Stimulation of rat liver glycogen synthesis by the adenosine kinase inhibitor 5-iodotubercidin

5-Iodotubercidin inhibits SARS-CoV-2 RNA synthesis

Coronavirus disease 2019 (COVID-19) is a newly emerged infectious disease caused by a novel coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The rapid global emergence of SARS-CoV-2 highlights the importance and urgency for potential drugs to control the pandemic. The functional importance of RNA-dependent RNA polymerase (RdRp) in the viral life cycle, combined with structural conservation and absence of closely related homologs in humans, makes it an attractive target for designing antiviral drugs. Nucleos(t)ide analogs (NAs) are still the most promising broad-spectrum class of viral RdRp inhibitors. In this study, using our previously developed cell-based SARS-CoV-2 RdRp report system, we screened 134 compounds in the Selleckchemicals NAs library. Four candidate compounds, Fludarabine Phosphate, Fludarabine, 6-Thio-20-Deoxyguanosine (6-Thio-dG), and 5-Iodotubercidin, exhibit remarkable potency in inhibiting SARS-CoV-2 RdRp. Among these four compounds, 5-Iodotubercidin exhibited the strongest inhibition upon SARS-CoV-2 RdRp, and was resistant to viral exoribonuclease activity, thus presenting the best antiviral activity against coronavirus from a different genus. Further study showed that the RdRp inhibitory activity of 5-Iodotubercidin is closely related to its capacity to inhibit adenosine kinase (ADK).

Crystal structure of ADP-dependent glucokinase from Methanocaldococcus jannaschii in complex with 5-iodotubercidin reveals phosphoryl transfer mechanism

ADP-dependent glucokinase (ADPGK) is an alternative novel glucose phosphorylating enzyme in a modified glycolysis pathway of hyperthermophilic Archaea. In contrast to classical ATP-dependent hexokinases, ADPGK utilizes ADP as a phosphoryl group donor. Here, we present a crystal structure of archaeal ADPGK from Methanocaldococcus jannaschii in complex with an inhibitor, 5-iodotubercidin, d-glucose, inorganic phosphate, and a magnesium ion. Detailed analysis of the architecture of the active site allowed for confirmation of the previously proposed phosphorylation mechanism and the crucial role of the invariant arginine residue (Arg197). The crystal structure shows how the phosphate ion, while mimicking a β-phosphate group, is positioned in the proximity of the glucose moiety by arginine and the magnesium ion, thus providing novel insights into the mechanism of catalysis. In addition, we demonstrate that 5-iodotubercidin inhibits human ADPGK-dependent T cell activation-induced reactive oxygen species (ROS) release and downstream gene expression, and as such it may serve as a model compound for further screening for hADPGK-specific inhibitors.

Identification of 5-Iodotubercidin as a genotoxic drug with anti-cancer potential

Tumor suppressor p53, which is activated by various stress and oncogene activation, is a target for anti-cancer drug development. In this study, by screening panels of protein kinase inhibitors and protein phosphatase inhibitors, we identified 5-Iodotubercidin as a strong p53 activator. 5-Iodotubercidin is purine derivative and is used as an inhibitor for various kinases including adenosine kinase. We found that 5-Iodotubercidin could cause DNA damage, verified by induction of DNA breaks and nuclear foci positive for γH2AX and TopBP1, activation of Atm and Chk2, and S15 phosphorylation and up-regulation of p53. As such, 5-Iodotubercidin induces G2 cell cycle arrest in a p53-dependent manner. Itu also induces cell death in p53-dependent and -independent manners. DNA breaks were likely generated by incorporation of 5-Iodotubercidin metabolite into DNA. Moreover, 5-Iodotubercidin showed anti-tumor activity as it could reduce the tumor size in carcinoma xenograft mouse models in p53-dependent and -independent manners. These findings reveal 5-Iodotubercidin as a novel genotoxic drug that has chemotherapeutic potential.

