Proteomic characterization of high-density lipoprotein particles in patients with non-alcoholic fatty liver disease

High-density lipoproteins and non-alcoholic fatty liver disease

Background and aims

Non-alcoholic fatty liver disease (NAFLD), a high incidence liver pathology, is associated with a ∼1.5-fold higher cardiovascular disease risk. This phenomenon is generally attributed to the NAFLD-associated increase in circulating levels of pro-atherogenic apolipoprotein B100-containing small dense low-density lipoprotein and plasma hypertriglyceridemia. However, also a significant reduction in cholesterol transported by anti-atherogenic high-density lipoproteins (HDL) is frequently observed in subjects suffering from NAFLD as compared to unaffected people. In this review, we summarize data regarding the relationship between NAFLD and plasma HDL-cholesterol levels, with a special focus on highlighting potential causality between the NAFLD pathology and changes in HDL metabolism.

Methods and results

Publications in PUBMED describing the relationship between HDL levels and NAFLD susceptibility and/or disease severity, either in human clinical settings or genetically-modified mouse models, were critically reviewed for subsequent inclusion in this manuscript. Furthermore, relevant literature describing effects on lipid loading in cultured hepatocytes of models with genetic alterations related to HDL metabolism have been summarized.

Conclusions

Although in vitro observations suggest causality between HDL formation by hepatocytes and protection against NAFLD-like lipid accumulation, current literature remains inconclusive on whether relative HDL deficiency is actually driving the development of fatty liver disease in humans. In light of the current obesity pandemic and the associated marked rise in NAFLD incidence, it is of clear scientific and societal interest to gain further insight into the relationship between HDL-cholesterol levels and fatty liver development to potentially uncover the therapeutic potential of pharmacological HDL level and/or function modulation.

Proteomic and functional analysis of HDL subclasses in humans and rats: a proof-of-concept study

BACKGROUND

The previous study investigated whether the functions of small, medium, and large high density lipoprotein (S/M/L-HDL) are correlated with protein changes in mice. Herein, the proteomic and functional analyses of high density lipoprotein (HDL) subclasses were performed in humans and rats.

METHODS

After purifying S/M/L-HDL subclasses from healthy humans (n = 6) and rats (n = 3) using fast protein liquid chromatography (FPLC) with calcium silica hydrate (CSH) resin, the proteomic analysis by mass spectrometry was conducted, as well as the capacities of cholesterol efflux and antioxidation was measured.

RESULTS

Of the 120 and 106 HDL proteins identified, 85 and 68 proteins were significantly changed in concentration among the S/M/L-HDL subclasses in humans and rats, respectively. Interestingly, it was found that the relatively abundant proteins in the small HDL (S-HDL) and large HDL (L-HDL) subclasses did not overlap, both in humans and in rats. Next, by searching for the biological functions of the relatively abundant proteins in the HDL subclasses via Gene Ontology, it was displayed that the relatively abundant proteins involved in lipid metabolism and antioxidation were enriched more in the medium HDL (M-HDL) subclass than in the S/L-HDL subclasses in humans, whereas in rats, the relatively abundant proteins associated with lipid metabolism and anti-oxidation were enriched in M/L-HDL and S/M-HDL, respectively. Finally, it was confirmed that M-HDL and L-HDL had the highest cholesterol efflux capacity among the three HDL subclasses in humans and rats, respectively; moreover, M-HDL exhibited higher antioxidative capacity than S-HDL in both humans and rats.

CONCLUSIONS

The S-HDL and L-HDL subclasses are likely to have different proteomic components during HDL maturation, and results from the proteomics-based comparison of the HDL subclasses may explain the associated differences in function.

HDL Composition, Heart Failure, and Its Comorbidities

Although research on high-density lipoprotein (HDL) has historically focused on atherosclerotic coronary disease, there exists untapped potential of HDL biology for the treatment of heart failure. Anti-oxidant, anti-inflammatory, and endothelial protective properties of HDL could impact heart failure pathogenesis. HDL-associated proteins such as apolipoprotein A-I and M may have significant therapeutic effects on the myocardium, in part by modulating signal transduction pathways and sphingosine-1-phosphate biology. Furthermore, because heart failure is a complex syndrome characterized by multiple comorbidities, there are complex interactions between heart failure, its comorbidities, and lipoprotein homeostatic mechanisms. In this review, we will discuss the effects of heart failure and associated comorbidities on HDL, explore potential cardioprotective properties of HDL, and review novel HDL therapeutic targets in heart failure.

