Emerging role of AMP-activated protein kinase in endocrine control of metabolism in the liver

Accurate Quantification of Ten Methylated Purine Nucleosides by Highly Sensitive and Stable Isotope-Diluted UHPLC-MS/MS

The dynamic landscape of cellular nucleotides/nucleosides associated with RNA metabolism, particularly in diseases like cancer, has spurred intensive interest. Here, we report a robust stable isotope-diluted UHPLC-ESI-MS/MS method for accurate quantification of 12 purine ribonucleosides, including 10 methylated purine nucleosides. By the use of thermally decomposable ammonium bicarbonate (NH4HCO3) as a mobile phase additive for UHPLC-MS/MS detection, the ESI-MS/MS signal responses of these target compounds were enhanced by 1.7-24.5 folds. Noteworthily, three methylated guanosine isomers (m1G, m2G, and m7G) and two methylated adenosine isomers (m1A and m6A) that are indistinguishable directly by mass spectrometry were well resolved with optimal UHPLC separation. Combined with methanol extraction and solid-phase extraction (SPE) pretreatment, the method quantified intracellular concentrations of three modified nucleosides (Gm, m1G, and m2G), which would otherwise be undetectable because of significant suppression of their signals by the interfering cellular matrix. Nine purine nucleosides were simultaneously quantified in 293T cells, and their concentrations ranged by 4 orders of magnitude. Overall, the method presents high recovery rates over 90% for endogenous modified purine nucleosides in cultured cells, along with good precision, linearity, and LOD ranging from 0.30 fmol to 0.37 pmol per 5 × 105 cells. The developed UHPLC-MS/MS method holds potential for screening purine nucleosides as diagnostic and prognostic biomarkers and for quantifying purine epigenetic nucleosides post-cell metabolome analysis, thereby providing a valuable analytical tool for intracellular nucleoside quantification in future clinical research.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly simulates hepatic glucose production (HGP), but for reasons which are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course and relevance (to metabolic flux) of glucagon mediated molecular events in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a 6-fold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group glucose remained at basal while in the other glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) but only partially sustained increase in hepatic cAMP over 4h, a continued elevation in G6P, and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis and HGP increased rapidly, peaking at 30 min, then returned to baseline over the next 3h (although glucagons stimulatory effect on HGP was sustained relative to the hyperglycemic control group). Hepatic gluconeogenic flux did not increase due to lack of glucagon effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, and downregulation of genes involved in extracellular matrix assembly and development.

AMPK and the Endocrine Control of Metabolism

Complex multicellular organisms require a coordinated response from multiple tissues to maintain whole-body homeostasis in the face of energetic stressors such as fasting, cold, and exercise. It is also essential that energy is stored efficiently with feeding and the chronic nutrient surplus that occurs with obesity. Mammals have adapted several endocrine signals that regulate metabolism in response to changes in nutrient availability and energy demand. These include hormones altered by fasting and refeeding including insulin, glucagon, glucagon-like peptide-1, catecholamines, ghrelin, and fibroblast growth factor 21; adipokines such as leptin and adiponectin; cell stress-induced cytokines like tumor necrosis factor alpha and growth differentiating factor 15, and lastly exerkines such as interleukin-6 and irisin. Over the last 2 decades, it has become apparent that many of these endocrine factors control metabolism by regulating the activity of the AMPK (adenosine monophosphate-activated protein kinase). AMPK is a master regulator of nutrient homeostasis, phosphorylating over 100 distinct substrates that are critical for controlling autophagy, carbohydrate, fatty acid, cholesterol, and protein metabolism. In this review, we discuss how AMPK integrates endocrine signals to maintain energy balance in response to diverse homeostatic challenges. We also present some considerations with respect to experimental design which should enhance reproducibility and the fidelity of the conclusions.

AMPK inhibits liver gluconeogenesis: fact or fiction?

Is there a role for AMPK in the control of hepatic gluconeogenesis and could targeting AMPK in liver be a viable strategy for treating type 2 diabetes? These are frequently asked questions this review tries to answer. After describing properties of AMPK and different small-molecule AMPK activators, we briefly review the various mechanisms for controlling hepatic glucose production, mainly via gluconeogenesis. The different experimental and genetic models that have been used to draw conclusions about the role of AMPK in the control of liver gluconeogenesis are critically discussed. The effects of several anti-diabetic drugs, particularly metformin, on hepatic gluconeogenesis are also considered. We conclude that the main effect of AMPK activation pertinent to the control of hepatic gluconeogenesis is to antagonize glucagon signalling in the short-term and, in the long-term, to improve insulin sensitivity by reducing hepatic lipid content.

The Regulatory Roles of Polysaccharides and Ferroptosis-Related Phytochemicals in Liver Diseases

Liver disease is a global health burden with high morbidity and mortality worldwide. Liver injuries can develop into severe end-stage diseases, such as cirrhosis or hepatocellular carcinoma, without valid treatment. Therefore, identifying novel drugs may promote liver disease treatment. Phytochemicals, including polysaccharides, flavonoids, alkaloids, and terpenes, are abundant in foods and medicinal plants and have various bioactivities, such as antioxidation, immunoregulation, and tumor killing. Recent studies have shown that many natural polysaccharides play protective roles in liver disease models in vitro and in vivo, such as fatty liver disease, alcoholic liver disease, drug-induced liver injury, and liver cancer. The mechanisms of liver disease are complex. Notably, ferroptosis, a new type of cell death driven by iron and lipid peroxidation, is considered to be the key mechanism in many hepatic pathologies. Therefore, polysaccharides and other types of phytochemicals with activities in ferroptosis regulation provide novel therapeutic strategies for ferroptosis-related liver diseases. This review summarizes our current understanding of the mechanisms of ferroptosis and liver injury and compelling preclinical evidence of natural bioactive polysaccharides and phytochemicals in treating liver disease.

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

Temperature, heat shock proteins and growth regulation of the bone tissue

Ambient heat modulates the elongation of bones in mammals, and the mechanism of such a plasticity has not been studied completely. The influence of heat on growth and development of bone depends on its values. Five zones of temperature influence on the bone tissue with different biological effects have been distinguished : a) under-threshold thermal zone < 36.6 ºС, insufficient amount of heat is a limiting factor for osteogenesis; b) normal temperature zone 36.6‒37.5 ºС, the processes of breakdown and development of bone in this temperature range is balanced; b) zone of mild thermal shock 39‒41 ºС, the processes of functioning of osteoblasts, osteocytes and formation of the bone tissue intensify; d) the zone of sublethal thermal shock > 42 ºС, growth of bone slows; e) zone of non-critical shock > 50 ºС, bone tissue cells die. We propose a model of the mechanism of influence of heat shock on bone growth. Mild heat shock is a type of stress to which membrane enzymes adenylyl cyclase and cAMP-protein kinase react. Protein kinase A phosphorylates the gene factors of thermal shock proteins, stress proteins and enzymes of energy-generating processes – glycolysis and lipolysis. Heat shock protein HSP70 activates alkaline phosphatase and promotes the process of mineralization of the bone tissue. In the cells, there is intensification in syntheses of insulin-like growth factor-I, factors of mitogenic action, signals of intensification of blood circulation (NO) and synthesis of somatotropin. The affinity between insulin-like growth factor I and its acid-labile subunit decreases, leading to increased free and active insulin-like growth factor I. Against the background of acceleration of the capillarization process, energy generation and the level of stimulators of growth of bone tissue, mitotic and functional activities of producer cells of the bone – osteoblasts and osteocytes – activate. The generally known Allen’s rule has been developed and expanded: “Warm-blooded animals of different species have longer distal body parts (tails) if after birth the young have developed in the conditions of higher temperature”. The indicated tendency is realized through increased biosynthesis of heat shock proteins and other stimulators of growth processes in the bone tissue.

