Role of AMP-activated protein kinase in the regulation by glucose of islet beta cell gene expression

Opposing effects on regulated insulin secretion of acute vs chronic stimulation of AMP-activated protein kinase

AIMS/HYPOTHESIS

Although targeted in extrapancreatic tissues by several drugs used to treat type 2 diabetes, the role of AMP-activated protein kinase (AMPK) in the control of insulin secretion is still debatable. Previous studies have used pharmacological activators of limited selectivity and specificity, and none has examined in primary pancreatic beta cells the actions of the latest generation of highly potent and specific activators that act via the allosteric drug and metabolite (ADaM) site.

METHODS

AMPK was activated acutely in islets isolated from C57BL6/J mice, and in an EndoC-βH3 cell line, using three structurally distinct ADaM site activators (991, PF-06409577 and RA089), with varying selectivity for β1- vs β2-containing complexes. Mouse lines expressing a gain-of-function mutation in the γ1 AMPK subunit (D316a) were generated to examine the effects of chronic AMPK stimulation in the whole body, or selectively in the beta cell.

RESULTS

Acute (1.5 h) treatment of wild-type mouse islets with 991, PF-06409577 or RA089 robustly stimulated insulin secretion at high glucose concentrations (p<0.01, p<0.05 and p<0.001, respectively), despite a lowering of glucose-induced intracellular free Ca2+ dynamics in response to 991 (AUC, p<0.05) and to RA089 at the highest dose (25 μmol/l) at 5.59 min (p<0.05). Although abolished in the absence of AMPK, the effects of 991 were observed in the absence of the upstream kinase, liver kinase B1, further implicating 'amplifying' pathways. In marked contrast, chronic activation of AMPK, either globally or selectively in the beta cell, achieved using a gain-of-function mutant, impaired insulin release in vivo (p<0.05 at 15 min following i.p. injection of 3 mmol/l glucose) and in vitro (p<0.01 following incubation of islets with 17 mmol/l glucose), and lowered glucose tolerance (p<0.001).

CONCLUSIONS/INTERPRETATION

AMPK activation exerts complex, time-dependent effects on insulin secretion. These observations should inform the design and future clinical use of AMPK modulators.

The Glucagon-Like Adipokinetic Hormone in Drosophila melanogaster - Biosynthesis and Secretion

Metabolic homeostasis requires the precise regulation of circulating sugar titers. In mammals, homeostatic control of circulating sugar titers requires the coordinated secretion and systemic activities of glucagon and insulin. Metabolic homeostasis is similarly regulated in Drosophila melanogaster through the glucagon-like adipokinetic hormone (AKH) and the Drosophila insulin-like peptides (DILPs). In flies and mammals, glucagon and AKH are biosynthesized in and secreted from specialized endocrine cells. KATP channels borne on these cells respond to fluctuations in circulating glucose titers and thereby regulate glucagon secretion. The influence of glucagon in the pathogenesis of type 2 diabetes mellitus is now recognized, and a crucial mechanism that regulates glucagon secretion was reported nearly a decade ago. Ongoing efforts to develop D. melanogaster models for metabolic syndrome must build upon this seminal work. These efforts make a critical review of AKH physiology timely. This review focuses on AKH biosynthesis and the regulation of glucose-responsive AKH secretion through changes in CC cell electrical activity. Future directions for AKH research in flies are discussed, including the development of models for hyperglucagonemia and epigenetic inheritance of acquired metabolic traits. Many avenues of AKH physiology remain to be explored and thus present great potential for improving the utility of D. melanogaster in metabolic research.

Selenium alleviates the negative effect of heat stress on myogenic differentiation of C2C12 cells with the response of selenogenome

With the globe warming, heat stress (HS) has frequently affected animal production. Selenium (Se) is an essential trace element for animals and exerts most of its biological functions through selenoproteins. We previously demonstrated that the damage to C2C12 cells by HS accompanied with the response of selenoprotein encoding genes and proteins. The objective of this study was to investigate whether selenium supplementation (sodium selenite, SS and selenomethionine, SeMet) could alleviate the negative effect of heat stress on the differentiation of C2C12 cells, and interpret the potential corresponding selenoproteins response. The differentiated cells were cultured for 4 and 8 days under different condition: at 37 °C, 41.5 °C and 41.5 °C with 0.5 μmol Se/L SS or SeMet, and the HSP70, cell apoptosis, selenoproteins and cell differentiation-related gene or protein were detected. The result showed that HS up-regulated (P < 0.05) mRNA and protein levels of HSP70 and gene expression of AMPKα1 and AMPKα2, and down-regulated (P < 0.05) mRNA or protein levels of MYOGENIN and MYOD. Meanwhile, up to 15 and 17 selenoprotein genes expression were significantly changed response to 4-and 8-days HS challenge, respectively. Relative to the HS group, SS and SeMet supplementation down-regulated the mRNA and protein abundance of HSP70 to different degrees, and partly recovered (P < 0.05) the mRNA or protein abundance of MYOGENIN and MYOD at 4th and 8th day. Especially, 16 and 10 selenoprotein genes expression in cells affected by HS were altered by SS and SeMet supplementation, respectively. Both SS and SeMet supplementation modestly increased (P < 0.05) protein levels of GPX1 and SELENON in cells under HS. In summary, Se supplementation partly alleviated the negative impact of HS on myogenic differentiation of C2C12 cells and the process may associate with the alternation of selenoprotein expression pattern, and SeMet exhibits better effect than SS.

AMPK Profiling in Rodent and Human Pancreatic Beta-Cells under Nutrient-Rich Metabolic Stress

Chronic exposure of pancreatic β-cells to elevated nutrient levels impairs their function and potentially induces apoptosis. Like in other cell types, AMPK is activated in β-cells under conditions of nutrient deprivation, while little is known on AMPK responses to metabolic stresses. Here, we first reviewed recent studies on the role of AMPK activation in β-cells. Then, we investigated the expression profile of AMPK pathways in β-cells following metabolic stresses. INS-1E β-cells and human islets were exposed for 3 days to glucose (5.5–25 mM), palmitate or oleate (0.4 mM), and fructose (5.5 mM). Following these treatments, we analyzed transcript levels of INS-1E β-cells by qRT-PCR and of human islets by RNA-Seq; with a special focus on AMPK-associated genes, such as the AMPK catalytic subunits α1 (Prkaa1) and α2 (Prkaa2). AMPKα and pAMPKα were also evaluated at the protein level by immunoblotting. Chronic exposure to the different metabolic stresses, known to alter glucose-stimulated insulin secretion, did not change AMPK expression, either in insulinoma cells or in human islets. Expression profile of the six AMPK subunits was marginally modified by the different diabetogenic conditions. However, the expression of some upstream kinases and downstream AMPK targets, including K-ATP channel subunits, exhibited stress-specific signatures. Interestingly, at the protein level, chronic fructose treatment favored fasting-like phenotype in human islets, as witnessed by AMPK activation. Collectively, previously published and present data indicate that, in the β-cell, AMPK activation might be implicated in the pre-diabetic state, potentially as a protective mechanism.

Glycomacropeptide Prevents Iron/Ascorbate-Induced Oxidative Stress, Inflammation and Insulin Sensitivity with an Impact on Lipoprotein Production in Intestinal Caco-2/15 Cells

Background. Metabolic Syndrome (MetS), a major worldwide concern for the public health system, refers to a cluster of key metabolic components, and represents a risk factor for diabetes and cardiovascular diseases. As oxidative stress (OxS) and inflammation are the major triggers of insulin sensitivity (IS), a cardinal MetS feature, the principal aim of the present work is to determine whether glycomacropeptide (GMP), a milk-derived bioactive peptide, exerts beneficial effects on their expression. Methods. Fully differentiated intestinal Caco-2/15 cells are used to evaluate the preventive action of 2 mg/mL GMP against OxS and inflammation induced by the mixture iron-ascorbate (Fe/Asc) (200 μM:2 mM). The potency of GMP of decreasing the production of lipoproteins, including chylomicrons (CM), very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL) is also assessed. Results. The administration of GMP significantly reduces malondialdehyde, a biomarker of lipid peroxidation, and raises superoxide dismutase 2 and glutathione peroxidase via the induction of the nuclear factor erythroid 2–related factor 2, a transcription factor, which orchestrates cellular antioxidant defenses. Similarly, GMP markedly lowers the inflammatory agents tumor necrosis factor-α and cyclooxygenase-2 via abrogation of the nuclear transcription factor-kB. Moreover, GMP-treated cells show a down-regulation of Fe/Asc-induced mitogen activated protein kinase pathway, suggesting greater IS. Finally, GMP decreases the production of CM, VLDL, and LDL. Conclusions. Our results highlight the effectiveness of GMP in attenuating OxS, inflammation and lipoprotein biogenesis, as well as improving IS, the key components of MetS. Further investigation is needed to elucidate the mechanisms mediating the preventive action of GMP.

