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

Duration of morning hyperinsulinemia determines hepatic glucose uptake and glycogen storage later in the day

The second-meal phenomenon refers to the improvement in glucose tolerance seen following a second identical meal. We previously showed that 4 h of morning hyperinsulinemia, but not hyperglycemia, enhanced hepatic glucose uptake (HGU) and glycogen storage during an afternoon hyperinsulinemic-hyperglycemic (HIHG) clamp. Our current aim was to determine if the duration or pattern of morning hyperinsulinemia is important for the afternoon response to a HIHG clamp. To determine this, the same total amount of insulin was administered either over 2 h in the first (Ins2h-A) or second (Ins2h-B) half of the morning or over the entire 4 h (Ins4h) of the morning. In the 4-h afternoon period, all three groups had 4x-basal insulin, 2x-basal glycemia, and portal glucose infusion to expose the liver to the primary postprandial regulators of hepatic glucose metabolism. During the afternoon clamp, there was a marked increase in HGU and hepatic glycogen synthesis in the Ins4h group compared with the Ins2h-A and Ins2h-B groups, despite matched hepatic glucose loads and total insulin infusion rates. Thus, the longer duration (Ins4h) of lower hyperinsulinemia in the morning seems to be the key to much greater liver glucose uptake during the afternoon clamp.NEW & NOTEWORTHY Morning insulin exposure primes the liver for increased hepatic glucose uptake and glycogen storage during a subsequent hyperinsulinemic-hyperglycemic clamp. This study addressed whether the pattern and/or duration of insulin delivery in the morning influences insulin's ensuing priming effect. We found that despite receiving equal total doses of insulin in the morning, a prolonged, lower rate of morning insulin delivery improved afternoon liver glucose metabolism more effectively than a shorter, higher rate of delivery.

Duration of Morning Hyperinsulinemia Determines Hepatic Glucose Uptake and Glycogen Storage Later in the Day

The second meal phenomenon refers to the improvement in glucose tolerance seen following a second identical meal. We previously showed that 4 hours of morning hyperinsulinemia, but not hyperglycemia, enhanced hepatic glucose uptake (HGU) and glycogen storage during an afternoon hyperinsulinemic-hyperglycemic (HIHG) clamp. Our current aim was to determine if the duration or pattern of morning hyperinsulinemia is important for the afternoon response to a HIHG clamp. To determine this, we administered the same total amount of insulin either over 2h in the first (Ins2h-A) or second (Ins2h-B) half of the morning, or over the entire 4h (Ins4h) of the morning. In the 4h afternoon period, all three groups had 4x-basal insulin, 2x-basal glycemia, and portal glucose infusion to expose the liver to the primary postprandial regulators of hepatic glucose metabolism. During the afternoon clamp, there was a marked increase in HGU and hepatic glycogen synthesis in the Ins4h group compared to the Ins2h-A and Ins2h-B groups, despite matched hepatic glucose loads and total insulin infusion rates. Thus, the longer duration (Ins4h) of lower hyperinsulinemia in the morning seems to be the key to much greater liver glucose uptake during the afternoon clamp.

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

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

The GCKR-P446L gene variant predisposes to raised blood cholesterol and lower blood glucose in the P446L mouse-a model for GCKR rs1260326

OBJECTIVES

The Glucokinase Regulatory Protein GKRP, encoded by GCKR, enables acute regulation of liver glucokinase to support metabolic demand. The common human GCKR rs1260326:Pro446 > Leu variant within a large linkage disequilibrium region associates with pleiotropic traits including lower Type 2 diabetes risk and raised blood triglycerides and cholesterol. Whether the GCKR-P446 > L substitution is causal to the raised lipids is unknown. We determined whether mouse GKRP phenocopies the human GKRP:P446 > L substitution and studied a GKRP:P446L knockin mouse to identify physiological consequences to P446 > L.

METHODS

GKRP-deficient hepatocytes were transfected with adenoviral vectors for human or mouse GKRP:446 P or 446 L for cellular comprehensive analysis including transcriptomics consequent to P446 > L. Physiological traits in the diet-challenged P446L mouse were compared with pleiotropic associations at the human rs1260326 locus. Transcriptomics was compared in P446L mouse liver with hepatocytes overexpressing glucokinase or GKRP:446 P/L.

RESULTS

1. P446 > L substitution in mouse or human GKRP similarly compromises protein expressivity of GKRP:446 L, nuclear sequestration of glucokinase and counter-regulation of gene expression. 2. The P446L knockin mouse has lower liver glucokinase and GKRP protein similar to human liver homozygous for rs1260326-446 L. 3. The diet-challenged P446L mouse has lower blood glucose, raised blood cholesterol and altered hepatic cholesterol homeostasis consistent with relative glucokinase-to-GKRP excess, but not raised blood triglycerides.

CONCLUSIONS

Mouse GKRP phenocopies the human GKRP:P446 > L substitution despite the higher affinity for glucokinase of human GKRP. The diet-challenged P446L mouse replicates several traits found in association with the rs1260326 locus on chromosome 2 including raised blood cholesterol, lower blood glucose and lower liver glucokinase and GKRP protein but not raised blood triglycerides.

Catestatin induces glycogenesis by stimulating the phosphoinositide 3-kinase-AKT pathway

AIM

Defects in hepatic glycogen synthesis contribute to post-prandial hyperglycaemia in type 2 diabetic patients. Chromogranin A (CgA) peptide Catestatin (CST: hCgA352-372 ) improves glucose tolerance in insulin-resistant mice. Here, we seek to determine whether CST induces hepatic glycogen synthesis.