The ups and downs of caloric restriction and fasting: from molecular effects to clinical application

Age-associated diseases are rising to pandemic proportions, exposing the need for efficient and low-cost methods to tackle these maladies at symptomatic, behavioral, metabolic, and physiological levels. While nutrition and health are closely intertwined, our limited understanding of how diet precisely influences disease often precludes the medical use of specific dietary interventions. Caloric restriction (CR) has approached clinical application as a powerful, yet simple, dietary modulation that extends both life- and healthspan in model organisms and ameliorates various diseases. However, due to psychological and social-behavioral limitations, CR may be challenging to implement into real life. Thus, CR-mimicking interventions have been developed, including intermittent fasting, time-restricted eating, and macronutrient modulation. Nonetheless, possible side effects of CR and alternatives thereof must be carefully considered. We summarize key concepts and differences in these dietary interventions in humans, discuss their molecular effects, and shed light on advantages and disadvantages.

Protective effects of the imidazoline-like drug lnp599 in a marmoset model of obesity-induced metabolic disorders

BACKGROUND/OBJECTIVES

Overweight and obesity are undoubtable risk factors for type 2 diabetes and cardiovascular diseases and significantly contribute to the global morbi-mortality. We previoulsy reported that LNP599, a pharmacological imidazoline-like activator of hepatic AMPK/adiponectin signaling, protects against the development of adiposity and obesity and the associated cardio-metabolic disorders, suggesting that it may be a suitable drug candidate for a therapeutic approach targeting the development of obesity at very early stages. The objective of the present study was to evaluate the metabolic effects of LNP599 in a model of diet-induced overweight and metabolic disorders in a nonhuman primate, the common marmoset (Callithrix jacchus), and more particularly to establish the impact of the compound on cholesterol homeostasis, i.e., HDL and LDL/VLDL lipoproteins.

METHODS

Marmosets were fed normal (NC) or hypercaloric (HC) chow during 16 weeks. Diet-induced changes in body weight and metabolism were assessed. Effects of LNP599 were evaluated in a subset of HC animals (HC-LNP) receiving the compound at a daily dose of 10 mg/kg over the 16 weeks.

RESULTS

HC-feeding induced significant overweight associated with a marked dyslipidemia (hypertriglyceridemia, hypercholesterolemia, and reduced HDL over LDL/VLDL cholesterol ratio). LNP599 blunted the diet-induced body weight gain and largely protected against the development of hypertriglyceridemia. Total cholesterol was unchanged but the ratio of HDL over LDL/VLDL cholesterol was more than doubled.

CONCLUSIONS

The profile of metabolic troubles obtained upon enriched diet mimicked the disorders associated with spontaneous obesity in marmosets. HC marmosets represent an experimental model of high clinical relevance to study the pathophysiology of obesity and related dyslipidemia and to evaluate the effects of emerging therapies targeting these disorders. Our data confirm the preventing effects of LNP599 in a nonhuman primate model and demonstrate for the first time the high potency of this drug in promoting HDL-cholesterol.

Metformin exerts anti-cancerogenic effects and reverses epithelial-to-mesenchymal transition trait in primary human intrahepatic cholangiocarcinoma cells

Intrahepatic cholangiocarcinoma (iCCA) is a highly aggressive cancer with marked resistance to chemotherapeutics without therapies. The tumour microenvironment of iCCA is enriched of Cancer-Stem-Cells expressing Epithelial-to-Mesenchymal Transition (EMT) traits, being these features associated with aggressiveness and drug resistance. Treatment with the anti-diabetic drug Metformin, has been recently associated with reduced incidence of iCCA. We aimed to evaluate the anti-cancerogenic effects of Metformin in vitro and in vivo on primary cultures of human iCCA. Our results showed that Metformin inhibited cell proliferation and induced dose- and time-dependent apoptosis of iCCA. The migration and invasion of iCCA cells in an extracellular bio-matrix was also significantly reduced upon treatments. Metformin increased the AMPK and FOXO3 and induced phosphorylation of activating FOXO3 in iCCA cells. After 12 days of treatment, a marked decrease of mesenchymal and EMT genes and an increase of epithelial genes were observed. After 2 months of treatment, in order to simulate chronic administration, Cytokeratin-19 positive cells constituted the majority of cell cultures paralleled by decreased Vimentin protein expression. Subcutaneous injection of iCCA cells previously treated with Metformin, in Balb/c-nude mice failed to induce tumour development. In conclusion, Metformin reverts the mesenchymal and EMT traits in iCCA by activating AMPK-FOXO3 related pathways suggesting it might have therapeutic implications.

The Cardiometabolic Health Benefits of Sauna Exposure in Individuals with High-Stress Occupations. A Mechanistic Review

Components of the metabolic syndrome (i.e., hypertension, insulin resistance, obesity, atherosclerosis) are a leading cause of death in the United States and result in low-grade chronic inflammation, excessive oxidative stress, and the eventual development of cardiometabolic diseases (CMD). High-stress occupations (HSO: firefighters, police, military personnel, first responders, etc.) increase the risk of developing CMD because they expose individuals to chronic and multiple stressors (i.e., sleep deprivation, poor nutrition habits, lack of physical activity, psychological stress). Interestingly, heat exposure and, more specifically, sauna bathing have been shown to improve multiple markers of CMD, potentially acting as hormetic stressors, at the cellular level and in the whole organism. Therefore, sauna bathing might be a practical and alternative intervention for disease prevention for individuals with HSO. The purpose of this review is to detail the mechanisms and pathways involved in the response to both acute and chronic sauna bathing and collectively present sauna bathing as a potential treatment, in addition to current standard of care, for mitigating CMD to both clinicians and individuals serving in HSO.

Four sesquiterpene glycosides from loquat (Eriobotrya japonica) leaf ameliorates palmitic acid-induced insulin resistance and lipid accumulation in HepG2 Cells via AMPK signaling pathway

Insulin resistance (IR), caused by impaired insulin signal and decreased insulin sensitivity, is generally responsible for the pathophysiology of type 2 diabetes mellitus (T2DM). Sesquiterpene glycosides (SGs), the exclusive natural products from loquat leaf, have been regarded as potential lead compounds owing to their high efficacy in hypoglycemia and hypolipidemia. Here, we evaluated the beneficial effects of four single SGs isolated from loquat leaf, including SG1, SG2, SG3 and one novel compound SG4 against palmitic acid-induced insulin resistance in HepG2 cells. SG1, SG3 and SG4 could significantly enhance glucose uptake of insulin-resistant HepG2 cells at non-cytotoxic concentration. Meanwhile, Oil Red O staining showed the decrease of both total cholesterol and triglyceride content, suggesting the amelioration of lipid accumulation by SGs in insulin-resistant HepG2 cells. Further investigations found that the expression levels of phosphorylated AMPK, ACC, IRS-1, and Akt were significantly up-regulated after SGs treatment, on the contrary, the expression levels of SREBP-1 and FAS were significantly down-regulated. Notably, AMPK inhibitor Compound C (CC) blocked the regulative effects, while AMPK activator AICAR mimicked the effects of SGs in PA-treated insulin-resistant HepG2 cells. In conclusion, SGs (SG4>SG1≈SG3>SG2) improved lipid accumulation in insulin-resistant HepG2 cells through the AMPK signaling pathway.

Vitamin D - A host directed autophagy mediated therapy for tuberculosis

According to the WHO report 2019, Tuberculosis (TB) is an ancient disease of humanity that is curable. TB has caused significant morbidity and mortality even in 2018. The etiological agent of TB, Mycobacterium tuberculosis (MTB) exploits its virulence factors to escape from host immunity and therapeutic drugs. Host Directed Therapy (HDT) is an adjunctive therapy where repurposed drugs, small molecules, vitamins, cytokines, and monoclonal antibodies are used to overcome the pathogen exploited pathways in the host. One of the HDTs, i.e. induction of autophagy is a highly regulated intracellular self-degradative process in which pathogens are sequestered in double-layered autophagosomes and targeted to the lysosome for degradation. Apart from the pathogen clearance, autophagy involves the release of nutrients during starvation, removal of damaged organelles and aggregated proteins, antigen presentation, tumor suppression, and anti-aging mechanisms. Xenophagy is a type of selective autophagy against microbes induced by ubiquitin receptors (p62/SQSTM1, NDP52, NBR1, OPTN, Parkin and Smurf proteins) after pathogen recognition. ULK1/2, Beclin-1, ATG5-ATG12-ATG16 L and LC-II-PE complexes along with two nutrient-sensing protein complexes, mTOR and AMPK activate autophagy mechanisms to limit infection. Pattern Recognition Receptors (PRRs) such as TLR2, recognize lipopolysaccharide (LPS) of MTB and triggers vitamin D₃ activating enzymes. Activated vitamin D₃ induces the synthesis of antimicrobial peptide, LL-37, which further enhances xenophagy. Apart from vitamin D, few micronutrients such as zinc and iron also regulate autophagy. In this review, we discuss current knowledge, advances and perspectives of autophagy against TB.

