Inhibition Of Glycogen Synthesis Research Articles

Airborne PM2.5-Induced Hepatic Insulin Resistance by Nrf2/JNK-Mediated Signaling Pathway.

Animal and epidemiological studies have suggested that exposure to airborne particulate matter (PM) with an aerodynamic diameter less than 2.5 μm (PM2.5) is associated with the risk of developing type 2 diabetes. However, the mechanism underlying this risk is poorly understood. In the present study, we investigated the effects of PM2.5 exposure on glucose homeostasis and related signaling pathways in mice. Wild-type and nuclear factor erythroid 2-related factor 2 (Nrf2) knockout (Nrf2−/−) C57BL/6 male mice were exposed to either ambient concentrated PM2.5 or filtered air (FA) for 12 weeks through a whole-body PM exposure system. At the end of the exposure, we assessed liver damage, and performed metabolic studies, gene expressions, as well as molecular signal transductions to determine the signaling pathways involving oxidative responses, insulin signaling, and glucose metabolism. Our results indicated that PM2.5 exposure for 12 weeks caused significant liver damage as evidenced by elevated levels of aminotransferase (AST) and alanine aminotransferase (ALT). Furthermore, PM2.5 exposure induced impaired glucose tolerance and inhibited glycogen synthesis, leading to hepatic insulin resistance indicated by higher glucose levels, higher area under the curve (AUC), and homeostasis model assessment of insulin resistance (HOMA-IR) values. We further found that PM2.5 exposure significantly increased the expressions of Nrf2 and Nrf2-regulated antioxidant genes. Moreover, PM2.5 exposure activated the c-Jun N-terminal kinase (JNK) signaling pathway and increased insulin receptor substrate-1 (IRS-1) phosphorylation at Ser307, but reduced protein kinase B phosphorylation at Ser473. Taken together, our study demonstrated PM2.5 exposure triggered Nrf2-mediated oxidative responses and activated the JNK-mediated inhibitory signaling pathway, resulting in hepatic insulin resistance.

A Computational Model of Hepatic Energy Metabolism: Understanding Zonated Damage and Steatosis in NAFLD.

In non-alcoholic fatty liver disease (NAFLD), lipid build-up and the resulting damage is known to occur more severely in pericentral cells. Due to the complexity of studying individual regions of the sinusoid, the causes of this zone specificity and its implications on treatment are largely ignored. In this study, a computational model of liver glucose and lipid metabolism is presented which treats the sinusoid as the repeating unit of the liver rather than the single hepatocyte. This allows for inclusion of zonated enzyme expression by splitting the sinusoid into periportal to pericentral compartments. By simulating insulin resistance (IR) and high intake diets leading to the development of steatosis in the model, we identify key differences between periportal and pericentral cells accounting for higher susceptibility to pericentral steatosis. Secondly, variation between individuals is seen in both susceptibility to steatosis and in its development across the sinusoid. Around 25% of obese individuals do not show excess liver fat, whilst 16% of lean individuals develop NAFLD. Furthermore, whilst pericentral cells tend to show higher lipid levels, variation is seen in the predominant location of steatosis from pericentral to pan-sinusoidal or azonal. Sensitivity analysis was used to identify the processes which have the largest effect on both total hepatic triglyceride levels and on the sinusoidal location of steatosis. As is seen in vivo, steatosis occurs when simulating IR in the model, predominantly due to increased uptake, along with an increase in de novo lipogenesis. Additionally, concentrations of glucose intermediates including glycerol-3-phosphate increased when simulating IR due to inhibited glycogen synthesis. Several differences between zones contributed to a higher susceptibility to steatosis in pericentral cells in the model simulations. Firstly, the periportal zonation of both glycogen synthase and the oxidative phosphorylation enzymes meant that the build-up of glucose intermediates was less severe in the periportal hepatocyte compartments. Secondly, the periportal zonation of the enzymes mediating β-oxidation and oxidative phosphorylation resulted in excess fats being metabolised more rapidly in the periportal hepatocyte compartments. Finally, the pericentral expression of de novo lipogenesis contributed to pericentral steatosis when additionally simulating the increase in sterol-regulatory element binding protein 1c (SREBP-1c) seen in NAFLD patients in vivo. The hepatic triglyceride concentration was predicted to be most sensitive to inter-individual variation in the activity of enzymes which, either directly or indirectly, determine the rate of free fatty acid (FFA) oxidation. The concentration was most strongly dependent on the rate constants for β-oxidation and oxidative phosphorylation. It also showed moderate sensitivity to the rate constants for processes which alter the allosteric inhibition of β-oxidation by acetyl-CoA. The predominant sinusoidal location of steatosis meanwhile was most sensitive variations in the zonation of proteins mediating FFA uptake or triglyceride release as very low density lipoproteins (VLDL). Neither the total hepatic concentration nor the location of steatosis showed strong sensitivity to variations in the lipogenic rate constants.