Adenosine Monophosphate-Activated Protein Kinase (AMPK) as a New Target for Antidiabetic Drugs: A Review on Metabolic, Pharmacological and Chemical Considerations

In view of the epidemic nature of type 2 diabetes and the substantial rate of failure of current oral antidiabetic drugs the quest for new therapeutics is intensive. The adenosine monophosphate-activated protein kinase (AMPK) is an important regulatory protein for cellular energy balance and is considered a master switch of glucose and lipid metabolism in various organs, especially in skeletal muscle and liver. In skeletal muscles, AMPK stimulates glucose transport and fatty acid oxidation. In the liver, it augments fatty acid oxidation and decreases glucose output, cholesterol and triglyceride synthesis. These metabolic effects induced by AMPK are associated with lowering blood glucose levels in hyperglycemic individuals. Two classes of oral antihyperglycemic drugs (biguanidines and thiazolidinediones) have been shown to exert some of their therapeutic effects by directly or indirectly activating AMPK. However, side effects and an acquired resistance to these drugs emphasize the need for the development of novel and efficacious AMPK activators. We have recently discovered a new class of hydrophobic D-xylose derivatives that activates AMPK in skeletal muscles in a non insulin-dependent manner. One of these derivatives (2,4;3,5-dibenzylidene-D-xylose-diethyl-dithioacetal) stimulates the rate of hexose transport in skeletal muscle cells by increasing the abundance of glucose transporter-4 (GLUT-4) in the plasma membrane through activation of AMPK. This compound reduces blood glucose levels in diabetic mice and therefore offers a novel strategy of therapeutic intervention strategy in type 2 diabetes. The present review describes various classes of chemically-related compounds that activate AMPK by direct or indirect interactions and discusses their potential for candidate antihyperglycemic drug development.

Effects of 5-iodotubercidin on hepatic fatty acid metabolism mediated by the inhibition of acetyl-CoA carboxylase

Diverse mechanisms of action have been proposed for 5-iodotubercidin, although it is widely used as an adenosine kinase inhibitor that consequently interferes with the metabolism of adenosine and adenine nucleotides. Incubation of rat hepatocytes with iodotubercidin produced important effects on lipid metabolism. (i) Both acetyl-CoA carboxylase and fatty acid synthesis de novo were inhibited in parallel by iodotubercidin, with no change in the activity of fatty acid synthase. The inhibition of both activities showed a comparable dependence on iodotubercidin concentration and was accompanied by a similar decrease (about 60%) in the intracellular malonyl-CoA concentration. (ii) Iodotubercidin stimulated palmitate oxidation, although octanoate oxidation was unaffected. However, this effect can be attributed to the decrease of malonyl-CoA concentration and the concomitant relief of the inhibition of carnitine palmitoyltransferase I, because the activity of this enzyme was found unaltered when determined in cells permeabilized with digitonin. (iii) Iodotubercidin also inhibited cholesterol synthesis de novo. Results, thus, indicate that iodotubercidin increases fatty acid oxidation activity of the liver at the expense of lipogenesis, and we suggest that these effects on fatty acid metabolism are mediated by the inhibition of acetyl-CoA carboxylase, probably due to a more than twice increase in the AMP/ATP ratio and the concomitant stimulation of the AMP-activated protein kinase.

Stimulation of AMP-activated protein kinase (AMPK) is associated with enhancement of Glut1-mediated glucose transport

In cells expressing only the Glut1 isoform of glucose transporters, we have shown that glucose transport is markedly stimulated in response to hypoxia or inhibition of oxidative phosphorylation, conditions that would be expected to cause a stimulation of AMP-activated protein kinase (AMPK) activity. In the present study we tested the hypothesis that the stimulation of AMPK activity might be accompanied by an enhancement of Glut1-mediated glucose transport. Exposure of Clone 9 cells, 3T3-L1 preadipocytes, and C(2)C(12) myoblasts (cells that express only the Glut1 isoform) to 5-aminoimidazole-4-carboxamideribonucleoside (AICAR), an adenosine analog that stimulates AMPK activity, resulted in a marked increase in the rate of glucose transport (ranging from four- to sixfold) that was accompanied by activation of AMPK. This stimulation of AMPK activity was associated with an increase in the phosphorylation of threonine 172 on the activation loop of its alpha subunit, with the predominant change being in the alpha-2 isoform. Exposure of Clone 9 cells to 5-iodotubercidin, an inhibitor of adenosine kinase, abolished the accumulation of AICAR-5'-monophosphate (ZMP), stimulation of AMPK, and the enhancement of glucose transport in response to AICAR. There was no significant increase in the content of Glut1 in plasma membranes of Clone 9 cells exposed to AICAR. We conclude that stimulation of AMPK activity is associated with enhancement of Glut1-mediated glucose transport, and that the glucose transport response is mediated by activation of Glut1 transporters preexisting in the plasma membrane.

