Flux through hepatic pyruvate carboxylase and phosphoenolpyruvate carboxykinase detected by hyperpolarized 13C magnetic resonance

Myocardial energy shortage and unmet anaplerotic needs in the fasted long-chain acyl-CoA dehydrogenase knockout mouse

AIMS

The aim of this animal study is to assess fasting-induced changes in myocardial substrate metabolism and energy status as a consequence of mitochondrial long-chain fatty acid β-oxidation deficiency, using magnetic resonance spectroscopy (MRS).

METHODS AND RESULTS

Carbon-13 ((13)C) MRS of hyperpolarized [1-(13)C]pyruvate was used to assess in vivo pyruvate dehydrogenase (PDH) activity in fed and fasted wild-type (WT) mice and long-chain acyl-CoA dehydrogenase knockout (LCAD KO) mice. PDH activity decreased after fasting in both genotypes, but was 2.7-fold higher in fasted LCAD KO mice compared with fasted WT mice. Incorporation of the (13)C label into the myocardial malate and aspartate pools in fasted LCAD KO mice demonstrates enhanced activity of anaplerotic pathways in fasted LCAD KO hearts. These findings were corroborated by ex vivo assays revealing partially depleted pools of citric acid cycle intermediates in fasted LCAD KO myocardium, suggesting an increased, but unmet need for anaplerosis. The in vivo myocardial energy status, assessed using phosphorous-31 ((31)P) MRS, was lower in fasted LCAD KO mice than in fasted WT mice.

CONCLUSION

This study revealed that the heart of fasted LCAD KO mice has an elevated reliance on glucose oxidation, in combination with an unmet demand for myocardial anaplerosis. Due to a lack of substrate availability, the sustained myocardial glucose uptake and PDH activity in LCAD KO mice are ineffective to maintain metabolic homeostasis during fasting, which is reflected by an impaired myocardial energy status in fasted LCAD KO mice.

Hyperpolarized butyrate: a metabolic probe of short chain fatty acid metabolism in the heart

Purpose

Butyrate, a short chain fatty acid, was studied as a novel hyperpolarized substrate for use in dynamic nuclear polarization enhanced magnetic resonance spectroscopy experiments, to define the pathways of short chain fatty acid and ketone body metabolism in real time.

Methods

Butyrate was polarized via the dynamic nuclear polarization process and subsequently dissolved to generate an injectable metabolic substrate. Metabolism was initially assessed in the isolated perfused rat heart, followed by evaluation in the in vivo rat heart.

Results

Hyperpolarized butyrate was generated with a polarization level of 7% and was shown to have a T₁ relaxation time of 20 s. These physical characteristics were sufficient to enable assessment of multiple steps in its metabolism, with the ketone body acetoacetate and several tricarboxylic acid cycle intermediates observed both in vitro and in vivo. Metabolite to butyrate ratios of 0.1–0.4% and 0.5–2% were observed in vitro and in vivo respectively, similar to levels previously observed with hyperpolarized [2-¹³C]pyruvate.

Conclusions

In this study, butyrate has been demonstrated to be a suitable hyperpolarized substrate capable of revealing multi-step metabolism in dynamic nuclear polarization experiments and providing information on the metabolism of fatty acids not currently achievable with other hyperpolarized substrates. Magn Reson Med 71:1663–1669, 2014. © 2013 Wiley Periodicals, Inc.