METHODS

We determined liver glycogen, glucose-6-phosphate (G6P), uridine diphosphate glucose (UDPG) and glycogen synthase (GYS2) activities; plasma insulin, glucagon, noradrenaline and adrenaline levels in wild-type (WT) as well as in CST knockout (CST-KO) mice; glycogen synthesis and glycogenolysis in primary hepatocytes. We also analysed phosphorylation signals of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), phosphatidylinositol-dependent kinase-1 (PDK-1), GYS2, glycogen synthase kinase-3β (GSK-3β), AKT (a kinase in AKR mouse that produces Thymoma)/PKB (protein kinase B) and mammalian/mechanistic target of rapamycin (mTOR) by immunoblotting.

RESULTS

CST stimulated glycogen accumulation in fed and fasted liver and in primary hepatocytes. CST reduced plasma noradrenaline and adrenaline levels. CST also directly stimulated glycogenesis and inhibited noradrenaline and adrenaline-induced glycogenolysis in hepatocytes. In addition, CST elevated the levels of UDPG and increased GYS2 activity. CST-KO mice had decreased liver glycogen that was restored by treatment with CST, reinforcing the crucial role of CST in hepatic glycogenesis. CST improved insulin signals downstream of IR and IRS-1 by enhancing phospho-AKT signals through the stimulation of PDK-1 and mTORC2 (mTOR Complex 2, rapamycin-insensitive complex) activities.

CONCLUSIONS

CST directly promotes the glycogenic pathway by (a) reducing glucose production, (b) increasing glycogen synthesis from UDPG, (c) reducing glycogenolysis and (d) enhancing downstream insulin signalling.

Microsporidia infection upregulates host energy metabolism but maintains ATP homeostasis

Microsporidia are a group of obligate intracellular parasites which lack mitochondria and have highly reduced genomes. Therefore, they are unable to produce ATP via the tricarboxylic acid (TCA) cycle and oxidative phosphorylation. Instead, they have evolved strategies to obtain and manipulate host metabolism to acquire nutrients. However, little is known about how microsporidia modulate host energy metabolisms. Here, we present the first targeted metabolomics study to investigate changes in host energy metabolism as a result of infection by a microsporidian. Metabolites of silkworm embryo cell (BmE) were measured 48 h post infection by Nosema bombycis. Thirty metabolites were detected, nine of which were upregulated and mainly involved in glycolysis (glucose 6-phosphate, fructose 1,6-bisphosphate) and the TCA cycle (succinate, α-ketoglutarate, cis-aconitate, isocitrate, citrate, fumarate). Pathway enrichment analysis suggested that the upregulated metabolites could promote the synthesization of nucleotides, fatty acids, and amino acids by the host. ATP concentration in host cells, however, was not significantly changed by the infection. This ATP homeostasis was also found in Encephalitozoon hellem infected mouse macrophage RAW264.7, human monocytic leukemia THP-1, human embryonic kidney 293, and human foreskin fibroblast cells. These findings suggest that microsporidia have evolved strategies to maintain levels of ATP in the host while stimulating metabolic pathways to provide additional nutrients for the parasite.

Postprandial Glycogen Content Is Increased in the Hepatocytes of Human and Rat Cirrhotic Liver

Chronic hepatitises of various etiologies are widespread liver diseases in humans. Their final stage, liver cirrhosis (LC), is considered to be one of the main causes of hepatocellular carcinoma (HCC). About 80-90% of all HCC cases develop in LC patients, which suggests that cirrhotic conditions play a crucial role in the process of hepatocarcinogenesis. Carbohydrate metabolism in LC undergoes profound disturbances characterized by altered glycogen metabolism. Unfortunately, data on the glycogen content in LC are few and contradictory. In this study, the material was obtained from liver biopsies of patients with LC of viral and alcohol etiology and from the liver tissue of rats with CCl₄-induced LC. The activity of glycogen phosphorylase (GP), glycogen synthase (GS), and glucose-6-phosphatase (G6Pase) was investigated in human and rat liver tissue by biochemical methods. Total glycogen and its labile and stable fractions were measured in isolated individual hepatocytes, using the cytofluorometry technique of PAS reaction in situ. The development of LC in human and rat liver was accompanied by an increase in fibrous tissue (20- and 8.8-fold), an increase in the dry mass of hepatocytes (by 25.6% and 23.7%), and a decrease in the number of hepatocytes (by 50% and 28%), respectively. The rearrangement of the liver parenchyma was combined with changes in glycogen metabolism. The present study showed a significant increase in the glycogen content in the hepatocytes of the human and the rat cirrhotic liver, by 255% and 210%, respectively. An increased glycogen content in cells of the cirrhotic liver can be explained by a decrease in glycogenolysis due to a decreased activity of G6Pase and GP.

Identification of C-β-d-Glucopyranosyl Azole-Type Inhibitors of Glycogen Phosphorylase That Reduce Glycogenolysis in Hepatocytes: In Silico Design, Synthesis, in Vitro Kinetics, and ex Vivo Studies

