Role of glucose 6-phosphate in the translocation of glycogen synthase in rat hepatocytes

Fernández‐Novell, Josep M.; Ariño, Joaquín; Vilaró, Senén; Bellido, David; Guinovart, Joan J.

doi:10.1042/bj2880497

Cited by 49 publications

(45 citation statements)

References 9 publications

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“…In a recent study [22] we presented evidence supporting the hypothesis that in healthy rat hepatocytes translocation of the enzyme was triggered by an increase in Glc6P levels. In diabetic hepatocytes Glc6P accumulates more slowly than in healthy cells upon incubation with glucose.…”

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confidence: 56%

“…Enzymes. Glycogen synthase (EC 2.4.1.1 1); glucose-6-phosphate dehydrogenase (EC 1.1.1.49).the general hypothesis that glucose 6-phosphate (Glc6P) constitutes the signal responsible for both the activation [20, 211 and the translocation of glycogen synthase induced by glucose [22].In this study we have addressed the question of whether the glucose-triggered translocation of glycogen synthase is altered in the diabetic state. Our results indicate that in diabetic hepatocytes glucose is still able to translocate glycogen synthase by increasing Glc6P levels.…”

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confidence: 99%

“…the general hypothesis that glucose 6-phosphate (Glc6P) constitutes the signal responsible for both the activation [20, 211 and the translocation of glycogen synthase induced by glucose [22].…”

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Effects of Glucose on the Activation and Translocation of Glycogen Synthase in Diabetic Rat Hepatocytes

Fernández‐Novell

Ariño

Guinovart

1994

European Journal of Biochemistry

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Incubation of hepatocytes from diabetic rats with glucose results in the translocation of glycogen synthase from soluble fractions to fractions which sediment at 9200 g. The extent of the translocation correlates positively with the intracellular concentration of glucose 6-phosphate. No difference was found between healthy and diabetic rats in the capacity of glycogen synthase to translocate to pellets in response to an increase in glucose 6-phosphate. In diabetic hepatocytes, glycogen synthase in the supernatant fractions was not activated upon incubation of the cells with glucose, whereas this sugar was able to activate the enzyme found in the fractions that could be pelleted. In the 9200-g pellets, the glycogen synthase activity ratio (-glucose 6-phosphate/+glucose 6-phosphate) from both healthy and diabetic animals correlated with the intracellular glucose 6-phosphate levels. In the supernatants, the glycogen synthase activity ratio from healthy cells also correlated with glucose 6-phosphate levels. In contrast, in diabetic cells the activation state of the soluble enzyme remained essentially unchanged despite the accumulation of glucose 6-phosphate.Glycogen deposition from glucose is impaired in diabetic rat hepatocytes in which little net synthesis of glycogen from glucose occurs [l-61. The molecular bases for this defect are unknown. This defect is not due to a lack of glycogen synthase, the key enzyme in the control of glycogen synthesis, since total glycogen synthase activity is either unaltered [7, 81 or even increased [9-111 in diabetic rat liver. Glucose is widely believed to stimulate the synthesis of glycogen by promoting the interconversion of glycogen synthase from the more phosphorylated forms to the dephosphorylated forms. This process is catalyzed in vitro by several protein phosphatases. Glycogen synthase phosphatase activity in the liver [ 3 , 7,8,[12][13][14] and in other tissues [15] is lowered in insulindeficient animals, it has been suggested that the decrease in synthase phosphatase which involves the glycogen-bound form of the enzyme [14, 161, could be the basis for the defect in the deposition of glycogen from glucose in the liver of diabetic animals. Alternatively, this impairment has been attributed to a decrease in hepatic glucose uptake or glucose phosphorylation by glucokinase or both, which are two wellknown alterations of liver glucose metabolism in the diabetic state [17, 181.Recently we have shown that in healthy rat hepatocytes, glucose, in addition to activating glycogen synthase, triggers changes in the intracellular distribution of this enzyme, measured as the amount both of total glycogen synthase activity and of immunoreactive enzyme sedimenting at about 10000 g [19]. Evidence obtained in our laboratory supports Correspondence to J. J. Guinovart, Departament Bioquimica. Facultat Quimica, Marti i Franqubs 1, 7" planta, E-08028 Barcelona, Spain ~~ Abbreviation. Glc6P, glucose 6-phosphate. Enzymes. Glycogen synthase (EC 2.4.1.1 1); glucose-6-phosphate dehydrogenase (EC 1.1...

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Effects of Glucose on the Activation and Translocation of Glycogen Synthase in Diabetic Rat Hepatocytes

Fernández‐Novell

Ariño

Guinovart

1994

European Journal of Biochemistry

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“…2) suggests that inactivation of phosphorylase and activation of synthase cannot fully explain the glycogenic action of PTG. We therefore tested the effects of PTG expression on the compartmentation of glycogen synthase, using a similar approach as used previously (26,28) to study glucose-6-P-mediated translocation. Expression of PTG caused a 5-fold increase in the distribution of total glycogen synthase in the pellet (Fig.…”

Section: Effects Of Ptg Are Additive With Glucokinase Expression But mentioning

confidence: 99%

The Glycogenic Action of Protein Targeting to Glycogen in Hepatocytes Involves Multiple Mechanisms Including Phosphorylase Inactivation and Glycogen Synthase Translocation

