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

Disrupted metabolic flux balance between pyruvate dehydrogenase and pyruvate carboxylase in human fatty liver

Hepatic metabolism involving pyruvate carboxylase (PC) and pyruvate dehydrogenase (PDH) may be abnormal in fatty livers. In this study, [13C]bicarbonate production from [1-13C1]pyruvate in the liver and glycerol glyceroneogenesis were examined in relation to hepatic fat content using hyperpolarized [1-13C1]pyruvate and oral [U-13C3]glycerol. After an overnight fast, 15 subjects with a range of hepatic fat content received hyperpolarized [1-13C1]pyruvate intravenously to assess its conversion to [1-13C1]lactate and [13C]bicarbonate in the liver. They also received oral [U-13C3]glycerol, followed by venous blood sampling to examine glucose and the glycerol backbone of the triglycerides released primarily from the liver. From hyperpolarized [1-13C1]pyruvate, participants with high intrahepatic fat fraction produced higher [1-13C1]lactate and lower [13C]bicarbonate than those with low liver fat. The fraction of plasma triglycerides derived from oral [U-13C3]glycerol via the TCA cycle was similar between groups. The fraction of plasma [5,6-13C2]glucose, which reflects PC flux, decreased in subjects with fatty liver. In contrast, the fraction of [4,5-13C2]glucose + [6-13C1]glucose, which can be produced via either PC or PDH, was comparable between groups. The study results suggest a shift in pyruvate metabolism in fatty liver, with a decreased metabolic flux ratio of PC/PDH. The methodology in this study provides insights into fatty liver metabolism of human subjects inaccessible previously and is applicable to advanced liver diseases such as cirrhosis and hepatomas.

Regional quantification of metabolic liver function using hyperpolarized [1-13C] pyruvate MRI

Assessment of liver function is essential before partial hepatectomy to predict the risk of post hepatectomy liver failure, a severe and life-threatening complication. Traditional methods have focused on expected future liver remnant (FLR) volume estimation. However, liver volume does not always correlate with function. We suggest that metabolism might be a better surrogate for function than volume. Therefore, we aimed to investigate the metabolic changes in a porcine model of partial portal vein ligation (PVL) using hyperpolarized magnetic resonance imaging (HP-MRI). Specifically, we sought to quantify and compare the pyruvate metabolism in the FLR and the deportalized liver (DL).Six pigs underwent PVL. HP-MRI with [1-13C] pyruvate was performed at baseline, post-surgery, and 1 week after surgery. Metabolic conversion was quantified with kinetic modelling of the rate constants of pyruvate to lactate (kPL) and pyruvate to alanine (kPA). Mean kPL was increased in FLR compared to DL at post-surgery and 1 week after surgery (P = 0.002), while kPA was unaltered (P = 0.761). These findings indicate a metabolic shift towards glycolysis in the FLR. This non-invasive metabolic imaging technique could serve as a powerful tool for evaluation of regional liver function prior to partial hepatectomy and consequently improve patient outcomes.

Analyses of mitochondrial metabolism in diseases: a review on 13C magnetic resonance tracers

Metabolic diseases such as obesity, type 2 diabetes, and cardiovascular diseases have become a global health concern due to their widespread prevalence and profound impact on life expectancy, healthcare expenditures, and the overall economy. Devising effective treatment strategies and management plans for these diseases requires an in-depth understanding of the pathophysiology of the metabolic abnormalities associated with each disease. Mitochondrial dysfunction is intricately linked to a wide range of metabolic abnormalities and is considered an important biomarker for diseases. However, assessing mitochondrial functions in viable tissues remains a challenging task, with measurements of oxygen consumption rate (OCR) and ATP production being the most widely accepted approaches for evaluating the health of mitochondria in tissues. Measurements of cellular metabolism using carbon-13 (or 13C) tracers have emerged as a viable method for characterizing mitochondrial metabolism in a variety of organelles ranging from cultured cells to humans. Information on metabolic activities and mitochondrial functions can be obtained from magnetic resonance (MR) analyses of 13C-labeled metabolites in tissues and organs of interest. Combining novel 13C tracer technologies with advanced analytical and imaging tools in nuclear magnetic resonance spectroscopy (NMR) and magnetic resonance imaging (MRI) offers the potential to detect metabolic abnormalities associated with mitochondrial dysfunction. These capabilities would enable accurate diagnosis of various metabolic diseases and facilitate the assessment of responses to therapeutic interventions, hence improving patient health and optimizing clinical outcomes.

The mitochondrial dicarboxylate carrier mediates in vivo hepatic gluconeogenesis

Hepatic gluconeogenesis (GNG) is essential for maintaining euglycemia during prolonged fasting. However, GNG becomes pathologically elevated and drives chronic hyperglycemia in type 2 diabetes (T2D). Lactate/pyruvate is a major GNG substrate known to be imported into mitochondria for GNG. Yet, the subsequent mitochondrial carbon export mechanisms required to supply the extra-mitochondrial canonical GNG pathway have not been genetically delineated. Here, we evaluated the role of the mitochondrial dicarboxylate carrier (DiC) in mediating GNG from lactate/pyruvate. We generated liver-specific DiC knockout (DiC LivKO) mice. During lactate/pyruvate tolerance tests, DiC LivKO decreased plasma glucose excursion and 13C-lactate/-pyruvate flux into hepatic and plasma glucose. In a Western diet (WD) feeding model of T2D, acute DiC LivKO after induction of obesity decreased lactate/pyruvate-driven GNG, hyperglycemia, and hyperinsulinemia. Our results show that mitochondrial carbon export through the DiC mediates GNG and that the DiC contributes to impaired glucose homeostasis in a mouse model of T2D.

Hyperpolarized [2-13C, 3-2H3]Pyruvate Detects Hepatic Gluconeogenesis In Vivo

The feasibility of hyperpolarized [2-13C, 3-2H3]pyruvate for probing gluconeogenesis in vivo was investigated in this study. Whereas hyperpolarized [1-13C]pyruvate has clear access to metabolic pathways that convert pyruvate to lactate, alanine, and bicarbonate, its utility for assessing pyruvate carboxylation and gluconeogenesis has been limited by technical challenges, including spectral overlap and an obscure enzymatic step that decarboxylates the labeled carbon. To achieve unambiguous detection of gluconeogenic products, the carbonyl carbon in pyruvate was labeled with 13C. To prolong the T1 relaxation time, [2-13C, 3-2H3]pyruvate was synthesized and dissolved with D2O after dynamic nuclear polarization. The T1 of [2-13C, 3-2H3]pyruvate in D2O could be improved by 76.9% (79.6 s at 1 T and 74.5 s at 3 T) as compared to [2-13C]pyruvate in water. Hyperpolarized [2-13C, 3-2H3]pyruvate with D2O dissolution was applied to rat livers in vivo under normal feeding and fasting conditions. A gluconeogenic product, [2-13C]phosphoenolpyruvate, was observed at 149.9 ppm from fasted rats only, highlighting the utility of [2-13C, 3-2H3]pyruvate in detecting key gluconeogenic enzyme activities such as pyruvate carboxylase and phosphoenolpyruvate carboxykinase in vivo.

