Inactivation Of Phosphorylase Research Articles

Genome-wide identification of the PP1 and PPP1R3 gene family in oyster Crassostrea gigas: Unraveling their roles in glycogen metabolism

High glycogen in oysters contributes to their flavor, quality, and hardiness, making it a crucial target trait for breeding purposes. Until now, the upstream regulatory mechanism for glycogen synthesis has remained unclear. Protein phosphatase-1 regulatory subunit 3 (PPP1R3) is a member of a family that directs the protein phosphatase-1 (PP1) to glycogen, resulting in the activation of glycogen synthase (GS) and the inactivation of glycogen phosphorylase (GP). However, their functions in molluscan species have remained unclear until now. In our study, we verified the function of the PP1α/PPP1R3D pathways in the glycogen metabolism process in Crassostrea gigas. Initially, we conducted a phylogenetic analysis and identified two PP1 and PPP1R3 members in molluscan species, emphasizing copy number variations in eukaryotic animals. Additionally, we identified distinct substitutions of conserved motifs specific to bivalves, Cephalopoda, or Gastropods, revealing functional diversity in PP1 and PPP1R3. Our primary focus was on PP1α/PPP1R3D in the oyster C.gigas, detected in the muscle glycogen fraction of bivalve species, with mRNA expression showing a positive correlation with glycogen content. By manipulating CgPPP1R3D expression in oysters through adenovirus-mediated overexpression or siRNA-mediated downregulation, we established a direct positive correlation between CgPPP1R3D expression levels and glycogen content. Colocation analysis confirmed protein interactions between CgPP1α and CgPPP1R3D. Furthermore, to elucidate the correlation between gene expression and glycogen accumulation, we conducted an association analysis and identified a cluster of significant SNP loci upstream of CgPPP1R3D significantly related to glycogen content. The high polymorphism observed may be linked to differential expressions of CgPPP1R3D in oyster individuals with high and low glycogen levels. These findings serve as a crucial reference for enhancing glycogen traits in molluscan species.

Role of nitration in control of phosphorylase and glycogenolysis in mouse skeletal muscle.

Phosphorylase is one of the most carefully studied proteins in history, but knowledge of its regulation during intense muscle contraction is incomplete. Tyrosine nitration of purified preparations of skeletal muscle phosphorylase results in inactivation of the enzyme and this is prevented by antioxidants. Whether an altered redox state affects phosphorylase activity and glycogenolysis in contracting muscle is not known. Here, we investigate the role of the redox state in control of phosphorylase and glycogenolysis in isolated mouse fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) muscle preparations during repeated contractions. Exposure of crude muscle extracts to H2O2 had little effect on phosphorylase activity. However, exposure of extracts to peroxynitrite (ONOO-), a nitrating/oxidizing agent, resulted in complete inactivation of phosphorylase (half-maximal inhibition at ∼200 µM ONOO-), which was fully reversed by the presence of an ONOO- scavanger, dithiothreitol (DTT). Incubation of isolated muscles with ONOO- resulted in nitration of phosphorylase and marked inhibition of glycogenolysis during repeated contractions. ONOO- also resulted in large decreases in high-energy phosphates (ATP and phosphocreatine) in the rested state and following repeated contractions. These metabolic changes were associated with decreased force production during repeated contractions (to ∼60% of control). In contrast, repeated contractions did not result in nitration of phosphorylase, nor did DTT or the general antioxidant N-acetylcysteine alter glycogenolysis during repeated contractions. These findings demonstrate that ONOO- inhibits phosphorylase and glycogenolysis in living muscle under extreme conditions. However, nitration does not play a significant role in control of phosphorylase and glycogenolysis during repeated contractions.NEW & NOTEWORTHY Here we show that exogenous peroxynitrite results in nitration of phosphorylase as well as inhibition of glycogenolysis in isolated intact mouse skeletal muscle during short-term repeated contractions. However, repeated contractions in the absence of exogenous peroxynitrite do not result in nitration of phosphorylase or affect glycogenolysis, nor does the addition of antioxidants alter glycogenolysis during repeated contractions. Thus phosphorylase is not subject to redox control during repeated contractions.

