The 3H-leucine tracer: its use in kinetic studies of plasma lipoproteins

Unravelling lipoprotein metabolism with stable isotopes: tracing the flow

Dysregulated lipoprotein metabolism is a major cause of atherosclerotic cardiovascular disease (ASCVD). Use of stable isotope tracers and compartmental modelling have provided deeper understanding of the mechanisms underlying lipid disorders in patients at high risk of ASCVD, including familial hypercholesterolemia (FH), elevated lipoprotein(a) [Lp(a)] and metabolic syndrome (MetS). In patients with FH, deficiency in low-density lipoprotein (LDL) receptor activity not only impairs the catabolism of LDL, but also induces hepatic overproduction and decreases catabolism of triglyceride-rich lipoproteins (TRLs). Patients with elevated Lp(a) are characterized by increased hepatic secretion of Lp(a) particles. Atherogenic dyslipidemia in MetS patients relates to a combination of overproduction of very-low density lipoprotein-apolipoprotein (apo) B-100, decreased catabolism of apoB-100-containing particles, and increased catabolism of high-density lipoprotein-apoA-I particles, as well as to impaired clearance of TRLs in the postprandial state. Kinetic studies show that weight loss, fish oils, statins and fibrates have complementary modes of action that correct atherogenic dyslipidemia. Defining the kinetic mechanisms of action of proprotein convertase subtilisin/kexin type 9 and angiopoietin-like 3 inhibitors on lipid and lipoprotein mechanism in dyslipidemic subjects will further our understanding of these therapies in decreasing the development of ASCVD. "Everything changes but change itself. Everything flows and nothing remains the same... You cannot step twice into the same river, for other waters and yet others go flowing ever on." Heraclitus (c.535- c. 475 BCE).

SRM-based measurements of proprotein convertase subtilisin/kexin type 9 and lipoprotein(a) kinetics in nonhuman primate serum

Aim: PCSK9 and Lp(a) have been identified as potential biomarkers for cardiovascular disease. The ability to measure protein turnover rates will provide insights into the dynamic properties of these proteins and lead to better understanding of their biological roles. We aimed to implement the stable isotope-labeled tracers ([2H3]-leucine) and develop a novel LC-selected reaction monitoring (SRM) mass spectrometry (MS) method to study the kinetics of PCSK9 and Lp(a). Results: A sensitive method using immunoaffinity enrichment coupled with LC-SRM MS was developed to measure the production and degradation rates of PCSK9 and Lp(a) in naive nonhuman primate serum. Comparable results were obtained from two different routes of tracer administration. Conclusion: Immunoaffinity enrichment coupled with LC-SRM MS demonstrated success in in vivo kinetic measurements of proteins with relatively slow turnover rate (Lp[a]) or low abundance (PCSK9) in serum.

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

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

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

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

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.

New model for kinetic studies of HDL metabolism in humans

BACKGROUND

The aim of the study was to develop a new model for kinetic studies of Apolipoprotein A-I of HDL (Apo A-I-HDL) labelled with stable isotope by using HDL subclasses isolated with fast protein liquid chromatography (FPLC).

MATERIALS AND METHODS

Apo A-I-HDL kinetics were studied by infusing [5.5.5-(2)H(3)]-leucine for 14 h in six healthy subjects. Prebeta(1) and alphaHDL were separated by FPLC and total HDL by ultracentrifugation (HDL-UC).

RESULTS

The tracer-to-tracee ratios were higher in prebeta(1) HDL than in HDL-UC or alphaHDL. Leucine enrichments found in HDL-UC were higher compared with alphaHDL, suggesting that HDL-UC were composed of a mixture of Apo A-I-alphaHDL and Apo A-I-prebeta(1) HDL. Kinetic analysis of data obtained from FPLC was achieved using a multicompartmental model, including a conversion between prebeta(1) and alphaHDL compartments. The production rate of prebeta(1) HDL was 7.72 +/- 2.86 mg kg(-1) d(-1) (mean +/- SD). Prebeta(1) HDL were converted to alphaHDL at a rate of 96.24 +/- 42.99 pool d(-1), and the synthesis rate of prebeta(1) HDL from alphaHDL was 10-fold slower: 7.09 +/- 4.51 pool d(-1). Apo A-I-FCR of HDL-UC was estimated using a one-compartment model (0.165 +/- 0.074 pool d(-1)), and was higher but not significantly compared with FCR of Apo A-I-alphaHDL (0.112 +/- 0.026 pool d(-1)) calculated with the new model.

CONCLUSIONS

This study reports for the first time a model involving enrichments of Apo A-I in prebeta(1) and alphaHDL which allowed the measure of Apo A-I cycling within HDL fraction and will aid better understanding of kinetics of HDL in humans.

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.

