinase by AICAR

Age represents a major risk factor for multiple organ failure, including cardiac dysfunction, in patients with sepsis. AMP-activated protein kinase (AMPK) is a crucial regulator of energy homeostasis that controls mitochondrial biogenesis by activation of peroxisome proliferator-activated receptor-γ coactivator-1α and disposal of defective organelles by autophagy. We investigated whether AMPK dysregulation contributes to age-dependent cardiac injury in young (2-3 mo) and mature adult (11-13 mo) male mice subjected to sepsis by cecal ligation and puncture and whether AMPK activation by 5-amino-4-imidazole carboxamide riboside affords cardioprotective effects. Plasma proinflammatory cytokines and myokine follistatin were similarly elevated in vehicle-treated young and mature adult mice at 18 h after sepsis. However, despite equivalent troponin I and T levels compared with similarly treated young mice, vehicle-treated mature adult mice exhibited more severe cardiac damage by light and electron microscopy analyses with more marked intercellular edema, inflammatory cell infiltration, and mitochondrial derangement. Echocardiography revealed that vehicle-treated young mice exhibited left ventricular dysfunction after sepsis, whereas mature adult mice exhibited a reduction in stroke volume without apparent changes in load-dependent indexes of cardiac function. At molecular analysis, phosphorylation of the catalytic subunits AMPK-α₁/α₂ was associated with nuclear translocation of peroxisome proliferator-activated receptor-γ coactivator-1α in vehicle-treated young but not mature adult mice. Treatment with 5-amino-4-imidazole carboxamide riboside ameliorated cardiac architecture derangement in mice of both ages. These cardioprotective effects were associated with attenuation of the systemic inflammatory response and amelioration of cardiac dysfunction in young mice only, not in mature adult animals. NEW & NOTEWORTHY Our data suggest that sepsis-induced cardiac dysfunction manifests with age-dependent characteristics, which are associated with a distinct regulation of AMP-activated protein kinase-dependent metabolic pathways. Consistent with this age-related deterioration, pharmacological activation of AMP-activated protein kinase may afford cardioprotective effects allowing a partial recovery of cardiac function in young but not mature age.

Kahweol inhibits lipid accumulation and induces Glucose-uptake through activation of AMP-activated protein kinase (AMPK)

Weight loss ≥ 5 percent is sufficient to significantly reduce health risks for obese people; therefore, development of novel weight loss compounds with reduced toxicity is urgently required. After screening of natural compounds with anti-adipogenesis properties in 3T3-L1 cells, we determined that kahweol, a coffee-specific diterpene, inhibited adipogenesis. Kahweol reduced lipid accumulation and expression levels of adipogenesis and lipid accumulation-related factors. Levels of phosphorylated AKT and phosphorylated JAK2, that induce lipid accumulation, decreased in kahweol-treated cells. Particularly, kahweol treatment significantly increased AMP-activated protein kinase (AMPK) activation. We revealed that depletion of AMPK alleviated reduction in lipid accumulation from kahweol treatment, suggesting that inhibition of lipid accumulation by kahweol is dependent on AMPK activation. We detected more rapid reduction in blood glucose levels in mice administrated kahweol than in control mice. We suggest that kahweol has anti-obesity effects and should be studied further for possible therapeutic applications.

Prior treatment with the AMPK activator AICAR induces subsequently enhanced glucose uptake in isolated skeletal muscles from 24-month-old rats

5' AMP-activated protein kinase (AMPK) activation may be part of the exercise-induced process that enhances insulin sensitivity. Independent of exercise, acute prior treatment of skeletal muscles isolated from young rats with a pharmacological AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), causes subsequently improved insulin-stimulated glucose uptake (GU). However, efficacy of a single prior AICAR exposure on insulin-stimulated GU in muscles from old animals has not been studied. The purpose of this study was to determine whether brief, prior exposure to AICAR (3.5 h before GU assessment) leads to subsequently increased GU in insulin-stimulated skeletal muscles from old rats. Epitrochlearis muscles from 24-month-old male rats were isolated and initially incubated ±AICAR (60 min), followed by incubation without AICAR (3 h), then incubation ±insulin (50 min). Muscles were assessed for GU (via 3-O-methyl-[³H]-glucose accumulation) and site-specific phosphorylation of key proteins involved in enhanced GU, including AMPK, Akt, and Akt substrate of 160 kDa (AS160), via Western blotting. Prior ex vivo AICAR treatment resulted in greater GU by insulin-stimulated muscles from 24-month-old rats. Prior AICAR treatment also resulted in greater phosphorylation of AMPK (T172) and AS160 (S588, T642, and S704). Glucose transporter type 4 (GLUT4) protein abundance was unaffected by prior AICAR and/or insulin treatment. These findings demonstrate that skeletal muscles from older rats are susceptible to enhanced insulin-stimulated GU after brief activation of AMPK by prior AICAR. Consistent with earlier research using muscles from young rodents, increased phosphorylation of AS160 is implicated in this effect, which was not attributable to altered GLUT4 glucose transporter protein abundance.

Deregulation of Hexokinase II Is Associated with Glycolysis, Autophagy, and the Epithelial-Mesenchymal Transition in Tongue Squamous Cell Carcinoma under Hypoxia

The glycolytic enzyme Hexokinase (HKII) participates in tumor glycolysis and the progression of various cancers, but its clinicopathological effect on the progression of tongue squamous cell carcinoma (TSCC) and its role in glycolysis, autophagy, and the epithelial-mesenchymal transition of TSCC in a hypoxic microenvironment remain unknown. Our results showed that HKII expression was dramatically increased in TSCC tissues and that its upregulation was significantly associated with the presence of pathological differentiation, lymph node metastasis, and clinical stage. The level of autophagy-specific protein LC3, EMT-related proteins, and the migration and invasion capabilities of TSCC cells all increased under hypoxia. Moreover, hypoxia increased the glucose consumption and lactate production of TSCC cells, and we demonstrated that the expression of the glycolytic key gene HKII was significantly higher than in that of the control group. Notably, the downregulation of HKII resulted in a significant decrease of TSCC cell glucose consumption lactate production and autophagic activity during hypoxia. HKII knockdown blocked the migratory and invasive capacity of TSCC cells and we specifically determined that the EMT ability decreased. Therefore, our findings revealed that the upregulation of HKII enhanced glycolysis and increased autophagy and the epithelial-mesenchymal transition of tongue squamous cell carcinoma under hypoxia.

