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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 8 3949-3955
Copyright © 2004 by The Endocrine Society

Rate of Production of Plasma and Very-Low-Density Lipoprotein (VLDL) Apolipoprotein C-III Is Strongly Related to the Concentration and Level of Production of VLDL Triglyceride in Male Subjects with Different Body Weights and Levels of Insulin Sensitivity

Jeffrey S. Cohn, Bruce W. Patterson, Kris D. Uffelman, Jean Davignon and George Steiner

Clinical Research Institute of Montreal (J.S.C., J.D.), Quebec, Canada, H2W 1R7; the Washington University School of Medicine (B.W.P.), St. Louis, Missouri 63110; and the Toronto General Hospital (K.D.U., G.S.), Ontario, Canada M5G 2C4

Address all correspondence and requests for reprints to: Jeffrey S. Cohn, Ph.D., Hyperlipidemia and Atherosclerosis Research Group, Clinical Research Institute of Montréal, 110 Pine Avenue West, Québec, Canada, H2W 1R7. E-mail: cohnj{at}ircm.qc.ca.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Overweight individuals with reduced insulin sensitivity often have mild to moderate hypertriglyceridemia. To investigate the role of apolipoprotein (apo)C-III metabolism in the etiology of hypertriglyceridemia in these individuals, we investigated 10 male subjects with different body weights (body mass index, 24–34 kg/m2) and insulin sensitivity (homeostasis model assessment, 4.7–35.0). Total plasma and very-low-density lipoprotein (VLDL) apoC-III kinetics, as well as VLDL triglyceride (TG) and VLDL apoB kinetics, were measured with iv injected stable isotopes. The apoC-III, TG, and apoB levels in VLDL ranged from 2.9–18.2 mg/dl, 0.49–2.89 mmol/liter, and 6.7–29.3 mg/dl, respectively. Mean production rates (PRs) were: VLDL apoC-III, 20.2 ± 4.1 µmol/d (range, 8.0–44.8); VLDL TG, 26.9 ± 4.6 mmol/d (range, 10.2–51.1); and VLDL apoB, 4.4 ± 0.8 µmol/d (range, 1.5–9.1). VLDL apoC-III PRs were significantly correlated with body mass index, homeostasis model assessment, and plasma TG (r = 0.66, P < 0.05; r = 0.80, P < 0.01; r = 0.95, P < 0.001, respectively). Similar correlations were found for plasma apoC-III PRs (r = 0.70, P < 0.05; r = 0.67, P < 0.05; r = 0.80, P < 0.01, respectively). Fractional catabolic rates (FCRs) were not significantly related to metabolic variables. VLDL TG levels were strongly related to VLDL apoC-III levels (r = 0.99, P < 0.001) and VLDL apoC-III PRs (r = 0.94, P < 0.001). VLDL apoC-III levels were more strongly correlated with VLDL TG PRs (r = 0.81, P < 0.01) than with VLDL TG FCRs or VLDL apoB FCRs (r = –0.53, P = 0.12; r = –0.37, P = 0.29). These results suggest that increased hepatic production of VLDL apoC-III is characteristic of subjects with higher body weights and lower levels of insulin sensitivity and is strongly related to the plasma concentration and level of production of VLDL TG.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
INCREASED FOOD INTAKE and a sedentary lifestyle have led to an exponential increase in the number of men and women who are overweight and at increased risk of coronary artery disease (1). These individuals often have a mild to moderate elevation in plasma triglyceride (TG) concentration, the presence in plasma of small dense low-density lipoproteins, and reduced levels of high-density lipoprotein (HDL) cholesterol (2, 3). Typically, they also have increased hepatic production of very-low-density lipoproteins (VLDL) and increased secretion of VLDL TG in response to increased flux of fatty acids from the intestine, from adipose tissue, and from de novo hepatic lipogenesis (4, 5, 6). Changes in plasma TG and fatty acid metabolism are thus a central feature of this atherogenic metabolic dyslipidemia (2).

Apolipoprotein (apo)C-III is a 8.8-kDa glycoprotein (reviewed in Refs. 7 and 8), which is synthesized by the liver and, to a lesser extent, by the intestine. It is produced as a 99-amino-acid peptide but is secreted from tissues in its mature form as a 79-amino-acid protein after intracellular removal of its signal peptide. It is found as a nonglycosylated isoform (apoC-III0) or as a glycosylated isoform, containing either one or two moles of sialic acid (apoC-III1 and apoC-III2, respectively). ApoC-III is associated with apoB-containing and apoA-I-containing lipoproteins in the blood and has the ability to exchange between TG-rich lipoproteins (TRL) and HDL. In normolipidemic subjects, the majority of plasma apoC-III is bound to HDL; whereas in hypertriglyceridemic subjects, the majority is bound to TRL (9, 10).

