Gluconeogenesis and its regulation

AMPK inhibits liver gluconeogenesis: fact or fiction?

Is there a role for AMPK in the control of hepatic gluconeogenesis and could targeting AMPK in liver be a viable strategy for treating type 2 diabetes? These are frequently asked questions this review tries to answer. After describing properties of AMPK and different small-molecule AMPK activators, we briefly review the various mechanisms for controlling hepatic glucose production, mainly via gluconeogenesis. The different experimental and genetic models that have been used to draw conclusions about the role of AMPK in the control of liver gluconeogenesis are critically discussed. The effects of several anti-diabetic drugs, particularly metformin, on hepatic gluconeogenesis are also considered. We conclude that the main effect of AMPK activation pertinent to the control of hepatic gluconeogenesis is to antagonize glucagon signalling in the short-term and, in the long-term, to improve insulin sensitivity by reducing hepatic lipid content.

PCK1 dysregulation in cancer: Metabolic reprogramming, oncogenic activation, and therapeutic opportunities

The last few decades have witnessed an advancement in our understanding of multiple cancer cell pathways related to metabolic reprogramming. One of the most important cancer hallmarks, including aerobic glycolysis (the Warburg effect), the central carbon pathway, and multiple-branch metabolic pathway remodeling, enables tumor growth, progression, and metastasis. Phosphoenolpyruvate carboxykinase 1 (PCK1), a key rate-limiting enzyme in gluconeogenesis, catalyzes the conversion of oxaloacetate to phosphoenolpyruvate. PCK1 expression in gluconeogenic tissues is tightly regulated during fasting. In tumor cells, PCK1 is regulated in a cell-autonomous manner rather than by hormones or nutrients in the extracellular environment. Interestingly, PCK1 has an anti-oncogenic role in gluconeogenic organs (the liver and kidneys), but a tumor-promoting role in cancers arising from non-gluconeogenic organs. Recent studies have revealed that PCK1 has metabolic and non-metabolic roles in multiple signaling networks linking metabolic and oncogenic pathways. Aberrant PCK1 expression results in the activation of oncogenic pathways, accompanied by metabolic reprogramming, to maintain tumorigenesis. In this review, we summarize the mechanisms underlying PCK1 expression and regulation, and clarify the crosstalk between aberrant PCK1 expression, metabolic rewiring, and signaling pathway activation. In addition, we highlight the clinical relevance of PCK1 and its value as a putative cancer therapeutic target.

Mcy protein, a potential antidiabetic agent: evaluation of carbohydrate metabolic enzymes and antioxidant status

The objective of the present study is to elucidate the long-term effects of anti-hyperglycemic active principle, Mcy protein (MCP), isolated from the fruits of Momordica cymbalaria on carbohydrate metabolism and oxidative stress in experimental diabetic rats. We used streptozotocin induced diabetic rats for the current studies. Our studies showed that MCP (2.5mg/kg.b.w) treatment significantly normalized the deranged activities of critical carbohydrate metabolizing enzymes, hexokinase, glucose-6-phosphate dehydrogenase, glucose-6-phosphatase and fructose-1,6-bis phosphatase. In addition MCP showed inhibitory activity on α-glucosidase and aldose reductase enzymes in in vitro assays. Further MCP treatment improved the antioxidant defensive mechanism by preventing deleterious oxidative products of cellular metabolism, which initiates the lipid peroxidation and by normalizing the antioxidant enzyme (catalase, superoxide dismutase, glutathione peroxidase) activities. Additional structural studies using circular dichroism spectroscopy indicate that MCP contains majorly α-helix. Our findings suggest MCP regulates blood glucose and better manage diabetes mellitus associated complications by regulating carbohydrate metabolism and by protecting from the deleterious effects of oxidative stress.

Riboflavin supplementation improves energy metabolism in mice exposed to acute hypoxia

This study investigated the effects of riboflavin on energy metabolism in hypoxic mice. Kunming mice were fed diets containing riboflavin at doses of 6, 12, 24 and 48 mg/kg, respectively for 2 weeks before exposure to a simulated altitude of 6000 m for 8 h. Changes of riboflavin status and energy metabolism were assessed biochemically. Simultaneously, a (1)H nuclear magnetic resonance (NMR) based metabolomic technique was used to track the changes of plasma metabolic profiling. It was found that the content of hepatic riboflavin was decreased and erythrocyte glutathione activation coefficient was elevated significantly under hypoxic condition. Meanwhile, increased plasma pyruvate, lactate, beta-hydroxybutyrate and urea, as well as decreased plasma carnitine were observed. Riboflavin supplementation improved riboflavin status remarkably in hypoxic mice and decreased plasma levels of pyruvate, free fatty acids and beta-hydroxybutyrate significantly. Plasma carnitine was increased in response to riboflavin supplementation. Results obtained from (1)H NMR analysis were basically in line with the data from biochemical assays and remarkable changes in plasma taurine, choline and some other metabolites were also indicated. It was concluded that riboflavin requirement was increased under acute hypoxic condition and riboflavin supplementation was effective in improving energy metabolism in hypoxic mice.