Allosteric Site at the Biotin Carboxylase Dimer Interface Mediates Activation and Inhibition in Staphylococcus aureus Pyruvate Carboxylase

Allosteric regulation of the essential anaplerotic enzyme, pyruvate carboxylase (PC), is vital for metabolic homeostasis. PC catalyzes the bicarbonate- and ATP-dependent carboxylation of pyruvate to form oxaloacetate. Dysregulation of PC activity can impact glucose and redox metabolism, which contributes to the pathogenicity of many diseases. To maintain homeostasis, PC is allosterically activated by acetyl-CoA and allosterically inhibited by l-aspartate. In this study, we further characterize the molecular basis of allosteric regulation in Staphylococcus aureus PC (SaPC) using slowly/nonhydrolyzable dethia analogues of acetyl-CoA and site-directed mutagenesis of residues at the biotin carboxylase homodimer interface. The dethia analogues fully activate SaPC but demonstrate significantly reduced binding affinities relative to acetyl-CoA. Residues Arg21, Lys46, and Glu418 of SaPC are located at the biotin carboxylase dimer interface and play a critical role in both allosteric activation and inhibition. A structure of R21A SaPC in complex with acetyl-CoA reveals an intact molecule of acetyl-CoA bound at the allosteric site, offering new molecular insights into the acetyl-CoA binding site. This study demonstrates that the biotin carboxylase domain dimer interface is a critical allosteric site in PC, serving as a convergence point for allosteric activation by acetyl-CoA and inhibition by l-aspartate.

"Metformin Impairs Intestinal Fructose Metabolism"

Objective

To investigate the effects of metformin on intestinal carbohydrate metabolism in vivo.

Method

Male mice preconditioned with a high-fat, high-sucrose diet were treated orally with metformin or a control solution for two weeks. Fructose metabolism, glucose production from fructose, and production of other fructose-derived metabolites were assessed using stably labeled fructose as a tracer.

Results

Metformin treatment decreased intestinal glucose levels and reduced incorporation of fructose-derived metabolites into glucose. This was associated with decreased intestinal fructose metabolism as indicated by decreased enterocyte F1P levels and diminished labeling of fructose-derived metabolites. Metformin also reduced fructose delivery to the liver. Proteomic analysis revealed that metformin coordinately down-regulated proteins involved carbohydrate metabolism including those involved in fructolysis and glucose production within intestinal tissue.

Conclusion

Metformin reduces intestinal fructose metabolism, and this is associated with broad-based changes in intestinal enzyme and protein levels involved in sugar metabolism indicating that metformin's effects on sugar metabolism are pleiotropic.

Oxidation of hyperpolarized [1-13 C]pyruvate in isolated rat kidneys

Kidneys play a central role in numerous disorders but current imaging methods have limited utility to probe renal metabolism. Hyperpolarized (HP) 13 C magnetic resonance imaging is uniquely suited to provide metabolite-specific information about key biochemical pathways and it offers the further advantage that renal imaging is practical in humans. This study evaluated the feasibility of hyperpolarization examinations in a widely used model for analysis of renal physiology, the isolated kidney, which enables isolation of renal metabolism from the effects of other organs and validation of HP results by independent measurements. Isolated rat kidneys were supplied with either HP [1-13 C]pyruvate only or HP [1-13 C]pyruvate plus octanoate. Metabolic activity in both groups was confirmed by stable renal oxygen consumption. HP [1-13 C]pyruvate was readily metabolized to [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]alanine, detectable seconds after HP [1-13 C]pyruvate was injected. Octanoate suppressed but did not eliminate the production of HP [13 C]bicarbonate from [1-13 C]pyruvate. Steady-state flux analyses using non-HP 13 C substrates validated the utilization of HP [1-13 C]pyruvate, as observed by HP 13 C NMR. In the presence of octanoate, lactate is generated from a tricarboxylic acid cycle intermediate, oxaloacetate. The isolated rat kidney may serve as an excellent model for investigating and establishing new HP 13 C metabolic probes for future kidney imaging applications.

Magnetic Resonance Imaging and Spectroscopy Methods to Study Hepatic Glucose Metabolism and Their Applications in the Healthy and Diabetic Liver

The liver plays an important role in whole-body glucose homeostasis by taking up glucose from and releasing glucose into the blood circulation. In the postprandial state, excess glucose in the blood circulation is stored in hepatocytes as glycogen. In the postabsorptive state, the liver produces glucose by breaking down glycogen and from noncarbohydrate precursors such as lactate. In metabolic diseases such as diabetes, these processes are dysregulated, resulting in abnormal blood glucose levels. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are noninvasive techniques that give unique insight into different aspects of glucose metabolism, such as glycogenesis, glycogenolysis, and gluconeogenesis, in the liver in vivo. Using these techniques, liver glucose metabolism has been studied in regard to a variety of interventions, such as fasting, meal intake, and exercise. Moreover, deviations from normal hepatic glucose metabolism have been investigated in both patients with type 1 and 2 diabetes, as well as the effects of antidiabetic medications. This review provides an overview of current MR techniques to measure hepatic glucose metabolism and the insights obtained by the application of these techniques in the healthy and diabetic liver.

Multi-nuclear magnetic resonance spectroscopy: state of the art and future directions

With the development of heteronuclear fluorine, sodium, phosphorus, and other probes and imaging technologies as well as the optimization of magnetic resonance imaging (MRI) equipment and sequences, multi-nuclear magnetic resonance (multi-NMR) has enabled localize molecular activities in vivo that are central to a variety of diseases, including cardiovascular disease, neurodegenerative pathologies, metabolic diseases, kidney, and tumor, to shift from the traditional morphological imaging to the molecular imaging, precision diagnosis, and treatment mode. However, due to the low natural abundance and low gyromagnetic ratios, the clinical application of multi-NMR has been hampered. Several techniques have been developed to amplify the NMR sensitivity such as the dynamic nuclear polarization, spin-exchange optical pumping, and brute-force polarization. Meanwhile, a wide range of nuclei can be hyperpolarized, such as 2H, 3He, 13C, 15 N, 31P, and 129Xe. The signal can be increased and allows real-time observation of biological perfusion, metabolite transport, and metabolic reactions in vivo, overcoming the disadvantages of conventional magnetic resonance of low sensitivity. HP-NMR imaging of different nuclear substrates provides a unique opportunity and invention to map the metabolic changes in various organs without invasive procedures. This review aims to focus on the recent applications of multi-NMR technology not only in a range of preliminary animal experiments but also in various disease spectrum in human. Furthermore, we will discuss the future challenges and opportunities of this multi-NMR from a clinical perspective, in the hope of truly bridging the gap between cutting-edge molecular biology and clinical applications.

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.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans

PURPOSE

This study is to investigate time-resolved ¹³ C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-¹³ C]pyruvate in the human brain.

METHODS

Time-resolved ¹³ C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-¹³ C]pyruvate. ¹³ C MRS with low flip-angle excitations and a multichannel ¹³ C/¹ H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of ¹³ C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise ¹³ C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval.

RESULTS

[1-¹³ C]Lactate relative to the total ¹³ C signal (tC) was 0.21-0.24 in all slices. [¹³ C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- were calculated as 0.014-0.018 s^-1 and 0.0043-0.0056 s^-1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1-¹³ C]pyruvate.

CONCLUSIONS

Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-¹³ C]pyruvate.

[13C]bicarbonate labelled from hyperpolarized [1-13C]pyruvate is an in vivo marker of hepatic gluconeogenesis in fasted state

Hyperpolarized [1-¹³C]pyruvate enables direct in vivo assessment of real-time liver enzymatic activities by ¹³C magnetic resonance. However, the technique usually requires the injection of a highly supraphysiological dose of pyruvate. We herein demonstrate that liver metabolism can be measured in vivo with hyperpolarized [1-¹³C]pyruvate administered at two- to three-fold the basal plasma concentration. The flux through pyruvate dehydrogenase, assessed by ¹³C-labeling of bicarbonate in the fed condition, was found to be saturated or partially inhibited by supraphysiological doses of hyperpolarized [1-¹³C]pyruvate. The [¹³C]bicarbonate signal detected in the liver of fasted rats nearly vanished after treatment with a phosphoenolpyruvate carboxykinase (PEPCK) inhibitor, indicating that the signal originates from the flux through PEPCK. In addition, the normalized [¹³C]bicarbonate signal in fasted untreated animals is dose independent across a 10-fold range, highlighting that PEPCK and pyruvate carboxylase are not saturated and that hepatic gluconeogenesis can be directly probed in vivo with hyperpolarized [1-¹³C]pyruvate.

