Contribution of very low-density lipoprotein triglyceride fatty acids to postabsorptive free fatty acid flux in obese humans

Objective

In the fasting state, plasma free fatty acids (FFA) are thought to derive almost exclusively from adipose tissue lipolysis. However, there are mixed reports as to whether the spillover of fatty acids (FA) from very low-density lipoprotein triglyceride (VLDL–TG) hydrolysis contributes significantly to the plasma FFA pool. Because substantial VLDL–TG fatty acid spillover into the plasma FFA pool would profoundly impact the interpretation of isotope dilution measures of FFA flux, we investigated the contribution of VLDL–TG spillover to plasma FFA appearance.

Materials/Methods

Eighteen obese adults (15 women) participated in these studies. Each volunteer received a primed, continuous infusion of their own ex-vivo labeled ([1-¹⁴C]triolein) VLDL–TG and a continuous infusion of [U-¹³C]oleate (8 nmol · kg fat free mass^− 1 · min^− 1) to measure VLDL–TG and FFA rate of appearance (Ra), respectively. The presence of ¹⁴C-oleate in the plasma FFA–oleate pool was used to calculate the contribution of spillover from VLDL–TG–oleate to the plasma FFA–oleate Ra.

Results

The spillover rate of VLDL–TG–oleate into plasma FFA–oleate was 6 ± 2 μmol/min (7% ± 2% of [¹⁴C]oleate from VLDL–TG) and FFA–oleate flux was 240 ± 61 μmol/min. Thus, only 3% ± 1% of total plasma FFA–oleate appearance could be accounted for by VLDL–TG spillover.

Conclusion

The contribution of VLDL–TG spillover to the total plasma FFA pool is negligible and will not materially affect the interpretation of FFA flux measures as an index of adipose tissue lipolysis.     

Differential influence of different dietary fatty acids on very low-density lipoprotein secretion when delivered to hepatocytes in chylomicron remnants

The influence of dietary fats carried in chylomicron remnants on the hepatic secretion of very low-density lipoprotein (VLDL) was investigated using chylomicron remnant-like particles (CRLPs) and cultured rat hepatocytes as the experimental model. Chylomicron remnant-like particles containing triacylglycerol (TG) from palm, olive, or corn (enriched in saturated, monounsaturated, or n-6 polyunsaturated fatty acids) oil, respectively, were incubated with cultured hepatocytes for 5 hours. The medium was then removed and replaced with medium without CRLPs; and the secretion of TG, cholesterol, and apolipoprotein B48 during the following 16 hours was determined. Secretion of TG into the d less than 1.050-g/mL fraction containing VLDL was unaffected by olive CRLPs, but was significantly increased in cells exposed to palm or corn CRLPs in comparison with both olive CRLPs and control incubations without CRLPs. Secretion of apolipoprotein B48, however, was not changed by any of the CRLP types. Apolipoprotein B messenger RNA levels were decreased by olive and corn CRLPs, and 3-hydroxy-3-methylglutaryl coenzyme A reductase messenger RNA abundance was increased by palm CRLPs; but expression of other genes involved in the regulation of VLDL secretion was unaffected. These findings demonstrate that CRLPs enriched in saturated fatty acids or n-6 polyunsaturated fatty acids increase the secretion of TG in VLDL, possibly because of the secretion of larger particles, whereas those enriched in monounsaturated fatty acids have no effect. Thus, different dietary fats have differential effects on VLDL secretion directly when delivered to the liver in chylomicron remnants.     

Intake of trans fatty acids and low-density lipoprotein size in a Costa Rican population

Intervention studies show that dietary composition altered low-density lipoprotein (LDL) particle size, but population studies are scarce, and the potential effects of trans fatty acids (FA) on LDL size are unknown. Trans FA intake has been associated with a more atherogenic lipid profile and increased coronary heart disease (CHD). We examined the association between dietary intake, including trans FA and LDL size, in 414 randomly selected subjects living in Puriscal, Costa Rica. Dietary intake was assessed by a validated food frequency questionnaire (FFQ). Women had larger LDL size (A) compared with men (263 v 261), and large LDL particles were correlated with increased intake (% energy) of protein (P =.005), animal fat (P =.041), trans FA (P <.0001), and decreased intake of carbohydrate (P =.052) in sex-, age-, and total energy intake-adjusted models. The correlation between trans FA intake and large LDL was significant in multivariate models that included dietary and nondietary factors; a 1% difference in trans FA was associated with a 2.44 A increase in LDL size (P =.004). In sum, it is possible that the effects of dietary factors, such as intake of trans FA on CHD are mediated through their effects on LDL size.     

