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Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2005-1895
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The Journal of Clinical Endocrinology & Metabolism Vol. 91, No. 3 973-979
Copyright © 2006 by The Endocrine Society

High-Density Lipoprotein (HDL) Transport in the Metabolic Syndrome: Application of a New Model for HDL Particle Kinetics

Juying Ji, Gerald F. Watts, Anthony G. Johnson, Dick C. Chan, Esther M. M. Ooi, Kerry-Anne Rye, Adrian P. Serone and P. Hugh R. Barrett

Lipoprotein Research Unit (J.J., G.F.W., D.C.C., E.M.M.O., P.H.R.B.), School of Medicine and Pharmacology, University of Western Australia, The West Australian Institute for Medical Research, Perth, WA 6847, Australia; GlaxoSmithKline R & D (A.G.J.), King of Prussia, Pennsylvania 19406; Lipid Research Group (K.-A.R.), The Heart Research Institute, Sydney, Australia; Department of Medicine (K.-A.R.), University of Sydney, Sydney, Australia; Department of Medicine (K.-A.R.), University of Melbourne, Melbourne, Australia; and James Lance GlaxoSmithKline Medicines Research Unit (A.P.S.), Prince of Wales Hospital, Sydney, NSW 2052, Australia

Address all correspondence and requests for reprints to: Professor G. F. Watts, School of Medicine and Pharmacology, Royal Perth Hospital, Box X2213 GPO, Perth, WA 6847, Australia. E-mail: gfwatts{at}cyllene.uwa.edu.au.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: Reduced high density lipoprotein (HDL) concentration in the metabolic syndrome (MetS) is associated with increased risk of diabetes and cardiovascular disease and is related to defects in the kinetics of HDL apolipoprotein (apo) A-I and A-II.

Objective: The objective of the study was to investigate HDL apoA-I and apoA-II kinetics in nondiabetic men with MetS and lean controls by developing a model that describes the kinetics of lipoprotein (Lp)A-I and LpA-I:A-II particles.

Design: Twenty-three MetS men and 10 age-matched lean controls were investigated. ApoA-I and apoA-II tracer/tracee ratios were studied after iv d3-leucine administration using gas chromatography mass spectrometry.

Results: Compared with lean subjects, MetS subjects had accelerated catabolism of LpA-I (P < 0.001), LpA-I:A-II (P = 0.005), and apoA-II (P = 0.005); the production rate of LpA-I was also significantly elevated in MetS, so that the dominant changes in plasma concentrations were reduction in LpA-I:A-II (P < 0.001) and apoA-II (P < 0.05). Increased catabolism of LpA-I and LpA-I:A-II was directly related to increased waist circumference, hypertriglyceridemia, low HDL-cholesterol, small HDL particle size, hyperinsulinemia, and low phospholipid transfer protein (PLTP) activity; overproduction of LpA-I was significantly associated with increased waist circumference, insulin resistance, and low PLTP activity.

Conclusions: MetS men exhibit hypercatabolism of the two major HDL lipoprotein particles, LpA-I and LpA-I:A-II, but selective overproduction of LpA-I maintains a normal plasma concentration of LpA-I. These kinetic perturbations are probably related to central obesity, insulin resistance, hypertriglyceridemia, and low plasma PLTP activity.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
THE METABOLIC SYNDROME (MetS) portends diabetes and cardiovascular disease (CVD) (1). Dyslipoproteinemia, reflected by elevated plasma triglyceride and reduced high-density lipoprotein (HDL) concentrations, is a cardinal feature of MetS that independently predicts CVD (2) and is accordingly a therapeutic target for risk reduction.

Diverse evidence indicates that reduction in plasma HDL-cholesterol increases the risk of CVD. Apolipoprotein A-I (apoA-I) and apoA-II are the major apolipoproteins of HDL. The antiatherogenicity of apoA-I is well supported by experimental and clinical studies (3), with compelling but less consistent evidence available for apoA-II (4). Plasma HDL-cholesterol concentration is directly correlated with apoA-I concentration, which is physiologically dependent chiefly on apoA-I fractional catabolic rate (FCR) (5, 6). However, in normolipidemic individuals, plasma apoA-II levels are mainly determined by apoA-II production rate (PR) (5, 6, 7), which when elevated may decrease apoA-I FCR (7).