Can et al. demonstrate the ability to use hyperpolarized [1-13C]pyruvate at nearphysiological concentrations to directly assess liver enzymatic activities by 13C magnetic resonance. While in the fed state, the normalized [13C]bicarbonate signal produced from hyperpolarized [1-13C]pyruvate derives from PDH activity, which is saturated at supraphysiological doses, it results from PEPCK in the fasted state and is dose-independent, allowing non-invasive in vivo detection of hepatic gluconeogenesis.”

In Vivo Magnetic Resonance Spectroscopy of Hyperpolarized [1-13 C]Pyruvate and Proton Density Fat Fraction in a Guinea Pig Model of Non-Alcoholic Fatty Liver Disease Development After Life-Long Western Diet Consumption

BACKGROUND

Alterations in glycolysis are central to the increasing incidence of non-alcoholic fatty liver disease (NAFLD), highlighting a need for in vivo, non-invasive technologies to understand the development of hepatic metabolic aberrations.

PURPOSE

To use hyperpolarized magnetic resonance spectroscopy (MRS) and proton density fat fraction (PDFF) magnetic resonance imaging (MRI) techniques to investigate the effects of a chronic, life-long exposure to the Western diet (WD) in an animal model resulting in NAFLD; to investigate the hypothesis that exposure to the WD will result in NAFLD in association with altered pyruvate metabolism.

STUDY TYPE

Prospective.

ANIMAL MODEL

Twenty-eight male guinea pigs weaned onto a control diet (N = 14) or WD (N = 14).

FIELD STRENGTH/SEQUENCE

3 T; T1-weighted gradient echo, T2-weighted spin-echo, three-dimensional gradient multi-echo fat-water separation (IDEAL-IQ), and broadband point-resolved spectroscopy (PRESS) chemical-shift sequences.

ASSESSMENT

Median PDFF was calculated in the liver and hind limbs. [1-¹³ C]pyruvate dynamic MRS in the liver was quantified by the time-to-peak (TTP) for each metabolite. Animals were euthanized and tissue was analyzed for lipid and cholesterol concentration and enzyme level and activity.

STATISTICAL TESTS

Unpaired Student's t-tests were used to determine differences in measurements between the two diet groups. The Pearson correlation coefficient was calculated to determine correlations between measurements.

RESULTS

Life-long WD consumption resulted in significantly higher liver PDFF and elevated triglyceride content in the liver. The WD group exhibited a decreased TTP for lactate production, and ex vivo analysis highlighted increased liver lactate dehydrogenase (LDH) activity.

DATA CONCLUSION

PDFF MRI results suggest differential fat deposition patterns occurring in animals fed a life-long WD characteristic of lean, or lacking excessive subcutaneous fat, NAFLD. The decreased liver lactate TTP and increased ex vivo LDH activity suggest lipid accumulation occurs in association with a shift from oxidative metabolism to anaerobic glycolytic metabolism in WD-exposed livers.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.

Schulte R, Laustsen C. Comprehensive Literature Review of Hyperpolarized Carbon-13 MRI: The Road to Clinical Application

This review provides a comprehensive assessment of the development of hyperpolarized (HP) carbon-13 metabolic MRI from the early days to the present with a focus on clinical applications. The status and upcoming challenges of translating HP carbon-13 into clinical application are reviewed, along with the complexity, technical advancements, and future directions. The road to clinical application is discussed regarding clinical needs and technological advancements, highlighting the most recent successes of metabolic imaging with hyperpolarized carbon-13 MRI. Given the current state of hyperpolarized carbon-13 MRI, the conclusion of this review is that the workflow for hyperpolarized carbon-13 MRI is the limiting factor.

The use of hyperpolarised 13C-MRI in clinical body imaging to probe cancer metabolism

Metabolic reprogramming is one of the hallmarks of cancer and includes the Warburg effect, which is exhibited by many tumours. This can be exploited by positron emission tomography (PET) as part of routine clinical cancer imaging. However, an emerging and alternative method to detect altered metabolism is carbon-13 magnetic resonance imaging (MRI) following injection of hyperpolarised [1-13C]pyruvate. The technique increases the signal-to-noise ratio for the detection of hyperpolarised 13C-labelled metabolites by several orders of magnitude and facilitates the dynamic, noninvasive imaging of the exchange of 13C-pyruvate to 13C-lactate over time. The method has produced promising preclinical results in the area of oncology and is currently being explored in human imaging studies. The first translational studies have demonstrated the safety and feasibility of the technique in patients with prostate, renal, breast and pancreatic cancer, as well as revealing a successful response to treatment in breast and prostate cancer patients at an earlier stage than multiparametric MRI. This review will focus on the strengths of the technique and its applications in the area of oncological body MRI including noninvasive characterisation of disease aggressiveness, mapping of tumour heterogeneity, and early response assessment. A comparison of hyperpolarised 13C-MRI with state-of-the-art multiparametric MRI is likely to reveal the unique additional information and applications offered by the technique.

Differentiation of Heterogeneous Mouse Liver from HCC by Hyperpolarized 13C Magnetic Resonance

The clinical characterization of small hepatocellular carcinoma (HCC) lesions in the liver and differentiation from heterogeneous inflammatory or fibrotic background is important for early detection and treatment. Metabolic monitoring of hyperpolarized 13C-labeled substrates has been suggested as a new avenue for diagnostic magnetic resonance. The metabolism of hyperpolarized [1-13C]pyruvate was monitored in mouse precision-cut liver slices (PCLS) of aged MDR2-KO mice, which served as a model for heterogeneous liver and HCC that develops similarly to the human disease. The relative in-cell activities of lactate dehydrogenase (LDH) to alanine transaminase (ALT) were found to be 0.40 ± 0.06 (n = 3) in healthy livers (from healthy mice), 0.90 ± 0.27 (n = 3) in heterogeneously inflamed liver, and 1.84 ± 0.46 (n = 3) in HCC. Thus, the in-cell LDH/ALT activities ratio was found to correlate with the progression of the disease. The results suggest that the LDH/ALT activities ratio may be useful in the assessment of liver disease. Because the technology used here is translational to both small liver samples that may be obtained from image-guided biopsy (i.e., ex vivo investigation) and to the intact liver (i.e., in a noninvasive MRI scan), these results may provide a path for differentiating heterogeneous liver from HCC in human subjects.

Elevated Brain Glutamate Levels in Bipolar Disorder and Pyruvate Carboxylase-Mediated Anaplerosis

In vivo ¹H magnetic resonance spectroscopy studies have found elevated brain glutamate or glutamate + glutamine levels in bipolar disorder with surprisingly high reproducibility. We propose that the elevated glutamate levels in bipolar disorder can be explained by increased pyruvate carboxylase-mediated anaplerosis in brain. Multiple independent lines of evidence supporting increased pyruvate carboxylase-mediated anaplerosis as a common mechanism underlying glutamatergic hyperactivity in bipolar disorder and the positive association between bipolar disorder and obesity are also described.