Gluconeogenesis and energy expenditure after a high-protein, carbohydrate-free diet

BACKGROUND

High-protein diets have been shown to increase energy expenditure (EE).

OBJECTIVE

The objective was to study whether a high-protein, carbohydrate-free diet (H diet) increases gluconeogenesis and whether this can explain the increase in EE.

DESIGN

Ten healthy men with a mean (+/-SEM) body mass index (in kg/m(2)) of 23.0 +/- 0.8 and age of 23 +/- 1 y received an isoenergetic H diet (H condition; 30%, 0%, and 70% of energy from protein, carbohydrate, and fat, respectively) or a normal-protein diet (N condition; 12%, 55%, and 33% of energy from protein, carbohydrate, and fat, respectively) for 1.5 d according to a randomized crossover design, and EE was measured in a respiration chamber. Endogenous glucose production (EGP) and fractional gluconeogenesis were measured via infusion of [6,6-(2)H(2)]glucose and ingestion of (2)H(2)O; absolute gluconeogenesis was calculated by multiplying fractional gluconeogenesis by EGP. Body glycogen stores were lowered at the start of the intervention with an exhaustive glycogen-lowering exercise test.

RESULTS

EGP was lower in the H condition than in the N condition (181 +/- 9 compared with 226 +/- 9 g/d; P < 0.001), whereas fractional gluconeogenesis was higher (0.95 +/- 0.04 compared with 0.64 +/- 0.03; P < 0.001) and absolute gluconeogenesis tended to be higher (171 +/- 10 compared with 145 +/- 10 g/d; P = 0.06) in the H condition than in the N condition. EE (resting metabolic rate) was greater in the H condition than in the N condition (8.46 +/- 0.23 compared with 8.12 +/- 0.31 MJ/d; P < 0.05). The increase in EE was a function of the increase in gluconeogenesis (DeltaEE = 0.007 x Deltagluconeogenesis - 0.038; r = 0.70, R(2) = 0.49, P < 0.05). The contribution of Deltagluconeogenesis to DeltaEE was 42%; the energy cost of gluconeogenesis was 33% (95% CI: 16%, 50%).

CONCLUSIONS

Forty-two percent of the increase in energy expenditure after the H diet was explained by the increase in gluconeogenesis. The cost of gluconeogenesis was 33% of the energy content of the produced glucose.

Training enhanced hepatic gluconeogenesis: the importance for glucose homeostasis during exercise

Endurance training has long been known to improve the individual's resistance to exercise-induced hypoglycemia. Traditionally attributed to a reduction in glucose uptake subsequent to enhanced fat oxidation, this issue has only recently been directly addressed. This paper briefly reviews the evidence for reduced glucose uptake versus enhanced glucose production in the improved hypoglycemic resistance following training. While whole body glucose removal and production may be reduced following training, this has only been demonstrated under exercising conditions in which glycemia demonstrates little deviation from rest. Under exercise conditions where untrained animals demonstrate substantial reductions in blood glucose, training enhanced hypoglycemic resistance has been shown to result entirely from enhanced glucose production via gluconeogenesis. Using the in situ perfused liver preparation, the authors have provided direct evidence for a training enhanced hepatic gluconeogenic capacity. The site of adaptation within the gluconeogenic pathway has now been constrained to below the level of the triose phosphates. Lack of evidence for suppressed skeletal muscle glucose uptake following training, a uniform observation for humans and rats, is also discussed. It is concluded that the improved hepatic gluconeogenic capacity of endurance trained individuals, at least in rats, is critical to their demonstrated resistance to exercise-induced hypoglycemia.

