Purification and characterization of the glycogen-bound protein phosphatase from rat liver

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

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

Scalable selection of hepatocyte- and hepatocyte precursor-like cells from culture of differentiating transgenically modified murine embryonic stem cells

Potential therapeutic applications of embryonic stem cell (ESC)-derived hepatocytes are limited by their relatively low output in differentiating ESC cultures, as well as by the danger of contamination with tumorigenic undifferentiated ESCs. To address these problems, we developed transgenic murine ESC clones possessing bicistronic expression vector that contains the alpha-fetoprotein gene promoter driving a cassette for the enhanced green "live" fluorescent reporter protein (eGFP) and a puromycin resistance gene. Under established culture conditions these clones allowed for both monitoring of differentiation and for puromycin selection of hepatocyte-committed cells in a suspension mass culture of transgenic ESC aggregates ("embryoid bodies" [EBs]). When plated on fibronectin, the selected eGFP-positive cells formed colonies, in which intensely proliferating hepatocyte precursor-like cells gave rise to morphologically differentiated cells expressing alpha-1-antitrypsin, alpha-fetoprotein, and albumin. A number of cells synthesized glycogen and in some of the cells cytokeratin 18 microfilaments were detected. Major hepatocyte marker genes were expressed in the culture, along with the gene and protein expression of stem/progenitor markers, suggesting the features of both hepatocyte precursors and more advanced differentiated cells. When cultured in suspension, the EB-derived puromycin-selected cells formed spheroids capable of outgrowing on an adhesive substrate, resembling the behavior of fetal mouse hepatic progenitor cells. The established system based on the highly efficient selection/purification procedure could be suitable for scalable generation of ESC-derived hepatocyte- and hepatocyte precursor-like cells and offers a potential in vitro source of cells for transplantation therapy of liver diseases, tissue engineering, and drug and toxicology screening.

Protein phosphatase type-1 and glycogen bind to a domain in the skeletal muscle regulatory subunit containing conserved hydrophobic sequence motif

This study identifies a 100-residue domain within the rabbit skeletal muscle regulatory subunit (PP1G) that binds both type-1 protein phosphatase (PP1C) and glycogen. An N-terminal portion of PP1G was cloned by RT-PCR, and different sized fragments were expressed in bacteria as glutathione S-transferase (GST) fusion proteins. A GST-PP1G fusion containing residues 51-240 bound both PPIC and glycogen, whereas GST alone or fusions containing residues 51-140 or 241-360 bound neither PP1C nor glycogen. The PPIC in whole cell lysates or partially purified PP1C from skeletal muscle, or a complex of PP1C-MCLR-biotin, all bound more effectively than Mn(2+)-activated, recombinant PP1C purified from bacteria. Binding was enhanced by increasing the ionic strength and was disrupted by ethylene glycol, consistent with hydrophobic interactions being critical for stable association. Phosphorylation of the GST-PP1G fusion by cAMP-dependent protein kinase prevented completely association of PP1C. This domain of PP1G, from residues 141-240, contains two sequence motifs of hydrophobic residues: Gx8FEKx10W and DxFxFxIxL, that are conserved among the known glycogen-binding PP1 regulatory subunits. These segments are predicted to form an alpha helix and a beta sheet, and we propose that they are the sites for association with PP1C and glycogen, respectively.

Characterization of a ribosomal inhibitory polypeptide of protein phosphatase-1 from rat liver

About 4% of the spontaneous phosphorylase phosphatase activity in a rat liver extract was associated with the ribosomal fraction and stemmed from both protein phosphatase-1 (PP-1) and protein phosphatase-2A (PP-2A). However, after repeated washing, only PP-1 remained bound to the ribosomes. The activity of ribosome-associated PP-1 (PP-1R) was partially latent and could be increased 2-3-fold by incubation with trypsin and an additional 50% by incubation with low concentrations of exogenous type-1 catalytic subunit. In contrast, incubation of the ribosomal fraction with MgATP resulted in a 50% drop in the activity of PP-1R. We have purified from a ribosomal extract a basic polypeptide (pI > or = 10.5) of 23 kDa that potently inhibited PP-1. This ribosomal inhibitor of PP-1, termed RIPP-1, was at least 30-times less efficient in inhibiting other major Ser/Thr protein phosphatases (PP-2A, PP-2B and PP-2C). RIPP-1 was identified as a non-competitive inhibitor of PP-1 with a substrate-dependent potency. The lowest Ki (approximately 20 nM) was obtained with phosphorylase and myelin basic protein as substrates. Besides instantaneously inhibiting the type-1 catalytic subunit, RIPP-1 also converted the catalytic subunit in a time-dependent manner (t 1/2 = 45 min at 25 degrees C) into a less active conformation. Unlike the inhibition, this slow inactivation was not reversed by the removal of RIPP-1. We propose that RIPP-1 accounts, at least in part, for the latency of PP-1R.