The 9p21 Locus and its Potential Role in Atherosclerosis Susceptibility; Molecular Mechanisms and Clinical Implications.

Cardiovascular disease (CVD) is the leading cause of global mortality. Although extensive efforts have been made to identify valid biomarkers for CVD risk, relatively few are of proven clinical utility. It is recognized that genetic factors play a major role in determining the susceptibility to CVD. Recent genome-wide-association-studies have demonstrated common genetic variants in a region on chromosome 9p21 associated with an increased risk of CVD. Several genetic-polymorphisms have been identified in this region that are highly associated with CVD, and these are clustered around the gene loci for CDKN2B (coding for p15ink4b), CDKN2A (coding for p16ink4a and p14ARF) and the 3' end of CDKN2BAS, which has been termed antisense noncoding RNA in the INK4 locus (ANRIL). Targeted deletion of the 9p21 locus reduces the cardiac expression of CDKN2A/B and is the most frequent mechanism for methylthioadenosine phosphorylase inactivation, leading to a less stable plaque phenotype in the artery. Further investigations will be essential to explore the clinical utility of emerging-markers in larger and multicenter setting in order to establish their values for risk stratification or prediction a chance of future CVD events. The aim of the current review was to provide an overview of the possible molecular mechanisms by which the chromosome 9p21 locus may confer CVD risk, and the consequential clinical implications with particular emphasis on preclinical/clinical trials on genetic changes of this locus and CVD risk.

Myocardial glycophagy — A specific glycogen handling response to metabolic stress is accentuated in the female heart

Cardiac metabolic stress is a hallmark of many cardiac pathologies, including diabetes. Cardiac glycogen mis-handling is a frequent manifestation of various cardiopathologies. Diabetic females have a higher risk of heart disease than males, yet sex disparities in cardiac metabolic stress settings are not well understood. Oestrogen acts on key glycogen regulatory proteins. The goal of this study was to evaluate sex-specific metabolic stress-triggered cardiac glycogen handling responses. Male and female adult C57Bl/6J mice were fasted for 48h. Cardiac glycogen content, particle size, regulatory enzymes, signalling intermediates and autophagic processes were evaluated. Female hearts exhibited 51% lower basal glycogen content than males associated with lower AMP-activated-kinase (AMPK) activity (35% decrease in pAMPK:AMPK). With fasting, glycogen accumulated in female hearts linked with decreased particle size and upregulation of Akt and AMPK signalling, activation of glycogen synthase and inactivation of glycogen phosphorylase. Fasting did not alter glycogen content or regulatory proteins in male hearts. Expression of glycogen autophagy marker, starch-binding-protein-domain-1 (STBD1), was 63% lower in female hearts than males and increased by 69% with fasting in females only. Macro-autophagy markers, p62 and LC3BII:I ratio, increased with fasting in male and female hearts. This study identifies glycogen autophagy (‘glycophagy’) as a potentially important component of the response to cardiac metabolic stress. Glycogen autophagy occurs in association with a marked and selective accumulation of glycogen in the female myocardium. Our findings suggest that sex-specific differences in glycogen handling may have cardiopathologic consequences in various settings, including diabetic cardiomyopathy.

Role of methylthioadenosine phosphorylase (MTAP) in hepatic stellate cell activation

The activation of hepatic stellate cells (HSCs) is a key event of hepatic fibrosis, and the polyamine metabolic pathway has been implicated to affects hepatic fibrosis. Methylthioadenosine phosphorylase (MTAP) the rate-limiting enzyme in the methionine and adenine salvage pathway catalyzes the phosphorylation of 5'-deoxy-5′-(methylthio)adenosine (MTA), which is a by-product of polyamine synthesis.