Adaptation to chronic continuous hypoxia potentiates Akt/HK2 anti-apoptotic pathway during brief myocardial ischemia/reperfusion insult

Adaptation to chronic hypoxia represents a potential cardioprotective intervention reducing the extent of acute ischemia/reperfusion (I/R) injury, which is a major cause of death worldwide. The main objective of this study was to investigate the anti-apoptotic Akt/hexokinase 2 (HK2) pathway in hypoxic hearts subjected to I/R insult. Hearts isolated from male Wistar rats exposed either to continuous normobaric hypoxia (CNH; 10% O₂) or to room air for 3 weeks were perfused according to Langendorff and subjected to 10 min of no-flow ischemia and 10 min of reperfusion. The hearts were collected either after ischemia or after reperfusion and used for protein analyses and quantitative fluorescence microscopy. The CNH resulted in increased levels of HK1 and HK2 proteins and the total HK activity after ischemia compared to corresponding normoxic group. Similarly, CNH hearts exhibited increased ischemic level of Akt protein phosphorylated on Ser⁴⁷³. The CNH also strengthened the interaction of HK2 with mitochondria and prevented downregulation of mitochondrial creatine kinase after reperfusion. The Bax/Bcl-2 ratio was significantly lower after I/R in CNH hearts than in normoxic ones, suggesting a lower probability of apoptosis. In conclusion, the Akt/HK2 pathway is likely to play a role in the development of a cardioprotective phenotype of CNH by preventing the detachment of HK2 from mitochondria at reperfusion period and decreases the Bax/Bcl-2 ratio during I/R insult, thereby lowering the probability of apoptosis activation in the mitochondrial compartment.

Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training

Background

The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice.

Methods

Young (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution.

Results

The main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles.

Conclusions

The findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1397-4) contains supplementary material, which is available to authorized users.

Regulation and function of AMPK in physiology and diseases

5′-adenosine monophosphate (AMP)-activated protein kinase (AMPK) is an evolutionarily conserved serine/threonine kinase that was originally identified as the key player in maintaining cellular energy homeostasis. Intensive research over the last decade has identified diverse molecular mechanisms and physiological conditions that regulate the AMPK activity. AMPK regulates diverse metabolic and physiological processes and is dysregulated in major chronic diseases, such as obesity, inflammation, diabetes and cancer. On the basis of its critical roles in physiology and pathology, AMPK is emerging as one of the most promising targets for both the prevention and treatment of these diseases. In this review, we discuss the current understanding of the molecular and physiological regulation of AMPK and its metabolic and physiological functions. In addition, we discuss the mechanisms underlying the versatile roles of AMPK in diabetes and cancer.

Preconditioning mice with activators of AMPK ameliorates ischemic acute kidney injury in vivo

This study had two objectives: 1) to determine whether preconditioning cultured proximal tubular cells (PTCs) with pharmacological activators of AMP-activated protein kinase (AMPK) protects these cells from apoptosis induced by metabolic stress in vitro and 2) to assess the effects of preconditioning mice with these agents on the severity of ischemic acute renal kidney injury (AKI) in vivo. We demonstrate that preconditioning PTCs with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) or A-769662 reduces apoptosis of PTCs induced by subsequent stress. We also show that the reduction in cell death during metabolic stress associated with pretreatment by AMPK activators is associated with an increase in the cytosolic level of ATP, which is mediated by an increase in the rate of glycolysis. In addition, we provide evidence that the effect of AMPK activators on glycolysis is mediated, at least in part, by an increased uptake of glucose, and by the induction of hexokinase II (HK II) expression. Our data also show that the increased in HK II expression associated with preconditioning with AMPK activators is mediated by the activation (phosphorylation) of the cAMP-response element binding protein (CREB). We also provide entirely novel evidence that that A-79662 is substantially more effective than AICAR in mediating these alterations in PTCs in vitro. Finally, we demonstrate that preconditioning mice with AICAR or A-769662 substantially reduces the severity of renal dysfunction and tubular injury in a model of ischemic AKI in vivo and that the efficacy of AICAR and A-768662 in ameliorating ischemic AKI in vivo is comparable.

Cytosolic calcium transients are a determinant of contraction-induced HSP72 transcription in single skeletal muscle fibers

The intrinsic activating factors that induce transcription of heat shock protein 72 (HSP72) in skeletal muscle following exercise remain unclear. We hypothesized that the cytosolic Ca(2+) transient that occurs with depolarization is a determinant. We utilized intact, single skeletal muscle fibers from Xenopus laevis to test the role of the cytosolic Ca(2+) transient and several other exercise-related factors (fatigue, hypoxia, AMP kinase, and cross-bridge cycling) on the activation of HSP72 transcription. HSP72 and HSP60 mRNA levels were assessed with real-time quantitative PCR; cytosolic Ca(2+) concentration ([Ca(2+)]cyt) was assessed with fura-2. Both fatiguing and nonfatiguing contractions resulted in a significant increase in HSP72 mRNA. As expected, peak [Ca(2+)]cyt remained tightly coupled with peak developed tension in contracting fibers. Pretreatment with N-benzyl-p-toluene sulfonamide (BTS) resulted in depressed peak developed tension with stimulation, while peak [Ca(2+)]cyt remained largely unchanged from control values. Despite excitation-contraction uncoupling, BTS-treated fibers displayed a significant increase in HSP72 mRNA. Treatment of fibers with hypoxia (Po2: <3 mmHg) or AMP kinase activation had no effect on HSP72 mRNA levels. These results suggest that the intermittent cytosolic Ca(2+) transient that occurs with skeletal muscle depolarization provides a sufficient activating stimulus for HSP72 transcription. Metabolic or mechanical factors associated with fatigue development and cross-bridge cycling likely play a more limited role.

Ursolic acid increases energy expenditure through enhancing free fatty acid uptake and β-oxidation via an UCP3/AMPK-dependent pathway in skeletal muscle

SCOPE

Ursolic acid (UA) is a triterpenoid compound with multifold biological functions. Our previous studies have reported that UA protects against high-fat diet-induced obesity and improves insulin resistance (IR). However, the potential mechanisms are still undefined. Free fatty acid (FFA) metabolism in skeletal muscle plays a central role in obesity and IR. Therefore, in this study, we investigated the effect and the potential mechanisms of UA on skeletal muscle FFA metabolism.

METHODS AND RESULTS

In diet-induced obese rats, 0.5% UA supplementation for 6 weeks markedly reduced body weight, increased energy expenditure, decreased FFA level in serum and skeletal muscle and triglyceride content in skeletal muscle. In vitro, the data provided directly evidence that UA significantly increased fluorescently labeled FFA uptake and (3) H-labeled palmitic acid β-oxidation. UA-activated AMP-activated protein kinase (AMPK) and downstream targets were involved in the increase of FFA catabolism. Moreover, upregulated uncoupling protein 3 (UCP3) by UA contributed to AMPK activation via elevating adenosine monophosphate/adenosine triphosphate ratio.

CONCLUSION

UA increases FFA burning through enhancing skeletal muscle FFA uptake and β-oxidation via an UCP3/AMPK-dependent pathway, which provides a novel perspective on the biological function of UA against obesity and IR.

