Cholesterol efflux analyses using stable isotopes and mass spectrometry

A new method for measuring cholesterol efflux capacity uses stable isotope-labeled, not radioactive-labeled, cholesterol

The incidence of cardiovascular events correlates inversely with cholesterol efflux capacity (CEC) more than with HDL-cholesterol level. The measurement of CEC is used to qualify cardiovascular disease risk and is conventionally performed with radioisotope (RI)-labeled cholesterol. Here, we established a CEC measurement technique using stable isotope-labeled cholesterol as an alternative, and we compared the new method with RI and fluorescence (boron dipyrromethene difluoride-cholesterol) methods in cells and in patient serum. We incubated J774 cells labeled with [d ₇]cholesterol ([d ₇]C) with patient serum depleted of apoB, and [d ₇]C extracted from the culture medium was quantified by liquid chromatography/quadrupole time-of-flight mass spectrometry. [d ₇]C efflux increased with greater apoB-depleted serum concentration and longer incubation time. The assay coefficient of variation (CV) of five consecutive measurements of three sets of samples ranged from 7.3% to 9.5%, and the interassay CV determined by measuring three samples four times ranged from 4.1% to 8.5%, both indicating good precision. We then measured CEC levels of 41 outpatients with serum HDL-cholesterol levels between 36 and 94 mg/dl (mean: 61.7 ± 18.0 mg/dl); in the presence of cAMP, we observed a significant, positive correlation between CEC levels determined with the stable isotope and RI methods that was stronger than the correlation between measurements obtained by the fluorescence and RI methods (r = 0.73, P < 0.0001 vs. r = 0.55, P < 0.001). Therefore, our stable isotope method can be considered useful as a non-RI method and thus deserves evaluation in future clinical studies.

Transfer of Functional Cargo in Exomeres

Exomeres are a recently discovered type of extracellular nanoparticle with no known biological function. Herein, we describe a simple ultracentrifugation-based method for separation of exomeres from exosomes. Exomeres are enriched in Argonaute 1-3 and amyloid precursor protein. We identify distinct functions of exomeres mediated by two of their cargo, the β-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) that α2,6- sialylates N-glycans, and the EGFR ligand, amphiregulin (AREG). Functional ST6Gal-I in exomeres can be transferred to cells, resulting in hypersialylation of recipient cell-surface proteins including β1-integrin. AREG-containing exomeres elicit prolonged EGFR and downstream signaling in recipient cells, modulate EGFR trafficking in normal intestinal organoids, and dramatically enhance the growth of colonic tumor organoids. This study provides a simplified method of exomere isolation and demonstrates that exomeres contain and can transfer functional cargo. These findings underscore the heterogeneity of nanoparticles and should accelerate advances in determining the composition and biological functions of exomeres.

Development and Validation of Apolipoprotein AI-Associated Lipoprotein Proteome Panel for the Prediction of Cholesterol Efflux Capacity and Coronary Artery Disease

BACKGROUND

Cholesterol efflux capacity (CEC) is a measure of HDL function that, in cell-based studies, has demonstrated an inverse association with cardiovascular disease. The cell-based measure of CEC is complex and low-throughput. We hypothesized that assessment of the lipoprotein proteome would allow for precise, high-throughput CEC prediction.

METHODS

After isolating lipoprotein particles from serum, we used LC-MS/MS to quantify 21 lipoprotein-associated proteins. A bioinformatic pipeline was used to identify proteins with univariate correlation to cell-based CEC measurements and generate a multivariate algorithm for CEC prediction (pCE). Using logistic regression, protein coefficients in the pCE model were reweighted to yield a new algorithm predicting coronary artery disease (pCAD).

RESULTS

Discovery using targeted LC-MS/MS analysis of 105 training and test samples yielded a pCE model comprising 5 proteins (Spearman r = 0.86). Evaluation of pCE in a case–control study of 231 specimens from healthy individuals and patients with coronary artery disease revealed lower pCE in cases (P = 0.03). Derived within this same study, the pCAD model significantly improved classification (P &lt; 0.0001). Following analytical validation of the multiplexed proteomic method, we conducted a case–control study of myocardial infarction in 137 postmenopausal women that confirmed significant separation of specimen cohorts in both the pCE (P = 0.015) and pCAD (P = 0.001) models.