Protective effect of pretreatment with propofol against tumor necrosis factor-α-induced hepatic insulin resistance

Insulin resistance is common in critically ill patients and seriously affects their prognosis. The anesthetic propofol (2,6-diisopropylphenol) has been shown to cause insulin resistance in rats; however, the specific mechanism underlying this phenomenon remains unknown. Thus, the aim of the present study was to determine the molecular mechanism through which propofol influences insulin resistance in the liver. The current study assessed the effects of propofol on the phosphorylation level of key enzymes involved in the insulin signaling pathway, as well as the glycogen content in primary mouse hepatocytes. Propofol administration was demonstrated to considerably reduce the phosphorylation levels of Akt (Ser473) and glycogen synthase kinase (GSK)-3β (Ser9) in the primary mouse hepatocytes. In addition, propofol was shown to downregulate the phosphoinositide 3-kinase (PI3K)/Akt/GSK-3β signaling pathway and inhibit glycogen synthesis in hepatocytes. Thus, the present results indicated that propofol induced insulin resistance in primary mouse hepatocytes. Notably, pretreatment with propofol in tumor necrosis factor (TNF)-α-induced primary mouse hepatocytes with insulin resistance was demonstrated to alleviate the inhibitory effects of TNF-α on the PI3K/Akt/GSK-3β signaling pathway and glycogen synthesis. These results indicated that propofol exerts a protective effect against insulin resistance in primary mouse hepatocytes induced by TNF-α, indicating that propofol therapy may be clinically feasible to alleviate insulin resistance in critically ill patients.

Inducible Metabolic Adaptation Promotes Mesenchymal Stem Cell Therapy for Ischemia

Objective— Ischemic tissue is an environment with limited oxygen and nutrition availability. The poor retention of mesenchymal stem cells (MSC) in ischemic tissues greatly limits their therapeutic potential. The aim of this study was to determine whether and how inducible metabolic adaptation enhances MSC survival and therapy under ischemia. Approach and Results— MSC were subjected to glycogen synthase 1–specific small interfering RNA or vehicle treatment, and then sublethal hypoxic preconditioning (HP) was applied to induce glycogenesis. The treated cells were subjected to ischemic challenge. The results exhibited that HP of MSC induced glycogen storage and stimulated glycogen catabolism and cellular ATP production, thereby preserving cell viability in long-term ischemia. In vivo study using the mouse limb ischemia model transplanted with HP or control MSC into the ischemic thigh muscles revealed a significant increased retention of MSC with glycogen storage associated with improved limb salvage, perfusion recovery and angiogenesis in the ischemic muscles. In contrast, glycogen synthesis inhibition significantly abolished these improvements. Further molecular analysis indicated that phosphoinositide 3-kinase/AKT, hypoxia-inducible factor-1, and glycogen synthase kinase-3β regulated expression of glycogenesis genes, including glucose transporter 1, hexokinase, phosphoglucomutase 1, glycogen synthase 1, and glycogen phosphorylase, thereby regulating glycogen metabolism of stem cell during HP. Conclusions— HP-induced glycogen storage improves MSC survival and therapy in ischemic tissues. Thus, inducible metabolic adaptation in stem cells may be considered as a novel strategy for potentiating stem cell therapy for ischemia.

