Ser/Thr phosphatases: One of the key regulators of insulin signaling

Protein phosphorylation is an important post-translational modification that regulates several cellular processes including insulin signaling. The evidences so far have already portrayed the importance of balanced actions of kinases and phosphatases in regulating the insulin signaling cascade. Therefore, elucidating the role of both kinases and phosphatases are equally important. Unfortunately, the role of phosphatases is less studied as compared to kinases. Since brain responds to insulin and insulin signaling is reported to be crucial for many neuronal processes, it is important to understand the role of neuronal insulin signaling regulators. Ser/Thr phosphatases seem to play significant roles in regulating neuronal insulin signaling. Therefore, in this review, we discussed the involvement of Ser/Thr phosphatases in regulating insulin signaling and insulin resistance in neuronal system at the backdrop of the same phosphatases in peripheral insulin sensitive tissues.

Increased frequency of rare missense PPP1R3B variants among Danish patients with type 2 diabetes

Background

PPP1R3B has been suggested as a candidate gene for monogenic forms of diabetes as well as type 2 diabetes (T2D) due to its association with glycaemic trait and its biological role in glycogen synthesis.

Objectives

To study if rare missense variants in PPP1R3B increase the risk of maturity onset diabetes of the young (MODY), T2D or affect measures of glucose metabolism.

Method

Targeted resequencing of PPP1R3B was performed in 8,710 samples; MODY patients with unknown etiology (n = 54), newly diagnosed patients with T2D (n = 2,930) and population-based control individuals (n = 5,726, of whom n = 4,569 had normal glucose tolerance). All population-based sampled individuals were examined using an oral glucose tolerance test.

Results

Among n = 396 carriers, we identified twenty-three PPP1R3B missense mutations, none of which segregated with MODY. The burden of likely deleterious PPP1R3B variants was significantly increased with a total of 17 carriers among patients with T2D (0.58% (95% CI: 0.36–0.93)) compared to 18 carriers among non-diabetic individuals (0.31% (95% CI: 0.20–0.49)), resulting in an increased risk of T2D (OR (95% CI) = 2.57 (1.14–5.79), p = 0.02 (age and sex adjusted)). Furthermore, carriers with diabetes had less abdominal fat and a higher serum concentration of LDL-cholesterol compared to patients with T2D without rare missense PPP1R3B variants. In addition, non-diabetic carriers had a higher birth weight compared to non-carriers.

Conclusion

Rare missense PPP1R3B variants may predispose to T2D.

Approaches to Study Phosphatases

Phosphatases play key roles in normal physiology and diseases. Studying phosphatases has been both essential and challenging, and the application of conventional genetic and biochemical methods has led to crucial but still limited understanding of their mechanisms, substrates, and exclusive functions within highly intricate networks. With the advances in technologies such as cellular imaging and molecular and chemical biology in terms of sensitive tools and methods, the phosphatase field has thrived in the past years and has set new insights for cell signaling studies and for therapeutic development. In this review, we give an overview of the existing interdisciplinary tools for phosphatases, give examples on how they have been applied to increase our understanding of these enzymes, and suggest how they-and other tools yet barely used in the phosphatase field-might be adapted to address future questions and challenges.

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.

Glycogen phosphorylase inhibitors: a patent review (2008 - 2012)

INTRODUCTION

Glycogen phosphorylase (GP) is the enzyme responsible for the synthesis of glucose-1-phosphate, the source of energy for muscles and the rest of the body. The binding of different ligands in catalytic or allosteric sites assures activation and deactivation of the enzyme. A description of the regulation mechanism and the implications in glycogen metabolism are given.

AREAS COVERED

Deregulation of GP has been observed in diseases such as diabetes mellitus or cancers. Therefore, it appears as an attractive therapeutic target for the treatment of such pathologies. Numbers of inhibitors have been published in academic literature or patented in the last two decades. This review presents the main patent claims published between 2008 and 2012.

