Glycerol not lactate is the major net carbon source for gluconeogenesis in mice during both short and prolonged fasting

An Organism-Level Quantitative Flux Model of Energy Metabolism in Mice

Mammalian tissues feed on nutrients in the blood circulation. At the organism-level, mammalian energy metabolism comprises of oxidation, storage, interconverting, and releasing of circulating nutrients. Though much is known about the individual processes and nutrients, a holistic and quantitative model describing these processes for all major circulating nutrients is lacking. Here, by integrating isotope tracer infusion, mass spectrometry, and isotope gas analyzer measurement, we developed a framework to systematically quantify fluxes through these metabolic processes for 10 major circulating energy nutrients in mice, resulting in an organism-level quantitative flux model of energy metabolism. This model revealed in wildtype mice that circulating nutrients have more dominant metabolic cycling fluxes than their oxidation fluxes, with distinct partition between cycling and oxidation flux for individual circulating nutrients. Applications of this framework in obese mouse models showed on a per animal basis extensive elevation of metabolic cycling fluxes in ob/ob mice, but not in diet-induced obese mice. Thus, our framework describes quantitatively the functioning of energy metabolism at the organism-level, valuable for revealing new features of energy metabolism in physiological and disease conditions.

Highlights

A flux model of energy metabolism integrating 13 C labeling of metabolites and CO 2 Circulating nutrients have characteristic partition between oxidation and storageCirculating nutrients' total cycling flux outweighs their total oxidation fluxCycling fluxes are extensively elevated in ob/ob but not in diet-induced obese mice.

Control of Physiologic Glucose Homeostasis via the Hypothalamic Modulation of Gluconeogenic Substrate Availability

The brain augments glucose production during fasting, but the mechanisms are poorly understood. Here, we show that Cckbr-expressing neurons in the ventromedial hypothalamic nucleus (VMNCckbr cells) prevent low blood glucose during fasting through sympathetic nervous system (SNS)-mediated augmentation of adipose tissue lipolysis and substrate release. Activating VMNCckbr neurons mobilized gluconeogenic substrates without altering glycogenolysis or gluconeogenic enzyme expression. Silencing these cells (CckbrTetTox animals) reduced fasting blood glucose, impaired lipolysis, and decreased circulating glycerol (but not other gluconeogenic substrates) despite normal insulin, counterregulatory hormones, liver glycogen, and liver gluconeogenic gene expression. Furthermore, β3-adrenergic adipose tissue stimulation in CckbrTetTox animals restored lipolysis and blood glucose. Hence, VMNCckbr neurons impact blood glucose not by controlling islet or liver physiology, but rather by mobilizing gluconeogenic substrates. These findings establish a central role for hypothalamic and SNS signaling during normal glucose homeostasis and highlight the importance of gluconeogenic substrate mobilization during physiologic fasting.

Comprehensive quantification of metabolic flux during acute cold stress in mice

Cold-induced thermogenesis (CIT) is widely studied as a potential avenue to treat obesity, but a thorough understanding of the metabolic changes driving CIT is lacking. Here, we present a comprehensive and quantitative analysis of the metabolic response to acute cold exposure, leveraging metabolomic profiling and minimally perturbative isotope tracing studies in unanesthetized mice. During cold exposure, brown adipose tissue (BAT) primarily fueled the tricarboxylic acid (TCA) cycle with fat in fasted mice and glucose in fed mice, underscoring BAT's metabolic flexibility. BAT minimally used branched-chain amino acids or ketones, which were instead avidly consumed by muscle during cold exposure. Surprisingly, isotopic labeling analyses revealed that BAT uses glucose largely for TCA anaplerosis via pyruvate carboxylation. Finally, we find that cold-induced hepatic gluconeogenesis is critical for CIT during fasting, demonstrating a key functional role for glucose metabolism. Together, these findings provide a detailed map of the metabolic rewiring driving acute CIT.

Measurement of gluconeogenesis by 2H2O labeling and mass isotopomer distribution analysis

The gluconeogenesis pathway, which converts nonsugar molecules into glucose, is critical for maintaining glucose homeostasis. Techniques that measure flux through this pathway are invaluable for studying metabolic diseases such as diabetes that are associated with dysregulation of this pathway. We introduce a new method that measures fractional gluconeogenesis by heavy water labeling and gas chromatographic-mass spectrometric analysis. This technique circumvents cumbersome benchwork or inference of positionality from mass spectra. The enrichment and pattern of deuterium label on glucose is quantified by use of mass isotopomer distribution analysis, which informs on how much of glucose-6-phosphate-derived glucose comes from the gluconeogenesis (GNG) pathway. We use an in vivo model of the GNG pathway that is based on previously published models but offers a new approach to calculating GNG pathway and subpathway contributions using combinatorial probabilities. We demonstrated that this method accurately quantifies fractional GNG through experiments that perturb flux through the pathway and by probing analytical sensitivity. While this method was developed in mice, the results suggest that it is translatable to humans in a clinical setting.

Gluconeogenesis in the Yolk Sac Membrane: Enzyme Activity, Gene Expression, and Metabolites During Layer Chicken Development

Yolk sac membranes of layer eggs were collected daily (n = 7-9) from day three of incubation to day three post-hatch, and mRNA expression and activities were quantified for key gluconeogenesis enzymes (glucose-6-phosphatase, fructose-1,6-bisphosphatase, cytosolic and mitochondrial phosphoenolpyruvate carboxykinases, and pyruvate carboxylase). Lactate, triglycerides, non-esterified fatty acids, glycogen, and glucose in the yolk sac membrane, and blood glucose levels were also measured. The mRNA expression and activity were detected for all enzymes. Differences in expression levels and enzyme activities seemed to reflect the embryo's developmental environment and physiological demands at different developmental stages. During the first week to the mid-second week of incubation, the expression and activity of gluconeogenic enzymes and lactate concentrations were high, suggesting an active period of gluconeogenesis from lactate, reflecting possible hypoxia in the embryo before completed formation of the chorioallantoic capillaries. From the mid-second week to mid-third week, when embryos were in an aerobic state, the triglyceride and non-esterified fatty acid contents increased in the yolk sac. Triglycerides from yolk lipids are typically hydrolyzed to produce non-esterified fatty acids as an energy source, whereas the glycerol skeleton is used for gluconeogenesis. In the late third week, when embryos were considered to re-enter an anaerobic state, the mRNA expression and enzyme activity of only glucose-6-phosphatase were high and the amount of glycogen in the yolk sac was reduced. Therefore, it is suggested that gluconeogenesis activity is low during this period, and the carbohydrates stored in the yolk sac membrane are secreted into the blood as energy for hatching. This study confirmed the role of the yolk sac membrane as a vital gluconeogenic organ during chicken egg incubation.