Several C-β-d-glucopyranosyl azoles have recently been uncovered as among the most potent glycogen phosphorylase (GP) catalytic site inhibitors discovered to date. Toward further exploring their translational potential, ex vivo experiments have been performed for their effectiveness in reduction of glycogenolysis in hepatocytes. New compounds for these experiments were predicted in silico where, for the first time, effective ranking of GP catalytic site inhibitor potencies using the molecular mechanics-generalized Born surface area (MM-GBSA) method has been demonstrated. For a congeneric training set of 27 ligands, excellent statistics in terms of Pearson (R_P) and Spearman (R_S) correlations (both 0.98), predictive index (PI = 0.99), and area under the receiver operating characteristic curve (AU-ROC = 0.99) for predicted versus experimental binding affinities were obtained, with ligand tautomeric/ionization states additionally considered using density functional theory (DFT). Seven 2-aryl-4(5)-(β-d-glucopyranosyl)-imidazoles and 2-aryl-4-(β-d-glucopyranosyl)-thiazoles were subsequently synthesized, and kinetics experiments against rabbit muscle GPb revealed new potent inhibitors with best K_i values in the low micromolar range (5c = 1.97 μM; 13b = 4.58 μM). Ten C-β-d-glucopyranosyl azoles were then tested ex vivo in mouse primary hepatocytes. Four of these (5a-c and 9d) demonstrated significant reduction of glucagon stimulated glycogenolysis (IC₅₀ = 30-60 μM). Structural and predicted physicochemical properties associated with their effectiveness were analyzed with permeability related parameters identified as crucial factors. The most effective ligand series 5 contained an imidazole ring, and the calculated pK_a (Epik: 6.2; Jaguar 5.5) for protonated imidazole suggests that cellular permeation through the neutral state is favored, while within the cell, there is predicted more favorable binding to GP in the protonated form.

Role of glycogen phosphorylase in liver glycogen metabolism

Liver glycogen is synthesized after a meal in response to an increase in blood glucose concentration in the portal vein and endocrine and neuroendocrine signals, and is degraded to glucose between meals to maintain blood glucose homeostasis. Glycogen degradation and synthesis during the diurnal cycle are mediated by changes in the activities of phosphorylase and glycogen synthase. Phosphorylase is regulated by phosphorylation of serine-14. Only the phosphorylated form of liver phosphorylase (GPa) is catalytically active. Interconversion between GPa and GPb (unphosphorylated) is dependent on the activities of phosphorylase kinase and of phosphorylase phosphatase. The latter comprises protein phosphatase-1 in conjunction with a glycogen-targeting protein (G-subunit) of the PPP1R3 family. At least two of six G-subunits (GL and PTG) expressed in liver are involved in GPa dephosphorylation. GPa to GPb interconversion is dependent on the conformational state of phosphorylase which can be relaxed (R) or tense (T) depending on the concentrations of allosteric effectors such as glucose, glucose 6-phosphate and adenine nucleotides and on the acetylation state of lysine residues. The G-subunit, GL, encoded by PPP1R3B gene is expressed at high levels in liver and can function as a phosphorylase phosphatase and a synthase phosphatase and has an allosteric binding site for GPa at the C-terminus which inhibits synthase phosphatase activity. GPa to GPb conversion is a major upstream event in the regulation of glycogen synthesis by glucose, its downstream metabolites and extracellular signals such as insulin and neurotransmitters.

Hepatic glucose sensing and integrative pathways in the liver

The hepatic glucose-sensing system is a functional network of enzymes and transcription factors that is critical for the maintenance of energy homeostasis and systemic glycemia. Here we review the recent literature on its components and metabolic actions. Glucokinase (GCK) is generally considered as the initial postprandial glucose-sensing component, which acts as the gatekeeper for hepatic glucose metabolism and provides metabolites that activate the transcription factor carbohydrate response element binding protein (ChREBP). Recently, liver receptor homolog 1 (LRH-1) has emerged as an upstream regulator of the central GCK-ChREBP axis, with a critical role in the integration of hepatic intermediary metabolism in response to glucose. Evidence is also accumulating that O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and acetylation can act as glucose-sensitive modifications that may contribute to hepatic glucose sensing by targeting regulatory proteins and the epigenome. Further elucidation of the components and functional roles of the hepatic glucose-sensing system may contribute to the future treatment of liver diseases associated with deregulated glucose sensors.

Glycogen phosphorylase inhibitor N-(3,5-dimethyl-Benzoyl)-N'-(β-D-glucopyranosyl)urea improves glucose tolerance under normoglycemic and diabetic conditions and rearranges hepatic metabolism

Glycogen phosphorylase (GP) catalyzes the breakdown of glycogen and largely contributes to hepatic glucose production making GP inhibition an attractive target to modulate glucose levels in diabetes. Hereby we present the metabolic effects of a novel, potent, glucose-based GP inhibitor (KB228) tested in vitro and in vivo under normoglycemic and diabetic conditions. KB228 administration enhanced glucose sensitivity in chow-fed and obese, diabetic mice that was a result of higher hepatic glucose uptake. Besides improved glucose sensitivity, we have observed further unexpected metabolic rearrangements. KB228 administration increased oxygen consumption that was probably due to the overexpression of uncoupling protein-2 (UCP2) that was observed in animal and cellular models. Furthermore, KB228 treatment induced mammalian target of rapamycin complex 2 (mTORC2) in mice. Our data demonstrate that glucose based GP inhibitors are capable of reducing glucose levels in mice under normo and hyperglycemic conditions. Moreover, these GP inhibitors induce accommodation in addition to GP inhibition - such as enhanced mitochondrial oxidation and mTORC2 signaling – to cope with the glucose influx and increased glycogen deposition in the cells, however the molecular mechanism of accommodation is unexplored.

Liver glycogen loading dampens glycogen synthesis seen in response to either hyperinsulinemia or intraportal glucose infusion

The purpose of this study was to determine the effect of liver glycogen loading on net hepatic glycogen synthesis during hyperinsulinemia or hepatic portal vein glucose infusion in vivo. Liver glycogen levels were supercompensated (SCGly) in two groups (using intraportal fructose infusion) but not in two others (Gly) during hyperglycemic-normoinsulinemia. Following a 2-h control period during which fructose infusion was stopped, there was a 2-h experimental period in which the response to hyperglycemia plus either 4× basal insulin (INS) or portal vein glucose infusion (PoG) was measured. Increased hepatic glycogen reduced the percent of glucose taken up by the liver that was deposited in glycogen (74 ± 3 vs. 53 ± 5% in Gly+INS and SCGly+INS, respectively, and 72 ± 3 vs. 50 ± 6% in Gly+PoG and SCGly+PoG, respectively). The reduction in liver glycogen synthesis in SCGly+INS was accompanied by a decrease in both insulin signaling and an increase in AMPK activation, whereas only the latter was observed in SCGly+PoG. These data indicate that liver glycogen loading impairs glycogen synthesis regardless of the signal used to stimulate it.