A comparison of chronic AICAR treatment-induced metabolic adaptations in red and white muscles of rats

The signaling molecule 5'-AMP-activated protein kinase plays a pivotal role in metabolic adaptations. Treatment with 5-aminoimidazole-4-carboxamide-1-β-D-ribofranoside (AICAR) promotes the expression of metabolic regulators and components involved in glucose uptake, mitochondrial biogenesis, and fatty acid oxidation in skeletal muscle cells. Our aim was to determine whether AICAR-induced changes in metabolic regulators and components were more prominent in white or red muscle. Rats were treated with AICAR (1 mg/g body weight/day) for 14 days, resulting in increased expression levels of nicotinamide phosphoribosyltransferase (NAMPT), peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), glucose transporter 4 proteins, and enhanced mitochondrial biogenesis. These changes were more prominent in white rather than red gastrocnemius muscle or were only observed in the white gastrocnemius. Our results suggest that AICAR induces the expression of metabolic regulators and components, especially in type II (B) fibers.

AMP-activated protein kinase (AMPK)α2 plays a role in determining the cellular fate of glucose in insulin-resistant mouse skeletal muscle

AIMS/HYPOTHESIS

We determined whether: (1) an acute lipid infusion impairs skeletal muscle AMP-activated protein kinase (AMPK)α2 activity, increases inducible nitric oxide synthase (iNOS) and causes peripheral insulin resistance in conscious, unstressed, lean mice; and (2) restoration of AMPKα2 activity during the lipid infusion attenuates the increase in iNOS and reverses the defect in insulin sensitivity in vivo.

METHODS

Chow-fed, 18-week-old C57BL/6J male mice were surgically catheterised. After 5 days they received: (1) a 5 h infusion of 5 ml kg(-1) h(-1) Intralipid + 6 U/h heparin (Lipid treatment) or saline (Control); (2) Lipid treatment or Control, followed by a 2 h hyperinsulinaemic-euglycaemic clamp (insulin clamp; 4 mU kg(-1) min(-1)); and (3) infusion of the AMPK activator, 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) (1 mg kg(-1) min(-1)), or saline during Lipid treatment, followed by a 2 h insulin clamp. In a separate protocol, mice producing a muscle-specific kinase-dead AMPKα2 subunit (α2-KD) underwent an insulin clamp to determine the role of AMPKα2 in insulin-mediated muscle glucose metabolism.

RESULTS

Lipid treatment decreased AMPKα2 activity, increased iNOS abundance/activation and reduced whole-body insulin sensitivity in vivo. AICAR increased AMPKα2 activity twofold; this did not suppress iNOS or improve whole-body or tissue-specific rates of glucose uptake during Lipid treatment. AICAR caused a marked increase in insulin-mediated glycogen synthesis in skeletal muscle. Consistent with this latter result, lean α2-KD mice exhibited impaired insulin-stimulated glycogen synthesis even though muscle glucose uptake was not affected.

CONCLUSIONS/INTERPRETATION

Acute induction of insulin resistance via lipid infusion in healthy mice impairs AMPKα2, increases iNOS and causes insulin resistance in vivo. However, these changes do not appear to be interrelated. Rather, a functionally active AMPKα2 subunit is required for insulin-stimulated muscle glycogen synthesis.

AMPK regulation of fatty acid metabolism and mitochondrial biogenesis: implications for obesity

Skeletal muscle plays an important role in regulating whole-body energy expenditure given it is a major site for glucose and lipid oxidation. Obesity and type 2 diabetes are causally linked through their association with skeletal muscle insulin resistance, while conversely exercise is known to improve whole body glucose homeostasis simultaneously with muscle insulin sensitivity. Exercise activates skeletal muscle AMP-activated protein kinase (AMPK). AMPK plays a role in regulating exercise capacity, skeletal muscle mitochondrial content and contraction-stimulated glucose uptake. Skeletal muscle AMPK is also thought to be important for regulating fatty acid metabolism; however, direct genetic evidence in this area is currently lacking. This review will discuss the current paradigms regarding the influence of AMPK in regulating skeletal muscle fatty acid metabolism and mitochondrial biogenesis at rest and during exercise, and highlight the potential implications in the development of insulin resistance.

Effects of irbesartan on gene expression revealed by transcriptome analysis of left atrial tissue in a porcine model of acute rapid pacing in vivo

BACKGROUND

Atrial fibrillation (AF) is characterized by electrical and structural remodeling of the atria with atrial fibrosis being one hallmark. Angiotensin II (AngII) is a major contributing factor and blockage of its type I receptor (AT1R) prevents remodeling to some extent. Here we explored the effects of the AT1R antagonist irbesartan on global gene expression and profibrotic signaling pathways after induction of rapid atrial pacing (RAP) in vivo in pigs.

METHODS AND RESULTS

Microarray-based RNA profiling was used to screen left atrial (LA) tissue specimens for differences in atrial gene expression in a model of acute RAP. RAP caused an overall expression profile that reflected AngII-induced ROS production, tissue remodeling, and energy depletion. Of special note, the mRNA levels of EDN1, SGK1, and CTGF encoding pro-endothelin, stress- and glucocorticoid activated kinase-1, and of connective tissue growth factor were identified to be significantly increased after 7h of rapid pacing. These specific expression changes were additionally validated by RT-qPCR or immunoblot analyses in LA, RA, and partly in LV samples. All RAP-induced differential gene expression patterns were partially attenuated in the presence of irbesartan. Similar results were obtained after RAP of HL-1 cardiomyocytes in vitro. Furthermore, exogenously added endothelin-1 (ET1) induced CTGF expression concomitant to the transcriptional activation of SGK1 in HL-1 cells.

CONCLUSIONS

RAP provokes substantial changes in atrial and ventricular myocardial gene expression that could be partly reversed by irbesartan. ET1 contributes to AF-dependent atrial fibrosis by synergistic activity with AngII to stimulate SGK1 expression and enhance phosphorylation of the SGK1 protein which, in turn, induces CTGF. The latter has been consistently associated with tissue fibrosis. These findings suggest ETR antagonists as being beneficial in AF treatment.

Iron deficiency causes a shift in AMP-activated protein kinase (AMPK) subunit composition in rat skeletal muscle

Background

As a cellular energy sensor, the 5’AMP-activated protein kinase (AMPK) is activated in response to energy stresses such as hypoxia and muscle contraction. To determine effects of iron deficiency on AMPK activation and signaling, as well as the AMPK subunit composition in skeletal muscle, rats were fed a control (C=50-58 mg/kg Fe) or iron deficient (ID=2-6 mg/kg Fe) diet for 6–8 wks.

