Kinetic model for production and metabolism of very low density lipoprotein triglycerides. Evidence for a slow production pathway and results for normolipidemic subjects

Using measures of metabolic flux to align screening and clinical development: Avoiding pitfalls to enable translational studies

Screening campaigns, especially those aimed at modulating enzyme activity, often rely on measuring substrate→product conversions. Unfortunately, the presence of endogenous substrates and/or products can limit one's ability to measure conversions. As well, coupled detection systems, often used to facilitate optical readouts, are subject to interference. Stable isotope labeled substrates can overcome background contamination and yield a direct readout of enzyme activity. Not only can isotope kinetic assays enable early screening, but they can also be used to follow hit progression in translational (pre)clinical studies. Herein, we consider a case study surrounding lipid biology to exemplify how metabolic flux analyses can connect stages of drug development, caveats are highlighted to ensure reliable data interpretations. For example, when measuring enzyme activity in early biochemical screening it may be enough to quantify the formation of a labeled product. In contrast, cell-based and in vivo studies must account for variable exposure to a labeled substrate (or precursor) which occurs via tracer dilution and/or isotopic exchange. Strategies are discussed to correct for these complications. We believe that measures of metabolic flux can help connect structure-activity relationships with pharmacodynamic mechanisms of action and determine whether mechanistically differentiated biophysical interactions lead to physiologically relevant outcomes. Adoption of this logic may allow research programs to (i) build a critical bridge between primary screening and (pre)clinical development, (ii) elucidate biology in parallel with screening and (iii) suggest a strategy aimed at in vivo biomarker development.

Investigation of human apoB48 metabolism using a new, integrated non-steady-state model of apoB48 and apoB100 kinetics

BACKGROUND

Triglyceride-rich lipoproteins and their remnants have emerged as major risk factors for cardiovascular disease. New experimental approaches are required that permit simultaneous investigation of the dynamics of chylomicrons (CM) and apoB48 metabolism and of apoB100 in very low-density lipoproteins (VLDL).

METHODS

Mass spectrometric techniques were used to determine the masses and tracer enrichments of apoB48 in the CM, VLDL₁ and VLDL₂ density intervals. An integrated non-steady-state multicompartmental model was constructed to describe the metabolism of apoB48- and apoB100-containing lipoproteins following a fat-rich meal, as well as during prolonged fasting.

RESULTS

The kinetic model described the metabolism of apoB48 in CM, VLDL₁ and VLDL₂ . It predicted a low level of basal apoB48 secretion and, during fat absorption, an increment in apoB48 release into not only CM but also directly into VLDL₁ and VLDL₂ . ApoB48 particles with a long residence time were present in VLDL, and in subjects with high plasma triglycerides, these lipoproteins contributed to apoB48 measured during fasting conditions. Basal apoB48 secretion was about 50 mg day^-1 , and the increment during absorption was about 230 mg day^-1 . The fractional catabolic rates for apoB48 in VLDL₁ and VLDL₂ were substantially lower than for apoB48 in CM.

DISCUSSION

This novel non-steady-state model integrates the metabolic properties of both apoB100 and apoB48 and the kinetics of triglyceride. The model is physiologically relevant and provides insight not only into apoB48 release in the basal and postabsorptive states but also into the contribution of the intestine to VLDL pool size and kinetics.

Kinetic Studies to Elucidate Impaired Metabolism of Triglyceride-rich Lipoproteins in Humans

To develop novel strategies for prevention and treatment of dyslipidemia, it is essential to understand the pathophysiology of dyslipoproteinemia in humans. Lipoprotein metabolism is a complex system in which abnormal concentrations of various lipoprotein particles can result from alterations in their rates of production, conversion, and/or catabolism. Traditional methods that measure plasma lipoprotein concentrations only provide static estimates of lipoprotein metabolism and hence limited mechanistic information. By contrast, the use of tracers labeled with stable isotopes and mathematical modeling, provides us with a powerful tool for probing lipid and lipoprotein kinetics in vivo and furthering our understanding of the pathogenesis of dyslipoproteinemia.

Atorvastatin plus omega-3 fatty acid ethyl ester decreases very-low-density lipoprotein triglyceride production in insulin resistant obese men

AIM

To test the effect of atorvastatin (ATV) and ATV plus ω-3 FAEEs on VLDL-TG metabolism in obese, insulin resistant men.

METHODS

We carried out a 6-week randomized, placebo-controlled study to examine the effect of ATV (40 mg/day) and ATV plus ω-3 FAEEs (4 g/day) on VLDL-TG metabolism in 36 insulin resistant obese men. VLDL-TG kinetics were determined using d5 -glycerol, gas chromatography-mass spectrometry and compartmental modelling.

RESULTS

Compared with the placebo, ATV significantly decreased VLDL-TG concentration (-40%, p < 0.001) by increasing VLDL-TG fractional catabolic rate (FCR) (+47%, p < 0.01). ATV plus ω-3 FAEEs lowered VLDL-TG concentration to a greater degree compared with placebo (-46%, p < 0.001) or ATV monotherapy (-13%, p = 0.04). This was achieved by a reduction in VLDL-TG production rate (PR) compared with placebo (-32%, p = 0.008) or ATV (-20%, p = 0.03) as well as a reciprocal increase in VLDL-TG FCR (+42%, p < 0.05) compared with placebo.

CONCLUSION

In insulin resistant, dyslipidaemic, obese men, ATV improves VLDL-TG metabolism by increasing VLDL-TG FCR. The addition of 4 g/day ω-3 FAEE to statin therapy provides further TG-lowering by lowering VLDL-TG PR.

A computational model for the analysis of lipoprotein distributions in the mouse: translating FPLC profiles to lipoprotein metabolism

Disturbances of lipoprotein metabolism are recognized as indicators of cardiometabolic disease risk. Lipoprotein size and composition, measured in a lipoprotein profile, are considered to be disease risk markers. However, the measured profile is a collective result of complex metabolic interactions, which complicates the identification of changes in metabolism. In this study we aim to develop a method which quantitatively relates murine lipoprotein size, composition and concentration to the molecular mechanisms underlying lipoprotein metabolism. We introduce a computational framework which incorporates a novel kinetic model of murine lipoprotein metabolism. The model is applied to compute a distribution of plasma lipoproteins, which is then related to experimental lipoprotein profiles through the generation of an in silico lipoprotein profile. The model was first applied to profiles obtained from wild-type C57Bl/6J mice. The results provided insight into the interplay of lipoprotein production, remodelling and catabolism. Moreover, the concentration and metabolism of unmeasured lipoprotein components could be determined. The model was validated through the prediction of lipoprotein profiles of several transgenic mouse models commonly used in cardiovascular research. Finally, the framework was employed for longitudinal analysis of the profiles of C57Bl/6J mice following a pharmaceutical intervention with a liver X receptor (LXR) agonist. The multifaceted regulatory response to the administration of the compound is incompletely understood. The results explain the characteristic changes of the observed lipoprotein profile in terms of the underlying metabolic perturbation and resultant modifications of lipid fluxes in the body. The Murine Lipoprotein Profiler (MuLiP) presented here is thus a valuable tool to assess the metabolic origin of altered murine lipoprotein profiles and can be applied in preclinical research performed in mice for analysis of lipid fluxes and lipoprotein composition.

