Effect of fructose 1-phosphate on the activation of liver glycogen synthase

Ketohexokinase C blockade ameliorates fructose-induced metabolic dysfunction in fructose-sensitive mice

Increasing evidence suggests a role for excessive intake of fructose in the Western diet as a contributor to the current epidemics of metabolic syndrome and obesity. Hereditary fructose intolerance (HFI) is a difficult and potentially lethal orphan disease associated with impaired fructose metabolism. In HFI, the deficiency of aldolase B results in the accumulation of intracellular phosphorylated fructose, leading to phosphate sequestration and depletion, increased adenosine triphosphate (ATP) turnover, and a plethora of conditions that lead to clinical manifestations such as fatty liver, hyperuricemia, Fanconi syndrome, and severe hypoglycemia. Unfortunately, there is currently no treatment for HFI, and avoiding sugar and fructose has become challenging in our society. In this report, through use of genetically modified mice and pharmacological inhibitors, we demonstrate that the absence or inhibition of ketohexokinase (Khk), an enzyme upstream of aldolase B, is sufficient to prevent hypoglycemia and liver and intestinal injury associated with HFI. Herein we provide evidence for the first time to our knowledge of a potential therapeutic approach for HFI. Mechanistically, our studies suggest that it is the inhibition of the Khk C isoform, not the A isoform, that protects animals from HFI.

Inclusion of low amounts of fructose with an intraportal glucose load increases net hepatic glucose uptake in the presence of relative insulin deficiency in dog

The effect of small amounts of fructose on net hepatic glucose uptake (NHGU) during hyperglycemia was examined in the presence of insulinopenia in conscious 42-h fasted dogs. During the study, somatostatin (0.8 microg.kg(-1).min(-1)) was given along with basal insulin (1.8 pmol.kg(-1).min(-1)) and glucagon (0.5 ng.kg(-1).min(-1)). After a control period, glucose (36.1 micromol.kg(-1).min(-1)) was continuously given intraportally for 4 h with (2.2 micromol.kg(-1).min(-1)) or without fructose. In the fructose group, the sinusoidal blood fructose level (nmol/ml) rose from <16 to 176 +/- 11. The infusion of glucose alone (the control group) elevated arterial blood glucose (micromol/ml) from 4.3 +/- 0.3 to 11.2 +/- 0.6 during the first 2 h after which it remained at 11.6 +/- 0.8. In the presence of fructose, glucose infusion elevated arterial blood glucose (micromol/ml) from 4.3 +/- 0.2 to 7.4 +/- 0.6 during the first 1 h after which it decreased to 6.1 +/- 0.4 by 180 min. With glucose infusion, net hepatic glucose balance (micromol.kg(-1).min(-1)) switched from output (8.9 +/- 1.7 and 13.3 +/- 2.8) to uptake (12.2 +/- 4.4 and 29.4 +/- 6.7) in the control and fructose groups, respectively. Average NHGU (micromol.kg(-1).min(-1)) and fractional glucose extraction (%) during last 3 h of the test period were higher in the fructose group (30.6 +/- 3.3 and 14.5 +/- 1.4) than in the control group (15.0 +/- 4.4 and 5.9 +/- 1.8). Glucose 6-phosphate and glycogen content (micromol glucose/g) in the liver and glucose incorporation into hepatic glycogen (micromol glucose/g) were higher in the fructose (218 +/- 2, 283 +/- 25, and 109 +/- 26, respectively) than in the control group (80 +/- 8, 220 +/- 31, and 41 +/- 5, respectively). In conclusion, small amounts of fructose can markedly reduce hyperglycemia during intraportal glucose infusion by increasing NHGU even when insulin secretion is compromised.

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog

Intraportal infusion of small amounts of fructose markedly augmented net hepatic glucose uptake (NHGU) during hyperglycemic hyperinsulinemia in conscious dogs. In this study, we examined whether the inclusion of catalytic amounts of fructose with a glucose load reduces postprandial hyperglycemia and the pancreatic beta-cell response to a glucose load in conscious 42-h-fasted dogs. Each study consisted of an equilibration (-140 to -40 min), control (-40 to 0 min), and test period (0-240 min). During the latter period, glucose (44.4 micromol x kg(-1) x min(-1)) was continuously given intraduodenally with (2.22 micromol x kg(-1) x min(-1)) or without fructose. The glucose appearance rate in portal vein blood was not significantly different with or without the inclusion of fructose (41.3 +/- 2.7 vs. 37.3 +/- 8.3 micromol x kg(-1) x min(-1), respectively). In response to glucose infusion without the inclusion of fructose, the net hepatic glucose balance switched from output to uptake (from 10 +/- 2 to 11 +/- 4 micromol x kg(-1) x min(-1)) by 30 min and averaged 17 +/- 6 micromol x kg(-1) x min(-1). The fractional extraction of glucose by the liver during the infusion period was 7 +/- 2%. Net glycogen deposition was 2.44 mmol glucose equivalent/kg body wt; 49% of deposited glycogen was synthesized via the direct pathway. Net hepatic lactate production was 1.4 mmol/kg body wt. Arterial blood glucose rose from 4.1 +/- 0.2 to 7.3 +/- 0.4 mmol/l, and arterial plasma insulin rose from 42 +/- 6 to 258 +/- 66 pmol/l at 30 min, after which they decreased to 7.0 +/- 0.5 mmol/l and 198 +/- 66 pmol/l, respectively. Arterial plasma glucagon decreased from 54 +/- 7 to 32 +/- 3 ng/l. In response to intraduodenal glucose infusion in the presence of fructose, net hepatic glucose balance switched from 9 +/- 1 micromol x kg(-1) x min(-1) output to 12 +/- 3 and 28 +/- 5 micromol x kg(-1) x min(-1) uptake by 15 and 30 min, respectively. The average NHGU (28 +/- 5 micromol x kg(-1) x min(-1)) and fractional extraction during infusion period (12 +/- 2%), net glycogen deposition (3.68 mmol glucose equivalent/kg body wt), net hepatic lactate production (3.27 mmol/kg), and glycogen synthesis via the direct pathway (68%) were significantly higher (P < 0.05) compared to that in the absence of fructose. The increases in arterial blood glucose (from 4.4 +/- 0.1 to 6.4 +/- 0.2 mmol/l at 30 min) and arterial plasma insulin (from 48 +/- 6 to 126 +/- 30 pmol/l at 30 min) were significantly smaller (P < 0.05). In summary, the inclusion of small amounts of fructose with a glucose load augmented NHGU, increased hepatic glycogen synthesis via the direct pathway, and augmented hepatic glycolysis. As a result, postprandial hyperglycemia and insulin release by the pancreatic beta-cell were reduced. In conclusion, catalytic amounts of fructose have the ability to improve glucose tolerance.