Fructose 2,6-bisphosphate is essential for glucose-regulated gene transcription of glucose-6-phosphatase and other ChREBP target genes in hepatocytes

Glucose metabolism in the liver activates the transcription of various genes encoding enzymes of glycolysis and lipogenesis and also G6pc (glucose-6-phosphatase). Allosteric mechanisms involving glucose 6-phosphate or xylulose 5-phosphate and covalent modification of ChREBP (carbohydrate-response element-binding protein) have been implicated in this mechanism. However, evidence supporting an essential role for a specific metabolite or pathway in hepatocytes remains equivocal. By using diverse substrates and inhibitors and a kinase-deficient bisphosphatase-active variant of the bifunctional enzyme PFK2/FBP2 (6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase), we demonstrate an essential role for fructose 2,6-bisphosphate in the induction of G6pc and other ChREBP target genes by glucose. Selective depletion of fructose 2,6-bisphosphate inhibits glucose-induced recruitment of ChREBP to the G6pc promoter and also induction of G6pc by xylitol and gluconeogenic precursors. The requirement for fructose 2,6-bisphosphate for ChREBP recruitment to the promoter does not exclude the involvement of additional metabolites acting either co-ordinately or at downstream sites. Glucose raises fructose 2,6-bisphosphate levels in hepatocytes by reversing the phosphorylation of PFK2/FBP2 at Ser32, but also independently of Ser32 dephosphorylation. This supports a role for the bifunctional enzyme as the phosphometabolite sensor and for its product, fructose 2,6-bisphosphate, as the metabolic signal for substrate-regulated ChREBP-mediated expression of G6pc and other ChREBP target genes.

Postprandial and fasting hepatic glucose fluxes in long-standing type 1 diabetes

OBJECTIVE

Intravenous insulin infusion partly improves liver glucose fluxes in type 1 diabetes (T1D). This study tests the hypothesis that continuous subcutaneous insulin infusion (CSII) normalizes hepatic glycogen metabolism.

RESEARCH DESIGN AND METHODS

T1D with poor glycemic control (T1Dp; HbA(1c): 8.5 ± 0.4%), T1D with improved glycemic control on CSII (T1Di; 7.0 ± 0.3%), and healthy humans (control subjects [CON]; 5.2 ± 0.4%) were studied. Net hepatic glycogen synthesis and glycogenolysis were measured with in vivo (13)C magnetic resonance spectroscopy. Endogenous glucose production (EGP) and gluconeogenesis (GNG) were assessed with [6,6-(2)H(2)]glucose, glycogen phosphorylase (GP) flux, and gluconeogenic fluxes with (2)H(2)O/paracetamol.

RESULTS

When compared with CON, net glycogen synthesis was 70% lower in T1Dp (P = 0.038) but not different in T1Di. During fasting, T1Dp had 25 and 42% higher EGP than T1Di (P = 0.004) and CON (P < 0.001; T1Di vs. CON: P = NS). GNG was 74 and 67% higher in T1Dp than in T1Di (P = 0.002) and CON (P = 0.001). In T1Dp, GP flux (7.0 ± 1.6 μmol ⋅ kg(-1) ⋅ min(-1)) was twofold higher than net glycogenolysis, but comparable in T1Di and CON (3.7 ± 0.8 and 4.9 ± 1.0 μmol ⋅ kg(-1) ⋅ min(-1)). Thus T1Dp exhibited glycogen cycling (3.5 ± 2.0 μmol ⋅ kg(-1) ⋅ min(-1)), which accounted for 47% of GP flux.

CONCLUSIONS

Poorly controlled T1D not only exhibits augmented fasting gluconeogenesis but also increased glycogen cycling. Intensified subcutaneous insulin treatment restores these abnormalities, indicating that hepatic glucose metabolism is not irreversibly altered even in long-standing T1D.

Impact of a glycogen phosphorylase inhibitor and metformin on basal and glucagon-stimulated hepatic glucose flux in conscious dogs

The effects of a glycogen phosphorylase inhibitor (GPI) and metformin (MT) on hepatic glucose fluxes (μmol · kg(-1) · min(-1)) in the presence of basal and 4-fold basal levels of plasma glucagon were investigated in 18-h fasted conscious dogs. Compared with the vehicle treatment, GPI infusion suppressed net hepatic glucose output (NHGO) completely (-3.8 ± 1.3 versus 9.9 ± 2.8) despite increased glucose 6-phosphate (G-6-P) neogenesis from gluconeogenic precursors (8.1 ± 1.1 versus 5.5 ± 1.1). MT infusion did not alter those parameters. In response to a 4-fold rise in plasma glucagon levels, in the vehicle group, plasma glucose levels were increased 2-fold, and NHGO was increased (43.9 ± 5.7 at 10 min and 22.7 ± 3.4 at steady state) without altering G-6-P neogenesis (3.7 ± 1.5 and 5.5 ± 0.5, respectively). In the GPI group, there was no increase in NHGO due to decreased glucose-6-phosphatase flux associated with reduced G-6-P concentration. A lower G-6-P concentration was the result of increased net glycogenesis without altering G-6-P neogenesis. In the MT group, the increment in NHGO (22.2 ± 4.4 at 10 min and 12.1 ± 3.6 at steady state) was approximately half of that of the vehicle group. The lesser NHGO was associated with reduced glucose-6-phosphatase flux but a rise in G-6-P concentration and only a small incorporation of plasma glucose into glycogen. In conclusion, the inhibition of glycogen phosphorylase a activity decreases basal and glucagon-induced NHGO via redirecting glucose 6-phosphate flux from glucose toward glycogen, and MT decreases glucagon-induced NHGO by inhibiting glucose-6-phosphatase flux and thereby reducing glycogen breakdown.