PS6 - 4. Toward selective targeting of muscular AMPK: in search of a small molecule to change AMPK cellular localization

The energy-sensor AMP-activated protein kinase (AMPK) cycles between a glycogen-bound and a free state. The muscle-specific regulatory AMPKβ2 subunit carries a high affinity carbohydrate-binding module (CBM). Upon energy stress, such as exercise, AMPK localization at glycogen allows for rapid inhibition of glycogen synthesis, whereas cytosolic AMPK is capable of stimulating insulin-independent glucose uptake.

Pancreastatin is an endogenous peptide that regulates glucose homeostasis

Pancreastatin (PST) is a regulatory peptide containing 49 amino acids, first isolated from porcine pancreas. Intracellular and extracellular processing of the prohormone Chromogranin A (Chga) results various bioactive peptides of which PST has dysglycemic activity. PST regulates glucose, lipid, and protein metabolism in liver and adipose tissues. It also regulates the secretion of leptin and expression of leptin and uncoupling protein 2 in adipose tissue. In Chga knockout mice, PST induces gluconeogenesis in the liver. PST reduces glucose uptake in mice hepatocytes and adipocytes. In rat hepatocytes, PST induces glycogenolysis and glycolysis and inhibits glycogen synthesis. In rat adipocytes, PST inhibits lactate production and lipogenesis. These metabolic effects are confirmed in humans. In the dual signaling mechanism of PST receptor, mostly PST activates Gαq/11 protein leads to the activation of phospholipase C β3-isoform, therefore increasing cytoplasmic free calcium and stimulating protein kinase C. PST inhibits the cell growth in rat HTC hepatoma cells, mediated by nitric oxide and cyclic GMP production. Elevated levels of PST correlating with catecholamines have been found in gestational diabetes and essential hypertension. Rise in the blood PST level in Type 2 diabetes suggests that PST is a negative regulator of insulin sensitivity and glucose homeostasis.

Parallel induction of cell proliferation and inhibition of cell differentiation in hepatic progenitor cells by hepatitis B virus X gene

Increasing evidence has shown that normal stem cells may contribute to the development and progression of cancer by acting as cancer-initiating cells. The hepatitis B virus X (HBX) protein has been implicated in the hepatitis B virus (HBV)-associated liver carcinogenesis. However, the role of HBX in hepatic progenitor cells (HPCs) is poorly understood. In this study, we aimed to determine the role of HBX in regulating HPC proliferation and differentiation. Using MTT analysis, we showed that HPCs infected with adenovirus expressing HBX (Ad-HBX) grew more rapidly compared to HPCs infected with adenovirus expressing green fluorescent protein (Ad-GFP). To reveal the mechanism for the increased cell number after HBX treatment, we searched for possible alterations in the cell cycle and apoptosis by flow cytometry. We found that HBX treatment resulted in an increase in the S phase cell cycle fraction and a decrease in apoptosis. In addition, we examined the differentiation of HPCs infected with Ad-HBX and found that the HBX expression in HP14.5 cells led to an increased expression of early progenitor markers and a decreased expression of late hepatocyte markers. Furthermore, HBX inhibited glycogen synthesis in HP14.5 cells, indicating that HBX is capable of inhibiting terminal hepatic differentiation. Therefore, our results strongly suggest that HBX plays an important role in regulating HPC proliferation and differentiation. This is the potential mechanism of HBX-mediated liver carcinogenesis.