Results

Their respective hematocrits were 47.5% ± 1.0 and 16.5% ± 0.6. Iron deficiency resulted in 28.3% greater muscle fatigue (p<0.01) in response to 10 min of stimulation (1 twitch/sec) and was associated with a greater reduction in phosphocreatine (C: Resting 24.1 ± 0.9 μmol/g, Stim 13.1 ± 1.5 μmol/g; ID: Resting 22.7 ± 1.0 μmol/g, Stim 3.2 ± 0.7 μmol/g; p<0.01) and ATP levels (C: Resting 5.89 ± 0.48 μmol/g, Stim 6.03 ± 0.35 μmol/g; ID: Resting 5.51 ± 0.20 μmol/g, Stim 4.19 ± 0.47 μmol/g; p<0.05). AMPK activation increased with stimulation in muscles of C and ID animals. A reduction in Cytochrome c and other iron-dependent mitochondrial proteins was observed in ID animals (p<0.01). The AMPK catalytic subunit (α) was examined because both isoforms are known to play different roles in responding to energy challenges. In ID animals, AMPKα2 subunit protein content was reduced to 71.6% of C (p<0.05), however this did not result in a significant difference in resting AMPKα2 activity. AMPKα1 protein was unchanged, however an overall increase in AMPKα1 activity was observed (C: 0.91 pmol/mg/min; ID: 1.63 pmol/mg/min; p<0.05). Resting phospho Acetyl CoA Carboxylase (pACC) was unchanged. In addition, we observed significant reductions in the β2 and γ3 subunits of AMPK in response to iron deficiency.

Conclusions

This study indicates that chronic iron deficiency causes a shift in the expression of AMPKα, β, and γ subunit composition. Iron deficiency also causes chronic activation of AMPK as well as an increase in AMPKα1 activity in exercised skeletal muscle.

An expanded role for AMP-activated protein kinase: regulator of myocardial protein degradation

Rudolph Schoenheimer's concept of the "dynamic state of body constituents" has existed since the 1940s, but the idea that heart muscle cells renew themselves from within is relatively new. Many studies have elucidated the interaction of metabolic pathways for energy provision and contraction of the heart, and work in the field has uncovered novel metabolic regulators of enzyme action. However, the impact of myocardial energy metabolism on myocardial protein turnover has received little attention. Here, we review recent findings that identify metabolic signals as regulators of myocardial protein turnover and seek to broaden the role of energy substrate metabolism from a provider of ATP to a regulator of self-renewal of the cardiomyocyte.

The role of AMP-activated protein kinase in the coordination of skeletal muscle turnover and energy homeostasis

The AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that acts as a sensor of cellular energy status switch regulating several systems including glucose and lipid metabolism. Recently, AMPK has been implicated in the control of skeletal muscle mass by decreasing mTORC1 activity and increasing protein degradation through regulation of ubiquitin-proteasome and autophagy pathways. In this review, we give an overview of the central role of AMPK in the control of skeletal muscle plasticity. We detail particularly its implication in the control of the hypertrophic and atrophic signaling pathways. In the light of these cumulative and attractive results, AMPK appears as a key player in regulating muscle homeostasis and the modulation of its activity may constitute a therapeutic potential in treating muscle wasting syndromes in humans.

Dietary supplementation with vitamin E and C attenuates dexamethasone-induced glucose intolerance in rats

Glucocorticoid excess induces marked insulin resistance and glucose intolerance. A recent study has shown that antioxidants prevent dexamethasone (DEX)-induced insulin resistance in cultured adipocytes. The purpose of this investigation was to examine the effects of dietary vitamin E and C (Vit E/C) supplementation on DEX-induced glucose intolerance in rats. We hypothesized that feeding rats a diet supplemented with Vit E/C would improve glucose tolerance and restore insulin signaling in skeletal muscle, adipose, and liver and prevent alterations in AMPK signaling in these tissues. Male Wistar rats received either a control or Vit E/C-supplemented diet (0.5 g/kg diet each of L-ascorbate and DL-all rac-alpha-tocopherol) for 9 days prior to, and during, 5 days of daily DEX treatment (subcutaneous injections 0.8 mg/g body wt). DEX treatment resulted in increases in the glucose and insulin area under the curve (AUC) during an intraperitoneal glucose tolerance test. The glucose, but not insulin, AUC was lowered with Vit E/C supplementation. Improvements in glucose tolerance occurred independent of a restoration of PKB phosphorylation in tissues of rats stimulated with an intraperitoneal injection of insulin but were associated with increases in AMPK signaling in muscle and reductions in AMPK signaling and the expression of fatty acid oxidation enzymes in liver. There were no differences in mitochondrial enzymes in triceps muscles between groups. This study is the first to report that dietary Vit E/C supplementation can partially prevent DEX-induced glucose intolerance in rats.

AMP-activated protein kinase regulates E3 ligases in rodent heart

RATIONALE

The degradation of proteins by the ubiquitin proteasome system (UPS) is required for the maintenance of cellular homeostasis in the heart. An important regulator of metabolic homeostasis is AMP-activated protein kinase (AMPK). AMPK activation inhibits protein synthesis and activates autophagy, but whether AMPK plays a role in regulating protein breakdown through the UPS in the heart is not known.

OBJECTIVE

To determine whether AMPK enhances UPS-mediated protein degradation by directly regulating the ubiquitin ligases Atrogin-1 and muscle RING finger protein 1 (MuRF1) in the heart.

METHODS AND RESULTS

Nutrient deprivation and pharmacological or genetic activation of AMPK increased mRNA expression and protein levels of Atrogin-1 and MuRF1 and consequently enhanced protein degradation in neonatal cardiomyocytes. Inhibition of AMPK abrogated these effects. Using gene reporter and chromatin immunoprecipitation assays, we found that AMPK regulates MuRF1 expression by acting through the myocyte enhancer factor 2 (MEF2). We further validated these findings in vivo using MEF2-LacZ reporter mice. Furthermore, we demonstrated in adult cardiomyocytes that MuRF1 is necessary for AMPK-mediated proteolysis through the UPS in the heart. Consequently, MuRF1 knockout mice were protected from severe cardiac dysfunction during fasting.

CONCLUSIONS

AMPK regulates the transcription of Atrogin-1 and MuRF1 and enhances UPS-mediated protein degradation in heart. Specifically, AMPK regulates MuRF1 through the transcription factor MEF2. The absence of MuRF1 in the heart preserves cardiac function during fasting. The results strengthen the hypothesis that AMPK serves as a modulator of intracellular protein degradation in the heart.

AMP-activated protein kinase and metabolic control

AMP-activated protein kinase AMP-activated protein kinase (AMPK AMPK ), a phylogenetically conserved serine/threonine protein kinase, is a major regulator of cellular and whole-body energy homeostasis that coordinates metabolic pathways in order to balance nutrient supply with energy demand. It is now recognized that pharmacological activation of AMPK improves blood glucose homeostasis, lipid profile, and blood pressure in insulin-resistant rodents. Indeed, AMPK activation mimics the beneficial effects of physical activity or those of calorie restriction calorie restriction by acting on multiple cellular targets. In addition, it is now demonstrated that AMPK is one of the probable (albeit indirect) targets of major antidiabetic drugs drugs including the biguanides (metformin metformin ) and thiazolidinedione thiazolidinedione s, as well as of insulin-sensitizing adipokines (e.g., adiponectin adiponectin ). Taken together, such findings highlight the logic underlying the concept of targeting the AMPK pathway for the treatment of metabolic syndrome and type 2 diabetes.

