Glucose 6-phosphate produced by gluconeogenesis and by glucokinase is equally effective in activating hepatic glycogen synthase

Effects of a Novel Glucokinase Activator, Dorzagliatin, on Glycemic Control and Glucose Fluctuation in Drug-Naïve Patients with Type 2 Diabetes Mellitus

Aim

The prevalence rate of type 2 diabetes mellitus (T2DM) has been increasing and a large proportion of patients still do not achieve adequate or sustainable glycemic control on the basis of previous hypoglycemic treatment. In this present study, we explored whether dorzagliatin, a novel glucokinase activator (GKA), could improve glycemic control and lessen glucose fluctuation in drug-naïve patients with T2DM.

Methods

A self-comparative observational study of 25 drug-naïve patients with T2DM (aged 18-75 years and HbA1c of 7.5%-11.0%) treated with dorzagliatin 75 mg twice daily for 52 weeks. Before and after dorzagliatin intervention, the serum levels of hemoglobin A1c (HbA1c), insulin, and C-peptide were measured repeatedly during fasting and after a mixed meal. The continuous glucose monitoring (CGM) device was also used to obtain 24-hour glucose profiles and assess the changes in glycemic variability parameters.

Results

After 52 weeks of treatment with dorzagliatin, a numerally greater reduction in HbA1c of 1.03% from the baseline was observed in patients with T2DM, accompanied by significant improvement in insulin resistance and insulin secretion. Moreover, the standard deviation of blood glucose (SDBG) and the mean amplitude of glycemic excursion (MAGE) derived from CGM data were significantly decreased after dorzagliatin therapy. The 24-h glucose variation profile showed that study patients had obviously lower mean glucose levels during the postprandial period from the baseline to week 52, an effect also demonstrated by the significant decrease in the incremental area under glucose concentration versus time curve for 2 h (iAUC0-2 h) after meals.

Conclusions

This study suggests that dorzagliatin therapy could effectively improve glycemic control and glucose fluctuation in drug-naïve patients with T2DM.

The role of metabolic reprogramming of tumor-associated macrophages in shaping the immunosuppressive tumor microenvironment

Macrophages are potent immune effector cells in innate immunity and exert dual-effects in the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) make up a significant portion of TME immune cells. Similar to M1/M2 macrophages, TAMs are also highly plastic, and their functions are regulated by cytokines, chemokines and other factors in the TME. The metabolic changes in TAMs are significantly associated with polarization towards a protumour or antitumour phenotype. The metabolites generated via TAM metabolic reprogramming in turn promote tumor progression and immune tolerance. In this review, we explore the metabolic reprogramming of TAMs in terms of energy, amino acid and fatty acid metabolism and the potential roles of these changes in immune suppression.

Targeting human Glucokinase for the treatment of type 2 diabetes: an overview of allosteric Glucokinase activators

Diabetes mellitus is a worldwide impacting disorder and the ratio through which the number of diabetic patients had increased worldwide, puts medical professionals to serious stress for its effective management. Due to its polygenic origin and involvement of multiple genes to its pathophysiology, leads to understanding of this ailment more complex. It seems that current interventions, such as dietary changes, life style changes and drug therapy such as oral hypoglycaemics and insulin, are unable to halt the trend. There are various novel and emerging targets on which the researchers are paying attention to combat with this ailment successfully. Human glucokinase (GK) enzyme is one of these novel and emerging targets for management of diabetes. Its availability in the pancreas and liver cells makes this target more lucrative. GK's presence in the pancreatic and hepatic cells plays a very important function for the management of glucose homoeostasis. Small molecules that activate GK allosterically provide an alternative strategy for restoring/improving glycaemic regulation, especially in type 2 diabetic patients. Although after enduring many setbacks in the development of the GK activators, interest has been renewed especially due to introduction of novel dual acting GK activator dorzagliatin, and a novel hepato-selective GK activator, TTP399. This review article has been formulated to discuss importance of GK in glucose homeostasis, recent updates on small molecules of GK activators, clinical status of GK activators and challenges in development of GK activators.

Abnormalities in microbial composition and function in infants with necrotizing enterocolitis: A single-center observational study

OBJECTIVE

To investigate the features and functions of the intestinal microbiota in neonates with necrotizing enterocolitis (NEC) in a single center in China.

METHODS

We collected clinical information and stool samples from 19 participants in our center, including 9 infants with necrotizing enterocolitis and 10 control infants. DNA was extracted from the samples, and 16S rRNA gene sequencing was used to analyse the participants' gut microbiota. Functional prediction was achieved using PICRUSt2.

RESULTS

Alpha diversity analysis found that similar levels of bacterial richness and diversity were found in the gut microbiota of infants with NEC and control infants (P = 0.1800), whereas beta diversity analysis suggested that the overall structures of the gut microbiota were significantly different (P = 0.0020). The Mann-Whitney U test of bacterial composition and abundance analysis revealed that the abundance levels of Proteobacteria (P = 0.03049) and Firmicutes (P = 0.01011) significantly differed between the two groups at the phylum level. Proteobacteria was the most abundant phylum in the NEC group. At the genus level, the abundance levels of Enterococcus (P = 0.0003), Streptococcaceae (P = 0.0109) and Lactobacillales (P = 0.0171) were significantly decreased in infants with NEC. Furthermore, the linear discriminant analysis effect size (LEfSe) method showed 12 bacterial taxa with significant differences in relative abundances in the two groups. Interestingly, members of Proteobacteria were enriched in NEC samples. In addition, functional prediction suggested that the microbial changes observed in infants with NEC resulted in a decline in galactose metabolism, the pentose phosphate pathway, fructose and mannose metabolism, amino sugar and nucleotide sugar metabolism, glycolysis/gluconeogenesis, starch and sucrose metabolism, and phosphotransferase system (PTS) pathways (P < 0.05).

