Effects of dietary protein on lipid metabolism in high fructose fed humans.

BACKGROUND & AIMS: It has been reported that a high protein diet improves insulin sensitivity and reduces ectopic lipids in animals and humans with the metabolic syndrome. We therefore tested the hypothesis that a high dietary protein content may stimulate whole body lipid oxidation and alter post-prandial triglyceride (TG) after fructose ingestion. METHODS: The post-prandial metabolism of 8 young males was studied after two 6-day periods of hyper-energetic, high fructose diet (HiFruD), and after two 6-day periods of hyper-energetic high fructose high protein diet (HiFruHiProD). The order with which these periods were applied was randomized. At the end of each period, either a low protein, (13)C fructose test meal (Fru meal) or a high protein, (13)C fructose test meal (HiPro Fru meal) was administered. This resulted in the monitoring of metabolic parameters at 4 occasions in random order: a) with Fru meal ingested after HiFruD, b) with HiPro Fru meal ingested after HiFruD, c) with Fru meal ingested after HiFruHiProD or d) with HiPro Fru meal ingested after HiFruHiProD. On each occasion, post-prandial TG concentrations were monitored, energy expenditure and substrate metabolism were measured by indirect calorimetry, and fructose-induced gluconeogenesis was evaluated by measuring plasma (13)C-labeled glucose. RESULTS: TG responses to fructose ingestion were significantly higher after a hyper-energetic HiFruHiProD and after HiPro Fru meals than after a Fru meal ingested after a hyper-energetic HiFruD. Compared to low protein meals, high protein meals increased post-prandial energy expenditure, inhibited post-prandial lipid oxidation, and enhanced fructose-induced gluconeogenesis. These effects were similar with HiFruD and HiFruHiProD. CONCLUSIONS: Dietary proteins did not increase lipid oxidation and increased fructose-induced post-prandial TG in healthy humans fed an hyper-energetic, high fructose diet.

Cloning, functional expression, and chromosomal localization of the human pancreatic islet glucose-dependent insulinotropic polypeptide receptor.

Glucose-dependent insulinotropic polypeptide (GIP) is a hormone secreted by the endocrine K-cells from the duodenum that stimulates glucose-induced insulin secretion. Here, we present the molecular characterization of the human pancreatic islet GIP receptor. cDNA clones for the GIP receptor were isolated from a human pancreatic islet cDNA library. They encoded two different forms of the receptor, which differed by a 27-amino acid insertion in the COOH-terminal cytoplasmic tail. The receptor protein sequence was 81% identical to that of the rat GIP receptor. When expressed in Chinese hamster lung fibroblasts, both forms of the receptor displayed high-affinity binding for GIP (180 and 600 pmol/l). GIP binding was displaced by < 20% by 1 mumol/l glucagon, glucagon-like peptide (GLP-I)(7-36) amide, vasoactive intestinal peptide, and secretin. However exendin-4 and exendin-(9-39) at 1 mumol/l displaced binding by approximately 70 and approximately 100% at 10 mumol/l. GIP binding to both forms of the receptor induced a dose-dependent increase in intracellular cAMP levels (EC50 values of 0.6-0.8 nmol/l) but no elevation of cytoplasmic calcium concentrations. Interestingly, both exendin-4 and exendin-(9-39) were antagonists of the receptor, inhibiting GIP-induced cAMP formation by up to 60% when present at a concentration of 10 mumol/l. Finally, the physical and genetic chromosomal localization of the receptor gene was determined to be on 19q13.3, close to the ApoC2 gene. These data will help study the physiology and pathophysiology of the human GIP receptor.

Decreased glucose-induced thermogenesis after weight loss in obese subjects: a predisposing factor for relapse of obesity?

