Glucose induces the translocation of glycogen synthase to the cell cortex in rat hepatocytes

Self Cite

Glucose 6-phosphate (Glc-6-P) produced in cultured hepatocytes by direct phosphorylation of glucose or by gluconeogenesis from dihydroxyacetone (DHA) was equally effective in activating glycogen synthase (GS). However, glycogen accumulation was higher in hepatocytes incubated with glucose than in those treated with DHA. This difference was attributed to decreased futile cycling through GS and glycogen phosphorylase (GP) in the glucose-treated hepatocytes, owing to the partial inactivation of GP induced by glucose. Our results indicate that the gluconeogenic pathway and the glucokinasemediated phosphorylation of glucose deliver their common product to the same Glc-6-P pool, which is accessible to liver GS. As observed in the treatment with glucose, incubation of cultured hepatocytes with DHA caused the translocation of GS from a uniform cytoplasmic distribution to the hepatocyte periphery and a similar pattern of glycogen deposition. We hypothesize that Glc-6-P has a major role in glycogen metabolism not only by determining the activation state of GS but also by controlling its subcellular distribution in the hepatocyte.

Section: Discussionmentioning

confidence: 99%

Glucose 6-Phosphate Produced by Gluconeogenesis and by Glucokinase Is Equally Effective in Activating Hepatic Glycogen Synthase

Gomis

Favre

Garcı́a-Rocha

et al. 2003

Self Cite

“…In isolated hepatocytes, incubation with glucose induces GS activation and intracellular translocation to 1 To whom correspondence should be addressed: Blegdamsvej 3, 2200 Copenhagen N, Denmark. the cell periphery (24). In contrast, in the absence of glucose, GS has been shown to be mainly located inside the nucleus of both cultured liver and muscle cells; however, following addition of glucose GS translocates to the cytosol (25).…”

mentioning

confidence: 93%

Dual Regulation of Muscle Glycogen Synthase during Exercise by Activation and Compartmentalization

Prats

Helge

Nordby

et al. 2009

Glycogen synthase (GS) is considered the rate-limiting enzyme in glycogenesis but still today there is a lack of understanding on its regulation. We have previously shown phosphorylation-dependent GS intracellular redistribution at the start of glycogen re-synthesis in rabbit skeletal muscle (Prats, C., Cadefau, J. A., Cussó, R., Qvortrup, K., Nielsen, J. N., Wojtaszewki, J. F., Wojtaszewki, J. F., Hardie, D. G., Stewart, G., Hansen, B. F., and Ploug, T. (2005) J. Biol. Chem. 280, 23165-23172). In the present study we investigate the regulation of human muscle GS activity by glycogen, exercise, and insulin. Using immunocytochemistry we investigate the existence and relevance of GS intracellular compartmentalization during exercise and during glycogen re-synthesis. The results show that GS intrinsic activity is strongly dependent on glycogen levels and that such regulation involves associated dephosphorylation at sites 2؉2a, 3a, and 3a ؉ 3b. Furthermore, we report the existence of several glycogen metabolism regulatory mechanisms based on GS intracellular compartmentalization. After exhausting exercise, epinephrine-induced protein kinase A activation leads to GS site 1b phosphorylation targeting the enzyme to intramyofibrillar glycogen particles, which are preferentially used during muscle contraction. On the other hand, when phosphorylated at sites 2؉2a, GS is preferentially associated with subsarcolemmal and intermyofibrillar glycogen particles. Finally, we verify the existence in human vastus lateralis muscle of the previously reported mechanism of glycogen metabolism regulation in rabbit tibialis anterior muscle. After overnight low muscle glycogen level and/or in response to exhausting exercise-induced glycogenolysis, GS is associated with spherical structures at the I-band of sarcomeres.

“…The fact that several enzymes of glycogen metabolism are directly associated with the glycogen particle has long been appreciated (2,3). More recently, it has been shown that glucokinase (hexokinase IV) and glycogen synthase are sequestered in the nucleus under basal conditions and that both enzymes are translocated to the cytosol upon exposure to nutrients (7,8,11,13,14,28). Furthermore, hepatic overexpression of hexokinase I, an enzyme that is activated by binding to porin in mitochondrial membranes (29), has no effect on activation of glycogen synthase or glycogen deposition, while overexpression of glucokinase impacts both parameters, suggesting that the subcellular localization of these enzymes determines their impact on glycogenesis (9, 10).…”

Section: Figmentioning

confidence: 99%

Overexpression of Protein Targeting to Glycogen (PTG) in Rat Hepatocytes Causes Profound Activation of Glycogen Synthesis Independent of Normal Hormone- and Substrate-mediated Regulatory Mechanisms

Berman

O’Doherty

Anderson

et al. 1998

Protein targeting to glycogen (PTG), also known as PPP1R5, is a widely expressed member of a growing family of proteins that target protein phosphatase-1 (PP-1) to glycogen particles. Because PTG also binds to glycogen synthase and phosphorylase kinase, it has been suggested that it serves as a "scaffold" for efficient activation of glycogen synthesis. However, very little is known about the metabolic effects of PTG. In this study, we have used recombinant adenovirus to overexpress PTG in primary rat hepatocytes, a cell type with high glycogenic capacity. We find that overexpression of PTG potently activates glycogen synthesis in cultured hepatocytes. Surprisingly, the glycogenic effect of PTG is observed even in the complete absence of carbohydrates or insulin in the culture medium. Furthermore, glycogenolytic agents such as forskolin or glucagon are largely ineffective at activating glycogen degradation in PTG overexpressing hepatocytes, even though large increases in cAMP levels are demonstrated. These metabolic effects of PTG overexpression are accompanied by a 3.6-fold increase in glycogen synthase activation state and a 40% decrease in glycogen phosphorylase activity. Our results are consistent with a model in which PTG overexpression "locks" the hepatocyte in a glycogenic mode, presumably via its ability to promote interaction of enzymes of glycogen metabolism with PP-1.