Several lines of evidence support the concept that one of the major metabolic effects of apoC-III is to increase the concentration of plasma TG. For example, it has been shown that: 1) plasma and VLDL apoC-III levels are positively correlated with plasma TG concentration (9, 10, 11); 2) individuals with apoC-III gene polymorphisms have increased susceptibility to hypertriglyceridemia (HTG) (12, 13, 14); 3) subjects with an inherited deficiency of apoC-III have low plasma TG levels (15, 16); 4) overexpression of the human apoC-III gene in transgenic mice results in HTG (17, 18, 19), and 5) murine apoC-III gene deletion results in hypotriglyceridemia (20). The HTG effect of apoC-III is believed to be due to its ability to inhibit TRL catabolism by: 1) inhibiting lipoprotein lipase (21, 22) and/or hepatic lipase (23); 2) interfering with the interaction of TRL with LPL (24); and/or 3) inhibiting the interaction of TRL with hepatic lipoprotein receptors (25, 26, 27, 28).

Previous studies have demonstrated that overweight and/or insulin-resistant individuals have an increase in VLDL TG (29, 30, 31, 32) and/or VLDL apoB production (33, 34, 35, 36, 37, 38). In viscerally obese men with increased levels of VLDL apoB concentration, Chan et al. (39) found that increased levels of VLDL apoB production were strongly correlated with elevated levels of plasma apoC-III concentration. We, in turn, have shown that increased levels of plasma and VLDL apoC-III in patients with fasting HTG are strongly dependent on rates of apoC-III production (40). This has led us to hypothesize that increased apoC-III production may be a characteristic feature of overweight and/or insulin-resistant subjects and may play an important role in the onset and development of their dyslipidemia. To provide support for this concept, we have investigated the plasma kinetics of VLDL and total plasma apoC-III in a group of men (n = 10) with varying body weights and levels of insulin sensitivity. At the same time, we have measured VLDL apoB and VLDL TG production to investigate the relationship between these kinetic parameters.


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

Ten healthy men were recruited for the present study through an advertisement in the University of Toronto newspaper. Half the subjects were chosen because they had normal body weight; and half were chosen because they were overweight, i.e. body mass index (BMI) more than 27.5. They were 36–60 yr old, with BMIs ranging from 24–34 kg/m2. They were not diabetic, based on their fasting glucose and insulin levels. They did not have any serious medical condition and were not taking medications known to affect glucose or lipid metabolism. All subjects gave written informed consent before their participation in the study, which was approved by the Toronto Hospital Committee for Research on Human Subjects.

Study design

After a 12-h overnight fast, subjects were admitted to the Metabolic Investigation Unit of the Toronto Hospital. They remained fasted for the duration of the study but had free access to drinking water. An iv line was inserted into each forearm (one for infusing and one for taking blood samples). At approximately 0800 h, a baseline blood sample (20 ml) was taken, followed by a bolus of 100 µmol/kg of [1,1,2,3,3-2H] glycerol (98% enriched; Cambridge Isotope Laboratories, Andover, MA). Immediately thereafter, a bolus of 10 µmol/kg [D3]L-leucine was injected, followed by a 12-h constant infusion of [D3]L-leucine (10 µmol/kg·h) given by peristaltic pump. Blood samples (20 ml) were drawn into Vacutainer tubes containing Na2EDTA (Becton Dickinson, Rutherford, NJ) at regular intervals (5 min, 15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 9 h, 10 h, 11 h, and 12 h). Blood samples were centrifuged within 30 min (1500 rpm, 20 min, 4 C) to separate plasma from blood cells, and preservative (10 µl) containing 1.67 µg aprotinin and 0.2 mg sodium azide was added (40).

