Activation of AMP-activated protein kinase (AMPK) inhibits protein synthesis: a potential strategy to prevent the development of cardiac hypertrophy

AMPK Promotes Larval Metamorphosis of Mytilus coruscus

Metamorphosis is a critical process in the transition from planktonic life to benthic life for marine invertebrates, which is accompanied by a large amount of energy consumption. Previous studies have proved that AMP-activated protein kinase (AMPK), as a vital energy regulator, plays a prominent role in mediating the growth and development of terrestrial animals. However, its function in the growth and development of marine invertebrates, especially in metamorphosis, remains elusive. This study explored the function of AMPK in the larval metamorphosis of Mytilus coruscus. The full-length cDNA of AMPK genes in M. coruscus was cloned and characterized, which is composed of three subunits, McAMPKα, McAMPKβ, and McAMPKγ. Pharmacological tests demonstrated that through the application of an AMPK activator, AMP substantially enhanced the larval metamorphosis rate (p < 0.05). By contrast, the larval metamorphosis rate decreased significantly after being treated with the AMPK inhibitor Compound C (p < 0.05). McAMPK gene knock-down resulted in a reduction in McAMPK gene expression (p < 0.05), and the larval metamorphosis of M. coruscus was significantly restrained (p < 0.05). These results indicated that AMPK signaling is vital in the larval metamorphosis of M. coruscus, which advances further understanding in exploring the molecular mechanisms in the metamorphosis of marine invertebrate larvae.

Left Ventricular Hypertrophy in Diabetic Cardiomyopathy: A Target for Intervention

Heart failure is an important manifestation of diabetic heart disease. Before the development of symptomatic heart failure, as much as 50% of patients with type 2 diabetes mellitus (T2DM) develop asymptomatic left ventricular dysfunction including left ventricular hypertrophy (LVH). Left ventricular hypertrophy (LVH) is highly prevalent in patients with T2DM and is a strong predictor of adverse cardiovascular outcomes including heart failure. Importantly regression of LVH with antihypertensive treatment especially renin angiotensin system blockers reduces cardiovascular morbidity and mortality. However, this approach is only partially effective since LVH persists in 20% of patients with hypertension who attain target blood pressure, implicating the role of other potential mechanisms in the development of LVH. Moreover, the pathophysiology of LVH in T2DM remains unclear and is not fully explained by the hyperglycemia-associated cellular alterations. There is a growing body of evidence that supports the role of inflammation, oxidative stress, AMP-activated kinase (AMPK) and insulin resistance in mediating the development of LVH. The recognition of asymptomatic LVH may offer an opportune target for intervention with cardio-protective therapy in these at-risk patients. In this article, we provide a review of some of the key clinical studies that evaluated the effects of allopurinol, SGLT2 inhibitor and metformin in regressing LVH in patients with and without T2DM.

Cinacalcet as a surrogate therapy for diabetic cardiomyopathy in rats through AMPK-mediated promotion of mitochondrial and autophagic function

Decreased activity of AMP-activated protein kinase (AMPK) is implicated in the pathogenesis of diabetic cardiomyopathy (DCM). Recent evidence suggests a crosstalk between cinacalcet and AMPK activation. This study investigated the effects of cinacalcet on cardiac remodeling and dysfunction in type 2 diabetic rats (T2DM). High fat diet for 4 weeks combined with single intraperitoneal injection of streptozotocin (30 mg/kg) was used to induce type 2 diabetes in rats. Diabetic rats were either orally treated with vehicle, 5 or 10 mg/kg cinacalcet for 4 weeks. Control rats were fed standard chow diet and intraperitoneally injected with citrate buffer. T2DM rats showed lower body weight (BW), hyperglycemia and dyslipidemia, along with increased heart weight (HW) and HW/BW ratio. Masson's trichrome stained cardiac sections revealed massive fibrosis in T2DM rats. There were increased TGF-β1 and hydroxyproline levels, coupled with up-regulation of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in hearts of T2DM rats. These alterations were associated with redox imbalance and impaired cardiac functions. Decreased phosphorylation of AMPK at threonine172 residue was found in T2DM hearts. Cinacalcet for 4 weeks significantly activated AMPK and alleviated cardiac remodeling and dysfunction in a dose-dependent manner, without affecting blood glucose, serum calcium and phosphorus levels. Cinacalcet increased the mitochondrial DNA content, and expressions of PGC-1α, UCP-3, beclin-1 and LC3-II/LC3-I ratio. Cinacalcet decreased the pro-apoptotic Bax, while increased the anti-apoptotic Bcl-2 in cardiac tissue of T2DM rats. These findings might highlight cinacalcet as an alternative therapy to combat the development and progression of DCM.

RNA-sequencing reveals the metabolism regulation mechanism of sheep skeletal muscle under nutrition deprivation stress

Although the skeletal muscle is one of the main sites of metabolism, little is known about the molecular mechanisms involving its response to nutrition stress. The aim of the study was to screen the transcriptome of sheep muscle to identify the metabolism-related genes under nutrition deprivation stress. Ten healthy adult female Small-tailed Han sheep with similar age and weight were randomly divided into a normal group and fasted group. After 3 days, three sheep were randomly selected from each group and the semitendinosus samples were subjected to RNA-sequencing (RNA-seq) and a series of analyses and function annotations. Compared with the normal group, 391 differentially expressed genes (DEGs) were identified in the fasted group that had obvious weight loss, including 278 down-regulated and 113 up-regulated genes. Gene Ontology enrichment annotation classified 228 DEGs in the metabolic process, 11 of which were new genes and only Sheep_newGene_4578 had been annotated by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway. The results of Clusters of Orthologous Groups annotation indicated that 11, 9, and 4 DEGs were respectively classified in lipid transport and metabolism, amino acid transport and metabolism, and carbohydrate transport and metabolism. In addition, KEGG enrichment analysis showed that there were not only pathways which were directly related to metabolisms such as protein digestion and absorption pathway, fatty acid metabolism pathway, and biosynthesis pathway of unsaturated fatty acids, but also PI3K-AKT pathway, AMPK pathway, MAPK pathway, and FoxO pathway which were important to metabolism among the top 20 pathways with the lowest significant Q value. The MCODE analysis of protein-protein interaction revealed that two identified subnetworks with top score were closely associated with metabolism. The correlation analysis showed that the mRNA levels of most of DEGs that might be related in the two subnetworks were significantly correlated respectively, and the mRNA levels of most of 10 metabolism-related DEGs including Sheep_newGene_4578 were significantly correlated. Finally, 16 random and 10 metabolism-related DEGs were chosen for confirmation by quantitative real-time PCR, demonstrating the same expression change as determined by RNA-seq. In conclusion, multiple interrelated metabolism-related DEGs in skeletal muscle contributed to the response of sheep to nutritional deprivation stress.

Radix Puerarin Extract (Puerarin) Could Improve Meat Quality of Heat-Stressed Beef Cattle Through Changing Muscle Antioxidant Ability and Fiber Characteristics

The experiment was conducted to investigate the effects of dietary supplementation with Puerarin on meat quality, muscle antioxidant ability, and muscle fiber characteristics of beef cattle under a hot environment in summer. Thirty-two 15 ± 1.5-month-old Jinjiang bulls (291.65 ± 8.84 kg) were randomly divided into four groups with dietary Puerarin at 0 (control), 200 (Pue200), 400 (Pue400), and 800 (Pue800) mg/kg in the feed concentrate (n = 8). The feeding trial lasted for 60 days after a 10-day adaptation period (July 1-September 8); the average values of temperature, relative humidity and temperature, and humidity index were 30.68°C, 68.05%, and 81.81, respectively. The growth performance on day 40 of the experiment period was calculated. After 60 days' experimental period, four Jinjiang cattle per treatment from the control group, Pue400 group, and Pue800 group were slaughtered. Compared with the control group, the Pue400 and Pue800 groups improved the growth performance of beef cattle; the Pue800 group elevated the activities of superoxide dismutase, total antioxidant capacity, and glutathione peroxidase in the M. longissimus thoracis (LT) muscle. In the control group, the cell membrane was incomplete, and most of the mitochondria were elongated and in a fission state, while in the Pue400 and Pue800 groups, the cell membrane was clear and complete, and the mitochondria presented with round and oval shapes. Compared with the control group, the Pue400 and Pue800 groups reduced the shear force of the LT muscle, and the Pue400 group decreased the muscle fiber diameter and the myosin heavy-chain (MyHC)-IIb gene expression. Furthermore, the Pue400 and Pue800 groups decreased the ratio of AMP/ATP, the Pue800 group reduced the AMP-activated protein kinase α2 mRNA expression, and the Pue400 group improved the nuclear respiratory factor 1 mRNA expression. These results indicated that dietary supplementation with Puerarin might be beneficial to the meat quality of heat-stressed beef cattle by improving muscle antioxidant ability and reducing the MyHC-IIb muscle fiber composition. Based on the results of this study, we recommended 400 mg/kg Puerarin in the feed concentrate of beef cattle (~300 kg) for mitigation of heat stress.