Methylthioadenosine phosphorylase inactivation depends on gene deletion in laryngeal squamous cell carcinoma

Methylthioadenosine phosphorylase (MTAP) is an essential enzyme for the methionine and adenosine salvage pathway in normal cells, frequently inactivated in many different human cancers. MTAP status could be important for tumour cell sensitivity to adjuvant chemotherapy. To our knowledge, there have been no reports to date on MTAP status in laryngeal carcinoma. A series of 31 laryngeal squamous cell carcinomas was investigated for MTAP mRNA expression using reverse transcription and quantitative polymerase chain reaction (qPCR), as well as for MTAP gene deletion and/or promoter hypermethylation using qPCR and methylation-specific PCR, respectively. Low MTAP mRNA expression was found in 32% of cases, and was associated with MTAP gene deletion (in 70%; P<0.001) but not with MTAP promoter hypermethylation, indicating that, in this tumour, gene deletion is the main mechanism for MTAP inactivation. Neither low mRNA expression nor gene deletion was associated with any of the clinicopathological parameters investigated. Given the significance of MTAP status for cell sensitivity to different chemotherapeutic regimens, our results suggest that determination of MTAP inactivation should be taken into consideration in managing laryngeal squamous cell carcinomas.

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.

Helpful introduction to signal transduction

![Figure][1] Structure and Function in Cell Signalling By John Nelson Wiley (2008) 410 pages, ISBN 978-0-470-02551-2, £34.95 (paperback) When studying cell signaling, it is easy to get overwhelmed with the complexity of each pathway and with the crosstalk between them. Teaching the

AMP-activated Protein Kinase Phosphorylates R5/PTG, the Glycogen Targeting Subunit of the R5/PTG-Protein Phosphatase 1 Holoenzyme, and Accelerates Its Down-regulation by the Laforin-Malin Complex

R5/PTG is one of the glycogen targeting subunits of type 1 protein phosphatase, a master regulator of glycogen synthesis. R5/PTG recruits the phosphatase to the places where glycogen synthesis occurs, allowing the activation of glycogen synthase and the inactivation of glycogen phosphorylase, thus increasing glycogen synthesis and decreasing its degradation. In this report, we show that the activity of R5/PTG is regulated by AMP-activated protein kinase (AMPK). We demonstrate that AMPK interacts physically with R5/PTG and modifies its basal phosphorylation status. We have also mapped the major phosphorylation sites of R5/PTG by mass spectrometry analysis, observing that phosphorylation of Ser-8 and Ser-268 increased upon activation of AMPK. We have recently described that the activity of R5/PTG is down-regulated by the laforin-malin complex, composed of a dual specificity phosphatase (laforin) and an E3-ubiquitin ligase (malin). We now demonstrate that phosphorylation of R5/PTG at Ser-8 by AMPK accelerates its laforin/malin-dependent ubiquitination and subsequent proteasomal degradation, which results in a decrease of its glycogenic activity. Thus, our results define a novel role of AMPK in glycogen homeostasis.

Effect of proline on thermal inactivation, denaturation and aggregation of glycogen phosphorylase b from rabbit skeletal muscle

It has been shown that the relatively low concentrations of proline (0.1 M) have a slight accelerating effect on thermal aggregation of glycogen phosphorylase b (Ph b) from rabbit skeletal muscle registered by the accumulaton of the aggregated protein. The suppression of Ph b aggregation at high proline concentrations is mainly due to the protective action of proline on the stage of unfolding of the Ph b molecule. The enhancement of Ph b stability in the presence of the high concentrations of proline was demonstrated by the data on differential scanning calorimetry, analytical ultracentrifugation and thermoinactivation kinetics. The construction of the protein aggregate size versus time plots allowed the acceleration of the stage of Ph b aggregation in the presence of high concentrations of proline to be demonstrated. The obtained results are consistent with the predictions of the crowding theory.

Inactivation of rabbit muscle glycogen phosphorylase b by peroxynitrite revisited: Does the nitration of Tyr 613 in the allosteric inhibition site control enzymatic function?