Roles of malate and aspartate in gluconeogenesis in various physiological and pathological states

Gluconeogenesis, a pathway for glucose synthesis from non-carbohydrate substances, begins with the synthesis of oxaloacetate (OA) from pyruvate and intermediates of citric acid cycle in hepatocyte mitochondria. The traditional view is that OA does not cross the mitochondrial membrane and must be shuttled to the cytosol, where most enzymes involved in gluconeogenesis are compartmentalized, in the form of malate. Thus, the possibility of transporting OA in the form of aspartate has been ignored. In the article is shown that malate supply to the cytosol increases only when fatty acid oxidation in the liver is activated, such as during starvation or untreated diabetes. Alternatively, aspartate synthesized from OA by mitochondrial aspartate aminotransferase (AST) is transported to the cytosol in exchange for glutamate via the aspartate-glutamate carrier 2 (AGC2). If the main substrate for gluconeogenesis is an amino acid, aspartate is converted to OA via urea cycle, therefore, ammonia detoxification and gluconeogenesis are simultaneously activated. If the main substrate is lactate, OA is synthesized by cytosolic AST, glutamate is transported to the mitochondria through AGC2, and nitrogen is not lost. It is concluded that, compared to malate, aspartate is a more suitable form of OA transport from the mitochondria for gluconeogenesis.

The SLC25A47 locus controls gluconeogenesis and energy expenditure

Mitochondria provide essential metabolites and adenosine triphosphate (ATP) for the regulation of energy homeostasis. For instance, liver mitochondria are a vital source of gluconeogenic precursors under a fasted state. However, the regulatory mechanisms at the level of mitochondrial membrane transport are not fully understood. Here, we report that a liver-specific mitochondrial inner-membrane carrier SLC25A47 is required for hepatic gluconeogenesis and energy homeostasis. Genome-wide association studies found significant associations between SLC25A47 and fasting glucose, HbA1c, and cholesterol levels in humans. In mice, we demonstrated that liver-specific depletion of SLC25A47 impaired hepatic gluconeogenesis selectively from lactate, while significantly enhancing whole-body energy expenditure and the hepatic expression of FGF21. These metabolic changes were not a consequence of general liver dysfunction because acute SLC25A47 depletion in adult mice was sufficient to enhance hepatic FGF21 production, pyruvate tolerance, and insulin tolerance independent of liver damage and mitochondrial dysfunction. Mechanistically, SLC25A47 depletion leads to impaired hepatic pyruvate flux and malate accumulation in the mitochondria, thereby restricting hepatic gluconeogenesis. Together, the present study identified a crucial node in the liver mitochondria that regulates fasting-induced gluconeogenesis and energy homeostasis.

Whole grains-derived functional ingredients against hyperglycemia: targeting hepatic glucose metabolism

Type 2 diabetes mellitus (T2DM) is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycemia. However, concerns have been raised about the safety and efficacy of current hypoglycemic drugs due to undesirable side effects. Increasing studies have shown that whole grains (WG) consumption is inversely associated with the risk of T2DM and its subsequent complications. Thus, dietary strategies involving functional components from the WG provide an intriguing approach to restoring and maintaining glucose homeostasis. This review provides a comprehensive understanding of the major functional components derived from WG and their positive effects on glucose homeostasis, demonstrates the underlying molecular mechanisms targeting hepatic glucose metabolism, and discusses the unclear aspects according to the latest viewpoints and current research. Improved glycemic response and insulin resistance were observed after consumption of WG-derived bioactive ingredients, which are involved in the integrated, multi-factorial, multi-targeted regulation of hepatic glucose metabolism. Promotion of glucose uptake, glycolysis, and glycogen synthesis pathways, while inhibition of gluconeogenesis, contributes to amelioration of abnormal hepatic glucose metabolism and insulin resistance by bioactive components. Hence, the development of WG-based functional food ingredients with potent hypoglycemic properties is necessary to manage insulin resistance and T2DM.

Systematic diet composition swap in a mouse genome-scale metabolic model reveals determinants of obesogenic diet metabolism in liver cancer

Dietary nutrient availability and gene expression, together, influence tissue metabolic activity. Here, we explore whether altering dietary nutrient composition in the context of mouse liver cancer suffices to overcome chronic gene expression changes that arise from tumorigenesis and western-style diet (WD). We construct a mouse genome-scale metabolic model and estimate metabolic fluxes in liver tumors and non-tumoral tissue after computationally varying the composition of input diet. This approach, called Systematic Diet Composition Swap (SyDiCoS), revealed that, compared to a control diet, WD increases production of glycerol and succinate irrespective of specific tissue gene expression patterns. Conversely, differences in fatty acid utilization pathways between tumor and non-tumor liver are amplified with WD by both dietary carbohydrates and lipids together. Our data suggest that combined dietary component modifications may be required to normalize the distinctive metabolic patterns that underlie selective targeting of tumor metabolism.