AMPK α1 mediates the protective effect of adiponectin against insulin resistance in INS-1 pancreatic β cells

The fat-derived protein adiponectin is known to reverse the effects of insulin resistance and to lower blood glucose levels. The AMP-activated protein kinase (AMPK) signalling pathway plays a central role in metabolism and energy homeostasis. Here, to investigate the role of AMPK in the protective effect of adiponectin against insulin resistance, we established the model of high-glucose (HG)- and high-lipid (HL)-induced insulin resistance in INS-1 pancreatic β cells. We found that 25mM of glucose and 0.4mM of palmitic acid treatment significantly increased cell apoptosis and impaired insulin secretion in INS-1 cells. However, recombinant human adiponectin dramatically reduced HG- and/or HL-induced cell apoptosis and greatly improved insulin secretion. Interestingly, adiponectin treatment also activated AMPK signalling pathway by increasing the phosphorylation of Thr172 in the AMPK α subunit; 10μM of compound C, a potent AMPK inhibitor, blocked the protective effects of adiponectin against HG/HL-induced insulin resistance. Furthermore, knockout experiments by CRISPR/Cas9 technology showed that AMPK α1, but not AMPK α2, is involved in the protective effects of adiponectin. Taken together, adiponectin reversed the effects of insulin resistance via AMPK α1, which provides a novel insight into the protective mechanism of adiponectin and may be used as a new strategy for the treatment of type 2 diabetes. SIGNIFICANCE OF THE STUDY: Adiponectin can reverse the effects of insulin resistance and lower blood glucose levels. Here, adiponectin reduced HG/HL-induced cell apoptosis and greatly improved insulin secretion. These effects were blocked by AMPK inhibitor, compound C. Specifically, we found that AMPK α1, but not AMPK α2, mediates the protective effects of adiponectin, which provides a novel insight into the protective mechanism of adiponectin against insulin resistance.

Manipulation and Measurement of AMPK Activity in Pancreatic Islets

The role of the energy sensor AMPK-activated protein kinase (AMPK) in the insulin-secreting β-cell remains unclear and a subject of intense research. With this chapter, we aim to provide a detailed description of the methods that our group routinely applies to the study of AMPK function in mouse and human pancreatic islets. Thus, we provide detailed protocols to isolate and/or culture mouse and human islets, to modulate and measure AMPK activity in isolated islets, and to evaluate its impact on islet function.

Insulin signaling in LepR cells modulates fat and glucose homeostasis independent of leptin

Hypothalamic neurons detect changes in circulating hormones such as leptin and insulin and put forward outputs to sustain energy and glucose homeostasis. Because leptin and insulin receptors colocalize in ~40-60% of neurons in the hypothalamus, we characterized the metabolic phenotype of mice with selective deletion of the insulin receptor (InsR) in LepR cells. LR^ΔInsR mice presented no difference in body weight and insulin levels but increased fat mass. In the light phase, LR^ΔInsR mice exhibited increased food intake, locomotor activity, carbon dioxide production, and respiratory exchange rate. These mice showed reduced fat oxidation and reduced expression of cluster of differentiation 36 and AMP-activated protein kinase-α₁ in the liver, increased glucose oxidation in the light phase, and overall reduced basal glucose levels. To verify the impact of InsR deletion in LepR cells in obesity, we generated ob/ ob InsR^fl, ob/ ob LR^cre, and ob/ ob LR^ΔInsR mice. The ob/ ob LR^ΔInsR mice had higher body weight, fat mass, and expression of genes related to fat metabolism in the liver. No difference in food intake despite increased neuropeptide Y and agouti-related peptide expression, and no difference in energy expenditure, fat, or glucose oxidation was found in ob/ ob LR^ΔInsR compared with LR^cre or LR^ΔInsR controls. Remarkably, basal glucose levels were reduced, and the expression of genes associated with glucose metabolism in the liver was higher. Insulin signaling in LepR cells is required for the proper fat and glucose oxidation. These effects are independent of leptin given that the leptin-deficient ob/ ob LR^ΔInsR mice also presented reduced glycemia and higher adiposity. The mechanisms underlying these responses remain to be unveiled.

Nutritional Modulation of AMPK-Impact upon Metabolic-Inflammation

Nutritional status provides metabolic substrates to activate AMP-Activated Protein Kinase (AMPK), the energy sensor that regulates metabolism. Recent evidence has demonstrated that AMPK has wider functions with respect to regulating immune cell metabolism and function. One such example is the regulatory role that AMPK has on NLRP3-inlflammasome and IL-1β biology. This in turn can result in subsequent negative downstream effects on glucose, lipid and insulin metabolism. Nutrient stress in the form of obesity can impact AMPK and whole-body metabolism, leading to complications such as type 2 diabetes and cancer risk. There is a lack of data regarding the nature and extent that nutrient status has on AMPK and metabolic-inflammation. However, emerging work elucidates to a direct role of individual nutrients on AMPK and metabolic-inflammation, as a possible means of modulating AMPK activity. The posit being to use such nutritional agents to re-configure metabolic-inflammation towards more oxidative phosphorylation and promote the resolution of inflammation. The complex paradigm will be discussed within the context of if/how dietary components, nutrients including fatty acids and non-nutrient food components, such as resveratrol, berberine, curcumin and the flavonoid genistein, modulate AMPK dependent processes relating to inflammation and metabolism.

Treatment with AICAR inhibits blastocyst development, trophectoderm differentiation and tight junction formation and function in mice

STUDY QUESTION

What is the impact of adenosine monophosphate-activated protein kinase (AMPK) activation on blastocyst formation, gene expression, and tight junction formation and function?

SUMMARY ANSWER

AMPK activity must be tightly controlled for normal preimplantation development and blastocyst formation to occur.

WHAT IS KNOWN ALREADY

AMPK isoforms are detectable in oocytes, cumulus cells and preimplantation embryos. Cultured embryos are subject to many stresses that can activate AMPK.

STUDY DESIGN, SIZE, DURATION

Two primary experiments were carried out to determine the effect of AICAR treatment on embryo development and maintenance of the blastocoel cavity. Embryos were recovered from superovulated mice. First, 2-cell embryos were treated with a concentration series (0-2000 μM) of AICAR for 48 h until blastocyst formation would normally occur. In the second experiment, expanded mouse blastocysts were treated for 9 h with 1000 μM AICAR.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Outcomes measured included development to the blastocyst stage, cell number, blastocyst volume, AMPK phosphorylation, Cdx2 and blastocyst formation gene family expression (mRNAs and protein measured using quantitative RT-PCR, immunoblotting, immunofluorescence), tight junction function (FITC dextran dye uptake assay), and blastocyst ATP levels. The reversibility of AICAR treatment was assessed using Compound C (CC), a well-known inhibitor of AMPK, alone or in combination with AICAR.

MAIN RESULTS AND THE ROLE OF CHANCE

Prolonged treatment with AICAR from the 2-cell stage onward decreases blastocyst formation, reduces total cell number, embryo diameter, leads to loss of trophectoderm cell contacts and membrane zona occludens-1 staining, and increased nuclear condensation. Treatment with CC alone inhibited blastocyst development only at concentrations that are higher than normally used. AICAR treated embryos displayed altered mRNA and protein levels of blastocyst formation genes. Treatment of blastocysts with AICAR for 9 h induced blastocyst collapse, altered blastocyst formation gene expression, increased tight junction permeability and decreased CDX2. Treated blastocysts displayed three phenotypes: those that were unaffected by treatment, those in which treatment was reversible, and those in which effects were irreversible.

