Quantifying carbon sources for de novo lipogenesis in wild-type and IRS-1 knockout brown adipocytes

Metabolic flux analysis in adipose tissue reprogramming

Obesity is a growing epidemic in the United States and worldwide and is associated with insulin resistance and cardiovascular disease, among other comorbidities. Understanding of the pathology that links overnutrition to these disease processes is ongoing. Adipose tissue is a heterogeneous organ comprised of multiple different cell types and it is likely that dysregulated metabolism within these cell populations disrupts both inter- and intracellular interactions and is a key driver of human disease. In recent years, metabolic flux analysis, which offers a precise quantification of metabolic pathway fluxes in biological systems, has emerged as a candidate strategy for uncovering the metabolic changes that stoke these disease processes. In this mini review, we discuss metabolic flux analysis as an experimental tool, with a specific emphasis on mass spectrometry with isotope tracing as this is the technique most frequently used for metabolic flux analysis in adipocytes. Furthermore, we examine existing literature that uses metabolic flux analysis to further our understanding of adipose tissue biology. Our group has a specific interest in understanding the role of white adipose tissue inflammation in the progression of cardiometabolic disease, as we know that in obesity the accumulation of pro-inflammatory adipose tissue macrophages is associated with significant morbidity, so we use this as a paradigm throughout our review for framing the application of these experimental techniques. However, there are many other biological applications to which they can be applied to further understanding of not only adipose tissue biology but also systemic homeostasis.

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

Experimental Evolution Reveals Unifying Systems-Level Adaptations but Diversity in Driving Genotypes

Genotype-fitness maps of evolution have been well characterized for biological components, such as RNA and proteins, but remain less clear for systems-level properties, such as those of metabolic and transcriptional regulatory networks. Here, we take multi-omics measurements of 6 different E. coli strains throughout adaptive laboratory evolution (ALE) to maximal growth fitness. The results show the following: (i) convergence in most overall phenotypic measures across all strains, with the notable exception of divergence in NADPH production mechanisms; (ii) conserved transcriptomic adaptations, describing increased expression of growth promoting genes but decreased expression of stress response and structural components; (iii) four groups of regulatory trade-offs underlying the adjustment of transcriptome composition; and (iv) correlates that link causal mutations to systems-level adaptations, including mutation-pathway flux correlates and mutation-transcriptome composition correlates. We thus show that fitness landscapes for ALE can be described with two layers of causation: one based on system-level properties (continuous variables) and the other based on mutations (discrete variables). IMPORTANCE Understanding the mechanisms of microbial adaptation will help combat the evolution of drug-resistant microbes and enable predictive genome design. Although experimental evolution allows us to identify the causal mutations underlying microbial adaptation, it remains unclear how causal mutations enable increased fitness and is often explained in terms of individual components (i.e., enzyme rate) as opposed to biological systems (i.e., pathways). Here, we find that causal mutations in E. coli are linked to systems-level changes in NADPH balance and expression of stress response genes. These systems-level adaptation patterns are conserved across diverse E. coli strains and thus identify cofactor balance and proteome reallocation as dominant constraints governing microbial adaptation.

Integration of Liver Glycogen and Triglyceride NMR Isotopomer Analyses Provides a Comprehensive Coverage of Hepatic Glucose and Fructose Metabolism

Dietary glucose and fructose are both efficiently assimilated by the liver but a comprehensive measurement of this process starting from their conversion to sugar phosphates, involvement of the pentose phosphate pathway (PPP), and conversion to glycogen and lipid storage products, remains incomplete. Mice were fed a chow diet supplemented with 35 g/100 mL drinking water of a 55/45 fructose/glucose mixture for 18 weeks. On the final night, the sugar mixture was enriched with either [U-13C]glucose or [U-13C]fructose, and deuterated water (2H2O) was also administered. 13C-isotopomers representing newly synthesized hepatic glucose-6-phosphate (glucose-6-P), glycerol-3-phosphate, and lipogenic acetyl-CoA were quantified by 2H and 13C NMR analysis of post-mortem liver glycogen and triglyceride. These data were applied to a metabolic model covering glucose-6-P, PPP, triose-P, and de novo lipogenesis (DNL) fluxes. The glucose supplement was converted to glucose-6-P via the direct pathway, while the fructose supplement was metabolized by the liver to gluconeogenic triose-P via fructokinase-aldolase-triokinase. Glucose-6-P from all carbohydrate sources accounted for 40-60% of lipogenic acetyl-CoA and 10-12% was oxidized by the pentose phosphate pathway (PPP). The yield of NADPH from PPP flux accounted for a minority (~30%) of the total DNL requirement. In conclusion, this approach integrates measurements of glucose-6-P, PPP, and DNL fluxes to provide a holistic and informative assessment of hepatic glucose and fructose metabolism.

Metabolic flux analysis: a comprehensive review on sample preparation, analytical techniques, data analysis, computational modelling, and main application areas

Metabolic flux analysis (MFA) quantitatively describes cellular fluxes to understand metabolic phenotypes and functional behaviour after environmental and/or genetic perturbations. In the last decade, the application of stable isotopes became extremely important to determine and integrate in vivo measurements of metabolic reactions in systems biology. 13C-MFA is one of the most informative methods used to study central metabolism of biological systems. This review aims to outline the current experimental procedure adopted in 13C-MFA, starting from the preparation of cell cultures and labelled tracers to the quenching and extraction of metabolites and their subsequent analysis performed with very powerful software. Here, the limitations and advantages of nuclear magnetic resonance spectroscopy and mass spectrometry techniques used in carbon labelled experiments are elucidated by reviewing the most recent published papers. Furthermore, we summarise the most successful approaches used for computational modelling in flux analysis and the main application areas with a particular focus in metabolic engineering.

A simple method for quantifying de novo lipogenesis rate and substrate selection in cell cultures by 13 C NMR isotopomer analysis of the crude lipid fraction

PURPOSE

De novo lipogenesis (DNL) is critical for cell growth and maintenance, and acetyl-CoA precursors can be derived from different substrates. We developed a ¹³ C NMR analysis of lipid extracts from cultured microglia cells administered with [U-¹³ C]glucose that informs overall lipogenic activity as well as the contribution of glucose to lipogenic acetyl-CoA.

METHODS

BV-2 microglial cell line cultured with glucose and glutamine was provided with [U-¹³ C]glucose and unlabeled glutamine for 24 h and studied in either the presence or absence of lipopolysaccharide (LPS). Cells were then extracted for lipids and the crude lipid fraction was analyzed by ¹³ C NMR. ¹³ C-isotopomer signals in the fatty acid ω - 1 and ω - 2 signals representing consecutive or non-consecutive enrichment of the fatty acid chain by [1,2-¹³ C₂ ]acetyl-CoA were quantified and applied to a probabilistic model of acetyl-CoA precursor and fatty acid enrichment.

