Estimation of glucose-alanine-lactate-glutamine cycles in postabsorptive humans: role of skeletal muscle

Evaluating a Model of Added Sugar Intake Based on Amino Acid Carbon Isotope Ratios in a Controlled Feeding Study of U.S. Adults

Previous studies suggest that amino acid carbon stable isotope ratios (CIRAAs) may serve as biomarkers of added sugar (AS) intake, but this has not been tested in a demographically diverse population. We conducted a 15-day feeding study of U.S. adults, recruited across sex, age, and BMI groups. Participants consumed personalized diets that resembled habitual intake, assessed using two consecutive 7-day food records. We measured serum (n = 99) CIRAAs collected at the end of the feeding period and determined correlations with diet. We used forward selection to model AS intake using participant characteristics and 15 CIRAAs. This model was internally validated using bootstrap optimism correction. Median (25th, 75th percentile) AS intake was 65.2 g/day (44.7, 81.4) and 9.5% (7.2%, 12.4%) of energy. The CIR of alanine had the highest, although modest, correlation with AS intake (r = 0.32, p = 0.001). Serum CIRAAs were more highly correlated with animal food intakes, especially the ratio of animal to total protein. The AS model included sex, body weight and 6 CIRAAs. This model had modest explanatory power (multiple R2 = 0.38), and the optimism-corrected R2 was lower (R2 = 0.15). Further investigations in populations with wider ranges of AS intake are warranted.

Lactic Acid Regulation: A Potential Therapeutic Option in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic, persistent autoimmune disease that causes severe joint tissue damage and irreversible disability. Cumulative evidence suggests that patients suffering from RA for long durations are at risk of functional damage to cardiovascular, kidney, lung, and other tissues. This seriously affects the quality of work and life of patients. To date, no clear etiology of RA has been found. Recent studies have revealed that the massive proliferation of synoviocytes and immune cells requires a large amount of energy supply. Rapid energy supply depends on the anaerobic glucose metabolic pathway in both RA animal models and clinical patients. Anaerobic glycolysis can increase intracellular lactic acid (LA) content. LA induces the overexpression of monocarboxylate transporters (MCTs) in cell membranes. MCTs rapidly transport LA from the intracellular to the intercellular or articular cavity. Hence, a relatively high accumulation of LA could be formed in the intercellular and articular cavities of inflammatory joints. Moreover, LA contributes to the migration and activation of immune cells. Immune cells proliferate and secrete interleukins (IL) including IL-1, IL-2, IL-13, IL-17, and other inflammatory factors. These inflammatory factors enhance the immune inflammatory response of the body and aggravate the condition of RA patients. In this paper, the effects of LA on RA pathogenesis will be summarized from the perspective of the production, transport, and metabolism of synoviocytes and immune cells. Additionally, the drugs involved in the production, transport, and metabolism of LA are highlighted.

Chemoresistant Cancer Cell Lines Are Characterized by Migratory, Amino Acid Metabolism, Protein Catabolism and IFN1 Signalling Perturbations

Simple Summary

While chemoresistance remains a major barrier to improving the outcomes for patients with ovarian cancer, the molecular features, and associated biological functions, which underpin chemoresistance in ovarian cancer remain poorly understood. In this study we aimed to provide insight into the proteins and metabolites, and their associated biological pathways, which play a role in conferring chemoresistance to ovarian cancer. Through mass spectrometry analysis comparing the proteome and metabolome of chemosensitive vs chemoresistant ovarian cancer cell lines we revealed numerous perturbations in signalling and metabolic pathways in chemoresistant cells. Further comparison to primary cells taken from patients with chemoresistant or chemosensitive disease identified a shared dysregulation in cytokine and type 1 interferon signalling. Our research sets the foundation for a deeper understanding of the proteomic and metabolomic features of chemoresistance and identifies type 1 interferon signalling as a common feature of chemoresistance.

Abstract

Chemoresistance remains the major barrier to effective ovarian cancer treatment. The molecular features and associated biological functions of this phenotype remain poorly understood. We developed carboplatin-resistant cell line models using OVCAR5 and CaOV3 cell lines with the aim of identifying chemoresistance-specific molecular features. Chemotaxis and CAM invasion assays revealed enhanced migratory and invasive potential in OVCAR5-resistant, compared to parental cell lines. Mass spectrometry analysis was used to analyse the metabolome and proteome of these cell lines, and was able to separate these populations based on their molecular features. It revealed signalling and metabolic perturbations in the chemoresistant cell lines. A comparison with the proteome of patient-derived primary ovarian cancer cells grown in culture showed a shared dysregulation of cytokine and type 1 interferon signalling, potentially revealing a common molecular feature of chemoresistance. A comprehensive analysis of a larger patient cohort, including advanced in vitro and in vivo models, promises to assist with better understanding the molecular mechanisms of chemoresistance and the associated enhancement of migration and invasion.