Modulation of glycogen and breast meat processing ability by nutrition in chickens: Effect of crude protein level in 2 chicken genotypes1

The aim of the study was to evaluate the impact of 2 isoenergetic growing diets with different CP (17 vs. 23%) on the performance and breast meat quality of 2 lines of chicken divergently selected for abdominal fatness [i.e., fat and lean (LL) lines]. Growth performance, breast and abdominal fat yields, breast meat quality parameters (pH, color, drip loss), and muscle glycogen storage at death were measured. Increased dietary CP resulted in increased BW, increased breast meat yield, and reduced abdominal fatness at slaughter regardless of genotype (P < 0.001). By contrast, dietary CP affected glycogen storage and the related meat quality parameters only in the LL chickens. Giving LL chickens the low-CP diet led to reduced concentration of muscle glycogen (P < 0.01), and as a result, breast meat with a higher (P < 0.001) ultimate pH, decreased (P < 0.001) lightness, and reduced (P < 0.001) drip loss during storage. The decreased muscle glycogen content observed in LL receiving the low-CP diet compared with the high-CP diet occurred concomitantly with greater phosphorylation amount for the α-catalytic subunit of adenosine monophosphate-activated protein kinase and glycogen synthase. This was consistent with the reduced muscle glycogen content observed in LL fed the low-CP diet because adenosine monophosphate-activated protein kinase inhibits glycogen synthesis through its action on glycogen synthase. Our results demonstrated that nutrition is an effective means of modulating breast meat properties in the chicken. The results also highlighted the need to take into account interaction with the genetic background of the animal to select nutritional strategies to improve meat quality traits in poultry.

Neurodegeneration and functional impairments associated with glycogen synthase accumulation in a mouse model of Lafora disease

Lafora disease (LD) is caused by mutations in either the laforin or malin gene. The hallmark of the disease is the accumulation of polyglucosan inclusions called Lafora Bodies (LBs). Malin knockout (KO) mice present polyglucosan accumulations in several brain areas, as do patients of LD. These structures are abundant in the cerebellum and hippocampus. Here, we report a large increase in glycogen synthase (GS) in these mice, in which the enzyme accumulates in LBs. Our study focused on the hippocampus where, under physiological conditions, astrocytes and parvalbumin-positive (PV+) interneurons expressed GS and malin. Although LBs have been described only in neurons, we found this polyglucosan accumulation in the astrocytes of the KO mice. They also had LBs in the soma and some processes of PV+ interneurons. This phenomenon was accompanied by the progressive loss of these neuronal cells and, importantly, neurophysiological alterations potentially related to impairment of hippocampal function. Our results emphasize the relevance of the laforin–malin complex in the control of glycogen metabolism and highlight altered glycogen accumulation as a key contributor to neurodegeneration in LD.

Effect of Thiamin Derivative on Carbohydrate Metabolism at Rest in Mice

Thiamin is known to activate carbohydrate metabolism by the activation of pyruvate dehydrogenase (PDH). We previously reported that thiamin tetrahydrofurfuryl disulfide (TTFD), which is a derivative of thiamin, inhibits oxidation of exogenous glucose at rest, but not during exercise. Furthermore, concentrations of muscle and liver glycogen tended to be lower after TTFD ingestion during recovery from endurance exercise. Therefore, it could be hypothesized that administration of a thiamin derivative inhibited glycogen synthesis from glucose in muscle and liver and changed liver cytosolic glucose-induced lipogenesis via pentosephosphate pathway, which involves the other thiamin sensitive transketolase (TK) reaction. PURPOSE: To investigate the effect of TTFD on carbohydrate metabolism in liver and skeletal muscles at rest. METHODS: Seven-week-old male ICR mice were randomly assigned to a TTFD ingestion group or a water ingestion group. Mice orally ingested glucose (0.8 mg/g BW) plus water or TTFD (0.1 mg/g BW). Concentrations of blood substrates, muscle and liver glycogen and liver TK activity were measured at rest. RESULTS: No significant effect of TTFD was found in concentrations of blood lactate and glucose at rest. Blood free fatty acid concentrations were significantly lower in TTFD group than in water group (P<0.01). Concentration of muscle glycogen was significantly lower in TTFD mice than in control mice (P<0.05), but no significant difference was found in liver glycogen. Liver TK activity was lower in TTFD group (P<0.05). Therefore, single treatment of TTFD and glucose might suppress liver glucose-induced lipogenesis via pentose phosphate pathway. CONCLUSIONS: These results indicate that TTFD, a derivative of thiamin, which has been known to activate glucose metabolism, decreased glycogen concentration in skeletal muscle and blood free fatty acid concentration at rest.