Glucagon changes substrate preference in gluconeogenesis

Fasting hyperglycemia in diabetes mellitus is caused by unregulated glucagon secretion that activates gluconeogenesis (GNG) and increases the use of pyruvate, lactate, amino acids, and glycerol. Studies of GNG in hepatocytes, however, tend to test a limited number of substrates at nonphysiologic concentrations. Therefore, we treated cultured primary hepatocytes with three identical substrate mixtures of pyruvate/lactate, glutamine, and glycerol at serum fasting concentrations, where a different U-13C- or 2-13C-labeled substrate was substituted in each mix. In the absence of glucagon stimulation, 80% of the glucose produced in primary hepatocytes incorporated either one or two 13C-labeled glycerol molecules in a 1:1 ratio, reflecting the high overall activity of this pathway. In contrast, glucose produced from 13C-labeled pyruvate/lactate or glutamine rarely incorporated two labeled molecules. While glucagon increased the glycerol and pyruvate/lactate contributions to glucose carbon by 1.6- and 1.8-fold, respectively, the glutamine contribution to glucose carbon was increased 6.4-fold in primary hepatocytes. To account for substrate 13C carbon loss during metabolism, we also performed a metabolic flux analysis, which confirmed that the majority of glucose carbon produced by primary hepatocytes was from glycerol. In vivo studies using a PKA-activation mouse model that represents elevated glucagon activity confirmed that most circulating lactate carbons originated from glycerol, but very little glycerol was derived from lactate carbons, reflecting glycerol's importance as a carbon donor to GNG. Given the diverse entry points for GNG substrates, hepatic glucagon action is unlikely to be due to a single mechanism.

Hepatic glycerol shunt and glycerol-3-phosphate phosphatase control liver metabolism and glucodetoxification under hyperglycemia

OBJECTIVE

Glycerol-3-phosphate (Gro3P) phosphatase (G3PP) hydrolyzes Gro3P to glycerol that exits the cell, thereby operating a "glycerol shunt", a metabolic pathway that we identified recently in mammalian cells. We have investigated the role of G3PP and the glycerol shunt in the regulation of glucose metabolism and lipogenesis in mouse liver.

METHODS

We generated hepatocyte-specific G3PP-KO mice (LKO), by injecting AAV8-TBG-iCre to male G3PP^fl/fl mice. Controls received AAV8-TBG-eGFP. Both groups were fed chow diet for 10 weeks. Hyperglycemia (16-20 mM) was induced by glucose infusion for 55 h. Hepatocytes were isolated from normoglycemic mice for ex vivo studies and targeted metabolomics were measured in mice liver after glucose infusion.

RESULTS

LKO mice showed no change in body weight, food intake, fed and fasted glycemia but had increased fed plasma triglycerides. Hepatic glucose production from glycerol was increased in fasted LKO mice. LKO mouse hepatocytes displayed reduced glycerol production, elevated triglyceride and lactate production at high glucose concentration. Hyperglycemia in LKO mice led to increased liver weight and accumulation of triglycerides, glycogen and cholesterol together with elevated levels of Gro3P, dihydroxyacetone phosphate, acetyl-CoA and some Krebs cycle intermediates in liver. Hyperglycemic LKO mouse liver showed elevated expression of proinflammatory cytokines and M1-macrophage markers accompanied by increased plasma triglycerides, LDL/VLDL, urea and uric acid and myocardial triglycerides.

CONCLUSIONS

The glycerol shunt orchestrated by G3PP acts as a glucose excess detoxification pathway in hepatocytes by preventing metabolic disturbances that contribute to enhanced liver fat, glycogen storage, inflammation and lipid build-up in the heart. We propose G3PP as a novel therapeutic target for hepatic disorders linked to nutrient excess.

Differential Metabolism of Glycerol Based on Oral versus Intravenous Administration in Humans

Glycerol can be metabolized to glucose via gluconeogenesis or lactate via glycolysis. It is unknown if glycerol is metabolized similarly in the portal and systemic circulations in humans. Eight metabolically healthy overnight-fasted individuals received equimolar amounts of 13C3-glycerol orally and intravenously on two separate occasions with serial blood draws over four hours. Serum samples underwent liquid chromatography–mass spectrometry analysis. Oral 13C3-glycerol administration led to higher average serum glucose enrichment than intravenous administration (5.02 ± 1.43 versus 4.07 ± 0.79%, p = 0.009). In contrast, intravenous 13C3-glycerol administration yielded higher average serum lactate enrichment than oral administration (5.67 ± 0.80 versus 4.85 ± 1.30%, p = 0.032). Peak serum glucose enrichment was also higher with oral administration (9.37 ± 2.93 versus 7.12 ± 1.28%, p = 0.010). Glycerol metabolism across the portal and systemic circulations is not congruent. Orally administered labeled glycerol led to greater labeled glucose production, while intravenously administration yielded greater lactate production. These data support direct glycerol to lactate conversion in humans.

Tracing metabolic flux in vivo: basic model structures of tracer methodology

Molecules in living organisms are in a constant state of turnover at varying rates, i.e., synthesis, breakdown, oxidation, and/or conversion to different compounds. Despite the dynamic nature of biomolecules, metabolic research has focused heavily on static, snapshot information such as the abundances of mRNA, protein, and metabolites and/or (in)activation of molecular signaling, often leading to erroneous conclusions regarding metabolic status. Over the past century, stable, non-radioactive isotope tracers have been widely used to provide critical information on the dynamics of specific biomolecules (metabolites and polymers including lipids, proteins, and DNA), in studies in vitro in cells as well as in vivo in both animals and humans. In this review, we discuss (1) the historical background of the use of stable isotope tracer methodology in metabolic research; (2) the importance of obtaining kinetic information for a better understanding of metabolism; and (3) the basic principles and model structures of stable isotope tracer methodology using ¹³C-, ¹⁵N-, or ²H-labeled tracers.

Tagging biomolecules with stable isotopes of specific atoms can reveal details of the molecular inter-conversions of metabolism. The masses of the tracer isotopes used are greater than those of the more common atomic forms. This allows their movement through different metabolic pathways to be detected using mass spectrometry and modeling. Il-Young Kim at Gachon University School of Medicine in South Korea and colleagues focus their review on the use of stable, non-radioactive isotope tracers, especially, of carbon, nitrogen, and hydrogen, to study metabolism in live humans and other animals. They cover the basic model structures of tracer methodology that serve as the fundamental basis for various tracer methods available and the most recent applications. Their procedure is especially useful for monitoring the rates of metabolic inter-conversions, which can reveal aspects of health and disease.