Role of Lactate in Inflammatory Processes: Friend or Foe

During an inflammatory process, shift in the cellular metabolism associated with an increase in extracellular acidification are well-known features. This pH drop in the inflamed tissue is largely attributed to the presence of lactate by an increase in glycolysis. In recent years, evidence has accumulated describing the role of lactate in inflammatory processes; however, there are differences as to whether lactate can currently be considered a pro- or anti-inflammatory mediator. Herein, we review these recent advances on the pleiotropic effects of lactate on the inflammatory process. Taken together, the evidence suggests that lactate could exert differential effects depending on the metabolic status, cell type in which the effects of lactate are studied, and the pathological process analyzed. Additionally, various targets, including post-translational modifications, G-protein coupled receptor and transcription factor activation such as NF-κB and HIF-1, allow lactate to modulate signaling pathways that control the expression of cytokines, chemokines, adhesion molecules, and several enzymes associated with immune response and metabolism. Altogether, this would explain its varied effects on inflammatory processes beyond its well-known role as a waste product of metabolism.

The carbon isotope ratios of nonessential amino acids identify sugar-sweetened beverage (SSB) consumers in a 12-wk inpatient feeding study of 32 men with varying SSB and meat exposures

BACKGROUND

The carbon isotope ratios (CIRs) of individual amino acids (AAs) may provide more sensitive and specific biomarkers of sugar-sweetened beverages (SSBs) than total tissue CIR. Because CIRs turn over slowly, long-term controlled-feeding studies are needed in their evaluation.

OBJECTIVE

We assessed the responses of plasma and RBC CIRAA's to SSB and meat intake in a 12-wk inpatient feeding study.

METHODS

Thirty-two men (aged 46.2 ± 10.5 y) completed the feeding study at the National Institute of Diabetes and Digestive and Kidney Diseases in Phoenix, Arizona. The effects of SSB, meat, and fish intake on plasma and RBC CIRAA's were evaluated in a balanced factorial design with each dietary variable either present or absent in a common weight-maintaining, macronutrient-balanced diet. Fasting blood samples were collected biweekly from baseline. Dietary effects on the postfeeding CIR of 5 nonessential AAs (CIRNEAA's) and 4 essential AAs (CIREAA's) were analyzed using multivariable regression.

RESULTS

In plasma, 4 of 5 CIRNEAA's increased with SSB intake. Of these, the CIRAla was the most sensitive (β = 2.81, SE = 0.38) to SSB intake and was not affected by meat or fish intake. In RBCs, all 5 CIRNEAA's increased with SSBs but had smaller effect sizes than in plasma. All plasma CIREAA's increased with meat intake (but not SSB or fish intake), and the CIRLeu was the most sensitive (β = 1.26, SE = 0.23). CIRs of leucine and valine also increased with meat intake in RBCs. Estimates of turnover suggest that CIRAA's in plasma, but not RBCs, were in equilibrium with the diets by the end of the study.

CONCLUSIONS

The results of this study in men support CIRNEAA's as potential biomarkers of SSB intake and suggest CIREAA's as potential biomarkers of meat intake in US diets. This trial was registered at clinicaltrials.gov/ct2/show/NCT01237093 as NCT01237093.

Regulation of Postabsorptive and Postprandial Glucose Metabolism by Insulin-Dependent and Insulin-Independent Mechanisms: An Integrative Approach

Glucose levels in blood must be constantly maintained within a tight physiological range to sustain anabolism. Insulin regulates glucose homeostasis via its effects on glucose production from the liver and kidneys and glucose disposal in peripheral tissues (mainly skeletal muscle). Blood levels of glucose are regulated simultaneously by insulin-mediated rates of glucose production from the liver (and kidneys) and removal from muscle; adipose tissue is a key partner in this scenario, providing nonesterified fatty acids (NEFA) as an alternative fuel for skeletal muscle and liver when blood glucose levels are depleted. During sleep at night, the gradual development of insulin resistance, due to growth hormone and cortisol surges, ensures that blood glucose levels will be maintained within normal levels by: (a) switching from glucose to NEFA oxidation in muscle; (b) modulating glucose production from the liver/kidneys. After meals, several mechanisms (sequence/composition of meals, gastric emptying/intestinal glucose absorption, gastrointestinal hormones, hyperglycemia mass action effects, insulin/glucagon secretion/action, de novo lipogenesis and glucose disposal) operate in concert for optimal regulation of postprandial glucose fluctuations. The contribution of the liver in postprandial glucose homeostasis is critical. The liver is preferentially used to dispose over 50% of the ingested glucose and restrict the acute increases of glucose and insulin in the bloodstream after meals, thus protecting the circulation and tissues from the adverse effects of marked hyperglycemia and hyperinsulinemia.

Dietary Energy Partition: The Central Role of Glucose

Humans have developed effective survival mechanisms under conditions of nutrient (and energy) scarcity. Nevertheless, today, most humans face a quite different situation: excess of nutrients, especially those high in amino-nitrogen and energy (largely fat). The lack of mechanisms to prevent energy overload and the effective persistence of the mechanisms hoarding key nutrients such as amino acids has resulted in deep disorders of substrate handling. There is too often a massive untreatable accumulation of body fat in the presence of severe metabolic disorders of energy utilization and disposal, which become chronic and go much beyond the most obvious problems: diabetes, circulatory, renal and nervous disorders included loosely within the metabolic syndrome. We lack basic knowledge on diet nutrient dynamics at the tissue-cell metabolism level, and this adds to widely used medical procedures lacking sufficient scientific support, with limited or nil success. In the present longitudinal analysis of the fate of dietary nutrients, we have focused on glucose as an example of a largely unknown entity. Even most studies on hyper-energetic diets or their later consequences tend to ignore the critical role of carbohydrate (and nitrogen disposal) as (probably) the two main factors affecting the substrate partition and metabolism.