Regulation of the expression of the avian uncoupling protein 3 by isoproterenol and fatty acids in chick myoblasts: possible involvement of AMPK and PPARalpha?

The avian uncoupling protein 3 (UCP3), mainly expressed in muscle tissue, could be involved in fatty acid (FA) metabolism, limitation of reactive oxygen species production, and/or nonshivering thermogenesis. We recently demonstrated that UCP3 mRNA expression was increased by isoproterenol (Iso), a β-agonist, in chicken Pectoralis major. This upregulation was associated with changes in FA metabolism and variations in the activation of AMP-activated protein kinase (AMPK) and in the expression of the transcription factors peroxisome proliferator-activated receptor (PPAR)α, PPARβ/δ, and PPARγ coactivator-1α (PGC-1α). The aim of the present study was to elucidate the mechanisms involving AMPK and PPARα in UCP3 regulation in primary cultures of chick myoblasts. Avian UCP3 mRNA expression, associated with p38 mitogen-activated protein kinase (p38 MAPK) activation, was increased by Iso and/or FAs. The PKA pathway mediated the effects of Iso on UCP3 expression. FA stimulation also led to AMPK activation. Furthermore, the direct involvement of AMPK on UCP3 regulation was shown by using 5-aminoimidazole-4-carboxyamide ribonucleoside and Compound C. The use of the p38 MAPK inhibitor SB202190, which was associated with AMPK activation, also dramatically enhanced UCP3 mRNA expression. Finally the PPARα agonist WY-14643 strongly increased UCP3 mRNA expression. This study highlights the control of UCP3 expression by the β-adrenergic system and FA in chick myoblasts and demonstrates that its expression is directly regulated by AMPK and by PPARα. Overexpression of avian UCP3 might modulate energy utilization or limit oxidative stress when mitochondrial metabolism of FA is triggered by catecholamines.

Identification of differentially expressed genes in chickens differing in muscle glycogen content and meat quality

Background

The processing ability of poultry meat is highly related to its ultimate pH, the latter being mainly determined by the amount of glycogen in the muscle at death. The genetic determinism of glycogen and related meat quality traits has been established in the chicken but the molecular mechanisms involved in variations in these traits remain to be fully described. In this study, Chicken Genome Arrays (20 K) were used to compare muscle gene expression profiles of chickens from Fat (F) and Lean (L) lines that exhibited high and low muscle glycogen content, respectively, and of individuals exhibiting extremely high (G+) or low (G-) muscle glycogen content originating from the F₂ cross between the Fat and Lean lines. Real-time RT-PCR was subsequently performed to validate the differential expression of genes either selected from the microarray analysis or whose function in regulating glycogen metabolism was well known.

Results

Among the genes found to be expressed in chicken P. major muscle, 197 and 254 transcripts appeared to be differentially expressed on microarrays for the F vs. L and the G+ vs. G- comparisons, respectively. Some involved particularly in lipid and carbohydrate metabolism were selected for further validation studies by real-time RT-PCR. We confirmed that, as in mammals, the down-regulation of CEBPB and RGS2 coincides with a decrease in peripheral adiposity in the chicken, but these genes are also suggested to affect muscle glycogen turnover through their role in the cAMP-dependent signalling pathway. Several other genes were suggested to have roles in the regulation of glycogen storage in chicken muscle. PDK4 may act as a glycogen sensor in muscle, UGDH may compete for glycogen synthesis by using UDP-glucose for glucoronidation, and PRKAB1, PRKAG2, and PHKD may impact on glycogen turnover in muscle, through AMP-activated signalling pathways.

Conclusions

This study is the first stage in the understanding of molecular mechanisms underlying variations in poultry meat quality. Large scale analyses are now required to validate the role of the genes identified and ultimately to find molecular markers that can be used for selection or to optimize rearing practices.

High dietary intake of medium-chain fatty acids during pregnancy in rats prevents later-life obesity in their offspring

We investigated the effects of dietary fatty acids of different chain lengths during pregnancy in the rat on the susceptibility of offspring to later-life obesity and the underlying mechanisms. Pregnant rats were fed three different diets: standard (STD), high medium-chain fatty acids (MCFA); and high long-chain fatty acids (LCFA). The male offspring were assigned to three groups: STD control, MCFA and LCFA according to the maternal diets and suckled by dams fed with STD during pregnancy and lactation. After weaning, the offspring were fed with STD from 3 to 8 weeks of age. At the age of 8 weeks, rats in three groups: high-fat diet (HFD) control, MCFA and LCFA were fed with HFD until 14 weeks of age in an attempt to induce obesity, and rats in the HFD control group were selected randomly from the STD control group. Body weight and body fat content were decreased in the MCFA group accompanied by down-regulated mRNA expression of fatty acid synthase and acetyl-coA carboxylase 1, and increased mRNA and protein expression of adenosine monophosphate (AMP)-activated protein kinase (AMPK), carnitine palmitoyltransferase 1 and uncoupling protein 3 compared with the corresponding controls at 3, 8 and 14 weeks of age. The results suggested that the MCFA diet during pregnancy prevented later-life obesity in the offspring when they were exposed to HFD in later life, which might be related to programming of the expression of genes involved in fatty acid metabolism.

AMP-activated protein kinase regulates the expression of monocarboxylate transporter 4 in skeletal muscle

AIMS

The aim of this study was to determine the effect of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), an AMP-activated protein kinase (AMPK) activator, on monocarboxylate transporter 4 (MCT4) expression in rat skeletal muscle and a prototypic embryonal rhabdomyosarcoma cell line (RD cells).

MAIN METHODS

We examined the alteration in Glucose transporter 4 (GLUT4) and MCT4 mRNA levels by quantitative real-time PCR. Alteration in GLUT4 and MCT4 protein levels was examined by Western blotting.

KEY FINDINGS

In an in vivo study, AICAR increased MCT4 mRNA and protein levels in a fiber-type specific manner. In an in vitro study, AICAR increased MCT4 mRNA and protein levels. Moreover, AICAR-induced MCT4 expression was blocked by Compound C, an AMPK inhibitor.

SIGNIFICANCE

In this study, we found that AMPK activation induced expression of MCT4 in RD cells and rat skeletal muscle in a fiber-type specific manner. These results indicate the possible involvement of an AMPK-mediated pathway associated with MCT4 expression in skeletal muscle.