RESULTS

Glucose contributed 72 ± 2% of lipogenic acetyl-CoA while DNL from all sources accounted for 16 ± 2% of lipid turnover. With LPS, there was a significant decrease in glucose contribution (59 ± 4%, p < 0.05) while DNL was unchanged (11 ± 3%).

CONCLUSIONS

A simple ¹³ C NMR analysis of the crude lipid fractions of BV-2 cells administered with [U-¹³ C]glucose informs DNL activity and the contribution of glucose to the acetyl-CoA precursors. While DNL was preserved in the presence of LPS, there was redirection of lipogenic acetyl-CoA sources from glucose to other substrates. Thus, in the present article, we describe a novel and simple ¹³ C NMR analysis approach to disclose the overall lipogenic activity and substrate contribution to DNL, suitable for evaluating DNL rates in cell cultures.

Glutamine Homeostasis and Its Role in the Adaptive Strategies of the Blind Mole Rat, Spalax

Oxidative metabolism is fine-tuned machinery that combines two tightly coupled fluxes of glucose and glutamine-derived carbons. Hypoxia interrupts the coordination between the metabolism of these two nutrients and leads to a decrease of the system efficacy and may eventually cause cell death. The subterranean blind mole rat, Spalax, is an underexplored, underground, hypoxia-tolerant mammalian group which spends its life under sharply fluctuating oxygen levels. Primary Spalax cells are an exceptional model to study the metabolic strategies that have evolved in mammals inhabiting low-oxygen niches. In this study we explored the metabolic frame of glutamine (Gln) homeostasis in Spalax skin cells under normoxic and hypoxic conditions and their impacts on the metabolism of rat cells. Targeted metabolomics employing liquid chromatography and mass spectrometry (LC-MS) was used to track the fate of heavy glutamine carbons (¹³C₅ Gln) after 24 h under normoxia or hypoxia (1% O₂). Our results indicated that large amounts of glutamine-originated carbons were detected as proline (Pro) and hydroxyproline (HPro) in normoxic Spalax cells with a further increase under hypoxia, suggesting a strategy for reduced Gln carbons storage in proteins. The intensity of the flux and the presence of HPro suggests collagen as a candidate protein that is most abundant in animals, and as the primary source of HPro. An increased conversion of αKG to 2 HG that was indicated in hypoxic Spalax cells prevents the degradation of hypoxia-inducible factor 1α (HIF-1α) and, consequently, maintains cytosolic and mitochondrial carbons fluxes that were uncoupled via inhibition of the pyruvate dehydrogenase complex. A strong antioxidant defense in Spalax cells can be attributed, at least in part, to the massive usage of glutamine-derived glutamate for glutathione (GSH) production. The present study uncovers additional strategies that have evolved in this unique mammal to support its hypoxia tolerance, and probably contribute to its cancer resistance, longevity, and healthy aging.

mTOR Signaling and SREBP Activity Increase FADS2 Expression and Can Activate Sapienate Biosynthesis

Cancer cells display an increased plasticity in their lipid metabolism, which includes the conversion of palmitate to sapienate via the enzyme fatty acid desaturase 2 (FADS2). We find that FADS2 expression correlates with mammalian target of rapamycin (mTOR) signaling and sterol regulatory element-binding protein 1 (SREBP-1) activity across multiple cancer types and is prognostic in some cancer types. Accordingly, activating mTOR signaling by deleting tuberous sclerosis complex 2 (Tsc2) or overexpression of SREBP-1/2 is sufficient to increase FADS2 mRNA expression and sapienate metabolism in mouse embryonic fibroblasts (MEFs) and U87 glioblastoma cells, respectively. Conversely, inhibiting mTOR signaling decreases FADS2 expression and sapienate biosynthesis in MEFs with Tsc2 deletion, HUH7 hepatocellular carcinoma cells, and orthotopic HUH7 liver xenografts. In conclusion, we show that mTOR signaling and SREBP activity are sufficient to activate sapienate metabolism by increasing FADS2 expression. Consequently, targeting mTOR signaling can reduce sapienate metabolism in vivo.

Modulation of dysregulated cancer metabolism by plant secondary metabolites: A mechanistic review

Several signaling pathways and basic metabolites are responsible for the control of metabolism in both normal and cancer cells. As emerging hallmarks of cancer metabolism, the abnormal activities of these pathways are of the most noticeable events in cancer. This altered metabolism expedites the survival and proliferation of cancer cells, which have attracted a substantial amount of interest in cancer metabolism. Nowadays, targeting metabolism and cross-linked signaling pathways in cancer has been a hot topic to investigate novel drugs against cancer. Despite the efficiency of conventional drugs in cancer therapy, their associated toxicity, resistance, and high-cost cause limitations in their application. Besides, considering the numerous signaling pathways cross-linked with cancer metabolism, discovery, and development of multi-targeted and safe natural compounds has been a high priority. Natural secondary metabolites have exhibited promising anticancer effects by targeting dysregulated signaling pathways linked to cancer metabolism. The present review reveals the metabolism and cross-linked dysregulated signaling pathways in cancer. The promising therapeutic targets in cancer, as well as the critical role of natural secondary metabolites for significant anticancer enhancements, have also been highlighted to find novel/potential therapeutic agents for cancer treatment.

Biomarker selection and imaging design in cancer: A link with biochemical pathways for imminent engineering

Malignant cells reprogram metabolic pathways to meet the demands of growth and proliferation. These altered manners of metabolism are now identified as hallmarks of cancer. Studies have revealed tumor cells alter specific pathways such as glycolysis, fatty acid synthesis and amino acid synthesis to support their proliferation. In this review, we provide a theoretical framework to understand metabolic reprogramming and the mechanisms accompanying distorted metabolism to tumor progression. How these alterations will be assisting in cancer diagnostics and advances in standard techniques in marker identification and imagining are also discussed.

Determining contributions of exogenous glucose and fructose to de novo fatty acid and glycerol synthesis in liver and adipose tissue

The de novo synthesis of triglyceride (TG) fatty acids (FA) and glycerol can be measured with stable isotope tracers. However, these methods typically do not inform the contribution of a given substrate to specific pathways on these synthetic processes. We integrated deuterated water (²H₂O) measurement of de novo lipogenesis (DNL) and glycerol-3-phosphate (GLY) synthesis from all substrates with a ¹³C nuclear magnetic resonance (NMR) method that quantifies TG FA and glycerol enrichment from a specific [U-¹³C]precursor. This allowed the [U-¹³C]precursor contribution to DNL and GLY to be estimated. We applied this method in mice to determine the contributions of fructose and glucose supplemented in the drinking water to DNL and GLY in liver, mesenteric adipose tissue (MAT) and subcutaneous adipose tissue (SCAT). In liver, fructose contributed significantly more to DNL of saturated fatty acids (SFA) and oleate as well as to GLY compared to glucose. Moreover, its contribution to SFA synthesis was significantly higher compared to that of oleate. MAT and SCAT had lower fractional rates of total DNL and GLY compared to liver and glucose was utilized more predominantly than fructose for TG synthesis in these tissues. This novel ²H₂O/¹³C integrated method revealed for the first time, tissue specific selection of substrates for DNL, particularly fructose in regard to glucose in liver. Also, this approach was able to resolve the distribution of specific FAs into the TG sn2 and sn1,3 sites. This stable isotope integrated approach yielded information so far uncovered by other lipidomic tools and should powerfully assist in other nutritional, pathological or environmental contexts.

