Thematic review series: patient-oriented research. What we have learned about VLDL and LDL metabolism from human kinetics studies

Directly Measuring Atherogenic Lipoprotein Kinetics in Zebrafish with the Photoconvertible LipoTimer Reporter

Lipoprotein kinetics are a crucial factor in understanding lipoprotein metabolism since a prolonged time in circulation can contribute to the atherogenic character of apolipoprotein-B (ApoB)-containing lipoproteins (B-lps). Here, we report a method to directly measure lipoprotein kinetics in live developing animals. We developed a zebrafish geneticly encoded reporter, LipoTimer, in which endogenous ApoBb.1 is fused to the photoconvertible fluorophore Dendra2 which shift its emission profile from green to red upon UV exposure. By quantifying the red population of ApoB-Dendra2 over time, we found that B-lp turnover in wild-type larvae becomes faster as development proceeds. Mutants with impaired B-lp uptake or lipolysis present with increased B-lp levels and half-life. In contrast, mutants with impaired B-lp triglyceride loading display slightly fewer and smaller-B-lps, which have a significantly shorter B-lp half-life. Further, we showed that chronic high-cholesterol feeding is associated with a longer B-lp half-life in wild-type juveniles but does not lead to changes in B-lp half-life in lipolysis deficient apoC2 mutants. These data support the hypothesis that B-lp lipolysis is suppressed by the flood of intestinal-derived B-lps that follow a high-fat meal.

Lipid Profiling of Serum and Lipoprotein Fractions in Response to Pitavastatin Using an Animal Model of Familial Hypercholesterolemia

Statins are widely used for the treatment of atherosclerotic cardiovascular diseases. They inhibit cholesterol biosynthesis in the liver and cause pleiotropic effects, including anti-inflammatory and antioxidant effects. To develop novel therapeutic drugs, the effect of blood-borne lipid molecules on the pleiotropic effects of statins must be elucidated. Myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits, an animal model for hypercholesterolemia, are suitable for the determination of lipid molecules in the blood in response to statins because their lipoprotein metabolism is similar to that of humans. Herein, lipid molecules were investigated by lipidome analysis in response to pitavastatin using WHHLMI rabbits. Various lipid molecules in the blood were measured using a supercritical fluid chromatography triple quadrupole mass spectrometry. Cholesterol and cholesterol ester blood concentrations decreased by reducing the secretion of very low density lipoproteins from the liver. Independent of the inhibition effects of cholesterol biosynthesis, the concentrations of some lipids with anti-inflammation and antioxidant effects (phospholipid molecules with n-6 fatty acid side chains, lysophosphatidylcholines, phosphatidylethanolamine plasmalogens, and ceramide molecules) were significantly altered. These findings may lead to further investigation of the mechanism of statin action.

Targeting ATP-Citrate Lyase in Hyperlipidemia and Metabolic Disorders

Chronic overnutrition and a sedentary lifestyle promote imbalances in metabolism, often manifesting as risk factors for life-threating diseases such as atherosclerotic cardiovascular disease (ASCVD) and nonalcoholic fatty liver disease (NAFLD). Nucleocytosolic acetyl-coenzyme A (CoA) has emerged as a central signaling node used to coordinate metabolic adaptations in response to a changing nutritional status. ATP-citrate lyase (ACL) is the enzyme primarily responsible for the production of extramitochondrial acetyl-CoA and is thus strategically positioned at the intersection of nutrient catabolism and lipid biosynthesis. Here, we discuss recent findings from preclinical studies, as well as Mendelian and clinical randomized trials, demonstrating the importance of ACL activity in metabolism, and supporting its inhibition as a potential therapeutic approach to treating ASCVD, NAFLD, and other metabolic disorders.

The metabolism of lipoprotein (a): an ever-evolving story

Lipoprotein (a) [Lp(a)] is characterized by apolipoprotein (a) [apo(a)] covalently bound to apolipoprotein B 100. It was described in human plasma by Berg et al. in 1963 and the gene encoding apo(a) (LPA) was cloned in 1987 by Lawn and colleagues. Epidemiologic and genetic studies demonstrate that increases in Lp(a) plasma levels increase the risk of atherosclerotic cardiovascular disease. Novel Lp(a) lowering treatments highlight the need to understand the regulation of plasma levels of this atherogenic lipoprotein. Despite years of research, significant uncertainty remains about the assembly, secretion, and clearance of Lp(a). Specifically, there is ongoing controversy about where apo(a) and apoB-100 bind to form Lp(a); which apoB-100 lipoproteins bind to apo(a) to create Lp(a); whether binding of apo(a) is reversible, allowing apo(a) to bind to more than one apoB-100 lipoprotein during its lifespan in the circulation; and how Lp(a) or apo(a) leave the circulation. In this review, we highlight past and recent data from stable isotope studies of Lp(a) metabolism, highlighting the critical metabolic uncertainties that exist. We present kinetic models to describe results of published studies using stable isotopes and suggest what future studies are required to improve our understanding of Lp(a) metabolism.

