Quantitation of glycogen/glucose-1-P cycling in liver

Prediabetes linked to excess glucagon in transgenic mice with pancreatic active AKT1

Protein kinase B/AKT has three isoforms (AKT1-3) and is renowned for its central role in the regulation of cell growth and proliferation, due to its constitutive activation in various cancers. AKT2, which is highly expressed in insulin-responsive tissues, has been identified as a primary regulator of glucose metabolism as Akt2 knockout mice (Akt2(-/-)) are glucose-intolerant and insulin-resistant. However, the role of AKT1 in glucose metabolism is not as clearly defined. We previously showed that mice with myristoylated Akt1 (AKT1(Myr)) expressed through a bicistronic Pdx1-TetA and TetO-MyrAkt1 system were susceptible to islet cell carcinomas, and in this study we characterized an early onset, prediabetic phenotype. Beginning at weaning (3 weeks of age), the glucose-intolerant AKT1(Myr) mice exhibited non-fasted hyperglycemia, which progressed to fasted hyperglycemia by 5 months of age. The glucose intolerance was attributed to a fasted hyperglucagonemia, and hepatic insulin resistance detectable by reduced phosphorylation of the insulin receptor following insulin injection into the inferior vena cava. In contrast, treatment with doxycycline diet to turn off the transgene caused attenuation of the non-fasted and fasted hyperglycemia, thus affirming AKT1 hyperactivation as the trigger. Collectively, this model highlights a novel glucagon-mediated mechanism by which AKT1 hyperactivation affects glucose homeostasis and provides an avenue to better delineate the molecular mechanisms responsible for diabetes mellitus and the potential association with pancreatic cancer.

Pathogenesis of fasting and postprandial hyperglycemia in type 2 diabetes: implications for therapy

The objective of this research is to gain a greater understanding of the cause of fasting and postprandial hyperglycemia in people with type 2 diabetes. Endogenous glucose production is excessive before eating and fails to appropriately suppress after eating in people with type 2 diabetes. This is due in part to impaired insulin-induced suppression of endogenous glucose production, which is observed early in the evolution of type 2 diabetes. Increased rates of gluconeogenesis and perhaps glycogenolysis contribute to hepatic insulin resistance. Insulin-induced stimulation of hepatic glucose uptake and hepatic glycogen synthesis are reduced in people with type 2 diabetes primarily due to decreased uptake of extracellular glucose presumably because of inadequate activation of hepatic glucokinase. Delayed insulin secretion results in higher peak glucose concentrations particularly when suppression of glucagon is impaired, whereas insulin resistance prolongs the duration of hyperglycemia, which can be marked when both hepatic and extra-hepatic insulin resistance are present. The premise of these studies, as well as those performed by many other investigators, is that an understanding of the pathogenesis of type 2 diabetes will enable the development of targeted therapies that are directed toward correcting specific metabolic defects in a given individual. I, as well as many other investigators, believe that such therapies are likely to be more effective and to have a lower risk than would occur if everyone were treated the same regardless of the underlying cause of their hyperglycemia. While we do not yet have sufficient knowledge to truly individualize therapy, in my opinion this approach will be the norm in the not too distant future.

Changes in hepatic glycogen cycling during a glucose load in healthy humans

AIMS/HYPOTHESIS

Glycogen cycling, i.e. simultaneous glycogen synthesis and glycogenolysis, affects estimates of glucose fluxes using tracer techniques and may contribute to hyperglycaemia in diabetic conditions. This study presents a new method for quantifying hepatic glycogen cycling in the fed state. Glycogen is synthesised from glucose by the direct and indirect (gluconeogenic) pathways. Since glycogen is also synthesised from glycogen, i.e. glycogen-->glucose 1-phosphate-->glycogen, that synthesised through the direct and indirect pathways does not account for 100% of glycogen synthesis. The percentage contribution of glycogen cycling to glycogen synthesis then equals the difference between the sum of the percentage contributions of the direct and indirect pathways and 100.