Glycerol's contribution to lactate production outside of a glucose intermediate in fasting humans

BACKGROUND

Glycerol is a well-recognized substrate for new glucose production via gluconeogenesis in the liver. However, its carbon contribution to the glycolytic intermediate lactate is not known in humans.

METHODS

Here we infused stable isotope tracers 13C3-glycerol and 6,6-D2-glucose into six metabolically healthy individuals after an overnight fast to study glycerol metabolism and measure glucose rate of appearance. Serum samples underwent liquid chromatography-mass spectrometry analysis.

RESULTS

Glycerol and glucose rates of appearance were 2.21 ± 1.42 μmol/kg/min and 7.81 ± 1.15 μmol/kg/min, respectively. Under steady-state conditions, the 13C enrichment for lactate was significantly higher than that of glucose (2.90 ± 0.52% versus 1.53 ± 0.78%, p = 0.017), suggesting direct glycerol to lactate metabolism. The percentage of lactate derived from glycerol was also significantly higher than the percentage of glucose (13.88 ± 2.69% versus 6.50 ± 2.59%, p = 0.005).

CONCLUSION

Given that lactate itself is a carbon source for gluconeogenesis and tricycarboxylic cycle intermediates, glycerol's ability to donate carbons to lactate may make it quantitatively more important to intermediary metabolism than currently appreciated.

Protein and Sport: Alternative Sources and Strategies for Bioactive and Sustainable Sports Nutrition

Nutrition and sport play an important role in achieving a healthy lifestyle. In addition to the intake of nutrients derived from the normal diet, some sport disciplines require the consumption of supplements that contribute positively to improved athletic performance. Protein intake is important for many aspects related to health, and current evidence suggests that some athletes require increased amounts of this nutrient. On the other hand, society's demand for more environmentally friendly products, focus on the search for alternative food sources more sustainable. This review aims to summarize the latest research on novel strategies and sources for greener and functional supplementation in sport nutrition. Alternative protein sources such as insects, plants or mycoproteins have proven to be an interesting substrate due to their high added value in terms of bioactivity and sustainability. Protein hydrolysis has proven to be a very useful technology to revalue by-products, such as collagen, by producing bioactive peptides beneficial on athletes performance and sport-related complications. In addition, it has been observed that certain amino acids from plant sources, as citrulline or theanine, can have an ergogenic effect for this target population. Finally, the future perspectives of protein supplementation in sports nutrition are discussed. In summary, protein supplementation in sports nutrition is a very promising field of research, whose future perspective lies with the search for alternatives with greater bioactive potential and more sustainable than conventional sources.

Genistein Alleviates High-Fat Diet-Induced Obesity by Inhibiting the Process of Gluconeogenesis in Mice

Genistein is an isoflavone phytoestrogen that has been shown to improve obesity; however, the underlying molecular mechanisms involved therein have not been clearly elucidated. In this study, we administered genistein to high-fat diet-induced obese mice to investigate its effect on hepatic gluconeogenesis. The results showed that genistein treatment significantly inhibited body weight gain, hyperglycemia, and adipose and hepatic lipid deposition in high-fat diet-induced obese mice. Glucose tolerance test (GTT), insulin tolerance test (ITT) and pyruvate tolerance test (PTT) showed that genistein treatment significantly inhibited gluconeogenesis and improved insulin resistance in obese mice. In addition, this study also found that genistein could promote the expression of miR-451 in vitro and in vivo, and the dual-luciferase reporter system showed that G6pc (glucose-6-phosphatase) may be a target gene of miR-451. Both genistein treatment and in vivo injection of miR-451 agomir significantly inhibited gluconeogenesis and inhibited the expression of G6pc and Gk (glycerol kinase, a known target gene of miR-451). In conclusion, genistein may inhibit gluconeogenesis in obese mice by regulating the expression of Gk and G6pc through miR-451. These results may provide insights into the functions of miR-451 and food-derived phytoestrogens in ameliorating and preventing gluconeogenesis-related diseases.

Sepsis: a failing starvation response

Sepsis is involved in ~ 20% of annual global deaths. Despite decades of research, the current management of sepsis remains supportive rather than curative. Clinical trials in sepsis have mainly been focused on targeting the inflammatory pathway, but without success. Recent data indicate that metabolic dysregulation takes place in sepsis, and targeting metabolic pathways might hold much promise for the management of sepsis. Sepsis yields a strong starvation response, including the release of high-energy metabolites such as lactate and free fatty acids. However, the activity of two major transcription factors, GR and PPARα, is downregulated in hepatocytes, leading to the accumulation and toxicity of metabolites that, moreover, fail to be transformed into useful molecules such as glucose and ketones. We review the literature and suggest mechanisms and potential therapeutic targets that might prevent or revert the fatal metabolic dysregulation in sepsis.

Metformin, phenformin, and galegine inhibit complex IV activity and reduce glycerol-derived gluconeogenesis

Metformin is the most commonly prescribed drug for the treatment of type 2 diabetes mellitus, yet the mechanism by which it lowers plasma glucose concentrations has remained elusive. Most studies to date have attributed metformin’s glucose-lowering effects to inhibition of complex I activity. Contrary to this hypothesis, we show that inhibition of complex I activity in vitro and in vivo does not reduce plasma glucose concentrations or inhibit hepatic gluconeogenesis. We go on to show that metformin, and the related guanides/biguanides, phenformin and galegine, inhibit complex IV activity at clinically relevant concentrations, which, in turn, results in inhibition of glycerol-3-phosphate dehydrogenase activity, increased cytosolic redox, and selective inhibition of glycerol-derived hepatic gluconeogenesis both in vitro and in vivo.

