Metabolism of apolipoprotein B in large triglyceride-rich very low density lipoproteins of normal and hypertriglyceridemic subjects

Measuring VLDL1 secretion in humans with an intravenous fat emulsion test

Tracer techniques to assess very-low-density lipoprotein (VLDL) secretion in humans are expensive, are time consuming, and require mathematical models to estimate VLDL kinetics. Here, we describe an alternative, time- and cost-efficient protocol to directly determine VLDL1 secretion with an intravenous (i.v.) lipid emulsion test that does not require tracers and compartmental modeling. We describe steps for intralipid infusion, blood sampling, and removal of intralipid from plasma samples, followed by density gradient ultracentrifugation to isolate VLDL1 fraction and measure the secretion rate. For complete details on the use and execution of this protocol, please refer to Bjorkegren et al. (1996),1 Al-Shayji et al. (2007),2 and Metz et al. (2022).3.

Leptin increases hepatic triglyceride export via a vagal mechanism in humans

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

Metabolism of triglyceride-rich lipoproteins in health and dyslipidaemia

Accumulating evidence points to the causal role of triglyceride-rich lipoproteins and their cholesterol-enriched remnants in atherogenesis. Genetic studies in particular have not only revealed a relationship between plasma triglyceride levels and the risk of atherosclerotic cardiovascular disease, but have also identified key proteins responsible for the regulation of triglyceride transport. Kinetic studies in humans using stable isotope tracers have been especially useful in delineating the function of these proteins and revealing the hitherto unappreciated complexity of triglyceride-rich lipoprotein metabolism. Given that triglyceride is an essential energy source for mammals, triglyceride transport is regulated by numerous mechanisms that balance availability with the energy demands of the body. Ongoing investigations are focused on determining the consequences of dysregulation as a result of either dietary imprudence or genetic variation that increases the risk of atherosclerosis and pancreatitis. The identification of molecular control mechanisms involved in triglyceride metabolism has laid the groundwork for a 'precision-medicine' approach to therapy. Novel pharmacological agents under development have specific molecular targets within a regulatory framework, and their deployment heralds a new era in lipid-lowering-mediated prevention of disease. In this Review, we outline what is known about the dysregulation of triglyceride transport in human hypertriglyceridaemia.

Hepatitis C Virus Uses Host Lipids to Its Own Advantage

Lipids and lipoproteins constitute indispensable components for living not only for humans. In the case of hepatitis C virus (HCV), the option of using the products of our lipid metabolism is “to be, or not to be”. On the other hand, HCV infection, which is the main cause of chronic hepatitis, cirrhosis and hepatocellular carcinoma, exerts a profound influence on lipid and lipoprotein metabolism of the host. The consequences of this alternation are frequently observed as hypolipidemia and hepatic steatosis in chronic hepatitis C (CHC) patients. The clinical relevance of these changes reflects the fact that lipids and lipoprotein play a crucial role in all steps of the life cycle of HCV. The virus circulates in the bloodstream as a highly lipidated lipo-viral particle (LVP) that defines HCV hepatotropism. Thus, strict relationships between lipids/lipoproteins and HCV are indispensable for the mechanism of viral entry into hepatocytes, viral replication, viral particles assembly and secretion. The purpose of this review is to summarize the tricks thanks to which HCV utilizes host lipid metabolism to its own advantage.

Apolipoprotein B48 metabolism in chylomicrons and very low-density lipoproteins and its role in triglyceride transport in normo- and hypertriglyceridemic human subjects

BACKGROUND

Renewed interest in triglyceride-rich lipoproteins as causative agents in cardiovascular disease mandates further exploration of the integrated metabolism of chylomicrons and very low-density lipoproteins (VLDL).

METHODS

Novel tracer techniques and an integrated multi-compartmental model were used to determine the kinetics of apoB48- and apoB100-containing particles in the chylomicron and VLDL density intervals in 15 subjects with a wide range of plasma triglyceride levels.

RESULTS

Following a fat-rich meal, apoB48 appeared in the chylomicron, VLDL₁ and VLDL₂ fractions in all subjects. Chylomicrons cleared rapidly from the circulation but apoB48-containing VLDL accumulated, and over the day were 3-fold higher in those with high versus low plasma triglyceride. ApoB48-containing particles were secreted directly into both the chylomicron and VLDL fractions at rates that were similar across the plasma triglyceride range studied. During fat absorption, whilst most triglyceride entered the circulation in chylomicrons, the majority of apoB48 particles were secreted into the VLDL density range.

CONCLUSION

The intestine secretes apoB48-containing particles not only as chylomicrons but also directly into the VLDL₁ and VLDL₂ density ranges both in the basal state and during dietary lipid absorption. Over the day, apoB48-containing particles appear to comprise about 20-25% of circulating VLDL and, especially in those with elevated triglycerides, form part of a slowly cleared 'remnant' particle population, thereby potentially increasing CHD risk. These findings provide a metabolic understanding of the potential consequences for increased CHD risk when slowed lipolysis leads to the accumulation of remnants, especially in individuals with hypertriglyceridemia.

Relative effect of hypertriglyceridemia on non-HDLC and apolipoprotein B as cardiovascular disease risk markers

BACKGROUND

Non-high density lipoprotein cholesterol (non-HDLC) represents the cholesterol in triglyceride-rich lipoproteins (TRL) and low-density lipoproteins (LDL). Apolipoprotein B (apoB) reflects the number of TRL and LDL particles. In hypertriglyceridemia (HTG), there is triglyceride (TG) enrichment of TRLs, and also a substantial increase of cholesterol in larger TRLs that considerably augments the non-HDLC value. Therefore, in HTG, non-HDLC could increase disproportionately with respect to apoB.

OBJECTIVE

We aimed to compare the relative effect of the full range of mild, moderate, and severe HTG on the status of non-HDLC and apoB as cardiovascular disease (CVD) risk markers.

METHODS

Analysis of lipid profile data from 4347 patients in a Lipid Clinic cohort with baseline fasting lipid profiles documented prior to starting lipid-lowering medications. The correlation between non-HDLC and apoB was assessed in intervals of increasing TG. Non-HDLC and apoB were analyzed at each TG level using comparative CVD risk equivalent categories and assessed for divergence and discordance.

RESULTS

With increasing TG levels: (1) the correlation between non-HDLC and apoB diminished progressively, (2) non-HDLC levels increased continuously, whereas apoB levels plateaued after an initial increase up to TG of ~ 4.0-5.0 mmol/L (~354-443 mg/dL), (3) there was divergence in the stratification of non-HDLC and apoB into CVD risk equivalent categories.