CONCLUSIONS

Development of a proteomic pCE provides a reproducible high-throughput alternative to traditional cell-based CEC assays. The pCAD model improves stratification of case and control cohorts and, with further studies to establish clinical validity, presents a new opportunity for the assessment of cardiovascular health.

Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol Transport in Mammalian Cells

The mechanisms underlying sterol transport in mammalian cells are poorly understood. In particular, how cholesterol internalized from HDL is made available to the cell for storage or modification is unknown. Here, we describe three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and facilitate its removal from the plasma membrane. The crystal structure of the central domain of Aster-A broadly resembles the sterol-binding fold of mammalian StARD proteins, but sequence differences in the Aster pocket result in a distinct mode of ligand binding. The Aster N-terminal GRAM domain binds phosphatidylserine and mediates Aster recruitment to plasma membrane-ER contact sites in response to cholesterol accumulation in the plasma membrane. Mice lacking Aster-B are deficient in adrenal cholesterol ester storage and steroidogenesis because of an inability to transport cholesterol from SR-BI to the ER. These findings identify a nonvesicular pathway for plasma membrane to ER sterol trafficking in mammals.

A Rabbit Model for Testing Helper-Dependent Adenovirus-Mediated Gene Therapy for Vein Graft Atherosclerosis

Coronary artery bypass vein grafts are a mainstay of therapy for human atherosclerosis. Unfortunately, the long-term patency of vein grafts is limited by accelerated atherosclerosis. Gene therapy, directed at the vein graft wall, is a promising approach for preventing vein graft atherosclerosis. Because helper-dependent adenovirus (HDAd) efficiently transduces grafted veins and confers long-term transgene expression, HDAd is an excellent candidate for delivery of vein graft-targeted gene therapy. We developed a model of vein graft atherosclerosis in fat-fed rabbits and demonstrated long-term (≥20 weeks) persistence of HDAd genomes after graft transduction. This model enables quantitation of vein graft hemodynamics, wall structure, lipid accumulation, cellularity, vector persistence, and inflammatory markers on a single graft. Time-course experiments identified 12 weeks after transduction as an optimal time to measure efficacy of gene therapy on the critical variables of lipid and macrophage accumulation. We also used chow-fed rabbits to test whether HDAd infusion in vein grafts promotes intimal growth and inflammation. HDAd did not increase intimal growth, but had moderate-yet significant-pro-inflammatory effects. The vein graft atherosclerosis model will be useful for testing HDAd-mediated gene therapy; however, pro-inflammatory effects of HdAd remain a concern in developing HDAd as a therapy for vein graft disease.

Tandem Mass Spectrometry of Sphingolipids: Applications for Diagnosis of Sphingolipidoses

In recent years, mass spectrometry (MS) has become the dominant technology in lipidomic analysis. It is widely used in diagnosis and research of lipid metabolism disorders including those characterized by impairment of lysosomal functions and storage of nondegraded-degraded substrates. These rare diseases, which include sphingolipidoses, have severe and often fatal clinical consequences. Modern MS methods have contributed significantly to achieve a definitive diagnosis, which is essential in clinical practice to begin properly targeted patient care. Here we summarize MS and tandem MS methods used for qualitative and quantitative analysis of sphingolipids (SL) relative to the diagnostic process for sphingolipidoses and studies focusing on alterations in cell functions due to these disorders. This review covers the following topics: Tandem MS is sensitive and robust in determining the composition of sphingolipid classes in various biological materials. Its ability to establish SL metabolomic profiles using MS bench-top analyzers, significantly benefits the first stages of a diagnosis as well as metabolic studies of these disorders. It can thus contribute to a better understanding of the biological significance of SL.