Assessment of hepatic pyruvate carboxylase activity using hyperpolarized [1-13 C]-l-lactate

PURPOSE

To evaluate the utility of hyperpolarized [1-¹³ C]-l-lactate to detect hepatic pyruvate carboxylase activity in vivo under fed and fasted conditions.

METHODS

[1-¹³ C]-labeled sodium L-lactate was polarized using a dynamic nuclear polarizer. Polarization level and the T₁ were measured in vitro in a 3 Telsa MR scanner. Two groups of healthy rats (fasted vs. fed) were prepared for in vivo studies. Each rat was anesthetized and intravenously injected with 60-mM hyperpolarized [1-¹³ C]-l-lactate, immediately followed by dynamic acquisition of ¹³ C (carbon-13) MR spectra from the liver at 3 Tesla. The dosage-dependence of the ¹³ C-products was also investigated by performing another injection of an equal volume of 30-mM hyperpolarized [1-¹³ C]-l-lactate.

RESULTS

T₁ and liquid polarization level of [1-¹³ C]-l-lactate were estimated as 67.8 s and 40.0%, respectively. [1-¹³ C]pyruvate and [1-¹³ C]alanine, [¹³ C]bicarbonate ( HCO 3 - ) and [1-¹³ C]aspartate were produced from hyperpolarized [1-¹³ C]-l-lactate in rat liver. Smaller HCO 3 - and larger aspartate were measured in the fed group compared to the fasted group. Pyruvate and alanine production were increased in proportion to the lactate concentration, whereas the amount of HCO 3 - and aspartate production was consistent between 30-mM and 60-mM lactate injections.

CONCLUSION

This study demonstrates that a unique biomarker of pyruvate carboxylase flux, the appearance of [1-¹³ C]aspartate from [1-¹³ C]-l-lactate, is sensitive to nutritional state and may be monitored in vivo at 3 Tesla. Because [¹³ C] HCO 3 - is largely produced by pyruvate dehydrogenase flux, these results suggest that the ratio of [1-¹³ C]aspartate and [¹³ C] HCO 3 - (aspartate/ HCO 3 - ) reflects the saturable pyruvate carboxylase/pyruvate dehydrogenase enzyme activities.

Metabolic Imaging and Biological Assessment: Platforms to Evaluate Acute Lung Injury and Inflammation

Pulmonary inflammation is a hallmark of several pulmonary disorders including acute lung injury and acute respiratory distress syndrome. Moreover, it has been shown that patients with hyperinflammatory phenotype have a significantly higher mortality rate. Despite this, current therapeutic approaches focus on managing the injury rather than subsiding the inflammatory burden of the lung. This is because of the lack of appropriate non-invasive biomarkers that can be used clinically to assess pulmonary inflammation. In this review, we discuss two metabolic imaging tools that can be used to non-invasively assess lung inflammation. The first method, Positron Emission Tomography (PET), is widely used in clinical oncology and quantifies flux in metabolic pathways by measuring uptake of a radiolabeled molecule into the cells. The second method, hyperpolarized ¹³C MRI, is an emerging tool that interrogates the branching points of the metabolic pathways to quantify the fate of metabolites. We discuss the differences and similarities between these techniques and discuss their clinical applications.

Differentiation of Heterogeneous Mouse Liver from HCC by Hyperpolarized 13C Magnetic Resonance

The clinical characterization of small hepatocellular carcinoma (HCC) lesions in the liver and differentiation from heterogeneous inflammatory or fibrotic background is important for early detection and treatment. Metabolic monitoring of hyperpolarized 13C-labeled substrates has been suggested as a new avenue for diagnostic magnetic resonance. The metabolism of hyperpolarized [1-13C]pyruvate was monitored in mouse precision-cut liver slices (PCLS) of aged MDR2-KO mice, which served as a model for heterogeneous liver and HCC that develops similarly to the human disease. The relative in-cell activities of lactate dehydrogenase (LDH) to alanine transaminase (ALT) were found to be 0.40 ± 0.06 (n = 3) in healthy livers (from healthy mice), 0.90 ± 0.27 (n = 3) in heterogeneously inflamed liver, and 1.84 ± 0.46 (n = 3) in HCC. Thus, the in-cell LDH/ALT activities ratio was found to correlate with the progression of the disease. The results suggest that the LDH/ALT activities ratio may be useful in the assessment of liver disease. Because the technology used here is translational to both small liver samples that may be obtained from image-guided biopsy (i.e., ex vivo investigation) and to the intact liver (i.e., in a non-invasive MRI scan), these results may provide a path for differentiating heterogeneous liver from HCC in human subjects.

Differentiation of Heterogeneous Mouse Liver from HCC by Hyperpolarized 13C Magnetic Resonance

The clinical characterization of small hepatocellular carcinoma (HCC) lesions in the liver and differentiation from heterogeneous inflammatory or fibrotic background is important for early detection and treatment. Metabolic monitoring of hyperpolarized 13C-labeled substrates has been suggested as a new avenue for diagnostic magnetic resonance. The metabolism of hyperpolarized [1-13C]pyruvate was monitored in mouse precision-cut liver slices (PCLS) of aged MDR2-KO mice, which served as a model for heterogeneous liver and HCC that develops similarly to the human disease. The relative in-cell activities of lactate dehydrogenase (LDH) to alanine transaminase (ALT) were found to be 0.40 ± 0.06 (n = 3) in healthy livers (from healthy mice), 0.90 ± 0.27 (n = 3) in heterogeneously inflamed liver, and 1.84 ± 0.46 (n = 3) in HCC. Thus, the in-cell LDH/ALT activities ratio was found to correlate with the progression of the disease. The results suggest that the LDH/ALT activities ratio may be useful in the assessment of liver disease. Because the technology used here is translational to both small liver samples that may be obtained from image-guided biopsy (i.e., ex vivo investigation) and to the intact liver (i.e., in a non-invasive MRI scan), these results may provide a path for differentiating heterogeneous liver from HCC in human subjects.

Differential contribution of pyruvate carboxylation to anaplerosis and cataplerosis during non-gluconeogenic and gluconeogenic conditions in HepG2 cells

Pyruvate carboxylase (PC) is an anaplerotic enzyme that supplies oxaloacetate to mitochondria enabling the maintenance of other metabolic intermediates consumed by cataplerosis. Using liquid chromatography mass spectrometry (LC-MS) to measure metabolic intermediates derived from uniformly labeled ¹³C₆-glucose or [3-¹³C]l-lactate, we investigated the contribution of PC to anaplerosis and cataplerosis in the liver cell line HepG2. Suppression of PC expression by short hairpin RNA lowered incorporation of ¹³C glucose incorporation into tricarboxylic acid cycle intermediates, aspartate, glutamate and sugar derivatives, indicating impaired cataplerosis. The perturbation of these biosynthetic pathways is accompanied by a marked decrease of cell viability and proliferation. In contrast, under gluconeogenic conditions where the HepG2 cells use lactate as a carbon source, pyruvate carboxylation contributed very little to the maintenance of these metabolites. Suppression of PC did not affect the percent incorporation of ¹³C-labeled carbon from lactate into citrate, α-ketoglutarate, malate, succinate as well as aspartate and glutamate, suggesting that under gluconeogenic condition, PC does not support cataplerosis from lactate.