Kinetic studies to investigate lipoprotein metabolism

To develop novel strategies for the prevention and treatment of dyslipidaemia, it is essential to understand the pathophysiology of dyslipoproteinaemia in humans. Lipoprotein metabolism is a complex system in which abnormal concentrations of various lipoprotein particles can result from alterations in their rates of production, conversion and/or catabolism. Traditional methods that measure plasma lipoprotein concentrations only provide static estimates of lipoprotein metabolism and hence limited mechanistic information. By contrast, the use of tracers labelled with stable isotopes and mathematical modelling provides a powerful tool for probing lipid and lipoprotein kinetics in vivo and furthering understanding of the pathogenesis of dyslipoproteinaemia.

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.

VLDL-TG kinetics: a dual isotope study for quantifying VLDL-TG pool size, production rates, and fractional oxidation in humans

Very-low-density lipoproteins (VLDLs) are large, complex particles containing both surface proteins (e.g., ApoB100) and core lipids, e.g., cholesterol and triglycerides (TG). Whereas ApoB100 kinetics have been thoroughly studied, accurate measurement of VLDL-TG kinetics have proven difficult due to either complex mathematics or laborious procedures. The present study was therefore designed to measure VLDL-TG kinetics by dual isotope ex vivo labeled VLDL-TG tracers and well-established kinetics equations (bolus injection or the primed continuous infusion). Ten healthy Caucasian men [age, 23 +/- 3 yr old (mean +/- SD); body mass index, 24.7 +/- 1.3 kg/m(2)] were included in the study. VLDL-TG rate of appearance (Ra) was measured using a dual-tracer technique ([9,10-(3)H]-labeled VLDL-TG and [1-(14)C]-labeled VLDL-TG) to allow comparison of various bolus decay curve fits with the Ra obtained by the primed continuous infusion (PCI; considered the gold standard). In addition, VLDL-TG fatty acid oxidation was measured as (14)CO(2) in exhaled breath, using the hyamine trapping technique. Following a bolus injection, tracer decay was better described by a biexponential than a monoexponential fit (r(2) = 0.99 +/- 0.01 vs. 0.97 +/- 0.04, respectively, P = 0.01). VLDL-TG Ra calculated using the PCI correlated significantly with the biexponential fit (rho = 0.62, P < 0.05), whereas this was not the case for the monoexponential fit (rho = -0.18, P = not significant). VLDL-TG Ra using the best fit of the bolus injection method (biexponential) was less than values obtained by the constant infusion technique [biexponential, 34.3 (range, 27.1-69.6) vs. PCI, 44.4 (range, 33.0-72.7), P < 0.05]. Fractional oxidation of VLDL-TG was 37.2 +/- 8.8% at 240 min corresponding to 198.8 +/- 55.9 kcal/day or 10.6 +/- 3.3% of resting energy expenditure (REE). Our data demonstrate that VLDL-TG Ra measured by a biexponential fit to a bolus decay curve correlates well with VLDL-TG Ra measured by a primed continuous infusion, and therefore that a "second" peripheral VLDL-TG compartment with rapid exchange of TG exists. VLDL-TG volume of distribution is therefore greater than previously anticipated. Finally our data supports that VLDL-TG contributes quantitatively to REE.

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.

Impact of body composition on very-low-density lipoprotein-triglycerides kinetics

Upper body obese (UBO) subjects have greater cardiovascular disease risk than lower body obese (LBO) or lean subjects. Obesity is also associated with hypertriglyceridemia that may involve greater production and impaired removal of very-low-density lipoprotein (VLDL)-triglycerides (TG). In these studies, we assessed the impact of body composition on basal VLDL-TG production, VLDL-TG oxidation, and VLDL-TG storage. VLDL-TG kinetics were assessed in 10 UBO, 10 LBO, and 10 lean women using a bolus injection of [1-(14)C]VLDL-TG. VLDL-TG oxidation was measured by (14)CO(2) production (hyamine trapping) and VLDL-TG adipose tissue storage by fat biopsies. Insulin sensititvity was assessed by the hyperinsulinemic-euglycemic clamp technique and body composition by dual X-ray absorptiometry in combination with computed tomography. Hepatic VLDL-TG production was significantly greater in UBO than in lean women [(mumol/min) UBO: 64.8 (SD 40.0) vs. LBO: 42.5 (SD 25.6) vs. lean: 31.8 (SD 13.3), P = 0.04], whereas VLDL-TG oxidation was similar in the three groups and averaged 20% of resting energy expenditure [(mumol/min) UBO: 38.3 (SD 26.5) vs. LBO: 23.5 (SD 13.5) vs. lean: 21.1 (SD 9.7), P = 0.09]. In UBO women, more VLDL-TG was deposited in upper body subcutaneous fat [VLDL-TG redeposition in abdominal adipose tissue (mumol/min): UBO: 5.0 (SD 2.9) vs. LBO: 4.0 (SD 3.2) vs. lean: 1.3 (SD 1.0), ANOVA P = 0.01]; in LBO women, more VLDL-TG was deposited in femoral fat [VLDL-TG redeposition in femoral adipose tissue (mumol/min): UBO: 5.1 (SD 3.1) vs. LBO: 5.8 (SD 4.3) vs. lean: 2.3 (SD 1.5), ANOVA P = 0.04]. Only a small proportion of VLDL-TG (8-16%) was partitioned into redeposition in either group. We found that elevated VLDL-TG production without concomitant increased clearance via oxidation and adipose tissue redeposition contributes to hypertriglyceridemia in UBO women.

Systematic error of serum triglyceride measurements during three decades and the effect of fasting on serum triglycerides in population studies

BACKGROUND

An uncontrolled systematic error in serum biomarkers may be a serious problem when comparing their trends both within and between populations. The aim of the study was to assess which factors are responsible for systematic errors in the measurement of serum triglycerides (Tg) and the effect of fasting on serum triglycerides in Finnish population surveys.

METHODS

Data on precision and accuracy during 30 years for serum triglycerides were documented from participation in 492 rounds of five different external quality assessment (EQA) programs. Data on fasting and health status from questionnaires were combined from three population surveys comprising 27,131 participants.

