In vivo MRSI of hyperpolarized [1-(13)C]pyruvate metabolism in rat hepatocellular carcinoma

Enhancing Cancer Diagnosis with Real-Time Feedback: Tumor Metabolism through Hyperpolarized 1-13C Pyruvate MRSI

This review article discusses the potential of hyperpolarized (HP) ¹³C magnetic resonance spectroscopic imaging (MRSI) as a noninvasive technique for identifying altered metabolism in various cancer types. Hyperpolarization significantly improves the signal-to-noise ratio for the identification of ¹³C-labeled metabolites, enabling dynamic and real-time imaging of the conversion of [1-¹³C] pyruvate to [1-¹³C] lactate and/or [1-¹³C] alanine. The technique has shown promise in identifying upregulated glycolysis in most cancers, as compared to normal cells, and detecting successful treatment responses at an earlier stage than multiparametric MRI in breast and prostate cancer patients. The review provides a concise overview of the applications of HP [1-¹³C] pyruvate MRSI in various cancer systems, highlighting its potential for use in preclinical and clinical investigations, precision medicine, and long-term studies of therapeutic response. The article also discusses emerging frontiers in the field, such as combining multiple metabolic imaging techniques with HP MRSI for a more comprehensive view of cancer metabolism, and leveraging artificial intelligence to develop real-time, actionable biomarkers for early detection, assessing aggressiveness, and interrogating the early efficacy of therapies.

Unraveling the Sweet Secrets of HCC: Glucometabolic Rewiring in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the primary form of liver cancer. It causes ∼ 800 000 deaths per year, which is expected to increase due to increasing rates of obesity and metabolic dysfunction associated steatotic liver disease (MASLD). Current therapies include immune checkpoint inhibitors, tyrosine kinase inhibitors, and monoclonal antibodies, but these therapies are not satisfactorily effective and often come with multiple side effects and recurrences. Metabolic reprogramming plays a significant role in HCC progression and is often conserved between tumor types. Thus, targeting rewired metabolic pathways could provide an attractive option for targeting tumor cells alone or in conjunction with existing treatments. Therefore, there is an urgent need to identify novel targets involved in cancer-mediated metabolic reprogramming in HCC. In this review, we provide an overview of molecular rewiring and metabolic reprogramming of glucose metabolism in HCC to understand better the concepts that might widen the therapeutic window against this deadly cancer.

PFKFB3-mediated Pro-glycolytic Shift in Hepatocellular Carcinoma Proliferation

BACKGROUND & AIMS

Metabolic reprogramming, in particular, glycolytic regulation, supports abnormal survival and growth of hepatocellular carcinoma (HCC) and could serve as a therapeutic target. In this study, we sought to identify glycolytic regulators in HCC that could be inhibited to prevent tumor progression and could also be monitored in vivo, with the goal of providing a theragnostic alternative to existing therapies.

METHODS

An orthotopic HCC rat model was used. Tumors were stimulated into a high-proliferation state by use of off-target liver ablation and were compared with lower-proliferating controls. We measured in vivo metabolic alteration in tumors before and after stimulation, and between stimulated tumors and control tumors using hyperpolarized 13C magnetic resonance imaging (MRI) (h13C MRI). We compared metabolic alterations detected by h13C MRI to metabolite levels from ex vivo mass spectrometry, mRNA levels of key glycolytic regulators, and histopathology.

RESULTS

Glycolytic lactate flux increased within HCC tumors 3 days after tumor stimulation, correlating positively with tumor proliferation as measured with Ki67. This was associated with a shift towards aerobic glycolysis and downregulation of the pentose phosphate pathway detected by mass spectrometry. MRI-measured lactate flux was most closely coupled with PFKFB3 expression and was suppressed with direct inhibition using PFK15.

CONCLUSIONS

Inhibition of PFKFB3 prevents glycolytic-mediated HCC proliferation, trackable by in vivo h13C MRI.

Hyperpolarized carbon 13 MRI in liver diseases: Recent advances and future opportunities

Hyperpolarized carbon-13 magnetic resonance imaging (HP ¹³ C MRI) is a recently translated metabolic imaging technique. With dissolution dynamic nuclear polarization (d-DNP), more than 10 000-fold signal enhancement can be readily reached, making it possible to visualize real-time metabolism and specific substrate-to-metabolite conversions in the liver after injecting carbon-13 labelled probes. Increasing evidence suggests that HP ¹³ C MRI is a potential tool in detecting liver abnormalities, predicting disease progression and monitoring response treatment. In this review, we will introduce the recent progresses of HP ¹³ C MRI in diffuse liver diseases and liver malignancies and discuss its future opportunities from a clinical perspective, hoping to provide a comprehensive overview of this novel technique in liver diseases and highlight its scientific and clinical potential in the field of hepatology.

Profiling Carbohydrate Metabolism in Liver and Hepatocellular Carcinoma with [13C]-Glycerate Probes

The interplay between glycolysis and gluconeogenesis is central to carbohydrate metabolism. Here, we describe novel methods to assess carbohydrate metabolism using [¹³C]-probes derived from glycerate, a molecule whose metabolic fate in mammals remains underexplored. Isotope-based studies were conducted via NMR and mass spectrometry analyses of freeze-clamped liver tissue extracts after [2,3-¹³C₂]glycerate infusion. The ex vivo investigations were correlated with in vivo measurements using hyperpolarized [1-¹³C]glycerate. Application of [¹³C]glycerate to N-nitrosodiethylamine (DEN)-treated rats provided further assessments of intermediary carbohydrate metabolism in hepatocellular carcinoma. This method afforded direct analyses of control versus DEN tissues, and altered ratios of ¹³C metabolic products as well as unique glycolysis intermediates were observed in the DEN liver/tumor. Isotopomer studies showed increased glycerate uptake and altered carbohydrate metabolism in the DEN rats.

Current Trends in Non-Invasive Imaging of Interactions in the Liver Tumor Microenvironment Mediated by Tumor Metabolism

Simple Summary

Deregulated tumor metabolism is known to shape the tumor microenvironment and directly affect the local immune response, promoting tumor growth, metastasis, and resistance to treatment. However, the metabolic profile or tumor cells, and therefore, the composition of their microenvironment, are highly variable among patients and even within the same tumor, resulting in heterogeneous response rates to oncologic therapies, making patient selection a key issue. This review article focuses on non-invasive imaging techniques that aim to visualize the crosstalk between tumor cells and their microenvironment in liver cancer mediated by tumor metabolism. In addition to improved tumor detection, such imaging tools may be able to provide a more accurate characterization of the individual tumor and ultimately improve understanding, as well as guide personalized treatment regimens for patients with liver cancer.

Abstract

With the increasing understanding of resistance mechanisms mediated by the metabolic reprogramming in cancer cells, there is a growing clinical interest in imaging technologies that allow for the non-invasive characterization of tumor metabolism and the interactions of cancer cells with the tumor microenvironment (TME) mediated through tumor metabolism. Specifically, tumor glycolysis and subsequent tissue acidosis in the realms of the Warburg effect may promote an immunosuppressive TME, causing a substantial barrier to the clinical efficacy of numerous immuno-oncologic treatments. Thus, imaging the varying individual compositions of the TME may provide a more accurate characterization of the individual tumor. This approach can help to identify the most suitable therapy for each individual patient and design new targeted treatment strategies that disable resistance mechanisms in liver cancer. This review article focuses on non-invasive positron-emission tomography (PET)- and MR-based imaging techniques that aim to visualize the crosstalk between tumor cells and their microenvironment in liver cancer mediated by tumor metabolism.

