Loss of the hepatic glycogen-binding subunit (GL) of protein phosphatase 1 underlies deficient glycogen synthesis in insulin-dependent diabetic rats and in adrenalectomized starved rats

Protein Phosphatase 1 Regulatory Subunit 3: A Prognostic Biomarker in Stomach Adenocarcinoma

Purpose

This study aimed to determine the potential application of the protein phosphatase 1 regulatory subunit 3 (PPP1R3B) gene as a prognostic marker in stomach adenocarcinoma (STAD), as well as its potential mediating biological processes and pathways.

Materials and Methods

Differential expression analyses were performed using the TIMER2.0 and UALCAN databases. Complete RNA-seq data and other relevant clinical and survival data were acquired from The Cancer Genome Atlas (TCGA). Univariate survival analyses, Cox regression modelling, and Kaplan–Meier curves were implemented to investigate the associations between PPP1R3B gene expression and clinical pathologic features. A genome wide gene set enrichment analysis (GSEA) was conducted to define the underlying molecular mechanisms mediating the observed associations between the PPP1R3B gene and STAD development.

Results

We found that PPP1R3B was overexpressed in STAD tissues, and that higher PPP1R3B expression correlated with worse prognoses in patients with STAD. Comprehensive survival analyses suggested that PPP1R3B might be an independent predictive factor for survival time in patients with STAD. The prognostic relationship between PPP1R3B and STAD was also verified using Kaplan–Meier curves. Patients with higher PPP1R3B levels had a shorter clinical survival time on average. Additionally, a GSEA demonstrated that PPP1R3B might be involved in multiple biological processes and pathways.

Conclusion

Our findings demonstrate that the PPP1R3B gene has utility as a potential molecular marker for STAD prognoses.

Embryonic Steroids Control Developmental Programming of Energy Balance

Glucose is a major energy source for growth. At birth, neonates must change their energy source from maternal supply to its own glucose production. The mechanism of this transition has not been clearly elucidated. To evaluate the possible roles of steroids in this transition, here we examine the defects associated with energy production of a mouse line that cannot synthesize steroids de novo due to the disruption of its Cyp11a1 (cytochrome P450 family 11 subfamily A member 1) gene. The Cyp11a1 null embryos had insufficient blood insulin and failed to store glycogen in the liver since embryonic day 16.5. Their blood glucose dropped soon after maternal deprivation, and the expression of hepatic gluconeogenic and glycogenic genes were reduced. Insulin was synthesized in the mutant fetal pancreas but failed to be secreted. Maternal glucocorticoid supply rescued the amounts of blood glucose, insulin, and liver glycogen in the fetus but did not restore expression of genes for glycogen synthesis, indicating the requirement of de novo glucocorticoid synthesis for glycogen storage. Thus, our investigation of Cyp11a1 null embryos reveals that the energy homeostasis is established before birth, and fetal steroids are required for the regulation of glycogen synthesis, hepatic gluconeogenesis, and insulin secretion at the fetal stage.

Protein Phosphatase 1 Regulatory Subunit 3B Genotype at rs4240624 Has a Major Effect on Gallbladder Bile Composition

The protein phosphatase 1 regulatory subunit 3B (PPP1R3B) gene is a target of farnesoid X receptor (FXR), which is a major regulator of bile acid metabolism. Both PPP1R3B and FXR have been suggested to take part in glycogen metabolism, which may explain the association of PPP1R3B gene variants with altered hepatic computed tomography attenuation. We analyzed the effect of PPP1R3B rs4240624 variant on bile acid composition in individuals with obesity. The study cohort consisted of 242 individuals from the Kuopio Obesity Surgery Study (73 men, 169 women, age 47.6 ± 9.0 years, body mass index 43.2 ± 5.4 kg/m²) with PPP1R3B genotype and liver RNA sequencing (RNA-seq) data available. Fasting plasma and gallbladder bile samples were collected from 50 individuals. Bile acids in plasma did not differ based on the PPP1R3B rs4240624 genotype. However, the concentration of total bile acids (109 ± 55 vs. 35 ± 19 mM; P = 1.0 × 10^-5) and all individual bile acids (also 7α-hydroxy-4-cholesten-3-one [C4]) measured from bile were significantly lower in those with the AG genotype compared to those with the AA genotype. In addition, total cholesterol (P = 0.011) and phospholipid (P = 0.001) levels were lower in individuals with the AG genotype, but cholesterol saturation index did not differ, indicating that the decrease in cholesterol and phospholipid levels was secondary to the change in bile acids. Liver RNA-seq data demonstrated that expression of PPP1R3B, tankyrase (TNKS), Homo sapiens chromosome 8 clone RP11-10A14.5 (AC022784.1 [LOC157273]), Homo sapiens chromosome 8 clone RP11-375N15.1 (AC021242.1), and Homo sapiens chromosome 8, clone RP11-10A14 (AC022784.6) associated with the PPP1R3B genotype. In addition, genes enriched in transmembrane transport and phospholipid binding pathways were associated with the genotype. Conclusion: The rs4240624 variant in PPP1R3B has a major effect on the composition of gallbladder bile. Other transcripts in the same loci may be important mediators of the variant effect.