RESULTS

The mean annual accuracy (bias) of the Tg methods from all EQAs during 1978-2007 was -1.54% (95% CI -2.25, -0.83). The mean relative change in triglyceride concentration per fasting hour was -3.7% (95% CI -4.2, -3.1) in all subjects. A minimum serum Tg concentration was seen in men and women who had fasted for at least 8 and 7 h, respectively.

CONCLUSIONS

The mean bias in serum Tg analyses has been very small throughout the 30-year period. Fasting has a considerable effect on triglyceride levels, but they can be converted either to fasting or non-fasting levels using specific factors.

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.

Fatty liver, insulin resistance, and dyslipidemia

After recently being recognized as a feature of the metabolic syndrome, fatty liver has evolved as a key player in the pathogenesis of dyslipidemia. Development of nonalcoholic fatty liver disease comes from an imbalance between the influx and production of fatty acids and the use of fatty acids for oxidation or secretion as very low density lipoprotein (VLDL) triglycerides. Previously, we have shown a strong relationship between increased liver fat and overproduction of large VLDL particles. We observed recently that in patients with high liver fat, insulin was unable to regulate VLDL production. The result is increased concentrations of VLDL particles in the circulation. Consequently, changes are seen in the metabolism of other lipoproteins that interact with VLDL particles, the net result being decreased high-density lipoprotein cholesterol and increased formation of small, dense low-density lipoprotein. In this article, we review recent findings on the development of fatty liver and its role in the diabetic dyslipidemia pathogenesis.

Measurement of endogenous and exogenous triacylglycerol kinetics in the fed and fasted states

Emerging evidence has shown that an abnormal postprandial accumulation of dietary fat is atherogenic. However, there is a lack of data describing the mechanisms for accumulation of triacylglycerol (TAG) in the postprandial period. There is therefore a need to establish a specific measure of the kinetics of endogenous and exogenous TAG in the postprandial period.

Exploring the lipoprotein composition using Bayesian regression on serum lipidomic profiles

MOTIVATION

Serum lipids have been traditionally studied in the context of lipoprotein particles. Today's emerging lipidomics technologies afford sensitive detection of individual lipid molecular species, i.e. to a much greater detail than the scale of lipoproteins. However, such global serum lipidomic profiles do not inherently contain any information on where the detected lipid species are coming from. Since it is too laborious and time consuming to routinely perform serum fractionation and lipidomics analysis on each lipoprotein fraction separately, this presents a challenge for the interpretation of lipidomic profile data. An exciting and medically important new bioinformatics challenge today is therefore how to build on extensive knowledge of lipid metabolism at lipoprotein levels in order to develop better models and bioinformatics tools based on high-dimensional lipidomic data becoming available today.

RESULTS

We developed a hierarchical Bayesian regression model to study lipidomic profiles in serum and in different lipoprotein classes. As a background data for the model building, we utilized lipidomic data for each of the lipoprotein fractions from 5 subjects with metabolic syndrome and 12 healthy controls. We clustered the lipid profiles and applied a regression model within each cluster separately. We found that the amount of a lipid in serum can be adequately described by the amounts of lipids in the lipoprotein classes. In addition to improved ability to interpret lipidomic data, we expect that our approach will also facilitate dynamic modelling of lipid metabolism at the individual molecular species level.

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

To gain insight into the mechanisms regulating plasma lipid homeostasis, FFA, VLDL-triglyceride (TG), and VLDL-apolipoprotein B-100 (apoB-100) kinetics are commonly assessed using stable isotope-labeled tracer methods. The reproducibility of these measurements, which is critical for the experimental design, is unknown. Therefore, we investigated the repeatability of plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in eight healthy men using stable isotope-labeled tracer techniques. There were no systematic differences in plasma FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics between the two studies. Intraindividual day-to-day variability for various outcome variables ranged from 15% to 25%, and almost all of this was of biological origin. The most robust outcome variables were FFA rate of appearance and hepatic VLDL-TG and VLDL-apoB-100 secretion rates; the least robust were VLDL-TG and VLDL-apoB-100 plasma clearance rates and mean residence times. Overall, physiologically meaningful differences in mean values (i.e., 25-30% in magnitude) can be obtained with a sample size of 6-10 subjects for paired studies and 12-20 subjects per group for cross-sectional studies, assuming a type I error rate of 0.05 and a type II error rate of 0.20 (i.e., 80% power). These findings will be useful for future studies investigating FFA, VLDL-TG, and VLDL-apoB-100 kinetics with the methods described.

Delayed secretory pathway contributions to VLDL-triglycerides from plasma NEFA, diet, and de novo lipogenesis in humans

Newly synthesized triglyceride (TG) may exit the liver immediately as VLDL-TG or be stored and secreted after a delay. We quantified the contributions from plasma NEFA, diet, and de novo lipogenesis (DNL) to VLDL-TG via immediate and delayed pathways in five lean, normolipidemic subjects; six obese, hypertriglyceridemic (HPTG) nondiabetics; and six obese, HPTG diabetics. Intravenous [(2)H(31)]palmitate and [1-(13)C(1)] acetate and oral [(2)H(35)]stearate were administered for 30 h preceding an overnight fast. [1,2,3,4-(13)C(4)]palmitate was infused during the subsequent 12 h fast. Contributions from plasma NEFA via the immediate pathway were 64 +/- 15, 33 +/- 6, and 58 +/- 2% in control, HPTG, and diabetic HPTG, respectively. Delayed pool fractional contributions were as follows: dietary FA, 2.0 +/- 0.9, 2.5 +/- 1, and 12 +/- 2%; DNL, 3 +/- 0.3, 14 +/- 3, and 13 +/- 4%; delayed NEFA, 15 +/- 4, 20 +/- 4, and 30 +/- 3%. VLDL-TG production rates and absolute input rates from the delayed pool were significantly higher in HPTG and diabetic HPTG than in controls. In conclusion, we provide direct kinetic evidence for a hepatic TG storage pool in humans and document its metabolic sources. The turnover time and sources of this pool differ in diabetic HPTG and nondiabetic HPTG, with potential therapeutic implications.

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.