CONCLUSIONS

Non-HDLC and apoB should not be viewed as interchangeable CVD risk markers in the presence of severe HTG. This has never been tested. With increasing HTG severity, discordance between non-HDLC and apoB can cause clinically important divergence in CVD risk categorization.

Calculated Non-HDL Cholesterol Includes Cholesterol in Larger Triglyceride-Rich Lipoproteins in Hypertriglyceridemia

Context

Calculated non–high-density lipoprotein (HDL) cholesterol (non-HDLC) should selectively include cholesterol from atherogenic lipoproteins to be a reliable risk marker of cardiovascular disease. In hypertriglyceridemia (HTG), there is increased abundance of larger and less atherogenic triglyceride-rich lipoproteins (TRL), namely, larger very-low-density lipoproteins (VLDL), and chylomicrons.

Objective

We aim to demonstrate that serum triglyceride (TG) level has a substantial impact on non-HDLC’s ability to represent cholesterol from atherogenic lipoproteins, even though TG is not part of the calculation for non-HDLC.

Design

Analysis of lipid profile data

Settings

Lipid Clinic patient cohort, and Biochemistry Laboratory patient cohort

Patients or Other Participants

7,492 patients in the Lipid Clinic cohort with baseline lipid profiles documented prior to starting lipid-lowering medications and 156,311 lipid profiles from The Ottawa Hospital Biochemistry Laboratory cohort.

Intervention

None

Main Outcome Measure

Our modeling process includes derivation of TG-interval–specific lipoprotein composition factor (LCF) for TRL, which represents the mass ratio of cholesterol to TG in TRL. A high LCF indicates that the TRLs are mainly the cholesterol-rich atherogenic remnant lipoproteins. A low LCF indicates that the TRLs are mainly the TG-rich larger VLDL and chylomicrons.

Results

As serum TG increases, there is progressive decline in the LCF for TRL, which indicates that the calculated non-HDLC level reflects progressive inclusion of cholesterol from larger TRL. This is shown in both cohorts.

Conclusions

Calculated non-HDLC is influenced by TG level. As TG increases, non-HDLC gradually includes more cholesterol from larger TRL, which are less atherogenic than LDL and remnant lipoproteins.

Triglycerides and triglyceride-rich lipoproteins in the development and progression of atherosclerosis

PURPOSE OF REVIEW

In this review, we intend to show the heterogenicity of the triglyceride group, including the triglyceride-rich lipoproteins and its subparticles, apolipoproteins, and its role in atherogenesis through epidemiological and genetic studies, observing the association of these various components and subclasses with subclinical atherosclerosis and cardiovascular events. Also, we reevaluated the moment of blood collection for the triglyceride measurement and its repercussion in atherosclerosis. Finally, we present the current scenario and new insights about the pharmacologic treatment of hypertriglyceridemia.

RECENT FINDINGS

Recent studies have been observed, a correlation between cardiovascular disease and triglyceride components (as apolipoproteins A-V, C-I, C-III) as well as proteins involved in the metabolism pathway, such as the angiopoietin-like proteins. Also, the triglyceride-rich lipoproteins, also known as remnants, were recently associated with atherogenesis. Another important topic addressed is about nonfasting triglyceride level, which has been postulated as a better predictor of cardiovascular events than fasting collection.

SUMMARY

Regarding hypertriglyceridemia treatment, the drug therapy was updated, as the omega-3 polyunsaturated fatty acids were tested in primary prevention as eicosapentaenoic acid and docosahexaenoic acid combination resulted in no benefit, whereas the administration of icosapent ethyl in secondary prevention and high-risk patients showed a robust decrease of the cardiovascular outcomes.

N-3 polyunsaturated fatty acids improve lipoprotein particle size and concentration in Japanese patients with type 2 diabetes and hypertriglyceridemia: a pilot study

Background

Patients with type 2 diabetes are at high risk for cardiovascular disease. Although hydroxymethylglutaryl-CoA reductase inhibitors (statins) can reduce cardiovascular events, residual risk remains even after target low-density lipoprotein cholesterol (LDL-C) levels have been achieved. Lipoprotein particle size and fraction changes are thought to contribute to such risks. The purpose of this study was to evaluate the effects of n-3 polyunsaturated fatty acids (n-3 PUFAs), predominantly eicosapentaenoic acid and docosahexaenoic acid, on lipoprotein particle size, concentration, and glycemic control in Japanese patients with type 2 diabetes and hypertriglyceridemia.

Methods

This was a multicenter, prospective, open-label, single arm study. We enrolled 14 patients with type 2 diabetes and hypertriglyceridemia treated with statins and dipeptidyl peptidase-4 inhibitors with glycated hemoglobin (HbA1c) < 8.0%, LDL-C < 120 mg/dL, and fasting triglyceride ≥150 mg/dL. After a 12-week observation period, they were treated with 4 g/day n-3 PUFAs for 12 weeks. Lipoprotein particle sizes, concentrations, lipoprotein insulin resistance (LPIR) scores, lipid profiles, HbA1c, and fasting plasma glucose (FPG) were measured before and after treatment. Lipoprotein profiles were measured by nuclear magnetic resonance spectroscopy. Data were analyzed using Wilcoxon signed-rank tests.

Results

Concentrations of total cholesterol (P < 0.001), LDL-C (P = 0.003), and triglyceride (P < 0.001) decreased following n-3 PUFA administration. N-3 PUFAs decreased the size of very low-density lipoprotein (VLDL; P < 0.001) particles, but did not affect LDL or high-density lipoprotein (HDL) particles. The concentration of large LDL increased, whereas small LDL decreased, causing the large to small LDL ratio to increase significantly (P = 0.042). Large VLDL and chylomicron concentrations significantly decreased, as did the large to small VLDL ratio (all P < 0.001). FPG levels unchanged, whereas HbA1c levels slightly increased. LPIR scores improved significantly (P = 0.001).

Conclusions

N-3 PUFAs partly improved atherogenic lipoprotein particle size and concentration, and produced less atherogenic lipoprotein subclass ratios in patients that achieved target LDL-C levels and glycemic control. These results suggest that n-3 PUFAs may reduce residual cardiovascular risk factors in statin-treated patients with type 2 diabetes and hypertriglyceridemia.

Trial registration

The study was registered at UMIN-ID: UMIN000013776.