Diverse mechanisms of growth inhibition by luteolin, resveratrol, and quercetin in MIA PaCa-2 cells: a comparative glucose tracer study with the fatty acid synthase inhibitor C75

The rationale of this dose matching/dose escalating study was to compare a panel of flavonoids-luteolin, resveratrol, and quercetin-against the metabolite flux-controlling properties of a synthetic targeted fatty acid synthase inhibitor drug C75 on multiple macromolecule synthesis pathways in pancreatic tumor cells using [1,2-(13)C(2)]-d-glucose as the single precursor metabolic tracer. MIA PaCa-2 pancreatic adenocarcinoma cells were cultured for 48 h in the presence of 0.1% DMSO (control), or 50 or 100 μM of each test compound, while intracellular glycogen, RNA ribose, palmitate and cholesterol as well as extra cellular (13)CO(2), lactate and glutamate production patterns were measured using gas chromatography/mass spectrometry (GC/MS) and stable isotope-based dynamic metabolic profiling (SiDMAP). The use of 50% [1,2-(13)C(2)]-d-glucose as tracer resulted in an average of 24 excess (13)CO(2) molecules for each 1,000 CO(2) molecule in the culture media, which was decreased by 29 and 33% (P < 0.01) with 100 μM C75 and luteolin treatments, respectively. Extracellular tracer glucose-derived (13)C-labeled lactate fractions (Σm) were between 45.52 and 47.49% in all cultures with a molar ratio of 2.47% M + 1/Σm lactate produced indirectly by direct oxidation of glucose in the pentose cycle in control cultures; treatment with 100 μM C75 and luteolin decreased this figure to 1.80 and 1.67%. The tracer glucose-derived (13)C labeled fraction (Σm) of ribonucleotide ribose was 34.73% in controls, which was decreased to 20.58 and 8.45% with C75, 16.15 and 6.86% with luteolin, 27.66 and 19.25% with resveratrol, and 30.09 and 25.67% with quercetin, respectively. Luteolin effectively decreased nucleotide precursor synthesis pentose cycle flux primarily via the oxidative branch, where we observed a 41.74% flux (M + 1/Σm) in control cells, in comparison with only a 37.19%, 32.74%, or a 26.57%, 25.47% M + 1/Σm flux (P < 0.001) after 50 or 100 μM C75 or luteolin treatment. Intracellular de novo fatty acid palmitate (C16:0) synthesis was severely and equally blocked by C75 and luteolin treatments indicated by the 5.49% (control), 2.29 or 2.47% (C75) and 2.21 or 2.73% (luteolin) tracer glucose-derived (13)C-labeled fractions, respectively. On the other hand there was a significant 192 and 159% (P < 0.001), and a 103 and 117% (P < 0.01) increase in tracer glucose-derived cholesterol after C75 or luteolin treatment. Only resveratrol and quercetin at 100 μM inhibited tracer glucose-derived glycogen labeling (Σm) and turnover by 34.8 and 23.8%, respectively. The flavonoid luteolin possesses equal efficacy to inhibit fatty acid palmitate de novo synthesis as well as nucleotide RNA ribose turnover via the oxidative branch of the pentose cycle in comparison with the targeted fatty acid synthase inhibitor synthetic compound C75. Luteolin is also effective in stringently controlling glucose entry and anaplerosis in the TCA cycle, while it promotes less glucose flux towards cholesterol synthesis than that of C75. In contrast, quercetin and resveratrol inhibit glycogen synthesis and turnover as their underlying mechanism of controlling tumor cell proliferation. Therefore the flavonoid luteolin controls fatty and nucleic acid syntheses as well as energy production with pharmacological strength, which can be explored as a non-toxic natural treatment modality for pancreatic cancer.