The endocrinological component and signaling pathways associated to cardiac hypertrophy

Although myocardial growth corresponds to an adaptive response to maintain cardiac contractile function, the cardiac hypertrophy is a condition that occurs in many cardiovascular diseases and typically precedes the onset of heart failure. Different endocrine factors such as thyroid hormones, insulin, insulin-like growth factor 1 (IGF-1), angiotensin II (Ang II), endothelin (ET-1), catecholamines, estrogen, among others represent important stimuli to cardiomyocyte hypertrophy. Thus, numerous endocrine disorders manifested as changes in the local environment or multiple organ systems are especially important in the context of progression from cardiac hypertrophy to heart failure. Based on that information, this review summarizes experimental findings regarding the influence of such hormones upon signalling pathways associated with cardiac hypertrophy. Understanding mechanisms through which hormones differentially regulate cardiac hypertrophy could open ways to obtain therapeutic approaches that contribute to prevent or delay the onset of heart failure related to endocrine diseases.

l-Theanine regulates glucose, lipid, and protein metabolism via insulin and AMP-activated protein kinase signaling pathways

l-Theanine is an important component found in tea and has positive effects on nutrient absorption and transport. However, whether l-theanine can regulate glucose, lipid, and protein metabolism remains unknown. This study aims to investigate the effects of l-theanine on glucose, lipid, and protein metabolism in male Sprague-Dawley rats and characterize the underlying mechanisms. Compared to the control group, l-theanine increased the contents of hepatic and muscle glycogen, serum total protein (TP), and albumin (Alb), lowered the serum low-density lipoprotein cholesterol (LDL-C) level, decreased the activity of acetyl-CoA carboxylase (ACC), and enhanced carnitine palmitoyl transferase-1 (CPT-1) activity in the liver. Additionally, l-theanine upregulated the mRNA expression of phosphofructokinase (PFKL), CPT-1, insulin receptor (INSR), insulin receptor substrate (IRS), and liver kinase B1 (LKB1) and downregulated the mRNA expression of phosphoenolpyruvate carboxykinase 1 (PCK1), glucose-6-phosphatase catalytic subunit (G6PC), fatty acid synthase (FAS), and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR). Moreover, l-theanine upregulated the expression of PFKL, glycogen synthase 2 (GYS2), ribosomal protein S6 (S6), INSR, IRS, and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) proteins; downregulated the expression of FAS, sterol regulatory element binding protein-1c (SREBP-1c), and HMGCR proteins; enhanced the phosphorylation of the mammal target of rapamycin (mTOR), ribosomal protein S6 kinase (p70S6K), protein kinase B (AKT), and AMP-activated protein kinase (AMPK); and decreased the phosphorylation of glycogen synthase kinase 3β (GSK-3β) and ACC1. Furthermore, 100 mg kg-1l-theanine was more effective at eliciting these effects than 200 and 400 mg kg-1l-theanine. In conclusion, l-theanine can regulate glucose, lipid, and protein metabolism via insulin and AMPK and their downstream signaling pathways.

Myocardial Energy Metabolism in Non-ischemic Cardiomyopathy

As the most metabolically demanding organ in the body, the heart must generate massive amounts of energy adenosine triphosphate (ATP) from the oxidation of fatty acids, carbohydrates and other fuels (e.g., amino acids, ketone bodies), in order to sustain constant contractile function. While the healthy mature heart acts omnivorously and is highly flexible in its ability to utilize the numerous fuel sources delivered to it through its coronary circulation, the heart’s ability to produce ATP from these fuel sources becomes perturbed in numerous cardiovascular disorders. This includes ischemic heart disease and myocardial infarction, as well as in various cardiomyopathies that often precede the development of overt heart failure. We herein will provide an overview of myocardial energy metabolism in the healthy heart, while describing the numerous perturbations that take place in various non-ischemic cardiomyopathies such as hypertrophic cardiomyopathy, diabetic cardiomyopathy, arrhythmogenic cardiomyopathy, and the cardiomyopathy associated with the rare genetic disease, Barth Syndrome. Based on preclinical evidence where optimizing myocardial energy metabolism has been shown to attenuate cardiac dysfunction, we will discuss the feasibility of myocardial energetics optimization as an approach to treat the cardiac pathology associated with these various non-ischemic cardiomyopathies.

Retinoid X receptor agonists attenuates cardiomyopathy in streptozotocin-induced type 1 diabetes through LKB1-dependent anti-fibrosis effects

Diabetic cardiac fibrosis increases ventricular stiffness and facilitates the occurrence of diastolic dysfunction. Retinoid X receptor (RXR) plays an important role in cardiac development and has been implicated in cardiovascular diseases. In the present study, we investigated the effects of RXR agonist treatment on streptozotocin (STZ)-induced diabetic cardiomyopathy (DCM) and the underlying mechanism. Sprague-Dawley (SD) rats induced by STZ injection were treated with either RXR agonist bexarotene (Bex) or vehicle alone. Echocardiography was performed to determine cardiac structure and function. Cardiac fibroblasts (CFs) were treated with high glucose (HG) with or without the indicated concentration of Bex or the RXR ligand 9-cis-retinoic acid (9-cis-RA). The protein abundance levels were measured along with collagen, body weight (BW), blood biochemical indexes and transforming growth factor-β (TGF-β) levels. The effects of RXRα down-regulation by RXRα small interfering RNA (siRNA) were examined. The results showed that bexarotene treatment resulted in amelioration of left ventricular dysfunction by inhibiting cardiomyocyte apoptosis and myocardial fibrosis. Immunoblot with heart tissue homogenates from diabetic rats revealed that bexarotene activated liver kinase B1 (LKB1) signaling and inhibited p70 ribosomal protein S6 kinase (p70S6K). The increased collagen levels in the heart tissues of DCM rats were reduced by bexarotene treatment. Treatment of CFs with HG resulted in significantly reduced LKB1 activity and increased p70S6K activity. RXRα mediated the antagonism of 9-cis-RA on HG-induced LKB1/p70S6K activation changes in vitro. Our findings suggest that RXR agonist ameliorates STZ-induced DCM by inhibiting myocardial fibrosis via modulation of the LKB1/p70S6K signaling pathway. RXR agonists may serve as novel therapeutic agents for the treatment of DCM.

Empagliflozin Ameliorates Obesity-Related Cardiac Dysfunction by Regulating Sestrin2-Mediated AMPK-mTOR Signaling and Redox Homeostasis in High-Fat Diet-Induced Obese Mice

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) have favorable cardiovascular outcomes in patients with diabetes. However, whether SGLT2i can improve obesity-related cardiac dysfunction is unknown. Sestrin2 is a novel stress-inducible protein that regulates AMPK-mammalian target of rapamycin (mTOR) and suppresses oxidative damage. The aim of this study was to determine whether empagliflozin (EMPA) improves obesity-related cardiac dysfunction via regulating Sestrin2-mediated pathways in diet-induced obesity. C57BL/6J mice and Sestrin2 knockout mice were fed a high-fat diet (HFD) for 12 weeks and then treated with or without EMPA (10 mg/kg) for 8 weeks. Treating HFD-fed C57BL/6J mice with EMPA reduced body weight and whole-body fat and improved metabolic disorders. Furthermore, EMPA improved myocardial hypertrophy/fibrosis and cardiac function and reduced cardiac fat accumulation and mitochondrial injury. Additionally, EMPA significantly augmented Sestrin2 levels and increased AMPK and endothelial nitric oxide synthase phosphorylation, but inhibited Akt and mTOR phosphorylation. These beneficial effects were partially attenuated in HFD-fed Sestrin2 knockout mice. Intriguingly, EMPA treatment enhanced the Nrf2/HO-1-mediated oxidative stress response, suggesting antioxidant and anti-inflammatory activity. Thus, EMPA improved obesity-related cardiac dysfunction via regulating Sestrin2-mediated AMPK-mTOR signaling and maintaining redox homeostasis. These findings provide a novel mechanism for the cardiovascular protection of SGLT2i in obesity.