Electronic supplementary material

The online version of this article (10.1186/s12944-018-0706-8) contains supplementary material, which is available to authorized users.

CETP (Cholesteryl Ester Transfer Protein) Inhibition With Anacetrapib Decreases Production of Lipoprotein(a) in Mildly Hypercholesterolemic Subjects

OBJECTIVE

Lp(a) [lipoprotein (a)] is composed of apoB (apolipoprotein B) and apo(a) [apolipoprotein (a)] and is an independent risk factor for cardiovascular disease and aortic stenosis. In clinical trials, anacetrapib, a CETP (cholesteryl ester transfer protein) inhibitor, causes significant reductions in plasma Lp(a) levels. We conducted an exploratory study to examine the mechanism for Lp(a) lowering by anacetrapib.

APPROACH AND RESULTS

We enrolled 39 participants in a fixed-sequence, double-blind study of the effects of anacetrapib on the metabolism of apoB and high-density lipoproteins. Twenty-nine patients were randomized to atorvastatin 20 mg/d, plus placebo for 4 weeks, and then atorvastatin plus anacetrapib (100 mg/d) for 8 weeks. The other 10 subjects were randomized to double placebo for 4 weeks followed by placebo plus anacetrapib for 8 weeks. We examined the mechanisms of Lp(a) lowering in a subset of 12 subjects having both Lp(a) levels >20 nmol/L and more than a 15% reduction in Lp(a) by the end of anacetrapib treatment. We performed stable isotope kinetic studies using ²H₃-leucine at the end of each treatment to measure apo(a) fractional catabolic rate and production rate. Median baseline Lp(a) levels were 21.5 nmol/L (interquartile range, 9.9-108.1 nmol/L) in the complete cohort (39 subjects) and 52.9 nmol/L (interquartile range, 38.4-121.3 nmol/L) in the subset selected for kinetic studies. Anacetrapib treatment lowered Lp(a) by 34.1% (P≤0.001) and 39.6% in the complete and subset cohort, respectively. The decreases in Lp(a) levels were because of a 41% reduction in the apo(a) production rate, with no effects on apo(a) fractional catabolic rate.

CONCLUSIONS

Anacetrapib reduces Lp(a) levels by decreasing its production.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00990808.

Some dietary factors can modulate the effect of the zinc transporters 8 polymorphism on the risk of metabolic syndrome

There are conflicting data on the impact of zinc transporter 8 (ZNT8) gene variations on the metabolic syndrome (MetS). Hence, the effects of the interaction between rs13266634 and dietary factors on the risk of MetS were investigated in this study. Subjects of this nested case-control study were selected from the participants in Tehran Lipid and Glucose Study. Each of the cases (n = 817) was individually matched with a control. Dietary patterns were determined using factor analysis. The ZNT8 rs13266634 were genotyped by the Tetra-refractory mutation system-polymerase chain reaction analysis. Two dietary patterns were extracted. There were no significant interactions between the ZNT8 SNP and the dietary patterns on the risk of MetS or its components. An interaction was observed between rs13266634 and the omega-3 fatty acid intakes on the risk of MetS in subjects with the CC genotype (P interaction < 0.01). Zinc modified the association of the ZNT8 variant with high fasting blood sugar (P interaction = 0.05) in CC genotype carriers. An interaction was also observed between rs13266634 and salty snacks at the risk of abdominal obesity (P interaction < 0.05). Our findings suggest an interaction between omega-3 fatty acids, zinc, salty snacks and rs13266634, which may affect the risk of MetS or its components.

Effects of PCSK9 Inhibition With Alirocumab on Lipoprotein Metabolism in Healthy Humans

Supplemental Digital Content is available in the text.

Background:

Alirocumab, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9), lowers plasma low-density lipoprotein (LDL) cholesterol and apolipoprotein B100 (apoB). Although studies in mice and cells have identified increased hepatic LDL receptors as the basis for LDL lowering by PCSK9 inhibitors, there have been no human studies characterizing the effects of PCSK9 inhibitors on lipoprotein metabolism. In particular, it is not known whether inhibition of PCSK9 has any effects on very low-density lipoprotein or intermediate-density lipoprotein (IDL) metabolism. Inhibition of PCSK9 also results in reductions of plasma lipoprotein (a) levels. The regulation of plasma Lp(a) levels, including the role of LDL receptors in the clearance of Lp(a), is poorly defined, and no mechanistic studies of the Lp(a) lowering by alirocumab in humans have been published to date.

Methods:

Eighteen (10 F, 8 mol/L) participants completed a placebo-controlled, 2-period study. They received 2 doses of placebo, 2 weeks apart, followed by 5 doses of 150 mg of alirocumab, 2 weeks apart. At the end of each period, fractional clearance rates (FCRs) and production rates (PRs) of apoB and apo(a) were determined. In 10 participants, postprandial triglycerides and apoB48 levels were measured.

Results:

Alirocumab reduced ultracentrifugally isolated LDL-C by 55.1%, LDL-apoB by 56.3%, and plasma Lp(a) by 18.7%. The fall in LDL-apoB was caused by an 80.4% increase in LDL-apoB FCR and a 23.9% reduction in LDL-apoB PR. The latter was due to a 46.1% increase in IDL-apoB FCR coupled with a 27.2% decrease in conversion of IDL to LDL. The FCR of apo(a) tended to increase (24.6%) without any change in apo(a) PR. Alirocumab had no effects on FCRs or PRs of very low-density lipoproteins-apoB and very low-density lipoproteins triglycerides or on postprandial plasma triglycerides or apoB48 concentrations.

Conclusions:

Alirocumab decreased LDL-C and LDL-apoB by increasing IDL- and LDL-apoB FCRs and decreasing LDL-apoB PR. These results are consistent with increases in LDL receptors available to clear IDL and LDL from blood during PCSK9 inhibition. The increase in apo(a) FCR during alirocumab treatment suggests that increased LDL receptors may also play a role in the reduction of plasma Lp(a).

Clinical Trial Registration:

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01959971.

Hepatitis C virus G1b infection decreases the number of small low-density lipoprotein particles

AIM: To investigate how hepatitis C virus (HCV) G1b infection influences the particle number of lipoproteins.

METHODS: The numbers of lipoprotein particles in fasting sera from 173 Japanese subjects, 82 with active HCV G1b infection (active HCV group) and 91 with cleared HCV infection (SVR group), were examined. Serum lipoprotein was fractionated by high-performance liquid chromatography into twenty fractions. The cholesterol and triglyceride concentrations in each fraction were measured using LipoSEARCH. The number of lipoprotein particles in each fraction was calculated using a newly developed algorithm, and the relationship between chronic HCV G1b infection and the lipoprotein particle number was determined by multiple linear regression analysis.