Specific features of glycogen metabolism in the liver

Although the general pathways of glycogen synthesis and glycogenolysis are identical in all tissues, the enzymes involved are uniquely adapted to the specific role of glycogen in different cell types. In liver, where glycogen is stored as a reserve of glucose for extrahepatic tissues, the glycogen-metabolizing enzymes have properties that enable the liver to act as a sensor of blood glucose and to store or mobilize glycogen according to the peripheral needs. The prime effector of hepatic glycogen deposition is glucose, which blocks glycogenolysis and promotes glycogen synthesis in various ways. Other glycogenic stimuli for the liver are insulin, glucocorticoids, parasympathetic (vagus) nerve impulses and gluconeogenic precursors such as fructose and amino acids. The phosphorolysis of glycogen is mainly mediated by glucagon and by the orthosympathetic neurotransmitters noradrenaline and ATP. Many glycogenolytic stimuli, e.g. adenosine, nucleotides and NO, also act indirectly, via secretion of eicosanoids from non-parenchymal cells. Effectors often initiate glycogenolysis cooperatively through different mechanisms.

5-Iodotubercidin and proglycosyn: a comparison of two glycogenic compounds in hepatocytes from fasted rats

5-Iodotubercidin (Itu) and proglycosyn (Pro) have similar glycogenic properties. To compare their mechanisms of action, we tested them in hepatocytes from fasted rats. We show that both compounds are similar in that they stimulated glycogen synthesis, increased the concentration of synthase a, decreased that of phosphorylase a and lowered the concentration of F-2,6-P2 in the presence of glucose, lactate-pyruvate and amino acids. However, when amino acids were absent, Pro was the better stimulator of glycogenesis than Itu and in combination they elevated glycogen and synthase a concentrations synergistically. Further they differ in that (1) Itu enhanced the levels of cyclic AMP whereas Pro did not; (2) Pro depressed glucose production from gluconeogenic substrates, whereas Itu stimulated this process; (3) the inhibition of F-2,6-P2 formation and glycolysis by Pro became much weaker than that by Itu when glucose concentrations were raised from 10 to 20 mM. Inhibition of glycolysis but not that of glycogen synthesis was partly due to a phosphorylated metabolite of Itu. The present study indicates that despite their similar glycogenic effects, Itu and Pro do not share a common mechanism of action. Further, the inhibition of glycolysis and F-2,6-P2 formation by Itu cannot be explained if it acts solely as a general inhibitor of protein kinases.

Effects of C-1-substituted glucose analogue on the activation states of glycogen synthase and glycogen phosphorylase in rat hepatocytes

A series of glucose-analogue inhibitors of glycogen phosphorylase b (GPb) has been designed, synthesized and investigated in crystallographic binding and kinetic studies. The aim is to produce a compound that may exert more effective control over glycogen metabolism than the parent glucose molecule and which could alleviate hyperglycaemia in Type-II diabetes. N-Acetyl-beta-D-glucopyranosylamine (1-GlcNAc) has a Ki for muscle GPb in crude extracts of 30 microM, 367-fold lower than that of beta-D-glucose [Board, Hadwen and Johnson (1995) Eur. J. Biochem. 228, 753-761]. In the current work, the effects of 1-GlcNAc on the activation states of GP and glycogen synthase (GS) in cell-free preparations and in isolated hepatocytes are reported. In gel-filtered extracts of liver, which lack ATP for kinase activity, 1-GlcNAc produced a rapid and time-dependent inactivation of GP with a subsequent activation of GS. Effects of 1-GlcNAc on both enzymes were stronger than those of glucose, with 0.8 mM 1-GlcNAc being equipotent with 50 mM glucose. At 1 mM, 1-GlcNAc enhanced the dephosphorylation of exogenous GPa by liver extracts (600%) and by muscle extracts (75%). This represents an approximately 500-fold improvement on glucose for the liver activity and 40-fold for the muscle activity. In whole hepatocytes, 1-GlcNAc showed an approximately 5-fold enhancement of glucose effects for GP inactivation but failed to elicit activation of GS. Glucose-induced activation of GS in whole hepatocytes was reversed by subsequent addition of 1-GlcNAc. However, when GS activation was achieved via the adenosine analogue and kinase inhibitor, 5'-iodotubercidin (ITU), subsequent addition of 1-GlcNAc allowed continued activation of GS. Phosphorylation of 1-GlcNAc in rat hepatocytes was established using radiolabelled material. The rate of phosphorylation was 1.60 nmol/min per 10(6) cells at 20 mM 1-GlcNAc but was reduced by the presence of 50 microM ITU (0.775 nmol/min per 10(6) cells). It is suggested that the phosphorylated derivative of 1-GlcNAc formed in hepatocytes is 1-GlcNAc 6-phosphate and that the presence of this species is responsible for the failure of 1-GlcNAc to activate GS. The relative importance of the reduction in concentration of GPa versus increased glucose 6-phosphate levels for activation of GS is discussed.