An α2-adrenoceptor agonist: Dexmedetomidine induces protective cardiomyocyte hypertrophy through mitochondrial-AMPK pathway

Aims: Dexmedetomidine (Dex) as a highly selective α₂-adrenoceptor agonist, was widely used anesthetic in perioperative settings, whether Dex induces cardiac hypertrophy during perioperative administration is unknown. Methods: The effects of Dex on cardiac hypertrophy were explored using the transverse aortic constriction model and neonatal rat cardiomyocytes. Results: We reported that Dex induces cardiomyocyte hypertrophy with activated ERK, AKT, PKC and inactivated AMPK in both wild-type mice and primary cultured rat cardiomyocytes. Additionally, pre-administration of Dex protects against transverse aortic constriction induced-heart failure in mice. We found that Dex up-regulates the activation of ERK, AKT, and PKC via suppression of AMPK activation in rat cardiomyocytes. However, suppression of mitochondrial coupling efficiency and membrane potential by FCCP blocks Dex induced AMPK inactivation as well as ERK, AKT, and PKC activation. All of these effects are blocked by the α₂-adrenoceptor antagonist atipamezole. Conclusion: The present study demonstrates Dex preconditioning induces cardiac hypertrophy that protects against heart failure through mitochondria-AMPK pathway in perioperative settings.

FNDC5/Irisin inhibits pathological cardiac hypertrophy

Cardiac hypertrophy is a common pathophysiological process in various cardiovascular diseases, which still has no effective therapies. Irisin is a novel myokine mainly secreted by skeletal muscle and is believed to be involved in the regulation of energy metabolism. In the present study, we found that irisin expression was elevated in hypertrophic murine hearts and serum. Moreover, angiotension II-induced cardiomyocyte hypertrophy was attenuated after irisin administration and aggravated after irisin knockdown in vitro. Next, we generated transverse aortic constriction (TAC)-induced cardiac hypertrophy murine model and found that cardiac hypertrophy and fibrosis were significantly attenuated with improved cardiac function assessed by echocardiography after irisin treatment. Mechanistically, we demonstrated that FNDC5 was cleaved into irisin, at least partially, in a disintegrin and metalloproteinase (ADAM) family-dependent manner. ADAM10 was the candidate enzyme responsible for the cleavage. Further, we found irisin treatment activated AMPK and subsequently inhibited activation of mTOR. AMPK inhibition ablated the protective role of irisin administration. In conclusion, we find irisin is secreted in an ADAM family-dependent manner, and irisin treatment improves cardiac function and attenuates pressure overload-induced cardiac hypertrophy and fibrosis mainly through regulating AMPK-mTOR signaling.

Dietary wild bitter gourd displays selective androgen receptor modulator like activity and improves the muscle decline of orchidectomized mice

Loss of skeletal muscle mass and strength is often associated with disability and poor quality of life. Selective Androgen Receptor Modulators (SARMs) are under development as potential treatment. This study aims at examining the potential of wild bitter gourd (BG) as a SARM and its effects on the muscle decline induced by orchiectomy. In the cell-based androgen receptor (AR) transactivation assay, the BGP extract showed weak agonistic and antagonistic activities, resembling those of some SARMs. Male C57BL/6J mice were sham-operated (Sham group) or castrated (Cast groups) and fed a modified AIN-93G high sucrose diet supplemented without (Cast group) or with 5% lyophilized BG powder (Cast + BGP) or with testosterone propionate (7 mg TP per kg diet, Cast + TP) for 23 weeks. In contrast to the Cast + TP group, the BGP supplementation did not affect the serum testosterone concentration, and prostate and seminal vesicle mass. Both TP and BGP supplementation increased the weight of androgen responsive muscles, bulbocavernosus (BC) and levator ani (LA) (p < 0.05). The grip strength and the performance on a rotarod of the Cast + BGP group were comparable to those of the Cast + TP group (p > 0.05). The number of succinate dehydrogenase (SDH)-positive fibers of the Cast + BGP group was not significantly different from that of the Sham and Cast + TP groups (p > 0.05). The BGP supplementation up-regulated the Pgc1α, Ucp2 or Ucp3 gene expressions in skeletal muscles of castrated mice (p < 0.05). BGP showed some characteristics of the SARM and might improve skeletal muscle function through the up-regulation of mitochondrial biogenic genes and oxidative capacity, and ameliorated the castration-induced decline of skeletal muscle function in mice.

Satellite cell activation and mTOR signaling pathway response to resistance and combined exercise in elite weight lifters

PURPOSE

Our aim was to compare the effects of a single exercise training mode (resistance exercise) with a combined exercise training (resistance and plyometric exercise) mode on satellite cell activity and anabolic signaling at the molecular level.

METHODS

Eighteen male weight lifters (20 ± 4 years, BMI 27 ± 6 kg/m²) were randomly assigned to either a series of resistance exercise or a series of combined exercise group. The intensity of the exercise was set at 60% of their 1 RM weight and subjects completed three sets each of six repetitions. The combined exercise group performed three different types of resistance exercise alternating with three different types of plyometric exercise, whereas the resistance exercise group performed only the three different types of resistance exercise which was repeated twice. Muscle biopsies were obtained the vastus lateralis muscle immediately before and 3 h after one bout of exercise.

RESULTS

Exercise induced increases in satellite cell activation and myofibrillar protein synthesis following both exercise modes, but the resistance exercise group was superior compared to the combined exercise group in satellite cell activity expressed by Ki67/CD56 (165 vs 232%) and PI3K/Akt protein expression (121 vs 157%), mTOR protein expression (117 vs 288%), p70S6K protein expression (253 vs 809%), and 4E-BP1 protein expression (70 vs 139%) of anabolic signaling pathway.

CONCLUSIONS

These results suggest that the previous findings showing a greater effect of combined as opposed to a single exercise mode could be the effect of a greater training volume rather than a true-training effect of a combined exercise program.

Piperine Attenuates Pathological Cardiac Fibrosis Via PPAR-γ/AKT Pathways

Mitogen-activated protein kinases (MAPKs) and AMP­activated protein kinase α (AMPKα) play critical roles in the process of cardiac hypertrophy. Previous studies have demonstrated that piperine activates AMPKα and reduces the phosphorylation of extracellular signal-regulated kinase (ERK). However, the effect of piperine on cardiac hypertrophy remains completely unknown. Here, we show that piperine-treated mice had similar hypertrophic responses as mice treated with vehicle but exhibited significantly attenuated cardiac fibrosis after pressure overload or isoprenaline (ISO) injection. Piperine inhibited the transformation of cardiac fibroblasts to myofibroblasts induced by transforming growth factor-β (TGF-β) or angiotensin II (Ang II) in vitro. This anti-fibrotic effect was independent of the AMPKα and MAPK pathway. Piperine blocked activation of protein kinase B (AKT) and, downstream, glycogen synthase kinase 3β (GSK3β). The overexpression of constitutively active AKT or the knockdown of GSK3β completely abolished the piperine-mediated protection of cardiac fibroblasts. The cardioprotective effects of piperine were blocked in mice with constitutively active AKT. Pretreatment with GW9662, a specific inhibitor of peroxisome proliferator activated receptor-γ (PPAR-γ), reversed the effect elicited by piperine in vitro. In conclusion, piperine attenuated cardiac fibrosis via the activation of PPAR-γ and the resultant inhibition of AKT/GSK3β.

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Piperine attenuated cardiac fibrosis induced by pressure overload or isoprenaline in mice.

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Piperine inhibited the transformation of cardiac fibroblasts to myofibroblasts via the attenuation of the AKT/GSK3β pathway.

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Piperine acted as an agonist of PPAR-γ in cardiac fibroblasts.

Cardiac fibrosis plays key roles in the processes of cardiac remodeling and heart failure. The lack of therapy against cardiac fibrosis contributes to the high morbidity and mortality caused by HF. We found that piperine attenuates cardiac fibrosis via the activation of PPAR-γ and the resultant inhibition of AKT/GSK3β. Our study provides evidence for the application of piperine in the treatment of cardiac fibrosis.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

(ADP-ribose) polymerase 1 and AMP-activated protein kinase mediate progressive dopaminergic neuronal degeneration in a mouse model of Parkinson's disease

Genetic and epidemiologic evidence suggests that cellular energy homeostasis is critically associated with Parkinson's disease (PD) pathogenesis. Here we demonstrated that genetic deletion of Poly (ADP-ribose) polymerase 1 completely blocked 6-hydroxydopamine-induced dopaminergic neurodegeneration and related PD-like symptoms. Hyperactivation of PARP-1 depleted ATP pools in dopaminergic (DA) neurons, thereby activating AMP-activated protein kinase (AMPK). Further, blockade of AMPK activation by viral infection with dominant-negative AMPK strongly inhibited DA neuronal atrophy with moderate suppression of nuclear translocation of apoptosis-inhibiting factor (AIF), whereas overactivation of AMPK conversely strengthened the 6-OHDA-induced DA neuronal degeneration. Collectively, these results suggest that manipulation of PARP-1 and AMPK signaling is an effective therapeutic approach to prevent PD-related DA neurodegeneration.