CONCLUSIONS

Our study shows the compositional and functional alterations of the intestinal microbiota in NEC, which will help demonstrate the relationship between the gut microbiota and NEC pathogenesis.

Gut microbiota associated with cryptococcal meningitis and dysbiosis caused by anti-fungal treatment

The gut microbiota is a dynamic and highly diverse microbial ecosystem that affects many aspects of the host's physiology. An improved understanding of the gut microbiota could lead to better strategies for the diagnosis and therapy of cryptococcal meningitis (CM), but the impact of Cryptococcus infection and anti-fungal treatment on the gut microbiota has rarely been studied. We characterized the diversity and composition of the gut microbiota in CM patients at diagnosis and healthy controls (HCs) using metagenomic sequencing and determined the effects of anti-fungal drugs. We found that CM patients had distinct bacterial and fungal compositions compared with HCs, with eight differentially abundant fungal and 72 differentially abundant bacterial species identified between the two groups. CM patients showed an increased abundance of Enterococcus avium, Leuconostoc mesenteroides, and Weissella cibaria, and a decreased abundance of Prevotella spp. compared with HCs. However, anti-fungal treatment only led to minor changes in the intestinal microbiota. Moreover, both positive and negative correlations existed in fungal, bacterial, and clinical indicators. Our study suggests that the Cryptococcus neoformans infection caused a distinct dysbiosis of the gut microbiota and contributes valuable information implying potential links between the CM and gut microbiota.

Beyond energy storage: roles of glycogen metabolism in health and disease

Beyond storing and supplying energy in the liver and muscles, glycogen also plays critical roles in cell differentiation, signaling, redox regulation, and stemness under various physiological and pathophysiological conditions. Such versatile functions have been revealed by various forms of glycogen storage diseases. Here, we outline the source of carbon flux in glycogen metabolism and discuss how glycogen metabolism guides CD8⁺ T-cell memory formation and maintenance. Likewise, we review how this affects macrophage polarization and inflammatory responses. Furthermore, we dissect how glycogen metabolism supports tumor development by promoting tumor-repopulating cell growth in hypoxic tumor microenvironments. This review highlights the essential role of the gluconeogenesis-glycogenesis-glycogenolysis-PPP metabolic chain in redox homeostasis, thus providing insights into potential therapeutic strategies against major chronic diseases including cancer.

Glycogen metabolism regulates macrophage-mediated acute inflammatory responses

Our current understanding of how sugar metabolism affects inflammatory pathways in macrophages is incomplete. Here, we show that glycogen metabolism is an important event that controls macrophage-mediated inflammatory responses. IFN-γ/LPS treatment stimulates macrophages to synthesize glycogen, which is then channeled through glycogenolysis to generate G6P and further through the pentose phosphate pathway to yield abundant NADPH, ensuring high levels of reduced glutathione for inflammatory macrophage survival. Meanwhile, glycogen metabolism also increases UDPG levels and the receptor P2Y₁₄ in macrophages. The UDPG/P2Y₁₄ signaling pathway not only upregulates the expression of STAT1 via activating RARβ but also promotes STAT1 phosphorylation by downregulating phosphatase TC45. Blockade of this glycogen metabolic pathway disrupts acute inflammatory responses in multiple mouse models. Glycogen metabolism also regulates inflammatory responses in patients with sepsis. These findings show that glycogen metabolism in macrophages is an important regulator and indicate strategies that might be used to treat acute inflammatory diseases.

Heat-Shock Protein 70-Mediated Heat Preconditioning Attenuates Hepatic Carbohydrate and Oxidative Disturbances in Rats With Type 1 Diabetes

OBJECTIVES

Heat preconditioning and heat-shock protein (HSP) synthesis have significant cytoprotective effects against the development of cellular injury caused by the application of a subsequent stressor, which were found to depend on the time period between the stressors. We aimed to determine the most efficient recovery time (6 h or 24 h) following heat-stress exposure and prior application of diabetic streptozotocin (STZ) on the moderation of carbohydrate and oxidative metabolic disturbances caused by diabetes.

METHODS

Experiment animals (Wistar rats) were exposed to acute heat stress at 41±1°C for 45 min, followed by 6-h or 24-h recovery times at room temperature before sacrifice or STZ administration.

RESULTS

Our findings indicate that acute heat stress with 6-h or 24-h recovery periods results in a significant rise in the hepatic heat-shock protein 70 (HSP70) levels (even more so after 24 h), glycogen breakdown and stable glycemia, followed by reduced glycolytic and gluconeogenic activity (after 24 h) (glucose-6-phosphatase, fructose-1,6-bisphosphatase); stimulates antioxidative activity (glutathione peroxidase, glutathione reductase) (after 6 h); and decreases glutathione and catalase activity (after 24 h). Heat preconditioning (with 6-h and 24-h recovery periods) prior to STZ-induced diabetes increases HSP70 levels and causes lower serum glucose levels, higher glycogen and glucose-6-phosphate levels, lower glucose-6-phosphatase levels and glycogen phosphorylase and hexokinase levels but also elevates glutathione reductase and glutathione peroxidase activity compared to untreated STZ animals.