The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

Stable isotope-based methods have proved to be immensely valuable for ecological studies ranging in focus from animal movements to species interactions and community structure. Nevertheless, the use of these methods is dependent on assumptions about the incorporation and turnover of isotopes within animal tissues, which are oftentimes not explicitly acknowledged and vetted. Thus, the purpose of this review is to provide an overview of the estimation of stable isotope turnover rates in animals, and to highlight the importance of these estimates for ecological studies in terrestrial, freshwater, and marine systems that may use a wide range of stable isotopes. Specifically, we discuss 1) the factors that contribute to variation in turnover among individuals and across species, which influences the use of stable isotopes for diet reconstructions, 2) the differences in turnover among tissues that underlie so-called ‘isotopic clocks’, which are used to estimate the timing of dietary shifts, and 3) the use of turnover rates to estimate nutritional requirements and reconstruct histories of nutritional stress from tissue isotope signatures. As we discuss these topics, we highlight recent works that have effectively used estimates of turnover to design and execute informative ecological studies. Our concluding remarks suggest several steps that will improve our understanding of isotopic turnover and support its integration into a wider range of ecological studies.

Dynamics of Individual Fatty Acids in Muscle Fat Stores and Membranes of a Songbird and Its Functional and Ecological Importance

Although tissue fatty acid (FA) composition has been linked to whole-animal performance (e.g., aerobic endurance, metabolic rate, postexercise recovery) in a wide range of animal taxa, we do not adequately understand the pace of changes in FA composition and its implications for the ecology of animals. Therefore, we used a C₄ to C₃ diet shift experiment and compound-specific δ¹³C analysis to estimate the turnover rates of FAs in the polar and neutral fractions of flight muscle lipids (corresponding to membranes and lipid droplets) of exercised and sedentary zebra finches (Taeniopygia guttata). Turnover was fastest for linoleic acid (LA; 18:2n6) and palmitic acid (PA; 16:0), with 95% replacement times of 10.8-17.7 d in the polar fraction and 17.2-32.8 d in the neutral fraction, but was unexpectedly slow for the long-chain polyunsaturated FAs (LC-PUFAs) arachidonic acid (20:4n6) and docosahexaenoic acid (22:6n3) in the polar fraction, with 95% replacement in 64.9-136.5 d. Polar fraction LA and PA turnover was significantly faster in exercised birds (95% replacement in 8.5-13.3 d). Our results suggest that FA turnover in intramuscular lipid droplets is related to FA tissue concentrations and that turnover does not change in response to exercise. In contrast, we found that muscle membrane FA turnover is likely driven by a combination of selective LC-PUFA retention and consumption of shorter-chain FAs in energy metabolism. The unexpectedly fast turnover of membrane-associated FAs in muscle suggests that songbirds during migration could substantially remodel their membranes within a single migration stopover, and this may have substantial implications for how the FA composition of diet affects energy metabolism of birds during migration.

Hepatocyte-specific Sirt6 deficiency impairs ketogenesis

Sirt6 is an NADH (NAD⁺)-dependent deacetylase with a critical role in hepatic lipid metabolism. Ketogenesis is controlled by a signaling network of hepatic lipid metabolism. However, how Sirt6 functions in ketogenesis remains unclear. Here, we demonstrated that Sirt6 functions as a mediator of ketogenesis in response to a fasting and ketogenic diet (KD). The KD-fed hepatocyte-specific Sirt6 deficiency (HKO) mice exhibited impaired ketogenesis, which was due to enhanced Fsp27 (fat-specific induction of protein 27), a protein known to regulate lipid metabolism. In contrast, overexpression of Sirt6 in mouse primary hepatocytes promoted ketogenesis. Mechanistically, Sirt6 repressed Fsp27β expression by interacting with Crebh (cAMP response element-binding protein H) and preventing its recruitment to the Fsp27β gene promoter. The KD-fed HKO mice also showed exacerbated hepatic steatosis and inflammation. Finally, Fsp27 silencing rescued hypoketonemia and other metabolic phenotypes in KD-fed HKO mice. Our data suggest that the Sirt6-Crebh-Fsp27 axis is pivotal for hepatic lipid metabolism and inflammation. Sirt6 may be a pharmacological target to remedy metabolic diseases.

Actionable Metabolic Pathways in Heart Failure and Cancer-Lessons From Cancer Cell Metabolism

Recent advances in cancer cell metabolism provide unprecedented opportunities for a new understanding of heart metabolism and may offer new approaches for the treatment of heart failure. Key questions driving the cancer field to understand how tumor cells reprogram metabolism and to benefit tumorigenesis are also applicable to the heart. Recent experimental and conceptual advances in cancer cell metabolism provide the cardiovascular field with the unique opportunity to target metabolism. This review compares cancer cell metabolism and cardiac metabolism with an emphasis on strategies of cellular adaptation, and how to exploit metabolic changes for therapeutic benefit.

Restricting glycolysis impairs brown adipocyte glucose and oxygen consumption

During thermogenic activation, brown adipocytes take up large amounts of glucose. In addition, cold stimulation leads to an upregulation of glycolytic enzymes. Here we have investigated the importance of glycolysis for brown adipocyte glucose consumption and thermogenesis. Using siRNA-mediated knockdown in mature adipocytes, we explored the effect of glucose transporters and glycolytic enzymes on brown adipocyte functions such as consumption of glucose and oxygen. Basal oxygen consumption in brown adipocytes was equally dependent on glucose and fatty acid oxidation, whereas isoproterenol (ISO)-stimulated respiration was fueled mainly by fatty acids, with a significant contribution from glucose oxidation. Knockdown of glucose transporters in brown adipocytes not only impaired ISO-stimulated glycolytic flux but also oxygen consumption. Diminishing glycolytic flux by knockdown of the first and final enzyme of glycolysis, i.e., hexokinase 2 (HK2) and pyruvate kinase M (PKM), respectively, decreased glucose uptake and ISO-stimulated oxygen consumption. HK2 knockdown had a more severe effect, which, in contrast to PKM knockdown, could not be rescued by supplementation with pyruvate. Hence, brown adipocytes rely on glucose consumption and glycolytic flux to achieve maximum thermogenic output, with glycolysis likely supporting thermogenesis not only by pyruvate formation but also by supplying intermediates for efferent metabolic pathways.