Production, Purification, and Identification of Cholest-4-en-3-one Produced by Cholesterol Oxidase from Rhodococcus sp. in Aqueous/Organic Biphasic System

Cholest-4-en-3-one has positive uses against obesity, liver disease, and keratinization. It can be applied in the synthesis of steroid drugs as well. Most related studies are focused on preparation of cholest-4-en-3-one by using whole cells as catalysts, but production of high-quality cholest-4-en-3-one directly from cholesterol oxidase (COD) using an aqueous/organic two-phase system has been rarely explored. This study set up an enzymatic reaction system to produce cholest-4-en-3-one. We developed and optimized the enzymatic reaction system using COD from COX5-6 (a strain of Rhodococcus) instead of whole-cell biocatalyst. This not only simplifies and accelerates the production but also benefits the subsequent separation and purification process. Through extraction, washing, evaporation, column chromatography, and recrystallization, we got cholest-4-en-3-one with purity of 99.78%, which was identified by nuclear magnetic resonance, mass spectroscopy, and infrared spectroscopy. In addition, this optimized process of cholest-4-en-3-one production and purification can be easily scaled up for industrial production, which can largely decrease the cost and guarantee the purity of the product.

Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

Cholesterol efflux from THP-1 macrophages is impaired by the fatty acid component from lipoprotein hydrolysis by lipoprotein lipase

Lipoprotein lipase (LPL) is an extracellular lipase that primarily hydrolyzes triglycerides within circulating lipoproteins. Macrophage LPL contributes to atherogenesis, but the mechanisms behind it are poorly understood. We hypothesized that the products of lipoprotein hydrolysis generated by LPL promote atherogenesis by inhibiting the cholesterol efflux ability by macrophages. To test this hypothesis, we treated human THP-1 macrophages with total lipoproteins that were hydrolyzed by LPL and we found significantly reduced transcript levels for the cholesterol transporters ATP binding cassette transporter A1 (ABCA1), ABCG1, and scavenger receptor BI. These decreases were likely due to significant reductions for the nuclear receptors liver-X-receptor-α, peroxisome proliferator activated receptor (PPAR)-α, and PPAR-γ. We prepared a mixture of free fatty acids (FFA) that represented the ratios of FFA species within lipoprotein hydrolysis products, and we found that the FFA mixture also significantly reduced cholesterol transporters and nuclear receptors. Finally, we tested the efflux of cholesterol from THP-1 macrophages to apolipoprotein A-I, and we found that the treatment of THP-1 macrophages with the FFA mixture significantly attenuated cholesterol efflux. Overall, these data show that the FFA component of lipoprotein hydrolysis products generated by LPL may promote atherogenesis by inhibiting cholesterol efflux, which partially explains the pro-atherogenic role of macrophage LPL.

Differential regulation of ABCA1 and macrophage cholesterol efflux by elaidic and oleic acids

Trans fatty acid consumption is associated with an increased risk of coronary heart disease. This increased risk has been attributed to decreased levels of HDL cholesterol and increased levels of LDL cholesterol. However, the mechanism by which trans fatty acid modulates cholesterol transit remains poorly defined. ATP-binding cassette transporter A1 (ABCA1)-mediated macrophage cholesterol efflux is the rate-limiting step initiating apolipoprotein A-I lipidation. In this study, elaidic acid, the most abundant trans fatty acid in partially hydrogenated vegetable oil, was shown to stabilize macrophage ABCA1 protein levels in comparison to that of its cis fatty acid isomer, oleic acid. The mechanism responsible for the disparate effects of oleic and elaidic acid on ABCA1 levels was through accelerated ABCA1 protein degradation in cells treated with oleic acid. In contrast, no apparent differences were observed in ABCA1 mRNA levels, and only minor changes were observed in Liver X receptor/Retinoic X receptor promoter activity in cells treated with elaidic and oleic acid. Efflux of both tracers and cholesterol mass revealed that elaidic acid slightly increased ABCA1-mediated cholesterol efflux, while oleic acid led to decreased ABCA1-mediated efflux. In conclusion, these studies show that cis and trans structural differences in 18 carbon n-9 monoenoic fatty acids variably impact cholesterol efflux through disparate effects on ABCA1 protein degradation.