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.

The Carbon Isotope Ratios of Serum Amino Acids in Combination with Participant Characteristics can be Used to Estimate Added Sugar Intake in a Controlled Feeding Study of US Postmenopausal Women

BACKGROUND

The carbon isotope ratio (CIR) is a proposed biomarker for added sugar (AS) intake in the United States; however, because the CIR is also associated with meat intake in most populations the need for specificity remains. The CIR of amino acids (AAs) has the potential to differentiate sugars from meat intakes, because essential AAs must derive from dietary protein whereas certain nonessential AAs can be synthesized from sugars.

OBJECTIVES

We tested whether serum CIR-AAs in combination with participant characteristics could meet a prespecified biomarker criterion for AS intake in the Nutrition and Physical Activity Assessment Study Feeding Study (NPAAS-FS) of the Women's Health Initiative, a population in which the whole-serum CIR was not associated with AS intake.

METHODS

Postmenopausal women (n = 145) from Seattle, WA, were provided with individualized diets that approximated their habitual food intakes for 2 wk. Dietary intakes from consumed foods were characterized over the feeding period using the Nutrition Data System for Research. The CIR of 7 AAs-Ala, Gly, Val, Leu, Ile, Pro, and Phe-were measured in fasting serum collected at the end of the 2-wk feeding period, using gas chromatography-combustion isotope ratio mass spectrometry. Biomarker models were evaluated using regression R2 ≥ 0.36 as a major biomarker criterion, based on the benchmark R2 values of well-established recovery biomarkers in the NPAAS-FS.

RESULTS

AS intake was associated with CIR-Ala (ρ = 0.32; P < 0.0001). A model of AS intake based on CIR-Ala, CIR-Gly, CIR-Ile, smoking, leisure physical activity, and body weight met the biomarker criterion (R2 = 0.37). Biomarker-estimated AS intake was not associated with meat or animal protein intake.

CONCLUSIONS

Results support serum CIR-AAs in combination with participant characteristics as potential biomarkers of AS intake in US populations, including those with low AS intake.The Women's Health Initiative is registered at clinicaltrials.gov (NCT00000611).

Regulation of hepatic mitochondrial oxidation by glucose-alanine cycling during starvation in humans

In order to determine whether the glucose-alanine cycle regulates rates of hepatic mitochondrial oxidation in humans, we applied positional isotopomer NMR tracer analysis (PINTA) to assess rates of hepatic mitochondrial oxidation and pyruvate carboxylase flux in healthy volunteers following both an overnight (12 hours) and a 60-hour fast. Following the 60-hour fast, rates of endogenous glucose production and mitochondrial oxidation decreased, whereas rates of hepatic pyruvate carboxylase flux remained unchanged. These reductions were associated with reduced rates of alanine turnover, assessed by [3-13C]alanine, in a subgroup of participants under similar fasting conditions. In order to determine whether this reduction in alanine turnover was responsible for the reduced rates of hepatic mitochondrial oxidation, we infused unlabeled alanine into another subgroup of 60-hour fasted subjects to increase rates of alanine turnover, similar to what was measured after a 12-hour fast, and found that this perturbation increased rates of hepatic mitochondrial oxidation. Taken together, these studies demonstrate that 60 hours of starvation induce marked reductions in rates of hepatic mitochondrial oxidation, which in turn can be attributed to reduced rates of glucose-alanine cycling, and reveal a heretofore undescribed role for glucose-alanine in the regulation of hepatic mitochondrial oxidation in humans.

Mangosteen Concentrate Drink Supplementation Promotes Antioxidant Status and Lactate Clearance in Rats after Exercise

High-strength or long-duration exercise can lead to significant fatigue, oxidative stress, and muscle damage. The purpose of this study was to examine the effect of mangosteen concentrate drink (MCD) supplementation on antioxidant capacity and lactate clearance in rats after running exercise. Forty rats were divided into five groups: N, non-treatment; C, control; or supplemented with MCD, including M1, M5, and M10 (0.9, 4.5, and 9 mL/day) for 6 weeks. The rats were subjected to 30 min running and exhaustive-running tests using a treadmill. The blood lactate; triglyceride; cholesterol and glucose levels; hepatic and muscular malonaldehyde (MDA) levels; and antioxidant enzymes, including superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT), were analyzed. The results of this study demonstrated that MCD supplementation can increase GPx and CAT activities, alleviate oxidative stress in muscle, and increase lactate clearance, and is thereby beneficial to reduced muscle fatigue after exercise.