Differential regulation of glycogenolysis by mutant protein phosphatase-1 glycogen-targeting subunits

PTG and G(L) are hepatic protein phosphatase-1 (PP1) glycogen-targeting subunits, which direct PP1 activity against glycogen synthase (GS) and/or phosphorylase (GP). The C-terminal 16 amino residues of G(L) comprise a high affinity binding site for GP that regulates bound PP1 activity against GS. In this study, a truncated G(L) construct lacking the GP-binding site (G(L)tr) and a chimeric PTG molecule containing the C-terminal site (PTG-G(L)) were generated. As expected, GP binding to glutathione S-transferase (GST)-G(L)tr was reduced, whereas GP binding to GST-PTG-G(L) was increased 2- to 3-fold versus GST-PTG. In contrast, PP1 binding to all proteins was equivalent. Primary mouse hepatocytes were infected with adenoviral constructs for each subunit, and their effects on glycogen metabolism were investigated. G(L)tr expression was more effective at promoting GP inactivation, GS activation, and glycogen accumulation than G(L). Removal of the regulatory GP-binding site from G(L)tr completely blocked the inactivation of GS seen in G(L)-expressing cells following a drop in extracellular glucose. As a result, G(L)tr expression prevented glycogen mobilization under 5 mm glucose conditions. In contrast, equivalent overexpression of PTG or PTG-G(L) caused a similar increase in glycogen-targeted PP1 levels and GS dephosphorylation. Surprisingly, GP dephosphorylation was significantly reduced in PTG-G(L)-overexpressing cells. As a result, PTG-G(L) expression permitted glycogenolysis under 5 mm glucose conditions that was prevented in PTG-expressing cells. Thus, expression of constructs that contained the high affinity GP-binding site (G(L) and PTG-G(L)) displayed reduced glycogen accumulation and enhanced glycogenolysis compared with their respective controls, albeit via different mechanisms.

Cytoplasmic Escherichia coli ADP sugar pyrophosphatase binds to cell membranes in response to extracellular signals as the cell population density increases

ADP sugar pyrophosphatase (AspP) is a member of the 'Nudix' (Nucleoside diphosphate linked to some other moiety X) hydrolase family of enzymes that catalyzes the hydrolytic breakdown of ADP-glucose (ADPG) linked to glycogen biosynthesis. In a previous work, we showed that AspP activity is strongly enhanced by both glucose-1,6-bisphosphate and nucleotide-sugars, and by macromolecular crowding. In this work, we show that AspP binds to cell membranes as the bacterial population density increases, c. 30% of the total enzyme remaining membrane associated as glycogen depletes during the stationary phase. This process is not dependent on the stationary transcription factor RpoS, the producer of the bacterial quorum-sensing autoinducer 2 (LuxS), the presence of glycogen granules or glucose availability, but is stimulated by small soluble heat-labile molecule(s) occurring in cell-free spent supernatants of stationary cultures that are acid stabile and base labile. These data further point to AspP as a highly regulated enzyme, and provide a first set of evidences indicating that glycogen metabolism is subjected to regulation by intercellular communication in Escherichia coli.

Glucokinase and molecular aspects of liver glycogen metabolism

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.

Inhibition of the interaction between protein phosphatase 1 glycogen-targeting subunit and glycogen phosphorylase increases glycogen synthesis in primary rat hepatocytes

In Type 2 diabetes, increased glycogenolysis contributes to the hyperglycaemic state, therefore the inhibition of GP (glycogen phosphorylase), a key glycogenolytic enzyme, is one of the possibilities to lower plasma glucose levels. Following this strategy, a number of GPis (GP inhibitors) have been described. However, certain critical issues are associated with their mode of action, e.g. an impairment of muscle function. The interaction between GP and the liver glycogen targeting subunit (termed GL) of PP1 (protein phosphatase 1) has emerged as a new potential anti-diabetic target, as the disruption of this interaction should increase glycogen synthesis, potentially providing an alternative approach to counteract the enhanced glycogenolysis without inhibiting GP activity. We identified an inhibitor of the GL–GP interaction (termed GL–GPi) and characterized its mechanism of action in comparison with direct GPis. In primary rat hepatocytes, at elevated glucose levels, the GL–GPi increased glycogen synthesis similarly to direct GPis. Direct GPis significantly reduced the cellular GP activity, caused a dephosphorylation of the enzyme and decreased the amounts of GP in the glycogen-enriched fraction; the GL–GPi did not influence any of these parameters. Both mechanisms increased glycogen accumulation at elevated glucose levels. However, at low glucose levels, only direct GPis led to increased glycogen amounts, whereas the GL–GPi allowed the mobilization of glycogen because it did not block the activity of GP. Due to this characteristic, GL–GPi in comparison with GPis could offer an advantageous risk/benefit profile circumventing the potential downsides of a complete prevention of glycogen breakdown while retaining glucose- lowering efficacy, suggesting that inhibition of the GL–GP interaction may provide an attractive novel approach for rebalancing the disturbed glycogen metabolism in diabetic patients.