Glucose availability but not changes in pancreatic hormones sensitizes hepatic AMPK activity during nutritional transition in rodents

The cellular energy sensor AMP-activated protein kinase (AMPK) is a metabolic regulator that mediates adaptation to nutritional variations to maintain a proper energy balance in cells. We show here that suckling-weaning and fasting-refeeding transitions in rodents are associated with changes in AMPK activation and the cellular energy state in the liver. These nutritional transitions were characterized by a metabolic switch from lipid to glucose utilization, orchestrated by modifications in glucose levels and the glucagon/insulin ratio in the bloodstream. We therefore investigated the respective roles of glucose and pancreatic hormones on AMPK activation in mouse primary hepatocytes. We found that glucose starvation transiently activates AMPK, whereas changes in glucagon and insulin levels had no impact on AMPK. Challenge of hepatocytes with metformin-induced metabolic stress strengthened both AMPK activation and cellular energy depletion under limited-glucose conditions, whereas neither glucagon nor insulin altered AMPK activation. Although both insulin and glucagon induced AMPKα phosphorylation at its Ser^485/491 residue, they did not affect its activity. Finally, the decrease in cellular ATP levels in response to an energy stress was additionally exacerbated under fasting conditions and by AMPK deficiency in hepatocytes, revealing metabolic inflexibility and emphasizing the importance of AMPK for maintaining hepatic energy charge. Our results suggest that nutritional changes (i.e. glucose availability), rather than the related hormonal changes (i.e. the glucagon/insulin ratio), sensitize AMPK activation to the energetic stress induced by the dietary transition during fasting. This effect is critical for preserving the cellular energy state in the liver.

Ginsenoside Rk3 ameliorates high-fat-diet/streptozocin induced type 2 diabetes mellitus in mice via the AMPK/Akt signaling pathway

Ginsenoside Rk3 (G-Rk3) is a main active ingredient of ginsenosides. Several recent studies demonstrated that ginsenosides have potential anti-type 2 diabetes mellitus (T2DM) properties. To evaluate the anti-T2DM effect of G-Rk3 and verify its potential mechanism, a high-fat-diet/streptozocin (HFD/STZ) induced model of T2DM in C57BL/6 mice and a high glucose induced insulin resistance model of HepG2 cells were applied in this research. Our analysis indicated that G-Rk3 reduced HFD/STZ induced hyperglycemia, and serum insulin and inflammation levels, and ameliorated glucose tolerance and insulin resistance, and prevented liver histological changes. Furthermore, it also significantly reduced lipid accumulation as shown by lower TG, LDL-C and TC serum concentrations and Oil Red O staining in liver tissues. The hypoglycemic effect of G-Rk3 seemed to be partially mediated via the inhibition of hepatic gluconeogenesis, which was supported by the activated p-Akt, p-FoxO1 and GLUT2 and inhibited FoxO1, PEPCK and G6pase protein expressions in the liver as well as increased glucose uptake in high glucose induced HepG2 cells. The gene expressions of hepatic gluconeogenesis were also down-regulated by G-Rk3 in HFD/STZ induced T2DM mice. In addition, G-Rk3 suppressed HFD/STZ induced lipid accumulation by regulating related gene and protein expressions such as p-ACC, FAS and SREBP-1, which are the downstream targets of AMPK. AMPK and Akt inhibitors significantly reversed G-Rk3 mediated hepatic gluconeogenesis and lipid accumulation. Thus, our study is the first to illustrate that G-Rk3 mediates hepatic gluconeogenesis and lipid accumulation via activating the AMPK/Akt signaling pathway in HFD/STZ induced T2DM mice.

Targeted Determination of Tissue Energy Status by LC-MS/MS

Intracellular nucleotides and acyl-CoAs are metabolites that are central to the regulation of energy metabolism. They set the cellular energy charge and redox state, act as allosteric regulators, modulate signaling and transcription factors, and thermodynamically activate substrates for oxidation or biosynthesis. Unfortunately, no method exists to simultaneously quantify these biomolecules in tissue extracts. A simple method was developed using ion-pairing reversed-phase high-performance liquid chromatography–electrospray-ionization tandem mass spectrometry (HPLC-ESI-MS/MS) to simultaneously quantify adenine nucleotides (AMP, ADP, and ATP), pyridine dinucleotides (NAD⁺ and NADH), and short-chain acyl-CoAs (acetyl, malonyl, succinyl, and propionyl). Quantitative analysis of these molecules in mouse liver was achieved using stable-isotope-labeled internal standards. The method was extensively validated by determining the linearity, accuracy, repeatability, and assay stability. Biological responsiveness was confirmed in assays of liver tissue with variable durations of ischemia, which had substantial effects on tissue energy charge and redox state. We conclude that the method provides a simple, fast, and reliable approach to the simultaneous analysis of nucleotides and short-chain acyl-CoAs.

Effects of imidazoline-like drugs on liver and adipose tissues, and their role in preventing obesity and associated cardio-metabolic disorders

BACKGROUND/OBJECTIVES

We previously observed that selective agonists of the sympatho-inhibitory I₁ imidazoline receptors (LNP ligands) have favorable effects on several cardiovascular and metabolic disorders defining the metabolic syndrome, including body weight. The objectives of this study were to explore the effects of LNPs on adiposity and the mechanisms involved, and to evaluate their impact on metabolic homeostasis.

METHODS

Young Zucker fa/fa rats were treated with LNP599 (10 mg/kg/day) for 12 weeks. Effects on body weight, adiposity (regional re-distribution, morphology, and function of adipose tissues), cardiovascular and metabolic homeostasis, and liver function were evaluated. Direct effects on insulin and AMP-activated protein kinase (AMPK) signaling were studied in human hepatoma HepG2 cells.

RESULTS

LNP599 treatment limited the age-dependent remodeling and inflammation of subcutaneous, epididymal, and visceral adipose tissues, and prevented total fat deposits and the development of obesity. Body-weight stabilization was not related to reduced food intake but rather to enhanced energy expenditure and thermogenesis. Cardiovascular and metabolic parameters were also improved and were significantly correlated with body weight but not with plasma norepinephrine. Insulin and AMPK signaling were enhanced in hepatic tissues of treated animals, whereas blood markers of hepatic disease and pro-inflammatory cytokine levels were reduced. In cultured HepG2 cells, LNP ligands phosphorylated AMPK and the downstream acetyl-CoA carboxylase and prevented oleic acid-induced intracellular lipid accumulation. They also significantly potentiated insulin-mediated AKT activation and this was independent from AMPK.

CONCLUSIONS

Selective I₁ imidazoline receptor agonists protect against the development of adiposity and obesity, and the associated cardio-metabolic disorders. Activation of I₁ receptors in the liver, leading to stimulation of the cellular energy sensor AMPK and insulin sensitization, and in adipose tissues, leading to improvement of morphology and function, are identified as peripheral mechanisms involved in the beneficial actions of these ligands.