Molecular correlates of MRS-based 31 phosphocreatine muscle resynthesis rate in healthy adults

Dynamic phosphorus MRS (³¹ P-MRS) is a method used for in vivo studies of skeletal muscle energetics including measurements of phosphocreatine (PCr) resynthesis rate during recovery of submaximal exercise. However, the molecular events associated with the PCr resynthesis rate are still under debate. We assessed vastus lateralis PCr resynthesis rate from ³¹ P-MRS spectra collected from healthy adults as part of the CALERIE II study (caloric restriction), and assessed associations between PCr resynthesis and muscle mitochondrial signature transcripts and proteins (NAMPT, NQO1, PGC-1α, and SIRT1). Regression analysis indicated that higher concentration of nicotinamide phosphoribosyltransferase (NAMPT) protein, a mitochondrial capacity marker, was associated with faster PCr resynthesis. However, PCr resynthesis was not associated with greater physical fitness (VO₂ peak) or messenger ribonucleic acid levels of mitochondrial function markers such as NQO1, PGC-1α, and SIRT1, suggesting that the impact of these molecular signatures on PCr resynthesis may be minimal in the context of an acute exercise bout. Together, these findings suggest that ³¹ P-MRS based PCr resynthesis may represent a valid non-invasive surrogate marker of mitochondrial NAMPT in human skeletal muscle.

Metabolic Alterations Associated with Early-Stage Hepatocellular Carcinoma and Their Correlation with Aging and Enzymatic Activity in Patients with Viral Hepatitis-Induced Liver Cirrhosis: A Preliminary Study

Liver cirrhosis (LC) can develop hepatocellular carcinoma (HCC). However, noninvasive early diagnosis of HCCs in the cirrhotic liver is still challenging. We aimed to quantify the hepatic metabolites in normal control (NC), cirrhotic liver without HCC, cirrhotic liver with HCC (CLH), and early-stage HCC groups using proton magnetic resonance spectroscopy (¹H-MRS) with a long echo-time (TE) and to assess the potential association between the levels of hepatic metabolites in these four groups and aging and enzymatic activity. Thirty NCs, 30 viral hepatitis-induced LC patients without HCC, and 30 viral hepatitis-induced LC patients with HCC were included in this study. ¹H-MRS measurements were performed on a localized voxel of the normal liver parenchyma (n = 30) from NCs, cirrhotic liver parenchyma (n = 30) from LC patients without HCC, and each of the cirrhotic liver parenchyma (n = 30) and HCC (n = 30) from the same patients in the CLH group. Generalized estimating equations were used to evaluate potential risk factors for changes in metabolite levels. Potential associations between metabolite levels and age and serum enzymatic activities were assessed by correlation analysis. The levels of lactate+triglyceride (Lac+TG) and choline (Cho) in HCC were significantly higher compared to those in LC and CLH. A potential risk factor for changes in the Lac+TG and Cho levels was age, specifically 60–80 years of age. In particular, the Lac+TG level was associated with a high odds ratio of HCC in males aged 60–80 years. The Lac+TG and Cho concentrations were positively correlated with lactate dehydrogenase and alkaline phosphatase activities, respectively. Our findings suggested that ¹H-MRS measurement with a long TE was useful in quantifying hepatic Lac+TG and Cho levels, where higher Lac+TG and Cho levels were most likely associated with HCC-related metabolism in the viral hepatitis-induced cirrhotic liver. Further, the level of Lac+TG in HCC was highly correlated with older age and lactate dehydrogenase activity.

A metabolite-specific 3D stack-of-spiral bSSFP sequence for improved lactate imaging in hyperpolarized [1-13 C]pyruvate studies on a 3T clinical scanner

PURPOSE

The balanced steady-state free precession sequence has been previously explored to improve the efficient use of nonrecoverable hyperpolarized ¹³C magnetization, but suffers from poor spectral selectivity and long acquisition time. The purpose of this study was to develop a novel metabolite-specific 3D bSSFP ("MS-3DSSFP") sequence with stack-of-spiral readouts for improved lactate imaging in hyperpolarized [1-¹³ C]pyruvate studies on a clinical 3T scanner.

METHODS

Simulations were performed to evaluate the spectral response of the MS-3DSSFP sequence. Thermal ¹³C phantom experiments were performed to validate the MS-3DSSFP sequence. In vivo hyperpolarized [1-¹³ C], pyruvate studies were performed to compare the MS-3DSSFP sequence with metabolite-specific gradient echo ("MS-GRE") sequences for lactate imaging.

RESULTS

Simulations, phantom, and in vivo studies demonstrate that the MS-3DSSFP sequence achieved spectrally selective excitation on lactate while minimally perturbing other metabolites. Compared with MS-GRE sequences, the MS-3DSSFP sequence showed approximately a 2.5-fold SNR improvement for lactate imaging in rat kidneys, prostate tumors in a mouse model, and human kidneys.

CONCLUSIONS

Improved lactate imaging using the MS-3DSSFP sequence in hyperpolarized [1-¹³ C]pyruvate studies was demonstrated in animals and humans. The MS-3DSSFP sequence could be applied for other clinical applications such as in the brain or adapted for imaging other metabolites such as pyruvate and bicarbonate.

Phospholipids are A Potentially Important Source of Tissue Biomarkers for Hepatocellular Carcinoma: Results of a Pilot Study Involving Targeted Metabolomics

Background: Hepatocellular carcinoma (HCC) pathogenesis involves the alteration of multiple liver-specific metabolic pathways. We systematically profiled cancer- and liver-related classes of metabolites in HCC and adjacent liver tissues and applied supervised machine learning to compare their potential yield for HCC biomarkers. Methods: Tumor and corresponding liver tissue samples were profiled as follows: Bile acids by ultra-performance liquid chromatography (LC) coupled to tandem mass spectrometry (MS), phospholipids by LC-MS/MS, and other small molecules including free fatty acids by gas chromatography—time of flight MS. The overall classification performance of metabolomic signatures derived by support vector machine (SVM) and random forests machine learning algorithms was then compared across classes of metabolite. Results: For each metabolite class, there was a plateau in classification performance with signatures of 10 metabolites. Phospholipid signatures consistently showed the highest discrimination for HCC followed by signatures derived from small molecules, free fatty acids, and bile acids with area under the receiver operating characteristic curve (AUC) values of 0.963, 0.934, 0.895, 0.695, respectively, for SVM-generated signatures comprised of 10 metabolites. Similar classification performance patterns were observed with signatures derived by random forests. Conclusion: Membrane phospholipids are a promising source of tissue biomarkers for discriminating between HCC tumor and liver tissue.

A regional bolus tracking and real-time B1 calibration method for hyperpolarized 13 C MRI

PURPOSE

Acquisition timing and B1 calibration are two key factors that affect the quality and accuracy of hyperpolarized 13 C MRI. The goal of this project was to develop a new approach using regional bolus tracking to trigger Bloch-Siegert B1 mapping and real-time B1 calibration based on regional B1 measurements, followed by dynamic imaging of hyperpolarized 13 C metabolites in vivo.

