Application of the reverse dept polarization-transfer pulse sequence to monitor in vitro and in vivo metabolism of 13C-ethanol by 1H-NMR spectroscopy

(13) C magnetic resonance spectroscopy measurements with hyperpolarized [1-(13) C] pyruvate can be used to detect the expression of transgenic pyruvate decarboxylase activity in vivo

PURPOSE

Dissolution dynamic nuclear polarization can increase the sensitivity of the (13) C magnetic resonance spectroscopy experiment by at least four orders of magnitude and offers a novel approach to the development of MRI gene reporters based on enzymes that metabolize (13) C-labeled tracers. We describe here a gene reporter based on the enzyme pyruvate decarboxylase (EC 4.1.1.1), which catalyzes the decarboxylation of pyruvate to produce acetaldehyde and carbon dioxide.

METHODS

Pyruvate decarboxylase from Zymomonas mobilis (zmPDC) and a mutant that lacked enzyme activity were expressed using an inducible promoter in human embryonic kidney (HEK293T) cells. Enzyme activity was measured in the cells and in xenografts derived from the cells using (13) C MRS measurements of the conversion of hyperpolarized [1-(13) C] pyruvate to H(13) CO3-.

RESULTS

Induction of zmPDC expression in the cells and in the xenografts derived from them resulted in an approximately two-fold increase in the H(13) CO3-/[1-(13) C] pyruvate signal ratio following intravenous injection of hyperpolarized [1-(13) C] pyruvate.

CONCLUSION

We have demonstrated the feasibility of using zmPDC as an in vivo reporter gene for use with hyperpolarized (13) C MRS. Magn Reson Med 76:391-401, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

New advances in MR-compatible bioartificial liver

MR-compatible bioartificial liver (BAL) studies have been performed for 30 years and are reviewed. There are two types of study: (i) metabolism and drug studies using multinuclear MRS; primarily short-term (< 8 h) studies; (ii) the use of multinuclear MRS and MRI to noninvasively define the features and functions of BAL systems for long-term liver tissue engineering. In the latter, these systems often undergo not only modification of the perfusion system, but also the construction of MR radiofrequency probes around the bioreactor. We present novel MR-compatible BALs and the use of multinuclear MRS ((13)C, (19)F, (31)P) for the noninvasive monitoring of their growth, metabolism and viability, as well as (1)H MRI methods for the determination of flow profiles, diffusion, cell distribution, quality assurance and bioreactor integrity. Finally, a simple flexible coil design and circuit, and life support system, are described that can make almost any BAL MR-compatible.

In vivo detection of intermediate metabolic products of [1-(13) C]ethanol in the brain using (13) C MRS

In this study, in vivo (13) C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1-(13) C]ethanol. After [1-(13) C]ethanol had been administered systemically to rats, (13) C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. (13) C-labeled bicarbonate HCO 3- (161.0 ppm) was also detected. Saturating acetaldehyde C1 at 207.0 ppm was found to have no effect on the ethanol C1 (57.7 ppm) signal intensity after extensive signal averaging, providing direct in vivo evidence that direct metabolism of alcohol by brain tissue is minimal. To compare the labeling of brain metabolites by ethanol with labeling by glucose, in vivo time course data were acquired during intravenous co-infusion of [1-(13) C]ethanol and [(13) C(6) ]-D-glucose. In contrast with labeling by [(13) C(6) ]-D-glucose, which produced doublets of carboxylic/amide carbons with a J coupling constant of 51 Hz, the simultaneously detected glutamate and glutamine singlets were labeled by [1-(13) C]ethanol. As (13) C labels originating from ethanol enter the brain after being converted into [1-(13) C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered (13) C-labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo (13) C MRS of rat brain following intragastric administration of (13) C-labeled ethanol is demonstrated.

Replication of N2-Ethyldeoxyguanosine DNA Adducts in the Human Embryonic Kidney Cell Line 293

N(2)-Ethyldeoxyguanosine (N(2)-ethyldGuo) is a DNA adduct formed by reaction of the exocyclic amine of dGuo with the ethanol metabolite acetaldehyde. Because ethanol is a human carcinogen, we assessed the biological consequences of replication of template N(2)-ethyldGuo, in comparison to the well-studied adduct O(6)-ethyldeoxyguanosine (O(6)-ethyldGuo). Single chemically synthesized N(2)-ethyldGuo or O(6)-ethyldGuo adducts were placed site specifically in the suppressor tRNA gene of the mutation reporting shuttle plasmid pLSX. N(2)-EthyldGuo and O(6)-ethyldGuo were both minimally mutagenic in double-stranded pLSX replicated in human 293 cells; however, the placement of deoxyuridines on the complementary strand at 5'- and 3'-positions flanking the adduct resulted in 5- and 22-fold enhancements of the N(2)-ethyldGuo- and O(6)-ethyldGuo-induced mutant fractions, respectively. The fold increase in the N(2)-ethyldGuo-induced mutant fraction in deoxyuridine-containing plasmids was similar after replication in 293T cells, a mismatch repair deficient variant of 293 cells, indicating that postreplication mismatch repair has little role in modulating N(2)-ethyldGuo-mediated mutagenesis. The mutation spectrum generated by N(2)-ethyldGuo consisted primarily of single base deletions and adduct site-targeted transversions, in contrast to the exclusive production of adduct site-targeted transitions by O(6)-ethyldGuo. The yield of progeny plasmids after replication in 293 cells was reduced by the presence of N(2)-ethyldGuo in parental plasmids with or without deoxyuridine to 39 or 19%, respectively. Taken together, these data indicate that N(2)-ethyldGuo in DNA exerts its principal biological activity by blocking translesion DNA synthesis in human cells, resulting in either failure of replication or frameshift deletion mutations.

