1H NMR spectroscopic studies of lipid extracts from human fatty liver

Expanding the Molecular Disturbances of Lipoproteins in Cardiometabolic Diseases: Lessons from Lipidomics

The increasing global burden of cardiometabolic diseases highlights the urgent clinical need for better personalized prediction and intervention strategies. Early diagnosis and prevention could greatly reduce the enormous socio-economic burden posed by these states. Plasma lipids including total cholesterol, triglycerides, HDL-C, and LDL-C have been at the center stage of the prediction and prevention strategies for cardiovascular disease; however, the bulk of cardiovascular disease events cannot be explained sufficiently by these lipid parameters. The shift from traditional serum lipid measurements that are poorly descriptive of the total serum lipidomic profile to comprehensive lipid profiling is an urgent need, since a wealth of metabolic information is currently underutilized in the clinical setting. The tremendous advances in the field of lipidomics in the last two decades has facilitated the research efforts to unravel the lipid dysregulation in cardiometabolic diseases, enabling the understanding of the underlying pathophysiological mechanisms and identification of predictive biomarkers beyond traditional lipids. This review presents an overview of the application of lipidomics in the study of serum lipoproteins in cardiometabolic diseases. Integrating the emerging multiomics with lipidomics holds great potential in moving toward this goal.

The role of hepatic lipid composition in obesity-related metabolic disease

Obesity is a primary antecedent to non-alcoholic fatty liver disease whose cardinal feature is excessive hepatic lipid accumulation. Although total hepatic lipid content closely associates with hepatic and systemic metabolic dysfunction, accumulating evidence suggests that the composition of hepatic lipids may be more discriminatory. This review summarises cross-sectional human studies using liver biopsy/lipidomics and proton magnetic resonance spectroscopy to characterise hepatic lipid composition in people with obesity and related metabolic disease. A comprehensive literature search identified 26 relevant studies published up to 31st March 2021 which were included in the review. The available evidence provides a consistent picture showing that people with hepatic steatosis possess elevated saturated and/or monounsaturated hepatic lipids and a reduced proportion of polyunsaturated hepatic lipids. This altered hepatic lipid profile associates more directly with metabolic derangements, such as insulin resistance, and may be exacerbated in non-alcoholic steatohepatitis. Further evidence from lipidomic studies suggests that these deleterious changes may be related to defects in lipid desaturation and elongation, and an augmentation of the de novo lipogenic pathway. These observations are consistent with mechanistic studies implicating saturated fatty acids and associated bioactive lipid intermediates (ceramides, lysophosphatidylcholines and diacylglycerol) in the development of hepatic lipotoxicity and wider metabolic dysfunction, whilst monounsaturated fatty acids and polyunsaturated fatty acids may exhibit a protective role. Future studies are needed to prospectively determine the relevance of hepatic lipid composition for hepatic and non-hepatic morbidity and mortality; and to further evaluate the impact of therapeutic interventions such as pharmacotherapy and lifestyle interventions.

Alcohol-Induced Hepatic Steatosis: A Comparative Study to Identify Possible Indicator(s) of Alcoholic Fatty Liver Disease

Background

Fatty liver is an early sign of both nonalcoholic and alcoholic fatty liver diseases. Ethanol feeding using a Lieber-DeCarli liquid diet (LD) model which contains 35% fat to rats or mice is a well-established model for alcoholic fatty liver. However, LD diet alone can also induce fatty liver and its differential metabolic profile may be able to differentiate steatosis induced by LD versus LD plus ethanol.

Purpose

We investigated the lipidomic differences in the livers of Sprague-Dawley (SD) rats fed a pellet diet (PD), LD and liquid ethanol diet (LED) for six weeks.

Study Design

Male Sprague Dawley rats were fed with nonalcoholic diets PD, LD or LED (ethanol in LD) for six weeks. Lipids were extracted and analyzed by nuclear magnetic resonance (NMR)- based metabolomics. The NMR data obtained was analyzed by multivariate Principal Component Analysis (PCA) and Spotfire DecisionSite 9.0 software to compare PD versus LD and LD versus LED groups.

Results

PCA of the NMR spectral data of livers of both comparisons showed a clear separation of PD from LD group and LD from LED group indicating differences in lipid profiles which corresponded with changes in total lipid weights. LD showed increases for cholesterol, esterified cholesterol, cholesterol acetate and triglycerides with decreases for fatty acyl chain, diallylic and allylic protons, while the LED showed increases in esterified cholesterol, cholesterol acetate, fatty acid methyl esters, allylic protons and some triglyceride protons with decreases in free cholesterol and phosphatidylcholine (PC).