In contrast to other plasma lipoproteins, HDLs are widely heterogeneous. According to apolipoprotein composition, HDLs can be separated into lipoprotein A-I (LpA-I) particles, containing apoA-I alone, and LpA-I:A-II particles, containing both apoA-I and apoA-II (8). These particles are subject to modifications by several lipases and lipid transfer proteins, particularly lipoprotein lipase (LPL), hepatic lipase (HL), cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP) (9). Some studies suggests that LpA-I is more effective than LpA-I:A-II in promoting cellular cholesterol efflux in vitro, supporting the former’s primary antiatherogenic role (10). An angiographic study has, however, suggested that the plasma concentration, and by implication the kinetics, of LpA-I:A-II particles may be the major determinant of cardiovascular risk in type 2 diabetes (11). In these patients, LpA-I:A-II particles may also have a greater cholesterol efflux capacity in vitro than LpA-I particles.

Previous studies of HDL kinetics in subjects with impaired glucose tolerance and type 2 diabetes have consistently demonstrated elevation in apoA-I FCR (12, 13, 14). This abnormality is related chiefly to a remodelling of the HDL particle as a result of hypertriglyceridemia. Two small reports in similar subjects have also suggested that impaired glucose tolerance is not associated with changes in HDL apoA-II kinetics (12, 15). However, observations in larger patient groups have shown that hypertriglyceridemia and abdominal obesity, two features of MetS, are associated with accelerated apoA-II catabolism (14, 16). Although no investigations have specifically studied the impact of MetS on LpA-I and LpA-I:A-II particle kinetics, a small study in postmenopausal women has indicated that intraabdominal fat lowers plasma HDL levels by increasing the FCR of apoA-I in LpA-I (17).

Our major hypothesis in the present study was that MetS accelerates the catabolism of apoA-I in both LpA-I and LpA-I:A-II and apoA-II in LpA-I:A-II, based on the notion that in the presence of insulin resistance and hypertriglyceridemia, all HDL lipoprotein particles would be remodeled and hypercatabolized because of altered LPL, HL, CETP, and PLTP activities (9). We also aimed to explore corresponding changes in PRs and their impact on plasma lipoprotein concentrations. We extend our previous case-control comparison of apoA-I kinetics in MetS men by examining the kinetics of apoA-I in LpA-I and LpA-I:A-II particles in a larger sample population (18). To investigate this hypothesis, we generated, from previously published radiokinetic studies (19, 20), a new model for HDL particle metabolism that we also validated with an independent source of stable isotope data (21).


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Twenty-three men with MetS were recruited, as previously described (18). MetS was defined as at least three of the following on a weight-maintenance diet: waist circumference more than 102 cm, triglycerides higher than 1.7 mmol/liter, HDL-cholesterol less than 1.05 mmol/liter, blood pressure at least 130/85 mm Hg, and glucose greater than 6.1 mmol/liter. Exclusions included cholesterol greater than 7 mmo/liter, triglycerides greater than 4.5 mmo/liter, diabetes mellitus, alcohol more than 30 g/d, use of lipid regulators, apolipoprotein E2/E2, proteinuria, creatinemia (>120 µmol/liter), and hypothyroidism. We also studied 10 healthy age-matched, lean men [body mass index (BMI), 19–25 kg/m2], with waist circumference no more than 94 cm, triglycerides less than 1.7 mmol/liter, HDL-cholesterol at least 1.1 mmol/liter, and blood pressure less than 140/90 mm Hg. All subjects provided informed written consent, as approved by the Ethics Committees of the South Eastern Sydney Area Health Service and Royal Perth Hospital.