STC1 and PTHrP Modify Carbohydrate and Lipid Metabolism in Liver of a Teleost Fish

Stanniocalcin 1 (STC1) and parathyroid hormone-related protein (PTHrP) are calciotropic hormones in vertebrates. Here, a recently hypothesized metabolic role for these hormones is tested on European sea bass treated with: (i) teleost PTHrP(1-34), (ii) PTHrP(1-34) and anti-STC1 serum (pro-PTHrP groups), (iii) a PTHrP antagonist PTHrP(7-34) or (iv) PTHrP(7-34) and STC1 (pro-STC1 groups). Livers were analysed using untargeted metabolic profiling based on proton nuclear magnetic resonance (¹H-NMR) spectroscopy. Concentrations of branched-chain amino acid (BCAA), alanine, glutamine and glutamate increased in pro-STC1 groups suggesting their mobilization from the muscle to the liver for degradation and gluconeogenesis from alanine and glutamine. In addition, only STC1 treatment decreased the concentrations of succinate, fumarate and acetate, indicating slowing of the citric acid cycle. In the pro-PTHrP groups the concentrations of glucose, erythritol and lactate decreased, indicative of gluconeogenesis from lactate. Taurine, trimethylamine, trimethylamine N-oxide and carnitine changed in opposite directions in the pro-STC1 versus the pro-PTHrP groups, suggesting opposite effects, with STC1 stimulating lipogenesis and PTHrP activating lipolysis/β-oxidation of fatty acids. These findings suggest a role for STC1 and PTHrP related to strategic energy mechanisms that involve the production of glucose and safeguard of liver glycogen reserves for stressful situations.

Altered Plasma Amino Acids and Lipids Associated With Abnormal Glucose Metabolism and Insulin Resistance in Older Adults

CONTEXT AND OBJECTIVES

Glucose metabolism becomes progressively impaired with older age. Fasting glucose and insulin resistance are risk factors for premature death and other adverse outcomes. We aimed to identifying plasma metabolites associated with altered glucose metabolism and insulin resistance in older community-dwelling adults.

PARTICIPANTS AND METHODS

A targeted metabolomics approach was used to identify plasma metabolites associated with impaired fasting plasma glucose, 2-hour plasma glucose on oral glucose tolerance testing, and homeostatic model assessment insulin resistance (HOMA-IR) in 472 participants who participated in the Baltimore Longitudinal Study of Aging, with a mean (SD) age of 70.7 (9.9) years.

RESULTS

We measured 143 plasma metabolites. In ordinal logistic regression analyses, using a false discovery rate of 5% and adjusting for potential confounders, we found that alanine, glutamic acid, and proline were significantly associated with increased odds of abnormal fasting plasma glucose. Phosphatidylcholine (diacyl C34:4, alkyl-acyl C32:1, C32:2, C34:2, C34:3, and C36:3) was associated with decreased odds of abnormal fasting plasma glucose. Glutamic and acid phosphatidylcholine alkyl-acyl C34:2 were associated with increased and decreased odds of 2-hour plasma glucose, respectively. Glutamic acid was associated with increased odds of higher tertiles of HOMA-IR. Glycine; phosphatidylcholine (diacyl C32:0, alkyl-acyl C32:1, C32:2, C34:1, C34:2, C34:3, C36:2, C36:3, C40:5, C40:6, C42:3, C42:4, and C42:5); sphingomyelin C16:0, C24:1, and C26:1; and lysophosphatidylcholine C18:1 were associated with decreased odds of abnormal HOMA-IR.

CONCLUSIONS

Targeted metabolomics identified four plasma amino acids and 16 plasma lipid species, primarily containing polyunsaturated fatty acids, that were associated with abnormal glucose metabolism and insulin resistance in older adults.

Efficacy and safety of oral insulin compared to subcutaneous insulin: a systematic review and meta-analysis

INTRODUCTION

A systematic review and meta-analysis of interventional studies was conducted to compare the efficacy and safety of oral insulin versus subcutaneous (SC) insulin in diabetic patients.

METHODS

Medline, Scopus, ISI Web of Knowledge and Cochrane Central Register of Controlled Trials were searched. Two independent reviewers evaluated studies for eligibility and quality and extracted the data. The primary outcomes were fasting blood glucose (FBG), 1h and 2h postprandial blood glucose, HbA1c, AUC of insulin, C max and T max of insulin, and T max of glucose infusion rate. Secondary outcomes were adverse events.

RESULTS

Eleven studies (n = 373) met the inclusion criteria. Meta-analyses showed that there is no significant difference between oral and SC insulin in controlling HbA1c, FBG, 1 and 2 h postprandial blood glucose and producing C max of insulin (P > 0.05); however oral insulin had faster action as indicated by the shorter T max, compared to SC insulin (P < 0.05). The most included studies were varied in their methodological quality.

CONCLUSION

This systematic review and meta-analysis showed that oral insulin is comparable to SC insulin with regard to glycemic efficacy and safety. However, is necessary to conduct additional studies in which oral insulin administered to large number of patients for long enough periods of time.

