The effect of Ho3+ doping on 13C dynamic nuclear polarization at 5 T

Dissolution dynamic nuclear polarization was introduced in 2003 as a method for producing hyperpolarized 13C solutions suitable for metabolic imaging. The signal to noise ratio for the imaging experiment depends on the maximum polarization achieved in the solid state. Hence, optimization of the DNP conditions is essential. To acquire maximum polarization many parameters related to sample preparation can be modulated. Recently, it was demonstrated that Ho3+, Dy3+, Tb3+, and Gd3+ complexes enhance the polarization at 1.2 K and 3.35 T when using the trityl radical as the primary paramagnetic center. Here, we have investigated the influence of Ho-DOTA on 13C solid state DNP at 1.2 K and 5 T. We have performed 13C DNP on [1-13C] sodium acetate in 1 : 1 (v/v) water/glycerol with 15 mM trityl OX063 radicals in the presence of a series of Ho-DOTA concentrations (0, 0.5, 1, 2, 3, 5 mM). We have found that adding a small amount of Ho-DOTA in the sample preparation not only enhances the 13C polarization but also decreases the buildup time. The optimum Ho-DOTA concentration was 2 mM. In addition, the microwave sweep spectrum changes character in a manner that suggests both the cross effect and thermal mixing are active mechanisms for trityl radical at 5 T and 1.2 K.

Sensitivity enhancement for detection of hyperpolarized 13 C MRI probes with 1 H spin coupling introduced by enzymatic transformation in vivo

PURPOSE

Although ¹ H spin coupling is generally avoided in probes for hyperpolarized (HP) ¹³ C MRI, enzymatic transformations of biological interest can introduce large ¹³ C-¹ H couplings in vivo. The purpose of this study was to develop and investigate the application of ¹ H decoupling for enhancing the sensitivity for detection of affected HP ¹³ C metabolic products.

METHODS

A standalone ¹ H decoupler system and custom concentric ¹³ C/¹ H paddle coil setup were integrated with a clinical 3T MRI scanner for in vivo ¹³ C MR studies using HP [2-¹³ C]dihydroxyacetone, a novel sensor of hepatic energy status. Major ¹³ C-¹ H coupling J_CH  = ∼150 Hz) is introduced after adenosine triphosphate-dependent enzymatic transformation of HP [2-¹³ C]dihydroxyacetone to [2-¹³ C]glycerol-3-phosphate in vivo. Application of WALTZ-16 ¹ H decoupling for elimination of large ¹³ C-¹ H couplings was first tested in thermally polarized glycerol phantoms and then for in vivo HP MR studies in three rats, scanned both with and without decoupling.

RESULTS

As configured, ¹ H-decoupled ¹³ C MR of thermally polarized glycerol and the HP metabolic product [2-¹³ C]glycerol-3-phosphate was achieved at forward power of approximately 15 W. High-quality 3-s dynamic in vivo HP ¹³ C MR scans were acquired with decoupling duty cycle of 5%. Application of ¹ H decoupling resulted in sensitivity enhancement of 1.7-fold for detection of metabolic conversion of [2-¹³ C]dihydroxyacetone to HP [2-¹³ C]glycerol-3-phosphate in vivo.

CONCLUSIONS

Application of ¹ H decoupling provides significant sensitivity enhancement for detection of HP ¹³ C metabolic products with large ¹ H spin couplings, and is therefore expected to be useful for preclinical and potentially clinical HP ¹³ C MR studies. Magn Reson Med 80:36-41, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

The NQO1 bioactivatable drug, β-lapachone, alters the redox state of NQO1+ pancreatic cancer cells, causing perturbation in central carbon metabolism