New hepatic targets for glycaemic control in diabetes

Type-2 diabetes is associated with impaired glucose clearance by the liver in the postprandial state, and with elevated glucose production in the post-absorptive state. New targets within the liver are currently being investigated for development of antihyperglycaemic drugs for type-2 diabetes. They include glucokinase, which catalyses the first step in glucose metabolism, the glucagon receptor, and enzymes of gluconeogenesis and/or glycogenolysis such as glucose 6-phosphatase, fructose 1,6-bisphosphatase and glycogen phosphorylase. Preclinical studies with candidate drugs on animal models or cell-based assays suggest that these targets have the potential for pharmacological glycaemic control. Data from clinical studies is awaited. Further work is required for better understanding of the implications of targeting these sites in terms of possible side-effects or tachyphylaxis. The advantage of combined targeting of two or more sites within the liver for minimizing side-effects and tachyphylaxis caused by single-site targeting is discussed.

Bioactivity of glycogen phosphorylase inhibitors that bind to the purine nucleoside site

The bioactivity in hepatocytes of glycogen phosphorylase inhibitors that bind to the active site, the allosteric activator site and the indole carboxamide site has been described. However, the pharmacological potential of the purine nucleoside inhibitor site has remained unexplored. We report the chemical synthesis and bioactivity in hepatocytes of four new olefin derivatives of flavopiridol (1-4) that bind to the purine site. Flavopiridol and 1-4 counteracted the activation of phosphorylase in hepatocytes caused by AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolised to an AMP analogue. Unlike an indole carboxamide inhibitor, the analogues 1 and 4 suppressed the basal rate of glycogenolysis in hepatocytes by allosteric inhibition rather than by inactivation of phosphorylase, and accordingly caused negligible stimulation of glycogen synthesis. However, they counteracted the stimulation of glycogenolysis by dibutyryl cAMP by both allosteric inhibition and inactivation of phosphorylase. Cumulatively, the results show key differences between purine site and indole carboxamide site inhibitors in terms of (i) relative roles of dephosphorylation of phosphorylase-a as compared with allosteric inhibition, (ii) counteraction of the efficacy of the inhibitors on glycogenolysis by dibutyryl-cAMP and (iii) stimulation of glycogen synthesis.

Iminosugars as Potential Inhibitors of Glycogenolysis: Structural Insights into the Molecular Basis of Glycogen Phosphorylase Inhibition

Iminosugars DAB (5), isofagomine (9), and several N-substituted derivatives have been identified as potent inhibitors of liver glycogen phosphorylase a (IC(50) = 0.4-1.2 microM) and of basal and glucagon-stimulated glycogenolysis (IC(50) = 1-3 microM). The X-ray structures of 5, 9, and its N-3-phenylpropyl analogue 8 in complex with rabbit muscle glycogen phosphorylase (GPb) shows that iminosugars bind tightly at the catalytic site in the presence of the substrate phosphate and induce conformational changes that characterize the R-state conformation of the enzyme. Charged nitrogen N1 is within hydrogen-bonding distance with the carbonyl oxygen of His377 (5) and in ionic contact with the substrate phosphate oxygen (8 and 9). Our findings suggest that the inhibitors function as oxocarbenium ion transition-state analogues. The conformational change to the R state provides an explanation for previous findings that 5, unlike inhibitors that favor the T state, promotes phosphorylation of GPb in hepatocytes with sequential inactivation of glycogen synthase.

Stimulation of gluconeogenesis by intravenous lipids in preterm infants: response depends on fatty acid profile

In preterm infants, both hypo- and hyperglycemia are a frequent problem. Intravenous lipids can affect glucose metabolism by stimulation of gluconeogenesis by providing glycerol, which is a gluconeogenic precursor, and/or free fatty acids (FFA), which are stimulants of the rate of gluconeogenesis. In 25 preterm infants, glucose production and gluconeogenesis were measured using stable isotope techniques during a 6-h infusion of glucose only, glucose plus glycerol, or glucose plus an intravenous lipid emulsion. Two lipid emulsions differing in FFA composition were used: Intralipid ( approximately 60% polyunsaturated FFA) and Clinoleic (approximately 60% monounsaturated FFA). The rate of glucose infusion was 22 micromol x kg(-1) x min(-1) in all groups. During the study infusion, the FFA concentrations were higher in both lipid groups vs. the glycerol group (P < 0.001). Compared with baseline, the glucose production rate increased in the Intralipid group, whereas it decreased in the other groups (P = 0.002) due to a significant increase in gluconeogenesis in the Intralipid group (P = 0.016). The plasma glucose concentration was significantly higher during Intralipid infusion vs. the other groups (P = 0.046). Our conclusion was that Intralipid enhanced glucose production by increasing gluconeogenesis in preterm infants. This can be ascribed to the stimulatory effect of FFA in addition to any effect of glycerol alone. The lack of stimulation of gluconeogenesis in the Clinoleic vs. the Intralipid group suggests that different classes of fatty acids exert different effects on glucose kinetics in preterm infants.

The role of glucose 6-phosphate in mediating the effects of glucokinase overexpression on hepatic glucose metabolism

