Effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats

Knockdown of ACOT4 alleviates gluconeogenesis and lipid accumulation in hepatocytes

Acyl-CoA thioesterase 4 (ACOT4) has been reported to be related to acetyl-CoA carboxylase activity regulation; However, its exact functions in liver lipid and glucose metabolism are still unclear. Here, we discovered explored the regulatory roles of ACOT4 in hepatic lipid and glucose metabolism in vitro. We found that the expression level of ACOT4 was significantly increased in the hepatic of db/db and ob/ob mice as well as obese mice fed a high fat diet. Adenovirus-mediated overexpression of ACOT4 promoted gluconeogenesis and high-glucose/high-insulin-induced lipid accumulation and impaired insulin sensitivity in primary mouse hepatocytes, whereas ACOT4 knockdown notably suppressed gluconeogenesis and decreased the triglycerides accumulation in hepatocytes. Furthermore, ACOT4 knockdown increased insulin-induced phosphorylation of AKT and GSK-3β in primary mouse hepatocytes. Mechanistically, we found that upregulation of ACOT4 expression inhibited AMP-activated protein kinase (AMPK) activity, and its knockdown had the opposite effect. However, activator A769662 and inhibitor compound C of AMPK suppressed the impact of the change in ACOT4 expression on AMPK activity. Our data indicated that ACOT4 is related to hepatic glucose and lipid metabolism, primarily via the regulation of AMPK activity. In conclusion, ACOT4 is a potential target for the therapy of non-alcoholic fatty liver (NAFLD) and type 2 diabetes.

Gestational Diabetes Mellitus-Associated Hyperglycemia Impairs Glucose Transporter 3 Trafficking in Trophoblasts Through the Downregulation of AMP-Activated Protein Kinase

AMP-activated protein kinase (AMPK) is an important regulator of glucose metabolism, and glucose transporter 3 (GLUT3) is an efficient glucose transporter in trophoblasts. Whether placental AMPK and GLUT3 respond accordingly to gestational diabetes mellitus (GDM) remains uncertain. Here, we explored the regulatory role of AMPK in the GLUT3-dependent uptake of glucose by placental trophoblasts and the viability of the cells. In this study, the level of glycolysis in normal and GDM-complicated placentas was assessed by LC-MS/MS. The trophoblast hyperglycemia model was induced by the incubation of HTR8/SVneo cells with a high glucose concentration. GDM animal models were generated with db/ + mice and C57BL/6J mice fed a high-fat diet, and AMPK was manipulated by the oral administration of metformin. The uptake of glucose by trophoblasts was assessed using 2-NBDG or 2-deoxy-D-[³H] glucose. The results showed that GDM is associated with impaired glycolysis, AMPK activity, GLUT3 expression in the plasma membrane (PM) and cell survival in the placenta. Hyperglycemia induced similar changes in trophoblasts, and these changes were rescued by AMPK activation. Both hyperglycemic db/ + and high-fat diet-induced GDM mice exhibited a compromised AMPK–GLUT3 axis and suppressed cell viability in the placenta as well as excessive fetal growth, and all of these effects were partially alleviated by metformin. Taken together, our findings support the notion that AMPK activation upregulates trophoblast glucose uptake by stimulating GLUT3 translocation, which is beneficial for viability. Thus, the modulation of glucose metabolism in trophoblasts by targeting AMPK might ameliorate the adverse intrauterine environment caused by GDM.

Hepatic Injury Induced by Dietary Energy Level via Lipid Accumulation and Changed Metabolites in Growing Semi-Fine Wool Sheep

Liver injury threatens the overall health of an organism, as it is the core organ of the animal body. Liver metabolism is affected by numerous factors, with dietary energy level being a crucial one. Therefore, the present study aimed to evaluate hepatic injury and to describe its metabolic mechanism in ruminants fed diets with different dietary energy levels. A total of 25 Yunnan semi-fine wool sheep were fed diets with five dietary metabolic energy levels and were randomly assigned to five groups as follows: low energy (LE), medium–low energy (MLE), medium energy (ME), medium–high energy (MHE), and high energy (HE). The results revealed that the average optical density (AOD) of lipid droplets in the LE, MLE, and HE groups was higher than that in the ME and MHE groups. The enzyme activity of alanine aminotransferase (ALT) was the lowest in the ME group. An increase in dietary energy level promoted the superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activity and altered the malondialdehyde (MDA) and protein carbonyl (PCO) concentration quadratically. In addition, both high and low dietary energy levels upregulated the mRNA abundance of proinflammatory cytokine interleukin (IL)-1β, nuclear factor-kappa B (NF-κB), and tumor necrosis factor (TNF)-α. Metabonomic analysis revealed that 142, 77, 65, and 108 differential metabolites were detected in the LE, MLE, MHE, and HE groups, compared with ME group respectively. These metabolites were involved in various biochemical pathways, such as glycolipid, bile acid, and lipid metabolism. In conclusion, both high and low dietary energy levels caused hepatic injury. Section staining and metabonomic results revealed that hepatic injury might be caused by altered metabolism and lipid accumulation induced by lipid mobilization.

BHBA regulates the expressions of lipid synthesis and oxidation genes in sheep hepatocytes through the AMPK pathway

Pregnancy toxemia (PT) is the most frequent metabolic disease of sheep during late pregnancy, which can lead to enormous economic losses in sheep farm industry. However, the underlying mechanism of PT in sheep has not been fully elucidated. High levels of β-hydroxy butyric acid (BHBA) exist in PT sheep. The AMP-activated protein kinase (AMPK) pathway plays a major role in regulating liver function. The aim of this study was to explore the effects of gradient concentrations of BHBA on lipid metabolism of sheep hepatocytes and the underlying molecular mechanism in vitro. The results showed that 0.6, 1.2 mmol/L BHBA could activate AMPKα, promoted the expressions of peroxisome proliferator-activated receptor alpha (PPARα) and its target genes, and inhibited the expressions of sterol regulatory element binding protein-1c (SREBP-1c) as well as its downstream genes. When the concentration of BHBA was beyond 1.2 mmol/L, the expressions of the above-mentioned proteins and genes were just the opposite. However, the expressions of adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) did not change significantly. The levels of very low density lipoprotein (VLDL), triglyceride (TG) and cholesterol (T-CHOL) showed a gradually increasing trend with the increase of BHBA concentration. According to the results above, it demonstrates that high levels of BHBA can inhibit the expression of the AMPK pathway and cause lipid metabolism disorders in sheep hepatocytes, which may lead to the occurrence of PT.

A spotlight on underlying the mechanism of AMPK in diabetes complications

OBJECTIVE

Type 2 diabetes (T2D) is one of the centenarian metabolic disorders and is considered as a stellar and leading health issue worldwide. According to the International Diabetes Federation (IDF) Diabetes Atlas and National Diabetes Statistics, the number of diabetic patients will increase at an exponential rate from 463 to 700 million by the year 2045. Thus, there is a great need for therapies targeting functions that can help in maintaining the homeostasis of glucose levels and improving insulin sensitivity. 5' adenosine monophosphate-activated protein kinase (AMPK) activation, by various direct and indirect factors, might help to overcome the hurdles (like insulin resistance) associated with the conventional approach.