Study design and clinical protocols

All subjects were admitted to the metabolic ward in the morning after a 12-h fast. They were studied semirecumbent and allowed water only. Venous blood was collected for biochemical measurements. Body weight was measured, after bladder emptying, in light clothing, and height without shoes. Arterial blood pressure was recorded supine using a Dinamap1846 SX/P monitor (Critikon, Tampa, FL). Dietary intake was assessed using at least two 24-h dietary diaries and DIET 4 Nutrient Calculation Software (Xyris Software, Highgate Hill, Australia).

A primed infusion of d3-leucine (1 mg/kg bolus and 1 mg/kg·h infusion) was administered iv for 6 h into an antecubital vein via a Teflon cannula. Blood samples were taken at baseline and at 15, 30, and 45 min and 1, 2, 4, 6, 8, 10, 12, and 16 h after isotope injection, with additional fasting blood samples (24, 48, 72, and 96 h) collected in the morning on the following 4 d (18).

Isolation and measurement of isotopic enrichment of apoA-I and apoA-II

ApoB-containing lipoproteins were precipitated from plasma and HDL isolated by ultracentrifugation as previously described (18). ApoA-I and apoA-II were isolated from HDL fraction by SDS-PAGE nonreducing gel electrophoresis and blotted onto polyvinylidene difluoride membrane; apoA-I and apoA-II bands were excised from the membrane, hydrolyzed with 6 M HCl at 110 C for 16 h, and dried for derivatization by an oxazolinone method (18). Plasma-free leucine was isolated by cation-exchange chromatography and isotopic enrichment determined using GCMS, as previously described (18). Tracer to tracee ratios were derived from isotopic ratios.

Quantification of apoA-I, apoA-II, LpA-I, and LpA-I:A-II and other analyses

Aliquots of plasma from 4 separate days were pooled for HDL apoA-I and apoA-II measurements, assayed as total plasma apoA-I and apoA-II concentrations, assuming that more than 90% of apoA-I and apoA-II resides in HDL (5). Total plasma apoB, apoA-I, and apoA-II concentrations were determined by immunonephelometry (Dade Behring, Chicago, IL), with interassay coefficient of variation (CV) of less than 4.3%. LpA-I concentration was measured by differential electroimmuoassay on ready-to-use plates (Sebia, Moulineaux, France), with interassay CV less than 5% (22). LpA-I:A-II concentration was calculated as total apoA-I minus LpA-I (10, 11, 20).

Plasma total cholesterol, HDL-cholesterol, triglycerides, and glucose concentrations were determined by enzymatic methods as previously described (18). HDL2 (1.063–1.125 g/liter) and HDL3 (1.125–1.21 g/liter) were isolated by ultracentrifugation from 0.5 ml plasma, and cholesterol concentrations were measured. HDL particle size was estimated as the HDL-cholesterol/protein ratio, where protein = apoA-I + apoA-II concentrations (14). Plasma insulin was estimated by RIA (DiaSorini, Saluggia, Italy) and insulin resistance by homeostasis model assessment (HOMA) scores. Plasma adiponectin concentration was determined using an enzyme immunoassay kit (R&D Systems, Minneapolis, MN) with interassay CV less than 7%.

Plasma CETP activity was analyzed by an exogenous assay, measuring the transfer of radiolabeled [3H]cholesteryl ester from exogenous donor (HDL3) to acceptor [low-density lipoprotein (LDL)], with interassay CV less than 5%. PLTP activity was determined by measuring the transfer of radiolabeled [14C]phosphatidylcholine from vesicles (small unilamellar vesicles) to isolated HDL, precipitating the vesicles with a MnCl2/heparin solution and counting the [14C]phosphatidylcholine remaining in the supernatant, with interassay CV less than 10%.