Extreme Contrast of Postprandial Remnant-Like Particles Formed in Abetalipoproteinemia and Homozygous Familial Hypobetalipoproteinemia

BACKGROUND

Familial hypobetalipoproteinemia (FHBL) and abetalipoproteinemia (ABL) are rare inherited forms of hypolipidemia. Their differential diagnosis is important for predicting of the prognosis and selecting appropriate therapy.

MATERIALS AND METHODS

Genetic analysis was performed in two patients with primary hypocholesterolemia born from consanguineous parents. The oral fat tolerance test (OFTT) was performed in one patient with FHBL (apoB-87.77) and one with ABL as well as in four normal control subjects. After overnight fasting, blood samples were drawn. Serum lipoprotein and remnant-like particle (RLP) fractions were determined by HPLC analysis.

RESULTS

Both patients with homozygous FHBL were asymptomatic probably because of preserved levels of fat-soluble vitamins, especially vitamin E. The patients with FHBL were homozygous because of novel apoB-83.52 and apoB-87.77 mutations, and although one of them (apoB-87.77) had fatty liver disease, microscopic findings suggesting nonalcoholic steatohepatitis were absent. Fasting apoB-48 and RLP-triglyceride levels in the patient with homozygous FHBL, which were similar to those in normal control subjects, increased after OFTT both in normal control subjects and the patient with FHBL but not in the patient with ABL, suggesting that the fat load administered was absorbed only in the patient with FHBL.

CONCLUSION

Although lipid levels in the patients with homozygous FHBL and ABL were comparable, fasting, postoral fat loading of apoB-48, as well as RLP-triglyceride levels, may help in the differential diagnosis of FHBL and ABL and provide a prompt diagnosis using genetic analysis in the future.

Practical immunoaffinity-enrichment LC-MS for measuring protein kinetics of low-abundance proteins

BACKGROUND

For a more complete understanding of pharmacodynamic, metabolic, and pathophysiologic effects, protein kinetics, such as production rate and fractional catabolic rate, can offer substantially more information than protein concentration alone. Kinetic experiments with stable isotope tracers typically require laborious sample preparation and are most often used for studying abundant proteins. Here we describe a practical methodology for measuring isotope enrichment into low-abundance proteins that uses an automated procedure and immunoaffinity enrichment (IA) with LC-MS. Low-abundance plasma proteins cholesteryl ester transfer protein (CETP) and proprotein convertase subtilisin/kexin type 9 (PCSK9) were studied as examples.

METHODS

Human participants (n = 39) were infused with [(2)H(3)]leucine, and blood samples were collected at multiple time points. Sample preparation and analysis were automated and multiplexed to increase throughput. Proteins were concentrated from plasma by use of IA and digested with trypsin to yield proteotypic peptides that were analyzed by microflow chromatography-mass spectrometry to measure isotope enrichment.

RESULTS

The IA procedure was optimized to provide the greatest signal intensity. Use of a gel-free method increased throughput while increasing the signal. The intra- and interassay CVs were <15% at all isotope enrichment levels studied. More than 1400 samples were analyzed in <3 weeks without the need for instrument stoppages or user interventions.

CONCLUSIONS

The use of automated gel-free methods to multiplex the measurement of isotope enrichment was applied to the low-abundance proteins CETP and PCSK9.

The complex fate in plasma of gadolinium incorporated into high-density lipoproteins used for magnetic imaging of atherosclerotic plaques

We have previously reported enhancing the imaging of atherosclerotic plaques in mice using reconstituted high density lipoproteins (HDL) as nanocarriers for the MRI contrast agent gadolinium (Gd). This study focuses on the underlying mechanisms of Gd delivery to atherosclerotic plaques. HDL, LDL, and VLDL particles containing Gd chelated to phosphatidyl ethanolamine (DTPA-DMPE) and a lipidic fluorophore were used to demonstrate the transfer of Gd-phospholipids among plasma lipoproteins in vitro and in vivo. To determine the basis of this transfer, the roles of phospholipid transfer protein (PLTP) and lipoprotein lipase (LpL) in mediating the migration of Gd-DTPA-DMPE among lipoproteins were investigated. The results indicated that neither was an important factor, suggesting that spontaneous transfer of Gd-DTPA-DMPE was the most probable mechanism. Finally, two independent mouse models were used to quantify the relative contributions of HDL and LDL reconstituted with Gd-DTPA-DMPE to plaque imaging enhancement by MR. Both sets of results suggested that Gd-DTPA-DMPE originally associated with LDL was about twice as effective as that injected in the form of Gd-HDL, and that some of Gd-HDL's effectiveness in vivo is indirect through transfer of the imaging agent to LDL. In conclusion, the fate of Gd-DTPA-DMPE associated with a particular type of lipoprotein is complex, and includes its transfer to other lipoprotein species that are then cleared from the plasma into tissues.