MATERIALS AND RESULTS

A thorough review and analysis was conducted using various database including MEDLINE and EMBASE databases, with Google scholar using various keywords. This extensive review concluded that various drugs (plant-based, synthetic indirect/direct activators) are available, showing tremendous potential in maintaining the homeostasis of glucose and lipid metabolism, without causing insulin resistance, and improving insulin sensitivity. Moreover, these drugs have an effect against diabetes and are therapeutically beneficial in the treatment of diabetes-associated complications (neuropathy and nephropathy) via mechanism involving inhibition of nuclear translocation of SMAD4 (SMAD family member) expression and association with peripheral nociceptive neurons mediated by AMPK.

CONCLUSION

From the available information, it may be concluded that various indirect/direct activators show tremendous potential in maintaining the homeostasis of glucose and lipid metabolism, without resulting in insulin resistance, and may improve insulin sensitivity, as well. Therefore, in a nut shell, it may be concluded that the regulation of APMK functions by various direct/indirect activators may bring promising results. These activators may emerge as a novel therapy in diabetes and its associated complications.

Regulation of AMPK Activity by CRBN Is Independent of the Thalidomide-CRL4CRBN Protein Degradation Axis

Cereblon (CRBN), a primary target of immune-modulatory imide drugs (IMiDs), functions as a substrate receptor in the CUL4-RBX1-DDB1-CRBN (known as CRL4^CRBN) E3 ubiquitin ligase complex. Binding of IMiDs to CRBN redirects the CRL4^CRBN E3 ubiquitin ligase to recruit or displace its substrates. Interaction between CRBN and the AMPK α subunit leads to CRL4^CRBN-dependent degradation of the γ subunit and inhibits AMPK activity. However, the effect of thalidomide on the function of CRBN as a negative regulator of AMPK through interaction with the α subunit remains unclear. Here, we show that thalidomide does not affect AMPK activation or the binding affinity between CRBN and the AMPK α subunit. Thalidomide had no effect on AMPK activity independent of CRBN expression. The N-terminal region and C-terminal tail of CRBN, which is distinct from the IMiD binding site, were critical for interaction with the AMPK α subunit. The present results suggest that CRL4^CRBN negatively regulates AMPK through a pathway independent from the CRBN-IMiD binding region.

AICAr, a Widely Used AMPK Activator with Important AMPK-Independent Effects: A Systematic Review

5-Aminoimidazole-4-carboxamide ribonucleoside (AICAr) has been one of the most commonly used pharmacological modulators of AMPK activity. The majority of early studies on the role of AMPK, both in the physiological regulation of metabolism and in cancer pathogenesis, were based solely on the use of AICAr as an AMPK-activator. Even with more complex models of AMPK downregulation and knockout being introduced, AICAr remained a regular starting point for many studies focusing on AMPK biology. However, there is an increasing number of studies showing that numerous AICAr effects, previously attributed to AMPK activation, are in fact AMPK-independent. This review aims to give an overview of the present knowledge on AMPK-dependent and AMPK-independent effects of AICAr on metabolism, hypoxia, exercise, nucleotide synthesis, and cancer, calling for caution in the interpretation of AICAr-based studies in the context of understanding AMPK signaling pathway.

AICAr, a Widely Used AMPK Activator with Important AMPK-Independent Effects: A Systematic Review

5-Aminoimidazole-4-carboxamide ribonucleoside (AICAr) has been one of the most commonly used pharmacological modulators of AMPK activity. The majority of early studies on the role of AMPK, both in the physiological regulation of metabolism and in cancer pathogenesis, were based solely on the use of AICAr as an AMPK-activator. Even with more complex models of AMPK downregulation and knockout being introduced, AICAr remained a regular starting point for many studies focusing on AMPK biology. However, there is an increasing number of studies showing that numerous AICAr effects, previously attributed to AMPK activation, are in fact AMPK-independent. This review aims to give an overview of the present knowledge on AMPK-dependent and AMPK-independent effects of AICAr on metabolism, hypoxia, exercise, nucleotide synthesis, and cancer, calling for caution in the interpretation of AICAr-based studies in the context of understanding AMPK signaling pathway.

Are Nutraceuticals Beneficial in Chronic Kidney Disease?

Chronic kidney disease (CKD) is a worldwide health problem in which prevalence is constantly rising. The pathophysiology of CKD is complicated and has not been fully resolved. However, elevated oxidative stress is considered to play a vital role in the development of this disease. CKD is also thought to be an inflammatory disorder in which uremic toxins participate in the development of the inflammatory milieu. A healthy, balanced diet supports the maintenance of a good health status as it helps to reduce the risk of the development of chronic diseases, including chronic kidney disease, diabetes mellitus, and hypertension. Numerous studies have demonstrated that functional molecules and nutrients, including fatty acids and fiber as well as nutraceuticals such as curcumin, steviol glycosides, and resveratrol not only exert beneficial effects on pro-inflammatory and anti-inflammatory pathways but also on gut mucosa. Nutraceuticals have attracted great interest recently due to their potential favorable physiological effects on the human body and their safety. This review presents some nutraceuticals in which consumption could exert a beneficial impact on the development and progression of renal disease as well cardiovascular disease.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

Metabolic and transcriptome responses of RNAi-mediated AMPKα knockdown in Tribolium castaneum

Background

The AMP-activated protein kinase (AMPK) is an intracellular fuel sensor for lipid and glucose metabolism. In addition to the short-term regulation of metabolic enzymes by phosphorylation, AMPK may also exert long-term effects on the transcription of downstream genes through the regulation of transcription factors and coactivators. In this study, RNA interference (RNAi) was conducted to investigate the effects of knockdown of TcAMPKα on lipid and carbohydrate metabolism in the red flour beetle, Tribolium castaneum, and the transcriptome profiles of dsTcAMPKα-injected and dsEGFP-injected beetles under normal conditions were compared by RNA-sequencing.

Results

RNAi-mediated suppression of TcAMPKα increased whole-body triglyceride (TG) level and the ratio between glucose and trehalose, as was confirmed by in vivo treatment with the AMPK-activating compound, 5-Aminoimidazole-4-carboxamide1-β-D-ribofuranoside (AICAR). A total of 1184 differentially expressed genes (DEGs) were identified between dsTcAMPKα-injected and dsEGFP-injected beetles. These include genes involved in lipid and carbohydrate metabolism as well as insulin/insulin-like growth factor signaling (IIS). Real-time quantitative polymerase chain reaction analysis confirmed the differential expression of selected genes. Interestingly, metabolism-related transcription factors such as sterol regulatory element-binding protein 1 (SREBP1) and carbohydrate response element-binding protein (ChREBP) were also significantly upregulated in dsTcAMPKα-injected beetles.

Conclusions

AMPK plays a critical role in the regulation of beetle metabolism. The findings of DEGs involved in lipid and carbohydrate metabolism provide valuable insight into the role of AMPK signaling in the transcriptional regulation of insect metabolism.