Plasma apolipoprotein C-III transport in centrally obese men: associations with very low-density lipoprotein apolipoprotein B and high-density lipoprotein apolipoprotein A-I metabolism

CONTEXT: Apolipoprotein (apo) C-III is associated with hypertriglyceridemia and progression of cardiovascular disease. Plasma apoC-III is elevated in centrally obese men, and we hypothesized that the kinetics of apoC-III are disturbed in these subjects. OBJECTIVE: We developed a compartmental model to determine very low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) apoC-III metabolic parameters in centrally obese men and investigated the associations with VLDL-apoB and HDL-apoA-I kinetics. STUDY DESIGN: Apolipoprotein kinetics was determined using stable isotope techniques and compartmental modelling in 39 centrally obese and 12 nonobese men. RESULTS: Compared with nonobese subjects, centrally obese subjects had increased plasma apoC-III concentration (160 +/- 5 mg/liter vs. 103 +/- 9 mg/liter, P < 0.001), reflecting increased concentrations of both VLDL-apoC-III and HDL-apoC-III. These related to increased production rate (PR) of VLDL-apoC-III (2.12 +/- 0.14 vs. 1.56 +/- 0.29 mg/kg x d, P < 0.05) and reduced fractional catabolic rate (FCR) of both VLDL- and HDL-apoC-III (0.70 +/- 0.02 pools/d vs. 0.82 +/- 0.05 pools/d, P < 0.05). In centrally obese men, VLDL-apoC-III concentration was significantly (P < 0.05) associated with VLDL-apoB concentration and PR as well as HDL-apoA-I FCR and PR and inversely with VLDL-apoB FCR. HDL-apoC-III concentration was significantly (P < 0.05) associated with the concentrations of both VLDL-apoB and HDL-apoA-I, the FCR, and the PR of HDL-apoA-I and inversely with the VLDL-apoB FCR. In multiple regression analysis, both VLDL-apoC-III and HDL-apoC-III concentrations were significantly associated with HDL-apoA-I FCR. CONCLUSIONS: In centrally obese men, elevated VLDL-apoC-III and HDL-apoC-III concentrations are a consequence of elevated production and decreased catabolism of VLDL-apoC-III and reduced catabolism of HDL-apoC-III, respectively. These defects are associated with disturbances in VLDL-apoB and HDL-apoA-I metabolism.     

Kinetics of in vitro lipolysis of human very low-density lipoprotein by lipoprotein lipase

BACKGROUND AND AIM: An initial step in the catabolism of triglyceride-rich lipoprotein involves the hydrolysis of the triglyceride moiety by lipoprotein lipase (LPL). As differences in the lipolytic behaviour of very low-density lipoprotein (VLDL) particles have been observed, it is possible that different VLDL particles have a different affinity to the enzyme, which means that their fate may partially depend on the LPL-mediated hydrolysis of their triglyceride content. Our aim was to determine whether variation in VLDL chemical composition affects their properties as a substrate for LPL. METHODS AND RESULTS: Isolated VLDL was incubated in vitro with bovine LPL to determine substrate affinity. Under optimal assay conditions, free fatty acids were measured and the kinetic indicators for in vitro triglyceride hydrolysis (Km and Vmax) were calculated. VLDL cholesterol (VLDL-C), VLDL-apoB and the cholesterol/triglyceride ratio were assessed and the triglyceride/protein and triglyceride/apoB ratios were calculated as lipoprotein size estimators. VLDL-C, VLDL-apoB and the VLDL-C/triglyceride ratio positively correlated with Km: r = 0.52, p < 0.01; r = 0.52, p < 0.03; r = 0.69, p < 0.001 respectively. No correlation was found between the VLDL-triglyceride/protein or the VLDL-triglyceride/apoB ratios and Km (r = -0.20, and -0.06 respectively, p = not significant). Of the subjects' anthropometric characteristics, only the waist/hip ratio significantly correlated with Km: r = 0.63, p < 0.01. CONCLUSIONS: In the present study, we investigated the substrate function of VLDL particles in vitro. Enzyme affinity seems to be associated with cholesterol-triglyceride content or the number of VLDL particles rather than particle size. It may be expected that VLDL with a low cholesterol/triglyceride ratio will be efficiently lypolised by LPL, thus leading to the formation of a smaller particle with atherogenic potential.     