Hydralazine protects against renal ischemia-reperfusion injury in rats

In this study, we investigated whether hydralazine could reduce renal ischemia and reperfusion (I/R) injury in rats. Renal I/R was induced by a 70-min occlusion of the bilateral renal arteries and a 24-h reperfusion, which was confirmed by the increased the mortality, the levels of blood urea nitrogen (BUN), blood creatinine (Cr), renal tissue NO and the visible histological damage of the kidneys. Apoptosis was evaluated by terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) staining. Furthermore, the serum levels of malonaldehyde (MDA), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) were significantly elevated in renal I/R group, while the superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) levels were suppressed. However, intragastric pretreatment with hydralazine at doses of 7.5-30 mg/kg before renal I/R significantly limited the increase in mortality, BUN, Cr, oxidative stress, inflammatory factors, histological damage and apoptosis in the kidneys. In addition, hydralazine also increased p-AKT, Bcl-2 expression and decreased iNOS, Bax, cleaved caspase-3 expression in the kidneys. In conclusion, hydralazine reduced renal I/R injury probably via inhibiting NO production by iNOS/NO pathway, inhibiting oxidative stress, inflammatory response and apoptosis by a mitochondrial-dependent pathway.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

PCB126 Inhibits the Activation of AMPK-CREB Signal Transduction Required for Energy Sensing in Liver

3,3',4,4',5-pentachlorobiphenyl (PCB126), a dioxin-like PCB, elicits toxicity through a wide array of noncarcinogenic effects, including metabolic syndrome, wasting, and nonalcoholic fatty-liver disease. Previously, we reported decreases in the transcription of several enzymes involved in gluconeogenesis, before the early onset of lipid accumulation. Hence, this study was aimed at understanding the impact of resultant decreases gluconeogenic enzymes on growth, weight, and metabolism in the liver, upon extended exposure. Male Sprague Dawley rats (75-100 g), fed a defined AIN-93G diet, were injected (ip) with single dose of soy oil (5 ml/kg body weight; n = 14) or PCB126 (5 µmol/kg; n = 15), 28 days, prior euthanasia. A subset of rats from each group were fasted for 12 h (vehicle [n = 6] and PCB126 [n = 4]). Rats only showed significant weight loss between days 14 and 28 (p < .05) and some mortality (p = .0413). As in our previous studies, the expression levels of enzymes involved in gluconeogenesis (Pepck-c, G6Pase, Sds, Pc, and Ldh-A) and glycogenolysis (Pygl) were strongly downregulated. The decreased expression of these enzymes in PCB126-treated rats after a 12 h fast decreased hepatic glucose production from glycogen and gluconeogenic substrates, exacerbating the hypoglycemia. Additionally, PCB126 caused hepatic steatosis and decreased the expression of the transcription factor Pparα and its targets, necessary for fatty-acid oxidation. The observed metabolic disruption across multiple branches of fasting metabolism resulted from inhibition in the activation of enzyme AMPK and transcription factor CREB signaling, necessary for "sensing" energy-deprivation and the induction of enzymes that respond to the PCB126 triggered fuel crisis in liver.

Follicle-stimulating hormone enhances hepatic gluconeogenesis by GRK2-mediated AMPK hyperphosphorylation at Ser485 in mice

AIMS/HYPOTHESIS

Increased serum follicle-stimulating hormone (FSH) is correlated with fasting hyperglycaemia. However, the underlying mechanism remains unclear. Because excessive hepatic gluconeogenesis is a major cause of fasting hyperglycaemia the present study investigated whether FSH increases hepatic gluconeogenesis in mice.

METHODS

Ovariectomised mice supplemented with oestradiol (E2) to maintain normal levels of serum E2 (OVX+E2 mice) were injected with low or high doses of FSH. We knocked out Crtc2, a crucial factor in gluconeogenesis, and Fshr to discern their involvement in FSH signalling. To evaluate the role of the G-protein-coupled receptor (GPCR) kinase 2 (GRK2), which could affect glucose metabolism and interact directly with non-GPCR components, a specific GRK2 inhibitor was used. The pyruvate tolerance test (PTT), quantification of PEPCK and glucose-6-phosphatase (G6Pase), key enzymes of gluconeogenesis, GRK2 and phosphorylation of AMP-activated protein kinase (AMPK) were examined to evaluate the level of gluconeogenesis in the liver. A nonphosphorylatable mutant of AMPK Ser485 (AMPK S485A) was transfected into HepG2 cells to evaluate the role of AMPK Ser485 phosphorylation.

RESULTS

FSH increased fasting glucose (OVX+E2+high-dose FSH 8.18 ± 0.60 mmol/l vs OVX+E2 6.23 ± 1.33 mmol/l), the PTT results, and the transcription of Pepck (also known as Pck1; 2.0-fold increase) and G6pase (also known as G6pc; 2.5-fold increase) in OVX+E2 mice. FSH also enhanced the promoter luciferase activities of the two enzymes in HepG2 cells. FSH promoted the membrane translocation of GRK2, which is associated with increased AMPK Ser485 and decreased AMPK Thr172 phosphorylation, and enhanced the nuclear translocation of cyclic AMP-regulated transcriptional coactivator 2 (CRTC2). GRK2 could bind with AMPK and induce Ser485 hyperphosphorylation. Furthermore, either the GRK2 inhibitor or AMPK S485A blocked FSH-regulated AMPK Thr172 dephosphorylation and gluconeogenesis. Additionally, the deletion of Crtc2 or Fshr abolished the function of FSH in OVX+E2 mice.

CONCLUSIONS/INTERPRETATION

The results indicate that FSH enhances CRTC2-mediated gluconeogenesis dependent on AMPK Ser485 phosphorylation via GRK2 in the liver, suggesting an essential role of FSH in the pathogenesis of fasting hyperglycaemia.

Anti-tumoral effects of exercise on hepatocellular carcinoma growth

Regular physical exercise has many beneficial effects, including antitumor properties, and is associated with a reduced risk of developing hepatocellular carcinoma (HCC). Less is known about the impact of exercise on HCC growth and progression. Here, we investigated the effects of exercise on HCC progression and assessed whether any beneficial effects would be evident under sorafenib treatment and could be mimicked by metformin. American Cancer Institute rats with orthotopic syngeneic HCC derived from Morris Hepatoma‐3924A cells were randomly assigned to exercise (Exe) and sedentary groups, or sorafenib±Exe groups or sorafenib±metformin groups. The Exe groups ran on a motorized treadmill for 60 minutes/day, 5 days/week for 4 weeks. Tumor viable area was decreased by exercise, while cell proliferation and vascular density were reduced. Exercise increased the expression of phosphatase and tensin homolog deleted from chromosome 10 and increased the phosphorylation of adenosine monophosphate‐activated protein kinase, while the phosphorylation of protein kinase B, S6 ribosomal protein, and signal transducer and activator of transcription 3 were decreased. Transcriptomic analysis suggested major effects of exercise were on nontumoral liver rather than tumor tissue. Exercise demonstrated similar effects when combined with sorafenib. Moreover, similar effects were observed in the group treated with sorafenib+metformin, revealing an exercise‐mimicking effect of metformin. Conclusion: Exercise attenuates HCC progression associated with alterations in key signaling pathways, cellular proliferation, tumor vascularization, and necrosis. These beneficial effects are maintained when combined with sorafenib and can be mimicked by metformin. (Hepatology Communications 2018;2:607‐620)

Lactoferrin attenuates high-fat diet-induced hepatic steatosis and lipid metabolic dysfunctions by suppressing hepatic lipogenesis and down-regulating inflammation in C57BL/6J mice

Lactoferrin was reported to exert modulatory effects on lipid metabolism, but the regulatory mechanisms remain unclear.

Formaldehyde alters triglyceride synthesis and very low-density lipoprotein secretion in a time-dependent manner

Formaldehyde is a common indoor air pollutant that is toxic to the liver. This study aimed to investigate the effects of formaldehyde on triglyceride metabolism in human hepatocellular carcinoma cells (HepG2). Cell viability was detected using a MTT (3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide) assay. Following treatment with different concentrations of formaldehyde for 24 and 48h, the intra and extra-hepatocellular triglyceride (TG) content was determined using a chemical-enzymatic method; Western blotting was used to detect the levels of fatty acid synthesis and VLDL-related proteins. Our results showed that cell viability significantly decreased after formaldehyde treatment (0.5-12.5mM, 24/48h). Extracellular TG levels in the hepatocytes increased after formaldehyde treatment at 0.004mM-0.1mM for 24h. SREBP-1c, ACC, FASN, and MTP, CES3 and DGAT1 proteins increased significantly after 24h of formaldehyde treatment. Intracellular TG levels decreased for 48h treatment of formaldehyde. AMPKα increased significantly in all tested groups and p-AMPK increased significantly after 0.1mM formaldehyde treatment for 48h. Our results indicated that short-term formaldehyde exposure balances triglyceride metabolism by promoting hepatocellular TG synthesis and VLDL secretion; Long-term formaldehyde disturbs the TG metabolism balance in the hepatocytes.