LARGE SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

Our study investigates the effects of AICAR treatment on early development. While AICAR does increase AMPK activity and this is demonstrated in our study, AICAR is not a natural regulator of AMPK activity and some outcomes may result from off target non-AMPK AICAR regulated events. To support our results, blastocyst developmental outcomes were confirmed with two other well-known small molecule activators of AMPK, metformin and phenformin.

WIDER IMPLICATIONS OF THE FINDINGS

Metformin, an AMPK activator, is widely used to treat type II diabetes and polycystic ovarian disorder (PCOS). Our results indicate that early embryonic AMPK levels must be tightly regulated to ensure normal preimplantation development. Thus, use of metformin should be carefully considered during preimplantation and early post-embryo transfer phases of fertility treatment cycles.

STUDY FUNDING AND COMPETING INTEREST(S)

Canadian Institutes of Health Research (CIHR) operating funds. There are no competing interests.

Dictyostelium AMPKα regulates aggregate size and cell-type patterning

Starved Dictyostelium cells aggregate into groups of nearly 10⁵ cells. AMPK is a highly conserved serine/threonine protein kinase consisting of a catalytic and two regulatory subunits. As multi-cellular development in Dictyostelium is initiated upon starvation, we explored the role of the energy sensor, AMPK, which shows significant similarity to human AMPK and is expressed throughout development. Deletion of the ampkα gene results in the formation of numerous small-sized aggregates that develop asynchronously to form few fruiting bodies with small sori and long stalks. On the other hand, ampkα^OE cells form fruiting bodies with small stalks and large sori when compared with wild-type, Ax2. A minimum of 5% ampkα⁻ cells in a chimaera with Ax2 cells was sufficient to reduce the aggregate size. Also, the conditioned media collected from ampkα⁻ cells triggered Ax2 cells to form smaller aggregates. The starved ampkα⁻ cells showed low glucose levels and formed large aggregates when glucose was supplied exogenously. Interestingly, ampkα⁻ cells exhibit abnormal cell-type patterning with increased prestalk region and a concomitant reduction of prespore region. In addition, there was a loss of distinct prestalk/prespore boundary in the slugs.

AMPK and Friends: Central Regulators of β Cell Biology

If left unchecked, prediabetic hyperglycemia can progress to diabetes and often life-threatening attendant secondary complications. Central to the process of glucose homeostasis are pancreatic β cells, which sense elevations in plasma glucose and additional dietary components and respond by releasing the appropriate quantity of insulin, ensuring the arrest of hepatic glucose output and glucose uptake in peripheral tissues. Given that β cell failure is associated with the transition from prediabetes to diabetes, improved β cell function ('compensation') has a central role in preventing type 2 diabetes mellitus (T2DM). Recent data have shown that both insulin secretion and β cell mass dynamics are regulated by the liver kinase B1-AMP-activated kinase (LKB1-AMPK) pathway and related kinases of the AMPK family; thus, an improved understanding of the biological roles of AMPK in the β cell is now of considerable interest.

AICAR suppresses TNF-α-induced complement factor B in RPE cells

Age related macular degeneration is the leading cause of blindness in the developed world. Although its precise cause remains elusive, dysfunction of the retinal pigment epithelium (RPE) and dysregulation of complement have been implicated in its pathogenesis. The goal of this study was to evaluate the role of an AMP-dependent kinase (AMPK) activator, 5-aminoimidazole-4-carboxamide riboside (AICAR), on tumor necrosis factor alpha (TNF-α) induction of complement factor B (CFB) in RPE cells. We found that AICAR inhibited TNF-α-induced CFB expression in ARPE-19 and human primary RPE cells in a dose-dependent fashion. Treatment of cells with dipyridamole, which blocks AICAR cellular uptake abolished these effects. In contrast, the adenosine kinase inhibitor, 5-iodotubericidin, which inhibits the conversion of AICAR to the direct activator of AMPK, ZMP, did not reverse the effects on TNF-α-induced CFB expression, suggesting AMPK-independent effects. Indeed, knockout of AMPK in RPE cells using Clustered Regularly Interspaced Palindromic Repeats (CRISPR)/Cas9 did not abolish the inhibitory effects of AICAR on RPE CFB expression. Collectively, our results suggest that AICAR can suppress TNF-α-induced CFB expression in RPE cells in an AMPK-independent mechanism, and could be used as a therapeutic target in certain complement over-activation scenarios.

Phosphorylation of carboxypeptidase B1 protein regulates β-cell proliferation

A reduction in pancreatic islet β-cells leads to the onset of diabetes. Hence, the identification of the mechanisms inducing β-cell proliferation is important for developing a treatment course against the disease. It has been well established that post-translational modifications (PTMs) of proteins affect their functionality. In addition, PTMs have been suggested to play important roles in organ regeneration. Therefore, in this study, we investigated PTMs associated with pancreatic regeneration using two-dimensional electrophoresis. Four carboxypeptidase B1 (CPB1) proteins were identified at different isoelectric points, with the same molecular weight. The motif of CPB1 PTMs was identified by mass spectrophotometry, and the downregulation of CPB1 phosphorylation in pancreatectomy was confirmed. The dephosphorylation of CPB1 induced β-cell proliferation. We thus surmise that the altered PTM of CPB1 is associated with pancreatic regeneration.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Metformin prevents glucotoxicity by alleviating oxidative and ER stress-induced CD36 expression in pancreatic beta cells

AIM/HYPOTHESIS

Cluster determinant 36 (CD36), a fatty acid transporter, was reported to have a pivotal role in glucotoxicity-induced beta cell dysfunction. However, little is known about how glucotoxicity influences CD36 expression, and it is unknown whether this action can be counteracted by metformin. In the present study, we showed that metformin counteracts glucotoxicity by alleviating oxidative and endoplasmic reticulum (ER) stress-induced CD36 expression.

METHODS

We used primary rat islets as well as INS-1 cells for 72h to 24h with 30mM glucose, respectively. Thapsigargin was used as strong ER stressor, and Sulfo-N-succinimidyl oleate (SSO) and RNA interference were chosen for CD36 inhibition. Free fatty acid uptake was measured by radioisotope tracing technique.

RESULTS

Exposure of isolated rat islets to high glucose (HG) for 3days decreased insulin and pancreatic duodenal homeobox1 (Pdx1) mRNA expression, with the suppression of glucose-stimulated insulin secretion (GSIS) along with elevation of reactive oxygen species (ROS) levels. Incubation with metformin restored insulin and Pdx1 mRNA expression with significant improvements in GSIS and decrease in ROS production. HG exposure in INS-1 cells increased free fatty acid uptake via induction of CD36 along with impaired insulin and Pdx1 mRNA expression. Moreover, thapsigargin also increased the induction of CD36 expression. Metformin blocked HG- and thapsigargin-induced CD36 expression. In addition, the simultaneous inhibition of intracellular ROS production by metformin or CD36 activation by SSO or CD36 siRNA significantly decreased the apoptotic response in HG-treated INS-1 cells.

CONCLUSION/INTERPRETATION

In conclusion, metformin conferred protection against HG-induced apoptosis of pancreatic beta cells, largely by interfering with ROS production, and inhibited the CD36-mediated free fatty acid influx. This report provides evidence that the inhibition of CD36 may have potential therapeutic effects against hyperglycemia-induced beta cell damage in diabetes.