Plasma kinetics of an LDL-like nanoemulsion and lipid transfer to HDL in subjects with glucose intolerance

OBJECTIVE

Glucose intolerance is frequently associated with an altered plasma lipid profile and increased cardiovascular disease risk. Nonetheless, lipid metabolism is scarcely studied in normolipidemic glucose-intolerant patients. The aim of this study was to investigate whether important lipid metabolic parameters, such as the kinetics of LDL free and esterified cholesterol and the transfer of lipids to HDL, are altered in glucose-intolerant patients with normal plasma lipids.

METHODS

Fourteen glucose-intolerant patients and 15 control patients were studied; none of the patients had cardiovascular disease manifestations, and they were paired for age, sex, race and co-morbidities. A nanoemulsion resembling a LDL lipid composition (LDE) labeled with 14C-cholesteryl ester and ³H-free cholesterol was intravenously injected, and blood samples were collected over a 24-h period to determine the fractional clearance rate of the labels by compartmental analysis. The transfer of free and esterified cholesterol, triglycerides and phospholipids from the LDE to HDL was measured by the incubation of the LDE with plasma and radioactivity counting of the supernatant after chemical precipitation of non-HDL fractions.

RESULTS

The levels of LDL, non-HDL and HDL cholesterol, triglycerides, apo A1 and apo B were equal in both groups. The 14C-esterified cholesterol fractional clearance rate was not different between glucose-intolerant and control patients, but the ³H-free-cholesterol fractional clearance rate was greater in glucose-intolerant patients than in control patients. The lipid transfer to HDL was equal in both groups.

CONCLUSION

In these glucose-intolerant patients with normal plasma lipids, a faster removal of LDE free cholesterol was the only lipid metabolic alteration detected in our study. This finding suggests that the dissociation of free cholesterol from lipoprotein particles occurs in normolipidemic glucose intolerance and may participate in atherogenic signaling.

Culinary plants and their potential impact on metabolic overload

Contemporary human behavior has led a large proportion of the population to metabolic overload and obesity. Postprandial hyperlipidemia and hyperglycemia evoke redox imbalance in the short term and lead to complex chronic disease in the long term with repeated occurrence. Complex diseases are best prevented with complex components of plants; thus, current nutrition research has begun to focus on the development of plant-based functional foods and dietary supplements for health and well-being. Furthermore, given the wide range of species, parts, and secondary metabolites, culinary plants can contribute significant variety and complexity to the human diet. Although understanding the health benefits of culinary plants has been one of the great challenges in nutritional science due to their inherent complexity, it is an advantageous pursuit. This review will address the challenges and opportunities relating to studies of the health benefits of culinary plants, with an emphasis on obesity attributed to metabolic overload.

Diagnostic markers based on a computational model of lipoprotein metabolism

Background

Dyslipidemia is an important risk factor for cardiovascular disease and type II diabetes. Lipoprotein diagnostics, such as LDL cholesterol and HDL cholesterol, help to diagnose these diseases. Lipoprotein profile measurements could improve lipoprotein diagnostics, but interpretational complexity has limited their clinical application to date. We have previously developed a computational model called Particle Profiler to interpret lipoprotein profiles. In the current study we further developed and calibrated Particle Profiler using subjects with specific genetic conditions. We subsequently performed technical validation and worked at an initial indication of clinical usefulness starting from available data on lipoprotein concentrations and metabolic fluxes. Since the model outcomes cannot be measured directly, the only available technical validation was corroboration. For an initial indication of clinical usefulness, pooled lipoprotein metabolic flux data was available from subjects with various types of dyslipidemia. Therefore we investigated how well lipoprotein metabolic ratios derived from Particle Profiler distinguished reported dyslipidemic from normolipidemic subjects.

Results

We found that the model could fit a range of normolipidemic and dyslipidemic subjects from fifteen out of sixteen studies equally well, with an average 8.8% ± 5.0% fit error; only one study showed a larger fit error. As initial indication of clinical usefulness, we showed that one diagnostic marker based on VLDL metabolic ratios better distinguished dyslipidemic from normolipidemic subjects than triglycerides, HDL cholesterol, or LDL cholesterol. The VLDL metabolic ratios outperformed each of the classical diagnostics separately; they also added power of distinction when included in a multivariate logistic regression model on top of the classical diagnostics.

Conclusions

In this study we further developed, calibrated, and corroborated the Particle Profiler computational model using pooled lipoprotein metabolic flux data. From pooled lipoprotein metabolic flux data on dyslipidemic patients, we derived VLDL metabolic ratios that better distinguished normolipidemic from dyslipidemic subjects than standard diagnostics, including HDL cholesterol, triglycerides and LDL cholesterol. Since dyslipidemias are closely linked to cardiovascular disease and diabetes type II development, lipoprotein metabolic ratios are candidate risk markers for these diseases. These ratios can in principle be obtained by applying Particle Profiler to a single lipoprotein profile measurement, which makes clinical application feasible.