EXPERT OPINION

Good inhibitors with interesting IC50 and in vivo results are presented. However, such therapeutic strategy raises questions and some answers are proposed to bring new insights in the field.

Synthesis of tartaric acid analogues of FR258900 and their evaluation as glycogen phosphorylase inhibitors

Di-O-cinnamoylated, -p-coumaroylated, and -feruloylated d-, l- and meso-tartaric acids were synthesized as analogues of the natural product FR258900, a glycogen phosphorylase (GP) inhibitor with in vivo antihyperglycaemic activity. The new compounds inhibited rabbit muscle GP in the low micromolar range, and bound to the allosteric site of the enzyme. The best inhibitor was 2,3-di-O-feruloyl meso-tartaric acid and had Ki values of 2.0μM against AMP (competitive) and 3.36μM against glucose-1-phosphate (non-competitive).

Challenges and opportunities in the development of protein phosphatase-directed therapeutics

Protein phosphatases have both protective and promoting roles in the etiology of diseases. A prominent example is the existence of oncogenic as well as tumor-suppressing protein phosphatases. A few protein phosphatase activity modulators are already applied in therapies. These were however not developed in target-directed approaches, and the recent discovery of phosphatase involvement followed their application in therapy. Nevertheless, these examples demonstrate that small molecules can be generated that modulate the activity of protein phosphatases and are beneficial for the treatment of protein phosphorylation diseases. We describe here strategies for the development of activators and inhibitors of protein phosphatases and clarify some long-standing misconceptions concerning the druggability of these enzymes. Recent developments suggest that it is feasible to design potent and selective protein phosphatase modulators with a therapeutic potential.

Glycogen and its metabolism: some new developments and old themes

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

Defective glycogenesis contributes toward the inability to suppress hepatic glucose production in response to hyperglycemia and hyperinsulinemia in zucker diabetic fatty rats

OBJECTIVE

Examine whether normalizing net hepatic glycogenesis restores endogenous glucose production and hepatic glucose phosphorylation in response to diabetic levels of plasma glucose and insulin in Zucker diabetic fatty rats (ZDF).

RESEARCH DESIGN AND METHODS

Hepatic glucose and intermediate fluxes (µmol · kg(-1) · min(-1)) were measured with and without a glycogen phosphorylase inhibitor (GPI) using [2-(3)H]glucose, [3-(3)H]glucose, and [U-(14)C]alanine in 20 h-fasted conscious ZDF and their lean littermates (ZCL) under clamp conditions designed to maintain diabetic levels of plasma glucose and insulin.

RESULTS

With infusion of GPI into ZDF (ZDF-GPI+G), compared with vehicle infused ZDF (ZDF-V), high glycogen phosphorylase a activity was decreased and low synthase I activity was increased to that of ZCL. Low net glycogenesis from plasma glucose rose to 75% of ZCL levels (4 ± 1 in ZDF-V, 18 ± 1 in ZDF-GPI+G, and 24 ± 2 in ZCL) and phosphoenolpyruvate 260% (4 ± 2 in ZDF-V, 16 ± 1 in ZDF+GPI-G, and 6 ± 2 in ZCL). High endogenous glucose production was suppressed with GPI infusion but not to that of ZCL (46 ± 4 in ZDF-V, 18 ± 4 in ZDF-GPI+G, and -8 ± 3 in ZCL). This was accompanied by reduction of the higher glucose-6-phosphatase flux (75 ± 4 in ZDF-V, 41 ± 4 in ZDF-GPI+G, and 86 ± 12 in ZCL) and no change in low glucose phosphorylation or total gluconeogenesis.

CONCLUSIONS

In the presence of hyperglycemic-hyperinsulinemia in ZDF, reduced glycogenic flux partially contributes to a lack of suppression of hepatic glucose production by failing to redirect glucose-6-phosphate flux from production of glucose to glycogen but is not responsible for a lower rate of glucose phosphorylation.

