Absolute Quantification of Phosphor-Containing Metabolites in the Liver Using 31 P MRSI and Hepatic Lipid Volume Correction at 7T Suggests No Dependence on Body Mass Index or Age

Reproducibility of absolute quantification of adenosine triphosphate and inorganic phosphate in the liver with localized 31P-magnetic resonance spectroscopy at 3-T using different coils

Concentrations of the key metabolites of hepatic energy metabolism, adenosine triphosphate (ATP) and inorganic phosphate (Pi), can be altered in metabolic disorders such as diabetes mellitus. 31Phosphorus (31P)-magnetic resonance spectroscopy (MRS) is used to noninvasively measure hepatic metabolites, but measuring their absolute molar concentrations remains challenging. This study employed a 31P-MRS method based on the phantom replacement technique for quantifying hepatic 31P-metabolites on a 3-T clinical scanner. Two surface coils with different size and geometry were used to check for consistency in terms of repeatability and reproducibility and absolute concentrations of metabolites. Day-to-day (n = 8) and intra-day (n = 6) reproducibility was tested in healthy volunteers. In the day-to-day study, mean absolute concentrations of γ-ATP and Pi were 2.32 ± 0.24 and 1.73 ± 0.26 mM (coefficient of variation [CV]: 7.3% and 8.8%) for the single loop, and 2.32 ± 0.42 and 1.73 ± 0.27 mM (CVs 6.7% and 10.6%) for the quadrature coil, respectively. The intra-day study reproducibility using the quadrature coil yielded CVs of 4.7% and 6.8% for γ-ATP and Pi without repositioning, and 6.3% and 7.1% with full repositioning of the volunteer. The results of the day-to-day data did not differ between coils and visits. Both coils robustly yielded similar results for absolute concentrations of hepatic 31P-metabolites. The current method, applied with two different surface coils, can be readily utilized in long-term and interventional studies. In comparison with the single loop coil, the quadrature coil also allows measurements at a greater distance between the coil and liver, which is relevant for studying people with obesity.

Increased GH/IGF-I Axis Activity Relates to Lower Hepatic Lipids and Phosphor Metabolism

CONTEXT

Non-alcoholic fatty liver disease (NAFLD) is a leading causes of liver-related morbidity and mortality. While data on acromegaly, a state of chronic growth hormone (GH)/insulin-like growth factor I (IGF-I) excess, suggest an inverse relationship with intrahepatic lipid (IHL) content, less is known about the impact of the GH/IGF-I axis on IHL, lipid composition, and phosphor metabolites in individuals without disorders of GH secretion.

OBJECTIVE

The aim was to investigate the relation between activity of the GH/IGF-I axis and IHL content and phosphor metabolism.

METHODS

We performed a cross-sectional study in 59 otherwise metabolically healthy individuals (30 females), of which 16 met the criteria of NAFLD with IHL of ≥5.6%. The GH/IGF-I axis was evaluated in a fasting state and during an oral glucose tolerance test (OGTT). Insulin sensitivity was estimated by validated indices. IHL, lipid composition (unsaturation index), and phosphate metabolites were analyzed by using 1H/31P magnetic resonance spectroscopy.

RESULTS

In the overall cohort (40.6 ± 15 years; body mass index: 24.5 ± 3 kg/m2; IGF-I: 68.0 ± 17% upper limit of normal), fasting GH (R = -0.31; P = .02), GH during oral glucose tolerance test (R = -0.51; P < .01), and IGF-I (R = -0.28; P = .03) inversely correlated with IHL. GH levels during OGTT were significantly lower in NAFLD than in controls (47.7 [22; 143] ng/mL/min vs 16.8 [7; 32] ng/mL/min; P = .003). GH/IGF-I axis activity correlated with lipid composition and with phosphor metabolites. In multiple regression analysis, the GH/IGF-I axis activity was a strong predictor for IHL and lipid composition independent from insulin sensitivity.

CONCLUSION

GH/IGF-I axis activity impacts hepatic lipid and phosphate metabolism in individuals without disorders in GH secretion. Lower GH axis activity is associated with higher IHL and an unfavorable lipid composition, probably mediated by changes in hepatic energy metabolism.