MATERIALS AND METHODS

The indirect and direct pathways were measured independently in nine healthy volunteers who had fasted overnight. They ingested (2)H(2)O (5 ml/kg body water) and were infused with [5-(3)H]glucose and acetaminophen (paracetamol; 1 g) during hyperglycaemic clamps (7.8 mmol/l) lasting 8 h. The percentage contribution of the indirect pathway was calculated from the ratio of (2)H enrichments at carbon 5 to that at carbon 2, and the contribution of the direct pathway was determined from the (3)H-specific activity, relative to plasma glucose, of the urinary glucuronide excreted between 2 and 4, 4 and 6, and 6 and 8 h.

RESULTS

Glucose infusion rates increased (p<0.01) to approximately 50 mumol kg(-1) min(-1). Plasma insulin and the insulin : glucagon ratio rose approximately 3.6- and approximately 8.3-fold (p<0.001), respectively. From the difference between 100% and the sum of the direct (2-4 h, 54+/-6%; 4-6 h, 59+/-5%; 6-8 h, 63+/-4%) and indirect (32+/-3, 38+/-4, 36+/-3%) pathways, glycogen cycling was seen to be decreased (p<0.05) from 14+/-4% (2-4 h) to 4+/-3% (4-6 h) and 1+/-3% (6-8 h).

CONCLUSIONS/INTERPRETATION

This method allows measurement of hepatic glycogen cycling in the fed state and demonstrates that glycogen cycling occurs most in the early hours after glucose loading subsequent to a fast.

Higher insulin concentrations are required to suppress gluconeogenesis than glycogenolysis in nondiabetic humans

To determine the mechanism(s) by which insulin inhibits endogenous glucose production (EGP) in nondiabetic humans, insulin was infused at rates of 0.25, 0.375, or 0.5 mU. kg(-1). min(-1) and glucose was clamped at approximately 5.5 mmol/l. EGP, gluconeogenesis, and uridine-diphosphoglucose (UDP)-glucose flux were measured using [3-(3)H]glucose, deuterated water, and the acetaminophen glucuronide methods, respectively. An increase in insulin from approximately 75 to approximately 100 to approximately 150 pmol/l ( approximately 12.5 to approximately 17 to approximately 25 microU/ml) resulted in progressive (ANOVA; P < 0.02) suppression of EGP (13.1 +/- 1.3 vs. 11.7 +/- 1.03 vs. 6.4 +/- 2.15 micromol x kg(-1) x min(-1)) that was entirely due to a progressive decrease (ANOVA; P < 0.05) in the contribution of glycogenolysis to EGP (4.7 +/- 1.7 vs. 3.4 +/- 1.2 vs. -2.1 +/- 1.3 micro mol x kg(-1) x min(-1)). In contrast, both the contribution of gluconeogenesis to EGP (8.4 +/- 1.0 vs. 8.3 +/- 1.1 vs. 8.5 +/- 1.3 micro mol x kg(-1) x min(-1)) and UDP-glucose flux (5.0 +/- 0.4 vs. 5.0 +/- 0.3 vs. 4.0 +/- 0.5 micro mol x kg(-1) x min(-1)) remained unchanged. The contribution of the direct (extracellular) pathway to UDP-glucose flux was minimal and constant during all insulin infusions. We conclude that higher insulin concentrations are required to suppress the contribution of gluconeogenesis of EGP than are required to suppress the contribution of glycogenolysis to EGP in healthy nondiabetic humans. Since suppression of glycogenolysis occurred without a decrease in UDP-glucose flux, this implies that insulin inhibits EGP, at least in part, by directing glucose-6-phosphate into glycogen rather than through the glucose-6-phosphatase pathway.