There is increasing evidence that sequence-specific formation of 3-nitrotyrosine (3-NT) may cause functional changes in target proteins. Recently, the nitration of Tyr residues in glycogen phosphorylase b (Ph-b) was implicated in the age-associated decline of protein function [Sharov et al., Exp. Gerontol. 41 (2006) 407–416]; in another report, the nitration of one specific residue, Tyr 613, located in the allosteric inhibition site was hypothesized as a rationale for peroxynitrite inactivation [Dairou et al., J. Mol. Biol. 372 (2007) 1009–1021]. In this study, we have optimized the analysis of in-gel Ph-b digests by high performance liquid chromatography-electro spray ionization-tandem mass spectrometry, in order to achieve a quantitative analysis of nitration of individual Tyr residues at a high coverage of Tyr-containing sequences (92%). Our data do not confirm the role of Tyr 613 nitration in the control of enzymatic function. Furthermore, we show here that the enzymatic activity of Ph-b does not directly correlate with the protein nitration levels, and that the modification of Cys and, potentially, other amino acid residues can better rationalize Ph-b inactivation by peroxynitrite.

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.

Molecular Recognition of the Protein Phosphatase 1 Glycogen Targeting Subunit by Glycogen Phosphorylase

Disrupting the interaction between glycogen phosphorylase and the glycogen targeting subunit (G(L)) of protein phosphatase 1 is emerging as a novel target for the treatment of type 2 diabetes. To elucidate the molecular basis of binding, we have determined the crystal structure of liver phosphorylase bound to a G(L)-derived peptide. The structure reveals the C terminus of G(L) binding in a hydrophobically collapsed conformation to the allosteric regulator-binding site at the phosphorylase dimer interface. G(L) mimics interactions that are otherwise employed by the activator AMP. Functional studies show that G(L) binds tighter than AMP and confirm that the C-terminal Tyr-Tyr motif is the major determinant for G(L) binding potency. Our study validates the G(L)-phosphorylase interface as a novel target for small molecule interaction.

Acetylcholine exerts additive and permissive but not synergistic effects with insulin on glycogen synthesis in hepatocytes

Parasympathetic (cholinergic) innervation is implicated in the stimulation of hepatic glucose uptake by portal vein hyperglycaemia. We determined the direct effects of acetylcholine on hepatocytes. Acute exposure to acetylcholine mimicked insulin action on inactivation of phosphorylase, stimulation of glycogen synthesis and suppression of phosphoenolpyruvate carboxykinase mRNA levels but with lower efficacy and without synergy. Pre-exposure to acetylcholine had a permissive effect on insulin action similar to glucocorticoids and associated with increased glucokinase activity. It is concluded that acetylcholine has a permissive effect on insulin action but cannot fully account for the rapid stimulation of glucose uptake by the portal signal.

Stimulation of glycogen synthesis and inactivation of phosphorylase in hepatocytes by serotonergic mechanisms, and counter-regulation by atypical antipsychotic drugs

Intraportal infusion of serotonin (5-hydroxytryptamine, 5-HT) or inhibitors of its cellular uptake stimulate hepatic glucose uptake in vivo by either direct or indirect mechanisms. The aims of this study were to determine the direct effects of 5-HT in hepatocytes and to test the hypothesis that atypical antipsychotic drugs that predispose to type 2 diabetes counter-regulate the effects of 5-HT. Rat hepatocytes were studied in short-term primary culture. Serotonin (5-HT) stimulated glycogen synthesis at nanomolar concentrations but inhibited it at micromolar concentrations. The stimulatory effect was mimicked by alpha-methyl-5-HT, a mixed 5-HT1/5-HT2 receptor agonist, whereas the inhibition was counteracted by a 5-HT2B/2C receptor antagonist. alpha-Methyl-5-HT stimulated glycogen synthesis additively with insulin, but unlike insulin, did not stimulate glucose phosphorylation and glycolysis, nor did it cause Akt (protein kinase B) phosphorylation. Stimulation of glycogen synthesis by alpha-methyl-5-HT correlated with depletion of phosphorylase a. This effect could not be explained by elevated levels of glucose 6-phosphate, which causes inactivation of phosphorylase, but was explained, at least in part, by decreased phosphorylase kinase activity in situ. The antipsychotic drugs clozapine and olanzapine, which bind to 5-HT receptors, counteracted the effect of alpha-methyl-5-HT on phosphorylase inactivation. This study provides evidence for both stimulation and inhibition of glycogen synthesis in hepatocytes by serotonergic mechanisms. The former effects are associated with the inactivation of phosphorylase and are counteracted by atypical antipsychotic drugs that cause hepatic insulin resistance. Antagonism of hepatic serotonergic mechanisms may be a component of the hepatic dysregulation caused by antipsychotic drugs that predispose to type 2 diabetes.