Many cancer treatments, such as those for managing recalcitrant tumors like pancreatic ductal adenocarcinoma, cause off-target toxicities in normal, healthy tissue, highlighting the need for more tumor-selective chemotherapies. β-Lapachone is bioactivated by NAD(P)H:quinone oxidoreductase 1 (NQO1). This enzyme exhibits elevated expression in most solid cancers and therefore is a potential cancer-specific target. β-Lapachone's therapeutic efficacy partially stems from the drug's induction of a futile NQO1-mediated redox cycle that causes high levels of superoxide and then peroxide formation, which damages DNA and causes hyperactivation of poly(ADP-ribose) polymerase, resulting in extensive NAD⁺/ATP depletion. However, the effects of this drug on energy metabolism due to NAD⁺ depletion were never described. The futile redox cycle rapidly consumes O₂, rendering standard assays of Krebs cycle turnover unusable. In this study, a multimodal analysis, including metabolic imaging using hyperpolarized pyruvate, points to reduced oxidative flux due to NAD⁺ depletion after β-lapachone treatment of NQO1+ human pancreatic cancer cells. NAD⁺-sensitive pathways, such as glycolysis, flux through lactate dehydrogenase, and the citric acid cycle (as inferred by flux through pyruvate dehydrogenase), were down-regulated by β-lapachone treatment. Changes in flux through these pathways should generate biomarkers useful for in vivo dose responses of β-lapachone treatment in humans, avoiding toxic side effects. Targeting the enzymes in these pathways for therapeutic treatment may have the potential to synergize with β-lapachone treatment, creating unique NQO1-selective combinatorial therapies for specific cancers. These findings warrant future studies of intermediary metabolism in patients treated with β-lapachone.

Using a novel NQO1 bioactivatable drug, beta-lapachone (ARQ761), to enhance chemotherapeutic effects by metabolic modulation in pancreatic cancer

Novel, tumor-selective therapies are needed to increase the survival rate of pancreatic cancer patients. K-Ras-mutant-driven NAD(P)H:quinone oxidoreductase 1 (NQO1) is over-expressed in pancreatic tumor versus associated normal tissue, while catalase expression is lowered compared to levels in associated normal pancreas tissue. ARQ761 undergoes a robust, futile redox cycle in NQO1+ cancer cells, producing massive hydrogen peroxide (H2 O2 ) levels; normal tissues are spared by low NQO1 and high catalase expression. DNA damage created by ARQ761 in pancreatic cancer cells "hyperactivates" PARP1, causing metabolic catastrophe and NAD ± keresis cell death. NQO1: catalase levels (high in tumor, low in normal tissue) are an attractive therapeutic window to treat pancreatic cancer. Based on a growing body of literature, we are leading a clinical trial to evaluate the combination of ARQ761 and chemotherapy in patients with pancreatic cancer.

Hyperpolarized δ-[1-13 C]gluconolactone as a probe of the pentose phosphate pathway

The pentose phosphate pathway (PPP) is thought to be upregulated in trauma (to produce excess NADPH) and in cancer (to provide ribose for nucleotide biosynthesis), but simple methods for detecting changes in flux through this pathway are not available. MRI of hyperpolarized ¹³ C-enriched metabolites offers considerable potential as a rapid, non-invasive tool for detecting changes in metabolic fluxes. In this study, hyperpolarized δ-[1-¹³ C]gluconolactone was used as a probe to detect flux through the oxidative portion of the pentose phosphate pathway (PPP_ox ) in isolated perfused mouse livers. The appearance of hyperpolarized (HP) H¹³ CO₃^- within seconds after exposure of livers to HP-δ-[1-¹³ C]gluconolactone demonstrates that this probe rapidly enters hepatocytes, becomes phosphorylated, and enters the PPP_ox pathway to produce HP-H¹³ CO₃^- after three enzyme catalyzed steps (6P-gluconolactonase, 6-phosphogluconate dehydrogenase, and carbonic anhydrase). Livers perfused with octanoate as their sole energy source show no change in production of H¹³ CO₃^- after exposure to low levels of H₂ O₂ , while livers perfused with glucose and insulin showed a twofold increase in H¹³ CO₃^- after exposure to peroxide. This indicates that flux through the PPP_ox is stimulated by H₂ O₂ in glucose perfused livers but not in livers perfused with octanoate alone. Subsequent perfusion of livers with non-polarized [1,2-¹³ C]glucose followed by ¹ H NMR analysis of lactate in the perfusate verified that flux through the PPP_ox is indeed low in healthy livers and modestly higher in peroxide damaged livers. We conclude that hyperpolarized δ-[1-¹³ C]gluconolactone has the potential to serve as a metabolic imaging probe of this important biological pathway.