Methods for measuring glycogen cycling

Simultaneous synthesis and breakdown of glycogen is called glycogen cycling. The extent of hyperglycemia and decreased glycogen stores in diabetes mellitus may relate in part to the extent cycling occurs. Four methods have been introduced to estimate its extent in liver in humans. 1) In the fasted state, the rate of net hepatic glycogenolysis, i.e., glycogen breakdown minus synthesis, is estimated using NMR, and the rate of glycogenolysis is estimated from deuterium labeling of blood glucose on (2)H(2)O ingestion. 2) The rate of glycogen synthesis is estimated from the rate of labeling of carbon 1 of glycogen on [1-(13)C]glucose infusion, monitored by NMR, and the rate of breakdown from the rate of disappearance of that labeling on unlabeled glucose infusion. 3) The rate of synthesis from glucose-1-P, formed by glycogenolysis, is measured by the decrease in the (3)H/(14)C ratio in acetaminophen glucuronide on acetaminophen and [2-(3)H,6-(14)C]galactose administration. 4) The rate of synthesis is estimated from the dilution of label from labeled galactose in its conversion to the acetaminophen glucuronide, and the rate of glycogenolysis is estimated from the amount of label in blood glucose. In the first method, the fate of glucose-6-P is assumed to be only to glycogen and glucose. In the second, only glucose-6-P molecules formed by breakdown that are not cycled back to glycogen are measured. In the third, (3)H is assumed to be removed completely during cycling, and only the molecules cycled back to glycogen are measured. In the fourth, galactose conversion to glucose is assumed to be via glycogen. Quantitations in all four methods depend on assuming the order in which the molecules deposited in glycogen are released.

Type 2 diabetes impairs splanchnic uptake of glucose but does not alter intestinal glucose absorption during enteral glucose feeding: additional evidence for a defect in hepatic glucokinase activity

We have previously reported that splanchnic glucose uptake, hepatic glycogen synthesis, and hepatic glucokinase activity are decreased in people with type 2 diabetes during intravenous glucose infusion. To determine whether these defects are also present during more physiological enteral glucose administration, we studied 11 diabetic and 14 nondiabetic volunteers using a combined organ catheterization-tracer infusion technique. Glucose was infused into the duodenum at a rate of 22 micromol. kg(-1). min(-1) while supplemental glucose was given intravenously to clamp glucose at approximately 10 mmol/l in both groups. Endogenous hormone secretion was inhibited with somatostatin, and insulin was infused to maintain plasma concentrations at approximately 300 pmol/l (i.e., twofold higher than our previous experiments). Total body glucose disappearance, splanchnic, and leg glucose extractions were markedly lower (P < 0.01) in the diabetic subjects than in the nondiabetic subjects. UDP-glucose flux, a measure of glycogen synthesis, was approximately 35% lower (P < 0.02) in the diabetic subjects than in the nondiabetic subjects. This was entirely accounted for by a decrease (P < 0.01) in the contribution of extracellular glucose because the contribution of the indirect pathway to hepatic glycogen synthesis was similar between groups. Neither endogenous and splanchnic glucose productions nor rates of appearance of the intraduodenally infused glucose in the portal vein differed between groups. In summary, both muscle and splanchnic glucose uptake are impaired in type 2 diabetes during enteral glucose administration. The defect in splanchnic glucose uptake appears to be due to decreased uptake of extracellular glucose, implying decreased glucokinase activity. Thus, abnormal hepatic and muscle (but not gut) glucose metabolism are likely to contribute to postprandial hyperglycemia in people with type 2 diabetes.

Gluconeogenesis and the Cori cycle in 12-, 20-, and 40-h-fasted humans

Six subjects were infused with [U-13C]glucose (0.03-0.05 mg . kg-1 . min-1) starting 8-9 h after a meal, and the production of glucose, the recycling of glucose (the Cori cycle), the dilution of glucose by unlabeled carbon into the hepatic lactate-pyruvate pool, and gluconeogenesis were determined in these fasted volunteers by use of mass isotopomer analysis and equations previously described [J. A. Tayek and J. Katz. Am. J. Physiol. 272 (Endocrinol. Metab. 35): E476-E484, 1997]. A primed continuous 11-h infusion was started at 6:00 AM, and the above parameters were calculated after 3 h (for the 12-h fast) and at the end of the infusion (for the 20-h fast). Another group of five subjects was fasted for 40 h, and the above parameters were calculated as before. At 12, 20, and 40 h of fasting, respectively, blood glucose was 93 +/- 2, 83 +/- 2, and 71 +/- 2 (SE) mg/dl; glucose production was 2.3, 1.8, and 1.77 mg . kg-1 . min-1; the recycling of labeled carbon was 8, 15, and 15%, and that of glucose molecules (Cori cycle) was 18, 35, and 36%; the contribution of gluconeogenesis to glucose production was 41, 71, and 92% or 0.96, 1.29, and 1.64 mg . kg-1 . min-1; and the contribution of other sources to glucose production was 1.37, 0.53, and 0.15 mg . kg-1 . min-1. The recycling of glucose is important in prolonged fasting for the maintenance of plasma glucose concentration. We demonstrate here that gluconeogenesis can be easily measured and that it accounts for approximately 90% of glucose production after a 40-h fast.