Energy expenditure, insulin, and VLDL-triglyceride production in humans

Hypertriglyceridemia is considered a cardiovascular risk factor in diabetic and nondiabetic subjects. In this study, we aimed to determine potential regulators of very low density lipoprotein-triglyceride (TG) production. VLDL-TG kinetics were measured in 13 men and 12 women [body mass index [mean (range)]: 24.8 (20.2-35.6) kg/m(2)]. VLDL-TG production was assessed from the plasma decay of a bolus injection of ex vivo labeled VLDL particles ([1-(14)C]triolein-VLDL-TG). Similar VLDL-TG production (micromol/min) was found in men and women. VLDL-TG production was not significantly correlated with palmitate flux ([9,10-(3)H]palmitate) (r = 0.09, P = 0.67) or palmitate concentration (r = -0.29, P = 0.2) but was correlated significantly with fasting insulin concentration (r = 0.46, P < 0.05) and resting energy expenditure (REE) (r = 0.45, P < 0.05). The latter correlation improved when adjusted for sex. The best multivariate model with VLDL-TG production as the dependent variable and REE, body composition, hormones, and substrate levels as independent variables included fasting insulin (P = 0.02) and REE (P = 0.02) (r(2) = 0.32, P < 0.001). We conclude that VLDL kinetics are similar in men and women and that REE and plasma insulin are significant independent predictors of VLDL-TG production. FFA availability and body fat distribution are unrelated to VLDL production. We suggest that REE plays a greater role in VLDL-TG production than previously anticipated. REE and insulin should be taken into account when VLDL-TG production comparisons between groups are made.

Diabetic dyslipidaemia

PURPOSE OF REVIEW

Diabetic dyslipidaemia is a cluster of plasma lipid and lipoprotein abnormalities that are metabolically interrelated. The increase of large type 1 very low density lipoprotein particles in type 2 diabetes initiates a sequence of events that generates atherogenic remnants, small dense low-density lipoprotein and small dense high-density lipoprotein particles. Thus, it is of great importance to elucidate the mechanisms behind the overproduction of large very low density lipoprotein particles in diabetic dyslipidaemia. This review discusses the pathophysiology of very low density lipoprotein metabolism in type 2 diabetes and recent concepts of lipid management of diabetic dyslipidaemia.

RECENT FINDINGS

Results indicate that triglyceride and apolipoprotein B production in types 1 and 2 very low density lipoprotein are significantly correlated, suggesting a coupling of the two processes governing the metabolism of these lipoprotein subpopulations. Insulin resistance, hyperglycaemia, and liver fat were associated with excess hepatic production of type 1 but not type 2 very low density lipoprotein particles. These data provide support for the independent regulation of types 1 and 2 very low density lipoprotein apolipoprotein B production.

SUMMARY

Recent data suggest that the assembly of very low density lipoprotein is fundamentally altered in type 2 diabetes, explaining the overproduction of large type 1 very low density lipoprotein as well as the inability of insulin to suppress production of type 1 very low density lipoprotein in type 2 diabetes. Future discoveries hopefully will delineate the regulatory steps to allow more targeted treatment of diabetic dyslipidaemia.

Metabolism of apoB lipoproteins of intestinal and hepatic origin during constant feeding of small amounts of fat

We aimed to identify mechanisms by which apolipoprotein B-48 (apoB-48) could have an atherogenic role by simultaneously studying the metabolism of postprandial apoB-48 and apoB-100 lipoproteins. The kinetics of apoB-48 and apoB-100, each in four density subfractions of VLDL and intermediate density lipoprotein (IDL), were studied by stable isotope labeling in a constantly fed state with half-hourly administration of almond oil in five postmenopausal women. A non-steady-state, multicompartmental model was used. Despite a much lower production rate, VLDL and IDL apoB-48 shared a similar secretion pattern with apoB-100: both were directly secreted into all fractions with similar percentage mass distributions. Fractional catabolic rates (FCRs) of apoB-48 and apoB-100 were similar in VLDL and IDL. We identified a fast turnover compartment of light VLDL that had a residence time of <30 min for apoB-48 and apoB-100. Finally, a high secretion rate of apoB-48 was associated with a slow FCR of VLDL and IDL apoB-100. In conclusion, the intestine secretes a spectrum of apoB lipoproteins, similar to what the liver secretes, albeit with a much lower secretion rate. Once in plasma, intestinal and hepatic triglyceride-rich lipoproteins have similar rates of clearance and participate interactively in similar metabolic pathways, with high apoB-48 production inhibiting the clearance of apoB-100.

Treatment with high-dose simvastatin reduces secretion of apolipoprotein B-lipoproteins in patients with diabetic dyslipidemia

HMG-CoA reductase inhibitors (statins) are effective lipid-altering drugs for the treatment of dyslipidemia in patients with type 2 diabetes mellitus. We conducted a randomized, double-blind, placebo-controlled, crossover design trial to determine the effects of simvastatin, 80 mg/day, on plasma lipid and lipoprotein levels and on the metabolism of apolipoprotein B (apoB) in VLDL, intermediate density lipoprotein (IDL), and LDL and of triglycerides (TGs) in VLDL. Simvastatin therapy decreased TG, cholesterol, and apoB significantly in VLDL, IDL, and LDL. These effects were associated with reduced production of LDL-apoB, mainly as a result of reduced secretion of apoB-lipoproteins directly into the LDL density range. Statin therapy also reduced hepatic production of VLDL-TG. There were no effects of simvastatin on the fractional catabolic rates of VLDL-apoB or -TG or LDL-apoB. The basis for decreased VLDL-TG secretion during simvastatin treatment is not clear, but recent studies suggest that statins may activate peroxisomal proliferator-activated receptor alpha (PPARalpha). Activation of PPARalpha could lead to increased hepatic oxidation of fatty acids and less synthesis of TG for VLDL assembly.

A new combined multicompartmental model for apolipoprotein B-100 and triglyceride metabolism in VLDL subfractions

The use of stable isotopes in conjunction with compartmental modeling analysis has greatly facilitated studies of the metabolism of the apolipoprotein B (apoB)-containing lipoproteins in humans. The aim of this study was to develop a multicompartment model that allows us to simultaneously determine the kinetics of apoB and triglyceride (TG) in VLDL(1) and VLDL(2) after a bolus injection of [(2)H(3)]leucine and [(2)H(5)]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Here, we describe the model and present the results of its application in a fasting steady-state situation in 17 subjects with lipid values representative of a Western population. Analysis of the correlations showed that plasma TG was determined by the VLDL(1) and VLDL(2) apoB and TG fractional catabolic rate. Furthermore, the model showed a linear correlation between VLDL(1) TG and apoB production. A novel observation was that VLDL TG entered the circulation within 21 min after its synthesis, whereas VLDL apoB entered the circulation after 33 min. These observations are consistent with a sequential assembly model of VLDL and suggest that the TG is added to a primordial apoB-containing particle in the liver.

Measuring very low density lipoprotein-triglyceride kinetics in man in vivo: how different the various methods really are

PURPOSE OF REVIEW

The purpose of this article is to briefly outline the methods that are currently available for the determination of very low density lipoprotein-triglyceride (VLDL-TG) kinetics in man in vivo.

