Signalling pathways involved in the stimulation of glycogen synthesis by insulin in rat hepatocytes

Several amino acids were found to stimulate glycogen synthesis and lipogenesis, and to inhibit ketogenesis in isolated rat hepatocytes. When hepatocytes were incubated in the presence of 20 mM-glucose, the amino acids could be classified in decreasing order of efficiency as follows: glutamine and proline, alanine, aminoisobutyric acid, asparagine and histidine for stimulation of glycogen synthesis; glutamine, proline and alanine for stimulation of lipogenesis; proline and glutamine for inhibition of ketogenesis. The study of the time course revealed that the rates were not linear and were preceded by a lag period. In all conditions studied, glutamine and proline were found to have similar quantitative effects on glycogen synthesis and lipid metabolism. However, their effects differ qualitatively. Indeed, the effects of proline on glycogen synthesis, lipogenesis and glutamate and aspartate content were faster. Moreover, proline increased the hydroxybutyrate/acetoacetate ratio, whereas glutamine did not change it. Incubation of hepatocytes with aminoisobutyric acid or under hypo-osmotic conditions, which increased cell volume and mimicked the amino acid-induced stimulation of glycogen synthesis, had little effect on lipogenesis. In hepatocytes incubated without glucose, ketogenesis was inhibited, in decreasing order of efficiency, by alanine, asparagine, glutamine and proline. Under these conditions, glutamine increased, alanine decreased and asparagine did not affect the concentration of malonyl-CoA. This indicates that the latter cannot be responsible for the inhibition of ketogenesis by alanine and asparagine.

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Stimulation of glycogen synthesis and lipogenesis and inhibition of ketogenesis by various amino acids in isolated rat hepatocytes

Clinical Nutrition ◽

10.1016/0261-5614(90)90114-8 ◽

1990 ◽

Vol 9 (1) ◽

pp. XIV

Author(s):

A. Baquet ◽

A. Lavoinne ◽

L. Hue

Keyword(s):

Amino Acids ◽

Glycogen Synthesis ◽

Rat Hepatocytes ◽

Isolated Rat Hepatocytes ◽

Isolated Rat ◽

Stimulation Of

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The contribution of pyruvate cycling to loss of [6-3H]glucose during conversion of glucose to glycogen in hepatocytes: effects of insulin, glucose and acinar origin of hepatocytes

Biochemical Journal ◽

10.1042/bj2890255 ◽

1993 ◽

Vol 289 (1) ◽

pp. 255-262 ◽

Cited By ~ 14

Author(s):

L Agius ◽

D Tosh ◽

M Peak

Keyword(s):

Guinea Pig ◽

High Glucose ◽

Glucose Concentration ◽

Phosphoenolpyruvate Carboxykinase ◽

Glycogen Synthesis ◽

Rat Hepatocytes ◽

Human Hepatocytes ◽

Pyruvate Cycling ◽

Stimulation Of ◽

In Cells

1. During conversion of [6-3H,U-14C]glucose to glycogen in liver, loss of 6-3H can occur either by cycling via pyruvate (between glycolysis and gluconeogenesis) or by other mechanisms. We used mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase, to determine the extent to which pyruvate cycling contributes to loss of 6-3H during glucose conversion to glycogen in hepatocytes. 2. Mercaptopicolinate increased the 3H/14C ratio in glycogen during incubation of rat, guinea pig, pig and human hepatocytes with [6-3H,U-14C]glucose. The increase in the 3H/14C ratio in glycogen caused by mercaptopicolinate was greater in periportal than in perivenous rat hepatocytes, indicating that cycling of glucose via pyruvate is more prominent in cells with a higher gluconeogenic relative to glycolytic capacity. 3. The effect of mercaptopicolinate on the 3H/14C ratio in glycogen was observed both in the absence and in the presence of insulin, indicating that stimulation of glycogen synthesis by insulin is not associated with inhibition of pyruvate cycling. In rat and guinea pig but not in pig hepatocytes, the effects of mercaptopicolinate on the 3H/14C ratio in glycogen were greater at 10-15 mM glucose than at 30 mM glucose, suggesting diminished cycling via pyruvate at high glucose concentrations. 4. Insulin increased the loss of 6-3H during stimulation of conversion of glucose to glycogen in hepatocytes from all species. This was due in part to an increase in pyruvate cycling and in part to other mechanisms that are not inhibited by mercaptopicolinate. 5. These results suggest that pyruvate cycling is a significant, but not exclusive, component of the loss of 6-3H in the hepatocyte during glucose conversion to glycogen. The extent of pyruvate cycling is dependent on the acinar origin of the hepatocytes and on the glucose concentration and presence of insulin.