Arteriovenous Blood Metabolomics: A Readout of Intra-Tissue Metabostasis

The human circulatory system consists of arterial blood that delivers nutrients to tissues, and venous blood that removes the metabolic by-products. Although it is well established that arterial blood generally has higher concentrations of glucose and oxygen relative to venous blood, a comprehensive biochemical characterization of arteriovenous differences has not yet been reported. Here we apply cutting-edge, mass spectrometry-based metabolomic technologies to provide a global characterization of metabolites that vary in concentration between the arterial and venous blood of human patients. Global profiling of paired arterial and venous plasma from 20 healthy individuals, followed up by targeted analysis made it possible to measure subtle (<2 fold), yet highly statistically significant and physiologically important differences in water soluble human plasma metabolome. While we detected changes in lactic acid, alanine, glutamine, and glutamate as expected from skeletal muscle activity, a number of unanticipated metabolites were also determined to be significantly altered including Krebs cycle intermediates, amino acids that have not been previously implicated in transport, and a few oxidized fatty acids. This study provides the most comprehensive assessment of metabolic changes in the blood during circulation to date and suggests that such profiling approach may offer new insights into organ homeostasis and organ specific pathology.

Stable Isotope Ratios as Biomarkers of Diet for Health Research

Diet is a leading modifiable risk factor for chronic disease, but it remains difficult to measure accurately due to the error and bias inherent in self-reported methods of diet assessment. Consequently, there is a pressing need for more objective biomarkers of diet for use in health research. The stable isotope ratios of light elements are a promising set of candidate biomarkers because they vary naturally and reproducibly among foods, and those variations are captured in molecules and tissues with high fidelity. Recent studies have identified valid isotopic measures of short- and long-term sugar intake, meat intake, and fish intake in specific populations. These studies provide a strong foundation for validating stable isotopic biomarkers in the general US population. Approaches to improve specificity for specific foods are needed; for example, by modeling intake using multiple stable isotope ratios or by isolating and measuring specific molecules linked to foods of interest.

Applying (1)H NMR Spectroscopy to Detect Changes in the Urinary Metabolite Levels of Chinese Half-Pipe Snowboarders after Different Exercises

Monitoring physical training is important for the health and performance of athletes, and real-time assessment of fatigue is crucial to improve training efficiency. The relationship between key biomarkers and exercise has been reported. The aim of this study was to determine the effects of different levels of training exercises on the urine metabolome. (1)H NMR-based metabolomics analysis was performed on urine samples from half-pipe snowboarders, and spectral profiles were subjected to PCA and PLS-DA. Our results show that metabolic profiles varied during different stages of exercises. Lactate, alanine, trimethylamine, malonate, taurine, and glycine levels decreased while TMAO and phenylalanine levels increased in the stage with higher amount and intensity of exercise. Although the amount of exercise was reduced in subsequent stage, no significant variations of metabolic profile were found. Metabolic changes induced by training level were analyzed with related metabolic pathway. Studying metabolome changes can provide a better understanding of the physiology of athletes and could aid in adjusting training.

Plasma lactate and diabetes risk in 8045 participants of the atherosclerosis risk in communities study

PURPOSE

Determinants of oxidative capacity, such as fitness and level of adiposity, are strongly associated with type 2 diabetes. Whether decreased oxidative capacity itself is a cause or consequence of insulin resistance and diabetes is unknown.

METHODS

We examined the association of plasma lactate, a marker of oxidative capacity, with incident diabetes in 8045 participants from the Atherosclerosis Risk in Communities (ARIC) Study with no history of subclinical or diagnosed diabetes at baseline (1996-1998). Incident diabetes was self-reported during annual telephone calls.

RESULTS

During a median follow-up of 12 years, there were 1513 new cases of diabetes. In Cox proportional hazards models, baseline plasma lactate (per 10 mg/dL) was significantly associated with diabetes (hazard ratio, 1.20; 95% confidence interval, 1.01-1.43), even after adjustment for diabetes risk factors, fasting glucose, and insulin. The upper quartile of baseline lactate (≥ 8.1 mg/dL) was also significantly associated with diabetes risk (hazard ratio, 1.20; 95% confidence interval, 1.02-1.41) compared with the lowest quartile (≤ 5.1 mg/dL). Significant associations persisted among persons without insulin resistance (homeostatic model assessment insulin resistance index < 2.6 U) (P-trend < .01).

CONCLUSIONS

These findings suggest that low oxidative capacity may precede diabetes. Future studies should evaluate the physiological origins of elevated lactate to better understand its possible role in the pathogenesis of diabetes.

The carbon isotope ratio of alanine in red blood cells is a new candidate biomarker of sugar-sweetened beverage intake

An objective dietary biomarker would help clarify the contribution of sugar-sweetened beverage (SSB) intake to obesity and chronic disease risk. Previous studies have proposed the carbon isotope ratio (δ(13)C) as a biomarker of SSB intake but found associations that were of modest size and confounded by other components of the diet. We investigated whether the δ(13)C values of nonessential amino acids (δ(13)CNEAA) in RBCs could provide valid biomarkers that are more specific to SSBs. We assessed the associations of RBC δ(13)CNEAA with SSB intake in a study population of 68 Yup'ik people, using gas chromatography/combustion/isotope ratio mass spectrometry to measure δ(13)CNEAA and four 24-h dietary recalls to assess intake. Among RBC nonessential amino acids, alanine δ(13)C (δ(13)Calanine) was strongly correlated with intake of SSBs, added sugar, and total sugar (r = 0.70, 0.59, and 0.57, respectively; P < 0.0001) but uncorrelated with other dietary sources of elevated δ(13)C. We also evaluated whether sweetener intake could be noninvasively assessed using hair δ(13)Calanine in a subset of the study population (n = 30). Hair δ(13)Calanine was correlated with RBC δ(13)Calanine (r = 0.65; P < 0.0001) and showed similar associations with SSB intake. These results show that δ(13)Calanine in RBCs provides a valid and specific biomarker of SSB intake for the Yup'ik population and suggest RBCs and hair δ(13)Calanine as candidate biomarkers of SSB intake for validation in the general U.S. population. Ultimately, these biomarkers could clarify our understanding of whether and how SSB intake contributes to chronic disease.