Biochemical analyses

VLDLs were separated from fresh plasma samples (5 ml) by ultracentrifugation (d = 1.006 g/ml, 39,000 rpm, 16 h, 4 C, 50.4 Ti rotor). Total plasma lipoprotein fractions (d < 1.21 g/ml) were also prepared by ultracentrifugation from plasma samples taken at 1 h, 2 h, 4 h, 6 h, 7 h, 10 h, 11 h, and 12 h. VLDL and d-less-than-1.21-g/ml fractions were returned to their original plasma volume with saline and were stored frozen at –70 C together with aliquotted plasma samples. Glucose was measured enzymatically using a Glucose Analyzer II (Beckman Instruments, Fullerton, CA). Insulin was measured by RIA (Pharmacia Diagnostic, Uppsala, Sweden). Insulin sensitivity was estimated using the homeostasis model assessment (HOMA) index (41). Cholesterol and TG were measured enzymatically on plasma and VLDL samples using commercially available kits (Roche Diagnostics, Dorval, Quebec, Canada). VLDL apoB was measured by electroimmunoassay (42). Plasma and VLDL apoC-III concentrations were measured by ELISA (10).

Stable isotope analyses

VLDL samples were delipidated with diethyl ether/methanol in acid-washed glass tubes to separate VLDL lipids from proteins. VLDL lipid extracts were dried under nitrogen, and VLDL-TG was separated by thin-layer chromatography on 20 x 20-cm silica gel plates (06–600A gel G TLC; Fisher Scientific, Hampton, NH) using heptane:isopropyl ether:acetic acid (80:18:2). VLDL-TG was recovered and converted to a heptafluorobutyryl derivative, as previously described (43). Enrichment of [1,1,2,3,3-2H5]glycerol was measured by electron impact ionization gas chromatography/mass spectrometry (GC/MS) using a 30m x 0.25 mm, 0.25-m film DB-17 capillary column (Supelco, Bellefonte, PA) on a model 5973 quadrupole GC/MS (Hewlett-Packard). Glycerol (m/z 467) and its m+5 isotopomer (m/z 472) were measured, and glycerol tracer:tracee ratios were determined by calibration of measured m+5/m+0 ratios for standards of known isotopic enrichment. VLDL-TG fractional catabolic rates (FCR) were estimated (in terms of pools/d) from the monoexponential decay slope of VLDL-TG [2H5]glycerol enrichment curves, as previously described (43). VLDL-TG production (in mmol/d) was calculated by multiplying plasma VLDL-TG concentration (mmol/liter) by plasma vol (0.045 liter/kg body weight) and by FCR (pools/d). VLDL-TG concentration was taken to be the average of 5 measurements made at 0 h, 3 h, 6 h, 9 h, and 12 h during the infusion experiment.

VLDL samples were loaded onto 4–20% sodium dodecyl sulfate polyacrylamide gels for separating apoB-100 by gradient gel electrophoresis (44). VLDL apoC-III and total plasma apoC-III (d < 1.21 g/ml) were isolated by preparative isoelectric focusing (IEF) on 7.5% polyacrylamide-urea (8 M) gels (pH gradient 4–7) (45). Before IEF separation, total plasma lipoprotein samples (d <1.21 g/ml) were preincubated with cysteamine (ß-mercaptoethylamine, Sigma-Aldrich, St. Louis, MO) at a ratio of 6 mg for every milligram of protein for 4 h at 37 C. The aim of cysteamine treatment was to separate apoA-II from apoC-III1. Cysteamine treatment caused an amino group to bind to the single-cysteine residue of apoA-II, causing cysteamine-modified apoA-II to be more positively charged and to migrate to a higher position after electrophoresis (45). Coumassie blue staining was used to identify the position of apo in gels after electrophoresis; apoB-100 and apoC-III bands, as well as blank (nonprotein containing) gel slices, were excised from SDS-PAGE and from IEF gels. The band corresponding to the major isoform of apoC-III, monosialylated apoC-III (apoC-III1), was excised and analyzed in all cases. Each slice was added to a borosilicate sample vial containing 600 µl of 6 N HCl, and an internal standard of 250 ng norleucine (Sigma-Aldrich) dissolved in 50 µl double-distilled water (45). Gel slices were hydrolyzed at 110 C for 24 h, cooled to –20 C for 20 min, and centrifuged at 3500 rpm for 5 min. Free amino acids in the hydrolysate were separated from precipitated polyacrylamide, purified by cation exchange chromatography using AG 50 W-X8 resin (Bio-Rad, Richmond, CA), and derivatized by treatment with 200 µl acetyl chloride-acidified 1-propanol (1:5 vol/vol) for 1 h at 100 C, and 50 µl of heptaflurobutyric anhydride (Supelco) for 20 min at 60 C (40). Plasma amino acids were also separated by cation exchange chromatography and derivatized to allow for the determination of plasma leucine isotopic enrichment. Enrichment of samples with deuterium-labeled leucine was measured by 5988 GC/MS (Hewlett-Packard) using negative chemical ionization and methane as the moderator gas. Selective ion monitoring at m/z = 352 and 349 (ionic species corresponding to derivatized deuterium-labeled and derivatized nondeuterium-labeled leucine, respectively) was performed, and tracer to tracee ratios were derived from isotopic ratios for each sample according to the formula derived by Cobelli et al. (46). Tracer to tracee ratios were corrected for background leucine in gel slices (i.e. trace amounts of leucine introduced during the amino acid purification and derivatization procedures) by estimating the amount of leucine in processed blank gel slices relative to the norleucine internal standard (29). Background leucine represented (mean ± SD) 2.7 ± 0.8% of total leucine recovered for apoB-100 samples and 9.2 ± 3.9% of total leucine recovered for apoC-III samples. Stable isotope enrichment curves for apoC-III and apoB were fitted to a three-compartment model using SAAM II computer software (SAAM II Institute, Seattle, WA). Kinetic analysis allowed for the determination of apo FCR. Production rates (PRs) of apo (in µmol/d) were calculated by multiplying FCRs (pools/d) by apo pool sizes [plasma concentration (µmol/liter) multiplied by plasma volume (0.045 liter/kg body weight], where VLDL apoB and apoC-III levels and plasma apoC-III levels were taken to be the average of five measurements made at 0 h, 3 h, 6 h, 9 h, and 12 h during the infusion experiment.