A comprehensive analysis of myocardial substrate preference emphasizes the need for a synchronized fluxomic/metabolomic research design

The heart oxidizes fatty acids, carbohydrates, and ketone bodies inside the tricarboxylic acid (TCA) cycle to generate the reducing equivalents needed for ATP production. Competition between these substrates makes it difficult to estimate the extent of pyruvate oxidation. Previously, hyperpolarized pyruvate detected propionate-mediated activation of carbohydrate oxidation, even in the presence of acetate. In this report, the optimal concentration of propionate for the activation of glucose oxidation was measured in mouse hearts perfused in Langendorff mode. This study was performed with a more physiologically relevant perfusate than the previous work. Increasing concentrations of propionate did not cause adverse effects on myocardial metabolism, as evidenced by unchanged O₂ consumption, TCA cycle flux, and developed pressures. Propionate at 1 mM was sufficient to achieve significant increases in pyruvate dehydrogenase flux (3×), and anaplerosis (6×), as measured by isotopomer analysis. These results further demonstrate the potential of propionate as an aid for the correct estimation of total carbohydrate oxidative capacity in the heart. However, liquid chromotography/mass spectroscopy-based metabolomics detected large changes (~30-fold) in malate and fumarate pool sizes. This observation leads to a key observation regarding mass balance in the TCA cycle; flux through a portion of the cycle can be drastically elevated without changing the O₂ consumption.

A general chemical shift decomposition method for hyperpolarized (13) C metabolite magnetic resonance imaging

Metabolic imaging with hyperpolarized carbon-13 allows sequential steps of metabolism to be detected in vivo. Potential applications in cancer, brain, muscular, myocardial, and hepatic metabolism suggest that clinical applications could be readily developed. A primary concern in imaging hyperpolarized nuclei is the irreversible decay of the enhanced magnetization back to thermal equilibrium. Multiple methods for rapid imaging of hyperpolarized substrates and their products have been proposed with a multi-point Dixon method distinguishing itself as a robust protocol for imaging [1-(13) C]pyruvate. We describe here a generalized chemical shift decomposition method that incorporates a single-shot spiral imaging sequence plus a spectroscopic sequence to retain as much spin polarization as possible while allowing detection of metabolites that have a wide range of chemical shift values. The new method is demonstrated for hyperpolarized [1-(13) C]pyruvate, [1-(13) C]acetoacetate, and [2-(13) C]dihydroxyacetone. Copyright © 2016 John Wiley & Sons, Ltd.

Accelerated chemical shift imaging of hyperpolarized (13) C metabolites

PURPOSE

Chemical shift imaging (CSI) has long been considered the gold standard method for in vivo hyperpolarized (13) C metabolite imaging because of its high sensitivity. However, CSI requires a large number of excitations so it is desirable to reduce the number of RF excitations and the total acquisition time.

METHODS

Centric phase encoding and three-dimensional compressed sensing methods were adopted into a CSI acquisition to improve efficiency and reduce the number of excitations required for imaging hyperpolarized metabolites. The new method was implemented on a GE MR750W scanner for routine real time metabolic imaging experiments.

RESULTS

Imaging results from phantoms and in vivo animals using hyperpolarized (13) C tracers demonstrate that when the entire CSI dataset is treated as a single object, compressed sensing can be satisfactorily applied to spectroscopic CSI. Centric k-space trajectory data collection also greatly improves the acquisition efficiency. This combination of compressed sensing CSI and acquisition time reduction was used to perform a hyperpolarized (13) C dynamic study.

CONCLUSION

Compressed sensing can be satisfactorily applied to conventional CSI in hyperpolarized (13) C metabolite MR imaging to reduce the number of RF excitations and accelerate the imaging speed to take advantage of conventional CSI in providing high sensitivity and a large spectral bandwidth. Magn Reson Med 76:1033-1038, 2016. © 2016 Wiley Periodicals, Inc.