Impact of peripheral ketolytic deficiency on hepatic ketogenesis and gluconeogenesis during the transition to birth

Preservation of bioenergetic homeostasis during the transition from the carbohydrate-laden fetal diet to the high fat, low carbohydrate neonatal diet requires inductions of hepatic fatty acid oxidation, gluconeogenesis, and ketogenesis. Mice with loss-of-function mutation in the extrahepatic mitochondrial enzyme CoA transferase (succinyl-CoA:3-oxoacid CoA transferase, SCOT, encoded by nuclear Oxct1) cannot terminally oxidize ketone bodies and develop lethal hyperketonemic hypoglycemia within 48 h of birth. Here we use this model to demonstrate that loss of ketone body oxidation, an exclusively extrahepatic process, disrupts hepatic intermediary metabolic homeostasis after high fat mother's milk is ingested. Livers of SCOT-knock-out (SCOT-KO) neonates induce the expression of the genes encoding peroxisome proliferator-activated receptor γ co-activator-1a (PGC-1α), phosphoenolpyruvate carboxykinase (PEPCK), pyruvate carboxylase, and glucose-6-phosphatase, and the neonate's pools of gluconeogenic alanine and lactate are each diminished by 50%. NMR-based quantitative fate mapping of (13)C-labeled substrates revealed that livers of SCOT-KO newborn mice synthesize glucose from exogenously administered pyruvate. However, the contribution of exogenous pyruvate to the tricarboxylic acid cycle as acetyl-CoA is increased in SCOT-KO livers and is associated with diminished terminal oxidation of fatty acids. After mother's milk provokes hyperketonemia, livers of SCOT-KO mice diminish de novo hepatic β-hydroxybutyrate synthesis by 90%. Disruption of β-hydroxybutyrate production increases hepatic NAD(+)/NADH ratios 3-fold, oxidizing redox potential in liver but not skeletal muscle. Together, these results indicate that peripheral ketone body oxidation prevents hypoglycemia and supports hepatic metabolic homeostasis, which is critical for the maintenance of glycemia during the adaptation to birth.

Noninvasive identification and assessment of functional brown adipose tissue in rodents using hyperpolarized ¹³C imaging

OBJECTIVE

The recent identification of functional depots of brown adipose tissue (BAT) in adult humans has potential implications for the treatment of obesity. In order to evaluate new therapies aimed at inducing the production of more BAT or activating BAT in humans, it will be important to develop noninvasive methods to assess the functional state of the tissue in vivo. In this study, we investigate the feasibility of using hyperpolarized (13)C imaging to noninvasively identify functional, activated BAT in an in vivo rodent model, in less than 1 min, following an infusion of pre-polarized [1-(13)C] pyruvate.

DESIGN

Hyperpolarized (13)C imaging was used to monitor BAT metabolic conversion of pre-polarized [1-(13)C] pyruvate in rats during baseline and norepinephrine (NE)-stimulated conditions.

RESULTS

Activated BAT, stimulated by NE injection, can be detected in rats by increased conversion of pre-polarized [1-(13)C] pyruvate into its downstream products (13)C bicarbonate and [1-(13)C] lactate. The colocalization of the (13)C signal to interscapular BAT was validated using hematoxylin-eosin histological staining.

CONCLUSION

The radiation-free nature and recent translation into the clinic of the hyperpolarized (13)C-imaging test may potentially facilitate trials of therapeutics targeting BAT activation in humans.

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.

Understanding metabolic regulation and its influence on cell physiology

Metabolism impacts all cellular functions and plays a fundamental role in biology. In the last century, our knowledge of metabolic pathway architecture and the genomic landscape of disease has increased exponentially. Combined with these insights, advances in analytical methods for quantifying metabolites and systems approaches to analyze these data now provide powerful tools to study metabolic regulation. Here we review the diverse mechanisms cells use to adapt metabolism to specific physiological states and discuss how metabolic flux analyses can be applied to identify important regulatory nodes to understand normal and pathological cell physiology.

Assessment of early diabetic renal changes with hyperpolarized [1-(13) C]pyruvate

BACKGROUND

This experimental study explores a novel magnetic resonance imaging/spectroscopic (MRI/MRS) method that measures changes in renal metabolism in a diabetic rat model. This hyperpolarized metabolic MRI/MRS method allows monitoring of metabolic processes in seconds by >10 000-fold enhancement of the MR signal. The method has shown that the conversion of pyruvate to bicarbonate, i.e. pyruvate dehydrogenase (PDH) activity, is significantly altered in the myocardium already at the onset of diabetes, and the predominant Warburg effect is a valuable cancer maker via the lactate dehydrogenase (LDH) activity. We hypothesize that a similar change in PDH and LDH could be found in the early diabetic kidney.