Energy sensing by the AMP-activated protein kinase and its effects on muscle metabolism

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status, and a regulator of energy balance at both the cellular and whole body levels. Although ubiquitously expressed, its function is best understood in skeletal muscle. AMPK contains sites that reversibly bind AMP or ATP, with an increase in cellular AMP:ATP ratio (signalling a fall in cellular energy status) switching on the kinase. In muscle, AMPK activation is therefore triggered by sustained contraction, and appears to be particularly important in the metabolic changes that occur in the transition from resistance to endurance exercise. Once activated, AMPK switches on catabolic processes that generate ATP, while switching off energy-requiring processes not essential in the short term. Thus, it acutely activates glucose uptake (by promoting translocation of the transporter GLUT4 to the membrane) and fatty acid oxidation, while switching off glycogen synthesis and protein synthesis (the later via inactivation of the mammalian target-of-rapamycin pathway). Prolonged AMPK activation also causes some of the chronic adaptations to endurance exercise, such as increased GLUT4 expression and mitochondrial biogenesis. AMPK contains a glycogen-binding domain that causes a sub-fraction to bind to the surface of the glycogen particle, and it can inhibit glycogen synthesis by phosphorylating glycogen synthase. We have shown that AMPK is inhibited by exposed non-reducing ends in glycogen. We are working on the hypothesis that this ensures that glycogen synthesis is rapidly activated when glycogen becomes depleted after exercise, but is switched off again as soon as glycogen stores are replenished.

Glycogen synthesis in brain and astrocytes is inhibited by chronic lithium treatment

Lithium is a drug widely used to treat bipolar disorder. It has been shown to inhibit the total activity of phosphoglucomutase (PGM) from rat brains. In this work, we show that lithium inhibits in vitro PGM activity in the cortex, hippocampus, striatum, brainstem and cerebellum. As a compensatory effect, chronic lithium treatment of Wistar rats for 6 weeks caused a 1.6-fold upregulation of cortex PGM activity. No difference was observed in the other areas tested. Another effect of chronic lithium administration was a drastic reduction of glycogen content in rat brains, as PGM activity is essential for its synthesis. In a primary culture of astrocytes, which are the main cellular components of the brain that produce glycogen, administration of 1 mM lithium for 3 days markedly reduced the steady state of glycogen content. In agreement with this result, lithium did not cause insulin-like effects as previously observed in hepatocytes where lithium activated glycogen synthesis. Reduction of glycogen content was due to inhibition of glycogen synthesis, as incorporation of [ 14U −C]-glucose into glycogen was impaired by lithium. Consistent with these results, incubation of glucose-starved astrocytes with lithium did not stimulate dephosphorylation of glycogen synthase, which normally occurs with re-feeding of glucose. Furthermore, in a chronically treated astrocyte culture, glycogen synthase was phosphorylated constitutively. Our results indicate that chronic lithium treatment can inhibit glycogen synthesis in brain suggesting that this effect might contribute to lithium's therapeutic effect.

O‐GlcNacylation of Glycogenin Inhibits Glycogen Synthesis

It is becoming evident that increased cellular glucose concentration leads to an increase in UDP‐ß‐N‐Acetylglucosamine (UDP‐GlcNac) concentration and O‐GlcNacylation of many proteins. Since glycogenin self‐glycosylates with UDP‐glucose, we investigated whether UDP‐GlcNac serves as its substrate and possible effects of this modification on glycogen synthesis. Incubation of purified glycogen synthase, glycogenin, and apo‐glycogenin with UDPGlcNac led to incorporation of ~0.5 mole of GlcNac per mole of glycogenin. The presence of incorporated GlcNac into glycogenin was confirmed by HPAEC‐PAD and by radiolabeling with galactosyl transferase. The apparent KM for UDP‐GlcNac was 227 μM. Mutating Tyrosine 194 to Phenylalanine eliminated the ability of glycogenin to autoglycosylate with either substrate confirming that GlcNac was added to the same site as glucose. Our results suggest that GlcNac is added to Tyr194 either as a first residue or to the growing oligosaccharide. Furthermore, we observed that recombinant glycogenin modified with O‐GlcNac was still able to autoglucosylate. However, O‐GlcNac modified glycogenin was resistant to the addition of glucose by glycogen synthase. These results suggest that O‐GlcNacylation of glycogenin may inhibit glycogen synthase activity resulting in inhibition of glycogen synthesis. Supported by sabbatical leave to BSK and RISE grant: GM‐071638.

