Overexpression of Protein Targeting to Glycogen in Cultured Human Muscle Cells Stimulates Glycogen Synthesis Independent of Glycogen and Glucose 6-Phosphate Levels

Lerín, Carlos; Montell, E.; Berman, Hal K.; Newgard, Christopher B.; Gómèz-Foix, Anna M.

doi:10.1074/jbc.m006251200

Cited by 34 publications

(42 citation statements)

References 27 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Previous work demonstrated that transient PTG overexpression in cell lines and rat liver in vivo markedly increased glycogen stores (4,24,37,50), but this is the first report of a transgenic mouse line stably overexpressing PTG. At 6 mo of age, PTG overexpression in adipose tissue resulted in a 400-fold increase in glycogen levels, demonstrating a long-term alteration in glycogen metabolism resulting from the genetic manipulation of an endogenous adipocytic protein.…”

Section: Discussionmentioning

confidence: 79%

“…One such protein, termed protein targeting to glycogen (PTG), was first identified from 3T3-L1 adipocytes (57) and remains the only PP1 glycogen-targeting subunit reported to be expressed in adipocytes. Importantly, as reported by several groups, overexpression of PTG in a variety of cell types in vitro and in rodent liver in vivo markedly enhanced cellular glycogen levels (17,20,22,24,37,50,57,72).…”

mentioning

confidence: 76%

“…Previously, transient overexpression of the PP1 glycogen-targeting subunit PTG had been shown to increase intracellular glycogen levels in a number of cell types and rodent livers (17,20,22,24,37,50,57,72). PTG, originally identified in the 3T3-L1 adipocyte cell line, is the only reported PP1 glycogen-targeting subunit expressed in adipose tissue (57).…”

Section: Ap2-driven Overexpression Of Ptg In Adipose Tissue Of Transgmentioning

confidence: 99%

See 2 more Smart Citations

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Jurczak

Danos²,

Rehrmann³

et al. 2007

American Journal of Physiology-Endocrinology and Metabolism

View full text Add to dashboard Cite

-Adipocytes express the rate-limiting enzymes required for glycogen metabolism and increase glycogen synthesis in response to insulin. However, the physiological function of adipocytic glycogen in vivo is unclear, due in part to the low absolute levels and the apparent biophysical constraints of adipocyte morphology on glycogen accumulation. To further study the regulation of glycogen metabolism in adipose tissue, transgenic mice were generated that overexpressed the protein phosphatase-1 (PP1) glycogen-targeting subunit (PTG) driven by the adipocyte fatty acid binding protein (aP2) promoter. Exogenous PTG was detected in gonadal, perirenal, and brown fat depots, but it was not detected in any other tissue examined. PTG overexpression resulted in a modest redistribution of PP1 to glycogen particles, corresponding to a threefold increase in the glycogen synthase activity ratio. Glycogen synthase protein levels were also increased twofold, resulting in a combined greater than sixfold enhancement of basal glycogen synthase specific activity. Adipocytic glycogen levels were increased 200-to 400-fold in transgenic animals, and this increase was maintained to 1 yr of age. In contrast, lipid metabolism in transgenic adipose tissue was not significantly altered, as assessed by lipogenic rates, weight gain on normal or high-fat diets, or circulating free fatty acid levels after a fast. However, circulating and adipocytic leptin levels were doubled in transgenic animals, whereas adiponectin expression was unchanged. Cumulatively, these data indicate that murine adipocytes are capable of storing far higher levels of glycogen than previously reported. Furthermore, these results were obtained by overexpression of an endogenous adipocytic protein, suggesting that mechanisms may exist in vivo to maintain adipocytic glycogen storage at a physiological set point.insulin; glycogen synthesis; lipogenesis; protein phosphatase-1; targeting subunit CIRCULATING ENERGY SOURCES are maintained in a narrow homeostatic range across a wide variety of physiological conditions through the coordinate regulation of energy uptake, consumption, storage, and release by the principal metabolic tissues, namely adipose tissue, liver, and skeletal muscle. During times of energy deficit, adipose tissue is a primary site for energy provision via the hydrolysis of stored triglyceride to release free fatty acid (FFA) for ATP production and glycerol for hepatic gluconeogenesis. Conversely, following a meal, dietary lipid is stored by adipose tissue, and glucose is disposed of as glycogen by the skeletal muscle and to a lesser extent by the liver. Alternately, glucose can be utilized postprandially by the liver and adipocytes for de novo lipogenesis and long-term storage as triglyceride. However, adipocytes also store glucose as glycogen, albeit at substantially lower rates than in skeletal muscle and liver, so the physiological role of adipocytic glycogen metabolism remains unclear.In addition to its role in lipid metabolism, adipose tissue functions as an...