Stable isotope turnover of apolipoproteins of high-density lipoproteins in humans

Amino acid precursors labelled with stable isotopes have been successfully used to explore the metabolism of the apolipoproteins of HDL. Some methodological and mathematical modelling problems remain, mainly related to amino acid recycling in a plasma protein such as apolipoprotein A-I with a long residence time (the reciprocal of the fractional catabolic rate) of 4-5 days. Apolipoprotein A-I, apolipoprotein E, and apolipoprotein A-IV in triglyceride-rich lipoproteins (containing chylomicrons, VLDL, and remnants) exhibit more complex kinetics. The small amounts of apolipoprotein A-I and of apolipoprotein A-IV in the triglyceride-rich lipoproteins have a residence time similar to that of the apolipoprotein A-I of HDL. In contrast, the apolipoprotein E in triglyceride-rich lipoproteins has been found to have an average residence time of 0.11 days. Diets low in saturated fat and cholesterol, which lower HDL levels, do so by decreasing the secretion of apolipoprotein A-I, with apolipoprotein A-II kinetics unaffected. Individuals with impaired glucose tolerance have a decreased residence time of apolipoprotein A-I but no change in secretion rate or in apolipoprotein A-II kinetics. This suggests a link between insulin resistance and the risk of atherosclerosis. In heterozygous familial hypercholesterolemia, both the fractional catabolic rate and the secretion rate of apolipoprotein A-I are increased, resulting in no change in the plasma level. Stable isotope studies have strengthened the evidence that triglyceride enrichment of HDL increases its catabolism Laboratory.

Human apolipoprotein (Apo) B-48 and ApoB-100 kinetics with stable isotopes

The kinetics of apolipoprotein (apo) B-100 and apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within IDL and LDL were examined with a primed-constant infusion of (5,5,5-(2)H(3)) leucine in the fed state (hourly feeding) in 19 subjects after consumption of an average American diet (36% fat). Lipoproteins were isolated by ultracentrifugation and apolipoproteins by SDS gels, and isotope enrichment was assessed by gas chromatography/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. The pool sizes (PS) of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were 17+/-10, 273+/-167, and 3325+/-1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73+/-3.48 versus 5.02+/-2.07 pools/d, respectively, P=0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07+/-7.28 and 0.27+/-0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1. 3+/-0.90 versus 20.06+/-6.53 mg. kg(-1). d(-1), P<0.0001). TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and by apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). These data indicate that (1) the FCRs of TRL apoB-48 and VLDL apoB-100 are similar in the fed state, (2) TRL apoB-48 PS is correlated with TRL apoB-48 PR, (3) VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and PR of TRL apoB-48, and (4) LDL apoB-100 PS is correlated with LDL FCR.

Development of compartmental models in stable-isotope experiments: application to lipid metabolism

Kinetic experiments are of great importance in lipid research because they further the understanding of lipid metabolism in vivo and help to explain the physiopathology of lipid disorders in humans. At present, due to species specificity, no valid animal model can efficiently replace a study in humans to explore lipid metabolism, and the use of radioactive tracers is restricted in humans. Thus, stable-isotope tracer kinetic studies have become an important component of research programs to achieve in humans a quantitative understanding of the dynamics of metabolic processes in vivo. The aim of this review is to describe the practical aspects of compartmental model development in stable-isotope experiments. The recent development of computer hardware and modeling software has dramatically facilitated the task of the modeler in his or her calculations. In the current review, we show that the model may be considered an integral component of the experimental design and that model development must obey strict rules to provide a rigorous solution. The main difficulties of model development in tracer experiments, such as experiment design, model identifiability, data expression, comparison of models, or tracer recycling, are presented with extensive references. We have paid particular attention to kinetic modeling in stable-isotope experiments because they have shown the greatest development in recent years.

Development of kinetic models in the nonlinear world of molecular cell biology

Increasingly, successful research on metabolic systems relies on teams of specialists. Because of the enormous complexity of these systems, many experimental groups have sought collaborations with theoreticians for data analysis and modeling. Predictably, cultural differences in scientific approach, methodology, assumptions, and language have led to some persistent difficulties in communication across the experiment-theory frontier. This report attempts to diagnose some of these difficulties from the perspective of 30 years' experience in both experimental and theoretical biology, and to suggest guidelines for effective collaboration between experimentalists and theorists. As these collaborations move to the level of cellular and molecular biology, effective communication will become all the more important because the simple linear rate laws of radiotracer and stable-isotope kinetics will no longer suffice. This is because every form of regulation and control, hallmarks of metabolic systems, results in nonlinear kinetics. To advance this transition to nonlinear cellular and molecular metabolic models and to facilitate communication between experimental and theoretical collaborators, a general procedure for incorporating control mechanisms in metabolic rate laws is developed based on the familiar rapid-equilibrium assumption of classical enzyme kinetics.