Lipoprotein enrichment in orange insoluble particulate matter reproducibly appearing in cryoprecipitate

BACKGROUND AND OBJECTIVE

Cryoprecipitate prepared from two whole blood donations from the same donor contained insoluble orange particulate material (OPM). We sought to identify the OPM.

MATERIALS AND METHODS

OPM was recovered from the blood product by centrifugation, dissolved in sodium dodecyl sulphate (SDS) and analysed by SDS-polyacrylamide gel electrophoresis and immunoblotting.

RESULTS

Solubilized OPM was enriched in apolipoproteins B and E, but not apolipoprotein A1, immunoglobulin G or albumin, suggesting lipoprotein enrichment in OPM. Subsequent clinical laboratory blood tests confirmed low-density lipoprotein hyperlipidaemia with normal triglyceride levels. Further, cryoprecipitate production from this donor was prevented by implementation of national predominantly male plasma policies.

CONCLUSION

Cryoprecipitate produced from hyperlipidaemic donors may contain insoluble particles that render it inappropriate for transfusion.

Development of apolipoprotein B antisense molecules as a therapy for hyperlipidemia

As new studies demonstrate that lower levels of low-density lipoprotein cholesterol (LDL-C) reduce cardiovascular disease, and as goals for LDL-C in high-risk individuals are reduced further and further, reaching those goals becomes more difficult for a significant percentage of the population. New therapeutic approaches to lower LDL-C would, therefore, be advantageous, particularly in those who are most likely to suffer cardiovascular disease-associated morbidity and mortality. Mouse and human genetic models suggest that decreasing hepatic apolipoprotein B (apoB) production may be a therapeutic approach for the treatment of dyslipidemia. Because antisense oligonucleotides naturally distribute to the liver and can specifically inhibit synthesis of proteins from their messenger RNAs, antisense oligonucleotides represent a potential approach for decreasing the biosynthesis of apoB, and thereby, the production of both very low density lipoprotein (VLDL) and LDL. Newly developed apoB antisense approaches have produced results in animal models and humans, providing proof of concept regarding reductions in LDL-C concentrations. Surprisingly, despite prior experience with inhibitors of microsomal triglyceride transfer protein, which also inhibits the secretion of VLDL, apoB antisense-mediated reduction in VLDL secretion does not appear to cause marked steatosis. The mechanisms whereby two different approaches for inhibiting apoB and triglyceride secretion have different effects on hepatic triglycerides are currently being examined.

Chronic inflammation in obesity and the metabolic syndrome

The increasing incidence of obesity and the metabolic syndrome is disturbing. The activation of inflammatory pathways, used normally as host defence, reminds the seriousness of this condition. There is probably more than one cause for activation of inflammation. Apparently, metabolic overload evokes stress reactions, such as oxidative, inflammatory, organelle and cell hypertrophy, generating vicious cycles. Adipocyte hypertrophy, through physical reasons, facilitates cell rupture, what will evoke an inflammatory reaction. Inability of adipose tissue development to engulf incoming fat leads to deposition in other organs, mainly in the liver, with consequences on insulin resistance. The oxidative stress which accompanies feeding, particularly when there is excessive ingestion of fat and/or other macronutrients without concomitant ingestion of antioxidant-rich foods/beverages, may contribute to inflammation attributed to obesity. Moreover, data on the interaction of microbiota with food and obesity brought new hypothesis for the obesity/fat diet relationship with inflammation. Beyond these, other phenomena, for instance psychological and/or circadian rhythm disturbances, may likewise contribute to oxidative/inflammatory status. The difficulty in the management of obesity/metabolic syndrome is linked to their multifactorial nature where environmental, genetic and psychosocial factors interact through complex networks.

Genetic determinants of apolipoprotein B-100 kinetics

PURPOSE OF REVIEW

We review stable isotope tracer studies of apolipoprotein B-100 (apoB) kinetics concerning genetic polymorphisms and mutations that affect human lipoprotein metabolism.

RECENT FINDINGS

In obese men, the allelic combination of the apoB signal peptide, SP24, and cholesteryl ester transfer protein, CETP B1B1, is independently associated with lower VLDL apoB secretion. Microsomal triglyceride transfer protein -493G/T carriers have reduced IDL apoB and LDL apoB production as compared with controls. Mutations in cholesterol transporters (ATP-binding cassette transporter G8 and Niemann-Pick C1 Like 1) are associated with reduced VLDL apoB secretion and increased LDL apoB production and catabolism. The ATP-binding cassette transporter G8 400K variant is a significant, independent predictor of VLDL apoB secretion. Mutations in lipases (lipoprotein lipase and hepatic lipase) and transfer proteins (lecithin-cholesterol acyltransferase and cholesteryl ester transfer protein) alter their functional activity, which impact on VLDL and LDL kinetics.