Statistical analysis

The statistical significance of differences between mean values was assessed by t test using SigmaStat software (Jandel Scientific, Corte Madera, CA). Pearson correlation coefficients (r) were calculated to describe the correlation between different kinetic and mass parameters.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Clinical characteristics of the study subjects are shown in Table 1Go. The group as a whole had a mean age of 49.0 ± 2.9 yr, mean body weight of 83.6 ± 2.8 kg, and mean BMI of 27.1 ± 0.9. Mean plasma TG concentration was 1.61 ± 0.28 mmol/liter, ranging from 0.74 to 3.31 mmol/liter). Three subjects had HTG, on the basis that they had a plasma TG concentration greater than 2.0 mmol/liter. Subjects had normal glucose and insulin concentrations, and their insulin sensitivity (measured by HOMA) had a 7-fold range from 4.7–35.0. BMI was significantly correlated with HOMA (r = 0.77, P < 0.01) and with VLDL apoB concentration (r = 0.82, P < 0.01). Positive correlations were also found between BMI and VLDL TG (r = 0.54, P = 0.11) and between BMI and VLDL apoC-III (r = 0.50, P = 0.14), though these relationships did not reach statistical significance.


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  		TABLE 1. Subject characteristics

 
Isotopic enrichment of VLDL TG, VLDL apoB, VLDL apoC-III, and total plasma apoC-III was determined for all 10 subjects, and enrichment curves were constructed by plotting tracer-to-tracee ratios against time. Enrichment curves were obtained typical of those found in previous studies from our laboratories (32, 40, 43). Kinetic parameters were derived by analyzing individual enrichment curves. Mean results are shown in Table 2Go, together with mean plasma concentrations of VLDL TG, VLDL apoB, VLDL apoC-III, and plasma apoC-III. PRs are given in units of millimoles or micromoles per day, independent of body weight. PRs (mg/kg·d) for VLDL apoB, VLDL apoC-III, and plasma apoC-III were: 28.6 ± 4.7, 2.1 ± 0.4, and 2.9 ± 0.3, respectively. VLDL component concentrations varied over a 4- to 7-fold range, VLDL PRs varied over a 5- to 6-fold range, whereas FCRs varied over a 2- to 5-fold range.


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  		TABLE 2. VLDL TG, apoB, apoC-III, and total plasma apoC-III kinetic parameters of male subjects

 
The relationship between rates of apoC-III production (VLDL and total plasma apoC-III) and metabolic parameters (i.e. BMI, HOMA, and plasma TG) are shown in Fig. 1Go. A statistically significant positive relationship was found in each case. The strongest correlation was found between VLDL apoC-III production and plasma TG concentration (r = 0.95, P < 0.001). Relationships between other VLDL kinetic parameters and metabolic variables (including waist circumference) are shown in Table 3Go. VLDL TG and VLDL apoB PRs were also positively correlated with BMI, HOMA, and waist circumference. Plasma TG concentration was significantly related to VLDL TG but not to VLDL apoB production. None of the metabolic variables were found to have statistically significant correlations with VLDL or plasma FCRs (Table 3Go).