In vitro and in vivo suppression of gluconeogenesis by inhibition of pyruvate carboxylase

The mechanism of inhibition of gluconeogenesis by phenylalkanoic acids was studied in vitro and in vivo. In vitro production of 14CO2 from labeled glucose or palmitate was not inhibited at 4 mM, a concentration of phenylacetic acid that inhibited gluconeogenesis from lactate/pyruvate. In vitro studies with isolated mitochondria showed that the CoA ester of phenylacetic acid was formed. The parent phenylalkanoic acid had no effect on purified pyruvate carboxylase activity, but phenylacetyl CoA ester decreased pyruvate carboxylation in a concentration-dependent manner. Phenylacetic acid inhibited gluconeogenesis in isolated rat liver cells from 10 mM lactate/1 mM pyruvate (decreased 39%, P < 0.05), but not 10 mM L-glutamine or [14C]aspartate, showing that the inhibition of gluconeogenesis occurred at the level of pyruvate carboxylase. A 20 mg bolus with infusion of 1 mg/min of phenylpropionic acid decreased blood glucose levels of normal [110 +/- 12 to 66 +/- 11 mg/dL, N = 7, P < 0.05 (unpaired Student's t-test vs control)] and streptozocin diabetic rats [295 +/- 14 to 225 +/- 12 mg/dL, N = 7, P < 0.01 (paired t-test vs basal)]. Hepatic glucose production in control and diabetic rats was suppressed under conditions where liver glycogen was depleted, indicating that gluconeogenesis had been inhibited in vivo. The results suggest the possibility that the inappropriate overproduction of glucose can be controlled by inhibitors of pyruvate carboxylase. This class of inhibitors may be useful in the treatment of non-insulin-dependent diabetes mellitus.

Adaptations in glucose homeostasis during chronic nutritional deprivation in rats: hepatic resistance to both insulin and glucagon

The role of glucagon in glucose homeostasis during chronic malnutrition was studied in weanling-littermate rats either fed ad libitum or restricted to 60% of ad libitum intake for 8 weeks. Fasting glucose and insulin levels were lower in malnourished rats, and their response to glucagon (0.02 mg/kg intravenous [IV]) after a 16-hour fast was significantly less than in control littermates for both glucose (P = .039) and insulin (P = .008). During euglycemic glucose clamp studies at identical plasma glucose (PG) levels, insulin suppression of hepatic glucose production (HGP) was impaired in malnourished rats, indicating insulin resistance (mean +/- SE HGP: 48 +/- 5 v 32 +/- 10 mumol.kg-1.min-1 for controls, P = .028). Glucose disposal was not significantly different in the two groups. However, after IV glucagon, the increase in HGP was markedly impaired in malnourished rats (P = .0004), with the total amount of glucose produced by the liver over 15 minutes being 1,397 +/- 114 mumol/kg as compared with 2,031 +/- 118 in controls (P = .0047). The impaired response was not due to defective glycogenolysis, because the release of glucose from prelabeled glycogen in response to glucagon injection contributed only 6% to 8% of the overall increase in glucose output from the liver, and was not different in the two groups. Furthermore, liver glycogen stores were virtually exhausted after the 16-hour fast, without glucagon injection. Glucagon receptor affinity and number were not affected by malnutrition. It is concluded that (1) chronic malnutrition is associated with hepatic resistance to both insulin and glucagon, (2) the glucagon resistance is not due to impaired glycogenolysis, and (3) it is mediated by a postreceptor defect.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of protein supplementation on lactate accumulation during submaximal and maximal exercise

Eleven subjects performed a graded exercise test after 1 week of protein supplementation (PRO) or glucose polymer placebo (CON), randomly assigned in a double blind fashion. The exercise consisted of 3-min graded exercise bouts separated by 10 min of active recovery at zero pedal resistance. Subjects then performed a 30-sec Wingate test (WIN) to assess performance during supramaximal exercise. Blood samples were obtained in the last 15 sec of each exercise and recovery period. PRO resulted in a decrease in blood lactate following 120% VO2max and WIN, an increase in blood alanine at all time points, and lower postexercise muscle lactate and glycogen. Resting muscle GPT activity was 47% higher during the PRO trial. Mean power output during the WIN did not differ between PRO and CON. The WIN fatigue index was not significantly different between PRO and CON. The increased alanine may reflect increased transamination of pyruvate, thereby reducing the accumulation of lactate, which in turn had a marginal effect on performance during supramaximal exercise.

Cloning of a cDNA encoding phosphoenolpyruvate carboxykinase from Haemonchus contortus

Biochemical and metabolic data have led to the conclusion that the enzyme phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) contributes to a critical point of divergence in energy conservation pathways between mammals and nematodes. To facilitate the determination of the molecular basis for host vs parasite differences in PEPCK, we have cloned a cDNA encoding this enzyme from a parasitic nematode of ruminants, Haemonchus contortus. H. contortus PEPCK was cloned by functional complementation of a PEPCK-, malic enzyme- strain of Escherichia coli (E1786) using an egg stage H. contortus cDNA library in lambda ZAPII. Selection was for growth on malate as the sole carbon source (malate+ phenotype). We isolated a plasmid, pPEPCK, which reproducibly confers a malate+ phenotype in E1786. The sequence of the 2.0-kb EcoRI insert of pPEPCK predicts a 612-amino acid protein which shows about 74% similarity to Drosophila melanogaster and chicken PEPCK. Extracts of E1786[pPEPCK], but not E1786, contain IDP- or GDP-dependent PEPCK enzyme activity. Sequence analysis revealed that the open reading frame (ORF) in pPEPCK lacked a 5' initiation codon and was probably expressed as an in-frame fusion protein with beta-galactosidase. A strategy combining library screening with PCR analysis of positive clones led to the identification of a clone encoding 6 additional NH2-terminal amino acids, including a Met, which, by comparison with known PEPCK amino acid sequences, is likely to be the translation initiation site.