SUMMARY

Mutations in genes that regulate intrahepatic apoB assembly and lipid substrate availability to the liver impact on VLDL apoB secretion. Lipoprotein tracer studies can provide functional insight into the potential impact of genetic polymorphisms in regulating apoB metabolism in humans.

Optimization of N-methyl-N-[tert-butyldimethylsilyl]trifluoroacetamide as a derivatization agent for determining isotopic enrichment of glycerol in very-low density lipoproteins

Stable isotope kinetic studies play an important role in the study of very-low density lipoprotein (VLDL) metabolism, including basic and clinical research. Today, [1,1,2,3,3-(2)H(5)]glycerol is the most cost-effective alternative to measure glycerol and triglyceride kinetics. Recycling of glycerol from glycolysis and gluconeogenesis may lead to incompletely labelled tracer molecules. Many existing methods for the measurement of glycerol isotopic enrichment involve the production of glycerol derivatives that result in fragmentation of the glycerol molecule after ionization. It would be favourable to measure the intact tracer molecule since incompletely labelled tracer molecules may be measured as fully labelled. The number of methods available to measure the intact tracer in biological samples is limited. The aim of this project was to develop a gas chromatography/mass spectrometry (GC/MS) method for glycerol enrichment that measures the intact glycerol backbone and is suitable for electron ionization (EI), which is widely available. A previously published method for N-methyl-N-[tert-butyldimethylsilyl]trifluoroacetamide (MTBSTFA) derivatization was significantly improved; we produced a stable derivative and increased recovery 27-fold in standards. We used the optimized MTBSTFA method in VLDL-triglyceride and found that further modification was required to take matrix effects into account. We now have a robust method to measure glycerol isotopic enrichment by GC/EI-MS that can be used to rule out the known problem of tracer recycling in studies of VLDL kinetics.

Nutritional systems biology modeling: from molecular mechanisms to physiology

The use of computational modeling and simulation has increased in many biological fields, but despite their potential these techniques are only marginally applied in nutritional sciences. Nevertheless, recent applications of modeling have been instrumental in answering important nutritional questions from the cellular up to the physiological levels. Capturing the complexity of today's important nutritional research questions poses a challenge for modeling to become truly integrative in the consideration and interpretation of experimental data at widely differing scales of space and time. In this review, we discuss a selection of available modeling approaches and applications relevant for nutrition. We then put these models into perspective by categorizing them according to their space and time domain. Through this categorization process, we identified a dearth of models that consider processes occurring between the microscopic and macroscopic scale. We propose a "middle-out" strategy to develop the required full-scale, multilevel computational models. Exhaustive and accurate phenotyping, the use of the virtual patient concept, and the development of biomarkers from "-omics" signatures are identified as key elements of a successful systems biology modeling approach in nutrition research--one that integrates physiological mechanisms and data at multiple space and time scales.

Characterization of metabolic interrelationships and in silico phenotyping of lipoprotein particles using self-organizing maps

Plasma lipid concentrations cannot properly account for the complex interactions prevailing in lipoprotein (patho)physiology. Sequential ultracentrifugation (UCF) is the gold standard for physical lipoprotein isolations allowing for subsequent analyses of the molecular composition of the particles. Due to labor and cost issues, however, the UCF-based isolations are usually done only for VLDL, LDL, and HDL fractions; sometimes with the addition of intermediate density lipoprotein (IDL) particles and the fractionation of HDL into HDL(2) and HDL(3) (as done here; n = 302). We demonstrate via these data, with the lipoprotein lipid concentration and composition information combined, that the self-organizing map (SOM) analysis reveals a novel data-driven in silico phenotyping of lipoprotein metabolism beyond the experimentally available classifications. The SOM-based findings are biologically consistent with several well-known metabolic characteristics and also explain some apparent contradictions. The novelty is the inherent emergence of complex lipoprotein associations; e.g., the metabolic subgrouping of the associations between plasma LDL cholesterol concentrations and the structural subtypes of LDL particles. Importantly, lipoprotein concentrations cannot pinpoint lipoprotein phenotypes. It would generally be beneficial to computationally enhance the UCF-based lipoprotein data as illustrated here. Particularly, the compositional variations within the lipoprotein particles appear to be a fundamental issue with metabolic and clinical corollaries.