METHODS

The proposed approach was implemented on a system which allows real-time data processing and real-time control on the sequence. Real-time center frequency calibration upon the bolus arrival was also added. The feasibility of applying the proposed framework for in vivo hyperpolarized 13 C imaging was tested on healthy rats, tumor-bearing mice and a healthy volunteer on a clinical 3T scanner following hyperpolarized [1-13 C]pyruvate injection. Multichannel receive coils were used in the human study.

RESULTS

Automatic acquisition timing based on either regional bolus peak or bolus arrival was achieved with the proposed framework. Reduced blurring artifacts in real-time reconstructed images were observed with real-time center frequency calibration. Real-time computed B1 scaling factors agreed with real-time acquired B1 maps. Flip angle correction using B1 maps results in a more consistent quantification of metabolic activity (i.e, pyruvate-to-lactate conversion, kPL ). Experiment recordings are provided to demonstrate the real-time actions during the experiment.

CONCLUSIONS

The proposed method was successfully demonstrated on animals and a human volunteer, and is anticipated to improve the efficient use of the hyperpolarized signal as well as the accuracy and robustness of hyperpolarized 13 C imaging.

13C Pyruvate Transport Across the Blood-Brain Barrier in Preclinical Hyperpolarised MRI

Hyperpolarised MRI with Dynamic Nuclear Polarisation overcomes the fundamental thermodynamic limitations of conventional magnetic resonance, and is translating to human studies with several early-phase clinical trials in progress including early reports that demonstrate the utility of the technique to observe lactate production in human brain cancer patients. Owing to the fundamental coupling of metabolism and tissue function, metabolic neuroimaging with hyperpolarised [1-13C]pyruvate has the potential to be revolutionary in numerous neurological disorders (e.g. brain tumour, ischemic stroke, and multiple sclerosis). Through the use of [1-13C]pyruvate and ethyl-[1-13C]pyruvate in naïve brain, a rodent model of metastasis to the brain, or porcine brain subjected to mannitol osmotic shock, we show that pyruvate transport across the blood-brain barrier of anaesthetised animals is rate-limiting. We show through use of a well-characterised rat model of brain metastasis that the appearance of hyperpolarized [1-13C]lactate production corresponds to the point of blood-brain barrier breakdown in the disease. With the more lipophilic ethyl-[1-13C]pyruvate, we observe pyruvate production endogenously throughout the entire brain and lactate production only in the region of disease. In the in vivo porcine brain we show that mannitol shock permeabilises the blood-brain barrier sufficiently for a dramatic 90-fold increase in pyruvate transport and conversion to lactate in the brain, which is otherwise not resolvable. This suggests that earlier reports of whole-brain metabolism in anaesthetised animals may be confounded by partial volume effects and not informative enough for translational studies. Issues relating to pyruvate transport and partial volume effects must therefore be considered in pre-clinical studies investigating neuro-metabolism in anaesthetised animals, and we additionally note that these same techniques may provide a distinct biomarker of blood-brain barrier permeability in future studies.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Metabolic alterations in a rat model of hepatic ischaemia reperfusion injury: In vivo hyperpolarized 13 C MRS and metabolic imaging

BACKGROUND & AIMS

Despite a number of studies addressing the pathophysiology of hepatic IRI, a gold standard test for early diagnosis and evaluation of IRI remains elusive. This study investigated the metabolic alterations in a rat model of hepatic IRI using the in vivo hyperpolarized ¹³C MRS and metabolic imaging.

METHODS

Hyperpolarized ¹³ C MRS with IVIM-DWI was performed on the liver of 7 sham-operated control rats and 7 rats before and after hepatic IRI.

RESULTS

The hepatic IRI-induced rats showed significantly higher ratios of [1-¹³ C] alanine/pyruvate, [1-¹³ C] alanine/tC, [1-¹³ C] lactate/pyruvate and [1-¹³ C] lactate/tC compared with both sham-operated controls and rats before IRI, whereas [1-¹³ C] pyruvate/tC ratio was decreased in IRI-induced rats. In IVIM-DWI study, apparent diffusion coefficient (ADC), f and D values in rats after hepatic IRI were significantly lower than those of rats before IRI and sham-operated controls. The levels of [1-¹³ C] alanine and [1-¹³ C] lactate were negatively correlated with ADC, f and D values, whereas the level of [1-¹³ C] pyruvate was positively correlated with these values.

CONCLUSIONS

The levels of [1-¹³ C] alanine, [1-¹³ C] lactate and [1-¹³ C] pyruvate in conjunction with IVIM-DWI will be helpful to evaluate the hepatic IRI as well as these findings can be useful in understanding the biochemical mechanism associated with hepatic damage.

MiRSEA: Discovering the pathways regulated by dysfunctional MicroRNAs

Recent studies have shown that dysfunctional microRNAs (miRNAs) are involved in the progression of various cancers. Dysfunctional miRNAs may jointly regulate their target genes and further alter the activities of canonical biological pathways. Identification of the pathways regulated by a group of dysfunctional miRNAs could help uncover the pathogenic mechanisms of cancer and facilitate development of new drug targets. Current miRNA-pathway analyses mainly use differentially-expressed miRNAs to predict the shared pathways on which they act. However, these methods fail to consider the level of differential expression level, which could improve our understanding of miRNA function. We propose a novel computational method, MicroRNA Set Enrichment Analysis (MiRSEA), to identify the pathways regulated by dysfunctional miRNAs. MiRSEA integrates the differential expression levels of miRNAs with the strength of miRNA pathway associations to perform direct enrichment analysis using miRNA expression data. We describe the MiRSEA methodology and illustrate its effectiveness through analysis of data from hepatocellular cancer, gastric cancer and lung cancer. With these analyses, we show that MiRSEA can successfully detect latent biological pathways regulated by dysfunctional miRNAs. We have implemented MiRSEA as a freely available R-based package on CRAN (https://cran.r-project.org/web/packages/MiRSEA/).

A Metabolic Therapy for Malignant Glioma Requires a Clinical Measure

Cancers are "reprogrammed" to use a much higher rate of glycolysis (GLY) relative to oxidative phosphorylation (OXPHOS), even in the presence of adequate amounts of oxygenation. Originally identified by Nobel Laureate Otto Warburg, this hallmark of cancer has recently been termed metabolic reprogramming and represents a way for the cancer tissue to divert carbon skeletons to produce biomass. Understanding the mechanisms that underlie this metabolic shift should lead to better strategies for cancer treatments. Malignant gliomas, cancers that are very resistant to conventional treatments, are highly glycolytic and seem particularly suited to approaches that can subvert this phenotype.