Glucose-induced thermogenesis (GIT) after a 100-g oral glucose load was measured by continuous indirect calorimetry in 32 nondiabetic and diabetic obese subjects and compared to 17 young and 13 middle aged control subjects. The obese subjects were divided into three groups: A (n = 12) normal glucose tolerance, B (n = 13) impaired glucose tolerance, and C (n = 7) diabetics, and were studied before and after a body weight loss ranging from 9.6 to 33.5 kg consecutive to a 4 to 6 months hypocaloric diet. GIT, measured over 3 h and expressed as percentage of the energy content of the load, was significantly reduced in obese groups A and C (6.2 +/- 0.6, and 3.8 +/- 0.7%, respectively) when compared to their age-matched control groups: 8.6 +/- 0.7 (young) and 5.8 +/- 0.3% (middle aged). Obese group B had a GIT of 6.1 +/- 0.6% which was lower than that of the young control group but not different from the middle-aged control group. After weight loss, GIT in the obese was further reduced in groups A and B than before weight loss: ie, 3.4 +/- 0.6 (p less than 0.001), 3.7 +/- 0.5 (p less than 0.01) respectively, whereas in group C, weight loss induced no further diminution in GIT (3.8 +/- 0.6%). These results support the concept of a thermogenic defect after glucose ingestion in obese individuals which is not the consequence of their excess body weight but may be one of the factors favoring the relapse of obesity after weight loss.

Effect of amino acids and glucose on exercise-induced gut and skeletal muscle proteolysis in dogs.

The effect of amino acid and/or glucose administration before and during exercise on protein metabolism in visceral tissues and skeletal muscle was examined in mongrel dogs. The dogs were subjected to treadmill running (150 minutes at 10 km/h and 12% incline) and intravenously infused with a solution containing amino acids and glucose (AAG), amino acids (AA), glucose (G) or saline (S) in randomized order. The infusion was started 60 minutes before exercise and continued until the end of the exercise period. An arteriovenous-difference technique was used to estimate both tissue protein degradation and synthesis. When S was infused, the release of leucine (Leu) from the gut and phenylalanine (Phe) from the hindlimb significantly increased during exercise, thus indicating that exercise augmented proteolysis in these tissues. The balance of Leu across the gut during exercise demonstrated a net uptake with both AAG and AA, whereas a net release was observed for G and S. In addition, Leu uptake in the gut during the last 90 minutes of the exercise period tended to be greater with AAG versus AA (P = .06). Phe balance across the hindlimb during the late exercise period showed a significant release with S, AA, and G, whereas the balance with AAG did not show a significant release. These results suggest that exercise-induced proteolysis in the gut may be reduced by supplementation with AA, and this effect may be enhanced by concomitant G administration. However, in skeletal muscle, both AA and G may be required to prevent net protein degradation during exercise. G provided without AA did not achieve net protein synthesis in either tissue.

Unaltered glucose-induced thermogenesis in Graves' disease.

The thermogenic response to a 100-g oral glucose challenge was studied in 12 patients with Graves' disease using continuous indirect calorimetry. Seven hyperthyroid patients were reinvestigated under the same experimental conditions after medical therapy. The mean net increase in energy expenditure (delta EE) following the glucose challenge was the same in hyperthyroid patients as compared to a control group (delta EE = +0.15 +/- 0.02 and 0.15 +/- 0.01 kcal/min, respectively) and the treated patients (delta EE = +0.11 +/- 0.03 kcal/min ns). When expressed as a percentage of the energy content of the glucose challenge, the mean glucose induced thermogenesis was similar in all three groups: 7.0 +/- 1.0%, 7.4 +/- 0.5%, and 5.5 +/- 1.3% in hyperthyroid, control subjects, and treated patients, respectively. It is concluded that the high energy requirement of hyperthyroid patients is due primarily to an elevated resting energy expenditure. The postprandial thermogenesis in itself does not contribute to the elevated fuel utilization in Graves' disease.

Different transcriptional control of metabolism and extracellular matrix in visceral and subcutaneous fat of obese and rimonabant treated mice.