Pharmacological activation or overexpression of glucokinase in hepatocytes stimulates glucose phosphorylation, glycolysis and glycogen synthesis. We used an inhibitor of glucose 6-phosphate (Glc6P) hydrolysis, namely the chlorogenic derivative, 1-[2-(4-chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4,5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid (also known as S4048), to determine the contribution of Glc6P concentration, as distinct from glucokinase protein or activity, to the control of glycolysis and glycogen synthesis by glucokinase overexpression. The validity of S4048 for testing the role of Glc6P was supported by its lack of effect on glucokinase binding and its nuclear/cytoplasmic distribution. The stimulation of glycolysis by glucokinase overexpression correlated strongly with glucose phosphorylation, whereas glycogen synthesis correlated strongly with Glc6P concentration. Metabolic control analysis was used to determine the sensitivity of glycogenic flux to glucokinase or Glc6P at varying glucose concentrations (5-20 mm). The concentration control coefficient of glucokinase on Glc6P (1.4-1.7) was relatively independent of glucose concentration, whereas the flux control coefficients of Glc6P (2.4-1.0) and glucokinase (3.7-1.8) on glycogen synthesis decreased with glucose concentration. The high sensitivity of glycogenic flux to Glc6P at low glucose concentration is consistent with covalent modification by Glc6P of both phosphorylase and glycogen synthase. The high control strength of glucokinase on glycogenic flux is explained by its concentration control coefficient on Glc6P and the high control strength of Glc6P on glycogen synthesis. It is suggested that the regulatory strength of pharmacological glucokinase activators on glycogen metabolism can be predicted from their effect on the Glc6P content.

Kinetic and crystallographic studies of glucopyranose spirohydantoin and glucopyranosylamine analogs inhibitors of glycogen phosphorylase

Glycogen phosphorylase (GP) is currently exploited as a target for inhibition of hepatic glycogenolysis under high glucose conditions. Spirohydantoin of glucopyranose and N-acetyl-beta-D-glucopyranosylamine have been identified as the most potent inhibitors of GP that bind at the catalytic site. Four spirohydantoin and three beta-D-glucopyranosylamine analogs have been designed, synthesized and tested for inhibition of GP in kinetic experiments. Depending on the functional group introduced, the K(i) values varied from 16.5 microM to 1200 microM. In order to rationalize the kinetic results, we determined the crystal structures of the analogs in complex with GP. All the inhibitors bound at the catalytic site of the enzyme, by making direct and water-mediated hydrogen bonds with the protein and by inducing minor movements of the side chains of Asp283 and Asn284, of the 280s loop that blocks access of the substrate glycogen to the catalytic site, and changes in the water structure in the vicinity of the site. The differences observed in the Ki values of the analogs can be interpreted in terms of variations in hydrogen bonding and van der Waals interactions, desolvation effects, ligand conformational entropy, and displacement of water molecules on ligand binding to the catalytic site.

Glycogen synthesis correlates with androgen-dependent growth arrest in prostate cancer

BACKGROUND

Androgen withdrawal in normal prostate or androgen-dependent prostate cancer is associated with the downregulation of several glycolytic enzymes and with reduced glucose uptake. Although glycogen metabolism is known to regulate the intracellular glucose level its involvement in androgen response has not been studied.

METHODS

We investigated the effects of androgen on glycogen phosphorylase (GP), glycogen synthase (GS) and on glycogen accumulation in the androgen-receptor (AR) reconstituted PC3 cell line containing either an empty vector (PC3-AR-V) or vector with HPV-E7 (PC3-AR-E7) and the LNCaP cell line.

RESULTS

Androgen addition in PC3 cells expressing the AR mimics androgen ablation in androgen-dependent prostate cells. Incubation of PC3-AR-V or PC3-AR-E7 cells with the androgen R1881 induced G1 cell cycle arrest within 24 hours and resulted in a gradual cell number reduction over 5 days thereafter, which was accompanied by a 2 to 5 fold increase in glycogen content. 24 hours after androgen-treatment the level of Glucose-6-P (G-6-P) had increased threefold and after 48 hours the GS and GP activities increased twofold. Under this condition inhibition of glycogenolysis with the selective GP inhibitor CP-91149 enhanced the increase in glycogen content and further reduced the cell number. The androgen-dependent LNCaP cells that endogenously express AR responded to androgen withdrawal with growth arrest and increased glycogen content. CP-91149 further increased glycogen content and caused a reduction of cell number.

CONCLUSION

Increased glycogenesis is part of the androgen receptor-mediated cellular response and blockage of glycogenolysis by the GP inhibitor CP-91149 further increased glycogenesis. The combined use of a GP inhibitor with hormone therapy may increase the efficacy of hormone treatment by decreasing the survival of prostate cancer cells and thereby reducing the chance of cancer recurrence.

Increased potency and efficacy of combined phosphorylase inactivation and glucokinase activation in control of hepatocyte glycogen metabolism

Glucokinase and phosphorylase both have a high control strength over hepatocyte glycogen metabolism and are potential therapeutic targets for type 2 diabetes. We tested whether combined phosphorylase inactivation and glucokinase activation is a more effective strategy for controlling hepatic glycogen metabolism than single-site targeting. Activation of glucokinase by enzyme overexpression combined with selective dephosphorylation of phosphorylase-a by an indole carboxamide that favors the T conformation of phosphorylase caused a greater stimulation of glycogen synthesis than the sum of either treatment alone. This result is explained by the complementary roles of elevated glucose-6-phosphate (G6P; a positive modulator) and depleted phosphorylase-a (a negative modulator) in activating glycogen synthase and also by synergistic inactivation of phosphorylase-a by glucokinase activation and the indole carboxamide. Inactivation of phosphorylase-a by the indole carboxamide was counteracted by 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside, which is metabolized to an AMP analog; this effect was reversed by G6P. Our findings provide further evidence for the inverse roles of G6P and AMP in regulating the activation state of hepatic phosphorylase. It is proposed that dual targeting of glucokinase and phosphorylase-a enables improved potency and efficacy in controlling hepatic glucose metabolism.