Functional adiponectin resistance at the level of the skeletal muscle in mild to moderate chronic heart failure

BACKGROUND

Adiponectin is an antiinflammatory, insulin-sensitizing, and antiatherogenic adipocytokine that plays a fundamental role in energy homeostasis. In patients with chronic heart failure (CHF), high circulating adiponectin levels are associated with inverse outcome. Recently, adiponectin expression has been identified in human skeletal muscle fibers. We investigated the expression of adiponectin, the adiponectin receptors, and genes involved in the downstream lipid and glucose metabolism in the skeletal muscle of patients with CHF.

METHODS AND RESULTS

Muscle biopsies (vastus lateralis muscle) were obtained from 13 patients with CHF and 10 healthy subjects. mRNA transcript levels of adiponectin, adiponectin receptors (AdipoR1 and AdipoR2), and downstream adiponectin-related enzymes were quantified by real-time reverse transcriptase polymerase chain reaction. Adiponectin expression in the skeletal muscle of patients with CHF was 5-fold higher than in healthy subjects (P<0.001), whereas AdipoR1 was downregulated (P=0.005). In addition, the expression of the main genes involved in downstream pathway (peroxisome proliferator-activated receptor-alpha [PPAR-alpha] and both AMP-activated protein kinase-alpha1 and -alpha2 subunits) as well as their target genes in lipid (acyl-coenzyme A dehydrogenase C-14 to C-12 straight chain) and glucose metabolism (hexokinase-2) were significantly reduced in CHF. The strong positive correlation found between AdipoR1 and PPAR-alpha/AMP-activated protein kinase gene expression was confirmed in PPAR-alpha null mice, suggesting a cause-and-effect relationship. Immunohistochemical staining confirmed the presence of adiponectin in the skeletal muscle.

CONCLUSIONS

Despite increased adiponectin expression in the skeletal muscle, patients with CHF are characterized by downregulation of AdipoR1 that is most probably linked to deactivation of the PPAR-alpha/AMP-activated protein kinase pathway. These facts suggest functional adiponectin resistance at the level of the skeletal muscle in CHF.

AMPK-mediated regulation of transcription in skeletal muscle

Skeletal muscle phenotype plays a critical role in human performance and health, and skeletal muscle oxidative capacity is a key determinant of exercise tolerance. More recently, defective muscle oxidative metabolism has been implicated in a number of conditions associated with the metabolic syndrome, cardiovascular disease and muscle-wasting disorders. AMPK (AMP-activated protein kinase) is a critical regulator of cellular and organismal energy balance. AMPK has also emerged as a key regulator of skeletal muscle oxidative function, including metabolic enzyme expression, mitochondrial biogenesis and angiogenesis. AMPK mediates these processes primarily through alterations in gene expression. The present review examines the role of AMPK in skeletal muscle transcription and provides an overview of the known transcriptional substrates mediating the effects of AMPK on skeletal muscle phenotype.

AMPK in Health and Disease

The function and survival of all organisms is dependent on the dynamic control of energy metabolism, when energy demand is matched to energy supply. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals that control energy balance through the regulation of multiple biochemical pathways in all eukaryotes. In this review, we begin with the discovery of the AMPK family and discuss the recent structural studies that have revealed the molecular basis for AMP binding to the enzyme's gamma subunit. AMPK's regulation involves autoinhibitory features and phosphorylation of both the catalytic alpha subunit and the beta-targeting subunit. We review the role of AMPK at the cellular level through examination of its many substrates and discuss how it controls cellular energy balance. We look at how AMPK integrates stress responses such as exercise as well as nutrient and hormonal signals to control food intake, energy expenditure, and substrate utilization at the whole body level. Lastly, we review the possible role of AMPK in multiple common diseases and the role of the new age of drugs targeting AMPK signaling.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside increases myocardial glucose uptake during reperfusion and induces late pre-conditioning: potential role of AMP-activated protein kinase

Late pre-conditioning protects against myocardial ischaemic-reperfusion injury. AMP-activated protein kinase (AMPK) is activated by exercise and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). Early pre-conditioning involves AMPK activation and increased myocardial glucose uptake. The aim of the present study was to determine whether AICAR activates myocardial AMPK and induces late pre-conditioning and whether myocardial glucose uptake during reperfusion was modulated. Twenty-four hours after AICAR treatment or exercise, Wistar rats were subjected to ischaemia and reperfusion in a Langendorff model and compared to control rats. AMPK activity increased immediately 2.5-fold in AICAR-treated animals (P < 0.01) and twofold in exercised animals (P < 0.05). AICAR and exercise reduced infarct size by 60% and 50% (both P < 0.01), respectively, and increased myocardial glucose uptake during reperfusion (AICAR; 45%, P < 0.05, exercise; 40%, P < 0.05). In conclusion, AICAR induces late pre-conditioning and increases myocardial glucose uptake during reperfusion in rat hearts. AICAR and exercise activate AMPK, suggesting a role of AMPK in the signalling mechanisms behind late pre-conditioning.

AMP-activated kinase may suppress NADPH oxidase activation in vascular tissues

Activation of AMP-activated kinase (AMPK) suppresses NF-kappaB-mediated transcription in endothelial cells exposed to palmitate or TNF-alpha; it also impedes angiotensin II-driven proliferation in vascular smooth muscle cells. These phenomena become predictable if we postulate that AMPK can inhibit activation of NADPH oxidase. Such an effect would make sense from a homeostatic perspective, and moreover there is direct evidence that AMPK suppresses NADPH oxidase activation in neutrophils. New evidence that sub-pathological levels of peroxynitrite can activate AMPK suggest that this enzyme may act as an "early warning signal" for oxidant stress; inhibiting NADPH oxidase would constitute a rational feedback response to such a signal.

Heme oxygenase-1 induction remodels adipose tissue and improves insulin sensitivity in obesity-induced diabetic rats

Obesity-associated inflammation causes insulin resistance. Obese adipose tissue displays hypertrophied adipocytes and increased expression of the cannabinoid-1 receptor. Cobalt protoporphyrin (CoPP) increases heme oxygenase-1 (HO-1) activity, increasing adiponectin and reducing inflammatory cytokines. We hypothesize that CoPP administration to Zucker diabetic fat (ZDF) rats would improve insulin sensitivity and remodel adipose tissue. Twelve-week-old Zucker lean and ZDF rats were divided into 4 groups: Zucker lean, Zucker lean-CoPP, ZDF, and ZDF-CoPP. Control groups received vehicle and treatment groups received CoPP (2 mg/kg body weight) once weekly for 6 weeks. Serum insulin levels and glucose response to insulin injection were measured. At 18 weeks of age, rats were euthanized, and aorta, kidney, and subcutaneous and visceral adipose tissues were harvested. HO-1 expression was measured by Western blot analysis and HO-1 activity by serum carbon monoxide content. Adipocyte size and cannabinoid-1 expression were measured. Adipose tissue volumes were determined using MRI. CoPP significantly increased HO-1 activity, phosphorylated AKT and phosphorylated AMP kinase, and serum adiponectin in ZDF rats. HO-1 induction improved hyperinsulinemia and insulin sensitivity in ZDF rats. Subcutaneous and visceral adipose tissue volumes were significantly decreased in ZDF rats. Adipocyte size and cannabinoid-1 expression were both significantly reduced in ZDF-CoPP rats in subcutaneous and visceral adipose tissues. This study demonstrates that HO-1 induction improves insulin sensitivity, downregulates the peripheral endocannabinoid system, reduces adipose tissue volume, and causes adipose tissue remodeling in a model of obesity-induced insulin resistance. These findings suggest HO-1 as a potential therapeutic target for obesity and its associated health risks.