N-acetyl-beta-D-glucopyranosylamine: a potent T-state inhibitor of glycogen phosphorylase. A comparison with alpha-D-glucose

Structure-based drug design has led to the discovery of a number of glucose analogue inhibitors of glycogen phosphorylase that have an increased affinity compared to alpha-D-glucose (Ki = 1.7 mM). The best inhibitor in the class of N-acyl derivatives of beta-D-glucopyranosylamine, N-acetyl-beta-D-glucopyranosylamine (1-GlcNAc), has been characterized by kinetic, ultracentrifugation, and crystallographic studies. 1-GlcNAc acts as a competitive inhibitor for both the b (Ki = 32 microM) and the a (Ki = 35 microM) forms of the enzyme with respect to glucose 1-phosphate and in synergism with caffeine, mimicking the binding of glucose. Sedimentation velocity experiments demonstrated that 1-GlcNAc was able to induce dissociation of tetrameric phosphorylase a and stabilization of the dimeric T-state conformation. Co-crystals of the phosphorylase b-1-GlcNAc-IMP complex were grown in space group P4(3)2(1)2, with native-like unit cell dimensions, and the complex structure has been refined to give a crystallographic R factor of 18.1%, for data between 8 and 2.3 A resolution. 1-GlcNAc binds tightly at the catalytic site of T-state phosphorylase b at approximately the same position as that of alpha-D-glucose. The ligand can be accommodated in the catalytic site with very little change in the protein structure and stabilizes the T-state conformation of the 280s loop by making several favorable contacts to Asn 284 of this loop. Structural comparisons show that the T-state phosphorylase b-1-GlcNAc-IMP complex structure is overall similar to the T-state phosphorylase b-alpha-D-glucose complex structure. The structure of the 1-GlcNAc complex provides a rational for the biochemical properties of the inhibitor.

Demonstration of a glycogen/glucose 1-phosphate cycle in hepatocytes from fasted rats. Selective inactivation of phosphorylase by 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride

In search for a nonmetabolized, superior glucose analogue to study the mechanism of glucose-induced glycogen synthesis, we have tested 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride, which inhibits muscle phosphorylase beta 10-fold better than dose glucose (Street, I.P., Armstrong, C.R., and Withers, S.G. (1986) Biochemistry 25, 6021-6027). In a gel-filtered liver extract, 0.6 mM analogue and 10 mM glucose equally accelerated the inactivation of phosphorylase and shortened the latency before the activation of glycogen synthase. The analogue was not measurably defluorinated or phosphorylated by intact hepatocytes, as monitored by 19F NMR. When added to isolated hepatocytes, 10 mM analogue inactivated phosphorylase more extensively than did 50 mM glucose, but unlike glucose, it did not result in the activation of glycogen synthase. Therefore, the binding of glucose to phosphorylase alpha can account for the inactivation of phosphorylase, but the metabolism of glucose (probably to Glc-6-P) appears to be required to achieve activation of glycogen synthase. The livers of overnight-fasted, anesthetized mice contained appreciable amounts of both phosphorylase alpha and glycogen synthase alpha, without net glycogen accumulation. Likewise, hepatocytes isolated from fasted rats and incubated with 10 mM glucose contained 41% of phosphorylase and 32% of glycogen synthase in the alpha form, and these values remained stable for 1 h, while glycogen accumulated at only 22% of the rate expected from the glycogen synthase activity. The addition of 10 mM analogue decreased phosphorylase alpha to 10% without significant change in glycogen synthase alpha (38%), but with a 4-fold increased rate of glycogen accumulation. These findings imply that synthase alpha is fully active in the liver of the fasted animal and that the absence of net glycogen synthesis is due to continuous glycogenolysis by phosphorylase alpha.

Purification of the hepatic glycogen-associated form of protein phosphatase-1 by microcystin-Sepharose affinity chromatography

The form of protein phosphatase-1 associated with hepatic glycogen (PP1G) was purified to near homogeneity from rat liver by affinity chromatography on microcystin-Sepharose and gel-filtration. The enzyme is a heterodimer consisting of the catalytic subunit of PP1 (the alpha and beta isoforms) complexed to a 33 kDa glycogen-binding (GL) subunit. The GL subunit binds phosphorylase a with high affinity, and is responsible for the enhanced dephosphorylation of glycogen synthase by PP1G and its allosteric inhibition by phosphorylase a.