Mechanisms of hepatic very low-density lipoprotein overproduction in insulin resistance

An important complication of insulin-resistant states, such as obesity and type 2 diabetes, is an atherogenic dyslipidemia profile characterized by hypertriglyceridemia, low plasma high-density lipoproteins (HDL) cholesterol and a small, dense low-density lipoprotein (LDL) particle profile. The physiological basis of this metabolic dyslipidemia appears to be hepatic overproduction of apoB-containing very low-density lipoprotein (VLDL) particles. This has focused attention on the mechanisms that regulate VLDL secretion in insulin-resistant states. Recent studies in animal models of insulin resistance, particularly the fructose-fed hamster, have enhanced our understanding of these mechanisms, and certain key factors have recently been identified that play important roles in hepatic insulin resistance and dysregulation of the VLDL secretory process. This review focuses on these recent developments as well as on the hypothesis that an interaction between enhanced flux of free fatty acids from peripheral tissues to liver, chronic up-regulation of de novo lipogenesis by hyperinsulinemia and attenuated insulin signaling in the liver may be critical to the VLDL overproduction state observed in insulin resistance. It should be noted that the focus of this review is on molecular mechanisms of the hypertriglyceridemic state associated with insulin resistance and not that observed in association with insulin deficiency (e.g., in streptozotocin-treated animals), which appears to have a different etiology and is related to a catabolic defect rather than secretory overproduction of triglyceride-rich lipoproteins.     

极低密度脂蛋白受体与糖尿病及其并发症的关系

极低密度脂蛋白受体（VLDLR）是一种跨膜脂蛋白受体，它与糖尿病的脂代谢紊乱有着密切的联系。糖尿病及其并发症出现时VLDLR各亚型的表达变化及VLDLR基因治疗高脂血症的初见成效逐渐引起了人们对VLDLR的重视。     

Hypopituitarsim is associated with triglyceride enrichment of very low-density lipoprotein

The dyslipidemia associated with hypopituitarism may contribute to increased vascular mortality. The atherogenic potential of lipoproteins is determined not only by concentration but also by their composition. We therefore studied very low-density lipoprotein composition and apolipoprotein B kinetics in 16 hypopituitary subjects and 16 controls. Hypopituitarism was associated with reduced high-density lipoprotein cholesterol (0.98[0.82-1.18] vs. 1.35[1.15-1.41] mmol/liter, P < 0.001) and increased triglyceride concentrations (1.64[1.09-2.77] vs. 1.12[0.66-1.67] mmol/liter, P = 0.01). Total (P = 0.76) and low-density lipoprotein cholesterol (P = 0.56) concentrations were similar. Very low-density lipoprotein- triglyceride was significantly increased (1.48[1.02-2.55] vs. 0.9[0.31-2.30] mmol/liter, P = 0.004), but very low-density lipoprotein cholesterol levels were similar (P = 0.93). The molar ratios of very low-density lipoprotein-triglyceride:apolipoprotein B (6193[4283-9566] vs. 3599[3188-6854], P = 0.005) and very low-density lipoprotein-triglyceride:cholesterol (2.8[1.98-3.78] vs. 1.6[1.44-2.80], P < 0.003) were significantly increased; very low-density lipoprotein-cholesterol:apolipoprotein B molar ratios (P = 0.93) were similar. Very low-density lipoprotein apolipoprotein B fractional synthetic rate (a measure of apolipoprotein B catabolism, P = 0.42) and pool size (P = 0.63) were similar. The very low-density lipoprotein apolipoprotein B absolute synthetic rate (a measure of apolipoprotein B synthesis) tended to be higher in hypopituitarism (17.7[2.91-19.50] vs. 26.6[19.64-28.05] mg/kg per day, P = 0.24) but failed to reach statistical significance. The absolute synthetic rate, and hence very low-density lipoprotein production, correlated with very low-density lipoprotein triglyceride:apolipoprotein B ratio (P = 0.02, Rs = 0.63), suggesting that triglyceride enrichment of very low-density lipoprotein is important in the mechanism underlying very low-density lipoprotein overproduction in hypopituitarism. Because triglyceride-enriched lipoproteins are proatherogenic, this may contribute to the vascular mortality observed in hypopituitarism. The reasons for these observations are unknown; GH deficiency or routine endocrine replacement may be important.     