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

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

Flavonoids and type 2 diabetes: Evidence of efficacy in clinical and animal studies and delivery strategies to enhance their therapeutic efficacy

Diabetes mellitus type 2 (T2DM) is a metabolic disorder develops due to the overproduction of free radicals where oxidative stress could contribute it. Possible factors are defective insulin signals, glucose oxidation, and degradation of glycated proteins as well as alteration in glutathione metabolism which induced hyperglycemia. Previous studies revealed a link between T2DM with oxidative stress, inflammation and insulin resistance which are assumed to be regulated by numerous cellular networks such as NF-κB, PI3K/Akt, MAPK, GSK3 and PPARγ. Flavonoids are ubiquitously present in the nature and classified according to their chemical structures for example, flavonols, flavones, flavan-3-ols, anthocyanidins, flavanones, and isoflavones. Flavonoids indicate poor bioavailability which could be improved by employing various nano-delivery systems against the occurrences of T2DM. These bioactive compounds exert versatile anti-diabetic activities via modulating targeted cellular signaling networks, thereby, improving glucose metabolism, α -glycosidase, and glucose transport or aldose reductase by carbohydrate metabolic pathway in pancreatic β-cells, hepatocytes, adipocytes and skeletal myofibres. Moreover, anti-diabetic properties of flavonoids also encounter diabetic related complications. This review article has designed to shed light on the anti-diabetic potential of flavonoids, contribution of oxidative stress, evidence of efficacy in clinical, cellular and animal studies and nano-delivery approaches to enhance their therapeutic efficacy. This article might give some new insights for therapeutic intervention against T2DM in near future.

Flavonoid derivatives synthesis and anti-diabetic activities

In high fat diet-induced obese mice, the flavonoid derivative of tiliroside, Fla-CN, has antihyperglycemic effects, can improve insulin sensitivity, ameliorate metabolic lipid disorders, and benefits certain disorders characterized by insulin resistance. Fla-CN is a novel lead compound to discovery anti-diabetic and anti-obesity drugs. The present study reported the optimization of Fla-CN to obtain a new derivative, 10b, which has improved glucose consumption at the nanomolar level (EC₅₀ = 0.3 nM) in insulin resistant (IR) HepG2 cells. 10b also increased the glycogen content and glucose uptake, and concurrently inhibited gluconeogenesis in HepG2 cells. Western blotting showed that 10b markedly enhanced the phosphorylation of AMPK (AMP-activated protein kinase) and AS160 (protein kinase B substrate of 160 kDa) and reduced the levels of the gluconeogenesis key enzymes PEPCK (phosphoenolpyruvate carboxykinase) and G6P (glucose 6-phosphatase) in HepG2 cells. The potential molecular mechanism of 10b may be activation of the AMPK/AS160 and AMPK/PEPCK/G6P pathways. We concluded that 10b might be a valuable candidate to discover anti-diabetic drugs.

The Role of AMPK/mTOR Modulators in the Therapy of Acute Myeloid Leukemia

Differentiation therapy of acute promyelocytic leukemia with all-trans retinoic acid represents the most successful pharmacological therapy of acute myeloid leukemia (AML). Numerous studies demonstrate that drugs that inhibit mechanistic target of rapamycin (mTOR) and activate AMP-kinase (AMPK) have beneficial effects in promoting differentiation and blocking proliferation of AML. Most of these drugs are already in use for other purposes; rapalogs as immunosuppressants, biguanides as oral antidiabetics, and 5-amino-4-imidazolecarboxamide ribonucleoside (AICAr, acadesine) as an exercise mimetic. Although most of these pharmacological modulators have been widely used for decades, their mechanism of action is only partially understood. In this review, we summarize the role of AMPK and mTOR in hematological malignancies and discuss the possible role of pharmacological modulators in proliferation and differentiation of leukemia cells.

Dietary Supplementation of Vine Tea Ameliorates Glucose and Lipid Metabolic Disorder via Akt Signaling Pathway in Diabetic Rats

A traditional Chinese tea with many pharmacological effects, vine tea (VT) is considered a potential dietary supplement to improve type 2 diabetes (T2D). To investigate the effect and mechanism of VT on glucose and lipid metabolic disorders in T2D rats, Wistar rats fed a normal diet served as the normal control, while rats fed a high-fat diet combined with low-dose streptozotocin (STZ)-induced T2D were divided into three groups: The model group (MOD); the positive control group (MET, metformin at 200 mg/kg/d); and the VT-treated group (VT500, allowed to freely drink 500 mg/L VT). After four weeks of intervention, biochemical metrics indicated that VT significantly ameliorated hyperglycemia, hyperlipidemia and hyperinsulinemia in T2D rats. Metabolomics research indicated that VT regulated the levels of metabolites closely related to glucose and lipid metabolism and promoted glycogen synthesis. Furthermore, VT had a significant influence on the expression of key genes involved in the Akt signaling pathway, inhibited gluconeogenesis through the Akt/Foxo1/Pck2 signaling pathway, and reduced fatty acid synthesis via the SREBP1c/Fasn signaling pathways. In conclusion, VT has great potential as a dietary supplement to ameliorate glucose and lipid metabolic disorders via the Akt signaling pathway in T2D rats.

Molecular imaging of diabetes and diabetic complications: Beyond pancreatic β-cell targeting

Diabetes is a chronic non-communicable disease affecting over 400 million people worldwide. Diabetic patients are at a high risk of various complications, such as cardiovascular, renal, and other diseases. The pathogenesis of diabetes (both type 1 and type 2 diabetes) is associated with a functional impairment of pancreatic β-cells. Consequently, most efforts to manage and prevent diabetes have focused on preserving β-cells and their function. Advances in imaging techniques, such as magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography, and single-photon-emission computed tomography, have enabled noninvasive and quantitative detection and characterization of the population and function of β-cells in vivo. These advantages aid in defining and monitoring the progress of diabetes and determining the efficacy of anti-diabetic therapies. Beyond β-cell targeting, molecular imaging of biomarkers associated with the development of diabetes, e.g., lymphocyte infiltration, insulitis, and metabolic changes, may also be a promising strategy for early detection of diabetes, monitoring its progression, and occurrence of complications, as well as facilitating exploration of new therapeutic interventions. Moreover, molecular imaging of glucose uptake, production and excretion in specified tissues is critical for understanding the pathogenesis of diabetes. In the current review, we summarize and discuss recent advances in noninvasive imaging technologies for imaging of biomarkers beyond β-cells for early diagnosis of diabetes, investigation of glucose metabolism, and precise diagnosis and monitoring of diabetic complications for better management of diabetic patients.

l-Alanine activates hepatic AMP-activated protein kinase and modulates systemic glucose metabolism

Objective

AMP activated protein kinase (AMPK) is recognized as an important nutrient sensor contributing to regulation of cellular, tissue, and systemic metabolism. We aimed to identify specific amino acids which could modulate AMPK and determine effects on cellular and systemic metabolism.