Real-time hyperpolarized 13C magnetic resonance detects increased pyruvate oxidation in pyruvate dehydrogenase kinase 2/4-double knockout mouse livers

The pyruvate dehydrogenase complex (PDH) critically regulates carbohydrate metabolism. Phosphorylation of PDH by one of the pyruvate dehydrogenase kinases 1-4 (PDK1-4) decreases the flux of carbohydrates into the TCA cycle. Inhibition of PDKs increases oxidative metabolism of carbohydrates, so targeting PDKs has emerged as an important therapeutic approach to manage various metabolic diseases. Therefore, it is highly desirable to begin to establish imaging tools for noninvasive measurements of PDH flux in rodent models. In this study, we used hyperpolarized (HP) ¹³C-magnetic resonance spectroscopy to study the impact of a PDK2/PDK4 double knockout (DKO) on pyruvate metabolism in perfused livers from lean and diet-induced obese (DIO) mice and validated the HP observations with high-resolution ¹³C-nuclear magnetic resonance (NMR) spectroscopy of tissue extracts and steady-state isotopomer analyses. We observed that PDK-deficient livers produce more HP-bicarbonate from HP-[1-¹³C]pyruvate than age-matched control livers. A steady-state ¹³C-NMR isotopomer analysis of tissue extracts confirmed that flux rates through PDH, as well as pyruvate carboxylase and pyruvate cycling activities, are significantly higher in PDK-deficient livers. Immunoblotting experiments confirmed that HP-bicarbonate production from HP-[1-¹³C]pyruvate parallels decreased phosphorylation of the PDH E1α subunit (pE1α) in liver tissue. Our findings indicate that combining real-time hyperpolarized ¹³C NMR spectroscopy and ¹³C isotopomer analysis provides quantitative insights into intermediary metabolism in PDK-knockout mice. We propose that this method will be useful in assessing metabolic disease states and developing therapies to improve PDH flux.

Enzymatic and nonenzymatic protein acetylations control glycolysis process in liver diseases

Impaired glycolysis has pathologic effects on the occurrence and progression of liver diseases, and it appears that glycolysis is increased to different degrees in different liver diseases. As an important post-translational modification, reversible lysine acetylation regulates almost all cellular processes, including glycolysis. Lysine acetylation can occur enzymatically with acetyltransferases or nonenzymatically with acetyl-coenzyme A. Accompanied by the progression of liver diseases, there seems to be a temporal and spatial variation between enzymatic and nonenzymatic acetylations in the regulation of glycolysis. Here, we summarize the most recent findings on the functions and targets of acetylation in controlling glycolysis in the different stages of liver diseases. In addition, we discuss the differences and causes between enzymatic and nonenzymatic acetylations in regulating glycolysis throughout the progression of liver diseases. Then, we review these new discoveries to provide the potential implications of these findings for therapeutic interventions in liver diseases.-Li, J., Wang, T., Xia, J., Yao, W., Huang, F. Enzymatic and nonenzymatic protein acetylations control glycolysis process in liver diseases.

Glucagon up-regulates hepatic mitochondrial pyruvate carrier 1 through cAMP-responsive element-binding protein; inhibition of hepatic gluconeogenesis by ginsenoside Rb1

BACKGROUND AND PURPOSE

Hepatic mitochondrial pyruvate carrier (MPC) transports pyruvate into mitochondria. This study investigated the involvement of MPC1 in hepatic glucagon response, in order to identify a possible pharmacological intervention.

EXPERIMENTAL APPROACH

The correlation between hepatic glucagon response and MPC1 induction was investigated in fasted mice and primary hepatocytes. The effects of ginsenoside Rb1 on MPC1 function were observed.

KEY RESULTS

Glucagon challenge raised blood glucose with hepatic MPC1 induction, and inhibition of MPC induction coincided with a reduced rise in blood glucose. cAMP-responsive element-binding protein (CREB) knockdown blocked glucagon-induced MPC1 expression, while CREB overexpression increased MPC1 expression. Luciferase reporter, chromatin immunoprecipitation assay, and promoter mutation confirmed that CREB increased MPC1 transcription through gene promoter induction. CREB regulated transcription co-activator 2 nuclear translocation was also required for CREB to promote MPC1 induction. Glucagon shifted mitochondrial pyruvate towards carboxylation for gluconeogenesis via the opposite regulation of pyruvate dehydrogenase and carboxylase with respect to MPC1 induction. MPC1 induction was necessary for glucagon to promote pyruvate-driven hepatic glucose production (HGP), but glucagon failed to influence HGP from other gluconeogenic substrates routed into the tricarboxylic acid cycle, independent of MPC. Rb1 blocked cAMP signalling by inhibiting AC activity and deactivated CREB by dephosphorylation, possibly contributing to inhibiting MPC1 induction to reduce HGP.

CONCLUSIONS AND IMPLICATIONS

CREB transcriptionally up-regulates MPC1 to provide pyruvate for gluconeogenesis. Rb1 reduced cAMP formation which consequently reduced CREB-mediated MPC1 induction and thereby might contribute to limiting pyruvate-dependent HGP. These results suggest a therapeutic strategy to reduce hyperglycaemia in diabetes.

Loss of BCAA Catabolism during Carcinogenesis Enhances mTORC1 Activity and Promotes Tumor Development and Progression

Tumors display profound changes in cellular metabolism, yet how these changes aid the development and growth of tumors is not fully understood. Here we use a multi-omic approach to examine liver carcinogenesis and regeneration, and find that progressive loss of branched-chain amino acid (BCAA) catabolism promotes tumor development and growth. In human hepatocellular carcinomas and animal models of liver cancer, suppression of BCAA catabolic enzyme expression led to BCAA accumulation in tumors, though this was not observed in regenerating liver tissues. The degree of enzyme suppression strongly correlated with tumor aggressiveness, and was an independent predictor of clinical outcome. Moreover, modulating BCAA accumulation regulated cancer cell proliferation in vitro, and tumor burden and overall survival in vivo. Dietary BCAA intake in humans also correlated with cancer mortality risk. In summary, loss of BCAA catabolism in tumors confers functional advantages, which could be exploited by therapeutic interventions in certain cancers.

A new hyperpolarized 13C ketone body probe reveals an increase in acetoacetate utilization in the diabetic rat heart

Emerging studies have recently shown the potential importance of ketone bodies in cardio-metabolic health. However, techniques to determine myocardial ketone body utilization in vivo are lacking. In this work, we developed a novel method to assess myocardial ketone body utilization in vivo using hyperpolarized [3-¹³C]acetoacetate and investigated the alterations in myocardial ketone body metabolism in diabetic rats. Within a minute upon injection of [3-¹³C]acetoacetate, the production of [5-¹³C]glutamate and [1-¹³C] acetylcarnitine can be observed real time in vivo. In diabetic rats, the production of [5-¹³C]glutamate was elevated compared to controls, while [1-¹³C]acetylcarnitine was not different. This suggests an increase in ketone body utilization in the diabetic heart, with the produced acetyl-CoA channelled into the tricarboxylic acid cycle. This observation was corroborated by an increase activity of succinyl-CoA:3-ketoacid-CoA transferase (SCOT) activity, the rate-limiting enzyme of ketone body utilization, in the diabetic heart. The increased ketone body oxidation in the diabetic hearts correlated with cardiac hypertrophy and dysfunction, suggesting a potential coupling between ketone body metabolism and cardiac function. Hyperpolarized [3-¹³C]acetoacetate is a new probe with potential for non-invasive and real time monitoring of myocardial ketone body oxidation in vivo, which offers a powerful tool to follow disease progression or therapeutic interventions.

Cardiac metabolic modulation upon low-carbohydrate low-protein ketogenic diet in diabetic rats studied in vivo using hyperpolarized 13 C pyruvate, butyrate and acetoacetate probes

AIM

To investigate the effects of long-term low-carbohydrate low-protein ketogenic diet (KD) on cardiac metabolism and diabetic cardiomyopathy status in lean diabetic Goto-Kakizaki (GK) rats.