Quantitative dynamics of the link between cellular metabolism and histone acetylation

Acetylation on the tails of histones plays an important role in controlling transcription initiation. Although the steady-state abundances of histone acetyl groups have been reported, the rate at which histones are acetylated and deacetylated on a residue-specific basis has not been quantitatively established. We added [(13)C]glucose to human cells and monitored the dynamic incorporation of (13)C-labeled acetyl groups onto specific histone lysines with quantitative mass spectrometry. We determined the turnover of acetylation to be generally slower than phosphorylation, but fast relative to methylation, and that the rate varied depending on the histone, the residue modified, and also the neighboring modifications. Cells were also treated with a deacetylase inhibitor to determine the rate due to histone acetyltransferase activity alone and in the absence of deacetylase activity. Introduction of (13)C-labeled glucose also resulted in the incorporation of (13)C into alanine, which allowed us to partition histones into existing and newly synthesized protein categories. Newly synthesized histones were slower to accumulate histone modifications, especially modifications associated with silent chromatin. Finally, we applied our new approaches to find that quiescent fibroblasts exhibited lower levels of labeled acetyl accumulation compared with proliferating fibroblasts. This suggests that acetylation rates can be modulated in cells in different biological states and that these changes can be detected with the approach presented here. The methods we describe can be broadly applied to defining the turnover of histone acetylation in other cell states such as during cellular reprogramming and to quantify non-histone protein acetylation dynamics.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Predicting metabolic adaptation, body weight change, and energy intake in humans

Complex interactions between carbohydrate, fat, and protein metabolism underlie the body's remarkable ability to adapt to a variety of diets. But any imbalances between the intake and utilization rates of these macronutrients will result in changes in body weight and composition. Here, I present the first computational model that simulates how diet perturbations result in adaptations of fuel selection and energy expenditure that predict body weight and composition changes in both obese and nonobese men and women. No model parameters were adjusted to fit these data other than the initial conditions for each subject group (e.g., initial body weight and body fat mass). The model provides the first realistic simulations of how diet perturbations result in adaptations of whole body energy expenditure, fuel selection, and various metabolic fluxes that ultimately give rise to body weight change. The validated model was used to estimate free-living energy intake during a long-term weight loss intervention, a variable that has never previously been measured accurately.

Oral insulin: the rationale for this approach and current developments

Insulin remains the most effective and durable hypoglycemic agent for the treatment of diabetes. The addition of an effective oral insulin dosage form to the antidiabetes armamentarium may have significant benefits in terms of fostering compliance and adherence among patients, as well as physiologic advantages due to the fact that such a dosage form replicates the natural route of insulin secretion and absorption through the portal vein and targets the liver directly. Several companies have developed technological platforms that protect polypeptides and proteins from enzymatic hydrolysis, enable their transport across the epithelial lining, and promote their absorption from the gastrointestinal tract. A review of the potential physiological rationale and advantages, as well as of current pertinent technologies used specifically with insulin, is herewith provided.

Rhabdomyosarcoma cells show an energy producing anabolic metabolic phenotype compared with primary myocytes

BACKGROUND

The functional status of a cell is expressed in its metabolic activity. We have applied stable isotope tracing methods to determine the differences in metabolic pathways in proliferating Rhabdomysarcoma cells (Rh30) and human primary myocytes in culture. Uniformly 13C-labeled glucose was used as a source molecule to follow the incorporation of 13C into more than 40 marker metabolites using NMR and GC-MS. These include metabolites that report on the activity of glycolysis, Krebs' cycle, pentose phosphate pathway and pyrimidine biosynthesis.

RESULTS

The Rh30 cells proliferated faster than the myocytes. Major differences in flux through glycolysis were evident from incorporation of label into secreted lactate, which accounts for a substantial fraction of the glucose carbon utilized by the cells. Krebs' cycle activity as determined by 13C isotopomer distributions in glutamate, aspartate, malate and pyrimidine rings was considerably higher in the cancer cells than in the primary myocytes. Large differences were also evident in de novo biosynthesis of riboses in the free nucleotide pools, as well as entry of glucose carbon into the pyrimidine rings in the free nucleotide pool. Specific labeling patterns in these metabolites show the increased importance of anaplerotic reactions in the cancer cells to maintain the high demand for anabolic and energy metabolism compared with the slower growing primary myocytes. Serum-stimulated Rh30 cells showed higher degrees of labeling than serum starved cells, but they retained their characteristic anabolic metabolism profile. The myocytes showed evidence of de novo synthesis of glycogen, which was absent in the Rh30 cells.