Application of (13)C-filtered (1)H NMR to evaluate drug action on gluconeogenesis and glycogenolysis simultaneously in isolated rat hepatocytes

The effects of two inhibitors of hepatic glucose production, AICAR (5-aminoimidazole-4-carboxamide riboside) and metformin, whose precise mechanisms of action are a matter of some controversy, have been investigated in isolated rat hepatocytes by application of a novel NMR-based method whereby effects on metabolic flow from the two glucose-producing pathways, glycogenolysis and gluconeogenesis, and also lactate production, can be studied simultaneously. Hepatocytes were pre-incubated for 24 h with 15 mM 1-(13)C-glucose to load the cells with labeled glycogen, which under subsequent glycogenolytic conditions would yield predominantly 1-(13)C glucose and 3-(13)C-lactate, followed (after washing) by incubation in media with 2-(13)C-glycerol, which under subsequent gluconeogenic conditions would yield 2,5-(13)C-glucose, or if metabolized to lactate, 2-(13)C-lactate. Glucose production was then stimulated by glucagon for 3 h in the absence or presence of the inhibitors and then incubation media were analyzed by (13)C-HSQC (heteronuclear single quantum coherence)-filtered (1)H NMR spectra. The results show that metformin only inhibits glucose production by inhibition of gluconeogenesis, but also that it increases lactate production from both glycogenolysis and from glycerol, whereas, and contrary to expectations, AICAR inhibits glucose production by inhibiting both gluconeogenesis and glycogenolysis, and also increases lactate production from glycerol. The data show that application of this methodology can be used to answer important questions about drug action on hepatic metabolism that are not readily accessible by alternative means.

Heteronuclear cross polarization for enhanced sensitivity of in vivo 13C MR spectroscopy on a clinical 1.5 T MR system

The potential of heteronuclear ¿1H-13C¿ cross polarization was studied for optimization of the signal-to-noise ratio in in vivo 13C MR spectroscopy at the clinical field strength of 1.5 T. Experiments on the human calf showed a significant chemical-shift selective signal enhancement on triglyceride signals of 3.9 by heteronuclear cross polarization, compared to a standard pulse-acquire sequence. Studies on a neonatal piglet brain showed an enhancement by cross polarization of 2.2 for the detection of 13C-1-glucose. This enhancement allowed a fourfold improvement in time resolution in dynamic 13C MR of 13C-1-glucose inflow in piglet brain. Phantom experiments demonstrated the efficiency of this technique for interleaved detection of two spectral regions. Tests with a volume coil showed the feasibility of signal enhancement by cross polarization over a large volume of interest.

The formation and stability of imidazolidinone adducts from acetaldehyde and model peptides. A kinetic study with implications for protein modification in alcohol abuse

The kinetics of the reaction of acetaldehyde (AcH) with the alpha-amino group of several di- and tripeptides to form 2-methylimidazolidin-4-one adducts were determined at pH 7, 4, 37 degrees C, using reverse phase HPLC to separate peptides from adducts. The imidazolidin-4-one structure of the adducts was confirmed by 13C NMR spectroscopy. The reaction of val-gly-gly with AcH was shown to follow second-order kinetics over a wide range of concentrations of both reactants, with k2 = 0.734 +/- 0.032 M(-1) min(-1). Under conditions similar to those in the liver of an alcoholic during chronic ethanol oxidation ([Ach]o = 50-910 microm; [free peptide alpha-amino groups]o = 1.5 mM), the reaction proceeded until effectively all of the AcH had been consumed. The side chain of the N-terminal amino acid was shown not to have a marked effect on the rate of imidazolidinone formation. The decomposition of the imidazolidinone adduct of val-gly-gly and AcH was observed at 60-100 degrees C. Extrapolation of an Arrhenius plot to 37 degrees C provided an estimate of K(obs) of 0.002 h-1 (t1/2 approximately 14 days). Based on these kinetic studies, it is concluded that imidazolidinone adducts of AcH with proteins may be present in the liver and, possibly, in the blood of alcoholics.

Modification of proteins and other biological molecules by acetaldehyde: adduct structure and functional significance

1. Chronic ethanol consumption is a major cause of liver disease. The modification of hepatic proteins by acetaldehyde (AcH), the primary metabolite of ethanol, has for some time been suggested as one of the major events initiating alcoholic liver disease. 2. These alterations in protein structure are believed to affect liver cell function, and may serve to activate the immune system. 3. This review considers the interaction between AcH and macromolecules and its functional implications.