CONCLUSIONS

Based on our findings, heat preconditioning and HSP70 induction in rats with type 1 diabetes attenuates STZ-induced metabolic alterations in hepatic carbohydrate metabolism and oxidative states. These changes are more evident at 24 h recovery post-acute heat stress, based on the most evident accumulation of HSP70 in this time frame.

In ovo feeding of L-arginine alters energy metabolism in post-hatch broilers

This study aimed to investigate the effects of in ovo feeding (IOF) of L-arginine (Arg) on energy metabolism in post-hatch broilers. A total of 720 eggs was randomly assigned to 3 treatments: 1) non-injected control group, 2) 0.75% NaCl diluent-injected control group, and 3) 1.0% Arg solution-injected group. At 17.5 d of incubation, 0.6 mL of each solution was injected into the amniotic fluid of each egg of injected groups. After hatching, 80 male chicks were randomly assigned to each treatment group with 8 replicates per group. The results showed that IOF of Arg increased glycogen and glucose concentrations in the liver and pectoral muscle of broilers at hatch (P < 0.05). The plasma glucose and insulin levels were higher in the Arg group than in the non-injected and diluent-injected control groups (P < 0.05). Meanwhile, IOF of Arg enhanced the hepatic glucose-6-phosphatase (G6P) activity at hatch (P < 0.05). There was no difference in hexokinase (HK) or phosphofructokinase (PFK) enzyme activities in the pectoral muscle in all groups. Further, IOF of Arg increased the phosphoenolpyruvate carboxykinase (PEPCK) and fructose-1,6-bisphosphatase (FBP) mRNA expressions at hatch (P < 0.05). In addition, broilers in the Arg group had a higher mRNA expression of glycogen synthase and a lower expression of glycogen phosphorylase in the liver and pectoral muscles than in the non-injected controls at hatch (P < 0.05). In conclusion, IOF of Arg solution enhanced liver and pectoral muscle energy reserves at hatch, which might be considered as an effective strategy for regulating early energy metabolism in broilers.

Immunochemical Study of the Effect of F2Glc on Glycogen Synthase Translocation and Glycogen Synthesis in Isolated Rat Hepatocytes

The compound 2-deoxy-2-fluoro-α-D-glucopyranosyl fluoride (F₂Glc), which is a nonmetabolized superior glucose analogue, is a potent inhibitor of glycogen phosphorylase and pharmacological properties are reported. Glycogen phosphorylase (GP) and glycogen synthase (GS) are responsible of the degradation and synthesis, respectively, of glycogen which is a polymer of glucose units that provides a readily available source of energy in mammals. GP and GS are two key enzymes that modulate cellular glucose and glycogen levels; therefore, these proteins are suggested as potential targets for the treatment of diseases related to glycogen metabolism disorders. We studied by Western Blot technique that F₂Glc decreased GP activity, and we also showed that F₂Glc did not affect GS activity and its translocation from a uniform cytosolic distribution to the hepatocyte periphery, which is crucial for glycogen synthesis, using immunoblotting and immunofluorescence labeling techniques. F₂Glc specifically inhibits glycogenolysis pathway and permits a greater deposition of glycogen. These observations open up the possibility of further develop drugs that act specifically on GP. The ability to selectively inhibit GP, which is a key enzyme for the release of glucose from the hepatic glycogen reserve, may represent a new approach for the treatment of hyperglycemia in type 2 diabetes.

Prevalent role of the insulin receptor isoform A in the regulation of hepatic glycogen metabolism in hepatocytes and in mice

AIMS/HYPOTHESIS

In the postprandial state, the liver regulates glucose homeostasis by glucose uptake and conversion to glycogen and lipids. Glucose and insulin signalling finely regulate glycogen synthesis through several mechanisms. Glucose uptake in hepatocytes is favoured by the insulin receptor isoform A (IRA), rather than isoform B (IRB). Thus, we hypothesised that, in hepatocytes, IRA would increase glycogen synthesis by promoting glucose uptake and glycogen storage.

METHODS

We addressed the role of insulin receptor isoforms on glycogen metabolism in vitro in immortalised neonatal hepatocytes. In vivo, IRA or IRB were specifically expressed in the liver using adeno-associated virus vectors in inducible liver insulin receptor knockout (iLIRKO) mice, a model of type 2 diabetes. The role of IR isoforms in glycogen synthesis and storage in iLIRKO was subsequently investigated.

RESULTS

In immortalised hepatocytes, IRA, but not IRB expression induced an increase in insulin signalling that was associated with elevated glycogen synthesis, glycogen synthase activity and glycogen storage. Similarly, elevated IRA, but not IRB expression in the livers of iLIRKO mice induced an increase in glycogen content.

CONCLUSIONS/INTERPRETATION

We provide new insight into the role of IRA in the regulation of glycogen metabolism in cultured hepatocytes and in the livers of a mouse model of type 2 diabetes. Our data strongly suggest that IRA is more efficient than IRB at promoting glycogen synthesis and storage. Therefore, we suggest that IRA expression in the liver could provide an interesting therapeutic approach for the regulation of hepatic glucose content and glycogen storage.