RESULTS: The median number of low-density lipoprotein (LDL) particles was significantly lower in the active HCV group [1182 nmol/L, interquartile range (IQR): 444 nmol/L] than in the SVR group (1363 nmol/L, IQR: 472 nmol/L, P < 0.001), as was that of high-density lipoprotein (HDL) particles (14168 nmol/L vs 15054 nmol/L, IQR: 4114 nmol/L vs 3385 nmol/L, P = 0.042). The number of very low-density lipoprotein (VLDL) particles was similar between the two groups. Among the four LDL sub-fractions, the number of large LDL particles was similar between the two groups. However, the numbers of medium (median: 533.0 nmol/L, IQR: 214.7 nmol/L vs median: 633.5 nmol/L, IQR: 229.6 nmol/L, P < 0.001), small (median: 190.9 nmol/L, IQR: 152.4 nmol/L vs median: 263.2 nmol/L, IQR: 159.9 nmol/L; P < 0.001), and very small LDL particles (median: 103.5 nmol/L, IQR: 66.8 nmol/L vs median: 139.3 nmol/L, IQR: 67.3 nmol/L, P < 0.001) were significantly lower in the active HCV group than in the SVR group, respectively. Multiple linear regression analysis indicated an association between HCV G1b infection and the decreased numbers of medium, small, and very small LDL particles. However, active HCV infection did not affect the number of large LDL particles or any sub-fractions of VLDL and HDL particles.

CONCLUSION: HCV G1b infection decreases the numbers of medium, small, and very small LDL particles.

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

Apolipoprotein B-100-containing lipoprotein metabolism in subjects with lipoprotein lipase gene mutations

OBJECTIVE

We investigated the impact of lipoprotein lipase (LPL) gene mutations on apolipoprotein B (apoB)-100 metabolism.

METHODS AND RESULTS

We studied 3 subjects with familial LPL deficiency; 14 subjects heterozygous for the LPL gene mutations Gly188Glu, Trp64Stop, and Ile194Thr; and 10 control subjects. Very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL)-apoB-100 kinetics were determined in the fed state using stable isotope methods and compartmental modeling. Compared with controls, familial LPL deficiency had markedly elevated plasma triglycerides and lower VLDL-apoB-100 fractional catabolic rate (FCR), IDL-apoB-100 FCR, VLDL-to-IDL conversion, and VLDL-apoB-100 production rate (P<0.01). Compared with controls, Gly188Glu had higher plasma triglyceride and VLDL- and IDL-apoB-100 concentrations and lower VLDL- and IDL-apoB-100 FCR (P<0.05). Plasma triglycerides were not different, but IDL-apoB-100 concentration and production rate and VLDL-to-IDL conversion were lower in Trp64Stop compared with controls (P<0.05). No differences between controls and Ile194Thr were observed.

CONCLUSIONS

Our results confirm that hypertriglyceridemia is a key feature of familial LPL deficiency. This is due to impaired VLDL- and IDL-apoB-100 catabolism and VLDL-to-IDL conversion. Single-allele mutations of the LPL gene result in modest to elevated plasma triglycerides. The changes in plasma triglycerides and apoB-100 kinetics are attributable to the effects of the LPL genotype.

HCV and the hepatic lipid pathway as a potential treatment target

Atherosclerosis has been described as a liver disease of the heart [1]. The liver is the central regulatory organ of lipid pathways but since dyslipidaemias are major contributors to cardiovascular disease and type 2 diabetes rather than liver disease, research in this area has not been a major focus for hepatologists. Virus-host interaction is a continuous co-evolutionary process [2] involving the host immune system and viral escape mechanisms [3]. One of the strategies HCV has adopted to escape immune clearance and establish persistent infection is to make use of hepatic lipid pathways. This review aims to: • update the hepatologist on lipid metabolism • review the evidence that HCV exploits hepatic lipid pathways to its advantage • discuss approaches to targeting host lipid pathways as adjunctive therapy.

Reduction in systemic and VLDL triacylglycerol concentration after a 3-month Mediterranean-style diet in high-cardiovascular-risk subjects

The first results of the PREDIMED (PREvencion con Dieta MEDiterranea) randomized trial, after 3-month intervention, showed that the Mediterranean Diet (MD), supplemented with either virgin olive oil (VOO) or nuts, reduced systolic blood pressure, serum cholesterol and triacylglycerol (TG) concentrations and increased high-density lipoprotein (HDL)-cholesterol when compared to a control (low-fat diet) group. Serum TG levels are an independent risk factor for coronary heart disease and are strongly determined by very low-density lipoprotein (VLDL) composition, which can be specifically modified by dietary lipid source. Within the context of the PREDIMED study, we assessed the VLDL composition in 50 participants after 3 months of intake of two MD, supplemented with VOO or nuts, compared with a low-fat diet. Total and low-density lipoprotein cholesterol concentrations were reduced in subjects on the MD+nuts, whereas HDL-cholesterol increased after consumption of the MD+VOO. Serum TG concentrations were significantly lowered in both intervention groups (either the MD+nuts or MD+VOO). However, only the MD+VOO reduced the VLDL-cholesterol and VLDL-TG content and the TG/apolipoprotein B ratio in VLDL, which was used to estimate particle size. Although VLDL-TG fatty acids were very slightly modified, VLDL-TG molecular species in VLDL after consumption of the MD+nuts were characterized by a higher presence of linoleic acid (18:2, n-6), whereas after the intake of MD+VOO, they were rich in oleic acid (18:1, n-9). Therefore, we conclude that the reduction in systemic TG concentrations observed after consumption of the MD may be explained by reduction of the lipid core of VLDL and a selective modification of the molecular species composition in the particle.

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.