Endurance exercise training raises high-density lipoprotein cholesterol and lowers small low-density lipoprotein and very low-density lipoprotein independent of body fat phenotypes in older men and women

Endurance exercise training improves plasma lipoprotein and lipid profiles and reduces cardiovascular disease risk. However, the effect of endurance exercise training, independent of diet and body fat phenotypes, on plasma lipoprotein subfraction particle concentration, size, and composition as measured by nuclear magnetic resonance (NMR) spectroscopy is not known. We hypothesized that 24 weeks of endurance exercise training would independently improve plasma lipoprotein and lipid profiles as assessed by both conventional and novel NMR measurement techniques. One hundred sedentary, healthy 50- to 75-year-olds following a standardized diet were studied before and after 24 weeks of aerobic exercise training. Lipoprotein and lipid analyses, using both conventional and NMR measures, were performed at baseline and after 24 weeks of exercise training. Relative and absolute maximum oxygen consumption increased 15% with exercise training. Most lipoprotein and lipid measures improved with 24 weeks of endurance exercise training, and these changes were consistently independent of baseline body fat and body fat changes with training. For example, with exercise training, total cholesterol, triglycerides, and low-density lipoprotein cholesterol (LDL-C) decreased significantly (2.1+/-1.8 mg/dL, P=.001; -17+/-3.5 mg/dL, P<.0001; and -0.7+/-1.7 mg/dL, P<.0001, respectively), and high-density lipoprotein cholesterol subfractions (HDL3-C and HDL2-C) increased significantly (1.9+/-0.5 mg/dL, P=.01, and 1.2+/-0.3 mg/dL, P=.02, respectively). Particle concentrations decreased significantly for large and small very low-density lipoprotein particles (-0.7+/-0.4 nmol/L, P<.0001, and -1.1+/-1.7 nmol/L, P<.0001, respectively), total, medium, and very small LDL particles (-100+/-26 nmol/L, P=.01; -26+/-7.0 nmol/L, P=.004; and -103+/-27 nmol/L, P=.02, respectively), and small HDL particles (-0.03+/-0.4 micromol/L, P=.007). Mean very low-density lipoprotein particle size also decreased significantly (-1.7+/-0.9 nm, P<.0001) and mean HDL particle size increased significantly with exercise training (0.1+/-0.0 nm, P=.04). These results show that 24 weeks of endurance exercise training induced favorable changes in plasma lipoprotein and lipid profiles independent of diet and baseline or change in body fat.     

