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

0021-972X/04/$15.00/0 Printed in U.S.A. The Journal of Clinical Endocrinology & Metabolism 89(8):3949 –3955 Copyright © 2004 by The Endocrine Society doi: 10.1210/jc.2003-032056 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 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-lowdensity 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 ⴞ 4.1 ␮mol/d (range, 8.0 – 44.8); VLDL TG, 26.9 ⴞ 4.6 mmol/d (range, 10.2–51.1); and VLDL apoB, 4.4 ⴞ 0.8 ␮mol/d (range, 1.5–9.1). VLDL apoC-III PRs were significantly correlated with body mass index, homeostasis I NCREASED 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 – 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 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. JCEM is published monthly by The Endocrine Society (http://www. endo-society.org), the foremost professional society serving the endocrine community. model assessment, and plasma TG (r ⴝ 0.66, P < 0.05; r ⴝ 0.80, P < 0.01; r ⴝ 0.95, P < 0.001, respectively). Similar correlations were found for plasma apoC-III PRs (r ⴝ 0.70, P < 0.05; r ⴝ 0.67, P < 0.05; r ⴝ 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 ⴝ 0.99, P < 0.001) and VLDL apoC-III PRs (r ⴝ 0.94, P < 0.001). VLDL apoC-III levels were more strongly correlated with VLDL TG PRs (r ⴝ 0.81, P < 0.01) than with VLDL TG FCRs or VLDL apoB FCRs (r ⴝ ⴚ0.53, P ⴝ 0.12; r ⴝ ⴚ0.37, P ⴝ 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. (J Clin Endocrinol Metab 89: 3949 –3955, 2004) 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 –11); 2) individuals with apoC-III gene polymorphisms have increased susceptibility to hypertriglyceridemia (HTG) (12–14); 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 (17–19), 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 3949 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 3950 J Clin Endocrinol Metab, August 2004, 89(8):3949 –3955 Subjects and Methods 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 ␮mol/kg of [1,1,2,3,3-2H] glycerol (98% enriched; Cambridge Isotope Laboratories, Andover, MA). Immediately thereafter, a bolus of 10 ␮mol/kg [D3]l-leucine was injected, followed by a (...truncated)