Improved cholesterol phenotype analysis by a model relating lipoprotein life cycle processes to particle size

Increased plasma cholesterol is a known risk factor for cardiovascular disease. Lipoprotein particles transport both cholesterol and triglycerides through the blood. It is thought that the size distribution of these particles codetermines cardiovascular disease risk. New types of measurements can determine the concentration of many lipoprotein size-classes but exactly how each small class relates to disease risk is difficult to clear up. Because relating physiological process status to disease risk seems promising, we propose investigating how lipoprotein production, lipolysis, and uptake processes depend on particle size. To do this, we introduced a novel model framework (Particle Profiler) and evaluated its feasibility. The framework was tested using existing stable isotope flux data. The model framework implementation we present here reproduced the flux data and derived lipoprotein size pattern changes that corresponded to measured changes. It also sensitively indicated changes in lipoprotein metabolism between patient groups that are biologically plausible. Finally, the model was able to reproduce the cholesterol and triglyceride phenotype of known genetic diseases like familial hypercholesterolemia and familial hyperchylomicronemia. In the future, Particle Profiler can be applied for analyzing detailed lipoprotein size profile data and deriving rates of various lipolysis and uptake processes if an independent production estimate is given.

High-carbohydrate diets affect the size and composition of plasma lipoproteins in hamsters (Mesocricetus auratus)

High-carbohydrate diets reduce plasma low-density lipoprotein (LDL)-cholesterol but also provoke the appearance of an atherogenic lipoprotein profile (ALP). Characterized by high plasma triglyceride, small dense LDL, and reduced high-density lipoprotein (HDL) cholesterol, an ALP is associated with insulin resistance. Despite extensive use of the fructose-fed hamster as a model of insulin resistance, little is known about changes that occur in the physical properties of circulating lipoproteins. Therefore, we investigated the metabolic and physical properties of lipoproteins in hamsters fed high-carbohydrate diets of varying complexity (60% carbohydrate as chow, cornstarch, or fructose) for 2 wk. Hamsters fed the high-fructose diet showed significantly increased very- low-density lipoprotein (VLDL)-triglyceride (92.3%), free cholesterol (68.6%), and phospholipid (95%), whereas apolipoprotein B levels remained unchanged. Median diameter of circulating VLDL was larger in fructose-fed hamsters (63 nm) than in cornstarch-fed hamsters. Fructose feeding induced a 42.5% increase LDL-triglyceride concurrent with a 20% reduction in LDL-cholesteryl ester. Compositional changes were associated with reduced LDL diameter. In contrast, fructose feeding caused elevations in all HDL fractions. The physical properties of apolipoprotein-B-containing lipoprotein fractions are similar between fructose-fed hamsters and humans with ALP. However, metabolism of high-density lipoprotein appears to differ in the 2 species.

Distinct patterns of heparin affinity chromatography VLDL1 and VLDL2 subfractions in the different dyslipidaemias

Very low density lipoprotein (VLDL) 1 and 2 were fractionated by heparin affinity chromatography into a bound and an unbound fraction and the different subfractions were quantified in 17 normolipidaemic (NL), 13 hypercholesterolaemic (HC), 10 hypertriglyceridaemic (HTG) and 11 combined hyperlipidaemic subjects (CHL). Unbound VLDL1 and VLDL2 were, respectively, 1.9- and 2.2-fold richer in triglycerides than bound VLDL1 and VLDL2. In HTG and CHL the concentration of all the VLDL subfractions was increased and plasma triglyceride level was correlated to unbound VLDL1 and to bound VLDL1 (respectively, r=0.86 (p<0.001) and r=0.77 (p<0.01) in HTG and r=0.73 (p<0.001) and r=0.62 (p<0.05) in CHL). In HC unbound VLDL2 and bound VLDL2 concentration were increased compared to NL and in CHL, the concentration of bound VLDL2 was particularly increased (3.2-fold compared to NL (p<0.001)). In both HC and CHL bound VLDL2 concentration was correlated to low density lipoprotein cholesterol (LDL-C) concentration (respectively, r=0.67 (p<0.01) and r=0.62 (p<0.05)). In hypertriglyceridaemic states the intravascular accumulation of both unbound and bound VLDL1 appears as the determinant of plasma triglyceride concentration, whereas in moderately hypercholesterolaemic states the concentration of bound VLDL2 is strikingly correlated to LDL-C concentration, suggesting that these two species are linked metabolically, e.g. bound VLDL2 contain the precursor pool of LDL.

Removal of triacylglycerols from chylomicrons and VLDL by capillary beds: the basis of lipoprotein remnant formation

The triacylglycerol content of chylomicrons and VLDL (very-low-density lipoprotein) compete for the same lipolytic pathway in the capillary beds. Although chylomicron triacylglycerols appear to be the favoured substrate for lipoprotein lipase, VLDL particles compete in numbers. Methods to quantify the specific triacylglycerol removal from VLDL and chylomicrons may involve endogenous labelling of the triacylglycerol substrate with stable isotopes in combination with arteriovenous blood sampling in humans. Arteriovenous quantification of remnant lipoproteins suggests that adipose tissue with its high lipoprotein lipase activity is a principal site for generation of remnant lipoproteins. Under circumstances of reduced efficiency in the removal of triacylglycerols from lipoproteins, there is accumulation of remnant lipoproteins, which are potentially atherogenic.

Lipoprotein metabolism in chronic renal insufficiency

Chronic renal insufficiency (CRI) is associated with a characteristic dyslipidemia. Findings in children with CRI largely parallel those in adults. Moderate hypertriglyceridemia, increased triglyceride-rich lipoproteins (TRL) and reduced high-density lipoproteins (HDL) are the most usual findings, whereas total and low-density lipoprotein cholesterol (LDL-C) remain normal or modestly increased. Qualitative abnormalities in lipoproteins are common, including small dense LDL, oxidized LDL, and cholesterol-enriched TRL. Measures of lipoprotein lipase and hepatic lipase activity are reduced, and concentrations of apolipoprotein C-III are markedly elevated. Still an active area of research, major pathophysiological mechanisms leading to the dyslipidemia of CRI include insulin resistance and nonnephrotic proteinuria. Sources of variability in the severity of this dyslipidemia include the degree of renal impairment and the modality of dialysis. The benefits of maintaining normal body weight and physical activity extend to those with CRI. In addition to multiple hypolipidemic pharmaceuticals, fish oils are also effective as a triglyceride-lowering agent, and the phosphorous binding agent sevelamer also lowers LDL-C. Emerging classes of hypolipidemic agents and drugs affecting sensitivity to insulin may impact future treatment. Unfortunately, cardiovascular benefit has not been convincingly demonstrated by any trial designed to study adults or children with renal disease. Therefore, it is not possible at this time to endorse general recommendations for the use of any agent to treat dyslipidemia in children with chronic kidney disease.