Loss of hepatic AMP-activated protein kinase impedes the rate of glycogenolysis but not gluconeogenic fluxes in exercising mice

Pathologies including diabetes and conditions such as exercise place an unusual demand on liver energy metabolism, and this demand induces a state of energy discharge. Hepatic AMP-activated protein kinase (AMPK) has been proposed to inhibit anabolic processes such as gluconeogenesis in response to cellular energy stress. However, both AMPK activation and glucose release from the liver are increased during exercise. Here, we sought to test the role of hepatic AMPK in the regulation of in vivo glucose-producing and citric acid cycle-related fluxes during an acute bout of muscular work. We used ²H/¹³C metabolic flux analysis to quantify intermediary metabolism fluxes in both sedentary and treadmill-running mice. Additionally, liver-specific AMPK α1 and α2 subunit KO and WT mice were utilized. Exercise caused an increase in endogenous glucose production, glycogenolysis, and gluconeogenesis from phosphoenolpyruvate. Citric acid cycle fluxes, pyruvate cycling, anaplerosis, and cataplerosis were also elevated during this exercise. Sedentary nutrient fluxes in the postabsorptive state were comparable for the WT and KO mice. However, the increment in the endogenous rate of glucose appearance during exercise was blunted in the KO mice because of a diminished glycogenolytic flux. This lower rate of glycogenolysis was associated with lower hepatic glycogen content before the onset of exercise and prompted a reduction in arterial glucose during exercise. These results indicate that liver AMPKα1α2 is required for maintaining glucose homeostasis during an acute bout of exercise.

Glucagon's effect on liver protein metabolism in vivo

The postprandial state is characterized by a storage of nutrients in the liver, muscle, and adipose tissue for later utilization. In the case of a protein-rich meal, amino acids (AA) stimulate glucagon secretion by the α-cell. The aim of the present study was to determine the impact of the rise in glucagon on AA metabolism, particularly in the liver. We used a conscious catheterized dog model to recreate a postprandial condition using a pancreatic clamp. Portal infusions of glucose, AA, and insulin were used to achieve postprandial levels, while portal glucagon infusion was either maintained at the basal level or increased by three-fold. The high glucagon infusion reduced the increase in arterial AA concentrations compared with the basal glucagon level (-23%, P < 0.05). In the presence of high glucagon, liver AA metabolism shifted toward a more catabolic state with less protein synthesis (-36%) and increased urea production (+52%). Net hepatic glucose uptake was reduced modestly (-35%), and AA were preferentially used in gluconeogenesis, leading to lower glycogen synthesis (-54%). The phosphorylation of AMPK was increased by the high glucagon infusion (+40%), and this could be responsible for increasing the expression of genes related to pathways producing energy and lowering those involved in energy consumption. In conclusion, the rise in glucagon associated with a protein-rich meal promotes a catabolic utilization of AA in the liver, thereby, opposing the storage of AA in proteins.

Effect of curcumin on glycerol-induced acute kidney injury in rats

The aim of this study was to investigate the protective role and underlying mechanisms of curcumin on glycerol-induced acute kidney injury (AKI) in rats. Glycerol (10 ml/kg BW, 50% v/v in sterile saline, i.m.) was used to induce AKI, followed by curcumin (200 mg/kg/day, p.o.) administration for 3 days. To confirm renal damage and the effects of curcumin on AKI, serum BUN, Scr, and CK as well as renal SOD, MDA, GSH-Px were measured. Additionally, morphological changes were identified by H&E staining and transmission electron microscopy. The expression of several factors including chemotactic factor MCP-1, proinflammatory cytokines including TNF-α and IL-6, as well as the kidney injury markers, as Kim-1 and Lipocalin-2 were also assessed using q-PCR. Finally, cell apoptosis in renal tissue was detected using in situ TUNEL apoptosis fluorescence staining and expression of proteins associated with apoptotic, oxidative stress and lipid oxidative related signaling pathways were detected using immunohistochemical staining and western blot. The results showed that curcumin exerts renoprotective effects by inhibiting oxidative stress in rhabdomyolysis-induced AKI through regulation of the AMPK and Nrf2/HO-1 signaling pathways, and also ameliorated RM-associated renal injury and cell apoptosis by activating the PI3K/Akt pathway.

AMPK activation: Role in the signaling pathways of neuroinflammation and neurodegeneration

Adenosine monophosphate-activated protein kinase (AMPK) is an evolutionarily conserved sensor of cellular energy status and has been reported to be involved in chronic inflammatory disorders. AMPK is expressed in immune cells, such as dendritic cells, macrophages, lymphocytes and neutrophils, and is an important regulator of inflammatory responses through the regulation of complex signaling networks in part by inhibiting downstream cascade pathways, such as nuclear factor kB, which is a key regulator of innate immunity and inflammation, as well as acting as a negative regulator of toll-like receptors. Recent data suggest that AMPK dysregulation may participate in neurodegenerative diseases, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and neuropathies. However, there are conflicting reports on the benefits or detrimental effects of AMPK in distinct pathological conditions. This paper offers a review of the recent literature on the pharmacological modulation of the AMPK system as a potential molecular target in the management of neurodegenerative diseases.

Regulation of energy metabolism during social interactions in rainbow trout: a role for AMP-activated protein kinase

Rainbow trout (Oncorhynchus mykiss) confined in pairs form social hierarchies in which subordinate fish typically experience fasting and high circulating cortisol levels, resulting in low growth rates. The present study investigated the role of AMP-activated protein kinase (AMPK) in mediating metabolic adjustments associated with social status in rainbow trout. After 3 days of social interaction, liver AMPK activity was significantly higher in subordinate than dominant or sham (fish handled in the same fashion as paired fish but held individually) trout. Elevated liver AMPK activity in subordinate fish likely reflected a significantly higher ratio of phosphorylated AMPK (phospho-AMPK) to total AMPK protein, which was accompanied by significantly higher AMPKα₁ relative mRNA abundance. Liver ATP and creatine phosphate concentrations in subordinate fish also were elevated, perhaps as a result of AMPK activity. Sham fish that were fasted for 3 days exhibited effects parallel to those of subordinate fish, suggesting that low food intake was an important trigger of elevated AMPK activity in subordinate fish. Effects on white muscle appeared to be influenced by the physical activity associated with social interaction. Overall, muscle AMPK activity was significantly higher in dominant and subordinate than sham fish. The ratio of phospho-AMPK to total AMPK protein in muscle was highest in subordinate fish, while muscle AMPKα₁ relative mRNA abundance was elevated by social dominance. Muscle ATP and creatine phosphate concentrations were high in dominant and subordinate fish at 6 h of interaction and decreased significantly thereafter. Collectively, the findings of the present study support a role for AMPK in mediating liver and white muscle metabolic adjustments associated with social hierarchy formation in rainbow trout.

The sweet tooth of the circadian clock

The endogenous circadian clock is a key regulator of daily metabolic processes. On the other hand, circadian clocks in a broad range of tissues can be tuned by extrinsic and intrinsic metabolic cues. The bidirectional interaction between circadian clocks and metabolism involves both transcriptional and post-translational mechanisms. Nuclear receptors exemplify the transcriptional programs that couple molecular clocks to metabolism. The post-translational modifications of the core clock machinery are known to play a key role in metabolic entrainment of circadian clocks. O-linked N-acetylglucosamine modification (O-GlcNAcylation) of intracellular proteins is a key mediator of metabolic response to nutrient availability. This review highlights our current understanding of the role of protein O-GlcNAcylation in mediating metabolic input and output of the circadian clock.