Leptin increases hepatic triglyceride export via a vagal mechanism in humans

Recombinant human leptin (metreleptin) reduces hepatic lipid content in patients with lipodystrophy and overweight patients with non-alcoholic fatty liver disease and relative hypoleptinemia independent of its anorexic action. In rodents, leptin signaling in the brain increases very-low-density lipoprotein triglyceride (VLDL-TG) secretion and reduces hepatic lipid content via the vagus nerve. In this randomized, placebo-controlled crossover trial (EudraCT Nr. 2017-003014-22), we tested whether a comparable mechanism regulates hepatic lipid metabolism in humans. A single metreleptin injection stimulated hepatic VLDL-TG secretion (primary outcome) and reduced hepatic lipid content in fasted, lean men (n = 13, age range 20-38 years) but failed to do so in metabolically healthy liver transplant recipients (n = 9, age range 26-62 years) who represent a model for hepatic denervation. In an independent cohort of lean men (n = 10, age range 23-31 years), vagal stimulation by modified sham feeding replicated the effects of metreleptin on VLDL-TG secretion. Therefore, we propose that leptin has anti-steatotic properties that are independent of food intake by stimulating hepatic VLDL-TG export via a brain-vagus-liver axis.

In Vitro 31P MR Chemical Shifts of In Vivo-Detectable Metabolites at 3T as a Basis Set for a Pilot Evaluation of Skeletal Muscle and Liver 31P Spectra with LCModel Software

Most in vivo ³¹P MR studies are realized on 3T MR systems that provide sufficient signal intensity for prominent phosphorus metabolites. The identification of these metabolites in the in vivo spectra is performed by comparing their chemical shifts with the chemical shifts measured in vitro on high-field NMR spectrometers. To approach in vivo conditions at 3T, a set of phantoms with defined metabolite solutions were measured in a 3T whole-body MR system at 7.0 and 7.5 pH, at 37 °C. A free induction decay (FID) sequence with and without ¹H decoupling was used. Chemical shifts were obtained of phosphoenolpyruvate (PEP), phosphatidylcholine (PtdC), phosphocholine (PC), phosphoethanolamine (PE), glycerophosphocholine (GPC), glycerophosphoetanolamine (GPE), uridine diphosphoglucose (UDPG), glucose-6-phosphate (G6P), glucose-1-phosphate (G1P), 2,3-diphosphoglycerate (2,3-DPG), nicotinamide adenine dinucleotide (NADH and NAD⁺), phosphocreatine (PCr), adenosine triphosphate (ATP), adenosine diphosphate (ADP), and inorganic phosphate (Pi). The measured chemical shifts were used to construct a basis set of ³¹P MR spectra for the evaluation of ³¹P in vivo spectra of muscle and the liver using LCModel software (linear combination model). Prior knowledge was successfully employed in the analysis of previously acquired in vivo data.

Non-Invasive Analysis of Human Liver Metabolism by Magnetic Resonance Spectroscopy

The liver is a key node of whole-body nutrient and fuel metabolism and is also the principal site for detoxification of xenobiotic compounds. As such, hepatic metabolite concentrations and/or turnover rates inform on the status of both hepatic and systemic metabolic diseases as well as the disposition of medications. As a tool to better understand liver metabolism in these settings, in vivo magnetic resonance spectroscopy (MRS) offers a non-invasive means of monitoring hepatic metabolic activity in real time both by direct observation of concentrations and dynamics of specific metabolites as well as by observation of their enrichment by stable isotope tracers. This review summarizes the applications and advances in human liver metabolic studies by in vivo MRS over the past 35 years and discusses future directions and opportunities that will be opened by the development of ultra-high field MR systems and by hyperpolarized stable isotope tracers.

Concentration of Gallbladder Phosphatidylcholine in Cholangiopathies: A Phosphorus-31 Magnetic Resonance Spectroscopy Pilot Study

BACKGROUND

Biliary phosphatidylcholine (PtdC) concentration plays a role in the pathogenesis of bile duct diseases. In vivo phosphorus-31 magnetic resonance spectroscopy (³¹ P-MRS) at 7 T offers the possibility to assess this concentration noninvasively with high spectral resolution and signal intensity.

PURPOSE

Comparison of PtdC levels of cholangiopathic patient groups to a control group using a measured T₁ relaxation time of PtdC in healthy subjects.

STUDY TYPE

Case control.

SUBJECTS

Two patient groups with primary sclerosing cholangitis (PSC, 2f/3 m; age: 43 ± 7 years) and primary biliary cholangitis (PBC, 4f/2 m; age: 57 ± 6 years), and a healthy control group (CON, 2f/3 m; age: 38 ± 7 years). Ten healthy subjects for the assessment of the T₁ relaxation time of PtdC.

