Gluconeogenesis is associated with high rates of tricarboxylic acid and pyruvate cycling in fasting northern elephant seals

Animals that endure prolonged periods of food deprivation preserve vital organ function by sparing protein from catabolism. Much of this protein sparing is achieved by reducing metabolic rate and suppressing gluconeogenesis while fasting. Northern elephant seals ( Mirounga angustirostris) endure prolonged fasts of up to 3 mo at multiple life stages. During these fasts, elephant seals maintain high levels of activity and energy expenditure associated with breeding, reproduction, lactation, and development while maintaining rates of glucose production typical of a postabsorptive mammal. Therefore, we investigated how fasting elephant seals meet the requirements of glucose-dependent tissues while suppressing protein catabolism by measuring the contribution of glycogenolysis, glycerol, and phosphoenolpyruvate (PEP) to endogenous glucose production (EGP) during their natural 2-mo postweaning fast. Additionally, pathway flux rates associated with the tricarboxylic acid (TCA) cycle were measured specifically, flux through phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate cycling. The rate of glucose production decreased during the fast (F1,13= 5.7, P = 0.04) but remained similar to that of postabsorptive mammals. The fractional contributions of glycogen, glycerol, and PEP did not change with fasting; PEP was the primary gluconeogenic precursor and accounted for ∼95% of EGP. This large contribution of PEP to glucose production occurred without substantial protein loss. Fluxes through the TCA cycle, PEPCK, and pyruvate cycling were higher than reported in other species and were the most energetically costly component of hepatic carbohydrate metabolism. The active pyruvate recycling fluxes detected in elephant seals may serve to rectify gluconeogeneic PEP production during restricted anaplerotic inflow in these fasting-adapted animals.

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The impact of obesity, sex, and diet on hepatic glucose production in cats

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.90771.2008 ◽

2009 ◽

Vol 296 (4) ◽

pp. R936-R943 ◽

Cited By ~ 28

Author(s):

Saskia Kley ◽

Margarethe Hoenig ◽

John Glushka ◽

Eunsook S. Jin ◽

Shawn C. Burgess ◽

...

Keyword(s):

Phosphoenolpyruvate Carboxykinase ◽

Citrate Synthase ◽

Hepatic Glucose Production ◽

Glucose Production ◽

Endogenous Glucose Production ◽

Hepatic Glucose Metabolism ◽

Peripheral Insulin Resistance ◽

Pyruvate Cycling ◽

Hepatic Glucose ◽

The Impact

Obesity is a risk factor for type 2 diabetes in cats. The risk of developing diabetes is severalfold greater for male cats than for females, even after having been neutered early in life. The purpose of this study was to investigate the role of different metabolic pathways in the regulation of endogenous glucose production (EGP) during the fasted state considering these risk factors. A triple tracer protocol using 2H2O, [U-13C3]propionate, and [3,4-13C2]glucose was applied in overnight-fasted cats (12 lean and 12 obese; equal sex distribution) fed three different diets. Compared with lean cats, obese cats had higher insulin ( P < 0.001) but similar blood glucose concentrations. EGP was lower in obese cats ( P < 0.001) due to lower glycogenolysis and gluconeogenesis (GNG; P < 0.03). Insulin, body mass index, and girth correlated negatively with EGP ( P < 0.003). Female obese cats had ∼1.5 times higher fluxes through phosphoenolpyruvate carboxykinase ( P < 0.02) and citrate synthase ( P < 0.05) than male obese cats. However, GNG was not higher because pyruvate cycling was increased 1.5-fold ( P < 0.03). These results support the notion that fasted obese cats have lower hepatic EGP compared with lean cats and are still capable of maintaining fasting euglycemia, despite the well-documented existence of peripheral insulin resistance in obese cats. Our data further suggest that sex-related differences exist in the regulation of hepatic glucose metabolism in obese cats, suggesting that pyruvate cycling acts as a controlling mechanism to modulate EGP. Increased pyruvate cycling could therefore be an important factor in modulating the diabetes risk in female cats.