Glycogen metabolism has a key role in the cancer microenvironment and provides new targets for cancer therapy

Metabolic reprogramming is a hallmark of cancer cells and contributes to their adaption within the tumour microenvironment and resistance to anticancer therapies. Recently, glycogen metabolism has become a recognised feature of cancer cells since it is upregulated in many tumour types, suggesting that it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells under stress conditions such as hypoxia, glucose deprivation and anticancer treatment. The various methods to detect glycogen in tumours in vivo as well as pharmacological modulators of glycogen metabolism are also reviewed. Finally, we discuss the therapeutic value of targeting glycogen metabolism as a strategy for combinational approaches in cancer treatment.

Glycogen metabolism in cancer

Since its identification more than 150 years ago, there has been an extensive characterisation of glycogen metabolism and its regulatory pathways in the two main glycogen storage organs of the body, i.e. liver and muscle. In recent years, glycogen metabolism has also been demonstrated to be upregulated in many tumour types, suggesting it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells. The various methods to detect glycogen in tumours in vivo are also reviewed. Finally, we discuss the targeting of glycogen metabolism as a strategy for cancer treatment.

Glucose-6-phosphate-mediated activation of liver glycogen synthase plays a key role in hepatic glycogen synthesis

The liver responds to an increase in blood glucose levels in the postprandial state by uptake of glucose and conversion to glycogen. Liver glycogen synthase (GYS2), a key enzyme in glycogen synthesis, is controlled by a complex interplay between the allosteric activator glucose-6-phosphate (G6P) and reversible phosphorylation through glycogen synthase kinase-3 and the glycogen-associated form of protein phosphatase 1. Here, we initially performed mutagenesis analysis and identified a key residue (Arg(582)) required for activation of GYS2 by G6P. We then used GYS2 Arg(582)Ala knockin (+/R582A) mice in which G6P-mediated GYS2 activation had been profoundly impaired (60-70%), while sparing regulation through reversible phosphorylation. R582A mutant-expressing hepatocytes showed significantly reduced glycogen synthesis with glucose and insulin or glucokinase activator, which resulted in channeling glucose/G6P toward glycolysis and lipid synthesis. GYS2(+/R582A) mice were modestly glucose intolerant and displayed significantly reduced glycogen accumulation with feeding or glucose load in vivo. These data show that G6P-mediated activation of GYS2 plays a key role in controlling glycogen synthesis and hepatic glucose-G6P flux control and thus whole-body glucose homeostasis.

Effects of a glucokinase activator on hepatic intermediary metabolism: study with 13C-isotopomer-based metabolomics

GKAs (glucokinase activators) are promising agents for the therapy of Type 2 diabetes, but little is known about their effects on hepatic intermediary metabolism. We monitored the fate of (13)C-labelled glucose in both a liver perfusion system and isolated hepatocytes. MS and NMR spectroscopy were deployed to measure isotopic enrichment. The results demonstrate that the stimulation of glycolysis by GKA led to numerous changes in hepatic metabolism: (i) augmented flux through the TCA (tricarboxylic acid) cycle, as evidenced by greater incorporation of (13)C into the cycle (anaplerosis) and increased generation of (13)C isotopomers of citrate, glutamate and aspartate (cataplerosis); (ii) lowering of hepatic [Pi] and elevated [ATP], denoting greater phosphorylation potential and energy state; (iii) stimulation of glycogen synthesis from glucose, but inhibition of glycogen synthesis from 3-carbon precursors; (iv) increased synthesis of N-acetylglutamate and consequently augmented ureagenesis; (v) increased synthesis of glutamine, alanine, serine and glycine; and (vi) increased production and outflow of lactate. The present study provides a deeper insight into the hepatic actions of GKAs and uncovers the potential benefits and risks of GKA for treatment of diabetes. GKA improved hepatic bioenergetics, ureagenesis and glycogenesis, but decreased gluconeogenesis with a potential risk of lactic acidosis and fatty liver.

A novel mechanism for regulating hepatic glycogen synthesis involving serotonin and cyclin-dependent kinase-5

Hepatic autonomic nerves regulate postprandial hepatic glucose uptake, but the signaling pathways remain unknown. We tested the hypothesis that serotonin (5-hydroxytryptamine [5-HT]) exerts stimulatory and inhibitory effects on hepatic glucose disposal. Ligands of diverse 5-HT receptors were used to identify signaling pathway(s) regulating glucose metabolism in hepatocytes. 5-HT had stimulatory and inhibitory effects on glycogen synthesis in hepatocytes mediated by 5-HT1/2A and 5-HT2B receptors, respectively. Agonists of 5-HT1/2A receptors lowered blood glucose and increased hepatic glycogen after oral glucose loading and also stimulated glycogen synthesis in freshly isolated hepatocytes with greater efficacy than 5-HT. This effect was blocked by olanzapine, an antagonist of 5-HT1/2A receptors. It was mediated by activation of phosphorylase phosphatase, inactivation of glycogen phosphorylase, and activation of glycogen synthase. Unlike insulin action, it was not associated with stimulation of glycolysis and was counteracted by cyclin-dependent kinase (cdk) inhibitors. A role for cdk5 was supported by adaptive changes in the coactivator protein p35 and by elevated glycogen synthesis during overexpression of p35/cdk5. These results support a novel mechanism for serotonin stimulation of hepatic glycogenesis involving cdk5. The opposing effects of serotonin, mediated by distinct 5-HT receptors, could explain why drugs targeting serotonin function can cause either diabetes or hypoglycemia in humans.