Mycobacterial heat shock protein 65 mediated metabolic shift in decidualization of human endometrial stromal cells

Successful implantation is dependent on the appropriate decidualization of endometrial stromal cells for the establishment of pregnancy in women. Mycobacterial heat shock protein 65 (HSP65) is involved in pathogenesis of the genital tuberculosis (GTB), one of the common causes of infertility in emerging countries. Though implantation failure appears to be the major cause, understanding the status of decidualizaiton process in women diagnosed with GTB has not been thoroughly addressed. We, therefore, explored the effect of HSP65 protein on the endometrial cell metabolism during in vitro decidualization. In order to identify the cellular metabolism of decidual cells with and without HSP65 treatment, proton NMR based characterization of metabolites extracted from cells and culture media were performed. In presence of HSP65, significant reduction in the decidual phenotype of endometrial stromal cells and prolactin expression is suggestive of impairment in decidualization. The intracellular and extracellular metabolic changes in HSP65 treated endometrial stromal cells produced a distinct pattern, reflecting the interaction between the protein and cellular metabolism. HSP65 mediated dysregulation in cellular metabolism is associated with poor decidualization. Besides enriching the present knowledge on metabolic changes underlying stromal cells decidualization, these findings assist in identifying potential molecular causes for decidualization failure in GTB women.

Comprehensive analysis of glucose and xylose metabolism in Escherichia coli under aerobic and anaerobic conditions by 13C metabolic flux analysis

Glucose and xylose are the two most abundant sugars derived from the breakdown of lignocellulosic biomass. While aerobic glucose metabolism is relatively well understood in E. coli, until now there have been only a handful of studies focused on anaerobic glucose metabolism and no ¹³C-flux studies on xylose metabolism. In the absence of experimentally validated flux maps, constraint-based approaches such as MOMA and RELATCH cannot be used to guide new metabolic engineering designs. In this work, we have addressed this critical gap in current understanding by performing comprehensive characterizations of glucose and xylose metabolism under aerobic and anaerobic conditions, using recent state-of-the-art techniques in ¹³C metabolic flux analysis (¹³C-MFA). Specifically, we quantified precise metabolic fluxes for each condition by performing parallel labeling experiments and analyzing the data through integrated ¹³C-MFA using the optimal tracers [1,2-¹³C]glucose, [1,6-¹³C]glucose, [1,2-¹³C]xylose and [5-¹³C]xylose. We also quantified changes in biomass composition and confirmed turnover of macromolecules by applying [U-¹³C]glucose and [U-¹³C]xylose tracers. We demonstrated that under anaerobic growth conditions there is significant turnover of lipids and that a significant portion of CO₂ originates from biomass turnover. Using knockout strains, we also demonstrated that β-oxidation is critical for anaerobic growth on xylose. Quantitative analysis of co-factor balances (NADH/FADH₂, NADPH, and ATP) for different growth conditions provided new insights regarding the interplay of energy and redox metabolism and the impact on E. coli cell physiology.

The two faces of reactive oxygen species (ROS) in adipocyte function and dysfunction

White adipose tissue (WAT) is actively involved in the regulation of whole-body energy homeostasis via storage/release of lipids and adipokine secretion. Current research links WAT dysfunction to the development of metabolic syndrome (MetS) and type 2 diabetes (T2D). The expansion of WAT during oversupply of nutrients prevents ectopic fat accumulation and requires proper preadipocyte-to-adipocyte differentiation. An assumed link between excess levels of reactive oxygen species (ROS), WAT dysfunction and T2D has been discussed controversially. While oxidative stress conditions have conclusively been detected in WAT of T2D patients and related animal models, clinical trials with antioxidants failed to prevent T2D or to improve glucose homeostasis. Furthermore, animal studies yielded inconsistent results regarding the role of oxidative stress in the development of diabetes. Here, we discuss the contribution of ROS to the (patho)physiology of adipocyte function and differentiation, with particular emphasis on sources and nutritional modulators of adipocyte ROS and their functions in signaling mechanisms controlling adipogenesis and functions of mature fat cells. We propose a concept of ROS balance that is required for normal functioning of WAT. We explain how both excessive and diminished levels of ROS, e.g. resulting from over supplementation with antioxidants, contribute to WAT dysfunction and subsequently insulin resistance.

Fundamentals of cancer metabolism

Tumors reprogram pathways of nutrient acquisition and metabolism to meet the bioenergetic, biosynthetic, and redox demands of malignant cells. These reprogrammed activities are now recognized as hallmarks of cancer, and recent work has uncovered remarkable flexibility in the specific pathways activated by tumor cells to support these key functions. In this perspective, we provide a conceptual framework to understand how and why metabolic reprogramming occurs in tumor cells, and the mechanisms linking altered metabolism to tumorigenesis and metastasis. Understanding these concepts will progressively support the development of new strategies to treat human cancer.

Mitochondria in Cancer Energy Metabolism: Culprits or Bystanders?

Cancer is a disease characterized by uncontrolled growth. Metabolic demands to sustain rapid proliferation must be compelling since aerobic glycolysis is the first as well as the most commonly shared characteristic of cancer. During the last decade, the significance of metabolic reprogramming of cancer has been at the center of attention. Nonetheless, despite all the knowledge gained on cancer biology, the field is not able to reach agreement on the issue of mitochondria: Are damaged mitochondria the cause for aerobic glycolysis in cancer? Warburg proposed the damaged mitochondria theory over 80 years ago; the field has been testing the theory equally long. In this review, we will discuss alterations in metabolic fluxes of cancer cells, and provide an opinion on the damaged mitochondria theory.

Cancer-Specific Production of N-Acetylaspartate via NAT8L Overexpression in Non-Small Cell Lung Cancer and Its Potential as a Circulating Biomarker

In order to identify new cancer-associated metabolites that may be useful for early detection of lung cancer, we performed a global metabolite profiling of a non-small cell lung cancer (NSCLC) line and immortalized normal lung epithelial cells from the same patient. Among several metabolites with significant cancer/normal differences, we identified a unique metabolic compound, N-acetylaspartate (NAA), in cancer cells-undetectable in normal lung epithelium. NAA's cancer-specific detection was validated in additional cancer and control lung cells as well as selected NSCLC patient tumors and control tissues. NAA's cancer specificity was further supported in our analysis of NAA synthetase (gene symbol: NAT8L) gene expression levels in The Cancer Genome Atlas: elevated NAT8L expression in approximately 40% of adenocarcinoma and squamous cell carcinoma cases (N = 577), with minimal expression in all nonmalignant lung tissues (N = 74). We then showed that NAT8L is functionally involved in NAA production of NSCLC cells through siRNA-mediated suppression of NAT8L, which caused selective reduction of intracellular and secreted NAA. Our cell culture experiments also indicated that NAA biosynthesis in NSCLC cells depends on glutamine availability. For preliminary evaluation of NAA's clinical potential as a circulating biomarker, we developed a sensitive NAA blood assay and found that NAA blood levels were elevated in 46% of NSCLC patients (N = 13) in comparison with age-matched healthy controls (N = 21) among individuals aged 55 years or younger. Taken together, these results indicate that NAA is produced specifically in NSCLC tumors through NAT8L overexpression, and its extracellular secretion can be detected in blood. Cancer Prev Res; 9(1); 43-52. ©2015 AACR.