Kinetic analyses: rationale and model development

Figure 1Go shows the compartment model developed and used to describe HDL apoA-I and apoA-II leucine tracer/tracee ratio data, based on the SAAM II program (SAAM Institute, Seattle, WA). Plasma leucine tracer data were modeled using a four-compartment system: compartment 1 is connected to compartment 5, a compartment that accounts for the synthesis and secretion of apoA-I and apoA-II into plasma; compartments 6 and 7 represent apoA-I associated with the LpA-I and LpA-I:A-II HDL particles, respectively; and compartment 8 describes the plasma kinetics of HDL apoA-II. Although apoA-I and apoA-II are secreted from both the liver and intestine, a single compartment was used to account for the process of synthesis and secretion into plasma.


Figure 1
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  		FIG. 1. Compartment model describing apoA-I in LpA-I and LpA-I:A-II particles and apoA-II tracer kinetics. Leucine tracer is injected into plasma, compartment 2, and distributes to extravascular compartments 1, 3, and 4. Compartments 1–4 are required to describe leucine tracer kinetics observed in plasma. Compartment 1 is connected to an intracellular (hepatic, enterocytic) delay compartment (compartment 5) that accounts for the synthesis, assembly, and secretion of apoA-I and apoA-II. Compartments 6 and 7 describe the plasma kinetics of HDL apoA-I associated with LpA-I and LpA-I:A-II particles, respectively. Plasma kinetics of HDL apoA-II, which represents the apoA-II component of LpA-I:A-II, is described by compartment 8.

 
The LpA-I/LpA-I:A-II section of the model was developed using plasma HDL apoA-I and apoA-II tracer data in conjunction with the findings of two published studies (19, 20). Zech et al. (19) developed a compartment model, using exogenous radiotracer data, that identified two plasma pools of apoA-I. Furthermore, the kinetics of one of the apoA-I compartments was similar to that of apoA-II. More recently, Rader et al. (20) observed that the FCR of LpA-I was greater than that of LpA-I:A-II and that the FCR of HDL apoA-II was similar to apoA-I in LpA-I:A-II particles. Using this information, we constructed and tested a model that assumed the kinetics of apoA-I in the LpA-I:A-II particle were the same as HDL apoA-II (19). The fractional rate constants out of compartments 7 and 8 [k (0, 7) and k (0, 8)] were therefore constrained to be equal. The kinetics of compartments 7 and 8 were determined by the fit to the HDL apoA-II tracer data. The mass of apoA-I in the LpA-I and LpA-I:A-II compartments (compartments 6 and 7) was fixed using the LpA-I immunoassay data. In fitting the model to the tracer data, the HDL apoA-II tracer data determined the kinetics parameters of compartments 8 and 7. The HDL apoA-I tracer data represented the sum of the kinetics of apoAI in the LpA-I and LpA-I:A-II particles, where the kinetic parameters of LpA-I:A-II were defined by the kinetics of apoA-II. We validated this assumption using apoA-I enrichment data obtained in LpA-I and LpA-I:A-II particles, separated from plasma by immunoaffinity chromatography, obtained after infusion of d3-leucine in five male subjects with mixed hyperlipidemia (21). Adhering to the principle of parsimony, the model shown in Fig. 1Go provided good fits (weighted residual sum of squares distributed randomly with mean zero); precision of parameters was high with individual coefficients of variation (SD/mean) for LpA-I and LpAI:A-II FCRs of less than 21.6 and 10.3%, respectively. In addition, we used the Akaike information criteria test to compare different model structures. The mean FCRs for LpA-I and LpAI:A-II particles in the five male subjects were 0.48 ± 0.06 and 0.23 ± 0.01 pools/d, respectively. Consistent with this, the predicted (simulated) tracer curves for the LpA-I and LpA-I:A-II particles (Fig. 2Go) showed faster kinetics of LpA-I particles relative to LpA-I:A-II particles in MetS and lean subjects.


Figure 2
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  		FIG. 2. Fit of compartment model to HDL apoA-I and apoA-II enrichment data. A and B, Tracer data in two MetS subjects; C and D, tracer data in two lean controls. The symbols represent the enrichment data, and the solid lines through the data show the best fit of the model to the tracer data. The dashed lines, labeled LpA-I and LpA-I:A-II, show the predicted tracer time course for LpA-I and LpA-I:A-II particles, respectively. The faster rise and fall of the LpA-I tracer curve compared with the LpA-I:A-II curve is a reflection of the higher FCR of the LpA-I particles.