MATERIALS AND METHODS

Diabetic GK rats were fed with KD for 62 weeks. Cardiac function and metabolism were assessed using magnetic resonance imaging and ¹³ C magnetic resonance spectroscopy (¹³ C-MRS), at rest and under dobutamine stress. ¹³ C-MRS was performed following injection of hyperpolarized [3-¹³ C]acetoacetate, [1-¹³ C]butyrate or [1-¹³ C]pyruvate to assess ketone body, short-chain fatty acid or glucose utilization, respectively. Protein expression and cardiomyocyte structure were determined via Western blotting and histology, respectively.

RESULTS

KD lowered blood glucose, triglyceride and insulin levels while increasing blood ketone body levels. In KD-fed diabetic rats, myocardial ketone body and glucose oxidation were lower than in chow-fed diabetic rats, while myocardial glycolysis and short-chain fatty acid oxidation were unaltered. Dobutamine stress revealed an increased cardiac preload and reduced cardiac compliance in KD-fed diabetic rats. Dobutamine-induced stimulation of myocardial glycolysis was more enhanced in KD-fed diabetic rats than in chow-fed diabetic rats, which was potentially facilitated via an upregulation in basal expression of proteins involved in glucose transport and glycolysis in the hearts of KD-fed rats. The metabolic profile induced by KD was accompanied by cardiac hypertrophy, a trend for increased myocardial lipid and collagen content, and an increased marker of oxidative stress.

CONCLUSION

KD seems to exacerbate diabetic cardiomyopathy in GK rats, which may be associated with maladaptive cardiac metabolic modulation and lipotoxicity.

Real-time ALT and LDH activities determined in viable precision-cut mouse liver slices using hyperpolarized [1-13 C]pyruvate-Implications for studies on biopsied liver tissues

Precision-cut liver slices (PCLS) are widely used in liver research as they provide a liver model with all liver cell types in their natural architecture. The purpose of this study was to demonstrate the use of PCLS for hyperpolarized metabolic investigation in a mouse model, for potential future application in liver biopsy cores. Fresh normal liver was harvested from six mice. 500 μm PCLS were prepared and placed in a 10 mm NMR tube in an NMR spectrometer and perfused continuously. ³¹ P spectra were acquired to evaluate the presence of adenosine triphosphate (ATP) and validate viability in all samples. Hyperpolarized [1-¹³ C]pyruvate was flushed into the NMR tube in the spectrometer. Consecutive ¹³ C NMR spectra were acquired immediately after the injection using both non-selective (five injections, two livers) and selective RF excitation (six injections, three livers). The ³¹ P spectra showed the characteristic signals of ATP, confirming the viability of the PCLS for more than 2.5 h in the spectrometer. After each of the [1-¹³ C]pyruvate injections, both [1-¹³ C]lactate and [1-¹³ C]alanine signals were detected. Selective RF excitation aimed at both [1-¹³ C]lactate and [1-¹³ C]alanine enabled better visualization and quantification of the metabolic activity. Using this acquisition approach only the newly formed metabolites are observed upon excitation, and their intensities relative to those of hyperpolarized pyruvate enable quantification of metabolite production rates. This rate of lactate and alanine production appeared to be constant throughout the measurement time, with alanine production about 2.3 times higher than lactate. In summary, the viability of PCLS in an NMR spectrometer was demonstrated and hyperpolarized [1-¹³ C]pyruvate metabolism was recorded. This study opens up the possibility of evaluating alanine aminotransferase (ALT) and lactate dehydrogenase (LDH) activities in human liver biopsies, while preserving the tissue architecture and viability. In healthy, well-perfused liver slices the ratio of ALT to LDH activity is about 2.3.

Hyperpolarized [1-13 C] pyruvate as a possible diagnostic tool in liver disease

Introduction of hyperpolarized magnetic resonance in preclinical studies and lately translation to patients provides new detailed in vivo information of metabolic flux in organs. Hyperpolarized magnetic resonance based on 13 C enriched pyruvate is performed without ionizing radiation and allows quantification of the pyruvate conversion products: alanine, lactate and bicarbonate in real time. Thus, this methodology has a promising potential for in vivo monitoring of energetic alterations in hepatic diseases. Using 13 C pyruvate, we investigated the metabolism in the porcine liver before and after intravenous injection of glucose. The overall mean lactate to pyruvate ratio increased significantly after the injection of glucose whereas the bicarbonate to pyruvate ratio was unaffected, representative of the levels of pyruvate entering the tricarboxylic acid cycle. Similarly, alanine to pyruvate ratio did not change. The increased lactate to pyruvate ratio over time showed an exponential correlation with insulin, glucagon and free fatty acids. Together, these data, obtained by hyperpolarized 13 C magnetic resonance spectroscopy and by blood sampling, indicate a hepatic metabolic shift in glucose utilization following a glucose challenge. Our findings demonstrate the capacity of hyperpolarized 13 C magnetic resonance spectroscopy for quantifying hepatic substrate metabolism in accordance with well-known physiological processes. When combined with concentration of blood insulin, glucagon and free fatty acids in the blood, the results indicate the potential of hyperpolarized magnetic resonance spectroscopy as a future clinical method for quantification of hepatic substrate metabolism.

Metabolic Remodeling during Liver Regeneration

Liver disease is linked to a decreased capacity of hepatocytes to divide. In addition, cellular metabolism is important for tissue homeostasis and regeneration. Since metabolic changes are a hallmark of liver disease, we investigated the connections between metabolism and cell division. We determined global metabolic changes at different stages of liver regeneration using a combination of integrated transcriptomic and metabolomic analyses with advanced functional redox in vivo imaging. Our data indicate that blocking hepatocyte division during regeneration leads to mitochondrial dysfunction and downregulation of oxidative pathways. This resulted in an increased redox ratio and hyperactivity of alanine transaminase allowing the production of alanine and α-ketoglutarate from pyruvate when mitochondrial functions are impaired. Our data suggests that during liver regeneration, cell division leads to hepatic metabolic remodeling. Moreover, we demonstrate that hepatocytes are equipped with a flexible metabolic machinery able to adapt dynamically to changes during tissue regeneration.

Proteomic Analysis Reveals the Renoprotective Effect of Tribulus terrestris against Obesity-Related Glomerulopathy in Rats

Tribulus terrestris L. (Zygophyllaceae) (TT) is usually used as a cardiotonic, diuretic, and aphrodisiac, as well as for herbal post-stroke rehabilitation in traditional Chinese medicine. However, little is known about the renoprotective effects of TT on obesity-related glomerulopathy (ORG). In this study, 340 monomeric compounds were identified from TT extracts obtained with ethyl acetate combined with 50% methanol. In vitro, IC₅₀ of TT was 912.01 mg/L, and the appropriate concentration of TT against oxidized-low density lipoprotein (ox-LDL) induced human renal glomerular endothelial cells (HRGECs) was 4 mg/L. TT significantly increased the viability (63.2%) and migration (2.33-fold increase) of HRGECs. ORG model rats were induced by a chronic high-fat diet (45%) for 20 weeks and were then treated with TT extract (2.8 g/kg/d) for 8 weeks. Subsequently, the kidneys were removed and their differentially expressed protein profile was identified using two-dimensional electrophoresis coupled with matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF)-TOF MS. Molecular categorization and functional analysis of bioinformatic annotation suggested that excessive energy metabolism, decreased response to stress and low immunity were the potential etiologies of ORG. After TT administration for 8 weeks, body weight, blood pressure, serum cystatin C and cholesterol were decreased. Additionally, TT significantly enhanced the resistance of rats to ORG, decreased energy consumption and the hemorrhagic tendency, and improved the response to acute phase reactants and immunity. In conclusion, TT may play a protective role against ORG in rats.