Specificities of Human Hepatocellular Carcinoma Developed on Non-Alcoholic Fatty Liver Disease in Absence of Cirrhosis Revealed by Tissue Extracts ¹H-NMR Spectroscopy

There is a rising incidence of non-alcoholic fatty liver disease (NAFLD) as well as of the frequency of Hepato-Cellular Carcinoma (HCC) associated with NAFLD. To seek for putative metabolic pathways specific of the NAFLD etiology, we performed comparative metabolomics between HCC associated with NAFLD and HCC associated with cirrhosis. The study included 28 pairs of HCC tissue versus distant Non-Tumoral Tissue (NTT) collected from patients undergoing hepatectomy. HCC was associated with cirrhosis (n = 9), normal liver (n = 6) and NAFLD (n = 13). Metabolomics was performed using 1H-NMR Spectroscopy on tissue extracts and combined to multivariate statistical analysis. In HCC compared to NTT, statistical models showed high levels of lactate and phosphocholine, and low level of glucose. Shared and Unique Structures (SUS) plots were performed to remove the impact of underlying disease on the metabolic profile of HCC. HCC-cirrhosis was characterized by high levels of β-hydroxybutyrate, tyrosine, phenylalanine and histidine whereas HCC-NAFLD was characterized by high levels of glutamine/glutamate. In addition, the overexpression glutamine/glutamate on HCC-NAFLD was confirmed by both Glutamine Synthetase (GS) immuno-staining and NMR-spectroscopy glutamine quantification. This study provides evidence of metabolic specificities of HCC associated with non-cirrhotic NAFLD versus HCC associated with cirrhosis. These alterations could suggest activation of glutamine synthetase pathway in HCC-NAFLD and mitochondrial dysfunction in HCC-cirrhosis, that may be part of specific carcinogenic processes.

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

Until recently, molecular imaging using magnetic resonance (MR) has been limited by the modality's low sensitivity, especially with non-proton nuclei. The advent of hyperpolarized (HP) MR overcomes this limitation by substantially enhancing the signal of certain biologically important probes through a process known as external nuclear polarization, enabling real-time assessment of tissue function and metabolism. The metabolic information obtained by HP MR imaging holds significant promise in the clinic, where it could play a critical role in disease diagnosis and therapeutic monitoring. This review will provide a comprehensive overview of the developments made in the field of hyperpolarized MR, including advancements in polarization techniques and delivery, probe development, pulse sequence optimization, characterization of healthy and diseased tissues, and the steps made towards clinical translation.

Plasma metabolomic analysis of human hepatocellular carcinoma: Diagnostic and therapeutic study

Many hepatocellular carcinoma (HCC) patients suffer from late stages when diagnosed, leading to dismal prospects for cure. Improving the diagnosis and treatment of HCC remains a challenge. In this work, NMR-based metabolomic techniques were used to investigate the metabolic alterations of HCC patients from different pathological backgrounds. Metabolic improvement of clinical surgical treatments or transcatheter arterial chemoembolization (TACE) for recurrent or metastatic HCC was also evaluated. HCC was characterized by enhanced lipid metabolism and high consumption in response to liver injury. Expectedly, higher consumption of glucose and lactate production in TACE group confirmed that recurrent or metastatic HCC is more active in citric acid cycle and oxidative phosphorylation. However, TACE or surgical treatments did not immediately improve the metabolic profiles of HCC patients. Combining multivariate statistical analyses with univariate t-test, a series of characteristic metabolites were identified and served as biomarkers for discrimination of HCC patients in different pathological backgrounds. The relative metabolic pathway analyses help to get insight into the underlying biochemical mechanism and extend clinical relevance. Furthermore, algorithm of support vector classification was used to identify HCC and control subjects, and diagnostic sensitivity and specificity reached to 100% and 81.08% respectively by receiver operating characteristic analysis. It is concluded that NMR-based metabolomic analysis of plasma can provide a powerful approach to discover diagnostic and therapeutic biomarkers, and subsequently contribute to clinical disease management.

Multiparametric human hepatocellular carcinoma characterization and therapy response evaluation by hyperpolarized (13) C MRSI

Individual tumor characterization and treatment response monitoring based on current medical imaging methods remain challenging. This work investigates hyperpolarized (13) C compounds in an orthotopic rat hepatocellular carcinoma (HCC) model system before and after transcatheter arterial embolization (TAE). HCC ranks amongst the top six most common cancer types in humans and accounts for one-third of cancer-related deaths worldwide. Early therapy response monitoring could aid in the development of personalized therapy approaches and novel therapeutic concepts. Measurements with selectively (13) C-labeled and hyperpolarized urea, pyruvate and fumarate were performed in tumor-bearing rats before and after TAE. Two-dimensional, slice-selective MRSI was used to obtain spatially resolved maps of tumor perfusion, cell energy metabolic conversion rates and necrosis, which were additionally correlated with immunohistochemistry. All three injected compounds, taken together with their respective metabolites, exhibited similar signal distributions. TAE induced a decrease in blood flow into the tumor and thus a decrease in tumor to muscle and tumor to liver ratios of urea, pyruvate and its metabolites, alanine and lactate, whereas conversion rates remained stable or increased on TAE in tumor, muscle and liver tissue. Conversion from fumarate to malate successfully indicated individual levels of necrosis, and global malate signals after TAE suggested the washout of fumarase or malate itself on necrosis. This study presents a combination of three (13) C compounds as novel candidate biomarkers for a comprehensive characterization of genetically and molecularly diverse HCC using hyperpolarized MRSI, enabling the simultaneous detection of differences in tumor perfusion, metabolism and necrosis. If, as in this study, bolus dynamics are not required and qualitative perfusion information is sufficient, the desired information could be extracted from hyperpolarized fumarate and pyruvate alone, acquired at higher fields with better spectral separation. Copyright © 2016 John Wiley & Sons, Ltd.

Time-course metabolic changes in high-fat diet-induced obesity rats: A pilot study using hyperpolarized (13)C dynamic MRS

The purpose of this study was to investigate the time-course metabolic changes based on hyperpolarized (13)C magnetic resonance spectroscopy (MRS) in high-fat diet (HFD)-induced obesity rats and the correlation between metabolic and serum enzyme levels. Sprague-Dawley rats were fed either HFD (60% fat) or normal diet (10% fat) for 6weeks. A HyperSense DNP was used to hyperpolarize [1-(13)C] pyruvic acid and the hyperpolarized (13)C MRS was examined every 2weeks in the course of 6weeks using a 3T GE MR750 scanner. The body weight of HFD-induced obese rats was significantly increased compared to normal rats at the 6th week after the onset of feeding (p=0.05). Simultaneously, the HFD-induced obese rats showed significantly increased levels of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and low-density lipoprotein (LDL)-cholesterol compared to normal rats (p≤0.05). In the dynamic (13)C MR spectra acquired at the 6th week, the obese rats showed significantly increased ratios of [1-(13)C] lactate/[1-(13)C] pyruvate and [1-(13)C] alanine/[1-(13)C] pyruvate (p=0.05). The (13)C spectral outcomes are positively correlated with the enzyme levels of ALT and LDH in the HFD-induced obesity. The [1-(13)C] lactate and [1-(13)C] alanine are potentially considered as noninvasive biomarkers for the HFD-induced obesity.

Speeding up dynamic spiral chemical shift imaging with incoherent sampling and low-rank matrix completion

PURPOSE

To improve the temporal and spatial resolution of dynamic ¹³ C spiral chemical shift imaging via incoherent sampling and low-rank matrix completion (LRMC).

METHODS

Spiral CSI data were both simulated and acquired in rats, and undersampling was implemented retrospectively and prospectively by pseudorandomly omitting a fraction of the spiral interleaves. Undersampled data were reconstructed with both LRMC and a conventional inverse nonuniform fast Fourier transform (iNUFFT) and compared with fully sampled data.