Dapper1 attenuates hepatic gluconeogenesis and lipogenesis by activating PI3K/Akt signaling

Studies have shown that hepatic insulin resistance, a disorder of glucose and lipid metabolism, plays a vital role in type 2 diabetes (T2D). To clarify the function of Dapper1 in glucose and lipid metabolism in the liver, we investigated the relationships between Dapper1 and adenosine triphosphate (ATP)- and Ca²⁺-mediated activation of PI3K/Akt. We observed a reduction in hepatic Dapper1 in db/db (mice that are homozygous for a spontaneous diabetes mutation) and HFD-induced diabetic mice with T2D. Hepatic overexpression of Dapper1 improved hyperglycemia, insulin resistance, and fatty liver. It also increased Akt (pAkt) signaling and repressed both gluconeogenesis and lipogenesis. Conversely, Ad-shDapper1-induced knockdown of hepatic Dapper1 promoted gluconeogenesis and lipogenesis. Furthermore, Dapper1 activated PI3K p110α/Akt in an insulin-independent manner by inducing ATP production and secretion in vitro. Blockade of P2 ATP receptors, the downstream phospholipase C (PLC), or the inositol triphosphate receptor (IP3R all reduced the Dapper1-induced increase in cytosolic free calcium and Dapper1-mediated PI3K/Akt activation, as did removal of calcium in the medium. In conclusion, Dapper1 attenuates hepatic gluconeogenesis and lipogenesis in T2D.

Inhibition of expression of the circadian clock gene Period causes metabolic abnormalities including repression of glycometabolism in Bombyx mori cells

Abnormalities in the circadian clock system are known to affect the body’s metabolic functions, though the molecular mechanisms responsible remain uncertain. In this study, we achieved continuous knockdown of B. mori Period (BmPer) gene expression in the B. mori ovary cell line (BmN), and generated a Per-KD B. mori model with developmental disorders including small individual cells and slow growth. We conducted cell metabolomics assays by gas chromatography/liquid chromatography-mass spectrometry and showed that knockdown of BmPer gene expression resulted in significant inhibition of glycometabolism. Amino acids that used glucose metabolites as a source were also down-regulated, while lipid metabolism and nucleotide metabolism were significantly up-regulated. Metabolite correlation analysis showed that pyruvate and lactate were closely related to glycometabolism, as well as to metabolites such as aspartate, alanine, and xanthine in other pathways. Further validation experiments showed that the activities of the key enzymes of glucose metabolism, hexokinase, phosphofructokinase, and citrate synthase, were significantly decreased and transcription of their encoding genes, as well as that of pyruvate kinase, were also significantly down-regulated. We concluded that inhibition of the circadian clock gene BmPer repressed glycometabolism, and may be associated with changes in cellular amino acid metabolism, and in cell growth and development.

Liver AMP-Activated Protein Kinase Is Unnecessary for Gluconeogenesis but Protects Energy State during Nutrient Deprivation

AMPK is an energy sensor that protects cellular energy state by attenuating anabolic and promoting catabolic processes. AMPK signaling is purported to regulate hepatic gluconeogenesis and substrate oxidation; coordination of these processes is vital during nutrient deprivation or pathogenic during overnutrition. Here we directly test hepatic AMPK function in regulating metabolic fluxes that converge to produce glucose and energy in vivo. Flux analysis was applied in mice with a liver-specific deletion of AMPK (L-KO) or floxed control littermates to assess rates of hepatic glucose producing and citric acid cycle (CAC) fluxes. Fluxes were assessed in short and long term fasted mice; the latter condition is a nutrient stressor that increases liver AMP/ATP. The flux circuit connecting anaplerosis with gluconeogenesis from the CAC was unaffected by hepatic AMPK deletion in short and long term fasting. Nevertheless, depletion of hepatic ATP was exacerbated in L-KO mice, corresponding to a relative elevation in citrate synthase flux and accumulation of branched-chain amino acid-related metabolites. L-KO mice also had a physiological reduction in flux from glycogen to G6P. These results demonstrate AMPK is unnecessary for maintaining gluconeogenic flux from the CAC yet is critical for stabilizing liver energy state during nutrient deprivation.

Targeting of AMP-activated protein kinase: prospects for computer-aided drug design

INTRODUCTION

Dysregulation of energy homeostasis has been implicated in a number of human chronic diseases including diabetes, obesity, cancer, and inflammation. Given the functional attributes as a central regulator of energy homeostasis, AMP-activated protein kinase (AMPK) is emerging as a therapeutic target for these diseases, and lines of evidence have highlighted the need for rational and robust screening systems for identifying specific AMPK modulators with a therapeutic potential for preventing and/or curing these diseases. Areas covered: Here, the authors review the recent advances in the understanding of three-dimensional structures of AMPK in relationship with the regulatory mechanisms, potentials of AMPK as a therapeutic target in human chronic diseases, and prospects of computer-based drug design for AMPK. Expert opinion: Accumulating information of AMPK structure has provided us with deep insight into the molecular basis underlying the regulatory mechanisms, and further discloses several structural domains, which can be served for a target site for computer-based drug design. Molecular docking and simulations provides useful information about the binding sites between potent drugs and AMPK as well as a rational screening format to discover isoform-specific AMPK modulators. For these reasons, the authors suggest that computer-aided virtual screening methods hold promise as a rational approach for discovering more specific AMPK modulators.

AMPK-dependent regulation of GLP1 expression in L-like cells

This study examined whether AMPK, an evolutionarily conserved sensor of cellular energy status, determines the production of glucagon-like peptide-1 (GLP1). A negative relation existed between phosphorylation of AMPKα and the expression and secretion of GLP1 during changes in energy status in STC-1 cells, an L-like cell line. High concentration of glucose (25 mmol/L) decreased AMPKα phosphorylation, whereas it stimulated the expression and secretion of GLP1 relative to 5.6 mmol/L glucose. Serum starvation upregulated AMPKα phosphorylation, whereas it reduced GLP1 production significantly. Stimulation of AMPK phosphorylation by AICAR and overexpression of wild-type AMPKα1, constitutively active AMPKα1 plasmids, or AMPKα1 lentivirus particles suppressed proglucagon mRNA and protein contents in STC-1 cells. Inactivation of AMPK by Compound C, AMPKα1 siRNA or kinase-inactive AMPKα1 mutant increased the expression and secretion of GLP1. Our results suggest that AMPKα1 may link energy supply with the production of GLP1 in L-like cells.

Rhodiola crenulata extract regulates hepatic glycogen and lipid metabolism via activation of the AMPK pathway

Background

Metabolic syndrome may lead to many complications, such as nonalcoholic fatty liver disease (NAFLD). A natural and effective therapeutic agent for patients with NAFLD is urgently needed. In a previous study, we showed that Rhodiola crenulata root extract (RCE) regulated hepatic gluconeogenesis through activation of AMPK signaling. However, the manner in which RCE regulates hepatic lipid and glycogen metabolism remains unclear. The current study was conducted to investigate the effects of RCE on hepatic glycogen and lipid metabolism, as well as the mechanisms underlying such effects.

Methods

Human hepatoma HepG2 cells were treated with RCE for 6 h under high glucose conditions, after which glycogen synthesis, lipogenesis, and relative gene expression were examined. In addition, lipogenesis-related genes were investigated in vivo.

Results

RCE significantly increased glycogen synthesis and inhibited lipogenesis, while regulating genes related to these processes, including glycogen synthase kinase 3β (GSK3β), glycogen synthase (GS), fatty acid synthase (FAS), CCAAT/enhancer-binding protein (C/EBP), and sterol regulatory element-binding protein 1c (SREBP-1c). However, the effects caused by RCE were neutralized by compound C, an AMPK antagonist. Further studies showed that expression levels of lipogenic genes decreased at the protein and mRNA levels in the rat liver.

Conclusions

Our results demonstrate that RCE regulates hepatic glycogen and lipid metabolism through the AMPK signaling pathway. These results suggest that RCE is a potential intervention for patients with NAFLD.