Portal venous 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion overcomes hyperinsulinemic suppression of endogenous glucose output

AMP-activated protein kinase (AMPK) plays a key role in regulating metabolism, serving as a metabolic master switch. The aim of this study was to assess whether increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-beta-D-ribosyl-5-monophosphate, in the liver would create a metabolic response consistent with an increase in whole-body metabolic need. Dogs had sampling (artery, portal vein, hepatic vein) and infusion (vena cava, portal vein) catheters and flow probes (hepatic artery, portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and hyperinsulinemic-euglycemic or -hypoglycemic clamp periods (0-150 min). At t = 0 min, somatostatin was infused and glucagon was replaced in the portal vein at basal rates. An intraportal hyperinsulinemic (2 mU . kg(-1) . min(-1)) infusion was also initiated at this time. Glucose was clamped at hypoglycemic or euglycemic levels in the presence (H-AIC, n = 6; E-AIC, n = 6) or absence (H-SAL, n = 6; E-SAL, n = 6) of a portal venous 5-aminoimidazole-4-carboxamide-ribofuranoside (AICAR) infusion (1 mg . kg(-1) . min(-1)) initiated at t = 60 min. In the presence of intraportal saline, glucose was infused into the vena cava to match glucose levels seen with intraportal AICAR. Glucagon remained fixed at basal levels, whereas insulin rose similarly in all groups. Glucose fell to 50 +/- 2 mg/dl by t = 60 min in hypoglycemic groups and remained at 105 +/- 3 mg/dl in euglycemic groups. Endogenous glucose production (R(a)) was similarly suppressed among groups in the presence of euglycemia or hypoglycemia before t = 60 min and remained suppressed in the H-SAL and E-SAL groups. However, intraportal AICAR infusion stimulated R(a) to increase by 2.5 +/- 1.0 and 3.4 +/- 0.4 mg . kg(-1) . min(-1) in the E-AIC and H-AIC groups, respectively. Arteriovenous measurement of net hepatic glucose output showed similar results. AICAR stimulated hepatic glycogen to decrease by 5 +/- 3 and 19 +/- 5 mg/g tissue (P < 0.05) in the presence of euglycemia and hypoglycemia, respectively. AICAR significantly increased net hepatic lactate output in the presence of hypoglycemia. Thus, intraportal AICAR infusion caused marked stimulation of both hepatic glucose output and net hepatic glycogenolysis, even in the presence of high levels of physiological insulin. This stimulation of glucose output by AICAR was equally marked in the presence of both euglycemia and hypoglycemia. However, hypoglycemia amplified the net hepatic glycogenolytic response to AICAR by approximately fourfold.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside causes acute hepatic insulin resistance in vivo

The infusion of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) causes a rise in tissue concentrations of the AMP analog 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranotide (ZMP), which mimics an elevation of cellular AMP levels. The purpose of this work was to determine the effect of raising hepatic ZMP levels on hepatic insulin action in vivo. Dogs had sampling and infusion catheters as well as flow probes implanted 16 days before an experiment. After an 18-h fast, blood glucose was 82 +/- 1 mg/dl and basal net hepatic glucose output 1.5 +/- 0.2 mg . kg(-1) . min(-1). Dogs received portal venous glucose (3.2 mg . kg(-1) . min(-1)), peripheral venous somatostatin, and basal portal venous glucagon infusions from -90 to 60 min. Physiological hyperinsulinemia was established with a portal insulin infusion (1.2 mU . kg(-1) . min(-1)). Peripheral venous glucose infusion was used to clamp arterial blood glucose at 150 mg/dl. Starting at t = 0 min, dogs received portal venous AICAR infusions of 0, 1, or 2 mg . kg(-1) . min(-1). Net hepatic glucose uptake was 2.4 +/- 0.5 mg . kg(-1) . min(-1) (mean of all groups) before t = 0 min. In the absence of AICAR, net hepatic glucose uptake was 1.9 +/- 0.4 mg . kg(-1) . min(-1) at t = 60 min. The lower-dose AICAR infusion caused a complete suppression of net hepatic glucose uptake (-1.0 +/- 1.7 mg . kg(-1) . min(-1) at t = 60 min). The higher AICAR dose resulted in a profound shift in hepatic glucose balance from net uptake to a marked net output (-6.1 +/- 1.9 mg . kg(-1) . min(-1) at t = 60 min), even in the face of hyperglycemia and hyperinsulinemia. These data show that elevations in hepatic ZMP concentrations, induced by portal venous AICAR infusion, cause acute hepatic insulin resistance. These findings have important implications for the targeting of AMP kinase for the treatment of insulin resistance, using AMP analogs.

The Glycogenic Action of Protein Targeting to Glycogen in Hepatocytes Involves Multiple Mechanisms Including Phosphorylase Inactivation and Glycogen Synthase Translocation

Expression of the glycogen-targeting protein PTG promotes glycogen synthase activation and glycogen storage in various cell types. In this study, we tested the contribution of phosphorylase inactivation to the glycogenic action of PTG in hepatocytes by using a selective inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a and sequential activation of glycogen synthase. Similar to CP-91194, graded expression of PTG caused a concentration-dependent inactivation of phosphorylase and activation of glycogen synthase. The latter was partially counter-acted by the expression of muscle phosphorylase and was not additive with the activation by CP-91149, indicating that it is in part secondary to the inactivation of phosphorylase. PTG expression caused greater stimulation of glycogen synthesis and translocation of glycogen synthase than CP-91149, and the translocation of synthase could not be explained by accumulation of glycogen, supporting an additional role for glycogen synthase translocation in the glycogenic action of PTG. The effects of PTG expression on glycogen synthase and glycogen synthesis were additive with the effects of glucokinase expression, confirming the complementary roles of depletion of phosphorylase a (a negative modulator) and elevated glucose 6-phosphate (a positive modulator) in potentiating the activation of glycogen synthase. PTG expression mimicked the inactivation of phosphorylase caused by high glucose and counteracted the activation caused by glucagon. The latter suggests a possible additional role for PTG on phosphorylase kinase inactivation.