 
An extravascular compartment has often been used to help describe the kinetics of HDL apoA-I and apoA-II, although the exact nature of the exchange compartment is unknown. Despite this, we tested whether adding an extravascular compartment would improve the fit of the model to the tracer data. The tracer data did not support the presence of such a compartment, and this was reflected by the poor precision of the model parameters and a larger value for the Akaike information criteria test. Although an extravascular compartment may exist, the present study does not permit us to resolve this, in part because we have added complexity to the model that accounts for HDL heterogeneity within the plasma pool. In addition, we explored possible precursor-product relationships between the LpA-I and LpA-I:A-II compartments, but the data did not support such pathways.

The FCRs of apoA-I in LpA-I and LpA-I:A-II, apoA-I, and apoA-II were estimated after simultaneously fitting the model to the apoA-I and apoA-II tracer/tracee ratio and LpA-I and LpA-I:A-II mass data. The corresponding PRs were calculated as the product of FCR and pool size, which equals the plasma concentration multiplied by plasma volume; plasma volume was estimated as 4.5% of body weight, with appropriate adjustment made for the effects of an increase in relative body weight. In the present study, the kinetics of apoA-I in LpA-I and LpA-I:A-II particles were denoted as the kinetics of LpA-I and LpA-I:A-II, respectively.

Statistical analyses

Statistical analyses were carried out using SPSS 11.5 (Chicago, IL). Skewed variables were logarithmically transformed. Groups were compared using independent t tests, with a priori hypotheses. Associations were examined by linear regression analyses.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Comparison of group characteristics

Compared with lean subjects, MetS subjects had significantly higher body weight, BMI, waist circumference, diastolic blood pressure, and HOMA score as well as significantly higher plasma concentrations of glucose, insulin, cholesterol, triglycerides, LDL-cholesterol, and apoB (Table 1Go). Conversely, MetS subjects had significantly lower plasma adiponectin, HDL-, HDL2-, and HDL3-cholesterol concentrations as well as lower HDL-cholesterol/protein ratio, indicative of smaller HDL particle size. PLTP activity was significantly lower in MetS compared with controls, but there was no significant difference in CETP activity. Mean daily dietary intake of energy (8109 ± 21 vs. 7037 ± 122 kJ), total fat (73.2 ± 4.0 vs. 59.9 ± 12.4 g), saturates (30.7 ± 2.1 vs. 24.6 ± 4.8 g), polyunsaturates (13.1 ± 0.8 vs. 12.2 ± 3.1 g), monounsaturates (29.5 ± 1.8 vs. 23.2 ± 5.3 g), carbohydrate (228.8 ± 12.9 vs. 198.5 ± 45.2 g), cholesterol (314.7 ± 39.3 vs. 218.3 ± 46.1 g), and alcohol (1.4 ± 0.9 vs. 8.1 ± 5.6 g) did not differ significantly between groups.


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  		TABLE 1. Clinical and biochemical characteristics of the subjects with the MetS and the lean controls

 
Kinetic differences for MetS vs. control group

Table 2Go shows kinetic characteristics of LpA-I, LpA-I:A-II, apoA-II, and plasma apoA-I (LpA-I plus LpA-I:A-II) in MetS and lean subjects. Both LpA-I FCR and PR were significantly increased by an average of 91 and 88%, respectively, in MetS subjects, accounting for the lack of group difference in plasma LpA-I concentration. LpA-I:A-II FCR was also significantly increased by 30% in MetS subjects, with no significant difference in the corresponding PR, accounting for their significantly lower LpA-I:A-II concentration compared with controls. Consistent with our model, apoA-II FCR was also significantly higher by 30% in the cases compared with controls; however, there was no significant group difference in apoA-II production, so that MetS subjects had significantly lower plasma concentration of apoA-II. Table 2Go also shows that although the FCR of plasma apoA-I was significantly increased in MetS, the corresponding PR was similar to controls, accounting for the significantly lower apoA-I concentration in MetS.