Restoration of hepatic glycogen deposition reduces hyperglycaemia, hyperphagia and gluconeogenic enzymes in a streptozotocin-induced model of diabetes in rats

AIMS/HYPOTHESIS

Glycogen deposition is impaired in diabetes, thus contributing to the development of hyperglycaemia. Several glucose-lowering strategies have attempted to increase liver glycogen deposition by modulating targets, which eventually trigger the activation of liver glycogen synthase (LGS). However, these targets also alter several other biological processes, and therefore their therapeutic use may be limited. Here we tested the approach of directly activating LGS and evaluated the potential of this strategy as a possible treatment for diabetes.

METHODS

In this study, we examined the efficacy of directly overproducing a constitutively active form of LGS in the liver to ameliorate streptozotocin-induced diabetes in rats.

RESULTS

Activated mutant LGS overproduction in the liver of streptozotocin-induced diabetic rats normalised liver glycogen content, despite low levels of glucokinase and circulating insulin. Moreover, this overproduction led to a decrease in food intake and in the production of the main gluconeogenic enzymes, glucose-6-phosphatase, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase. The resulting combined effect was a reduction in hyperglycaemia.

CONCLUSIONS/INTERPRETATION

The restoration of liver glycogen ameliorated diabetes and therefore is considered a potential strategy for the treatment of this disease.

A physiological increase in the hepatic glycogen level does not affect the response of net hepatic glucose uptake to insulin

To determine the effect of an acute increase in hepatic glycogen on net hepatic glucose uptake (NHGU) and disposition in response to insulin in vivo, studies were performed on two groups of dogs fasted 18 h. During the first 4 h of the study, somatostatin was infused peripherally, while insulin and glucagon were replaced intraportally in basal amounts. Hyperglycemia was brought about by glucose infusion, and either saline (n = 7) or fructose (n = 7; to stimulate NHGU and glycogen deposition) was infused intraportally. A 2-h control period then followed, during which the portal fructose and saline infusions were stopped, allowing NHGU and glycogen deposition in the fructose-infused animals to return to rates similar to those of the animals that received the saline infusion. This was followed by a 2-h experimental period, during which hyperglycemia was continued but insulin infusion was increased fourfold in both groups. During the initial 4-h glycogen loading period, NHGU averaged 1.18 +/- 0.27 and 5.55 +/- 0.53 mg x kg(-1) x min(-1) and glycogen synthesis averaged 0.72 +/- 0.24 and 3.98 +/- 0.57 mg x kg(-1) x min(-1) in the saline and fructose groups, respectively (P < 0.05). During the 2-h hyperinsulinemic period, NHGU rose from 1.5 +/- 0.4 and 0.9 +/- 0.2 to 3.1 +/- 0.6 and 2.5 +/- 0.5 mg x kg(-1) x min(-1) in the saline and fructose groups, respectively, a change of 1.6 mg x kg(-1) x min(-1) in both groups despite a significantly greater liver glycogen level in the fructose-infused group. Likewise, the metabolic fate of the extracted glucose (glycogen, lactate, or carbon dioxide) was not different between groups. These data indicate that an acute physiological increase in the hepatic glycogen content does not alter liver glucose uptake and storage under hyperglycemic/hyperinsulinemic conditions in the dog.

Oxidative stress and chronological aging in glycogen-phosphorylase-deleted yeast

Chronological aging in yeast resembles aging in mammalian, postmitotic tissues. Such chronological aging begins with entrance into the stationary phase after the nutrients are exhausted. Many changes in metabolism take place at this moment, and survival in this phase strongly depends on oxidative-stress resistance. In this study, hypo- and hyperglycogenic phenotypes of Saccharomyces cerevisiae strains with deletions of carbohydrate-metabolism enzymes were selected, and a comparison of their chronological longevities was made. Stress sensitivity, ROS, and apoptosis markers during aging were analyzed in the emerged candidates. Among the strains that accumulated greater amounts of glycogen, the deletion of glycogen phosphorylase, gph1delta, was unique in showing a shortened life span, stress intolerance, and higher levels of ROS during its survival. The transcription of superoxide dismutase genes during survival was three- to fourfold lower in gph1delta. Extra copies of SOD1/2 counteracted the stress sensitivity and the accelerated aging of gph1delta. In conclusion, the lack of gph1 produced a rapidly aging strain, which could be attributed, at least in part, to the weakened stress resistance associated with the decreased expression of both SODs. Gph1p seems to be a candidate in a scenario that could link early metabolic changes with other targets of the stress response during stationary-phase survival.

Hypermethylation of hepatic Gck promoter in ageing rats contributes to diabetogenic potential

AIMS/HYPOTHESIS

Hepatic glucokinase (GCK) is a key enzyme in glucose utilisation. Downregulation of its activity is associated with insulin resistance and type 2 diabetes mellitus. However, it is unknown whether hepatic Gck expression is influenced by age and is involved in ageing-mediated diabetes, and whether the degree of methylation of the hepatic Gck promoter is correlated with the transcription of Gck. To address the question, we evaluated hepatic Gck transcription and promoter methylation in young (14 weeks), adult (40 weeks) and aged (80 weeks) rats.

METHODS

Hepatic glycogen, Gck expression and the kinase activity of GCK were measured in three age groups. The CpG methylation status was determined by both bisulphite direct sequencing and clone sequencing of the PCR amplificates of Gck promoter. The causal relationship between Gck methylation and mRNA expression was confirmed by treating rat primary hepatocytes with 5-aza-2'-deoxycytidine (5-Aza-CdR).