CONCLUSION

The specific 13C isotopomer patterns showed that the major difference between the transformed and the primary cells is the shift from energy and maintenance metabolism in the myocytes toward increased energy and anabolic metabolism for proliferation in the Rh30 cells. The data further show that the mitochondria remain functional in Krebs' cycle activity and respiratory electron transfer that enables continued accelerated glycolysis. This may be a common adaptive strategy in cancer cells.

Acute depletion of plasma glutamine increases leucine oxidation in prednisone-treated humans

BACKGROUND, AIMS & METHODS

To determine whether depletion in plasma glutamine worsens the catabolic response to corticosteroids, seven healthy volunteers received oral prednisone for 6 days on two separate occasions, at least 2 weeks apart, and in random order. On the sixth day of each treatment course, they received 5 h intravenous infusions of L-[1-(14)C]-leucine and L-[1-(13)C]-glutamine in the postabsorptive state (1) under baseline conditions (prednisone only day) and (2) after 24h of treatment with phenylbutyrate (prednisone+phenylbutyrate day), a glutamine chelating agent.

RESULTS

Phenylbutyrate treatment was associated with (1) an approximately 15% decline in plasma glutamine concentration (627+/-39 vs. 530+/-31 micromol l(-1); P<0.05), (2) no change in leucine appearance rate, an index of protein breakdown (124+/-9 vs. 128+/-9 micromol kg(-1) h(-1); NS) nor in non-oxidative leucine disposal, an index of whole body protein synthesis (94+/-9 vs. 91+/-7 micromol kg(-1) h(-1); NS), and (3) a approximately 25% rise in leucine oxidation (30+/-1 vs. 38+/-2 micromol kg(-1) h(-1), P<0.05), despite an approximately 25% decline (P<0.05) in leucine concentration.

CONCLUSIONS

In a model of mild, stress-induced protein catabolism, depletion of plasma glutamine per se may worsen branched chain amino acid and protein wasting.

Computational model of in vivo human energy metabolism during semistarvation and refeeding

Changes in body weight and composition are the result of complex interactions among metabolic fluxes contributing to macronutrient balances. To better understand these interactions, a mathematical model was constructed that used the measured dietary macronutrient intake during semistarvation and refeeding as model inputs and computed whole body energy expenditure, de novo lipogenesis, and gluconeogenesis as well as turnover and oxidation of carbohydrate, fat, and protein. Published in vivo human data provided the basis for the model components that were integrated by fitting a few unknown parameters to the classic Minnesota human starvation experiment. The model simulated the measured body weight and fat mass changes during semistarvation and refeeding and predicted the unmeasured metabolic fluxes underlying the body composition changes. The resting metabolic rate matched the experimental measurements and required a model of adaptive thermogenesis. Refeeding caused an elevation of de novo lipogenesis that, along with increased fat intake, resulted in a rapid repletion and overshoot of body fat. By continuing the computer simulation with the prestarvation diet and physical activity, the original body weight and composition were eventually restored, but body fat mass was predicted to take more than one additional year to return to within 5% of its original value. The model was validated by simulating a recently published short-term caloric restriction experiment without changing the model parameters. The predicted changes in body weight, fat mass, resting metabolic rate, and nitrogen balance matched the experimental measurements, thereby providing support for the validity of the model.

Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony

Recent data underscore the importance of intertissue communication in the maintenance of normal glucose homeostasis. Important signals are conveyed by hormones, cytokines, and fuel substrates and are sensed through a variety of cellular mechanisms. The ability of tissues to sense and adapt to changes in metabolic status and fuel availability is altered in insulin-resistant states including type 2 diabetes. Here we review the roles of glucose and its metabolites as signaling molecules and the diverse physiologic mechanisms for glucose sensing.

Impact of glutamine supplementation on glucose homeostasis during and after exercise

The interaction of glutamine availability and glucose homeostasis during and after exercise was investigated, measuring whole body glucose kinetics with [3-3H]glucose and net organ balances of glucose and amino acids (AA) during basal, exercise, and postexercise hyperinsulinemic-euglycemic clamp periods in six multicatheterized dogs. Dogs were studied twice in random treatment order: once with glutamine (12 micromol.kg(-1).min(-1); Gln) and once with saline (Con) infused intravenously during and after exercise. Plasma glucose fell by 7 mg/dl with exercise in Con (P < 0.05), but it did not fall with Gln. Gln further stimulated whole body glucose production and utilization an additional 24% above a normal exercise response (P < 0.05). Net hepatic uptake of glutamine and alanine was greater with Gln than Con during exercise (P < 0.05). Net hepatic glucose output was increased sevenfold during exercise with Gln (P < 0.05) but not with Con. Net hindlimb glucose uptake was increased similarly during exercise in both groups (P < 0.05). During the postexercise hyperinsulinemic-euglycemic period, glucose production decreased to near zero with Con, but it did not decrease below basal levels with Gln. Gln increased glucose utilization by 16% compared with Con after exercise (P < 0.05). Furthermore, net hindlimb glucose uptake in the postexercise period was increased approximately twofold vs. basal with Gln (P < 0.05) but not with Con. Net hepatic uptake of glutamine during the postexercise period was threefold greater for Gln than Con (P < 0.05). In conclusion, glutamine availability modulates glucose homeostasis during and after exercise, which may have implications for postexercise recovery.