show abstract

Section: Discussionmentioning

confidence: 79%

mentioning

confidence: 76%

Section: Ap2-driven Overexpression Of Ptg In Adipose Tissue Of Transgmentioning

confidence: 99%

See 1 more Smart Citation

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Jurczak

Danos²,

Rehrmann³

et al. 2007

American Journal of Physiology-Endocrinology and Metabolism

View full text Add to dashboard Cite

show abstract

“…PTG has binding sites for both synthase and phosphorylase (22). However, the role of both proteins appears to be a stimulation of glycogen synthesis (21,23,24) and their properties do not appear to provide an adequate explanation for the last-in-first-out principle.…”

Section: Table II Calculated Degree Of Last-in-first-outmentioning

confidence: 99%

Partly Ordered Synthesis and Degradation of Glycogen in Cultured Rat Myotubes

Elsner

Quistorff

Hansen

et al. 2002

Journal of Biological Chemistry

View full text Add to dashboard Cite

The following questions concerning glycogen synthesis and degradation were examined in cultured rat myotubes. 1) Is synthesis and degradation of the individual glycogen molecule a strictly ordered process, with the last glucosyl unit incorporated into the molecule being the first to be released (the last-in-first-out principle), or is it a random process? 2) Are all glycogen molecules in skeletal muscle synthesized and degraded in phase (simultaneous order) or out of phase (sequential order)? Basal glycogen stores were minimized by fasting and were subsequently replenished in two intervals, the first (0 -0.5 h) with tritium-labeled and the second (0.5-3 h) with carbon-labeled glucose as precursor. Glycogen degradation was initiated by addition of forskolin. The kinetics of glycogen accumulation as well as degradation could be approximated by monoexponential equations with rate constants of 0.81 and 1.39 h ؊1 , respectively. The degradation of glycogen largely followed the lastin-first-out principle, particularly in the initial period. Analysis of the size of the glycogen molecules and the ␤-dextrin limit during glycogen accumulation and degradation showed that both synthesis and degradation of glycogen molecules are largely sequential and the small deviation from this order is most pronounced at the beginning of the accumulation and at the end of the degradation period. This pattern may reflect the number of synthase and phosphorylase molecules and fits well with the role of glycogen in skeletal muscle as a readily available energy store and with the known structure of the glycogen molecule. It is emphasized that the observed nonlinear relation between the change in glycogen concentration and release of label during glycogen degradation may have important practical consequences for interpretation of experimental data.Glycogen is a branched polymer of glucose of spherical geometry composed of ϳ53,000 glucosyl residues for the fully synthesized molecule (␤-glycogen). Glycogen plays a very dynamic role in the energy metabolism of skeletal muscle by serving as a readily available energy source during muscle contraction of high intensity (1, 2). The spherical form and branched structure of glycogen appear to be optimally suited to fast breakdown by the glycogen phosphorylase and the debranching enzyme (2). The basic structure of glycogen consists of layers (tiers) of branched glucosyl chains with the protein glycogenin as the core. Each B-chain consists of 13 glucosyl residues linked by ␣[134] glucosyl bonds and gives rise to two new B-chains by way of the ␣[136] branch points (3). This design principle is repeated until 11 layers of B-chains are formed and the synthesis is completed by the addition of one tier of A-chains of the same length as the B-chain, but without branch points (2, 4). Further synthesis is not possible for steric reasons (2).For the first eight tiers, the ␣[134] glucosyl bonds are synthesized by proglycogen synthase and the ␣[136] branch points by branching enzyme, and the molecule is defined as...