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  		TABLE 2. Kinetic characteristics of LpA-I, LpA-I:A-II, apoA-II and plasma apoA-I in subjects with the metabolic syndrome and lean controls

 
Kinetic associations in MetS group

In the MetS group, the concentrations of LpA-I and LpA-I:A-II were significantly positively correlated with their correspondent PRs (r = 0.614, P < 0.01; r = 0.416, P < 0.05, respectively). However, no significant associations were found between any HDL kinetic parameters and triglyceride, measures of insulin resistance, adiponectin, CETP and PLTP activities, or dietary intake.

Kinetic associations in combined group

In the combined group of MetS and lean controls, LpA-I FCR was positively associated (P < 0.05) with waist circumference, insulin, and triglycerides (r = 0.456, 0.646, and 0.603, respectively) but negatively associated with adiponectin concentration (r = –0.422, P < 0.05). Similar significant correlations were found between the LpA-I:A-II FCR and the above parameters. LpA-I PR was positively associated (P < 0.05) with waist circumference and insulin (r = 0.409 and 0.402). Plasma HDL2- and HDL3-cholesterol concentrations and HDL particle size were all significantly and inversely correlated (P < 0.05) with the FCRs of LpA-I and LpA-I:A-II. PLTP activity was significantly inversely correlated (P < 0.05) with the FCRs of LpA-I and LpA-I:A-II (r = –0.395 and –0.363) as well as with the PR of LpA-I (r = –0.429).


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
We present new findings showing that subjects with MetS have accelerated catabolism of HDL LpA-I and LpA-I:A-II particles. The PR of LpA-I matched the corresponding rate of catabolism, so that the dominant change in plasma concentration was reduction in LpA-I:A-II. This may represent a novel defect in HDL metabolism that, as discussed later, could account for accelerated risk of CVD in MetS. In pooled data, increased catabolism of LpA-I, LpA-I:A-II, and apo-AII were related to low plasma concentrations of HDL-cholesterol, to small HDL particle size, and to lower PLTP activity as well as to other features of MetS including waist circumference, hypertriglyceridemia, and insulin resistance. We also newly report that in MetS plasma, PLTP activity is decreased, and this may contribute to the alterations in HDL particle kinetics.

Our kinetic estimates of HDL lipoprotein particle turnover in control and insulin-resistant subjects concur with previous results (13, 14, 16). In our normolipidemic controls, the fractional turnover of LpA-I was marginally higher than LpA-I:A-II (20), but we newly demonstrate that this relative difference was substantially greater in MetS. Previous studies have generally shown that the FCRs of apoA-I and apoA-II are increased in subjects with abdominal obesity and hypertriglyceridemia (13, 14, 16). We extend earlier observations by investigating the effects of MetS without diabetes on the turnover of LpA-I and LpA-I:A-II particles. Radiokinetic data in normolipidemic subjects show that the plasma concentration of LpA-I is chiefly determined by the FCR of apoA-I, whereas the concentration of LpA-I:A-II is determined by the PR of apoA-II (7). We suggest that these relationships may not apply to MetS, where in the setting of accelerated catabolism of HDL particles, the steady-state plasma concentration of LpA-I and LpA-I:A-II are chiefly determined by their PRs.