Measuring changes in substrate utilization in the myocardium in response to fasting using hyperpolarized [1-(13)C]butyrate and [1-(13)C]pyruvate

Cardiac dysfunction is often associated with a shift in substrate preference for ATP production. Hyperpolarized (HP) ¹³C magnetic resonance spectroscopy (MRS) has the unique ability to detect real-time metabolic changes in vivo due to its high sensitivity and specificity. Here a protocol using HP [1-¹³C]pyruvate and [1-¹³C]butyrate is used to measure carbohydrate versus fatty acid metabolism in vivo. Metabolic changes in fed and fasted Sprague Dawley rats (n = 36) were studied at 9.4 T after tail vein injections. Pyruvate and butyrate competed for acetyl-CoA production, as evidenced by significant changes in [¹³C]bicarbonate (−48%), [1-¹³C]acetylcarnitine (+113%), and [5-¹³C]glutamate (−63%), following fasting. Butyrate uptake was unaffected by fasting, as indicated by [1-¹³C]butyrylcarnitine. Mitochondrial pseudoketogenesis facilitated the labeling of the ketone bodies [1-¹³C]acetoacetate and [1-¹³C]β-hydroxybutyryate, without evidence of true ketogenesis. HP [1-¹³C]acetoacetate was increased in fasting (250%) but decreased during pyruvate co-injection (−82%). Combining HP ¹³C technology and co-administration of separate imaging agents enables noninvasive and simultaneous monitoring of both fatty acid and carbohydrate oxidation. This protocol illustrates a novel method for assessing metabolic flux through different enzymatic pathways simultaneously and enables mechanistic studies of the changing myocardial energetics often associated with disease.

Metabolism of hyperpolarized [1-(13)C]pyruvate through alternate pathways in rat liver

The source of hyperpolarized (HP) [(13)C]bicarbonate in the liver during metabolism of HP [1-(13)C]pyruvate is uncertain and likely changes with physiology. Multiple processes including decarboxylation through pyruvate dehydrogenase or pyruvate carboxylase followed by subsequent decarboxylation via phosphoenolpyruvate carboxykinase (gluconeogenesis) could play a role. Here we tested which metabolic fate of pyruvate contributed to the appearance of HP [(13)C]bicarbonate during metabolism of HP [1-(13)C]pyruvate by the liver in rats after 21 h of fasting compared to rats with free access to food. The (13)C NMR of HP [(13)C]bicarbonate was observed in the liver of fed rats, but not in fasted rats where pyruvate carboxylation and gluconeogenesis was active. To further explore the relative fluxes through pyruvate carboxylase versus pyruvate dehydrogenase in the liver under typical conditions of hyperpolarization studies, separate parallel experiments were performed with rats given non-hyperpolarized [2,3-(13)C]pyruvate. (13)C NMR analysis of glutamate isolated from the liver of rats revealed that flux from injected pyruvate through pyruvate dehydrogenase was dominant under fed conditions whereas flux through pyruvate carboxylase dominated under fasted conditions. The NMR signal of HP [(13)C]bicarbonate does not parallel pyruvate carboxylase activity followed by subsequent decarboxylation reaction leading to glucose production. In the liver of healthy well-fed rats, the appearance of HP [(13)C]bicarbonate exclusively reflects decarboxylation of HP [1-(13)C]pyruvate via pyruvate dehydrogenase.

Mitochondrial metabolism mediates oxidative stress and inflammation in fatty liver

Mitochondria are critical for respiration in all tissues; however, in liver, these organelles also accommodate high-capacity anaplerotic/cataplerotic pathways that are essential to gluconeogenesis and other biosynthetic activities. During nonalcoholic fatty liver disease (NAFLD), mitochondria also produce ROS that damage hepatocytes, trigger inflammation, and contribute to insulin resistance. Here, we provide several lines of evidence indicating that induction of biosynthesis through hepatic anaplerotic/cataplerotic pathways is energetically backed by elevated oxidative metabolism and hence contributes to oxidative stress and inflammation during NAFLD. First, in murine livers, elevation of fatty acid delivery not only induced oxidative metabolism, but also amplified anaplerosis/cataplerosis and caused a proportional rise in oxidative stress and inflammation. Second, loss of anaplerosis/cataplerosis via genetic knockdown of phosphoenolpyruvate carboxykinase 1 (Pck1) prevented fatty acid-induced rise in oxidative flux, oxidative stress, and inflammation. Flux appeared to be regulated by redox state, energy charge, and metabolite concentration, which may also amplify antioxidant pathways. Third, preventing elevated oxidative metabolism with metformin also normalized hepatic anaplerosis/cataplerosis and reduced markers of inflammation. Finally, independent histological grades in human NAFLD biopsies were proportional to oxidative flux. Thus, hepatic oxidative stress and inflammation are associated with elevated oxidative metabolism during an obesogenic diet, and this link may be provoked by increased work through anabolic pathways.