Genetic Variation at PPP1R3B Increases Hepatic CT Attenuation and Interacts With Prandial Status on Plasma Glucose

CONTEXT

A common genetic variant near PPP1R3B (rs4841132G > A) has been associated with increased hepatic computed tomography (CT) attenuation and with plasma levels of glucose and liver enzymes.

OBJECTIVE

To elucidate the association of rs4841132 with hepatic CT attenuation, and to test if synergistic effects modify the association of the variant with plasma glucose and liver enzymes.

DESIGN

Population-based cohort study.

SETTING AND PARTICIPANTS

The Copenhagen City Heart Study and the Copenhagen General Population Study combined, totaling 107 192 individuals from the Danish general population. Hepatic CT scans were available in 6445 individuals.

MAIN OUTCOME MEASURES

Hepatic CT attenuation and plasma levels of glucose and liver enzymes.

RESULTS

The rs4841132 A-allele (rs4841132-A) was associated with higher hepatic CT attenuation (P = 5 × 10-6). The probability of carrying rs4841132-A increased with higher hepatic CT attenuation in the range above 65 Hounsfield units, but remained constant at the range below (P = 4 × 10-8 for nonlinearity). Rs4841132-A was associated with up to 0.17 mmol/L higher plasma glucose in fasting individuals, but with up to 0.17 mmol/L lower glucose in postprandial individuals (P = 6 × 10-5 for interaction between rs4841132 and time since last meal on plasma glucose). Finally, rs4841132-A was associated with up to 2 U/L higher plasma alanine transaminase (P = 3 × 10-6). This association was not modified by adiposity, alcohol intake, or steatogenic genetic risk.

CONCLUSIONS

Rs4841132-A associates with higher hepatic CT attenuation in a distinctly nonlinear manner, and its association with plasma glucose depends on prandial status. The overall association pattern supports that rs4841132-A promotes hepatic glycogen synthesis postprandially.

Regulation of Glucose Production in the Pathogenesis of Type 2 Diabetes

PURPOSE OF REVIEW

Increased glucose production associated with hepatic insulin resistance contributes to the development of hyperglycemia in T2D. The molecular mechanisms accounting for increased glucose production remain controversial. Our aims were to review recent literature concerning molecular mechanisms regulating glucose production and to discuss these mechanisms in the context of physiological experiments and observations in humans and large animal models.

RECENT FINDINGS

Genetic intervention studies in rodents demonstrate that insulin can control hepatic glucose production through both direct effects on the liver, and through indirect effects to inhibit adipose tissue lipolysis and limit gluconeogenic substrate delivery. However, recent experiments in canine models indicate that the direct effects of insulin on the liver are dominant over the indirect effects to regulate glucose production. Recent molecular studies have also identified insulin-independent mechanisms by which hepatocytes sense intrahepatic carbohydrate levels to regulate carbohydrate disposal. Dysregulation of hepatic carbohydrate sensing systems may participate in increased glucose production in the development of diabetes.

Association of PPP1R3B polymorphisms with blood lipid and C-reactive protein levels in a Chinese population (PPP1R3B C )

BACKGROUND

Genetic variants of PPP1R3B, a gene encoding a critical protein involved in hepatic glycogen metabolism, were recently reported to be associated with plasma levels of lipids and C-reactive protein (CRP) among populations of mostly European descent. The aim of the present study was to investigate whether the PPP1R3B variants are associated with plasma levels of lipids and inflammation factors in Chinese Han.