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Lipolysis and glycerol gluconeogenesis in simultaneously fasting and lactating northern elephant seals

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00403.2007 ◽

2007 ◽

Vol 293 (6) ◽

pp. R2376-R2381 ◽

Cited By ~ 25

Author(s):

Dorian S. Houser ◽

Cory D. Champagne ◽

Daniel E. Crocker

Keyword(s):

Adult Female ◽

Glucose Production ◽

Endogenous Glucose Production ◽

High Rate ◽

Constant Infusion ◽

Substantial Contribution ◽

Fasting Period ◽

Elephant Seals ◽

Glucoregulatory Hormones

Adult female elephant seals ( Mirounga angustirostris) combine long-term fasting with lactation and molting. Glycerol gluconeogenesis has been hypothesized as potentially meeting all of the glucose requirements of the seals during these fasts. To test this hypothesis, a primed constant infusion of [2-14C]glycerol was administered to 10 ten adult female elephant seals at 5 and 21–22 days postpartum and to 10 additional adult females immediately after the molt. Glycerol kinetics, rates of lipolysis, and the contribution of glycerol to glucose production were determined for each period. Plasma metabolite levels as well as insulin, glucagon, and cortisol were also measured. Glycerol rate of appearance was not significantly correlated with mass ( P = 0.14, r2 = 0.33) but was significantly related to the percentage of glucose derived from glycerol ( P < 0.01, r2 = 0.81) during late lactation. The contribution of glycerol to glucose production was <3% during each fasting period, suggesting a lower contribution to gluconeogenesis than is observed in other long-term fasting mammals. Because of a high rate of endogenous glucose production in fasting elephant seals, it is likely that glycerol gluconeogenesis still makes a substantial contribution to the substrate needs of glucose-dependent tissues. The lack of a relationship between glucoregulatory hormones and glycerol kinetics, glycerol gluconeogenesis, and metabolites supports the proposition that fasting elephant seals do not conform to the traditional insulin-glucagon model of substrate metabolism.

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Comparative gluconeogenesis in hepatocytes from salmonid fishes

Canadian Journal of Zoology ◽

10.1139/z86-166 ◽

1986 ◽

Vol 64 (5) ◽

pp. 1110-1115 ◽

Cited By ~ 52

Author(s):

Thomas P. Mommsen

Keyword(s):

Sockeye Salmon ◽

Phosphoenolpyruvate Carboxykinase ◽

Coho Salmon ◽

Glucose Production ◽

Isolated Hepatocytes ◽

Substrate Oxidation ◽

Endogenous Glucose Production ◽

Salmo Gairdneri ◽

Salmonid Fishes ◽

Isolated Mitochondria

Rates of gluconeogenic flux and substrate oxidation are assessed in isolated hepatocytes from three species of salmonid fishes: rainbow trout (Salmo gairdneri), coho salmon (Oncorhynchus kisutch), and sockeye salmon (Oncorhynchus nerka). Coho salmon displays the highest capacity for gluconeogenesis from lactate and alanine, but rates are well below those of eels. Enzyme compartmentation on isolated mitochondria shows that in trout and sockeye, phosphoenolpyruvate carboxykinase is almost entirely localized in the mitochondrion and the cytosol, respectively, while in the coho, 40% of phosphoenolpyruvate carboxykinase activity is associated with the cytosol. Freshly isolated salmonid hepatocytes are in negative glycogen balance. It is established here that at low (<250 μmol glucosyl units/g) glycogen concentrations a linear relationship exists between the rate of endogenous glucose production and the initial glycogen concentration. High rates of endogenous glycogen breakdown necessitate the use of radiotracers for determining gluconeogenic fluxes in fish hepatocytes. Rates of gluconeogenesis calculated from radiolabel experiments are compared with nonlabelled lactate and are determined not to be significantly different from each other. It is concluded that in fish hepatocytes, (i) radiotracer experiments give accurate estimates of gluconeogenesis, (ii) dilution of label at the oxalacetate level is insignificant, and, consequently, (iii) rates of 14CO2 production are a valid measure of true substrate oxidation.

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Adult male northern elephant seals maintain high rates of glucose production during extended breeding fasts

Journal of Comparative Physiology B ◽

10.1007/s00360-017-1098-1 ◽

2017 ◽

Vol 187 (8) ◽

pp. 1183-1192 ◽

Cited By ~ 3

Author(s):

Daniel E. Crocker ◽

Brian K. Wenzel ◽

Cory D. Champagne ◽

Dorian S. Houser

Keyword(s):

Adult Male ◽

Glucose Production ◽

Elephant Seals ◽

Northern Elephant Seals

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TGF-β1/Smad3 Pathway Targets PP2A-AMPK-FoxO1 Signaling to Regulate Hepatic Gluconeogenesis

Journal of Biological Chemistry ◽

10.1074/jbc.m116.764910 ◽

2017 ◽

Vol 292 (8) ◽

pp. 3420-3432 ◽

Cited By ~ 36

Author(s):