Conclusion

Our data suggest that altered lipid signature or PC levels could be an indicator to differentiate between nonalcoholic versus alcoholic fatty liver.

The neuropeptide TLQP-21 opposes obesity via C3aR1-mediated enhancement of adrenergic-induced lipolysis

OBJECTIVES

Obesity is characterized by excessive fat mass and is associated with serious diseases such as type 2 diabetes. Targeting excess fat mass by sustained lipolysis has been a major challenge for anti-obesity therapies due to unwanted side effects. TLQP-21, a neuropeptide encoded by the pro-peptide VGF (non-acronymic), that binds the complement 3a receptor 1 (C3aR1) on the adipocyte membrane, is emerging as a novel modulator of adipocyte functions and a potential target for obesity-associated diseases. The molecular mechanism is still largely uncharacterized.

METHODS

We used a combination of pharmacological and genetic gain and loss of function approaches. 3T3-L1 and mature murine adipocytes were used for in vitro experiments. Chronic in vivo experiments were conducted on diet-induced obese wild type, β1, β2, β3-adrenergic receptor (AR) deficient and C3aR1 knockout mice. Acute in vivo lipolysis experiments were conducted on Sprague Dawley rats.

RESULTS

We demonstrated that TLQP-21 does not possess lipolytic properties per se. Rather, it enhances β-AR activation-induced lipolysis by a mechanism requiring Ca2+ mobilization and ERK activation of Hormone Sensitive Lipase (HSL). TLQP-21 acutely potentiated isoproterenol-induced lipolysis in vivo. Finally, chronic peripheral TLQP-21 treatment decreases body weight and fat mass in diet induced obese mice by a mechanism involving β-adrenergic and C3a receptor activation without associated adverse metabolic effects.

CONCLUSIONS

In conclusion, our data identify an alternative pathway modulating lipolysis that could be targeted to diminish fat mass in obesity without the side effects typically observed when using potent pro-lipolytic molecules.

¹H and ³¹P NMR lipidome of ethanol-induced fatty liver

BACKGROUND

 Hepatic steatosis (fatty liver), an early and reversible stage of alcoholic liver disease, is characterized by triglyceride deposition in hepatocytes, which can advance to steatohepatitis, fibrosis, cirrhosis, and ultimately to hepatocellular carcinoma. In the present work, we studied altered plasma and hepatic lipid metabolome (lipidome) to understand the mechanisms and lipid pattern of early-stage alcohol-induced-fatty liver.

METHODS

 Male Fischer 344 rats were fed 5% alcohol in a Lieber-DeCarli diet. Control rats were pair-fed an equivalent amount of maltose-dextrin. After 1 month, animals were killed and plasma collected. Livers were excised for morphological, immunohistochemical, and biochemical studies. The lipids from plasma and livers were extracted with methyl-tert-butyl ether and analyzed by 750/800 MHz proton nuclear magnetic resonance (¹H NMR) and phosphorus (³¹P) NMR spectroscopy on a 600 MHz spectrometer. The NMR data were then subjected to multivariate statistical analysis.

RESULTS

 Hematoxylin and Eosin and Oil Red O stained liver sections showed significant fatty infiltration. Immunohistochemical analysis of liver sections from ethanol-fed rats showed no inflammation (absence of CD3 positive cells) or oxidative stress (absence of malondialdehyde reactivity or 4-hydroxynonenal positive staining). Cluster analysis and principal component analysis of ¹H NMR data of lipid extracts of both plasma and livers showed a significant difference in the lipid metabolome of ethanol-fed versus control rats. ³¹P NMR data of liver lipid extracts showed significant changes in phospholipids similar to ¹H NMR data. ¹H NMR data of plasma and liver reflected several changes, while comparison of ¹H NMR and ³¹P NMR data offered a correlation among the phospholipids.

CONCLUSIONS

 Our results show that alcohol consumption alters metabolism of cholesterol, triglycerides, and phospholipids that could contribute to the development of fatty liver. These studies also indicate that fatty liver precedes oxidative stress and inflammation. The similarities observed in plasma and liver lipid profiles offer a potential methodology for detecting early-stage alcohol-induced fatty liver disease by analyzing the plasma lipid profile.