METHODS

Five single nucleotide polymorphisms (SNPs) of the PPP1R3B gene were genotyped and their associations with plasma lipids and CRP were determined in 1636 Chinese Han from Shanghai.

RESULTS

Three SNPs, namely rs2126259, rs9987289, and rs19334, were significantly associated with plasma levels of low-density lipoprotein-cholesterol, and rs2126259 and rs9987289 were further significantly associated with total cholesterol. The remaining two SNPs (rs189798 and rs330919) were significantly associated with plasma CRP levels, but not with plasma lipid levels.

CONCLUSION

Genetic polymorphisms of the PPP1R3B gene may contribute to variations in plasma lipids and CRP levels among Chinese Han individuals.

Mutated PPP1R3B is recognized by T cells used to treat a melanoma patient who experienced a durable complete tumor regression

Adoptive cell therapy with tumor-infiltrating lymphocytes (TILs) represents an effective treatment for patients with metastatic melanoma. However, most of the Ag targets recognized by effective melanoma-reactive TILs remain elusive. In this study, patient 2369 experienced a complete response, including regressions of bulky liver tumor masses, ongoing beyond 7 y following adoptive TIL transfer. The screening of a cDNA library generated from the autologous melanoma cell line resulted in the isolation of a mutated protein phosphatase 1, regulatory (inhibitor) subunit 3B (PPP1R3B) gene product. The mutated PPP1R3B peptide represents the immunodominant epitope recognized by tumor-reactive T cells in TIL 2369. Five years following adoptive transfer, peripheral blood T lymphocytes obtained from patient 2369 recognized the mutated PPP1R3B epitope. These results demonstrate that adoptive T cell therapy targeting a tumor-specific Ag can mediate long-term survival for a patient with metastatic melanoma. This study also provides an impetus to develop personalized immunotherapy targeting tumor-specific, mutated Ags.

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.

Restoration of hepatic glycogen deposition reduces hyperglycaemia, hyperphagia and gluconeogenic enzymes in a streptozotocin-induced model of diabetes in rats

AIMS/HYPOTHESIS

Glycogen deposition is impaired in diabetes, thus contributing to the development of hyperglycaemia. Several glucose-lowering strategies have attempted to increase liver glycogen deposition by modulating targets, which eventually trigger the activation of liver glycogen synthase (LGS). However, these targets also alter several other biological processes, and therefore their therapeutic use may be limited. Here we tested the approach of directly activating LGS and evaluated the potential of this strategy as a possible treatment for diabetes.

METHODS

In this study, we examined the efficacy of directly overproducing a constitutively active form of LGS in the liver to ameliorate streptozotocin-induced diabetes in rats.

RESULTS

Activated mutant LGS overproduction in the liver of streptozotocin-induced diabetic rats normalised liver glycogen content, despite low levels of glucokinase and circulating insulin. Moreover, this overproduction led to a decrease in food intake and in the production of the main gluconeogenic enzymes, glucose-6-phosphatase, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase. The resulting combined effect was a reduction in hyperglycaemia.

CONCLUSIONS/INTERPRETATION

The restoration of liver glycogen ameliorated diabetes and therefore is considered a potential strategy for the treatment of this disease.

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.

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.

Abnormalities of glucose homeostasis and the hypothalamic-pituitary-adrenal axis in mice lacking hexose-6-phosphate dehydrogenase