Green

Aiston

Greenberg

et al. 2004

Journal of Biological Chemistry

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Expression of the glycogen-targeting protein PTG promotes glycogen synthase activation and glycogen storage in various cell types. In this study, we tested the contribution of phosphorylase inactivation to the glycogenic action of PTG in hepatocytes by using a selective inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a and sequential activation of glycogen synthase. Similar to CP-91194, graded expression of PTG caused a concentration-dependent inactivation of phosphorylase and activation of glycogen synthase. The latter was partially counteracted by the expression of muscle phosphorylase and was not additive with the activation by CP-91149, indicating that it is in part secondary to the inactivation of phosphorylase. PTG expression caused greater stimulation of glycogen synthesis and translocation of glycogen synthase than CP-91149, and the translocation of synthase could not be explained by accumulation of glycogen, supporting an additional role for glycogen synthase translocation in the glycogenic action of PTG. The effects of PTG expression on glycogen synthase and glycogen synthesis were additive with the effects of glucokinase expression, confirming the complementary roles of depletion of phosphorylase a (a negative modulator) and elevated glucose 6-phosphate (a positive modulator) in potentiating the activation of glycogen synthase. PTG expression mimicked the inactivation of phosphorylase caused by high glucose and counteracted the activation caused by glucagon. The latter suggests a possible additional role for PTG on phosphorylase kinase inactivation.Regulation of liver glycogen metabolism by hormones and substrates is mediated by changes in the phosphorylation state of glycogen synthase and phosphorylase and by subcellular translocation of these proteins (1-3). Dephosphorylation is catalyzed by PP1C 1 in association with glycogen-targeting proteins (4), of which three isoforms are expressed in liver, designated G L , PTG, and R6 (5). All isoforms have glycogensynthase phosphatase and phosphorylase phosphatase activity when assayed in vitro; however, the synthase phosphatase/ phosphorylase phosphatase ratio is higher for G L -PP1 than for PTG-PP1, suggesting that G L may be the predominant synthase phosphatase (5). G L (R4) is expressed predominantly in liver and to a lesser extent in heart and human skeletal muscle (6, 7), PTG is expressed in several tissues including liver and skeletal muscle (8 -10), and R6 is expressed ubiquitously (11). Both G L and PTG show adaptive changes in expression in rat liver in response to changes in insulin status (5), and the physiological role of these changes has been confirmed from studies (12, 13) in hepatocytes, which showed glycogen synthase activation and increased glycogen storage when G L and PTG were overexpressed.A unique property of G L is its allosteric binding site for phosphorylase a, which accounts for the inhibition of glycogensynthase phosphatase activity by low concentrations of phosphorylase a (5). This mechanism ...

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“…Furthermore, G6P binding to GS induces a conformational change, increasing its susceptibility to dephosphorylation (5,6). Finally, elevation of intracellular glucose metabolites induces the translocation of cytosolic GS to glycogen-containing fractions in primary hepatocytes and 3T3-L1 adipocytes (7)(8)(9). Thus, GS activity can be increased through dephosphorylation, translocation, and allosteric activation.…”

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Specific Desensitization of Glycogen Synthase Activation by Insulin in 3T3-L1 Adipocytes

Jensen¹,

Crosson²,

Kartha³

et al. 2000

Journal of Biological Chemistry

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A protocol was developed in 3T3-L1 adipocytes that resulted in the specific desensitization of glycogen synthase activation by insulin. Cells were pretreated for 15 min with 100 nM insulin, and then recovered for 1.5 h in the absence of hormone. Subsequent basal and insulininduced phosphorylation of the insulin receptor, IRS-1, MAPK, Akt kinase, and GSK-3 were similar in control and pretreated cells. Additionally, enhanced glucose transport and incorporation into lipid in response to insulin were unaffected. However, pretreatment reduced insulin-stimulated glycogen synthesis by over 50%, due to a nearly complete inhibition of glycogen synthase activation. Removal of extracellular glucose during the recovery period blocked the increase in glycogen levels, and restored insulin-induced glycogen synthase activation. Furthermore, incubation of pretreated 3T3-L1 adipocytes with glycogenolytic agents reversed the desensitization event. Separation of cellular lysates on sucrose gradients revealed that glycogen synthase was primarily located in the dense pellet fraction, with lesser amounts in the lighter fractions. Insulin induced glycogen synthase translocation from the lighter to the denser glycogen-containing fractions. Interestingly, insulin preferentially activated translocated enzyme while having little effect on the majority of glycogen synthase activity in the pellet fraction. In insulin-pretreated cells, glycogen synthase did not return to the lighter fractions during recovery, and thus did not move in response to the second insulin exposure. These results suggest that, in 3T3-L1 adipocytes, the translocation of glycogen synthase may be an important step in the regulation of glycogen synthesis by insulin. Furthermore, intracellular glycogen levels can regulate glycogen synthase activation, potentially through modulation of enzymatic localization.

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Role of glucose 6-phosphate in the translocation of glycogen synthase in rat hepatocytes

Cited by 49 publications

References 9 publications

Effects of Glucose on the Activation and Translocation of Glycogen Synthase in Diabetic Rat Hepatocytes

Effects of Glucose on the Activation and Translocation of Glycogen Synthase in Diabetic Rat Hepatocytes

The Glycogenic Action of Protein Targeting to Glycogen in Hepatocytes Involves Multiple Mechanisms Including Phosphorylase Inactivation and Glycogen Synthase Translocation

Specific Desensitization of Glycogen Synthase Activation by Insulin in 3T3-L1 Adipocytes

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