AMPK signaling pathway is rapidly activated by T3 and regulates the cardiomyocyte growth

Previous studies have indicated that AMP-activated protein kinase (AMPK) plays a critical role in the control of cardiac hypertrophy mediated by different stimuli such as thyroid hormone (TH). Although the classical effects of TH mediating cardiac hypertrophy occur by transcriptional mechanisms, recent studies have identified other responses to TH, which are more rapid and take place in seconds or minutes evidencing that TH rapidly modulates distinct signaling pathway, which might contribute to the regulation of cardiomyocyte growth. Here, we evaluated the rapid effects of TH on AMPK signaling pathway in cultured cardiomyocytes and determined the involvement of AMPK in T3-induced cardiomyocyte growth. We found for the first time that T3 rapidly activated AMPK signaling pathway. The use of small interfering RNA against AMPK resulted in increased cardiomyocyte hypertrophy while the pharmacological stimulation of AMPK attenuated this process, demonstrating that AMPK contributes to regulation of T3-induced cardiomyocyte growth.

Altered cardiac muscle mTOR regulation during the progression of cancer cachexia in the ApcMin/+ mouse

Cancer cachexia is a muscle wasting condition that occurs in response to a malignant growth in the body. The mechanisms regulating cardiac muscle mass with cachexia are not well understood. Using the Apc^Min/+ mouse model of colorectal cancer, we investigated how cachexia affects the regulation of 5′-adenosine monophosphate-activated protein kinase (AMPK), protein kinase B (Akt) and mammalian target of rapamycin (mTOR) signaling in the heart. Compared to age-matched C57BL/6 (BL6) mice, Apc^Min/+ body mass and heart mass were lower at 12 (11±5 and 8±3%, respectively) and 20 weeks (26±3 and 6±4%, respectively) of age (P<0.05). Diminished heart mass in the 20-week-old Apc^Min/+ mice coincided with a decreased rate of myofibrillar protein synthesis and increased AMPKα phosphorylation. Cachexia decreased mTOR phosphorylation and the phosphorylation of the mTOR substrates, S6 ribosomal protein and 4EBP1 independent of Akt activation. These changes in mTOR-related protein signaling were accompanied by modest increases in the amount of Beclin1 but not protein ubiquitination or cardiomyocyte apoptosis. Taken together, these data suggest that loss of cardiac mass during cachexia progression in the Apc^Min/+ mouse is associated with an Akt-independent suppression of anabolic signaling and evidence of increased autophagy.

Arctigenin alleviates ER stress via activating AMPK

AIM

To investigate the protective effects of arctigenin (ATG), a phenylpropanoid dibenzylbutyrolactone lignan from Arctium lappa L (Compositae), against ER stress in vitro and the underlying mechanisms.

METHODS

A cell-based screening assay for ER stress regulators was established. Cell viability was measured using MTT assay. PCR and Western blotting were used to analyze gene and protein expression. Silencing of the CaMKKβ, LKB1, and AMPKα1 genes was achieved by RNA interference (RNAi). An ATP bioluminescent assay kit was employed to measure the intracellular ATP levels.

RESULTS

ATG (2.5, 5 and 10 μmol/L) inhibited cell death and unfolded protein response (UPR) in a concentration-dependent manner in cells treated with the ER stress inducer brefeldin A (100 nmol/L). ATG (1, 5 and 10 μmol/L) significantly attenuated protein synthesis in cells through inhibiting mTOR-p70S6K signaling and eEF2 activity, which were partially reversed by silencing AMPKα1 with RNAi. ATG (1-50 μmol/L) reduced intracellular ATP level and activated AMPK through inhibiting complex I-mediated respiration. Pretreatment of cells with the AMPK inhibitor compound C (25 μmol/L) rescued the inhibitory effects of ATG on ER stress. Furthermore, ATG (2.5 and 5 μmol/L) efficiently activated AMPK and reduced the ER stress and cell death induced by palmitate (2 mmol/L) in INS-1 β cells.

CONCLUSION

ATG is an effective ER stress alleviator, which protects cells against ER stress through activating AMPK, thus attenuating protein translation and reducing ER load.

Evidence that AMP-activated protein kinase can negatively modulate ornithine decarboxylase activity in cardiac myoblasts

The responses of AMP-activated protein kinase (AMPK) and Ornithine decarboxylase (ODC) to isoproterenol have been examined in H9c2 cardiomyoblasts, AMPK represents the link between cell growth and energy availability whereas ODC, the key enzyme in polyamine biosynthesis, is essential for all growth processes and it is thought to have a role in the development of cardiac hypertrophy. Isoproterenol rapidly induced ODC activity in H9c2 cardiomyoblasts by promoting the synthesis of the enzyme protein and this effect was counteracted by inhibitors of the PI3K/Akt pathway. The increase in enzyme activity became significant between 15 and 30min after the treatment. At the same time, isoproterenol stimulated the phosphorylation of AMPKα catalytic subunits (Thr172), that was associated to an increase in acetyl coenzyme A carboxylase (Ser72) phosphorylation. Downregulation of both α1 and α2 isoforms of the AMPK catalytic subunit by siRNA to knockdown AMPK enzymatic activity, led to superinduction of ODC in isoproterenol-treated cardiomyoblasts. Downregulation of AMPKα increased ODC activity even in cells treated with other adrenergic agonists and in control cells. Analogue results were obtained in SH-SY5Y neuroblastoma cells transfected with a shRNA construct against AMPKα. In conclusion, isoproterenol quickly activates in H9c2 cardiomyoblasts two events that seem to contrast one another. The first one, an increase in ODC activity, is linked to cell growth, whereas the second, AMPK activation, is a homeostatic mechanism that negatively modulates the first. The modulation of ODC activity by AMPK represents a mechanism that may contribute to control cell growth processes.

The CPT1C 5'UTR contains a repressing upstream open reading frame that is regulated by cellular energy availability and AMPK

BACKGROUND

Translational control is utilized as a means of regulating gene expression in many species. In most cases, posttranscriptional regulatory mechanisms play an important role in stress response pathways and can lead to dysfunctional physiology if blocked by mutations. Carnitine Palmitoyltransferase 1 C (CPT1C), the brain-specific member of the CPT 1 family, has previously been shown to be involved in regulating metabolism in situations of energy surplus.

PRINCIPAL FINDINGS

Sequence analysis of the CPT1C mRNA revealed that it contains an upstream open reading frame (uORF) in the 5' UTR of its mRNA. Using CPT1C 5' UTR/luciferase constructs, we investigated the role of the uORF in translational regulation. The results presented here show that translation from the CPT1C main open reading frame (mORF) is repressed by the presence of the uORF, that this repression is relieved in response to specific stress stimuli, namely glucose deprivation and palmitate-BSA treatment, and that AMPK inhibition can relieve this uORF-dependent repression.

SIGNIFICANCE

The fact that the mORF regulation is relieved in response to a specific set of stress stimuli rather than general stress response, hints at an involvement of CPT1C in cellular energy-sensing pathways and provides further evidence for a role of CPT1C in hypothalamic regulation of energy homeostasis.

Exercise training does not correct abnormal cardiac glycogen accumulation in the db/db mouse model of type 2 diabetes

Substrate imbalance is a well-recognized feature of diabetic cardiomyopathy. Insulin resistance effectively limits carbohydrate oxidation, resulting in abnormal cardiac glycogen accumulation. Aims of the present study were to 1) characterize the role of glycogen-associated proteins involved in excessive glycogen accumulation in type 2 diabetic hearts and 2) determine if exercise training can attenuate abnormal cardiac glycogen accumulation. Control (db(+)) and genetically diabetic (db/db) C57BL/KsJ-lepr(db)/lepr(db) mice were subjected to sedentary or treadmill exercise regimens. Exercise training consisted of high-intensity/short-duration (10 days) and low-intensity/long-duration (6 wk) protocols. Glycogen levels were elevated by 35-50% in db/db hearts. Exercise training further increased (2- to 3-fold) glycogen levels in db/db hearts. Analysis of soluble and insoluble glycogen pools revealed no differential accumulation of one glycogen subspecies. Phosphorylation (Ser(640)) of glycogen synthase, an indicator of enzymatic fractional activity, was greater in db/db mice subjected to sedentary and exercise regimens. Elevated glycogen levels were accompanied by decreased phosphorylation (Thr(172)) of 5'-AMP-activated kinase and phosphorylation (Ser(79)) of its downstream substrate acetyl-CoA carboxylase. Glycogen concentration was not associated with increases in other glycogen-associated proteins, including malin and laforin. Novel observations show that exercise training does not correct diabetes-induced elevations in cardiac glycogen but, rather, precipitates further accumulation.