It is highly likely that the combination of central obesity, insulin resistance, and hypertriglyceridemia collectively account for the accelerated catabolism of HDL subspecies in MetS (12, 13, 14, 16). Low plasma adiponectin has been associated with depressed HDL-cholesterol levels (23), but in our study adiponectin was not as significantly nor consistently predictive of hypercatabolism of HDL species as insulin or HOMA score. Because of expansion in the very-low-density lipoprotein triglyceride pool and increased neutral lipid transfer by CETP, HDL particle composition is altered in MetS (24); cholesterol depletion and triglyceride enrichment, with hydrolysis by HL, result in apoA-I dissociation from the HDL particle, with subsequent recycling or catabolism by the kidney (5, 25). The impact of insulin resistance on these processes could partly be free fatty acid mediated involving stimulation of CETP-mediated transfer of cholesteryl esters and triglycerides between HDLs and apoB-containing lipoproteins (9). However, we failed to show significant changes in CETP activity in our insulin-resistant subjects, in agreement with other data (9). The faster catabolism of LpA-I compared with LpA-I:A-II in the present study may reflect several processes including the potential delay in HDL particle catabolism by apoA-II via inhibition of HL, CETP, and scavenger receptor class B type 1 (SR-BI) activities (4, 24). Consistent with this notion, triglyceride enrichment of HDLs has been shown to result in hypercatabolism of LpA-I relative to LpA-I:A-II particles (17). In contrast to other studies (26), we found that plasma PLTP activity was reduced in subjects with insulin resistance. Another report showed decreased PLTP mass, with a paradoxical increase in PLTP activity, in insulin-resistant hypertriglyceridemic subjects (27). PLTP synthesis is potentially controlled by peroxisome proliferator-activated receptor-{alpha} (28), which is in turn down-regulated in insulin resistance, and this could explain our lower PLTP activity in MetS. PLTP facilitates the transfer of phospholipids from triglyceride-rich lipoproteins to HDL during lipolysis and converts HDLs into smaller (pre-ß) and larger particles by altering their phospholipid composition (29). Our new findings that low PLTP activity correlates with high LpA-I and LpA-I:A-II FCRs concur with a previous report that apoA-I FCR is increased in PLTP knockout mice (30). In addition, evidence indicates that higher apoA-II/apoA-I ratios reduce PLTP-mediated HDL conversion into larger HDL particles and the release of pre-ß-HDL (31); therefore, LpA-I:A-II particles may interact less with PLTP compared with LpA-I, resulting in the preferred remodeling of LpA-I particles and potential elevation in the production of LpA-I in MetS.

The apparent increased production of LpA-I particles in MetS subjects who maintained normal plasma concentration of LpA-I may involve several mechanisms. These could include increased transfer of apoA-I to HDL from triglyceride-rich lipoproteins (5) and an effect of hyperinsulinemia that increases both apoA-I gene expression and activity of ATP-binding cassette transporter 1 (32). That apoA-II production was not higher in MetS subjects could account for the lack of significant difference in LpA-I:A-II production compared with controls (7). Dietary fat and alcohol intake can influence the production of apoA-I (5) but did not differ significantly between our groups. HL activity is increased in insulin resistance and is involved in the generation of pre-ß-HDL (9). Whether the increased LpA-I production reflects increased cycling of apoA-I in this HDL subfraction merits further examination (33). That the predominant effect of MetS was a reduction in plasma LpA-I:A-II concentration concurs with the reduction in HDL particle size (14) that involves remodeling of HDL by the concerted actions of LPL, HL, and CETP activities (9). The potential impact of genetic variations in these enzymes and lipid transfer proteins, as well as in ATP-binding cassette transporter 1, apoA-I, and lecithin cholesterol acyltransferase, on HDL kinetics in MetS merit investigation.

Our kinetic findings could be clinically important. Decreased plasma LpA-I:A-II concentration is a predictor of coronary events in population studies (34) and in type 2 diabetes is independently associated with angiographic coronary disease (11). In contrast to LpA-I, low plasma LpA-I:A-II concentrations together with low PLTP activity determine low cholesterol efflux capacity in diabetes (35). The ability of LpA-I:A-II to effect cellular cholesterol efflux has also been shown in several cellular models, consistent with the atheroprotection shown in apoA-II transgenic mice (4). LpA-I:A-II particles have a higher ratio of cholesteryl esters to free cholesterol than LpA-I (36) and may therefore be the major carrier for cholesteryl esters from peripheral tissues back to the liver. The antioxidant and antiinflammatory properties of HDL have mainly been studied with apoA-I (37), but there are also data supporting a role for apoA-II (38) and hence LpA-I:A-II. The absence of a balancing feedback overproduction of LpA-I:A-II particles compared with LpA-I in subjects with MetS could therefore have both qualitative and quantitative adverse implications for reverse cholesterol transport and atherosclerosis. Our data also emphasize the therapeutic value of peroxisome proliferator-activated receptor-{alpha} agonists, specifically fenofibrate, that significantly increase the expression of both apoA-I and apoA-II (28, 39).