FIELD STRENGTH/SEQUENCE

A 3D phase-encoded pulse-acquire ³¹ P-MRSI sequence for PtdC quantification and a 1D image-selected in vivo ³¹ P spectroscopy for T₁ estimation at 7 T, and a T2-weighted half-Fourier single-shot turbo spin echo MRI sequence for volumetry at 3 T.

ASSESSMENT

Calculation of gallbladder volumes and PtdC concentration in groups using hepatic gamma-adenosine triphosphate signal as an internal reference and correction for insufficient relaxation of PtdC with a T₁ value assessed in healthy subjects.

STATISTICAL TESTS

Group comparison of PtdC content and gallbladder volumes of the PSC/PBC and CON group using Student's t-tests with a significance level of 5%.

RESULTS

PtdC T₁ value of 357 ± 85 msec in the gallbladder. Significant lower PtdC content for the PSC group, and for the female subgroup of the PBC group compared to the CON group (PSC/CON: 5.74 ± 0.73 mM vs. 9.64 ± 0.97 mM, PBC(f)/CON: 5.77 ± 1.44 mM vs. 9.64 ± 0.97 mM). Significant higher gallbladder volumes of the patient groups compared to the CON group (PSC/CON: 66.3 ± 15.8 mL vs. 20.9 ± 2.2 mL, PBC/CON: 49.8 ± 18.2 mL vs. 20.9 ± 2.2 mL).

DATA CONCLUSION

This study demonstrated the application of a ³¹ P-MRSI protocol for the quantification of PtdC in the human gallbladder at 7 T. Observed differences in PtdC concentration suggest that this metabolite could serve as a biomarker for specific hepatobiliary disorders.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 3.

Feasibility of Hepatic Fat Quantification Using Proton Density Fat Fraction by Multi-Echo Chemical-Shift-Encoded MRI at 7T

Fat fraction quantification and assessment of its distribution in the hepatic tissue become more important with the growing epidemic of obesity, and the increasing prevalence of diabetes mellitus type 2 and non-alcoholic fatty liver disease. At 3Tesla, the multi-echo, chemical-shift-encoded magnetic resonance imaging (CSE-MRI)-based acquisition allows the measurement of proton density fat-fraction (PDFF) even in clinical protocols. Further improvements in SNR can be achieved by the use of phased array coils and increased static magnetic field. The purpose of the study is to evaluate the feasibility of PDFF imaging using a multi-echo CSE-MRI technique at ultra-high magnetic field (7Tesla). Thirteen volunteers (M/F) with a broad range of age, body mass index, and hepatic PDFF were measured at 3 and 7T by multi-gradient-echo MRI and single-voxel spectroscopy MRS. All measurements were performed in breath-hold (exhalation); the MRI protocols were optimized for a short measurement time, thus minimizing motion-related problems. 7T data were processed off-line using Matlab® (MRI:multi-gradient-echo) and jMRUI (MRS), respectively. For quantitative validation of the PDFF results, a similar protocol was performed at 3T, including on-line data processing provided by the system manufacturer, and correlation analyses between 7 and 3T data were performed off-line. The multi-echo CSE-MRI measurements at 7T with a phased-array coil configuration and an optimal post-processing yielded liver volume coverage ranging from 30 to 90% for high- and low-BMI subjects, respectively. PDFFs ranged between 1 and 20%. We found significant correlations between 7T MRI and -MRS measurements (R² ≅ 0.97; p < 0.005), and between MRI-PDFF at 7T and 3T fields (R² ≅ 0.94; p < 0.005) in the evaluated volumes. Based on the measurements and analyses performed, the multi-echo CSE-MRI method using a 32-channel coil at 7T showed its aptitude for MRI-based quantitation of PDFF in the investigated volumes. The results are the first step toward qMRI of the whole liver at 7T with further improvements in hardware.

In vivo absolute quantification of hepatic γ-ATP concentration in mice using 31 P MRS at 11.7 T