BACKGROUND: The visceral (VAT) and subcutaneous (SCAT) adipose tissues play different roles in physiology and obesity. The molecular mechanisms underlying their expansion in obesity and following body weight reduction are poorly defined. METHODOLOGY: C57Bl/6 mice fed a high fat diet (HFD) for 6 months developed low, medium, or high body weight as compared to normal chow fed mice. Mice from each groups were then treated with the cannabinoid receptor 1 antagonist rimonabant or vehicle for 24 days to normalize their body weight. Transcriptomic data for visceral and subcutaneous adipose tissues from each group of mice were obtained and analyzed to identify: i) genes regulated by HFD irrespective of body weight, ii) genes whose expression correlated with body weight, iii) the biological processes activated in each tissue using gene set enrichment analysis (GSEA), iv) the transcriptional programs affected by rimonabant. PRINCIPAL FINDINGS: In VAT, "metabolic" genes encoding enzymes for lipid and steroid biosynthesis and glucose catabolism were down-regulated irrespective of body weight whereas "structure" genes controlling cell architecture and tissue remodeling had expression levels correlated with body weight. In SCAT, the identified "metabolic" and "structure" genes were mostly different from those identified in VAT and were regulated irrespective of body weight. GSEA indicated active adipogenesis in both tissues but a more prominent involvement of tissue stroma in VAT than in SCAT. Rimonabant treatment normalized most gene expression but further reduced oxidative phosphorylation gene expression in SCAT but not in VAT. CONCLUSION: VAT and SCAT show strikingly different gene expression programs in response to high fat diet and rimonabant treatment. Our results may lead to identification of therapeutic targets acting on specific fat depots to control obesity.

Deciphering neuron-glia compartmentalization in cortical energy metabolism.

Energy demand is an important constraint on neural signaling. Several methods have been proposed to assess the energy budget of the brain based on a bottom-up approach in which the energy demand of individual biophysical processes are first estimated independently and then summed up to compute the brain's total energy budget. Here, we address this question using a novel approach that makes use of published datasets that reported average cerebral glucose and oxygen utilization in humans and rodents during different activation states. Our approach allows us (1) to decipher neuron-glia compartmentalization in energy metabolism and (2) to compute a precise state-dependent energy budget for the brain. Under the assumption that the fraction of energy used for signaling is proportional to the cycling of neurotransmitters, we find that in the activated state, most of the energy ( approximately 80%) is oxidatively produced and consumed by neurons to support neuron-to-neuron signaling. Glial cells, while only contributing for a small fraction to energy production ( approximately 6%), actually take up a significant fraction of glucose (50% or more) from the blood and provide neurons with glucose-derived energy substrates. Our results suggest that glycolysis occurs for a significant part in astrocytes whereas most of the oxygen is utilized in neurons. As a consequence, a transfer of glucose-derived metabolites from glial cells to neurons has to take place. Furthermore, we find that the amplitude of this transfer is correlated to (1) the activity level of the brain; the larger the activity, the more metabolites are shuttled from glia to neurons and (2) the oxidative activity in astrocytes; with higher glial pyruvate metabolism, less metabolites are shuttled from glia to neurons. While some of the details of a bottom-up biophysical approach have to be simplified, our method allows for a straightforward assessment of the brain's energy budget from macroscopic measurements with minimal underlying assumptions.

Brain glucose sensing and neural regulation of insulin and glucagon secretion.

Glucose homeostasis requires the tight regulation of glucose utilization by liver, muscle and white or brown fat, and glucose production and release in the blood by liver. The major goal of maintaining glycemia at ∼ 5 mM is to ensure a sufficient flux of glucose to the brain, which depends mostly on this nutrient as a source of metabolic energy. This homeostatic process is controlled by hormones, mainly glucagon and insulin, and by autonomic nervous activities that control the metabolic state of liver, muscle and fat tissue but also the secretory activity of the endocrine pancreas. Activation or inhibition of the sympathetic or parasympathetic branches of the autonomic nervous systems are controlled by glucose-excited or glucose-inhibited neurons located at different anatomical sites, mainly in the brainstem and the hypothalamus. Activation of these neurons by hyper- or hypoglycemia represents a critical aspect of the control of glucose homeostasis, and loss of glucose sensing by these cells as well as by pancreatic β-cells is a hallmark of type 2 diabetes. In this article, aspects of the brain-endocrine pancreas axis are reviewed, highlighting the importance of central glucose sensing in the control of counterregulation to hypoglycemia but also mentioning the role of the neural control in β-cell mass and function. Overall, the conclusions of these studies is that impaired glucose homeostasis, such as associated with type 2 diabetes, but also defective counterregulation to hypoglycemia, may be caused by initial defects in glucose sensing.