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-β-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.

The role of protein kinase B/Akt in insulin-induced inactivation of phosphorylase in rat hepatocytes

An insulin signalling pathway leading from activation of protein kinase B (PKB, also known as Akt) to phosphorylation (inactivation) of glycogen synthase kinase-3 (GSK-3) and activation of glycogen synthase is well characterised. However, in hepatocytes, inactivation of GSK-3 is not the main mechanism by which insulin stimulates glycogen synthesis. We therefore tested whether activation of PKB causes inactivation of glycogen phosphorylase. We used a conditionally active form of PKB, produced using recombinant adenovirus, to test the role of acute PKB activation in the control of glycogen phosphorylase and glycogen synthesis in hepatocytes. Conditional activation of PKB mimicked the inactivation of phosphorylase, the activation of glycogen synthase, and the stimulation of glycogen synthesis caused by insulin. In contrast, inhibition of GSK-3 caused activation of glycogen synthase but did not mimic the stimulation of glycogen synthesis by insulin. PKB activation and GSK-3 inhibition had additive effects on the activation of glycogen synthase, indicating convergent mechanisms downstream of PKB involving inactivation of either phosphorylase or GSK-3. Glycogen synthesis correlated inversely with the activity of phosphorylase-a, irrespective of whether this was modulated by insulin, by PKB activation or by a selective phosphorylase ligand, supporting an essential role for phosphorylase inactivation in the glycogenic action of insulin in hepatocytes. In hepatocytes, the acute activation of PKB, but not the inhibition of GSK-3, mimics the stimulation of glycogen synthesis by insulin. This is explained by a pathway downstream of PKB leading to inactivation of phosphorylase, activation of glycogen synthase, and stimulation of glycogen synthesis, independent of the GSK-3 pathway.

Reversible Phosphorylation and Kinase Cascades: the Work of Edwin G. Krebs

Reversible Phosphorylation and Kinase Cascades: the Work of Edwin G. Krebs

Corrigendum

CORRIGENDACorrigendumPublished Online:01 Jul 2005https://doi.org/10.1152/ajprenal.00165.2005Original articleMoreSectionsPDF (34 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Volume 284, March 2003 Volume 53, March 2003 Pages F433–F446: Knepper MA, Saidel GM, Hascall VC, and Dwyer T. “Concentration of solutes in the renal inner medulla: interstitial hyaluronan as a mechano-osmotic transducer.” ( http//:ajprenal.physiology.org/cgi/content/full/53/3/284; doi: 10.1152/ajprenal.00165.2005).On p. F440, in the legend for Fig. 5, the URL for a video clip showing contractions is stated incorrectly. The URL should have read http://ajprenal.physiology.org/cgi/content/full/284/3/F433/DC1.This article has no references to display. Download PDF Previous Back to Top FiguresReferencesRelatedInformationRelated ArticlesConcentration of solutes in the renal inner medulla: interstitial hyaluronan as a mechano-osmotic transducer 01 Mar 2003American Journal of Physiology-Renal PhysiologyCited ByStimulation of glycogen synthesis and inactivation of phosphorylase in hepatocytes by serotonergic mechanisms, and counter-regulation by atypical antipsychotic drugs20 June 2007 | Diabetologia, Vol. 50, No. 8 More from this issue > Volume 289Issue 1July 2005Pages F225-F225 Copyright & PermissionsCopyright © 2005 the American Physiological Societyhttps://doi.org/10.1152/ajprenal.00165.2005PubMed15802561History Published online 1 July 2005 Published in print 1 July 2005 Metrics

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.