Effects of Acipimox on the metabolism of free fatty acids and very low density lipoprotein triglyceride

The mechanism of triglyceride lowering by Acipimox, a nicotinic acid analogue, was examined in a group of five moderately hypertriglyceridemic male rhesus monkeys. Two experiments were designed to examine the effect of the drug on lipid and glucose metabolism in nondiabetic, insulin-resistant animals. A single dose of Acipimox (8 mg/kg) given with a meal lowered the plasma free fatty acids (FFA) significantly at 4 h (0.102±0.008 vs 0.154±0.020 g/l; ±SEM;P<0.03); however, FFA concentrations returned to control levels at 6 h. Chronic administration of Acipimox (16 mg/kg q. i. d.) for 2 months produced a 31% reduction in triglyceride concentration (P<0.05) and a significant decrease in low density lipoprotein (LDL)-cholesterol (P<0.04), without changes in insulin action as measured by the hyperinsulinemic euglycemic clamp. Fasting FFA concentrations were not significantly altered by chronic treatment (0.163±0.013 versus 0.140±0.034 g/l). Fatty acid metabolic studies indicated increases in FFA transport (203.7±59.1 versus 136.1±26.6 μEq/min;P<0.05), while FFA fractional clearance rate (FCR) was unchanged. Very low density lipoprotein triglyceride (VLDL-Tg) metabolic experiments, using [³H]glycerol, showed increases in production and FCR with the drug. Increased VLDL-Tg clearance, in spite of increased production of VLDL, appears to be the mechanism by which triglycerides are lowered upon chronic Acipimox administration.     

Low density lipoprotein non-esterified fatty acids and lipoprotein lipase in diabetes

OBJECTIVES: Fatty acid metabolism is disturbed in poorly controlled diabetes. Low density lipoprotein (LDL) oxidation, thought to be an atherogenic modification, is partly dependent on LDL fatty acid content whether it be in the form of cholesteryl ester, phospholipids, triglyceride or non-esterified fatty acid (NEFA). Lipoprotein lipase (LPL) is deficient in diabetic patients. Lipoprotein lipase bound to LDL may facilitate cholesterol accumulation in the artery wall through the attachment of LDL to the proteoglycans expressed on endothelial cells and collagen. The purpose of this study was to examine the degree of binding of fatty acids and lipoprotein lipase to LDL in type 2 diabetic patients and to examine the relationship between non-esterified fatty acids attached to LDL and LDL oxidisability. SUBJECTS AND METHODS: Eight type 2 diabetic patients and eight control subjects were examined fasting and at 4 and 6h following a high fat meal. Six control subjects were examined fasting and 30 min after intravenous heparin. LDL was isolated by sequential ultracentrifugation. Individual LDL non-esterified fatty acids were measured by gas-liquid chromatography following transmethylation. LPL and oxidised LDL were measured by ELISA. RESULTS: The diabetic patients had HbA1c of 7.8 +/- 0.5% confirming moderate diabetic control. There was a large increase in the mean non-esterified fatty acids on LDL from diabetic subjects (0.66 +/- 0.40 mg/mg versus 0.06 +/- 0.02 mg/mg LDL protein, p < 0.01). Mean LDL cholesterol ester fatty acids were also significantly increased in the diabetic subjects (1.47 +/- 0.58 mg/mg versus 0.57 +/- 0.40 mg/mg LDL protein, p < 0.01). There was a significant increase in oxidised LDL (31.2 +/- 24 mg/mg versus 7.7 +/- 4.5 mg/mg LDL protein, p < 0.01) and a significant correlation between postprandial non-esterified fatty acid and LDL oxidation (r = 0.69, p < 0.05). LPL was significantly increased on the LDL but not in the plasma of diabetic subjects. Acute elevation in non-esterified fatty acids produced by heparin in control subjects did not increase LDL non-esterified fatty acids. CONCLUSIONS: This study demonstrates that the disturbance in fatty acid metabolism found in type 2 diabetic subjects is associated with a significant increase in non-esterified fatty acids attached to LDL. This may account, at least in part, for the increased oxidation of the LDL and therefore its atherogenicity. The finding of an increase in the amount of LPL bound to LDL suggests an important mechanism to facilitate the uptake of diabetic LDL by endothelial proteoglycans and collagen in the atherosclerotic plaque.     