Measurement of ATP concentrations and synthesis in humans indicated abnormal hepatic energy metabolism in obesity, non-alcoholic fatty liver disease (NAFLD) and Type 2 diabetes. Further mechanistic studies on energy metabolism require the detailed phenotyping of specific mouse models. Thus, this study aimed to establish and evaluate a robust and fast single voxel ³¹ P MRS method to quantify hepatic γ-ATP concentrations at 11.7 T in three mouse models with different insulin sensitivities and liver fat contents (72-week-old C57BL/6 control mice, 72-week-old insulin resistant sterol regulatory-element binding protein-1c overexpressing (SREBP-1c⁺ ) mice and 10-12-week-old prediabetic non-obese diabetic (NOD) mice). Absolute quantification was performed by employing an external reference and a matching replacement ATP phantom with 3D image selected in vivo spectroscopy ³¹ P MRS. This single voxel ³¹ P MRS method non-invasively quantified hepatic γ-ATP within 17 min and the repeatability tests provided a coefficient of variation of 7.8 ± 1.1%. The mean hepatic γ-ATP concentrations were markedly lower in SREBP-1c⁺ mice (1.14 ± 0.10 mM) than in C57BL/6 mice (2.15 ± 0.13 mM; p < 0.0002) and NOD mice (1.78 ± 0.13 mM; p < 0.006, one-way ANOVA test). In conclusion, this method allows us to rapidly and precisely measure hepatic γ-ATP concentrations, and thereby to non-invasively detect abnormal hepatic energy metabolism in mice with different degrees of insulin resistance and NAFLD. Thus, this ³¹ P MRS will also be useful for future mechanistic as well as therapeutic translational studies in other murine models.

Gluconeogenesis, But Not Glycogenolysis, Contributes to the Increase in Endogenous Glucose Production by SGLT-2 Inhibition

OBJECTIVE

Recent studies indicate that sodium-glucose cotransporter 2 (SGLT-2) inhibition increases endogenous glucose production (EGP), potentially counteracting the glucose-lowering potency, and stimulates lipid oxidation and lipolysis. However, the acute effects of SGLT-2 inhibition on hepatic glycogen, lipid, and energy metabolism have not yet been analyzed. We therefore investigated the impact of a single dose of dapagliflozin (D) or placebo (P) on hepatic glycogenolysis, hepatocellular lipid (HCL) content and mitochondrial activity (kATP).

RESEARCH DESIGN AND METHODS

Ten healthy volunteers (control [CON]: age 30 ± 3 years, BMI 24 ± 1 kg/m², HbA_1c 5.2 ± 0.1%) and six patients with type 2 diabetes mellitus (T2DM: age 63 ± 4 years, BMI 28 ± 1.5 kg/m², HbA_1c 6.1 ± 0.5%) were investigated on two study days (CON-P vs. CON-D and T2DM-P vs. T2DM-D). ¹H/¹³C/³¹P MRS was performed before, 90-180 min (MR1), and 300-390 min (MR2) after administration of 10 mg dapagliflozin or placebo. EGP was assessed by tracer dilution techniques.

RESULTS

Compared with CON-P, EGP was higher in CON-D (10.0 ± 0.3 vs. 12.4 ± 0.5 μmol kg^-1 min^-1; P < 0.05) and comparable in T2DM-D and T2DM-P (10.1 ± 0.7 vs. 10.4 ± 0.5 μmol kg^-1 min^-1; P = not significant [n.s.]). A strong correlation of EGP with glucosuria was observed (r = 0.732; P < 0.01). The insulin-to-glucagon ratio was lower after dapagliflozin in CON-D and T2DM-D compared with baseline (P < 0.05). Glycogenolysis did not differ between CON-P and CON-D (-3.28 ± 0.49 vs. -2.53 ± 0.56 μmol kg^-1 min^-1; P = n.s.) or T2DM-P and T2DM-D (-0.74 ± 0.23 vs. -1.21 ± 0.33 μmol kg^-1 min^-1; P = n.s.), whereas gluconeogenesis was higher after dapagliflozin in CON-P compared with CON-D (6.7 ± 0.6 vs. 9.9 ± 0.6 μmol kg^-1 min^-1; P < 0.01) but not in T2DM. No significant changes in HCL and kATP were observed.

CONCLUSIONS

The rise in EGP after SGLT-2 inhibition is due to increased gluconeogenesis, but not glycogenolysis. Changes in glucagon and the insulin-to-glucagon ratio are not associated with an increased hepatic glycogen breakdown. HCL and kATP are not significantly affected by a single dose of dapagliflozin.