Reproducible Determination of High-Density Lipoprotein Proteotypes

High-density lipoprotein (HDL) is a heterogeneous mixture of blood-circulating multimolecular particles containing many different proteins, lipids, and RNAs. Recent advancements in mass spectrometry-based proteotype analysis show promise for the analysis of proteoforms across large patient cohorts. In order to create the required spectral libraries enabling these data-independent acquisition (DIA) strategies, HDL was isolated from the plasma of more than 300 patients with a multiplicity of physiological HDL states. HDL proteome spectral libraries consisting of 296 protein groups and more than 786 peptidoforms were established, and the performance of the DIA strategy was benchmarked for the detection of HDL proteotype differences between healthy individuals and a cohort of patients suffering from diabetes mellitus type 2 and/or coronary heart disease. Bioinformatic interrogation of the data using the generated spectral libraries showed that the DIA approach enabled robust HDL proteotype determination. HDL peptidoform analysis enabled by using spectral libraries allowed for the identification of post-translational modifications, such as in APOA1, which could affect HDL functionality. From a technical point of view, data analysis further shows that protein and peptide quantities are currently more discriminative between different HDL proteotypes than peptidoforms without further enrichment. Together, DIA-based HDL proteotyping enables the robust digitization of HDL proteotypes as a basis for the analysis of larger clinical cohorts.

Association of anti-oxidative capacity of HDL with subclinical atherosclerosis in subjects with and without non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) patients are at a substantial risk for developing cardiovascular disease (CVD). High-density lipoprotein (HDL) is well known to have protective effects against the development of atherosclerotic CVD. One of the major antiatherogenic effects of HDL is its anti-oxidative function.

OBJECTIVES

This study investigated the association of anti-oxidative capacity of HDL with subclinical atherosclerosis in NAFLD and non-NAFLD subjects.

METHODS

A total of 143 subjects including 51 NAFLD and 92 control subjects were included in this case-control study. HDL oxidative index (HOI) was determined spectrophotometrically using a cell-free method in the presence of a fluorescent substrate dichlorofluorescein diacetate (DCFDA). Paraoxonase 1 (PON1) activity, superoxide dismutase (SOD) activity, and malondialdehyde (MDA) plasma levels were assessed in both groups.

RESULTS

The NAFLD patients with impaired HDL anti-oxidative function (HOI  ≥ 1) had higher MDA levels, aspartate amino transferase (AST), liver stiffness (LS), and carotid intima-media thickness (cIMT) values compared to the controls. HDL oxidative index (HOI) was positively correlated with MDA levels and cIMT and negatively correlated with SOD activity.

CONCLUSIONS

Higher circulating levels of MDA were associated with the impaired anti-oxidative function of HDL in NAFLD. The impaired anti-oxidative capacity of HDL might be related to NAFLD severity and subclinical atherosclerosis in NAFLD patients.

Molecular Patterns of Extreme and Persistent Cholesterol Efflux Capacity

Objective: Cholesterol efflux capacity (CEC), the ability of extracellular acceptors to pick-up cholesterol from macrophages, is a clinically relevant cardiovascular biomarker. CEC is inversely associated with incident atherosclerotic cardiovascular disease events. However, CEC is only modestly associated with HDL-C (high-density lipoprotein cholesterol) levels, which may explain the failure of HDL-C raising therapies to improve atherosclerotic cardiovascular disease outcomes. Determinants of variation in CEC are not well understood. Thus, we sought to establish whether extreme high and low CEC is a robust persistent phenotype and to characterize associations with cholesterol, protein, and phospholipids across the particle size distribution. Approach and Results: CEC was previously measured in 2924 participants enrolled in the Dallas Heart Study, a multi-ethnic population-based study from 2000 to 2002. We prospectively recruited those who were below the 10th and above 90th percentile of CEC. Our study revealed that extreme low and high CEC are persistent, robust phenotypes after 15 years of follow-up. Using size exclusion chromatography, CEC to fractionated plasma depleted of apolipoprotein B (fraction-specific CEC) demonstrated significant differences in CEC patterns between persistent high and low efflux groups. Fraction-specific CEC was correlated with fraction-specific total phospholipid but not apolipoprotein A-I, cholesterol, or total protein. These correlations varied across the size distribution and differed among persistent high versus low efflux groups. Conclusions: Extreme high and low CEC are persistent and robust phenotypes. CEC patterns in fractionated plasma reveal marked variation across the size distribution. Future studies are warranted to determine specific molecular species linked to CEC in a size-specific manner.