Differential modulation of hepatic very low-density lipoprotein secretion by triacylglycerol-rich lipoproteins derived from different oleic-acid rich dietary oils

Minor components from dietary oils can modulate the atherogenic response of the TAG-rich lipoproteins (TRL) in which they are transported. In the present study we investigated the influence of TRL isolated from man after the intake of oleic acid-rich oils with different minor component compositions on VLDL secretion by rat primary hepatocytes. TRL were isolated from nine men after the intake of meals enriched with high-oleic sunflower oil (HOSO) or virgin olive oil (VOO) or VOO enriched with minor components (EVO). TRL were incubated with rat primary hepatocytes and the lipid accumulation was analysed in the cells and the secreted VLDL. The expression of genes for proteins related to hepatic lipid metabolism and VLDL production was also measured. Incubation of hepatocytes with TRL derived from HOSO as compared to VOO led to lower intracellular lipid accumulation and VLDL production despite higher mRNA expression for diacylglycerol-acyltransferase, microsomal TAG transfer protein, apoB and PPARalpha. When TRL derived from EVO were used there were no changes in VLDL secretion. These results suggest that incorporation of minor components from dietary high-oleic oils into TRL modulates the effect of these atherogenic particles on VLDL secretion.

Development of a novel method to determine very low density lipoprotein kinetics

Isotopic tracer methods of determining triglyceride-rich lipoprotein (TRL) kinetics are costly, time-consuming, and labor-intensive. This study aimed to develop a simpler and cost-effective method of obtaining TRL kinetic data, based on the fact that chylomicrons compete with large VLDL (VLDL(1); S(f) = 60-400) for the same catalytic pathway. Ten healthy subjects [seven men; fasting triglyceride (TG), 44.3-407.6 mg/dl; body mass index, 21-35 kg/m(2)] were given an intravenous infusion of a chylomicron-like TG emulsion (Intralipid; 0.1 g/kg bolus followed by 0.1 g/kg/h infusion) for 75-120 min to prevent the clearance of VLDL(1) by lipoprotein lipase. Multiple blood samples were taken during and after infusion for separation of Intralipid, VLDL(1), and VLDL(2) by ultracentrifugation. VLDL(1)-apolipoprotein B (apoB) and TG production rates were calculated from their linear increases in the VLDL(1) fraction during the infusion. Intralipid-TG clearance rate was determined from its exponential decay after infusion. The production rates of VLDL(1)-apoB and VLDL(1)-TG were (mean +/- SEM) 25.4 +/- 3.9 and 1,076.7 +/- 224.7 mg/h, respectively, and the Intralipid-TG clearance rate was 66.9 +/- 11.7 pools/day. Kinetic data obtained from this method agree with values obtained from stable isotope methods and show the expected relationships with indices of body fatness and insulin resistance (all P < 0.05). The protocol is relatively quick, inexpensive, and transferable to nonspecialist laboratories.

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.

Omega-3 fatty acids improve glucose tolerance and components of the metabolic syndrome in Alaskan Eskimos: the Alaska Siberia project

OBJECTIVES

To test the hypothesis that the unusually low prevalences of insulin resistance (IR), metabolic syndrome (MS) and diabetes (DM) in Alaskan Eskimos, compared to American Indians, is related to the traditional Eskimo diet, high in C20-C22 omega-3 fatty acids (FAs). To determine if the relatively low blood pressures, low serum triglycerides and high HDL cholesterol levels in Eskimos result from high omega-3 FA consumption.

STUDY DESIGN

Cross-sectional study.

METHODS

We measured plasma FA concentrations in 447 Norton Sound Eskimos (35-74 years of age) and screened for DM, CHD and associated risk factors. A dietary assessment (24-hr recall) was obtained for comparison the day before the blood sampling.

RESULTS

Plasma omega-3 FA concentrations were highly correlated with dietary omega-3 FAs and HDL levels and inversely correlated with plasma levels of insulin, 2-h insulin (OGTT), HOMI-IR, 2-h glucose (OGTT), triglyceride levels and diastolic blood pressure.

CONCLUSIONS

High consumption of omega-3 FAs positively affects components of the MS, insulin sensitivity and glucose tolerance. This finding suggests that high consumption of C20-C22 omega-3 FAs protects against the development of the MS and glucose intolerance.

The dietary alpha-linolenic acid to linoleic acid ratio does not affect the serum lipoprotein profile in humans

Alpha-linolenic acid [ALA, 18:3(n-3)] and linoleic acid [LA, 18:2(n-6)] have comparable effects on serum lipid and lipoprotein concentrations, but their effects on lipoprotein subclass distributions and particle sizes are unknown. It is also not known whether these effects are changed by the ALA:LA ratio in the diet. To address these questions, healthy subjects (n = 54) consumed a control diet providing 7% of energy (En%) as LA and 0.4 En% as ALA during a 4-wk run-in period. For the following 6 wk of intervention, each diet was consumed by 18 subjects: the control diet, a low-LA diet (3 En% LA, 0.4 En% ALA), or a high-ALA diet (7 En% LA, 1.1 En% ALA). The ALA:LA ratio for the control diet was 1:19 and was 1:7 for the other 2 diets. Compared with the control group, LDL cholesterol decreased significantly in the ALA group (-0.32 mmol/L, P = 0.024), as did total cholesterol, apolipoprotein (apo) B, and the total:HDL cholesterol ratio. None of the dietary interventions affected HDL cholesterol, apo A-1, or triacylglycerol concentrations. The decrease in total VLDL particle concentrations in the low-LA group was due mainly to a decrease in medium VLDL (-16 nmol/L, P = 0.018) and in the high-ALA group to a decrease in small VLDL (-14 nmol/L, P = 0.044). We conclude that the ALA:LA ratio does not affect the serum lipoprotein profile. Compared with the control and LA diets, ALA lowered LDL cholesterol concentrations, possibly caused by the decrease in small VLDL.

Omega-3 fatty acids alter lipoprotein subfraction distributions and the in vitro conversion of very low density lipoproteins to low density lipoproteins

The purpose of this study was to determine the effects of a fish oil concentrate (FOC) on the in vitro conversion of very low density lipoproteins (VLDL) to intermediate (IDL) and low density lipoproteins (LDL). Six hypertriglyceridemic patients were randomly allocated to receive either placebo (olive oil) or FOC (1 g/14 kg body weight/day) for 4 weeks in a crossover study with a 4-week washout period. The FOC provided 3 g of eicosapentaenoic + docosahexaenoic acid per 70 kg of body weight, and it lowered plasma triglyceride and VLDL cholesterol levels by 35% and 42%, respectively. Decreases in the largest particles (VLDL(1)) were primarily responsible, with no effect noted in smaller VLDL particles (VLDL(2) and VLDL(3)). The FOC increased LDL cholesterol levels by 25% (P < 0.06) but did not affect LDL particle size. VLDL(1) and VLDL(3) were incubated in vitro with human postheparin lipases. Although triglycerides from both types of VLDL were hydrolyzed to the same extent with both treatments, particles isolated during the FOC phase were more readily converted into IDL and LDL than were control particles. These data suggest that the marine omega3 fatty acids may enhance the propensity of VLDL to be converted to LDL, partly explaining the decreased VLDL and increased LDL levels in FOC-treated patients.