Hexose-6-phosphate dehydrogenase (EC 1.1.1.47) catalyzes the conversion of glucose 6-phosphate to 6-phosphogluconolactone within the lumen of the endoplasmic reticulum, thereby generating reduced nicotinamide adenine dinucleotide phosphate. Reduced nicotinamide adenine dinucleotide phosphate is a necessary cofactor for the reductase activity of 11beta-hydroxysteroid dehydrogenase type 1 (EC 1.1.1.146), which converts hormonally inactive cortisone to active cortisol (in rodents, 11-dehydrocorticosterone to corticosterone). Mice with targeted inactivation of hexose-6-phosphate dehydrogenase lack 11beta-hydroxysteroid dehydrogenase type 1 reductase activity, whereas dehydrogenase activity (corticosterone to 11-dehydrocorticosterone) is increased. We now report that both glucose output and glucose use are abnormal in these mice. Mutant mice have fasting hypoglycemia. In mutant primary hepatocytes, glucose output does not increase normally in response to glucagon. Mutant animals have lower hepatic glycogen content when fed and cannot mobilize it normally when fasting. As assessed by RT-PCR, responses of hepatic enzymes to fasting are blunted; enzymes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase) are not appropriately up-regulated, and expression of glucokinase, an enzyme required for glycolysis, is not suppressed. Corticosterone has attenuated effects on expression of these enzymes in cultured mutant primary hepatocytes. Mutant mice have increased sensitivity to insulin, as assessed by homeostatic model assessment values and by increased glucose uptake by the muscle. The hypothalamic-pituitary-adrenal axis is also abnormal. Circulating ACTH, deoxycorticosterone, and corticosterone levels are increased in mutant animals, suggesting decreased negative feedback on the hypothalamic-pituitary-adrenal axis. Comparison with other animal models of adrenal insufficiency suggests that many of the observed abnormalities can be explained by blunted intracellular corticosterone actions, despite elevated circulating levels of this hormone.

Expression and glycogenic effect of glycogen-targeting protein phosphatase 1 regulatory subunit GL in cultured human muscle

Glycogen-targeting PP1 (protein phosphatase 1) subunit G(L) (coded for by the PPP1R3B gene) is expressed in human, but not rodent, skeletal muscle. Its effects on muscle glycogen metabolism are unknown. We show that G(L) mRNA levels in primary cultured human myotubes are similar to those in freshly excised muscle, unlike subunits G(M) (gene PPP1R3A) or PTG (protein targeting to glycogen; gene PPP1R3C), which decrease strikingly. In cultured myotubes, expression of the genes coding for G(L), G(M) and PTG is not regulated by glucose or insulin. Overexpression of G(L) activates myotube GS (glycogen synthase), glycogenesis in glucose-replete and -depleted cells and glycogen accumulation. Compared with overexpressed G(M), G(L) has a more potent activating effect on glycogenesis, while marked enhancement of their combined action is only observed in glucose-replete cells. G(L) does not affect GP (glycogen phosphorylase) activity, while co-overexpression with muscle GP impairs G(L) activation of GS in glucose-replete cells. G(L) enhances long-term glycogenesis additively to glucose depletion and insulin, although G(L) does not change the phosphorylation of GSK3 (GS kinase 3) on Ser9 or its upstream regulator kinase Akt/protein kinase B on Ser473, nor its response to insulin. In conclusion, in cultured human myotubes, the G(L) gene is expressed as in muscle tissue and is unresponsive to glucose or insulin, as are G(M) and PTG genes. G(L) activates GS regardless of glucose, does not regulate GP and stimulates glycogenesis in combination with insulin and glucose depletion.

The twentieth century struggle to decipher insulin signalling

Following the discovery of insulin, it took the rest of the twentieth century to understand how this hormone regulates intracellular metabolism. What are the main discoveries that led to our current understanding of this process? And how is this new knowledge being exploited in an attempt to develop improved drugs to treat the epidemic of type-2 diabetes?

Arginine-vasopressin mediates central and peripheral glucose regulation in response to carotid body receptor stimulation with Na-cyanide

Hypoxic stimulation of the carotid body receptors (CBR) results in a rapid hyperglycemia with an increase in brain glucose retention. Previous work indicates that neurohypophysectomy inhibits this hyperglycemic response. Here, we show that systemic arginine vasopressin (AVP) induced a transient, but significant, increase in blood glucose levels and increased brain glucose retention, a response similar to that observed after CBR stimulation. Comparable results were obtained after intracerebral infusion of AVP. Systemic AVP-induced changes were maintained in hypophysectomized rats but were not observed after adrenalectomy. Glycemic changes after CBR stimulation were inhibited by pharmacological blockage of AVP V1a receptors with a V1a-selective receptor antagonist ([β-Mercapto-β,β-cyclopentamethylenepropionyl1,O-me-Tyr2, Arg8]-vasopressin). Importantly, local application of micro-doses of this antagonist to the liver was sufficient to abolish the hyperglycemic response after CBR stimulation. These results suggest that AVP is a mediator of the hyperglycemic reflex and cerebral glucose retention following CBR stimulation. We propose that hepatic activation of AVP V1a receptors is essential for this hyperglycemic response.