Plasma triglycerides and cardiovascular events in the Treating to New Targets and Incremental Decrease in End-Points through Aggressive Lipid Lowering trials of statins in patients with coronary artery disease

We determined the ability of in-trial measurements of triglycerides (TGs) to predict new cardiovascular events (CVEs) using data from the Incremental Decrease in End Points through Aggressive Lipid Lowering (IDEAL) and Treating to New Targets (TNT) trials. The trials compared atorvastatin 80 mg/day with moderate-dose statin therapy (simvastatin 20 to 40 mg/day in IDEAL and atorvastatin 10 mg/day in TNT) in patients with clinically evident coronary heart disease or a history of myocardial infarction. The outcome measurement in the present research was CVE occurring after the first year of the trial. After adjusting for age, gender, and study, risk of CVEs increased with increasing TGs (p <0.001 for trend across quintiles of TGs). Patients in the highest quintile had a 63% higher rate of CVEs than patients in the lowest quintile (hazard ratio 1.63, 95% confidence interval 1.46 to 1.81) and the relation of TGs to risk was apparent even within the normal range of TGs. The ability of TG measurements to predict risk decreased when high-density lipoprotein cholesterol and apolipoprotein B:apolipoprotein A-1 were included in the statistical analysis, and it was abolished with inclusion of further variables (diabetes, body mass index, glucose, hypertension, and smoking; (p = 0.044 and 0.621, respectively, for trend across quintiles of TGs). Similar results were obtained in patients in whom low-density lipoprotein cholesterol had been lowered to guideline-recommended levels. In conclusion, even slightly increased TG levels are associated with higher risk of recurrence of CVEs in statin-treated patients and should be considered a useful marker of risk.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

Overproduction of very low-density lipoproteins is the hallmark of the dyslipidemia in the metabolic syndrome

Insulin resistance is a key feature of the metabolic syndrome and often progresses to type 2 diabetes. Both insulin resistance and type 2 diabetes are characterized by dyslipidemia, which is an important and common risk factor for cardiovascular disease. Diabetic dyslipidemia is a cluster of potentially atherogenic lipid and lipoprotein abnormalities that are metabolically interrelated. Recent evidence suggests that a fundamental defect is an overproduction of large very low-density lipoprotein (VLDL) particles, which initiates a sequence of lipoprotein changes, resulting in higher levels of remnant particles, smaller LDL, and lower levels of high-density liporotein (HDL) cholesterol. These atherogenic lipid abnormalities precede the diagnosis of type 2 diabetes by several years, and it is thus important to elucidate the mechanisms involved in the overproduction of large VLDL particles. Here, we review the pathophysiology of VLDL biosynthesis and metabolism in the metabolic syndrome. We also review recent research investigating the relation between hepatic accumulation of lipids and insulin resistance, and sources of fatty acids for liver fat and VLDL biosynthesis. Finally, we briefly discuss current treatments for lipid management of dyslipidemia and potential future therapeutic targets.

Computational lipidology: predicting lipoprotein density profiles in human blood plasma

Monitoring cholesterol levels is strongly recommended to identify patients at risk for myocardial infarction. However, clinical markers beyond "bad" and "good" cholesterol are needed to precisely predict individual lipid disorders. Our work contributes to this aim by bringing together experiment and theory. We developed a novel computer-based model of the human plasma lipoprotein metabolism in order to simulate the blood lipid levels in high resolution. Instead of focusing on a few conventionally used predefined lipoprotein density classes (LDL, HDL), we consider the entire protein and lipid composition spectrum of individual lipoprotein complexes. Subsequently, their distribution over density (which equals the lipoprotein profile) is calculated. As our main results, we (i) successfully reproduced clinically measured lipoprotein profiles of healthy subjects; (ii) assigned lipoproteins to narrow density classes, named high-resolution density sub-fractions (hrDS), revealing heterogeneous lipoprotein distributions within the major lipoprotein classes; and (iii) present model-based predictions of changes in the lipoprotein distribution elicited by disorders in underlying molecular processes. In its present state, the model offers a platform for many future applications aimed at understanding the reasons for inter-individual variability, identifying new sub-fractions of potential clinical relevance and a patient-oriented diagnosis of the potential molecular causes for individual dyslipidemia.

Presecretory oxidation, aggregation, and autophagic destruction of apoprotein-B: a pathway for late-stage quality control

Hepatic secretion of apolipoprotein-B (apoB), the major protein of atherogenic lipoproteins, is regulated through posttranslational degradation. We reported a degradation pathway, post-ER pre secretory proteolysis (PERPP), that is increased by reactive oxygen species (ROS) generated within hepatocytes from dietary polyunsaturated fatty acids (PUFA). We now report the molecular processes by which PUFA-derived ROS regulate PERPP of apoB. ApoB exits the ER; undergoes limited oxidant-dependent aggregation; and then, upon exit from the Golgi, becomes extensively oxidized and converted into large aggregates. The aggregates slowly degrade by an autophagic process. None of the oxidized, aggregated material leaves cells, thereby preventing export of apoB-lipoproteins containing potentially toxic lipid peroxides. In summary, apoB secretory control via PERPP/autophagosomes is likely a key component of normal and pathologic regulation of plasma apoB levels, as well as a means for remarkably late-stage quality control of a secreted protein.