Hariom Yadav ◽

Samir Devalaraja ◽

Stephanie T. Chung ◽

Sushil G. Rane

Keyword(s):

Glucose Homeostasis ◽

Phosphoenolpyruvate Carboxykinase ◽

Metabolic Diseases ◽

Nuclear Translocation ◽

Glucose Production ◽

Endogenous Glucose Production ◽

Prolonged Fasting ◽

Hepatic Gluconeogenesis ◽

Endogenous Glucose ◽

Tgf Β1

Maintenance of glucose homeostasis is essential for normal physiology. Deviation from normal glucose levels, in either direction, increases susceptibility to serious medical complications such as hypoglycemia and diabetes. Maintenance of glucose homeostasis is achieved via functional interactions among various organs: liver, skeletal muscle, adipose tissue, brain, and the endocrine pancreas. The liver is the primary site of endogenous glucose production, especially during states of prolonged fasting. However, enhanced gluconeogenesis is also a signature feature of type 2 diabetes (T2D). Thus, elucidating the signaling pathways that regulate hepatic gluconeogenesis would allow better insight into the process of normal endogenous glucose production as well as how this process is impaired in T2D. Here we demonstrate that the TGF-β1/Smad3 signaling pathway promotes hepatic gluconeogenesis, both upon prolonged fasting and during T2D. In contrast, genetic and pharmacological inhibition of TGF-β1/Smad3 signals suppressed endogenous glucose production. TGF-β1 and Smad3 signals achieved this effect via the targeting of key regulators of hepatic gluconeogenesis, protein phosphatase 2A (PP2A), AMP-activated protein kinase (AMPK), and FoxO1 proteins. Specifically, TGF-β1 signaling suppressed the LKB1-AMPK axis, thereby facilitating the nuclear translocation of FoxO1 and activation of key gluconeogenic genes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. These findings underscore an important role of TGF-β1/Smad3 signaling in hepatic gluconeogenesis, both in normal physiology and in the pathophysiology of metabolic diseases such as diabetes, and are thus of significant medical relevance.

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Protein Feeding Promotes Redistribution of Endogenous Glucose Production to the Kidney and Potentiates Its Suppression by Insulin

Endocrinology ◽

10.1210/en.2008-0601 ◽

2009 ◽

Vol 150 (2) ◽

pp. 616-624 ◽

Cited By ~ 48

Author(s):

Bruno Pillot ◽

Maud Soty ◽

Amandine Gautier-Stein ◽

Carine Zitoun ◽

Gilles Mithieux

Keyword(s):

Insulin Sensitivity ◽

Glucose Uptake ◽

Phosphoenolpyruvate Carboxykinase ◽

Glucose Production ◽

Glycogen Storage ◽

Endogenous Glucose Production ◽

Diabetic Patients ◽

Type 2 Diabetic Patients ◽

Protein Feeding ◽

Endogenous Glucose

The aim of this study was to assess in rats the effect of protein feeding on the: 1) distribution of endogenous glucose production (EGP) among gluconeogenic organs, and 2) repercussion on the insulin sensitivity of glucose metabolism. We used gene expression analyses, a combination of glucose tracer dilution and arteriovenous balance to quantify specific organ release, and hyperinsulinemic euglycemic clamps to assess EGP and glucose uptake. Protein feeding promoted a dramatic induction of the main regulatory gluconeogenic genes (glucose-6 phosphatase and phosphoenolpyruvate carboxykinase) in the kidney, but not in the liver. As a consequence, the kidney glucose release was markedly increased, compared with rats fed a normal starch diet. Protein feeding ameliorated the suppression of EGP by insulin and the sparing of glycogen storage in the liver but had no effect on glucose uptake. Combined with the previously reported induction of gluconeogenesis in the small intestine, the present work strongly suggests that a redistribution of glucose production among gluconeogenic organs might occur upon protein feeding. This phenomenon is in keeping with the improvement of insulin sensitivity of EGP, most likely involving the hepatic site. These data shed a new light on the improvement of glucose tolerance, previously observed upon increasing the amount of protein in the diet, in type 2 diabetic patients. Protein feeding increases kidney gluconeogenesis without increasing global endogenous glucose production, and improves insulin suppression of the latter, likely at the hepatic site.