Differential expression of liver proteins between obesity-prone and obesity-resistant rats in response to a high-fat diet

Rodents respond to a chronic high-fat diet (HFD) in two ways: some readily become obese (obesity prone, OP) and others do not (obesity resistant, OR). Although several hypotheses have been proposed, the mechanisms underlying the inter-individual susceptibility to diet-induced obesity remain to be fully defined. In the present study, two-dimensional gel electrophoresis (2-DE) combined with matrix-assisted laser desorption ionisation time-of-flight MS was carried out for identification of differentially expressed liver proteins in OP and OR rats fed a HFD, in an attempt to discover marker proteins involved in susceptibility and/or resistance to obesity in rat liver. The 2-DE analysis demonstrated that forty spots from 380 visualised spots were differentially regulated between the groups. Among these forty spots, twelve were differentially expressed proteins between OP and OR rats, reaching statistical significance. Of these, five proteins have already been linked to obesity; however, seven proteins involved in obesity susceptibility or resistance were identified for the first time in the present study. In order to validate the proteomic results and gain insight into the metabolic changes between the OP and OR groups, we further confirmed the expression pattern of some proteins of interest by Western blot analysis. Combined results of proteomic analysis with Western blot analysis revealed that reduced lipogenesis and increased fat oxidation were achieved in the livers of OR rats. In conclusion, the present proteomic study is an important advance over the previous steps required for identification of OP and OR rats, and should prove valuable in the search for the pathogenesis of obesity in humans.

Identification of the human testis protein phosphatase 1 interactome

Protein phosphorylation is a critical regulatory mechanism in cellular signalling. To this end, PP1 is a major eukaryotic serine/threonine-specific phosphatase whose cellular functions, in turn, depend on complexes it forms with PP1 interacting proteins-PIPs. The importance of the testis/sperm-enriched variant, PP1γ2, in sperm motility and spermatogenesis has previously been shown. Given the key role of PIPs, it is imperative to identify the physiologically relevant PIPs in testis and sperm. Hence, we performed Yeast Two-Hybrid screens of a human testis cDNA library using as baits the different PP1 isoforms and also a proteomic approach aimed at identifying PP1γ2 binding proteins. To the best of our knowledge this is the largest data set of the human testis PP1 interactome. We report the identification of 77 proteins in human testis and 7 proteins in human sperm that bind PP1. The data obtained increased the known PP1 interactome by reporting 72 novel interactions. Confirmation of the interaction of PP1 with 5 different proteins was also further validated by co-immunoprecipitation or protein overlays. The data here presented provides important insights towards the function of these proteins and opens new possibilities for future research. In fact, such diversity in PP1 regulators makes them excellent targets for pharmacological intervention.

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.

Activation and Translocation of Glucokinase in Rat Primary Hepatocytes Monitored by High Content Image Analysis

In the liver, glucokinase (GK) regulatory protein (GKRP) negatively modulates the metabolic enzyme GK by locking it in an inactive state in the nucleus. Here, the authors established a high content screening assay in the 384-well microplate format to measure the nucleus-to-cytoplasm translocation of GK by reagents that destabilize the interaction between GK and GKRP. As a cellular model system, primary rat hepatocytes endogenously expressing both GK and GKRP at physiological levels were used. The GK translocation assay was robust, displayed limited day-to-day variability, and delivered good Z′ statistics. The increase of the glucose concentration in the extracellular medium from a low glucose situation (2.8 mM) to beyond its physiological set point value of 5 mM was found to drive GK from the nucleus into the cytoplasm. Likewise, both fructose (converted intracellularly into fructose-1-phosphate) and a known allosteric GK activator were found to induce the export of GK from the nucleus and to synergistically enhance the effects of medium or high glucose concentrations with respect to GK translocation. Transfer of the high content screening format to a semiautomated medium throughput screening platform enabled the profiling of large compound numbers with respect to allosteric activation of GK. ( Journal of Biomolecular Screening 2008:837-846)

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.