Methods

We performed an unbiased amino acid screen to identify activators of AMPK. Detailed analysis of cellular signaling and metabolism was performed in cultured hepatoma cells, and in vivo glucose metabolism and metabolomic patterns were assessed in both chow-fed mice and mice made obese by high-fat diet feeding.

Results

Alanine acutely activates AMP kinase in both cultured hepatic cells and in liver from mice treated in vivo with Ala. Oral alanine administration improves systemic glucose tolerance in both chow and high fat diet fed mice, with reduced efficacy of Ala in mice with reduced AMPK activity. Our data indicate that Ala activation of AMPK is mediated by intracellular Ala metabolism, which reduces TCA cycle metabolites, increases AMP/ATP ratio, and activates NH₃ generation.

Conclusions

Ala may serve as a distinct amino acid energy sensor, providing a positive signal to activate the beneficial AMPK signaling pathway.

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Unbiased amino acid screen identified alanine as a unique activator of AMP kinase.

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Alanine acutely activates AMP kinase in both cultured cells and in vivo.

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Alanine and NH₃ metabolism contribute to regulation of AMP kinase activation.

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Effects of Ala are reduced in absence of AMP kinase.

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Oral alanine improves glucose tolerance in vivo in both chow and HFD-fed mice.

Urine levels of 5-aminoimidazole-4-carboxamide riboside (AICAR) in patients with type 2 diabetes

AIMS

5-Aminoimidazole-4-carboxamide riboside (AICAR) is an endogenous activator of AMPK, a central regulator of energy homeostasis. Loss and/or reduction of AMPK signaling plays an important role in the development of insulin resistance in type 2 diabetes. The loss of AMPK in diabetes could be due to a loss of AICAR. The aim of this study was to characterize urine levels of AICAR in diabetes and determine whether an association exists with respect to late complications, e.g., retinopathy, nephropathy and neuropathy.

METHODS

Urine AICAR was measured by liquid chromatography tandem mass spectrometry in 223 patients consisting of 5 healthy controls, 63 patients with pre-diabetes, 29 patients with newly diagnosed type 2 diabetes and 126 patients with long-standing type 2 diabetes. For statistical analyses, nonparametric Kruskal-Wallis test, one-way ANOVA and multivariate regression analysis were performed to investigate the associations of urinary AICAR excretion within different groups and different clinical parameters.

RESULTS

The mean urine AICAR for all 223 patients was 694.7 ± 641.1 ng/ml. There was no significant difference in urine AICAR between the control and patients with diabetes (592.3 ± 345.1 vs. 697.1 ± 646.5 ng/ml). No association between any of the biochemical and/or clinical parameters measured and urine AICAR was found, with the exception of age of patient (R = - 0.34; p < 0.01) and estimated glomerular filtration rate (R = 0.19; p = 0.039). These results were confirmed additionally by linear regression analysis.

CONCLUSIONS

Clinical diabetes is not associated with a change in endogenous AICAR levels. Loss of AICAR may therefore not be a mechanism by which AMPK signaling is reduced in diabetes.

Dietary Flavonoids in the Prevention of T2D: An Overview

Type 2 diabetes (T2D) is a progressive metabolic disease that is increasing in prevalence globally. It is well established that insulin resistance (IR) and a progressive decline in functional β-cell mass are hallmarks of developing T2D. Obesity is a leading pathogenic factor for developing IR. Constant IR will progress to T2D when β-cells are unable to secret adequate amounts of insulin to compensate for decreased insulin sensitivity. Recently, a considerable amount of research has been devoted to identifying naturally occurring anti-diabetic compounds that are abundant in certain types of foods. Flavonoids are a group of polyphenols that have drawn great interest for their various health benefits. Results from many clinical and animal studies demonstrate that dietary intake of flavonoids might be helpful in preventing T2D, although cellular and molecular mechanisms underlying these effects are still not completely understood. This review discusses our current understanding of the pathophysiology of T2D and highlights the potential anti-diabetic effects of flavonoids and mechanisms of their actions.

Mitochondrial oxidative stress causes insulin resistance without disrupting oxidative phosphorylation

Mitochondrial oxidative stress, mitochondrial dysfunction, or both have been implicated in insulin resistance. However, disentangling the individual roles of these processes in insulin resistance has been difficult because they often occur in tandem, and tools that selectively increase oxidant production without impairing mitochondrial respiration have been lacking. Using the dimer/monomer status of peroxiredoxin isoforms as an indicator of compartmental hydrogen peroxide burden, we provide evidence that oxidative stress is localized to mitochondria in insulin-resistant 3T3-L1 adipocytes and adipose tissue from mice. To dissociate oxidative stress from impaired oxidative phosphorylation and study whether mitochondrial oxidative stress per se can cause insulin resistance, we used mitochondria-targeted paraquat (MitoPQ) to generate superoxide within mitochondria without directly disrupting the respiratory chain. At ≤10 μm, MitoPQ specifically increased mitochondrial superoxide and hydrogen peroxide without altering mitochondrial respiration in intact cells. Under these conditions, MitoPQ impaired insulin-stimulated glucose uptake and glucose transporter 4 (GLUT4) translocation to the plasma membrane in both adipocytes and myotubes. MitoPQ recapitulated many features of insulin resistance found in other experimental models, including increased oxidants in mitochondria but not cytosol; a more profound effect on glucose transport than on other insulin-regulated processes, such as protein synthesis and lipolysis; an absence of overt defects in insulin signaling; and defective insulin- but not AMP-activated protein kinase (AMPK)-regulated GLUT4 translocation. We conclude that elevated mitochondrial oxidants rapidly impair insulin-regulated GLUT4 translocation and significantly contribute to insulin resistance and that MitoPQ is an ideal tool for studying the link between mitochondrial oxidative stress and regulated GLUT4 trafficking.

High-glucose toxicity is mediated by AICAR-transformylase/IMP cyclohydrolase and mitigated by AMP-activated protein kinase in Caenorhabditis elegans

The enzyme AICAR-transformylase/IMP cyclohydrolase (ATIC) catalyzes the last two steps of purine de novo synthesis. It metabolizes 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), which is an AMP analogue, leading to activation of AMP-activated kinase (AMPK). We investigated whether the AICAR-ATIC pathway plays a role in the high glucose (HG)-mediated DNA damage response and AICAR-mediated AMPK activation, explaining the detrimental effects of glucose on neuronal damage and shortening of the lifespan. HG up-regulated the expression and activity of the Caenorhabditis elegans homologue of ATIC, C55F2.1 (atic-1), and increased the levels of reactive oxygen species and methylglyoxal-derived advanced glycation end products. Overexpression of atic-1 decreased the lifespan and head motility and increased neuronal damage under both standard and HG conditions. Inhibition of atic-1 expression, by RNAi, under HG was associated with increased lifespan and head motility and reduced neuronal damage, reactive oxygen species, and methylglyoxal-derived advanced glycation end product accumulation. This effect was independent of an effect on DNA damage or antioxidant defense pathways, such as superoxide dismutase (sod-3) or glyoxalase-1 (glod-4), but was dependent on AMPK and accumulation of AICAR. Through AMPK, AICAR treatment also reduced the negative effects of HG. The mitochondrial inhibitor rotenone abolished the AICAR/AMPK-induced amelioration of HG effects, pointing to mitochondria as a prime target of the glucotoxic effects in C. elegans We conclude that atic-1 is involved in glucotoxic effects under HG conditions, either by blocked atic-1 expression or via AICAR and AMPK induction.