Metformin improves cardiac function in a nondiabetic rat model of post-MI heart failure

Metformin is the first choice drug for the treatment of patients with diabetes, but its use is debated in patients with advanced cardiorenal disease. Epidemiological data suggest that metformin may reduce cardiac events, in patients both with and without heart failure. Experimental evidence suggests that metformin reduces cardiac ischemia-reperfusion injury. It is unknown whether metformin improves cardiac function (remodeling) in a long-term post-MI remodeling model. We therefore studied male, nondiabetic, Sprague-Dawley rats that were subjected to either myocardial infarction (MI) or sham operation. Animals were randomly allocated to treatment with normal water or metformin-containing water (250 mg·kg(-1)·day(-1)). At baseline, 6 wk, and 12 wk, metabolic parameters were analyzed and oral glucose tolerance tests (OGTT) were performed. Echocardiography and hemodynamic parameters were assessed 12 wk after MI. In the MI model, infarct size was significantly smaller after 12-wk metformin treatment (29.6 ± 3.2 vs. 38.0 ± 2.2%, P < 0.05). Moreover, metformin resulted in less left ventricular dilatation (6.0 ± 0.4 vs. 7.6 ± 0.6 mm, P < 0.05) and preservation of left ventricular ejection fraction (65.8 ± 3.7% vs. 48.6 ± 5.6%, P < 0.05) compared with MI control. The improved cardiac function was associated with decreased atrial natriuretic peptide mRNA levels in the metformin-treated group (50% reduction compared with MI, P < 0.05). Insulin resistance did not occur during cardiac remodeling (as indicated by normal OGTT) and fasting glucose levels and the pattern of the OGTT were not affected by metformin. Molecular analyses suggested that altered AMP kinase phosphorylation status and low insulin levels mediate the salutary effects of metformin. Altogether our results indicate that metformin may have potential to attenuate heart failure development after myocardial infarction, in the absence of diabetes and independent of systemic glucose levels.

AMPK activation enhances PPARα activity to inhibit cardiac hypertrophy via ERK1/2 MAPK signaling pathway

Activation of adenosine monophosphate-activated protein kinase (AMPK) has been shown to inhibit cardiac hypertrophy through peroxisome proliferators-activated receptor-α (PPARα) signaling pathway, but the detailed mechanism remains unclear. A rat model of cardiac hypertrophy created by transaortic constriction (TAC) was used to investigate the mechanism involved in regulation of PPARα activity by AMPK. It was observed that treatment with AICAR (5-aminoimidazole 1 carboxamide ribonucleoside), an AMPK activator, significantly inhibited cardiac hypertrophy in vivo and in vitro. Phosphorylated extracellular signal regulated protein kinase (phospho-ERK1/2) and phospho-p38 mitogen-activated protein kinase (MAPK) protein levels were significantly up-regulated, while PPARα protein level was down-regulated in TAC rats. AICAR treatment reversed the changes of PPARα and phospho-ERK1/2, but increased phospho-p38 MAPK protein level in TAC rats. Similar changes of PPARα and phospho-ERK1/2 protein levels were observed in the hypertrophied cardiomyocytes induced by phenylephrine treatment. Epidermal growth factor (EGF, ERK1/2 activator), but not SB203580 (p38 inhibitor) blocked the up-regulation of PPARα protein level induced by AICAR. Luciferase assay showed that AICAR increased PPARα transcriptional activity which was abrogated by EGF, but not by SB203580. These results demonstrate that AMPK activation enhances the activity of PPARα to inhibit cardiac hypertrophy through ERK1/2, but not p38 MAPK, signaling pathway.

Upregulation of SIRT1 deacetylase in phenylephrine-treated cardiomyoblasts

The sirtuin SIRT1 is an ubiquitous NAD(+) dependent deacetylase that plays a role in biological processes such as longevity and stress response. In cardiac models, SIRT1 is associated to protection against many stresses. However, the link between SIRT1 and heart hypertrophy is complex and not fully understood. This study focuses specifically on the response of SIRT1 to the α-adrenergic agonist phenylephrine in H9c2 cardiac myoblasts, a cell model of cardiac hypertrophy. After 24 and 48h of phenylephrine treatment, SIRT1 expression and deacetylase activity were significantly increased. SIRT1 upregulation by phenylephrine was not associated to changes in NAD(+) levels, but was blocked by inhibitors of AMP-activated Protein Kinase (AMPK) or by AMPK knockdown by siRNA. When SIRT1 was inhibited with sirtinol or downregulated by siRNA, H9c2 cell viability was significantly decreased following phenylephrine treatment, showing that SIRT1 improves cell survival under hypertrophic stress. We so then propose that the increase in SIRT1 activity and expression in H9c2 cells treated with phenylephrine is an adaptive response to the hypertrophic stress, suggesting that adrenergic stimulation of heart cells activates hypertrophic programming and at the same time also promotes a self-protecting and self-regulating mechanism.

Isoform-selective 5'-AMP-activated protein kinase-dependent preconditioning mechanisms to prevent postischemic leukocyte-endothelial cell adhesive interactions

We previously demonstrated that preconditioning induced by ethanol consumption at low levels [ethanol preconditioning (EPC)] or with 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR-PC) 24 h before ischemia-reperfusion prevents postischemic leukocyte-endothelial cell adhesive interactions (LEI) by a mechanism that is initiated by nitric oxide formed by endothelial nitric oxide synthase. Recent work indicates that 1) ethanol increases the activity of AMP-activated protein kinase (AMPK) and 2) AMPK phosphorylates endothelial nitric oxide synthase at the same activation site seen following EPC (Ser1177). In light of these observations, we postulated that the heterotrimeric serine/threonine kinase, AMPK, may play a role in triggering the development of the anti-inflammatory phenotype induced by EPC. Ethanol was administered to C57BL/6J mice by gavage in the presence or absence of AMPK inhibition. Twenty-four hours later, the numbers of rolling and adherent leukocytes in postcapillary venules of the small intestine were recorded using an intravital microscopic approach. Following 45 min of ischemia, LEI were recorded after 30 and 60 min of reperfusion or at equivalent time points in control animals. Ischemia-reperfusion induced a marked increase in LEI relative to sham-operated control mice. The increase in LEI was prevented by EPC, an effect that was lost with AMPK inhibition during the period of ethanol exposure. Studies conducted in AMPK α(1)- and α(2)-knockout mice suggest that the anti-inflammatory effects of AICAR are not dependent on which isoform of the catalytic α-subunit is present because a deficiency of either isoform results in a loss of protection. In sharp contrast, EPC appears to be triggered by an AMPK α(2)-isoform-dependent mechanism.

Tumor Suppressor Serine/Threonine Kinase LKB1 Expression, Not Kinase Activity, Increased in the Vascular Smooth Muscle Cells and Neointima in the Rat Carotid Artery Injury Model

BACKGROUND AND OBJECTIVES

Vascular smooth muscle cell (VSMC) proliferation is responsible for the restenosis of previously inserted coronary stents. Angiotensin II (Ang II) is known to regulate VSMC proliferation. LKB1, a serine/threonine kinase, interacts with the p53 pathway and acts as a tumor suppressor.

MATERIALS AND METHODS

We assessed the association of Ang II and the expression of LKB1 in primary cultured murine VSMCs and neointima of the Sprague Dawley rat carotid artery injury model. We created carotid balloon injuries and harvested the injured carotid arteries 14 days after the procedure.

RESULTS

Ang II increased LKB1 expression in a time-dependent manner and peaked at an Ang II concentration of 10(-7) mole/L in VSMCs. In the animal experiment, neointima was markedly increased after balloon injury compared to the control group. Immunohistochemical studies showed that LKB1 expression increased according to neointima thickness. Ang II augmented LKB1 expression after the injury. Western blot analysis of LKB1 with carotid artery lysate revealed the same pattern as LKB1 immunohistochemistry. Increased LKB1 expression started at 5 days after the balloon injury, and peaked at 14 days after the injury. Although LKB1 expression was increased after the injury, LKB1 kinase activity was not increased. Ang II or balloon-injury increased the expression of LKB1 although the LKB1 activity was reduced.

CONCLUSION

Ang II increased LKB1 expression in VSMCs and neointima. These findings were not kinase dependant.

Protein kinase A is a target for aging and the aging heart

PKA is an important mediator of signal transduction downstream of G-protein-coupled receptors and plays a key role in the regulation of metabolism and triglyceride storage. It is a ubiquitous cellular kinase that phosphorylates serine and threonine residues in response to cAMP. PKA consists of two regulatory subunits, RI and RII, that are activated by cAMP to release two catalytic subunits, Calpha and Cbeta. We have shown that C57/BL6J male mice lacking the regulatory RIIbeta subunit have extended lifespan and are resistant to age-related conditions including cardiac decline. In addition to being protected from diet-induced pathologies, PKA Cbeta null mutant mice are protected from age-related problems such as weight gain and enlarged livers, as well as cardiac dysfunction and hypertrophy. Several possible mechanisms for the age sparing effects of PKA inhibition are discussed including A kinase anchoring protein signaling, alterations in the beta-adrenergic pathway, and activation of AMPK. Since PKA is a major metabolic regulator of gene signaling, the human gene homologs are potential pharmacological targets for age-related conditions including heart disease associated with declining cardiac performance.