Glycogen metabolism

ATP= : adenosine triphosphate; cAMP= : cyclic adenosine monophosphate; GBE= : glycogen branching enzyme; GSK= : glycogen synthase kinase; PTG= : protein targeting to glycogen. Brain glycogen metabolism provides a typical example of metabolic coupling between neurons and astrocytes. Glycogen is almost exclusively localized in astrocytes and serves as an endogenous source of energy for these cells and for neurons. Neural activity leads to release of glutamate and other neurotransmitters that trigger mobilization of energy from glycogen in astrocytes. Glycogenolysis in astrocytes leads to the production of lactate, which serves as an energy substrate for the active neurons. Although the functional role of glycogen in the brain is incompletely understood, there is experimental evidence that glycogen is able to sustain neuronal activity during energy deprivation or in the setting of hypoxia, ischemia, hypoglycemia, or seizures. Whereas neurons also have the enzymatic machinery to synthesize glycogen, in normal conditions this process is actively inhibited, and this prevents glycogen- triggered damage of these cells. Abnormal glycogen metabolism has been typically associated with muscle disorders, but may also underlie some disorders affecting the CNS. Inability to inhibit glycogen synthesis in neurons is the basis for Lafora disease and impaired activity of glycogen branching enzyme occurs in adult polyglucosan body disease. The focus of this short review is on some physiologic aspects of brain glycogen metabolism and their clinical correlations. These topics have been comprehensively reviewed elsewhere.1–7 ### Glycogen metabolism in astrocytes. In adult brain, glycogen is predominantly present in astrocytes. Glycogen is a spherical and highly branched polymer comprising up to approximately 53,000 glycosyl units. It is synthesized from a core consisting of a peptide called glycogenin and proglycogen, which serves as a stable intermediate that is converted into macroglycogen, the storage form of glycogen.8,9 Glycogen synthesis from glucose depends on the activity of glycogen synthase, which catalyzes formation of α-1,4-glycosidic linkage of the polymer, and glycogen branching enzyme, which introduces the α-1,6-glycosidic branch points. Degradation occurs in the cytosol through …

Abstract C8: Short-chain fatty acids induce autophagy through inhibition of mTOR pathway

Abstract Short-chain fatty acid (SCFA), mainly propionate and butyrate are major by-products of the bacterial fermentation of undigested dietary fiber in the human large intestine. Evidence to date implicates propionate/butyrate as inhibitors of colon cancer cell proliferation and inducers of apoptotic cell death. In this study, we demonstrated that propionate/butyrate was able to induce another type of programmed cell death named autophagy in human colon cancer HCT116 cells. Propionate/butyrate-treated cells exhibited extensive characteristics of autophagy, namely the conversion of LC3-I into LC3-II, reduced expression of p62/ SQSTM1; redistribution of GFPLC3 fusion protein from a diffuse pattern towards autophagosomes, which became visible as cytoplasmic puncta; and increased acidic vesicular organelle development as manifested by dramatically increased quantity of strongly stained punctate areas reflected by LysoTracker staining and monodansylcadaverine (MDC) incorporation. Propionate/butyrate-induced autophagy was based on inhibition of its negative regulator-mTOR pathway as evidenced by dephosphorylation of p70S6K and 4E-BP1 and was marked by increased phosphorylation of AMP kinase, two events known to be linked to autophagy. Activation of AMPK was associated with a fluctuation of intracellular ATP, during which the level of ATP transiently increased following propionate/butyrate exposure, reaching maximum at 5 hours, and then declined in slow but gradual manner. The decline of ATP may be due to compromised mitochondria membrane potential and enhanced cellular reactive oxygen species generation. We also showed that this autophagic increase was associated with a series of changes in metabolic pathways, in which energy-demanding program (fatty acids composition and glycogen synthesis inhibition) was downregulated whereas energy-producing program (mitochondria biogenesis) was initiated to cope with the energy crisis. Finally, we showed that increased autophagy and mTOR inhibition by propionate/butyrate are not dependent on p53 activation. Altogether, this study demonstrates autophagy as a novel programmed cell death pathway for propionate/butyrate induced cell death. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C8.