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Stimulation of glycogen synthesis and lipogenesis by glutamine in isolated rat hepatocytes

Biochemical Journal ◽

10.1042/bj2480429 ◽

1987 ◽

Vol 248 (2) ◽

pp. 429-437 ◽

Cited By ~ 68

Author(s):

A Lavoinne ◽

A Baquet ◽

L Hue

Keyword(s):

Glycogen Phosphorylase ◽

Glycogen Synthase ◽

Glycogen Synthesis ◽

Lactate Production ◽

Rat Hepatocytes ◽

Diabetic Rats ◽

Purine Analogues ◽

Isolated Rat Hepatocytes ◽

Body Production ◽

Stimulation Of

Glutamine stimulated glycogen synthesis and lactate production in hepatocytes from overnight-fasted normal and diabetic rats. The effect, which was half-maximal with about 3 mM-glutamine, depended on glucose concentration and was maximal below 10 mM-glucose. beta-2-Aminobicyclo[2.2.1.]heptane-2-carboxylic acid, an analogue of leucine, stimulated glutaminase flux, but inhibited the stimulation of glycogen synthesis by glutamine. Various purine analogues and inhibitors of purine synthesis were found to inhibit glycogen synthesis from glucose, but they did not abolish the stimulatory effect of glutamine on glycogen synthesis. The correlation between the rate of glycogen synthesis and synthase activity suggested that the stimulation of glycogen synthesis by glutamine depended solely on the activation of glycogen synthase. This activation of synthase was not due to a change in total synthase, nor was it caused by a faster inactivation of glycogen phosphorylase, as was the case after glucose. It could, however, result from a stimulation of synthase phosphatase, since, after the addition of 1 nM-glucagon or 10 nM-vasopressin, glutamine did not interfere with the inactivation of synthase, but did promote its subsequent re-activation. Glutamine was also found to inhibit ketone-body production and to stimulate lipogenesis.

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Multiple signalling pathways involved in the stimulation of fatty acid and glycogen synthesis by insulin in rat epididymal fat cells

Biochemical Journal ◽

10.1042/bj3110595 ◽

1995 ◽

Vol 311 (2) ◽

pp. 595-601 ◽

Cited By ~ 72

Author(s):

S K Moule ◽

N J Edgell ◽

G I Welsh ◽

T A Diggle ◽

E J Foulstone ◽

...

Keyword(s):

Fatty Acid ◽

Protein Kinase ◽

Pyruvate Dehydrogenase ◽

Glycogen Synthase ◽

Glycogen Synthesis ◽

Signalling Pathways ◽

Fat Cells ◽

Acetyl Coa Carboxylase ◽

Acetyl Coa ◽

Stimulation Of

We have investigated the signalling pathways involved in the stimulation of glycogen and fatty acid synthesis by insulin in rat fat cells using wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and rapamycin, which blocks activation of p70 ribosomal S6 protein kinase (p70S6K). Insulin produced a decrease in the activity of glycogen synthase kinase-3 which is likely to be important in the observed stimulation of glycogen synthase. Both of these actions were found to be sensitive to inhibition by wortmannin. Activation of three processes is involved in the stimulation of fatty acid synthesis from glucose by insulin, namely glucose uptake, acetyl-CoA carboxylase and pyruvate dehydrogenase. Whereas wortmannin largely abolished the effects of insulin on glucose utilization and acetyl-CoA carboxylase activity, it was without effect on the stimulation of pyruvate dehydrogenase. Although epidermal growth factor stimulated mitogen-activated protein kinase to a greater extent than insulin, it was unable to mimic the effect of insulin on glycogen synthase, glycogen synthase kinase-3, glucose utilization, acetyl-CoA carboxylase or pyruvate dehydrogenase. Rapamycin also failed to have any appreciable effect on stimulation of these parameters by insulin, although it did block the effect of insulin on p70S6K. We conclude that the activity of phosphatidylinositol 3-kinase is required for the effects of insulin on glycogen synthesis, glucose uptake and acetyl-Co-AN carboxylase, but is not involved in signalling to pyruvate dehydrogenase. Activation of mitogen-activated protein kinase or p70S6K, however, does not appear to be sufficient to bring about the stimulation of fatty acid or glycogen synthesis. Altogether is seems likely that at least four distinct signalling pathways are involved in the effects of insulin on rat fat cells.

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