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221–232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.

Lipoprotein transport in the metabolic syndrome: methodological aspects of stable isotope kinetic studies

The metabolic syndrome encapsulates visceral obesity, insulin resistance, diabetes, hypertension and dyslipidaemia. Dyslipidaemia is a cardinal feature of the metabolic syndrome that accelerates the risk of cardiovascular disease. It is usually characterized by high plasma concentrations of triacylglycerol (triglyceride)-rich and apoB (apolipoprotein B)-containing lipoproteins, with depressed concentrations of HDL (high-density lipoprotein). However, lipoprotein metabolism is complex and abnormal plasma concentrations can result from alterations in the rates of production and/or catabolism of these lipoprotein particles. Our in vivo understanding of kinetic defects in lipoprotein metabolism in the metabolic syndrome has been achieved chiefly by ongoing developments in the use of stable isotope tracers and mathematical modelling. This review deals with the methodological aspects of stable isotope kinetic studies. The design of in vivo turnover studies requires considerations related to stable isotope tracer administration, duration of sampling protocol and interpretation of tracer data, all of which are critically dependent on the kinetic properties of the lipoproteins under investigation. Such models provide novel insight that further understanding of metabolic disorders and effects of treatments. Future investigations of the pathophysiology and therapy of the dyslipoproteinaemia of the metabolic syndrome will require the development of novel kinetic methodologies. Specifically, new stable isotope techniques are required for investigating in vivo the turnover of the HDL subpopulation of particles, as well as the cellular efflux of cholesterol into the extracellular space and its subsequent transport in plasma and metabolic fate in the liver.

Comparison of a high-carbohydrate and a high-monounsaturated fat, olive oil-rich diet on the susceptibility of LDL to oxidative modification in subjects with Type 2 diabetes mellitus

AIMS

To compare the effects of a high-carbohydrate (CHO) diet and a high-monounsaturated fatty acid (MUFA) diet on LDL oxidative resistance in free-living individuals with Type 2 diabetes mellitus.

METHODS

Twenty-two men and women out-patients with Type 2 diabetes, with mean age 61 years and in fair metabolic control (HbA1c<8.0%), were enrolled at a university hospital lipid clinic in a randomized, crossover feeding trial comparing two isocaloric diets for 6 weeks each: CHO (fat, 28% energy) and MUFA (fat, 40% energy) based on virgin olive oil. Outcome measurements were changes in LDL susceptibility to oxidation, body weight, glycaemic control, and lipoprotein profiles.

RESULTS

Planned and observed diets were well matched. Participants preferred the MUFA diet over the CHO diet. The lag time of conjugated diene formation during Cu2+-induced LDL oxidation was similar after the CHO and MUFA diets (36.4 +/- 12.2 min and 36.0 +/- 13.7 min, respectively). Body weight, glycaemic control, total triglycerides, and total, LDL- and HDL-cholesterol levels also were similar after the two diets. Compared with the CHO diet, the MUFA diet lowered VLDL-cholesterol by 35% (P=0.023) and VLDL triglyceride by 16% (P=0.016).

CONCLUSIONS

Natural food-based high-CHO and high-MUFA diets have similar effects on LDL oxidative resistance and metabolic control in subjects with Type 2 diabetes. A MUFA diet is a good alternative to high-CHO diets for nutrition therapy of diabetes because it also has a beneficial effect on the lipid profile and superior patient acceptance.

Influence of apolipoprotein E genotype on the response to caloric restriction in type 2 diabetic patients with hyperlipidaemia

AIMS

Hyperlipidaemia is a major predisposing factor to atherosclerosis in patients with type 2 diabetes. Apolipoprotein (apo) E polymorphism influences lipoprotein metabolism, and the present study was undertaken to explore the relation, in type 2 diabetics, between apo E genotype and the plasma lipid response to dietary therapy.

METHODS

The subjects were 104 patients with type 2 diabetes and hyperlipidaemia, and the difference, due to apo E genotype, in dietary response was followed for 4-6 weeks. The caloric intake was maintained in the range 20-25 kcal/kg, and the medications for diabetes were not changed during the follow-up period.

RESULTS

Plasma total cholesterol (TC) and triglyceride (TG) levels were significantly lowered by the dietary treatment in patients with apo E genotypes of epsilon3/3, epsilon2/3 and epsilon3/4; however, the lipid levels in patients with epsilon2/4 did not respond to the diet.

CONCLUSIONS

apo E genotype should be taken into consideration in the treatment of hyperlipidaemia in diabetic patients.

Comparative lipid and lipoprotein metabolism

This review summarizes a number of recent reports in several areas of lipid and lipoprotein metabolism. Absorption of dietary lipids, cholesterol synthesis, and biliary cholesterol metabolism are mentioned only briefly to be complete. Comparative aspects of lipoprotein metabolism, however, are detailed in an effort to integrate the myriad metabolic events which characterize these important lipid transport particles. Where comparative information is known, those aspects of lipoprotein metabolism that may be protective against atherogenesis in certain mammalian species are also described. Efforts to understand atherogenic resistance comparatively in animals lends a better understanding of the metabolic events leading to coronary artery disease in humans. They also provide an important basis for understanding lipid metabolism in numerous veterinary species.

Diabetes mellitus and coronary artery disease: therapeutic considerations

Diabetes mellitus affects about 6% of the U.S. population and represents a significant public health challenge, with numbers of those affected increasing every year. The most common cause of death in these patients is macrovascular disease, with coronary disease being the predominant form. The pathophysiology of coronary disease in patients with diabetes is complex and involves elements of hyperglycemia, dyslipidemia, hyperinsulinemia, as well as a procoagulant vascular milieu. First-generation trials looking at revascularization of multivessel disease in patients with diabetes have had long clinical follow-up periods and seem to consistently favor coronary bypass grafting over percutaneous interventions; however, newer trials that include the use of stents and glycoprotein IIb/IIIa inhibitor therapy as part of the latter strategy have raised some interesting questions, so that the issue remains controversial and by no means settled.