Production of hyperpolarized 13CO2 from [1-13C]pyruvate in perfused liver does reflect total anaplerosis but is not a reliable biomarker of glucose production

In liver, 13CO2 can be generated from [1-13C] pyruvate via pyruvate dehydrogenase or anaplerotic entry of pyruvate into the TCA cycle followed by decarboxylation at phosphoenolpyruvate carboxykinase (PEPCK), the malic enzyme, isocitrate dehydrogenase, or α-ketoglutarate dehydrogenase. The purpose of this study was to determine the relative importance of these pathways in production of hyperpolarized (HP) 13CO2 after administration of hyper-polarized pyruvate in livers supplied with a fatty acid plus substrates for gluconeogenesis. Isolated mouse livers were perfused with a mixture of thermally-polarized 13C-enriched pyruvate, lactate and octanoate in various combinations prior to exposure to HP pyruvate. Under all perfusion conditions, HP malate, aspartate and fumarate were detected within ~ 3 s showing that HP [1-13C]pyruvate is rapidly converted to [1-13C]oxaloacetate which can subsequently produce HP 13CO2 via decarboxylation at PEPCK. Measurements using HP [2-13C]pyruvate allowed the exclusion of reactions related to TCA cycle turnover as sources of HP 13CO2. Direct measures of O2 consumption, ketone production, and glucose production by the intact liver combined with 13C isotopomer analyses of tissue extracts yielded a comprehensive profile of metabolic flux in perfused liver. Together, these data show that, even though the majority of HP 13CO2 derived from HP [1-13C]pyruvate in livers exposed to fatty acids reflects decarboxylation of [4-13C]oxaloacetate (PEPCK) or [4-13C]malate (malic enzyme), the intensity of the HP 13CO2 signal is not proportional to glucose production because the amount of pyruvate returned to the TCA cycle via PEPCK and pyruvate kinase is variable, depending upon available substrates.

Kinetic Modeling and Constrained Reconstruction of Hyperpolarized [1-13C]-Pyruvate Offers Improved Metabolic Imaging of Tumors

Hyperpolarized [1-(13)C]-pyruvate has shown tremendous promise as an agent for imaging tumor metabolism with unprecedented sensitivity and specificity. Imaging hyperpolarized substrates by magnetic resonance is unlike traditional MRI because signals are highly transient and their spatial distribution varies continuously over their observable lifetime. Therefore, new imaging approaches are needed to ensure optimal measurement under these circumstances. Constrained reconstruction algorithms can integrate prior information, including biophysical models of the substrate/target interaction, to reduce the amount of data that is required for image analysis and reconstruction. In this study, we show that metabolic MRI with hyperpolarized pyruvate is biased by tumor perfusion and present a new pharmacokinetic model for hyperpolarized substrates that accounts for these effects. The suitability of this model is confirmed by statistical comparison with alternates using data from 55 dynamic spectroscopic measurements in normal animals and murine models of anaplastic thyroid cancer, glioblastoma, and triple-negative breast cancer. The kinetic model was then integrated into a constrained reconstruction algorithm and feasibility was tested using significantly undersampled imaging data from tumor-bearing animals. Compared with naïve image reconstruction, this approach requires far fewer signal-depleting excitations and focuses analysis and reconstruction on new information that is uniquely available from hyperpolarized pyruvate and its metabolites, thus improving the reproducibility and accuracy of metabolic imaging measurements.

Hyperpolarized magnetic resonance as a sensitive detector of metabolic function

Hyperpolarized magnetic resonance allows for noninvasive measurements of biochemical reactions in vivo. Although this technique provides a unique tool for assaying enzymatic activities in intact organs, the scope of its application is still elusive for the wider scientific community. The purpose of this review is to provide key principles and parameters to guide the researcher interested in adopting this technology to address a biochemical, biomedical, or medical issue. It is presented in the form of a compendium containing the underlying essential physical concepts as well as suggestions to help assess the potential of the technique within the framework of specific research environments. Explicit examples are used to illustrate the power as well as the limitations of hyperpolarized magnetic resonance.