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Involvement of liver and skeletal muscle in sucrose-induced insulin resistance: dose-response studies

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1994.266.5.r1637 ◽

1994 ◽

Vol 266 (5) ◽

pp. R1637-R1644 ◽

Cited By ~ 18

Author(s):

M. J. Pagliassotti ◽

K. A. Shahrokhi ◽

M. Moscarello

Keyword(s):

Insulin Resistance ◽

Skeletal Muscle ◽

Body Weight ◽

Dose Response ◽

Phosphoenolpyruvate Carboxykinase ◽

Glycogen Synthase ◽

Glucose Production ◽

Endogenous Glucose Production ◽

Male Rats ◽

Glycogen Accumulation

The ability of dietary sucrose to induce insulin resistance independent of changes in body weight is controversial. In the present study male rats were fed a high-starch (ST) diet (starch 68% of total kcal) ad libitum for 2 wk and then were fed equicalorically either the ST diet or a high-sucrose (SU) diet (sucrose 68% of total kcal) for 8 wk. Euglycemic, hyperinsulinemic (0, 1.2, 4.1, 8, 15 mU.kg-1.min-1, n = 6-8/group per dose) clamps were then used to establish dose-response relationships for glucose kinetics and metabolism. Body weight (513 +/- 3 g) and composition were similar between groups after the 8-wk dietary period. Glucose infusion rates (GIR; mg.kg-1.min-1) were significantly less in SU (0.9 +/- 5.8 +/- 0.6, 14.8 +/- 1.3, and 18 +/- 1.1) than in ST rats (4.1 +/- 0.9, 12.3 +/- 1.2, 22.6 +/- 1.5, and 25.9 +/- 1.8) at 1.2, 4.1, 8, and 15 mU.kg-1.min-1, respectively. Impaired suppression of endogenous glucose production accounted for 46, 43, 23, and 0% of the reduction in GIR in SU rats at 1.2, 4.1, 8, and 15 mU.kg-1.min-1, respectively. Despite basal hyperinsulinemia (38 +/- 2 microU/ml in SU vs. 26 +/- 2 microU/ml in ST rats), liver phosphoenolpyruvate carboxykinase (PEPCK) activity was 50% higher in SU than in ST rats and remained elevated in SU rats (by 30-40%) at the two lower insulin doses. No skeletal muscle glycogen accumulation occurred in SU rats at any of the insulin doses, and glycogen synthase I activity was significantly lower in SU rats at the two highest insulin doses.(ABSTRACT TRUNCATED AT 250 WORDS)

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The involvement of pyruvate cycling in the metabolism of aspartate and glycerate by the perfused rat kidney

Biochemical Journal ◽

10.1042/bj2370691 ◽

1986 ◽

Vol 237 (3) ◽

pp. 691-698 ◽

Cited By ~ 6

Author(s):

R C Scaduto ◽

E J Davis

Keyword(s):

Phosphoenolpyruvate Carboxykinase ◽

Rat Kidney ◽

Lactate Production ◽

Glucose Production ◽

Kidney Cortex ◽

Acid Oxidation ◽

Minor Effect ◽

Major Pathway ◽

Pyruvate Cycling ◽

Reducing Potential

The metabolism of glycerate and aspartate was investigated in perfused rat kidneys. The major pathway active for aspartate metabolism and NH3 production was found to include transamination, and not the purine nucleotide cycle. Pyruvate cycling was identified as a means by which reducing potential is generated in the cytosol for glucose and lactate production from these substrates. Inhibition of mitochondrial pyruvate transport caused an inhibition of glucose production, accumulation of lactate and pyruvate in the perfusate, and a decrease in the [lactate]/[pyruvate] ratio in kidneys perfused with aspartate. These data indicate a role of mitochondrial pyruvate transport in the provision of cytosolic reducing potential. With either aspartate or glycerate, 3-mercaptopicolinic acid (3-MPA) suppressed glucose synthesis and caused accumulation of malate plus fumarate within the kidney. Glucose production from glycerate was much less sensitive to the presence of 3-MPA than was glucose production from aspartate, illustrating a phosphoenolpyruvate carboxykinase (PEPCK)-independent pathway for the cycling of pyruvate. In aspartate-perfused kidneys, the presence of 3-MPA, at concentrations that completely blocked glucose accumulation in the perfusate, did not affect the rate of NH3 production and had only a minor effect on the rate of aspartate uptake. These data allow for an estimation of the rate of pyruvate formation from aspartate of about 1 mumol/min per kidney under conditions of complete PEPCK inhibition. Thus a PEPCK-independent pathway is operative for amino acid oxidation and pyruvate formation in perfused kidneys. The NADP-linked, but not the NAD-linked, ‘malic’ enzyme activity of the kidney cortex was found to be sufficient to catalyse this estimated rate of pyruvate formation.

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