Review of magnetic resonance spectroscopy in the liver and the pancreas

Proton magnetic resonance spectroscopy is a powerful tool for in vivo biochemical characterization of normal and abnormal tissues. The initial application in the abdomen was the measurement of fat concentration in the liver using chemical shift imaging. The success of chemical shift imaging in providing a semiquantitative measure of liver fat concentration led to the application of the more quantitative single-voxel volume-selective spectroscopy of the liver. This single-voxel volume-selective spectroscopic technique is able to characterize the different lipids and metabolites present in the liver and the pancreas, providing information about the ratio of unsaturated and saturated lipids. The purposes of this article were to review the spectroscopic techniques and to discuss some of the clinical applications of these techniques in the abdomen.

Changes in hepatic lipids of mice infected with cysticerci ofTaenia crassiceps

Nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the effect of infection withTaenia crassicepscysticerci on the lipid profile of mouse liver. Chloroform/methanol extracts of livers from infected mice showed lower concentrations of phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, total glycerophospholipid, triacylglycerol, total fatty acid (FA) and all measured FA components than those from controls. Furthermore, the ratios obtained on dividing concentrations of the FA components by that of total FA demonstrate that the concentration decreases caused by infection are less for polyunsaturated fatty acids (FAs) than for other FAs. Extracts ofT. crassicepsdisplayed a similar lipid profile to that of host liver but contained a lower lipid content and a shorter average FA chain length.

Proton nuclear magnetic resonance analysis of hepatic bile from donors and recipients in human liver transplantation

The current shortage of donor organs in liver transplantation has led experienced transplant centers to use more "marginal" grafts. The development of a reliable technique of bile collection gives access to hepatic bile from donors and recipients for bile analysis to characterize the grafts. Proton nuclear magnetic resonance analysis has been applied to the study of bile for more than 30 years, showing encouraging results. This is the first study where proton nuclear magnetic resonance analysis has been applied to hepatic bile from selected liver grafts to evaluate its potential role in graft assessment. Hepatic bile was collected from eight liver donors (four with normal and four with steatotic grafts) during organ retrieval and four transplant recipients (two with good early graft function and two with primary dysfunction) immediately after graft reperfusion. A Varian Unity+ NMR spectrometer, operating at 11.7 Tesla (500 MHz for 1H), was used to obtain the proton nuclear magnetic resonance spectra. The results showed that the hepatic bile from steatotic grafts collected before transplantation had more intense phosphatidylcholine head group resonance than bile from normal grafts. It also showed slower clearance of University of Wisconsin solution in grafts with subsequent primary graft dysfunction, suggesting a slower recovery of bile secretion. These preliminary findings suggest that proton nuclear magnetic resonance analysis might help to differentiate the characteristics of bile acids and biliary lipids from normal and steatotic grafts. The monitoring of the resonance signal of University of Wisconsin solution washout, bile acid, and biliary lipid secretion may help to predict the development of primary graft dysfunction and avoid the need for retransplantation.

Influence of preculture on the prefreeze and postthaw characteristics of hepatocytes

Recent studies performed in our laboratory have shown that a brief period of preculture prior to cryopreservation improves the postthaw viability of hepatocytes. The purpose of this investigation is to characterize specific metabolic and biochemical characteristics of the hepatocytes (both frozen and nonfrozen) to help elucidate the role of preculture on the postthaw viability. Fresh and thawed hepatocytes were cultured in a bioartificial liver (BAL) to determine albumin secretion as a function of time in culture. In addition, cell extracts were analyzed using nuclear magnetic resonance (NMR) spectroscopy to quantify changes in cell membrane composition and energetics as a function of time in culture prefreeze and postthaw. The results of these studies showed an increase in albumin concentration in the culture medium with time in culture for the period tested for both fresh and frozen and thawed hepatocytes. NMR spectroscopy of lipid extracts indicates that in vitro culture of hepatocytes results in an increase in cholesterol relative to membrane phospholipid. Moreover, the NMR results also indicate phospholipid interconversion, via specific lipases in cultured hepatocytes, and these changes are consistent with water permeability measurements performed previously. Significant changes in phosphoenergetics were also observed, with the net energy charge for the cells increasing significantly with time in culture. In addition, NMR spectra show increased levels of 6-phosphogluconate, another indicator of the cellular response to the stresses of isolation and ex vivo culture. These results suggest that energetic considerations may be a significant factor in the ability of hepatocytes to survive the stresses of freezing and thawing. Significant shifts in membrane phospholipids may also influence membrane permeability and postthaw survival.