The Paradoxical Effects of AMPK on Insulin Gene Expression and Glucose-Induced Insulin Secretion

The activation of AMP-activated protein kinase (AMPK) is known to repress the expression of the insulin gene and glucose-stimulated insulin secretion (GSIS). However, the mechanisms by which this occurs, as well as the effects of AMPK activation on glucolipotoxicity-induced β-cell dysfunction, have not been elucidated. To investigate the effects of 5-amino-4-imidazolecarboxamide ribonucleotide (AICAR) and peroxisome proliferator-activated receptorγ-coactivator-1α (PGC-1α) on β-cell-specific genes under glucolipotoxic conditions, we performed real-time PCR and measured insulin secretion by primary islets. To study these effects in vivo, we administered AICAR for 10 days (1 mg/g body weight) to 90% pancreatectomized hyperglycemic mice. The exposure of isolated rat and human islets to glucolipotoxic conditions and the overexpression of PGC-1α suppressed insulin and NEUROD1 mRNA expression. However, the expression of these genes was preserved by AICAR treatment and by PGC-1α inhibition. Exposure of isolated islets to glucolipotoxic conditions for 3 days decreased GSIS, which was also well maintained by AICAR treatment and by PGC-1α inhibition. The administration of AICAR to 90% pancreatectomized hyperglycemic mice improved glucose tolerance and insulin secretion. These results indicate that treatment of islets with an AMPK agonist under glucolipotoxic conditions protects against glucolipotoxicity-induced β-cell dysfunction. A better understanding of the functions of molecules such as PGC-1α and AMPK, which play key roles in intracellular fuel regulation, could herald a new era for the treatment of patients with type 2 diabetes mellitus by providing protection against glucolipotoxicity.

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.

Cell type-specific deletion in mice reveals roles for PAS kinase in insulin and glucagon production

AIMS/HYPOTHESIS

Per-Arnt-Sim kinase (PASK) is a nutrient-regulated domain-containing protein kinase previously implicated in the control of insulin gene expression and glucagon secretion. Here, we explore the roles of PASK in the control of islet hormone release, by generating mice with selective deletion of the Pask gene in pancreatic beta or alpha cells.

METHODS

Floxed alleles of Pask were produced by homologous recombination and animals bred with mice bearing beta (Ins1 (Cre); PaskBKO) or alpha (Ppg (Cre) [also known as Gcg]; PaskAKO) cell-selective Cre recombinase alleles. Glucose homeostasis and hormone secretion in vivo and in vitro, gene expression and islet cell mass were measured using standard techniques.

RESULTS

Ins1 (Cre)-based recombination led to efficient beta cell-targeted deletion of Pask. Beta cell mass was reduced by 36.5% (p < 0.05) compared with controls in PaskBKO mice, as well as in global Pask-null mice (38%, p < 0.05). PaskBKO mice displayed normal body weight and fasting glycaemia, but slightly impaired glucose tolerance, and beta cell proliferation, after maintenance on a high-fat diet. Whilst glucose tolerance was unaffected in PaskAKO mice, glucose infusion rates were increased, and glucagon secretion tended to be lower, during hypoglycaemic clamps. Although alpha cell mass was increased (21.9%, p < 0.05), glucagon release at low glucose was impaired (p < 0.05) in PaskAKO islets.

CONCLUSIONS/INTERPRETATION

The findings demonstrate cell-autonomous roles for PASK in the control of pancreatic endocrine hormone secretion. Differences between the glycaemic phenotype of global vs cell type-specific null mice suggest important roles for tissue interactions in the control of glycaemia by PASK.

Reduced AMPKα2 protein expression restores glucose-induced insulin secretion in islets from calorie-restricted rats

In a state of caloric restriction (CR), improved insulin action was associated with the activation of AMP-activated kinase (AMPK). Here, we verified whether AMPK was involved in impaired β-cell function in islets from rats subjected to CR for 21 days. Eight-week-old male rats were distributed into a control (CTL) group that was fed an isocaloric diet ad libitum or a CR group that received 60% of the food consumed by the CTL group. From days 18-21, CTL and CR rats were treated with sense (CTLS and CRS) or antisense (CTLAS and CRAS) AMPKα2 oligonucleotides. Caloric restriction was associated with decreased body weight, perigonadal fat pads and insulinaemia, while higher glucose tolerance was observed in CRS rats. Antisense treatment normalized insulinaemia and glucose tolerance in CRAS rats and increased cholesterolaemia in CRAS and CTLAS groups. These effects were associated with reduced pAMPK/AMPK protein expression in the liver of rats treated with antisense oligonucleotides. Additionally, CRS islets showed higher pAMPK/AMPK content and lower glucose-induced insulin release. As expected, antisense oligonucleotides against AMPKα2 efficiently reduced pAMPK/AMPK protein in CRAS and CTLAS islets. The lower AMPK content in CRAS islets normalized the insulin secretion in islets exposed to 16.7 mM glucose. In addition, CTLAS islets presented higher insulin secretion at 2.8 and 16.7 mM glucose. These findings support the hypothesis that higher AMPK protein expression is involved in impaired β-cell function in islets from rats subjected to CR for 21 days.

LKB1 and AMPKα1 are required in pancreatic alpha cells for the normal regulation of glucagon secretion and responses to hypoglycemia

AIMS/HYPOTHESIS

Glucagon release from pancreatic alpha cells is required for normal glucose homoeostasis and is dysregulated in both Type 1 and Type 2 diabetes. The tumour suppressor LKB1 (STK11) and the downstream kinase AMP-activated protein kinase (AMPK), modulate cellular metabolism and growth, and AMPK is an important target of the anti-hyperglycaemic agent metformin. While LKB1 and AMPK have emerged recently as regulators of beta cell mass and insulin secretion, the role of these enzymes in the control of glucagon production in vivo is unclear.

METHODS

Here, we ablated LKB1 (αLKB1KO), or the catalytic alpha subunits of AMPK (αAMPKdKO, -α1KO, -α2KO), selectively in ∼45% of alpha cells in mice by deleting the corresponding flox'd alleles with a preproglucagon promoter (PPG) Cre.

RESULTS

Blood glucose levels in male αLKB1KO mice were lower during intraperitoneal glucose, aminoimidazole carboxamide ribonucleotide (AICAR) or arginine tolerance tests, and glucose infusion rates were increased in hypoglycemic clamps (p < 0.01). αLKB1KO mice also displayed impaired hypoglycemia-induced glucagon release. Glucose infusion rates were also elevated (p < 0.001) in αAMPKα1 null mice, and hypoglycemia-induced plasma glucagon increases tended to be lower (p = 0.06). Glucagon secretion from isolated islets was sensitized to the inhibitory action of glucose in αLKB1KO, αAMPKdKO, and -α1KO, but not -α2KO islets.

CONCLUSIONS/INTERPRETATION

An LKB1-dependent signalling cassette, involving but not restricted to AMPKα1, is required in pancreatic alpha cells for the control of glucagon release by glucose.

Effect of 24 h Fasting on Gene Expression of AMPK, Appetite Regulation Peptides and Lipometabolism Related Factors in the Hypothalamus of Broiler Chicks

The 5’-adenosine monophosphate-activated protein kinase (AMPK) is a key part of a kinase-signaling cascade that acts to maintain energy homeostasis. The objective of this experiment was to investigate the possible effects of fasting and refeeding on the gene expression of hypothalamic AMPK, some appetitive regulating peptides and lipid metabolism related enzymes. Seven-day-old male broiler (Arbor Acres) chicks were allocated into three equal treatments: fed ad libitum (control); fasted for 24 h; fasted for 24 h and then refed for 24 h. Compared with the control, the hypothalamic gene expression of AMPKα2, AMPKβ1, AMPKβ2, AMPKγ1, Ste20-related adaptor protein β (STRADβ), mouse protein 25α (MO25α) and agouti-related peptide (AgRP) were increased after fasting for 24 h. No significant difference among treatments was observed in mRNA levels of AMPKα1, AMPKγ2, LKB1 and neuropeptide Y (NPY). However, the expression of MO25β, pro-opiomelanocortin (POMC), corticotropin-releasing hormone (CRH), ghrelin, fatty acid synthase (FAS), acetyl-CoA carboxylase α (ACCα), carnitine palmitoyltransferase 1 (CPT-1) and sterol regulatory element binding protein-1 (SREBP-1) were significantly decreased. The present results indicated that 24 h fasting altered gene expression of AMPK subunits, appetite regulation peptides and lipometabolism related factors in chick’s hypothalamus; the hypothalamic FAS signaling pathway might be involved in the AMPK regulated energy homeostasis and/or appetite regulation in poultry.