Catabolism of Branched Chain Amino Acids Contributes Significantly to Synthesis of Odd-Chain and Even-Chain Fatty Acids in 3T3-L1 Adipocytes

The branched chain amino acids (BCAA) valine, leucine and isoleucine have been implicated in a number of diseases including obesity, insulin resistance, and type 2 diabetes mellitus, although the mechanisms are still poorly understood. Adipose tissue plays an important role in BCAA homeostasis by actively metabolizing circulating BCAA. In this work, we have investigated the link between BCAA catabolism and fatty acid synthesis in 3T3-L1 adipocytes using parallel ¹³C-labeling experiments, mass spectrometry and model-based isotopomer data analysis. Specifically, we performed parallel labeling experiments with four fully ¹³C-labeled tracers, [U-¹³C]valine, [U-¹³C]leucine, [U-¹³C]isoleucine and [U-¹³C]glutamine. We measured mass isotopomer distributions of fatty acids and intracellular metabolites by GC-MS and analyzed the data using the isotopomer spectral analysis (ISA) framework. We demonstrate that 3T3-L1 adipocytes accumulate significant amounts of even chain length (C14:0, C16:0 and C18:0) and odd chain length (C15:0 and C17:0) fatty acids under standard cell culture conditions. Using a novel GC-MS method, we demonstrate that propionyl-CoA acts as the primer on fatty acid synthase for the production of odd chain fatty acids. BCAA contributed significantly to the production of all fatty acids. Leucine and isoleucine contributed at least 25% to lipogenic acetyl-CoA pool, and valine and isoleucine contributed 100% to lipogenic propionyl-CoA pool. Our results further suggest that low activity of methylmalonyl-CoA mutase and mass action kinetics of propionyl-CoA on fatty acid synthase result in high rates of odd chain fatty acid synthesis in 3T3-L1 cells. Overall, this work provides important new insights into the connection between BCAA catabolism and fatty acid synthesis in adipocytes and underscores the high capacity of adipocytes for metabolizing BCAA.

Reviving Lonidamine and 6-Diazo-5-oxo-L-norleucine to Be Used in Combination for Metabolic Cancer Therapy

Abnormal metabolism is another cancer hallmark. The two most characterized altered metabolic pathways are high rates of glycolysis and glutaminolysis, which are natural targets for cancer therapy. Currently, a number of newer compounds to block glycolysis and glutaminolysis are being developed; nevertheless, lonidamine and 6-diazo-5-oxo-L-norleucine (DON) are two old drugs well characterized as inhibitors of glycolysis and glutaminolysis, respectively, whose clinical development was abandoned years ago when the importance of cancer metabolism was not fully appreciated and clinical trial methodology was less developed. In this review, a PubMed search using the words lonidamine and 6-diazo-5-oxo-L-norleucine (DON) was undertaken to analyse existing information on the preclinical and clinical studies of these drugs for cancer treatment. Data show that they exhibit antitumor effects; besides there is also the suggestion that they are synergistic. We conclude that lonidamine and DON are safe and potentially effective drugs that need to be reevaluated in combination as metabolic therapy of cancer.

Non-targeted Tracer Fate Detection

Stable isotopes have been used to trace atoms through metabolism and quantify metabolic fluxes for several decades. Only recently non-targeted stable isotope labeling approaches have emerged as a powerful tool to gain insights into metabolism. However, the manual detection of isotopic enrichment for a non-targeted analysis is tedious and time consuming. To overcome this limitation, the non-targeted tracer fate detection (NTFD) algorithm for the automated metabolome-wide detection of isotopic enrichment has been developed. NTFD detects and quantifies isotopic enrichment in the form of mass isotopomer distributions (MIDs) in an automated manner, providing the means to trace functional groups, determine MIDs for metabolic flux analysis, or detect tracer-derived molecules in general. Here, we describe the algorithmic background of NTFD, discuss practical considerations for the freely available NTFD software package, and present potential applications of non-targeted stable isotope labeling analysis.

Integrated 13C-metabolic flux analysis of 14 parallel labeling experiments in Escherichia coli

The use of parallel labeling experiments for (13)C metabolic flux analysis ((13)C-MFA) has emerged in recent years as the new gold standard in fluxomics. The methodology has been termed COMPLETE-MFA, short for complementary parallel labeling experiments technique for metabolic flux analysis. In this contribution, we have tested the limits of COMPLETE-MFA by demonstrating integrated analysis of 14 parallel labeling experiments with Escherichia coli. An effort on such a massive scale has never been attempted before. In addition to several widely used isotopic tracers such as [1,2-(13)C]glucose and mixtures of [1-(13)C]glucose and [U-(13)C]glucose, four novel tracers were applied in this study: [2,3-(13)C]glucose, [4,5,6-(13)C]glucose, [2,3,4,5,6-(13)C]glucose and a mixture of [1-(13)C]glucose and [4,5,6-(13)C]glucose. This allowed us for the first time to compare the performance of a large number of isotopic tracers. Overall, there was no single best tracer for the entire E. coli metabolic network model. Tracers that produced well-resolved fluxes in the upper part of metabolism (glycolysis and pentose phosphate pathways) showed poor performance for fluxes in the lower part of metabolism (TCA cycle and anaplerotic reactions), and vice versa. The best tracer for upper metabolism was 80% [1-(13)C]glucose+20% [U-(13)C]glucose, while [4,5,6-(13)C]glucose and [5-(13)C]glucose both produced optimal flux resolution in the lower part of metabolism. COMPLETE-MFA improved both flux precision and flux observability, i.e. more independent fluxes were resolved with smaller confidence intervals, especially exchange fluxes. Overall, this study demonstrates that COMPLETE-MFA is a powerful approach for improving flux measurements and that this methodology should be considered in future studies that require very high flux resolution.

13C isotope-assisted methods for quantifying glutamine metabolism in cancer cells

Glutamine has recently emerged as a key substrate to support cancer cell proliferation, and the quantification of its metabolic flux is essential to understand the mechanisms by which this amino acid participates in the metabolic rewiring that sustains the survival and growth of neoplastic cells. Glutamine metabolism involves two major routes, glutaminolysis and reductive carboxylation, both of which begin with the deamination of glutamine to glutamate and the conversion of glutamate into α-ketoglutarate. In glutaminolysis, α-ketoglutarate is oxidized via the tricarboxylic acid cycle and decarboxylated to pyruvate. In reductive carboxylation, α-ketoglutarate is reductively converted into isocitrate, which is isomerized to citrate to supply acetyl-CoA for de novo lipogenesis. Here, we describe methods to quantify the metabolic flux of glutamine through these two routes, as well as the contribution of glutamine to lipid synthesis. Examples of how these methods can be applied to study metabolic pathways of oncological relevance are provided.