AMPK Re-Activation Suppresses Hepatic Steatosis but its Downregulation Does Not Promote Fatty Liver Development

Nonalcoholic fatty liver disease is a highly prevalent component of disorders associated with disrupted energy homeostasis. Although dysregulation of the energy sensor AMP-activated protein kinase (AMPK) is viewed as a pathogenic factor in the development of fatty liver its role has not been directly demonstrated. Unexpectedly, we show here that liver-specific AMPK KO mice display normal hepatic lipid homeostasis and are not prone to fatty liver development, indicating that the decreases in AMPK activity associated with hepatic steatosis may be a consequence, rather than a cause, of changes in hepatic metabolism. In contrast, we found that pharmacological re-activation of downregulated AMPK in fatty liver is sufficient to normalize hepatic lipid content. Mechanistically, AMPK activation reduces hepatic triglyceride content both by inhibiting lipid synthesis and by stimulating fatty acid oxidation in an LKB1-dependent manner, through a transcription-independent mechanism. Furthermore, the effect of the antidiabetic drug metformin on lipogenesis inhibition and fatty acid oxidation stimulation was enhanced by combination treatment with small-molecule AMPK activators in primary hepatocytes from mice and humans. Overall, these results demonstrate that AMPK downregulation is not a triggering factor in fatty liver development but in contrast, establish the therapeutic impact of pharmacological AMPK re-activation in the treatment of fatty liver disease.

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Hepatic AMPK deficiency is not sufficient to trigger fatty liver development

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Re-activation of downregulated AMPK in fatty liver normalizes hepatic lipid content

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Hepatic AMPK activation both inhibits lipogenesis and stimulates fatty acid oxidation

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AMPK activation modulates lipid metabolism via a transcription-independent mechanism

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Small-molecule AMPK activators enhance metformin effects on hepatic lipid metabolism

Nonalcoholic fatty liver disease is a highly prevalent component of metabolic syndrome, for which treatment options are limited. Downregulation of the energy sensor AMPK is viewed as a pathogenic factor in the development of fatty liver. However, we show here hepatic AMPK suppression is not sufficient to promote hepatic lipid accumulation, indicating that the decreases in AMPK activity associated with hepatic steatosis may be a consequence, rather than a cause, of changes in hepatic metabolism. In contrast, we found that pharmacological re-activation of downregulated AMPK in fatty liver is sufficient to normalize hepatic lipid content. Thus, these results establish the therapeutic impact of pharmacological AMPK re-activation in the treatment of fatty liver disease.

AMPK in skeletal muscle function and metabolism

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

Liver-Specific Activation of AMPK Prevents Steatosis on a High-Fructose Diet

AMP-activated protein kinase (AMPK) plays a key role in integrating metabolic pathways in response to energy demand. We identified a mutation in the γ1 subunit (γ1^D316A) that leads to activation of AMPK. We generated mice with this mutation to study the effect of chronic liver-specific activation of AMPK in vivo. Primary hepatocytes isolated from these mice have reduced gluconeogenesis and fatty acid synthesis, but there is no effect on fatty acid oxidation compared to cells from wild-type mice. Liver-specific activation of AMPK decreases lipogenesis in vivo and completely protects against hepatic steatosis when mice are fed a high-fructose diet. Our findings demonstrate that liver-specific activation of AMPK is sufficient to protect against hepatic triglyceride accumulation, a hallmark of non-alcoholic fatty liver disease (NAFLD). These results emphasize the clinical relevance of activating AMPK in the liver to combat NAFLD and potentially other associated complications (e.g., cirrhosis and hepatocellular carcinoma).

Nanoformulated copper/zinc superoxide dismutase exerts differential effects on glucose vs lipid homeostasis depending on the diet composition possibly via altered AMPK signaling

Evidence suggests that superoxide dismutase 1 (SOD1) promotes glucose vs lipid metabolism depending on the diet type. We recently reported that nanoformulated SOD1 (Nano) improved lipid metabolism without altering glucose homeostasis in high-fat (HF) diet-fed mice. Here, we sought to determine the effects and potential mechanisms of Nano in modulating glucose and lipid homeostasis in mice fed a normal chow diet (CD) vs HF diet. Mice were fed a CD or a HF diet (45%) for 10 wk and injected with Nano once every 2 days for 15 days. The fasting glucose level was lower (P < 0.05) in CD + Nano-treated mice compared to control. Conversely, blood glucose was not altered but serum triglycerides were lower in HF + Nano-treated mice. Genes involved in fatty acid synthesis were reduced by Nano in the skeletal muscle of CD but not of HF diet-fed mice. Adenosine monophosphate-activated protein kinase (AMPK), which promotes both glucose and lipid metabolism depending on the fuel availability, is activated by Nano in CD-fed mice. Moreover, Nano increased phosphorylation of ACC, a downstream target of AMPK, in both CD and HF diet-fed mice. Nano increased mitochondrial respiration in C2C12 myocytes in the presence of glucose or fatty acid, and this effect is inhibited by Compound C, an AMPK inhibitor. Our data suggest that Nano promotes glucose and lipid metabolism in CD and HF diet-fed mice, respectively, and this effect is mediated partly via AMPK signaling.

AMPK as a Therapeutic Target for Treating Metabolic Diseases

The AMP-activated protein kinase (AMPK) is a central regulator of multiple metabolic pathways and may have therapeutic importance for treating obesity, insulin resistance, type 2 diabetes (T2D), non-alcoholic fatty liver disease (NAFLD), and cardiovascular disease (CVD). Given the ubiquitous expression of AMPK, it has been a challenge to evaluate which tissue types may be most beneficially poised for mediating the positive metabolic effects of AMPK-centered treatments. In this review we evaluate the metabolic phenotypes of transgenic mouse models in which AMPK expression and function have been manipulated, and the impact this has on controlling lipid metabolism, glucose homeostasis, and inflammation. This information may be useful for guiding the development of AMPK-targeted therapeutics to treat chronic metabolic diseases.