Cell swelling and glycogen metabolism in hepatocytes from fasted rats

Cell swelling is known to increase net glycogen production from glucose in hepatocytes from fasted rats by activating glycogen synthase. Since both active glycogen synthase and phosphorylase are present in hepatocytes, suppression of flux through phosphorylase may also contribute to the net increase in glycogen synthesis by cell swelling. We have developed an isotopic procedure to estimate the fluxes through glycogen synthase and phosphorylase in intact hepatocytes and we have examined the effect of cell swelling on both enzyme fluxes. The following observations were made. (1) Hypotonic or glutamine-induced cell swelling increased net glycogen production by activating flux through glycogen synthase with little effect on phosphorylase flux. Proline, previously shown to increase glycogen synthesis more than could be accounted for by its ability to cause cell swelling, increased flux through glycogen synthase and inhibited phosphorylase flux. (2) Incorporation of [14C]glucose into glycogen preceded complete mixing of [14C]glucose with the intracellular pool of UDPglucose. It is concluded that cell swelling affects glycogen synthase only and that UDPglucose is compartmentalized.

Contributions of gluconeogenesis to glucose production in the fasted state

Healthy subjects ingested 2H2O and after 14, 22, and 42 h of fasting the enrichments of deuterium in the hydrogens bound to carbons 2, 5, and 6 of blood glucose and in body water were determined. The hydrogens bound to the carbons were isolated in formaldehyde which was converted to hexamethylenetetramine for assay. Enrichment of the deuterium bound to carbon 5 of glucose to that in water or to carbon 2 directly equals the fraction of glucose formed by gluconeogenesis. The contribution of gluconeogenesis to glucose production was 47 +/- 49% after 14 h, 67 +/- 41% after 22 h, and 93 +/- 2% after 42 h of fasting. Glycerol's conversion to glucose is included in estimates using the enrichment at carbon 5, but not carbon 6. Equilibrations with water of the hydrogens bound to carbon 3 of pyruvate that become those bound to carbon 6 of glucose and of the hydrogen at carbon 2 of glucose produced via glycogenolysis are estimated from the enrichments to be approximately 80% complete. Thus, rates of gluconeogenesis can be determined without corrections required in other tracer methodologies. After an overnight fast gluconeogenesis accounts for approximately 50% and after 42 h of fasting for almost all of glucose production in healthy subjects.

Effects of infused glucose on glycogen metabolism in healthy humans

In order to determine whether or not hepatic glycogen breakdown contributes to systemic glucose flux during glucose infusion, net carbohydrate oxidation (indirect calorimetry) and the total rate of glucose appearance (6,6(-2)H-glucose) were measured in six healthy women during infusion of U-13 C labelled glucose (22 mumol/kg/min). Glucose infusion completely suppressed endogeneous glucose production and increased net carbohydrate oxidation from 10.9 +/- 1.6 to 18.9 +/- 1.0 mumol/kg/min. To differentiate between the oxidation of endogenous (i.e. glycogen) and of exogenous carbohydrates, the 13CO2 production was measured and the oxidation of exogenous 13C labelled carbohydrate was calculated. For this purpose, the specific recovery factor in breath of 13CO2 issued from oxidation of uniformly labelled glucose was determined during infusions of equimolar amounts of 13C bicarbonate, 1-13C acetate and 2-13C acetate. The average recovery was 53.9 +/- 1.5%. The oxidation of exogenous carbohydrate was 20.9 +/- 0.7 mumol/kg/min. This value was slightly higher than net carbohydrate oxidation, indicating that no oxidation of endogenous, unlabelled carbohydrate, and, hence, no utilization of hepatic glycogen took place. These results indicate that (i) estimation of glucose oxidation from indirect calorimetry and tracer technology give concordant results when an appropriate factor of 13CO2 recovery in breath is used, and (ii) utilization of previously formed glycogen is inhibited during hyperglycaemia and hyperinsulinaemia.