INSPECTOR: free software for magnetic resonance spectroscopy data inspection, processing, simulation and analysis

In vivo magnetic resonance spectroscopy (MRS) is a powerful tool for biomedical research and clinical diagnostics, allowing for non-invasive measurement and analysis of small molecules from living tissues. However, currently available MRS processing and analytical software tools are limited in their potential for in-depth quality management, access to details of the processing stream, and user friendliness. Moreover, available MRS software focuses on selected aspects of MRS such as simulation, signal processing or analysis, necessitating the use of multiple packages and interfacing among them for biomedical applications. The freeware INSPECTOR comprises enhanced MRS data processing, simulation and analytical capabilities in a one-stop-shop solution for a wide range of biomedical research and diagnostic applications. Extensive data handling, quality management and visualization options are built in, enabling the assessment of every step of the processing chain with maximum transparency. The parameters of the processing can be flexibly chosen and tailored for the specific research problem, and extended confidence information is provided with the analysis. The INSPECTOR software stands out in its user-friendly workflow and potential for automation. In addition to convenience, the functionalities of INSPECTOR ensure rigorous and consistent data processing throughout multi-experiment and multi-center studies.

Effects of Thyroid Function on Phosphodiester Concentrations in Skeletal Muscle and Liver: An In Vivo NMRS Study

CONTEXT

Thyroid function is clinically evaluated by determination of circulating concentrations of thyrotropin (thyroid-stimulating hormone; TSH) and free thyroxine (fT4). However, a tissue-specific effector substrate of thyroid function is lacking. Energy-rich phosphorus-containing metabolites (PM) and phospholipids (PL) might be affected by thyroid hormone action and can be noninvasively measured by 31P nuclear magnetic resonance spectroscopy (NMRS).

OBJECTIVES

To measure the actions of peripheral thyroid hormones on PM and PL tissue concentrations.

DESIGN AND SETTING

A longitudinal, prospective pilot study was performed.

PARTICIPANTS

Nine patients with hyperthyroidism (HYPER) and 4 patients with hypothyroidism (HYPO) were studied at baseline and 3 months after treatment.

MAIN OUTCOME MEASURES

High-field 1H/31P NMRS was used to assess profiles of PM, PL, and flux through oxidative phosphorylase in liver and skeletal muscle, as well as ectopic tissue lipid content.

RESULTS

The concentrations of total skeletal muscle (m-) and hepatic (h-) phosphodiesters (PDE) and one of the PDE constituents, glycerophosphocholine (GPC), were lower in HYPER compared with HYPO (m-PDE: 1.4 ± 0.4 mM vs 7.4 ± 3.5 mM, P = 0.003; m-GPC: 0.9 ± 0.3 mM vs 6.7 ± 3.5 mM, P = 0.003; h-PDE: 4.4 ± 1.4 mM vs 9.9 ± 3.9 mM, P = 0.012; h-GPC: 2.2 ± 1.0 mM vs 5.1 ± 2.4 mM, P = 0.024). Both h-GPC (rho = -0.692, P = 0.018) and h-GPE (rho = -0.633, P = 0.036) correlated negatively with fT4. In muscle tissue, a strong negative association between m-GPC and fT4 (rho = -0.754, P = 0.003) was observed.

CONCLUSIONS

Thyroxine is closely negatively associated with the PDE concentrations in liver and skeletal muscle. Normalization of thyroid dysfunction resulted in a decline of PDE in hypothyroidism and an increase in hyperthyroidism. Thus, PDE might be a sensitive tool to estimate tissue-specific peripheral thyroid hormone action.

Increased ATP synthesis might counteract hepatic lipid accumulation in acromegaly

Patients with active acromegaly (ACRO) exhibit low hepatocellular lipids (HCL), despite pronounced insulin resistance (IR). This contrasts the strong association of IR with nonalcoholic fatty liver disease in the general population. Since low HCL levels in ACRO might be caused by changes in oxidative substrate metabolism, we investigated mitochondrial activity and plasma metabolomics/lipidomics in active ACRO. Fifteen subjects with ACRO and seventeen healthy controls, matched for age, BMI, sex, and body composition, underwent 31P/1H-7-T MR spectroscopy of the liver and skeletal muscle as well as plasma metabolomic profiling and an oral glucose tolerance test. Subjects with ACRO showed significantly lower HCL levels, but the ATP synthesis rate was significantly increased compared with that in controls. Furthermore, a decreased ratio of unsaturated-to-saturated intrahepatocellular fatty acids was found in subjects with ACRO. Within assessed plasma lipids, lipidomics, and metabolomics, decreased carnitine species also indicated increased mitochondrial activity. We therefore concluded that excess of growth hormone (GH) in humans counteracts HCL accumulation by increased hepatic ATP synthesis. This was accompanied by a decreased ratio of unsaturated-to-saturated lipids in hepatocytes and by a metabolomic profile, reflecting the increase in mitochondrial activity. Thus, these findings help to better understanding of GH-regulated antisteatotic pathways and provide a better insight into potentially novel therapeutic targets for treating NAFLD.