Metformin exerts its plasma glucose-lowering therapeutic effect primarily through inhibition of hepatic gluconeogenesis. However, the precise molecular mechanism by which metformin inhibits hepatic gluconeogenesis is still unclear. Although inhibition of mitochondrial complex I is frequently invoked as metformin’s primary mechanism of action, the metabolic effects of complex I inhibition have not been thoroughly evaluated in vivo. Here, we show that acute portal infusion of piericidin A, a potent and specific complex I inhibitor, does not reduce hepatic gluconeogenesis in vivo. In contrast, we show that metformin, phenformin, and galegine selectively inhibit hepatic gluconeogenesis from glycerol. Specifically, we show that guanides/biguanides interact with complex IV to reduce its enzymatic activity, leading to indirect inhibition of glycerol-3-phosphate (G3P) dehydrogenase (GPD2), increased cytosolic redox, and reduced glycerol-derived gluconeogenesis. We report that inhibition of complex IV with potassium cyanide replicates the effects of the guanides/biguanides in vitro by selectively reducing glycerol-derived gluconeogenesis via increased cytosolic redox. Finally, we show that complex IV inhibition is sufficient to inhibit G3P-mediated respiration and gluconeogenesis from glycerol. Taken together, we propose a mechanism of metformin action in which complex IV–mediated inhibition of GPD2 reduces glycerol-derived hepatic gluconeogenesis.

Multi-omics-based label-free metabolic flux inference reveals obesity-associated dysregulatory mechanisms in liver glucose metabolism

Glucose homeostasis is maintained by modulation of metabolic flux. Enzymes and metabolites regulate the involved metabolic pathways. Dysregulation of glucose homeostasis is a pathological event in obesity. Analyzing metabolic pathways and the mechanisms contributing to obesity-associated dysregulation in vivo is challenging. Here, we introduce OMELET: Omics-Based Metabolic Flux Estimation without Labeling for Extended Trans-omic Analysis. OMELET uses metabolomic, proteomic, and transcriptomic data to identify relative changes in metabolic flux, and to calculate contributions of metabolites, enzymes, and transcripts to the changes in metabolic flux. By evaluating the livers of fasting ob/ob mice, we found that increased metabolic flux through gluconeogenesis resulted primarily from increased transcripts, whereas that through the pyruvate cycle resulted from both increased transcripts and changes in substrates of metabolic enzymes. With OMELET, we identified mechanisms underlying the obesity-associated dysregulation of metabolic flux in the liver.

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We developed OMELET to infer metabolic flux from label-free multi-omic data

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Contributions of metabolites, enzymes, and transcripts for flux were inferred

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Gluconeogenic flux increased in fasting ob/ob mice by increased transcripts

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Increased pyruvate cycle fluxes were led by increased transcripts and substrates

Exogenous d ‐β‐hydroxybutyrate lowers blood glucose in part by decreasing the availability of L‐alanine for gluconeogenesis

Background

Interventions that induce ketosis simultaneously lower blood glucose and the explanation for this phenomenon is unknown. Additionally, the glucose‐lowering effect of acute ketosis is greater in people with type 2 diabetes (T2D). On the contrary, L‐alanine is a gluconeogenic substrate secreted by skeletal muscle at higher levels in people with T2D and infusing of ketones lower circulating L‐alanine blood levels. In this study, we sought to determine whether supplementation with L‐alanine would attenuate the glucose‐lowering effect of exogenous ketosis using a ketone ester (KE).

Methods

This crossover study involved 10 healthy human volunteers who fasted for 24 h prior to the ingestion of 25 g of d‐β‐hydroxybutyrate (βHB) in the form of a KE drink (ΔG^®) on two separate visits. During one of the visits, participants additionally ingested 2 g of L‐alanine to see whether L‐alanine supplementation would attenuate the glucose‐lowering effect of the KE drink. Blood L‐alanine, L‐glutamine, glucose, βHB, free fatty acids (FFA), lactate and C‐peptide were measured for 120 min after ingestion of the KE, with or without L‐alanine.

Findings

The KE drinks elevated blood βHB concentrations from negligible levels to 4.52 ± 1.23 mmol/L, lowered glucose from 4.97 ± SD 0.39 to 3.77 ± SD 0.40 mmol/L, and lowered and L‐alanine from 0.56 ± SD 0.88 to 0.41 ± SD 0.91 mmol/L. L‐alanine in the KE drink elevated blood L‐Alanine by 0.68 ± SD 0.15 mmol/L, but had no significant effect on blood βHB, L‐glutamine, FFA, lactate, nor C‐peptide concentrations. By contrast, L‐alanine supplementation significantly attenuated the ketosis‐induced drop in glucose from 28% ± SD 8% to 16% ± SD 7% (p < .01).

Conclusions

The glucose‐lowering effect of acutely elevated βHB is partially due to βHB decreasing L‐alanine availability as a substrate for gluconeogenesis.

Optimized 3D Culture of Hepatic Cells for Liver Organoid Metabolic Assays

The liver is among the principal organs for glucose homeostasis and metabolism. Studies of liver metabolism are limited by the inability to expand primary hepatocytes in vitro while maintaining their metabolic functions. Human hepatic three-dimensional (3D) organoids have been established using defined factors, yet hepatic organoids from adult donors showed impaired expansion. We examined conditions to facilitate the expansion of adult donor-derived hepatic organoids (HepAOs) and HepG2 cells in organoid cultures (HepGOs) using combinations of growth factors and small molecules. The expansion dynamics, gluconeogenic and HNF4α expression, and albumin secretion are assessed. The conditions tested allow the generation of HepAOs and HepGOs in 3D cultures. Nevertheless, gluconeogenic gene expression varies greatly between conditions. The organoid expansion rates are limited when including the TGFβ inhibitor A8301, while are relatively higher with Forskolin (FSK) and Oncostatin M (OSM). Notably, expanded HepGOs grown in the optimized condition maintain detectable gluconeogenic expression in a spatiotemporal distribution at 8 weeks. We present optimized conditions by limiting A8301 and incorporating FSK and OSM to allow the expansion of HepAOs from adult donors and HepGOs with gluconeogenic competence. These models increase the repertoire of human hepatic cellular tools available for use in liver metabolic assays.