Interactome analysis of AMP-activated protein kinase (AMPK)-α1 and -β1 in INS-1 pancreatic beta-cells by affinity purification-mass spectrometry

The heterotrimeric enzyme AMP-activated protein kinase (AMPK) is a major metabolic factor that regulates the homeostasis of cellular energy. In particular, AMPK mediates the insulin resistance that is associated with type 2 diabetes. Generally, cellular processes require tight regulation of protein kinases, which is effected through their formation of complex with other proteins and substrates. Despite their critical function in regulation and pathogenesis, there are limited data on the interaction of protein kinases. To identify proteins that interact with AMPK, we performed large-scale affinity purification (AP)-mass spectrometry (MS) of the AMPK-α1 and -β1 subunits. Through a comprehensive analysis, using a combination of immunoprecipitaion and ion trap mass spectrometry, we identified 381 unique proteins in the AMPKα/β interactomes: 325 partners of AMPK-α1 and 243 for AMPK-β1. Further, we identified 196 novel protein-protein interactions with AMPK-α1 and AMPK-β1. Notably, in our bioinformatics analysis, the novel interaction partners mediated functions that are related to the regulation of actin organization. Specifically, several such proteins were linked to pancreatic beta-cell functions, including glucose-stimulated insulin secretion, beta-cell development, beta-cell differentiation, and cell-cell communication.

Role of the mammalian target of rapamycin (mTOR) complexes in pancreatic β-cell mass regulation

Exquisite regulation of insulin secretion by pancreatic β-cells is essential to maintain metabolic homeostasis. β-Cell mass must be accordingly adapted to metabolic needs and can be largely modified under different situations. The mammalian target of rapamycin (mTOR) complexes has been consistently identified as key modulators of β-cell mass. mTOR can be found into two different complexes, mTORC1 and mTORC2. Under systemic insulin resistance, mTORC1/mTORC2 signaling in β-cells is needed to increase β-cell mass and insulin secretion. However, type 2 diabetes arises when these compensatory mechanisms fail, being the role of mTOR complexes still obscure in β-cell failure. In this chapter, we introduce the protein composition and regulation of mTOR complexes and their role in pancreatic β-cells. Furthermore, we describe their main signaling effectors through the review of numerous animal models, which indicate the essential role of mTORC1/mTORC2 in pancreatic β-cell mass regulation.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Connexin36 contributes to INS-1E cells survival through modulation of cytokine-induced oxidative stress, ER stress and AMPK activity

Cell-to-cell communication mediated by gap junctions made of Connexin36 (Cx36) contributes to pancreatic β-cell function. We have recently demonstrated that Cx36 also supports β-cell survival by a still unclear mechanism. Using specific Cx36 siRNAs or adenoviral vectors, we now show that Cx36 downregulation promotes apoptosis in INS-1E cells exposed to the pro-inflammatory cytokines (IL-1β, TNF-α and IFN-γ) involved at the onset of type 1 diabetes, whereas Cx36 overexpression protects against this effect. Cx36 overexpression also protects INS-1E cells against endoplasmic reticulum (ER) stress-mediated apoptosis, and alleviates the cytokine-induced production of reactive oxygen species, the depletion of the ER Ca(2+) stores, the CHOP overexpression and the degradation of the anti-apoptotic protein Bcl-2 and Mcl-1. We further show that cytokines activate the AMP-dependent protein kinase (AMPK) in a NO-dependent and ER-stress-dependent manner and that AMPK inhibits Cx36 expression. Altogether, the data suggest that Cx36 is involved in Ca(2+) homeostasis within the ER and that Cx36 expression is downregulated following ER stress and subsequent AMPK activation. As a result, cytokine-induced Cx36 downregulation elicits a positive feedback loop that amplifies ER stress and AMPK activation, leading to further Cx36 downregulation. The data reveal that Cx36 plays a central role in the oxidative stress and ER stress induced by cytokines and the subsequent regulation of AMPK activity, which in turn controls Cx36 expression and mitochondria-dependent apoptosis of insulin-producing cells.

In vivo activating transcription factor 3 silencing ameliorates the AMPK compensatory effects for ER stress-mediated β-cell dysfunction during the progression of type-2 diabetes

In obese Zucker diabetic fatty (ZDF) rats, ER stress is associated with insulin resistance and pancreatic β-cell dysfunction; however the exact mechanisms by which ER stress drives type-2 diabetes remain uncertain. Here, we investigated the role of ATF3 on the preventive regulation of AMPK against ER stress-mediated β-cell dysfunction during the end-stage progression of hyperglycemia in ZDF rats. The impaired glucose metabolism and β-cell dysfunction were significantly increased in late-diabetic phase 19-week-old ZDF rats. Although AMPK phosphorylation reduced in 6- and 12-week-old ZDF rats was remarkably increased at 19weeks, the increases of lipogenice genes, ATF3, and ER stress or ROS-mediated β-cell dysfunction were still remained, which were attenuated by in vivo-injection of chemical chaperon tauroursodeoxycholate (TUDCA), chronic AICAR, or antioxidants. ATF3 did not directly affect AMPK phosphorylation, but counteracts the preventive effects of AMPK for high glucose-induced β-cell dysfunction. Moreover, knockdown of ATF3 by delivery of in vivo-jetPEI ATF3 siRNA attenuated ER stress-mediated β-cell dysfunction and enhanced the beneficial effect of AICAR. Our data suggest that ATF3 may play as a counteracting regulator of AMPK and thus promote β-cell dysfunction and the development of type-2 diabetes and could be a potential therapeutic target in treating type-2 diabetes.

When less is more: the forbidden fruits of gene repression in the adult β-cell

Outside of the biological arena the term 'repression' often has a negative connotation. However, in the pancreatic β-cell a small group of genes, which are abundantly expressed in most if not all other mammalian tissues, are highly selectively repressed, with likely functional consequences. The two 'founder' members of this group, lactate dehydrogenase A (Ldha) and monocarboxylate transporter-1 (MCT-1/Slc16a1), are inactivated by multiple mechanisms including histone modifications and microRNA-mediated silencing. Their inactivation ensures that pyruvate and lactate, derived from muscle during exercise, do not stimulate insulin release inappropriately. Correspondingly, activating mutations in the MCT-1 promoter underlie 'exercise-induced hyperinsulinism' (EIHI) in man, a condition mimicked by forced over-expression of MCT-1 in the β-cell in mice. Furthermore, LDHA expression in the β-cell is upregulated in both human type 2 diabetes and in rodent models of the disease. Recent work by us and by others has identified a further ∼60 genes which are selectively inactivated in the β-cell, a list which we refine here up to seven by detailed comparison of the two studies. These genes include key regulators of cell proliferation and stimulus-secretion coupling. The present, and our earlier results, thus highlight the probable importance of shutting down a subset of 'disallowed' genes for the differentiated function of β-cells, and implicate previously unsuspected signalling pathways in the control of β-cell expansion and insulin secretion. Targeting of deregulated 'disallowed' genes in these cells may thus, in the future, provide new therapeutic avenues for type 2 diabetes.

Disruption of the cereblon gene enhances hepatic AMPK activity and prevents high-fat diet-induced obesity and insulin resistance in mice

A nonsense mutation in cereblon (CRBN) causes a mild type of mental retardation in humans. An earlier study showed that CRBN negatively regulates the functional activity of AMP-activated protein kinase (AMPK) in vitro by binding directly to the α1-subunit of the AMPK complex. However, the in vivo role of CRBN was not studied. For elucidation of the physiological functions of Crbn, a mouse strain was generated in which the Crbn gene was deleted throughout the whole body. In Crbn-deficient mice fed a normal diet, AMPK in the liver showed hyperphosphorylation, which indicated the constitutive activation of AMPK. Since Crbn-deficient mice showed significantly less weight gain when fed a high-fat diet and their insulin sensitivity was considerably improved, the functions of Crbn in the liver were primarily investigated. These results provide the first in vivo evidence that Crbn is a negative modulator of AMPK, which suggests that Crbn may be a potential target for metabolic disorders of the liver.