Growth and energy metabolism of Nile tilapia juveniles fed glycerol

The objective of this work was to evaluate the effect of inclusion of dietary glycerol in replacement to starch on the growth and energy metabolism of Nile tilapia juveniles. The experiment was carried out in a completely randomized design with four treatments (0, 5, 10, and 15% purified glycerol) and six replicates. Pelleted, isonitrogenous, and isocaloric diets were provided for 60 days. Growth performance parameters and muscle glucose and protein concentrations were not affected by dietary glycerol levels. The treatment with 15% glycerol presented higher levels of muscle and liver triglycerides. A quadratic effect of treatments on muscle and liver triglyceride concentrations was observed. The treatment with 0% glycerol presented higher hepatic glucose levels than the one with 15%. Treatments did not differ for concentrations of liver protein, as well as of plasma glucose, triglycerides, and protein. Treatments with 10 and 15% glycerol showed higher activity of the glucose-6-phosphate-dehydrogenase enzyme than the treatment with 5%; however, there were no significant differences in the hepatic activities of the malic and glycerol kinase enzymes. A linear positive effect of treatments was observed on the activity of the glycerol kinase enzyme in liver. Levels of glycerol inclusion above 10% in the diet of Nile tilapia juveniles characterize it as a lipogenic nutrient.

Glutamate reduces glucose utilization while concomitantly enhancing AQP9 and MCT2 expression in cultured rat hippocampal neurons.

The excitatory neurotransmitter glutamate has been reported to have a major impact on brain energy metabolism. Using primary cultures of rat hippocampal neurons, we observed that glutamate reduces glucose utilization in this cell type, suggesting alteration in mitochondrial oxidative metabolism. The aquaglyceroporin AQP9 and the monocarboxylate transporter MCT2, two transporters for oxidative energy substrates, appear to be present in mitochondria of these neurons. Moreover, they not only co-localize but they interact with each other as they were found to co-immunoprecipitate from hippocampal neuron homogenates. Exposure of cultured hippocampal neurons to glutamate 100 μM for 1 h led to enhanced expression of both AQP9 and MCT2 at the protein level without any significant change at the mRNA level. In parallel, a similar increase in the protein expression of LDHA was evidenced without an effect on the mRNA level. These data suggest that glutamate exerts an influence on neuronal energy metabolism likely through a regulation of the expression of some key mitochondrial proteins.

Thermogenesis in obese women: effect of fructose vs. glucose added to a meal.

To assess the effect of a fructose meal on resting energy expenditure (EE), indirect calorimetry was used in 23 women (10 lean and 13 obese) for 30 min before and 6 h after the ingestion of a mixed meal containing 20% protein, 33% fat, and either 75 g glucose or 75 g fructose as carbohydrate source (47%). Expressed as a percentage of the energy content of the meal, the thermogenic response to the fructose meal was significantly greater (10.2 +/- 0.5%) than that of the glucose meal (8.4 +/- 0.4%, P less than 0.01). This difference was still apparent when the lean and obese women were considered separately. The mean respiratory quotient during the 6-h postprandial period was significantly greater (P less than 0.01) for the fructose (0.85 +/- 0.01) than for the glucose meal (0.83 +/- 0.01) in the combined subjects. In addition, cumulative carbohydrate oxidation was significantly greater after the fructose than after the glucose meal (51.1 +/- 2.3 vs. 40.9 +/- 2.0 g/6 h, respectively, P less than 0.01). Only small changes were observed in postprandial plasma levels of glucose and insulin after the fructose meal, but the plasma levels of lactate increased more with fructose than with the glucose meal. These results suggest that there might be some advantages (higher thermogenesis and carbohydrate oxidations) in using fructose as part of the carbohydrate source in diet of people with obesity and/or insulin resistance.

Brain energy metabolism measured by (13) C magnetic resonance spectroscopy in vivo upon infusion of [3-(13) C]lactate.