Identification of the glycogenic compound 5-iodotubercidin as a general protein kinase inhibitor

Addition of micromolar concentrations of the adenosine derivative 5-iodotubercidin (Itu) initiates glycogen synthesis in isolated hepatocytes by causing inactivation of phosphorylase and activation of glycogen synthase [Flückiger-Isler and Walter (1993) Biochem. J. 292, 85-91]. We report here that Itu also antagonizes the effects of saturating concentrations of glucagon and vasopressin on these enzymes. The Itu-induced activation of glycogen synthase could not be explained by the removal of phosphorylase a (a potent inhibitor of the glycogen-associated synthase phosphatase). When tested on purified enzymes, Itu did not affect the activities of the major Ser/Thr-specific protein phosphatases (PP-1, PP-2A, PP-2B and PP-2C), but it inhibited various Ser/Thr-specific protein kinases as well as the tyrosine kinase activity of the insulin receptor (IC50 between 0.4 and 28 microM at 10-15 microM ATP). Tubercidin, which did not affect glycogen synthase or phosphorylase in liver cells, was 300 times less potent as a protein kinase inhibitor. Kinetic analysis of the inhibition of casein kinase-1 and protein kinase A showed that Itu acts as a competitive inhibitor with respect to ATP, and as a mixed-type inhibitor with respect to the protein substrate. We propose that Itu inactivates phosphorylase and activates glycogen synthase by inhibiting phosphorylase kinase and various glycogen synthase kinases. Consistent with the broad specificity of Itu in vitro, this compound decreased the phosphorylation level of numerous phosphopolypeptides in intact liver cells. Our data suggest that at least some of the biological effects of Itu can be explained by an inhibition of protein kinases.

Effect of proglycosyn and other phenolic compounds on glycogen metabolism in isolated hepatocytes. Potential role of glucuronidated metabolites

The mechanism by which proglycosyn (LY 177,507) stimulates glycogen synthesis in isolated hepatocytes [Harris, R. A., Yamanouchi, K., Roach, P. J., Yen, T. T., Dominiani, S. J. & Stephens, T. W. (1989) J. Biol. Chem. 264, 13674-13680] has been investigated. When incubated in the presence of hepatocytes, proglycosyn was metabolized to an O-demethylated glucuronidated derivative, as determined by fast-atom-bombardment mass spectrometry and enzymic analysis. This metabolite accumulated almost linearly inside the cells to reach a concentration of approximately 3 mumol/g protein after 50 min, without apparent release into the medium. In confirmation of previous work, proglycosyn decreased the level of phosphorylase a and increased that of synthase a in hepatocytes. Washing of cells incubated with proglycosyn for 30 min considerably decreased the concentration of the drug without significantly modifying the intracellular concentration of the metabolite and the activation state of glycogen synthase. Several compounds bearing structural analogy with proglycosyn were also tested for their effect on glycogen metabolism. At millimolar or submillimolar concentrations, resorcinol, m-anisidine, phenol, 3-hydroxyacetophenone, and 3-acetamidophenol, although not 4-acetamidophenol, stimulated the incorporation of [14C]glucose into glycogen, decreased the level of phosphorylase a and increased the level of synthase a. In the case of phenol, the effect on the glycogen enzymes paralleled the intracellular accumulation of phenylglucuronide. Furthermore, ethanol and D-galactosamine, which decreased the conversion of phenol to phenylglucuronide and the intracellular concentration of phenylglucuronide, counteracted the effect of phenol on the synthase and on the phosphorylase. From these results, it is suggested that the effect of proglycosyn and of simpler phenol derivatives is mediated by glucuronidated metabolites, which act on an intracellular target.