REGULAÇÃO GÊNICA DA VIA AMPK PELO EXERCÍCIO FÍSICO: REVISÃO SISTEMÁTICA E ANÁLISE IN SILICO

RESUMO Introdução: Novos estudos de regulação gênica do exercício físico por meio de técnicas pós-genômicas em ensaios de resistência (endurance) e força caracterizam a transcriptômica do exercício físico. Entre os genes afetados, destacamos a via da proteína quinase ativada por AMP (AMPK), cuja ativação ocorre durante o exercício como resultado das alterações dos níveis de fosfato energético da fibra muscular. Objetivo: Avaliar a via de sinalização da AMPK por revisão sistemática da expressão de genes e análise in silico. Método: Foi efetuada uma revisão sistemática para avaliar a regulação gênica da via de sinalização AMPK, caracterizando os genes estudados na literatura, as variações de regulação obtidas, na forma de fold change e tipos de exercício usados. Resultados: A via de sinalização AMPK mostrou 133 genes no repositório KEGG (Kyoto Encyclopedia of Genes and Genomes), os quais foram confrontados com a revisão sistemática da literatura, totalizando 65 genes. Dezessete genes apresentaram UR e 24 mostraram DR com relação ao seu respectivo controle. Além destes, 20 genes estavam presentes nos trabalhos, apresentando tanto UR e DR e quatro genes não apresentaram dados de regulação. Verificou-se regulação específica em função do tipo de exercício efetuado. Discussão: Dos 133 genes da via AMPK, 48,8% foram amostrados nos trabalhos revisados, indicando que uma parte significativa da via é regulada pelo exercício. O estudo apresentou a regulação gênica básica de dois mecanismos para a recuperação energética, a biogênese mitocondrial e o bloqueio da gliconeogênese. Conclusão: Este trabalho mostrou que o exercício atua ativamente na via de sinalização da AMPK, na importância da regulação via PGC-1α e no papel de outros genes, regulando a expressão de mais da metade dos genes amostrados.

Molecular mechanisms underlying metabolic syndrome: the expanding role of the adipocyte

The metabolic syndrome (MetS) is defined as a cluster of 3 or more metabolic and cardiovascular risk factors and represents a serious problem for public health. Altered function of adipose tissue has a significant impact on whole-body metabolism and represents a key driver for the development of these metabolic derangements, collectively referred as to MetS. In particular, increased visceral and ectopic fat deposition play a major role in the development of insulin resistance and MetS. A large body of evidence demonstrates that aging and MetS share several metabolic alterations. Of importance, molecular pathways that regulate lifespan affect key processes of adipose tissue physiology, and transgenic mouse models with adipose-specific alterations in these pathways show derangements of adipose tissue and other metabolic features of MetS, which highlights a causal link between dysfunctional adipose tissue and deleterious effects on whole-body homeostasis. This review analyzes adipose tissue-specific dysfunctions, including metabolic alterations that are related to aging, that have a significant impact on the development of MetS.-Armani, A., Berry, A., Cirulli, F., Caprio, M. Molecular mechanisms underlying metabolic syndrome: the expanding role of the adipocyte.

3,3'-Diindolylmethane Suppresses Adipogenesis Using AMPKα-Dependent Mechanism in 3T3-L1 Adipocytes and Caenorhabditis elegans

3,3'-diindolylmethane is a major in vivo metabolite of indole-3-carbinol, a bioactive compound found in cruciferous vegetables. Although 3,3'-diindolylmethane has been implicated to possess antitumorigenic and anti-inflammatory properties, the effect of 3,3'-diindolylmethane on adipogenesis has not been explored previously. Thus, the present study was conducted to determine if 3,3'-diindolylmethane affects adipogenesis using 3T3-L1 adipocytes and Caenorhabditis elegans. Treatment of 3,3'-diindolylmethane significantly reduced fat accumulation without affecting viability in 3T3-L1 adipocytes. 3,3'-diindolylmethane suppressed expression of peroxisome proliferator-activated receptor γ (PPARγ), CCAAT-enhancer-binding protein α (C/EBPα), fatty acid binding protein 4 (FABP4), and perilipin. In addition, 3,3'-diindolylmethane activated AMP-activated protein kinase α (AMPKα), which subsequently inactivated acetyl CoA carboxylase (ACC), resulting in reduced fat accumulation. These observations were further confirmed in C. elegans as treatment with 3,3'-diindolylmethane significantly reduced body fat accumulation, which was partly associated with aak-1, but not aak-2, orthologs of AMPKα catalytic subunits α1 and α2, respectively. The current results demonstrate that 3,3'-diindolylmethane, a biologically active metabolite of indole-3-carbinol, may prevent adipogenesis through the AMPKα-dependent pathway.

Multiple AMPK activators inhibit l-carnitine uptake in C2C12 skeletal muscle myotubes

Mutations in the gene that encodes the principal l-carnitine transporter, OCTN2, can lead to a reduced intracellular l-carnitine pool and the disease Primary Carnitine Deficiency. l-Carnitine supplementation is used therapeutically to increase intracellular l-carnitine. As AMPK and insulin regulate fat metabolism and substrate uptake, we hypothesized that AMPK-activating compounds and insulin would increase l-carnitine uptake in C2C12 myotubes. The cells express all three OCTN transporters at the mRNA level, and immunohistochemistry confirmed expression at the protein level. Contrary to our hypothesis, despite significant activation of PKB and 2DG uptake, insulin did not increase l-carnitine uptake at 100 nM. However, l-carnitine uptake was modestly increased at a dose of 150 nM insulin. A range of AMPK activators that increase intracellular calcium content [caffeine (10 mM, 5 mM, 1 mM, 0.5 mM), A23187 (10 μM)], inhibit mitochondrial function [sodium azide (75 μM), rotenone (1 μM), berberine (100 μM), DNP (500 μM)], or directly activate AMPK [AICAR (250 μM)] were assessed for their ability to regulate l-carnitine uptake. All compounds tested significantly inhibited l-carnitine uptake. Inhibition by caffeine was not dantrolene (10 μM) sensitive despite dantrolene inhibiting caffeine-mediated calcium release. Saturation curve analysis suggested that caffeine did not competitively inhibit l-carnitine transport. To assess the potential role of AMPK in this process, we assessed the ability of the AMPK inhibitor Compound C (10 μM) to rescue the effect of caffeine. Compound C offered a partial rescue of l-carnitine uptake with 0.5 mM caffeine, suggesting that AMPK may play a role in the inhibitory effects of caffeine. However, caffeine likely inhibits l-carnitine uptake by alternative mechanisms independently of calcium release. PKA activation or direct interference with transporter function may play a role.

In vivo PET imaging with [(18)F]FDG to explain improved glucose uptake in an apolipoprotein A-I treated mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by decreased HDL levels, as well as the level of apolipoprotein A-I (apoA-I), the main apolipoprotein of HDLs. Pharmacological elevation of HDL and apoA-I levels is associated with improved glycaemic control in patients with type 2 diabetes. This is partly due to improved glucose uptake in skeletal muscle.

METHODS

This study used kinetic modelling to investigate the impact of increasing plasma apoA-I levels on the metabolism of glucose in the db/db mouse model.