RESULTS

We have shown an age-associated decline in hepatic glycogen, Gck expression levels and the kinase activity of hepatic GCK. The eleven CpG sites studied displayed age-related progressive methylation changes in hepatic Gck promoter, which were confirmed by two methods: direct and clone sequencing. After 5-Aza-CdR treatment of rat primary hepatocytes, there was a fourfold increase in Gck expression.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that an age-related increase in methylation is negatively associated with hepatic Gck expression, suggesting that DNA methylation could be involved in increasing age-dependent susceptibility to hepatic insulin resistance and diabetes. Thus, the epigenetic modification of the hepatic Gck promoter may represent an important marker for diabetogenic potential during the ageing process.

Changes in carbohydrate metabolism during acclimation to a moderate hyperthermic environment in rats

We studied the influence of heat acclimation (1 to 48 h and 4 to 60 d at 35 +/- 1 degrees C) on certain hepatic carbohydrate-related enzymes and substrates in rats. The results showed a decrease of liver glycogen content and GPho-ase a activity during the period of short-term exposure, followed by normalization to the control level and stabilization to the new level in the period of long-term heat acclimation. Conversely, G-6-P-ase and F-1,6-BP-ase activities increased during the short-term period, followed by a decrease and stabilization to a new, lower level in the prolonged acclimation. The blood glucose level decreased during whole period of acclimation, whereas intermediate substrates increased during the short-term and stabilized at a new, higher level during prolonged acclimation. The time-dependent changes of duration of heat acclimation could be summarized in three phases: short-term heat exposure (1 to 24 h) with intensive glycogenolysis and gluconeogenesis to glucose; a period with temporary changes (24 h to 7 d) with tendency of normalization to control level, and prolonged heat acclimation (7 d to 60 d), which favors both direct and indirect glycogen synthesis.

Hepatic glycogen synthesis in the absence of glucokinase: the case of embryonic liver

Glucokinase (GK, hexokinase type IV) is required for the accumulation of glycogen in adult liver and hepatoma cells. Paradoxically, mammalian embryonic livers store glycogen successfully in the absence of GK. Here we address how mammalian embryonic livers, but not adult livers or hepatoma cells, manage to accumulate glycogen in the absence of this enzyme. Hexokinase type I or II (HKI, HKII) substitutes for GK in hepatomas and in embryonic livers. We engineered FTO2B cells, a hepatoma cell line in which GK is not expressed, to unveil the modifications required to allow them to accumulate glycogen. In the light of these results, we then examined glycogen metabolism in embryonic liver. Glycogen accumulation in FTO2B cells can be triggered through elevated expression of HKI or either of the protein phosphatase 1 regulatory subunits, namely PTG or G L. Between these two strategies to activate glycogen deposition in the absence of GK, embryonic livers choose to express massive levels of HKI and HKII. We conclude that although the GK/liver glycogen synthase tandem is ideally suited to store glycogen in liver when blood glucose is high, the substitution of HKI for GK in embryonic livers allows the HKI/liver glycogen synthase tandem to make glycogen independently of the glucose concentration in blood, although it requires huge levels of HK. Moreover, the physiological consequence of the HK isoform switch is that the embryonic liver safeguards its glycogen deposits, required as the main source of energy at birth, from maternal starvation.

The flavonoid silibinin decreases glucose-6-phosphate hydrolysis in perfused rat hepatocytes by an inhibitory effect on glucose-6-phosphatase

BACKGROUND/AIMS

The flavonoid silibinin has been reported to be beneficial in several hepatic disorders. Recent evidence also suggests that silibinin could be beneficial in the treatment of type 2 diabetes, owing to its anti-hyperglycemic properties. However, the mechanism(s) underlying these metabolic effects remains unknown.

METHODS

The effects of silibinin on liver gluconeogenesis were studied by titrating hepatocytes from starved rats with sub-saturating concentrations of various exogenous substrates in a perifusion system. Hepatocytes from fed rats were also used to investigate glycogenolysis from endogenous glycogen. The effect of silibinin on glucose-6-phosphatase kinetics was determined in intact and permeabilized rat liver microsomes.

RESULTS

Silibinin induced a dose-dependent inhibition of gluconeogenesis associated with a potent decrease in glucose-6-phosphate hydrolysis. This effect was demonstrated whatever the gluconeogenic substrates used, i.e. dihydroxyacetone, lactate/pyruvate, glycerol and fructose. In addition, silibinin decreased the glucagon-induced stimulation of both gluconeogenesis and glycogenolysis, this being associated with a reduction of glucose-6-phosphate hydrolysis. Silibinin inhibits glucose-6-phosphatase in rat liver microsomes in a concentration-dependent manner that could explain the decrease in glucose-6-phosphate hydrolysis seen in intact cells.

CONCLUSION

The inhibitory effect of silibinin on both hepatic glucose-6-phosphatase and gluconeogenesis suggests that its use may be interesting in treatment of type 2 diabetes.