RESULTS

Two-fold undersampling with LRMC reconstruction enabled a two-fold improvement in temporal or spatial resolution without significant artifacts or spatiotemporal distortion. Conversely, undersampling with iNUFFT reconstruction created strong artifacts that obscured the image. LRMC performed better at time points with strong metabolite signal.

CONCLUSION

Incoherent undersampling and LRMC provides a way to increase the spatiotemporal resolution of spiral CSI without degrading data integrity. Magn Reson Med 77:951-960, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Assessing inflammatory liver injury in an acute CCl4 model using dynamic 3D metabolic imaging of hyperpolarized [1-(13)C]pyruvate

To facilitate diagnosis and staging of liver disease, sensitive and non-invasive methods for the measurement of liver metabolism are needed. This study used hyperpolarized (13)C-pyruvate to assess metabolic parameters in a CCl4 model of liver damage in rats. Dynamic 3D (13)C chemical shift imaging data from a volume covering kidney and liver were acquired from 8 control and 10 CCl4-treated rats. At 12 time points at 5 s temporal resolution, we quantified the signal intensities and established time courses for pyruvate, alanine, and lactate. These measurements were compared with standard liver histology and an alanine transaminase (ALT) enzyme assay using liver tissue from the same animals. All CCl4-treated but none of the control animals showed histological liver damage and elevated ALT enzyme levels. In agreement with these results, metabolic imaging revealed an increased alanine/pyruvate ratio in liver of CCl4-treated rats, which is indicative of elevated ALT activity. Similarly, lactate/pyruvate ratios were higher in CCl4-treated compared with control animals, demonstrating the presence of inflammation. No significant differences in metabolite ratios were observed in kidney or vasculature. Thus this work shows that metabolic imaging using (13)C-pyruvate can be a successful tool to non-invasively assess liver damage in vivo.

The use of dynamic nuclear polarization (13)C-pyruvate MRS in cancer

In recent years there has been an immense development of new targeted anti-cancer drugs. For practicing precision medicine, a sensitive method imaging for non-invasive, assessment of early treatment response and for assisting in developing new drugs is warranted. Magnetic Resonance Spectroscopy (MRS) is a potent technique for non-invasive in vivo investigation of tissue chemistry and cellular metabolism. Hyperpolarization by Dynamic Nuclear Polarization (DNP) is capable of creating solutions of molecules with polarized nuclear spins in a range of biological molecules and has enabled the real-time investigation of in vivo metabolism. The development of this new method has been demonstrated to enhance the nuclear polarization more than 10,000-fold, thereby significantly increasing the sensitivity of the MRS with a spatial resolution to the millimeters and a temporal resolution at the subsecond range. Furthermore, the method enables measuring kinetics of conversion of substrates into cell metabolites and can be integrated with anatomical proton magnetic resonance imaging (MRI). Many nuclei and substrates have been hyperpolarized using the DNP method. Currently, the most widely used compound is (13)C-pyruvate due to favoring technicalities. Intravenous injection of the hyperpolarized (13)C-pyruvate results in appearance of (13)C-lactate, (13)C-alanine and (13)C-bicarbonate resonance peaks depending on the tissue, disease and the metabolic state probed. In cancer, the lactate level is increased due to increased glycolysis. The use of DNP enhanced (13)C-pyruvate has in preclinical studies shown to be a sensitive method for detecting cancer and for assessment of early treatment response in a variety of cancers. Recently, a first-in-man 31-patient study was conducted with the primary objective to assess the safety of hyperpolarized (13)C-pyruvate in healthy subjects and prostate cancer patients. The study showed an elevated (13)C-lactate/(13)C-pyruvate ratio in regions of biopsy-proven prostate cancer compared to noncancerous tissue. However, more studies are needed in order to establish use of hyperpolarized (13)C MRS imaging of cancer.

Application of flow sensitive gradients for improved measures of metabolism using hyperpolarized (13) c MRI

PURPOSE

To develop the use of bipolar gradients to suppress partial-volume and flow-related artifacts from macrovascular, hyperpolarized spins.

THEORY AND METHODS

Digital simulations were performed over a range of spatial resolutions and gradient strengths to determine the optimal bipolar gradient strength and duration to suppress flowing spins while minimizing signal loss from static tissue. In vivo experiments were performed to determine the efficacy of this technique to suppress vascular signal in the study of hyperpolarized [1-(13)C]pyruvate renal metabolism.

RESULTS

Digital simulations showed that in the absence of bipolar gradients, partial-volume artifacts from the vasculature were still present, causing underestimation of the apparent reaction rate of pyruvate to lactate (kP). The addition of a bipolar gradient with b = 32 s/mm(2) sufficiently suppressed the vascular signal without a substantial decrease in signal from static tissue. In vivo results corroborate digital simulations, with similar peak lactate signal to noise ratio (SNR) but substantially different kP in the presence of bipolar gradients.

CONCLUSION

The proposed approach suppresses signal from flowing spins while minimizing signal loss from static tissue, removing contaminating signal from the vasculature and increasing kinetic modeling accuracy without substantially sacrificing SNR or temporal resolution.

Simultaneous imaging of 13C metabolism and 1H structure: technical considerations and potential applications

Real-time imaging of (13)C metabolism in vivo has been enabled by recent advances in hyperpolarization. As a result of the inherently low natural abundance of endogenous (13)C nuclei, hyperpolarized (13)C images lack structural information that could be used to aid in motion detection and anatomical registration. Motion before or during the (13)C acquisition can therefore result in artifacts and misregistration that may obscure measures of metabolism. In this work, we demonstrate a method to simultaneously image both (1)H and (13)C nuclei using a dual-nucleus spectral-spatial radiofrequency excitation and a fully coincident readout for rapid multinuclear spectroscopic imaging. With the appropriate multinuclear hardware, and the means to simultaneously excite and receive on both channels, this technique is straightforward to implement requiring little to no increase in scan time. Phantom and in vivo experiments were performed with both Cartesian and spiral trajectories to validate and illustrate the utility of simultaneous acquisitions. Motion compensation of dynamic metabolic measurements acquired during free breathing was demonstrated using motion tracking derived from (1)H data. Simultaneous multinuclear imaging provides structural (1)H and metabolic (13)C images that are correlated both spatially and temporally, and are therefore amenable to joint (1)H and (13)C analysis and correction of structure-function images.

