In vivo investigation of hyperpolarized [1,3-13C2]acetoacetate as a metabolic probe in normal brain and in glioma.

Dysregulation in NAD+/NADH levels is associated with increased cell division and elevated levels of reactive oxygen species in rapidly proliferating cancer cells. Conversion of the ketone body acetoacetate (AcAc) to β-hydroxybutyrate (β-HB) by the mitochondrial enzyme β-hydroxybutyrate dehydrogenase (BDH) depends upon NADH availability. The β-HB-to-AcAc ratio is therefore expected to reflect mitochondrial redox. Previous studies reported the potential of hyperpolarized 13C-AcAc to monitor mitochondrial redox in cells, perfused organs and in vivo. However, the ability of hyperpolarized 13C-AcAc to cross the blood brain barrier (BBB) and its potential to monitor brain metabolism remained unknown. Our goal was to assess the value of hyperpolarized [1,3-13C2]AcAc in healthy and tumor-bearing mice in vivo. Following hyperpolarized [1,3-13C2]AcAc injection, production of [1,3-13C2]β-HB was detected in normal and tumor-bearing mice. Significantly higher levels of [1-13C]AcAc and lower [1-13C]β-HB-to-[1-13C]AcAc ratios were observed in tumor-bearing mice. These results were consistent with decreased BDH activity in tumors and associated with increased total cellular NAD+/NADH. Our study confirmed that AcAc crosses the BBB and can be used for monitoring metabolism in the brain. It highlights the potential of AcAc for future clinical translation and its potential utility for monitoring metabolic changes associated with glioma, and other neurological disorders

Genetic Manipulation of Iron Biomineralization Enhances MR Relaxivity in a Ferritin-M6A Chimeric Complex

Ferritin has gained significant attention as a potential reporter gene for in vivo imaging by magnetic resonance imaging (MRI). However, due to the ferritin ferrihydrite core, the relaxivity and sensitivity for detection of native ferritin is relatively low. We report here on a novel chimeric magneto-ferritin reporter gene – ferritin-M6A – in which the magnetite binding peptide from the magnetotactic bacteria magnetosome-associated Mms6 protein was fused to the C-terminal of murine h-ferritin. Biophysical experiments showed that purified ferritin-M6A assembled into a stable protein cage with the M6A protruding into the cage core, enabling magnetite biomineralisation. Ferritin-M6A-expressing C6-glioma cells showed enhanced (per iron) r2 relaxivity. MRI in vivo studies of ferritin-M6A-expressing tumour xenografts showed enhanced R2 relaxation rate in the central hypoxic region of the tumours. Such enhanced relaxivity would increase the sensitivity of ferritin as a reporter gene for non-invasive in vivo MRI-monitoring of cell delivery and differentiation in cellular or gene-based therapies

ENDOR Spectroscopy and DFT Calculations: Evidence for the Hydrogen-Bond Network Within α2 in the PCET of E. coli Ribonucleotide Reductase

Escherichia coli class I ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is composed of two subunits: α2 and β2. β2 contains a stable di-iron tyrosyl radical (Y[subscript 122]•) cofactor required to generate a thiyl radical (C[subscript 439]•) in α2 over a distance of 35 Å, which in turn initiates the chemistry of the reduction process. The radical transfer process is proposed to occur by proton-coupled electron transfer (PCET) via a specific pathway: Y[subscript 122] ⇆ W[subscript 48][?] ⇆ Y[subscript 356] in β2, across the subunit interface to Y[subscript 731] ⇆ Y[subscript 730] ⇆ C[subscript 439] in α2. Within α2 a colinear PCET model has been proposed. To obtain evidence for this model, 3-amino tyrosine (NH2Y) replaced Y[subscript 730] in α2, and this mutant was incubated with β2, cytidine 5′-diphosphate, and adenosine 5′-triphosphate to generate a NH2Y730• in D2O. [[superscript 2]H]-Electron–nuclear double resonance (ENDOR) spectra at 94 GHz of this intermediate were obtained, and together with DFT models of α2 and quantum chemical calculations allowed assignment of the prominent ENDOR features to two hydrogen bonds likely associated with C[subscript 439] and Y[subscript 731]. A third proton was assigned to a water molecule in close proximity (2.2 Å O–H···O distance) to residue 730. The calculations also suggest that the unusual g-values measured for NH[subscript 2]Y[subscript 730]• are consistent with the combined effect of the hydrogen bonds to Cys[subscript 439] and Tyr[subscript 731], both nearly perpendicular to the ring plane of NH[subscript 2]Y[subscript 730]. The results provide the first experimental evidence for the hydrogen-bond network between the pathway residues in α2 of the active RNR complex, for which no structural data are available.National Institutes of Health (U.S.) (NIH GM29595