AMPK is involved in the regulation of incretin receptors expression in pancreatic islets under a low glucose concentration

The precise role of AMP-activated protein kinase (AMPK), a target of metformin, in pancreatic β cells remains controversial, even though metformin was recently shown to enhance the expression of incretin receptors (GLP-1 and GIP receptors) in pancreatic β cells. In this study, we investigated the effect of AMPK in the regulation of incretin receptors expression in pancreatic islets. The phosphorylation of AMPK in the mouse islets was decreased by increasing glucose concentrations. We showed the expression of incretin receptors in bell-shaped response to glucose; Expression of the incretin receptors in the isolated islets showed higher levels under a medium glucose concentration (11.1 mM) than that under a low glucose concentration (2.8 mM), but was suppressed under a high glucose concentration (22.2 mM). Both treatment with an AMPK inhibitor and DN-AMPK expression produced a significant increase of the incretin receptors expression under a low glucose concentration. By contrast, in hyperglycemic db/db islets, the enhancing effect of the AMPK inhibitor on the expression of incretin receptors was diminished under a low glucose concentration. Taken together, AMPK is involved in the regulation of incretin receptors expression in pancreatic islets under a low glucose concentration.

Role of AMPK in pancreatic beta cell function

Pharmacological activation of AMP activated kinase (AMPK) by metformin has proven to be a beneficial therapeutic approach for the treatment of type II diabetes. Despite improved glucose regulation achieved by administration of small molecule activators of AMPK, the potential negative impact of enhanced AMPK activity on insulin secretion by the pancreatic beta cell is an important consideration. In this review, we discuss our current understanding of the role of AMPK in central functions of the pancreatic beta cell, including glucose-stimulated insulin secretion (GSIS), proliferation, and survival. In addition we discuss the controversy surrounding the role of AMPK in insulin secretion, underscoring the merits and caveats of methods used to date.

The human glucagon-like peptide-1 analogue liraglutide regulates pancreatic beta-cell proliferation and apoptosis via an AMPK/mTOR/P70S6K signaling pathway

Glucagon-like peptide-1 (GLP-1), an effective therapeutic agent for the treatment of diabetes, has been proven to protect pancreatic beta cells through many pathways. Recent evidence demonstrates that AMP-activated protein kinase (AMPK), as a metabolic regulator, coordinates beta-cell protein synthesis through regulation of the mammalian target of rapamycin (mTOR) signaling pathway. The purpose of the present study was to explore whether liraglutide, a human GLP-1 analogue, protects beta cells via AMPK/mTOR signaling. We evaluated INS-1 beta-cell line proliferation using the Cell Counting Kit-8, and examined the effect of GLP-1 on cellular ATP levels using an ATP assay kit. mTOR pathway protein expression levels were tested by Western blotting and glucolipotoxicity-induced cell apoptosis was evaluated by flow cytometry. Liraglutide increased beta-cell viability at an optimum concentration of 100 nmol/L in the presence of 11.1 or 30 mmol/L glucose. Liraglutide (100 nmol/L) activated mTOR and its downstream effectors, 70-kDa ribosomal protein S6 kinase and eIF4E-binding protein-1, in INS-1 cells. This effect was abated by pathway blockers: the AMPK activator AICAR and the mTOR inhibitor rapamycin. Furthermore, the effect of liraglutide on beta-cell proliferation was inhibited by AICAR and rapamycin. Liraglutide increased cellular ATP levels. In addition, liraglutide protected beta cells from glucolipotoxicity-induced apoptosis. This response was also prevented by rapamycin treatment. These results suggest that the enhancement of beta-cell proliferation by that GLP-1 receptor agonist liraglutide is mediated, at least in part, by AMPK/mTOR signaling. Liraglutide also prevents beta-cell glucolipotoxicity by activating mTOR.

Glucose-dependent regulation of AMP-activated protein kinase in MIN6 beta cells is not affected by the protein kinase A pathway

AMP-activated protein kinase (AMPK) is a sensor of cellular energy status. In pancreatic beta cells, glucose induces the dephosphorylation of Thr172 within the catalytic subunit and the inactivation of the AMPK complex. Here we demonstrate that glucose also activates protein kinase A (PKA), leading to the phosphorylation of AMPKα at Ser485 and Ser497. However, these modifications do not impair the phosphorylation of Thr172 by upstream kinases, and phosphorylation of Thr172 does not affect the phosphorylation of AMPKα by PKA either. Thus, although phosphorylation of Thr172 and Ser485/Ser497 are inversely correlated in response to glucose, they follow an independent regulation.

Fasting alters protein expression of AMP-activated protein kinase in the hypothalamus of broiler chicks (Gallus gallus domesticus)

An experiment was conducted to investigate the effects of fasting and re-feeding on hypothalamic 5'-AMP-activated protein kinase (AMPK) levels and (an)orexigenic neuropeptides. Male Arbor Acres chicks (7-day-old, n=160) were allocated to four equal treatment groups: control chicks (fed ad libitum for 48 h, C48), chicks that were fasted for 48 h (F48), chicks that were first fasted for 48 h and then re-fed for 24h (F48C24), and chicks that were fed ad libitum for 72h (C72). Fasting for 48 h significantly (P<0.05) increased the ratio of phosphorylated AMPKα to total AMPKα and phosphorylated LKB1 to total LKB1, whereas re-feeding for 24h reduced these ratios to that of the ad libitum fed C72 chicks. The gene expressions of agouti-related peptide (AgRP), neuropeptide Y (NPY), melanocortin receptor 4, melanin-concentrating hormone, prepro-orexins and carnitine palmitoyltransferase-1 were significantly (P<0.05) increased in the fasted chicks relative to the ad libitum fed C48 group. The gene expression of pro-opiomelanocortin (POMC), as well as cocaine- and amphetamine-regulated transcript (CART) was not affected by the nutritional status. Fasting significantly (P<0.05) decreased the mRNA levels of fatty acid synthase (FAS) and sterol regulatory element binding protein-1 (SREBP-1). The results suggest that the LKB1/AMPK signal pathway is involved in the energy homeostasis of fasted chicks, and its possible role in feed intake regulation might be mediated by the AgRP/NPY rather than the POMC/CART pathway.

Cross-talk between AMPK and mTOR in regulating energy balance

Energy balance is maintained by a complex homeostatic system involving some signaling pathways and "nutrient sensors" in multiple tissues and organs. Any defect associated with the pathways can lead to metabolic disorders including obesity, type 2 diabetes, and the metabolic syndrome. The 5'-adenosine monophosphate-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) appear to play a significant role in the intermediary metabolism of these diseases. AMPK is involved in the fundamental regulation of energy balance at the whole body level by responding to hormonal and nutrient signals in the central nervous system and peripheral tissues that modulate food intake and energy expenditure. Mammalian target of rapamycin (mTOR),is one of the downstream targets of AMPK functions as an intracellular nutrient sensor to control protein synthesis, cell growth, and metabolism. Recent research demonstrated the possible interplay between mTOR and AMPK signaling pathways. In this review, we will present current knowledge of AMPK and mTOR pathways in regulating energy balance and demonstrate the convergence between these two pathways.

Roles of Ca2+ ions in the control of ChREBP nuclear translocation

Carbohydrate-responsive element binding protein (ChREBP (MLXIPL)) is emerging as an important mediator of glucotoxity both in the liver and in the pancreatic β-cells. Although the regulation of its nuclear translocation and transcriptional activation by glucose has been the subject of intensive research, it is still not fully understood. We have recently uncovered a novel mechanism in the excitable pancreatic β-cell where ChREBP interacts with sorcin, a penta-EF-hand Ca(2)(+)-binding protein, and is sequestered in the cytosol at low glucose concentrations. Upon stimulation with glucose and activation of Ca(2)(+) influx, or application of ATP as an intracellular Ca(2)(+)-mobilising agent, ChREBP rapidly translocates to the nucleus. In sorcin-silenced cells, ChREBP is constitutively present in the nucleus, and both glucose and Ca(2)(+) are ineffective in stimulating further ChREBP nuclear shuttling. Whether an active Ca(2)(+)-sorcin element of ChREBP activation also exists in non-excitable cells is discussed.