Glutamine oxidation maintains the TCA cycle and cell survival during impaired mitochondrial pyruvate transport

Alternative modes of metabolism enable cells to resist metabolic stress. Inhibiting these compensatory pathways may produce synthetic lethality. We previously demonstrated that glucose deprivation stimulated a pathway in which acetyl-CoA was formed from glutamine downstream of glutamate dehydrogenase (GDH). Here we show that import of pyruvate into the mitochondria suppresses GDH and glutamine-dependent acetyl-CoA formation. Inhibiting the mitochondrial pyruvate carrier (MPC) activates GDH and reroutes glutamine metabolism to generate both oxaloacetate and acetyl-CoA, enabling persistent tricarboxylic acid (TCA) cycle function. Pharmacological blockade of GDH elicited largely cytostatic effects in culture, but these effects became cytotoxic when combined with MPC inhibition. Concomitant administration of MPC and GDH inhibitors significantly impaired tumor growth compared to either inhibitor used as a single agent. Together, the data define a mechanism to induce glutaminolysis and uncover a survival pathway engaged during compromised supply of pyruvate to the mitochondria.

Real-time detection of hepatic gluconeogenic and glycogenolytic states using hyperpolarized [2-13C]dihydroxyacetone

Glycogenolysis and gluconeogenesis are sensitive to nutritional state, and the net direction of flux is controlled by multiple enzymatic steps. This delicate balance in the liver is disrupted by a variety of pathological states including cancer and diabetes mellitus. Hyperpolarized carbon-13 magnetic resonance is a new metabolic imaging technique that can probe intermediary metabolism nondestructively. There are currently no methods to rapidly distinguish livers in a gluconeogenic from glycogenolytic state. Here we use the gluconeogenic precursor dihydroxyacetone (DHA) to deliver hyperpolarized carbon-13 to the perfused mouse liver. DHA enters gluconeogenesis at the level of the trioses. Perfusion conditions were designed to establish either a gluconeogenic or a glycogenolytic state. Unexpectedly, we found that [2-(13)C]DHA was metabolized within a few seconds to the common intermediates and end products of both glycolysis and gluconeogenesis under both conditions, including [2,5-(13)C]glucose, [2-(13)C]glycerol 3-phosphate, [2-(13)C]phosphoenolpyruvate (PEP), [2-(13)C]pyruvate, [2-(13)C]alanine, and [2-(13)C]lactate. [2-(13)C]Phosphoenolpyruvate, a key branch point in gluconeogenesis and glycolysis, was monitored in functioning tissue for the first time. Observation of [2-(13)C]PEP was not anticipated as the free energy difference between PEP and pyruvate is large. Pyruvate kinase is the only regulatory step of the common glycolytic-gluconeogenic pathway that appears to exert significant control over the kinetics of any metabolites of DHA. A ratio of glycolytic to gluconeogenic products distinguished the gluconeogenic from glycogenolytic state in these functioning livers.

Hyperpolarized 13C NMR detects rapid drug-induced changes in cardiac metabolism

PURPOSE

The diseased myocardium lacks metabolic flexibility and responds to stimuli differently compared with healthy hearts. Here, we report the use of hyperpolarized 13C NMR spectroscopy to detect sudden changes in cardiac metabolism in isolated, perfused rat hearts in response to adrenergic stimulation.

METHODS

Metabolism of hyperpolarized [1-(13)C]pyruvate was investigated in perfused rat hearts. The hearts were stimulated in situ by isoproterenol shortly after the administration of hyperpolarized [1-(13)C]pyruvate. The hyperpolarized 13C NMR results were corroborated with 1H NMR spectroscopy of tissue extracts.

RESULTS

Addition of isoproterenol to hearts after equilibration of hyperpolarized [1-(13)C]pyruvate into the existing lactate pool resulted in a sudden, rapid increase in hyperpolarized [1-(13)C]lactate signal within seconds after exposure to drug. The hyperpolarized H(13)CO3 (-) and hyperpolarized [1-(13)C]alanine signals were not affected by the isoproterenol-induced elevated cardiac workload. Separate experiments confirmed that the new hyperpolarized [1-(13)C]lactate signal that arises after stimulation by isoproterenol reflects a sudden increase in total tissue lactate derived from glycogen.