RECENT FINDINGS

A number of novel methodologies have been developed over the years for quantifying VLDL-TG production, clearance, and turnover rates. Besides the splanchnic arteriovenous balance technique, tracer methods with radioactive and, more recently, stable isotopes have been widely used. Most of the latter approaches utilize an isotopically labelled substrate, such as glycerol, fatty acid or acetate, which is eventually incorporated into a VLDL-TG moiety, and monitor the time course of change in specific activity or enrichment. A procedure of in vivo labelling of VLDL-TG with stable isotopes and use of the labelled VLDL-TG as a tracer has also been described in man. There is, however, considerable variability in estimates of VLDL-TG kinetics obtained by the various techniques, which cannot be readily attributed to normal physiological variation. Still, a large part of this discrepancy may be related to differences in VLDL-TG pool size within the normal range, which seem to account for approximately 40-50% of the total variance in VLDL-TG kinetics in both men and women.

SUMMARY

Several methods are available for quantifying VLDL-TG kinetics in man in vivo, varying in the selection of tracer, mode of administration and sampling, and data analysis. These inherent features, along with different pool sizes, result in multifold variable estimations of VLDL-TG kinetic parameters.

Stable isotope tracer dilution for quantifying very low-density lipoprotein-triacylglycerol kinetics in man

BACKGROUND & AIM

A number of approaches have been employed in the past to measure very low-density lipoprotein (VLDL) triacylglycerol (TG) kinetics in humans in vivo, varying in the selection of tracer and mode of administration. All, however, make use of labeled TG precursors and more or less complicated mathematical models to derive the kinetic parameters of interest. The aim of the present study was to develop a conceptually straightforward method, based on the traditional tracer infusion technique, for quantifying VLDL-TG production rates in man using stable isotopes.

METHOD

Our approach involves ingestion of [U-13C3]glycerol to endogenously label the glycerol in VLDL-TG, plasmapheresis, isolation of the newly 13C-labeled VLDL from plasma, and administration within the next 2-3 days via a primed constant autologous reinfusion. This procedure produces enough tracer for a priming dose plus 2-3 h of infusion. In the physiological conditions examined (basal and hyperglycemic states, fat- and carbohydrate-rich diets), with almost 3-fold ranging VLDL-TG pool sizes, a steady state in plasma VLDL-TG glycerol tracer-to-tracee ratio was readily achieved within 2 h. Consequently, calculations are made according to the isotope dilution principle, thus avoiding assumptions implicit in more complicated models.

CONCLUSION

The stable isotope VLDL-TG tracer dilution method offers an alternative and reliable tool for the determination of endogenous VLDL-TG kinetics in man under a variety of metabolic states.

Effect of weight loss on VLDL-triglyceride and apoB-100 kinetics in women with abdominal obesity

The effects of obesity and weight loss on lipoprotein kinetics were evaluated in six lean women [body mass index (BMI): 21 +/- 1 kg/m(2)] and seven women with abdominal obesity (BMI: 36 +/- 1 kg/m(2)). Stable isotope tracer techniques, in conjunction with compartmental modeling, were used to determine VLDL-triglyceride (TG) and apolipoprotein B-100 (apoB-100) secretion rates in lean women and in obese women before and after 10% weight loss. VLDL-TG and VLDL-apoB-100 secretion rates were similar in lean and obese women. Weight loss decreased the rate of VLDL-TG secretion by approximately 40% (from 0.41 +/- 0.05 to 0.23 +/- 0.03 micromol x kg fat-free mass(-1) x min(-1); P < 0.05). The relative decline in VLDL-TG produced from nonsystemic fatty acids, derived from intraperitoneal and intrahepatic TG, was greater (61 +/- 7%) than the decline in VLDL-TG produced from systemic fatty acids, predominantly derived from subcutaneous TG (25 +/- 8%; P < 0.05). Weight loss did not affect VLDL-apoB-100 secretion rate. We conclude that weight loss decreases the rate of VLDL-TG secretion in women with abdominal obesity, primarily by decreasing the availability of nonsystemic fatty acids. There is a dissociation in the effect of weight loss on VLDL-TG and apoB-100 metabolic pathways that may affect VLDL particle size.

Methods for measuring lipid metabolism in vivo

PURPOSE OF REVIEW

This review discusses diverse methods that have been used in several recent papers for the qualitative and quantitative analysis of lipids, studies of lipid oxidation, lipoprotein fractionation, and studies of lipid metabolism and metabolic kinetics using tracers. Papers for this review were selected on the basis of their timeliness, novelty, and/or their potential impact on diverse fields of lipid metabolism.

RECENT FINDINGS

Many methods used for studies of lipid metabolism employ advanced chromatographic and mass spectrometric techniques to characterize lipids. In particular, the use of stable isotopically labeled tracers has become increasingly important to study metabolic kinetics.

SUMMARY

Such developments in methodology will continue to advance studies of lipid metabolism in many areas of clinical interest, including heart disease, obesity, and diabetes.

Effect of nicotinic acid administration on hepatic very low density lipoprotein-triglyceride production

Our objective was to examine very low density lipoprotein-triglyceride (VLDL-TG) kinetics after chronic and acute administration of nicotinic acid (NA). Incorporation of [1,2,3,4-(13)C(4)]palmitate and [2-(13)C(1)]glycerol into VLDL-TG was measured in five healthy, normolipidemic women. Each subject was studied twice; the 4-day hospital stays were separated by 1 mo, during which time doses of NA were increased to 2 g/day (500 mg, 4 times/day). During posttreatment study, 500 mg of NA were administered acutely at 0800. Under baseline postabsorptive conditions, incorporation curves from (13)C-labeled free fatty acid (FFA) and (13)C-labeled glycerol were superimposable, and VLDL-TG kinetics were in agreement (t(1/2) = 1.4 +/- 0.3 and 1.3 +/- 0.3 h, and production rates = 27.2 +/- 6.1 and 28.5 +/- 5.3 g/day, respectively). In the postabsorptive state after chronic NA therapy, VLDL-TG concentrations and production rates were lower despite a trend toward elevated plasma FFA concentrations and fluxes. After the acute dose of NA, plasma FFA concentrations and flux fell dramatically, and there was a virtual halt to VLDL-TG production, which continued throughout the 6-h period after NA, despite a marked rebound overshoot in serum FFA concentrations and flux after hour 2. Plasma homocysteine concentrations increased 68% (P < 0.001) in the NA phase, consistent with chronic increased transmethylation demand. We conclude that 1) NA acutely and chronically decreases VLDL-TG production rate in normal women; 2) the acute effect on VLDL-TG production is associated with an initial suppression of lipolysis but persists for several hours after the antilipolytic action of NA has abated and is observed in the basal postabsorptive state, when lipolytic rates are not reduced; and 3) the effect of NA on VLDL-TG production, therefore, cannot be completely explained by its antilipolytic actions.