Inhibiting Glycine Decarboxylase Suppresses Pyruvate-to-Lactate Metabolism in Lung Cancer Cells

Glycine decarboxylase (GLDC) gene is frequently upregulated in various types of cancer including lung, prostate and brain. It catabolizes glycine to yield 5,10-methylenetetrahydrofolate, an important substrate in one-carbon metabolism for nucleotide synthesis. In this study, we used exon splicing modulating steric hindrance antisense oligonucleotide (shAON) to suppress GLDC expression and investigated its effect on pyruvate metabolism via hyperpolarized carbon-13 magnetic resonance spectroscopy (MRS). The MRS technique allows us to study in vivo metabolic flux in tumor tissues with/without GLDC-shAON intervention. Here, we show that GLDC-shAON treatment is able to suppress lung cancer cell growth and tumorigenesis, both in vitro and in vivo. The carbon-13 MRS results indicated that the conversion of pyruvate into lactate in GLDC-shAON-treated tumor tissues was significantly reduced, when compared with the control groups. This observation corroborated with the reduced activity of lactate dehydrogenase and pyruvate dehydrogenase in GLDC-shAON-treated lung cancer cells and tumor tissues. Glycolysis stress test showed that extracellular acidification rate was significantly suppressed after GLDC-shAON treatment. Besides lung cancer, the antitumor effect of GLDC-shAON was also observed in brain, liver, cervical, and prostate cancer cell lines. Furthermore, it enhanced the treatment efficacy of cisplatin in lung cancer cells. Taken together, our findings illustrate that pyruvate metabolism decreases upon GLDC inhibition, thereby starving cancer cells from critical metabolic fuels.

Detection of Bacteria-Specific Metabolism Using Hyperpolarized [2-13C]Pyruvate

The differentiation of bacterial infection from other causes of inflammation is difficult in clinical practice and is critical where patient outcomes rely heavily on early interventions. In addition to physical exam and laboratory markers, several imaging modalities are frequently employed, but these techniques generally target the host immune response, rather than the living microorganisms themselves. Here, we describe a method to detect bacteria-specific metabolism using hyperpolarized (HP) ¹³C magnetic resonance spectroscopy. This technology allows visualization of the real-time conversion of enriched ¹³C substrates to their metabolic products, identified by their distinct chemical shifts. We have identified the rapid metabolism of HP [2-¹³C]pyruvate to [1-¹³C]acetate as a metabolic signature of common bacterial pathogens. We demonstrate this conversion in representative Gram-negative and Gram-positive bacteria, namely, Escherichia coli and Staphylococcus aureus, and its absence in key mammalian cell types. Furthermore, this conversion was successfully modulated in three mutant strains, corresponding to deletions of relevant enzymes.

Imaging in Pharmacogenetics

An increasing collection of imaging technologies makes it possible to differentiate treatment responders from nonresponders based on genetic variation. This chapter will review some of the imaging technologies currently available in nuclear medicine to visualize drug absorption, distribution, metabolism, and elimination. Some of the commonly used techniques to detect radiation-emitting compounds are the two-dimensional scintigraphy and the three-dimensional single-photon emission computed tomography (SPECT) which both detect photons using a gamma camera, and the three-dimensional positron emission tomography (PET), which detect the decay of positron-emitting radionuclides. Current examples include visualization of functional effects of genetic variants, and these provide proof of concept for imaging in pharmacogenetics as a tool to improve efficacy and safety of drugs.

Non-invasive detection of divergent metabolic signals in insulin deficiency vs. insulin resistance in vivo

The type 2 diabetic phenotype results from mixed effects of insulin deficiency and insulin resistance, but the relative contributions of these two distinct factors remain poorly characterized, as do the respective roles of the gluconeogenic organs. The purpose of this study was to investigate localized in vivo metabolic changes in liver and kidneys of contrasting models of diabetes mellitus (DM): streptozotocin (STZ)-treated wild-type Zucker rats (T1DM) and Zucker diabetic fatty (ZDF) rats (T2DM). Intermediary metabolism was probed using hyperpolarized (HP) [1-13C]pyruvate MRI of the liver and kidneys. These data were correlated with gene expression data for key mediators, assessed using rtPCR. Increased HP [1-13C]lactate was detected in both models, in association with elevated gluconeogenesis as reflected by increased expression of phosphoenolpyruvate carboxykinase. In contrast, HP [1-13C]alanine diverged between the two models, increasing in ZDF rats, while decreasing in the STZ-treated rats. The differences in liver alanine paralleled differences in key lipogenic mediators. Thus, HP [1-13C]alanine is a marker that can identify phenotypic differences in kidneys and liver of rats with T1DM vs. T2DM, non-invasively in vivo. This approach could provide a powerful diagnostic tool for characterizing tissue metabolic defects and responses to treatment in diabetic patients with ambiguous systemic manifestations.

Roles of pyruvate carboxylase in human diseases: from diabetes to cancers and infection

Pyruvate carboxylase (PC), an anaplerotic enzyme, plays an essential role in various cellular metabolic pathways including gluconeogenesis, de novo fatty acid synthesis, amino acid synthesis, and glucose-induced insulin secretion. Deregulation of PC expression or activity has long been known to be associated with metabolic syndrome in several rodent models. Accumulating data in the past decade clearly showed that deregulation of PC expression is associated with type 2 diabetes in humans, while targeted inhibition of PC expression in a mouse model reduced adiposity and improved insulin sensitivity in diet-induced type 2 diabetes. More recent studies also show that PC is strongly involved in tumorigenesis in several cancers, including breast, non-small cell lung cancer, glioblastoma, renal carcinoma, and gall bladder. Systems metabolomics analysis of these cancers identified pyruvate carboxylation as an essential metabolic hub that feeds carbon skeletons of downstream metabolites of oxaloacetate into the biosynthesis of various cellular components including membrane lipids, nucleotides, amino acids, and the redox control. Inhibition or down-regulation of PC expression in several cancers markedly impairs their growth ex vivo and in vivo, drawing attention to PC as an anti-cancer target. PC has also exhibited a moonlight function by interacting with immune surveillance that can either promote or block viral infection. In certain pathogenic bacteria, PC is essential for infection, replication, and maintenance of their virulence phenotype.

Combining hyperpolarized 13 C MRI with a liver-specific gadolinium contrast agent for selective assessment of hepatocyte metabolism

PURPOSE

Hyperpolarized 13 C MRI is a powerful tool for studying metabolism, but can lack tissue specificity. Gadoxetate is a gadolinium-based MRI contrast agent that is selectively taken into hepatocytes. The goal of this project was to investigate whether gadoxetate can be used to selectively suppress the hyperpolarized signal arising from hepatocytes, which could in future studies be applied to generate specificity for signal from abnormal cell types.

METHODS

Baseline gadoxetate uptake kinetics were measured using T1 -weighted contrast enhanced imaging. Relaxivity of gadoxetate was measured for [1-13 C]pyruvate, [1-13 C]lactate, and [1-13 C]alanine. Four healthy rats were imaged with hyperpolarized [1-13 C]pyruvate using a three-dimensional (3D) MRSI sequence prior to and 15 min following administration of gadoxetate. The lactate:pyruvate ratio and alanine:pyruvate ratios were measured in liver and kidney.