CONCLUSION

These results suggest that hyperpolarized pyruvate and 13C MRS may be useful for detecting abnormal glycogen metabolism in intact tissues.

Propionate stimulates pyruvate oxidation in the presence of acetate

Flux through pyruvate dehydrogenase (PDH) in the heart may be reduced by various forms of injury to the myocardium, or by oxidation of alternative substrates in normal heart tissue. It is important to distinguish these two mechanisms because imaging of flux through PDH based on the appearance of hyperpolarized (HP) [(13)C]bicarbonate derived from HP [1-(13)C]pyruvate has been proposed as a method for identifying viable myocardium. The efficacy of propionate for increasing PDH flux in the setting of PDH inhibition by an alternative substrate was studied using isotopomer analysis paired with exams using HP [1-(13)C]pyruvate. Hearts from C57/bl6 mice were supplied with acetate (2 mM) and glucose (8.25 mM). (13)C NMR spectra were acquired in a cryogenically cooled probe at 14.1 Tesla. After addition of hyperpolarized [1-(13)C]pyruvate, (13)C NMR signals from lactate, alanine, malate, and aspartate were easily detected, in addition to small signals from bicarbonate and CO2. The addition of propionate (2 mM) increased appearance of HP [(13)C]bicarbonate >30-fold without change in O2 consumption. Isotopomer analysis of extracts from the freeze-clamped hearts indicated that acetate was the preferred substrate for energy production, glucose contribution to energy production was minimal, and anaplerosis was stimulated in the presence of propionate. Under conditions where production of acetyl-CoA is dominated by the availability of an alternative substrate, acetate, propionate markedly stimulated PDH flux as detected by the appearance of hyperpolarized [(13)C]bicarbonate from metabolism of hyperpolarized [1-(13)C]pyruvate.

Development and performance of a 129-GHz dynamic nuclear polarizer in an ultra-wide bore superconducting magnet

OBJECTIVE

We sought to build a dynamic nuclear polarization system for operation at 4.6 T (129 GHz) and evaluate its efficiency in terms of (13)C polarization levels using free radicals that span a range of ESR linewidths.

MATERIALS AND METHODS

A liquid helium cryostat was placed in a 4.6 T superconducting magnet with a 150-mm warm bore diameter. A 129-GHz microwave source was used to irradiate (13)C enriched samples. Temperatures close to 1 K were achieved using a vacuum pump with a 453-m(3)/h roots blower. A hyperpolarized (13)C nuclear magnetic resonance (NMR) signal was detected using a saddle coil and a Varian VNMRS console operating at 49.208 MHz. Samples doped with free radicals BDPA (1,3-bisdiphenylene-2-phenylallyl), trityl OX063 (tris{8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)dithiole-4-yl}methyl sodium salt), galvinoxyl ((2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy), 2,2-diphenylpicrylhydrazyl (DPPH) and 4-oxo-TEMPO (4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy) were assayed. Microwave dynamic nuclear polarization (DNP) spectra and solid-state (13)C polarization levels for these samples were determined.

RESULTS

(13)C polarization levels close to 50 % were achieved for [1-(13)C]pyruvic acid at 1.15 K using the narrow electron spin resonance (ESR) linewidth free radicals trityl OX063 and BDPA, while 10-20 % (13)C polarizations were achieved using galvinoxyl, DPPH and 4-oxo-TEMPO.

CONCLUSION

At this field strength free radicals with smaller ESR linewidths are still superior for DNP of (13)C as opposed to those with linewidths that exceed that of the (1)H Larmor frequency.