Oxidation of nonplasma fatty acids during exercise is increased in women with abdominal obesity

We evaluated plasma fatty acid availability and plasma and whole body fatty acid oxidation during exercise in five lean and five abdominally obese women (body mass index = 21 +/- 1 vs. 38 +/- 1 kg/m(2)), who were matched on aerobic fitness, to test the hypothesis that obesity alters the relative contribution of plasma and nonplasma fatty acids to total energy production during exercise. Subjects exercised on a recumbent cycle ergometer for 90 min at 54% of their peak oxygen consumption. Stable isotope tracer methods ([(13)C]palmitate) were used to measure fatty acid rate of appearance in plasma and the rate of plasma fatty acid oxidation, and indirect calorimetry was used to measure whole body substrate oxidation. During exercise, palmitate rate of appearance increased progressively and was similar in obese and lean groups between 60 and 90 min of exercise [3.9 +/- 0.4 vs. 4.0 +/- 0.3 micromol. kg fat free mass (FFM)(-1). min(-1)]. The rate of plasma fatty acid oxidation was also similar in obese and lean subjects (12.8 +/- 1.7 vs. 14.5 +/- 1.8 micromol. kg FFM(-1). min(-1); P = not significant). However, whole body fatty acid oxidation during exercise was 25% greater in obese than in lean subjects (21.9 +/- 1.2 vs. 17.5 +/- 1.6 micromol. kg FFM(-1). min(-1); P < 0.05). These results demonstrate that, although plasma fatty acid availability and oxidation are similar during exercise in lean and obese women, women with abdominal obesity use more fat as a fuel by oxidizing more nonplasma fatty acids.

In vivo metabolism of ApoB, ApoA-I, and VLDL triglycerides in a form of hypobetalipoproteinemia not linked to the ApoB gene

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant disorder that may result from different mutations in the apolipoprotein B (apoB) gene or chromosome 2. However, linkage of FHBL to the apoB gene was ruled out in 2 kindreds reported to date, and the genetic and metabolic bases for FHBL remain unknown. One of the reported kindreds is our 40-member F kindred, in which we found linkage of FHBL to a novel susceptibility region on chromosome 3p21. 1-2. In addition to having low apoB levels, some, but not all, of the affected subjects in the F kindred also had low levels of high density lipoprotein (HDL) cholesterol and apoA-I. Our aim was to define the metabolic bases of the disorder in the F kindred. Therefore, we studied the in vivo kinetics of apoB and apoA-I and very low density lipoprotein (VLDL) triglycerides in 4 affected subjects and 5 normolipidemic relatives. Deuterated leucine and deuterated glycerol were used to label the apolipoproteins and triglycerides, respectively. Compartmental modeling was used to obtain the kinetic parameters. Affected subjects had (1) normal fractional catabolic rates (FCRs) for VLDL apoB, (2) increased FCRs for low density lipoprotein (LDL) apoB (0.050+/-0.009 versus 0. 030+/-0.006 pools per hour for normal subjects, P=0.005), and (3) decreased production rates of VLDL apoB (11.4+/-1.7 versus 25.6+/-4. 9 mg. kg(-1). d(-1), P=0.003), LDL apoB (7.8+/-1.3 versus 12.7+/-3.7 mg. kg(-1). d(-1), P=0.04), and VLDL triglycerides (8.2+/-4.5 versus 19.6+/-10.8 58 micromol. kg(-1). h(-1), P=0.09). These data differ from those obtained in previously studied FHBL heterozygotes bearing apoB-2 and apoB-9, 2 very short truncations of apoB. Low HDL cholesterol and apoA-I levels were caused by higher apoA-I FCRs (0. 035+/-0.005 versus 0.018+/-0.005 pools per hour in controls, P<0.01) without significant decrease in apoA-I production rates (18.7+/-2.7 versus 22.8+/-5.6 mg. kg(-1). d(-1)). In conclusion, decreased secretion of apoB-containing lipoproteins and hypercatabolism of LDL account for low apoB and cholesterol levels in this novel form of FHBL.

Decreased production rates of VLDL triglycerides and ApoB-100 in subjects heterozygous for familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant disorder characterized by low levels of apolipoprotein (apo) B and low-density lipoprotein (LDL) cholesterol. Decreased production rates of apoB have been demonstrated in vivo in FHBL heterozygotes. In the present study, we wished to investigate whether the transport of triglycerides was similarly affected in these subjects. Therefore, we studied the in vivo kinetics of very-low-density lipoprotein (VLDL) triglycerides and VLDL apoB-100 simultaneously in 7 FHBL heterozygotes from 2 well-characterized kindreds and 7 healthy normolipidemic subjects. In both kindreds, hypobetalipoproteinemia is caused by mutations in the 5' portion of the apoB gene specifying short truncations of apoB undetectable in plasma. A bolus injection of deuterated palmitate and a primed constant infusion of deuterated leucine were given simultaneously, and their incorporation into VLDL triglycerides and VLDL apoB, respectively, were determined by gas chromatography-mass spectrometry. Kinetic parameters were calculated by using compartmental modeling. VLDL apoB fractional catabolic rates (FCRs) in FHBL heterozygotes and controls were similar (11. 6+/-3.9 and 10.9+/-2.4 pools per day, respectively, P=0.72). On the other hand, FHBL heterozygotes had a 75% decrease in VLDL apoB production rates compared with normal subjects (5.8+/-1.8 versus 23.4+/-7.1 mg/kg per day, P<0.001). The decreased production rates of VLDL apoB accounts for the very low concentrations of plasma apoB found in heterozygotes from these kindreds (24% of normal). Mean VLDL triglyceride FCRs in FHBL subjects and controls were not significantly different (1.06+/-0.74 versus 0.89+/-0.50 pools per hour, respectively, P=0.61). There was a good correlation between VLDL apoB FCR and VLDL triglyceride FCR in the 2 groups (r=0.84, P<0. 001). VLDL triglyceride production rates were decreased by 60% in FHBL heterozygotes compared with controls (9.3+/-6.0 versus 23.0+/-9. 6 micromol/kg per hour, P=0.008). Thus, the hepatic secretion of VLDL triglycerides is reduced in FHBL heterozygotes but to a lesser extent than the decrease in apoB-100 secretion. This is probably achieved by the secretion of VLDL particles enriched with triglycerides.