The assembly of apoB-containing lipoproteins: a structural biology point of view

Atherosclerosis is a widespread disease caused by the deposition of lipids on arterial walls. Such lipid plaques in coronary arteries can be fatal. Although many factors related to diet, life-style, etc. contribute to the worsening of the ailment, the primary cause, the lipids in the circulatory system, come from a series of low-density lipoproteins. These lipoproteins are necessary for the transport of lipids to and from different organs. It would be valuable to medicine and the field of drug design if a more detailed understanding of the organization of lipid and protein in these molecules were available. Unfortunately because of heterogeneity in their size and lipid composition, all classes of the low-density serum lipoproteins appear to be not amenable to the most widely used method for obtaining detailed atomic data - X-ray crystallography. However there appears to be a recently identified homolog that is relatively homogeneous, and crystal structures have been obtained. Used as a molecular model, the homolog serves as a source of conformational information that might help to unravel the processes involved in the lipid loading of the low-density lipoproteins. The review attempts to give a brief summary of the structural biology of the serum low-density lipoproteins relative to the molecular model of lipovitellin.

Extra Virgin Olive Oils Increase Hepatic Fat Accumulation and Hepatic Antioxidant Protein Levels in APOE-/- Mice

We assessed the effects of Picual and Arbequina olive oil, rich and poor in polyphenols, respectively, on plasma lipid and glucose metabolism, hepatic fat content, and the hepatic proteome in female Apoe-/- mice. Both olive oils increased hepatic fat content and adipophilin levels (p < 0.05), though Picual olive oil significantly decreased plasma triglycerides (p < 0.05). Proteomics identified a range of hepatic antioxidant enzymes that were differentially regulated by both olive oils as compared with palm oil. We found a clear association between olive oil consumption and differential regulation of adipophilin and betaine homocysteine methyl transferase as modulators of hepatic triglyceride metabolism. Therefore, our "systems biology" approach revealed hitherto unrecognized insights into the triglyceride-lowering and anti-atherogenic mechanisms of extra virgin olive oils, wherein the up-regulation of a large array of anti-oxidant enzymes may offer sufficient protection against lesion development and diminish oxidative stress levels instigated by hepatic steatosis.

Serum concentration of small dense low-density lipoprotein-cholesterol during oral glucose tolerance test and oral fat tolerance test

BACKGROUND

Small dense low-density lipoprotein (sdLDL) is well known as an atherogenic lipoprotein. We developed a new assay to measure serum concentration of sdLDL-cholesterol (sdLDLC). Using this assay, we reported a unique circadian rhythm of sdLDLC. We determined whether a glucose intake and/or a fat intake affects on serum sdLDLC concentration and determined the modulators of serum sdLDLC concentration.

METHODS

Ten healthy volunteers were recruited to perform both a 75 g oral glucose tolerance test (OGTT) and an oral fat tolerance test (OFTT) to determine the effects of glucose and fat ingestion separately. Blood was measured for sdLDLC concentration and other valuables.

RESULTS

Serum concentrations of total cholesterol, LDLC, remnant-like particles-cholesterol (RLPC), and apolipoprotein B significantly decreased during OGTT (p<0.05). SdLDLC also decreased and was a minimum at 2 h after glucose ingestion and increased to the baseline by 3 h. The sdLDLC decrease was seen while serum insulin level was high. The change of sdLDLC during OGTT had greater inverse correlationship with that of serum insulin level (r=-0.74, p<0.01) than that of plasma glucose level (r=-0.69, p=0.04). After fat ingestion, triglyceride and RLPC increased remarkably (p<0.01) but sdLDLC, LDLC, apolipoprotein B, and insulin did not change significantly.

CONCLUSIONS

Serum concentration of sdLDLC was not affected by a fat intake but by a glucose intake. The change of sdLDLC was associated by that of serum insulin level, suggesting that insulin can be one of the key modulator of serum sdLDLC level as well as LDL metabolism.

Metabolic syndrome pathophysiology: the role of adipose tissue

Several pathophysiological explanations for the metabolic syndrome have been proposed involving insulin resistance, chronic inflammation and ectopic fat accumulation following adipose tissue saturation. However, current concepts create several paradoxes, including limited cardiovascular risk reduction with intensive glucose control in diabetics, therapies that result in weight gain (PPAR agonists), and presence of some of the metabolic traits among some lipodystrophies. We propose the functional failure of an organ, in this case, the adipose tissue as a model to interpret its manifestations and to reconcile some of the apparent paradox. A cornerstone of this model is the failure of the adipose tissue to buffer postprandial lipids. In addition, homeostatic feedback loops guide physiological and pathological adipose tissue activities. Fat turnover is determined by a complex equilibrium in which insulin is a main factor but not the only one. Chronically inadequate energy balance may be a key factor, stressing the system. In this situation, an adipose tissue functional failure occurs resulting in changes in systemic energy delivery, impaired glucose consumption and activation of self-regulatory mechanisms that extend their influence to whole body homeostasis system. These include changes in adipokines secretion and vascular effects. The functional capacity of the adipose tissue varies among subjects explaining the incomplete overlapping among the metabolic syndrome and obesity. Variations at multiple gene loci will be partially responsible for these interindividual differences. Two of those candidate genes, the adiponectin (APM1) and the perilipin (PLIN) genes, are discussed in more detail.