Metabolic impact of overexpression of liver glycogen synthase with serine-to-alanine substitutions in rat primary hepatocytes

To investigate the effect of elevation of liver glycogen synthase (GYS2) activity on glucose and glycogen metabolism, we performed adenoviral overexpression of the mutant GYS2 with six serine-to-alanine substitutions in rat primary hepatocytes. Cell-free assays demonstrated that the serine-to-alanine substitutions caused constitutive activity and electrophoretic mobility shift. In rat primary hepatocytes, overexpression of the mutant GYS2 significantly reduced glucose production by 40% and dramatically induced glycogen synthesis via the indirect pathway rather than the direct pathway. Thus, we conclude that elevation of glycogen synthase activity has an inhibitory effect on glucose production in hepatocytes by shunting gluconeogenic precursors into glycogen. In addition, although intracellular compartmentation of glucose-6-phosphate (G6P) remains unclear in hepatocytes, our results imply that there are at least two G6P pools via gluconeogenesis and due to glucose phosphorylation, and that G6P via gluconeogenesis is preferentially used for glycogen synthesis in hepatocytes.

Liver glyconeogenesis: a pathway to cope with postprandial amino acid excess in high-protein fed rats?

This paper provides molecular evidence for a liver glyconeogenic pathway, that is, a concomitant activation of hepatic gluconeogenesis and glycogenesis, which could participate in the mechanisms that cope with amino acid excess in high-protein (HP) fed rats. This evidence is based on the concomitant upregulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression, downregulation of glucose 6-phosphatase catalytic subunit (G6PC1) gene expression, an absence of glucose release from isolated hepatocytes and restored hepatic glycogen stores in the fed state in HP fed rats. These effects are mainly due to the ability of high physiological concentrations of portal blood amino acids to counteract glucagon-induced liver G6PC1 but not PEPCK gene expression. These results agree with the idea that the metabolic pathway involved in glycogen synthesis is dependent upon the pattern of nutrient availability. This nonoxidative glyconeogenic disposal pathway of gluconeogenic substrates copes with amino excess and participates in adjusting both amino acid and glucose homeostasis. In addition, the pattern of PEPCK and G6PC1 gene expression provides evidence that neither the kidney nor the small intestine participated in gluconeogenic glucose production under our experimental conditions. Moreover, the main glucose-6-phosphatase (G6Pase) isoform expressed in the small intestine is the ubiquitous isoform of G6Pase (G6PC3) rather than the G6PC1 isoform expressed in gluconeogenic organs.

Dynamic profiling of the glucose metabolic network in fasted rat hepatocytes using [1,2-13C2]glucose

Recent studies in metabolic profiling have underscored the importance of the concept of a metabolic network of pathways with special functional characteristics that differ from those of simple reaction sequences. The characterization of metabolic functions requires the simultaneous measurement of substrate fluxes of interconnecting pathways. Here we present a novel stable isotope method by which the forward and reverse fluxes of the futile cycles of the hepatic glucose metabolic network are simultaneously determined. Unlike previous radio-isotope methods, a single tracer [1,2-13C2]D-glucose and mass isotopomer analysis is used. Changes in fluxes of substrate cycles, in response to several gluconeogenic substrates, in isolated fasted hepatocytes from male Wistar rats were measured simultaneously. Incubation with these substrates resulted in a change in glucose-6-phosphatase/glucokinase and glycolytic/gluconeogenic flux ratios. Different net redistributions of intermediates in the glucose network were observed, resulting in distinct metabolic phenotypes of the fasted hepatocytes in response to each substrate condition. Our experimental observations show that the constraints of concentrations of shared intermediates, and enzyme kinetics of intersecting pathways of the metabolic network determine substrate redistribution throughout the network when it is perturbed. These results support the systems-biology notion that network analysis provides an integrated view of the physiological state. Interaction between metabolic intermediates and glycolytic/gluconeogenic pathways is a basic element of cross-talk in hepatocytes, and may explain some of the difficulties in genotype and phenotype correlation.

Subcellular localization of liver FBPase is modulated by metabolic conditions

In primary cultured hepatocytes, fructose-1,6-bisphosphatase (FBPase) localization is modulated by glucose, dihydroxyacetone (DHA) and insulin. In the absence of these substrates, FBPase was present in the cytoplasm, but the addition of glucose or DHA induced its translocation to the nucleus. As expected, we observed the opposite effect of glucose on glucokinase localization. The addition of insulin in the absence of glucose largely increased the amount of nuclear FBPase. Moreover, at high concentrations of glucose or DHA, FBPase shifted from the cytosol to the cell periphery and co-localized with GS. Interestingly, the synthesis of Glu-6-P and glycogen induced by DHA was not inhibited by insulin. These results indicate that FBPase is involved in glycogen synthesis from gluconeogenic precursors. Overall, these findings show that translocation may be a new integrative mechanism for gluconeogenesis and glyconeogenesis.

Glucose 6-phosphate causes translocation of phosphorylase in hepatocytes and inactivates the enzyme synergistically with glucose

The role of glucose 6-P (glucose 6-phosphate) in regulating the activation state of glycogen synthase and its translocation is well documented. In the present study, we investigated the effects of glucose 6-P on the activation state and compartmentation of phosphorylase in hepatocytes. Glucose 6-P levels were modulated in hepatocytes by glucokinase overexpression or inhibition with 5-thioglucose and the effects of AMP were tested using AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolized to an AMP analogue. Inhibition of glucokinase partially counteracted the effect of glucose both on the inactivation of phosphorylase and on the translocation of phosphorylase a from a soluble to a particulate fraction. The increase in glucose 6-P caused by glucokinase overexpression caused translocation of phosphorylase a to the pellet and had additive effects with glucose on inactivation of phosphorylase. It decreased the glucose concentration that caused half-maximal inactivation from 20 to 11 mM, indicating that it acts synergistically with glucose. AICAR activated phosphorylase and counteracted the effect of glucose 6-P on phosphorylase inactivation. However, it did not counteract translocation of phosphorylase by glucose 6-P. Glucose 6-P and AICAR had opposite effects on the activation state of glycogen synthase, but they had additive effects on translocation of the enzyme to the pellet. There was a direct correlation between the translocation of phosphorylase a and of glycogen synthase to the pellet, suggesting that these enzymes translocate in tandem. In conclusion, glucose 6-P causes both translocation of phosphorylase and inactivation, indicating a more complex role in the regulation of glycogen metabolism than can be explained from regulation of glycogen synthase alone.