The muscle fiber type-fiber size paradox: hypertrophy or oxidative metabolism?

An inverse relationship exists between striated muscle fiber size and its oxidative capacity. This relationship implies that muscle fibers, which are triggered to simultaneously increase their mass/strength (hypertrophy) and fatigue resistance (oxidative capacity), increase these properties (strength or fatigue resistance) to a lesser extent compared to fibers increasing either of these alone. Muscle fiber size and oxidative capacity are determined by the balance between myofibrillar protein synthesis, mitochondrial biosynthesis and degradation. New experimental data and an inventory of critical stimuli and state of activation of the signaling pathways involved in regulating contractile and metabolic protein turnover reveal: (1) higher capacity for protein synthesis in high compared to low oxidative fibers; (2) competition between signaling pathways for synthesis of myofibrillar proteins and proteins associated with oxidative metabolism; i.e., increased mitochondrial biogenesis via AMP-activated protein kinase attenuates the rate of protein synthesis; (3) relatively higher expression levels of E3-ligases and proteasome-mediated protein degradation in high oxidative fibers. These observations could explain the fiber type-fiber size paradox that despite the high capacity for protein synthesis in high oxidative fibers, these fibers remain relatively small. However, it remains challenging to understand the mechanisms by which contractile activity, mechanical loading, cellular energy status and cellular oxygen tension affect regulation of fiber size. Therefore, one needs to know the relative contribution of the signaling pathways to protein turnover in high and low oxidative fibers. The outcome and ideas presented are relevant to optimizing treatment and training in the fields of sports, cardiology, oncology, pulmonology and rehabilitation medicine.

A high-fructose diet worsens eccentric left ventricular hypertrophy in experimental volume overload

The development of left ventricular (LV) hypertrophy (LVH) can be affected by diet manipulation. Concentric LVH resulting from pressure overload can be worsened by feeding rats with a high-fructose diet. Eccentric LVH is a different type of hypertrophy and is associated with volume overload (VO) diseases. The impact of an abnormal diet on the development of eccentric LVH and on ventricular function in chronic VO is unknown. This study therefore examined the effects of a fructose-rich diet on LV eccentric hypertrophy, ventricular function, and myocardial metabolic enzymes in rats with chronic VO caused by severe aortic valve regurgitation (AR). Wistar rats were divided in four groups: sham-operated on control diet (SC; n = 13) or fructose-rich diet (SF; n = 13) and severe aortic regurgitation fed with the same diets [aortic regurgitation on control diet (ARC), n = 16, and aortic regurgitation on fructose-rich diet (ARF), n = 13]. Fructose-rich diet was started 1 wk before surgery, and the animals were euthanized 9 wk later. SF and ARF had high circulating triglycerides. ARC and ARF developed significant LV eccentric hypertrophy after 8 wk as expected. However, ARF developed more LVH than ARC. LV ejection fraction was slightly lower in the ARF compared with ARC. The increased LVH and decreased ejection fraction could not be explained by differences in hemodynamic load. SF, ARC, and ARF had lower phosphorylation levels of the AMP kinase compared with SC. A fructose-rich diet worsened LV eccentric hypertrophy and decreased LV function in a model of chronic VO caused by AR in rats. Normal animals fed the same diet did not develop these abnormalities. Hypertriglyceridemia may play a central role in this phenomenon as well as AMP kinase activity.

AMP-activated protein kinase inhibits IGF-I signaling and protein synthesis in vascular smooth muscle cells via stimulation of insulin receptor substrate 1 S794 and tuberous sclerosis 2 S1345 phosphorylation

AMP-activated protein kinase (AMPK) inhibits IGF-I actions, but the mechanism by which AMPK functions is undefined. This study identified signaling events that were induced by AMPK that mediated inhibition of IGF-I-stimulated phosphoinosotide-3-kinase (PI3K) pathway activation. The AMPK activator metformin stimulated AMPK Thr172 phosphorylation and inhibited IGF-I-stimulated phosphorylation of Akt/tuberous sclerosis 2 (TSC2)/mammalian target of rapamycin (mTOR)/p70S6 kinase (p70S6K). Expression of constitutively active forms of AMPK suppressed IGF-I-stimulated activation of Akt/TSC2/mTOR/p70S6K and protein synthesis, whereas AMPK knockdown resulted in enhanced responses to IGF-I. To determine the mechanism by which AMPK inhibited IGF-I signaling, the role of insulin receptor substrate-1 (IRS-1) was examined. Both metformin and constitutively activated AMPK enhanced phosphorylation of IRS-1 Ser794, which led to decreased IRS-1 tyrosine phosphorylation and recruitment of the p85 subunit of PI3K. Overexpression of IRS-1 S794A was associated with increased IGF-I-stimulated IRS-1 tyrosine phosphorylation, p85 association, and protein synthesis. To determine whether other signaling molecules mediated the effect of AMPK, TSC2 function was examined. Cells overexpressing TSC2/S1345A (the site of AMPK phosphorylation) were less responsive to metformin-induced inhibition of p70S6 kinase. These findings are relevant to whole animal physiology because administration of metformin to mice resulted in inhibition of IGF-I-stimulated phosphorylation of Akt/mTOR/p70S6K. In conclusion, AMPK functions to inhibit IGF-I-stimulated PI3K pathway activation through stimulation of IRS-1 serine 794 phosphorylation. Because IGF-I is an important stimulant of the anabolic response, this effect of AMPK could account for part of its inhibitory effect on protein synthesis, thus allowing more efficient energy use by other cellular processes.

Cardiac and metabolic changes in long-term high fructose-fat fed rats with severe obesity and extensive intramyocardial lipid accumulation

Metabolic syndrome and obesity-related diseases are affecting more and more people in the Western world. The basis for an effective treatment of these patients is a better understanding of the underlying pathophysiology. Here, we characterize fructose- and fat-fed rats (FFFRs) as a new animal model of metabolic syndrome. Sprague-Dawley rats were fed a 60 kcal/100 kcal fat diet with 10% fructose in the drinking water. After 6, 12, 18, 24, 36, and 48 wk of feeding, blood pressure, glucose tolerance, plasma insulin, glucose, and lipid levels were measured. Cardiac function was examined by in vivo pressure volume measurements, and intramyocardial lipid accumulation was analyzed by confocal microscopy. Cardiac AMP-activated kinase (AMPK) and hepatic phosphoenolpyruvate carboxykinase (PEPCK) levels were measured by Western blotting. Finally, an ischemia-reperfusion study was performed after 56 wk of feeding. FFFRs developed severe obesity, decreased glucose tolerance, increased serum insulin and triglyceride levels, and an initial increased fasting glucose, which returned to control levels after 24 wk of feeding. The diet had no effect on blood pressure but decreased hepatic PEPCK levels. FFFRs showed significant intramyocardial lipid accumulation, and cardiac hypertrophy became pronounced between 24 and 36 wk of feeding. FFFRs showed no signs of cardiac dysfunction during unstressed conditions, but their hearts were much more vulnerable to ischemia-reperfusion and had a decreased level of phosphorylated AMPK at 6 wk of feeding. This study characterizes a new animal model of the metabolic syndrome that could be beneficial in future studies of metabolic syndrome and cardiac complications.

Differential AMPK phosphorylation sites associated with phenylephrine vs. antihypertrophic effects of adenosine agonists in neonatal rat ventricular myocytes

Stimulation of cardiac AMP-activated protein kinase (AMPK) has been demonstrated in both prohypertrophic and antihypertrophic settings, although the reasons for such discrepant results are not well understood. We determined how AMPK is regulated in response to phenylephrine-induced cardiomyocyte hypertrophy and assessed whether AMPK activity may be a factor underlying the antihypertrophic effect of adenosine receptor agonists. The role of AMPK in hypertrophic responses was determined by assessing the effect of the AMPK activator 5-aminoimidazole-4-carboxyamide ribonucleoside on three hypertrophic indexes, including protein synthesis, cell surface area, and fetal gene expression. The changes in phosphorylation of the catalytic alpha-subunit of AMPK at two different sites, Thr(172) and Ser(485/491), in response to phenylephrine and adenosine receptor agonists were also examined. 5-Aminoimidazole-4-carboxyamide ribonucleoside completely abolished phenylephrine-induced increases in protein synthesis, cell surface area, and fetal gene expression. AMPK phosphorylation time course studies revealed that phenylephrine induced a time-dependent activation at site Ser(485/491), in contrast to adenosine receptor agonists, which demonstrated rapid AMPK phosphorylation at Thr(172). Furthermore, the phosphorylation at Ser(485/491) by phenylephrine was not affected by the addition of adenosine receptor agonists, although, conversely, phosphorylation of AMPK at Thr(172) by adenosine receptor agonists was abrogated by the addition of phenylephrine. We propose from these results that cardiomyocyte hypertrophic and antihypertrophic responses, at least with respect to inhibition of phenylephrine-induced hypertrophy by adenosine receptor agonists, are mediated by multisite AMPK regulation. The latter are reflected by increased phosphorylation at Ser(485/491) and at Thr(172), associated with prohypertrophic and antihypertrophic responses, respectively.