The aspartate metabolism pathway is differentiable in human hepatocellular carcinoma: transcriptomics and (13) C-isotope based metabolomics

Hepatocellular carcinoma (HCC), the primary form of human adult liver malignancy, is a highly aggressive tumor with average survival rates that are currently less than a year following diagnosis. Although bioinformatic analyses have indicated differentially expressed genes and cancer related mutations in HCC, integrated genetic and metabolic pathway analyses remain to be investigated. Herein, gene (i.e. messenger RNA, mRNA) enrichment analysis was performed to delineate significant alterations of metabolic pathways in HCC. The objective of this study was to investigate the pathway of aspartate metabolism in HCC of humans. Coupled with transcriptomic (i.e. mRNA) and NMR based metabolomics of human tissue extracts, we utilized liquid chromatography mass spectrometry based metabolomics analysis of stable [U-(13) C6 ]glucose metabolism or [U-(13) C5 ,(15) N2 ]glutamine metabolism of HCC cell culture. Our results indicated that aspartate metabolism is a significant and differentiable metabolic pathway of HCC compared with non-tumor liver (p value < 0.0001). In addition, branched-chain amino acid metabolism (p value < 0.0001) and tricarboxylic acid metabolism (p value < 0.0001) are significant and differentiable. Statistical analysis of measurable NMR metabolites indicated that at least two of the group means were significantly different for the metabolites alanine (p value = 0.0013), succinate (p value = 0.0001), lactate (p value = 0.0114), glycerophosphoethanolamine (p value = 0.015), and inorganic phosphate (p value = 0.0001). However, (13) C isotopic enrichment analysis of these metabolites revealed less than 50% isotopic enrichment with either stable [U-(13) C6 ]glucose metabolism or [U-(13) C5 ,(15) N2 ]glutamine. This may indicate the differential account of total metabolite pool versus de novo metabolites from a (13) C labeled substrate. The ultimate translation of these findings will be to determine putative enzyme activity via (13) C labeling, to investigate targeted therapeutics against these enzymes, and to optimize the in vivo performance of (13) C MRI techniques.

Optimal variable flip angle schemes for dynamic acquisition of exchanging hyperpolarized substrates

In metabolic MRI with hyperpolarized contrast agents, the signal levels vary over time due to T1 decay, T2 decay following RF excitations, and metabolic conversion. Efficient usage of the nonrenewable hyperpolarized magnetization requires specialized RF pulse schemes. In this work, we introduce two novel variable flip angle schemes for dynamic hyperpolarized MRI in which the flip angle is varied between excitations and between metabolites. These were optimized to distribute the magnetization relatively evenly throughout the acquisition by accounting for T1 decay, prior RF excitations, and metabolic conversion. Simulation results are presented to confirm the flip angle designs and evaluate the variability of signal dynamics across typical ranges of T1 and metabolic conversion. They were implemented using multiband spectral-spatial RF pulses to independently modulate the flip angle at various chemical shift frequencies. With these schemes we observed increased SNR of [1-(13)C]lactate generated from [1-(13)C]pyruvate, particularly at later time points. This will allow for improved characterization of tissue perfusion and metabolic profiles in dynamic hyperpolarized MRI.

Tissue metabolomics of hepatocellular carcinoma: tumor energy metabolism and the role of transcriptomic classification

Hepatocellular carcinoma (HCC) is one of the commonest causes of death from cancer. A plethora of metabolomic investigations of HCC have yielded molecules in biofluids that are both up- and down-regulated but no real consensus has emerged regarding exploitable biomarkers for early detection of HCC. We report here a different approach, a combined transcriptomics and metabolomics study of energy metabolism in HCC. A panel of 31 pairs of HCC tumors and corresponding nontumor liver tissues from the same patients was investigated by gas chromatography-mass spectrometry (GCMS)-based metabolomics. HCC was characterized by ∼2-fold depletion of glucose, glycerol 3- and 2-phosphate, malate, alanine, myo-inositol, and linoleic acid. Data are consistent with a metabolic remodeling involving a 4-fold increase in glycolysis over mitochondrial oxidative phosphorylation. A second panel of 59 HCC that had been typed by transcriptomics and classified in G1 to G6 subgroups was also subjected to GCMS tissue metabolomics. No differences in glucose, lactate, alanine, glycerol 3-phosphate, malate, myo-inositol, or stearic acid tissue concentrations were found, suggesting that the Wnt/β-catenin pathway activated by CTNNB1 mutation in subgroups G5 and G6 did not exhibit specific metabolic remodeling. However, subgroup G1 had markedly reduced tissue concentrations of 1-stearoylglycerol, 1-palmitoylglycerol, and palmitic acid, suggesting that the high serum α-fetoprotein phenotype of G1, associated with the known overexpression of lipid catabolic enzymes, could be detected through metabolomics as increased lipid catabolism.

CONCLUSION

Tissue metabolomics yielded precise biochemical information regarding HCC tumor metabolic remodeling from mitochondrial oxidation to aerobic glycolysis and the impact of molecular subtypes on this process.

Perfusion and diffusion sensitive 13C stimulated-echo MRSI for metabolic imaging of cancer

Metabolic imaging with hyperpolarized [1-(13)C]-pyruvate can rapidly probe tissue metabolic profiles in vivo and has been shown to provide cancer imaging biomarkers for tumor detection, progression, and response to therapy. This technique uses a bolus injection followed by imaging within 1-2 minutes. The observed metabolites include vascular components and their generation is also influenced by cellular transport. These factors complicate image interpretation, especially since [1-(13)C]lactate, a metabolic product that is a biomarker of cancer, is also produced by red blood cells. It would be valuable to understand the distribution of metabolites between the vasculature, interstitial space, and intracellular compartments. The purpose of this study was to better understand this compartmentalization by using a perfusion and diffusion-sensitive stimulated-echo acquisition mode (STEAM) MRSI acquisition method tailored to hyperpolarized substrates. Our results in mouse models showed that among metabolites, the injected substrate (13)C-pyruvate had the largest vascular fraction overall while (13)C-alanine had the smallest vascular fraction. We observed a larger vascular fraction of pyruvate and lactate in the kidneys and liver when compared to back muscle and prostate tumor tissue. Our data suggests that (13)C-lactate in prostate tumor tissue voxels was the most abundant labeled metabolite intracellularly. This was shown in STEAM images that highlighted abnormal cancer cell metabolism and suppressed vascular (13)C metabolite signals.

Hyperpolarized 13C metabolic imaging using dissolution dynamic nuclear polarization

This article describes the basic physics of dissolution dynamic nuclear polarization (dissolution-DNP), and the impact of the resulting highly nonequilibrium spin states, on the physics of magnetic resonance imaging (MRI) detection. The hardware requirements for clinical translation of this technology are also presented. For studies that allow the use of externally administered agents, hyperpolarization offers a way to overcome normal magnetic resonance sensitivity limitations, at least for a brief T(1)-dependent observation window. A 10,000-100,000-fold signal-to-noise advantage provides an avenue for real-time measurement of perfusion, metabolite transport, exchange, and metabolism. The principles behind these measurements, as well as the choice of agent, and progress toward the application of hyperpolarized (13)C metabolic imaging in oncology, cardiology, and neurology are reviewed.

Multimodal assessment of in vivo metabolism with hyperpolarized [1-13C]MR spectroscopy and 18F-FDG PET imaging in hepatocellular carcinoma tumor-bearing rats

Abnormalities of tumor metabolism can be exploited for molecular imaging. PET imaging of (18)F-FDG is a well-established method using the avid glucose uptake of tumor cells. (13)C MR spectroscopic imaging (MRSI) of hyperpolarized [1-(13)C]pyruvate and its metabolites, meanwhile, represents a new method to study energy metabolism by visualizing, for example, the augmented lactate dehydrogenase activity in tumor cells. Because of rapid signal loss, this method underlies strict temporal limitations, and the acquisition of data-encoding spatial, temporal, and spectral information within this time frame-is challenging. The object of our study was to compare spectroscopic images with (18)F-FDG PET images for visualizing tumor metabolism in a rat model.