Simultaneous steady-state and dynamic 13C NMR can differentiate alternative routes of pyruvate metabolism in living cancer cells

Metabolic reprogramming facilitates cancer cell growth, so quantitative metabolic flux measurements could produce useful biomarkers. However, current methods to analyze flux in vivo provide either a steady-state overview of relative activities (infusion of (13)C and analysis of extracted metabolites) or a dynamic view of a few reactions (hyperpolarized (13)C spectroscopy). Moreover, although hyperpolarization has successfully quantified pyruvate-lactate exchanges, its ability to assess mitochondrial pyruvate metabolism is unproven in cancer. Here, we combined (13)C hyperpolarization and isotopomer analysis to quantify multiple fates of pyruvate simultaneously. Two cancer cell lines with divergent pyruvate metabolism were incubated with thermally polarized [3-(13)C]pyruvate for several hours, then briefly exposed to hyperpolarized [1-(13)C]pyruvate during acquisition of NMR spectra using selective excitation to maximize detection of H[(13)C]O3(-) and [1-(13)C]lactate. Metabolites were then extracted and subjected to isotopomer analysis to determine relative rates of pathways involving [3-(13)C]pyruvate. Quantitation of hyperpolarized H[(13)C]O3(-) provided a single definitive metabolic rate, which was then used to convert relative rates derived from isotopomer analysis into quantitative fluxes. This revealed that H[(13)C]O3(-) appearance reflects activity of pyruvate dehydrogenase rather than pyruvate carboxylation followed by subsequent decarboxylation reactions. Glucose substantially altered [1-(13)C]pyruvate metabolism, enhancing exchanges with [1-(13)C]lactate and suppressing H[(13)C]O3(-) formation. Furthermore, inhibiting Akt, an oncogenic kinase that stimulates glycolysis, reversed these effects, indicating that metabolism of pyruvate by both LDH and pyruvate dehydrogenase is subject to the acute effects of oncogenic signaling on glycolysis. The data suggest that combining (13)C isotopomer analyses and dynamic hyperpolarized (13)C spectroscopy may enable quantitative flux measurements in living tumors.

Metabolic characteristics of human hearts preserved for 12 hours by static storage, antegrade perfusion, or retrograde coronary sinus perfusion

OBJECTIVE

Machine perfusion of donor hearts is a promising strategy to increase the donor pool. Antegrade perfusion is effective but can lead to aortic valve incompetence and nonnutrient flow. Experience with retrograde coronary sinus perfusion of donor hearts has been limited. We tested the hypothesis that retrograde perfusion could support myocardial metabolism over an extended donor ischemic interval.

METHODS

Human hearts from donors that were rejected or not offered for transplantation were preserved for 12 hours in University of Wisconsin Machine Perfusion Solution by: (1) static hypothermic storage; (2) hypothermic antegrade machine perfusion; or (3) hypothermic retrograde machine perfusion. Myocardial oxygen consumption (MVO2), and lactate accumulation were measured. Ventricular tissue was collected for proton and phosphorus 31 magnetic resonance spectroscopy (MRS) to evaluate the metabolic state of the myocardium. Myocardial water content was determined at the end of the experiment.

RESULTS

Stable perfusion parameters were maintained throughout the perfusion period with both perfusion techniques. Lactate/alanine ratios were lower in perfused hearts compared with static hearts (P<.001). Lactate accumulation (antegrade 2.0±0.7 mM, retrograde 1.7±0.1 mM) and MVO2 (antegrade 0.25±0.2 mL, retrograde 0.26±0.3 mL O2/min/100 g) were similar in machine-perfused groups. High-energy phosphates were better preserved in both perfused groups (P<.05). Left ventricular myocardial water content was increased in retrograde perfused hearts (80.2±0.8%) compared with both antegrade perfused hearts (76.6±0.8%, P=.02) and static storage hearts (76.7±1%, P=.02).

CONCLUSIONS

Machine perfusion by either the antegrade or the retrograde technique can support myocardial metabolism over long intervals. Machine perfusion seems promising for long-term preservation of human donor hearts.

Influence of deuteration in the glassing matrix on 13C dynamic nuclear polarization

Replacement of protons by deuterons in the glassing solvents led to 2-3-fold improvement of the (13)C dynamic nuclear polarization (DNP) solid-state NMR signal for samples doped with large electron spin resonance (ESR) linewidth free radicals galvinoxyl, DPPH, and 4-oxo-TEMPO. Meanwhile, the reverse effect is observed for (13)C DNP using small ESR linewidth free radicals BDPA and trityl OX063.