AMP-activated protein kinase, stress responses and cardiovascular diseases

AMPK (AMP-activated protein kinase) is one of the key players in maintaining intracellular homoeostasis. AMPK is well known as an energy sensor and can be activated by increased intracellular AMP levels. Generally, the activation of AMPK turns on catabolic pathways that generate ATP, while inhibiting cell proliferation and biosynthetic processes that consume ATP. In recent years, intensive investigations on the regulation and the function of AMPK indicates that AMPK not only functions as an intracellular energy sensor and regulator, but is also a general stress sensor that is important in maintaining intracellular homoeostasis during many kinds of stress challenges. In the present paper, we will review recent literature showing that AMPK functions far beyond its proposed energy sensor and regulator function. AMPK regulates ROS (reactive oxygen species)/redox balance, autophagy, cell proliferation, cell apoptosis, cellular polarity, mitochondrial function and genotoxic response, either directly or indirectly via numerous downstream pathways under physiological and pathological conditions.

Hyperthermia with mild electrical stimulation protects pancreatic β-cells from cell stresses and apoptosis

Induction of heat shock protein (HSP) 72 improves metabolic profiles in diabetic model mice. However, its effect on pancreatic β-cells is not known. The current study investigated whether HSP72 induction can reduce β-cell stress signaling and apoptosis and preserve β-cell mass. MIN6 cells and db/db mice were sham-treated or treated with heat shock (HS) and mild electrical stimulation (MES) (HS+MES) to induce HSP72. Several cellular markers, metabolic parameters, and β-cell mass were evaluated. HS+MES treatment or HSP72 overexpression increased HSP72 protein levels and decreased tumor necrosis factor (TNF)-α-induced Jun NH(2)-terminal kinase (JNK) phosphorylation, endoplasmic reticulum (ER) stress, and proapoptotic signal in MIN6 cells. In db/db mice, HS+MES treatment for 12 weeks significantly improved insulin sensitivity and glucose homeostasis. Upon glucose challenge, a significant increase in insulin secretion was observed in vivo. Compared with sham treatment, levels of HSP72, insulin, pancreatic duodenal homeobox-1, GLUT2, and insulin receptor substrate-2 were upregulated in the pancreatic islets of HS+MES-treated mice, whereas JNK phosphorylation, nuclear translocation of forkhead box class O-1, and nuclear factor-κB p65 were reduced. Apoptotic signals, ER stress, and oxidative stress markers were attenuated. Thus, HSP72 induction by HS+MES treatment protects β-cells from apoptosis by attenuating JNK activation and cell stresses. HS+MES combination therapy may preserve pancreatic β-cell volume to ameliorate glucose homeostasis in diabetes.

Glucose-induced posttranslational activation of protein phosphatases PP2A and PP1 in yeast

The protein phosphatases PP2A and PP1 are major regulators of a variety of cellular processes in yeast and other eukaryotes. Here, we reveal that both enzymes are direct targets of glucose sensing. Addition of glucose to glucose-deprived yeast cells triggered rapid posttranslational activation of both PP2A and PP1. Glucose activation of PP2A is controlled by regulatory subunits Rts1, Cdc55, Rrd1 and Rrd2. It is associated with rapid carboxymethylation of the catalytic subunits, which is necessary but not sufficient for activation. Glucose activation of PP1 was fully dependent on regulatory subunits Reg1 and Shp1. Absence of Gac1, Glc8, Reg2 or Red1 partially reduced activation while Pig1 and Pig2 inhibited activation. Full activation of PP2A and PP1 was also dependent on subunits classically considered to belong to the other phosphatase. PP2A activation was dependent on PP1 subunits Reg1 and Shp1 while PP1 activation was dependent on PP2A subunit Rts1. Rts1 interacted with both Pph21 and Glc7 under different conditions and these interactions were Reg1 dependent. Reg1-Glc7 interaction is responsible for PP1 involvement in the main glucose repression pathway and we show that deletion of Shp1 also causes strong derepression of the invertase gene SUC2. Deletion of the PP2A subunits Pph21 and Pph22, Rrd1 and Rrd2, specifically enhanced the derepression level of SUC2, indicating that PP2A counteracts SUC2 derepression. Interestingly, the effect of the regulatory subunit Rts1 was consistent with its role as a subunit of both PP2A and PP1, affecting derepression and repression of SUC2, respectively. We also show that abolished phosphatase activation, except by reg1Δ, does not completely block Snf1 dephosphorylation after addition of glucose. Finally, we show that glucose activation of the cAMP-PKA (protein kinase A) pathway is required for glucose activation of both PP2A and PP1. Our results provide novel insight into the complex regulatory role of these two major protein phosphatases in glucose regulation.

Glucose-induced O₂ consumption activates hypoxia inducible factors 1 and 2 in rat insulin-secreting pancreatic beta-cells

Background

Glucose increases the expression of glycolytic enzymes and other hypoxia-response genes in pancreatic beta-cells. Here, we tested whether this effect results from the activation of Hypoxia-Inducible-factors (HIF) 1 and 2 in a hypoxia-dependent manner.

Methodology/Principal Findings

Isolated rat islets and insulin-secreting INS-1E cells were stimulated with nutrients at various pO₂ values or treated with the HIF activator CoCl₂. HIF-target gene mRNA levels and HIF subunit protein levels were measured by real-time RT-PCR, Western Blot and immunohistochemistry. The formation of pimonidazole-protein adducts was used as an indicator of hypoxia. In INS-1E and islet beta-cells, glucose concentration-dependently stimulated formation of pimonidazole-protein adducts, HIF1 and HIF2 nuclear expression and HIF-target gene mRNA levels to a lesser extent than CoCl₂ or a four-fold reduction in pO₂. Islets also showed signs of HIF activation in diabetic Lepr^db/db but not non-diabetic Lepr^db/+ mice. In vitro, these glucose effects were reproduced by nutrient secretagogues that bypass glycolysis, and were inhibited by a three-fold increase in pO₂ or by inhibitors of Ca²⁺ influx and insulin secretion. In INS-1E cells, small interfering RNA-mediated knockdown of Hif1α and Hif2α, alone or in combination, indicated that the stimulation of glycolytic enzyme mRNA levels depended on both HIF isoforms while the vasodilating peptide adrenomedullin was a HIF2-specific target gene.

Conclusions/Significance

Glucose-induced O₂ consumption creates an intracellular hypoxia that activates HIF1 and HIF2 in rat beta-cells, and this glucose effect contributes, together with the activation of other transcription factors, to the glucose stimulation of expression of some glycolytic enzymes and other hypoxia response genes.

AMP-activated protein kinase (AMPK) mediates nutrient regulation of thioredoxin-interacting protein (TXNIP) in pancreatic beta-cells

Thioredoxin-interacting protein (TXNIP) regulates critical biological processes including inflammation, stress and apoptosis. TXNIP is upregulated by glucose and is a critical mediator of hyperglycemia-induced beta-cell apoptosis in diabetes. In contrast, the saturated long-chain fatty acid palmitate, although toxic to the beta-cell, inhibits TXNIP expression. The mechanisms involved in the opposing effects of glucose and fatty acids on TXNIP expression are unknown. We found that both palmitate and oleate inhibited TXNIP in a rat beta-cell line and islets. Palmitate inhibition of TXNIP was independent of fatty acid beta-oxidation or esterification. AMP-activated protein kinase (AMPK) has an important role in cellular energy sensing and control of metabolic homeostasis; therefore we investigated its involvement in nutrient regulation of TXNIP. As expected, glucose inhibited whereas palmitate stimulated AMPK. Pharmacologic activators of AMPK mimicked fatty acids by inhibiting TXNIP. AMPK knockdown increased TXNIP expression in presence of high glucose with and without palmitate, indicating that nutrient (glucose and fatty acids) effects on TXNIP are mediated in part via modulation of AMPK activity. TXNIP is transcriptionally regulated by carbohydrate response element-binding protein (ChREBP). Palmitate inhibited glucose-stimulated ChREBP nuclear entry and recruitment to the Txnip promoter, thereby inhibiting Txnip transcription. We conclude that AMPK is an important regulator of Txnip transcription via modulation of ChREBP activity. The divergent effects of glucose and fatty acids on TXNIP expression result in part from their opposing effects on AMPK activity. In light of the important role of TXNIP in beta-cell apoptosis, its inhibition by fatty acids can be regarded as an adaptive/protective response to glucolipotoxicity. The finding that AMPK mediates nutrient regulation of TXNIP may have important implications for the pathophysiology and treatment of diabetes.