Adipocyte-specific mTORC2 deficiency impairs BAT and iWAT thermogenic capacity without affecting glucose uptake and energy expenditure in cold-acclimated mice

Deletion of mechanistic target of rapamycin complex 2 (mTORC2) essential component rapamycin insensitive companion of mTOR (Rictor) by a Cre recombinase under control of the broad, nonadipocyte-specific aP2/FABP4 promoter impairs thermoregulation and brown adipose tissue (BAT) glucose uptake on acute cold exposure. We investigated herein whether adipocyte-specific mTORC2 deficiency affects BAT and inguinal white adipose tissue (iWAT) signaling, metabolism, and thermogenesis in cold-acclimated mice. For this, 8-wk-old male mice bearing Rictor deletion and therefore mTORC2 deficiency in adipocytes (adiponectin-Cre) and littermates controls were either kept at thermoneutrality (30 ± 1°C) or cold-acclimated (10 ± 1°C) for 14 days and evaluated for BAT and iWAT signaling, metabolism, and thermogenesis. Cold acclimation inhibited mTORC2 in BAT and iWAT, but its residual activity is still required for the cold-induced increases in BAT adipocyte number, total UCP-1 content and mRNA levels of proliferation markers Ki67 and cyclin 1 D, and de novo lipogenesis enzymes ATP-citrate lyase and acetyl-CoA carboxylase. In iWAT, mTORC2 residual activity is partially required for the cold-induced increases in multilocular adipocytes, mitochondrial mass, and uncoupling protein 1 (UCP-1) content. Conversely, BAT mTORC1 activity and BAT and iWAT glucose uptake were upregulated by cold independently of mTORC2. Noteworthy, the impairment in BAT and iWAT total UCP-1 content and thermogenic capacity induced by adipocyte mTORC2 deficiency had no major impact on whole body energy expenditure in cold-acclimated mice due to a compensatory activation of muscle shivering. In conclusion, adipocyte mTORC2 deficiency impairs, through different mechanisms, BAT and iWAT total UCP-1 content and thermogenic capacity in cold-acclimated mice, without affecting glucose uptake and whole body energy expenditure.NEW & NOTEWORTHY BAT and iWAT mTORC2 is inhibited by cold acclimation, but its residual activity is required for cold-induced increases in total UCP-1 content and thermogenic capacity, but not glucose uptake and mTORC1 activity. The impaired BAT and iWAT total UCP-1 content and thermogenic capacity induced by adipocyte mTORC2 deficiency are compensated by activation of muscle shivering in cold-acclimated mice.

Liquid Chromatography-Mass Spectrometry for Clinical Metabolomics: An Overview

Metabolomics is a discipline that offers a comprehensive analysis of metabolites in biological samples. In the last decades, the notable evolution in liquid chromatography and mass spectrometry technologies has driven an exponential progress in LC-MS-based metabolomics. Targeted and untargeted metabolomics strategies are important tools in health and medical science, especially in the study of disease-related biomarkers, drug discovery and development, toxicology, diet, physical exercise, and precision medicine. Clinical and biological problems can now be understood in terms of metabolic phenotyping. This overview highlights the current approaches to LC-MS-based metabolomics analysis and its applications in the clinical research.

AccuCor2: isotope natural abundance correction for dual-isotope tracer experiments

Stable isotope labeling techniques have been widely applied in the field of metabolomics and proteomics. Before the measured mass spectral data can be used for quantitative analysis, it must be accurately corrected for isotope natural abundance and tracer isotopic impurity. Despite the increasing popularity of dual-isotope tracing strategy such as 13C-15N or 13C-2H, there are no accurate tools for correcting isotope natural abundance for such experiments in a resolution-dependent manner. Here, we present AccuCor2 as an R-based tool to perform the correction for 13C-15N or 13C-2H labeling experiments. Our method uses a newly designed algorithm to construct the correction matrices that link labeling pattern and measured mass fractions, then use non-negative least-squares to solve the labeling patterns. Our results show that the dual-isotope experiments often require a mass resolution that is high enough to resolve 13C and 15N or 13C and 2H. Otherwise, the labeling pattern is not solvable. However, this mass resolution may not be sufficiently high to resolve other non-tracer elements such as oxygen or sulfur from the tracer elements. Therefore, we design AccuCor2 to perform the correction based on the actual mass resolution of the measurements. Using both simulated and experimental data, we show that AccuCor2 performs accurate and resolution-dependent correction for dual-isotope tracer data.

Starvation Ketosis and the Kidney

BACKGROUND

The remarkable ability of the body to adapt to long-term starvation has been critical for survival of primitive man. An appreciation of these processes can provide the clinician better insight into many clinical conditions characterized by ketoacidosis.

SUMMARY

The body adapts to long-term fasting by conserving nitrogen, as the brain increasingly utilizes keto acids, sparing the need for glucose. This shift in fuel utilization decreases the need for mobilization of amino acids from the muscle for purposes of gluconeogenesis. Loss of urinary nitrogen is initially in the form of urea when hepatic gluconeogenesis is dominant and later as ammonia reflecting increased glutamine uptake by the kidney. The carbon skeleton of glutamine is utilized for glucose production and regeneration of consumed HCO3-. The replacement of urea with NH4+ provides the osmoles needed for urine flow and waste product excretion. Over time, the urinary loss of nitrogen is minimized as kidney uptake of filtered ketone bodies becomes more complete. Adjustments in urine Na+ serve to minimize kidney K+ wasting and, along with changes in urine pH, minimize the likelihood of uric acid precipitation. There is a sexual dimorphism in response to starvation. Key Message: Ketoacidosis is a major feature of common clinical conditions to include diabetic ketoacidosis, alcoholic ketoacidosis, salicylate intoxication, SGLT2 inhibitor therapy, and calorie sufficient but carbohydrate-restricted diets. Familiarity with the pathophysiology and metabolic consequences of ketogenesis is critical, given the potential for the clinician to encounter one of these conditions.

Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis

The reactions of the tricarboxylic acid (TCA) cycle allow the controlled combustion of fat and carbohydrate. In principle, TCA cycle intermediates are regenerated on every turn and can facilitate the oxidation of an infinite number of nutrient molecules. However, TCA cycle intermediates can be lost to cataplerotic pathways that provide precursors for biosynthesis, and they must be replaced by anaplerotic pathways that regenerate these intermediates. Together, anaplerosis and cataplerosis help regulate rates of biosynthesis by dictating precursor supply, and they play underappreciated roles in catabolism and cellular energy status. They facilitate recycling pathways and nitrogen trafficking necessary for catabolism, and they influence redox state and oxidative capacity by altering TCA cycle intermediate concentrations. These functions vary widely by tissue and play emerging roles in disease. This article reviews the roles of anaplerosis and cataplerosis in various tissues and discusses how they alter carbon transitions, and highlights their contribution to mechanisms of disease. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

In vivo2H/13C flux analysis in metabolism research

Identifying the factors and mechanisms that regulate metabolism under normal and diseased states requires methods to quantify metabolic fluxes of live tissues within their physiological milieu. A number of recent developments have expanded the reach and depth of isotope-based in vivo flux analysis, which have in turn challenged existing dogmas in metabolism research. First, minimally invasive techniques of intravenous isotope infusion and sampling have advanced in vivo metabolic tracer studies in animal models and human subjects. Second, recent breakthroughs in analytical instrumentation have expanded the scope of isotope labeling measurements and reduced sample volume requirements. Third, innovative modeling approaches and publicly available software tools have facilitated rigorous analysis of sophisticated experimental designs involving multiple tracers and expansive metabolomics datasets. These developments have enabled comprehensive in vivo quantification of metabolic fluxes in specific tissues and have set the stage for integrated multi-tissue flux assays.

Loss of FOXO transcription factors in the liver mitigates stress-induced hyperglycemia

Objective

Stress-induced hyperglycemia is associated with poor outcomes in nearly all critical illnesses. This acute elevation in glucose after injury or illness is associated with increased morbidity and mortality, including multiple organ failure. Stress-induced hyperglycemia is often attributed to insulin resistance as controlling glucose levels via exogenous insulin improves outcomes, but the mechanisms are unclear. Forkhead box O (FOXO) transcription factors are direct targets of insulin signaling in the liver that regulate glucose homeostasis via direct and indirect pathways. Loss of hepatic FOXO transcription factors reduces hyperglycemia in chronic insulin resistance; however, the role of FOXOs in stress-induced hyperglycemia is unknown.

Methods

We subjected mice lacking FOXO transcription factors in the liver to a model of injury known to cause stress-induced hyperglycemia. Glucose, insulin, glycerol, fatty acids, cytokines, and adipokines were assessed before and after injury. Liver and adipose tissue were analyzed for changes in glycogen, FOXO target gene expression, and insulin signaling.

Results

Stress-induced hyperglycemia was associated with reduced hepatic insulin signaling and increased hepatic FOXO target gene expression while loss of FOXO1, 3, and 4 in the liver attenuated hyperglycemia and prevented hyperinsulinemia. Mechanistically, the loss of FOXO transcription factors mitigated the stress-induced hyperglycemia response by directly altering gene expression and glycogenolysis in the liver and indirectly suppressing lipolysis in adipose tissue. Reductions were associated with decreased IL-6, TNF-α, and follistatin and increased FGF21, suggesting that cytokines and FOXO-regulated hepatokines contribute to the stress-induced hyperglycemia response.

Conclusions

This study implicates FOXO transcription factors as a predominant driver of stress-induced hyperglycemia through means that include cross-talk between the liver and adipose, highlighting a novel mechanism underlying acute hyperglycemia and insulin resistance in stress.

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Liver forkhead box O (FOXO) target gene expression is increased in critical illness.

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Loss of FOXO1, 3, and 4 in the liver mitigates stress-induced hyperglycemia (SIH).

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Hepatic FOXO drives SIH via direct and indirect means in the liver and adipose.

In Vivo Estimates of Liver Metabolic Flux Assessed by 13C-Propionate and 13C-Lactate Are Impacted by Tracer Recycling and Equilibrium Assumptions

Isotope-based assessment of metabolic flux is achieved through a judicious balance of measurements and assumptions. Recent publications debate the validity of key assumptions used to model stable isotope labeling of liver metabolism in vivo. Here, we examine the controversy surrounding estimates of liver citric acid cycle and gluconeogenesis fluxes using a flexible modeling platform that enables rigorous testing of standard assumptions. Fasted C57BL/6J mice are infused with [¹³C₃]lactate or [¹³C₃]propionate isotopes, and hepatic fluxes are regressed using models with gradually increasing complexity and relaxed assumptions. We confirm that liver pyruvate cycling fluxes are incongruent between different ¹³C tracers in models with conventional assumptions. When models are expanded to include more labeling measurements and fewer constraining assumptions, however, liver pyruvate cycling is significant, and inconsistencies in hepatic flux estimates using [¹³C₃]lactate and [¹³C₃]propionate isotopes emanate, in part, from peripheral tracer recycling and incomplete isotope equilibration within the citric acid cycle.

Fasting Hormones Synergistically Induce Amino Acid Catabolism Genes to Promote Gluconeogenesis

BACKGROUND & AIMS

Gluconeogenesis from amino acids (AAs) maintains glucose homeostasis during fasting. Although glucagon is known to regulate AA catabolism, the contribution of other hormones to it and the scope of transcriptional regulation dictating AA catabolism are unknown. We explored the role of the fasting hormones glucagon and glucocorticoids in transcriptional regulation of AA catabolism genes and AA-dependent gluconeogenesis.

METHODS

We tested the RNA expression of AA catabolism genes and glucose production in primary mouse hepatocytes treated with fasting hormones (glucagon, corticosterone) and feeding hormones (insulin, fibroblast growth factor 19). We analyzed genomic data of chromatin accessibility and chromatin immunoprecipitation in mice and primary mouse hepatocytes. We performed chromatin immunoprecipitation in livers of fasted mice to show binding of cAMP responsive element binding protein (CREB) and the glucocorticoid receptor (GR).

RESULTS

Fasting induced the expression of 31 genes with various roles in AA catabolism. Of them, 15 were synergistically induced by co-treatment of glucagon and corticosterone. Synergistic gene expression relied on the activity of both CREB and GR and was abolished by treatment with either insulin or fibroblast growth factor 19. Enhancers adjacent to synergistically induced genes became more accessible and were bound by CREB and GR on fasting. Akin to the gene expression pattern, gluconeogenesis from AAs was synergistically induced by glucagon and corticosterone in a CREB- and GR-dependent manner.