Effect of stress on glucoregulation in physiology and diabetes

To examine the glucoregulatory responses to stress and their impact on diabetes, we used the following models of stress: A) Hypoglycemia; B) Epinephrine infusion; C) intracerebroventricular (ICV) injection of carbachol, an analog of acetylcholine. A) Hypoglycemia induces release of all counterregulatory hormones. During acute hypoglycemia, glucose production increases initially mainly due to glucagon release but eventually also due to a very large increment in catecholamines. In newborn dogs, neither epinephrine nor glucagon respond to a decrease in plasma glucose. This lack of a safeguard against hypoglycemia may indicate that the brain in pups is less dependent on a normal supply of glucose as a fuel, than in adult dogs. Counterregulation is enhanced when the effects of endogenous opiates are blocked by naloxone, indicating that endogenous opiates play a regulatory role during hypoglycemia. However, beta-endorphins which can be released with epinephrine during various stress situations, potentiate the peripheral effect of epinephrine. Glucoregulatory responses, even to slight changes in plasma glucose, are greatly enhanced during glucocorticoid treatment. This apparently reflects the greater sensitivity of the liver to glucagon. In diabetic dogs, similar to human diabetics, the glucagon response is abolished and the response of the catecholamines is partially decreased. On the basis of histological studies, we proposed that the deficient glucagon response in diabetes could be related to an increase in the somatostatin-glucagon ratio in the diabetic pancreas. This ratio is further augmented when normoglycemia is maintained with insulin. In response to a decrease in plasma glucose, there is a biphasic increment in glucose production in normal dogs, which is missing in diabetes. When normoglycemia is restored in diabetic dogs with phlorizin treatment, the second but not the first increment in glucose production is restored. We postulated, therefore, that the toxic effect of hyperglycemia, in addition to the lack of glucagon response, is the main reason why in diabetes, glucose production cannot respond promptly to a decrease in plasma glucose. The low rate of metabolic clearance of glucose seen in diabetes in the post-absorptive state, also reflects, at least in part, the toxic effect of glucose, because with acute normalization of glucose with phlorizin, metabolic glucose clearance substantially improves. Hyperglycemia is the main reason for the decreased number of glucose transporters in diabetic muscle. B) Epinephrine infusion in normal dogs mimics some effects of stress, in that it increases glucose production, inhibits metabolic glucose clearance and increases lipolysis. These metabolic effects of epinephrine are independent of glucagon release.(ABSTRACT TRUNCATED AT 400 WORDS)

Food deprivation alters liver glycogen metabolism and endocrine responses to hemorrhage

Liver glycogen content, blood glucose, insulin, glucagon, and epinephrine were determined during 1 h hemorrhagic hypotension at 60 mmHg and 23 h thereafter in fed and two groups of 24-h food-deprived rats receiving either no infusion or 30% glucose intravenously during hemorrhage. Liver glycogen content was reduced by greater than 90% after 24-h food deprivation. Fed and food-deprived rats given glucose developed similar and substantial elevations of blood glucose during hemorrhage, whereas changes in blood glucose were modest in food-deprived rats given no infusion. In fed rats, liver glycogen was reduced by 60% during the 1-h bleed, but within 2 h after hemorrhage repletion of liver glycogen content commenced. By 6 h, approximately 75% of the glycogen lost during hemorrhage had been restored, and 23 h after hemorrhage liver glycogen content was six times greater compared with nonbled controls. Although glycogen levels increased after hemorrhage in food-deprived animals, the increase was negligible compared with that found in fed rats. Infusion of glucose during hemorrhage or adrenergic blockade after hemorrhage did not alter glycogen repletion in food-deprived rats. Posthemorrhage fed animals had high levels of insulin, glucagon, and epinephrine during hemorrhage, whereas insulin levels remained low in food-deprived rats despite exogenously induced hyperglycemia. It is concluded that rapid and substantial glycogen repletion can occur even immediately poststress. The conditions seem to be related to the nutritional state at the time of the insult.