Increased sensitivity of glycogen synthesis to phosphorylase-a and impaired expression of the glycogen-targeting protein R6 in hepatocytes from insulin-resistant Zucker fa/fa rats

Hepatic insulin resistance in the leptin-receptor defective Zucker fa/fa rat is associated with impaired glycogen synthesis and increased activity of phosphorylase-a. We investigated the coupling between phosphorylase-a and glycogen synthesis in hepatocytes from fa/fa rats by modulating the concentration of phosphorylase-a. Treatment of hepatocytes from fa/fa rats and Fa/? controls with a selective phosphorylase inhibitor caused depletion of phosphorylase-a, activation of glycogen synthase and stimulation of glycogen synthesis. The flux-control coefficient of phosphorylase on glycogen synthesis was glucose dependent and at 10 mm glucose was higher in fa/fa than Fa/? hepatocytes. There was an inverse correlation between the activities of glycogen synthase and phosphorylase-a in both fa/fa and Fa/? hepatocytes. However, fa/fa hepatocytes had a higher activity of phosphorylase-a, for a corresponding activity of glycogen synthase. This defect was, in part, normalized by expression of the glycogen-targeting protein, PTG. Hepatocytes from fa/fa rats had normal expression of the glycogen-targeting proteins G(L) and PTG but markedly reduced expression of R6. Expression of R6 protein was increased in hepatocytes from Wistar rats after incubation with leptin and insulin. Diminished hepatic R6 expression in the leptin-receptor defective fa/fa rat may be a contributing factor to the elevated phosphorylase activity and/or its high control strength on glycogen synthesis.

A novel glycogen-targeting subunit of protein phosphatase 1 that is regulated by insulin and shows differential tissue distribution in humans and rodents

Stimulation of glycogen-targeted protein phosphatase 1 (PP1) activity by insulin contributes to the dephosphorylation and activation of hepatic glycogen synthase (GS) leading to an increase in glycogen synthesis. The glycogen-targeting subunits of PP1, GL and R5/PTG, are downregulated in the livers of diabetic rodents and restored by insulin treatment. We show here that the mammalian gene PPP1R3E encodes a novel glycogen-targeting subunit of PP1 that is expressed in rodent liver. The phosphatase activity associated with R3E is slightly higher than that associated with R5/PTG and it is downregulated in streptozotocin-induced diabetes by 60-70% and restored by insulin treatment. Surprisingly, although mRNA for R3E is most highly expressed in rat liver and heart muscle, with only low levels in skeletal muscle, R3E mRNA is most abundant in human skeletal muscle and heart tissues with barely detectable levels in human liver. This species-specific difference in R3E mRNA expression has similarities to the high level of expression of GL mRNA in human but not rodent skeletal muscle. The observations imply that the mechanisms by which insulin regulates glycogen synthesis in liver and skeletal muscle are different in rodents and humans.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Human skeletal muscle expresses a glycogen-targeting subunit of PP1 that is identical to the insulin-sensitive glycogen-targeting subunit G(L) of liver

Insulin has been previously shown to regulate the expression of the hepatic glycogen-targeting subunit, G(L), of protein phosphatase 1 (PP1) and is believed to control the activity of the PP1-G(L) complex by modulation of the level of phosphorylase a, which allosterically inhibits the activity of PP1-G(L). These mechanisms contribute to the ability of insulin to increase hepatic glycogen synthesis. Human G(L) shows >88% amino acid identity to its rat and mouse homologs, with complete conservation of the phosphorylase a binding site. G(L) is highly expressed in the liver and present at appreciable levels in heart tissue of all three species. Surprisingly, G(L) is highly expressed in human skeletal muscle while only being detected at very low levels in rat, mouse, and rabbit skeletal muscle. The amino acid sequence of G(L) predicted from the cDNA is identical in human liver and skeletal muscle and encoded by a gene on chromosome 8 at p23.1. The species-specific difference in the level of expression of G(L) mRNA and protein in skeletal muscle has important implications for understanding the mechanisms by which insulin regulates glycogen synthesis in human skeletal muscle and for questions regarding whether rodents are appropriate models for this purpose.