Avian renal proximal tubule urate secretion is inhibited by cellular stress-induced AMP-activated protein kinase

Urate is a potent antioxidant at high concentrations but it has also been associated with a wide variety of health risks. Plasma urate concentration is determined by ingestion, production, and urinary excretion; however, factors that regulate urate excretion remain uncertain. The objective of this study was to determine whether cellular stress, which has been shown to affect other renal transport properties, modulates urate secretion in the avian renal proximal tubule. Chick kidney proximal tubule epithelial cell primary culture monolayers were used to study the transepithelial transport of radiolabeled urate. This model allowed examination of the processes, such as multidrug resistance protein 4 (Mrp4, Abcc4), which subserve urate secretion in a functional, intact, homologous system. Our results show that the recently implicated urate efflux transporter, breast cancer resistance protein ( ABCG2), does not significantly contribute to urate secretion in this system. Exposure to a high concentration of zinc for 6 h induced a cellular stress response and a striking decrease in transepithelial urate secretion. Acute exposure to zinc had no effect on transepithelial urate secretion or isolated membrane vesicle urate transport, suggesting involvement of a cellular stress adaptation. Activation of AMP-activated protein kinase (AMPK), a candidate modulator of ATP-dependent urate efflux, by 5′-aminoimidazole-4-carboxamide 1-β-d-ribo-furanoside caused a decrease in urate secretion similar to that seen with zinc-induced cellular stress. This effect was prevented with the AMPK inhibitor compound C. Notably, the decrease in urate secretion seen with zinc-induced cellular stress was also prevented by compound C, implicating AMPK in regulation of renal uric acid excretion.

Metformin therapy in a hyperandrogenic anovulatory mutant murine model with polycystic ovarian syndrome characteristics improves oocyte maturity during superovulation

Background

Metformin, an oral biguanide traditionally used for the treatment of type 2 diabetes, is widely used for the management of polycystic ovary syndrome (PCOS)-related anovulation. Because of the significant prevalence of insulin resistance and glucose intolerance in PCOS patients, and their putative role in ovulatory dysfunction, the use of metformin was touted as a means to improve ovulatory function and reproductive outcomes in PCOS patients. To date, there has been inconsistent evidence to demonstrate a favorable effect of metformin on oocyte quality and competence in women with PCOS. Given the heterogeneous nature of this disorder, we hypothesized that metformin may be beneficial in mice with aberrant metabolic characteristics similar to a significant number of PCOS patients. The aim of this study was to gain insight into the in vitro and in vivo effects of metformin on oocyte development and ovulatory function.

Methods

We utilized metformin treatment in the transgenic ob/ob and db/db mutant murine models which demonstrate metabolic and reproductive characteristics similar to women with PCOS. Results: Metformin did not improve in vitro oocyte maturation nor did it have an appreciable effect on in vitro granulosa cell luteinization (progesterone production) in any genotype studied. Although both mutant strains have evidence of hyperandrogenemia, anovulation, and hyperinsulinemia, only db/db mice treated with metformin had a greater number of mature oocytes and total overall oocytes compared to control. There was no observed impact on body mass, or serum glucose and androgens in any genotype.

Conclusions

Our data provide evidence to suggest that metformin may optimize ovulatory performance in mice with a specific reproductive and metabolic phenotype shared by women with PCOS. The only obvious difference between the mutant murine models is that the db/db mice have elevated leptin levels raising the questions of whether their response to metformin is related to elevated leptin levels and/or if a subset of PCOS women with hyperleptinemia may be responsive to metformin therapy. Further study is needed to better define a subset of women with PCOS that may be responsive to metformin.

Per-arnt-sim (PAS) domain-containing protein kinase is downregulated in human islets in type 2 diabetes and regulates glucagon secretion

AIMS/HYPOTHESIS

We assessed whether per-arnt-sim (PAS) domain-containing protein kinase (PASK) is involved in the regulation of glucagon secretion.

METHODS

mRNA levels were measured in islets by quantitative PCR and in pancreatic beta cells obtained by laser capture microdissection. Glucose tolerance, plasma hormone levels and islet hormone secretion were analysed in C57BL/6 Pask homozygote knockout mice (Pask-/-) and control littermates. Alpha-TC1-9 cells, human islets or cultured E13.5 rat pancreatic epithelia were transduced with anti-Pask or control small interfering RNAs, or with adenoviruses encoding enhanced green fluorescent protein or PASK.

RESULTS

PASK expression was significantly lower in islets from human type 2 diabetic than control participants. PASK mRNA was present in alpha and beta cells from mouse islets. In Pask-/- mice, fasted blood glucose and plasma glucagon levels were 25 ± 5% and 50 ± 8% (mean ± SE) higher, respectively, than in control mice. At inhibitory glucose concentrations (10 mmol/l), islets from Pask-/- mice secreted 2.04 ± 0.2-fold (p < 0.01) more glucagon and 2.63 ± 0.3-fold (p < 0.01) less insulin than wild-type islets. Glucose failed to inhibit glucagon secretion from PASK-depleted alpha-TC1-9 cells, whereas PASK overexpression inhibited glucagon secretion from these cells and human islets. Extracellular insulin (20 nmol/l) inhibited glucagon secretion from control and PASK-deficient alpha-TC1-9 cells. PASK-depleted alpha-TC1-9 cells and pancreatic embryonic explants displayed increased expression of the preproglucagon (Gcg) and AMP-activated protein kinase (AMPK)-alpha2 (Prkaa2) genes, implying a possible role for AMPK-alpha2 downstream of PASK in the control of glucagon gene expression and release.

CONCLUSIONS/INTERPRETATION

PASK is involved in the regulation of glucagon secretion by glucose and may be a useful target for the treatment of type 2 diabetes.

The role of LKB1 and AMPK in cellular responses to stress and damage

The LKB1 and AMPK proteins participate in an energy sensing cascade that responds to depletion of ATP, serving as a master regulator of metabolism that inhibits anabolic processes and stimulates catabolic processes. However in recent years, LKB1 and AMPK have been implicated in a variety of other cellular processes, both cytoplasmic and nuclear, such as control of cell polarity and regulation of gene transcription. In this review, we summarize the most recent discoveries regarding participation of LKB1 and AMPK in signaling pathways that respond to cellular stress and damage, and the relevance of this signaling for disease and therapy.

AMP-activated protein kinase regulates glucagon secretion from mouse pancreatic alpha cells

AIM/HYPOTHESIS

AMP-activated protein kinase (AMPK), encoded by Prkaa genes, is emerging as a key regulator of overall energy homeostasis and the control of insulin secretion and action. We sought here to investigate the role of AMPK in controlling glucagon secretion from pancreatic islet alpha cells.

METHODS

AMPK activity was modulated in vitro in clonal alphaTC1-9 cells and isolated mouse pancreatic islets using pharmacological agents and adenoviruses encoding constitutively active or dominant negative forms of AMPK. Glucagon secretion was measured during static incubation by radioimmunoassay. AMPK activity was assessed by both direct phosphotransfer assay and by western (immuno-)blotting of the phosphorylated AMPK α subunits and the downstream target acetyl-CoA carboxylase 1. Intracellular free [Ca²(+)] was measured using Fura-Red.

RESULTS

Increasing glucose concentrations strongly inhibited AMPK activity in clonal pancreatic alpha cells. Forced increases in AMPK activity in alphaTC1-9 cells, achieved through the use of pharmacological agents including metformin, phenformin and A-769662, or via adenoviral transduction, resulted in stimulation of glucagon secretion at both low and high glucose concentrations, whereas AMPK inactivation inhibited both [Ca²(+)](i) increases and glucagon secretion at low glucose. Transduction of isolated mouse islets with an adenovirus encoding AMPK-CA under the control of the preproglucagon promoter increased glucagon secretion selectively at elevated glucose concentrations.

CONCLUSIONS/INTERPRETATION

AMPK is strongly regulated by glucose in pancreatic alpha cells, and increases in AMPK activity are sufficient and necessary for the stimulation of glucagon release in vitro. Modulation of AMPK activity in alpha cells may therefore provide a novel approach to controlling blood glucose concentrations.