Effect of metformin on the high-density lipoprotein proteome in youth with type 1 diabetes

Background

Youth with type 1 diabetes (T1D) have normal or elevated High‐Density Lipoprotein Cholesterol (HDL‐C), however, the function of HDL, partly mediated by the HDL proteome, may be impaired. Metformin can be used as an adjunct therapy in youth with T1D, but its effects on the HDL proteome are unknown.

Objective

To determine the effect of metformin on the HDL proteome.

Subjects

Youth (12–20 years old) with T1D who had a BMI > 90th percentile, HbA1c > 8.0% and Tanner stage 5.

Methods

Double‐blinded, placebo‐controlled randomized sub‐study. We examined the effects of metformin (n = 25) or placebo (n = 10) after 6 months on HDL proteome. Changes in HDL proteins were measured by data‐independent acquisition (DIA) mass spectrometry and compared between treatment groups. As a secondary outcome, associations between proteins of interest and the most studied function of HDL, the cholesterol efflux capacity (CEC), was examined.

Results

The relative abundance of 84 HDL‐associated proteins were measured. Two proteins were significantly affected by metformin treatment, peptidoglycan recognition protein 2 (PGRP2; +23.4%, p = .0058) and alpha‐2‐macroglobulin (A2MG; +29.8%, p = .049). Metformin did not significantly affect CEC. Changes in affected HDL proteins did not correlate with CEC.

Conclusions

Despite having little effect on HDL‐C, metformin increased PGRP2 and A2MG protein on HDL in youth with T1D, but had no significant effect on CEC. Further studies are needed to understand the impact of PGRP2 and A2MG on other HDL functions.

The role of high-density lipoprotein cholesterol, apolipoprotein A and paraoxonase-1 in the pathophysiology of neuroprogressive disorders

Lowered high-density lipoprotein (HDL) cholesterol has been reported in major depressive disorder, bipolar disorder, first episode of psychosis, and schizophrenia. HDL, its major apolipoprotein component, ApoA1, and the antioxidant enzyme paraoxonase (PON)1 (which is normally bound to ApoA1) all have anti-atherogenic, antioxidant, anti-inflammatory, and immunomodulatory roles, which are discussed in this paper. The paper details the pathways mediating the anti-inflammatory effects of HDL, ApoA1 and PON1 and describes the mechanisms leading to compromised HDL and PON1 levels and function in an environment of chronic inflammation. The molecular mechanisms by which changes in HDL, ApoA1 and PON1 might contribute to the pathophysiology of the neuroprogressive disorders are explained. Moreover, the anti-inflammatory actions of ApoM-mediated sphingosine 1-phosphate (S1P) signalling are reviewed as well as the deleterious effects of chronic inflammation and oxidative stress on ApoM/S1P signalling. Finally, therapeutic interventions specifically aimed at improving the levels and function of HDL and PON1 while reducing levels of inflammation and oxidative stress are considered. These include the so-called Mediterranean diet, extra virgin olive oil, polyphenols, flavonoids, isoflavones, pomegranate juice, melatonin and the Mediterranean diet combined with the ketogenic diet.