RESULTS

Treatment of db/db mice with apoA-I for 2 h significantly improved both glucose tolerance (AUC 2574 ± 70 mmol/l × min vs 2927 ± 137 mmol/l × min, for apoA-I and PBS, respectively; p < 0.05) and insulin sensitivity (AUC 388.8 ± 23.8 mmol/l × min vs 194.1 ± 19.6 mmol/l × min, for apoA-I and PBS, respectively; p < 0.001). ApoA-I treatment also increased glucose uptake by skeletal muscle in both an insulin-dependent and insulin-independent manner as evidenced by increased uptake of fludeoxyglucose ([(18)F]FDG) from plasma into gastrocnemius muscle in apoA-I treated mice, both in the absence and presence of insulin. Kinetic modelling revealed an enhanced rate of insulin-mediated glucose phosphorylation (k 3) in apoA-I treated mice (3.5 ± 1.1 × 10(-2) min(-1) vs 2.3 ± 0.7 × 10(-2) min(-1), for apoA-I and PBS, respectively; p < 0.05) and an increased influx constant (3.7 ± 0.6 × 10(-3) ml min(-1) g(-1) vs 2.0 ± 0.3 × 10(-3) ml min(-1) g(-1), for apoA-I and PBS, respectively; p < 0.05). Treatment of L6 rat skeletal muscle cells with apoA-I for 2 h indicated that increased hexokinase activity mediated the increased rate of glucose phosphorylation.

CONCLUSIONS/INTERPRETATION

These findings indicate that apoA-I improves glucose disposal in db/db mice by improving insulin sensitivity and enhancing glucose phosphorylation.

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.

Electroacupuncture Attenuates Hepatic Lipid Accumulation via Amp-Activated Protein Kinase (Ampk) Activation in Obese Rats

Background

Electroacupuncture (EA) may offer an effective alternative approach for the treatment of obesity. EA mobilizes energy stores, but its effect on hepatic lipid metabolism is unknown, and the underlying mechanisms remain unclear.

Objective

To examine the effect of EA on hepatic lipid accumulation in diet-induced obese (DIO) rats, and to explore potential underlying mechanisms.

Methods

Male Sprague-Dawley rats were fed a normal diet (control group, n=10) or a high-fat diet (HFD) for 12 weeks to induce obesity. Those exhibiting diet-induced obesity were subdivided into two groups, one receiving EA (DIO+EA group, n=10) and one left untreated (DIO group, n=10) and observed for a further 4 weeks. Body, liver and fat pad weight were measured, and liver injury was assessed histologically as well as by measuring serum values of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Hepatic triglyceride (TG) and total cholesterol were quantified by enzymatic colorimetric methods. Expression of liver AMP-activated protein kinase (AMPK), acetyl-coenzyme A carboxylase (ACC), and carnitine palmitoyltransferase (CPT-1) was measured by Western blotting.

Results

EA treatment led to a reduction in body, liver and fat pad weight in DIO rats. This was accompanied by decreases in hepatic TG and total cholesterol values, fatty droplet accumulation, and serum concentrations of ALT and AST. Furthermore, EA treatment restored phosphorylation levels of AMPK (Thr172) and ACC (Ser79) inhibited by HFD, and increased CPT-1 expression.

Conclusions

EA reduces HFD-induced hepatic lipid accumulation, an effect that appears to be mediated through AMPK signalling pathways. Our results shed new light on the mechanisms by which EA may reduce obesity.

HL156A, a novel AMP-activated protein kinase activator, is protective against peritoneal fibrosis in an in vivo and in vitro model of peritoneal fibrosis

HL156A is a novel AMP-activated protein kinase (AMPK) activator. We aimed to investigate the protective mechanism of HL156A against peritoneal fibrosis (PF) in in vivo and in vitro models. The rat PF model was induced by daily intraperitoneally injection of chlorhexidine (CHX) solution containing 0.1% CHX gluconate and 15% ethanol for 4 wk. The rats in the treatment group were treated with HL156A (1 mg·kg−1·day−1). Control rats were injected with vehicle alone. In vitro, cultured rat peritoneal mesothelial cells (RPMCs) were treated with either high glucose (HG; 50 mM), normal glucose (NG; 5 mM), NG+HL156A, or HG+HL156A. HL156A in supplemented rats ameliorated peritoneal calcification, cocoon formation, bowel obstruction, and PF. Immunohistochemistry showed reduced fibronectin accumulation in the peritoneum of HL156A-treated rats compared with those injected with CHX alone. HL156A treatment of RPMCs inhibited HG-induced myofibroblast transdifferentiation and markers of epithelial-mesenchymal transition (EMT). Moreover, HL156A ameliorated HG-induced transforming growth factor-β1, Smad3, Snail, and fibronectin expression in the RPMCs via AMPK upregulation. These results suggest that HL156A exhibits a protective effect in PF progression. Further research is warranted to seek the therapeutic potential of HL156A as an antifibrotic agent in peritoneal dialysis patients.

Glucose transporters: cellular links to hyperglycemia in insulin resistance and diabetes

Abnormal expression and/or function of mammalian hexose transporters contribute to the hallmark hyperglycemia of diabetes. Due to different roles in glucose handling, various organ systems possess specific transporters that may be affected during the diabetic state. Diabetes has been associated with higher rates of intestinal glucose transport, paralleled by increased expression of both active and facilitative transporters and a shift in the location of transporters within the enterocyte, events that occur independent of intestinal hyperplasia and hyperglycemia. Peripheral tissues also exhibit deregulated glucose transport in the diabetic state, most notably defective translocation of transporters to the plasma membrane and reduced capacity to clear glucose from the bloodstream. Expression of renal active and facilitative glucose transporters increases as a result of diabetes, leading to elevated rates of glucose reabsorption. However, this may be a natural response designed to combat elevated blood glucose concentrations and not necessarily a direct effect of insulin deficiency. Functional foods and nutraceuticals, by modulation of glucose transporter activity, represent a potential dietary tool to aid in the management of hyperglycemia and diabetes.

Inhibition of MEK1 Signaling Pathway in the Liver Ameliorates Insulin Resistance

Although mitogen-activated protein kinase kinase (MEK) is a key signaling molecule and a negative regulator of insulin action, it is still uncertain whether MEK can be a therapeutic target for amelioration of insulin resistance (IR) in type 2 diabetes (T2D) in vivo. To clarify whether MEK inhibition improves T2D, we examined the effect of continuous MEK inhibition with two structurally different MEK inhibitors, RO5126766 and RO4987655, in mouse models of T2D. RO5126766 and RO4987655 were administered via dietary admixture. Both compounds decreased blood glucose and improved glucose tolerance in doses sufficient to sustain inhibition of extracellular signal-regulated kinase (ERK)1/2 phosphorylation downstream of MEK in insulin-responsive tissues in db/db mice. A hyperinsulinemic-euglycemic clamp test showed increased glucose infusion rate (GIR) in db/db mice treated with these compounds, and about 60% of the increase was attributed to the inhibition of endogenous glucose production, suggesting that the liver is responsible for the improvement of IR. By means of adenovirus-mediated Mek1 shRNA expression, we confirmed that blood glucose levels are reduced by suppression of MEK1 expression in the liver of db/db mice. Taken together, these results suggested that the MEK signaling pathway could be a novel therapeutic target for novel antidiabetic agents.