Comparative in vivo metabolism of apolipoproteins E2 and E4 in heterozygous apoE2/4 subjects

Apolipoprotein E (apoE) exists in three common forms in humans: the wild-type apoE3 and two common genetic variants, apoE2 and apoE4. Although previous studies have examined the metabolism of the different apoE isoforms in human subjects, they have not involved direct comparison of two different isoforms in subjects heterozygous for the same two isoforms. We conducted this study to directly compare the catabolism of apoE2 and apoE4 in heterozygous E2/4 subjects in vivo. Iodine 131-labeled apoE2 and iodine 125-labeled apoE4 were simultaneously injected into three E4/2 heterozygous subjects. The mean residence time of apoE4 (0.40 +/- 0.01 day) was found to be one-third that of apoE2 (1.20 +/- 0.18 day). ApoE2 was present primarily in high-density lipoprotein, whereas apoE4 was present equally in very low density and high-density lipoprotein. In all lipoprotein subfractions, apoE4 was catabolized at a much faster rate than apoE2. In conclusion, E4 is catabolized three times faster than apoE2 in heterozygous E2/4 subjects, indicating that these two apoE isoproteins have distinct metabolic pathways.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Comparison of apolipoprotein B metabolism in familial defective apolipoprotein B and heterogeneous familial hypercholesterolemia

Both defective LDL receptors (familial hypercholesterolaemia, FH) and mutations in apolipoprotein B (apoB) on LDL (familial defective apoB, FDB) give rise to a phenotype of elevated LDL cholesterol. We sought to compare the metabolic basis of the two conditions by examining apoB turnover in FDB and FH subjects. A group comprising three heterozygous and one homozygous FDB subjects were compared with five FH heterozygotes and 17 control subjects using a deuterated leucine tracer. Kinetic parameters were derived by multicompartmental modelling. FH heterozygotes had a reduced delipidation rate for VLDL, which led to a moderate increase in plasma triglyceride. Compared with controls and FH, the FDB subjects converted 44% less IDL to LDL. The LDL FCR was reduced to a similar extent in FDB and FH. In all subjects LDL plasma levels appeared to be regulated by the LDL FCR and the rate of production of small VLDL. We conclude that disturbances in IDL metabolism provide the basis for understanding why FDB is less severe than FH. Our findings suggest that an apoB-LDL receptor interaction is important in the IDL to LDL conversion.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

The coffee diterpene cafestol increases plasma triacylglycerol by increasing the production rate of large VLDL apolipoprotein B in healthy normolipidemic subjects

BACKGROUND

Cafestol is a diterpene in unfiltered coffee that raises plasma triacylglycerol in humans.

OBJECTIVE

We studied whether cafestol increases plasma triacylglycerol by increasing the production rate or by decreasing the fractional catabolic rate of VLDL(1) [Svedberg flotation unit (S(f)) 60-400] apolipoprotein (apo) B. In addition, we studied the effect of cafestol on the composition of VLDL(1) and VLDL(2) (S(f) 20-60).

DESIGN

Eight healthy normolipidemic men were administered a daily dose of 75 mg cafestol for 2 wk. A bolus injection of 7 mg L-[5,5,5-(2)H(3)]leucine/kg body wt was given after a baseline period with no cafestol and again after treatment with cafestol. We derived kinetic constants to describe the metabolism of VLDL(1) apo B by using a multicompartmental model.

RESULTS

Cafestol significantly increased plasma triacylglycerol by 31% or 0.32 mmol/L (95% CI: 0.03, 0.61); the increase was due mainly to a nonsignificant rise in VLDL(1) triacylglycerol of 57% or 0.23 mmol/L (95% CI: -0.02, 0.48). Cafestol significantly increased the mean rate of VLDL(1) apo B production by 80% or 755 mg/d (95% CI: 0.2, 5353), whereas it did not significantly change the mean fractional catabolic rate of VLDL(1) apo B (mean increase of 3 pools/d; 95% CI: -4, 10]). Cafestol did not change the composition of VLDL(1). A significant increase in the ratio of VLDL(2) cholesteryl ester to triacylglycerol indicates that VLDL(2) became enriched with cholesteryl esters at the cost of triacylglycerol.

CONCLUSION

Cafestol increases plasma triacylglycerol by increasing the production rate of VLDL(1) apo B, probably via increased assembly of VLDL(1) in the liver.

Responses of plasma lipoproteins and sex hormones to the consumption of lean fish incorporated in a prudent-type diet in normolipidemic men

OBJECTIVE

The effects of lean fish on plasma lipoproteins, postheparin plasma lipolytic activities and sex hormones were examined in 11 normolipidemic male subjects.

METHODS

This study was a randomized crossover trial of two isoenergetic prudent-type diets, lean fish diet and beef, pork, veal, eggs and milk (nonfish) diet. Experimental diets provided approximately 11800 kJ--18% as proteins, 50% as carbohydrates, 32% as lipids [ratio of polyunsaturated to saturated fatty acids (P:S) of 1:1 compared with 0.5:1 in preexperimental diet], and 260 mg cholesterol/day.

RESULTS

Compared with the nonfish diet, the lean fish diet induced higher plasma total and LDL apolipoprotein (apo) B and apo B:apo A-1 ratio, indicating that the substitution of lean fish for beef, veal, pork, eggs and milk provides little benefits with regard to plasma apo B concentrations in a low-fat high P:S diet. Moreover, triglycerides:apo B and cholesterol:apo B ratios of VLDL were lower following the lean fish diet than the nonfish diet, suggesting the presence of smaller very low-density lipoprotein (VLDL) particles following the consumption of lean fish. Higher plasma concentrations of sex hormone-binding globulin (SHBG), HDL2 cholesterol and HDL2:HDL3 cholesterol ratio were found with the lean fish diet compared with the nonfish diet. Negative correlations between plasma postheparin lipoprotein lipase (LPL) activity and VLDL triglycerides (n = 11, r = -0.53, p = 0.02), and between plasma postheparin LPL activity and VLDL triglycerides:apo B ratio (n = 11, r = -0.64, p = 0.02) were also observed following the lean fish diet.

CONCLUSION

These results suggest that the effects of substituting lean fish for beef, veal, pork, eggs and milk on plasma lipoproteins may be partly associated with variations in plasma sex hormone status and plasma LPL activity in normolipidemic men.