Lipoprotein receptor SR-B1 deficiency enhances adipose tissue inflammation and reduces susceptibility to hepatic steatosis during diet-induced obesity in mice

Scavenger receptor class B type 1 (SR-B1) is a membrane lipoprotein receptor/lipid transporter involved in the pathogenesis of atherosclerosis, but its role in obesity and fatty liver development is unclear. Here, we determined the effects of SR-B1 deficiency on plasma metabolic and inflammatory parameters as well as fat deposition in adipose tissue and liver during obesity. To induce obesity, we performed high-fat diet (HFD) exposure for 12 weeks in male SR-B1 knock-out (SR-B1^-/-, n = 14) and wild-type (WT, n = 12) mice. Compared to HFD-fed WT mice, plasma from HFD-fed SR-B1^-/- animals exhibited increased total cholesterol, triglycerides (TG) and tumor necrosis factor-α (TNF-α) levels. In addition, hypertrophied adipocytes and macrophage-containing crown-like structures (CLS) were observed in adipose tissue from HFD-fed SR-B1 deficient mice. Remarkably, liver from obese SR-B1^-/- mice showed attenuated TG content, dysregulation in hepatic peroxisome proliferator-activated receptors (PPARs) expression, increased hepatic TG secretion, and altered hepatic fatty acid (FA) composition. In conclusion, we show that SR-B1 deficiency alters the metabolic environment of obese mice through modulation of liver and adipose tissue lipid accumulation. Our findings provide the basis for further elucidation of SR-B1's role in obesity and fatty liver, two major public health issues that increase the risk of advanced chronic diseases and overall mortality.

An Increased Plasma Level of ApoCIII-Rich Electronegative High-Density Lipoprotein May Contribute to Cognitive Impairment in Alzheimer's Disease

High-density lipoprotein (HDL) plays a vital role in lipid metabolism and anti-inflammatory activities; a dysfunctional HDL impairs cholesterol efflux pathways. To understand HDL's role in patients with Alzheimer's disease (AD), we analyzed the chemical properties and function. HDL from AD patients (AD-HDL) was separated into five subfractions, H1-H5, using fast-protein liquid chromatography equipped with an anion-exchange column. Subfraction H5, defined as the most electronegative HDL, was increased 5.5-fold in AD-HDL (23.48 ± 17.83%) in comparison with the control HDL (4.24 ± 3.22%). By liquid chromatography mass spectrometry (LC/MS^E), AD-HDL showed that the level of apolipoprotein (apo)CIII was elevated but sphingosine-1-phosphate (S1P)-associated apoM and anti-oxidative paraoxonase 1 (PON1) were reduced. AD-HDL showed a lower cholesterol efflux capacity that was associated with the post-translational oxidation of apoAI. Exposure of murine macrophage cell line, RAW 264.7, to AD-HDL induced a vibrant expression of ganglioside GM1 in colocalization with apoCIII on lipid rafts alongside a concomitant increase of tumor necrosis factor-α (TNF-α) detectable in the cultured medium. In conclusion, AD-HDL had a higher proportion of H5, an apoCIII-rich electronegative HDL subfraction. The associated increase in pro-inflammatory (apoCIII, TNF-α) components might favor Amyloid β assembly and neural inflammation. A compromised cholesterol efflux capacity of AD-HDL may also contribute to cognitive impairment.

Interaction between high-density lipoproteins and inflammation: Function matters more than concentration!

High-density lipoprotein (HDL) plays an important role in lipid metabolism and especially contributes to the reverse cholesterol transport pathway. Over recent years it has become clear that the effect of HDL on immune-modulation is not only dependent on HDL concentration but also and perhaps even more so on HDL function. This review will provide a concise general introduction to HDL followed by an overview of post-translational modifications of HDL and a detailed overview of the role of HDL in inflammatory diseases. The clinical potential of HDL and its main apolipoprotein constituent, apoA-I, is also addressed in this context. Finally, some conclusions and remarks that are important for future HDL-based research and further development of HDL-focused therapies are discussed.