Glycogen Metabolism and Brain Pathologies

Glucose is the main fuel for cell life, and supports a number of different processes in providing cells with energy. Excess glucose is polymerized into glycogen, which is an energy-glucose store. Alterations in glycogen content and/or synthesis have been reported in human neuropathologies, such as Alzheimers disease, epilepsies and cancer. Epileptic foci are hypometabolic during the interictal period, and probably hypermetabolic during crisis. Animal models of epilepsies are used for studying the reasons why neurons suddenly and temporally synchronize their activity. One model associates seizures of the “grand mal” type with cortical glycogen accumulation: induction of epileptiform crisis by methionine sulfoximine (MSO). The glycogen accumulation, observed in astrocytes only, occurs as soon as the preconvulsive period. High glycogen has also been demonstrated in primary cultures of astrocytes. Abnormal glycogen content has been characterized in various types of cancers, including gliomas. High invasion properties, spontaneous resistance to chemotherapeutic drugs, and a mean prognosis of 12 months characterize glioblastomas, the highest grade of gliomas that inevitably leads to death. The various therapeutic means, including surgery, chemicaland radio-therapies, and gene therapy have thus far been inefficient in significantly improving patient survival. Glycogen synthesis was targeted in cell lines from murine and human glioblastomas by an antisense glycogen synthase cDNA strategy; and the inhibition of glycogen synthesis in these cell lines decreases both in vitro and in vivo invasiveness. Glycogen can therefore be considered as putatively involved in at least two different pathologies of the brain, such as epilepsies and cancer. This abnormal glycogen content and synthesis can be proposed as putative diagnostic and therapeutic targets in brain pathologies. Keywords: Glycogen, Glycogen synthase, Central nervous system, Astrocytes, Epilepsy, Glioblastoma, Antisense, Neuropathology

Leptin and cardiovascular disease: response to therapeutic interventions.

Leptin and cardiovascular disease: response to therapeutic interventions.

Glycogen: a Trojan horse for neurons

Neural activity leads to the mobilization of energy from glycogen in astrocytes. A new paper reports that neurons have an ambivalent relationship with glycogen: they can synthesize it themselves, but that synthesis induces apoptosis. Presumably for this reason, neurons normally inhibit glycogen synthesis through two redundant pathways.

The control of hepatic glycogen metabolism in an in vitro model of sepsis

Culturing hepatocytes with a combination of LPS, TNF-alpha, IL-1beta and IFN-gamma resulted in an inhibition of glucose output from glycogen and prevented the repletion of glycogen in freshly cultured cells. The reduced glycogen mobilisation correlated with the lower cell glycogen content and reduced rate of glycogen synthesis from [U-(14)C]glucose rather than alterations in either total phosphorylase or phosphorylase a activity. There was no change in the percentage of glycogen exported as glucose nor the production of lactate plus pyruvate indicating that redistribution of the Gluc-6-P cannot explain the failure of the liver to export glucose. Although changes in glycogen mobilisation correlated with NO production, inhibition of NO synthase by inclusion of L-NMMA in the culture medium failed to prevent the inhibition of either glycogen accumulation or mobilisation by the proinflammatory cytokines, precluding the involvement of NO in this response. LPS plus cytokine treatment had no effect on total glycogen synthase activity although the activity ratio was lowered, indicative of increased phosphorylation. The inhibition of glycogen synthesis correlated with a fall in the intracellular concentrations of Gluc-6-P and UDP-glucose and in the absence of measured changes in kinase activity, it is suggested that the fall in Gluc-6-P reduces both substrate supply and glycogen synthase phosphatase activity. The fall in Gluc-6-P coincided with a reduction in total glucokinase and hexokinase activity within the cells, but no significant change in either the translocation of glucokinase or glucose-6-phosphatase activity. This demonstrates direct cytokine effects on glycogen metabolism independent of changes in glucoregulatory hormones.