METHODS

In a streptozotocin rat model of type 1 diabetes, hyperpolarized (13) C-MRI and blood oxygenation level-dependent (1) H-MRI was employed to investigate the changes in renal metabolism in the diabetic and the control kidneys in vivo.

RESULTS

The diabetic kidney showed a 149% increase in the lactate/pyruvate ratio compared with the control rat kidney, whereas the bicarbonate/pyruvate ratio was unchanged between the diabetic and the control rat kidneys, consistent with literature findings. These metabolic findings paralleled a reduced intrarenal oxygen availability as found by blood oxygenation level-dependent MRI.

DISCUSSION

Hyperpolarized (13) C-MRI shows promise in the diagnosis and monitoring of early renal changes associated with diabetes, with the pyruvate/lactate ratio as an imaging biomarker for regional renal changes.

In vivo hyperpolarized carbon-13 magnetic resonance spectroscopy reveals increased pyruvate carboxylase flux in an insulin-resistant mouse model

The pathogenesis of type 2 diabetes is characterized by impaired insulin action and increased hepatic glucose production (HGP). Despite the importance of hepatic metabolic aberrations in diabetes development, there is currently no molecular probe that allows measurement of hepatic gluconeogenic pathways in vivo and in a noninvasive manner. In this study, we used hyperpolarized carbon 13 ((13)C)-labeled pyruvate magnetic resonance spectroscopy (MRS) to determine changes in hepatic gluconeogenesis in a high-fat diet (HFD)-induced mouse model of type 2 diabetes. Compared with mice on chow diet, HFD-fed mice displayed higher levels of oxaloacetate, aspartate, and malate, along with increased (13)C label exchange rates between hyperpolarized [1-(13) C]pyruvate and its downstream metabolites, [1-(13)C]malate and [1-(13)C]aspartate. Biochemical assays using liver extract revealed up-regulated malate dehydrogenase activity, but not aspartate transaminase activity, in HFD-fed mice. Moreover, the (13) C label exchange rate between [1-(13)C]pyruvate and [1-(13)C]aspartate (k(pyr->asp)) exhibited apparent correlation with gluconeogenic pyruvate carboxylase (PC) activity in hepatocytes. Finally, up-regulated HGP by glucagon stimulation was detected by an increase in aspartate signal and k(pyr->asp), whereas HFD mice treated with metformin for 2 weeks displayed lower production of aspartate and malate, as well as reduced k(pyr->asp) and (13)C-label exchange rate between pyruvate and malate, consistent with down-regulated gluconeogenesis.

CONCLUSION

Taken together, we demonstrate that increased PC flux is an important pathway responsible for increased HGP in diabetes development, and that pharmacologically induced metabolic changes specific to the liver can be detected in vivo with a hyperpolarized (13)C-biomolecular probe. Hyperpolarized (13)C MRS and the determination of metabolite exchange rates may allow longitudinal monitoring of liver function in disease development.

Metabolic response of glioma to dichloroacetate measured in vivo by hyperpolarized (13)C magnetic resonance spectroscopic imaging

BACKGROUND

The metabolic phenotype that derives disproportionate energy via glycolysis in solid tumors, including glioma, leads to elevated lactate labeling in metabolic imaging using hyperpolarized [1-(13)C]pyruvate. Although the pyruvate dehydrogenase (PDH)-mediated flux from pyruvate to acetyl coenzyme A can be indirectly measured through the detection of carbon-13 ((13)C)-labeled bicarbonate, it has proven difficult to visualize (13)C-bicarbonate at high enough levels from injected [1-(13)C]pyruvate for quantitative analysis in brain. The aim of this study is to improve the detection of (13)C-labeled metabolites, in particular bicarbonate, in glioma and normal brain in vivo and to measure the metabolic response to dichloroacetate, which upregulates PDH activity.