Hyperglycemia induces key genetic and phenotypic changes in human liver epithelial HepG2 cells which parallel the Leprdb/J mouse model of non-alcoholic fatty liver disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is a growing global health concern. With a propensity to progress towards non-alcoholic steatohepatitis (NASH), cirrhosis, and hepatocellular carcinoma, NAFLD is an important link amongst a multitude of comorbidities including obesity, diabetes, and cardiovascular and kidney disease. As several in vivo models of hyperglycemia and NAFLD are employed to investigate the pathophysiology of this disease process, we aimed to characterize an in vitro model of hyperglycemia that was amenable to address molecular mechanisms and therapeutic targets at the cellular level. Utilizing hyperglycemic cell culturing conditions, we induced steatosis within a human hepatocyte cell line (HepG2 cells), as confirmed by electron microscopy. The deposition and accumulation of lipids within hyperglycemic HepG2 cells is significantly greater than in normoglycemic cells, as visualized and quantified by Nile red staining. Alanine aminotransferase (ALT) and alkaline phosphatase (ALP), diagnostic biomarkers for liver damage and disease, were found to be upregulated in hyperglycemic HepG2 cells as compared with normoglycemic cells. Suppression of CEACAM1, GLUT2, and PON1, and elevation of CD36, PCK1, and G6PK were also found to be characteristic in hyperglycemic HepG2 cells compared with normoglycemic cells, suggesting insulin resistance and NAFLD. These in vitro findings mirror the characteristic genetic and phenotypic profile seen in Lepr^db/J mice, a well-established in vivo model of NAFLD. In conclusion, we characterize an in vitro model displaying several key genetic and phenotypic characteristics in common with NAFLD that may assist future studies in addressing the molecular mechanisms and therapeutic targets to combat this disease.

Proteomics approach to investigate dynamic protein profile involved in high fat diet-induced fatty liver disease in rats

Nonalcoholic fatty liver disease (NAFLD) is a disorder of the liver found worldwide. The molecular mechanisms underlying NAFLD initiation and progression, however, remain poorly understood. In this study, fluorescence difference gel electrophoresis (DIGE) combined with mass spectrometry was performed to profile the intracellular processes in the rat liver at the proteome level when rats were fed a high-fat diet for 8 weeks. Dynamic changes of 27 protein spots were observed. Among them, upregulation of 14 spots and downregulation of 13 spots were observed during the eight weeks of the high fat diet-induction period. These spots were analyzed by matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF), and ultimately 24 proteins were identified with more than 95% confidence. Gene ontology (GO) annotation indicated that these proteins were implicated in the metabolism of carbohydrates, lipids, and amino acids. Four proteins were validated by western blot. Further functional studies on these dynamically changing proteins may lead to a better understanding of the mechanisms of high fat diet-induced fatty liver disease.

HDL flux is higher in patients with nonalcoholic fatty liver disease

Altered lipid metabolism and inflammation are involved in the pathogenesis of both nonalcoholic fatty liver disease (NAFLD) and cardiovascular disease (CVD). Even though high-density lipoprotein (HDL), a CVD protective marker, is decreased, whether HDL metabolism and function are perturbed in NAFLD are currently unknown. We examined the effect of NAFLD and disease severity on HDL metabolism and function in patients with biopsy-proven simple steatosis (SS), nonalcoholic steatohepatitis (NASH), and healthy controls. HDL turnover and HDL protein dynamics in SS (n = 7), NASH (n = 8), and healthy controls (n = 9) were studied in vivo. HDL maturation and remodeling, antioxidant, cholesterol efflux properties, and activities of lecithin-cholesterol ester acyltransferase and cholesterol ester transfer protein (CETP) were quantified using in vitro assays. All patients with NAFLD had increased turnover of both HDL cholesterol (HDLc; 0.16 ± 0.09 vs. 0.34 ± 0.18 days, P < 0.05) and apolipoprotein A1 (ApoAI) (0.26 ± 0.04 vs. 0.34 ± 0.06 days, P < 0.005) compared with healthy controls. The fractional catabolic rates of other HDL proteins, including ApoAII (and ApoAIV) were higher (P < 0.05) in patients with NAFLD who also had higher CETP activity, ApoAI/HDLc ratio (P < 0.05). NAFLD-induced alterations were associated with lower antioxidant (114.2 ± 46.6 vs. 220.5 ± 48.2 nmol·mL-1·min-1) but higher total efflux properties of HDL (23.4 ± 1.3% vs. 25.5 ± 2.3%) (both P < 0.05), which was more pronounced in individuals with NASH. However, no differences were observed in either HDL turnover, antioxidant, and cholesterol efflux functions of HDL or HDL proteins' turnover between subjects with SS and subjects with NASH. Thus, HDL metabolism and function are altered in NAFLD without any significant differences between SS and NASH.