Effect of lipid transfer proteins on lipoprotein lipase induced transformation of VLDL and HDL

Lipoprotein lipase-induced lipolysis of human plasma VLDL usually does not yield a complete conversion of VLDL to LDL due to insufficient loss of surface and core lipids and apolipoprotein E. In order to assess the role of lipid transfer proteins in this process human VLDL and apo E free HDL, in approximately physiologic proportions, and with sufficient albumin to bind all released fatty acids, were subjected to 90% lipolysis of triglycerides in 2 h by lipoprotein lipase in the presence or absence of partially purified human cholesteryl ester and phospholipid transfer proteins. Lipoprotein lipase caused a partial transfer of VLDL unesterified cholesterol (16%) and phospholipid (11%), apo E (19%) and almost complete transfer of apo CII and CIII to HDL. VLDL remnants possessed excess apo E and surface and core lipids when compared to plasma LDL, and densities ranging from that of VLDL/IDL to LDL. With addition of the lipid transfer proteins to the lipolysis incubation there was an increased transfer of phospholipid and unesterified cholesterol (2-fold) and apo E (1.6-fold) to HDL over that for lipoprotein lipase incubations. The source of transferred material was primarily from remnants which isolated in the LDL density range in lipoprotein lipase incubations. This transfer resulted in LDL-like particles which had a smaller particle size but lighter density compared to those in lipoprotein lipase incubation. Transfer of cholesteryl esters to VLDL from HDL in exchange for triglyceride was absent or substantially reduced in incubations containing lipoprotein lipase and lipid transfer proteins compared to incubations with only lipid transfer proteins. It is concluded that during rapid lipolysis lipid transfer proteins promote the loss of phospholipid, unesterified cholesterol and apo E from VLDL remnants but do not promote the transfer of cholesteryl ester from HDL to VLDL.

The role of stable isotopes in the investigation of plasma lipoprotein metabolism

The last 5 years have seen promising beginnings of the application of stable-isotope-based methods to the study of lipoprotein metabolism. Many aspects of plasma lipid transport make it an attractive system for investigating by this means. While early efforts borrowed from standard techniques of generating and interpreting kinetic parameters (FSRs), the shortcomings of these procedures as applied to lipoproteins are now appreciated. Lipoprotein heterogeneity requires that multicompartmental analysis and relatively long-term studies be employed if the information obtained is going to be a useful adjunct to that produced by radio-iodinated lipoprotein tracers. Both approaches-stable-isotope-labeled endogenous tracer and ex vivo radio-iodinated lipoprotein experiments--must be considered complementary. The first provides direct information on lipoprotein synthesis pathways while the latter is superior at following interconversions and catabolic events. Excellent agreement has been demonstrated where the methods have been used simultaneously to estimate the same kinetic parameter. Many of the questions that have arisen from decades of radioactive tracer studies relate to the nature and rate of lipoprotein synthesis, e.g. what kinds of particles are produced by the liver when different diets are taken, and how are the synthetic pathways altered in dyslipidaemic states? Endogenous tracers can address these issues, and methods are presently available which provide the means for measuring the production of all apolipoproteins and for estimating cholesterol and fatty acid biosynthesis. When techniques are developed to examine triglyceride, cholesteryl ester and phospholipid production then a much clearer picture of how lipoproteins are assembled in vivo will emerge. This kind of information will be essential to an understanding of regulation of plasma lipid transport and the subtle changes that occur in those at risk for coronary heart disease.

Effect of L-carnitine treatment on very low density lipoprotein kinetics in the hyperlipidemic rabbit

This study examined the hypolipidemic effect of 4 weeks of L-carnitine treatment (170 mg/kg b.w./day) in New Zealand White rabbits fed a high fat diet (5% corn oil/0.5% cholesterol). Specifically, [3H] glycerol and [125I] very low density lipoprotein (VLDL) turnover studies were conducted to examine the effect of treatment on VLDL kinetics. The masses of plasma VLDL-triglycerides (VLDL-TG) and VLDL-apoprotein B (VLDL-apoB) were significantly increased by the high-fat diet. Four weeks of treatment with L-carnitine significantly reduced these masses. Kinetic analysis indicated that fat feeding reduced the fractional catabolic rates (FCRs) of VLDL-TG and VLDL-apoB relative to chow-fed controls. The transport of these VLDL components was not altered by the diet. L-carnitine treatment had no effect on the FCRs of VLDL-TG and VLDL-apoB or on the transport of VLDL-apoB. Yet, treatment significantly lowered the transport of VLDL-TG. These data indicate that the lipid-lowering effect of L-carnitine in this animal model was due, in part, to a decrease in the transport and not due to an alteration in the fractional catabolic rate of VLDL-TG.

Trafficking of dietary fat in lean rats

Despite increasing interest in the role that fuel partitioning plays in determining body composition, the relative importance of oxidative versus storage pathways in the clearance of dietary fat remains unclear. A widely held view is that the primary destination of chylomicron triglyceride fatty acids (TGFA) is adipose tissue, and the primary source of lipid fuel for skeletal muscle is non-esterified fatty acids (NEFA). An alternate view is that muscle, not adipose tissue, is the primary site of TGFA clearance. This view is supported by estimates of the total lipoprotein lipase content of muscle and adipose tissue. To directly study the partitioning of dietary fat between oxidation and storage, 14C-labeled oleic acid was fed to Sprague Dawley rats and its metabolic rate followed over 30 days. Two hours after ingestion, more than 3.5 times as much label was found in skeletal muscle tissue (2.42 +/- 0.45 nmols) and CO2 (0.25 +/- 0.01 nmols) than was found in adipose tissue (0.71 +/- 0.14 nmols). Intramuscular triglyceride was the lipid class most extensively labeled. After skeletal muscle, liver was the next most important site of TGFA clearance. Surprisingly a substantial quantity of label remained associated with the GI tract even 24 hours after ingestion. Between 2 and 10 days following ingestion there was a net decline in the 14C content of muscle, liver and GI tract, associated with a net rise in the 14C content of adipose tissue. These findings demonstrate: 1) the importance of skeletal muscle and liver in whole organism TGFA clearance, 2) the importance of intramuscular partitioning of lipid fuels between direct oxidation and storage as TG, 3) the potentially important role of the GI tract in the delivery of dietary fat to the circulation 10-24 hours following ingestion, and 4) the stability of adipose tissue as a storage site. The complex nature of the tissue-specific clearance of TGFA over time is perhaps better described by the term "trafficking" than by the more commonly used term "partitioning." Future studies of TGFA clearance combined with sampling of relevant tissues over time will provide insight into the specific roles that abnormalities in liver, muscle and adipose tissue TGFA metabolism play in the development of hypertriglyceridemic disorders and states of increased or reduced body weight.