Modulation of polyamine metabolic flux in adipose tissue alters the accumulation of body fat by affecting glucose homeostasis

The continued rise in obesity despite public education, awareness and policies indicates the need for mechanism-based therapeutic approaches to help control the disease. Our data, in conjunction with other studies, suggest an unexpected role for the polyamine catabolic enzyme spermidine/spermine-N1-acetyltransferase (SSAT) in fat homeostasis. Our previous studies showed that deletion of SSAT greatly exaggerates weight gain and that the transgenic overexpression suppresses weight gain in mice on a high-fat diet. This discovery is substantial but the underlying molecular linkages are only vaguely understood. Here, we used a comprehensive systems biology approach, on white adipose tissue (WAT), to discover that the partition of acetyl-CoA towards polyamine catabolism alters glucose homeostasis and hence, fat accumulation. Comparative proteomics and antibody-based expression studies of WAT in SSAT knockout, wild type and transgenic mice identified nine proteins with an increasing gradient across the genotypes, all of which correlate with acetyl-CoA consumption in polyamine acetylation. Adipose-specific SSAT knockout mice and global SSAT knockout mice on a high-fat diet exhibited similar growth curves and proteomic patterns in their WAT, confirming that attenuated consumption of acetyl-CoA in acetylation of polyamines in adipose tissue drives the obese phenotype of these mice. Analysis of protein expression indicated that the identified changes in the levels of proteins regulating acetyl-CoA consumption occur via the AMP-activated protein kinase pathway. Together, our data suggest that differential expression of SSAT markedly alters acetyl-CoA levels, which in turn trigger a global shift in glucose metabolism in adipose tissue, thus affecting the accumulation of body fat.

In vivo HIF-mediated reductive carboxylation is regulated by citrate levels and sensitizes VHL-deficient cells to glutamine deprivation

Hypoxic and VHL-deficient cells use glutamine to generate citrate and lipids through reductive carboxylation (RC) of α-ketoglutarate. To gain insights into the role of HIF and the molecular mechanisms underlying RC, we took advantage of a panel of disease-associated VHL mutants and showed that HIF expression is necessary and sufficient for the induction of RC in human renal cell carcinoma (RCC) cells. HIF expression drastically reduced intracellular citrate levels. Feeding VHL-deficient RCC cells with acetate or citrate or knocking down PDK-1 and ACLY restored citrate levels and suppressed RC. These data suggest that HIF-induced low intracellular citrate levels promote the reductive flux by mass action to maintain lipogenesis. Using [(1-13)C]glutamine, we demonstrated in vivo RC activity in VHL-deficient tumors growing as xenografts in mice. Lastly, HIF rendered VHL-deficient cells sensitive to glutamine deprivation in vitro, and systemic administration of glutaminase inhibitors suppressed the growth of RCC cells as mice xenografts.

Ketone body metabolism and cardiovascular disease

Ketone bodies are metabolized through evolutionarily conserved pathways that support bioenergetic homeostasis, particularly in brain, heart, and skeletal muscle when carbohydrates are in short supply. The metabolism of ketone bodies interfaces with the tricarboxylic acid cycle, β-oxidation of fatty acids, de novo lipogenesis, sterol biosynthesis, glucose metabolism, the mitochondrial electron transport chain, hormonal signaling, intracellular signal transduction pathways, and the microbiome. Here we review the mechanisms through which ketone bodies are metabolized and how their signals are transmitted. We focus on the roles this metabolic pathway may play in cardiovascular disease states, the bioenergetic benefits of myocardial ketone body oxidation, and prospective interactions among ketone body metabolism, obesity, metabolic syndrome, and atherosclerosis. Ketone body metabolism is noninvasively quantifiable in humans and is responsive to nutritional interventions. Therefore, further investigation of this pathway in disease models and in humans may ultimately yield tailored diagnostic strategies and therapies for specific pathological states.

Using multiple tracers for 13C metabolic flux analysis

(13)C-Metabolic flux analysis ((13)C-MFA) is a powerful technique for quantifying intracellular metabolic fluxes in living cells. These in vivo fluxes provide important information on the physiology of cells in culture, which can be used for metabolic engineering purposes and serve as inputs for systems biology modeling. The (13)C-MFA technique consists of several steps: (1) selecting appropriate tracers for a given system of interest, (2) performing isotopic labeling experiments, (3) measuring isotopic labeling distributions in metabolic products, (4) estimating metabolic fluxes using least-squares regression, and (5) evaluating the goodness of fit and computing confidence intervals for estimated fluxes. In this chapter, we provide guidelines for performing (13)C-MFA studies using multiple isotopic tracers, a technique that is especially useful for elucidating fluxes in complex biological systems where multiple carbon sources are present. Here, as an example, we describe key steps and decision points for designing (13)C-MFA studies for microbes grown on mixtures of glucose and xylose. The general concepts described in this chapter are applicable to many other biological systems. For example, the same procedures can be applied to design (13)C-MFA studies in mammalian cells, which are generally grown in complex media containing multiple substrates such as glucose and amino acids.

Parallel labeling experiments and metabolic flux analysis: Past, present and future methodologies

Radioactive and stable isotopes have been applied for decades to elucidate metabolic pathways and quantify carbon flow in cellular systems using mass and isotope balancing approaches. Isotope-labeling experiments can be conducted as a single tracer experiment, or as parallel labeling experiments. In the latter case, several experiments are performed under identical conditions except for the choice of substrate labeling. In this review, we highlight robust approaches for probing metabolism and addressing metabolically related questions though parallel labeling experiments. In the first part, we provide a brief historical perspective on parallel labeling experiments, from the early metabolic studies when radioisotopes were predominant to present-day applications based on stable-isotopes. We also elaborate on important technical and theoretical advances that have facilitated the transition from radioisotopes to stable-isotopes. In the second part of the review, we focus on parallel labeling experiments for (13)C-metabolic flux analysis ((13)C-MFA). Parallel experiments offer several advantages that include: tailoring experiments to resolve specific fluxes with high precision; reducing the length of labeling experiments by introducing multiple entry-points of isotopes; validating biochemical network models; and improving the performance of (13)C-MFA in systems where the number of measurements is limited. We conclude by discussing some challenges facing the use of parallel labeling experiments for (13)C-MFA and highlight the need to address issues related to biological variability, data integration, and rational tracer selection.