Pharmacokinetic characteristics of the obese overfed rat

The present study was undertaken to examine the appropriateness of the obese overfed rat and the obese Zucker rat as animal models for evaluating drug disposition changes in human obesity. It was found that 11 of 12 characteristics that control or influence drug clearance and volume of drug distribution in obese humans were qualitatively reproduced in the obese overfed rat. In contrast, existing literature shows that the obese Zucker rat resembles the obese human in only five of 12 characteristics, with meaningful discrepancies in fat-free mass, creatinine clearance, and thyroid function. Perhaps of greatest significance were changes in hepatic cytochrome P-450, which increased in proportion to total body mass in the obese overfed rat but remain unchanged in the Zucker rat. Although P-450 status in human obesity is unknown, the overfed rat model provides an opportunity for examining increased oxidative drug elimination that appears as an established feature of human obesity. In conclusion, the obese overfed rat appears to be superior to the obese Zucker rat as an animal model for evaluating the pharmacological consequences of human obesity, particularly those in which reproducing drug pharmacokinetics is an important consideration.     

Clearance of coagulation factor VIII in very low-density lipoprotein receptor knockout mice

Low-density lipoprotein receptor-related protein (LRP) contributes to factor VIII (FVIII) catabolism in vivo. Besides LRP, FVIII also interacts with very low-density lipoprotein receptor (VLDLR) in vitro. We investigated the physiological role of VLDLR in FVIII catabolism, using knockout mouse models for VLDLR and LRP, alone and in combination. VLDLR(-/-) mice displayed normal plasma FVIII, whereas VLDLR(-/-) LRP(-) double-knockout mice had slightly increased FVIII compared with LRP-deficient mice. Remarkably, VLDLR deficiency slightly accelerated FVIII clearance. Adenovirus-mediated overexpression of VLDLR did not lower plasma FVIII in LRP-deficient mice. We conclude that VLDLR does not act in concert with LRP in FVIII clearance in vivo.     

Reduced oxidized low-density lipoprotein after weight reduction in obese premenopausal women.

BACKGROUND: Several studies support the hypothesis that oxidation of low-density lipoprotein (LDL) promotes atherogenesis. Obesity is one of the risk factors of atherosclerosis, but it is not known whether obesity is related to LDL oxidation. OBJECTIVE AND DESIGN: We investigated the effect of weight reduction and subsequent weight maintenance program on LDL oxidation in 77 obese premenopausal women (BMI 29-46 kg/m(2)). Another group of seven obese women served as a control group. Oxidized LDL was measured as baseline concentration of conjugated dienes in LDL lipids (ox-LDL). The weight reduction was performed in 12 weeks, using a very-low-energy diet. RESULTS: The mean weight loss was 13 kg (92 vs 79 kg). During weight reduction, the concentration of LDL cholesterol decreased by 11%, the concentration of ox-LDL decreased by 40%, and the ratio of ox-LDL to LDL by 33%. The concentration of LDL antioxidant capacity (LDL-TRAP) decreased by 8%, but the decrease was caused by the decrease in LDL. The concentration of LDL, ox-LDL or LDL-TRAP did not change in the control group. The weight reduction correlated with the decrease of ox-LDL. During the subsequent 9 month weight maintenance programme, the concentrations of serum LDL (10%), ox-LDL (11%), LDL-TRAP (29%), and the ratio of LDL-TRAP to LDL (21%) decreased. CONCLUSION: This study strengthens the evidence that the risk of atherogenesis is influenced favourably by weight reduction in obese women. This risk reduction is associated with a reduced oxidation of LDL.     

The very low density lipoprotein receptor A second lipoprotein receptor that may mediate uptake of fatty acids into muscle and fat cells

The isolation of a cDNA highly homologous to that of the low-density lipoprotein (LDL) receptor revealed the presence of a lipoprotein receptor that specifically binds apolipoprotein-E-containing lipoproteins, including very low density lipoprotein (VLDL), intermediate-density lipoprotein, and β-migrating VLDL. This new receptor, designated VLDL receptor, consists of five domains that resemble those of the LDL receptor. The VLDL receptor mRNA is abundant in tissues performing active fatty acid metabolism, suggesting that the receptor may be responsible for the uptake of fatty acids in triglyceride-rich lipoproteins into muscle and fat cells.