METHODS

(13)C MRSI with IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation) chemical shift imaging in combination with single-shot spiral acquisition was used to obtain dynamic data from 23 rats bearing a subcutaneous hepatocellular carcinoma and from reference regions of the same animals. Static and dynamic analysis of (18)F-FDG PET images of the same animals was performed. The data were analyzed qualitatively (visual assessment) and quantitatively (magnitude and dynamics of (18)F-FDG uptake, (13)C MRSI dynamics, and physiologic parameters).

RESULTS

In most animals increased [1-(13)C]lactate signals in the tumor could be detected by simple display of integrated [1-(13)C]lactate images with corresponding enhanced (18)F-FDG uptake. Low [1-(13)C]pyruvate or [1-(13)C]lactate signals did not correlate with histologic or physiologic parameters. Significantly less pyruvate reached the tumors than the gastrointestinal tract, but in tumors a significantly higher amount of pyruvate was converted to lactate and alanine within seconds after intravenous administration.

CONCLUSION

This study reveals that PET and (13)C MRSI can be used to visualize increased glycolytic flux in malignant tissue. The combination of signals will allow the quantitative dissection of substrate metabolism, with respect to uptake and downstream metabolic pathways. Although hyperpolarized [1-(13)C]pyruvate increases the sensitivity of MR imaging, signal-to-noise ratio constraints still apply for spatially and temporally resolved (13)C MRSI, emphasizing the need for further MR methodologic development. These first imaging data suggest the feasibility of (13)C MRSI for future clinical use.

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.

A rapid method for direct detection of metabolic conversion and magnetization exchange with application to hyperpolarized substrates

In this work, we present a new MR spectroscopy approach for directly observing nuclear spins that undergo exchange, metabolic conversion, or, generally, any frequency shift during a mixing time. Unlike conventional approaches to observe these processes, such as exchange spectroscopy (EXSY), this rapid approach requires only a single encoding step and thus is readily applicable to hyperpolarized MR in which the magnetization is not replenished after T(1) decay and RF excitations. This method is based on stimulated-echoes and uses phase-sensitive detection in conjunction with precisely chosen echo times in order to separate spins generated during the mixing time from those present prior to mixing. We are calling the method Metabolic Activity Decomposition Stimulated-echo Acquisition Mode or MAD-STEAM. We have validated this approach as well as applied it in vivo to normal mice and a transgenic prostate cancer mouse model for observing pyruvate-lactate conversion, which has been shown to be elevated in numerous tumor types. In this application, it provides an improved measure of cellular metabolism by separating [1-(13)C]-lactate produced in tissue by metabolic conversion from [1-(13)C]-lactate that has flowed into the tissue or is in the blood. Generally, MAD-STEAM can be applied to any system in which spins undergo a frequency shift.

Fast volumetric imaging of ethanol metabolism in rat liver with hyperpolarized [1-(13) C]pyruvate

Rapid volumetric imaging of hyperpolarized (13) C compounds allows the real-time measurement of metabolic activity and can be useful in distinguishing between normal and diseased tissues. This work extends a fast two-dimensional undersampled spiral MRSI sequence to provide volumetric coverage, acquiring a 16 × 16 × 12 matrix with a nominal isotropic resolution of 5 mm in 4.5 s. The rapid acquisition enables a high temporal resolution for dynamic imaging. This dynamic three-dimensional MRSI method was used to investigate hyperpolarized [1-(13) C]pyruvate metabolism modulated by the administration of ethanol in rat liver. A significant increase in the pyruvate to lactate conversion was observed in the liver as a result of the greater availability of NADH (nicotinamide adenine dinucleotide, reduced form) from ethanol metabolism.

Whole genome expression profile in neuroblastoma cells exposed to 1-methyl-4-phenylpyridine

Mitochondrial dysfunction and subsequent energy failure is a contributing factor to degeneration of the substantia nigra pars compacta associated with Parkinson's disease (PD). In this study, we investigate molecular events triggered by cell exposure to the mitochondrial toxin 1-methyl-4-phenylpyridine (MPP+) using whole genome-expression microarray, Western Blot and metabolic studies. The data show that MPP+ (500 μM) obstructs mitochondrial respiration/oxidative phosphorylation (OXPHOS) in mouse neuroblastoma Neuro-2a cells, juxtaposing accelerated glucose consumption and production of lactic acid. While additional glucose concentrations restored viability in the presence of MPP+ (500 μM), the loss of OXPHOS was sustained, suggesting that compensatory anaerobic metabolic systems were fulfilling required energy needs. Under these conditions, MPP+ initiated significant changes to the transcription of 439 genes of which 287 DAVID IDs were identified and subsequent functional annotation clusters identified. Prominent changes were as follows; MPP+ initiated loss of mRNA for mitochondrial encoded 3-hydroxybutyratedehydrogenase, type 2(Bdh2), tv1, NADH dehydrogenase 4,5 genes, cytochrome b and NADH dehydrogenase (ubiquinone) flavoprotein 3, concomitant to rise in a mitochondrial fission gene; ganglioside-induced differentiation-associated-protein 1 (GDAP1). The negative changes to OXPHOS components were accompanied by protective forces within the mitochondria espousing elevated ratio of anti/pro-apoptotic processes. These included a loss of apoptotic Bcl-2/adenovirus E1B 19-kDa-interacting protein (BNIP3) and family with sequence similarity 162, member A (FAM162a) and rise of heat shock protein 1 and Lon peptidase 1. There were no changes indicative of free radical damage (e.g. SOD, GSH-Px), rather MPP+ initiated significant elevation in G protein signaling components (which trigger catabolic processes) and anaerobic metabolic systems involving carboxylic acid/transamination reactions (e.g. glutamate oxaloacetate transaminase 1 (GOT1), glutamic pyruvate-alanine transaminase 2 (GPT2), cystathionase and redox proteins such as cytochrome b5 reductase 1 and ferredoxin reductase. Counter-intuitively, the data show reduction of mRNA in glycolytic processes [DAVID enrichment score 9.96 p value 1.90E-19], some corroborated by Western Blot, bringing in to question the sources of lactate observed in the presence of MPP+. Examining this aspect, the data show that diverse carboxylic acids (succinate, oxaloacetate and a-ketoglutarate) are capable of contributing to the lactate pool in addition to phosph(enolpyruvate) or pyruvate in the absence of glucose by this cell line. In conclusion, these findings show that MPP+ negatively affects the transcriptome involved with complex I, but initiated an elevation of G protein signaling and anaerobic metabolic systems involved with nitrogen/carboxylic acid metabolism. Future research will be required to elucidate the survival pathways that drive anaerobic substrate level phosphorylation, and define functional ramification to the loss of mitochondrial FAM162a and BNIP3 proteins.

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.

Promising application of dynamic nuclear polarization for in vivo (13)C MR imaging

Use of hyperpolarized (13)C in magnetic resonance (MR) imaging is a new technique that enhances signal tens of thousands-fold. Recent in vivo animal studies of metabolic imaging that used hyperpolarized (13)C demonstrated its potential in many applications for disease indication, metabolic profiling, and treatment monitoring. We review the basic physics for dynamic nuclear polarization (DNP) and in vivo studies reported in prostate cancer research, hepatocellular carcinoma research, diabetes and cardiac applications, brain metabolism, and treatment response as well as investigations of various DNP (13)C substrates.