N-3 fatty acids from fish oil. Effects on plasma lipoproteins and hypertriglyceridemic patients

In the experimental studies reported in this review, dietary n-3 fatty acids from fish and fish oil had profound hypolipidemic effects in normal subjects and in hypertriglyceridemic patients with combined hyperlipidemia (type II-b) and types IV and V hyperlipidemia. In these carefully controlled metabolic experiments, dramatic reductions occurred in plasma triglycerides and to a lesser extent in plasma total cholesterol. Reductions in VLDL, chylomicrons, remnants, LDL, apo B, and apo E were also noted. HDL changes varied from subject to subject. These plasma lipoprotein changes occurred in subjects with non-insulin-dependent diabetes mellitus as well, without deterioration of diabetic control. Similar results are reported in two other papers in this volume. Fish oil did not cause deterioration of diabetic control. Whereas the mechanism of the hypolipidemic action of the n-6 rich vegetable oils containing linoleic acid such as corn or safflower oil still remains obscure, the mechanism of the hypolipidemic action of the n-3 fatty acids in fish oil is well documented. The synthesis of triglyceride and VLDL in the liver is greatly reduced by n-3 fatty acids. At the same time, the turnover of VLDL in plasma is shortened. In another study, LDL production was decreased. Combined with other dietary manipulations, such as a reduction in saturated fat and dietary cholesterol, the use of n-3 fatty acids to treat hyperlipidemia, especially hypertriglyceridemia, appears to have a well-supported rationale. Fish oil combined with a low cholesterol, low saturated fat diet has been shown to produce complementary effects. Total plasma cholesterol and LDL cholesterol were lowered by the low cholesterol, low saturated fat diet, whereas plasma triglyceride and VLDL were decreased by the fish oil. In most situations, the use of fish oil supplements should be regarded as pharmacologic therapy, particularly effective in severe hypertriglyceridemic states (e.g., chylomicronemia). However, a lifelong diet rich in fish may be protective against atherosclerosis as well. Further studies are required to delineate exact doses and precise indications for the use of fish oil in different types of hyperlipidemias and to differentiate the effects, if any, of the two major n-3 fatty acids in fish oil, EPA and DHA. The hypolipidemic effects of n-3 fatty acids coupled with their known antithrombotic actions (secondary to changes in prostaglandin secretion, platelet function, inhibition of growth factors, and enhancement of endothelial-derived relaxation factor) appear to have an important potential role in the control of coronary heart disease and other atherosclerotic disorders. Moreover, fish oil may prevent the "chylomicronemia" syndrome of type V hyperlipidemia.

Stabilisation of plasma substrate concentrations: A model for conducting metabolic studies

Metabolic studies involving changes in plasma substrate concentrations are frequently carried out after an overnight fast. This condition, however, is a transition between the post-prandial period and the beginning of starvation, and thus is associated with rapid changes in the plasma concentration of many substrates. Such alterations might interfere with the interpretation of modifications in plasma concentrations resulting from experimental manipulations. Infusion of glucose at a rate of 250 mg kg(-1) h(-1) for 1h and subsequently at 162 mg kg(-1) h(-1) together with amino-acids at 50 mg kg(-1) h(-1) is used to stabilise the plasma concentration of most substrates within 3 h, a condition which is maintained for the subsequent 5 h or more. This study offers a model which is more suitable for many metabolic investigations than overnight fasting and which takes little time or expense to prepare.

Phenotypic expression of heterozygous lipoprotein lipase deficiency in the extended pedigree of a proband homozygous for a missense mutation

Familial lipoprotein lipase (LPL) deficiency is a rare genetic disorder accompanied by well-characterized manifestations. The phenotypic expression of heterozygous LPL deficiency has not been so clearly defined. We studied the pedigree of a proband known to be homozygous for a mutation resulting in nonfunctional LPL. Hybridization of DNA from 126 members with allele-specific probes detected 29 carriers of the mutant allele. Adipose tissue LPL activity, measured previously, was reduced by 50% in carriers, but did not reliably distinguish them from noncarriers. Carriers were prone to the expression of a form of familial hypertriglyceridemia characterized by increased plasma triglyceride, VLDL cholesterol and apolipoprotein B, and decreased LDL and HDL cholesterol concentrations. These manifestations were age modulated, with conspicuous differences between carriers and noncarriers observed only after age 40. Several noncarriers exhibited similar lipid abnormalities, but without the inverse relationship between VLDL cholesterol and LDL cholesterol noted among carriers. In addition to age and carrier status, the potentially reversible conditions, obesity, hyperinsulinemia and lipid-raising drug use were contributory. Thus heterozygous lipoprotein lipase deficiency, together with age-related influences, may account for a form of familial hypertriglyceridemia.

Increased turnover of very low density lipoprotein triglyceride during treatment with cholestyramine in familial hypercholesterolaemia

Kinetics of very low density lipoprotein (VLDL) triglyceride were determined in seven patients with heterozygous familial hypercholesterolaemia, using a 3H-glycerol technique. The study was repeated after 5-7 weeks of therapy with the bile acid-binding resin, cholestyramine. The rate of synthesis of VLDL triglyceride was increased by 85% (P less than 0.05) during resin therapy. Simultaneously, the fractional catabolic rate of VLDL was increased by 40% (P less than 0.02), so that only a moderate increase in plasma concentration was observed. Repeated measurements of VLDL size by electron microscopy (before, 1 week, and 5-7 weeks after initiation of therapy) indicated that a transient increase in VLDL size occurred in response to cholestyramine. The results are consistent with a stimulatory effect of bile acid sequestrants on VLDL triglyceride production and indicate that, in most subjects, a compensatory increase in VLDL triglyceride removal occurs.

Presence of very low density lipoprotein compositional abnormalities in type 1 (insulin-dependent) diabetic patients; effects of blood glucose optimisation

Plasma lipoprotein compositional abnormalities were investigated in eight normolipidaemic (plasma cholesterol less than 5.70 mmol/l; triglyceride less than 2.03 mmol/l) young male Type 1 (insulin-dependent) diabetic patients (before and after a short period of optimised blood glucose control) and in nine healthy control subjects, matched for sex, age and body mass index. Free and esterified cholesterol, triglyceride, phospholipids were assayed in all lipoprotein classes (VLDL, IDL, LDL) and in HDL subclasses (HDL2 and HDL3); apoB was measured only in very low density lipoproteins (VLDL). All VLDL constituents were increased in the diabetic group, the differences being more striking for apoB (6.0 +/- 1.1 mg/dl vs 2.0 +/- 0.1 mg/dl, p less than 0.02), free cholesterol (0.27 +/- 0.04 mmol/l vs 0.13 +/- 0.02 mmol/l, p less than 0.02) and esterified cholesterol (0.32 +/- 0.08 mmol/l vs 0.13 +/- 0.01 mmol/l, p less than 0.05). Also HDL subfractions showed differences between the two groups: all HDL2 constituents were increased, while in HDL3 only triglyceride was significantly increased (0.11 +/- 0.01 mmol/l vs 0.08 +/- 0.004 mmol/l, p less than 0.02). After two weeks of optimised blood glucose control all VLDL constituents were reduced and particularly: esterified cholesterol (-39%, p less than 0.02), free cholesterol (-37%, p less than 0.05), apoB (-35%, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)