Parallel labeling experiments with [1,2-(13)C]glucose and [U-(13)C]glutamine provide new insights into CHO cell metabolism

We applied a parallel labeling strategy using two isotopic tracers, [1,2-(13)C]glucose and [U-(13)C]glutamine, to determine metabolic fluxes in Chinese hamster ovary (CHO) cells. CHO cells were grown in parallel cultures over a period of six days with glucose and glutamine feeding. On days 2 and 5, isotopic tracers were introduced and (13)C-labeling of intracellular metabolites was measured by gas chromatography-mass spectrometry (GC-MS). Metabolites in glycolysis pathway reached isotopic steady state for [1,2-(13)C]glucose within 1.5h, and metabolites in the TCA cycle reached isotopic steady state for [U-(13)C]glutamine within 3h. Combined analysis of multiple data sets produced detailed flux maps at two key metabolic phases, exponential growth phase (day 2) and early stationary phase (day 5). Flux results revealed significant rewiring of intracellular metabolism in the transition from growth to non-growth, including changes in oxidative pentose phosphate pathway, anaplerosis, amino acid metabolism, and fatty acid biosynthesis. At the growth phase, de novo fatty acid biosynthesis correlated well with the lipid requirements for cell growth. However, surprisingly, at the non-growth phase the fatty acid biosynthesis flux remained high even though no new lipids were needed for cell growth. Additionally, we identified a discrepancy in the estimated TCA cycle flux obtained using traditional stoichiometric flux balancing and (13)C-metabolic flux analysis. Our results suggested that CHO cells produced additional metabolites from glucose that were not captured in previous metabolic models. Follow-up experiments with [U-(13)C]glucose confirmed that additional metabolites were accumulating in the medium that became M+3 and M+6 labeled.

Cancer-associated isocitrate dehydrogenase mutations inactivate NADPH-dependent reductive carboxylation

Isocitrate dehydrogenase (IDH) is a reversible enzyme that catalyzes the NADP(+)-dependent oxidative decarboxylation of isocitrate (ICT) to α-ketoglutarate (αKG) and the NADPH/CO(2)-dependent reductive carboxylation of αKG to ICT. Reductive carboxylation by IDH1 was potently inhibited by NADP(+) and, to a lesser extent, by ICT. IDH1 and IDH2 with cancer-associated mutations at the active site arginines were unable to carry out the reductive carboxylation of αKG. These mutants were also defective in ICT decarboxylation and converted αKG to 2-hydroxyglutarate using NADPH. These mutant proteins were thus defective in both of the normal reactions of IDH. Biochemical analysis of heterodimers between wild-type and mutant IDH1 subunits showed that the mutant subunit did not inactivate reductive carboxylation by the wild-type subunit. Cells expressing the mutant IDH are thus deficient in their capacity for reductive carboxylation and may be compromised in their ability to produce acetyl-CoA under hypoxia or when mitochondrial function is otherwise impaired.

Aerobic glycolysis: meeting the metabolic requirements of cell proliferation

Warburg's observation that cancer cells exhibit a high rate of glycolysis even in the presence of oxygen (aerobic glycolysis) sparked debate over the role of glycolysis in normal and cancer cells. Although it has been established that defects in mitochondrial respiration are not the cause of cancer or aerobic glycolysis, the advantages of enhanced glycolysis in cancer remain controversial. Many cells ranging from microbes to lymphocytes use aerobic glycolysis during rapid proliferation, which suggests it may play a fundamental role in supporting cell growth. Here, we review how glycolysis contributes to the metabolic processes of dividing cells. We provide a detailed accounting of the biosynthetic requirements to construct a new cell and illustrate the importance of glycolysis in providing carbons to generate biomass. We argue that the major function of aerobic glycolysis is to maintain high levels of glycolytic intermediates to support anabolic reactions in cells, thus providing an explanation for why increased glucose metabolism is selected for in proliferating cells throughout nature.

Combined transcriptomic-(1)H NMR metabonomic study reveals that monoethylhexyl phthalate stimulates adipogenesis and glyceroneogenesis in human adipocytes

Adipose tissue is a major storage site for lipophilic environmental contaminants. The environmental metabolic disruptor hypothesis postulates that some pollutants can promote obesity or metabolic disorders by activating nuclear receptors involved in the control of energetic homeostasis. In this context, monoethylhexyl phthalate (MEHP) is of particular concern since it was shown to activate the peroxisome proliferator-activated receptor γ (PPARγ) in 3T3-L1 murine preadipocytes. In the present work, we used an untargeted, combined transcriptomic-¹H NMR-based metabonomic approach to describe the overall effect of MEHP on primary cultures of human subcutaneous adipocytes differentiated in vitro. MEHP stimulated rapidly and selectively the expression of genes involved in glyceroneogenesis, enhanced the expression of the cytosolic phosphoenolpyruvate carboxykinase, and reduced fatty acid release. These results demonstrate that MEHP increased glyceroneogenesis and fatty acid reesterification in human adipocytes. A longer treatment with MEHP induced the expression of genes involved in triglycerides uptake, synthesis, and storage; decreased intracellular lactate, glutamine, and other amino acids; increased aspartate and NAD, and resulted in a global increase in triglycerides. Altogether, these results indicate that MEHP promoted the differentiation of human preadipocytes to adipocytes. These mechanisms might contribute to the suspected obesogenic effect of MEHP.

Using an untargeted combined transcriptomic-¹H NMR-based metabonomic approach, we describe the overall effect of monoethyl-hexyl phthalate (MEHP) on primary cultures of human subcutaneous adipocytes differentiated in vitro. MEHP rapidly and selectively stimulated glyceroneogenesis, a metabolic pathway involved in the control of fatty acid release from adipose tissue. A longer treatment with MEHP promoted the differentiation of human preadipocytes to adipocytes. These mechanisms might contribute to an obesogenic effect of MEHP.

De novo lipogenesis in the differentiating human adipocyte can provide all fatty acids necessary for maturation

The primary products of de novo lipogenesis (DNL) are saturated fatty acids, which confer adverse cellular effects. Human adipocytes differentiated with no exogenous fat accumulated triacylglycerol (TG) in lipid droplets and differentiated normally. TG composition showed the products of DNL (saturated fatty acids from 12:0 to 18:0) together with unsaturated fatty acids (particularly 16:1n-7 and 18:1n-9) produced by elongation/desaturation. There was parallel upregulation of expression of genes involved in DNL and in fatty acid elongation and desaturation, suggesting coordinated control of expression. Enzyme products (desaturation ratios, elongation ratios, and total pathway flux) were also correlated with mRNA levels. We used (13)C-labeled substrates to study the pathway of DNL. Glucose (5 mM or 17.5 mM in the medium) provided less than half the carbon used for DNL (42% and 47%, respectively). Glutamine (2 mM) provided 9-10%, depending upon glucose concentration. In contrast, glucose provided most (72%) of the carbon of TG-glycerol. Pathway analysis using mass isotopomer distribution analysis (MIDA) revealed that the pathway for conversion of glucose to palmitate is complex. DNL in human fat cells is tightly coupled with further modification of fatty acids to produce a range of saturated and unsaturated fatty acids consistent with normal maturation.