Phosphorylase regulates the association of glycogen synthase with a proteoglycogen substrate in hepatocytes

Changes in the glucosylation state of the glycogen primer, glycogenin, or its association with glycogen synthase are potential sites for regulation of glycogen synthesis. In this study we found no evidence for hormonal control of the glucosylation state of glycogenin in hepatocytes. However, using a modified glycogen synthase assay that separates the product into acid-soluble (glycogen) and acid-insoluble (proteoglycogen) fractions we found that insulin and glucagon increase and decrease, respectively, the association of glycogen synthase with an acid-insoluble substrate. The latter fraction had a higher affinity for UDP-glucose and accounted for between 5 and 21% of total activity depending on hormonal conditions. Phosphorylase overexpression mimicked the effect of glucagon. It is concluded that phosphorylase activation or overexpression causes dissociation of glycogen synthase from proteoglycogen causing inhibition of initiation of glycogen synthesis.

Nuclear localization of liver FBPase isoenzyme in kidney and liver

Nuclear localization has been observed for glycolytic enzymes but not for key gluconeogenic enzymes. We report our findings on the intracellular localization of liver FBPase in rat liver and kidney, the main organs in the endogenous glucose production. Immunofluorescence and confocal analysis revealed that FBPase was present in the cytosol and, unexpectedly, inside the nucleus of hepatocytes and proximal cells of the nephron. Additionally, FBPase was found in the plasma membrane area of adjacent hepatocytes where glycogen is synthesized and in the apical region of proximal kidney cells. This subcellular distribution in multiple compartments suggests the presence of different localization signals on FBPase for diverse metabolic functions.

Control of glycogen deposition

Traditionally, glycogen synthase (GS) has been considered to catalyze the key step of glycogen synthesis and to exercise most of the control over this metabolic pathway. However, recent advances have shown that other factors must be considered. Moreover, the control of glycogen deposition does not follow identical mechanisms in muscle and liver. Glucose must be phosphorylated to promote activation of GS. Glucose-6-phosphate (Glc-6-P) binds to GS, causing the allosteric activation of the enzyme probably through a conformational rearrangement that simultaneously converts it into a better substrate for protein phosphatases, which can then lead to the covalent activation of GS. The potency of Glc-6-P for activation of liver GS is determined by its source, since Glc-6-P arising from the catalytic action of glucokinase (GK) is much more effective in mediating the activation of the enzyme than the same metabolite produced by hexokinase I (HK I). As a result, hepatic glycogen deposition from glucose is subject to a system of control in which the 'controller', GS, is in turn controlled by GK. In contrast, in skeletal muscle, the control of glycogen synthesis is shared between glucose transport and GS. The characteristics of the two pairs of isoenzymes, liver GS/GK and muscle GS/HK I, and the relationships that they establish are tailored to suit specific metabolic roles of the tissues in which they are expressed. The key enzymes in glycogen metabolism change their intracellular localization in response to glucose. The changes in the intracellular distribution of liver GS and GK triggered by glucose correlate with stimulation of glycogen synthesis. The translocation of GS, which constitutes an additional mechanism of control, causes the orderly deposition of hepatic glycogen and probably represents a functional advantage in the metabolism of the polysaccharide.

Inactivation of phosphorylase is a major component of the mechanism by which insulin stimulates hepatic glycogen synthesis

Multiple signalling pathways are involved in the mechanism by which insulin stimulates hepatic glycogen synthesis. In this study we used selective inhibitors of glycogen synthase kinase-3 (GSK-3) and an allosteric inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a, to determine the relative contributions of inactivation of GSK-3 and dephosphorylation of phosphorylase a as alternative pathways in the stimulation of glycogen synthesis by insulin in hepatocytes. GSK-3 inhibitors (SB-216763 and Li+) caused a greater activation of glycogen synthase than insulin (90% vs. 40%) but a smaller stimulation of glycogen synthesis (30% vs. 150%). The contribution of GSK-3 inactivation to insulin stimulation of glycogen synthesis was estimated to be less than 20%. Dephosphorylation of phosphorylase a with CP-91149 caused activation of glycogen synthase and translocation of the protein from a soluble to a particulate fraction and mimicked the stimulation of glycogen synthesis by insulin. The stimulation of glycogen synthesis by phosphorylase inactivation cannot be explained by either inhibition of glycogen degradation or activation of glycogen synthase alone and suggests an additional role for translocation of synthase. Titrations with the phosphorylase inactivator showed that stimulation of glycogen synthesis by insulin can be largely accounted for by inactivation of phosphorylase over a wide range of activities of phosphorylase a. We conclude that a signalling pathway involving dephosphorylation of phosphorylase a leading to both activation and translocation of glycogen synthase is a critical component of the mechanism by which insulin stimulates hepatic glycogen synthesis. Selective inactivation of phosphorylase can mimic insulin stimulation of hepatic glycogen synthesis.