Protein phosphatase 1 – targeted in many directions

Protein phosphatase 1 (PP1) is a major eukaryotic protein serine/threonine phosphatase that regulates an enormous variety of cellular functions through the interaction of its catalytic subunit (PP1c) with over fifty different established or putative regulatory subunits. Most of these target PP1c to specific subcellular locations and interact with a small hydrophobic groove on the surface of PP1c through a short conserved binding motif – the RVxF motif – which is often preceded by further basic residues. Weaker interactions may subsequently enhance binding and modulate PP1 activity/specificity in a variety of ways. Several putative targeting subunits do not possess an RVxF motif but nevertheless interact with the same region of PP1c. In addition, several ‘modulator’ proteins bind to PP1c but do not possess a domain targeting them to a specific location. Most are potent inhibitors of PP1c and possess at least two sites for interaction with PP1c, one of which is identical or similar to the RVxF motif.Regulation of PP1c in response to extracellular and intracellular signals occurs mostly through changes in the levels, conformation or phosphorylation status of targeting subunits. Understanding of the mode of action of PP1c complexes may facilitate development of drugs that target particular PP1c complexes and thereby modulate the phosphorylation state of a very limited subset of proteins.

Hepatic glycogen metabolism in type 1 diabetes after long-term near normoglycemia

We tested the impact of long-term near normoglycemia (HbA(1c) <7% for >1 year) on glycogen metabolism in seven type 1 diabetic and seven matched nondiabetic subjects after a mixed meal. Glycemic profiles (6.2 +/- 0.10 vs. 5.9 +/- 0.07 mmol/l; P < 0.05) of diabetic patients were approximated to that of nondiabetic subjects by variable insulin infusion. Rates of hepatic glycogen synthesis and breakdown were calculated from the glycogen concentration time curves between 7:30 P.M. and 8:00 A.M. using in vivo (13)C nuclear magnetic resonance spectroscopy. Glucose production was determined with D-[6,6-(2)H(2)]glucose, and the hepatic uridine-diphosphate glucose pool was sampled with acetaminophen. Glycogen synthesis and breakdown as well as glucose production were identical in diabetic and healthy subjects: 7.3 +/- 0.9 vs. 7.1 +/- 0.7, 4.2 +/- 0.5 vs. 3.8 +/- 0.3, and 8.7 +/- 0.5 vs. 8.4 +/- 0.7 micromol x kg(-1) x min(-1), respectively. Although portal vein insulin concentrations were doubled, the flux through the indirect pathway of glycogen synthesis remained higher in type 1 diabetic subjects: approximately 70 vs. approximately 50%; P < 0.05. In conclusion, combined long- and short-term intensified insulin substitution normalizes rates of hepatic glycogen synthesis but not the contribution of gluconeogenesis to glycogen synthesis in type 1 diabetes.

The level of the glycogen targetting regulatory subunit R5 of protein phosphatase 1 is decreased in the livers of insulin-dependent diabetic rats and starved rats

Hepatic glycogen synthesis is impaired in insulin-dependent diabetic rats owing to defective activation of glycogen synthase by glycogen-bound protein phosphatase 1 (PP1). The identification of three glycogen-targetting subunits in liver, G(L), R5/PTG and R6, which form complexes with the catalytic subunit of PP1 (PP1c), raises the question of whether some or all of these PP1c complexes are subject to regulation by insulin. In liver lysates of control rats, R5 and R6 complexes with PP1c were found to contribute significantly (16 and 21% respectively) to the phosphorylase phosphatase activity associated with the glycogen-targetting subunits, G(L)-PP1c accounting for the remainder (63%). In liver lysates of insulin-dependent diabetic and of starved rats, the phosphorylase phosphatase activities of the R5 and G(L) complexes with PP1c were shown by specific immunoadsorption assays to be substantially decreased, and the levels of R5 and G(L) were shown by immunoblotting to be much lower than those in control extracts. The phosphorylase phosphatase activity of R6-PP1c and the concentration of R6 protein were unaffected by these treatments. Insulin administration to diabetic rats restored the levels of R5 and G(L) and their associated activities. The regulation of R5 protein levels by insulin was shown to correspond to changes in the level of the mRNA, as has been found for G(L). The in vitro glycogen synthase phosphatase/phosphorylase phosphatase activity ratio of R5-PP1c was lower than that of G(L)-PP1c, suggesting that R5-PP1c may function as a hepatic phosphorylase phosphatase, whereas G(L)-PP1c may be the major hepatic glycogen synthase phosphatase. In hepatic lysates, more than half the R6 was present in the glycogen-free supernatant, suggesting that R6 may have lower affinity for glycogen than R5 and G(L)