Increase in the concentration of carbon 18 monounsaturated fatty acids in the liver with hepatitis C: analysis in transgenic mice and humans

Steatosis is one of the histologic characteristics of chronic hepatitis C and is well reproduced in a transgenic mouse model for hepatocellular carcinoma (HCC) in which the core protein of hepatitis C virus (HCV) plays a pivotal role in inducing steatosis and HCC. In the present study, the lipid composition in the liver of the HCV core gene transgenic mice as well as in those of chronic hepatitis C patients was determined. The concentration of carbon 18 monounsaturated (C18:1) fatty acids, such as oleic and vaccenic acids, which are known to increase membrane fluidity leading to higher cell division rates, significantly increased in the livers of transgenic mice compared to nontransgenic control mice. The concentration of C18:1 fatty acids also significantly increased in the livers of chronic hepatitis C patients compared to subjects without HCV infection. These results suggest that HCV may affect a specific pathway in the lipid metabolism and cause steatosis in the liver.

Nonalcoholic steatohepatitis

NASH is an important form of chronic liver disease that is increasingly recognized. The diagnosis is secured by biopsy findings with similarities to alcoholic hepatitis in a patient with a confirmed history of abstinence. Obesity is a major risk factor, but the disease also occurs in the nonobese. In 20% to 40% of patients the disease can progress to various stages of fibrosis and ultimately cause cirrhosis and death from end-stage liver disease. For this reason, recognition of NASH is important, and establishing the diagnosis provides a further impetus for performing a liver biopsy as part of the evaluation of unexplained liver abnormalities. The mainstay of treatment is weight reduction in the obese. For those individuals who are not obese, continued observation is the only option currently available. Patients who develop decompensated cirrhosis should be considered for liver transplantation unless advanced age or other underlying medical illnesses are a problem. With the increasing knowledge about the pathophysiology of hepatic steatosis, it is hoped that better diagnostic tests for specific causes of NASH will be available and lead to efficacious therapy.

Nonalcoholic steatohepatitis

NASH is a form of chronic liver disease that is defined by biopsy findings and has the appearance of alcoholic hepatitis. Although this disease was once thought to be a problem of women, diabetics, and the obese, more recent studies have identified a significant proportion of patients who do not fit these risk factors. In a fraction of patients, the disease can progress to various stages of fibrosis leading ultimately to cirrhosis and death from end-stage liver disease. For this reason, recognition of NASH is important and provides a further impetus for performing a liver biopsy as part of the evaluation of unexplained liver biochemical abnormalities. The mainstay of treatment is weight reduction in the obese. For those individuals who are not obese, continued observation is the only available option at this point. With increasing knowledge about the pathophysiology of hepatic steatosis, perhaps more specific diagnostic tests for the cause of the disease in specific patients will be available and will guide appropriate therapy.

Proton MR spectroscopy in quantitative in vivo determination of fat content in human liver steatosis

To demonstrate that the lipid volume fraction in liver steatosis can be accurately estimated with in vivo hydrogen-1 magnetic resonance (MR) spectroscopy, the authors developed a calibration procedure based on in vitro MR spectroscopy of lipid extracts from steatotic liver specimens. The lipid volume fractions determined with the calibration procedure were compared with the results of histomorphometry and with calibrated computed tomographic (CT) data. The volume fraction of fat determined with MR spectroscopy was in good agreement with the CT results, whereas histomorphometry underestimated the amount of hepatic fat. The results indicate that determination of the fat volume fraction in steatotic liver can be achieved noninvasively with MR spectroscopy.

Structural studies of NMR detected lipids in myocardial ischemia

Lipid-induced abnormalities in myocardial function have been implicated in a number of ischemic events including the accumulation of lipids in human myocardium following myocardial infarction. Although animal models have shown the source of these lipids to be triglycerides, the specific species involved has not been identified. In order to better understand the mechanism(s) defining this lipid accumulation, it follows that the identification of the lipids involved may be important in achieving this aim. Therefore, this study examined the use of NMR probes for delineating the biochemical makeup of the increased 1H NMR observed lipid signal following myocardial infarction. Specifically, the present study demonstrated the utility of the spin-echo pulse sequence for the study of alterations in myocardial lipids following ischemic injury. Spin-echo spectra allowed the analysis of subsets of lipids within the large lipid pool inherent in most myocardium. The analyses of the chemical shifts of the lipid resonances provided a simple yet powerful means for deducing lipid class associated with the ischemic injury and suggested the species arises predominantly from saturated lipids. The examination of the CH2/CH3 NMR ratio provided additional information regarding the species involved, however, because the spin-echo technique was utilized, which may distort certain signal intensities, caution must be exercised in interpreting the specific species involved. With this in mind, a tentative assignment has been given to octanoic acid. Finally, a temperature dependence of the lipid signals was noted and determined to be unique for spin-echo lipid.(ABSTRACT TRUNCATED AT 250 WORDS)