Cardiac-specific deletion of LKB1 leads to hypertrophy and dysfunction

LKB1 encodes a serine/threonine kinase, which functions upstream of the AMP-activated protein kinase (AMPK) superfamily. To clarify the role of LKB1 in heart, we generated and characterized cardiac myocyte-specific LKB1 knock-out (KO) mice using alpha-myosin heavy chain-Cre deletor strain. LKB1-KO mice displayed biatrial enlargement with atrial fibrillation and cardiac dysfunction at 4 weeks of age. Left ventricular hypertrophy was observed in LKB1-KO mice at 12 weeks but not 4 weeks of age. Collagen I and III mRNA expression was elevated in atria at 4 weeks, and atrial fibrosis was seen at 12 weeks. LKB1-KO mice displayed cardiac dysfunction and atrial fibrillation and died within 6 months of age. Indicative of a prohypertrophic environment, the phosphorylation of AMPK and eEF2 was reduced, whereas mammalian target of rapamycin (mTOR) phosphorylation and p70S6 kinase phosphorylation were increased in both the atria and ventricles of LKB1-deficient mice. Consistent with vascular endothelial growth factor mRNA and protein levels being significantly reduced in LKB1-KO mice, these mice also exhibited a reduction in capillary density of both atria and ventricles. In cultured cardiac myocytes, LKB1 silencing induced hypertrophy, which was ameliorated by the expression of a constitutively active form AMPK or by treatment with the inhibitor of mTOR, rapamycin. These findings indicate that LKB1 signaling in cardiac myocytes is essential for normal development of the atria and ventricles. Cardiac hypertrophy and dysfunction in LKB1-deficient hearts are associated with alterations in AMPK and mTOR/p70S6 kinase/eEF2 signaling and with a reduction in vascular endothelial growth factor expression and vessel rarefaction.

AMP-activated protein kinase enhances the expression of muscle-specific ubiquitin ligases despite its activation of IGF-1/Akt signaling in C2C12 myotubes

Two muscle-specific ubiquitin ligases (UL), muscle atrophy F box (MAFbx) and muscle RING finger 1 (MuRF1), are crucial for myofibrillar protein breakdown. The insulin like growth factor-1 (IGF-1) pathway inhibits muscle UL expression through Akt-mediated inhibition of FoxO transcription factors, while AMP-activated protein kinase (AMPK) promotes UL expression. The underlying cellular mechanism, however, remains obscure. In this study, the effect of AMPK and its interaction with IGF-1 on ubiquitin ligases expression was investigated. C2C12 myotubes were treated with 0, 0.1, 0.3, and 1.0 mM 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) in the presence or absence of 50 ng/ml IGF-1. IGF-1 activated Akt, which enhanced phosphorlytion of FoxO3a at Thr 318/321 and reduced the expression of UL. Intriguingly, though activation of AMPK by 0.3 and 1.0 mM AICAR synergized IGF-1-induced Akt activation, the expression of UL was not attenuated, but strengthened by AMPK activation. AICAR treatment decreased FoxO3a phosphorylation at 318/321 in the cytoplasm and induced FoxO3 nuclear relocation. mTOR inhibition increased basal MAFbx expression and reversed the inhibitory effect of IGF-1 on UL expression. In conclusion, our data show that AMPK activation by AICAR stimulates UL expression despite the activation of Akt signaling, which may be due to the possible antagonistic effect of FoxO phosphorylation by AMPK on phosphorylation by Akt. In addition, AMPK inhibition of mTOR may provide an additional explanation for the enhancement of UL expression by AMPK.

Antroquinonol displays anticancer potential against human hepatocellular carcinoma cells: a crucial role of AMPK and mTOR pathways

5'AMP-activated protein kinase (AMPK) and the mammalian target of rapamycin (mTOR) are two serine/threonine protein kinases responsible for cellular energy homeostasis and translational control, respectively. Evidence suggests that these two kniases are potential targets for cancer chemotherapy against hepatocellular carcinoma (HCC). Antroquinonol that is isolated from Antrodia camphorate, a well-known Traditional Chinese Medicine for treatment of liver diseases, displayed effective anticancer activity against both HBV DNA-positive and -negative HCC cell lines. The rank order of potency against HCCs is HepG2>HepG2.2.15>Mahlavu>PLC/PRF/5>SK-Hep1>Hep3B. Antroquinonol completely abolished cell-cycle progression released from double-thymidine-block synchronization and caused a subsequent apoptosis. The data were supported by down-regulation and reduced nuclear translocation of G1-regulator proteins, including cyclin D1, cyclin E, Cdk4 and Cdk2. Further analysis showed that the mRNA expressions of the G1-regulator proteins were not modified by antroquinonol, indicating an inhibition of translational but not transcriptional levels. Antroquinonol induced the assembly of tuberous sclerosis complex (TSC)-1/TSC2, leading to the blockade of cellular protein synthesis through inhibition of protein phosphorylation including mTOR (Ser(2448)), p70(S6K) (Thr(421)/Ser(424) and Thr(389)) and 4E-BP1 (Thr(37)/Thr(46) and Thr(70)). Furthermore, the AMPK activity was elevated by antroquinonol. Compound C, a selective AMPK inhibitor, significantly reversed antroquinonol-mediated effects suggesting the crucial role of AMPK. Besides, the loss of mitochondrial membrane potential and depletion of mitochondrial content indicated the mitochondrial stress caused by antroquinonol. In summary, the data suggest that antroquinonol displays anticancer activity against HCCs through AMPK activation and inhibition of mTOR translational pathway, leading to G1 arrest of the cell-cycle and subsequent cell apoptosis.

Changes in muscle mass with mechanical load: possible cellular mechanismsThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Understanding the mechanisms that regulate skeletal muscle mass has remained a focus of numerous researchers for many years. Recent investigations have begun to elucidate cellular signaling mechanisms that regulate skeletal muscle hypertrophy, with significant effort being focused on the Akt/mammalian target of rapamycin (mTOR) signaling pathway. The Akt/mTOR pathway plays a major role in regulating the initiation of protein synthesis after the onset of mechanical loading of skeletal muscle. Although a number of downstream substrates for Akt/mTOR have been elucidated, very little is known about the upstream mechanisms that mechanical load employs to activate the Akt/mTOR signaling pathway. Thus, the purpose of this review is to discuss potential mechanisms that may contribute to the activation of the Akt/mTOR signaling mechanism in mechanically loaded skeletal muscle.

AMP-activated protein kinase: a core signalling pathway in the heart

Over the past decade, AMP-activated protein kinase (AMPK) has emerged as an important intracellular signalling pathway in the heart. Activated AMPK stimulates the production of ATP by regulating key steps in both glucose and fatty acid metabolism. It has an inhibitory effect on cardiac protein synthesis. AMPK also interacts with additional intracellular signalling pathways in a coordinated network that modulates essential cellular processes in the heart. Evidence is accumulating that AMPK may protect the heart from ischaemic injury and limit the development of cardiac myocyte hypertrophy to various stimuli. Heart AMPK is activated by hormones, cytokines and oral hypoglycaemic drugs that are used in the treatment of type 2 diabetes. The tumour suppressor LKB1 is the major regulator of AMPK activity, but additional upstream kinases and protein phosphatases also contribute. Mutations in the regulatory gamma2 subunit of AMPK lead to an inherited syndrome of hypertrophic cardiomyopathy and ventricular pre-excitation, which appears to be due to intracellular glycogen accumulation. Future research promises to elucidate the molecular mechanisms responsible for AMPK activation, novel downstream AMPK targets, and the therapeutic potential of targeting AMPK for the prevention and treatment of myocardial ischaemia or cardiac hypertrophy.

AICAR preconditioning prevents postischemic leukocyte rolling and adhesion: role of K(ATP) channels and heme oxygenase

OBJECTIVE

We previously demonstrated that pharmacologic activation of AMP-activated protein kinase (AMPK) with 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) 24 hours prior to (AICAR preconditioning; AICAR-PC) ischemia/reperfusion (I/R) prevents postischemic leukocyte-endothelial cell adhesive interactions (LEI) by a mechanism initiated by endothelial nitric oxide synthase (eNOS)-dependent NO production during the period of AICAR-PC. The major aim of this study was to examine the role of ATP-sensitive potassium (K(ATP)) channels and heme oxygenase as mediators of the antiadhesive effects of AICAR-PC during I/R 24 hours later.