Hepatic glycogen accurately reflected by acetaminophen glucuronide in dogs refed after fasting

To validate a method to "biochemically biopsy" the immediate precursor of intrahepatic glycogen [uridyl diphosphate (UDP)-glucose] using acetaminophen and to assess how fasting affects the direct and indirect pathways of glycogen synthesis, dogs were fasted overnight (group 1, n = 5) or for 2.5 days (group 2, n = 5) and then given a 4-h duodenal infusion of unlabeled glucose, [3-3H]glucose, and [U-14C]lactate to label hepatic glycogen via the direct and indirect pathways, respectively, and [1-13C]galactose to measure intrahepatic UDP-glucose flux. After 3 h for equilibration, acetaminophen was given and urine was collected for acetaminophen glucuronide. Multiple liver biopsies were obtained. The mean 3H/14C ratios of glucose derived from glycogen (10.4 +/- 4.1 and 1.1 +/- 0.3 for groups 1 and 2, respectively) and glucose derived from acetaminophen glucuronide (11.5 +/- 4.0 and 1.0 +/- 0.1 for groups 1 and 2, respectively) were similar. Fasting significantly increased UDP-glucose flux, the rate of glycogen synthesis, and the contribution of the indirect pathway. We conclude that, in dogs, 1) no functional hepatic zonation exists with regard to acetaminophen glucuronidation and liver glycogen synthesis and 2) with appropriate choice of isotopic tracers and study design, UDP-glucose flux can accurately reflect rates of hepatic glycogen synthesis.

Demonstration of a glycogen/glucose 1-phosphate cycle in hepatocytes from fasted rats. Selective inactivation of phosphorylase by 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride

In search for a nonmetabolized, superior glucose analogue to study the mechanism of glucose-induced glycogen synthesis, we have tested 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride, which inhibits muscle phosphorylase beta 10-fold better than dose glucose (Street, I.P., Armstrong, C.R., and Withers, S.G. (1986) Biochemistry 25, 6021-6027). In a gel-filtered liver extract, 0.6 mM analogue and 10 mM glucose equally accelerated the inactivation of phosphorylase and shortened the latency before the activation of glycogen synthase. The analogue was not measurably defluorinated or phosphorylated by intact hepatocytes, as monitored by 19F NMR. When added to isolated hepatocytes, 10 mM analogue inactivated phosphorylase more extensively than did 50 mM glucose, but unlike glucose, it did not result in the activation of glycogen synthase. Therefore, the binding of glucose to phosphorylase alpha can account for the inactivation of phosphorylase, but the metabolism of glucose (probably to Glc-6-P) appears to be required to achieve activation of glycogen synthase. The livers of overnight-fasted, anesthetized mice contained appreciable amounts of both phosphorylase alpha and glycogen synthase alpha, without net glycogen accumulation. Likewise, hepatocytes isolated from fasted rats and incubated with 10 mM glucose contained 41% of phosphorylase and 32% of glycogen synthase in the alpha form, and these values remained stable for 1 h, while glycogen accumulated at only 22% of the rate expected from the glycogen synthase activity. The addition of 10 mM analogue decreased phosphorylase alpha to 10% without significant change in glycogen synthase alpha (38%), but with a 4-fold increased rate of glycogen accumulation. These findings imply that synthase alpha is fully active in the liver of the fasted animal and that the absence of net glycogen synthesis is due to continuous glycogenolysis by phosphorylase alpha.