The Pathogenesis of Nonalcoholic Fatty Liver Disease: Interplay between Diet, Gut Microbiota, and Genetic Background

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in the world, and it comprises a spectrum of hepatic abnormalities from simple hepatic steatosis to steatohepatitis, fibrosis, cirrhosis, and liver cancer. While the pathogenesis of NAFLD remains incompletely understood, a multihit model has been proposed that accommodates causal factors from a variety of sources, including intestinal and adipose proinflammatory stimuli acting on the liver simultaneously. Prior cellular and molecular studies of patient and animal models have characterized several common pathogenic mechanisms of NAFLD, including proinflammation cytokines, lipotoxicity, oxidative stress, and endoplasmic reticulum stress. In recent years, gut microbiota has gained much attention, and dysbiosis is recognized as a crucial factor in NAFLD. Moreover, several genetic variants have been identified through genome-wide association studies, particularly rs738409 (Ile748Met) in PNPLA3 and rs58542926 (Glu167Lys) in TM6SF2, which are critical risk alleles of the disease. Although a high-fat diet and inactive lifestyles are typical risk factors for NAFLD, the interplay between diet, gut microbiota, and genetic background is believed to be more important in the development and progression of NAFLD. This review summarizes the common pathogenic mechanisms, the gut microbiota relevant mechanisms, and the major genetic variants leading to NAFLD and its progression.

Protective Effects of Curcumin on Renal Oxidative Stress and Lipid Metabolism in a Rat Model of Type 2 Diabetic Nephropathy

PURPOSE

Diabetic nephropathy is a serious complication of type 2 diabetes mellitus, and delaying the development of diabetic nephropathy in patients with diabetes mellitus is very important. In this study, we investigated inflammation, oxidative stress, and lipid metabolism to assess whether curcumin ameliorates diabetic nephropathy.

MATERIALS AND METHODS

Animals were divided into three groups: Long-Evans-Tokushima-Otsuka rats for normal controls, Otsuka-Long-Evans-Tokushima Fatty (OLETF) rats for the diabetic group, and curcumin-treated (100 mg/kg/day) OLETF rats. We measured body and epididymal fat weights, and examined plasma glucose, adiponectin, and lipid profiles at 45 weeks. To confirm renal damage, we measured albumin-creatinine ratio, superoxide dismutase (SOD), and malondialdehyde (MDA) in urine samples. Glomerular basement membrane thickness and slit pore density were evaluated in the renal cortex tissue of rats. Furthermore, we conducted adenosine monophosphate-activated protein kinase (AMPK) signaling and oxidative stress-related nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signaling to investigate mechanisms of lipotoxicity in kidneys.

RESULTS

Curcumin ameliorated albuminuria, pathophysiologic changes on the glomerulus, urinary MDA, and urinary SOD related with elevated Nrf2 signaling, as well as serum lipid-related index and ectopic lipid accumulation through activation of AMPK signaling.

CONCLUSION

Collectively, these findings indicate that curcumin exerts renoprotective effects by inhibiting renal lipid accumulation and oxidative stress through AMPK and Nrf2 signaling pathway.

Unraveling the actions of AMP-activated protein kinase in metabolic diseases: Systemic to molecular insights

AMP-activated protein kinase (AMPK) plays a critical role both in sensing and regulating cellular energy state. In experimental animals, its activation has been shown to reduce the risk of obesity and diabetes-related co-morbidities such as insulin resistance, the metabolic syndrome and atherosclerotic cardiovascular disease. However, in humans, AMPK activation alone often does not completely resolve these conditions. Thus, an improved understanding of AMPK action and regulation in metabolic and other diseases is needed. Herein, we provide a brief description of the enzymatic regulation of AMPK and review its role in maintaining energy homeostasis. We then discuss tissue-specific actions of AMPK that become distorted during such conditions as obesity, type 2 diabetes and certain cancers. Finally, we explore recent findings regarding the interactions of AMPK with mammalian target of rapamycin complex 1 and the lysosome and discuss how changes in these relationships during overnutrition may lead to AMPK dysfunction. A more thorough understanding of AMPK's molecular interactions during diseases of overnutrition may provide key insights for the development of AMPK-based combinatorial treatments for metabolic disease.

AMP-activated protein kinase and its multifaceted regulation of hepatic metabolism

PURPOSE OF REVIEW

The current review summarizes recent advancements in our mechanistic and physiological understanding of the energy sensing AMP-activated protein kinase (AMPK) and its regulation of select aspects of hepatic metabolism.

RECENT FINDINGS

A highly conserved serine/threonine kinase, AMPK governs a multitude of cellular process to activate catabolic and inhibit anabolic pathways. Recent work has provided clarity as to the importance and contribution of the AMPK signaling cascade to various aspects of cellular metabolism, including lipid homeostasis, hepatic glucose production, mitochondrial metabolism, and autophagy.

SUMMARY

With more than 60 confirmed substrates, the physiological significance of AMPK signaling has been difficult to ascertain. The generation of targeted knock-in mutations on key AMPK substrates has begun to shed light on this complex system. Future studies are needed to further decipher the complexity, significance, and potential therapeutic targeting of hepatic AMPK signaling.

The Deep Correlation between Energy Metabolism and Reproduction: A View on the Effects of Nutrition for Women Fertility

In female mammals, mechanisms have been developed, throughout evolution, to integrate environmental, nutritional and hormonal cues in order to guarantee reproduction in favorable energetic conditions and to inhibit it in case of food scarcity. This metabolic strategy could be an advantage in nutritionally poor environments, but nowadays is affecting women's health. The unlimited availability of nutrients, in association with reduced energy expenditure, leads to alterations in many metabolic pathways and to impairments in the finely tuned inter-relation between energy metabolism and reproduction, thereby affecting female fertility. Many energetic states could influence female reproductive health being under- and over-weight, obesity and strenuous physical activity are all conditions that alter the profiles of specific hormones, such as insulin and adipokines, thus impairing women fertility. Furthermore, specific classes of nutrients might affect female fertility by acting on particular signaling pathways. Dietary fatty acids, carbohydrates, proteins and food-associated components (such as endocrine disruptors) have per se physiological activities and their unbalanced intake, both in quantitative and qualitative terms, might impair metabolic homeostasis and fertility in premenopausal women. Even though we are far from identifying a "fertility diet", lifestyle and dietary interventions might represent a promising and invaluable strategy to manage infertility in premenopausal women.

A cellular model to study drug-induced liver injury in nonalcoholic fatty liver disease: Application to acetaminophen

Obesity and nonalcoholic fatty liver disease (NAFLD) can increase susceptibility to hepatotoxicity induced by some xenobiotics including drugs, but the involved mechanisms are poorly understood. For acetaminophen (APAP), a role of hepatic cytochrome P450 2E1 (CYP2E1) is suspected since the activity of this enzyme is consistently enhanced during NAFLD. The first aim of our study was to set up a cellular model of NAFLD characterized not only by triglyceride accumulation but also by higher CYP2E1 activity. To this end, human HepaRG cells were incubated for one week with stearic acid or oleic acid, in the presence of different concentrations of insulin. Although cellular triglycerides and the expression of lipid-responsive genes were similar with both fatty acids, CYP2E1 activity was significantly increased only by stearic acid. CYP2E1 activity was reduced by insulin and this effect was reproduced in cultured primary human hepatocytes. Next, APAP cytotoxicity was assessed in HepaRG cells with or without lipid accretion and CYP2E1 induction. Experiments with a large range of APAP concentrations showed that the loss of ATP and glutathione was almost always greater in the presence of stearic acid. In cells pretreated with the CYP2E1 inhibitor chlormethiazole, recovery of ATP was significantly higher in the presence of stearate with low (2.5mM) or high (20mM) concentrations of APAP. Levels of APAP-glucuronide were significantly enhanced by insulin. Hence, HepaRG cells can be used as a valuable model of NAFLD to unveil important metabolic and hormonal factors which can increase susceptibility to drug-induced hepatotoxicity.