METHODS

An optimized protocol for chemical shift imaging and high concentration of hyperpolarized [1-(13)C]pyruvate were used to improve measurements of lactate and bicarbonate in C6 glioma-transplanted rat brains. Hyperpolarized [1-(13)C]pyruvate was injected before and 45 min after dichloroacetate infusion. Metabolite ratios of lactate to bicarbonate were calculated to provide improved metrics for characterizing tumor metabolism.

RESULTS

Glioma and normal brain were well differentiated by lactate-to-bicarbonate ratio (P = .002, n = 5) as well as bicarbonate (P = .0002) and lactate (P = .001), and a stronger response to dichloroacetate was observed in glioma than in normal brain.

CONCLUSION

Our results clearly demonstrate for the first time the feasibility of quantitatively detecting (13)C-bicarbonate in tumor-bearing rat brain in vivo, permitting the measurement of dichloroacetate-modulated changes in PDH flux. The simultaneous detection of lactate and bicarbonate provides a tool for a more comprehensive analysis of glioma metabolism and the assessment of metabolic agents as anti-brain cancer drugs.

Troubleshooting and improving the mouse and rat isolated perfused liver preparation

INTRODUCTION

Isolated perfused liver (IPL) model is not only widely performed in rats but is also used in mouse liver, although a detailed description of this procedure is absent. A comparison of the different techniques used on rats and mice will be discussed in this article association with a detailed description of the surgical and technical aspects needed to obtain and maintain the integrity of the livers during the organ isolation and perfusion.

METHODS

The surgical procedures, the IPL set-up, and the evaluation of hepatic function and damage will be described in relation to both rats and mice. In particular, the heparin dosage and administration, the portal vein cannulation avoiding portal leakage, the use of suprahepatic caval vein output, and the insertion of a cannula for bile collection will be reported. For the settings, the perfusion circuit, the perfusion solution, the temperature and the flow rate will be described, with particular regard to the balance between perfusion pressure and oxygen delivery. The monitoring of liver integrity by measuring oxygen concentration and calculating oxygen delivery rate and oxygen uptake rate, and recommendations for the collection of perfusate and bile samples will be considered. Accurate pH measurement with normalization, and the perfusion portal pressure assay by a calibrated water manometer will be also reported.

RESULTS AND DISCUSSION

This work analyzes the parameters crucial to performing a correct IPL both in rat and mouse, comparing our experience with the equivalent practice from other laboratories. An updated example of IPL applications in liver toxicology and pharmacology, physiology and pathophysiology, and liver graft preservation will be briefly presented, underlining how this technique provides essential information allowing a more accurate planning of the in vivo studies.

Using metabolomics to assess myocardial metabolism and energetics in heart failure

There is a long history of investigation into the metabolism of the failing heart. Congestive heart failure is marked both by severe disruptions in myocardial energy supply and an inability of the heart to efficiently uptake and oxidize fuels. Despite the many advancements in our understanding, there are still even more outstanding questions in the field. Metabolomics has the power to assist our understanding of the metabolic derangements which accompany myocardial dysfunction. Metabolomic investigations in animal models of heart failure have already highlighted several novel, potentially important pathways of substrate selection and toxicity. Metabolomic biomarker studies in humans, already successfully applied to other forms of cardiovascular disease, have the potential to improve diagnosis and patient care. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Impact of Gd3+ on DNP of [1-13C]pyruvate doped with trityl OX063, BDPA, or 4-oxo-TEMPO