Uncoupling proteins in heart failure

The development of heart failure is associated with alterations in the expression of a wide variety of structural, signaling, and metabolic proteins. One such effect is the downregulation of uncoupling proteins in the setting of heart failure. This group of proteins regulates the mitochondrial membrane potential and therefore plays a role in mitochondrial energy metabolism as well as reactive oxygen species generation by the mitochondria. This review discusses the role of uncoupling proteins in regulating oxidant stress and implications with respect to the pathogenesis of heart failure and potential treatments.

Relationship between AMPK and the transcriptional balance of clock-related genes in skeletal muscle

Circadian clocks coordinate physiological, behavioral, and biochemical events with predictable daily environmental changes by a self-sustained transcriptional feedback loop. CLOCK and ARNTL are transcriptional activators that regulate Per and Cry gene expression. PER and CRY inhibit their own transcription, and their turnover allows this cycle to restart. The transcription factors BHLHB2 and BHLHB3 repress Per activation, whereas orphan nuclear receptors of the NR1D and ROR families control Arntl expression. Here we show the AMP-activated protein kinase (AMPK)gamma(3) subunit is involved in the regulation of peripheral circadian clock function. AMPKgamma3 knockout (Prkag3(-/-)) mice or wild-type littermates were injected with saline or an AMPK activator, 5-amino-4-imidazole-carboxamide riboside (AICAR), and white glycolytic gastrocnemius muscle was removed for gene expression analysis. Genes involved in the regulation of circadian rhythms (Cry2, Nr1d1, and Bhlhb2) were differentially regulated in response to AICAR in wild-type mice but remained unaltered in Prkag3(-/-) mice. Basal expression of Per1 was higher in Prkag3(-/-) mice compared with wild-type mice. Distinct diurnal changes in the respiratory exchange ratio (RER) between the light and dark phase of the day were observed in wild-type mice but not Prkag3(-/-) mice. In summary, the expression profile of clock-related genes in skeletal muscle in response to AICAR, as well as the diurnal shift in energy utilization, is impaired in AMPKgamma(3) subunit knockout mice. Our results indicate AMPK heterotrimeric complexes containing the AMPKgamma(3) subunit may play a specific role in linking circadian oscillators and energy metabolism in skeletal muscle.

Skeletal muscle hexokinase: regulation in mammalian hibernation

Skeletal muscle hexokinase (HK) from Richardson's ground squirrels was analyzed to determine how the enzyme is regulated during hibernation, a state of cold torpor. The HK II isozyme dominated in muscle and ~15% of total HK was bound to the insoluble fraction. HK maximum activity was 33% lower in hibernator muscle and the enzyme showed a significantly higher K ( m ) ATP (by 80%) and a lower K ( i ) for glucose-6-P (by 40%) than euthermic HK (assayed at 22 degrees C). However, 5 degrees C assay significantly reduced K ( m ) glucose of hibernator HK. Stimulation of AMP-dependent protein kinase (AMPK) in hibernator extracts elevated the HK activity and reduced K ( m ) ATP, but did not affect euthermic HK. Stimulation of protein phosphatases significantly lowered the HK activity in both situations. AMPK-dependent phosphorylation was confirmed by immunopreciptiation of (32)P-labeled HK. DEAE-Sephadex ion exchange chromatography revealed two peaks of HK in hibernator muscle extracts (low and high phosphate forms), whereas only a single peak of phospho-HK was present in euthermic muscle. We conclude that differential control of muscle HK in euthermic versus hibernating states is derived from two main regulatory influences, reversible protein phosphorylation and temperature effects on kinetic properties.

Muscle unloading-induced metabolic remodeling is associated with acute alterations in PPARδ and UCP-3 expression

A number of physiological changes follow prolonged skeletal muscle unloading as occurs in spaceflight, bed rest, and hindlimb suspension (HLS) and also in aging. These include muscle atrophy, fiber type switching, and loss of the ability to switch between lipid and glucose usage, or metabolic inflexibility. The signaling and genomic events that precede these physiological manifestations have not been investigated in detail, particularly in regard to loss of metabolic flexibility. Here we used gene arrays to determine the effects of 24-h HLS on metabolic remodeling in mouse muscle. Acute unloading resulted in differential expression of a number of transcripts in soleus and gastrocnemius muscle, including many involved in lipid and glucose metabolism. These include the peroxisome proliferator-activated receptors (PPARs). In contrast to Ppar-α and Ppar-γ, which were downregulated by acute HLS, Ppar-δ was upregulated concomitant with increased expression of its downstream target, uncoupling protein-3 ( Ucp-3). However, differential expression of Ppar-δ was both acute and transient in nature, suggesting that regulation of PPARδ may represent an adaptive, compensatory response aimed at regulating fuel utilization and maintaining metabolic flexibility.

AMP-activated protein kinase phosphorylates transcription factors of the CREB family

AMP-activated protein kinase (AMPK) has been identified as a regulator of gene transcription, increasing mitochondrial proteins of oxidative metabolism as well as hexokinase expression in skeletal muscle. In mice, muscle-specific knockout of LKB1, a component of the upstream kinase of AMPK, prevents contraction- and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR)-induced activation of AMPK in skeletal muscle, and the increase in hexokinase II protein that is normally observed with chronic AICAR activation of AMPK. Since previous reports show a cAMP response element in the promoter region of the hexokinase II gene, we hypothesized that the cAMP-response element (CRE) binding protein (CREB) family of transcription factors could be targets of AMPK. Using radioisotopic kinase assays, we found that recombinant and rat liver and muscle AMPK phosphorylated CREB1 at the same site as cAMP-dependent protein kinase (PKA). AMPK was also found to phosphorylate activating transcription factor 1 (ATF1), CRE modulator (CREM), and CREB-like 2 (CREBL2), but not ATF2. Treatment of HEK-293 cells stably transfected with a CREB-driven luciferase reporter with AICAR increased luciferase activity approximately threefold over a 24-h time course. This increase was blocked with compound C, an AMPK inhibitor. In addition, AICAR-induced activation of AMPK in incubated rat epitrochlearis muscles resulted in an increase in both phospho-acetyl-CoA carboxylase and phospho-CREB. We conclude that CREB and related proteins are direct downstream targets for AMPK and are therefore likely involved in mediating some effects of AMPK on expression of genes having a CRE in their promoters.