RESULTS

Overall, the hyperpolarized signal decreased approximately 60% as a result of pre-injection of gadoxetate. In liver, the lactate:pyruvate and alanine:pyruvate ratios decreased 42% and 78%, respectively (P < 0.05) following gadoxetate administration. In kidneys, these ratios did not change significantly. Relaxivity of gadoxetate for [1-13 C]alanine was 12.6 times higher than relaxivity of gadoxetate for [1-13 C]pyruvate, explaining the greater selective relaxation effect on alanine.

CONCLUSIONS

The liver-specific gadolinium contrast-agent gadoxetate can selectively suppress normal hepatocyte contributions to hyperpolarized 13 C MRI signals. Magn Reson Med 77:2356-2363, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Hyperpolarized Sodium [1-13C]-Glycerate as a Probe for Assessing Glycolysis In Vivo

Hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) provides unprecedented opportunities to obtain clinical diagnostic information through in vivo monitoring of metabolic pathways. The continuing advancement of this field relies on the identification of molecular probes that can effectively interrogate pathways critical to disease. In this report, we describe the synthesis, development, and in vivo application of sodium [1-¹³C]-glycerate ([¹³C]-Glyc) as a novel probe for evaluating glycolysis using hyperpolarized ¹³C MRS. This agent was prepared by a concise synthetic route and formulated for dynamic nuclear polarization. [¹³C]-Glyc displayed a high level of polarization and long spin-lattice relaxation time-both of which are necessary for future clinical investigations. In vivo spectroscopic studies with hyperpolarized [¹³C]-Glyc in rat liver furnished metabolic products, [¹³C]-labeled pyruvate and lactate, originating from glycolysis. The levels of production and relative intensities of these metabolites were directly correlated with the induced glycolytic state (fasted versus fed groups). This work establishes hyperpolarized [¹³C]-Glyc as a novel agent for clinically relevant ¹³C MRS studies of energy metabolism and further provides opportunities for evaluating intracellular redox states in biochemical investigations.

Detection of localized changes in the metabolism of hyperpolarized gluconeogenic precursors 13 C-lactate and 13 C-pyruvate in kidney and liver

PURPOSE

The purpose of this study was to characterize tissue-specific alterations in metabolism of hyperpolarized (HP) gluconeogenic precursors 13 C-lactate and 13 C-pyruvate by rat liver and kidneys under conditions of fasting or insulin-deprived diabetes.

METHODS

Seven normal rats were studied by MR spectroscopic imaging of both HP 13 C-lactate and 13 C-pyruvate in both normal fed and 24 h fasting states, and seven additional rats were scanned after induction of diabetes by streptozotocin (STZ) with insulin withdrawal. Phosphoenolpyruvate carboxykinase (PEPCK) expression levels were also measured in liver and kidney tissues of the STZ-treated rats.

RESULTS

Multiple sets of significant signal modulations were detected, with graded intensity in general between fasting and diabetic states. An approximate two-fold reduction in the ratio of 13 C-bicarbonate to total 13 C signal was observed in both organs in fasting. The ratio of HP lactate-to-alanine was markedly altered, ranging from a liver-specific 54% increase in fasting, to increases of 69% and 92% in liver and kidney, respectively, in diabetes. Diabetes resulted in a 40% increase in renal lactate signal. STZ resulted in 5.86-fold and 2.73-fold increases in PEPCK expression in liver and kidney, respectively.

CONCLUSION

MRI of HP 13 C gluconeogenic precursors may advance diabetes research by clarifying organ-specific roles in abnormal diabetic metabolism. Magn Reson Med 77:1429-1437, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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.

Simultaneous in vivo assessment of cardiac and hepatic metabolism in the diabetic rat using hyperpolarized MRS

Understanding and assessing diabetic metabolism is vital for monitoring disease progression and improving treatment of patients. In vivo assessments, using MRI and MRS, provide non-invasive and accurate measurements, and the development of hyperpolarized ¹³ C spectroscopy in particular has been demonstrated to provide valuable metabolic data in real time. Until now, studies have focussed on individual organs. However, diabetes is a systemic disease affecting multiple tissues in the body. Therefore, we have developed a technique to simultaneously measure metabolism in both the heart and liver during a single acquisition. A hyperpolarized ¹³ C MRS protocol was developed to allow acquisition of metabolic data from the heart and liver during a single scan. This protocol was subsequently used to assess metabolism in the heart and liver of seven control male Wistar rats and seven diabetic rats (diabetes was induced by three weeks of high-fat feeding and a 30 mg/kg injection of streptozotocin). Using our new acquisition, we observed decreased cardiac and hepatic pyruvate dehydrogenase flux in our diabetic rat model. These diabetic rats also had increased blood glucose levels, decreased insulin, and increased hepatic triglycerides. Decreased production of hepatic [1-¹³ C]alanine was observed in the diabetic group, but this change was not present in the hearts of the same diabetic animals. We have demonstrated the ability to measure cardiac and hepatic metabolism simultaneously, with sufficient sensitivity to detect metabolic alterations in both organs. Further, we have non-invasively observed the different reactions of the heart and liver to the metabolic challenge of diabetes.

Inhibition of Pyruvate Dehydrogenase Kinase 2 Protects Against Hepatic Steatosis Through Modulation of Tricarboxylic Acid Cycle Anaplerosis and Ketogenesis

Hepatic steatosis is associated with increased insulin resistance and tricarboxylic acid (TCA) cycle flux, but decreased ketogenesis and pyruvate dehydrogenase complex (PDC) flux. This study examined whether hepatic PDC activation by inhibition of pyruvate dehydrogenase kinase 2 (PDK2) ameliorates these metabolic abnormalities. Wild-type mice fed a high-fat diet exhibited hepatic steatosis, insulin resistance, and increased levels of pyruvate, TCA cycle intermediates, and malonyl-CoA but reduced ketogenesis and PDC activity due to PDK2 induction. Hepatic PDC activation by PDK2 inhibition attenuated hepatic steatosis, improved hepatic insulin sensitivity, reduced hepatic glucose production, increased capacity for β-oxidation and ketogenesis, and decreased the capacity for lipogenesis. These results were attributed to altered enzymatic capacities and a reduction in TCA anaplerosis that limited the availability of oxaloacetate for the TCA cycle, which promoted ketogenesis. The current study reports that increasing hepatic PDC activity by inhibition of PDK2 ameliorates hepatic steatosis and insulin sensitivity by regulating TCA cycle anaplerosis and ketogenesis. The findings suggest PDK2 is a potential therapeutic target for nonalcoholic fatty liver disease.

Detection of inflammatory cell function using (13)C magnetic resonance spectroscopy of hyperpolarized [6-(13)C]-arginine

Myeloid-derived suppressor cells (MDSCs) are highly prevalent inflammatory cells that play a key role in tumor development and are considered therapeutic targets. MDSCs promote tumor growth by blocking T-cell-mediated anti-tumoral immune response through depletion of arginine that is essential for T-cell proliferation. To deplete arginine, MDSCs express high levels of arginase, which catalyzes the breakdown of arginine into urea and ornithine. Here, we developed a new hyperpolarized (13)C probe, [6-(13)C]-arginine, to image arginase activity. We show that [6-(13)C]-arginine can be hyperpolarized, and hyperpolarized [(13)C]-urea production from [6-(13)C]-arginine is linearly correlated with arginase concentration in vitro. Furthermore we show that we can detect a statistically significant increase in hyperpolarized [(13)C]-urea production in MDSCs when compared to control bone marrow cells. This increase was associated with an increase in intracellular arginase concentration detected using a spectrophotometric assay. Hyperpolarized [6-(13)C]-arginine could therefore serve to image tumoral MDSC function and more broadly M2-like macrophages.

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

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

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.