ANGPTL4 mediates shuttling of lipid fuel to brown adipose tissue during sustained cold exposure

Brown adipose tissue (BAT) activation via cold exposure is increasingly scrutinized as a potential approach to ameliorate cardio-metabolic risk. Transition to cold temperatures requires changes in the partitioning of energy substrates, re-routing fatty acids to BAT to fuel non-shivering thermogenesis. However, the mechanisms behind the redistribution of energy substrates to BAT remain largely unknown. Angiopoietin-like 4 (ANGPTL4), a protein that inhibits lipoprotein lipase (LPL) activity, is highly expressed in BAT. Here, we demonstrate that ANGPTL4 is part of a shuttling mechanism that directs fatty acids derived from circulating triglyceride-rich lipoproteins to BAT during cold. Specifically, we show that cold markedly down-regulates ANGPTL4 in BAT, likely via activation of AMPK, enhancing LPL activity and uptake of plasma triglyceride-derived fatty acids. In contrast, cold up-regulates ANGPTL4 in WAT, abolishing a cold-induced increase in LPL activity. Together, our data indicate that ANGPTL4 is an important regulator of plasma lipid partitioning during sustained cold.

DOI:http://dx.doi.org/10.7554/eLife.08428.001

The body stores energy in the form of fat molecules. Most of these molecules are stored in white fat cells. Other fat cells, the so-called brown fat cells, consume fats and produce heat to maintain body temperature in cold conditions. The capacity of brown fat cells to consume fats has led researchers to investigate whether brown fat cells might be a key to combat obesity.

When an organism is cold, fat is shuttled to the brown fat cells. An enzyme called lipoprotein lipase is involved in a process that allows these fat molecules to be taken up by brown fat cells. However, it was not clear exactly how this process works.

A protein called Angiopoietin-like 4 (ANGPTL4) inhibits the activity of lipoprotein lipase in white fat cells and is also found at high levels in brown fat cells. Here, Dijk et al. used genetic and biochemical approaches to study the role of ANGPTL4 in the fat cells of mice. The experiments show that when mice are exposed to cold, the levels of ANGPTL4 decrease in the brown fat cells. This allows the activity of lipoprotein lipase to increase so that these cells are able to take up more fat molecules.

However, the opposite happens in white fat cells during cold exposure. The levels of ANGPTL4 increase, which decreases the activity of lipoprotein lipase in white fat cells to allow fat molecules to be shuttled specifically to the brown fat cells. Further experiments suggest that the opposite regulation of ANGPTL4 in brown and white fat cells could be due to a protein called AMPK. This protein is found at higher levels in brown fat cells than in white fat cells and is produced by brown fat cells during cold exposure.

Taken together, Dijk et al. show that organs and cells work together to ensure that fat molecules are appropriately distributed to cells in need of energy, such as to brown fat cells during cold. How these findings could be used to stimulate fat consumption by brown fat cells in humans remains open for further investigation.

DOI:http://dx.doi.org/10.7554/eLife.08428.002

Pharmacological induction of mitochondrial biogenesis as a therapeutic strategy for the treatment of type 2 diabetes

Defects in mitochondrial oxidative function have been associated with the onset of type 2 diabetes. Although the causal relationship between mitochondrial dysfunction and diabetes has not been fully established, numerous studies indicate that improved glucose homeostasis achieved via lifestyle interventions, such as exercise or calorie restriction, is tightly associated with increased mitochondrial biogenesis and oxidative function. Therefore, it is conceivable that potentiating mitochondrial biogenesis by pharmacological means could constitute an efficacious therapeutic strategy that would particularly benefit those diabetic patients who cannot adhere to comprehensive programs based on changes in lifestyle or that require a relatively rapid improvement in their diabetic status. In this review, we discuss several pharmacological targets and drugs that modulate mitochondrial biogenesis as well as their potential use as treatments for insulin resistance and diabetes.

Enhancement of insulin-mediated rat muscle glucose uptake and microvascular perfusion by 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside

BACKGROUND

Insulin-induced microvascular recruitment is important for optimal muscle glucose uptake. 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR, an activator of AMP-activated protein kinase), can also induce microvascular recruitment, at doses that do not acutely activate glucose transport in rat muscle. Whether low doses of AICAR can augment physiologic insulin action is unknown. In the present study we used the euglycemic hyperinsulinemic clamp to assess whether insulin action is augmented by low dose AICAR.

METHODS

Anesthetized rats were studied during saline infusion or euglycemic insulin (3 mU/kg/min) clamp for 2 h in the absence or presence of AICAR for the last hour (5 mg bolus followed by 3.75 mg/kg/min). Muscle glucose uptake (R'g) was determined radioisotopically with (14)C-2-deoxyglucose and muscle microvascular perfusion by contrast-enhanced ultrasound with microbubbles.

RESULTS

AICAR did not affect blood glucose, or lower leg R'g, although it significantly (p < 0.05) increased blood lactate levels and augmented muscle microvascular blood volume via a nitric oxide synthase dependent pathway. Insulin increased femoral blood flow, whole body glucose infusion rate (GIR), R'g, hindleg glucose uptake, and microvascular blood volume. Addition of AICAR during insulin infusion increased lactate production, further increased R'g in Type IIA (fast twitch oxidative) and IIB (fast twitch glycolytic) fiber containing muscles, and hindleg glucose uptake, but decreased R'g in the Type I (slow twitch oxidative) fiber muscle. AICAR also decreased GIR due to inhibition of insulin-mediated suppression of hepatic glucose output. AICAR augmented insulin-mediated microvascular perfusion.

CONCLUSIONS

AICAR, at levels that have no direct effect on muscle glucose uptake, augments insulin-mediated microvascular blood flow and glucose uptake in white fiber type muscles. Agents targeted to endothelial AMPK activation are promising insulin sensitizers, however, the decrease in GIR and the propensity to increase blood lactate cautions against AICAR as an acute insulin sensitizer.

Electroacupuncture inhibits weight gain in diet-induced obese rats by activating hypothalamic LKB1-AMPK signaling

Background

Electroacupuncture (EA) is reported to be an effective treatment for obesity, but its mechanism is unclear. This study aimed to investigate the relationship between hypothalamic LKB1-AMPK-ACC signaling and EA.

Methods

Fifty male Sprague–Dawley rats were divided into two groups fed either chow (chow-fed group) or high-fat diet (HF group). After 4 weeks of feeding, obese rats in the HF group (defined as weighing 20 % or more than rats in the chow-fed group) were randomly allocated into an EA or Diet-induced obesity (DIO) group. The EA group was given EA on bilateral ST25–ST36 for 4 weeks, while the DIO group received no further intervention. Body weight of the chow-fed, DIO, and EA groups were measured weekly. mRNA and protein levels of the hypothalamic LKB1-AMPK-ACC signaling pathway were detected using real-time (RT)-PCR and western blot, respectively.

Results

After 4 weeks of EA treatment, the weight growth trend of rats in the EA group was inhibited compared with those in the DIO group. RT-PCR and western blotting showed that EA upregulated the transcription of Adenosine 5′-monophosphate -activated protein kinase α2 (AMPKα2), promoted protein expression of Liver kinase B1 (LKB1) and AMPKα1, and inhibited acetyl-CoA carboxylase (ACC) protein expression in the hypothalamus.

Conclusions

This study suggests that hypothalamic LKB1-AMPK-ACC signaling plays an important role in EA treatment for obesity.