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  		FIG. 1. Relationship between plasma TG, HOMA, BMI, and rates of apoC-III production (VLDL and total plasma) in male subjects (n = 10). Linear regression lines are shown for each relationship, together with their associated correlation coefficients (r) and their statistical significance.

 

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  		TABLE 3. Relationship between VLDL kinetic parameters and metabolic variables

 
To determine the relationship between the concentration and kinetics of different VLDL components, univariate analysis was carried out, and correlation coefficients were determined. For ease of interpretation, they are presented schematically in Fig. 2Go. Strong positive and statistically significant correlations were found between VLDL apoC-III PRs, VLDL TG PRs, plasma concentrations of VLDL apoC-III, and plasma concentrations of VLDL TG. Not shown is the statistically significant correlation between rates of VLDL TG production and VLDL apoC-III production (r = 0.79, P < 0.01) and the positive correlation between rates of VLDL TG production and plasma concentrations of VLDL apoC-III (r = 0.81, P < 0.01). VLDL apoB PRs and VLDL apoB concentrations were also positively related to the aforementioned parameters, although to a less significant extent (Fig. 2Go). Not shown are the positive, though weaker, relationships between VLDL apoC-III production and VLDL apoB production (r = 0.49, P = 0.15) and between VLDL TG production and VLDL apoB production (r = 0.51, P = 0.13), which did not reach statistical significance. At the same time, plasma VLDL levels were inversely, though not significantly, correlated with FCRs. In view of the capacity of apoC-III to inhibit lipoprotein lipase (21, 22) and inhibit the hepatic uptake of TRL remnants (24, 25, 26, 27, 28), strong negative correlations were expected between VLDL apoC-III levels and VLDL FCRs. These negative correlations between VLDL apoC-III concentration and VLDL TG fractional catabolism (r = –0.53, r = 0.12), between VLDL apoC-III concentration and VLDL apoB fractional catabolism (r = –0.37, r = 0.29), and between VLDL apoC-III concentration and VLDL apoC-III fractional catabolism (r = –0.46, r = 0.18) did not, however, reach statistical significance.



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  		FIG. 2. Schematic diagram showing the relationship (i.e. correlation coefficient) between circulating VLDL TG, VLDL apoB, and VLDL apoC-III concentrations and their respective rates of production and fractional catabolism. Statistically significant relationships are shown in bold: *, P < 0.05; **, P < 0.01; ***, P < 0.001.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
In the present study, male subjects with higher body weights and lower levels of insulin sensitivity (i.e. higher HOMA index) were found to have higher rates of production of VLDL TG, VLDL apoB, and VLDL apoC-III. Neither BMI, waist circumference, or HOMA index were significantly correlated with fractional rates of VLDL catabolism. Rates of plasma and VLDL apoC-III production were strongly correlated with the plasma concentration and level of production of VLDL TG. These results suggest that increased apoC-III production is a characteristic feature of the mild to moderately elevated TG levels often found in overweight and insulin-resistant subjects.

Previous studies have demonstrated that overweight and/or insulin-resistant individuals have an increase in VLDL TG (29, 30, 31, 32) and/or VLDL apoB production (33, 34, 35, 36, 37, 38). Hepatic VLDL overproduction induced by insulin resistance is due to several factors, including: 1) increased flux of fatty acids from extrahepatic tissues to the liver; 2) increased remnant lipoprotein uptake; 3) increased hepatic de novo fatty acid synthesis; 4) preferential esterification vs. oxidation of fatty acids; and 5) reduced posttranslational degradation of apo B and overexpression of microsomal TG transfer protein (47). Based on the present data, it is evident that increased VLDL production by the liver is associated with increased hepatic production of VLDL apoC-III. This could be due to a direct effect of glucose and/or insulin status on the synthesis and secretion of apoC-III. Insulin has been shown to decrease apoC-III transcription in mice as well as in cultured human hepatocytes (48). Insensitivity to insulin would therefore be expected to result in increased apoC-III transcription. A negative insulin response element has been identified in the apoC-III gene promoter and a single bp change at either the –482 or the –455 site of the apoC-III gene is sufficient to abolish insulin responsiveness (49). In the European Atherosclerosis Research Study II, young male carriers of the rare T-482 variant of the apoC-III insulin response element were found to have significantly elevated glucose and insulin levels in response to an oral glucose tolerance test (50). In middle-aged men and women from the Ely study, the T-482 variant was associated with 33% lower 30-min oral glucose tolerance test insulin levels and 10% higher 30-min nonesterified fatty acid levels (51). In addition, homozygosity for the C-455 variant has been associated with increased apoC-III levels and an elevated risk for coronary artery disease (odds ratio = 2.5) (52). On the other hand, Lee et al. (53) have recently shown that, if diabetics and nondiabetics are matched for plasma TG level, then diabetic status per se is not associated with higher VLDL apoC-III levels. This argues against a direct effect of diabetic status on apoC-III production and supports the concept that increased apoC-III production in overweight and/or insulin-resistant individuals is a secondary effect driven by increased formation and secretion of VLDL.