Hyperpolarized [1-(13)C]pyruvate has become an important diagnostic tracer of normal and aberrant cellular metabolism for in vitro and in vivo NMR spectroscopy (MRS) and imaging (MRI). In pursuit of achieving high NMR signal enhancements in dynamic nuclear polarization (DNP) experiments, we have performed an extensive investigation of the influence of Gd(3+) doping, a parameter previously reported to improve hyperpolarized NMR signals, on the DNP of this compound. [1-(13)C]Pyruvate samples were doped with varying amounts of Gd(3+) and fixed optimal concentrations of free radical polarizing agents commonly used in fast dissolution DNP: trityl OX063 (15 mM), 4-oxo-TEMPO (40 mM), and BDPA (40 mM). In general, we have observed three regions of interest, namely, (i) a monotonic increase in DNP-enhanced nuclear polarization P(dnp) upon increasing the Gd(3+) concentration until a certain threshold concentration c(1) (1-2 mM) is reached, (ii) a region of roughly constant maximum P(dnp) from c(1) until a concentration threshold c(2) (4-5 mM), and (iii) a monotonic decrease in P(dnp) at Gd(3+) concentration c > c(2). Of the three free radical polarizing agents used, trityl OX063 gave the best response to Gd(3+) doping, with a 300% increase in the solid-state nuclear polarization, whereas addition of the optimum Gd(3+) concentration on BDPA and 4-oxo-TEMPO-doped samples only yielded a relatively modest 5-20% increase in the base DNP-enhanced polarization. The increase in P(dnp) due to Gd(3+) doping is ascribed to the decrease in the electronic spin-lattice relaxation T(1e) of the free radical electrons, which plays a role in achieving lower spin temperature T(s) of the nuclear Zeeman system. These results are discussed qualitatively in terms of the spin temperature model of DNP.

Hyperpolarized (13)C-pyruvate magnetic resonance imaging in cancer diagnostics

INTRODUCTION

The use of hyperpolarized molecules allows one to obtain information about metabolism in both cells and animals; such a task represents a tremendous advancement with respect to the results achieved so far with in vivo NMR techniques. Pyruvate appears an excellent tumor biomarker as it allows the attainment of early diagnosis, stadiation and monitoring of response to therapy.

AREAS COVERED

As pyruvate conversion to lactate in the glycolytic pathway is highly enhanced in tumor cells, the 1-(13)C-lactate levels after administration of hyperpolarized 1-(13)C-pyruvate are markedly higher in tumor tissues and depend on the type and grade of the tumor. This review covers the most recent research results (both in vitro and in vivo) about the use of hyperpolarized 1-(13)C-pyruvate for tumor localization, stadiation and for monitoring the response to therapy. The technique may find application in clinics, especially when other imaging modalities are of difficult applicability.

EXPERT OPINION

While (13)C-pyruvate has been shown to be the candidate of choice for metabolic imaging, high expectations are present in the scientific community to see if other hyperpolarized substrates could provide more specific and sensitive biomarkers. The use of hyperpolarized molecules will have a tremendous impact in the armory of diagnostic tools.

Isotope enhanced approaches in metabolomics

The rapidly growing area of "metabolomics," in which a large number of metabolites from body fluids, cells or tissue are detected quantitatively, in a single step, promises immense potential for a number of disciplines including early disease diagnosis, therapy monitoring, systems biology, drug discovery and nutritional science. Because of its ability to detect a large number of metabolites in intact biological samples reproducibly and quantitatively, nuclear magnetic resonance (NMR) spectroscopy has emerged as one of the most powerful analytical techniques in metabolomics. NMR spectroscopy of biological samples with isotope labeling of metabolites using nuclei such as (2)H, (13)C, (15)N and (31)P, either in vivo or ex vivo, has dramatically improved our ability to identify low concentrated metabolites and trace important metabolic pathways. Considering the somewhat limited sensitivity and high complexity of NMR spectra of biological samples, efforts have been made to increase sensitivity and selectivity through isotope labeling methods, which pave novel avenues to unravel biological complexity and understand cellular functions in health and various disease conditions. This chapter describes current developments in isotope labeling of metabolites in vivo as well as ex vivo, and their potential metabolomics applications.