METHODS

Intravital fluorescence microscopy was used to quantify LEI in the small intestine of AICAR-preconditioned C57BL/6J mice treated with K(ATP) channel or heme oxygenase inhibitors during I/R 24 hours after AICAR-PC in separate experiments.

RESULTS

I/R induced marked increases in LEI relative to sham control mice, proadhesive responses that were prevented by AICAR-PC 24 hours prior to I/R. The effects of AICAR-PC to prevent postischemic LEI were abolished by K(ATP) channel or heme oxygenase inhibition during I/R.

DISCUSSION/CONCLUSION

Our results indicate that the antiadhesive effects of AICAR-PC are mediated by K(ATP) channel- and heme oxygenase-dependent mechanisms during I/R.

Increase of macrophage migration inhibitory factor (MIF) expression in cardiomyocytes during chronic hypoxia

BACKGROUND

Macrophage migration inhibitory factor (MIF) might play an important role in the myocardium during chronic hypoxia because MIF protects the heart during myocardial ischemia by activating 5'-adenosine monophosphate activated protein kinase (AMPK).

METHODS

We investigated 35 infants with cyanotic or acyanotic cardiac defects and H9c2 embryonic rat cardiomyocytes to examine the effect of chronic hypoxia on the expression of MIF in vivo and in vitro, respectively.

RESULTS

We found out an increase of endogenous cardiac MIF expression positively correlated with degree of hypoxia. Also, AMPK activation was elevated while MIF expression was increased in cells exposed to long periods of hypoxia in vitro. There was no significant difference in the growth ratio of cells cultivated in long periods of hypoxia and normoxia.

CONCLUSIONS

The expression of MIF is significantly increased in cardiomyocytes exposed to chronic hypoxia, and the activation of AMPK was increased accordingly.

AMPKalpha2 counteracts the development of cardiac hypertrophy induced by isoproterenol

As AMP-activated protein kinase (AMPK) controls protein translation, an anti-hypertrophic effect of AMPK has been suggested. However, there is no genetic evidence to confirm this hypothesis. We investigated the contribution of AMPKalpha2 in the control of cardiac hypertrophy by using AMPKalpha2-/- mice submitted to isoproterenol. The isoproterenol-induced cardiac hypertrophy, measured by left ventricular mass and histological examination, was significantly higher in AMPKalpha2-/- than in WT animals. Moreover, the intensification of cardiac hypertrophy found in AMPKalpha2-/- mice can be linked to the abnormal basal overstimulation of the p70 ribosomal S6 protein kinase, an enzyme known to regulate protein translation and cell growth. In conclusion, this work shows that AMPKalpha2 plays a role of brake for the development of cardiac hypertrophy.

Thromboxane receptor activates the AMP-activated protein kinase in vascular smooth muscle cells via hydrogen peroxide

Thromboxane A2 receptor (TPr) stimulation induces cellular hypertrophy in vascular smooth muscle cells (VSMCs); however, regulation of VSMC hypertrophy remains poorly understood. Here we show that TPr stimulation activates AMP-activated kinase (AMPK), which in turn limits TPr-induced protein synthesis in VSMCs. Exposure of cultured VSMCs to either TPr agonists, IBOP and U46619, or exogenous hydrogen peroxide (H2O2) caused time- and dose-dependent AMPK activation, as evidenced by increased phosphorylation of both AMPK-Thr172 and acetyl-coenzyme A carboxylase-Ser79, a downstream enzyme of AMPK, whereas SQ29548, a selective TPr antagonist, significantly attenuated TPr-enhanced AMPK activation. In parallel, both IBOP and U46619 significantly increased the production of reactive oxygen species such as H2O2. Furthermore, adenoviral overexpression of catalase (an H2O2 scavenger) abolished, whereas superoxide dismutase (which catalyzes H2O2 formation) enhanced, IBOP-induced AMPK activation, suggesting that TPr-activated AMPK was mediated by H2O2. Consistently, exposure of VSMCs to either TPr agonists or exogenous H2O2 dose-dependently increased the phosphorylation of LKB1 (at serines 428 and 307), an AMPK kinase, as well as coimmunoprecipitation of AMPK with LKB1. In addition, direct mutagenesis of either Ser428 or Ser307 of LKB1 into alanine, like the kinase-dead LKB1 mutant, abolished both TPr-stimulated AMPK activation and coimmunoprecipitation. Finally, genetic inhibition of AMPK significantly accentuated IBOP-enhanced protein synthesis, whereas adenoviral overexpression of constitutively active AMPK abolished IBOP-enhance protein synthesis in VSMCs. We conclude that TPr stimulation triggers reactive oxygen species-mediated LKB1-dependent AMPK activation, which in return inhibits cellular protein synthesis in VSMCs.

Activation of AMP-activated protein kinase by human EGF receptor 2/EGF receptor tyrosine kinase inhibitor protects cardiac cells

The human EGF receptor (HER) 2 receptor tyrosine kinase is a survival factor for human cardiomyocytes, and its inhibition may explain the increased incidence of cardiomyopathy associated with the anti-HER2 monoclonal antibody trastuzumab (Genentech, South San Francisco, CA), particularly in patients with prior exposure to cardiotoxic chemotherapies e.g., anthracyclines. Here, we show that GW2974 (HER2/EGF receptor tyrosine kinase inhibitor), but not trastuzumab, activates AMP-activated protein kinase (AMPK), initiating a metabolic stress response in human cardiomyocytes that protects against TNFalpha-induced cell death. GW2974 stimulates calcium dependent fatty acid oxidation in vitro and in the myocardium of GW2974-treated rodents. Calcium chelation or siRNA-targeted AMPK knockdown blocks GW2974 induced fatty acid oxidation. In addition, inhibition of AMPK by a specific inhibitor resulted in increased killing of cardiomyocytes. Elucidating the effects of HER2-targeted therapies on AMPK may predict for risk of cardiomyopathy and provide a novel HER2-targeted strategy designed to protect myocardium from the pro-apoptotic effects of pro-inflammatory cytokines released in response to cardiac injury by chemotherapy or acute ischemia.

Long-term activation of adenosine monophosphate-activated protein kinase attenuates pressure-overload-induced cardiac hypertrophy

Recent in vitro studies suggest that adenosine monophosphate (AMP)-activated protein kinase (AMPK) exerts inhibitory effects on cardiac hypertrophy. However, it is unclear whether long-term activation of AMPK will affect cardiac hypertrophy in vivo. In these reports, we investigate the in vivo effects of long-term AMPK activation on cardiac hypertrophy and the related molecular mechanisms. To examine the effects of AMPK activation in the development of pressure overload-induced cardiac hypertrophy, we administered 5-aminoimidazole 1 carboxamide ribonucleoside (AICAR, 0.5 mg/g body wt), a specific activator of AMPK, to rats with transaortic constriction (TAC) for 7 weeks. We found that long-term AMPK activation attenuated cardiac hypertrophy, and improved cardiac function in rats subjected to TAC. Furthermore, long-term AMPK activation attenuated protein synthesis, diminished calcineurin-nuclear factor of activated T cells (NFAT) and nuclear factor kappaB (NF-kappaB) signaling in pressure overload-induced hypertrophic hearts. Our in vitro experiments further proved that activation of AMPK by infection of AdAMPK blocked cardiac hypertrophy and NFAT, NF-kappaB, and MAPK signal pathways. The present study demonstrates for the first time that pharmacological activation of AMPK inhibits cardiac hypertrophy in through blocking signaling transduction pathways that are involved in cardiac growth. It presents a potential therapy strategy to inhibit pathological cardiac hypertrophy by increasing the activity of AMPK.

Involvement of AMPK signaling cascade in capsaicin-induced apoptosis of HT-29 colon cancer cells

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is activated during ATP-depleting metabolic states, such as hypoxia, heat shock, oxidative stress, and exercise. As a highly conserved heterotrimeric kinase that functions as a major metabolic switch to maintain energy homeostasis, AMPK has been shown to exert as an intrinsic regulator of mammalian cell cycle. Moreover, AMPK cascade has emerged as an important pathway implicated in cancer control. In this article, we have investigated the effects of capsaicin on apoptosis in relation to AMPK activation in colon cancer cell. Capsaicin-induced apoptosis was revealed by the presence of nucleobodies in the capsaicin-treated HT-29 colon cancer cells. Concomitantly, the activation of AMPK and the increased expression of the inactive form of acetyl-CoA carboxylase (ACC) were detected in capsaicin-treated colon cancer cells. We showed that both capsaicin and 5'-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR), an AMPK activator possess the AMPK-activating capacity as well as apoptosis-inducing properties. Evidence of the association between AMPK activation and the increased apoptosis in HT-29 colon cancer cells by capsaicin treatment, and further findings of the correlation of the activated AMPK and the elevated apoptosis by cotreatment of AICAR and capsaicin support AMPK as an important component of apoptosis, as well as a possible target of cancer control.