Stimulating effects of low-dose fructose on insulin-stimulated hepatic glycogen synthesis in humans

Fructose has been shown to have a catalytic effect on glucokinase activity in vitro; however, its effects on hepatic glycogen metabolism in humans is unknown. To address this question, we used (13)C nuclear magnetic resonance (NMR) spectroscopy to noninvasively assess rates of hepatic glycogen synthesis and glycogenolysis under euglycemic (approximately 5 mmol/l) hyperinsulinemic conditions (approximately 400 pmol/l) with and without a low-dose infusion of fructose (approximately 3.5 micromol. kg(-1). min(-1)). Six healthy overnight-fasted subjects were infused for 4 h with somatostatin (0.1 micromol. kg(-1). min(-1)) and insulin (240 pmol. m(-2). min(-1)). During the initial 120 min, [1-(13)C]glucose was infused to assess glycogen synthase flux followed by an approximately 120-min infusion of unlabeled glucose to assess rates of glycogen phosphorylase flux. Acetaminophen was given to assess the percent contribution of the direct and indirect (gluconeogenic) pathways of glycogen synthesis by the (13)C enrichment of plasma UDP-glucuronide and C-1 of glucose. In the control studies, the flux through glycogen synthase and glycogen phosphorylase was 0.31 +/- 0.06 and 0.17 +/- 0.04 mmol/l per min, respectively, and the rate of net hepatic glycogen synthesis was 0.14 +/- 0.05 mmol/l per min. In the fructose studies, the glycogen synthase flux increased 2.5-fold to 0.79 +/- 0.16 mmol/l per min (P = 0.018 vs. control), whereas glycogen phosphorylase flux remained unchanged (0.24 +/- 0.06; P = 0.16 vs. control). The infusion of fructose resulted in a threefold increase in rates of net hepatic glycogen synthesis (0.54 +/- 0.12 mmol/l per min; P = 0.008 vs. control) without affecting the pathways of hepatic glycogen synthesis (direct pathway approximately 60% in both groups). We conclude that during euglycemic hyperinsulinemia, a low-dose fructose infusion causes a threefold increase in net hepatic glycogen synthesis exclusively through stimulation of glycogen synthase flux. Because net hepatic glycogen synthesis has been shown to be diminished in patients with poorly controlled type 1 and type 2 diabetes, stimulation of hepatic glycogen synthesis by this mechanism may be of potential therapeutic value.

The structure, role, and regulation of type 1 protein phosphatases

Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.

Control of glycogen synthesis in health and disease

Investigations in our laboratory have shown that the activity of glycogen synthase phosphatase in the liver is shared by at least two functionally distinct proteins: a G-component, which is tightly associated with glycogen particles, and a soluble S-component. Most preparations of glycogen synthase-b that are isolated from the liver of fed glucagon-treated animals require the presence of both components in order to be converted to synthase-a. The G-component is subject to control mechanisms that do not affect the S-component. Its activity is strongly inhibited by phosphorylase-a. This feature explains why glycogen synthesis and glycogenolysis do not normally occur simultaneously, except in the glycogen-depleted liver, where a futile cycle may occur. Experiments in vitro have shown that a minimal glycogen concentration is required to ensure the interaction between the G-component and phosphorylase-a. The G-component is also selectively inhibited by Ca2+, and the magnitude of this inhibition depends markedly on the glycogen concentration. The latter inhibition is probably one of the mechanisms by which cyclic adenosine monophosphate (cAMP)-independent glycogenolytic agents achieve the inactivation of glycogen synthase in the liver. Glucocorticoid hormones and insulin are required for the induction and/or maintenance of the G-component in the liver. During the development of the fetal rat, glucocorticoids induce the G-component in the liver. This is an essential event in the glucocorticoid-triggered deposition of glycogen in the fetal liver. A functional adrenal cortex is also required in the adult animal to prevent a loss of the capacity for hepatic glycogen storage during starvation. The latter capacity depends on the concentration of functional G-component in the liver. Chronic diabetes causes a similar functional loss. However, the effect of glucocorticoids is not mediated by a putative secretion of insulin.