CONCLUSIONS

Transcriptional regulation of AA catabolism genes during fasting is widespread and is driven by glucagon (via CREB) and corticosterone (via GR). Glucose production in hepatocytes is also synergistically augmented, showing that glucagon alone is insufficient in fully activating gluconeogenesis.

An Ethanolic Extract of Artemisia dracunculus L. Enhances the Metabolic Benefits of Exercise in Diet-induced Obese Mice

PURPOSE

The purpose of this study was to determine the effect of an ethanolic extract of Artemisia dracunculus L. (5011) combined with exercise on in vivo glucose and fat metabolism in diet-induced obese male mice.

METHODS

After 8 wk of high-fat diet (HFD) feeding, 52 mice were randomly allocated to a voluntary wheel running group (HFD Ex), a 5011 + HFD sedentary group (5011 Sed), a 5011 + HFD Ex (5011 Ex), or an HFD sedentary group (HFD Sed) for 4 wk. Real-time energy expenditure and substrate utilization were measured by indirect calorimetry. A stable isotope glucose tolerance test was performed before and after the 4-wk wheel running period to determine changes in endogenous glucose production and glucose disposal. We also performed an analysis of genes and proteins associated with the early response to exercise and exercise adaptations in skeletal muscle and liver.

RESULTS

When compared with HFD Ex mice, 5011 Ex mice had increased fat oxidation during speed- and distance-matched wheel running bouts. Both HFD Ex and 5011 Ex mice had reduced endogenous glucose during the glucose tolerance test, whereas only the 5011 Sed and the 5011 Ex mice had improved glucose disposal after the 4-wk experimental period when compared with HFD Sed and HFD Ex mice. 5011 Ex mice had increased Pgc1-α and Tfam expression in skeletal muscle when compared with HFD Ex mice, whereas Pdk4 expression was reduced in the liver of HFD Ex and 5011 Ex mice.

CONCLUSIONS

Our study demonstrates that 5011, an ethanolic extract of A. dracunculus L., with a history of medicinal use, enhances the metabolic benefits of exercise to improve in vivo fat and glucose metabolism.

In Vivo and In Vitro Quantification of Glucose Kinetics: From Bedside to Bench

Like other substrates, plasma glucose is in a dynamic state of constant turnover (i.e., rates of glucose appearance [Ra glucose] into and disappearance [Rd glucose] from the plasma) while staying within a narrow range of normal concentrations, a physiological priority. Persistent imbalance of glucose turnover leads to elevations (i.e., hyperglycemia, Ra>Rd) or falls (i.e., hypoglycemia, Ra Collapse

Key Words

• Diabetes mellitus
• Insulin resistance
• Metabolic fluxomics
• Stable isotope tracers

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MESH Headings

• Animals
• Diabetes Mellitus/metabolism
• Gluconeogenesis
• Glucose/biosynthesis
• Glycogenolysis
• Humans
• Hyperglycemia/metabolism
• Hypoglycemia/metabolism
• Insulin Resistance
• Isotopes/administration & dosage
• Isotopes/metabolism
• Kinetics
• Liver/metabolism
• Metabolic Flux Analysis

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Grants

• 2018R1D1A1B07051053 National Research Foundation of Korea
• 2019H1D3A1A01071043 National Research Foundation of Korea
• 2020R1I1A1A01074380 National Research Foundation of Korea
• 2014M3A9D5A01073886 National Research Foundation of Korea
• 2017R1D1A1B03032988 National Research Foundation of Korea
• Ministry of Education
• Ministry of Sciences and ICT

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Affiliation(s)

Il-Young Kim Department of Molecular Medicine, College of Medicine, Gachon University, Incheon, Seoul, Korea Korea Mouse Metabolic Phenotyping Center, Lee Gil Ya Cancer and Diabetes Institute, Gachon University, Incheon, Seoul, Korea
Sanghee Park Department of Molecular Medicine, College of Medicine, Gachon University, Incheon, Seoul, Korea Korea Mouse Metabolic Phenotyping Center, Lee Gil Ya Cancer and Diabetes Institute, Gachon University, Incheon, Seoul, Korea
Yeongmin Kim Department of Health Sciences and Technology, Gachon Advanced Institute for Health Sciences & Technology (GAIHST), Gachon University, Incheon, Seoul, Korea
Yewon Chang Department of Health Sciences and Technology, Gachon Advanced Institute for Health Sciences & Technology (GAIHST), Gachon University, Incheon, Seoul, Korea
Cheol Soo Choi Department of Molecular Medicine, College of Medicine, Gachon University, Incheon, Seoul, Korea Korea Mouse Metabolic Phenotyping Center, Lee Gil Ya Cancer and Diabetes Institute, Gachon University, Incheon, Seoul, Korea
Sang-Hoon Suh Department of Physical Education, Yonsei University, Seoul, Korea
Robert R. Wolfe Department of Geriatrics, the Center for Translational Research in Aging & Longevity, Donald W. Reynolds Institute on Aging, University of Arkansas for Medical Sciences, Little Rock, AR, USA

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Tracking the carbons supplying gluconeogenesis

As the burden of type 2 diabetes mellitus (T2DM) grows in the 21st century, the need to understand glucose metabolism heightens. Increased gluconeogenesis is a major contributor to the hyperglycemia seen in T2DM. Isotope tracer experiments in humans and animals over several decades have offered insights into gluconeogenesis under euglycemic and diabetic conditions. This review focuses on the current understanding of carbon flux in gluconeogenesis, including substrate contribution of various gluconeogenic precursors to glucose production. Alterations of gluconeogenic metabolites and fluxes in T2DM are discussed. We also highlight ongoing knowledge gaps in the literature that require further investigation. A comprehensive analysis of gluconeogenesis may enable a better understanding of T2DM pathophysiology and identification of novel targets for treating hyperglycemia.