The brain uses lactate produced by glycolysis as an energy source. How lactate originated from the blood stream is used to fuel brain metabolism is not clear. The current study measures brain metabolic fluxes and estimates the amount of pyruvate that becomes labeled in glial and neuronal compartments upon infusion of [3-(13) C]lactate. For that, labeling incorporation into carbons of glutamate and glutamine was measured by (13) C magnetic resonance spectroscopy at 14.1 T and analyzed with a two-compartment model of brain metabolism to estimate rates of mitochondrial oxidation, glial pyruvate carboxylation, and the glutamate-glutamine cycle as well as pyruvate fractional enrichments. Extracerebral lactate at supraphysiological levels contributes at least two-fold more to replenish the neuronal than the glial pyruvate pools. The rates of mitochondrial oxidation in neurons and glia, pyruvate carboxylase, and glutamate-glutamine cycles were similar to those estimated by administration of (13) C-enriched glucose, the main fuel of brain energy metabolism. These results are in agreement with primary utilization of exogenous lactate in neurons rather than astrocytes. © 2014 Wiley Periodicals, Inc.

A Cellular Perspective on Brain Energy Metabolism and Functional Imaging.

The energy demands of the brain are high: they account for at least 20% of the body's energy consumption. Evolutionary studies indicate that the emergence of higher cognitive functions in humans is associated with an increased glucose utilization and expression of energy metabolism genes. Functional brain imaging techniques such as fMRI and PET, which are widely used in human neuroscience studies, detect signals that monitor energy delivery and use in register with neuronal activity. Recent technological advances in metabolic studies with cellular resolution have afforded decisive insights into the understanding of the cellular and molecular bases of the coupling between neuronal activity and energy metabolism and point at a key role of neuron-astrocyte metabolic interactions. This article reviews some of the most salient features emerging from recent studies and aims at providing an integration of brain energy metabolism across resolution scales.

Liver Glucokinase A456V induces potent hypoglycemia without dyslipidemia through a paradoxical induction of the catalytic subunit of glucose-6-phosphatase

Recent reports point out the importance of the complex GK-GKRP in controlling glucose and lipid homeostasis. Several GK mutations affect GKRP binding, resulting in permanent activation of the enzyme. We hypothesize that hepatic overexpression of a mutated form of GK, GKA456V, described in a patient with persistent hyperinsulinemic hypoglycemia of infancy (PHHI) and could provide a model to study the consequences of GK-GKRP deregulation in vivo. GKA456V was overexpressed in the liver of streptozotocin diabetic mice. Metabolite profiling in serum and liver extracts, together with changes in key components of glucose and lipid homeostasis, were analyzed and compared to GK wild-type transfected livers. Cell compartmentalization of the mutant but not the wild-type GK was clearly affected in vivo, demonstrating impaired GKRP regulation. GKA456V overexpression markedly reduced blood glucose in the absence of dyslipidemia, in contrast to wild-type GK-overexpressing mice. Evidence in glucose utilization did not correlate with increased glycogen nor lactate levels in the liver. PEPCK mRNA was not affected, whereas the mRNA for the catalytic subunit of glucose-6-phosphatase was upregulated ~4 folds in the liver of GKA456V-treated animals, suggesting that glucose cycling was stimulated. Our results provide new insights into the complex GK regulatory network and validate liver-specific GK activation as a strategy for diabetes therapy.

Brain glucose sensing in homeostatic and hedonic regulation.

Glucose homeostasis as well as homeostatic and hedonic control of feeding is regulated by hormonal, neuronal, and nutrient-related cues. Glucose, besides its role as a source of metabolic energy, is an important signal controlling hormone secretion and neuronal activity, hence contributing to whole-body metabolic integration in coordination with feeding control. Brain glucose sensing plays a key, but insufficiently explored, role in these metabolic and behavioral controls, which when deregulated may contribute to the development of obesity and diabetes. The recent introduction of innovative transgenic, pharmacogenetic, and optogenetic techniques allows unprecedented analysis of the complexity of central glucose sensing at the molecular, cellular, and neuronal circuit levels, which will lead to a new understanding of the pathogenesis of metabolic diseases.