Deepening our understanding of HDL proteome

Introduction: High-density lipoprotein (HDL) particles are heterogeneous and their proteome is complex and distinct from HDL cholesterol. However, it is largely unknown whether HDL proteins are associated with cardiovascular protection. Areas covered: HDL isolation techniques and proteomic analyses are reviewed. A list of HDL proteins reported in 37 different studies was compiled and the effects of different isolation techniques on proteins attributed to HDL are discussed. Mass spectrometric techniques used for HDL analysis and the need for precise and robust methods for quantification of HDL proteins are discussed. Expert opinion: Proteins associated with HDL have the potential to be used as biomarkers and/or help to understand HDL functionality. To achieve this, large cohorts must be studied using precise quantification methods. Key factors in HDL proteome quantification are the isolation methodology and the mass spectrometry technique employed. Isolation methodology affects what proteins are identified in HDL and the specificity of association with HDL particles needs to be addressed. Shotgun proteomics yields imprecise quantification, but the majority of HDL studies relied on this approach. Few recent studies used targeted tandem mass spectrometry to quantify HDL proteins, and it is imperative that future studies focus on the application of these precise techniques.

The association between lecithin-cholesterol acyltransferase activity and fatty liver index

Background

Non-alcoholic fatty liver disease is a frequent ailment with known complications, including those within the cardiovascular system. Associations between several indicators of high-density lipoprotein metabolism and function with clinical and laboratory parameters for the assessment of fatty liver index, a surrogate marker of non-alcoholic fatty liver disease, were evaluated.

Methods

The study comprised 130 patients classified according to fatty liver index values: fatty liver index < 30, fatty liver index 30–59 (the intermediate group) and fatty liver index ⩾ 60. Lecithin–cholesterol acyltransferase and cholesteryl ester transfer protein activities were determined. Paraoxonase 1 concentration and its activity, paraoxonase 3 concentration and high-density lipoprotein subclass distribution were assessed.

Results

Increased lecithin–cholesterol acyltransferase activity correlated with increased fatty liver index ( P < 0.001). Paraoxonase 3 concentration was lower in the fatty liver index ⩾ 60 group compared with the fatty liver index < 30 group ( P < 0.05). Cholesteryl ester transfer protein activity, paraoxonase 1 concentration and its activity did not significantly differ across the fatty liver index groups. The relative proportion of small-sized high-density lipoprotein 3 subclass was higher in the fatty liver index ⩾ 60 group compared with the other two fatty liver index groups ( P < 0.01). Lecithin–cholesterol acyltransferase activity positively associated with the fatty liver index ⩾ 60 group and remained significant after adjustment for other potential confounders. Only the triglyceride concentration remained significantly associated with lecithin–cholesterol acyltransferase activity when the parameters that constitute the fatty liver index equation were examined.

Conclusions

Higher lecithin–cholesterol acyltransferase activity is associated with elevated fatty liver index values. Significant independent association between triglycerides and lecithin–cholesterol acyltransferase activity might indicate a role of hypertriglyceridaemia in alterations of lecithin–cholesterol acyltransferase activity in individuals with elevated fatty liver index.

The Mexican consensus on nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) affects nearly one third of the population worldwide. Mexico is one of the countries whose population has several risk factors for the disease and its prevalence could surpass 50%. If immediate action is not taken to counteract what is now considered a national health problem, the medium-term panorama will be very bleak. This serious situation prompted the Asociación Mexicana de Gastroenterología and the Asociación Mexicana de Hepatología to produce the Mexican Consensus on Fatty Liver Disease. It is an up-to-date and detailed review of the epidemiology, pathophysiology, clinical forms, diagnosis, and treatment of the disease, whose aim is to provide the Mexican physician with a useful tool for the prevention and management of nonalcoholic fatty liver disease.