A somewhat unexpected finding in the present study was the lack of a statistically significant negative correlation between VLDL apoC-III levels and fractional rates of catabolism of VLDL TG, VLDL apoB, or VLDL apoC-III. This was surprising, considering that apoC-III has been shown to inhibit lipoprotein lipase (21, 22) and hepatic uptake of remnant lipoproteins (24, 25, 26, 27, 28). Mice overexpressing either human or murine apoC-III develop severe HTG, which is largely due to reduced plasma clearance of VLDL TG (17, 18, 19). Initially it was thought that delayed clearance of VLDL TG was due to the low amount of apoE, relative to apoC-III, on VLDL particles (54). However, cross-breeding of apoE knockout mice with transgenic mice overexpressing human apoC-III resulted in a massive accumulation of TG-rich VLDL, indicating that the absolute amount of apoC-III, rather than the extent of apoC-III-induced displacement of apoE, leads to reduced VLDL catabolism (28). In contrast, the present results point to an effect of apoC-III on hepatic VLDL TG production. This is supported by the results of Aalto-Setala et al. (18), who carried out experiments with iv injected Triton and showed that in vivo VLDL TG production, though not apoB production, was higher in human apoC-III transgenic mice compared with controls. In the presence of oleic acid, primary hepatocytes from apoC-III transgenic animals secreted 2-fold more TG (though not apoB) compared with control cells (18). In addition, cultured hepatoma cells transfected with human apoC-III have been found to secrete increased amounts of VLDL TG (55). Increase in TG secretion is observed only in cells treated with exogenous oleate and is confined to large VLDL (VLDL1, Sf > 100) (Philip Links and Zemin Yao, personal communication). The mechanism whereby apoC-III can stimulate cellular VLDL secretion remains to be established; however, apoC-III may: 1) act at the level of the endoplasmic reticulum to aid in the formation of nascent apoB-containing particles; 2) help to transport lipid to maturing VLDL particles in the Golgi compartment; or 3) assist in the final assembly and secretion of VLDL in secretory vesicles. In fact, apoC-III synthesis and secretion by the liver may be an important determinant of hepatic TG output and also human HTG, as supported by the observation that: 1) subjects in the present study with higher VLDL TG levels and higher rates of VLDL TG production had higher levels of VLDL apoC-III production; 2) patients with fasting HTG have significantly increased rates of plasma and VLDL apoC-III production (40); and 3) fibric acid drugs, which have a pronounced plasma TG-lowering effect, can directly down-regulate the transcription of the apoC-III gene (56) and also reduce VLDL apoC-III (57) and VLDL TG production (58).

In conclusion, the present study has shown that increased body weight and reduced insulin sensitivity in male subjects are associated with increased levels of VLDL apoC-III production, which is, in turn, linked to increased VLDL TG production. This finding lends support to the concept that apoC-III can affect the rate or extent of hepatic TG secretion. The mechanism by which apoC-III can regulate TG and/or VLDL secretion is however unknown, and additional experiments at the cellular level are clearly warranted.


   Acknowledgments
 
We are grateful for the excellent technical assistance of Hélène Jacques and Michel Tremblay.


   Footnotes
 
This work was supported by a grant from the Canadian Heart and Stroke Foundation awarded to G.S. The salary of J.S.C. was funded by a Canadian Institutes of Health Research (CIHR)/Pfizer Rx&D Investigator Award (DSC-64146), and work in his laboratory was funded by a CIHR operating grant (MOP-14684).

Abbreviations: apo, Apolipoprotein(s); BMI, body mass index; FCR, fractional catabolic rate; GC-MS, gas chromatography-mass spectrometry; HDL, high-density lipoprotein; HOMA, homeostasis model assessment; HTG, hypertriglyceridemia; IEF, isoelectric focusing; PR, production rate; TG, triglyceride; TRL, triglyceride-rich lipoprotein; VLDL, very-low-density lipoprotein.

Received November 30, 2003.

Accepted April 6, 2004.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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