Glycogen turnover in the isolated working rat heart

The isolated working rat heart was adapted for simultaneous determination of glycogen synthesis and degradation using a dual isotope technique. After prelabeling of glycogen with [U-14C]glucose, glycogenolysis was determined continuously from the washout of 14CO2 plus [14C]lactate. Glycogen synthesis was determined during the same period from incorporation of [5-3H]glucose. In the absence of added hormones, hearts were predominantly glycogenolytic (1.5 mumol/min/g, dry weight), and there was simultaneous synthesis (11% of the rate of glycogenolysis). The percentage of glucose taken up by the heart that could traverse the glycogen pool as a consequence of glycogen turnover was minor (5%). Insulin (10 milliunits/ml) predictably stimulated glycogen synthesis (3.6-fold) and nearly abolished glycogenolysis. Addition of glucagon (1 microgram/ml) increased contractile performance and initially stimulated glycogenolysis (3.8-fold) until glycogen was largely depleted. Net tritium incorporation was unaffected by glucagon. Both hormones stimulated glycolytic flux from exogenous glucose (3H2O from [5-3H]glucose) as well as total glycolytic flux (3H2O plus glycogenolysis). The initial stimulation in total glycolytic flux with glucagon was largely from glycogen, explaining the lag in stimulation from exogenous glucose. The relationship between the specific radioactivity and amount of glycogen remaining after different degrees of glycogenolysis suggests that the preference of glycogenolysis for newly synthesized glycogen is only partial.

Liver glycogen turnover in fed and fasted humans

Whether liver glycogen synthesis and breakdown occur simultaneously during net glycogen synthesis was assessed in fed and fasted healthy humans. The peak intensity of the carbon-1 (C1) resonance of the glycosyl units of glycogen was monitored with 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by unlabeled glucose infusion. The C1 peak intensity increased almost linearly during the [1-13C]glucose infusion, reflecting a near linear rate of glycogen synthesis. When switched to unlabeled glucose, the C1 peak intensity reached a plateau in the fasted subjects and declined in the fed subjects, reflecting active glycogenolysis during a time of net glycogen synthesis. We conclude that liver glycogen synthesis and degradation occur simultaneously in humans under conditions of net glycogen synthesis. The relative turnover rate was significantly higher in the fed (57 +/- 3%) than in the fasted state (31 +/- 8%; P < 0.01). The results indicate that glycogen may regulate its rate of breakdown and that liver glycogen turnover may be an important factor in limiting the accumulation of liver glycogen in humans.

Measuring glucose and fructose-6-phosphate cycling in liver in vivo

Approaches measuring futile cycling of glucose and fructose-6-phosphate (fructose-6-P) in liver in vivo depend on assumptions about the fates of hydrogens bound to specific carbons of glucose. Thus, 3H of [2-3H]glucose has been assumed to be completely removed after its conversion to glucose-6-P, [3-3H]glucose after its conversion to fructose-1,6-bisP, and [6-3H]glucose not at all. Previous measurements have shown that these assumptions are incompletely fulfilled. Corrections to estimates of cycling can be made when detritiations of [2-3H]glucose and [3-3H]glucose are not complete, and detritiation of [6-3H]glucose occurs. How the corrections can be made is presented using data previously reported on giving labeled glucoses to humans after an overnight fast and on infusing a glucose load. Estimates of glucose cycling nearly double, and that of fructose-6-P cycling almost triples. Estimates of hepatic glucose production as measured with [6-3H]glucose decrease. Correction of estimates of cycling under other conditions may very well be similarly affected. Thus, rates of glucose and fructose-6-P cycling appear to be substantially more than previously estimated. Quantitation under a given condition requires measurements to be made of the extent to which assumptions as to the fate of labeled hydrogen of the glucoses are fulfilled. The uncertain extent of exchange of label catalyzed by transaldolase and detritiation in the pentose cycle, the failure of fructose-6-P cycling to be expressed through detritiation of 3H from [3-3H]glucose, and possible isotope effects still limit the confidence that can be placed in such estimates.