Conformational control of intramolecular electron transfer processes

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METB-14. DOWN-REGULATION OF ACETATE METABOLISM TOWARDS FATTY ACIDS IN IDH1 MUTANT GLIOMA

Abstract Acetate has recently been identified as a major alternative source of nutrients for glioblastoma and brain metastases. After cellular uptake, acetate is converted to acetyl-CoA, a key metabolic intermediate that fuels the TCA cycle and is an essential building block for the biosynthesis of fatty acids. Interestingly, the potential role of acetate in lower-grade glioma harboring the isocitrate dehydrogenase 1 mutation has not yet been elucidated. The goal of this study was therefore to investigate the role of acetate in fatty acid biosynthesis using a well-characterized genetically-engineered cell model that overexpresses either wild-type IDH1 (IDHwt) or mutant IDH1 (IDHmut): an immortalized normal human astrocyte (NHA)-based model. We used 1H and 13C magnetic resonance spectroscopy to quantify the flux of [1,2-13C]-acetate to 13C fatty acids. Our results indicated that the total levels of fatty acids were not significantly different between IDHmut and IDHwt NHA cells. However, the flux of 13C-labeled acetate towards fatty acids was significantly reduced by ~60% in IDHmut NHA cells relative to IDHwt NHA cells. To investigate this disconnect and understand the underlying biological mechanisms, we performed cell biological assays. Surprisingly, this decrease in acetate metabolism was associated with a drop in fatty acid synthase and ATP citrate lyase expressions, two enzymes involved in fatty acid synthesis from acetyl-CoA, in IDHmut NHA cells, whereas expression of acetyl-CoA synthase (AceS1), the cytosolic enzyme converting acetate to acetyl-CoA, was not altered. A significant drop in lipid droplet accumulation was also observed in IDHmut NHA cells as indicated using a spectrophotometric assay. Taken together, this points to alternate sources for fatty acids (e.g. glucose, glutamine, uptake from serum) in IDH1mut cells and suggests that fatty acids are preferentially directed towards cell membrane assembly. It also highlights the unique metabolic reprogramming of mutant IDH1 cells

Solvent accessibility in the distal heme pocket of the nitrosyl d(1)-heme complex of Pseudomonas stutzeri cd(1) nitrite reductase.

In nitrite reductase (cd(1) NIR), the c-heme mediates electron transfer to the catalytic d(1)-heme where nitrite (NO(2)(-)) is reduced to nitric oxide (NO). An interesting feature of this enzyme is the relative lability of the reaction product NO bound to the d(1)-heme. Marked differences in the c- to d(1)-heme electron-transfer rates were reported for cd(1) NIRs from different sources, such as Pseudomonas stutzeri (P. stutzeri) and Pseudomonas aeruginosa (P. aeruginosa). The three-dimensional structure of the P. aeruginosa enzyme has been determined, but that of the P. stutzeri enzyme is still unknown. The difference in electron transfer rates prompted a comparison of the structural properties of the d(1)-heme pocket of P. stutzeri cd(1) NIR with those of the P. aeruginosa wild type enzyme (WT) and its Y10F using their nitrosyl d(1)-heme complexes. We applied high field pulse electron paramagnetic resonance (EPR) techniques that detect nuclear spins in the close environment of the spin bearing Fe(II)-NO entity. We observed similarities in the rhombic g-tensor and detected a proximal histidine ligand with (14)N hyperfine and quadrupole interactions also similar to those of P. aeruginosa WT and Y10F mutant complexes. In contrast, we also observed significant differences in the H-bond network involving the NO ligand and a larger solvent accessibility for P. stutzeri attributed to the absence of this tyrosine residue. For P. aeruginosa, cd(1) NIR domain swapping allows Tyr(10) to become H-bonded to the bound NO substrate. These findings support a previous suggestion that the large difference in the c- to d(1)-heme electron transfer rates between the two enzymes is related to solvent accessibility of their d(1)-heme pockets

MR Studies of Glioblastoma Models Treated with Dual PI3K/mTOR Inhibitor and Temozolomide:Metabolic Changes Are Associated with Enhanced Survival.