Analysis of mammalian fatty acyl-coenzyme A species by mass spectrometry and tandem mass spectrometry

Acyl-CoAs are intermediates of numerous metabolic processes in eukaryotic cells, including beta-oxidation within mitochondria and peroxisomes, and the biosynthesis/remodeling of lipids (e.g. mono-, di-, and triglycerides, phospholipids and sphingolipids). Investigations of lipid metabolism have been advanced by the ability to quantitate acyl-CoA intermediates via liquid chromatography coupled to electrospray ionization-tandem mass spectrometric detection (LC-ESI-MS/MS), which is presently one of the most sensitive and specific analytical methods for both lipids and acyl-CoAs. This review of acyl-CoA analysis by mass spectrometry focuses on mammalian samples and long-chain analytes (i.e. palmitoyl-CoA), particularly reports of streamlined methodology, improved recovery, or expansion of the number of acyl chain-lengths amenable to quantitation.

Myc-dependent mitochondrial generation of acetyl-CoA contributes to fatty acid biosynthesis and histone acetylation during cell cycle entry

Cell reprogramming from a quiescent to proliferative state requires coordinate activation of multiple -omic networks. These networks activate histones, increase cellular bioenergetics and the synthesis of macromolecules required for cell proliferation. However, mechanisms that coordinate the regulation of these interconnected networks are not fully understood. The oncogene c-Myc (Myc) activates cellular metabolism and global chromatin remodeling. Here we tested for an interconnection between Myc regulation of metabolism and acetylation of histones. Using [(13)C(6)]glucose and a combination of GC/MS and LC/ESI tandem mass spectrometry, we determined the fractional incorporation of (13)C-labeled 2-carbon fragments into the fatty acid palmitate, and acetyl-lysines at the N-terminal tail of histone H4 in myc(-/-) and myc(+/+) Rat1A fibroblasts. Our data demonstrate that Myc increases mitochondrial synthesis of acetyl-CoA, as the de novo synthesis of (13)C-labeled palmitate was increased 2-fold in Myc-expressing cells. Additionally, Myc induced a forty percent increase in (13)C-labeled acetyl-CoA on H4-K16. This is linked to the capacity of Myc to increase mitochondrial production of acetyl-CoA, as we show that mitochondria provide 50% of the acetyl groups on H4-K16. These data point to a key role for Myc in directing the interconnection of -omic networks, and in particular, epigenetic modification of proteins in response to proliferative signals.

Quantifying reductive carboxylation flux of glutamine to lipid in a brown adipocyte cell line

We previously reported that glutamine was a major source of carbon for de novo fatty acid synthesis in a brown adipocyte cell line. The pathway for fatty acid synthesis from glutamine may follow either of two distinct pathways after it enters the citric acid cycle. The glutaminolysis pathway follows the citric acid cycle, whereas the reductive carboxylation pathway travels in reverse of the citric acid cycle from alpha-ketoglutarate to citrate. To quantify fluxes in these pathways we incubated brown adipocyte cells in [U-(13)C]glutamine or [5-(13)C]glutamine and analyzed the mass isotopomer distribution of key metabolites using models that fit the isotopomer distribution to fluxes. We also investigated inhibitors of NADP-dependent isocitrate dehydrogenase and mitochondrial citrate export. The results indicated that one third of glutamine entering the citric acid cycle travels to citrate via reductive carboxylation while the remainder is oxidized through succinate. The reductive carboxylation flux accounted for 90% of all flux of glutamine to lipid. The inhibitor studies were compatible with reductive carboxylation flux through mitochondrial isocitrate dehydrogenase. Total cell citrate and alpha-ketoglutarate were near isotopic equilibrium as expected if rapid cycling exists between these compounds involving the mitochondrial membrane NAD/NADP transhydrogenase. Taken together, these studies demonstrate a new role for glutamine as a lipogenic precursor and propose an alternative to the glutaminolysis pathway where flux of glutamine to lipogenic acetyl-CoA occurs via reductive carboxylation. These findings were enabled by a new modeling tool and software implementation (Metran) for global flux estimation.

Global metabolic effects of glycerol kinase overexpression in rat hepatoma cells

Glycerol kinase has several diverse activities in mammalian cells. Glycerol kinase deficiency is a complex, single-gene, inborn error of metabolism wherein no genotype-phenotype correlation has been established. Since glycerol kinase has been suggested to exhibit additional activities than glycerol phosphorylation, expression level perturbation in this enzyme may affect cellular physiology globally. To investigate this possibility, we conducted metabolic investigations of wild-type and two glycerol kinase-overexpressing H4IIE rat hepatoma cell lines constructed in this study. The glycerol kinase-overexpressing cell lines exhibited a significantly higher consumption of carbon sources per cell, suggesting excess carbon expenditure. Furthermore, we quantified intracellular metabolic fluxes by employing stable isotope 13C labeling with a mathematically designed substrate mixture, gas chromatography-mass spectrometry, and comprehensive isotopomer balancing. This flux analysis revealed that the pentose phosphate pathway flux in the glycerol kinase-overexpressing cell lines was 2-fold higher than that in the wild-type, in addition to subtler flux changes in other pathways of carbohydrate metabolism. Furthermore, the activity and transcript level of the lipogenic enzyme glucose-6-phosphate dehydrogenase, the rate-limiting enzyme of the pentose phosphate pathway, were also about 2-fold higher than that of the wild-type; these data corroborate the flux analysis results. This study shows that glycerol kinase affects carbon metabolism globally, possibly through its additional functions, and highlights glycerol kinase's multifaceted role in cellular physiology.

Metabolic flux analysis in a nonstationary system: fed-batch fermentation of a high yielding strain of E. coli producing 1,3-propanediol

Metabolic fluxes estimated from stable-isotope studies provide a key to understanding cell physiology and regulation of metabolism. A limitation of the classical method for metabolic flux analysis (MFA) is the requirement for isotopic steady state. To extend the scope of flux determination from stationary to nonstationary systems, we present a novel modeling strategy that combines key ideas from isotopomer spectral analysis (ISA) and stationary MFA. Isotopic transients of the precursor pool and the sampled products are described by two parameters, D and G parameters, respectively, which are incorporated into the flux model. The G value is the fraction of labeled product in the sample, and the D value is the fractional contribution of the feed for the production of labeled products. We illustrate the novel modeling strategy with a nonstationary system that closely resembles industrial production conditions, i.e. fed-batch fermentation of Escherichia coli that produces 1,3-propanediol (PDO). Metabolic fluxes and the D and G parameters were estimated by fitting labeling distributions of biomass amino acids measured by GC/MS to a model of E. coli metabolism. We obtained highly consistent fits from the data with 82 redundant measurements. Metabolic fluxes were estimated for 20 time points during course of the fermentation. As such we established, for the first time, detailed time profiles of in vivo fluxes. We found that intracellular fluxes changed significantly during the fed-batch. The intracellular flux associated with PDO pathway increased by 10%. Concurrently, we observed a decrease in the split ratio between glycolysis and pentose phosphate pathway from 70/30 to 50/50 as a function of time. The TCA cycle flux, on the other hand, remained constant throughout the fermentation. Furthermore, our flux results provided additional insight in support of the assumed genotype of the organism.