Hepatic ABCG5/G8 overexpression reduces apoB-lipoproteins and atherosclerosis when cholesterol absorption is inhibited

We previously reported that liver-specific overexpression of ABCG5/G8 in mice is not atheroprotective, suggesting that increased biliary cholesterol secretion must be coupled with decreased intestinal cholesterol absorption to increase net sterol loss from the body and reduce atherosclerosis. To evaluate this hypothesis, we fed low density lipoprotein receptor-knockout (LDLr-KO) control and ABCG5/G8-transgenic (ABCG5/G8-Tg)xLDLr-KO mice, which overexpress ABCG5/G8 only in liver, a Western diet containing ezetimibe to reduce intestinal cholesterol absorption. On this dietary regimen, liver-specific ABCG5/G8 overexpression increased hepatobiliary cholesterol concentration and secretion rates (1.5-fold and 1.9-fold, respectively), resulting in 1.6-fold increased fecal cholesterol excretion, decreased hepatic cholesterol, and increased (4.4-fold) de novo hepatic cholesterol synthesis versus LDLr-KO mice. Plasma lipids decreased (total cholesterol, 32%; cholesteryl ester, 32%; free cholesterol, 30%), mostly as a result of reduced non-high density lipoprotein-cholesterol and apolipoprotein B (apoB; 36% and 25%, respectively). ApoB-containing lipoproteins were smaller and lipid-depleted in ABCG5/G8-TgxLDLr-KO mice. Kinetic studies revealed similar 125I-apoB intermediate density lipoprotein/LDL fractional catabolic rates, but apoB production rates were decreased 37% in ABCG5/G8-TgxLDLr-KO mice. Proximal aortic atherosclerosis decreased by 52% (male) and 59% (female) in ABCG5/G8-TgxLDLr-KO versus LDLr-KO mice fed the Western/ezetimibe diet. Thus, increased biliary secretion, resulting from hepatic ABCG5/G8 overexpression, reduces atherogenic risk in LDLr-KO mice fed a Western diet containing ezetimibe. These findings identify distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism and atherosclerosis.

Studying apolipoprotein turnover with stable isotope tracers: correct analysis is by modeling enrichments

Lipoprotein kinetic parameters are determined from mass spectrometry data after administering mass isotopes of amino acids, which label proteins endogenously. The standard procedure is to model the isotopic content of the labeled precursor amino acid and of proteins of interest as tracer-to-tracee ratio (TTR). It is shown here that even though the administered tracer alters amino acid mass and turnover, apolipoprotein synthesis is unaltered and hence the apolipoprotein system is in a steady state, with the total (labeled plus unlabeled) masses and fluxes remaining constant. The correct model formulation for apolipoprotein kinetics is shown to be in terms of tracer enrichment, not of TTR. The needed mathematical equations are derived. A theoretical error analysis is carried out to calculate the magnitude of error in published results using TTR modeling. It is shown that TTR modeling leads to a consistent underestimation of the fractional synthetic rate. In constant-infusion studies, the bias error percent is shown to equal approximately the plateau enrichment, generally <10%. It is shown that, in bolus studies, the underestimation error can be larger. Thus, for mass isotope studies with endogenous tracers, apolipoproteins are in a steady state and the data should be fitted by modeling enrichments.

Thematic review series: Patient-Oriented Research. Design and analysis of lipoprotein tracer kinetics studies in humans

Lipoprotein tracer kinetics studies have for many years provided new and important knowledge of the metabolism of lipoproteins. Our understanding of kinetics defects in lipoprotein metabolism has resulted from the use of tracer kinetics studies and mathematical modeling. This review discusses all aspects of the performance of kinetics studies, including the development of hypotheses, experimental design, statistical considerations, tracer administration and sampling schedule, and the development of compartmental models for the interpretation of tracer data. In addition to providing insight into new metabolic pathways, such models provide quantitative information on the effect of interventions on lipoprotein metabolism. Compartment models are useful tools to describe experimental data but can also be used to aid in experimental design and hypothesis generation. The SAAM II program provides an easy-to-use interface with which to develop and test compartmental models against experimental models. The development of a model requires that certain checks be performed to ensure that the model describes the experimental data and that the model parameters can be estimated with precision. In addition to methodologic aspects, several compartment models of apoprotein and lipid metabolism are reviewed.