Mitochondrial metabolism mediates oxidative stress and inflammation in fatty liver

Mitochondria are critical for respiration in all tissues; however, in liver, these organelles also accommodate high-capacity anaplerotic/cataplerotic pathways that are essential to gluconeogenesis and other biosynthetic activities. During nonalcoholic fatty liver disease (NAFLD), mitochondria also produce ROS that damage hepatocytes, trigger inflammation, and contribute to insulin resistance. Here, we provide several lines of evidence indicating that induction of biosynthesis through hepatic anaplerotic/cataplerotic pathways is energetically backed by elevated oxidative metabolism and hence contributes to oxidative stress and inflammation during NAFLD. First, in murine livers, elevation of fatty acid delivery not only induced oxidative metabolism, but also amplified anaplerosis/cataplerosis and caused a proportional rise in oxidative stress and inflammation. Second, loss of anaplerosis/cataplerosis via genetic knockdown of phosphoenolpyruvate carboxykinase 1 (Pck1) prevented fatty acid-induced rise in oxidative flux, oxidative stress, and inflammation. Flux appeared to be regulated by redox state, energy charge, and metabolite concentration, which may also amplify antioxidant pathways. Third, preventing elevated oxidative metabolism with metformin also normalized hepatic anaplerosis/cataplerosis and reduced markers of inflammation. Finally, independent histological grades in human NAFLD biopsies were proportional to oxidative flux. Thus, hepatic oxidative stress and inflammation are associated with elevated oxidative metabolism during an obesogenic diet, and this link may be provoked by increased work through anabolic pathways.

Green Tea Extract Rich in Epigallocatechin-3-Gallate Prevents Fatty Liver by AMPK Activation via LKB1 in Mice Fed a High-Fat Diet

Supplementation with epigallocatechin-3-gallate has been determined to aid in the prevention of obesity. Decaffeinated green tea extract appears to restore a normal hepatic metabolic profile and attenuate high-fat diet (HFD)-induced effects, thereby preventing non-alcoholic fatty liver disease in mice. Mice were maintained on either a control diet (CD) or HFD for 16 weeks and supplemented with either water or green tea extract (50 mg/kg/day). The body mass increase, serum adiponectin level, and lipid profile were measured over the course of the treatment. Furthermore, the AMPK pathway protein expression in the liver was measured. From the fourth week, the weight gain in the CD + green tea extract (CE) group was lower than that in the CD + water (CW) group. From the eighth week, the weight gain in the HFD + water (HFW) group was found to be higher than that in the CW group. Moreover, the weight gain in the HFD + green tea extract (HFE) group was found to be lower than that in the HFW group. Carcass lipid content was found to be higher in the HFW group than that in the CW and HFE groups. Serum analysis showed reduced non-esterified fatty acid level in the CE and HFE groups as compared with their corresponding placebo groups. Increased adiponectin level was observed in the same groups. Increased VLDL-TG secretion was observed in the HFW group as compared with the CW and HFE groups. Increased protein expression of AdipoR2, SIRT1, pLKB1, and pAMPK was observed in the HFE group, which explained the reduced expression of ACC, FAS, SREBP-1, and ChREBP in this group. These results indicate that the effects of decaffeinated green tea extract may be related to the activation of AMPK via LKB1 in the liver of HFD-fed mice.

Distinct Time Course of the Decrease in Hepatic AMP-Activated Protein Kinase and Akt Phosphorylation in Mice Fed a High Fat Diet

AMP-activated protein kinase (AMPK) plays an important role in insulin resistance, which is characterized by the impairment of the insulin-Akt signaling pathway. However, the time course of the decrease in AMPK and Akt phosphorylation in the liver during the development of obesity and insulin resistance caused by feeding a high fat diet (HFD) remains controversial. Moreover, it is unclear whether the impairment of AMPK and Akt signaling pathways is reversible when changing from a HFD to a standard diet (SD). Male ddY mice were fed the SD or HFD for 3 to 28 days, or fed the HFD for 14 days, followed by the SD for 14 days. We examined the time course of the expression and phosphorylation levels of AMPK and Akt in the liver by immunoblotting. After 3 days of feeding on the HFD, mice gained body weight, resulting in an increased oil red O staining, indicative of hepatic lipid accumulation, and significantly decreased AMPK phosphorylation, in comparison with mice fed the SD. After 14 days on the HFD, systemic insulin resistance occurred and Akt phosphorylation significantly decreased. Subsequently, a change from the HFD to SD for 3 days, after 14 days on the HFD, ameliorated the impairment of AMPK and Akt phosphorylation and systemic insulin resistance. Our findings indicate that AMPK phosphorylation decreases early upon feeding a HFD and emphasizes the importance of prompt lifestyle modification for decreasing the risk of developing diabetes.

Imidazoline-like drugs improve insulin sensitivity through peripheral stimulation of adiponectin and AMPK pathways in a rat model of glucose intolerance

Altered adiponectin signaling and chronic sympathetic hyperactivity have both been proposed as key factors in the pathogenesis of metabolic syndrome. We recently reported that activation of I1 imidazoline receptors (I1R) improves several symptoms of the metabolic syndrome through sympathoinhibition and increases adiponectin plasma levels in a rat model of metabolic syndrome (Fellmann L, Regnault V, Greney H, et al. J Pharmacol Exp Ther 346: 370-380, 2013). The present study was designed to explore the peripheral component of the beneficial actions of I1R ligands (i.e., sympathoinhibitory independent effects). Aged rats displaying insulin resistance and glucose intolerance were treated with LNP509, a peripherally acting I1R agonist. Glucose tolerance, insulin sensitivity, and adiponectin signaling were assessed at the end of the treatment. Direct actions of the ligand on hepatocyte and adipocyte signaling were also studied. LNP509 reduced the area under the curve of the intravenous glucose tolerance test and enhanced insulin hypoglycemic action and intracellular signaling (Akt phosphorylation), indicating improved glucose tolerance and insulin sensitivity. LNP509 stimulated adiponectin secretion acting at I1R on adipocytes, resulting in increased plasma levels of adiponectin; it also enhanced AMPK phosphorylation in hepatic tissues. Additionally, I1R activation on hepatocytes directly enhanced AMPK phosphorylation. To conclude, I1R ligands can improve insulin sensitivity acting peripherally, independently of sympathoinhibition; stimulation of adiponectin and AMPK pathways at insulin target tissues may account for this effect. This may open a promising new way for the treatment of the metabolic syndrome.

Analysis of aquaporin expression in liver with a focus on hepatocytes

A deeper understanding of aquaporins (AQPs) expression and transcriptional regulation will provide useful information for liver pathophysiology. We established a complete AQPs mRNA expression profile in human and mouse liver, as well as protein localization of expressed AQPs. Additionally, the modulation of AQPs mRNA levels in response to various agents was determined in human HuH7 cells and in primary culture of mouse hepatocytes. AQP1, AQP3, AQP7, AQP8, and AQP9 mRNA and protein expressions were detected in human liver, while only AQP6 and AQP11 mRNAs were detected. We reported for the first time the localization of AQP3 in Kupffer cells, AQP7 in hepatocytes and endothelial cells, and AQP9 in cholangiocytes. In addition, we confirmed the localization of AQP1 in endothelial cells, and of AQP8 and AQP9 in hepatocytes. On HuH7 cells, we reported the presence of AQP4 mRNA, confirmed the presence of AQP3, AQP7, and AQP11 mRNAs, but not of AQP8 mRNA. On primary culture of murine hepatocytes, AQP1 and AQP7 mRNAs were identified, while the presence of AQP3, AQP8, AQP9, and AQP11 mRNAs was confirmed. At the protein level, murine endothelial liver cells expressed AQP1 and AQP9, while hepatocytes expressed AQP3, AQP7, AQP8, and AQP9, and macrophages expressed AQP3. Dexamethasone, forskolin, AICAR, rosiglitazone, octanoylated, and non-octanoylated ghrelin regulated some AQP expression in primary culture of murine hepatocytes and human HuH7 cells. Additional studies will be required to further assess the role of AQPs expression in human and murine liver and understand the transcriptional regulation of AQPs in hepatocytes under pathophysiological conditions.