We did not measure post-heparin lipases in plasma, but predict that a higher ratio of HL to LPL (13, 14) would contribute to accelerated catabolism of HDL particles. There is a gender difference in HDL metabolism. Our study was restricted to men, and whether our results also apply to women with MetS requires additional investigation. In our model, we assumed that apoA-I and apoA-II were secreted into plasma from similar compartments, and after association to form LpA-I:A-II particles, were catabolized from plasma at comparable rates. Experimental data suggest that apoA-I is secreted from both liver and enterocytes, whereas apo-AII may be secreted into plasma from the liver alone (5). ApoA-II may, however, be secreted in triglyceride-rich lipoproteins by the human intestine during lipid absorption (4, 5). Hence, additional studies should examine HDL kinetics in the postprandial state. Our compartment model is also supported by radiokinetic data showing the divergent metabolic pathways of apoA-I and apoA-II and of LpA-I and LpA-I:A-II (20, 21). However, direct measurement of isotopic enrichment in LpA-I and LpA-I:A-II particles would be required to formally corroborate our findings. The validity of our model for LpA-I:A-II is supported by the comparability of our normative kinetic data with those derived from exogenously radiolabeled techniques (5). Although our model was constructed from experiments employing these techniques, there is evidence supporting close agreement between data derived from exogenous and endogenous tracer studies (7).

In conclusion, we demonstrate, based on a new kinetic model, hypercatabolism of the two major HDL lipoprotein particles, LpA-I and LpA-I:A-II, in subjects with MetS. This was related to features of this syndrome including central obesity, insulin resistance, and hypertriglyceridemia. The corresponding plasma concentrations of LpA-I and LpA-I:A-II appeared to be differentially set by selective overproduction of LpA-I that could relate partly to a balancing feedback effect of hyperinsulinemia on the expression of apoA-I. In terms of static plasma concentrations, our findings also suggest that the atherogenicity of disturbed HDL metabolism in MetS may be related to decreased LpA-I:A-II. Therapeutic agents that reduce the catabolism of both LpA-I and LpA-I:A-II particles and that selectively increase the production of LpA-I:A-II particles may be particularly important in CVD prevention in this condition.


   Acknowledgments
 
We are grateful to Dr. V. Burke for statistical advice. We also thank Professor E. J. Schaefer for supplying data to validate the HDL kinetic model.


   Footnotes
 
This study was funded by research grants from GlaxoSmithKline. P.H.R.B. is a National Health and Medical Research Council Research Fellow and was also supported by the National Institutes of Health (NIH/NIBIB P41 EB-001975). K.-A.R. is a Principal Research Fellow of the National Heart Foundation of Australia. D.C.C. was supported by a postdoctoral fellowship from the Raine/National Heart Foundation of Australia.

First Published Online December 20, 2005

Abbreviations: apoA-I, Apolipoprotein A-I; BMI, body mass index; CETP, cholesteryl ester transfer protein; CV, coefficient of variation; CVD, cardiovascular disease; FCR, fractional catabolic rate; HDL, high-density lipoprotein; HL, hepatic lipase; HOMA, homeostasis assessment model; LDL, low-density lipoprotein; LpA-I, lipoprotein A-I; LPL, lipoprotein lipase; MetS, metabolic syndrome; PLTP, phospholipid transfer protein; PR, production rate.

Received August 23, 2005.

Accepted December 13, 2005.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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