Native cytosolic protein phosphatase-1 (PP-1S) containing modulator (inhibitor-2) is an active enzyme

In vitro, the modulator protein (inhibitor-2) slowly converts the catalytic subunit of protein phosphatase-1 (PP-1C) into an inactive 'MgATP-dependent form' that can be reactivated by the transient phosphorylation of modulator with GSK-3/FA. We report here that this modulator-induced inactivation of PP-1C can be blocked by addition (at pH 7.5) of either 0.3 mM NaF or 150 mM NaCl, or by raising the pH to 8.5. Making use of a combination of the latter conditions, we have partially purified a soluble modulator-associated form of PP-1 (PP-1S) from rabbit skeletal muscle as a spontaneously active enzyme that cannot be further activated by kinase GSK-3/FA. These observations argue against a role for the 'MgATP-dependent' form of PP-1S as an inactive reservoir of PP-1C. PP-1S was separated on aminohexyl Sepharose from another active, cytosolic species of PP-1, which appears to be a proteolytic product of the glycogen-bound PP-1G.

Insulin but not progesterone promotes the biosynthesis of glycogen in Xenopus laevis oocytes: implications on the control of glycogen synthase by phosphorylation, dephosphorylation

Insulin, the well-known hypoglycemic hormone, mimics progesterone in promoting the resumption of meiosis within the oocyte of Xenopus laevis. Both hormones exert their action through the inhibition of protein kinases and the activation of protein phosphatases. Because glycogen synthase is an enzyme regulated by a kinases/phosphatases cascade, we investigated the effect of insulin and progesterone on the regulation of glycogen synthesis and glycogen synthase throughout the oogenesis of Xenopus laevis oocytes. In this framework the maximal activity of synthase "a" is concomitant with the vitellogenic period characterized by a drastic increase in the amount of glycogen. Oocyte glycogen synthase is inhibited by cAMP-dependent phosphorylation and stimulated by 20 mM Mg2+. The magnesium effect is inhibited by mu molar concentrations of okadaic acid and suggests that oocyte glycogen synthase is activated by dephosphorylation achieved by protein phosphatase-1. The okadaic acid effect is itself thwarted by the specific inhibitor of protein kinase A, confirming the role of this kinase in the regulation of glycogen synthase. Finally, working on intact ripe oocytes, we showed that insulin but not progesterone increases glycogen synthesis and glycogen synthase "a" activity and lowers the rates of phosphorylation, especially in the glycogen-bound proteins.

On target with a new mechanism for the regulation of protein phosphorylation

There is overwhelming evidence that the reversible phosphorylation of proteins regulates most aspects of cell life. However, the broad specificities displayed by many protein phosphatases and kinases in vitro dictates that their activities be strictly regulated in vivo. Recent evidence indicates that a novel class of proteins, known as targetting subunits, specifies the location, catalytic and regulatory properties of protein phosphatases and kinases, and thereby plays a key role in ensuring the fidelity of protein phosphorylation.

Comparative characterization of liver glycogen metabolism in rat and guinea-pig

1. Guinea-pig liver contained more phosphorylase in the active (phosphorylated) form and less synthase in the active (dephosphorylated) form when compared with rat liver. 2. Activities of cyclic AMP-dependent protein kinase and Ca(2+)-dependent phosphorylase kinase were the same in rat and guinea-pig livers. 3. Activities of phosphorylase phosphatase and synthase phosphatase in the extract and glycogen plus microsomal fraction of guinea-pig liver were significantly lower than those of rat liver. 4. The existence of inhibitor-1 in the liver of guinea-pig can maintain a lower activity of type-1 protein phosphatase, especially when inhibitor-1 is phosphorylated by cyclic AMP-dependent protein kinase.

The structure, role, and regulation of type 1 protein phosphatases

Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.

Fluorescence studies on the interaction of inhibitor 2 and okadaic acid with the catalytic subunit of type 1 phosphoprotein phosphatases

Phosphatase inhibitor 2 was mutagenized and expressed in Escherichia coli to produce a protein with a single cysteinyl residue at position 129. The newly introduced sulfhydryl group was labeled with a maleimide derivative of coumarin (CPM). The resulting fluorescent inhibitor 2 molecule (CPM-I2) retains biological activity and binds to the catalytic subunit of type 1 phosphatase (PP1-C) with a Kd similar to the Ki of native I2 (2-3 nM). Fluorescence anisotropy data indicate that kinase FA (glycogen synthase kinase 3) does not dissociate the CPM-I2.PP1-C complex but rather causes a conformational change in the I2 molecule that is retained even after the CPM-I2 is displaced by an excess of native I2. The fluorescence data presented here also indicate that okadaic acid and I2 are competitive for binding to PP1-C, even after kinase FA treatment of the CPM-I2.PP1-C complex.