Kinetics, molecular basis, and differentiation of L-lactate transport in spermatogenic cells

Round spermatid energy metabolism is closely dependent on the presence of L-lactate in the external medium. This L-lactate has been proposed to be supplied by Sertoli cells in the seminiferous tubules. L-Lactate, in conjunction with glucose, modulates intracellular Ca(2+) concentration in round spermatids and pachytene spermatocytes. In spite of this central role of L-lactate in spermatogenic cell physiology, the mechanism of L-lactate transport, as well as possible differentiation during spermatogenesis, has not been studied in these cells. By measuring radioactive L-lactate transport and intracellular pH (pH(i)) changes with pH(i) fluorescent probes, we show that these cells transport L-lactate using monocarboxylate-H(+) transport (MCT) systems. RT-PCR, in situ mRNA hybridization, and immunocyto- and immunohistochemistry data show that pachytene spermatocytes express mainly the MCT1 and MCT4 isoforms of the transporter (intermediate- and low-affinity transporters, respectively), while round spermatids, besides MCT1 and MCT4, also show expression of the MCT2 isoform (high-affinity transporter). These molecular data are consistent with the kinetic data of L-lactate transport in these cells demonstrating at least two transport components for L-lactate. These separate transport components reflect the ability of these cells to switch between the generation of glycolytic L-lactate in the presence of external glucose and the use of L-lactate when this substrate is available in the external environment. The supply of these substrates is regulated by the hormonal control of Sertoli cell glycolytic activity.

Amino acid kinetics during the anhepatic phase of liver transplantation

Alanine and glutamine are interorgan nitrogen/carbon carriers for ureagenesis and gluconeogenesis, which are mainly but not necessarily only hepatic. The liver is central to alanine and glutamine metabolism, but most organs can produce and use them. We studied amino acid kinetics after liver removal to depict initial events of liver failure and to provide a model to study extrahepatic gluconeogenesis and nitrogen disposal in humans. We measured amino acid kinetics with [5,5,5-(2)H(3)]leucine and [3-(13)C]alanine or [1,2-(13)C(2)]glutamine tracers in 21 subjects during and after the anhepatic phase of liver transplantation: 12 were at 7 months posttransplantation, and 7 were healthy control subjects. Anhepatic leucine kinetics, including proteolysis, was unchanged. Alanine plasma and whole-body contents increased 3x and 2x, with a halved metabolic clearance and a doubled production, 2% greater than disposal. Free whole-body glutamine decreased 25% but increased 50% in plasma. Glutamine clearance was halved, and the production decreased by 25%, still 2% greater than disposal. Liver replacement decreased alanine and glutamine concentrations, leaving leucine unchanged. Liver removal caused doubled alanine fluxes, minor changes in glutamine, and no changes in leucine. The initial events after liver removal are an accumulation of three-carbon compounds, an acceleration of alanine turnover, and limited nitrogen storage in alanine and glutamine.

Role of glucose in the regulation of glutamine metabolism in health and in type 1 insulin-dependent diabetes

To determine the effect of glucose availability on glutamine metabolism, glutamine kinetics were assessed under conditions of hyperglycemia resulting from 1) intravenous infusion of 7.5% dextrose in healthy adults and 2) insulin deficiency in young adults with insulin-dependent diabetes mellitus (IDDM). Eight healthy adults and five young adults with IDDM were studied in the postabsorptive state by use of a primed continuous infusion of D-[U-(14)C]glucose, L-[5,5,5-(2)H(3)]leucine, and L-[3, 4-(13)C]glutamine. Whether resulting from insulin deficiency or dextrose infusion, the rise in plasma glucose was associated with increased glucose turnover (23.5 +/- 0.7 vs. 12.9 +/- 0.3 micromol. kg(-1). min(-1), P < 0.01 and 20.9 +/- 2.5 vs. 12.8 +/- 0.4 micromol. kg(-1). min(-1), P = 0.03, in health and IDDM, respectively). In both cases, high blood glucose failed to alter glutamine appearance rate (R(a)) into plasma [298 +/- 9 vs. 312 +/- 14 micromol. kg(-1). h(-1), not significant (NS) and 309 +/- 23 vs 296 +/- 26 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively] and the estimated fraction of glutamine R(a) arising from de novo synthesis (210 +/- 7 vs. 217 +/- 10 micromol. kg(-1). h(-1), NS and 210 +/- 16 vs. 207 +/- 21 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively). When compared with the euglycemic day, the apparent contribution of glucose to glutamine carbon skeleton increased when high plasma glucose resulted from intravenous dextrose infusion in healthy volunteers (10 +/- 0.8 vs. 4.8 +/- 0.3%, P < 0.01) but failed to do so when hyperglycemia resulted from insulin deficiency in IDDM. We conclude that 1) the contribution of glucose to the estimated rate of glutamine de novo synthesis does not increase when elevation of plasma glucose results from insulin deficiency, and 2) the transfer of carbon from glucose to glutamine may depend on insulin availability.