The current standard of care for glioblastoma (GBM) is surgical resection, radiotherapy, and treatment with temozolomide (TMZ). However, resistance to current therapies and recurrence are common. To improve survival, agents that target the PI3K signaling pathway, which is activated in approximately 88% of GBM, are currently in clinical trials. A challenge with such therapies is that tumor shrinkage is not always observed. New imaging methods are therefore needed to monitor response to therapy and predict survival. The goal of this study was to determine whether hyperpolarized (13)C magnetic resonance spectroscopic imaging (MRSI) and (1)H magnetic resonance spectroscopy (MRS) can be used to monitor response to the second-generation dual PI3K/mTOR inhibitor voxtalisib (XL765, SAR245409), alone or in combination with TMZ. We investigated GS-2 and U87-MG GBM orthotopic tumors in mice, and used MRI, hyperpolarized (13)C MRSI, and (1)H MRS to monitor the effects of treatment. In our study, (1)H MRS could not predict tumor response to therapy. However, in both our models, we observed a significantly lower hyperpolarized lactate-to-pyruvate ratio in animals treated with voxtalisib, TMZ, or combination therapy, when compared with controls. This metabolic alteration was observed prior to MRI-detectable changes in tumor size, was consistent with drug action, and was associated with enhanced animal survival. Our findings confirm the potential translational value of the hyperpolarized lactate-to-pyruvate ratio as a biomarker for noninvasively assessing the effects of emerging therapies for patients with GBM. Mol Cancer Ther; 15(5); 1113-22. ©2016 AACR

TMOD-20. EARLY DETECTION OF HDAC INHIBITION IN GLIOBLASTOMA USING ADVANCED HYPERPOLARIZED 13C MRSI

Abstract Today, there is no reliable noninvasive imaging method available to monitor glioblastoma (GBM) response to therapy and predict survival prior to tumor shrinkage. Dissolution Dynamic Nuclear Polarization (DNP) combined with hyperpolarized 13C Magnetic Resonance Spectroscopic Imaging (MRSI) is a novel imaging method that allows probing real-time tumor metabolism. Recent studies using 13C MRSI have shown decreased lactate production from pyruvate in GBM responsive to a dual PI3K/mTOR inhibitor and/or Temozolomide, mediated by lower expression of LDHA or PKM2 enzymes, respectively. SAHA, the histone deacetylase (HDAC) inhibitor, is a novel drug that inhibits cell proliferation by inducing cell cycle arrest followed by apoptosis. The goal of this study was to detect early HDAC inhibition using advanced 13C MRSI. Analysis of dynamic real-time cellular metabolic changes in SAHA-treated U87 live cells in bioreactors demonstrated a significant 37.7% decrease in hyperpolarized [1-13C]-lactate production. SAHA-treated cells also showed a 29.6% decrease of steady-state lactate level in cell extracts. Furthermore, we demonstrated a significant 30.3% decrease in hyperpolarized lactate-to-pyruvate ratio in SAHA-treated U87-bearing mice, which occurred prior to MRI-detectable changes in tumor size that was associated with enhanced animal survival. In order to mechanistically validate our findings, we tested the levels of expression of LDHA, MCT1 and MCT4, the main players in pyruvate-to-lactate interconversion. While expression of LDHA, the enzyme that catalyzes pyruvate-to-lactate conversion was independent of SAHA treatment, expression of both MCT1 and MCT4 transporters, that shuttle pyruvate and lactate in and out of the cell, are increased. We thus propose that increased MCT1/4 led to a decrease in lactate in response to HDAC inhibition and resulted in a reduced pyruvate-to-lactate conversion. Our findings confirm the potential translational value of the hyperpolarized lactate-to-pyruvate ratio as a biomarker for noninvasively assessing the early effects of emerging therapies for patients with GBM