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

Gene structure and expression of the targeting subunit, RGL, of the muscle-specific glycogen-associated type 1 protein phosphatase, PP1G

The type I phosphatase associated with glycogen, PP1G, plays an important role in glycogen metabolism. PP1G is targeted to glycogen by the R(GL) subunit, which regulates the function of the enzyme. We report the cloning and characterization of the gene as well as the pattern of expression of the R(GL) subunit from mouse. The gene covers more than 37 kb, is composed of four exons and three introns, and codes for a 1089 residue polypeptide with a calculated molecular weight of 121,000. The amino acid sequence has 60% identity with the human and rabbit R(GL). The 5' flanking region of the gene contains a TATA box, c-Myc sites, and a potential cAMP-responsive element. Muscle specific motifs, such as MyoD and MEF-2, were also found. The A-T rich 3'-UTR contained several polyadenylation signals, two associated with poly(A) down-stream consensus motifs. ARE elements, which regulate mRNA stability, were dispersed throughout the 3'-UTR. Northern analysis of poly(A) mRNA from various murine tissues indicates a major transcript of 7.5 kb in skeletal muscle and heart. Western analysis demonstrates that R(GL) protein is present in skeletal and cardiac muscle from mouse, rat, and rabbit but not in L6 myoblasts, L6 myotubes, 3T3 L1 fibroblasts, 3T3 L1 or rat primary adipocytes, confirming that expression of the gene is specific to striated muscle. Analysis of skeletal muscle from rats made diabetic by streptozotocin treatment reveals that the level of R(GL) protein is the same as in control animals, indicating that expression is not regulated by insulin.

Insulin control of glycogen metabolism in knockout mice lacking the muscle-specific protein phosphatase PP1G/RGL

The regulatory-targeting subunit (RGL), also called GM) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/RGL has been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of RGL. To investigate the function of the phosphatase, RGL knockout mice were generated. Animals lacking RGL show no obvious defects. The RGL protein is absent from the skeletal and cardiac muscle of null mutants and present at approximately 50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by approximately 50% in the RGL-deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant RGL mice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by approximately 90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and RGL null mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that RGL is genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/RGL is not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.

Activation of direct and indirect pathways of glycogen synthesis by hepatic overexpression of protein targeting to glycogen

Glycogen-targeting subunits of protein phosphatase-1, such as protein targeting to glycogen (PTG), direct the phosphatase to the glycogen particle, where it stimulates glycogenesis. We have investigated the metabolic impact of overexpressing PTG in liver of normal rats. After administration of PTG cDNA in a recombinant adenovirus, animals were fasted or allowed to continue feeding for 24 hours. Liver glycogen was nearly completely depleted in fasted control animals, whereas glycogen levels in fasted or fed PTG-overexpressing animals were 70% higher than in fed controls. Nevertheless, transgenic animals regulated plasma glucose, triglycerides, FFAs, ketones, and insulin normally in the fasted and fed states. Fasted PTG-overexpressing animals receiving an oral bolus of [U-(13)C]glucose exhibited a large increase in hepatic glycogen content and a 70% increase in incorporation of [(13)C]glucose into glycogen. However, incorporation of labeled glucose accounted for only a small portion of the glycogen synthesized in PTG-overexpressing animals, consistent with our earlier finding that PTG promotes glycogen synthesis from gluconeogenic precursors. We conclude that hepatic PTG overexpression activates both direct and indirect pathways of glycogen synthesis. Because of its ability to enhance glucose storage without affecting other metabolic indicators, the glycogen-targeting subunit may prove valuable in controlling blood glucose levels in diabetes.

Spatial Compartmentalization in the Regulation of Glucose Metabolism by Insulin

Despite intense investigation, major gaps remain in our understanding of the cellular mechanisms that underlie the actions of insulin, as well as the regulation of the enzymes and transport proteins crucial to the orderly control of glucose metabolism. In recent years, the compartmentalization of signaling molecules and metabolic enzymes has been suggested to play an important role in ensuring metabolic balance. We will discuss examples of recent findings, suggesting that spatial compartmentalization and protein translocation might be the keys to understanding the specificity of insulin in the regulation of glucose metabolism.

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.