Cellular energy sensing and signaling by AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is an energy sensing/signaling protein that, when activated, increases ATP production by stimulating glucose uptake and fatty acid oxidation while at the same time inhibiting ATP = consuming processes such as protein synthesis. Chronic activation of AMPK inhibits expression of lipogenic enzymes in the liver and enhances expression of mitochondrial oxidative enzymes in skeletal muscle. Deficiency of muscle LKB1, the upstream kinase of AMPK, results in greater fluctuation in energy charge during muscle contraction and decreased capacity for exercise at higher work rates. Because AMPK enhances both glucose uptake and fatty acid oxidation in skeletal muscle, it has become a target for prevention and treatment of type 2 diabetes and obesity.

"AMPing up" our understanding of the hypothalamic control of energy balance

AMP-activated protein kinase (AMPK) has emerged as a metabolic "fuel gauge," which oscillates between anabolic and catabolic processes that ultimately influence energy balance. A study in this issue of the JCI by Claret et al. now extends the role of AMPK in medial basal hypothalamic neurons (see the related article beginning on page 2325). These findings maintain AMPK signaling as a common cellular mechanism in proopiomelanocortin and neuropeptide Y/agouti-related protein neurons and links hypothalamic AMPK to coordinated energy and glucose homeostasis.

Nitric oxide increases GLUT4 expression and regulates AMPK signaling in skeletal muscle

Nitric oxide (NO) and 5'-AMP-activated protein kinase (AMPK) are involved in glucose transport and mitochondrial biogenesis in skeletal muscle. Here, we examined whether NO regulates the expression of the major glucose transporter in muscle (GLUT4) and whether it influences AMPK-induced upregulation of GLUT4. At low levels, the NO donor S-nitroso-N-penicillamine (SNAP, 1 and 10 microM) significantly increased GLUT4 mRNA ( approximately 3-fold; P < 0.05) in L6 myotubes, and cotreatment with the AMPK inhibitor compound C ablated this effect. The cGMP analog 8-bromo-cGMP (8-Br-cGMP, 2 mM) increased GLUT4 mRNA by approximately 50% (P < 0.05). GLUT4 protein expression was elevated 40% by 2 days treatment with 8-Br-cGMP, whereas 6 days treatment with 10 microM SNAP increased GLUT4 expression by 65%. Cotreatment of cultures with the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one prevented the SNAP-induced increase in GLUT4 protein. SNAP (10 microM) also induced significant phosphorylation of alpha-AMPK and acetyl-CoA carboxylase and translocation of phosphorylated alpha-AMPK to the nucleus. Furthermore, L6 myotubes exposed to 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) for 16 h presented an approximately ninefold increase in GLUT4 mRNA, whereas cotreatment with the non-isoform-specific NOS inhibitor N(G)-nitro-l-arginine methyl ester, prevented approximately 70% of this effect. In vivo, GLUT4 mRNA was increased 1.8-fold in the rat plantaris muscle 12 h after AICAR injection, and this induction was reduced by approximately 50% in animals cotreated with the neuronal and inducible nitric oxide synthases selective inhibitor 1-(2-trifluoromethyl-phenyl)-imidazole. We conclude that, in skeletal muscle, NO increases GLUT4 expression via a cGMP- and AMPK-dependent mechanism. The data are consistent with a role for NO in the regulation of AMPK, possibly via control of cellular activity of AMPK kinases and/or AMPK phosphatases.

AMP-activated protein kinase (AMPK) action in skeletal muscle via direct phosphorylation of PGC-1alpha

Activation of AMP-activated kinase (AMPK) in skeletal muscle increases glucose uptake, fatty acid oxidation, and mitochondrial biogenesis by increasing gene expression in these pathways. However, the transcriptional components that are directly targeted by AMPK are still elusive. The peroxisome-proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) has emerged as a master regulator of mitochondrial biogenesis; furthermore, it has been shown that PGC-1alpha gene expression is induced by exercise and by chemical activation of AMPK in skeletal muscle. Using primary muscle cells and mice deficient in PGC-1alpha, we found that the effects of AMPK on gene expression of glucose transporter 4, mitochondrial genes, and PGC-1alpha itself are almost entirely dependent on the function of PGC-1alpha protein. Furthermore, AMPK phosphorylates PGC-1alpha directly both in vitro and in cells. These direct phosphorylations of the PGC-1alpha protein at threonine-177 and serine-538 are required for the PGC-1alpha-dependent induction of the PGC-1alpha promoter. These data indicate that AMPK phosphorylation of PGC-1alpha initiates many of the important gene regulatory functions of AMPK in skeletal muscle.

AMP-activated protein kinase agonists increase mRNA content of the muscle-specific ubiquitin ligases MAFbx and MuRF1 in C2C12 cells

The hypothesis of the present study was that exposure of differentiated muscle cells to agonists of the AMP-activated protein kinase (AMPK) would increase the mRNA content of the muscle-specific ubiquitin ligases muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1). C(2)C(12) cells were incubated with incremental doses of 5-aminoimidazol-4-carboximide ribonucleoside (AICAR) or metformin for 24 h. Both MAFbx and MuRF1 mRNA increased dose dependently in response to these AMPK activators. AICAR, metformin, and 2-deoxy-d-glucose produced time-dependent alterations in ubiquitin ligase expression, typified by a biphasic pattern of expression marked by an acute repression followed by a sustained induction. AMPK-activating treatments in conjunction with dexamethasone produced a pronounced synergistic effect on ligase mRNA expression at later time points. This cooperative response occurred in the absence of a dexamethasone-dependent increase in AMPK expression or activity, as determined by immunoblotting for phosphorylation and expression of AMPKalpha and its downstream target acetyl-CoA carboxylase (ACC). These responses elicited by AMPK activation singly or in combination with dexamethasone did not extend to the mRNA expression of the UBR box family E3s UBR1/E3alphaI and UBR2/E3alphaII. Treatment with the AMPK inhibitor compound C prevented increases in MAFbx and MuRF1 mRNA in response to serum deprivation, as well as AICAR and dexamethasone treatment individually or jointly. Stimulation of AMPK activity in vivo via AICAR injection increased both MAFbx and MuRF1 mRNA in murine skeletal muscle. These data suggest that activation of AMPK in skeletal muscle results in a specific upregulation of MAFbx and MuRF1, responses that are reminiscent of the proposed atrophic transcriptional program executed under various conditions of skeletal muscle wasting. Therefore, AMPK may be a critical component of the intercalated network of signaling pathways governing skeletal muscle atrophy, where its input acts to modify anti- and proatrophic signals to influence gene expression in reaction to catabolic perturbations.