show abstract

“…Cellular overexpression of PTG resulted in a significant increase in glycogen accumulation in a variety of cell types (23,25,27,28). These changes were accompanied by the activation of glycogen synthase and/or inactivation of glycogen phosphorylase (25,28,29).…”

mentioning

confidence: 99%

Protein Targeting to Glycogen Overexpression Results in the Specific Enhancement of Glycogen Storage in 3T3-L1 Adipocytes

Greenberg

Meredith

Yan

et al. 2003

Journal of Biological Chemistry

View full text Add to dashboard Cite

Protein phosphatase-1 (PP1) plays an important role in the regulation of glycogen synthesis by insulin. Protein targeting to glycogen (PTG) enhances glycogen accumulation by increasing PP1 activity against glycogenmetabolizing enzymes. However, the specificity of PTG's effects on cellular dephosphorylation and glucose metabolism is unclear. Overexpression of PTG in 3T3-L1 adipocytes using a doxycycline-controllable adenoviral construct resulted in a 10 -20-fold increase in PTG levels and an 8-fold increase in glycogen levels. Inclusion of 1 g/ml doxycycline in the media suppressed PTG expression, and fully reversed all PTG-dependent effects. Infection of 3T3-L1 adipocytes with the PTG adenovirus caused a marked dephosphorylation and activation of glycogen synthase. The effects of PTG seemed specific, because basal and insulin-stimulated phosphorylation of a variety of signaling proteins was unaffected. Indeed, glycogen synthase was the predominant protein whose phosphorylation state was decreased in 32 P-labeled cells. PTG overexpression did not alter PP1 protein levels but increased PP1 activity 6-fold against phosphorylase in vitro. In contrast, there was no change in PP1 activity measured using myelin basic protein, suggesting that PTG overexpression specifically directed PP1 activity against glycogen-metabolizing enzymes. To investigate the metabolic consequences of altering PTG levels, glucose uptake and storage in 3T3-L1 adipocytes was measured. PTG overexpression did not affect 2-deoxy-glucose transport rates in basal and insulin-stimulated cells but dramatically enhanced glycogen synthesis rates under both conditions. Despite the large increases in cellular glucose flux upon PTG overexpression, basal and insulin-stimulated glucose incorporation into lipid were unchanged. Cumulatively, these data indicate that PTG overexpression in 3T3-L1 adipocytes discretely stimulates PP1 activity against glycogen synthase and phosphorylase, resulting in a marked and specific increase in glucose uptake and storage as glycogen.Insulin exerts its anabolic actions by promoting the uptake and storage of glucose and lipids in target tissues. Insulin coordinates the stimulation of the translocation of both glucose and fatty acid transporters to the cell surface (1, 2) and the modulation of the activity of key metabolic enzymes (3). In addition, insulin inhibits hepatic glycogenolysis and adipocytic triglyceride breakdown, further lowering excess glucose and free fatty acids levels in the bloodstream. Disruption of the complex interplay between lipid and carbohydrate metabolism results in the development of insulin resistance and type II diabetes (4, 5). However, the molecular mechanisms by which insulin potently regulates energy uptake and storage in vivo are not fully understood.Glycogen synthase activity is stimulated by insulin in liver, muscle, and adipose tissue via protein dephosphorylation, allosteric activation, and enzymatic translocation. Glycogen synthase is phosphorylated on up to nine residues by a variety of ki...

show abstract

Overexpression of Protein Targeting to Glycogen in Cultured Human Muscle Cells Stimulates Glycogen Synthesis Independent of Glycogen and Glucose 6-Phosphate Levels

Cited by 34 publications

References 27 publications

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Partly Ordered Synthesis and Degradation of Glycogen in Cultured Rat Myotubes

Protein Targeting to Glycogen Overexpression Results in the Specific Enhancement of Glycogen Storage in 3T3-L1 Adipocytes

Contact Info

Product

Resources

About