Effect of lanthanide ions on dynamic nuclear polarization enhancement and liquid-state T1 relaxation

In the dynamic nuclear polarization process, microwave irradiation facilitates exchange of polarization from a radical's unpaired electron to nuclear spins at cryogenic temperatures, increasing polarization by >10,000. Doping samples with Gd(3+) ions further increases the achievable solid-state polarization. However, on dissolution, paramagnetic lanthanide metals can be potent relaxation agents, decreasing liquid-state polarization. Here, the effects of lanthanide metals on the solid and liquid-state magnetic properties of [1-(13)C]pyruvate are studied. The results show that in addition to gadolinium, holmium increases not only the achievable polarization but also the rate of polarization. Liquid-state relaxation studies found that unlike gadolinium, holmium minimally affects T(1). Additionally, results reveal that linear contrast agents dissociate in pyruvic acid, greatly reducing liquid-state T(1). Although macrocyclic agents do not readily dissociate, they yield lower solid-state polarization. Results indicate that polarization with free lanthanides and subsequent chelation during dissolution produces the highest polarization enhancement while minimizing liquid-state relaxation.

Pericellular pH homeostasis is a primary function of the Warburg effect: inversion of metabolic systems to control lactate steady state in tumor cells

The Warburg effect describes a heightened propensity of tumor cells to produce lactic acid in the presence or absence of O(2) . A generally held notion is that the Warburg effect is related to energy. Using whole-genome, proteomic MALDI-TOF-MS and metabolite analysis, we investigated the Warburg effect in malignant neuroblastoma N2a cells. The findings show that the Warburg effect serves a functional role in regulating acidic pericellular pH (pHe), which is mediated by metabolic inversion or a fluctuating dominance between glycolytic-rate substrate level phosphorylation (SLP) and mitochondrial (mt) oxidative phosphorylation (OXPHOS) to control lactic acid production. The results also show that an alkaline pHe caused an elevation in SLP/OXPHOS ratio (approximately 98% SLP/OXPHOS); while the ratio was approximately 56% at neutral pHe and approximately 93% in acidic pHe. Acidic pHe paralleled greater expression of mitochondrial biogenesis and OXPHOS genes, such as complex III-V (Uqcr10, Atp5 and Cox7c), mt Fmc1, Romo1, Tmem 173, Tomm6, aldehyde dehydrogenase, mt Sod2 mt biogenesis component PPAR-γ co-activator 1 adjunct to loss of mt fission (Mff). Moreover, acidic pHe corresponded to metabolic efficiency evidenced by a rise in mTOR nutrient sensor GβL, its downstream target (Eif4ebp1), insulin modulators (Trib3 and Fetub) and loss of catabolic (Hadhb, Bdh1 and Pygl)/glycolytic processes (aldolase C, pyruvate kinase, Nampt and aldose-reductase). In contrast, alkaline pHe initiated loss of mitofusin 2, complex II-IV (Sdhaf1, Uqcrq, Cox4i2 and Aldh1l2), aconitase, mitochondrial carrier triple repeat 1 and mt biosynthetic (Coq2, Coq5 and Coq9). In conclusion, the Warburg effect might serve as a negative feedback loop that regulates the pHe toward a broad acidic range by altering lactic acid production through inversion of metabolic systems. These effects were independent of changes in O(2) concentration or glucose supply.

Metabolite kinetics in C6 rat glioma model using magnetic resonance spectroscopic imaging of hyperpolarized [1-(13)C]pyruvate

In addition to an increased lactate-to-pyruvate ratio, altered metabolism of a malignant glioma can be further characterized by its kinetics. Spatially resolved dynamic data of pyruvate and lactate from C6-implanted female Sprague-Dawley rat brain were acquired using a spiral chemical shift imaging sequence after a bolus injection of a hyperpolarized [1-(13)C]pyruvate. Apparent rate constants for the conversion of pyruvate to lactate in three different regions (glioma, normal appearing brain, and vasculature) were estimated based on a two-site exchange model. The apparent conversion rate constant was 0.018 ± 0.004 s(-1) (mean ± standard deviation, n = 6) for glioma, 0.009 ± 0.003 s(-1) for normal brain, and 0.005 ± 0.001 s(-1) for vasculature, whereas the lactate-to-pyruvate ratio, the metabolic marker used to date to identify tumor regions, was 0.36 ± 0.07 (mean ± SD), 0.24 ± 0.07, and 0.12 ± 0.02 for glioma, normal brain, and vasculature, respectively. The data suggest that the apparent conversion rate better differentiate glioma from normal brain (P = 0.001, n = 6) than the lactate-to-pyruvate ratio (P = 0.02).

Generating super stimulated-echoes in MRI and their application to hyperpolarized C-13 diffusion metabolic imaging

Stimulated-echoes in MR can be used to provide high sensitivity to motion and flow, creating diffusion and perfusion weighting as well as T(1) contrast, but conventional approaches inherently suffer from a 50% signal loss. The super stimulated-echo, which uses a specialized radio-frequency (RF) pulse train, has been proposed in order to improve the signal while preserving motion and T(1) sensitivity. This paper presents a novel and straightforward method for designing the super stimulated-echo pulse train using inversion pulse design techniques. This method can also create adiabatic designs with an improved response to RF transmit field variations. The scheme was validated in phantom experiments and shown in vivo to improve signal-to-noise ratio (SNR). We have applied a super stimulated-echo to metabolic MRI with hyperpolarized (13)C-labeled molecules. For spectroscopic imaging of hyperpolarized agents, several repetition times are required but only a single stimulated-echo encoding is feasible, which can lead to unwanted motion blurring. To address this, a super stimulated-echo preparation scheme was used in which the diffusion weighting is terminated prior to the acquisition, and we observed a SNR increases of 60% in phantoms and 49% in vivo over a conventional stimulated-echo. Experiments following injection of hyperpolarized [1-(13)C] -pyruvate in murine transgenic cancer models have shown improved delineation for tumors since signals from metabolites within tumor tissues are retained while those from the vasculature are suppressed by the diffusion preparation scheme.

Human Hepatocellular Carcinoma Metabolism: Imaging by Hyperpolarized 13C Magnetic Resonance Spectroscopy

PURPOSE

Most cancers exhibit high levels of aerobic glycolytic metabolism with diminished levels of mitochondrial oxidative phosphorylation even in the presence of normal or near-normal levels of oxygen ("Warburg effect"). However, technical challenges have limited the development of non-invasive in vivo imaging techniques for monitoring glycolytic metabolism of hepatocellular carcinoma (HCC) and quantitatively evaluating the impact of this effect on the growth and therapy of this disease. Thus, there is a critical need to develop non-invasive techniques for longitudinal assessment of the metabolism and treatment response of patients with unresectable HCCs.

PROCEDURES

This article discusses a novel method, "Hyperpolarized ¹³C MRS imaging", for achieving this objective and thus improving the prognosis of HCC patients. The primary objective has been to characterize in vivo metabolic biomarkers as determinants of HCC metabolism and treatment response of unresectable HCC tumors or viable HCC cells.

RESULTS

This innovative technique capitalizes on a new technology that increases the sensitivity of MRS detection of crucial metabolites in cancer cells.

CONCLUSION

It is anticipated that this innovative approach will lead to improved methods, both for the diagnosis and staging of HCCs and for the facilitation of the development of enzyme targeted therapies and other therapeutic interventions.